Category: Geochemistry, mineralogy, petrology and volcanology

Changing conditions of metamorphism since the Archaean

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Metamorphic petrologists have known since their branch of geology emerged that the intensity or ‘grade’ of metamorphism varies with position in an orogenic belt. This is easily visualised by the sequence mudstone-shale-slate-phyllite-schist-gneiss that results from a clay-rich starting material as metamorphic grade increases. Very roughly speaking, the sequence reflects burial, heat and pressure, and must have been controlled by temperature increasing with depth and pressure: the geothermal gradient. In turn, that depends on internal heat production, geothermal heat flow and the way in which heat is transferred through the deep crust: by thermal conduction or mechanical convection. A particular rock composition gives rise to different metamorphic mineral assemblages under different temperature and pressure conditions.

George Barrow was the first to recognise this in the Southern Highlands of Scotland as a series of zones marked by different index minerals. For instance, in once clay-rich sediments he recognised a succession of new minerals in the sequence chlorite; biotite; garnet; staurolite; kyanite; sillimanite in rocks of progressively higher metamorphic grade. Barrow found that once basaltic lavas interleaved with the sediments displayed zones with different characteristic minerals. Other metamorphic terrains, however, revealed different index minerals. Experimental mineralogy eventually showed that Barrow’s zones and others reflected a wide range of chemical reactions between minerals that reach equilibrium over different combinations of pressure and temperature. This enabled geologists to distinguish between metamorphism that had occurred under conditions of high-pressure and low-temperature, low-P and high-T and intermediate conditions (see diagram). This suggested that metamorphic rocks can form in areas with different heat flow and geothermal gradients. Geochemical means of assessing the actual temperatures and pressures at which particular rocks had reached mineralogical equilibrium, known as ‘thermobarometry’, now enable such variations to be assessed quantitatively.

The latest division in pressure-temperature space of different styles of metamorphism (colours) and the main mineral equilibria (dashed lines) that define them

It has long been suspected that the average T/P conditions revealed by metamorphic rocks have varied over geological time, as well as from place to place at any one time. A recent paper has analysed thermobarometric data from the earliest Archaean to recent times (Brown, M. et al. 2020, Evolution of geodynamics since the Archean: Significant change at the dawn of the Phanerozoic: Geology, v. 48, p. 488–492; DOI: 10.1130/G47417.1) They conclude that from the Archaean to the start of the Neoproterozoic the average P/T ratio was more than twice as high as it was in the following billion years. At about 2 Ga they suggests a relatively sudden decrease that correlates with what they regard as the first major assembly of continental crust: the Columbia (Nuna) supercontinent. The Mesoproterozoic Era, occupied by the disassembly of Columbia and the eventual creation of the Rodinia supercontinent, retained a high mean T/P. That began to decline with the break-up of Rodinia and a succession of tectonic cycles of ocean opening and closing during the Neoproterozoic and the Phanerozoic. This phase of truly modern plate textonics saw first the assembly of Gondwana and then the all-encompassing Pangaea, followed by its break up as we witness today. There are other correlations with the T/P variations, but they need not detain us.

The raw metamorphic data (564 points spanning 3.5 Ga) are by no means evenly spaced in time, and four dense clusters of points show a very wide spread of T/P – up to 2 orders of magnitude. Yet the authors have used locally weighted scat­terplot smoothing (LOWESS) to reduce this to a smoothed curve with a zone of uncertainty that is a great deal narrower than the actual spread of data. Frankly, I do not believe the impression of systematic change that this approach has produced, though I am not a statistician. To a lesser extent than me, it seems that neither does Peter Cawood, who comments on the paper in the same issue of Geology: more clearly than do the authors themselves.

Peter Cawood’s ‘take’ on the relationship between tectonic development and other important variables in the Earth-system with the estimate by Brown et al. of the mean metamorphic T/P (‘thermobaric’) variation through Earth history

Cawood’s view is that it was all due to a steady fall in mantle temperature and related broad changes in tectonic processes. But metamorphic rocks form in only the outermost 100 km of the Earth. The post-800 Ma examples include a much greater proportion of those formed under high- and ultrahigh pressures – blueschists and various kinds of eclogite – than do the earlier metamorphic belts. This weights the post-800 Ma record to lower mean T/P. Such rocks form in subduction zones and their high density might seem to doom them to complete resorption into the deep mantle. Yet large chunks now end up embedded in continents, interleaved with less extreme materials. Cawood suggests, as do others, that cooling of the mantle has enabled deeper break-off of subducted slabs to meet their end at the core-mantle boundary. The retained low T/P lithosphere since 800 Ma may have been sliced into the continents by increased underthrusting during continent-continent collisions that dominate the more modern orogenic-metamorphic belts.

See also:  Cawood, P.A. 2020 Earth Matters: A tempo to our planet’s evolution: Geology, v. 48, p. 525–526; DOI: 10.1130/focus052020.1

Volcanism and the Justinian Plague

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Between 541 and 543 CE, during the reign of the Roman Emperor Justinian, bubonic plague spread through countries bordering the Mediterranean Sea. This was a decade after Justinian’s forces had had begun to restore the Roman Empire’s lost territory in North Africa, Spain, Italy and the present-day Balkans by expeditions out of Byzantium (the Eastern Empire). At its height, the Plague of Justinian, was killing 5000 people each day in Constantinople, eventually to consume 20 to 40% of its population and between 25 to 50 million people across the empire. Like the European Black Death of the middle 14th century. The bacterium Yersinia pestis originated in Central Asia and is carried in the gut of fleas that live on rats. The ‘traditional’ explanation of both plagues was that plague spread westwards along the Silk Road and then with black rats that infested ship-borne grain cargoes. Plausible as that might seem, Yersinia pestis, fleas and rats have always existed and remain present to this day. Trade along the same routes continued unbroken for more than two millennia. Although plagues with the same agents recurred regularly, only the Plague of Justinian and the Black Death resulted in tens of million deaths over short periods. Some other factor seems likely to have boosted fatalities to such levels.

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Monk administering the last rites to victims of the Plague of Justinian

Five years before plague struck the Byzantine historian Procopius recorded a long period of fog and haze that continually reduced sunlight; typical features of volcanic aerosol veils. Following this was the coldest decade in the past 2300 years, as recorded by tree-ring studies. It coincides with documentary evidence of famine in China, Ireland, the Middle East and Scandinavia.. A 72 m long ice core extracted from the Colle Gnifetti glacier in the Swiss Alps in 2013 records the last two millennia of local climatic change and global atmospheric dust levels. Sampled by laser slicing, the core has yielded a time series of data at a resolution of months or better. In 536 an Icelandic volcano emitted ash and probably sulfur dioxide over 18 months during which summer temperature fell by about 2°C. A second eruption followed in 540 to 541. ‘Volcanic winter’ conditions lasted from 536 to 545, amplifying the evidence from tree-ring data from the 1990’s.

The Plague of Justinian coincided with the second ‘volcanic winter’ after several years of regional famine. This scenario is paralleled by the better documented Great Famine of 1315-17 that ended the two centuries of economic prosperity during the 11th to 13th centuries. The period was marked by extreme levels of crime, disease, mass death, and even cannibalism and infanticide. In a population weakened through malnutrition to an extent that we can barely imagine in modern Europe, any pandemic disease would have resulted in the most affected dying in millions. Another parallel with the Plague of Justinian is that it followed the ending of four centuries of the Medieval Warm Period, during which vast quantities of land were successfully brought under the plough and the European population had tripled. That ended with a succession of major, sulfur-rich volcanic eruption in Indonesia at the end of the 13th century that heralded the Little Ice Age. Although geologists generally concern themselves with the social and economic consequences of a volcano’s lava and ash in its immediate vicinity– the ‘Pompeii view’ – its potential for global catastrophe is far greater in the case of really large (and often remote) events.

Chemical data from the same ice core reveals the broad economic consequences of the mid-sixth century plague. Lead concentrations in the ice, deposited as airborne pollution from smelting of lead sulfide ore to obtain silver bullion, fell and remained at low levels for a century. The recovery of silver production for coinage is marked by a spike in glacial lead concentration in 640; another parallel with the Black Death, which was followed by a collapse in silver production, albeit only for 4 to 5 years.

Related article: Gibbons, A. 2018. Why 536 was ‘the worst year to be alive’. Science, v. 362,p. 733-734; DOI:10.1126/science.aaw0632

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Oceanic hydrothermal vents and the origin of life

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A range of indirect evidence has been used to suggest that life originated deep in the oceans around hydrothermal vents, such as signs of early organic matter in association with Archaean pillow lavas. One particularly persuasive observation is that a number of proteins and other cell chemicals are constructed around metal sulfide groups. Such sulfides are common around hydrothermal ‘smokers’ associated with oceanic rift systems. Moreover, Fischer-Tropsch reactions between carbon monoxide and hydrogen produce quite complex hydrocarbon molecules under laboratory conditions. Such hydrogenation of a carbon-bearing gas requires a catalyst, a commonly used one being chromium oxide (see Abiotic formation of hydrocarbons by oceanic hydrothermal circulation May 2004). It also turns out that fluids emitted by sea-floor hydrothermal systems are sometimes rich in free hydrogen, formed by the breakdown of olivine in ultramafic rocks to form hydroxylated minerals such as serpentine and talc. The fact that chromium is abundant in ultramafic rocks, in the form of its oxide chromite, elevates the possibility that Fischer-Tropsch reactions may have been a crucial part of the life-forming process on the early Earth. What is needed is evidence that such reactions do occur in natural settings.

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A white carbonate mound forming at the Lost City hydrothermal vent field on the Mid-Atlantic Ridge (Credit: Baross 2018)

One site on the mid-Atlantic ridge spreading centre, the Lost City vent field, operates because of serpentinisation of peridotites exposed on the ocean floor, to form carbonate-rich plumes and rocky towers; ‘white smokers’. So that is an obvious place to test the abiotic theory for the origin of life. Past analyses of the vents have yielded a whole range of organic molecules, including alkanes, formates, acetates and pyruvates, that are possible precursors for such a natural process. Revisiting Lost City with advanced analytical techniques has taken the quest a major step forward (Ménez, B. et al. 2018. Abiotic synthesis of amino acids in the recesses of the oceanic lithosphere. Nature, advance online publication; DOI: 10.1038/s41586-018-0684-z). The researchers from France and Kazakhstan focused on rock drilled from 170 m below the vent system, probably beyond the influence of surface contamination from living organisms. Using several methods they detected the nitrogen-containing amino acid tryptophan, and that alone. Had they detected other amino acids their exciting result would have been severely tempered by the possibility of surface organic contamination. The formation of tryptophan implies that its abiotic formation had to involve the reduction of elemental nitrogen (N2) to ammonia (NH3). Bénédicte Ménez and colleagues suggest that the iron-rich clay saponite, which is a common product of serpentine alteration at low temperatures, may have catalysed such reduction and amino-acid synthesis through Friedel–Crafts reactions. Fascinating as this discovery may be, it is just a step towards confirming life’s abiogenesis. It also permits speculation that similar evidence may be found elsewhere in the Solar System on rocky bodies, such as the moons Enceladus and Europa that orbit Saturn and Jupiter respectively. That is, if the rock base of hydrothermal systems thought to occur there can be reached.

Related article: Baross, J.A. 2018. The rocky road to biomolecules. Nature, v. 564, p. 42-43; DOI: 10.1038/d41586-018-07262-8.

Hot-spot track beneath the Greenland ice cap

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Around 63 Ma ago, during the Palaeocene Epoch, major igneous activity broke out in what are now both sides of the North Atlantic Ocean. After initial sputtering it culminated massively between 57 and 53 Ma. Relics are to be seen in Baffin Island, West and East Greenland, the Faeroes and north-western parts of the British Islands, in the form of flood basalts, dyke swarms and scattered remnants of central volcanoes. Offshore drilling on the North Atlantic’s continental shelves suggests that the volcanism extended over 1.3 million km2 and blurted out around 6.6 million km3 of magma. Not for nothing have the products of this event been categorised as a Large Igneous Province. Its formation took place before the North Atlantic existed. It began to form as this precursor magmatic paroxysm waned.  Continued basaltic magma production created the ocean floor each side of the mid-Atlantic Ridge system to divide North America and Greenland from northern Europe. Sea floor spreading continues, rising above sea level in Iceland, which is underlain by a large mantle plume.

The plume beneath Iceland may have been present at a fixed position in the mantle for tens of million years. A hot spot over which plate movements have shifted lithosphere to be heated in a similar way to a sheet of paper dragged slowly over a candle flame. The Iceland plume may have left a hot-spot track similar to that involved in the Hawaiian island chain. The ocean floor to the east and west of Iceland is shallower and forms broad rides at right angles to the trend of the Mid-Atlantic Ridge system, judged to be such tracks that are still warm and buoyant after formation over the plume. But are there traces of earlier passage of drifting lithosphere over the plume. A way to detect older hot-spot tracks is through variations in geothermal heat flow through the continental surface, a linear pattern raising suspicions of such trace of passage. There is no sign to the east beneath Europe, so what about to the west. Greenland, being mainly blanketed in ice, is not a good place to conduct such a search as it would involve deep drilling through the ice at huge cost for each hole. But there is a roundabout way of obtaining geothermal information without even setting foot on Greenland’s icy wastes.

The geomagnetic field measured at the surface records anomalies in rock magnetisation in the solid Earth beneath. Near-surface variations due to large variations in rock types that comprise the continental crust appear as sharp, high frequency signals. Aeromagnetic surveys over Greenland are characterised by such noisy patterns because the subsurface geology is extremely complicated. However, the underlying upper mantle beneath all continents is geologically quite bland, but being uniformly rich in iron it contains a high proportion of magnetic minerals such as magnetite (Fe3O4). The upper mantle should therefore leave a signal in the surface geomagnetic field, albeit a commensurately bland one. Like radio signals that span a large range of wavelengths, Earth properties that vary spatially, such as the geomagnetic field, may be analysed using filters. Once the high-frequency geomagnetic features of the crust are filtered out what should remain is a signal that reflects the magnetic structure of the upper mantle. It should be more or less featureless, yet beneath Greenland it isn’t.

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Estimated Curie depth variation below Greenland (left) converted to geothermal heat flow variation (right). (Credit: Martos et al. 2018; Figures 1b and 1c)

Magnetic anomalies are created by magnetisation induced in magnetic minerals in rocks by the Earth’s magnetic field. Yet minerals lose their ability to be magnetised at temperatures above a threshold known as the Curie point, which is 580 °C for magnetite, the most abundant magnetic mineral. Depending on the geothermal heat flow the Curie point is exceeded at some depth in the lithosphere. So magnetic anomalies can safely be assumed to be produced only by rocks above the so-called Curie depth. Yasmina Martos of the British Antarctic Survey (now at the University of Maryland) and scientists from Britain, the US and Spain used a complex procedure, including gravity data and a few direct measurements of heat flow below Greenland as well as filtered aeromagnetic data, to estimate the variation in Curie depth beneath the ice cap. (Martos, Y.M. et al. 2018. Geothermal heat flux reveals the Iceland hotspot track underneath Greenland. Geophysical Research Letters, v. 45, online publication; doi: 10.1029/2018GL078289). Using that as an inverse proxy for heat flow they were able to map the likely geothermal variation beneath the island. Rather than a random and narrow variation in depth, as would be expected for roughly uniform heat flow, the Curie depth varied in a non-random way by over 20 km, equivalent to roughly 20 mW m-2.

The shallowest Curie depth and highest estimated heat flow occurs in East Greenland around Scoresby Sund where the largest sequence of Palaeocene flood basalts occur. It is also on a line perpendicular to the mid-Atlantic Rift system that meets the active Iceland plume. Running north-west from Scoresby Sund is a zone of locally high estimated heat flow. Martos et al. suggest that this is the track of Greenland’s motion over the Iceland hot spot from about 80 Ma to the period of maximum on-shore volcanism and the start of sea-floor spreading at around 50 Ma.

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Volcano heading for the sea

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John Murray of The Open University, UK has been studying Europe’s largest active volcano Mount Etna on Sicily for most of his career. With a group of colleagues he installed high-precision GPS receivers at over 100 stations on the flanks of the mountain. This was to monitor any shifts in elevation and geographic position, which might be related to magmatic events within the volcano, such as inflation and contraction of the magma chamber. Measurements of position gathered annually since 2001 reveal a somewhat alarming picture (Murray, J.B. et al. 2018. Gravitational sliding of the Mt. Etna massif along a sloping basement. Bulletin of Volcanology, v. 80 online, open access; doi /10.1007/s00445-018-1209-1). The edifice is moving relentlessly ESE at 14 mm yr-1, on average, towards the Mediterranean Sea. Research by one of Murray’s co-authors, Benjamin van Wyk de Vries of the Université Clermont Auvergne, established that many volcanoes have associated signs of deformation due to their huge masses. Often, this is a matter of radial spreading that produces thrust-like faults at their base and in the basement on which they grew. In the case of Etna all the annual displacements on its flanks are skewed to the ESE. The researchers are able to show that this is not a case of flank instability that ultimately may result in lateral collapse but the entire volcano is slowly slipping sideways.

English: Mount Etna, Sicily, topped in snow It...
Mount Etna, Sicily, topped in snow (credit: Wikipedia)

An experimental mock up of the volcano– a cone and flanking layers of lava and pyroclastic rocks made of sand on a substrate of putty to represent underlying sedimentary strata – began to slide once it was tilted at a shallow angle. This suggests that the base of the volcano and igneous debris that it has emitted dips gently to the ESE. The underlying materials are poorly consolidated Quaternary sediments, which are likely to be rheologically weak. Geophysics shows that the NW side of the volcano rests on an almost horizontal plateau, the cone itself being above a spoon-like depression, probably produced by the cone’s mass, and the base dips seawards  in the SE sector. It is through this basement that magma makes its way to Etna’s summit vent system, probably along fractures.

The authors warn that such sliding volcanoes are prone to devastating sector collapse on the downslope side, although there are no signs that might be imminent. Yet it will almost certainly have an effect on eruptive activity as the magma conduits are continually changing. Future research needs to focus on periods when there is horizontal contraction on the volcano, as happens during lengthy periods of dormancy – the period for which there are data has been one of expansion.

Volcanism and sea level fall

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Most volcanic activity stems from the rise of hot, deep rock, usually within the mantle. Pressure suppresses partial melting, so as hot rock rises the greater the chance that it will begin to melt without any rise in its temperature. That is the reason why mantle plumes are associated with many volcanic centres within plates. Extension at oceanic ridges allows upper mantle to rise in linear belts below rift systems giving rise to shallow partial melting, mid-ocean ridge basalts and sea-floor spreading. These aren’t the only processes that can reduce pressure to induce such decompression melting; any means of uplift will do, provided the rate of uplift exceeds the rate of cooling at depth. As well as tectonic uplift and erosion, melting of thick ice sheets and major falls in sea level may result in unloading of the lithosphere.

During Messinian Stage of the late Miocene up to 3 km of evaporitic salt was deposited in the deepest parts of the Mediterranean Basin. One mechanism might have been faster evaporation of seawater than its resupply from the Atlantic through the Straits of Gibraltar, similar to the way in which salts is deposited below the Dead Sea. But the salt layer beneath the modern Mediterranean Sea bed has interleaved riverine sediments containing fossils of land plants. The Straits had closed and the Mediterranean Sea evaporated away. From about 6 to 5.3 Ma ago sea level fell by 3 to 5 km, only returning to normal when the Straits reopened to launch the huge Zanclean flood, with which the Pliocene of southern Europe and North Africa commenced. A team from the Universities of Geneva, Orleans and Paris and the Instituto de Ciencias de la Tierra Jaume Almera in Barcelona has tested the hypothesis that the Messinian Crisis affected volcanic activity in the area (Sternai, P. et al. 2017. Magmatic pulse driven by sea-level changes associated with the Messinian salinity crisis. Nature Geoscience, v. 10 online; doi:10.1038/ngeo3032).

From the record of salt precipitation, Pietro Sternai and colleagues, reckon that the main phase of unloading of the Mediterranean Basin began at around 5.6 Ma. Allowing for loading by the thick evaporites they calculated that the effect of the loss of water mass was equivalent to an unloading of 15 MPa in the deeper Eastern Mediterranean and 10 MPa in the west. Using standard pressure-temperature melting curves for the upper mantle, they then estimated that any magma chambers affected by the decrease in pressure could yield up to 17% more melt. Radiometrically dated lavas and igneous dykes within the Mediterranean region became more frequent and the number of events more than doubled during the time of main salt deposition.

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Stromboli, one of the most active volcanoes in the Mediterranean Basin (credit: Wikipedia)

In May 2017 a study of subglacial volcanoes in West Antarctica based on radar mapping of the solid surface identified 138, 91 of them previously unknown (van Wyk de Vries et al. 2017. A new volcanic province: an inventory of subglacial volcanoes in West Antarctica.  Geological Society, London, Special Publication 461) They lie within a buried rift system and are covered by thick ice. Only one volcano in Antarctica is known to be active, Erebus, which is part of the cluster. Most of the news items stemming from the publication mentioned the possibility that the buried volcanic tract could be adding to the instability of the West Antarctic Ice Sheet through heating up its base. The WAIS is the ice sheet most feared to collapse seawards leading to a rise of about 3 m in global sea level. If the 2 km thick WAIS did slide off its underlying crust it might possibly trigger reactivation of the volcanic cluster.

Wildfires and climate at the K-Pg boundary

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It is now certain that the Cretaceous-Palaeogene boundary 66 Ma ago coincided with the impact of a ~10 km diameter asteroid that produced the infamous Chicxulub crater north of Mexico’s Yucatán peninsula. Whether or not this was the trigger for the mass extinction of marine and terrestrial fauna and flora – the flood basalts of the Deccan Traps are still very much in the frame – the worldwide ejecta layer from Chicxulub coincides exactly with the boundary that separates the Mesozoic and Cenozoic Eras. As well as shocked quartz grains, anomalously high iridium concentrations and glass spherules the boundary layer contains abundant elemental carbon, which has been widely ascribed to soot released by vegetation that went up in flames on a massive scale. Atmospheric oxygen levels in the late Cretaceous were a little lower than those at present, or so recent estimates from carbon isotopes in Mesozoic to Recent ambers suggest (Tappert, R. et al. 2013. Stable carbon isotopes of C3 plant resins and ambers record changes in atmospheric oxygen since the Triassic. Geochimica et Cosmochimica Acta, v. 121, p. 240-262,) – other estimates put the level substantially above that in modern air. Whatever, global wildfires occurred within the time taken for the Chicxulub ejecta to settle from the atmosphere; probably a few years. It has been estimated that about 700 billion tonnes of soot were laid down, suggesting that most of the Cretaceous terrestrial biomass and even a high proportion of that in soils literally went up in smoke.

Charles Bardeen and colleagues at the University of Colorado, Boulder, have modelled the climatic and chemical effects of this aspect of the catastrophe (Bardeen, C.G. et al. 2017. On transient climate change at the Cretaceous−Paleogene boundary due to atmospheric soot injections. Proceedings of the National Academy of Sciences; doi:10.1073/pnas.1708980114). Despite the associated release of massive amounts of CO2 and water vapour by both the burning and the impact into seawater, giving increased impetus to the greenhouse effect, the study suggests that fine-grained soot would have lingered as an all enveloping pall in the stratosphere. Sunlight would have been blocked for over a year so that no photosynthesis would have been possible on land or in the upper ocean, the temperatures of the continent and ocean surfaces would have dropped by as much as 28 and 11 °C respectively to cause freezing temperatures at mid-latitudes. Moreover, absorption of solar radiation by the stratospheric soot layer would have increased the temperature of the upper atmosphere by several hundred degrees to destroy the ozone layer. Consequently, once the soot cleared the surface would have had a high ultraviolet irradiation for around a year.

The main implication of the modelling is a collapse in both green terrestrial vegetation and oceanic phytoplankton; most of the food chain would have been absent for long enough to wipe out those animals that depended on it entirely. While an enhanced greenhouse effect and increased acidification of the upper ocean through CO2 emissions by the Deccan flood volcanism would have placed gradually increasing and perhaps episodic stresses on the biosphere, the outcome of the Chicxulub impact would have been immediate and terrible.

More on mass extinctions and impacts here and here

The late-Ordovician mass extinction: volcanic connections

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The dominant feature of Phanerozoic stratigraphy is surely the way that many of the formally named major time boundaries in the Stratigraphic Column coincide with sudden shifts in the abundance and diversity of fossil organisms. That is hardly surprising since all the globally recognised boundaries between Eras, Periods and lesser divisions in relative time were, and remain, based on palaeontology. Two boundaries between Eras – the Palaeozoic-Mesozoic (Permian-Triassic) at 252 Ma and Mesozoic-Cenozoic (Cretaceous-Palaeogene) at 66 Ma – and a boundary between Periods – Triassic-Jurassic at 201 Ma – coincide with enormous declines in biological diversity. They are defined by mass extinctions involving the loss of up to 95 % of all species living immediately before the events. Two other extinction events that match up to such awesome statistics do not define commensurately important stratigraphic boundaries. The Frasnian Stage of the late-Devonian closed at 372 Ma with a prolonged series of extinctions (~20 Ma) that eliminated  at least 70% of all species that were alive before it happened. The last 10 Ma of the Ordovician period witnessed two extinction events that snuffed out about the same number of species. The Cambrian Period is marked by 3 separate events that in percentage terms look even more extreme than those at the end of the Ordovician, but there are a great many less genera known from Cambrian times than formed fossils during the Ordovician.

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Faunal extinctions during the Phanerozoic in relation to the Stratigraphic Column.

Empirical coincidences between the precise timing of several mass extinctions with that of large igneous events – mainly flood basalts – suggest a repeated volcanic connection with deterioration of conditions for life. That is the case for four of the Famous Five, the end-Ordovician die-off having been ascribed to other causes; global cooling that resulted in south-polar glaciation of the Gondwana supercontinent and/or an extra-solar gamma-ray burst (predicated on the preferential extinction of Ordovician near-surface, planktonic fauna such as some trilobite families). Neither explanation is entirely satisfactory, but new evidence has emerged that may support a volcanic trigger (Jones, D.S. et al. 2017. A volcanic trigger for the Late Ordovician mass extinction? Mercury data from south China and Laurentia. Geology, v. 45, p. 631-634; doi:10.1130/G38940.1). David Jones and his US-Japan colleagues base their hypothesis on several very strong mercury concentrations in thin sequences in the western US and southern China of late Ordovician marine sediments that precede, but do not exactly coincide with, extinction pulses. They ascribe these to large igneous events that had global effects, on the basis of similar Hg anomalies associated with extinction-related LIPs. Yet no such volcanic provinces have been recorded from that time-range of the Ordovician, although rift-related volcanism of roughly that age has been reported from Korea. That does not rule out the possibility as LIPs, such as the Ontong Java Plateau, are known from parts of the modern ocean floor that formed in the Mesozoic and Cenozoic. Ordovician ocean floor was subducted long ago.

The earlier Hg pulses coincide with evidence for late Ordovician glaciations over what is now Africa and eastern South America. The authors suggest that massive volcanism may then have increased the Earth’s albedo by blasting sulfates into the stratosphere. A similar effect may have resulted from chemical weathering of widely exposed flood basalts which draws down atmospheric CO2. The later pulses coincide with the end of Gondwanan glaciation, which may signify massive emanation of volcanic CO2 into the atmosphere and global warming. Despite being somewhat speculative, in the absence of evidence, a common link between the Big Five plus several other major extinctions and LIP volcanism would quieten their popular association with major asteroid and/or comet impacts currently being reinvigorated by drilling results from the K-Pg Chicxulub crater offshore of Mexico’s Yucatan Peninsula.

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Steam-bath Earth

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The Earth’s mantle probably contained a significant amount of water from the start. Its earliest history was one of intense bombardment, including the impact that formed the Moon. Together with the conversion of gravitational potential energy to heat while the core was settling out from the mantle, impacts would have kept its overall temperature high enough to prevent water vapour from condensing on the surface until such heat input ceased and heat loss by radiation allowed the surface rapidly to cool. The atmosphere would have been rich in water vapour. Evidence from zircons that are the earliest tangible materials yet recovered hint at the formation of Zr-rich magmas – probably granitic in the broad sense – about 100 Ma after the Moon-forming event (see EPN July 2001: Zircons’ window on the Hadean). Yet no trace of substantial granitic rocks that old have ever been found.

Don Baker and Kassandra Sofonio of McGill University in Montreal, Canada have considered processes other than partial melting or fractional crystallisation that may have been possible during the earliest Hadean. In particular they have looked at one thought once to be a contender in the genesis of granite and latterly sidelined (Baker, D.R. & Sofonio, K. 2017. A metasomatic mechanism for the formation of Earth’s earliest evolved crust. Earth and Planetary Science Letters, v. 463, p. 48-55; http://dx.doi.org/10.1016/j.epsl.2017.01.022 ). They heated powdered artificial samples that chemically resembled the Earth’s original silicate mantle in sealed double capsules – an inner part containing the silicate powder and an outer one containing water. The capsules were held at around 727°C for a time and then quenched. The outer part of each capsule was found to be a glass of roughly granite composition. The experimental design ensured that superheated water diffused across the inner-outer capsule wall. So the ‘granite’ must have formed by a metasomatic process – essentially preferential solution of its component elements in supercritical water – the experimental temperature being insufficient to partially melt the ultramafic charge in the inner capsule.

Baker and Sofonio conclude that degassing of this metasomatic fluid – silicate-rich ‘steam’ – may have produced substantial masses of sialic crust on the Earth’s surface. Removal of material produced in such a manner would also have extracted trace elements with an affinity for granite from the early mantle – so-called incompatible elements. The subsequent recycling of such granitic blobs back into the mantle may explain geochemical signs in >500 Ma younger Archaean crust – produced by ‘normal’ igneous processes – of incompatible-element enriched reservoirs in the Early mantle.

Archaean continents derived from Hadean oceanic crust

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As DNA is to tracing  human evolution and migration, so various isotope systems are to the evolution of the Earth. One of the most fruitful is the samarium-neodymium (Sm-Nd) system. The decay of 147Sm to 143Nd is used in dating rocks across the full range of Earth history, given coeval rocks with a suitable range of Sm/Nd ratios, because the decay has a long half life (1.06 x 1011 years). However, samarium has another radioactive isotope 147Sm with a half life that is a thousand times shorter (1.06 x 108 years). So it remains only as a minute proportion of the total Sm in rocks, most having decayed since it was formed in a pre-Solar System supernova. But its daughter isotope 142Nd is present in easily measurable quantities, having accumulated from 147Sm decay over the first few hundred million years of Earth’s history; i.e. during the Hadean and earliest Archaean Eons. It is this fact that allows geochemists to get an indirect ‘handle’ on events that took place in the Earth’s earliest, largely vanished history. The principle behind this approach is that when an ancient rock undergoes partial melting to produce a younger magma the rock that crystallizes from it inherits the relative proportions of Nd isotopes of its source and thereby carries a record of the earlier history.

English: An outcrop of metamorphosed volcanose...
Metamorphosed volcanosedimentary rocks from the Porpoise Cove locality, Nuvvuagittuq supracrustal belt, Canada. Possibly the oldest rocks on Earth. (credit: Wikipedia)

The eastern shore of Hudson Bay in Canada hosts the oldest tangible geology known, in form of some metamorphosed basaltic rocks dated at 4200 Ma old known as the Nuvvuagittuq Greenstone Belt – the only known Hadean rocks. They occur in a tiny (20 km2) patch associated with gneisses of tonalite-trondjhemits-granodiorite composition that are dated between 3760 and 3350 Ma. Engulfing both are younger (2800 to 2500 Ma) Archaean plutonic igneous rocks of felsic composition. Jonathan O’Neil and Richard Carlson of the University of Ottawa, Canada and the Carnegie Institution for Science, Washington DC, USA respectively, measured proportions of Nd isotopes in both sets of felsic igneous rocks (O’Neil, J. & Carlson, R.W. 2017. Building Archean cratons from Hadean mafic crust. Science, v. 355, p. 1199-1202; doi:10.1126/science.aah3823).

The oldest gneisses contained relative proportions of 142Nd commensurate with them having been formed by partial melting of the Hadean mafic rocks about a few hundred million years after they had been erupted to form the oldest known crust; no surprise there. However, the dominant components of the local continental crust that are about a billion years younger also contain about the same relative proportions of 142Nd. A reasonable conclusion is that the Archaean continental crust of NE Canada formed by repeated melting of mafic crust of Hadean age over a period of 1.5 billion years. The modern Earth continually replenishes its oceanic crust over about 200 Ma due to plate tectonics. During the Archaean mantle dynamics would have been driven faster by much higher internal heat production. Had this involved simply faster plate tectonics the outermost skin of mafic crust would have been resorbed into the mantle even faster. By the end of the Archaean (2500 Ma) barely any Hadean crust should have been available to produce felsic magmas. But clearly at least some did linger, adding more weight to the idea that plate tectonics did not operate during the Hadean and Archaean Eons. See Formation of continents without subduction below.

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Formation of continents without subduction

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The formulation of the theory of plate tectonics provided plausible explanations for the growth of continental crust over time, among many other fundamental Earth processes. Briefly expressed, once basalt capped oceanic lithosphere is forced downwards at plate boundaries where plates move towards one another, beyond a certain penetration cool, moist basalt undergoes a pressure-controlled change of state. Its chemical constituents reassemble into minerals more stable under elevated pressure. In doing so, one outcome involves dehydration reactions the other being that the bulk composition is recast mainly in the form of high-pressure pyroxene and the mineral garnet: the rock eclogite. The density of the basaltic cap increases above that of the mantle. Gravity acts to pull the subducting slab downwards, this slab-pull force being the main driver of plate motions globally. Water vapour and other fluids shed by dehydration reactions rise from the subducted slab into the wedge of overlying mantle to change its conditions of partial melting and the composition of the magma so produced. This is the source of arc magmatism that persists at the destructive plate margin to increase the volcanic pile’s thickness over time. When magma is able to pond at the base of the new crust its fractional crystallisation produces dense cumulates of high-temperature mafic silicates and residual melt that is both lighter and more enriched in silica. Residual magma rises to add to the middle and upper crust while the cumulate-rich lower crust becomes less gravitationally stable, eventually to spall downwards by delamination. Such a process helps to explain the bulk low density of continental crust built up over time together with the freeboard of continents relative to the ocean floor: a unique feature of the Earth compared with all other bodies in the Solar System. It also accounts for the vast bulk of continental crust having remained at the surface since it formed: it rarely gets subducted, if at all.

One suggested model for pre-plate tectonic continent formation (credit, Robert J Stern https://speakingofgeoscience.org/2013/04/28/when-did-plate-tectonics-begin-on-earth-and-what-came-before/)

Tangible signs that such subduction was taking place in the past – eclogites and other high-pressure, low-temperature metamorphosed basalts or blueschists – are only found after 800 Ma ago. Before that time evidence for plate tectonics is circumstantial. Some geologists have argued for a different style of subduction in earlier times, plates under riding others at low angles. Others have argued for a totally different style of tectonics in Earth early history, marked by changes in bulk chemical composition of the continental crust at the Archaean-Proterozoic boundary. A new twist comes from evidence in the Archaean Pilbara Craton of Western Australia (Johnson, T.E. et al. 2017. Earth’s first stable continents did not form by subduction. Nature, v. 543, p. 239-242; doi:10.1038/nature21383). The authors found that basalts dated at about 3.5 Ga have trace-element geochemistry with affinities to the primitive basalts of island arcs. That makes them a plausible source for slightly younger felsic plutonic rocks with a tonalite-trondhjemite-granodiorite (TTG) compositional range (characteristic of Archaean continental crust). If the basalts were partially melted to yield 30% of their mass as new magma the melt composition would match that of the TTG crust. This would be feasible at only 30 km depth given a temperature increase with depth of at least 25° C per kilometre; more than the average continent geothermal gradient today but quite plausible with the then higher heat production by less decayed radioactive isotopes of uranium, thorium and potassium 3.5 Ga ago. This would have required the basalts to have formed a 30 km thick crust. However, the basalts’ geochemistry requires their generation by partial melting of earlier more mafic basalts rather than directly from the mantle. That early Archaean mantle melting probably did generate vast amounts such primary magma is generally acknowledged and confirmed by the common occurrence of komatiitic lavas with much higher magnesium content than common basalts of modern constructive margins. In essence, Johnson et al. favour thermal reworking of primitive Archaean crust, rather than reworking in a plate tectonic cycle.

More on continental growth and plate tectonics

See also

When did Plate Tectonics begin on Earth, and what came before?

A ‘recipe’ for Earth’s accretion, without water

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The Earth continues to collect meteorites, the vast majority of which are about as old as our planet; indeed many are slightly older. So it has long been thought that Earth originally formed by gravitational accretion when the parental bodies of meteorites were much more abundant and evenly distributed. Meteorites fall in several classes, metallic (irons) and several kinds that contain silicate minerals, some with a metallic component (stony irons) others without, some with blebs or chondrules of once molten material (chondrites) and others that do not (achondrites), and more subtle divisions among these general groups. In the latter half of the 20th century geochemists and cosmochemists became able to compare the chemical characteristics of different meteorite classes with that of the Sun –from its radiation spectrum – and those of different terrestrial rocks – from direct analysis. The relative proportions of elements in chondrites turned out to match those in the Sun – inherited from the gas nebula from which it formed – better than did other classes. The best match with this primitive composition turned out to be the chemistry of carbonaceous chondrites that contain volatile organic molecules and water as well as silicates and sulfides. The average chemistry of one sub-class of carbonaceous chondrites (C1) has been chosen as a ‘standard of standards’ against which the composition of terrestrial rocks are compared in order that they can be assessed in terms of their formative processes relative to one another. For a while carbonaceous chondrites were reckoned to have formed the bulk of the Earth through homogeneous accretion: that is until analyses became more precise at increasingly lower concentrations. This view has shifted …

Geochemistry is a complex business(!), bearing in mind that rocks that can be analysed today predominantly come from the tiny proportion of Earth that constitutes the crust. The igneous rocks at the centre of wrangling how the whole Earth has evolved formed through a host of processes in the mantle and deep crust, which have operated since the Earth formed as a chemical system. To work out the composition of the primary source of crustal igneous rocks, the mantle, involves complex back calculations and modelling. It turns out that there may be several different kinds of mantle. To make matters worse, those mantle processes have probably changed considerably from time to time. To work back to the original formative processes for the planet itself faces the more recent discovery that different meteorite classes formed in different ways, different distances from the Sun and at different times in the early evolution of the pre-Solar nebula. Thankfully, some generalities about chemical evolution and the origin of the Earth can be traced using different isotopes of a growing suite of elements. For instance, lead isotopes have revealed when the Moon formed from Earth by a giant impact, and tungsten isotopes narrow-down the period when the Earth first accreted. Incidentally, the latest ideas on accretion involve a series of ‘embryo’ planets between the Moon and Mars in size.

An example of an E-type Chondrite (from the Ab...
An example of an enstatite chondrite (from the Abee fall) in the Gallery of Minerals at the Royal Ontario Museum. (Photo credit: Wikipedia)

Calculating from a compendium of isotopic data from various types of meteorite and terrestrial materials, Nicolas Dauphas of the University of Chicago has convincingly returned attention to a model of heterogeneous accretion of protoplanetary materials from different regions of the pre-Solar nebula (Dauphas, N. 2017. The isotopic nature of the Earth’s accreting material through time. Nature, v. 541, p. 521-524; doi:10.1038/nature20830). His work suggests that the first 60% of Earth’s accretion involved materials that were a mixture of meteorite types, half being a type known as enstatite chondrites. These meteorites are dry and contain grains of metallic iron-nickel alloy and iron sulfides set in predominant MgSiO3 the pyroxene enstatite. The Earth’s remaining bulk accumulated almost purely from enstatite-chondrite material. A second paper in the same issue of Nature (Fischer-Gödde, M. & Kleine, T. 2017. Ruthenium isotopic evidence for an inner Solar System origin of the late veneer. Nature, v. 541, p. 525-527; doi:10.1038/nature21045) reinforces the notion that the final addition was purely enstatite chondrite.

This is likely to cause quite a stir: surface rocks are nothing like enstatite chondrite and nor are rocks brought up from the upper mantle by volcanic activity or whose composition has been back-calculated from that of surface lavas; and where did the Earth’s water at the surface and in the mantle come from? It is difficult to escape the implication of a mantle dominated by enstatite chondrite From Dauphas’s analysis, for lots of other evidence from Earth materials seem to rule it out. One ‘escape route’ is that the enstatite chondrites that survived planetary accretion, which only make up 2% of museum collections, have somehow been changed during later times.  The dryness of enstatite chondrites and the lack of evidence for a late veneer of ‘moist’ carbonaceous chondrite in these analyses cuts down the options for delivery of water, the most vital component of the bulk Earth and its surface.  Could moister meteorites have contributed to the first 60% of accretion, or was  post-accretion cometary delivery to the surface able to be mixed in to the deep mantle? Nature’s News & Views reviewer, Richard Carlson of the Carnegie Institution for Science in Washington DC, offers what may be a grim outlook for professional meteoriticists: that perhaps “the meteorites in our collection are not particularly good examples of Earth’s building blocks” (Carlson, R.W. 2017. Earth’s building blocks. Nature, v. 541, p. 468-470; doi:10.1038/541468a).

Animation of how the Solar System may have formed.

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Oceans of magma, Moon formation and Earth’s ‘Year Zero’

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That the Moon formed and Earth’s geochemistry was reset by our planet’s collision with another, now vanished world, has become pretty much part of the geoscientific canon. It was but one of some unimaginably catastrophic events that possibly characterised the early Solar System and those around other stars. Since the mantle geochemistry of the Earth’s precursor was fundamentally transformed to that which underpinned all later geological events, notwithstanding the formation of the protoEarth about 4.57 Ga ago, I now think of the Moon-forming event as our homeworld’s ‘Year Zero’. It was the ‘beginning’ of which James Hutton reckoned there was ‘no vestige’. Any modern geochemist might comment, ‘Well, there must be some kind of signature!’, but what that might be and when it happened are elusive, to say the least. One way of looking for answers is, as with so many thorny issues these days, to make a mathematical model. James Connelly and Martin Bizzarro of the University of Copenhagen, Denmark, have designed one based on the fact that one of the volatile elements that must have been partially ‘blown off’ by such a collision is lead and, of course, that is an element with several isotopes that are daughters of long-term decay of radioactive uranium and thorium (Connelly, J.N. & Bizzarro, M. 2016. Lead isotope evidence for a young formation age of the Earth–Moon system. Earth and Planetary Science Letters, v. 452, p. 36-43. doi:10.1016/j.epsl.2016.07.010).

Artist’s impression of the impact of a roughly Mars-size planet with the proto-Earth to form an incandescent cloud, from part of which the Moon formed.
Artist’s impression of the impact of a roughly Mars-size planet with the proto-Earth to form an incandescent cloud, from part of which the Moon formed. A NASA animation of lunar history can be viewed here.

Loss of volatile daughter isotopes of Pb produced by the decay schemes of highly refractory isotopes of U and Th would have reset the U-Pb and Th-Pb isotopic systems and therefore the radiogenic ‘clocks’ that depend on them in the same way as melting or high-temperature metamorphism resets the simpler 87Rb-87Sr decay scheme. Each radioactive U isotope has a different decay rate that produces a different Pb isotope daughter (235U Þ 207Pb; 238U Þ 206Pb, so it is possible to devise means of using present-day values of ratios between Pb isotopes, such as 207Pb/206Pb, 206Pb/204Pb and 207Pb/204Pb, to work back to such a ‘closure’ time. In short, that is the approach used by Connelly and Bizzarro. The most complicated bit of that geochemical ruse is estimating values of the ratios for the Earth’s modern mantle and for the Solar system in general – a procedure based on what we can actually measure: lots of mantle-derived basalts and lots of meteorites. Cutting out some important caveats, the result of their model is quite a surprise: ‘Year Zero’ on their account was between 4426 and 4417 Ma years ago, which is astonishingly precise. And it is pretty close to the measured age of the of lunar Highland anorthosites – products of fractional crystallisation of the Moon’s early magma ocean – and also to that of the oldest zircons on Earth. But is also about 60 Ma later than previous estimates

The Connelly and Bizzarro paper follows hard on the heels of another with much the same objective  (Snape, J.F. and 8 others 2016. Lunar basalt chronology, mantle differentiation and implications for  determining the age of the Moon. Earth and Planetary Science Letters, v. 451, p. 149-158. doi.org/10.1016/j.epsl.2016.07.026). Once again omitting a great deal of argument, Snape and colleagues end up with an age for the isotopic resetting of the lunar mantle of 4376 Ma to the nearest 18 Ma; i.e. an age significantly different from that arrived at by Connelly and Bizzarro. So the answer to the question, ‘When was there a vestige of a beginning?’ is, ‘It depends on the model’… Thankfully, neither estimate for ‘Year Zero’ has much bearing on the big, practical questions, such as, ‘When did life form?’, ‘Has there always been plate tectonics?’

More on the origin of the early Solar System and formation of the Earth-Moon system

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Salt and Earth’s atmosphere

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It is widely known that glacial ice contains a record of Earth’s changing atmospheric composition in the form of bubbles trapped when the ice formed. That is fine for investigations going back about a million years, in particular those that deal with past climate change. Obviously going back to the composition of air tens or hundreds of million years ago cannot use such a handy, direct source of data, but has relied on a range of indirect proxies. These include the number of pores or stomata on fossil plant leaves for CO2, variations in sulfur isotopes for oxygen content and so on. Variation over time of the atmosphere’s content of oxygen has vexed geoscientists a great deal, partly because it has probably been tied to biological evolution: forming by some kind of oxygenic photosynthesis and being essential for the rise to dominance of eukaryotic animals such as ourselves. Its presence or absence also has had a large bearing on weathering and the associated dissolution or precipitation of a variety of elements, predominantly iron. Despite progressively more clever proxies to indicate the presence of oxygen, and intricate geochemical theory through which its former concentration can be modelled, the lack of an opportunity to calibrate any of the models has been a source of deep frustration and acrimony among researchers.

Yet as is often said, there are more ways of getting rid of cats than drowning them in butter. The search has been on for materials that trap air in much the same way as does ice, and one popular, if elusive target has been the bubbles in crystals of evaporite minerals. The trouble is that most halite deposits formed by precipitation of NaCl from highly concentrated brines in evaporating lakes or restricted marine inlets. As a result the bubbles contain liquids that do a grand job of preserving aqueous geochemistry but leave a lot of doubt as regards the provenance of gases trapped within them. For that to be a sample of air rather than gases once dissolved in trapped liquid, the salt needs to have crystallized above the water surface. That may be possible if salt forms from brines so dense that crystals are able to float, or perhaps where minerals such as gypsum form as soil moisture is drawn upwards by capillary action to form ‘desert roses’. A multinational team, led by Nigel Blamey of Brock University in Canada, has published results from Neoproterozoic halite whose chevron-like crystals suggest subaerial formation (Blamey, N.J.F. and 7 others, 2016. Paradigm shift in determining Neoproterozoic atmospheric oxygen. Geology, v. 44, p. 651-654). Multiple analyses of five halite samples from an ~815 Ma-old horizon in a drill core from the Neoproterozoic Canning Basin of Western Australia contained about 11% by volume of oxygen, compared with 25% from Cretaceous salt from China, 20% of late-Miocene age from Italy, and 19 to 22% from samples modern salt of the same type.

Salar de Atacama salt flat in the Chilean puna
Evaporite salts in the Salar de Atacama Chile (credit: Wikipedia)

Although the Neoproterozoic result is only about half that present in modern air, it contradicts results that stem from proxy approaches, which suggest a significant rise in atmospheric oxygenation from 2 to about 18% during the younger Cryogenian and Ediacaran Periods of the Neoproterozoic, when marine animal life made explosive developments at the time of repeated Snowball Earth events. Whether or not this approach can be extended back to the Great Oxygenation Event at around 2.3 Ga ago and before depends on finding evaporite minerals that fit stringent criteria for having formed at the surface: older deposits are known even from the Archaean.

Bury the beast in basalt

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Global warming cannot simply be reversed by turning off the tap of fossil fuel burning. Two centuries’ worth of accumulated anthropogenic carbon dioxide would continue to trap solar energy, even supposing that an immediate shutdown of emissions was feasible; a pure fantasy for any kind of society hooked on coal, oil and gas. It takes too long for natural processes to download CO2 from the atmosphere into oceans, living organic matter or, ultimately, back once more into geological storage. In the carbon cycle, it has been estimated that an individual molecule of the gas returns to one of these ‘sinks’ in about 30 to 95 years. But that is going on all the time for both natural and anthropogenic emissions. Despite the fact that annual human emissions are at present only about 4.5 % of the amount emitted by natural processes, clearly the drawdown processes in the carbon cycle are incapable of balancing them, at present. Currently the anthropogenic excess of CO2 over that in the pre-industrial atmosphere is more than 100 parts per million achieved in only 250 years or so. The record of natural CO2 levels measured in cores through polar ice caps suggests that natural processes would take between 5 to 20 thousand years to achieve a reduction of that amount.
Whatever happens as regards international pledges to reduce emissions, such as those reported by the Paris Agreement, so called ‘net-zero emissions’ leave the planet still a lot warmer than it would be in the ‘natural course of things’. This is why actively attempting to reduce atmospheric carbon dioxide may be the most important thing on the real agenda. The means of carbon sequestration that is most widely touted is pumping emissions from fossil fuel burning into deep geological storage (carbon capture and storage or CCS), but oddly that did not figure in the Paris Agreement, as I mentioned in EPN December 2015. In that post I noted that CCS promised by the actual emitters was not making much progress: a cost of US$50 to 100 per tonne sequestered makes most fossil fuel power stations unprofitable. Last week CCS hit the worlds headlines through reports that an Icelandic initiative to explore a permanent, leak-proof approach had made what appears to be a major breakthrough (Matter, J.M. and 17 others, 2016. Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide emissions. Science, v. 352, p. 1312-1314). EPN January 2009 discussed the method that has now been tested in Iceland. It stems from the common observation that some of the minerals in mafic and ultramafic igneous rocks tend to breakdown in the presence of carbon dioxide dissolved in slightly acid water. The minerals are olivine ([Fe,Mg]2SiO4)] and pyroxene ([Fe,Mg]CaSi2O6), from whose breakdown the elements calcium and magnesium combine with CO2 to form carbonates.
Iceland is not short of basalts, being on the axial ridge of the North Atlantic. Surprisingly for a country that uses geothermal power to generate electricity it is not short of carbon dioxide either, as the hot steam contains large quantities of it. In 2012 the CarbFix experiment began to inject a 2 km deep basalt flow with 220 t of geothermal CO2 ‘spiked’ with 14C to check where the gas had ended up This was in two phases, each about 3 months long. After 18 months the pump that extracted groundwater directly from the lave flow for continuous monitoring of changes in the tracer and pH broke down. The fault was due to a build up of carbonate – a cause for astonishment and rapid evaluation of the data gathered. In just 18 months 95% of the 14C in the injected CO2 had been taken up by carbonation reactions. A similar injection experiment into the Snake River flood basalts in Washington State, USA, is said to have achieved similar results (not yet published). A test would be to drill core from the target flow to see if any carbonates containing the radioactive tracer filled either vesicles of cracks in the rock – some press reports have shown Icelandic basalt cores that contain carbonates, but no evidence that they contain the tracer .
Although this seems a much more beneficial use of well-injection than fracking, the problem is essentially the same as reinjection of carbon dioxide into old oil and gas fields; the high cost. Alternatives might be to spread basaltic or ultramafic gravel over large areas so that it reacts with CO2 dissolved in rainwater or to lay bear fresh rocks of that kind by removal of soil cover.

Related articles:

Kintisch, E., 2016. Underground injections turn carbon dioxide to stone. Science, v. 352, p. 1262-1263.

In a first, Iceland power plant turns carbon emissions to stone. Phys.org

Tungsten isotopes provide a ‘vestige of a beginning’

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Apart from ancient detrital zircons no dated materials from the Earth’s crust come anywhere near the age when our home world formed, which incidentally was derived by indirect means. Hutton’s famous saying towards the close of the 18th century, ‘The result, therefore, of our present enquiry is, that we find no vestige of a beginning, – no prospect of an end’ seems irrefutable. Hardly surprising, you might think, considering the frantic pace of events that have reworked the geological record for four billion years and convincing evidence that not long after accretion the Moon-forming collision may have melted most of the early mantle. But there is a way of peering beyond even that definitive catastrophe. The metal tungsten, as anyone from the steel town of Rotherham will tell you, alloys very nicely with iron and makes it harder, stronger and more temperature resistant. Most of the Earth’s original complement of tungsten probably ended up in the core; it is a siderophile element. But traces can be detected in virtually any rock and, of course, in W-rich ore bodies. Its interest to modern-day geochemists lies in its naturally occurring isotopes, particularly 182W, a proportion of which forms by decay of a radioactive isotope of hafnium (182Hf). Or rather it did, for 182Hf has a half-life of about 9 million years. Only a vanishingly small amount from a nearby supernova that may have triggered  formation of the solar system remains undecayed.

Artistic impression of the early Earth before Moon formation. (Source: Creative Commons)
Artistic impression of the early Earth before Moon formation. (Source: Creative Commons)

A sign of the former presence of 182Hf in the early Earth comes from higher amounts of its daughter isotope 182W in some Archaean rocks (3.96 Ga) than in younger rocks. That excess is probably from undecayed  182Hf  in asteroidal masses that bombarded the Earth between 4.1 and 3.8 Ga. Now it turns out that some much younger flood basalts from the Ontong Java Plateau on the floor of the West Pacific Ocean (~120 Ma) and Baffin Island in northern Canada (~60 Ma) also contain anomalously high 182W/184W ratios (Rizo, H. et al. 2016. Preservation of Earth-forming events in the tungsten isotopic composition of modern flood basalts. Science, v. 352, p. 809-812; see also: Dahl, T.W. 2016. Identifying remnants of early Earth. Science, v. 352, p. 768-769). A different explanation is required for these occurrences. The flood basalts must have melted from chemically anomalous mantle, which originally contained undecayed 182Hf. The researchers have worked out that this heterogeneity stems from a silicate-rich planetesimal that had formed in the first 50 Ma of the solar system’s history, and was accreted to the Earth before the Moon-forming event – lunar rocks formed after 182Hf became extinct. That catastrophe and the succeeding 4.51 Ga of mantle convection failed to mix the ancient anomaly with the rest of the Earth.

So, when did plate tectonics start up?

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Tiny, 4.4 billion year old zircon grains extracted from much younger sandstones in Western Australia are the oldest known relics of the Earth system. But they don’t say much about early tectonic processes. For that, substantial exposures of rock are needed, of which the undisputedly oldest are the Acasta gneisses 300 km north of Yellowknife in Canada’s North West Territories, which have an age of slightly more than 4 Ga. The ‘world’s oldest rock’ has been something of a grail for geologists and isotope geochemists who have combed the ancient Archaean cratons for 5 decades. But since the discovery of metasediments with an age of 3.8 Ga in West Greenland during the 1970s they haven’t made much headway into the huge time gap between Earth’s accretion at 4.54 Ga and the oldest known rocks (the Hadean Eon).

The Deccan Traps shown as dark purple spot on ...
Continental cratons (orange) where very-old rocks are likely to lurk. (credit: Wikipedia)

There have been more vibrant research themes about the Archaean Earth system, specifically the issue of when our planet settled into its modern plate tectonic phase A sprinkling of work on reconstructing the deep structural framework of Archaean relics has convinced some that opposed motion of rigid, brittle plates was responsible for their geological architecture, whereas others have claimed signs of a more plastic and chaotic kind of deformation of the outer Earth. More effort has been devoted to using the geochemistry of all the dominant rocks found in the ancient cratons, seeking similarities with and differences from those of more recent vintage. There can be little doubt that the earliest processes did form crust whose density prevented or delayed it from being absorbed into the mantle. Even the 4.4 Ga zircons probably crystallized from magma that was felsic in composition. Once trapped by buoyancy at the surface and subsequently wrapped around by similarly low density materials continental crust formed as a more or less permanent rider on the Earth’s deeper dynamics. But did it all form by the same kinds of process that we know to be operating today?

Plate tectonics involves the perpetual creation of rigid slabs of basalt-capped oceanic lithosphere at oceanic rift systems and their motions and interactions, including those with continental crust. Ocean floor cools as it ages and becomes hydrated by seawater that enters it. The bulk of it is destined eventually to oppose, head-to-head, the motions of other such plates and to deform in some way. The main driving force for global tectonics begins when an old, cold plate does deform, breaks, bends and drives downwards. Increasing pressure on its cold, wet basaltic top transforms it into a denser form: from a wet basaltic mineralogy (feldspar+pyroxene+amphibole) to one consisting of anhydrous pyroxene and garnet (eclogite) from which watery fluid is expelled upwards. Eclogite’s density exceeds that of mantle peridotite and compels the whole slab of oceanic lithosphere to sink or subduct into the mantle, dragging the younger parts with it. This gravity-induced ‘slab pull’ sustains the sum total of all tectonic motion. The water rising from it induces the wedge of upper mantle above to melt partially, the resulting magma evolves to produce new felsic crust in island arcs whose destiny is to be plastered on to and enlarge older continental masses.

Relics of eclogites and other high-pressure, low-temperature versions of hydrated basalts incorporated into continents bear direct and unchallengeable witness to plate tectonics having operated back to about 800 Ma ago. Before that, evidence for plate tectonics is circumstantial and in need of special pleading. Adversarial to-ing and fro-ing seems to be perpetual, between geoscientists who see no reason to doubt that Earth has always behaved in this general fashion and others who see room for very different scenarios in the distant past. The non-Huttonian tendency suggests an early, more ductile phase when greater radioactive heat production in the mantle produced oceanic crust so fast that when it interacted with other slabs it was hot enough to resist metamorphic densification wherever it was forced down. Faster production of magma by the mantle without slab-pull could have produced a variety of ‘recycling’ turnover mechanisms that were not plate-tectonic.

One thing that geochemists have discovered is that the composition of Archaean continental crust is very different from that produced in later times. In 1985 Ross Taylor and Scott McLennan, then of the Australian National University, hit on the idea of using shales of different ages as proxies for the preceding continental crust from which they had been derived by long erosion. Archaean and younger shales differed in such a way that suggests that after 2.5 Ga (the end of the Archaean) vast amounts of feldspar were extracted from the continent-forming magmas. This left the later Precambrian and Phanerozoic upper crust depleted in the rare-earth element europium, which ended up in a mafic, feldspar-rich lower crust. On the other hand, no such mass fractionation had left such a signature before 2.5 Ga. Another ANU geochemist, now at the University of Maryland, Roberta Rudnick has subsequently carried this approach further, culminating in a recent paper (Tang, M., Chen, K and Rudnick, R.L. 2016. Archean upper crust transition from mafic to felsic marks the onset of plate tectonics. Science, v. 351, p. 372-375). This uses nickel, chromium and zinc concentrations in ancient igneous and sedimentary rocks to track the contribution of magnesium (the ‘ma’ in ‘mafic’) to the early continents. The authors found that between 3.0 to 2.5 Ga continental additions shifted from a dominant more mafic composition to one similar to that of later times by the end of the Archaean. Moreover, this accompanied a fivefold increase in the pace of continental growth. Such a spurt has long been suspected and widely suggested to mark to start of true plate tectonics: but an hypothesis bereft of evidence.

A better clue, in my opinion, came 30 years ago from a study of the geochemistry of actual crustal rocks that formed before and after 2.5 Ga (Martin, H. 1986. Effect of steeper Archean geothermal gradient on geochemistry of subduction-zone magmas. Geology, v. 14, p. 753-756). Martin showed that plutonic Archaean and post-Archaean felsic rocks of the continental crust lie in distinctly different fields on plots of their rare-earth element (REE) abundances. Archaean felsic plutonic rocks show a distinct trend of enrichment in light REE relative to heavy REE as measures of the degree of partial melting decreases, whereas the younger crustal rocks show almost constant, low values of heavy REE/light REE whatever the degree of melting. The conclusion he reached was that while in the post Archaean the source was consistent with modern subduction processes – i.e. partial melting of hydrated peridotite in the mantle wedge above subduction zones – but during the Archaean the source was hydrated, garnet-bearing amphibolite of basaltic composition, in the descending slab of subducted oceanic crust. Together with Taylor and McLennan’s lack of evidence for any fractional crystallization in Archaean continental growth, in contrast to that implicated in Post-Archaean times.

The geochemistry forces geologists to accept that a fundamental change took place in the generation and speed of continental growth at the end of the Archaean, marking a shift from a dominance of melting of oceanic, mafic crust to one where the upper mantle was the main source of felsic, low-density magmas. Yet, no matter how much we might speculate on indirect evidence, whether or not subduction, slab-pull and therefore plate tectonics dominated the Archaean remains an open question.

More on continental growth and plate tectonics

Deccan Trap sprung by bolide?

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English: Alvarez and K-T Boundary
Luis and Walter Alvarez at the end-Mesozoic Boundary (credit: Wikipedia)

It was 35 years back that father and son team Luis and Walter Alvarez upset a great many geoscientists by suggesting that a very thin layer of iridium-rich mud that contained glass spherules and shocked mineral grains was evidence for a large meteorite having struck Earth. They especially annoyed palaeontologists because of their claim that it occurred at the very top of the youngest Cretaceous and that the mud was spread far and wide in deep- and shallow-marine stratigraphic sequences and also in those of continental rocks. It marked the boundary between the Mesozoic and Cenozoic Eras and, of course, the demise of the dinosaurs and a great many more, less ‘sexy’ beasts. Luis was a physicist, his son a proper geologist and their co-researchers were chemists. It can hardly be said that they stole anyone’s thunder since the issue of mass extinctions was quiescent, yet their discovery ranks with that of Alfred Wegener; another interloper into the closed-shop geoscientific community. They got the same cold-shoulder treatment, but massive popular acclaim as well, even from a minority of geologists who welcomed their having shaken up their colleagues, 15 years after the last ‘big thing’: plate tectonics. And then the actual site of the impact was found by geophysicists in a sedimentary basin in the Gulf of Mexico off the small town of Chicxulub on the Yucatan peninsula.

Chicxulub impact - artist impression
Chicxulub impact – artist impression (credit: Wikipedia)

As they say, ‘the rest is history’ and a great many geoscientists didn’t just jump but pounced on this potential bandwagon. Central to this activity was the fact that, within error, the ages of the impact, the mass extinction and a vast pile of continental lavas in western India, the Deccan Traps, were more or less the same (around 66 Ma). Flood basalt events are just about as dramatic as mega-impacts because of their sheer scale, of the order of a million cubic kilometres; that they were exuded in a mere million years or so, but in only a few tens of stupendous lava flows; and they are far beyond the direct experience of humans, blurting out only every 30 Ma or so. This periodicity roughly tallies with mass extinctions, great and small, through the Mesozoic. There have been two large bands of enthusiasts engaged in the causality of the end-Mesozoic die-off – the extraterrestrials and the parochialists who favoured a more mundane, albeit cataclysmic snuffing-out. Mass extinctions in general have been repeatedly examined, and in recent years it has become clear that most of those since 250 Ma ago seem to be associated with basalt-flood events and are purely terrestrial in origin. As regards the event that ended the Mesozoic, it has proved difficult to resolve whether to point the finger at the Deccan Traps or the Chicxulub impact. Both might have severely damaged the biosphere in perhaps different ways, so a ‘double whammy’ has become a compromise solution.

The Western Ghat hills at Matheran in Maharash...
Deccan flood basalts forming the Western Ghats in Maharashtra, India (credit: Wikipedia)

Unsurprisingly, a lot of effort from different quarters has gone into charting the progress of the Deccan volcanism. Some dating seemed at one stage to place the bulk of the volcanism significantly before the mass extinction and impact, others had them spot on and there were even signs of an hiatus in eruptions at the critical juncture. The problem was geochronological precision of the argon-argon method of radiometric dating that is most used for rocks of basaltic composition: many labs cannot do better than an uncertainty of 1%, which is ±0.7 Ma for ages around the end of the Mesozoic, not far short of the entire duration of these huge events. Some Deccan samples have now been dated to a standard of ±0.1 Ma by the Ar-Ar lab at the Department of Earth and Planetary Sciences, University of California-Berkeley (Renne, P.R. et al. 2010. State shift in Deccan volcanism at the Cretaceous-Paleogene boundary, possibly induced by impact. Science, v. 350, p. 76-78). The results, between 65.5 to 66.5 Ma, nicely bracket the K/T (now K/Pg) boundary age of 66.04±0.04 Ma. It looks like the double whammy compromise is the hypothesis of choice. But there is more to mere dating.

Renne and colleagues plot the ages against their position in the volcanic stratigraphy of the Deccan Traps in two ways: against the estimated height from base in the pile and against the estimated volume of the erupted materials as it built up – the extent and thickness of successive flows varies quite a lot. The second plot provided a surprise. After the K/Pg event the mean rate of effusion – the limited number of individual flows capped by well-developed soils shows that the build-up was episodic – doubled from 0.4±0.2 to 0.9±0.3 km3 yr-1. Despite the much larger uncertainty in the extent and volume of individual lava Formations than that of their ages, this is clearly significant. Does it imply that the Chicxulub impact somehow affected the magma production from, the mantle plume beneath the Deccan? It had been suggested early in the debate that the antipodean position of the lava field relative to that of Chicxulub may indicate that the huge seismicity from the impact triggered the Deccan magma production. Few accepted that possibility when it first appeared. However, Renne and co. do think it deserves another look, at least at the possibility of some linked effect on the magmatism. Perhaps the magma chamber was somehow enlarged by increased global seismicity; other chambers could have been added; magma might have been ‘pumped’ out more efficiently, or a combination of such effects. The ‘plumbing’ of flood basalt piles is generally hidden, but huge dyke swarms in Precambrian times have been suggested as feeders to long-eroded flood basalts. Seismicity of the scale produced by asteroid impacts can do a lot of damage. The Chicxulub impactor at around 10 km diameter would have carried energy a million times greater than that of the largest thermonuclear bomb, equivalent to an earthquake of Magnitude 12.4 that would have been a thousand times more powerful than the largest recorded earthquake with tectonic causes. Extensional faulting sourced in this fashion in the Deccan area may have increased the pathways along which magma might blurt out.

Related articles

Duncan, R. 2015. Deadly combination. Nature, v. 527, p. 172-173.

Continental hot-spot track in eastern Australia

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cosgrove volcano track shown on map of australia
The Cosgrove volcanic track on a natural-colour image mosaic of Australia (credit: Drew Whitehouse, NCI National Facility VizLab)

It is sometimes forgotten that not only oceanic lithosphere provides evidence for hot spot tracks, probably because they are so obvious as island and seamount chains on bathymetric maps. They are not so clear on continents, either because of erosion of volcanoes or topography dominated by features that predate volcanism, but they account for about 20% of proposed tracks. Eastern Australia seems well endowed; four of them marked by a variety of volcanic structures that trend parallel to the Indo-Australian Plate’s NNE Cenozoic drift powered by the Southeast Indian Ridge that separates it from the Antarctic Plate. The timing of the volcanism along the proposed tracks is also highly persuasive. The longest of the tracks, extending about 2000 km SSW from Cape Hillsborough on the coast of central Queensland through New South Wales to Cosgrove in Victoria, is marked by sporadic volcanoes whose age decreases from Late Eocene in the north to Late Miocene in Victoria.

Unlike oceanic hot-spot tracks, those on continents are not continuous lines of volcanic occurrences. The Cosgrove track has several volcanic gaps, up to 650 km wide. This kind of patchy feature once encouraged hot-spot sceptics to question the tectonic affinities of what they regarded as fortuitous alignments. Where volcanic age trends consistent with the hypothesis emerged such doubts have faded into the background academic ‘noise’. In the case of the Cosgrove track all but one of the dates of volcanism tally quite well with the Cenozoic absolute motion of the Indo-Australian Plate and their position along the track (Davies, D.R. et al 2015. Lithospheric controls on magma composition along Earth’s longest continental hotspot track. Nature, v. 525, p. 511-514). Yet the objective of the authors, from the Australian National University and the University of Aberdeen in Britain, was not merely to establish the alignment as a hot-spot track, but to suggest what may have resulted in its marked patchiness.

The geochemistry of lavas from the volcanoes turns out to be of two fundamentally different types: ‘common-or-garden’ basalts in the case of Queensland and peculiar potassium-rich basalts containing the K-feldspathoid leucite in New South Wales and Victoria. Why these compositional differences occur where they do emerged very clearly when their positions were plotted on a new map of the thickness variations of the eastern Australian continental lithosphere. The ordinary basalts rest on the thinnest lithosphere (£110 km), whereas the leucitites are underlain by considerably thicker lithosphere (~135 km). This suggests that the rising mantle whose partial melting produced the magmas was halted at different depths, different geochemical ‘signatures’ of basalts depending on the pressure of melting. The most interesting outcome, albeit one based on an absence of evidence, is that the very large volcanic gaps along the track are each above much thicker lithosphere (>150 km). At those depths a rising mantle plume would be much less likely to begin melting.

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Roman concrete restrains magma

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Four million people in and around the Italian city of Naples on the shore of the Tyrrhenian Sea have always lived under a double threat of natural disaster. The one that immediately springs to most people’s mind is the huge volcano Vesuvius that looms over its eastern suburbs, for this was the source of the incandescent pyroclastic flow that overwhelmed Pompeii and Herculaneum in 79 CE. Less familiar outside Italy is a cluster of elliptical volcanic features directly to the west of the city: Campi Flegrei or the Phlegraean Fields. In fact the cluster is part of a vast, dormant caldera, half of which lies beneath the sea centred on the ancient Roman port of Puteoli (modern Pozzuoli). This volcanic collapse structure is about 10 km across; about as large as Vesuvius. Campi Flegrei is famous for its sulfur-rich fumaroles including the mythical crater home of Vulcan the god of fire, Solfatara.

The Bay of Naples with Vesuvius to the east of the city and Campi Flegrei to the west. (credit: Google Earth)
The Bay of Naples with Vesuvius to the east of the city and Campi Flegrei to the west. (credit: Google Earth)

Between 1970 and 1984 the ground around Pozzuoli rose more than 2 metres, which may be evidence that the deep seated magma chamber is inflating. Fears that this might presage an eruption in the near future stems from a curious feature affecting archaeological remains, such as upright pillars in the harbour area of Pozzuoli. At many different levels the stonework is pockmarked by curious holes that are the fossil borings of marine molluscs: at some stage the feet of the pillars descended below sea level. Together with historic records since the Roman era these borings help to establish the local ups and downs of the surface over the last two millennia in considerable detail. From a high of 4 m above sea level when the pillars were erected 194 BCE they slowly subsided to reach sea level around 300 CE when Puteoli ceased to be an important harbour and 4 metres below that around 900 CE. For the last millennium they have slowly risen until in 1538 more than 4 metres of inflation took place very rapidly. That was immediately followed by a small eruption of about 0.02 km3 of magma at Mount Nuovo, to the northeast of another recent crater now occupied by a lake: hence the fear surrounding the uplift in 1970-84. Campi Flegrei has a history of eruptions going back 40 thousand years, including two in the ‘super volcano’ category of 200 and 40 km3 that blanketed vast areas in pyroclastic ash.

One tantalising aspect of the ground inflation and deflation is that each cycle lasts of the order of a thousand years. Another seems to be that magma breaks to the surface very rapidly after a long period of inflation, as if whatever was keeping the magma chamber in a metastable state failed in a brittle fashion. Tiziana Vanorio and Waruntorn Kanitpanyacharoen of Stanford and Chulalonkorn universities in the US and Thailand have come up with a possible reason for such gradual crustal warping in volcanic areas and long-delayed eruption, for which Campi Flegrei is a model case (in fact the oscillations there are unsurpassed). Such long-term bending of the crust suggests abnormally strong rock near the surface. The co-workers analysed borehole cores that penetrated to the depth of small shallow earthquakes – in the metamorphic basement of the area – and found that the zone above the seismically active layer is not only stronger than the basement, but closely resembles a construction material to which Roman architecture owes its longevity (Vanorio, T. & Kanitpanyacharoen, W. 2015. Rock physics of fibrous rocks akin to Roman concrete explains uplifts at Campi Flegrei Caldera. Science, v. 349, p. 617-621).

Deutsch: Pozzuoli, Macellum
Mollusc-bored pillars in the Macellum (indoor market) of Pozzuoli (credit: Wikipedia)

Roman masons discovered that by mixing young, loose volcanic ash with lime mortar (calcium hydroxide) produced a strong concrete when cured. Specifically, the invention of concrete took place at Pozzuoli itself, using volcanic ash from Campi Flegrei and the product was known as pozzolana. Young ash from an explosive volcano is mainly shards of anhydrous silicate glass, which quickly react with water and calcium hydroxide to produce fibres of hydrous calc-silicate minerals, almost as in conventional cement curing, but without the need for heating limestone and clay to very high temperatures. The strength of pozzolano enabled Roman architects to build the great dome of the Pantheon in Rome, still the world’s largest unreinforced concrete dome. Moreover, the speed with which it sets by exothermic reactions enables its use below sea level. Vanorio and Kanitpanyacharoen found that the strong upper zone beneath Campi Flegrei is almost identical to pozzolano, and suggest that it formed as a result of calcium-rich hydrothermal fluids percolating through young pyroclastic rocks. The calcium derives from metamorphic basement rich in calc-silicate layers through which hot groundwater is driven as a result of heat from the underlying magma chamber. It seems the Campi Flegrei caldera has built its own containing dome. But that is perhaps a mixed blessing: the 1970-84 inflation seems now to be deflating and the flexible carapace may make using ground movements as means of predicting eruptions unreliable.

Intérieur du panteon à Rome
Interior view of the dome of the Pantheon in Rome (credit: Wikipedia)
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How far has geochemistry led geology?

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Granite pmg ss 2006
Thin section of a typical granite: clear white and grey grains are quarts (silica); striped black and white is feldspar; coloured minerals are micas (credit: Wikipedia)

In the Solar System the Earth is unique in having a surface split into two distinct categories according to their relative elevation; one covered by water, the other not. More than 60% of its surface – the ocean basins – falls between 2 to 11 km below sea level with a mean around 4 to 5 km deep. A bit less than 40% – land and the continental shelves – stands higher than 1 km below sea level up to almost 9 km above, with a mean around 1 km high. Between 1 and 2 km below sea level is represented by only around 3 % of the surface area. This combined hypsography and wetness is reckoned to have had a massive bearing on the course of climate and biological evolution, as far as allowing our own emergence. The Earth’s bimodal elevation stems from the near-surface rock beneath each division having different densities: continental crust is less dense than its oceanic counterpart, and there is very little crustal rock with an intermediate density. Gravitational equilibrium ensures that continents rise higher than oceans. That continents were underpinned mainly by rocks of granitic composition and density, roughly speaking, was well known by geologists at the close of the 19th century. What lay beneath the oceans didn’t fully emerge until after the advent of plate tectonics and the notion of simple basaltic magmas pouring out as plates became detached.

In 1915 Canadian geologist Norman Levi Bowen resolved previously acquired knowledge of the field relations, mineralogy and, to a much lesser extent, the chemistry of igneous rocks, predominantly those on the continents in a theory to account for the origin of continents. This involved a process of distillation or fractionation in which the high-temperature crystallisation of mafic (magnesium- and iron-rich) minerals from basaltic magma left a residual melt with lower Mg and Fe, higher amounts of alkalis and alkaline earth elements and especially enriched in SiO2 (silica). A basalt with ~50% silica could give rise to rocks of roughly granitic composition (~60% SiO2) – the ‘light’ rocks that buoy-up the continental surface – through Bowen’s hypothetical fractional crystallisation. Later authors in the 1930s, including Bowen’s teacher Reginald Aldworth Daly, came up with the idea that granites may form by basalt magma digesting older SiO2-rich rocks or by partially melting older crustal rocks as suggested by British geologist Herbert Harold Read. But, of course, this merely shifted the formation of silica-rich crust further back in time

A great deal of field, microscope and, more recently, geochemical lab time has been spent since on to-ing and fro-ing between these hypotheses, as well as on the petrology of basaltic magmas since the arrival of plate theory and the discovery of the predominance of basalt beneath ocean floors. By the 1990s one of the main flaws seen in Bowen’s hypothesis was removed, seemingly at a stroke. Surely, if a basalt magma split into a dense Fe- Mg-rich cumulate in the lower crust and a less dense, SiO2-rich residual magma in the upper continental crust the bulk density of that crust ought to remain the same as the original basalt. But if the dense part somehow fell back into the mantle what remained would be more able to float proud. Although a neat idea, outside of proxy indications that such delamination had taken place, it could not be proved.

Since the 1960s geochemical analysis has became steadily easier, quicker and cheaper, using predominantly X-ray fluorescence and mass-spectrometric techniques. So geochemical data steadily caught up with traditional analysis of thin sections of rock using petrological microscopes. Beginning in the late 1960s igneous geochemistry became almost a cottage industry and millions of rocks have been analysed. Recently, about 850 thousand multi-element analyses of igneous rocks have been archived with US NSF funding in the EarthChem library. A group from the US universities of Princeton, California – Los Angeles and Wisconsin – Madison extracted 123 thousand plutonic and 172 thousand volcanic igneous rocks of continental affinities from EarthChem to ‘sledgehammer’ the issue of continent formation into a unified theory (Keller, C.B. et al. 2015. Volcanic-plutonic parity and the differentiation of the continental crust. Nature, v. 523, p. 301-307).

In a nutshell, the authors compared the two divisions in this vast data bank; the superficial volcanic with the deep-crustal plutonic kinds of continental igneous rock. The gist of their approach is a means of comparative igneous geochemistry with an even longer pedigree, which was devised in 1909 by British geologist Alfred Harker. The Harker Diagram plots all other elements against the proportionally most variable major component of igneous rocks, SiO2. If the dominant process involved mixing of basalt magma with or partial melting of older silica-rich rocks such simple plots should approximate straight lines. It turns out – and this is not news to most igneous geochemists with far smaller data sets – that the plots deviate considerably from straight lines. So it seems that old Bowen was right all along, the differing deviations from linearity stemming from subtleties in the process of initial melting of mantle to form basalt and then its fractionation at crustal depths. Keller and colleagues found an unexpected similarity between the plutonic rocks of subduction-related volcanic arcs and those in zones of continental rifting. Both record the influence of water in the process, which lowers the crystallisation temperature of granitic magma so that it freezes before the bulk can migrate to the surface and extrude as lava. Previously. rift-related magmas had been thought to be drier than those formed in arcs so that silica-rich magma should tend to be extruded.

But there is a snag, the EarthChem archive hosts only data from igneous rocks formed in the Phanerozoic, most being less than 100 Ma old. It has long been known that continental crust had formed as far back as 4 billion years ago, and many geologists believe that most of the continental crust was in place by the end of the Precambrian about half a billion years ago. Some even reckon that igneous process may have been fundamentally different before 3 billion years ago(see: Dhuime, B., Wuestefeld, A. & Hawkesworth, C. J. 2015. Emergence of modern continental crust about 3 billion years ago.  Nature Geoscience, v. 8, p.552–555). So big-science data mining may flatter to deceive and leave some novel questions unanswered .

Related articles

Till, C. 2015. Big geochemistry. Nature, v. 523, p. 293-294.

 

Two happy events for plate tectonics

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In an era where fears of rising sea level and loss of land are growing it is a great pleasure to announce (albeit several years late) the birth of two new islands. They emerged close to the axis of the Red Sea in Yemeni territory as new members of the volcanic Zubair Islands during episodic eruptions that began on 18 December 2011. First to form was dubbed Sholan (‘One who is Blessed’ in Arabic – a girl’s name), which ceased to be active a month later. Further submarine volcanism began on 28 September 2013, with another island, Jadid (‘New’ in Arabic – a boy’s name), breaking surface in October 2013. The double event has been described in great detail by geoscientists based at King Abdullah University of Science and Technology, Saudi Arabia (Xu, W. 2015. Birth of two volcanic islands in the southern Red Sea. Nature Communications, DOI: 10.1038/ncomms8104. After rapid growth during their initial eruptive phases both islands underwent significant marine erosion once quiescent, but seem set to remain as part of the Zubair archipelago.

'Before and after' images of the Zubair archipelago in the southern Red Sea. (Left from Bing maps, right (February 2014) from Google Earth)
‘Before and after’ images of the Zubair archipelago in the southern Red Sea. (Left from Bing maps, right (February 2014) from Google Earth)

Analysis of small earthquakes that happened during the islands’ growth together with Interferometric iradar surveys that showed coincident ground movements among the islands suggest that both eruptions took place along an active north-south fracture system, probably part of axial rifting system of the Red Sea. In more detail, magma seems to have moved upwards along N-S fissures similar to those that now show up as dykes cutting lavas on the older islands in the area. The local fracture patterns are oblique to the main Red Sea Rift that trends NNW-SSE, possibly as a result of non-linear stress trajectories in the Arabia-Africa rifting. In almost all respects the volcanism and mechanism of intrusion and effusion closely resemble that reported recently from a terrestrial setting in the nearby Afar Depression. The slow spreading Red Sea Rift rarely manifests itself by volcanism, so these events reveal a previous unsuspected zone of active melting in the mantle beneath the Zubair archipelago.

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Magma rushed into largest layered intrusion

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Chances are that the platinum in the catalytic converter that helps prevent your car emitting toxic gases in its exhaust fumes came from a vast igneous intrusion in South Africa known as the Bushveldt complex. The world’s most important source of noble metals formed by repeated differentiation of huge volumes of mafic magma to form thin, dense layers rich in sulfides, platinum group metals and chromium ore set in very thick layers of barren gabbro and other mafic to ultramafic rock. The intrusion is exposed over an area the size of Ireland and formed about 2 billion years ago. Its 370 000 to 600 000 km3 volume suggests that it was the magma chamber that fed flood basalts that erosion has since eroded away. Successive pulses of basaltic magma built up a total thickness of about 8 kilometres of layered rock.

English: black Chromitite and grey anorthosite...
Layered igneous rocks in the Bushveld Complex (credit: Wikipedia)

The final product of the Bushveldt differentiation process was minute pockets of material of more felsic composition trapped within overwhelmingly larger amounts of gabbro. One of the elements that ended up in these roughly granitic inclusions was zirconium that mafic minerals are unable to accommodate while basaltic magma is crystallising. That formed minute crystals of the mineral zircon (ZrSiO4) in the residual pockets, which in turn locked up a variety of other elements, including uranium. Zircon can be dated using uranium’s radioactive decay to form lead isotopes, its refusal to enter chemical reactions after its crystallisation makes U/Pb dates of zircon among the most reliable available for geochronology and the precision of such dates has become increasingly exquisite as mass spectrometry has improved. So, the Bushveldt complex now has among the best records of magma chamber evolution (Zeh, A. et al. 2015. The Bushveld Complex was emplaced and cooled in less than one million years – results of zirconology, and geotectonic implications. Earth and Planetary Science Letters, v. 418, p. 103-114).

Like a number of younger large igneous provinces, the Bushveldt complex took a very short time to form, about 950 thousand years at 2055 Ma ago. That is from magma emplacement to final crystallization when the zircon ages were set, so the accumulation of magma probably took only 100 thousand years. This suggests that magma blurted into the lower crust at an average rate of around 5 cubic kilometers per year, and quite probably even faster if the magmatism was episodic. It requires a major stretch of the imagination to suggest that this could have occurred by some passive process. Instead, the authors have suggested that while a plume of mantle material rose from well below the lithosphere a large slab of lower lithosphere, formed from dense eclogite, broke off and literally fell into the deeper mantle. The resulting changes in stress in the lower lithosphere would have acted as a pump to drive the plume upwards, causing it to melt as pressure dropped, and to squirt magma into the overlying continental crust. Although the authors do not mention it, this is reminiscent of the idea of large igneous provinces having sufficient power to eject large masses from the Earth’s surface: the Verneshot theory, recently exhumed in late 2014. The main difference is that the originators of the Verneshot theory appealed to explosive gas release.

Glacial cycles and sea-floor spreading

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The London Review of Books recently published a lengthy review (Godfrey-Smith, P. 2015. The Ant and the Steam Engine. London Review of Books, v. 37, 19 February 2015 issue, p. 18-20) of the latest contribution to Earth System Science by James Lovelock, the man who almost singlehandedly created that popular paradigm through his Gaia concept of a self-regulating Earth (Lovelock, J. A Rough Ride to the Future. Allen Lane: London; ISBN 978 0 241 00476 0). Coincidentally, on 5 February 2015 Science published online a startling account of the inner-outer-inner synergism of Earth processes and climate (Crowley, J.W. et al. 2015. Glacial cycles drive variations in the production of oceanic crust. Science doi:10.1126/science.1261508). In fact serendipity struck twice: the following day a similar online article appeared in a leading geophysics journal (Tolstoy, M. 2015. Mid-ocean ridge eruptions as a climate valve. Geophysical Research Letters, doi:10.1002/2014GL063015)

Both articles centred on the most common topographic features on the ocean floor, abyssal hills. These linear features trend parallel to seafloor spreading centres and the magnetic stripes, which chart the progressive additions to oceanic lithosphere at constructive margins. Abyssal hills are most common around intermediate- and fast-spreading ridges and have been widely regarded as fault-tilt blocks resulting from extensional forces where cooling of the lithosphere causes it to sag towards the abyssal plains. However, some have suggested a possible link with variations in magma production beneath ridge axes as pressure due to seawater depth varied with rising and falling sea level through repeated glacial cycles. Mantle melting beneath ridges results from depressurization of rising asthenosphere: so-called ‘adiabatic’ melting. Pressure changes equivalent to sea-level fluctuations of around 100-130 m should theoretically have an effect on magma productivity, falls resulting in additional volumes of lava erupted on the ocean floor and thus bathymetric highs.

English: A close-up showing mid-ocean ridge to...
Formation of mid-ocean ridge topography, including abyssal hills that parallel the ridge axis. (credit: Wikipedia)

A test of this hypothesis would be see how the elevation of the sea floor adjacent to spreading axes changes with the age of the underlying crust. John Crowley and colleagues from Oxford and Harvard Universities and the Korea Polar Research Institute analysed new bathymetry across the Australian-Antarctic Ridge, whereas Maya Tolstoy of Columbia University performed similar work across the Southern East Pacific Rise. In both studies frequency analysis of changes in bathymetry through time, as calibrated by local magnetic stripes, showed significant peaks at roughly 23, 41 and 100 ka in the first study and at 100 ka in the second. These correspond to the well known Milankovitch periods due to precession, changing axial tilt and orbital eccentricity: persuasive support for a glacial control over mid-ocean ridge magmatism.

Enlarged by 100% & sharpened file with IrfanView.
Periodicities of astronomical forcing and global climate over the last million years (credit: Wikipedia)

An interesting corollary of the observations may be that pulses in sea-floor eruption rates emit additional carbon dioxide, which eventually percolates through the ocean to add to its atmospheric concentration, which would result in climatic warming. The maximum effect would correspond to glacial maxima when sea level reached its lowest, the reduction in pressure stimulating the greatest magmatism. One of the puzzling features of glacial cycles over the last million years, when the 100 ka eccentricity signal dominates, is the marked asymmetry of the sea-level record; slowly declining to a glacial maximum and then a rapid rise due to warming and melting as the Earth changed to interglacial conditions. Atmospheric CO2 concentrations recorded by bubbles in polar ice cores show a close correlation with sea-level change indicated by oxygen isotope data from oceanic sediments. So it is possible that build-up of polar ice caps in a roundabout way eventually reverse cooling once they reach their greatest thickness and extents, by modulating ocean-ridge volcanism and thereby the greenhouse effect.

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Verneshots (huge volcanic gas blasts) ten years on

Published on by zooks777Leave a comment

One of the most daring hypotheses of modern geosciences: is that of the ‘Verneshot’ reported by Earth Pages in 2004.  Jason Phipps Morgan and colleagues explored the possible consequences of a build-up of volatiles in plume-related magmas at the base of thick continental lithosphere beneath cratons, prior to the eruption of continental flood basalts. They suggested that pressure would eventually result in an explosive release at a lithospheric weak point, followed by collapse above the plume head that would propagate upwards, at hypersonic speeds. Modelling the forces involved, the authors of the novel idea considered that they would be sufficient to fling huge rock masses into orbit.  Verneshots might neatly explain the circumstances around mass extinctions, such as their coincidence with continental flood basalt events; large impact structures, most likely at the antipode of the event; global debris layers containing shocked rock, melt spherules; unusual element suites and compounds (including fullerenes); and enough toxic gas to cause biological devastation.

Ten years on, Verneshots are back, again in the prestigious journal Earth and Planetary Science Letters, and this time among the co-authors are Morgan père et fils (W. Jason a founder of plate tectonics, and Jason P. who launched the idea). This time the yet-to-be –accepted hypothesis comes with evidence of an extremely unusual and fortuitous kind (Vannucchi, P. et al. 2015. Direct evidence of ancient shock metamorphism at the site of the 1908 Tunguska event. Earth and Planetary Science Letters, v. 409, p. 168-174). The origin of the paper lies in an attempt to verify reports of shocked quartz in samples collected close to the centre of the 2000 km2 devastation that resulted from what is now accepted to have been a comet or asteroid air-burst explosion in June 1908 in the Tunguska region of Siberia. Apart from a disputed 300 m crater in the area, the Tunguska Event left no long-lived sign: it ‘merely’ knocked over millions of trees. However, its epicenter lay in a 10 km depression ringed by hills, that has been suggested to be a volcanic centre associated with the end-Permian Siberian Traps.

Trees knocked down and burned over hundreds of square km by the 1908Tunguska Event (credit: Leonid Alekseyevich Kulik deceased)
Trees knocked down and burned over hundreds of square km by the 1908 Tunguska Event (credit: Leonid Alekseyevich Kulik deceased)

The reported shocked quartz locality turned out to associated with an isolated occurrence of quartz-rich sand and rounded clasts of quartzite that contains sedimentary structures. The occurrence is surrounded by basalts of the Siberian Traps, yet is situated topographically above them. The quartzite is thought to be Permian terrestrial sandstone that commonly underlies much of the remaining extent of Siberian Traps.

Quartzite clasts do indeed contain shocked quartz, together with pseudotachylite glass veinlets, quartz and feldspar crystal growth on sedimentary grains and silica-rich glassy spherules. These features are not uniquely diagnostic of shock metamorphism, but are oddly absent from the surrounding Siberian Traps nearby, which suggests that whatever formed them predated the final eruptive stages of the end-Permian large igneous province. Indeed it would be unlikely that airburst of some extraterrestrial bolide in 1908 could produce the metamorphic features of the quartzites without setting ablaze the trees that it felled. A second possibility, that the Tunguska Depression is a Permo-Triassic impact crater and the quartzites being part of an associated central uplift runs into the unlikely coincidence of lying less than 5 km from the 1908 epicentre.

A third hypothesis is that the Tunguska Depression is a massive diatreme associated with a Verneshot. Another odd association lies 8 km to the south of the epicentre, a carbonatite that is one of many, along with smaller pipe-like structures all possibly linked to magmatic gas escape. The Tunguska Event, a mighty puzzle in its own right, may perhaps be eclipsed. Will silence return as it did after the original Verneshot hypothesis was published? Quite possibly, but another quirk about the Siberian Traps was reported by Earth Pages in mid-2014. In a contribution to a link between this massive end-Permian volcanic effusion and the Permian-Triassic mass extinction it was noted that in the Chinese sedimentary repository of evidence for the extinction there is an isolated spike in the abundance of nickel  that is almost certainly of volcanic origin, but only the one when repeated flood basalt events perhaps ought to have led to a series of nickel anomalies. One huge volcanic gas release as the Siberian Traps were building up?