Category: Geochemistry, mineralogy, petrology and volcanology

It is hardly contested these days that the massive Siberian Traps – the largest known continental flood basalt province – had something to do with the mass extinction at the Permian/Triassic boundary. Yet what actually produced sufficient, planet-wide environmental stress to slaughter up to 90% of all previously living species has not been pinned down. It was probably a combination of direct and indirect outcomes of the volcanic outpourings and several mechanisms have been suggested, such as: acid rain produced by SO₂ emissions from the magma; global warming as a result of volcanic CO₂ having accumulated in the atmosphere; a marked fall in the oxygen content of the atmosphere (see New twist for end-Permian extinctions in the May 2005 issue of EPN); increased phosphate fertilization of the oceans leading to anoxia and release of hydrogen sulfide gas.

Interestingly, the part of Siberia where the basalt floods took place is rich in coal measures and carbon-rich shales. Their thermal metamorphism by an overlying pile of lavas could conceivably have added huge amounts of CO₂ and methane to the atmosphere, creating strong greenhouse conditions: gas release from combustion and baking would have been almost instantaneous as each major flow came into contact with carbonaceous sediments. Yet direct evidence of widespread carbon combustion at the P/Tr boundary has not yet been demonstrated, although there are abundant gas-release structures in Siberia of around that age (Svenson, H. et al. 2009. Siberian gas venting and the end-Permian environmental crisis. Earth and Planetary Science Letters, v. 277, p. 490-500).

From a study of a near-continuous section of deep water marine sediments, whose ages range from Late Carboniferous to Cretaceous, something surprising has emerged. Silica-rich shales that span the P/Tr boundary show a major shift in d¹³C that matches the C-isotopic signature of the boundary elsewhere, and two lesser anomalies before the boundary event. At each C-isotope anomaly the shales also contain fly ash (Grasby, S.E. et al. 2011. Catastrophic dispersion of coal fly ash into oceans during the latest Permian extinction. Nature Geoscience, v. 4, p. 104-107), which forms today only in the rapid high-temperature combustion of coal in thermal power stations. It does not form from natural fires in underground and exposed coal seams that are caused by spontaneous combustion, usually ignited by rapid oxidation of pyrite in coal. The ash particles are smaller than 50 µm, and like similar sized, but denser, volcanic ash could easily be carried large distances. The Canadian team suggests that the fly ash formed when Siberian Trap basalts burned coals and organic-rich sediments, explosive release or explosive phases of the volcanism injecting them high in the atmosphere. Coal fly ash is not identifiable by normal microscopy, and its absence from the geological record may reflect that fact. Using organic petrography routinely on rocks from occurrences of the P/Tr and other boundary sequences should settle the matter.

Related Articles

• Coal-fired trigger of mass extinction (nature.com)

The bulk of igneous rocks found within and upon the crust formed by one of two fundamental processes of magma differentiation: calc-alkaline and tholeiitic, responsible for island arcs and ultimately continents forming and for generating oceanic crust and flood basalts. The parental material for both is basaltic magma, but the first leads to a decrease in iron in more fractionated magmas, whereas an increase in iron characterised the second. In the first case conditions favour iron entering igneous minerals, whereas in the second they urge crystallising minerals to exclude iron. The most likely explanation is that the calc-alkaline magmas of volcanic arcs devour electrons so that iron exists in the oxidised ferric or Fe³⁺ state and readily forms dense iron oxide minerals whose progressive removal makes the remaining magma less and less rich in iron. More reducing conditions that lack an abundant electron acceptor, primarily oxygen, make the formation of iron oxides less likely, and iron can build up in residual magmas. But how greater oxidation occurs in arc magmas than in those of the oceanic crust has several possible explanations. The most-widely assumed is that it happens because volcanic arcs lie above subduction zones where hydrated and therefore oxidised ocean floor descends into the mantle conferring oxygen to the products of partial melting. Another candidate is the depth at which fractional crystallisation takes place and there are other possibilities. The oxidation state of fundamental magmatic processes can be proxied by determining in rocks produced by fractionation the relative proportions of elements that behave differently in conditions of increased or decreased oxygen. One such pair is insensitive zinc and sensitive iron (Lee, C.-T.A. et al. 2010. The redox state of arc mantle using Zn/Fe systematics. Nature, v. 468, p. 681-685). The surprise is that the parent magmas of both calc-alkaline and tholeiitic fractionation series have identical Zn/Fe ratios, suggesting that both partially melt from mantle with much the same availability of oxygen. The Zn/Fe ratios differ in more evolved igneous rocks from the two series, suggesting that it is in the fractionating magma chambers that the distinctively different oxygenation occurs, not in the zone of mantle melting.

Related Articles

• Geologist’s discoveries resolve debate about oxygen in Earth’s mantle (scienceblog.com)

As the temperature and pressure affecting crustal rocks go up and down, as for instance in the thickening of crust when two continents collide and then erosion strips off the cover so that the rocks slowly rise, the rocks undergo progressive changes in their mineral content; in both cases they are metamorphosed. Rising intensity of conditions gives rise to a prograde metamorphic sequence, and when they wane retrograde metamorphism takes place as the elements that combine in minerals react to adjust to new conditions. In some cases it is possible to use the mineral assemblages, specifically the proportions of different elements that are shared between two or more minerals, to chart the changes in temperature and pressure. That reveals the path taken by the rock through temperature- and pressure space, which is effectively a measure of the crustal processes involved and the geothermal conditions under which they acted: a P-T path. Adding the timing to give a sort of movie to all the changes has been hit-or-miss up to now, and based on radiometric ages from igneous rocks formed and emplaced during the metamorphic evolution. Thanks to the finely targeted mass spectrometry that an ion microprobe an achieve, adding the ‘t’ dimension is now possible from the metamorphic rocks themselves (Sajeev, K. et al. 2010. Sensitive high-resolution ion microprobe U-Pb dating of prograde and retrograde ultrahigh-temperature  metamorphism exemplified by Sri Lankan granulites. Geology, v. 38, p. 971-974). Minerals based on the element zirconium (Zr), such as zircon and monazite are extremely resistant to the effects of temperature as regards the radioactive and radiogenic elements that they contain, specifically uranium (U) and thorium (Th) and the lead (Pb) isotopes that form when ²³⁵U, ²³⁸U and ²³²Th decay. Both these minerals become zoned as successive layers grow during metamorphism, and the ion microprobe can measure the isotopic composition on a later-by-layer and therefore event-by-event basis. The famous granulites (charnockites) of the island of Sri Lanka (Ceylon) reached the peak of their metamorphism (1050°C and 0.9 GPa) at ~570 Ma and began to retrogress about 20 Ma later around the start of the Cambrian. Previously it was not possible to separate metamorphic ages from those when the original rocks formed in the Archaean and early Neoproterozoic.  Such high temperatures are very difficult to attain in the crust under normal geothermal conditions unless extra heat is added by large volumes of basaltic magma ponding at the base of the crust during crustal thickening.

As any gardener knows, the element phosphorus is an essential plant nutrient or fertiliser, along with potassium and nitrogen plus a host of minor elements that are rarely mentioned as sufficient amounts are generally available in soils. The same necessities for life apply to oceans too, in which amounts vary a great deal from place to place and whose relative proportions have changed through geological time. For the oceans the main source of phosphorus is the continental crust, where the element resides mainly in the mineral apatite (Ca₅(PO₄)₃(F,Cl,OH)). This is not an easily dissolved mineral, which is why for agricultural fertiliser it is generally made available in the soluble form of calcium superphosphate (Ca(H₂PO₄)₂) that is produced by reaction between apatite and sulfuric acid. Since the land surface was colonised by plants about 450 Ma ago, biological processes made phosphorus more readily available to solution in river water by their break-down of apatite; supply by rivers to the ocean nowadays is of the order of 10⁹ kg y^-1. Ups and downs of P dissolved in ocean water though geological time would be expected to have influenced its overall biological productivity, controlled by photosynthetic phytoplankton and prokaryotes. Variations of carbon isotopes (δ¹³C) in organic and carbonate sediments are know to have occurred episodically since Archaean times, suggesting wide fluctuations in both bioproductivity and burial of dead organic matter. However, it has been hard to judge any geochemical reasons underpinning such variations. Since it is now clear that the common iron mineral goethite (FeOOH) ‘mops up’ many chemical species including phosphate ions by adsorption on its grain surfaces, measuring the P/Fe ratios in marine ironstones containing these minerals is a potential guide to the changing phosphorus concentration in the oceans (Planavsky, N.J. et al. 2010. The evolution of the marine phosphate reservoir. Nature, v. 467, p. 1088-1090).

The US-French-Canadian researchers charted P/Fe ratios in banded iron formations and ironstones precipitated around ocean-floor hydrothermal vents since the Archaean. What emerged were four episodes: from 2900 to 1700 Ma with generally low ratios; the Neoproterozoic from 750 to 635 Ma with much higher ratios; the Phanerozoic from Cambrian to Jurassic with low ratios and post-Cretaceous high ratios. There are several significant gaps in the record of ocean phosphate levels, notable one a billion years long from 750 to 1700 Ma. One factor that probably affected the variation is the way that dissolved silica (SiO₂) drives down the proportion of phosphate adsorbing onto goethite. The rapid evolution and expansion since the Cretaceous of diatoms that secrete silica probably reduced SiO₂ concentration in ocean water as their remains rained down to be buried on the ocean floor; that explains the high P/Fe ratios since about 100 Ma. Earlier Phanerozoic oceans are estimated to have had as much as seven times the present concentration of dissolved SiO₂, thereby explaining the low values of P/Fe in ironstones deposited in the 100-540 Ma range. From 1700 to 3000 Ma the low P/Fe suggests oceanic phosphorus levels equivalent to those from the Jurassic to Cambrian (but perhaps up to 4 times that, depending on the poorly constrained SiO₂ concentrations).

The Neoproterozoic phosphorus ‘spike’, at a time when dissolved SiO₂ would have been no different from that in earlier times, suggests a massive influx of phosphate to the oceans at that time. It coincides with the two greatest glacial epochs the Earth has experienced: ‘Snowball’ Earth when glacial ice existed at tropic latitudes. In themselves the massive glaciations offer an explanation for high phosphorus delivery from the continents through glacial erosion and massive run-off during melting. More exciting is that the P/Fe ‘spike’ occurred at a time of massive perturbations in stable carbon isotopes ascribed to huge explosions of phytoplankton and organic carbon burial, which would have been permitted by high dissolved phosphate in the oceans. A large increase in primary biological productivity, i.e. photosynthesis, would have boosted oxygen levels; a necessity for the emergence of metazoan life forms soon after the end of ‘Snowball’ Earth conditions. But that begs the question of how glacially ground-up apatite, abundant as it would have been together with vast amounts of other rock debris, came to be dissolved. In today’s oceans crystalline apatite is barely soluble. It seems that apatite’s solubility decreases as temperature rises, and increases with pH – in alkaline conditions. As well as being cold, Neoproterozoic ocean water around the time of the ‘Snowball’ Earths was saturated with carbonate ions that helped thick, almost pure limestones to form globally after each glaciation. That spells alkaline conditions favouring phosphate solution. The authors speculate that global geochemical conditions during the Cryogenian Period (850-635 Ma) may have fostered the origin of the metazoans. Maybe, but their data have a billion-year gap immediately before that Period, and genomic molecular clocks suggest that the root metazoans emerged as much as half a billion years earlier.
See also: Filippelli, G.M. 2010. Phosphorus and the gust of fresh air. Nature, v. 467, p.1052-1053.

As any gardener knows, the element phosphorus is an essential plant nutrient or fertiliser, along with potassium and nitrogen plus a host of minor elements that are rarely mentioned as sufficient amounts are generally available in soils. The same necessities for life apply to oceans too, in which amounts vary a great deal from place to place and whose relative proportions have changed through geological time. For the oceans the main source of phosphorus is the continental crust, where the element resides mainly in the mineral apatite (Ca₅(PO₄)₃(F,Cl,OH)). This is not an easily dissolved mineral, which is why for agricultural fertiliser it is generally made available in the soluble form of calcium superphosphate (Ca(H₂PO₄)₂) that is produced by reaction between apatite and sulfuric acid. Since the land surface was colonised by plants about 450 Ma ago, biological processes made phosphorus more readily available to solution in river water by their break-down of apatite; supply by rivers to the ocean nowadays is of the order of 10⁹ kg y^-1. Ups and downs of P dissolved in ocean water though geological time would be expected to have influenced its overall biological productivity, controlled by photosynthetic phytoplankton and prokaryotes. Variations of carbon isotopes (δ¹³C) in organic and carbonate sediments are know to have occurred episodically since Archaean times, suggesting wide fluctuations in both bioproductivity and burial of dead organic matter. However, it has been hard to judge any geochemical reasons underpinning such variations. Since it is now clear that the common iron mineral goethite (FeOOH) ‘mops up’ many chemical species including phosphate ions by adsorption on its grain surfaces, measuring the P/Fe ratios in marine ironstones containing these minerals is a potential guide to the changing phosphorus concentration in the oceans (Planavsky, N.J. et al. 2010. The evolution of the marine phosphate reservoir. Nature, v. 467, p. 1088-1090).

The US-French-Canadian researchers charted P/Fe ratios in banded iron formations and ironstones precipitated around ocean-floor hydrothermal vents since the Archaean. What emerged were four episodes: from 2900 to 1700 Ma with generally low ratios; the Neoproterozoic from 750 to 635 Ma with much higher ratios; the Phanerozoic from Cambrian to Jurassic with low ratios and post-Cretaceous high ratios. There are several significant gaps in the record of ocean phosphate levels, notable one a billion years long from 750 to 1700 Ma. One factor that probably affected the variation is the way that dissolved silica (SiO₂) drives down the proportion of phosphate adsorbing onto goethite. The rapid evolution and expansion since the Cretaceous of diatoms that secrete silica probably reduced SiO₂ concentration in ocean water as their remains rained down to be buried on the ocean floor; that explains the high P/Fe ratios since about 100 Ma. Earlier Phanerozoic oceans are estimated to have had as much as seven times the present concentration of dissolved SiO₂, thereby explaining the low values of P/Fe in ironstones deposited in the 100-540 Ma range. From 1700 to 3000 Ma the low P/Fe suggests oceanic phosphorus levels equivalent to those from the Jurassic to Cambrian (but perhaps up to 4 times that, depending on the poorly constrained SiO₂ concentrations).

The Neoproterozoic phosphorus ‘spike’, at a time when dissolved SiO₂ would have been no different from that in earlier times, suggests a massive influx of phosphate to the oceans at that time. It coincides with the two greatest glacial epochs the Earth has experienced: ‘Snowball’ Earth when glacial ice existed at tropic latitudes. In themselves the massive glaciations offer an explanation for high phosphorus delivery from the continents through glacial erosion and massive run-off during melting. More exciting is that the P/Fe ‘spike’ occurred at a time of massive perturbations in stable carbon isotopes ascribed to huge explosions of phytoplankton and organic carbon burial, which would have been permitted by high dissolved phosphate in the oceans. A large increase in primary biological productivity, i.e. photosynthesis, would have boosted oxygen levels; a necessity for the emergence of metazoan life forms soon after the end of ‘Snowball’ Earth conditions. But that begs the question of how glacially ground-up apatite, abundant as it would have been together with vast amounts of other rock debris, came to be dissolved. In today’s oceans crystalline apatite is barely soluble. It seems that apatite’s solubility decreases as temperature rises, and increases with pH – in alkaline conditions. As well as being cold, Neoproterozoic ocean water around the time of the ‘Snowball’ Earths was saturated with carbonate ions that helped thick, almost pure limestones to form globally after each glaciation. That spells alkaline conditions favouring phosphate solution. The authors speculate that global geochemical conditions during the Cryogenian Period (850-635 Ma) may have fostered the origin of the metazoans. Maybe, but their data have a billion-year gap immediately before that Period, and genomic molecular clocks suggest that the root metazoans emerged as much as half a billion years earlier.

See also: Filippelli, G.M. 2010. Phosphorus and the gust of fresh air. Nature, v. 467, p.1052-1053.

Geoscientists take it for granted that the Earth has a certain age (currently estimated at 4.54 Ga), but it is one divined from indirect evidence, lead isotopes in meteorites and ancient ores of lead derived from uranium. If ever geoscientists are to grasp the nature of the early planet the evidence would be geochemical, yet also second-hand because relics must lie somewhere in the mantle as the crust is constantly being changed. For decades it has been known that the mantle shows geochemical heterogeneity as a result of episodes of partial melting from which the oceanic and continental crust emerged. Even with such an ancient origin it seems intuitively likely that there should be some mantle that has not been interfered with. Now a group of geochemists from the US and Britain have presented evidence for just such ur-mantle (Jackson, M.G et al. 2010. Evidence for the survival of the oldest terrestrial mantle reservoir. Nature, v. 466, p. 853-856). Their data come from Cenozoic lavas collected on Baffin Island and in West Greenland, which gave an earlier clue for having melted from a truly antique source: they contain the highest ratio of helium produced in the Big Bang (3He) to that released by radioactive decay (4He). Repeated melting of the mantle gradually drives off, yet radioactive decay continually replenishes its complement of 4He, so the more reworked a mantle source for lavas is the lower its 3He/ 4He ratio. This notion is backed up by the lead and neodymium isotopes in the Baffin Island and West Greenland lavas and they suggest an age of formation of the mantle source between 4.45-4.55 Ga.

Convection over billions of years ensures a degree of mixing in the mantle, but such is the viscosity of the Earth that there is a good chance that some areas have remained unchanged, the more so if the bulk of magmatism involving deep mantle has been linked to narrow rising plumes. But what emerges from the rest of the geochemistry of these lavas? Provided they have not been contaminated by continental crust through which they have passed, it should be possible using models for the way different elements are contributed to or withheld from magma by mantle minerals to estimate the source mantle’s overall composition. The team did this, bearing in mind the uncertainties. Plotted relative to a ‘guestimate’ of the original bulk Earth based on the geochemistry of chondritic meteorites they sshoww a very good fit for those elements that are likely to be retained by mantle minerals during partial melting: the so-called ‘compatible’ elements. But the estimated source for the lavas seems to have been depleted in the ‘incompatible’ elements that are highly likely to enter magma as soon as partial melting starts. This pattern would be expected if the early mantle had undergone some kind of differentiation as a whole, and that would be a consequence of the entire mantle having been molten and then crystallising: some low-density minerals could preferentially have taken in incompatible elements and floated upwards to deplete those elements in the deep mantle. That is compatible with the idea of Moon formation as a result of a collision between the proto-Earth and a Mars-sized planet, which could have released sufficient energy in the form of heat tp completely melt the outermost Earth.

So the data reveal a great deal, especially that this ancient mantle may well have been the parent for all later mantle compositions as the Earth evolved by dominantly igneous processes. But they do not resolve the perennial debate as to whether the Earth accreted from a uniform mix of nebular material of which meteorites are relics, roughly the composition of chondrites, or heterogeneously from different materials that had condensed from incandescent vapour at different nebular temperatures at different times. Moon formation would have mixed up the latter efficiently in a mantle-wide magma ocean, so we may never know. However some of the oldest meteorites contain fragments of condensates that did form at different temperatures.

Geoscientists take it for granted that the Earth has a certain age (currently estimated at 4.54 Ga), but it is one divined from indirect evidence, lead isotopes in meteorites and ancient ores of lead derived from uranium. If ever geoscientists are to grasp the nature of the early planet the evidence would be geochemical, yet also second-hand because relics must lie somewhere in the mantle as the crust is constantly being changed. For decades it has been known that the mantle shows geochemical heterogeneity as a result of episodes of partial melting from which the oceanic and continental crust emerged. Even with such an ancient origin it seems intuitively likely that there should be some mantle that has not been interfered with. Now a group of geochemists from the US and Britain have presented evidence for just such ur-mantle (Jackson, M.G et al. 2010. Evidence for the survival of the oldest terrestrial mantle reservoir. Nature, v. 466, p. 853-856). Their data come from Cenozoic lavas collected on Baffin Island and in West Greenland, which gave an earlier clue for having melted from a truly antique source: they contain the highest ratio of helium produced in the Big Bang (³He) to that released by radioactive decay (⁴He). Repeated melting of the mantle gradually drives off, yet radioactive decay continually replenishes its complement of ⁴He, so the more reworked a mantle source for lavas is the lower its ³He/ ⁴He ratio. This notion is backed up by the lead and neodymium isotopes in the Baffin Island and West Greenland lavas and they suggest an age of formation of the mantle source between 4.45-4.55 Ga.

Convection over billions of years ensures a degree of mixing in the mantle, but such is the viscosity of the Earth that there is a good chance that some areas have remained unchanged, the more so if the bulk of magmatism involving deep mantle has been linked to narrow rising plumes. But what emerges from the rest of the geochemistry of these lavas? Provided they have not been contaminated by continental crust through which they have passed, it should be possible using models for the way different elements are contributed to or withheld from magma by mantle minerals to estimate the source mantle’s overall composition. The team did this, bearing in mind the uncertainties. Plotted relative to a ‘guestimate’ of the original bulk Earth based on the geochemistry of chondritic meteorites they sshoww a very good fit for those elements that are likely to be retained by mantle minerals during partial melting: the so-called ‘compatible’ elements. But the estimated source for the lavas seems to have been depleted in the ‘incompatible’ elements that are highly likely to enter magma as soon as partial melting starts. This pattern would be expected if the early mantle had undergone some kind of differentiation as a whole, and that would be a consequence of the entire mantle having been molten and then crystallising: some low-density minerals could preferentially have taken in incompatible elements and floated upwards to deplete those elements in the deep mantle. That is compatible with the idea of Moon formation as a result of a collision between the proto-Earth and a Mars-sized planet, which could have released sufficient energy in the form of heat tp completely melt the outermost Earth.

So the data reveal a great deal, especially that this ancient mantle may well have been the parent for all later mantle compositions as the Earth evolved by dominantly igneous processes. But they do not resolve the perennial debate as to whether the Earth accreted from a uniform mix of nebular material of which meteorites are relics, roughly the composition of chondrites, or heterogeneously from different materials that had condensed from incandescent vapour at different nebular temperatures at different times. Moon formation would have mixed up the latter efficiently in a mantle-wide magma ocean, so we may never know. However some of the oldest meteorites contain fragments of condensates that did form at different temperatures.

Kei Hirose, the discoverer in 2002 of a ultra high-pressure transformation of mantle mineralogy, has produced a highly readable review of the implications of his work for how the mantle functions (Hirose, K. 2010. The Earth’s missing ingredient. Scientific American, v. 302 (June 2010 issue), p. 58-65).

Seismology has long charted the occurrence of step-changes in mantle properties at a several more or less constant depths. Mantle above 410 km provide most of the samples available to geoscientists as inclusions in basalt lavas and is olivine-rich peridotite. From 410-660 km the elements forming olivine take on a different configuration more akin to the mineral spinel; also backed by some direct as well as theoretical/experimental evidence. At 660 km deep seismic properties change dramatically in a major transition zone. Experimental work in the 1970s with mantle chemical compositions at high pressures and temperatures showed that at greater depths the structure of magnesium silicates like olivine, pyroxene and spinel collapses to a denser form with very efficient packing of aoms that is the same as that of a broad group of minerals known as perovskites. That seemed to be the end of the matter. However, continued geophysical investigations and geochemical studies of basalts derived by partial melting of mantle rock teased out complexities in the once assumed simplicity of the mantle. In 1983 analysis of seismic records revealed a further step in physical properties of the deepest mantle (once designated the D layer) that forced a revision to recognise a transition at 2600 km deep, just 300 km above the core-mantle boundary. This now separates the 2000 km thick D’ layer from the lowest D” layer in the mantle. Subsequently, chemical heterogeneities in the deep mantle became a major puzzle.

Hirose and his team pushed experimental conditions to match the huge pressures below 2600 km and discovered a yet more efficient, hitherto unknown molecular configuration that arranges magnesium, silicon and oxygen into separate layers: dubbed ‘post-perovskite’ for want of a already known mineral structure. As well as a small (1.5%) increase in density, the mineralogical change unexpectedly releases rather than consumes heat energy. Such an exothermic process clearly had great implications for how the mantle works. If rock from higher levels finds its way down to and below the D’-D” transition, as might happen if subducted oceanic lithosphere slabs continue ever downwards, it gets an energy ‘kick’. Theoretical work revealed that the early Earth would have been too hot for post-perovskite to form. But once it had cooled below a threshold the phase change ‘snapped’ into existence: that must have significantly changed mantle dynamics. Convective motion in D” that brings material to the D’:D” boundary the post-perovskite to perovskite phase change produces a sharp decrease in density and an upward force. So, once D” formed plume formation and overall mantle convection would have increased. That impetus could not have been present before so that early Earth mantle dynamics were more sluggish. That would maintain a hotter core-mantle boundary, thereby slowing cooling of the liquid core and formation of the solid inner core. Moreover, the upper mantle would have been cooler than now, creating the paradox of less surface magmatism on the early Earth. Theoretically, development of D” should have been marked by a 20% increase in heat flow and a paroxysm of tectonics and crust formation. Was that linked with the formation of stable continental crust around 4 Ga, the spurt in continental growth in the late Archaean or some later event (Hirose suggests 2.3 Ga, but no major tectonic shift has that age)?

As well as tectonic implications, the affect of the D” layer on the pace of crystallisation of the solid inner core may have controlled increasing strength and stability of the geomagnetic field. Because only Earth’s strong magnetic field protects the surface from life-threatening cosmic rays and the solar wind, in a roundabout way post-perovskite possibly played a role in allowing the origin, evolution and survival of life on our home world. That possibility is pretty much the ultimate link between solid Earth and the biosphere: take note Gaians!
See also: Buffet, B.A. 2010. The enigmatic inner core. Science, v. 328, p. 982-983.

Ice sheets during the last glaciation reached more than 2 km in thickness over vast high-latitude areas of the Northern Hemisphere. Even though ice has less than half the density of continental crust, their sheer mass forced the lithosphere down into the asthenosphere by up to several hundred metres. The displaced asthenosphere resulted in a corresponding bulge around the glacial fringe. Continental flood basalts are about three times as dense as ice and reach thicknesses up to 2-3 km, so they would have produced even more subsidence, although set against that is the uplifting effect of reduced density of the crust as a result of magmatic heating. The loading effects of individual volcanoes are well known. Yet surprisingly, there have been few accounts of subsidence caused by CFB loading, and the prevailing view is that plume-related large igneous provinces are preceded by doming and even erosion. Geophysicists at the University of Colorado modelled the effects of plumes and CFB eruption and reverse the general view decisively (Leng, W. & Zhong, S 2010. Surface subsidence caused by mantle plumes and volcanic loading in large igneous provinces. Earth and Planetary Science Letters, v. 291, p. 207-214). They found that phase changes in the rising mantle plume at the 660 km deep discontinuity cause subsidence themselves, so that even before volcanism begins the surface subsides. This is borne out by preservation of basinal sediments beneath some CFB provinces, such as the Siberian and Deccan Traps. Effectively, flood basalts may fill shallow basins that they recreate and maintain due to their loading effect on the crust during successive eruptions. The high elevations of many ancient CFB provinces are a product of later tectonic processes rather than being ‘built’ by volcanism.

‘Microdating’ sedimentary sequences

There are few minerals amenable to radiometric dating that are found in all sedimentary rock types. To give ages that are stratigraphically useful they would have had to form authigenically while the sediment itself was accumulating – glauconite in ‘greensands’ is an example. Calibrated stratigraphy largely depends on dateable igneous minerals found in volcanic rocks interlayered with sediments, the most common being zircon that can be dated precisely using U-Pb methods. The vast bulk of high quality ages of this kind depend on being able to collect sufficient volcanic ash or lava to yield zircon grains. So only volcanic layers thicker than a few centimetres have been used, and they are haphazard in their occurrence in sedimentary sequences. Much thinner ash layers do occur more commonly and uniformly in sequences from arc-related sedimentary basins, and being able to date those would permit much better control over rates of sedimentation and correlation between different sequences. The key is being able to date zircons in thin section (Rasmussen, B. & Fletcher, I.R. 2010. Dating sedimentary rocks using in situ U-Pb geochronology of syneruptive zircon in ash-fall tiffs <1 mm thick. Geology, v. 38, p. 299-302). Rasmussen and Fletcher (Curtin University, Western Australia) applied ion-microprobe methods to polished this sections of diamond drill core through Archaean sediments of the Pilbara craton in Western Australia, specifically to date a thin sediment layer that contains spherules formed by a major asteroid impact. They were able to narrow its age down to that of a thin ash only 15 mm above the spherules, about 2632+7 Ma. Though with a specialised objective, they demonstrate that semi-continuous stable isotope data in sediments can be calibrated sufficiently precisely to allow global correlations

Several flood volcanism events seem to link to mass extinctions, and they have been seen as the culprits for global environmental change. Since flood volcanism is outside human experience, geologists have little conception of what they do other than amass up to millions of cubic kilometres of lavas both mafic and silicic. They all probably emitted CO₂ and contributed to global warming, but whether they are able to deliver sulfate and particulate aerosols to the stratosphere to trigger cooling is hard to judge. But it seems there is a proxy for their global influence (Peate, D. 2009. Global dispersal of Pb by large-volume silicic eruptions in the Paraná-Etendeka large igneous province. Geology, v. 37, p. 1071-1074). Lead is potentially a volatile element that would accompany large volcanic gas and dust emissions, and it also bears unique isotopic signatures. Lead isotope proportions in sediments in contemporaneous marine sediments could be matched with those of large igneous provinces (LIPs). Should their signature occur globally, then it would be a fair bet that the products of volcanism did reach cloud-free stratospheric altitudes, there to be mixed globally and to remain aloft for many years. Below the tropopause gas and dust would soon be rained out, so that signatures would remain local.

Dave Peate of the University of Iowa found that the ²⁰⁸Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁶Pb ratios of 132 Ma sediments from an Ocean Drilling Program core in the mid-Pacific fall in the same field as those of the Paraná-Etendeka large igneous province. The sediments occur just below and within a prominent δ¹³C anomaly that geochemists believe to signify a major change in the biosphere, and the site is almost at the antipode of the Paraná-Etendeka large igneous province. Sediments from below the shift in carbon isotopes show lead-isotope ratios that can be explained by derivation from the oceanic crust underlying them, whereas those that witness a profound change in the biosphere overlap with the field of the P-E LIP. Specifically, they match the lead ‘signature’ of silicic volcanics rather than basalts, and in particular those with low titanium contents. So it seems that in this case basalt floods may not have been implicated in global environmental change, but the much less voluminous but probably far more violent ignimbrite do seem likely culprits. There were more than 20 such events within an interval of less than 2 Ma that emitted >100 km³ of silicic magma, most exceeding 1000 km³.