Post-perovskite unveiled

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.

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