Variability of Stishovite Genesis under Terrestrial Conditions: Physicogeochemical Aspects

Abstract

A model of the genesis of stishovite and other SiO₂ phases in terrestrial matter is developed; it combines the physicochemical and geodynamic conditions of their formation. Based on the experimental data, a P–T diagram of SiO₂ polymorphs in combination with the boundaries of geospheres and geotherm was plotted. Stishovite and other SiO₂ phases of cosmic-impact synthesis were buried in the early Earth during the period of meteorite accretion (50 Ma). These SiO₂ phases are completely assimilated by melts of the pyrolite global magma ocean that existed for 500 Ma. By 2.0 Ga, the magma ocean crystallized, and the Earth’s crust, upper mantle, transition zone, and lower mantle with layer D” (with seismic boundaries between them) were formed. During this period, the main mass of the Earth’s core was separated, which completed by 2.7 Ga. As a result, the gravitational field intensified, which contributed to the fractional ultramafic–mafic evolution of mantle magmas with peritectic reactions of ringwoodite–akimotoite in the transition zone and bridgmanite in the lower mantle with melts and the formation of stishovite (shown experimentally at 20 and 26 GPa). These reactions in diamond-forming carbonate–silicate–carbon melts provided the formation of stishovite, which was captured as a paragenetic inclusion by diamonds and transported to the Earth’s surface by magmas. The genesis of stishovite under the terrestrial conditions is controlled by global mantle convection as well. The subduction of lithospheric plates to layer D'' near the liquid core was accompanied by the formation of stishovite, and then its transformation into poststishovite phases. When superplumes rise from layer D'' to the Earth’s crust, the peritectic reactions of postperovskite and bridgmanite, and then ringwoodite–akimotoite, with melts are likely to form stishovite and cause its subsequent transformation into low-pressure SiO₂ phases. With the emergence of the Earth’s crust, the impact-meteorite genesis of stishovite resumes. Stishovite that formed under the terrestrial conditions appears as an inclusion in ultradeep diamonds on the Earth’s surface. Stishovite of cosmic-impact synthesis is preserved in meteorite craters. In both cases, stishovite is a metastable phase.