Shock-induced minerals in meteorite provide prospecting tools for mineral physics

Over the last 40 years, a combination of seismic data from Earth's interior and experimentation at high pressures on materials of composition similar to that in Earth's mantle has led to an increasingly accurate picture of what succession of phases must be present at increasing depths. During the same time period, it has been discovered that many of these phases are present in minute amounts in meteorites that, before their arrival on Earth, were shocked to high pressures and temperatures during collisions between the asteroids from which the meteorites were derived. In a recent issue of PNAS, Chen et al. (1) reported a previously uncharacterized mineral from such a shocked meteorite and, by using diamond anvil experiments and synchrotron x-ray analysis, derived the stability conditions of that mineral and another closely related one discovered earlier this year from the same meteorite. Both of these phases are high-pressure polymorphs of the mineral chromite FeCr₂O₄, which is a common (although minor) mineral in peridotites, rocks of the upper mantle that are abundantly exposed at the surface and also found as inclusions (xenoliths) in volcanic rocks. Some of this volcanism (e.g., kimberlite) also carries diamonds and hence must be derived from a minimum depth of ≈120 km. The authors suggest that the newly discovered polymorphs of chromite, if found in natural rocks on Earth, could serve as anchor points for estimation of the depths of origin of rocks transported from great depth. The two polymorphs place minimum pressures of 12–13 GPa (CaFe₂O₄ structure) and 20 GPa (CaTi₂O₄ structure). These pressures are equivalent in Earth to depths of ≈400 km and ≈680 km, respectively. However, is there any reason to believe that any rocks presently at the surface have ever seen such pressures? …