High-pressure (Mg, Fe)₂SiO₄ phases in the Tenham chondritic meteorite

Abstract

THE high-pressure phase transformations of olivine (Mg, Fe)₂SiO₄ are of fundamental importance to the dynamics of the mantle^1–3. Experiments⁴ have demonstrated that at high pressure olivines transform to a solid solution series of (Mg, Fe) spinel (γ phase), while in olivines with Mg/Mg + Fe ratios greater than 0.85, ‘modified’ spinels with an orthorhombic structure (β phase) are formed⁵. The β phase in pure Mg₂SiO₄ is considered to have a stability field at pressures intermediate between those of olivine and the γ spinel phase, although the possibility that it forms as a metastable quench product from γ spinel has not been entirely ruled out. A central issue is the mechanism and hence the kinetics of these transformations but no such experimental work has been carried out. Details of the mechanisms involved may be inferred from experimental results, but available natural samples of high-pressure (Mg, Fe)₂SiO₄ polymorphs are confined to shock-produced phases in chondritic meteorites (for example, Tenham, Coorara, Catherwood and Coolamon^6–10). Binns et al.⁶ first reported the X-ray diffraction evidence for the existence of natural γ spinel in the Tenham chondrite where it occurs as small (≈ 100µm), purple grains within shock-produced veins. The natural γ spinel is termed ringwoodite. Shock-wave experiments by Jeanloz and Ahrens¹¹ have failed to produce a spinel phase, and recently Jeanloz¹² has claimed, from X-ray powder diffraction, IR and optical data that ringwoodite is a complex aggregate of goethite, majorite garnet and γ iron, mis-identified as the spinel phase by previous workers. Here we present definitive evidence from electron-probe microanalysis, transmission electron microscopy and diffraction on the true nature of the purple grains (termed ringwoodite) in the Tenham chondrite.