T17 Frontiers in Mineral Sciences: Mineral/Melt Energetics, Mineral Surface Chemistry, Mineral Nanoscience, and High-Pressure Mineralogy I

Sunday, 18 October 2009
8:00 AM - 12:00 PM
E145

TRIVALENT CATIONS IN MANTLE SILICATE PEROVSKITE

Paper number: T17-4
Program: Technical Sessions
Time: 8:45 AM – 9:00 AM

Authors

Abstract

In her 1999 Science paper, Alex Navrotsky addressed the importance of trivalent cations in mantle silicate perovskite. Here we report the effects of Fe3+ and Al3+ on Mg-silicate perovskite at high pressure. Perovskites with 0.09Fe2O3•0.91MgSiO3 and 0.05Fe2O3•0.05Al2O3•0.9MgSiO3 were synthesized above 50 GPa in the laser-heated diamond-cell. The spin state of Fe3+ was determined by synchrotron Mössbauer spectroscopy and X-ray emission spectroscopy at high-pressure. The effect of trivalent cation substitution on the unit cell volume of perovskite was investigated using synchrotron X-ray diffraction.

Above 50 GPa, Fe3+ and Al3+ enter the perovskite structure primarily by charge-coupled substitution. When Fe3+ exists alone in perovskite, it enters both the dodecahedral and octahedral sites. With compression, Fe3+ in the octahedral site undergoes a gradual change to the low spin state, while Fe3+ in the dodecahedral site remains high spin to at least 136 GPa. The spin transition results in a low bulk modulus for perovskite below 60 GPa.

When Fe3+ exists together with Al, at low pressure the majority of Fe3+ sits in the dodecahedral site in the high spin state. However, we found a rapid increase in the low spin Fe3+ fraction in the octahedral site of perovskite at ~70 GPa. Because low spin Fe3+ is similar in size to Al3+, more Fe3+ enters the octahedral site at higher pressure, while Al3+ must enter into the dodecahedral site. Above 80 GPa, approximately half of the Fe3+ exists in the octahedral site. As a result of the site mixing, the Al-bearing perovskite has a significantly smaller volume at lowermost mantle pressures.