Phase Transformations in Primitive Morb and Pyrolite Compositions to 25 Gpa and Some Geophysical Implications
- Murli H. Manghnani and
- Yasuhiko Syono
Published Online: 21 MAR 2013
Copyright © 1987 by Terra Scientific Publishing Company (TERRAPUB), Tokyo.
High-Pressure Research in Mineral Physics: A Volume in Honor of Syun-iti Akimoto
How to Cite
Irifunea, T. and Ringwood, A. E. (1987) Phase Transformations in Primitive Morb and Pyrolite Compositions to 25 Gpa and Some Geophysical Implications, in High-Pressure Research in Mineral Physics: A Volume in Honor of Syun-iti Akimoto (eds M. H. Manghnani and Y. Syono), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM039p0231
- Published Online: 21 MAR 2013
- Published Print: 1 JAN 1987
Print ISBN: 9780875900667
Online ISBN: 9781118664124
- Mineralogy and Crystal Chemistry;
- Phase transformations;
- High Pressure-High Temperature Research
The mineralogies adopted by primitive MORB and “pyrolite minus olivine” compositions have been studied over the pressure range of 4–25 GPa at 1200–1400°C. Both compositions crystallize to eclogite assemblages (garnet+clinopyroxene) with and without orthopyroxene between 4 and 10 GPa, and there is little change in the relative proportions of garnet and pyroxene over this range. The proportion of garnet, however, increases rather rapidly above 10 GPa as pyroxene dissolves in the garnet structure, so that pyroxene-free garnetites are formed at 14–16 GPa. A Ca-rich glassy phase+garnet are recovered from 20 GPa in both compositions. The glassy phase has a composition close to CaSiO3 with 2–8 mol% CaTiO3 and 5–10 mol% MgSiO3 in solid solution, and is believed to be the retrogressive transformation product of a Ca-rich perovskite formed on the release of pressure. The zero-pressure density of this Ca-rich perovskite is estimated as 4.31 g/cm3, which is substantially higher than that of MgSiO3 perovskite (4.11 g/cm3). The amount of CaSiO3-rich perovskite increases and residual garnet again becomes aluminum-rich with increasing pressure above 20 GPa. Significant amounts of ilmenite (at 22.5 GPa) and perovskite (at 24.5 GPa) forms of MgSiO3 are also found to coexist with garnet and CaSiO3-rich perovskite in the “pyrolite minus olivine” composition. Density calculations based on these experimental results confirm that the subducted oceanic crust is substantially denser than surrounding pyrolite throughout the mantle, except for a limited region between 670 and 710 km. In this depth interval, a large amount of garnet remains stable in the MORB composition, whereas pyrolite transforms to a denser assemblage (Mg,Fe)SiO3-perovskite plus magnesiowüstite. Seismic velocity and density profiles calculated for the pyrolite composition are consistent with those obtained from seismic observations for the upper mantle and transition zone.