HR: 0800h
AN: V41C-1413    [Abstracts]
TI: Minor and Trace Element Partitioning Among $\alpha$-, $\beta$- and $\gamma$-(Mg,Fe)$_{2}$SiO$_{4}$ in Fertile Peridotite KLB-1: a High Pressure Experimental Study
AU: * Davies, R M
EM: rdavies@amnh.org
AF: Dept. Earth & Plan. Sci., American Museum of Natural History, Central Park West at 79th, New York, NY 10024-5192
AU: Fei, Y
EM: y.fei@gl.ciw.edu
AF: Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd., NW Washington, DC 20015
AU: George, H E
EM: gharlow@amnh.org
AF: Dept. Earth & Plan. Sci., American Museum of Natural History, Central Park West at 79th, New York, NY 10024-5192
AU: George, H E
EM: gharlow@amnh.org
AF: Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd., NW Washington, DC 20015
AB: Syngenetic mineral inclusions in diamond provide direct sampling of the mantle where diamond crystallized. Petrological models for the upper mantle suggest a pyrolitic composition where olivine, (Mg,Fe)$_{2}$SiO$_{4}$, and its high-pressure polymorphs comprise as much as 60 vol.%. Consistent with this, olivine is the most common inclusion in diamonds from the upper mantle, and rare diamonds from the uppermost lower mantle contain mineral inclusions with peridotitic compositions. However no peridotitic diamonds have been recovered from the transition zone. The most likely reason is that the crystal structure signature of the transition zone minerals may be erased upon decompression. However, the partitioning behavior of the (Mg,Fe)$_{2}$SiO$_{4}$ polymorphs may provide independent evidence for the origin of the diamond. Crystal chemistry and partitioning among $\alpha$-, $\beta$- and $\gamma$-(Mg,Fe)$_{2}$SiO$_{4}$ crystallized from fertile peridotite KLB-1 are being studied in multianvil experiments at 12 to 21 GPa and 1300 to 1600$\deg$C. At 12 GPa olivine is the dominant phase, together with garnet (Grt), diopsidic clinopyroxene (Cpx) and minor Al, Cr-spinel. At 13 GPa, Grt becomes strongly majoritic (jumping from 3.0 Si per 12 oxygens at 12 GPa to 3.7 Si at 13 GPa) with a high MgSiO3 component. Between 13 and 15 GPa olivine is replaced by wadsleyite. Wadsleyite contains higher Na, Ni, Ti, Cr and Al than olivine. Partition coefficients between wadsleyite and olivine (D$^{Wad/Ol}$) range from 2 (Na) to 7 (Al). Olivine contains higher Mg-numbers (av. 90.0 Ol, 89.5 Wad) and Mn than wadsleyite. Both olivine and wadsleyite show a strong negative correlation between Si and Mg. Ca, Na, Al, Cr and Ti, however, exhibit different behavior between the two phases. In olivine, Mg is negatively correlated with Ca, Na, Al, Cr and Ti, while in wadsleyite, Mg is positively correlated with Na, Al, Cr and Ti. The experiments demonstrate that in a fertile peridotite 1) the former presence of wadsleyite can be determined by analysis of trace elements in olivine when preserved in the confines of a diamond and 2) the majorite content of garnet increases sharply as pressure is increased from 12 to 13 GPa.
DE: 3620 Crystal chemistry
DE: 3630 Experimental mineralogy and petrology
DE: 1025 Composition of the mantle
DE: 1212 Earth's interior--composition and state (8105)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting