HR: 0800h
AN: V41E-1522    [Abstracts]
TI: Tranzition zone origins for olivine inclusions in diamond? A multianvil experimental study
AU: * Davies, R M
EM: rdavies@amnh.org
AF: Dept. Earth & Plan. Sci., American Museum of Natural History, New York, NY 10024 United States
AU: Harlow, G E
EM: gharlow@amnh.org
AF: Dept. Earth & Plan. Sci., American Museum of Natural History, New York, NY 10024 United States
AB: Diamonds and their inclusions are deep Earth probes that record the composition, geochemistry and evolution of Earth's mantle. Studies of mineral inclusions in diamond demonstrate that some diamonds originate in the transition zone and uppermost part of the lower mantle (400 to 700 km). However, it is not possible to determine if inclusions with (Mg,Fe)₂SiO₄ compositions formed as olivine or the higher-pressure polymorphs wadsleyite and ringwoodite in the transition zone, because the high-pressure mineral structures revert to olivine upon decompression. Chemical fingerprinting appears to be the only way to recognize former single-phase wadsleyite and ringwoodite inclusions in diamond. The crystal chemistry and element partitioning among olivine, wadsleyite and ringwoodite and associated minerals crystallized from fertile peridotite KLB-1 are being studied in multianvil experiments at 10 to 20 GPa and 1300 to 1700°C. Run products contain (Mg,Fe)₂SiO₄ (55%), garnet/majorite garnet (33%), clinopyroxene/CaSi-perovskite (12%) and a relict Cr-spinel phase (1%), with modal abundances largely remaining constant across the P-T range. The average Mg number for (Mg,Fe)₂SiO₄ polymorphs decreases with increasing pressure: olivine (90.0), wadsleyite (89.7) and ringwoodite (88.3). The decrease in Mg number between phases is accompanied by an increase in Ni and Ti and a decrease in Mn and Ca. Partition coefficients between wadsleyite and olivine are approximately 3 (Al), 1.5 (Cr), 1.5 (Ti), 1.4 (Na), 1.1 (Ni), 0.9 (Mn) and 0.8 (Ca). Elements such as Al, Cr, Mn, Ca and Na have higher abundances in wadsleyite compared to ringwoodite (D^Rw/Wad: 2.5 (Ti), 1.2 (Ni), 0.95 (Cr), 0.7 (Al), 0.8 (Na), 0.5 (Mn), and 0.3 (Ca)). The experiments demonstrate that wadsleyite or ringwoodite may be precursor phases for some olivine inclusions in diamond.
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1042 Mineral and crystal chemistry (3620)
DE: 1065 Major and trace element geochemistry
DE: 3621 Mantle processes (1038)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005