HR: 0800h
AN: V51D-1517    [Abstracts]
TI: Melt-Rock Reactions in the Uppermost Sub-Arc Mantle Beneath Kamchatka: Evidence from Peridotite Xenoliths from Shiveluch Volcano
AU: * Bryant, J
EM: jbryant@geol.sc.edu
AF: University of South Carolina, Dept. of Geological Sciences, Columbia, SC 29208 United States
AU: Yogodzinski, G M
EM: gyogodzin@geol.sc.edu
AF: University of South Carolina, Dept. of Geological Sciences, Columbia, SC 29208 United States
AU: Churikova, T G
EM: churikova@pisem.net
AF: Institute of Volcanology & Seismology, Petropavlovsk, Kamchatsky, 683006 Russian Federation
AB: Ultramafic xenoliths from Shiveluch Volcano are predominantly spinel harzburgites with 55-90 modal percent olivine and 7-45 percent orthopyroxene (OPX). Abundant kink-banded olivine and textures that vary from protogranular to porphyroclastic, and granuloblastic, are consistent with a history of plastic deformation under conditions of mantle flow. Metasomatic OPX, phlogopite, clinopyroxene and amphibole, crosscut the xenoliths in mm-scale veins and form irregular patches that appear to replace the olivine-dominant primary mineralogy. Textural features suggest that high modal OPX in the harzburgites was produced by melt-rock reactions involving the replacement of olivine by OPX (e.g., OPX occurs mostly along grain boundaries between coarse olivine crystals). Primary mineral compositions are refractory, with olivine from FO89-94 and Cr# (Cr*100/Al+Cr) in spinel from 40-80. Equilibration temperatures and pressures, calculated using two-pyroxene thermometry and Ca-in-olivine barometry, are between 800-1000°C and 10-25kb. Our best estimate for the temperature and pressure of equilibration of the xenoliths, based on results from samples that have well developed two-pyroxene + olivine mineral assemblages, is approximately 900°C and 12-14kb. Oxygen barometry shows that the xenoliths are strongly oxidized (log (fO2)FMQ from 2.5-4.5) compared to abyssal and continental peridotites. This high fO2 may also reflect a history of melt-rock interaction beneath Shiveluch (e.g., Parkinson and Arculus, Chem. Geol., 1999). These results suggest that melt-rock reactions may play a strong role in creating and modifying the uppermost mantle and deepest crust beneath active subduction-related volcanic arcs.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1065 Major and trace element geochemistry
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 3621 Mantle processes (1038)
DE: 3651 Thermobarometry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005