HR: 10:20h
AN: V32A-01    [Abstracts]
TI: Subduction of Continental Crust into Earth's Transition Zone: an Experimental Investigation with Implications for the Fate of Continental Subduction
AU: * Wu, Y
EM: zmjin@cug.edu.cn
AF: The State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, 388 Lumo Road, Wuhan, 430074, China
AU: * Wu, Y
EM: zmjin@cug.edu.cn
AF: Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., Washington, DC 20018, United States
AU: Fei, Y
EM: y.fei@gl.ciw.edu
AF: Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., Washington, DC 20018, United States
AU: Jin, Z
EM: zmjin@cug.edu.cn
AF: The State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, 388 Lumo Road, Wuhan, 430074, China
AB: The occurrence of f ultra-high pressure metamorphic (UHPM) index minerals within rocks from continental collision zones indicate that continental rocks may descend into the Earth's upper mantle, perhaps even to the mantle transition zone, and leave specific geochemical signatures during the continental collision. The conceiving evidence of continental crust being subducted to depth >350 km has recently been considered with the discovery of former stishovite pseudomorphy in UHPM rocks from the Altyn Tagh, western China and relevant experiments at high pressure (>10-13 GPa). The idea that volumetrically abundant lithologies of continental crust, along with UHP eclogite could be subducted into the transition zone has also been supported by geochemical isotopic and seismic tomographical studies. In order to understand the fate of subducted continental crust, we conducted experimental studies up to 24GPa and 2073 K on natural UHPM gneisses powders in two bulk compositions corresponding to average upper continental crust and terrigenous rocks with piston-cylinder and multi-anvil apparatus. Phase identifications were based on raman spectra and microprobe analyses. Coesite, clinopyroxene, orthoclase and ganrnet with minor phengite or k-mica with unknown structure and epidote/lawsonite are found to be stable in runs up to 9GPa. At pressure between 9-14GPa, the charges mainly contain stishovite, cpx/jadeite, K-hollandite and garnet. Abundant mymekites consisting of K-hollandite and majoritic garnet (Si=3.18pfu at 14GPa and 1673K ) indicate the breakdown of biotite in starting materials. Rutile with α-PbO₂structure has also been identified as break down product of titanite. At pressures from 14 to 24GPa, the Si and Mg contents in garnet increase with increasing pressure, while Al and Fe and Ca contents decrease accordingly. Contents of Al and divalent cations in stishovite show slightly increase with increasing pressure. The composition of cpx becomes gradually less aluminous and higher in jadeite component with increasing pressure. The solubility of NaAlSi₃O₈ and CaAl₂Si₂O₈ component in K-hollandite, extending to 42 mol% and 11 mol% at 24GPa and 2073K respectively, shows a strong correlation with increasing temperature. Our experimental results suggest that the density of continental crust would be equal to or denser than that of pyrolite when those rocks have been transported to depth >300 km (9-10GPa), which would favor continental subduction into the lower part of the upper mantle.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3630 Experimental mineralogy and petrology
DE: 3654 Ultra-high pressure metamorphism
DE: 3924 High-pressure behavior
DE: 8412 Reactions and phase equilibria (1012, 3612)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting