HR: 14:55h
AN: V32E-06    [PDF]
TI: Deep Recycling of Sedimentary Lithologies in Subduction Zones: Geochemical and Physical Constraints from Phase Equilibria and Synchrotron-Based Multi-Anvil Experiments at 15-25 GPa
AU: * Rapp, R P
EM: rrapp@notes.cc.sunysb.edu
AF: Geodynamics Research Center, Ehime University 2-5 Bunkyo-cho, Matsuyama, 790-8577 Japan
AU: Nishiyama, N
AF: Geodynamics Research Center, Ehime University 2-5 Bunkyo-cho, Matsuyama, 790-8577 Japan
AU: Irifune, T
AF: Geodynamics Research Center, Ehime University 2-5 Bunkyo-cho, Matsuyama, 790-8577 Japan
AU: Inoue, T
AF: Geodynamics Research Center, Ehime University 2-5 Bunkyo-cho, Matsuyama, 790-8577 Japan
AU: Yamasaki, D
AF: Geodynamics Research Center, Ehime University 2-5 Bunkyo-cho, Matsuyama, 790-8577 Japan
AB: Ocean island basalts (OIBs) provide geochemical evidence for the presence of crustally-derived sedimentary material in the deep mantle plume source region for EM-type OIBs, and global seismic tomography provides us with dramatic images of subducted slabs, presumably carrying a sediment component, penetrating through the transition zone and into the lower mantle, in some cases to the core-mantle boundary. In an effort to better constrain the geochemical effects of deeply recycled sedimentary material in subduction zones, and their role in the petrogenesis of EM-type OIBs, we have undertaken a series of phase equlibria experiments in the multi-anvil apparatus at 10-25 GPa, using natural sediment lithologies as starting materials. The goal of these experiments is to identify the dominant phases in deeply subducted sediments, constrain their P-T stability limits, and to assess their role in crustal recycling and element redistribution in the deep mantle during subduction. The phase equilibria experiments were performed in a 2000-ton Kawai-type apparatus, using tungsten carbide cubes with 3 mm TEL and Cr-doped MgO and zirconia pressure media. A cylindrical lanthanum chromite heater was used, along with short ($<$ 1 mm), thick-walled, pressure-welded gold capsules to minimize thermal gradients and to retain the small amounts of water ($<$ 1 wt%) present in the starting material, and long run-durations (12-48 hours) in order to facilitate future analyses of the dominant phases for key trace elements using the ion microprobe. Our preliminary results at 10-25 GPa indicate that K-hollandite (KalSi$_{3}$O$_{3}$) and stishovite are the primary high-pressure phases in the sediment composition, with subordinate garnet and an as-yet-unidentified (possibly hydrous) Al-silicate phase present as well. These results suggest that K-hollandite is the primary repository for incompatible elements (e.g., La, Ce, Sr, Ba, Rb, etc., and the heat-producing elements K, U and Th) in sedimentary material recycled into the deep mantle via subduction. In-situ synchrotron-based multi-anvil experiments were performed on synthetic, pure K-hollandite at pressures up to 25 GPa to determine the equation of state parameters of this phase into the lower mantle. From these experiments, we derive the bulk modulus at room temperature for K-hollandite, K$_{0}$ = 183(2) GPa, and coefficient of thermal expansion, a$_{20.5}$ = 2.18(3) x 10$^{-5}$K$^{-1}$. Although the overall buoyancy of deeply subducted metasedimentary lithologies will be determined by the modal proportions of high-pressure phases, the calculated density of K-hollandite under lower mantle conditions is less than that of Mg- or Ca-perovskite, and approximately equal to that of magnesiowustite.
DE: 1025 Composition of the mantle
DE: 3630 Experimental mineralogy and petrology
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting