HR: 1340h
AN: V13A-1447    [Abstracts]
TI: Composition of fluids during serpentinite breakdown in subduction zones: Evidence for limited boron mobility
AU: Hermann, J
EM: joerg.hermann@anu.edu.au
AF: Australian National University, RSES Mills Rd., Canberra, ACT 0200 Australia
AU: * Tenthorey, E
EM: eric.tenthorey@anu.edu.au
AF: Australian National University, RSES Mills Rd., Canberra, ACT 0200 Australia
AB: Subduction of serpentinized oceanic lithosphere provides an important mechanism by which H$_{2}$O and possibly boron is introduced to the deep mantle. Although it has been suggested that release of H$_{2}$O during serpentinite dehydration may govern certain physical processes in subduction zones, the geochemical signature of this fluid and the olivine-rich residue have largely been ignored in terms of their importance to arc magma genesis and the recycling of oceanic crust, respectively. In this work, we present results from novel piston-cylinder experiments designed to characterize the mobility of B and other trace elements during dehydration of serpentinite. Fluids derived from the serpentine breakdown reaction were drained into a porous diamond trap, thereby segregating fluid and solid domains. Following each run, the olivine/opx residue and the quench precipitates within the diamond trap were analysed for trace elements using laser ablation ICP-MS (LA). Three experiments were conducted at 30 kbar and 750$\deg$C for a duration of one week. LA analyses show that elements highly mobile in aqueous solutions (Li, As, Cs, Ba, Rb, Pb) are strongly depleted in the olivine-opx residue indicating that they were dissolved in the aqueous fluid produced by serpentine break down. From these data, we determined the fluid/residue partition coefficients ($^{F/R}$D) to be 10-250. The surprising exception is B, which is only enriched in the fluid by a factor of 2 to 4 times, with respect to the residue, corresponding to $^{F/R}$D = 3-5. This result is in marked contrast to previous studies on B partitioning, which suggest that K$_{D}$ for boron is $>>$ 1 during reaction at high temperatures. The difference in B behaviour observed in our experiments is likely due to the involvement of olivine, which is capable of incorporating significant boron. The results from our experiments have widespread implications with regard to boron recycling throughout the Earth and suggest that high-B olivines from xenoliths or ophiolitic sections are a fingerprint for metasomatized or previously serpentized mantle Our results also suggest that significant B is recycled into the deep mantle, and that fluids migrating into the olivine-rich mantle wedge may undergo partial B removal during recrystallization processes.
DE: 3630 Experimental mineralogy and petrology
DE: 3660 Metamorphic petrology
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting