HR: 09:45h
AN: V51A-08    [PDF]
TI: B, Cl, Li Variability in the Subducted Oceanic Mantle and in Antigorite-Breakdown Fluids: Implications for Fluid Processes and Light Element Recycling
AU: * Scambelluri, M
EM: msca@dipteris.unige.it
AF: Dipartimento per lo Studio del Territorio e delle sue Risorse, University of Genova, C.so Europa 26, Genova, ITA 16132 Italy
AU: Muentener, O
AF: Geological Institute, University of Neuchatel, Rue E. Argand, Neuchatel, CHE 2007 Switzerland
AU: Ottolini, L
AF: CNR-IGG, V. Ferrata 1, Pavia, ITA 27100 Italy
AU: Pettke, T
AF: Institute of Isotope Geochemistry and Mineral Resources, ETH-Zentrum, Sonnegstr 5, Zurich, CHE 8092 Switzerland
AU: Vannucci, R
AF: Dipartimento di Scienze della Terra, University of Pavia, V. Ferrata 1, Pavia, ITA 27100 Italy
AB: We present an inventory of B, Cl, Li in a set of ultramafites featuring the evolutionary stages of the subducted oceanic mantle, and in the fluid inclusions formed at antigorite breakdown. Samples correspond to: non-subducted serpentinites (Northern Apennine); high-pressure antigorite serpentinites (Western Alps; Betic Cordillera); high-pressure olivine-opx rocks (Betic Cordillera). Analyses of Cl, B, Li in the rock forming minerals indicate that the hydrous mantle is an important light element carrier. The estimated bulk-rock B and Cl decrease from oceanic (46.7 B; 729 ppm Cl) to antigorite serpentinites (20 ppm B; 221 ppm Cl) to olivine-opx rocks (9.4 ppm B; 45 ppm Cl), suggesting release of oceanic Cl and B in subduction fluids, likely without inputs from external sources. Li is less abundant in oceanic serpentinites (1.26 ppm) and such concentrations are still preserved in high-pressure serpentinites. High Li contents in the olivine-opx rocks (4.9 ppm bulk Li) suggest income from associated crustal rocks. LAM ICP-MS analyses of fluid inclusions representing the antigorite-breakdown fluid in olivine-opx rocks, enabled an estimate of the trace element composition of this fluid. The inclusion compositions yield maximum average $^{fluid/rock}$D$_{B}$ and $^{fluid/rock}$D$_{Li}$ of 11 and 17.8, respectively. Model calculations of the B and Cl loss form subducting serpentinite suggest $^{fluid/rock}$D$_{B}$ and $^{fluid/rock}$D$_{Cl}$ of about 15-30 and 30-60, respectively, and $^{fluid/rock}$D$_{B}$ / $^{fluid/rock}$D$_{Cl}$ $<$ 1 (0.66-0.5). Cl thus preferentially partitions in fluids relative to B: the fluid B/Cl ratio progressively raises at increasing depths and temperature. Despite light element loss, appreciable B, Cl, Li are still stored in olivine-opx rocks beyond the antigorite stability. This permits a bulk storage of about 10 ppm B and 5 ppm Li, i.e. concentrations much higher than in normal mantle reservoirs. The oceanic mantle may thus retain B and Li deeper than the arc magma sources, potentially creating light element anomalies in the upper mantle.
DE: 1030 Geochemical cycles (0330)
DE: 1065 Trace elements (3670)
DE: 3660 Metamorphic petrology
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting