HR: 1340h
AN: V13A-1439    [Abstracts]
TI: Oxidized As (V) in fore-arc mantle serpentinites: Transfer of fluid-soluble elements from slabs to arc magmas
AU: * Hattori, K H
EM: Khattori@uottawa.ca
AF: University of Ottawa, Department of Earth Sciences, Ottawa, ON K1N 6N5 Canada
AU: Takahashi, Y
EM: takahasi@geol.sci.hiroshima-u.ac.jp
AF: Hiroshima University, Department of Earth and Planetary Systems, Hiroshima, 739-8526 Japan
AU: Guillot, S
EM: stephane.guillot@univ-lyon1.fr
AF: CNRS,UCB-Lyon, Lab. Dynamique de la Lithosphere, Villeurbanne, 69622 France
AU: Johanson, B
EM: bo.johanson@gsf.fi
AF: Geological Survey of Finland, Microprobe Lab., Espoo, 02151 Finland
AB: Fluids released from subducting slabs and sediments hydrate the overlying peridotites in mantle wedges. Such hydrated peridotites (serpentinites) are enriched in fluid-soluble elements, although insoluble elements show the refractory geochemical signature. The enrichment pattern of serpentinites is similar to that of arc magmas (Hattori & Guillot, 2003 in Geology). Arsenic is one of the most enriched soluble elements, reaching greater than 1000 times of the primitive mantle value. We examined the speciation and occurrence of As in serpentinites to understand how such a highly mobile element can be transferred from subducting slabs to arc magmas via mantle wedges. Our study used serpentinites associated with the Tso Morari eclogitic rocks in the Indus Suture Zone of Himalaya. They represent the hydrated peridotites at the base of mantle wedge beneath the margin of Eurasia and were exhumed from the depth of about 100 km during the active subduction of the Indian continental margin. The serpentinites are made up of antigorite, chromite, minor talc. It contains As ranging from 6 to 275 ppm and S up to 51 ppm, but most have S below detection limit, 4 ppm. X-ray absorption spectroscopy data show that As is mostly As(V) and combined with oxygen, although Minute grains of As-bearing sulphides and arsenides are identified in samples. The fractions of As(V), calculated from the X-ray absorption near-edge structure, are greater in samples with higher As contents. High proportion of As(V) in the fore-arc mantle serpentinites contasts with high As(III) in the serpentinite at the base of the Nidar ophiolite. The source of As (V) in the serpentinites is most likely As adsorped on Fe- and Mn-oxides in subducted sediments and slabs. It was released during the subduction of slab and sediments at low temperatures, \< $350\deg$ C, and shallow depths, \< 25 km. Continuous flux of water from slabs at the base of the mantle wedge likely maintained As in oxidized condition. Arsenic in the serpentinites is, then transported to deeper levels in the mantle by the downward movement of the host serpentinites along the subduction zone. Eventual dehydration of serpentinites discharges water and fluid-soluble elements, leading to partial melting and tne enrichment of these elements in partial melt
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 3670 Minor and trace element composition
DE: 1025 Composition of the mantle
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting