Element signatures of subduction-zone fluids. An experimental study of the element partitioning (Dfluid/rock) of natural partly altered igneous rocks from the ODP drilling site 1,256
The trace element signatures of fluids were investigated by leaching experiments on natural samples of partly altered mafic igneous rocks recovered from the drilling site 1,256 of ODP Leg 206 on the Cocos plate (Central America). Experiments with ultrapure water were performed at 400 °C/0.4 GPa and 500 °C/0.7 GPa. Both fluids and residual solids were examined to obtain the partition coefficients (Dfluid/rock) of various trace elements. Element partition coefficients (Dfluid/rock) obtained at 500 °C/0.7 GPa are significantly lower compared to results obtained at 400 °C/0.4 GPa, which is in contrast to observations at higher pressures (2.2–6 GPa) and temperatures between 700 and 1,400 °C (Kessel et al. in Earth Planet Sci Lett 237: 873–892, 2005a; Spandler et al. in Chem Geol 239: 228–249, 2007). This finding may indicate a considerable pressure effect on the leaching processes and strongly divergent fluid–rock interactions in the upper part of a subduction zone at 0.4–0.7 GPa compared to deeper subduction areas with higher pressures. Furthermore, this may be interpreted as one of the earliest fractionation processes during the subduction of crustal material.
KeywordsSubduction zones Trace elements Partition coefficients Leaching experiments Cocos plate
This publication is contribution No. 257 of the Sonderforschungsbereich 574 “Volatiles and Fluids in Subduction Zones” at the University of Kiel, supported by the Deutsche Forschungsgemeinschaft. We sincerely thank for assistance at the electron microprobe B. Mader (Kiel) and A. Fehler for his support in preparing the samples. Comments by the topic editor, Erwin Suess, and two anonymous referees are appreciated.
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