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Journal of Petrology Volume 42 Number 1 Pages 55-67 2001
© Oxford University Press 2001
Fluid and Element Cycling in Subducted Serpentinite: a Trace-Element Study of the Erro–Tobbio High-Pressure Ultramafites (Western Alps, NW Italy)
1DIPARTIMENTO PER LO STUDIO DEL TERRITORIO E DELLE SUE RISORSE, UNIVERSITÀ DI GENOVA, CORSO EUROPA 26, 16132 GENOVA, ITALY
2DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITÀ DI MILANO, VIA BOTTICELLI 23, MILANO, ITALY
The oceanic serpentinization of peridotites and the influence of such an alteration on element cycling during their subduction dewatering are here investigated in a mantle slice (Erro–Tobbio peridotite), first exposed to oceanic serpentinization and later involved in alpine subduction, partial dewatering and formation of a high-pressure olivine + titanian-clinohumite + diopside + antigorite assemblage in the peridotites and in veins. Previous work indicates that high-pressure veins include primary brines, representing a residue after crystallization of the vein assemblage and containing recycled oceanic Cl and alkalis. To reconstruct the main changes during oceanic peridotite serpentinization and subsequent subduction, we analysed samples in profiles from serpentinized oceanic peridotites to high-pressure serpentinites, and from high-pressure ultramafites to veins. Here we present results indicating that the main features of the oceanic serpentinization are immobility of rare earth elements (REE), considerable water increase, local CaO decrease and uptake of trace amounts of Sr, probably as a function of the intensity of alteration. Sr entered fine-grained Ca phases associated with serpentine and chlorite. Trace-element analyses of mantle clinopyroxenes and high-pressure diopsides (in country ultramafites and veins), highlight the close similarity in the REE compositions of the various clinopyroxenes, thereby indicating rock control on the vein fluids and lack of exotic components carried by externally derived fluids. Presence of appreciable Sr contents in vein-forming diopside indicates cycling of oceanic Sr in the high-pressure fluid. This, together with the recognition of pre-subduction Cl and alkalis in the vein fluid, indicates closed-system behaviour during eclogitization and internal cycling of exogenic components. Diopside and Ti-clinohumite are the high-pressure minerals acting as repositories for REE and Sr, and for high field strength elements (HFSE), respectively. The aqueous fluid equilibrated with such an assemblage is enriched in Cl and alkaline elements but strongly depleted in REE and HFSE (less than chondrite abundances). Sr is low [(0·2–1·6) x chondrites], although selectively enriched relative to light REE.
KEY WORDS: eclogite facies; fluid; trace elements; serpentinite; subduction
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