Fluid and Element Cycling in Subducted Serpentinite: a Trace-Element Study of the Erro-Tobbio High-Pressure Ultramafites (Western Alps, NW Italy) |
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| Authors: | SCAMBELLURI MARCO; RAMPONE ELISABETTA; PICCARDO GIOVANNI B |
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| Institution: | 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 |
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| Abstract: | The oceanic serpentinization of peridotites and the influenceof such an alteration on element cycling during their subductiondewatering are here investigated in a mantle slice (Erro–Tobbioperidotite), first exposed to oceanic serpentinization and laterinvolved in alpine subduction, partial dewatering and formationof a high-pressure olivine + titanian-clinohumite + diopside+ antigorite assemblage in the peridotites and in veins. Previouswork indicates that high-pressure veins include primary brines,representing a residue after crystallization of the vein assemblageand containing recycled oceanic Cl and alkalis. To reconstructthe main changes during oceanic peridotite serpentinizationand subsequent subduction, we analysed samples in profiles fromserpentinized oceanic peridotites to high-pressure serpentinites,and from high-pressure ultramafites to veins. Here we presentresults indicating that the main features of the oceanic serpentinizationare immobility of rare earth elements (REE), considerable waterincrease, local CaO decrease and uptake of trace amounts ofSr, probably as a function of the intensity of alteration. Srentered fine-grained Ca phases associated with serpentine andchlorite. Trace-element analyses of mantle clinopyroxenes andhigh-pressure diopsides (in country ultramafites and veins),highlight the close similarity in the REE compositions of thevarious clinopyroxenes, thereby indicating rock control on thevein fluids and lack of exotic components carried by externallyderived fluids. Presence of appreciable Sr contents in vein-formingdiopside indicates cycling of oceanic Sr in the high-pressurefluid. This, together with the recognition of pre-subductionCl and alkalis in the vein fluid, indicates closed-system behaviourduring eclogitization and internal cycling of exogenic components.Diopside and Ti-clinohumite are the high-pressure minerals actingas repositories for REE and Sr, and for high field strengthelements (HFSE), respectively. The aqueous fluid equilibratedwith such an assemblage is enriched in Cl and alkaline elementsbut strongly depleted in REE and HFSE (less than chondrite abundances).Sr is low (0·2–1·6) x chondrites], althoughselectively enriched relative to light REE. KEY WORDS: eclogite facies; fluid; trace elements; serpentinite; subduction |
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