Syros Metasomatic Tourmaline: Evidence for Very High-{delta}11B Fluids in Subduction Zones |
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Authors: | MARSCHALL HORST R; LUDWIG THOMAS; ALTHERR RAINER; KALT ANGELIKA; TONARINI SONIA |
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Institution: | 1 MINERALOGISCHES INSTITUT, UNIVERSITÄT HEIDELBERG IM NEUENHEIMER FELD 236, D-69120 HEIDELBERG, GERMANY
2 INSTITUT DE GÉOLOGIE, UNIVERSITÉ DE NEUCHÂTEL RUE EMILE ARGAND 11, CH-2007 NEUCHÂTEL, SWITZERLAND
3 ISTITUTO DI GEOSCIENZE E GEORISORSE, CNR VIA MORUZZI 1, I-56124 PISA, ITALY |
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Abstract: | High-pressure (HP) metamorphic blocks enclosed in a mafic toultramafic matrix from a mélange on the island of Syrosare rimmed by tourmaline-bearing reaction zones (blackwalls).The B isotopic composition of dravitic tourmaline within theseblackwalls was investigated in situ by secondary ion mass spectrometry.Boron in these tourmalines is unusually heavy, with 11B valuesexceeding +18 in all investigated samples and reaching an extremevalue of +28·4 in one sample. Blackwalls formed duringexhumation of the HP mélange at a depth of 2025km at temperatures of 400430°C, by influx of externalhydrous fluids. The compositions of the fluids are estimatedto be in the range of 100300 µg/g B with 11B valuesof +18 to +28 . The high 11B values cannot be explained by tourmalineformation from unmodified slab-derived fluids. However, suchfluids could interact with the material in the exhumation channelon their way from the dehydrating slab to the site of tourmalineformation in the blackwalls. This could produce exceptionallyhigh 11B values in the fluids, a case that is modelled in thisstudy. The model demonstrates that subduction fluids may beeffectively modified in both trace element and isotopic compositionduring their migration through the material overlying the subductingslab. Blackwall tourmaline from Syros has a large grain size(several centimetres), high abundance, and an exceptionallyhigh 11B value. The formation of tourmaline at the contact betweenmafic or felsic HP blocks and their ultramafic matrix involvedfluids released during dehydration reactions in the subductingslab. It forms a heavy-boron reservoir in hybrid rocks overlyingthe subducting slab, and may, thus, have a significant impacton the geochemical cycle of B and its isotopes in subductionzones. KEY WORDS: boron isotopes; tourmaline; subduction zone; fluid, high pressure |
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Keywords: | : boron isotopes tourmaline subduction zone fluid high pressure |
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