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Journal Articles Journal of Petrology Year : 2011

Low-temperature wollastonite formed by carbonate reduction: a marker of serpentinite redox conditions

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Abstract

In the Alpine blueschist- to eclogite-facies meta-ophiolitic units of northern Corsica, the contact between a serpentinite body and an immediately overlying siliceous marble is remarkable for the occurrence of wollastonite and, on the marble side, a dark halo around the serpentinite. The refolded, continuous contact is a rodingite-type reaction zone with a centimetre-thick nephritic selvage of diopside + andradite/grossular ± perovskite on the serpentinite side, followed towards the marble by a 1-5 cm thick zone of massive wollastonite (± grossular), followed by a 5-20 cm thick dark zone (the halo) consisting of wollastonite + quartz + graphitic material (± grossular ± diopside), with no carbonate. The transition to the overlying wollastonite-free, calcite + quartz-bearing layers is sharp. Considering the stability of calcite + quartz everywhere else in the regional metamorphic series, this low-temperature occurrence of wollastonite (c. 400-450°C) requires unusual conditions. A clue to its origin is the abundance of graphitic matter with the wollastonite within a few decimetres of the serpentinite body. We interpret this observation as evidence for local reduction of Ca-carbonate to form elemental carbon and wollastonite according to the reaction: SiO2 + CaCO3 + 2H2 = CaSiO3 + C + 2H2O. Thermochemical modelling of mineral stability in the successive reaction zones suggests a positive oxygen-fugacity gradient from the serpentinite to the marble (-6 < FMQ < -1), mediated through a CH4- and H2-bearing aqueous intergranular fluid.
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Dates and versions

hal-00828900 , version 1 (31-05-2013)

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Benjamin Malvoisin, Christian Chopin, Fabrice Brunet, Matthieu E. Galvez. Low-temperature wollastonite formed by carbonate reduction: a marker of serpentinite redox conditions. Journal of Petrology, 2011, 53 (1), pp.159-176. ⟨10.1093/petrology/egr060⟩. ⟨hal-00828900⟩
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