Titel: Redox conditions during deserpentinization in western Elba Island, Italy
Malte Kalter1, Wolfgang Bach2
1Freie Universität Berlin, Germany; 2Universität Bremen, Germany
Veranstaltung: GeoKarlsruhe 2021
The observation of oxidized arc melts has led to a discussion about the redox conditions during the dehydration reactions of serpentinites in subduction zones. The discussed range of oxygen fugacities (fO2) between+5 and -2 log units relatively to the QFM buffer allows sulfur to be present either as oxidized or reduced species.
This work investigates the development of the fO2 with serpentines form the western part of the island Elba in Italy. We compared observations of opaque mineral phases and silicates with thermodynamic models. The opaque mineral phases have previously shown to be a good indicator for the redox conditons during the hydration of ultramafic rocks.
The samples have faced different metamorphic grades during the contact metamorphism of the 6.9 Ma Mt. Capanne pluton up to the Amphibole-facies. The peak assemblage shows the paragenesis of prograde grown anthophyllite and olivine. The omnipresence of magnetite between 500 °C and 650 °C indicates an fO2 above the QFM buffer at these temperatures. However, the fO2 does not exceeded the Mt-Hm buffer because hematite has not formed. The maximum fO2 is 2 log units above the QFM buffer and limited due to the ubiquitous presence of pentlandite in the serpentinites. The most abundant paragenesis of pentlandite-magnetite-heazlewoodite and pentlandite-magnetite-pyrrhotite is in equilibrium with 0.01-0.1 mol/kg H2S. Combined with the low sulfur concentrations below 200 ppm in the bulk rock composition a loss of sulfur as a reduced species in the form of H2S is indicated.
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