The majoritic garnet-bearing websterites from Bardane (Western Gneiss Region, Norway) are unique examples of metasomatised mantle wedge that interacted with C-saturated COH subduction fluid phases at 200km depth. These samples represent slices of former Archean transition zone mantle that upwelled, melted and accreted to a thick cratonic lithosphere, where it cooled until the Middle Proterozoic (stages M1-M2). During the subsequent Caledonian to Scandian subduction cycle (stage M3) these depleted mantle rocks were dragged into deep portions of the supra-subduction mantle wedge, where the episodic/progressive infiltration of crustal fluids at increasing depth enhanced crystallisation of diamond and of majoritic garnet. In these rocks microdiamond occurs as daughter phase in polyphase, fluid-related, inclusions and majoritic garnet is present as vein mineral. We studied the peak (stage M3) mineral assemblage garnet+orthopyroxene+olivine±clinopyroxene±phlogopite, where garnet composition becomes majoritic with pressure increase from 3 to 6.5GPa and 800-1000°C temperature. Majoritic garnet contains polyphase inclusions with daughter Cr-spinel+phlogopite/K-amphibole+dolomite/magnesite+graphite/diamond. They witness COH fluid/mineral interaction closely related to the oxidation state of the rock and responsible for diamond formation. We determined the fO2 in the M3 assemblage starting from Fe3+ analyses in majoritic garnet, which is characterised by the progressive enrichment in Fe3+/-Fe up to 0.15. The fO2 values obtained for the Bardane mantle wedge garnet websterites are up to -2log units lower than the fayalite-quartz-magnetite (FMQ) buffer along a trend from arc lavas (FMQ+1.5-FMQ+3) to mantle wedge garnet peridotites equilibrated at 4-5GPa (FMQ-FMQ+2). The determination of oxygen fugacity of the hydrate-carbonate-bearing M3 mineral association enabled us to estimate the speciation of slab-derived metasomatic COH fluids responsible for polyphase inclusions precipitation. Such fluids are mixtures of silicates and H2O+CO2, where water is the dominant species of the COH component. The peculiar composition of majorite-hosted diamond-bearing polyphase inclusions from Bardane and the speciation of its COH component point to an "oxidised" silicate-rich aqueous fluid contaminant from the subducted slab to the mantle wedge.
The oxidation state of mantle wedge majoritic garnet websterites metasomatised by C-bearing subduction fluids
SCAMBELLURI M;
2010-01-01
Abstract
The majoritic garnet-bearing websterites from Bardane (Western Gneiss Region, Norway) are unique examples of metasomatised mantle wedge that interacted with C-saturated COH subduction fluid phases at 200km depth. These samples represent slices of former Archean transition zone mantle that upwelled, melted and accreted to a thick cratonic lithosphere, where it cooled until the Middle Proterozoic (stages M1-M2). During the subsequent Caledonian to Scandian subduction cycle (stage M3) these depleted mantle rocks were dragged into deep portions of the supra-subduction mantle wedge, where the episodic/progressive infiltration of crustal fluids at increasing depth enhanced crystallisation of diamond and of majoritic garnet. In these rocks microdiamond occurs as daughter phase in polyphase, fluid-related, inclusions and majoritic garnet is present as vein mineral. We studied the peak (stage M3) mineral assemblage garnet+orthopyroxene+olivine±clinopyroxene±phlogopite, where garnet composition becomes majoritic with pressure increase from 3 to 6.5GPa and 800-1000°C temperature. Majoritic garnet contains polyphase inclusions with daughter Cr-spinel+phlogopite/K-amphibole+dolomite/magnesite+graphite/diamond. They witness COH fluid/mineral interaction closely related to the oxidation state of the rock and responsible for diamond formation. We determined the fO2 in the M3 assemblage starting from Fe3+ analyses in majoritic garnet, which is characterised by the progressive enrichment in Fe3+/-Fe up to 0.15. The fO2 values obtained for the Bardane mantle wedge garnet websterites are up to -2log units lower than the fayalite-quartz-magnetite (FMQ) buffer along a trend from arc lavas (FMQ+1.5-FMQ+3) to mantle wedge garnet peridotites equilibrated at 4-5GPa (FMQ-FMQ+2). The determination of oxygen fugacity of the hydrate-carbonate-bearing M3 mineral association enabled us to estimate the speciation of slab-derived metasomatic COH fluids responsible for polyphase inclusions precipitation. Such fluids are mixtures of silicates and H2O+CO2, where water is the dominant species of the COH component. The peculiar composition of majorite-hosted diamond-bearing polyphase inclusions from Bardane and the speciation of its COH component point to an "oxidised" silicate-rich aqueous fluid contaminant from the subducted slab to the mantle wedge.
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