Subduction environments are complex systems, being the sites where the fluid phases metasomatise and re-fertilise the upper mantle by transferring elements from the slab to the mantle wedge. The coexistence of hydrous minerals with carbonates (phlogopite and magnesite) witnesses metasomatism by fluid phases enriched in CO2 and incompatible elements. Such fluids likely correspond to complex C–O–H solutions, released by dehydration and decarbonation reactions within the slab. Recent findings of OH-bearing minerals coexisting with carbonates and C polypmorphs [1,2], provide evidence that the fluid speciation is closely related to the oxidation state of the system. The oxygen fugacity (fO2) is conventionally used in petrology as a variable describing the oxidation state of a rock. In ultrahigh-pressure mantle peridotites fO2 can be estimated by the determination of ferric iron in garnet, where Fe3+ occurs in octahedral coordination. We studied metasomatised garnet websterites from Bardane (Western Gneiss Region, Norway), where inclusions of phlogopite + magnesite + graphite/diamond occur in orthopyroxene and majoritic garnets. These peridotites 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 in deep portions of the supra-subduction mantle wedge, where the infiltration of crustal fluids initiated diamond crystallisation. During this stage majoritic garnet crystallised at grain boundaries (M3-2) and in veins (M3-3) [3]. Evidence of C-O-H fluid influx is given by the precipitation of dolomite/magnesite, pyroxene, phlogopite, spinel and diamond/graphite in solid multiphase inclusions within orthopyroxene and garnet. We measured the Fe3+/ΣFe of M2-garnet and M3-majoritic garnet from both grain boundaries and veins with the "flank method" technique with electron microprobe at the Dipartimento di Scienze della Terra (University of Milano). The results indicate a progressive enrichment in Fe3+/ΣFe from M2-garnets (0.00-0.05) to M3-majoritic garnets (0.05-0.15). Both the majoritic garnets growing along the grain boundaries (M3-2) and those in the veins (M3-3) show a core-to-rim zonation with decreasing Fe3+/ΣFe from 0.15 to ≤0.05 and from 0.10 to 0.00, respectively. The fO2 calculations have been performed with an improved thermodynamic solution model for skiagite-bearing garnets, modelling a non-ideal mixing of Al and Fe3+, with the addition of Cr3+, on the octahedral site, and a non-ideal mixing on the dodecahedral site, with a symmetric regular solution model for reciprocal solid solutions of Ca–Fe2+–Mg–Al–Cr–Fe3+-garnet. The resulting fO2 is variably lower than the fayalite-quartz-magnetite (FMQ) buffer (≤ 1 log unit). However, these values are higher than fO2 of garnet peridotite xenoliths from sub-cratonic mantle equilibrated at similar pressure conditions (≤ FMQ-3) [4], but lower than mantle wedge garnet peridotites from the ultrahigh pressure Sulu belt (FMQ÷FMQ+2) [5]. The determination of oxygen fugacity of these hydrate–carbonate-bearing garnet peridotites enabled us to estimate the speciation of C–O–H metasomatic fluids derived from the subducting slab, responsible of polyphase inclusions precipitation, which result enriched in CO2. References. [1] van Roermund H.L.M., Carswell D.A., Drury M.R., et al. (2002) Geology, 30, 959–962; [2] Carswell D.A., van Roermund H.L.M. (2005) Eu J Miner 17, 31–42; [3] Scambelluri M., Pettke T., van Roermund H.L.M. (2008) Geology 36, 59–62; [4] Woodland A.B., Koch M. (2003) Earth Planet Scie Lett 214 295–310; [5] Malaspina N., Poli S., Fumagalli P. (in press) J Pet.

The oxidation state of majoritic garnets from Bardane (Western Norway) and fluid speciation in the deepest portions of the supra-subduction mantle wedge / N. Malaspina, M. Scambelluri, S. Poli, H.L.M. van Roermund. ((Intervento presentato al convegno Geoitalia tenutosi a Rimini nel 2009.

The oxidation state of majoritic garnets from Bardane (Western Norway) and fluid speciation in the deepest portions of the supra-subduction mantle wedge

N. Malaspina
Primo
;
S. Poli
Penultimo
;
2009

Abstract

Subduction environments are complex systems, being the sites where the fluid phases metasomatise and re-fertilise the upper mantle by transferring elements from the slab to the mantle wedge. The coexistence of hydrous minerals with carbonates (phlogopite and magnesite) witnesses metasomatism by fluid phases enriched in CO2 and incompatible elements. Such fluids likely correspond to complex C–O–H solutions, released by dehydration and decarbonation reactions within the slab. Recent findings of OH-bearing minerals coexisting with carbonates and C polypmorphs [1,2], provide evidence that the fluid speciation is closely related to the oxidation state of the system. The oxygen fugacity (fO2) is conventionally used in petrology as a variable describing the oxidation state of a rock. In ultrahigh-pressure mantle peridotites fO2 can be estimated by the determination of ferric iron in garnet, where Fe3+ occurs in octahedral coordination. We studied metasomatised garnet websterites from Bardane (Western Gneiss Region, Norway), where inclusions of phlogopite + magnesite + graphite/diamond occur in orthopyroxene and majoritic garnets. These peridotites 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 in deep portions of the supra-subduction mantle wedge, where the infiltration of crustal fluids initiated diamond crystallisation. During this stage majoritic garnet crystallised at grain boundaries (M3-2) and in veins (M3-3) [3]. Evidence of C-O-H fluid influx is given by the precipitation of dolomite/magnesite, pyroxene, phlogopite, spinel and diamond/graphite in solid multiphase inclusions within orthopyroxene and garnet. We measured the Fe3+/ΣFe of M2-garnet and M3-majoritic garnet from both grain boundaries and veins with the "flank method" technique with electron microprobe at the Dipartimento di Scienze della Terra (University of Milano). The results indicate a progressive enrichment in Fe3+/ΣFe from M2-garnets (0.00-0.05) to M3-majoritic garnets (0.05-0.15). Both the majoritic garnets growing along the grain boundaries (M3-2) and those in the veins (M3-3) show a core-to-rim zonation with decreasing Fe3+/ΣFe from 0.15 to ≤0.05 and from 0.10 to 0.00, respectively. The fO2 calculations have been performed with an improved thermodynamic solution model for skiagite-bearing garnets, modelling a non-ideal mixing of Al and Fe3+, with the addition of Cr3+, on the octahedral site, and a non-ideal mixing on the dodecahedral site, with a symmetric regular solution model for reciprocal solid solutions of Ca–Fe2+–Mg–Al–Cr–Fe3+-garnet. The resulting fO2 is variably lower than the fayalite-quartz-magnetite (FMQ) buffer (≤ 1 log unit). However, these values are higher than fO2 of garnet peridotite xenoliths from sub-cratonic mantle equilibrated at similar pressure conditions (≤ FMQ-3) [4], but lower than mantle wedge garnet peridotites from the ultrahigh pressure Sulu belt (FMQ÷FMQ+2) [5]. The determination of oxygen fugacity of these hydrate–carbonate-bearing garnet peridotites enabled us to estimate the speciation of C–O–H metasomatic fluids derived from the subducting slab, responsible of polyphase inclusions precipitation, which result enriched in CO2. References. [1] van Roermund H.L.M., Carswell D.A., Drury M.R., et al. (2002) Geology, 30, 959–962; [2] Carswell D.A., van Roermund H.L.M. (2005) Eu J Miner 17, 31–42; [3] Scambelluri M., Pettke T., van Roermund H.L.M. (2008) Geology 36, 59–62; [4] Woodland A.B., Koch M. (2003) Earth Planet Scie Lett 214 295–310; [5] Malaspina N., Poli S., Fumagalli P. (in press) J Pet.
2009
majorite, diamond, fO2, COH fluid speciation
Settore GEO/07 - Petrologia e Petrografia
The oxidation state of majoritic garnets from Bardane (Western Norway) and fluid speciation in the deepest portions of the supra-subduction mantle wedge / N. Malaspina, M. Scambelluri, S. Poli, H.L.M. van Roermund. ((Intervento presentato al convegno Geoitalia tenutosi a Rimini nel 2009.
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