Release of oxidizing fluids in subduction zones recorded by iron isotope zonation in garnet

Subduction zones are key regions of chemical and mass transfer between the Earth’s surface and mantle. During subduction, oxidized material is carried into the mantle and large amounts of water are released due to the breakdown of hydrous minerals such as lawsonite. Dehydration accompanied by the release of oxidizing species may play a key role in controlling redox changes in the subducting slab and overlying mantle wedge. Here we present measurements of oxygen fugacity, using garnet–epidote oxybarometry, together with analyses of the stable iron isotope composition of zoned garnets from Sifnos, Greece. We find that the garnet interiors grew under relatively oxidized conditions whereas garnet rims record more reduced conditions. Garnet δ56Fe increases from core to rim as the system becomes more reduced. Thermodynamic analysis shows that this change from relatively oxidized to more reduced conditions occurred during lawsonite dehydration. We conclude that the garnets maintain a record of progressive dehydration and that the residual mineral assemblages within the slab became more reduced during progressive subduction-zone dehydration. This is consistent with the hypothesis that lawsonite dehydration accompanied by the release of oxidizing species, such as sulfate, plays an important and measurable role in the global redox budget and contributes to sub-arc mantle oxidation in subduction zones

Activity-composition relations for the calculation of partial melting equilibria in metabasic rocks

A set of thermodynamic models is presented that, for the first time, allows partial melting equilibriato be calculated for metabasic rocks. The models consist of new activity–composition relations com-bined with end-member thermodynamic properties from the Holland &amp; Powell dataset, version 6.They allow for forward modelling in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3. In particular, new activity–composition relations are presented for silicate melt ofbroadly trondhjemitic–tonalitic composition, and for augitic clinopyroxene with Si–Al mixing on thetetrahedral sites, while existing activity–composition relations for hornblende are extended to includeK2O and TiO2. Calibration of the activity–composition relations was carried out with the aim ofreproducing major experimental phase-in/phase-out boundaries that define the amphibolite–granulitetransition, across a range of bulk compositions, at ≤13 kbar.</p

High‐grade metamorphism and partial melting of basic and intermediate rocks

Rocks of basic and intermediate bulk composition occur in orogenic terranes from all geological time periods and are thought to represent significant petrological components of the middle and lower continental crust. However, the former lack of appropriate thermodynamic models for silicate melt, amphibole and clinopyroxene that can be applied to such lithologies at high temperature has inhibited effective phase equilibrium modelling of their petrological evolution during amphibolite- and granulite facies metamorphism. In this work, we present phase diagrams calculated in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (NCKFMASHTO) compositional system for a range of natural basic and intermediate bulk compositions for conditions of 2–12 kbar and 600–1050 ∘C using newly parameterized activity–composition relationships detailed in a companion paper by Green et al. in this issue. Particular attention is given to mid-ocean ridge basalt (MORB) and diorite protolith bulk compositions. Calculated subsolidus mineral assemblages in all basic and intermediate rock types are modally dominated by hornblende and plagioclase, with variable proportions of epidote, clinopyroxene, garnet, biotite, muscovite, quartz, titanite or ilmenite present at different pressures. The H2O-saturated (wet) solidus has a negative P−T slope and occurs between ∼620–690 ∘C at mid- to lower-crustal pressures of 5–10 kbar. The lowest-T melts generated close to the wet solidus are calculated to have granitic major-element oxide compositions. Melting at higher temperature is attributed primarily to multivariate hydrate-breakdown reactions involving biotite and/or hornblende. Partial melt compositions calculated at 800–1050 ∘C for MORB show good correlation with analysed compositions of experimental glasses produced via hydrate-breakdown melting of natural and synthetic basic protoliths, with Niggli norms indicating that they would crystallize to trondhjemite or tonalite. Diorite is shown to be significantly more fertile than MORB and is calculated to produce high-T melts (>800 ∘C) of granodioritic composition. Subsolidus and suprasolidus mineral assemblages show no significant variation between different members of the basalt family, although the P−T conditions at which orthopyroxene stabilizes, thus defining the prograde amphibolite–granulite transition, is strongly dependent on bulk-rock oxidation state and water content. The petrological effects of open- and closed-system processes on the mineral assemblages produced during prograde metamorphism and preserved during retrograde metamorphism are also examined via a case-study analysis of a natural Archean amphibolite from the Lewisian Complex, northwest Scotland

Activity-composition relations for the calculation of partial melting equilibria in metabasic rocks

A set of thermodynamic models is presented that, for the first time, allows partial melting equilibriato be calculated for metabasic rocks. The models consist of new activityandndash;composition relations com-bined with end-member thermodynamic properties from the Holland andamp; Powell dataset, version 6.They allow for forward modelling in the system Na2Oandndash;CaOandndash;K2Oandndash;FeOandndash;MgOandndash;Al2O3andndash;SiO2andndash;H2Oandndash;TiO2andndash;Fe2O3. In particular, new activityandndash;composition relations are presented for silicate melt ofbroadly trondhjemiticandndash;tonalitic composition, and for augitic clinopyroxene with Siandndash;Al mixing on thetetrahedral sites, while existing activityandndash;composition relations for hornblende are extended to includeK2O and TiO2. Calibration of the activityandndash;composition relations was carried out with the aim ofreproducing major experimental phase-in/phase-out boundaries that define the amphiboliteandndash;granulitetransition, across a range of bulk compositions, at andle;13 kbar.</p

High‐grade metamorphism and partial melting of basic and intermediate rocks

Rocks of basic and intermediate bulk composition occur in orogenic terranes from all geological time periods and are thought to represent significant petrological components of the middle and lower continental crust. However, the former lack of appropriate thermodynamic models for silicate melt, amphibole and clinopyroxene that can be applied to such lithologies at high temperature has inhibited effective phase equilibrium modelling of their petrological evolution during amphibolite- and granulite facies metamorphism. In this work, we present phase diagrams calculated in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (NCKFMASHTO) compositional system for a range of natural basic and intermediate bulk compositions for conditions of 2–12 kbar and 600–1050 ∘C using newly parameterized activity–composition relationships detailed in a companion paper by Green et al. in this issue. Particular attention is given to mid-ocean ridge basalt (MORB) and diorite protolith bulk compositions. Calculated subsolidus mineral assemblages in all basic and intermediate rock types are modally dominated by hornblende and plagioclase, with variable proportions of epidote, clinopyroxene, garnet, biotite, muscovite, quartz, titanite or ilmenite present at different pressures. The H2O-saturated (wet) solidus has a negative P−T slope and occurs between ∼620–690 ∘C at mid- to lower-crustal pressures of 5–10 kbar. The lowest-T melts generated close to the wet solidus are calculated to have granitic major-element oxide compositions. Melting at higher temperature is attributed primarily to multivariate hydrate-breakdown reactions involving biotite and/or hornblende. Partial melt compositions calculated at 800–1050 ∘C for MORB show good correlation with analysed compositions of experimental glasses produced via hydrate-breakdown melting of natural and synthetic basic protoliths, with Niggli norms indicating that they would crystallize to trondhjemite or tonalite. Diorite is shown to be significantly more fertile than MORB and is calculated to produce high-T melts (&gt;800 ∘C) of granodioritic composition. Subsolidus and suprasolidus mineral assemblages show no significant variation between different members of the basalt family, although the P−T conditions at which orthopyroxene stabilizes, thus defining the prograde amphibolite–granulite transition, is strongly dependent on bulk-rock oxidation state and water content. The petrological effects of open- and closed-system processes on the mineral assemblages produced during prograde metamorphism and preserved during retrograde metamorphism are also examined via a case-study analysis of a natural Archean amphibolite from the Lewisian Complex, northwest Scotland