An experimental investigation of C–O–H fluid-driven carbonation of serpentinites under forearc conditions

The carbonation of serpentinites in the forearc region of the mantle wedge in subduction zones and of serpentinites within the subducting slab by fluids derived from prograde dehydration and decarbonation has important implications for the deep Earth carbon cycle. This study shows that the carbonation of serpentinites under the forearc can establish, over time, a significant reservoir for carbon within a partially hydrated mantle wedge and that carbonation of (ultra-) mafic rocks within the subducting slab contributes to C-transfer to greater depths and might supply carbon for arc volcanism or the deep mantle

The South Patagonian batholith : 150 my of granite magmatism on a plate margin

A new database of 70 U–Pb zircon ages (mostly determined by SHRIMP) indicates that the South Patagonian batholith resulted from the amalgamation of subduction-related plutons from the Late Jurassic to the Neogene. Construction of the batholith began with a voluminous, previously undetected, Late Jurassic bimodal body mainly composed of leucogranite with some gabbro, emplaced along its present eastern margin within a restricted time span (157 to 145 Ma). This episode is, at least in part, coeval with voluminous rhyolitic ignimbrites of the Tobífera Formation, deposited in the deep Rocas Verdes Basin east of the batholith; this was the last of several southwestward-migrating silicic volcanic episodes in Patagonia that commenced in an Early Jurassic extensional tectonic regime. The quasi-oceanic mafic floor of the basin was also contemporaneous with this Late Jurassic batholithic event, as indicated by mutually cross-cutting field relationships. Changes in subduction parameters then triggered the generation of earliest Cretaceous plutons (Cretaceous 1: 144–137 Ma) west of the Late Jurassic ones, a westward shift that culminated at 136–127 Ma (Cretaceous 2) along the present western margin of the batholith. Most mid- to Late Cretaceous (Cretaceous 3: 126–75 Ma) and Paleogene (67–40 Ma) granitoids are represented by geographically restricted plutons, mainly emplaced between the previously established margins of the batholith, and mostly in the far south; no associated volcanic rocks of similar age are known at present in this area. During the final Neogene stage of plutonism (25–15 Ma) a recurrence of coeval volcanism is recognized within and east of the batholith. Typical εNdt values for the granitoids vary from strongly negative (− 5) in the Late Jurassic, to progressively higher values for Cretaceous 1 (− 4), Cretaceous 2 (− 0.7), Cretaceous 3 (+ 2) and the Paleogene (+ 5), followed by lower and more variable ones in the Neogene (− 1 to + 5). These variations may reflect different modes of pluton emplacement: large crustal magma chambers developed in the early stages (Late Jurassic to Cretaceous 1), leading to widespread emplacement of plutons with a crustal signature, whereas the Cretaceous 2, Cretaceous 3 and Palaeogene parts of the batholith resulted from incremental assembly of small plutons generated at greater depths and with higher εNdt. This does not in itself justify the idea of a reduction in crustal character due to progressive exhaustion of fusible material in the crust through which the magmas passed

Crystallization of platinum-group minerals from silicate melts: Evidence from Cr-spinel-hosted inclusions in volcanic rocks

The formation of platinum-group minerals (PGM) during magma differentiation has been suggested to be an important process in primitive magma evolution, but decisive textural evidence is difficult to obtain because PGM tend to be very small and very rare. We have investigated Cr-spinel phenocrysts from two oxidized magmas (Siberian meimechite and Vanuatu [Ambae Island] arc picrite) and one reduced magma (Uralian [Russia] ankaramite) for PGM inclusions and their platinum-group element (PGE) contents. We observed Os-Ir and Pt- Fe alloys entrapped as inclusions in Cr-spinel in all three suites of lava. The alloys may occur in association with PGE-bearing sulfides and co-trapped silicate melt. Cr-spinel crystals also contain measurable amounts of Os, Ir, Ru, and Rh, which are at concentrations 2×-100× higher than mantle values. Thermodynamic models indicate that the arc picrite and ankaramite melts were probably both saturated with the observed PGM phases, whereas the Os-Ir alloy grain observed in the meimechite is not in equilibrium with the "bulk" melt. Our results demonstrate that PGM (alloys and sulfides) occur as liquidus phases in primitive (unfractionated) melts at high temperature and at a variety of redox conditions, and that Cr-spinel is a significant host of PGE, either in the crystal structure or as PGM inclusions