Scales of fluid-rock interaction and carbon mobility in the deeply underplated and HP-Metamorphosed Schistes Lustrés, Western Alps

Abstract

The Schistes Lustrés metasedimentary complex, exposed in the Italian and French Alps, is a regionally extensive unit of blueschist to eclogite facies marble, calc-schist and metapelite providing a record of sediment underplating and fluid-rock interaction at 40–65 km of a paleo-subduction zone. Limestones, carbonate-rich mudstones, and black shales in SE France and central Switzerland, herein used as a proxy for the unmetamorphosed Schistes Lustrés, bear a striking resemblance to sediments currently subducting into the E. Sunda margin.The Schistes Lustrés, with estimated P-T conditions ranging from ∼1 GPa and 350 °C to ∼2.3 GPa and 550 °C, have on a regional scale had their average carbonate δ18OVSMOW lowered to +20.4‰, relative to the average of +28.5‰ of their likely protoliths. This decrease in δ18O can be most easily explained by varying combinations of: (1) closed-system equilibration of carbonate O with O in silicate minerals in the same rocks (i.e., at the μm to cm scale); and (2) interaction between carbonates and H2O-rich fluids released during prograde metamorphic devolatilization of the metapelitic rocks in the section (at up to km scales). Regional-scale decrease in carbonate δ18O in the Schistes Lustrés could have occurred without the need for infiltration by fluids generated outside of the Schistes Lustrés complex. More dramatic reductions in δ18O (as low as +8‰; average = +15.5‰) at/near major tectonic contacts, inferred to be fossil transient subduction interfaces, likely reflect deformation-enhanced infiltration by fluids derived in mafic or ultramafic rocks at greater depths.Pressure solution and concomitant cleavage formation in the Schistes Lustrés presumably were associated with some carbonate removal, and higher-grade rocks show some evidence for decarbonation. However, within the Schistes Lustrés, C is deposited as carbonate in locally abundant veins and it has proven difficult to identify clear evidence of wholesale carbonate removal from the complex (i.e., at scales of kms to 10s of kms). This study demonstrates that large fractions of subducted sedimentary carbonate sections could persist to depths approaching those beneath volcanic fronts, if they are not accreted or underplated and depending on the extents to which they are infiltrated by H2O-rich fluids capable of driving both decarbonation reactions and carbonate dissolution. Sediments such as these could supply C for additions to arc source regions or, with further subduction, convey C into the deeper mantle. The C removal and deposition reported by others for sites of particularly high fluid flux, along large fault systems, in zones of high fracture/vein density, and in metasomatized contacts with mafic/ultramafic rocks, contrast with the relative retention and lower degrees of mobilization of C observed within km-scale packets of Schistes Lustrés away from zones of enhanced deformation and behaving as relatively closed systems. The fragmented and potentially lithologically biased nature of HP/UHP metamorphic suites greatly complicates attempts to derive fluid and C fluxes scalable to modern subduction margins.