Multi-stage carbonation and decarbonation in the mantle-wedge: stable-isotope compositions of carbonate phases in ultramaficrocks of the ulten zone (Eastern Alps, Italy).

The cycling of carbon (C) among Earth’s reservoirs, and especially the fate of C in subduction-related environments, are still under debate. Of particular importance are the origin and fluxes of C at convergent margins, in which CHO-bearing fluids released by the downgoing slabs introduce C into mantle-wedges where dolomite is stable and might act as a C-host. Orogenic ultramafic rocks of the Ulten Zone (UZ, Eastern Alps, Italy) consist of peridotites from a suprasubduction zone mantle wedge that were captured by the crustal portion of a subducting slab and subsequently exhumed during the Late Paleozoic Variscan orogeny (Scambelluri et al., 2006). These rocks were metasomatized by aqueous-carbonic fluids sourced by the crustal slab (Rampone & Morten, 2001; Scambelluri et al., 2006). Such metasomatism potentially enables large-scale C-mobility and transfer into the high-pressure eclogite-facies assemblage and, indeed, Sapienza et al. (2009) documented occurrence of dolomite in fine-grained garnet + amphibole peridotites. Using UZ peridotites, we follow up on these initial reports and present an integrated textural/chemical study aimed at obtaining new insights into the origin and fate of C in the mantle wedge. A survey of a large number of new samples (~100) reveals the occurrence of diverse carbonate phases in various microtextural settings (discrete grains to intergrowths to veins). Fine-grained garnet + amphibole peridotites that are feebly serpentinized contain, according to Sapienza et al. (2009), discrete dolomite grains, while in the serpentinized ones abundant calcite-brucite-intergrowths are closely related to serpentine veins. These intergrowths have been suggested to be dedolomitization products in mantle-derived rocks (Berg et al., 1986). Such decarbonation of mantle-wedge peridotite during serpentinization-related fluid-rock interactions results in the release of C and potentially enables large-scale C-mobility and transfer. Whole-rock analyses performed at the University of Ferrara reveal relatively uniform total-C concentrations (0.06 wt-% to 0.08 wt-%) and low δ13CVPDB of -17.2‰ to -11.1‰. Preliminary C and O isotope data for microsampled carbonates, obtained at Lehigh University, show more variable δ13C, with values ranging from -14 to -2‰, and δ18OVSMOW of +6 to +20‰. This isotopic variability and the overall low δ13C of the UZ carbonates will be discussed in light of textural evidence for carbonation and decarbonation reactions in the mantle wedge, and in the context of the multiple processes that are known to have modified the UZ mantle-wedge, based on trace element constraints (Scambelluri et al., 2006; Sapienza et al., 2009). This systematic study of textures and stable isotopic signatures of carbonates will enhance our understanding of possible C sources and crust-mantle interactions affecting the cycling of C in collisional subduction-zone settings. - Berg, G.W. (1986): Evidence for carbonate in the mantle. Nature, 324, 59-60. - Rampone, E. & Morten, L. (2001): Records of Crustal Metasomatism in the Garnet Peridotites of the Ulten Zone (Upper Austroalpine, Eastern Alps). J. Petrol., 42, 207-219. - Sapienza, G.T., Scambelluri, M., Braga, R. (2009): Dolomite-bearing orogenic garnet peridotites witness fluid-mediated carbon recycling in a mantle wedge (Ulten Zone, Eastern Alps, Italy). Contrib. Mineral. Petrol., 158, 401-420. - Scambelluri, M., Hermann, J., Morten, L., Rampone, E. (2006): Melt- versus fluid-induced metasomatism in spinel to garnet wedge peridotites (Ulten Zone, Eastern Italian Alps): clues from trace element and Li abundances. Contrib. Mineral. Petrol., 151, 372-394.