Tracing carbon sources and mobility in subduction zones: an approach using carbon contents and isotopes in orogenic Ulten Zone peridotites.

The fate of carbon (C) in subduction-related environments is still under debate and of particular importance for unravelling the origin and cycling of carbon within the Earth’s mantle. Mantle wedge-derived orogenic peridotites of the Ulten Zone (UZ) tectonic unit (Eastern Alps, Italy) have been captured by a subducting crustal slab and subsequently exhumed during the Late Paleozoic Variscan orogeny (Scambelluri et al., 2006). Besides common hydrous metasomatic phases (Marocchi et al., 2007), metasomatic dolomite has been optically detected in only a few samples in UZ peridotites (Sapienza etal., 2009). However, samples that are dolomite-free on thin-section scale have been shown to be compositionally indistinguishable from dolomite-bearing varieties (Sapienza et al., 2009). Thus, these rocks represent a natural laboratory to study the sources, storage and recycling of carbon in subduction-modified lithospheric mantle. We will present a systematic study of whole-rock (WR) C-content and C-isotopic compositions of UZ peridotites combined with their detailed petrography. Preliminary C-analyses have been carried out at the University of Ferrara using EA-IRMS. The peridotite samples investigated so far cover a wide range of mineral assemblage, grain size, texture and retrogression and do not contain optically detectable carbonate. Nevertheless, these samples display homogeneous C-concentrations (0.06 wt.% to 0.08 wt.%), likely residing in dolomite. The associated C-isotopic composition is light relative to the primitive mantle (~ -5 ‰) and characterized by high variability (δ13C from -17.17 ‰ to -11.12 ‰, relative to PDB). While these results confirm the presence of carbonate as an accessory phase in UZ peridotites and indicate that carbon, if mobilized from the subducting slab, can be stored within the mantle wedge, the isotope systematics require a recycling model that considers complex C-fractionation processes related to fluid-rock interaction and/or devolatilization. By integrating petrographic information with elemental and isotopic signatures, we aim to unravel the different carbon sources and the processes involved in the transfer of carbon to the mantle wedge in collisional subduction zone settings.