Fluid-Rock Interaction and Carbon Retention in Deeply Subducted Basalts

Estimates have been made of the recycling of C into the deep-Earth via subducted oceanic slabs with little validation through petrologic and geochemical studies in the full array of sedimentary, mafic, and ultramafic rock types. The averaged estimated proportions of C subduction in these lithologies are 1:2.4 for sediment to oceanic crust [1,2] and 1:1 for oceanic crust to slab mantle materials [1], with the flux in carbonated slab mantle the least constrained. In our study, we investigate the effects of prograde metamorphism, devolatilization, and fluid-rock interactions on the retention and isotopic evolution of C in oceanic crust metamorphosed over a wide range of estimated P-T (1.2-2.5 GPa, 350-610°C). Of particular interest are assessments of the abundance of C, identification of C representing primary mantle/magmatic processes and seafloor alteration, potential records of fluid-mediated C exchange with nearby metasedimentary rocks, and the extent of large-scale C mobilization by decarbonation and dissolution. The results of this study will elucidate the δ13C of mixing end-members in models of output CO2 flux in volcanic arcs and yield insight regarding C subduction flux into deeper parts of the mantle. Metamorphosed pillow basalts in the Tianshan, China (1.2-2.1 GPa, 480-580°C), allow study of the retention of C during dehydration and other fluid-rock interactions in deeply subducted mafic rocks. Tianshan eclogites show a continuum in δ13CVPDB (-14 to +2.5‰) among the compositions of finely disseminated and various vein carbonates. This continuum likely reflects varying influences of seafloor alteration, HP-metamorphic introduction of C with marine-carbonate δ13C, and fluid-mediated introduction of organic δ13C from metasedimentary rocks. The relationships for the Tianshan eclogites are similar to those for pillow breccia matrices and multiple vein generations in the Catalina Schist [2] and in contrast with those for lower- and equivalent-grade basalts reported by others [3,4]. A smaller dataset for HP/UHP-metamorphosed pillow basalts at Monviso, Italy shows a similar range in δ18OVSMOW (+11.7 to +13), with more uniform δ13CVPDB (-0.8 to +2.5‰), relative to the dataset for the Tianshan eclogites. The δ18O values for the Monviso samples reflect fluid sources and fluid-rock interaction comparable to those for the Tianshan rocks, whereas the δ13C values reveal smaller C contributions from fluids bearing C isotope signatures of organic sources. More recent fieldwork in the Western Alps has resulted in further detailed sampling of carbonate-bearing interstitial pillow material and breccia matrices in blueschist and eclogite facies metabasalts [5,6]. Work underway assesses the mineralogical and isotopic records of fluid infiltration and calc-silicate reactions during the HP/UHP metamorphism of these rocks. The field observations and isotopic records for carbonate occurrences in these and other HP/UHP metabasalts appear to reflect the retention of metabasaltic C to great depths in subduction zones, perhaps to those beneath volcanic fronts. [1] Dasgupta and Hirschmann (2010), Earth. Planet. Sci. Lett. 298:1-13 [2] Bebout (1995), Chem. Geol. 126:191-218 [3] Miller et al. (2001), Chem. Geol. 178:43-69 [4] Cartwright and Barnicoat (2003), J. Meta. Geol. 21:143-161 [5] Agard et al. (2003), Terra Nova 15:306-314 [6] Angiboust et al. (2012), J. Meta. Geol. 30:37-61