1/14/2024 0 Comments Inorganic carbon molar mass![]() Finally, a comparison with conventional isotopic models is provided. The dissolution process and the effect of different CO 2/aragonite ratios and of different run durations from 0.24 to 240 h are evaluated and compared with thermodynamic calculations that include consideration of aqueous solute speciation 26, 27, 28 or consider only gas speciation in a conventional graphite-saturated COH fluid model 29, 30. Control experiments are performed with oxalic acid di-hydrate (98.7% 12C) as the source of CO 2 instead of graphite. Starting materials of synthetic labelled CaCO 3 (99.4% 13C) and synthetic graphite (98.9% 12C) are used as analogues for natural “heavy” carbonate and “light” organic matter and to generate a maximum isotopic difference in experiments (close to pure 13C and 12C end-members). These conditions are selected on the basis of the predicted peak of CO 2 produced by oxidative dissolution of graphite in subduction zones (Supplementary Fig. We provide the quantitative chemical analysis of the volatile species and the measured carbon isotopic composition of CO 2 produced by dissolution in water of graphite and of aragonite at P = 3 GPa, T = 700 ☌ and at redox-controlled conditions buffered to fH 2 = FMQ (equivalent to fO 2 expressed as ∆FMQ = + 0.61 log units). In this work, we investigate the carbon isotopic exchange occurring in a model system representative of open-ocean sediments containing calcium carbonate + organic matter subducted at subarc depths and interacting with aqueous fluids rising from the underlying dehydrating oceanic lithosphere 25. However, processes of dissolution and of isotopic exchange involving organic and inorganic carbon beneath arcs are still not fully understood. As it is generally assumed that the carbon isotope signature of arc emissions reflects that of their source, relatively high δ 13C values would point out assimilation of shallow “crustal” limestones, while low δ 13C are usually attributed to subducted organic carbon 4. This is heavier than the major carbon isotopic composition signature of the upper mantle (δ 13C = −6.0 ± 2.5‰) 4, but significantly lighter than sedimentary carbonates (δ 13C ≈ 0‰ 24). The global arc average δ 13C is −2.8 to −3.3 ‰ 4. The interaction of subducted sediments with deep fluids 14, 19 produces dissolved carbon that is transferred from the slab to the overlying mantle wedge, prompting carbonation/metasomatism and/or partial melting 20, 21, 22, 23, and eventually returning to the surface via CO 2 emitted by arc volcanoes 4. In turn, fully crystalline graphite displays a sluggish rate of isotopic diffusion and may be unaffected by isotopic reset even in cases of intense metamorphism and fluid/rock interactions 8.Ĭarbonates and graphitic carbon derived from organic matter, formerly assumed to be refractory to dissolution and devolatilization during subduction 9, 10, are now thought to show a non-negligible solubility in subduction fluids at least at certain P– T– fO 2–pH conditions 11, 12, 13, 14, 15, 16, 17, 18. The latter is particularly problematic because sedimentary organic carbon and coexisting carbonate may exhibit substantial isotopic exchange with increasing metamorphic grade, in particular at temperatures >650 ☌ during prograde graphitization 3, 5, 6, 7. However, the applicability of simple mixing models hinges on assumptions that may be overly simplistic, including that the sedimentary “end-member” compositions are spatially and temporally invariant, and that isotopic equilibrium is attained during metamorphism. Stable carbon isotopes and mass-balance calculations relying on simple mixing models have been used extensively to determine the relative contribution of organic matter and marine carbonates in sedimentary rocks 2, in their metamorphic equivalents 3 and in volcanic arc emissions 4. Conversely organic matter, which is essentially phytoplankton debris in the open seafloor, is depleted at 13C due to fractionation effects induced by photosynthesis, such that δ 13C ≈ –20‰ VPDB 1. ![]() It displays carbon isotopic ratios comparable to bicarbonate ions dissolved in seawater, characterized by δ 13C ≈ 0‰ expressed as 13C/ 12C per mil difference normalized to the international Vienna-Pee Dee Belemnite (VPDB) standard. Calcium carbonate, chiefly in sediments deposited above the calcite compensation depth, is essentially calcite forming the shells of organisms such as coccolithophores. Modern open-ocean sediments are dominated by phytoplankton.
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