Transport of carbon and hydrogen in subducted oceanic crust: an experimental study to 5 GPa

Carbon and hydrogen residence time in the Earth''s interior is controlled by complex phase equilibria attained in the subduction zone environment. Carbonic fluid/melts are highly mobile, whereas graphite–diamond are refractory and sluggishly participate in subsolidus reactions; on the contrary carbonates although refractory are extremely reactive. Inclusions of diamond/graphite+carbonates +hydrates in both orogenic and subcratonic mantle remnants reveal that the fates of C and H are intimately related. Experiments were carried out on an altered MORB bulk composition at P from 2.2 to 5.0 GPa and T from 680 °C to 800 °C, in the presence of a fluid at variable C–O–H ratios and amounts. The role of variable redox conditions was explored buffering fH2 at NNO and HM equilibria using a double capsule technique. Amphibole breaks down at 2.5–2.6 GPa, epidote persists to 2.7 GPa, 730 °C and talc to 3.3 GPa, 800 °C. Graphite is ubiquitous above 2.0 GPa at both hydrogen fugacities. At PN2.0 GPa and fH2 buffered by NNO, dolomite was recovered at 3.0 GPa, 730 °C only, in an experiment with large amount of volatiles added. On the contrary, at fH2 buffered by HM, carbonate phase fields get wider with pressure: aragonite and/or dolomite are stable up to 2.4 GPa and at higher pressure they are replaced by coexisting magnesite and dolomite. Mg–calcite forms at 4.6–5.0 GPa, 800 °C. Unexpectedly, lawsonite was found to coexist with magnesite at temperatures as high as 700 °C at 3.3 GPa, and 730 °C at 4.2 GPa, revealing a thermal stability in C–O–H bearing systems exceeding by more than 30 °C that found in mafic assemblages in the presence of hydrous species only. Chemographic analysis reveals that pressure–temperature stability of lawsonite in basaltic compositions is promoted by CO2 addition and by the oceanic alteration processes. In C–O–H bearing systems, complex mass-balance relations govern the partitioning of volatiles between fluid, hydrates, carbonates, and graphite/diamond at fixed fH2. In natural systems, the relative amounts of ferric/ferrous iron in garnet and clinopyroxene versus C/carbonate control fluid speciation. The potential production of C–O–H fluid mixtures is evaluated and a heterogeneous oxidation of the subducting lithosphere is suggested as a source for distinct fluid populations. Upon mixing, these fluids promote carbon precipitation.