The transport of carbon and hydrogen in subducted oceanic crust: An experimental study to 5 GPa

摘要

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 fH_2 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 P > 2.0 GPa and fH_2 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 fH_2 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 CO_2 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 fH_2. 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.