The role of oxidation-reduction and C-H-O fluids in determining melting conditions and magma compositions in the upper mantle

Abstract

High pressure experimental studies of the melting of lherzolitic upper mantle in the absence of carbon and hydrogen have shown that the lherzolite solidus has a positive dP/dT and that the percentage melting increases quite rapidly above the solidus. In contrast, the presence of carbon and hydrogen in the mantle results in a region of ‘incipient’ melting at temperatures below the C,H-free solidus. In this region the presence or absence of melt and the composition of the melt are dependent on the amount and nature of volatiles, particularly the CO2, H2O, and CH4 contents of the potential C-H-O fluid. Under conditions of low\(f_{O_2 } \) (IW to IW + 1 log unit atP ∼ 20–35kb), fluids such as CH4+H2O and CH4+H2 inhibit melting, having a low solubility in silicate melts. Under these conditions, carbon and hydrogen are mobile elements in the upper mantle. At slightly higher oxygen fugacity (IW+2 log units,P∼20–35 kb) fluids in equilibrium with graphite or diamond in peridotite C-H-O are extremely water-rich. Carbon is thus not mobile in the mantle in this\(f_{O_2 } \) range and the melting and phase relations for the upper mantle lherzolite approximate closely to the peridotite-H2O system. Pargasitic amphibole is stable to solidus temperatures in fertile lherzolite compositions and causes a distinctive peridotite solidus, the ‘dehydration solidus’, with a marked change in slope (a ‘back bend’) at 29–30kb due to instability of pargasite at high pressure. Intersections of geothermal gradients with the peridotite-H2O solidi define the boundary between lithosphere (subsolidus) and asthenosphere (incipient melt region). This boundary is thus sensitive to changes in\(f_{O_2 } \) [affecting CH4:H2O:CO2 ratios] and to the amount of H2O and carbon (CO2, CH4) present. At higher\(f_{O_2 } \) conditions (IW + 3 log units), CO2-rich fluids occur at low pressures but there is a marked depression of the solidus at 20–21 kb due to intersection with the carbonation reaction, producing the low temperature solidus for dolomite amphibole lherzolite (T∼925°C, 21 to >31kb). Melting of dolomite (or magnesite) amphibole lherzolite yields primary sodic dolomitic carbonatite melt with low H2O content, in equilibrium with amphibole garnet lherzolite.The complexity of melting in peridotite-C-H-O provides possible explanations for a wide range of observations on lithosphere/asthenosphere relations, on mantle melt and fluid compositions, and on processes of mantle metasomatism and magma genesis in the upper mantle.