Fate of fluids at the base of subcratonic lithosphere: Experimental constraints at 5.5–7.8 GPa and 1150–1350 deg C

Abstract

Phase relations in the lherzolite-C-O-H-N system are studied experimentally at 5.5–7.8 GPa, 1150–1350 deg C, and oxygen fugacity (fO2) from 2.5 log units below to 3.5 log units above the iron-wüstite (IW) equilibrium, in 10- to 150-h runs. The two-capsule technique is applied to maintain hydrogen fugacity (fH2) at the IW (Fe-FeO), MMO (Mo-MoO2), and HM (Fe2O3-Fe3O4) equilibria. The mineral assemblage stable in the Fe0- and graphite-saturated lherzolite-C-O-H-N system, at 6.3–7.8 GPa and 1200–1350 deg C, consists of olivine, orthopyroxene, clinopyroxene, and garnet. The metal phase occurs either as iron carbide (Fe3C) or iron nitride (Fe3N) at low and high concentrations of nitrogen, respectively. Carbide and nitride phases contain progressively more Ni (5–6 to 25 wt.%) as fluids become more hydrous. Fluids equilibrated with lherzolite consist of CH4 and C2H6, minor amounts of other alkanes, H2O, NH3, and methaneimine (CH3N). Fluids with high nitrogen contents are mainly composed of NH3, N2, light alkanes, and water. As fO2 increases, Fe3C and Fe3N oxidize and silicate phases (olivine, orthopyroxene, and garnet) contain more FeO, while fluids become richer in H2O and more depleted in hydrocarbons (HCs). Fluids synthesized at lithospheric P = 5.5–6.3 GPa, T = 1150–1200 deg C and fO2 near the carbon-saturated water maximum (CW) contain up to 85 rel.% H2O and at least 14 rel.% of total HCs. The presence of HCs stable to oxidation suppresses water activity in fluids while the solidus of the lherzolite-C-O-H-N system at fO2 near CW becomes at least 150 deg C higher than that of the carbon-free lherzolite-H2O system. Further oxidation of HCs and C0 leads to partial carbonation of olivine and orthopyroxene and their replacement by magnesite and clinopyroxene. The total content of HCs (mainly C2-C13 alkanes and oxygenated HCs) in fluids from magnesite-bearing lherzolite at 5.5 GPa and 1200 deg C is never below 9 rel.% even in 150-h runs, while CO2 does not exceed 3–5 rel.%. Melt inclusions composed of Mg and Ca carbonates in olivine record the first batches of carbonate melt at these P-T and redox conditions. The solidus of the lherzolite-C-O-H-N system at 5.5–7.8 GPa with fO2 between metal and carbonate saturation is above the typical upper mantle temperatures at a heat flux of 40 mW/m2. Thus, the experiments demonstrate that HC-H2O fluids can ascend from metal-saturated asthenosphere to more oxidized lithospheric mantle in a typical thermal regime and supply volatiles for carbonatite metasomatism and generation of carbonate-silicate melts.