Oxygen thermobarometry of abyssal spinel peridotites; the redox state and C-O-H volatile composition of the Earth's sub-oceanic upper mantle

Abstract

The authors have applied the spinel peridotite oxygen barometer to abyssal spinel peridotites from the mid-Atlantic, central Indian, southwest Indian and American-Antarctic ocean ridge systems. The results indicate that the oxygen fugacity (F{sub O{sub 2}}) of the suboceanic mantle is on average 0.9 {plus minus}0.7 (n = 33; {plus minus} 1sd) log units below the Fayalite-Magnetite-Quartz (FMQ) f{sub O{sub 2}} buffer, in excellent agreement with f{sub O{sub 2}} estimates of MORB glasses (FMQ {minus}1.20 {plus minus} 0.63, n = 87; {plus minus} 1 sd). The agreement between MORBs and their mantle source region suggests that the rapidly quenched liquids have not undergone significant oxidation (by hydrogen degassing, for example) during their ascent and eruption. Their results also show that the suboceanic mantle is more reduced than the subcontinental mantle, for which the average value of log f{sub O{sub 2}} lies approximately at FMQ (log f{sub O{sub 2}} = FMQ + 0.24 {plus minus} 0.5, n = 54; {plus minus} 1 sd). Abyssal spinel peridotites from the Islas Orcadas fracture Zone (FZ), near the Bouvet Island hotspot, are the most reduced samples in our suite (log f{sub O{sub 2}} = FMQ {minus}1.67 to {minus}2.32), and they are compatible with a graphite-saturatedmore » fluid being present in this source region. In general, however, upper mantle f{sub O{sub 2}}'s are too high for graphite to be stable, and they estimate an activity of carbon relative to graphite of about 0.05 under the approximate conditions of MORB generation (P {approximately} 10 kb, T {approximately} 1,325{degree}C, log f{sub O{sub 2}} {approximately} FMQ {minus}0.9). Under these conditions an average CO{sub 2} content in the mantle of {approximately} 215 to 545 ppm would be consistent with the fluid compositions of MORB glasses.« less