HIGH-TEMPERATURE OXYGEN-ISOTOPE FRACTIONATION IN THE ENSTATITE-OLIVINE-BACO3 SYSTEM

Equilibrium oxygen isotopic fractionation factors between coexisting olivine and enstatite at mantle temperatures and pressures, 1000-1400-degrees-C and greater-than-or-equal-to 13 kbar, were determined from a series of silicate-witherite oxygen isotope exchange experiments. Silicate-witherite fractionations ranged from -2 parts per thousand at 1000-degrees-C to -1 parts per thousand at 1400-degrees-C, and showed a systematic jump in magnitude at 1200-degrees-C of approximately 0.5 parts per thousand which remains to be explained. Enstatite-witherite fractionations varied nonlinearly with respect to inverse temperature squared. The high-temperature pyroxene-witherite data suggest enstatite undergoes a reversible structural change as it approaches an incongruent breakdown reaction to olivine, quartz, and sanbornite, which may account for the observed nonlinearity. A least-squares fit to the low-temperature equilibrium enstatite-olivine fractionation data forced through the origin has the functional form 1000 ln alpha(en-ol)T(K) (1.41 +/- 0.43) X (6)/T2 (0.0 +/- 0.24), for temperatures greater than 1000-degrees-C (+/- 2sigma errors). At equilibrium under mantle conditions, the oxygen isotopic composition of enstatite is always heavier than, or equal to, that of coexisting olivine.