COMPARING MEASURED OLIVINE OH CONCENTRATIONS WITH THOSE ESTIMATED FROM AMPHIBOLE EQUILIBRIA IN NINE MANTLE XENOLITHS FROM AUSTRALIA, SOUTH AFRICA AND THE SOUTHWESTERN U.S.A

Determining values of H₂O activity (aH₂O) for mantle rocks will yield a better understanding of mantle processes that are controlled, in part, by the availability of H₂O (e.g., melting and deformation). Values of mantle aH₂O may, in some cases, be inferred from the hydroxyl (OH) contents of nominally anhydrous minerals (NAMs) contained in mantle xenoliths. However, mantle NAMs may suffer H₂O loss during emplacement on earth’s surface. H₂O-buffering equilibria may also be used to determine values of mantle aH₂O (e.g., Lamb & Popp, 2009, Am. Min., v 94, pp. 41-52). Comparison of values of aH₂O estimated using these two-methods may provide insight into possible H₂O loss from mantle NAMs.

Nine amphibole-bearing xenoliths from three different locations, Australia, South Africa and the southwestern U.S.A. have been used to estimate values of aH₂O from mineral equilibria. In addition to amphibole, all samples contain olivine and two-pyroxenes, eight samples contain spinel (sp), while one contains garnet (gt). Temperatures (T) and pressures (P) estimated from the compositions of co-existing mineral are ≈840 to 960˚C at a minimum P ≈ 9 kbar for six sp-bearing samples. Two sp-bearing samples record T ≈ 840-890˚C at P ≈ 21 kbar, with T ≈ 760˚C at P ≈ 29 kbar for the gt-bearing sample. Amphibole equilibria, in conjunction with these P-T estimates, yield low values of aH₂O for all sp-bearing samples (≈ 0.02 to 0.1), whereas aH₂O ≈ 0.5 for the gt-bearing sample.

OH concentrations of olivine in seven of the spinel-bearing xenoliths were measured using FTIR spectroscopy, (< 2 to 13 wt. ppm). These olivine OH concentrations can be compared to values of aH₂O estimated from amphibole equilibria given experimentally determined relations between ƒH₂O and olivine OH concentrations (Mosenfelder et al., 2006, Am. Min., v 91, pp. 285-294; Zhao et al. 2004, Cont. Min. Pet., v 147, pp. 155-161). The relatively low values of aH₂O estimated from amphibole equilibria yield predicted olivine OH concentrations that range from 1 and 16 wt. ppm. These results indicate that: 1) the measured olivine OH concentrations are generally consistent with the low values of aH₂O inferred from amphibole equilibria for spinel-bearing xenoliths and 2), low olivine OH concentrations in xenoliths may reflect mantle conditions rather than dehydration during ascent.