Amphibole equilibria in mantle rocks: Determining values of mantle aH2O and implications for mantle H2O contents
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abstract
H2O can affect the thermophysical properties of the mantle, and nominally anhydrous mantle minerals, such as olivine, pyroxenes, and garnet, may be an important reservoir of mantle H2O. However, the H2O content of nominally anhydrous mantle minerals now at the Earth's surface may not always reflect mantle values. It is, therefore, desirable to develop different techniques to estimate mantle H2O contents, or values of the activity of H2O (aH2O) at the conditions of equilibration in the mantle. To examine the potential of amphibole equilibria to determine values of mantle aH2O, the chemical compositions of co-existing amphibole, olivine, two-pyroxenes, and spinel from a mantle xenolith, sample DH101E of McGuire et al. (1991), were used to estimate values of pressure (P), temperature (T), and aH2O. A value of aH 2O was estimated from pargasite dehydration equilibria using chemical compositions of minerals as the basis for estimating activities of end-members in the natural phases (e.g., the activity of forsterite in olivine). These calculations were performed with the THERMOCALC software package and, at an estimated maximum T and P of 900 C and 20 kbar, they yield an estimated value of aH2O 0.02 for sample DH101E. The application of oxy-amphibole equilibrium, as described by Popp et al. (2006a, 2006b), using the composition of the amphibole in DH101E yields a value of the log of the hydrogen fugacity (fH2) of -1.37. This value of fH2 together with the estimated log fO2 of -9.9 yields a value of aH 2O 0.0005 for sample DH101E. The lower estimated aH2O compared to that estimated from dehydration equilibria may reflect a slight loss of H from amphibole in the post-formation environment, but both types of amphibole equilibria are consistent with a low value of a H2O. Values of mantle a H2O can be used to predict the H2O content of mantle olivines. At 900 C and 20 kbar, the olivine in a sample that equilibrates at aH2O <0.04, as estimated for sample DH101E, should contain <10 wt ppm H2O. This value is consistent with the lower end of the range of measured H2O contents of mantle olivines (4-400 wt ppm). Thus, estimates of values of aH2O from amphibole equilibria can produce useful predictions of both the activity of H2O as well as the H2O content of nominally anhydrous mantle minerals.
