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Amphibole equilibria in mantle rocks: Determining values of mantle a_H2O and implications for mantle H₂O contents

Department of Geology and Geophysics, Texas A&M University, College Station, Texas 77843, U.S.A.

Correspondence: ^* E-mail: lamb{at}geo.tamu.edu

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 (a_H2O) at the conditions of equilibration in the mantle. To examine the potential of amphibole equilibria to determine values of mantle a_H2O, 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 a_H2O.

A value of a_H2O 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 a_H2O0.02 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 (f_H2) of –1.37. This value of f_H2 together with the estimated log f_O2 of –9.9 yields a value of a_H2O 0.0005 for sample DH101E. The lower estimated a_H2O 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 a_H2O <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 a_H2O from amphibole equilibria can produce useful predictions of both the activity of H₂O as well as the H₂O content of nominally anhydrous mantle minerals.

Key Words: Amphibole • mantle H₂O activity • H₂O in mantle • amphibole equilibria • phase equilibria • mantle a_H2O • thermobarometry • thermodynamics

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