| Popis: |
H 2 O 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 H 2 O. However, the H 2 O 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 H 2 O contents, or values of the activity of H 2 O ( a H 2 O ) at the conditions of equilibration in the mantle. To examine the potential of amphibole equilibria to determine values of mantle a H 2 O , 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 H 2 O . A value of a H 2 O 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 H 2 O ≈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 ( f H 2 ) of −1.37. This value of f H 2 together with the estimated log f O 2 of −9.9 yields a value of a H 2 O ≈ 0.0005 for sample DH101E. The lower estimated a H 2 O 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 H 2 O Values of mantle a H 2 O can be used to predict the H 2 O content of mantle olivines. At 900 °C and 20 kbar, the olivine in a sample that equilibrates at a H 2 O 2 O. This value is consistent with the lower end of the range of measured H 2 O contents of mantle olivines (≈4–400 wt ppm). Thus, estimates of values of a H 2 O from amphibole equilibria can produce useful predictions of both the activity of H 2 O as well as the H 2 O content of nominally anhydrous mantle minerals. |
