An experimental study was carried out to investigate the equilibrium between Fe oxy-component and hydroxy-component in Ti-bearing calcic amphiboles, as described in the dehydrogenation/oxidation reaction
Fe2+ + OH- = Fe3+ + O2- + 1/2 H2,
for which the equilibrium constant (K) can be expressed as
where □ = H-vacancy on the O3 anion position, Φ is the activity coeficient term, and Kx represents the thermodynamic mole fraction term (i.e., the K expressed as mole fractions rather than activities).
The variation in Kx was quantified experimentally by annealing experiments on amphiboles of two different compositions: a mantle-derived kaersutite from Greenland, and a crustal pargasite from the Tschicoma Formation from the Jemez Mountains, New Mexico, volcanic complex. The conditions of the experiments ranged from 700.1000 °C, 1.10 kbar, and fH2 from that of the HM to GM solid buffer assemblages. The results, combined with similar data for a titanian pargasite from Vulcanʼs Throne, Arizona (Popp et al. 1995a), define the variation in log Kx as a function of T, P, and amphibole composition as given by the equation:
If the T, P, and amphibole composition are known, values of log Kx calculated from the equation predict the equilibrium logfH2 of any experiment to within ~0.1 to 0.3 log units. It is assumed that a similar uncertainty in log fH2 would also to apply to the conditions of formation of natural amphiboles in the same composition range. If log fO₂ at the time of equilibration can be estimated independently for natural samples (e.g., mantle-derived amphiboles), the H2O activity also can be estimated.
An alternate approach for estimating H2O activity from amphibole-bearing mantle rocks is to use a variety of H2O-buffering equilibria among end-member components in olivine, two-pyroxenes, amphibole, and other phases: e.g., 2 tr +2 fo = 5 en + 4 di + 2 H2O.
A self-consistent thermodynamic database (THERMOCALC, Holland and Powell 1990) can be used to determine the aH₂O of such univariant H2O-buffering equilibria as a function of P and T.
A mantle amphibole assemblage from Dish Hill (sample DH101-E, McGuire et al. 1991) was used to calculate aH₂O using the two different methods. The mean value of log aH₂O determined from seven different dehydration reactions is .1.70, with a 1σ range of ±0.50. That range of water activity is in good agreement with the value of log aH₂O = .1.90 ± 0.3 obtained using the dehydrogenation/oxidation equilibrium, along with an estimate of log fO₂.
The use of xenolith amphiboles to infer values of aH₂O in the mantle requires that the H content of the amphibole does not change significantly during ascent or eruption. Changes in H content have significantly different effects on the dehydration and dehydrogenation equilibria, such that, comparison of the aH₂O estimates from the two different methods may permit quantification of H loss.
© 2015 by Walter de Gruyter Berlin/Boston