Mantle H2O-Activity Estimated From Amphibole Equilibria

Determining values of H₂O activity (aH₂O) for mantle rocks will yield a better understanding of those mantle processes that are controlled, in part, by the availability of H₂O (e.g., melting and deformation). Two different types of amphibole equilibria can be used to estimate H₂O activity in the mantle. The first method relies on the equilibrium between iron oxy-component and hydroxy-component, as described in the dehydrogenation/ oxidation reaction Fe²⁺ + OH^- = Fe³⁺ + O^2- + 1/2 H₂, for which the equilibrium constant (Ke) can be expressed in terms of thermodynamic mole fractions ([ ] = H-vacancy on the O(3) anion position) as Ke = fH₂ (28.94) ((XFe³⁺)²(X[ ])²)/((XFe²⁺)²(XOH)²). The variation in Ke was quantified experimentally by annealing three different amphiboles (two mantle-derived kaersutitic amphiboles and a crustal pargasite) over the range 700-1000°C, 1-10 kbar, and fH₂ from that of the HM to GM solid buffer assemblages. The equation below relates log Ke to T, P, and amphibole composition: log Ke = 4.23 - 4380/T(K) + [1.37((Ti+Al_total apfu) - 2.49)] + [(88/T(K)) (P-1 (kbar))] If the T, P, and amphibole composition are known, values of log Ke calculated from the equation predict the equilibrium log fH₂ to within 0.20 to 0.40 log units. If log fO₂ at the time of equilibration can be independently estimated, the H₂O activity can also 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., 2tr + 2fo = 5en + 4di + 2H₂O). 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, CA (sample DH101-E, McGuire et al., 1991) was used to calculate aH₂O using the two different methods. The mean value of aH₂O determined from seven different dehydration reactions is 0.04, with a 1-standard-deviation range from 0.006 to 0.06. That range of water activity is in good agreement with the value of aH₂O = 0.013 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 during ascent or eruption. Changes in H-content have significantly different effects on the dehydration and dehydrogenation equilibria. Thus, comparison of the aH₂O estimates from these two different methods may permit quantification of H-loss.