H2O IN THE UPPER MANTLE: INSIGHTS FROM MINERAL EQUILIBRIA

H₂O plays an important role in various mantle processes (e.g., melting and deformation), and the H₂O content of the upper mantle has been estimated from the H₂O contents of nominally anhydrous minerals (NAMs). However, for many common NAMs, the relation between H₂O activity (aH₂O) and H₂O content is not well known, and certain NAMs may be prone to H₂O loss during emplacement on Earth’s surface. Thus, it may be advantageous to constrain the activities of various fluid species using mineral equilibria. Amphibole-bearing samples may be particularly useful because equilibria involving this phase can be used to estimate values of aH₂O and hydrogen fugacity (ƒH₂). Spinel equilibria can be applied to estimate values of oxygen fugacity (ƒO₂) that, when combined with ƒH₂, will provide a second estimate of aH₂O.

Examination of 11 amphibole-bearing peridotite xenoliths from the southwestern U.S.A. reveals various amphibole textures (disseminated grains, rimming spinel, and poikilitic), indicating that amphibole in subsets of these samples may have different origins, including those that crystallized from a magma. The compositions of co-existing pyroxenes ± garnet yield P-T conditions for the 11 samples ranging from 1.1 to 1.5 GPa, and 880 to 980˚C. Application of pargasite dehydration equilibria yields values of aH₂O ranging from 0.05 to 0.26. The compositions of coexisting spinel + olivine + opx yield DlogƒO₂(FMQ) of -1 to +0.7. Values of ƒH₂, estimated using amphibole dehydrogenation equilibria (6 to 91 bars) were combined with values of ƒO₂ to estimate aH₂O in nine samples (≈ 0.02 to 0.12). The difference between these two values of aH₂O, estimated using dehydration equilibria versus ƒH₂ + ƒO₂, is < 0.08 for all samples, suggesting that the amphiboles experienced little or no retrograde H-loss.

Low values of H₂O are indicated for all samples, even though amphibole is present and more than one amphibole forming process is represented. Thus, if an amphibole is stable in the mantle it may act to consume much of the available H, and equilibrate at low values of aH₂O. Mantle rocks at the appropriate P, T and bulk composition may, therefore, grow amphibole even when H is present in low amounts, and the presence of a significant oxy-component in the amphibole may be indicative of low aH₂O rather than elevated values of ƒO₂.