Fluids in Mantle Peridotites: Insights from Phase Equilibria

Water (H₂O and H-species in minerals) has a strong influence on many physio-chemical properties of upper mantle rocks (e.g. viscosity, melting temperature, thermal conductivity). Estimates of mantle water contents are often based on measuring the concentration of H in nominally anhydrous minerals (NAMs) contained in peridotites. However, these NAMs may suffer diffusive loss of H during emplacement at the Earth's surface, and the relationship between NAM H contents and H₂O fugacities are not well known for all mantle NAMs. In this study, fluid-buffering mineral equilibria are applied to estimate the fugacities of various fluid species.

The compositions of co-existing minerals from eight alpine style peridotites were determined via electron microprobe. These samples were from six different locations (New Caledonia; Bestiac and Lherz, France; Teroun de Tececou, France; Almklovdalen, Norway; and the Trinity Ophiolite, CA USA), and contain amphibole + olivine (ol) + orthopyroxene (opx) + clinopyroxene (cpx) + spinel (spl), and one sample contains garnet. Two pyroxene thermometry yields temperatures of 700 to 920°C at a pressure (P) of 15 kbar for the spinel-bearing samples and P ≈ 31 kbar for the garnet-bearing sample. Estimates of water activities (aH₂O), based on amphibole equilibria, range from 0.06 to 0.3, except for samples from New Caledonia and the Trinity Ophiolite which record values between 0.8 and 1.0. The compositions of co-existing spl + ol + opx yield oxygen fugacities (ƒO₂) ranging from -0.8 to 1.6 ΔlogƒO₂(FMQ).

Low values of aH₂O (< 0.3) are similar to values recorded in other amphibole-bearing peridotites, while high water activities (0.8 - 1) are atypical. Variations in values of both aH₂O and ƒO₂ indicate that the mantle may not be homogeneous with respect to fluid composition and content. Further, different values of aH₂O and ƒO₂, as recorded in different peridotite bodies, could also represent re-equilibration of minerals during unroofing and emplacement. The New Caledonia peridotites, for example, were deformed during shearing and this deformation apparently occurred in a relatively H₂O-rich environment. Ultimately, the record of high aH₂O in mantle derived peridotities from this study should permit future insight into the amount of H-loss experienced by NAM's during emplacement.