Application of Mineral Equilibria to Constrain the Nature of Mantle Fluids Using Mantle Xenoliths

Abstract

Mantle processes (e.g., melting and deformation) are controlled, in part, by the availability of H₂O. Therefore, determining values of the activity of H₂O (aH₂O) will yield a better understanding of these complex mantle processes. The amount of H₂O contained in the mantle is typically estimated from the H₂O contents of nominally anhydrous mantle minerals (NAMs). Mineral equilibria have also been used to estimate values of aH₂O as well as values of hydrogen (ƒH₂) and oxygen fugacities (ƒO₂) in samples from the Earth’s mantle. Unlike NAMs, which may be prone to H-loss via diffusion during emplacement on Earth’s surface, dehydration equilibria are relatively resistant to re-equilibration and therefore, may be a better approach at constraining the H₂O content of the upper mantle.

Equilibria between co-existing minerals have been used to estimate values of temperature (T), pressure (P), ƒO₂, ƒH₂ (where ƒ = fugacity) and aH₂O for twenty amphibole-bearing xenoliths from three different regions, Eastern Australia, Southwestern U.S.A, and South Africa. The xenoliths contain an assemblage of olivine + orthopyroxene + clinopyroxene + amphibole + spinel ± garnet. All of the samples record low values of aH₂O (<0.20), as inferred from amphibole dehydration equilibria at PT conditions that range from 1.1 to 3.0 GPa and 690 to 980°C, respectively. Values of ƒH₂ estimated using amphibole dehydrogenation equilibria, and values of ƒO₂ estimated using oxidation equilibria involving spinel + olivine + orthopyroxene were combined toyield a second estimate of aH₂O; values, which are consistent with the values of aH₂O, estimated using amphibole equilibria. These low values of aH₂O are also consistent with low H₂O contents in NAMS that have been measured using FTIR spectroscopy. Furthermore, values of ƒO₂ have been used to constrain values of the fugacities of other fluid species in the C-O-H system. Calculations based on C-O-H equilibria yield H₂O rich conditions, which are significantly different from values of aH₂O from amphibole equilibria suggesting that the fluid pressure is lower than the lithostatic pressure and that these samples equilibrated in a fluid-absent system.