Hunt, Lindsey E (2017-07). Application of Mineral Equilibria to Constrain the Nature of Mantle Fluids Using Mantle Xenoliths. Doctoral Dissertation.
Thesis
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 (fH2) and oxygen fugacities (fO2) 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), fO2, fH2 (where f = 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 fH2 estimated using amphibole dehydrogenation equilibria, and values of fO2 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 fO2 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.
