T258 Mineral Equilibria, Fluid Flow and Metamorphism (Posters): A Celebration of John Ferry's Career

Tuesday, 27 September 2016
9:00 AM - 6:30 PM
Exhibit Hall E/F

USING MINERAL EQUILIBRIA TO CONSTRAIN THE NATURE OF MANTLE FLUIDS

Paper number: T258-5
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

H2O plays an important role in many mantle processes (e.g., melting and deformation) and, therefore, it is important to determine values of the activity of H2O (aH2O) in samples from the earth’s mantle. The H2O content of the upper mantle has been estimated from the H2O contents of nominally anhydrous mantle minerals (NAMMs). However, for many common NAMMs, the relation between H2O activity and H2O content is not well known, and these mantle minerals may be prone to H2O loss during emplacement on Earth’s surface. Amphibole equilibria can also be applied to estimate values of aH2O, and this may be a more robust method to determine values of aH2O. We have, therefore, used mineral equilibria to estimate values of aH2O, hydrogen fugacity (ƒH2), and oxygen fugacity (ƒO2) in mantle samples.

The chemical compositions of olivine + orthopyroxene + clinopyroxene + amphibole + spinel were characterized in eleven amphibole-bearing mantle xenoliths from the southwestern U.S.A. These mineral chemistries were used to estimate values of temperature (T), pressure (P), aH2O, ƒO2 and ƒH2. Estimated P-T conditions for the 11 samples range from 1.1 to 1.6 GPa, and 900 to 1020˚C, respectively. Application of pargasite dehydration equilibria yields values of aH2O ranging from 0.03 to 0.18. The compositions of coexisting spinel + olivine + orthopyroxene yield DlogƒO2(FMQ) of -1 to +0.7. For nine of the samples, values of ƒH2 were estimated using amphibole dehydrogenation equilibria, which required determining the ratios of Fe2+ to Fe3+ in the amphiboles. Values of ƒH2 ranged from 6 to 117 bars, and these values were combined with values of ƒO2 to estimate aH2O using the relation 2H2O = 2H2 + O2. Values of aH2O using this method range from 0.02 to 0.12 for these nine samples.

A potential problem with estimating aH2O using amphibole equilibria is possible retrograde H-loss from amphibole. However, values of aH2O estimated from dehydration equilibria are less sensitive to this H-loss as compared to values of aH2O estimated from the combination of ƒH2 and ƒO2. Therefore, equilibria based on amphiboles that suffered H-loss should yield different values of aH2O when these two methods are applied to the same sample. This difference in values of aH2O is < 0.05 for all samples, suggesting that the amphiboles have experienced little or no H-loss.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-284074

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