Influence of temperature, composition, silica activity and oxygen fugacity on the H2O storage capacity of olivine at 8 GPa

Anthony C. Withers, Marc M. Hirschmann

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Olivine crystals were grown in the presence of a hydrous silicate fluid during multi-anvil experiments at 8 GPa and 1,000-1,600°C. Experiments were conducted both in a simple system (FeO-MgO-SiO2-H2O) and in a more complex system containing additional elements (CaO-Na2O-Al2 O3-Cr2O3-TiO2-FeO-MgO- SiO2-H2O). Silica activity was buffered by the presence of either pyroxene (high aSiO2) or ferropericlase (low aSiO2), and fO2 was buffered by the presence of Ni + NiO or Fe + FeO, or constrained by the presence of Fe2O3. Raman spectroscopy was used to identify pyroxene polymorphs in the run products. Clinoenstatite was present in the 1,000°C experiment, and enstatite in experiments at 1,400-1,520°C. The H2O content of olivine was measured using secondary ion mass spectroscopy, and infrared spectroscopy was used to investigate the nature of hydrous defects. The H2O storage capacity of olivine decreases with increasing temperature at 8 GPa. In contrast to previous experimental results at ≤2 GPa, no significant effect of varying oxygen fugacity is evident, but H2O storage capacity is enhanced under conditions of low silica activity. No significant growth of low wavenumber (<3,400 cm-1) peaks, generally associated with high fO 2 at low pressure, was observed in the FTIR spectra of olivine from the high fO2 experiments. Our experiments show that previous high pressure H2O storage capacity measurements for olivine synthesized under more oxidizing conditions than the Earth's mantle are not likely to be compromised by the fO2 of the experiments. However, the considerable effect of temperature on H2O storage capacity in olivine must be taken into account to avoid overestimation of the bulk upper mantle H2O storage capacity.

Original languageEnglish (US)
Pages (from-to)595-605
Number of pages11
JournalContributions to Mineralogy and Petrology
Issue number5
StatePublished - 2008

Bibliographical note

Funding Information:
Acknowledgments We thank David Bell and an anonymous reviewer for their careful reading and appraisal of this paper. We are indebted to C.L. Dodgson for reassuring us that our use of terminology relating to storage capacity is appropriate. Parts of this work were carried out in the University of Minnesota I.T. Characterization Facility, which receives partial support from NSF through the NNIN program. This work was supported by NSF EAR0456405 and OCE 0623550.


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