Effect of oxygen fugacity on OH dissolution in olivine under peridotite-saturated conditions: An experimental study at 1.5–7 GPa and 1100–1300 °C. (15th January 2016)

• Record Type: Journal Article
• Authors: Yang, Xiaozhi
• Is Part Of: Geochimica et cosmochimica acta. Volume 173(2016:Jan. 15)
• Page Start: 319
• Page End: 336
• Publication Date: 2016-01-15
• Subjects: Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2015.11.007
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: The dissolution of OH in olivine by experimental studies at simulated conditions has attracted increasing interest over the past three decades, and the influence of pressure, temperature and composition has been relatively well constrained. Oxygen fugacity is highly heterogeneous in the upper mantle, on both temporal and spatial scales, and is an important parameter in characterizing many chemical and physical processes in the mantle. However, less attention has been devoted to the effect of oxygen fugacity on OH dissolution in olivine, and the only few available reports on this topic have led to significant inconsistency and debate. In this study, the correlation between oxygen fugacity and OH solubility in Fe-bearing olivine has been systematically investigated by conducting experiments at 1.5–7 GPa and 1100–1300 °C and under peridotite- and fluid-saturated conditions, with natural gem-quality olivine single crystals and fresh peridotite xenoliths as starting materials and with oxygen fugacity controlled by the Fe–FeO, Ni–NiO and Fe2 O3 –Fe3 O4 oxygen buffer pairs. The water concentrations were determined by polarized analyses using a Fourier-transform infrared spectroscopy. The results show that, at all the experimental conditions, the OH bands at both high frequency (∼3650–3450 cm −1 ) and low frequency (∼3450–3100 cm −1 ) are prominent. The intensity of OH bands at ∼3355 and 3325 cm −1 increases positively with oxygen fugacity, suggesting a dominant role of Fe 3+ in their incorporation. Under otherwise identical conditions, the water content is gradually enhanced with increasing pressure, temperature or oxygen fugacity. The effect of oxygen fugacity on the enhancement of OH solubility appears not sensitive to temperature (1100–1300 °C) at a given pressure, but becomes progressively stronger with increasing pressure from 1.5 to 7 GPa given the temperature. Relative to oxygen fugacity buffers, the OH solubility is on average increased by ∼50% between Fe–FeO and Ni–NiO, independent of pressure and temperature. The enhanced OH solubility at relatively oxidizing conditions could be explained by the role of oxygen fugacity in changing the point defects of olivine. Experimental work concerning the storage capacity of water in olivine and in the upper mantle must consider the effect of oxygen fugacity. The OH solubility of olivine in the reducing deep upper mantle obtained from experiments under oxidizing conditions would be overestimated. The initial OH information of many natural olivines may have been modified by some recent secondary events, e.g., during their shallow residence and/or transport to the surface, and in principle, olivine equilibrated in the upper mantle may be D-depleted relative to its coexisting minerals. Some preliminary constraints on the water storage capacity in the upper mantle of the Earth and the Mars are also provided.