Experimental investigation of oxygen fugacity and the evolution of arc basaltic andesites
Abstract
Recent experimental studies have shown that oxygen fugacity and water concentrations in basaltic systems can influence their liquid line of descent, with water-rich and oxidizing conditions favoring the formation of more evolved melts during equilibrium crystallization. This study focuses on examining the influence of oxygen fugacity on equilibrium crystallization in natural basaltic andesites from Westdahl and Okmok volcanoes Aleutian arc, as well as synthetic and analog materials. A suite of melting and crystallization experiments run at 1 atmosphere under fO2 approximately along the Ni-NiO buffer on the 1997 Okmok basaltic andesite (52.4 wt. % SiO2) and a synthetic analog (51.5 wt. % SiO2) created from mixed oxides indicates plagioclase and clinopyroxene crystallize first between 1200 and 1250 oC, followed by olivine (~1200 oC), and oxides (~1175 oC). Similar experiments run on 1992 Westdahl basaltic andesite (54.2 wt. % SiO2) and a naturally glassy analog material (54.8 wt. % SiO2) indicate the liquidus is similar, between 1200 and 1250 oC with plagioclase crystallizing first, oxides following at 1200 oC, with clinopyroxene forming below about 1180 oC. In comparison with Okmok, the Westdahl composition did not crystallize olivine over the temperature range examined. Glass compositions compare well with MELTS models run for similar temperature ranges and fO2 set to Ni-NiO. Further experiments run along the QFM buffer will be compared to determine differences in crystallization sequence and melt evolution as a function of oxygen fugacity, between the different starting compositions studied.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V51A1668L
- Keywords:
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- 1012 GEOCHEMISTRY / Reactions and phase equilibria