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Lowenstern, J.B., 1992

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Lowenstern, J.B., 1992, The behavior and distribution of volatile metals and gases in two silicic magmatic systems: Pantelleria, Italy, and the Valley of Ten Thousand Smokes, Alaska: Palo Alto, California, Stanford University, Ph.D. dissertation, 163 p., illust., map.


Analyses of melt (glass) and sulfide inclusions trapped in phenocrysts were used to explore the distribution of volatiles in magmas prior to volcanic eruptions. Two contrasting magmatic systems were chosen: the pantellerites and trachytes of Pantelleria, Italy, and the metaluminous rhyolite, dacite, and andesite erupted in 1912 at the Valley of Ten Thousand Smokes (VTTS), Alaska. Water contents of melt inclusions from the pantellerites range from 1.4 to 2.1 wt.% H20 and increase with differentiation (determined by infrared spectroscopy). X-ray microprobe analyses show that some of these inclusions have high Cu contents (>100 ppm) relative to typical inclusions (20 ppm) and matrix glass (3 ppm). The Cu is found in bubbles interpreted to be a magmatic volatile phase that was trapped along with melt in the inclusions. This phase also contained Cl, S, and CO2 and was present in crystal-poor (<10%) magmas. Unlike Cu, Mo was enriched in the pantellerite melt (15-25 ppm), presumably due to its decreased volatility in magmas with low oxygen and H2O fugacities. Other factors contributing to the high concentrations of Mo include the presumed low activity coefficient of Mo oxides in peralkaline melts and the low modal percentage of Fe-Ti-oxide phenocrysts, which would normally sequester Mo. In the pantellerites (but not trachytes), Mo precipitated as MoS2 microphenocrysts, which are found included in all phenocryst phases as well as in the matrix glass. Thermodynamic calculations show that the precipitation of MoS2 can be adequately explained by the decrease in temperature during fractionation in the magma reservoir. The metaluminous magmas of the VTTS have low Mo concentrations (<3 ppm) and do not contain MoS2. Like the pantellerites, though, they appear to have been volatile-saturated during crystallization, as large, Cu-bearing, bubbles are found in melt (glass) inclusions. Moreover, the S, Cl and Cu contents of the phenocryst assemblage are significantly less than necessary to account for the compatible behavior of these elements during differentiation. Their abundances appear to have been controlled partially by a volatile phase present in the crystal-poor (<2%) rhyolitic magma. The concentrations of H2O (~4 wt.%) and CO2 (<50 ppm) in melt inclusions in quartz from the 1912 high-silica rhyolite are consistent with volatile saturation at pressures of ~1 kbar.

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