期刊
AMERICAN MINERALOGIST
卷 106, 期 2, 页码 216-225出版社
MINERALOGICAL SOC AMER
DOI: 10.2138/am-2020-7535
关键词
Rhyolitic magma; decompression; redox evolution; sulfur; water
资金
- [15K13587]
- [18H01296]
Experimental results show that magma decompression with water and sulfur causes slight reduction, with sulfur reacting with oxygen leading to magma reduction. Magma is reduced when a pre-exsolved fluid phase is present, while oxidation occurs in the absence of a pre-exsolved fluid phase.
We performed decompression experiments on sulfur-bearing hydrous rhyolite magma at a temperature of 800 degrees C to investigate redox evolution during magma decompression. The magma was continuously decompressed from 100 MPa to 10-50 MPa at rates of 10 and 100 MPa h(-1). The evolution of the ferric to total iron ratio (Fe3+/Fe-total) during decompression was investigated using XANES, and redox evolution was determined based on a thermodynamic calculation and measured Fe3+/Fe-total. Before decompression, the sample was buffered from NNO to NNO+1, and the pre-exsolved fluid phase and sulfide crystal coexisted. Sulfide crystals were found in all decompressed samples, and Fe3+/Fe-total showed a slight decrease with decompression. It was confirmed that the sample in a gold capsule was not influenced by the change in redox conditions outside of the capsule for the timescale of the decompression experiments; thus, the decompressed sample reflected the redox evolution in magma during decompression. Our experiments indicated that magma decompression causes a slight reduction when it includes water and sulfur. This evolution is qualitatively explained by sulfur degassing and fluid-melt redox equilibria. During the fluid-melt redox equilibria, magma is reduced if the existence of a pre-exsolved fluid phase is assumed, while the model calculation shows that magma is oxidized when it contains only water or no pre-exsolved fluid phases. This is because sulfur buffers the oxidation of magma through a reaction with oxygen in the fluid phase. Therefore, we inferred that the redox condition of magma is not oxidized during explosive volcanism with a pre-exsolved fluid phase and closed-system degassing. In contrast, if magma experiences open-system degassing, it may be oxidized, resulting in the breakdown of sulfide crystals as observed in some pyroclasts and lavas.
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