Deep pre-eruptive storage of silicic magmas feeding Plinian and dome-forming eruptions of central and northern Dominica (Lesser Antilles) inferred from volatile contents of melt inclusions

Abstract : Volatiles contribute to magma ascent through the sub-volcanic plumbing system. Here, we investigate melt inclusion compositions in terms of major and trace elements, as well as volatiles (H2O, CO2, SO2, F, Cl, Br, S) for Quaternary Plinian and dome-forming dacite and andesite eruptions in the central and the northern part of Dominica (Lesser Antilles arc). Melt inclusions, hosted in orthopyroxene, clinopyroxene and plagioclase are consistently rhyolitic. Post-entrapment crystallisation effects are limited, and negligible in orthopyroxene-hosted inclusions. Melt inclusions are among the most water-rich yet recorded (≤ 8 wt% H2O). CO2 contents are generally low (< 650 ppm), although in general the highest pressure melt inclusion contain the highest CO2. Some low-pressure (< 3 kbars) inclusions have elevated CO2 (up to 1100–1150 ppm), suggestive of fluxing of shallow magmas with CO2-rich fluids. CO2-trace element systematics indicate that melts were volatile-saturated at the time of entrapment and can be used for volatile-saturation barometry. The calculated pressure range (0.8–7.5 kbars) indicates that magmas originate from a vertically-extensive (3–27 km depth) storage zone within the crust that may extend to the sub-Dominica Moho (28 km). The vertically-extensive crustal system is consistent with mush models for sub-volcanic arc crust wherein mantle-derived mafic magmas undergo differentiation over a range of crustal depths. The other volatile range of composition for melt inclusions from the central part is F (75–557 ppm), Cl (1525–3137 ppm), Br (6.1–15.4 ppm) and SO2 (< 140 ppm), and for the northern part it’s F (92–798 ppm), Cl (1506–4428 ppm), Br (not determined) and SO2 (< 569; one value at 1015 ppm). All MIs, regardless of provenance, describe the same Cl/F correlation (8.3 ± 2.7), indicating that the magma source at depth is similar. The high H2O content of Dominica magmas has implications for hazard assessment.
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H. Balcone-Boissard, G. Boudon, J. Blundy, Caroline Martel, R. Brooker, et al.. Deep pre-eruptive storage of silicic magmas feeding Plinian and dome-forming eruptions of central and northern Dominica (Lesser Antilles) inferred from volatile contents of melt inclusions. Contributions to Mineralogy and Petrology, Springer Verlag, 2018, 173 (12), 24 p. ⟨10.1007/s00410-018-1528-4⟩. ⟨insu-01934695⟩

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