. The effect of pressure on sulphur speciation in mid to deepcrustal arc magmas and implications for the formation of porphyry copper deposits. Contributions to Mineralogy and Petrology, 171, [66]. DOI: 10.1007DOI: 10. /s00410-016-1274 Peer reviewed version Link to published version (if available): 10.1007/s00410-016-1274-4Link to publication record in Explore Bristol Research PDF-document This is the author accepted manuscript (AAM). The final published version (version of record) is available online via Springer at http://link.springer.com/article/10.1007/s00410-016-1274-4. Please refer to any applicable terms of use of the publisher. University of Bristol -Explore Bristol Research General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/pure/about/ebr-terms phases and sulphur speciation in the melt are strong functions of ƒO2, as expected, but also of pressure. At 0.5 GPa the anhydrite stability field is intersected at NNO≥+2, but at 1.0 and 1.5 GPa experiments at the same ƒO2 produce sulphides and the stability field of sulphate moves towards higher ƒO2 by ~1 log unit at 1.0 GPa and ~1.5 log units at 1.5 GPa. As a result, models that appeal to high oxidation state as an important control on the mobility of Cu (and other chalcophiles) during crustal differentiation must also consider the enhanced stability of sulphide in deep-to mid-

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 (H 2 O, CO 2 , SO 2 , 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% H 2 O). CO 2 contents are generally low (< 650 ppm), although in general the highest pressure melt inclusion contain the highest CO 2. Some low-pressure (< 3 kbars) inclusions have elevated CO 2 (up to 1100-1150 ppm), suggestive of fluxing of shallow magmas with CO 2-rich fluids. CO 2-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 SO 2 (< 140 ppm), and for the northern part it's F (92-798 ppm), Cl (1506-4428 ppm), Br (not determined) and SO 2 (< 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 H 2 O content of Dominica magmas has implications for hazard assessment.

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