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The degassing of CO2 and S from arc volcanoes is fundamentally important to global climate, eruption forecasting, ore deposits, and the cycling of volatiles through subduction zones. However, all existing thermodynamic/empirical models have difficulties reproducing CO2‐H2O‐S trends observed in melt inclusions and provide widely conflicting results regarding the relationships between pressure and CO2/SO2 in the vapor. In this study, we develop an open‐source degassing model, Sulfur_X, to track the evolution of S, CO2, H2O, and redox states in melt and vapor in ascending mafic‐intermediate magma. Sulfur_X describes sulfur degassing by parameterizing experimentally derived sulfur partition coefficients for two equilibria: RxnI. FeS (m) + H2O (v) → $\to $ H2S (v) + FeO (m), and RxnII. CaSO4(m) → $\to $ SO2 (v) + O2 (v) + CaO (m), based on the sulfur speciation in the melt (m) and co‐existing vapor (v). Sulfur_X is also the first to track the evolution of fO2 and sulfur and iron redox states accurately in the system using electron balance and equilibrium calculations. Our results show that a typical H2O‐rich (4.5 wt.%) arc magma with high initial S6+/ΣS ratio (>0.5) will degas much more (∼2/3) of its initial sulfur at high pressures (>200 MPa) than H2O‐poor ocean island basalts with low initial S6+/ΣS ratio (<0.1), which will degas very little sulfur until shallow pressures (<50 MPa). The pressure‐S relationship in the melt predicted by Sulfur_X provides new insights into interpreting the CO2/ST ratio measured in high‐T volcanic gases in the run‐up to the eruption. Plain Language Summary: Understanding how CO2 and S are emitted from volcanoes, called degassing, is important in interpreting the CO2/ST gas precursors to volcanic eruptions and quantifying the total amount of gases released into the atmosphere that are climatically important. However, existing models show significant discrepancies in predicting the behavior of sulfur during degassing. In this study, we employ a new approach to describe sulfur behavior during magma degassing and develop a new model, Sulfur_X, that successfully reproduces the distinct S, CO2, and H2O degassing behavior recorded in melts from different volcanoes. Sulfur_X shows that sulfur can either degas early at high pressure or late at low pressure during magma ascent to the surface, depending on the initial sulfur speciation and H2O contents in the magma. In addition, sulfur is one of the most commonly measured volcanic gas components used for volcano monitoring. Therefore, the predicted compositional evolution of co‐existing vapor by Sulfur_X during magma ascent bears directly on the interpretation of CO2/ST ratio measured in high‐T volcanic gases and the development of eruption forecast models. Key Points: Sulfur_X is a new open‐source magma degassing model that accurately predicts the volatile and redox evolution of ascending arc magmasSulfur_X shows that sulfur can start degassing in the lower crust or near‐surface, depending on the initial S6+/ΣS and H2O in the meltThe vapor compositions predicted by Sulfur_X can be used to interpret the CO2/ST ratios in high‐T volcanic gases, an eruption precursor [ABSTRACT FROM AUTHOR]

Primitive magmas in the Trans-Mexican Volcanic Belt (TMVB) span a wide geochemical range that includes calc-alkaline basalt and basaltic andesite, potassic shoshonites, and intraplate alkaline basalts, indicating that the subarc mantle wedge is chemically heterogeneous. The aim of this study is to experimentally constrain the origins of potassic lavas that have erupted along the volcanic front in the TMVB. We used a piston-cylinder apparatus to determine the P-T-[H.sub.2]O near-liquidus phase relations for two primitive potassic lavas: a hornblende trachybasalt (shoshonite) from Cerro La Pilita in the central TMVB and a high-K calc-alkaline basalt from Ayutla in the western TMVB. Experiments were conducted at mantle pressures (0.8-2.5 GPa) and temperatures (1,100-1,400°C) with 1.5-6 wt% [H.sub.2]O. Results show that both samples were last equilibrated with an olivine + clinopyroxene assemblage at upper mantle pressures. Integrating our results with trace element characteristics, we conclude that the potassic magmas formed by a complex, multistage process in which melts from the hottest part of the mantle wedge either reequilibrated with clinopyroxene-rich veins in the shallow upper mantle or caused melting of such veins by advective heating. We combine our results with previous experiments on TMVB lavas to provide an along-arc perspective of melt equilibration depths in the mantle wedge. The results suggest that although melts may initially form deep in the wedge, they commonly reequilibrate with heterogeneous mantle at shallower depths. Primitive, medium-K basaltic andesites in the TMVB form by reequilibration with harzburgite, which we infer to be a common lithology in the upper mantle, whereas some potassic magmas like the ones studied here form through reequilibration with or melting of veins of olivine + clinopyroxene ± phlogopite. Though somewhat rare at the volcanic front relative to the more abundant medium-K volcanic rocks, the potassic magmas are an important lava type for revealing mantle chemical heterogeneities. Keywords Subduction zone * Basalt * Shoshonite * Experimental phase equilibria * Mantle melting, Introduction The variability of Si[O.sub.2], alkalies, volatiles ([H.sub.2]O, C[O.sub.2]), and incompatible trace elements in primitive arc lavas worldwide indicates heterogeneities in both the distribution of slab-derived components and the extent [...]

Ian Stuart Edward Carmichael (b. 1930-d. 2011; Fig. 1), F.R.S., was one of the pre-eminent petrologists of the last century. He was the executive editor of Contributions to Mineralogy and [...]

Since late 1994, open-vent eruptive activity and degassing at Popocatepetl volcano, Mexico, have released large masses of C[O.sub.2] and S[O.sub.2]. Tephra and lava produced by these eruptions show evidence for mixing of mafic and silicic magmas shortly before eruption. We present the first measurements of dissolved C[O.sub.2] in the mafic magma end member based on analyses of olivine-hosted melt inclusions that were trapped at pressures as high as ~400 MPa (~15 km depth) beneath the volcano. We combine our data with thermodynamic models to show that degassing of mafic magma at ~150-350 MPa pressure can explain the C[O.sub.2]/S[O.sub.2] mass ratios (1-8) of volcanic gases released from the volcano during 1995-1997. Our results demonstrate that mafic magma recharge was responsible for the high measured fluxes of C[O.sub.2] and S[O.sub.2] from 1995 to 1997. The total S[O.sub.2] emission of 9 Mt during this period requires intrusion and degassing of a minimum of 0.8 [km.sup.3] of mafic magma. Only ~0.3% of this new mafic magma has been erupted in the form of mixed (hybrid) lava and tephra. Our results suggest that the ongoing eruption of Popocatepetl is essentially an intrusive event. More generally, we suggest that intrusion and deep degassing may explain the high gas fluxes at some other open-vent volcanoes rather than convection of magma in the uppermost parts of subvolcanic conduits.

The Ontong Java Plateau in the western Pacific is anomalous compared to other oceanic large igneous provinces in that it appears to have never formed a large subaerial plateau. Paleoeruption depths (at 122 Ma) estimated from dissolved [H.sub.2]O and C[O.sub.2] in submarine basaltic glass pillow rims vary from ~1100 m below sea level (mbsl) on the central part of the plateau to 2200-3000 mbsl on the northeastern edge. Our results suggest maximum initial uplift for the plateau of 2500-3600 m above the surrounding seafloor and 1500 [+ or -] 400 m of postemplacement subsidence since 122 Ma. Our estimates of uplift and subsidence for the plateau are significantly less than predictions from thermal models of oceanic lithosphere, and thus our results are inconsistent with formation of the plateau by a high-temperature mantle plume. Two controversial possibilities to explain the anomalous uplift and subsidence are that the plateau (1) formed as a result of a giant bolide impact, or (2) formed from a mantle plume but has a lower crust of dense garnet granulite and/or eclogite; neither of these possibilities is fully consistent with all available geological, geophysical, and geochemical data. The origin of the largest magmatic event on Earth in the past 200 m.y. thus remains an enigma. Keywords: large igneous province, basalt, volatiles, mantle plume, meteorite impact.

Although there is a growing body of data on [H.sub.2]O in arc magmas, there is still considerable uncertainty about the relationship between [H.sub.2]O and various incompatible elements during enrichment of the mantle wedge by subduction processes. We report data for [H.sub.2]O, other volatiles (C[O.sub.2], S, Cl), and trace elements in olivine-hosted melt inclusions from high-Mg basalts in central Mexico that exhibit varying degrees of subduction-related enrichment. Most melt inclusions were trapped at low pressure, but rare inclusions (Mg# 65-78, olivine hosts [Fo.sub.85-90]) trapped at upper to middle crustal pressures (1-6 kbar) contain high C[O.sub.2] (250-2120 ppm). The high-pressure inclusions indicate magmatic [H.sub.2]O contents from 1.3 to 5.2 wt%. Enrichment of [H.sub.2]O relative to Nb correlates positively with K/Nb, Ba/Nb, and La/Nb, indicating a clear link between [H.sub.2]O and trace element enrichment of the mantle wedge. Our results show that fluxing of the wedge with an [H.sub.2]O-rich component from the subducted slab is important in formation of magmas that are enriched in large ion lithophile (LILE) and light rare earth (LREE) elements relative to high field strength elements (HFSE). In contrast, magmas with low LILEs and LREEs relative to HFSEs have relatively low [H.sub.2]O, and must have formed largely by decompression melting of unmodified mantle. Our data for volcanoes Keywords: basalt, magma, subduction zones, trace elements, volatiles, water.