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Today, volcanic sulfur emissions into the atmosphere are measured spectroscopically from the ground, air and space. For eruptions prior to the satellite era, two main sulfur proxies are used, the rock and ice core records, as illustrated by Peccia et al. (2023, https://doi.org/10.1029/2023gl103334). The first approach is based on calculations of the sulfur content of the magma, while the second uses traces of sulfur deposited in ice. Both approaches have their limitations. For glaciochemistry, the volcano responsible for a sulfur anomaly is often unknown and the atmospheric pathway by which the sulfur reached the ice uncertain. The petrologic method relies, too, on uncertain estimates of eruption size and a number of geochemical assumptions that are hard to verify. A deeper knowledge of processes occurring both within magma bodies prior to eruption, and within volcanic plumes in the atmosphere is needed to further our understanding of the impacts of volcanism on climate. Volcanic emissions of sulfur gases during large eruptions can change the global climate. Today, advanced spectroscopic tools allow us to measure sulfur from the ground, air and space. But to understand the impacts of volcanic activity on climate back through history we need to estimate volcanic sulfur releases in other ways. Clues can be found in the pumice deposits found around a volcano and, perhaps more surprisingly, in the ice caps of the polar regions, which receive the eventual fallout from large eruptions. Both approaches have limitations that are addressed in a recent work by Peccia et al. (2023, https://doi.org/10.1029/2023gl103334) and that are at the focus of our commentary. The impact of volcanic activity on climate remains difficult to quantify for eruptions prior to the instrumental eraThe petrological and glaciochemical methods each provide answers, but are still fraught with large uncertaintiesWe need to further our understanding of the physicochemical processes occurring both within magma reservoirs and volcanic plumes The impact of volcanic activity on climate remains difficult to quantify for eruptions prior to the instrumental era The petrological and glaciochemical methods each provide answers, but are still fraught with large uncertainties We need to further our understanding of the physicochemical processes occurring both within magma reservoirs and volcanic plumes

Phase equilibrium experiments are essential for robustly and accurately constraining the intensive parameters of magma systems and their fractionation history, which is particularly true for A‐type granites crystallized from H2O‐rich melts and at reducing conditions. Here, we constrain the crystallization conditions of the ferroan (A‐type), Sn‐mineralized Qitianling granite of South China, which formed during a major event of crustal formation and reorganization in the Mesozoic. To characterize the magma system conditions and fractionation, we have carried out a series of experiments on three representative, amphibole‐bearing samples. The experiments were performed at 100–700 MPa (mainly at 200 and 300 MPa), at an fO2of ~NNO−1.3 (1.3 log unit below the Ni‐NiO buffer) or ~NNO+2.4, at 660 to 900 °C, and at variable H2Omelt(~3–9 wt %). They show that the Qitianling magmas last crystallized at ≥300–350 MPa, at a H2Omelt≥ 6.5–8.0 wt %, and that magmatic fO2was ~NNO−1.3 ± 0.5 at above‐solidus conditions. Amphibole texture in the rocks suggests an early crystallization of this mineral, hence that water‐rich (≥4 wt % H2O in melt) conditions prevailed early during the magmatic evolution, prior to amphibole crystallization. At all investigated conditions, amphibole crystallization requires at least 5–6 wt % dissolved H2O, being even absent in the more potassic composition. We interpret this as resulting from the elevated K2O content of the investigated compositions that inhibits amphibole crystallization in metaluminous granitic systems. The experimental liquid line of descent obtained at 200–300 MPa mimics the geochemical trend expressed by the pluton suggesting that fractionation occurred in the upper crustal reservoir. A‐type Qitianling magmas were produced at high temperature (>900 °C) and with initial H2Omelt≥ 4 wt %Crystallization of Qitianling granites occurred at ≥300–350 MPa, NNO−1.3 ± 0.5, and near‐solidus H2O melt≥ 6.5–8.0 wt %Fractionation of the magmas occurred at or close to the level of final emplacement and crystallization

Modes of magma emplacement may provide key information on geodynamic‐tectonic settings of magmatism, when the threshold of magma volume and duration of emplacement process is crossed, brittle/ductile crustal deformation behavior, and volumes and rates of magma transfer in the crust. The majority of upper‐crustal granitic intrusions are incrementally emplaced as laccoliths or sills, while midcrustal or deeper‐level granitic intrusions appear to be predominantly emplaced as sills or lopoliths, but relatively few examples have been studied. We present a case study on the Jurassic Qitianling pluton from South China, which was constructed in the upper part of the middle crust (at a depth of ~10–15 km). To characterize its emplacement mechanism and evolution, we have combined field and microscopic observations, Anisotropy of Magnetic Susceptibility measurements and Bouguer gravity anomaly modeling. The main findings are (1) the generally undeformed granite‐country rock contacts with a narrow thermal aureole, (2) irregularly wavy and gradational contacts between the main granite facies, (3) dominant subhorizontal magnetic fabrics in the granites with a scattered magnetic foliation, (4) rare occurrence of dykes, and (5) a subcircular surface and lopolith‐like pluton geometry. Accordingly, we suggest that the Qitianling pluton forms an ~25‐km wide and ~5‐km thick lopolith that was incrementally emplaced during a period of tectonic quiescence, most likely by under‐accretion of multiple magma sheets deflected from magma pulses that ascended through pipe‐like channels. The mineralogical and compositional variation of the pluton may be the result of in situ fractionation, fractionation at deeper crustal level, or variation in source‐derived melt compositions. The Jurassic Qitianling pluton was emplaced as a lopolith‐ to sill‐like body at the upper part of the middle crust (at ~10–15 km) during a period of tectonic quiescenceThe pluton was constructed by downward accretion of multiple magma pulses ascent along pipe‐like channelsAnisotropy of Magnetic Susceptibility is an efficient tool for identifying possible contacts between incrementally emplaced magma pulses

Gas bubbles form as magmas ascend in the crust and exsolve volatiles. These bubbles evolve chemically and physically as magma decompression and crystallization proceed. It is generally assumed that the gas remains in thermal equilibrium with the melt but the relationship between gas and melt redox state is debated. Here, using absorption spectroscopy, we report the composition of gases emitted from the lava lake of Kīlauea Volcano, Hawaii, and calculate equilibrium conditions for the gas emissions. Our observations span a transition between more and less vigorous-degassing regimes. They reveal a temperature range of up to 250 °C, and progressive oxidation of the gas, relative to solid rock buffers, with decreasing gas temperature. We suggest that these phenomena are the result of changing gas bubble size. We find that even for more viscous magmas, fast-rising bubbles can cool adiabatically, and lose the redox signature of their associated melts. This process can result in rapid changes in the abundances of redox-sensitive gas species. Gas composition is monitored at many volcanoes in support of hazard assessment but time averaging of observations can mask such variability arising from the dynamics of degassing. In addition, the observed redox decoupling between gas and melt calls for caution in using lava chemistry to infer the composition of associated volcanic gases. The redox state of volcanic gases and melts can become decoupled during magma ascent, according to observations of gas emissions from Kīlauea’s lava lake, Hawaii. Cooling of fast-rising bubbles changes the abundance of redox-sensitive gas species.

Despite claims to the contrary, the compositions of magnetite and ilmenite in the Bishop Tuff correctly record the changing conditions of Tand fO2in the magma reservoir. In relatively reduced (ΔΝΝΟ <1) siliceous magmas (e.g., Bishop Tuff, Taupo units), Ti behaves compatibly (DTi≈ 2−3.5), leading to a decrease in TiO2activity in the melt with cooling and fractionation. In contrast, FeTioxides are poorer in TiO2in more oxidized magmas (ΔΝΝΟ > 1, e.g., Fish Canyon Tuff, Pinatubo), and the d(aTiO2)/dTslope can be negative. Biotite, FeTi-oxides, liquid, and possibly plagioclase largely maintained equilibrium in the Bishop Tuff magma (unlike the pyroxenes, and cores of quartz, sanidine, and zircon) prior to and during a mixing event triggered by a deeper recharge, which, based on elemental diffusion profiles in minerals, took place at least several decades before eruption. Equilibrating phases and pumice compositions show evolving chemical variations that correlate well with mutually consistent temperatures based on the FeTi-oxides, sanidine-plagioclase, and Δ18Ο quartz-magnetite pairs. Early Bishop Tuff (EBT) temperatures are lower (700 to ~780 °C) than temperatures (780 to >820 °C) registered in Late Bishop Tuff (LBT), the latter defined here not strictly stratigraphically, but by the presence of orthopyroxene and reverse-zoned rims on quartz and sanidine. The claimed similarity in compositions, Zr-saturation temperatures and thermodynamically calculated temperatures (730−740 °C) between EBT and less evolved LBT reflect the use of glass inclusions in quartz cores in LBT that were inherited from the low-temperature rhyolitic part of the reservoir characteristic of the EBT. LBT temperatures as high as 820 °C, the preservation of orthopyroxene, and the presence of reverse-zoned minerals (quartz, sanidine, zircons) are consistent with magma recharge at the base of the zoned reservoir, heating the cooler rhyolitic melt, partly remelting cumulate mush, and introducing enough CO2(0.4−1.4 wt%, mostly contained in the exsolved fluid phase) to significantly lower H2O-activity in the system.

ABSTRACTViscosities of liquid albite (NaAlSi3O8) and a Himalayan leucogranite were measured near the glass transition at a pressure of one atmosphere for water contents of 0, 2·8 and 3·4 wt.%. Measured viscosities range from 1013·8Pa. s at 935 K to 109·0Pa. s at 1119 K for anhydrous granite, and from 1010·2Pa. s at 760 K to 1012·9Pa. s at 658 K for granite containing 3·4 wt.% H2O. The leucogranite is the first naturally occurring liquid composition to be investigated over the wide range of T-X(H2O) conditions which may be encountered in both plutonic and volcanic settings. At typical magmatic temperatures of 750°C, the viscosity of the leucogranite is 1011·0Pa. s for the anhydrous liquid, dropping to 106·5Pa. s for a water content of 3 wt.% H2O. For the same temperature, the viscosity of liquid NaAlSi3O8is reduced from 1012·2to 106·3Pa. s by the addition of 1·9 wt.% H2O. Combined with published high-temperature viscosity data, these results confirm that water reduces the viscosity of NaAlSi3O8liquids to a much greater degree than that of natural leucogranitic liquids. Furthermore, the viscosity of NaAlSi3O8liquid becomes substantially nonArrhenian at water contents as low as 1 wt.% H2O, while that of the leucogranite appears to remain close to Arrhenian to at least 3 wt.% H2O, and viscosity–temperature relationships for hydrous leucogranites must be nearly Arrhenian over a wide range of temperature and viscosity. Therefore, the viscosity of hydrous NaAlSi3O8liquid does not provide a good model for natural granitic or rhyolitic liquids, especially at lower temperatures and water contents.Qualitatively, the differences can be explained in terms of configurational entropy theory because the addition of water should lead to higher entropies of mixing in simple model compositions than in complex natural compositions. This hypothesis also explains why the water reduces magma viscosity to a larger degree at low temperatures, and is consistent with published viscosity data for hydrous liquid compositions ranging from NaAlSi3O8and synthetic haplogranites to natural samples. Therefore, predictive models of magma viscosity need to account for compositional variations in more detail than via simple approximations of the degree of polymerisation of the melt structure.

Kinetics and reaction paths of Fe3+reduction by H2in high-Fe and low-H2O silicate melts have been investigated at 800 ℃. Time-series experiments were performed in cold-seal pressure vessels at 50 bars of pure H2using rapid-heating and rapid-quench strategies. Within the first minutes of the experiments, a fast partitioning of Na occurred between the gas and the melt due to the reducing conditions. Kinetically decoupled from the Na partitioning, the progression of a front of Fe3+reduction within the quenched melt was observed and was identified as a diffusion-limited process. The growth of the reduced layer is accompanied by an increase in concentration of OH-groups suggesting that reduction operates through proton incorporation within the melt. As this growth rate is slightly faster than predicted from the diffusion of molecular H2O, a different and mobile waterderived species seems likely. One possible mechanism is the reduction of Fe3+by the transport of molecular H2. As this process is limited by the flux of H2, it will depend on both diffusivity and solubility of H2in the melt. Alternatively, migration of protons (H+) and electronic species within the melt could control the velocity of the reduction front. The increase in concentration of the reactionderived OH groups produces a water over saturation followed by partial dehydration of the melt. This dehydration leads to a change in the redox conditions within the gas that influences the Na partitioning between gas and melt.

The kinetics of Fe oxidation-reduction in two hydrous rhyolitic melts, one metaluminous and the other peralkaline, have been studied at 800°C, 2kb, for melt water contents from ~5 wt% to saturation and fO2between NNO-2 and NNO + 3 (NNO = nickel-nickel oxide redox buffer). The metaluminous melt (~1 wt% FeOT) reached redox equilibrium after 10 hours and the peralkaline one (~3 wt% FeOT) after 3 hours. The kinetics of Fe oxidation and reduction are similar and unaffected by the presence or absence of a hydrous fluid phase. No redox front is observable in the glass as the Fe3+/Fe2+ratio evolves, implying that the Fe redox kinetics in hydrous silicic melts is rate-limited neither by the diffusion of H2nor by the mobilities of divalent cations, as observed for anhydrous basaltic melts. We propose a two-step reaction mechanism that involves: (1) virtually instantaneous diffusion of H2in the sample, followed by (2) slower structural/chemical reorganizations around Fe atoms. The overall redox process involving iron and hydrogen in Fe-poor, H2O-rich melts is thus reaction-limited and obeys a first-order logarithmic rate law. The relatively slow kinetics of oxidation/ reduction explains why melt Fe3+/Fe2+can be readily quenched in laboratory experiments. Simulation of oxidation of magmas due to H2exchange with wall rocks is performed using these new kinetics laws and two DH2values extracted from the literature. We demonstrate that the metaluminous composition is not significantly modified whereas the peralkaline composition undergoes important and fast changes of Fe3+/Fe2+during short processes such as ascent prior to Plinian-style eruptions

Modern plate tectonic brings down oceanic crust along subduction zones where it either dehydrates or melts. Those hydrous fluids or melts migrate into the overlying mantle wedge trigerring its melting which produces arc magmas and thus additional continental crust. Nowadays, melting seems to be restricted to cases of young (<50 Ma) subducted plates. Slab melts are silicic and strongly sodic (trondhjemitic). They are produced at low temperatures (<1000°C) and under water excess conditions. Their interaction with mantle peridotite produces hydrous metasomatic phases such as amphibole and phlogopite that can be more or less sodium rich. Upon interaction the slab melt becomes less silicic (dacitic to andesitic), and Mg, Ni and Cr richer. Virtually all exposed slab melts display geochemical evidence of ingestion of mantle material. Modern slab melts are thus unlike Archean Trondhjemite–Tonalite–Granodiorite rocks (TTG), which suggests that both types of magmas were generated via different petrogenetic pathways which may imply an Archean tectonic model of crust production different from that of the present-day, subduction-related, one.

Abstract An experimental setup is described to use the Shaw membrane technique in cold-seal pressure vessels (CSPV), allowing constant partial pressures of hydrogen (PH2) and hence hydrogen and oxygen fugacities (ƒH2,ƒ?2) to be maintained in experimental charges for long durations (up to several weeks). The experimental setup permits the accurate determination of ƒH2 prevailing in gas pressurized vessels. In contrast to previous designs (CSPV and internally heated pressure vessels), osmotic equilibrium with this setup can be attained by diffusion of H2 from the membrane toward the pressure vessel (without adding H2 to the pressure medium before the heating up). PH2 and thus/H2 can be fixed at any desired value with a minimum precision of± 1 bar. Osmotic equilibrium is reached within less than 2 or less than 20 hours, with membranes made of Ag23Pd77 and Ag70Pd30 alloys, respectively (at 770°C, 1 kbar total pressure and PH2 up to 50 bars). The technique described here has been successfully used to calibrate the/o2 imposed by the Co-CoO solid buffer assemblage at 735°C and 780°C at 1 kbar.