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Compositions of plagioclase‐melt pairs are commonly used to constrain temperatures (T), dissolved water contents (H2O) and pressures (P) of pre‐eruptive magma storage and transport. However, previous plagioclase‐based thermometers, hygrometers, and barometers can have significant errors, leading to imprecise reconstructions of conditions during plagioclase growth. Here, we explore whether we can refine existing plagioclase‐based hygrothermobarometers with either plagioclase‐melt or melt‐only chemistry (±T/H2O), calibrated using random forest machine learning on experimental petrology data (n = 1,152). We find that both the plagioclase‐melt and melt‐only models return similar cross‐validation root‐mean‐square errors (RMSEs), as the melt holds most of the P‐T‐H2O information rather than the plagioclase. T/H2O‐dependent melt models have test set RMSEs of 25°C, 0.70 wt.% and 76 MPa for temperature, H2O content and pressure, respectively, while T/H2O‐independent models have RMSEs of 38°C, 0.97 wt.% and 91 MPa. The melt thermometer and hygrometer are applicable to a wide range of plagioclase‐bearing melts at temperatures between 664 and 1355°C, and with H2O concentrations up to 11.2 wt.%, while the melt barometer is suitable for pressures of ≤500 MPa. An updated plagioclase‐melt equilibrium model has also been calibrated, allowing the equilibrium anorthite content to be predicted with an error of 5.8 mol%. The new P‐T‐H2O‐An models were applied to matrix glasses and melt inclusions from the 1980 Mount St Helens (USA) and 2014–2015 Holuhraun (Iceland) eruptions, corroborating previous independent estimates and observations. Models are available at https://github.com/kyra‐cutler/Plag‐saturated‐melt‐P‐T‐H2O‐An, enabling assessment of plagioclase‐melt equilibrium and characterization of last‐equilibrated P‐T‐H2O conditions of plagioclase‐saturated magmas. Plain Language Summary Thermobarometry and hygrometry are common methods for reconstructing magma crystallization conditions (pressure (P), temperature (T) and dissolved water content (H2O)) prior to eruption. Plagioclase is a ubiquitous mineral found in a wide range of volcanic rocks and is often used to estimate P‐T‐H2O conditions. Here, we use machine learning‐based regression to calibrate new models, based on either plagioclase‐melt or melt‐only chemistry, to test whether we can improve the existing range of plagioclase‐based thermobarometers and hygrometers. We also develop an updated model to determine the equilibrium composition of a plagioclase crystal with a given normalized melt composition. We find that the plagioclase‐melt and melt‐only thermobarometers and hygrometers return very similar model errors due to the melt holding nearly all the P‐T‐H2O information. The models can be applied to a wide range of plagioclase‐bearing melts except for the barometer, which is only appropriate for upper crustal pressures of ≤500 MPa (≤13.8 km depth). Models are available at https://github.com/kyra‐cutler/Plag‐saturated‐melt‐P‐T‐H2O‐An, enabling assessment of plagioclase‐melt equilibrium and characterization of last‐equilibrated P‐T‐H2O conditions of plagioclase‐saturated magmas. Key Points Investigation into whether existing plagioclase‐based hygrothermobarometers can be refined using random forest machine learning The algorithm highlights that only melt composition is required to estimate intensive variables (P‐T‐H2O) of plagioclase‐saturated magmas New melt models show improvement in errors compared to thermodynamic‐based plagioclase‐melt hygrothermobarometers and equilibria models

The water-saturated phase relations have been determined for a primitive magnesian andesite (57 wt% SiO 2 , 9 wt% MgO) from the Mt. Shasta, CA region over the pressure range 200–800 MPa, temperature range of 915–1,070 °C, and oxygen fugacities varying from the nickel–nickel oxide (NNO) buffer to three log units above NNO (NNO+3). The phase diagram of a primitive basaltic andesite (52 wt% SiO 2 , 10.5 wt% MgO) also from the Mt. Shasta region (Grove et al. in Contrib Miner Petrol 145:515–533; 2003 ) has been supplemented with additional experimental data at 500 MPa. Hydrous phase relations for these compositions allow a comparison of the dramatic effects of dissolved H 2 O on the crystallization sequence. Liquidus mineral phase stability and appearance temperatures vary sensitively in response to variation in pressure and H 2 O content, and this information is used to calibrate magmatic barometers-hygrometers for primitive arc magmas. H 2 O-saturated experiments on both compositions reveal the strong dependence of amphibole stability on the partial pressure of H 2 O. A narrow stability field is identified where olivine and amphibole are coexisting phases in the primitive andesite composition above 500 MPa and at least until 800 MPa, between 975–1,025 °C. With increasing H 2 O pressure ( ), the temperature difference between the liquidus and amphibole appearance decreases, causing a change in chemical composition of the first amphibole to crystallize. An empirical calibration is proposed for an amphibole first appearance barometer-hygrometer that uses Mg# of the amphibole and : This barometer gives a minimum recorded by the first appearance of amphibole in primitive arc basaltic andesite and andesite. We apply this barometer to amphibole antecrysts erupted in mixed andesite and dacite lavas from the Mt. Shasta, CA stratocone. Both high H 2 O pressures (500–900 MPa) and high pre-eruptive magmatic H 2 O contents (10–14 wt% H 2 O) are indicated for the primitive end members of magma mixing that are preserved in the Shasta lavas. We also use these new experimental data to explore and evaluate the empirical hornblende barometer of Larocque and Canil ( 2010 ).

The genesis of intermediate intrusions is highly controversial, and one of the hot topics is whether they represent frozen melts or cumulates in the evolution of magmatic systems. Distinguishing accumulation from crystallization melt differentiated along the liquid line of descent is the key issue. The Paleoproterozoic intermediate intrusions in southern North China Craton provide an excellent case to decipher this issue. Multiple lines of evidence, including mineral textures, geochemistry as well as alphaMELTS modeling, indicate disequilibrium between whole‐rock and minerals, with melt extraction occurring at temperatures of 760°–820°C and with 10–40 wt.% of trapped melts. Effective water storage, revealed by amphibole and clinopyroxene hygrometers, plays a crucial role in promoting crystal‐melt segregation in pluton‐sized reservoirs in the upper crust. This study demonstrates that the accumulation in intermediate magmas can be identified even without evident complementary initial and extracted melts and provides deep insights into the genesis of intermediate continental crust. Plain Language Summary The genesis of intermediate rocks has long been controversial since they are analogs of the average composition of continental upper crust. The key topic is whether intermediate rocks represent frozen melts of intermediate magmas or crystal residues after melt extraction in the evolution of felsic magmatic systems. Here we carried out a comprehensive study on the Paleoproterozoic intermediate intrusions in the southern North China Craton, which is considered to have recorded the process of crystal accumulation and melt segregation in the shallow crust. Upon mineral texture, geochronologic, whole‐rock and mineral geochemical, and thermodynamic investigation, we propose that crystal accumulation and crystal‐melt segregation processes can be identified in intermediate magmas with the melt extraction temperatures and proportions of trapped melts recorded by zircon trace elements. Amphibole and clinopyroxene hygrometers revealed high water content of the melts, which likely decreased the viscosity of melts and facilitated effective cumulate‐melt segregation. This study provides a powerful reference to see through the accumulation in intermediate magmas, record the processes of crystal‐melt segregation, and definitively resolve the debate concerning the genesis of intermediate intrusions. Key Points Crystal accumulation in intermediate magmas can be identified by mineral texture and geochemistry The effective water storage system in the reservoirs is key to promoting crystal‐melt segregation Crystal‐melt segregation and accumulation are fundamental processes to form evolved continental upper crust

Apatite readily incorporates volatile and trace elements in its structure, and thus carries a record of pre-eruptive melt-fluid chemical and physical processes that play critical roles in magmatic evolution, eruption triggering, and eruptive style. However, the pressure ( P ), temperature ( T ), oxygen fugacity ( f O 2 ), and crystal-melt composition dependencies of apatite-melt elemental partition relations are only partially understood, notably for alkaline melts. Here, we report a comprehensive dataset for partitioning relations of volatiles (CO 2 , H 2 O, F, Cl, S) and 24 trace elements (including rare earth elements—REEs) between fluorapatite and phonolitic melts, based on in situ analyses of co-existing fluorapatite and melt inclusions in anorthoclase megacrystals from Erebus volcano (Antarctica). The trace monovalent cations (Li, K, Rb) have partition coefficients ( D ) of ≤ 0.02, lower than divalent cations ( D < 0.4 for Mg, Pb, Ba, Mn; D ≈ 5 for Sr) and trivalent cations ( D REE + Y ≈ 5–30, with Nd being the most compatible REE). We use the measured trace element partition coefficients to establish a lattice-strain model for fluorapatite and alkaline melts. Based on these observations along with experimental data from the literature, we propose a general model for estimating D REE + Y in fluorapatite and calc-alkaline/alkaline melts under a wide range of P – T conditions. We also use the lattice strain model and the Eu contents of apatite and the melt to develop a new Eu-in-apatite oxybarometer. Applying it to the Erebus fluorapatite and phonolitic melts, we find that f O 2 of the system was 0.5 log units below the QFM (quartz–fayalite–magnetite reaction) buffer, consistent with the low sulphur partition coefficient we determined for apatite, and with the reduced nature of the melt reported by previous studies. The melt inclusions we analysed are much drier than the calculated melt derived from apatite-melt hygrometry, implying hydrogen reequilibration in melt inclusions during magma ascent. This has implications for magma viscosity and density, and hence for the eruptive behaviour of Erebus, as well as other open-vent volcanoes. Our generalised REE lattice-strain model is widely applicable to investigations of magma differentiation and ore formation where fluorapatite is present.

A detailed mineral-scale study was conducted on pumices of the latest, dominantly explosive eruption epoch (56–30 ka) of Ciomadul, the youngest, long-dormant volcano in eastern-central Europe for characterizing the magma storage system and for understanding better the changes in eruption style from effusive to explosive. The mineral cargo of dacitic pumices enables us to constrain the conditions of the pre-recharge crystal mush, the recharge magmas and the post-recharge magma prior to eruptions. A careful evaluation of the results yielded by various thermometers, barometers, oxybarometers, chemometers and hygrometers as well as direct comparison with experimental data were necessary to select the appropriate techniques and therefore to constrain the conditions for the Ciomadul magmatic system. Beneath the volcano, a felsic crystal mush body is inferred at 8–12 km depth comprising slightly oxidized (0.5–1.6 ∆NNO), low-temperature (680–750 °C), highly crystalline magma. This zone is underlain by a deep magma storage zone with less evolved, hot (> 900 °C) magma at 16–40 km depth. The dominantly explosive volcanism after the effusive eruptions (160–90 ka) can be explained by the ascent of distinct recharge magmas. They contained high-Mg (MgO > 18 wt%) amphibole, which could have crystallized from ultrahydrous (H 2 O > 8 wt%) magma at near-liquidus conditions. The rates of amphibole overgrowth and microphenocryst formation require weeks to months for the magma mixing and the eruption events. The hybridized melt became more oxidized and contained dissolved water in around 5.5 wt% at temperature of 790–830 °C calculated from the re-equilibrated Fe-Ti oxides. These magma properties along with the degree of crystallinity (27–38 vol% crystals) favored rapid magma ascent and an explosive style eruption. Thus, the strongly hydrous nature of the recharge magma in addition to the crystallinity and H 2 O content of the pre-eruption magma plays an important role in controlling the eruption style.

Alaskan-type ultramafic–mafic intrusions in convergent-margin settings provide valuable information on melt-cumulate petrogenetic processes operating at depth in the sub-arc crust. Here, we report the compositions and textural relationships of cumulus and postcumulus minerals in a suite of clinopyroxenites and hornblendites from the peripheral zone of the Tulameen Alaskan-type intrusion in British Columbia, Canada. Mineral chemistry is used to establish magma storage conditions (P, T, f O 2 , H 2 O melt ) and to reconstruct the composition of cryptic residual liquids that equilibrated with the mineral phases and subsequently escaped the local mush system. Residual liquids in equilibrium with clinopyroxene (diopside) are metaluminous calc-alkaline basalt to andesite; melts equilibrated with amphibole (magnesio-hastingsite) are metaluminous to peraluminous calc-alkaline dacite to low-silica rhyolite. Thermobarometry yields a robust estimate of storage pressure of 400 ± 50 MPa (~ 15 km paleodepth) for the Tulameen magma reservoir and equilibration temperatures of 1130–960 °C for clinopyroxene and 950–850 °C for amphibole. The large cooling interval between the early crystallization of clinopyroxene and late appearance and continued crystallization of peritectic amphibole facilitated progressive extraction of residual liquids from clinopyroxene-rich cumulates, consistent with textural relationships, mass balance calculations and experimental petrology. Peritectic dacitic melts are hydrous (~ 6–8.3 wt% H 2 O melt ), oxidized ( f O 2  ~ NNO + 1.6 to NNO + 3.6 log units) and buoyantly mobile with low density (~ 2200 kg/m 3 ) and viscosity (~ 10 3 poise). Lower water contents likely reflect degassing of peritectic melts driven by amphibole crystallization; relatively high redox conditions are attributed to precursor fractionation of olivine and clinopyroxene preserved as cumulates in the core of the Tulameen intrusion. Peritectic amphibole crystallized in response to migration of a thermally buffered reaction front marking the stability limit of amphibole (≤ 950 °C) and driven by near-isobaric cooling. Pervasive infiltration of reactive dacitic liquids through the clinopyroxene mush formed intergranular/poikilitic amphibole and channelized flow was captured in part by cm-scale hornblendite segregations; aggregated melts formed in situ bodies of replacive hornblendite. The absence of orthopyroxene and rarity of plagioclase in the evolved ultramafic cumulates of Alaskan-type intrusions and similar arc-related rocks is attributed primarily to high H 2 O melt and oxygen fugacity in differentiated arc magmas.

Accurate measurement of water vapor in the climate-sensitive region near the tropopause is very challenging. Unexplained systematic discrepancies between measurements at low water vapor mixing ratios made by different instruments on airborne platforms have limited our ability to adequately address a number of relevant scientific questions on the humidity distribution, cloud formation and climate impact in that region. Therefore, during the past decade, the scientific community has undertaken substantial efforts to understand these discrepancies and improve the quality of water vapor measurements. This study presents a comprehensive intercomparison of airborne state-of-the-art in situ hygrometers deployed on board the DLR (German Aerospace Center) research aircraft HALO (High Altitude and LOng Range Research Aircraft) during the Midlatitude CIRRUS (ML-CIRRUS) campaign conducted in 2014 over central Europe. The instrument intercomparison shows that the hygrometer measurements agree within their combined accuracy (±10 % to 15 %, depending on the humidity regime); total mean values agree within 2.5 %. However, systematic differences on the order of 10 % and up to a maximum of 15 % are found for mixing ratios below 10 parts per million (ppm) H2O. A comparison of relative humidity within cirrus clouds does not indicate a systematic instrument bias in either water vapor or temperature measurements in the upper troposphere. Furthermore, in situ measurements are compared to model data from the European Centre for Medium-Range Weather Forecasts (ECMWF) which are interpolated along the ML-CIRRUS flight tracks. We find a mean agreement within ±10 % throughout the troposphere and a significant wet bias in the model on the order of 100 % to 150 % in the stratosphere close to the tropopause. Consistent with previous studies, this analysis indicates that the model deficit is mainly caused by too weak of a humidity gradient at the tropopause.

Geochemical and geophysical evidence from the literature shows that the Quaternary basalts erupted along the Snake River Plain (SRP) and adjacent to Yellowstone (YS) National Park have a mantle source that includes both the Yellowstone plume and subduction-modified lithosphere. Further evidence of the latter in the literature is seen in direct analyses of 1–3 wt% H 2 O in olivine-hosted melt inclusions in YS/SRP basalts. With independent constraints on H 2 O contents known, the goal of this study is to test application of a new olivine-melt thermometer and hygrometer (which is based on the simultaneous utilization of both D Mg oliv/liq and D Ni oliv/liq ) to a suite of 17 SRP/YS basalts to evaluate if similar water contents are obtained. Previous studies show that accurate results are best obtained when the thermometer/hygrometer is applied at the olivine liquidus, thus providing temperature and water content in the melt at the onset of olivine crystallization. However, this requires that the most Mg-rich olivine analyzed in each sample passes equilibrium tests (based on Mg–Fe 2+ and Mg–Mn exchange coefficients) when paired with a liquid composition that matches that for the whole rock. Sixteen of 17 YS/SRP samples from this study (94%) pass olivine-liquidus equilibrium tests, which shows that each erupted sample was initially crystal-free prior to growth of its observed phenocrysts. This observation, combined with textural evidence of rapid olivine growth, supports the hypothesis of phenocryst growth in the YS/SRP basalts during ascent. Application of the thermometer/hygrometer at the liquidus of each sample leads to temperatures that range from 1204 to 1126 °C and an average dissolved water content of 1.5 (± 0.8) wt%, which broadly matches H 2 O analyses from the literature. A comparison of SRP/YS thermometry/hygrometry results from this study with those obtained by the same methodology applied to 45 basalts (all of which pass olivine-liquidus equilibrium tests) from four other volcanic fields (from diverse tectonic settings) lead to water contents that match those analyzed directly in olivine-hosted melt inclusions from the literature. Moreover, a compilation of all results shows a strong correlation between H 2 O content and lnD Ca oliv/liq , consistent with evidence from the literature that the latter is strongly sensitive to dissolved water in the melt. One of the most surprising findings of this study is how many (16 of 17) of the SRP/YS basalts pass the olivine-liquidus equilibrium tests, given that the basaltic suite spans a range of composition (9–6 wt% MgO), reflecting the effects of crystal fractionation. This result raises the question of how efficient crystal fractionation occurred, leading to crystal-free melts prior to growth of observed phenocrysts. Here, it is proposed that fractionation may take place during ascent through sidewall crystallization along dike margins, and that this may be a relatively common mechanism by which basalts differentiate from high- to low-MgO compositions.

The AquaVIT-4 intercomparison of atmospheric hygrometers was conducted at the AIDA (Aerosol Interaction and Dynamics in the Atmosphere) climate simulation chamber of the Karlsruhe Institute of Technology (KIT), Germany, in March–April 2022, within the framework of the HEMERA H2020 EU project. The objectives were to document the performance of existing hygrometers and to support the development of novel methods for water vapor (H2O) measurements in the upper atmosphere. The AquaVIT-4 intercomparison involved seven hygrometers based on either infrared laser absorption spectroscopy or frost-point hygrometry techniques: four deployed on aircraft or stratospheric balloon platforms and three reference instruments. The simulated conditions in the AIDA chamber reproduced the characteristic atmospheric conditions of the upper troposphere–lower stratosphere (UTLS, altitude range ∼ 5–28 km) in the tropics and midlatitudes, spanning 20–600 hPa pressure, 190–245 K temperature, and 0.5–530 ppm H2O mixing ratio. The campaign was divided into two phases, each consisting of 4 measurement days: an “open intercomparison”, where the simulated conditions were known to the participants, and a “blind intercomparison”, where the conditions were coordinated by independent referees and unknown to the participating teams. Here we present a statistical analysis of the entire dataset, which allows us to assess the accuracy and limitations of each instrument. For the accuracy evaluation, two sets of reference measurements were defined: one for in situ instruments, located inside the AIDA vessel, and one for extractive instruments, sampling the chamber gas through a heated inlet. This distinction accounts for H2O desorption effects, which are most prominent at low pressures and low H2O concentrations. All instruments showed good agreement with the reference values in the range of H2O >2 ppm, with mean deviations within ± 7 % for H2O >10 ppm and ± 8 % between 2–10 ppm H2O. The largest differences were found for H2O <2 ppm, a rarely observed range in the atmosphere, though most of the instruments still achieved average deviations within ± 10 %. Overall, the results of AquaVIT-4 demonstrate the high accuracy and reliability of the four involved sensors for upper-atmospheric monitoring and research applications.

Macrocrysts (large crystals) in magmas offer a premier record of pre‐eruptive magma storage conditions encoded in their chemistry and texture. Careful study of macrocryst zoning can deconvolve the conditions of crystal growth and the relative time a magma spends in a given physical‐chemical state prior to eruption. Importantly, identifying discrete macrocryst zones requires consideration of both chemistry and texture simultaneously. Here, we employ a novel image segmentation approach to characterize zoning from 2D chemical maps of plagioclase macrocrysts. We apply the method to 15 volcanic eruptions, across three stratigraphic sections, to track statistical differences in crystal zoning through relative time at an arc volcano (Mount Liamuiga, Saint Kitts). Plagioclase from the 15 eruptions are described by 7 unique textural‐chemical zoning populations, which we term “zoning groups,” each of which has a unique An# fingerprint. Two of the studied stratigraphic sections overwhelmingly record low‐An# zoning groups (ZGs), whereas the other section records mostly high‐An# ZGs, suggestive of two distinct storage conditions. Using observations from equilibrium experiments relevant to Saint Kitts bulk magma compositions, we show that differences in melt H2O are the primary drivers of the An# variability. Negative whole rock K2O versus predicted H2O trends are suggestive of ubiquitous H2O‐saturated conditions throughout the middle and upper crust, with a correlation between H2O‐saturated storage pressure (Psat) and eruptive dynamics; magmas stored in the upper‐crust (0.48 ± 0.28–1.08 ± 0.45 kbar) produce larger‐volume, pumice‐rich eruptions compared to magmas stored in the middle crust (3.29 ± 0.87–3.88 ± 1.05 kbar) which generally produce smaller‐volume, centimeter to decimeter‐thick fall deposits. Plain Language Summary The physical and chemical conditions under which a magma, a mixture of liquid melt and solid crystals, is stored in the Earth's crust determines how it erupts on Earth's surface. A reliable record of these pre‐eruptive conditions is the complex zoning inside crystals, which consist of textural‐chemical patterns that are interpretable by petrologists. Here, we study a series of volcanic eruptions from Mount Liamuiga (Saint Kitts, Lesser Antilles) and focus on the mineral plagioclase which is present in all samples. A novel image segmentation approach was used to rapidly break down 624 crystals from 15 volcanic samples into coherent zones. By combining plagioclase zoning data with published experimental observations, we identified two distinct pre‐eruptive magma storage regions, namely a deeper (∼3.5 kbar) and a shallower (∼1 kbar) region. Over the entire pressure range, magmas are shown to coexist with a volatile phase, which has implications for the physical properties of the Earth's crust in which these magmas are stored. Shallower stored magmas produce larger‐volume, pumice‐rich eruptions compared to magmas stored deeper, which generally produce smaller‐volume centimeter‐thick to decimeter‐thick fall deposits. The information we gather is important for interpreting the monitoring signals of future volcanic unrest on Saint Kitts. Key Points Our image segmentation method identifies zoning features in 624 plagioclase crystals that are correlated across 15 volcanic eruptions Negatively correlated whole rock K2O and pre‐eruptive H2O demonstrates that H2O‐saturated conditions were ubiquitous prior to eruption Larger‐volume deposits record shallower storage (0.6–0.7 kbar) than the commonest centimetric fall deposits (3.3–3.9 kbar)