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Real-time monitoring of volcanic eruptions involving caldera-forming events are rare (see the Perspective by Sigmundsson). Anderson et al. used several types of geophysical observations to track the caldera-forming collapse at the top of Kīlauea Volcano, Hawai'i, during the 2018 eruption. Gansecki et al. used near–real-time lava composition analysis to determine when magma shifted from highly viscous, slow-moving lava to low-viscosity, fast-moving lava. Patrick et al. used a range of geophysical tools to connect processes at the summit to lava rates coming out of far-away fissures. Together, the three studies improve caldera-collapse models and may help improve real-time hazard responses. Science , this issue p. eaaz0147 , p. eaay9070 ; p. eaaz1822 ; see also p. 1200 Near–real-time chemical monitoring of lava during the Kīlauea eruption allowed forecasting of high-temperature eruptions. Changes in magma chemistry that affect eruptive behavior occur during many volcanic eruptions, but typical analytical techniques are too slow to contribute to hazard monitoring. We used rapid energy-dispersive x-ray fluorescence analysis to measure diagnostic elements in lava samples within a few hours of collection during the 2018 Kīlauea eruption. The geochemical data provided important information for field crews and civil authorities in advance of changing hazards during the eruption. The appearance of hotter magma was recognized several days before the onset of voluminous eruptions of fast-moving flows that destroyed hundreds of homes. We identified, in near real-time, interactions between older, colder, stored magma—including the unexpected eruption of andesite—and hotter magma delivered during dike emplacement.

During solidification of magma chambers as systems closed to chemical exchange with environs, the residual siliceous melt may follow a trend of rising, constant, or decreasing oxidation state, relative to reference buffers such as nickel + nickel oxide (NNO) or fayalite + magnetite + quartz. Titanomagnetite–hemoilmenite thermometry and oxybarometry on quenched volcanic suites yield temperature versus oxygen fugacity arrays of varied positive and negative slopes, the validity of which has been disputed for several years. We resolve the controversy by introducing a new recorder of magmatic redox evolution employing temperature- and redox-sensitive trace-element abundances in zircon. The zircon/melt partition coefficients of cerium and uranium vary oppositely in response to variation of magma redox state, but vary in tandem as temperature varies. Plots of U/Pr versus Ce4+/Ce3+ in zircon provide a robust test for change in oxidation state of the melt during zircon crystallisation from cooling magma, and the plots discriminate thermally induced from redox-induced variation of Ce4+/Ce3+ in zircon. Temperature-dependent lattice strain causes Ce4+/Ce3+ in zircon to increase strongly as zircon crystallises from cooling magma at constant Ce4+/Ce3+ ratio in the melt. We examine 19 zircon populations from igneous complexes in varied tectonic settings. Variation of zircon Ce4+/Ce3+ due to minor variation in melt oxidation state during crystallisation is resolvable in 11 cases but very subordinate to temperature dependence. In many zircon populations described in published literature, there is no resolvable change in redox state of the melt during tenfold variation of Ce4+/Ce3+ in zircons. Varied magmatic redox trends indicated by different slopes on plots of zircon U/Pr versus Ce4+/Ce3+ are corroborated by Fe–Ti-oxide-based T–ƒO2 trends of correspondingly varied slopes. Zircon and Fe–Ti-oxide compositions agree that exceptionally, H2O-rich arc magmas tend to follow a trend of rising oxidation state of the melt during late stages of fluid-saturated magmatic differentiation at upper-crustal pressures. We suggest that H2 and/or SO3 and/or Fe2+ loss from the melt to segregating fluid is largely responsible. Conversely, zircon and Fe–Ti-oxide compositions agree in indicating that H2O-poor magmas tend to follow a T–ƒO2 trend of decreasing oxidation state of the melt during late stages of magmatic differentiation at upper-crustal pressures, because the precipitating mineral assemblage has higher Fe3+/Fe2+ than coexisting rhyolitic melt. We present new evidence showing that the Fe–Ti-oxide oxybarometer calibration by Ghiorso and Evans (Am J Sci 308(9):957–1039, 2008) retrieves experimentally imposed values of ƒO2 in laboratory syntheses of Fe–Ti-oxide pairs to a precision of ± 0.2 log unit, over a large experimental temperature range, without systematic bias up to at least log ƒO2 ≈ NNO + 4.4. Their titanomagnetite–hemoilmenite geothermometer calibration has large systematic errors in application to Ti-poor oxides that precipitate from very oxidised magmas. A key outcome is validation of Fe–Ti-oxide-based values of melt TiO2 activity for use in Ti-in-zircon thermometry and Ti-in-quartz thermobarometry.

Fahlore crystals and aggregates from the Darasun gold deposit show complex rhythmic-oscillatory chemical zoning resulting from crystal growth and coupled dissolution-reprecipitation reactions. Formation temperatures of these zonal fahlores were determined using the Raabe & Sack (1984) geothermometer and results of ~2780 microprobe analyses. Fahlore is a nearly complete solid solution between tetrahedrite-(Zn) and tennantite-(Fe) with ratios of Sb/(Sb + As) of 0.02–0.85 and Fe/(Fe + Zn) of 0.15–1.00, and exhibits a negative interdependence between these ratios consistent with the thermochemical incompatibility of As and Zn in fahlores. Fahlore compositions in the deposit exhibit scale invariance. The crystallization temperatures for zonal growth crystals of tennantite-(Fe) are from 115 to 314°C, vary from grain to grain and from rhythm to rhythm in individual grains, are similar to those for zonal-heterogeneous fahlore aggregates, from 115 to 290°C, and are in agreement with temperatures obtained for fahlores using fahlore-sphalerite (177–395°C) and fahlore-bournonite-seligmannite (117–316°C) geothermometers, with all of these temperatures being lower than the 284–395°C homogenization temperatures of fluid inclusions in coexisting quartz. Although overlapping in temperatures the general sequence of fahlores precipitated with decreasing temperature was: tetrahedrite + sphalerite → tennantite + sphalerite → fahlore + bournonite-seligmannite → zoned tennantite-(Fe) grains → pseudomorphic zonal-heterogeneous fahlore aggregates. The reasonable temperatures suggest that oscillatory zoning was created at nearly constant temperature and Fe(Zn) –1 exchange potential. About 30–40% of the temperatures are considered to be unreasonable, because the successive layers in oscillatory zoned fahlores were far from osmotic Fe(Zn) –1 exchange equilibrium with each other and/or were not deposited at nearly constant temperature. Using fahlore as an example, it was established that oscillatory zoning in minerals can occur under isothermal conditions under conditions far from equilibrium. The formation of zoning even within the same grain can be caused by different processes. Fahlore is a natural dynamic self-organizing system with a critical point: its composition evolves spontaneously with slight fluctuations in system external parameters with the formation of oscillatory zoning.

The pressure dependence of the exchange of Cr between clinopyroxene and garnet in peridotite is applicable as a geobarometer for mantle-derived Cr-diopside xenocrysts and xenoliths. The most widely used calibration (Nimis and Taylor Contrib Miner Petrol 139: 541–554, 2000; herein NT00) performs well at pressures below 4.5 GPa, but has been shown to consistently underestimate pressures above 4.5 GPa. We have experimentally re-examined this exchange reaction over an extended pressure, temperature, and compositional range using multi-anvil, belt, and piston cylinder apparatuses. Twenty-nine experiments were completed between 3–7 GPa, and 1100–1400 °C in a variety of compositionally complex lherzolitic systems. These experiments are used in conjunction with several published experimental datasets to present a modified calibration of the widely-used NT00 Cr-in-clinopyroxene (Cr-in-cpx) single crystal geobarometer. Our updated calibration calculates P (GPa) as a function of T (K), CaCr Tschermak activity in clinopyroxene aCaCrTscpx, and Cr/(Cr + Al) (Cr#) in clinopyroxene. Rearranging experimental results into a 2n polynomial using multiple linear regression found the following expression for pressure:PGPa=11.03+-TKln(aCaCrTscpx)×0.001088+1.526×lnCr#cpxTKwhere Cr#cpx=CrCr+Al, aCaCrTscpx=Cr-0.81·Cr#cpx·Na+K, with all mineral components calculated assuming six oxygen anions per formula unit in clinopyroxene.Temperature (K) may be calculated through a variety of geothermometers, however, we recommend the NT00 single crystal, enstatite-in-clinopyroxene (en-in-cpx) geothermometer. The pressure uncertainty of our updated calibration has been propagated by incorporating all analytical and experimental uncertainties. We have found that pressure estimates below 4 GPa, between 4–6 GPa and above 6 GPa have associated uncertainties of 0.31, 0.35, and 0.41 GPa, respectively. Pressures calculated using our calibration of the Cr-in-cpx geobarometer are in good agreement between 2–7 GPa, and 900–1400 °C with those estimated from widely-used two-phase geobarometers based on the solubility of alumina in orthopyroxene coexisting with garnet. Application of our updated calibration to suites of well-equilibrated garnet lherzolite and garnet pyroxenite xenoliths and xenocrysts from the Diavik-Ekati kimberlite and the Argyle lamproite pipes confirm the accuracy and precision of our modified geobarometer, and show that PT estimates using our revised geobarometer result in systematically steeper paleogeotherms and higher estimates of the lithosphere‒asthenosphere boundary compared with the original NT00 calibration.

This study aims to establish a reliable method for accurately estimating reservoir temperature in the Patuha Geothermal Field (PGF), a vapor-dominated reservoir. The current power generation is in the southern region of PGF, known for its high potential, while the northern region, a largely underexplored area due to limited subsurface data, presents an intriguing challenge. To address this, multicomponent geothermometry was adopted to assess the northern reservoir temperature. This method utilizes comprehensive chemical analysis and mineral saturation indices (SI) for precise temperature assessments. Optimization identified key minerals for better SI clustering. Multicomponent geothermometry estimated the reservoir temperature at 245°C, while classical solute geothermometry (quartz, chalcedony, Na-K-Ca, Na-K, and K-Mg) provided a range of 220-260 °C. Despite similar results, multicomponent geothermometry is considered more accurate as it accounts for various processes during water ascent. The optimization identified quartz, anhydrite, actinolite, calcite, illite, kaolinite, wairakite, and epidote as minerals, providing better SI clustering. Thus, multicomponent geothermometry outperforms classical solute geothermometry in vapor-dominated reservoirs like PGF, mainly when fluids are not in complete chemical equilibrium with reservoir minerals. This method’s broader application potential for straightforward analysis using standard geochemical data without intricate analyses promises significant advancements in geothermal research.

Several geothermobarometric tools have focused on clinopyroxene due to its prevalence in igneous rocks, however clinopyroxene produced in high-silica igneous systems is high in iron and low in aluminum, causing existing geothermometers that depend on aluminum exchange to fail or yield overestimated temperatures. Here we present a new clinopyroxene-liquid geothermometer recommended for use in natural igneous systems with bulk SiO2 ≥ 70 wt%, which contain clinopyroxene with Mg# ≤ 65 and Al2O3 ≤ 7 wt%. (1) T (°C) = 300 [-1.89 - 0.601 (XCaTsCpx) - 0.186 (XDiHd2003Cpx) + 4.71 (XSiO2 liq) + 77.6 (XTiO2liq) + 10.9 (XFeOliq) + 33.6 (XMgOliq) + 15.5 (XCaOliq) + 15.6 (XKO0.5liq)] The new geothermometer lowers calculated temperatures by ∼85 °C on average relative to Putirka (2008, Eq. 33) and reduces the uncertainty by a factor of two (standard error of estimate ±20 °C). When applied to natural systems, we find this new clinopyroxene-liquid geothermometer reconciles many inconsistencies between experimental phase equilibria and preexisting geothermometry results for silicic volcanism, including those from the Bishop Tuff and Yellowstone caldera-forming and post-caldera rhyolites. We also demonstrate that clinopyroxene is not restricted to near-liquidus temperatures in rhyolitic systems; clinopyroxene can be stable over a broad temperature range, often down to the solidus. An Excel spreadsheet and Python notebook for calculating temperature with this new geothermometer may be downloaded from GitHub at http://bit.ly/cpxrhyotherm.

The study of active and fossil hydrothermal systems shows clay minerals to be a fundamental tool for the identification and characterization of hydrothermal alteration facies. The occurrence and composition of hydrothermal alteration facies could provide useful information on the physicochemical conditions of the hydrothermal activity affecting a rock volume. In particular, clay minerals (i.e., smectite group, chlorite, illite, kaoline group, pyrophyllite, biotite) are pivotal for extrapolating important parameters that strongly affect the development of water/rock interaction processes such as the temperature and pH of the hydrothermal environment. This work aims to give a general reference scheme concerning the occurrence of clay minerals in hydrothermal alteration paragenesis, their significance, and the information that can be deduced by their presence and chemical composition, with some examples from active and fossil hydrothermal systems around the world. The main mineralogical geothermometers based on chlorite and illite composition are presented, together with the use of hydrogen and oxygen isotope investigation of clay minerals in hydrothermal systems. These techniques provide a useful tool for the reconstruction of the origin and evolution of fluids involved in hydrothermal alteration. Finally, a list of oxygen and hydrogen fractionation factor equations between the main clay minerals and water is also provided.

This study elucidates the hydrochemical characteristics and formation mechanisms of the Nanyang thermal spring, situated in a typical low-medium temperature geothermal field in central China. Based on isotopic and hydrochemistry analyses, the thermal groundwater was found to originate from precipitation via the Shennongjia group of mountains. In addition, hydrochemical features of the thermal groundwater derived from the regional flow system show it is generally of Na-Cl type, with high values of total dissolved solids and minor elements (Sr, F). This is different from the shallow cold water of the local flow system which is characterized by Ca-HCO3 or Ca·Mg-HCO3 type. The Na-K-Mg geothermometer indicates that none of the thermal groundwater reached water–rock equilibrium during pumping, while the mixing model shows that Nanyang thermal spring comprises approximately 84.9% shallow cold groundwater and 15.1% deep thermal groundwater. Furthermore, the reservoir temperature was evaluated by chemical geothermometry and validated by fluid-mineral equilibria calculations. The results show that quartz geothermometers, along with a silica-enthalpy mixing model, can provide a consistent reservoir temperature of 136.4–145.0 °C (average 141.8 °C) considering mixing behavior. Finally, a conceptual model of the formation housing the thermal spring was developed to highlight the origin-source pathway. The conceptual model shows that the thermal groundwater originates from long-distance flow of precipitation recharged via a deep-seated fault and fracture zone (mainly the Jiuchong Fault) from the Shennongjia Mountains, and then flows upwards to the surface, mixing with the local shallow cold groundwater as it encounters an impermeable shale layer.

The temperature-dependent exchange of Ni and Mg between garnet and olivine in mantle peridotite is an important geothermometer for determining temperature variations in the upper mantle and the diamond potential of kimberlites. Existing calibrations of the Ni-in-garnet geothermometer show considerable differences in estimated temperature above and below 1100 °C hindering its confident application. In this study, we present the results from new synthesis experiments conducted on a piston cylinder apparatus at 2.25–4.5 GPa and 1100–1325 °C. Our experimental approach was to equilibrate a Ni-free Cr-pyrope-rich garnet starting mixture made from sintered oxides with natural olivine capsules (Niolv ≅ 3000 ppm) to produce an experimental charge comprised entirely of peridotitic pyrope garnet with trace abundances of Ni (10–100 s of ppm). Experimental runs products were analysed by wave-length dispersive electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We use the partition coefficient for the distribution of Ni between our garnet experimental charge and the olivine capsule lnDgrt/olvNi;NigrtNiolv, the Ca mole fraction in garnet (XgrtCa; Ca/(Ca + Fe + Mg)), and the Cr mole fraction in garnet (XgrtCr; Cr/(Cr + Al)) to develop a new formulation of the Ni-in-garnet geothermometer that performs more reliably on experimental and natural datasets than existing calibrations. Our updated Ni-in-garnet geothermometer is defined here as:T∘C=-8254.568XgrtCa×3.023+XgrtCr×2.307+lnDgrtolvNi-2.639-273±55where Dgrt/olvNi=NigrtNiolv, Ni is in ppm, XgrtCa = Ca/(Ca + Fe + Mg) in garnet, and XgrtCr= Cr/(Cr + Al) in garnet. Our updated Ni-in-garnet geothermometer can be applied to garnet peridotite xenoliths or monomineralic garnet xenocrysts derived from disaggregation of a peridotite source. Our calibration can be used as a single grain geothermometer by assuming an average mantle olivine Ni concentration of 3000 ppm. To maximise the reliability of temperature estimates made from our Ni-in-garnet geothermometer, we provide users with a data quality protocol method which can be applied to all garnet EPMA and LA-ICP-MS analyses prior to Ni-in-garnet geothermometry. The temperature uncertainty of our updated calibration has been rigorously propagated by incorporating all analytical and experimental uncertainties. We have found that our Ni-in-garnet temperature estimates have a maximum associated uncertainty of ± 55 °C. The improved performance of our updated calibration is demonstrated through its application to previously published experimental datasets and on natural, well-characterised garnet peridotite xenoliths from a variety of published datasets, including the diamondiferous Diavik and Ekati kimberlite pipes from the Lac de Gras kimberlite field, Canada. Our new calibration better aligns temperature estimates using the Ni-in-garnet geothermometer with those estimated by the widely used (Nimis and Taylor, Contrib Mineral Petrol 139:541–554, 2000) enstatite-in-clinopyroxene geothermometer, and confirms an improvement in performance of the new calibration relative to existing versions of the Ni-in-garnet geothermometer.

The groundwater flow patterns and temperature distribution of deep parent fluid in an area of Western Guangdong (China) have been reconstructed using chemical geothermometry and multicomponent mineral equilibrium (MME) based on water chemistry and stable isotopes. Thermal groundwater samples (three drill holes, nine springs) and groundwater samples of ‘normal’ temperature (one spring, eight wells) were collected and analyzed to characterize the studied hydrothermal system. The geothermal waters mainly contain the cation Na+ followed by Ca2+ and the anion HCO3− followed by Cl−. The stable isotope composition (δD, δ18O) indicates a meteoric origin for both geothermal water and ‘normal’ groundwater. The exchange temperature in the geothermal reservoir of Western Guangdong is estimated to be 162.6 °C using MME and K-Na-Ca geothermometry, while other chemical geothermometers (Na-K, K-Mg, silica and chalcedony) provide unsuitable results. The results indicate that the meteoric waters descend through the fissures and reach a maximum depth of about 2.3 km, where they are heated by a subjacent granitic body and become the deep parent fluid. The rise of deep geothermal fluid is controlled by thermodynamics, and the fluid is cooled by heat conduction and possibly mixing with shallow groundwater. Water–rock interaction affects the chemistry of the deep geothermal fluid, and mixing with shallow groundwater changes the fluid chemistry in the shallow subsurface. Seawater incursion is identified in the thermal groundwater, contributing to higher Na+ and Cl− contents in the water. This circulation mechanism probably dominates most of the hydrothermal system in the coastal area of Western Guangdong.