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Amphibole (Amp) plays a crucial role in the study of several earth and planetary processes. One of its most common applications is in thermobarometry, especially for volcanic-magmatic systems. However, many thermobarometers require the input of melt composition, which is not always available in volcanic products (e.g., partially crystallized melts or devitrified glasses), or show rather high errors for characterizing the depth of magma chambers. In this work, a new version of amphibole thermobarometry based on the selection of recently published high-quality experimental data is reported. It is valid for Mg-rich calcic amphiboles in magmatic equilibrium with calc-alkaline or alkaline melts across a wide P-T range (up to 2200 MPa and 1130 °C) and presents the advantage of being a single-phase model with relatively low errors (P ±12%, T ±22 °C, logfO2 ±0.3, H2O in the melt ±14%). A user-friendly spreadsheet (Amp-TB2.xlsx) for calculating the physico-chemical parameters from the composition of natural amphiboles is also reported. It gives warnings whenever the input composition is incorrect or diverges from that of the calibration data and includes diagrams for an easy graphical representation of the results.

The Magma Chamber Simulator (MCS) is a thermodynamic tool for modeling the evolution of magmatic systems that are open with respect to assimilation of partial melts or stoped blocks, magma recharge + mixing, and fractional crystallization. MCS is available for both PC and Mac. In the MCS, the thermal, mass, and compositional evolution of a multicomponent–multiphase composite system of resident magma, wallrock, and recharge reservoirs is tracked by rigorous self-consistent thermodynamic modeling. A Recharge–Assimilation (Assimilated partial melt or Stoped blocks)–Fractional Crystallization (RnASnFC; ntot ≤ 30) scenario is computed by minimization or maximization of appropriate thermodynamic potentials using the family of rhyolite- and pMELTS engines coupled to an Excel Visual Basic interface. In MCS, during isobaric cooling and crystallization, resident magma thermally interacts with wallrock that is in internal thermodynamic equilibrium. Wallrock partial melt above a user-defined percolation threshold is homogenized (i.e., brought in to chemical potential equilibrium) with resident magma. Crystals that form become part of a cumulate reservoir that remains thermally connected but chemically isolated from resident melt. Up to 30 instances (n ≤ 30) of magma mixing by recharge and/or bulk assimilation of stoped wallrock blocks can occur in a single simulation; each recharge magma or stoped block has a unique user-defined composition and thermal state. Recharge magmas and stoped blocks hybridize (equilibrate) with resident melt, yielding a single new melt composition and temperature. MCS output includes major and trace element concentrations and isotopic ratios (Sr, Nd, Hf, Pb, Os, and O as defaults) of wallrock, recharge magma/stoped blocks, resident magma melt, and cumulates. The chemical formulae of equilibrium crystalline phases in the cumulate reservoir, wallrock, and recharge magmas/stoped blocks are also output. Depending on the selected rhyolite-MELTS engine, the composition and properties of a possible supercritical fluid phase (H2O and/or CO2) are also tracked. Forward modeling of theoretical magma systems and suites of igneous rocks provides quantitative insight into key questions in igneous petrology such as mantle versus crustal contributions to terrestrial magmas, the record of magmatism preserved in cumulates and exsolved fluids, and the chronology of RASFC processes that may be recorded by crystal populations, melt inclusions, and whole rocks. Here, we describe the design of the MCS software that focuses on major element compositions and phase equilibria (MCS-PhaseEQ). Case studies that involve fractional crystallization, magma recharge + mixing, and crustal contamination of a depleted basalt that resides in average upper crust illustrate the major element and phase equilibria consequences of these processes and highlight the rich array of data produced by MCS. The cases presented here, which represent an infinitesimal fraction of possible RASFC processes and bulk compositions, show that the records of recharge and/or crustal contamination may be subtle and are not necessarily those that would be predicted using conventional intuition and simple mass balance arguments. Mass and energy constrained thermodynamic tools like the MCS quantify the open-system evolution of magmas and provide a systematic understanding of the petrology and geochemistry of open system magmatic processes. The trace element and isotope MCS computational tool (MCS-Traces) is described in a separate contribution (part II).

The Magma Chamber Simulator (MCS) is a thermodynamic model that computes the phase, thermal, and compositional evolution of a multiphase–multicomponent system of a Fractionally Crystallizing resident body of magma (i.e., melt ± solids ± fluid), linked wallrock that may either be assimilated as Anatectic melts or wholesale as Stoped blocks, and multiple Recharge reservoirs (RnASnFC system, where n is the number of user-selected recharge events). MCS calculations occur in two stages; the first utilizes mass and energy balance to produce thermodynamically constrained major element and phase equilibria information for an RnASnFC system; this tool is informally called MCS-PhaseEQ, and is described in a companion paper (Bohrson et al. 2020). The second stage of modeling, called MCS-Traces, calculates the RASFC evolution of up to 48 trace elements and seven radiogenic and one stable isotopic system (Sr, Nd, Hf, 3xPb, Os, and O) for the resident melt. In addition, trace element concentrations are calculated for bulk residual wallrock and each solid (± fluid) phase in the cumulate reservoir and residual wallrock. Input consists of (1) initial trace element concentrations and isotope ratios for the parental melt, wallrock, and recharge magmas/stoped wallrock blocks and (2) solid-melt and solid–fluid partition coefficients (optional temperature-dependence) for stable phases in the resident magma and residual wallrock. Output can be easily read and processed from tabulated worksheets. We provide trace element and isotopic results for the same example cases (FC, R2FC, AFC, S2FC, and R2AFC) presented in the companion paper. These simulations show that recharge processes can be difficult to recognize based on trace element data alone unless there is an independent reference frame of successive recharge events or if serial recharge magmas are sufficiently distinct in composition relative to the parental magma or magmas on the fractionation trend. In contrast, assimilation of wallrock is likely to have a notable effect on incompatible trace element and isotopic compositions of the contaminated resident melt. The magnitude of these effects depends on several factors incorporated into both stages of MCS calculations (e.g., phase equilibria, trace element partitioning, style of assimilation, and geochemistry of the starting materials). Significantly, the effects of assimilation can be counterintuitive and very different from simple scenarios (e.g., bulk mixing of magma and wallrock) that do not take account phase equilibria. Considerable caution should be practiced in ruling out potential assimilation scenarios in natural systems based upon simple geochemical “rules of thumb”. The lack of simplistic responses to open-system processes underscores the need for thermodynamical RASFC models that take into account mass and energy conservation. MCS-Traces provides an unprecedented and detailed framework for utilizing thermodynamic constraints and element partitioning to document trace element and isotopic evolution of igneous systems. Continued development of the Magma Chamber Simulator will focus on easier accessibility and additional capabilities that will allow the tool to better reproduce the documented natural complexities of open-system magmatic processes.

Magma reservoirs play a key role in controlling numerous processes in planetary evolution, including igneous differentiation and degassing, crustal construction, and volcanism. For decades, scientists have tried to understand what happens in these reservoirs, using an array of techniques such as field mapping/petrology/geochemistry/geochronology on plutonic and volcanic lithologies, geophysical imaging of active magmatic provinces, and numerical/experimental modeling. This review paper tries to follow this multi-disciplinary framework while discussing past and present ideas. We specifically focus on recent claims that magma columns within the Earth's crust are mostly kept at high crystallinity (\"mush zones\"), and that the dynamics within those mush columns, albeit modulated by external factors (e.g., regional stress field, rheology of the crust, pre-existing tectonic structure), play an important role in controlling how magmas evolve, degas, and ultimately erupt. More specifically, we consider how the chemical and dynamical evolution of magma in dominantly mushy reservoirs provides a framework to understand: (1) the origin of petrological gradients within deposits from large volcanic eruptions (\"ignimbrites\"); (2) the link between volcanic and plutonic lithologies; (3) chemical fractionation of magmas within the upper layers of our planet, including compositional gaps noticed a century ago in volcanic series (4) volatile migration and storage within mush columns; and (5) the occurrence of petrological cycles associated with caldera-forming events in long-lived magmatic provinces. The recent advances in understanding the inner workings of silicic magmatism are paving the way to exciting future discoveries, which, we argue, will come from interdisciplinary studies involving more quantitative approaches to study the crust-reservoir thermo-mechanical coupling as well as the kinetics that govern these open systems.

The compositions of volcanic materials are sensitive to physical conditions in the underlying magmatic system. When basaltic melts are saturated in olivine‐plagioclase‐augite prior to eruption, their compositions can be used to estimate the pressure at which they last equilibrated. We developed PyOPAM, an open‐source tool that runs in Python, and use this refreshed liquid‐barometer to investigate the relationship between final depths of magma storage and magma flux. We first tested PyOPAM using 312 experimental glasses compiled from literature and found that the 1σ uncertainty is 1.13 kbar (±3 km). PyOPAM was then applied to a data set of 13,400 analyses from Iceland, where suspected controls on magma flux are well constrained. Of these, 3807 analyses return robust pressure estimates, constraining final pre‐eruptive magma storage depths for 23 of the 30 Icelandic volcanic systems. Our results indicate that magma storage pressures on Iceland are linked to melt‐flux from the mantle. This finding is consistent with previous models linking storage depths and spreading rates on the global mid‐ocean ridge system. In addition, we provide clear evidence that the magma flux, rather than spreading rate alone, is the key control on the distribution of melt at spreading centers. Increased melt flux is associated with shoaling of pre‐eruptive storage depths, indicating that mantle melt fluxes dictate the long‐term stabilization of extensive magmatic storage regions at depths shallower than 10 km. Quantitative relationships between mantle melt flux and storage depths can be used to test computational models of transcrustal magmatic systems. Plain Language Summary An important part of monitoring volcanoes is understanding where magma is stored before an eruption. Some individual volcanic systems have been rigorously studied and their plumbing systems are well understood. However, the overall physical controls on the depths of magma storage under volcanic systems are not yet resolved. In a free python script (PyOPAM), we refreshed and streamlined a basaltic liquid barometer, that allows us to estimate the final depth of storage for a basaltic magma, ±1.13 kbar. We applied this tool to basaltic samples from Iceland. Of the original 13,400 analyses entered into PyOPAM, 3800 returned reliable estimates. We find that there is a relationship between the amount of magma being produced and moving through an area, and the final depth of storage for basaltic magma before eruption. With increasing magma production, magma storage depths decrease and shift into the shallow crust. When magma production is low, basaltic magma is stored in the lower crust or mantle. This pattern is modified by the temperature of the crust, as cold crust prevents shallow storage of basaltic magma. These findings can be applied to other basaltic volcanoes and magma systems throughout the world. Key Points We created pyOPAM, a python script for applying the OPAM barometer to basaltic liquids Using statistical filters we constrained the uncertainty to 1 σ of this barometer to 1.13 kbar We applied pyOPAM to Icelandic samples and find that final depths of basaltic magma storage is controlled by magma flux

Mount St. Helens (MSH) volcano, in the southern Washington Cascades arc, has produced dominantly dacitic to andesitic magmatic products over the last 300 ka. Basaltic to basaltic andesitic magmas erupted only during the relatively brief (ca. 2100-1800 yr B.P.) Castle Creek period from vents separated by no more than a few kilometers. They provide a unique perspective on the evolution of this volcano. Despite close temporal and spatial proximity, these mafic magmas define two distinct compositional lineages: (1) low-K tholeiites (LKT) and (2) basalts of \"oceanic island\" or intraplate affinity (or IPB). Both lack typical arc geochemical signatures and appear to derive from distinct mantle sources, neither of which has been significantly modified by slab-derived fluid or melt components. No true calc-alkalic basalts have erupted from MSH despite its obvious arc setting. Each lineage includes derivative lavas that range from ∼7 to 5 wt% MgO and ∼49 to 55 wt% SiO2, and both are slightly porphyritic with dominantly olivine and plagioclase, minor spinel, and trace clinopyroxene in some but not all samples. With respect to incompatible elements (e.g., K, La, Nb, Th, etc.), compositional trends for the two lineages are dramatically different and inconsistent with simple fractional crystallization processes. The data instead suggest that each lineage was produced dominantly by mixing between distinct parental LKT and IPB basaltic magmas and material of intermediate composition roughly similar to average MSH andesite. Mineralogical characteristics of macrocrysts in MSH basalts indicate that they do not represent equilibrium assemblages. Olivine compositions and textures in some samples implicate accumulation of crystals formed from multiple magmas, and evidence for magma mixing is reinforced by the rare presence of \"blebs\" of rhyolitic glass. These assemblages of crystals presumably are derived from different magmas and/or older MSH magmatic products (including crystal mush zones) within crustal conduit systems. Extrapolation of compositional trends (\"mixing arrays\") to higher MgO content implicates the involvement of three types of parental magma: primitive LKT as well as distinct nepheline- and hypersthene-normative IPBs (or ne-IPB and hy-IPB), variants of which have erupted repeatedly from monogenetic vents in this sector of the Cascades. Such magmas are interpreted to form from distinct lherzolitic mantle sources (less fertile, with lower clinopyroxene content for LKT) at depths on the order of 80 (LKT) and 50-60 (IPBs) kilometers, under near-anhydrous conditions, in response to decompression rather than flux-melting. We also report a set of self-consistent estimates of temperature, pressure, water content, magma density, and weight fraction of \"andesitic\" mixing component for samples of each lineage. These parameters are highly correlated and serve to constrain the structure of the magma feeder system beneath MSH. A dynamic continuum of melt compositions is likely present, controlled principally by temperature and density gradients within the system. We envisage that during Castle Creek time the most primitive basaltic magmas formed distinct reservoirs in the deep crust, with the ne-IPB variant near 28 km depth and the LKT variant near 23 km. More silicic members of these lineages appear to have evolved at depths between ∼20-15 km. We suggest that reservoir depths were controlled mainly by magma density that, in turn, is largely determined by the degree of mixing with \"andesitic\" components at crustal depths. This configuration implies a vertical magmatic plexus with connections extending well into the upper mantle. The sharp chemical distinction between the LKT and IPB mixing arrays suggests that the respective feeder systems were isolated and rarely interacted despite their close proximity. Finally, it appears that the presence of large, complex, and long-lived conduit systems beneath stratovolcanoes can act as \"magma traps,\" within which deeper-seated (mantle) inputs are prone to modification by interaction with stored magmas and their differentiation products. In contrast, the occurrence of relatively primitive basalts from monogenetic vents distal from stratovolcanoes implies that diverse basaltic magmas ascend beneath virtually the entire arc segment and that the true complexity of this \"mantle wind\" is locally masked by modifications within the crust.

Magmatic processes in the continental crust such as crustal convection, melt ascent, magma emplacement, and batholith formation are not well understood. We solve the conservation equations for mass, momentum, and energy for two‐phase flow of melt and solid in 2D, for a thick continental crust heated from below by one or several heat pulses. A simplified binary melting model is incorporated. We systematically vary (a) the retention number, characterizing melt mobility, (b) the intensity of heat pulses applied at the bottom, and (c) the density of the solidified evolved rock. Two characteristic modes are identified: (a) in the “batholith emplacement mode,” segregation is sufficiently strong allowing melts to separate from the convective flow. This melt freezes to form buoyant SiO2‐rich layers. (b) In the “convective recycling mode,” melts are formed in the lower crust, rise together with the hot rock with little segregation, freeze at shallow depth but are partly recycled back to the lower crust where they remelt. Phase‐change‐driven convection dominates. Mode (a) is favored by high heat input, multiple heat pulses, high melt mobility, and low density of the evolved rock. Mode (b) is favored by less intense heating, less melt mobility, and denser evolved rocks. A scaling law is derived based on the thermal, melt, and compositional Rayleigh numbers and the retention number. The Altiplano‐Puna low‐velocity zone (LVZ) could represent the batholith emplacement mode with buoyant and voluminous magmas causing intense volcanism. The Tibetan LVZ is not associated with intense volcanism and might represent the convective recycling mode. Key Points Two‐phase flow models of crustal magmatic systems identify two modes: batholith emplacement versus convective recycling of evolved rock High melt mobility, multiple heating pulses, and low density of solidified evolved rock favor batholith emplacement The Altiplano‐Puna low‐velocity zone (LVZ) is in the batholith emplacement mode and the Tibetan LVZ is in the convective recycling mode

South Sister volcano, Oregon Cascade Range, USA, has repeatedly erupted rhyolite since ca. 40 ka. The youngest such eruptions are the ca. 2 ka Rock Mesa and Devils Chain rhyolites, erupted several hundred years apart from two multi‐vent complexes separated by 3–6 km. Fe‐Mg interdiffusion models of orthopyroxene rims from both rhyolites produce timescales up to several‐thousand years, but dominantly decades‐to‐centuries. Notably, the timescales of step‐normal zoned orthopyroxene rims (i.e., normally zoned with a steep chemical gradient) from the Rock Mesa rhyolite are longer than those of reversely zoned crystals, whereas the Devils Chain produced mostly decadal timescales for both zoning types. Despite the proximity and broadly similar products of these episodes, their respective timescales indicate distinct sequences of events leading up to each eruption. The Rock Mesa timescales record centuries of magma chamber growth followed by decades of predominantly magma rejuvenation, reorganization, and destabilization. In contrast, the Devils Chain episode was preceded by a single episode of coupled rhyolite extraction, rejuvenation, and hybridization. Rare, high‐An plagioclase cores and evidence of reheating implicate cryptic emplacement of mafic magma at the base of the rhyolite reservoirs. However, the diffusion timescales do not unequivocally support a single magma recharge event that affected both. Fluid fluxing and the reorganization of melt into buoyant magma chambers likely provided the source of increasing pressurization that initiated each eruption after several decades. Geodetic models of ongoing deformation west of South Sister could consider these processes in addition to magma emplacement.

Merapi is the most active volcano in Central Java and even in Indonesia. Previous research in 1988,1998,2011 by using the gravity method shows an increasing amount of magmas, which are observed from changes in the dimensions of the magma chamber. The aim of this research is to observed the gravity changes nearby summit area after big eruption in 2010. In this research the summit area was close because of the activity of Merapi. The results of further studies in 2019 showed a large anomaly increase of 2 to 5 mGal in the southeast to the southwest. Gravity data in the peak area has not yet been acquired due to the high activity of Merapi. The subsurface interpretation related to changes in the dimensions of the magma reservoir cannot be done, but it can be expected increasing of mass at the southeast of Merapi towards the peak. The increasing amount of mass cause Merapi eruption in 2020.

The flux of eruptible magma into a magmatic plumbing system influences eruption size and timing. If magma transfer is possible between two hydraulically‐connected magma lenses, system destabilization can tap a larger magma volume than stored in any one melt lens. This study identifies two distinct magma reservoirs beneath Ambrym, a basaltic island volcano in Vanuatu, during the time period February 2019 to January 2022. Using InSAR time series and a data assimilation approach, we estimate pressure changes within two reservoirs (located 5–7 and 4–6 km b.s.l.). Furthermore, a theoretical model demonstrates that the reservoirs may not currently be hydraulically connected, despite evidence of physical mixing of magma derived from each reservoir during the December 2018 eruption. These findings further our understanding of how magmatic plumbing systems at basaltic calderas may change after rift‐zone eruptions. Plain Language Summary To improve eruption forecasting, it is important to put limits of the volume of eruptible magma stored beneath a volcano. There is evidence that regions of magma storage can be approximated by multiple “reservoirs” of magma beneath a single volcano. If magma can be transferred between these reservoirs, more magma may be available when an eruption occurs. We use models of ground displacements measured by satellite over 3 years to identify two reservoirs of magma at Ambrym volcano, located in Vanuatu, from 2019 to 2022. We then estimate how much magma entered or left these respective reservoirs during this time. Finally, we conclude that magma cannot be transferred between the two reservoirs efficiently, which controls the total amount of magma that can erupt at Ambrym. Key Points At least two deformation sources were active at Ambrym volcano between the 2018 and 2022 eruptions Lava lake drainage caused closed‐system activity in 2019, resulting in uplift from ≥0.122 km3/year of magma inflow into a tilted reservoir Parameter space exploration of a theoretical model indicates that the two distinct reservoirs are not efficiently hydraulically connected