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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 Kalba–Narym metallogenic belt is located in East Kazakhstan, which displays rare metal mineralization. The Kvartsevoye rare metal Li–Ta–Nb deposit is located in the north-western ore district. This study presents the results of geological, mineralogical, geochemical, and geochronological analyses of rare metal granite pegmatites. Rare metal mineralization belongs to a field of variably differentiated pegmatites, including barren, quartz–albite–muscovite, muscovite, and muscovite–quartz–albite microcline mineral associations. This study established that the rare metal mineralization is localized in the quartz–albite–muscovite zone. The main concentrator minerals of rare metals are spodumene for Li and tantalite–columbite for Ta and Nb. Ar/Ar dating of the muscovite allowed us to establish the age of mineralization during the period of 288–285 Ma. The present study enabled the linkage of rare metal mineralization with the differentiation processes of the granites of the Kalba complex.

An updated and expanded data set that consists of 214 plagioclase-liquid equilibrium pairs from 40 experimental studies in the literature is used to recalibrate the thermodynamic model for the plagioclase-liquid hygrometer of Lange et al. (2009); the updated model is applicable to metaluminous and alkaline magmas. The model is based on the crystal-liquid exchange reaction between the anorthite (CaAl2Si2O8) and albite (NaAlSi3O8) components, and all available volumetric and calorimetric data for the pure end-member components are used in the revised model. The activities of the crystalline plagioclase components are taken from Holland and Powell (1992). Of the 214 experiments, 107 are hydrous and 107 are anhydrous. Four criteria were applied for inclusion of experiments in the final data set: (1) crystallinities <30%; (2) pure-H2O fluid saturated; (3) compositional totals (including H2O component) of 97-101% for hydrous quenched glasses and 98.5-101 for anhydrous quenched glasses; and (4) melt viscosities ≤5.2 log10 Pa·s. The final data set spans a wide range in liquid composition (45-80 wt% SiO2; 1-10 wt% Na2O+K2O), plagioclase composition (An17-95), temperature (750-1244°C), pressure (0-350 MPa), and H2O content (0-8.3 wt%). The water solubility model of Zhang et al. (2007) was applied to all hydrous experiments. The standard error estimate on the hygrometer model is 0.35 wt% H2O, and all liquid compositions are fitted equally well. Application of the model as a thermometer recovers temperatures to within ±12°, on average. Tests of the hygrometer on anhydrous piston-cylinder experiments in the literature, not included in the regression, show that the model is accurate at all pressures where plagioclase is stable. Applications of the hygrometer are made to natural rhyolites (Bishop Tuff, Katmai, and TobaTuff) with reported H2O analyses in quartz-hosted melt inclusions from the literature; the results show agreement. Applications of the hygrometer/thermometer are additionally made to natural rhyolites from Iceland and Glass Mountain, California. The updated model can be downloaded either as a program in Excel format or as a MatLab script from the Data Repository.

Micron to sub‐micron sized ferromagnetic inclusions in rock forming silicate minerals may give rise to particularly stable remanent magnetizations. When a population of inclusions have a preferred crystallographic or shape orientation in a rock, the recorded paleomagnetic direction and intensity may be biased by magnetic anisotropy. To better understand this effect, we investigated plagioclase grains from oceanic gabbro dredged from the Mid‐Atlantic Ridge at 11°–17°N. The plagioclase grains contain abundant needle and lath shaped magnetite inclusions aligned along specific directions of the plagioclase lattice. Electron back scatter diffraction and anisotropy of magnetic remanence measurements are used to correlate the orientation distribution of the magnetite inclusions in the host plagioclase that contains multiple twin types (Manebach, Carlsbad, Albite, and Pericline) with the bulk magnetic anisotropy of the inclusion‐host assembly. In non‐modified plagioclase, the anisotropy ellipsoid of magnetic remanence has oblate shapes that parallels the plagioclase (010) plane. It is suggested that recrystallization of magnetite inclusions during hydrothermal overprint shifts the relative abundance of the inclusions pertaining to the different orientation classes. We show that the maximum axis of the anisotropy ellipsoid of magnetic remanence parallels the plagioclase [001] direction, which in turn controls the recorded remanent magnetization direction. Our results are relevant for paleointensity and paleodirection determinations and for the interpretation of magnetic fabrics. Plain Language Summary Understanding how the Earth's magnetic field has varied in the past depends on the recording fidelity of the remanent magnetization held within the magnetic minerals in rocks, with magnetite being the most common. Magnetite may occur as tiny inclusions in host minerals such as plagioclase, and when they do, they are particularly robust magnetic recorders. Plagioclase from mafic plutonic rocks often contains needle‐shaped magnetite inclusions whose orientations are fixed along specific crystallographic directions of the plagioclase, which leads to extreme magnetic anisotropy. The anisotropy can significantly bias magnetic recording by deflecting the magnetization direction into the magnetic foliation plane or the lineation direction, which may be at high angles from the magnetic field direction. By combining optical and electron microscopy with magnetic measurements of individual magnetite bearing plagioclase grains, we show that different types of crystallographic twinning in plagioclase dictates different crystallographic orientation directions of the magnetite inclusions; the resultant anisotropic distribution of magnetite crystals in turn controls the direction of the magnetic remanence. Key Points Plagioclase from oceanic gabbro contains needle shaped magnetite inclusions that render plagioclase grains ferromagnetic Most of the needle elongation directions lie within or near the plagioclase (010) plane leading to pronounced magnetic anisotropy The remanent magnetization direction of the magnetite‐bearing plagioclase is controlled by magnetic anisotropy

Granulites from Holsnøy (Bergen Arcs, Norway) maintained a metastable state until fluid infiltration triggered the kinetically delayed eclogitization. Interconnected hydrous eclogite-facies shear zones are surrounded by unreacted granulites. Macroscopically, the granulite-eclogite interface is sharp and there are no significant compositional changes in the bulk chemistry, indicating the fluid composition was quickly rock buffered. To better understand the link between deformation, fluid influx, and fluid-rock interaction one cm-wide shear zone at incipient eclogitization is studied here. Granulite and eclogite consist of garnet, pyroxene, and plagioclase. These nominally anhydrous minerals (NAMs) can incorporate H.sub.2O in the form of OH groups. H.sub.2O contents increase from granulite to eclogite, as documented in garnet from ~ 10 to ~ 50 [micro]g/g H.sub.2O, pyroxene from ~ 50 to ~ 310 [micro]g/g H.sub.2O, and granulitic plagioclase from ~ 10 to ~ 140 [micro]g/g H.sub.2O. Bowl-shape profiles are characteristic for garnet and pyroxene with lower H.sub.2O contents in grain cores and higher at the rims, which suggest a prograde water influx into the NAMs. Omphacite displays a H.sub.2O content range from ~ 150 to 425 [micro]g/g depending on the amount of hydrous phases surrounding the grain. The granulitic plagioclase first separates into a hydrous, more albite-rich plagioclase and isolated clinozoisite before being replaced by new fine-grained phases like clinozoisite, kyanite and quartz during ongoing fluid infiltration. Results indicate a twofold fluid influx with different mechanisms to act simultaneously at different scales and rates. Fast and more pervasive proton diffusion is recorded by NAMs that retain the major element composition of the granulite-facies equilibration where hydrogen decorates pre-existing defects in the crystal lattice and leads to OH increase. Contemporaneously, slower grain boundary-assisted aqueous fluid influx enables element transfer and results in progressive formation of new minerals, e.g., hydrous phases. Both mechanisms lead to bulk H.sub.2O increase from ~ 450 to ~ 2500 [micro]g/g H.sub.2O towards the shear zone and convert the system from rigid to weak. The incorporation of OH groups reduces the activation energy for creep, promotes formation of smaller grain sizes (phase separation of plagioclase), and synkinematic metamorphic mineral reactions. These processes are part of the transient weakening, which enhance the sensitivity of the rock to deform.

The Boam Li deposit in Uljin, South Korea, is a hard-rock Li deposit composed of albite-spodumene pegmatite and lepidolite-elbaite greisen. The timing of Li mineralization in the Boam deposit is poorly constrained due to a wide range of the mica K-Ar ages of the greisen from 176 to 127 Ma. We investigated the microstructure, mineral composition, and U-Pb age of zircons separated from the albite-spodumene pegmatite of the Boam Li deposit. Zircons are found as inclusions in spodumene and discrete grains intergrown with secondary albite, muscovite, columbite group mineral (CGM), microlite, and apatite, indicating that the zircon was saturated during the intrusion of pegmatite melt and subsequent greisenization of the Boam deposit. Most zircons have altered porous domains in cores mantled by overgrown rims with contrasting microstructures and chemical compositions. The porous domains in zircon cores contain Ca, Al, and Fe- bearing zircon solid solution, quartz, K-feldspar, muscovite, CGM, and microlite, which indicates pervasive alteration during greisenization. Higher U contents and Zr/Hf ratios of the core than the rim suggest that the melt from which the zircon cores crystallized was U-rich and less evolved. The overgrown rim domain with weak oscillatory zoning occurs in association with albite, muscovite, and apatite. Based on the microstructure and zircon compositions, we suggest that the SHRIMP U-Pb age of the zircons from the pegmatite indicates the timing of greisenization and Li mineralization in the Boam deposit. The zircon core and rim U-Pb isotope data constitute a single discordia line, yielding a weighted mean age of 172.4 [+ or -] 1.1 Ma (MSWD = 2.3). The age is consistent with the oldest mica K-Ar age of the lepidolite-elbaite greisen in Uljin.

Volcanic rocks commonly display complex textures acquired both in the magma reservoir and during ascent to the surface. While variations in mineral compositions, sizes and number densities are routinely analysed to reconstruct pre-eruptive magmatic histories, crystal shapes are often assumed to be constant, despite experimental evidence for the sensitivity of crystal habit to magmatic conditions. Here, we develop a new program ( ShapeCalc ) to calculate 3D shapes from 2D crystal intersection data and apply it to study variations of crystal shape with size for plagioclase microlites ( l  < 100 µm) in intermediate volcanic rocks. The smallest crystals tend to exhibit prismatic 3D shapes, whereas larger crystals ( l  > 5–10 µm) show progressively more tabular habits. Crystal growth modelling and experimental constraints indicate that this trend reflects shape evolution during plagioclase growth, with initial growth as prismatic rods and subsequent preferential overgrowth of the intermediate dimension to form tabular shapes. Because overgrowth of very small crystals can strongly affect the external morphology, plagioclase microlite shapes are dependent on the available growth volume per crystal, which decreases during decompression-driven crystallisation as crystal number density increases. Our proposed growth model suggests that the range of crystal shapes developed in a magma is controlled by the temporal evolution of undercooling and total crystal numbers, i.e., distinct cooling/decompression paths. For example, in cases of slow to moderate magma ascent rates and quasi-continuous nucleation, early-formed crystals grow larger and develop tabular shapes, whereas late-stage nucleation produces smaller, prismatic crystals. In contrast, rapid magma ascent may suppress nucleation entirely or, if stalled at shallow depth, may produce a single nucleation burst associated with tabular crystal shapes. Such variation in crystal shapes have diagnostic value and are also an important factor to consider when constructing CSDs and models involving magma rheology.

Silicate weathering as an important negative feedback can regulate the Earth’s climate over time, but much debate concerns its response strength to each climatic factor and its evolution with land surface reorganisation. Such discrepancy arises from lacking weathering proxy validation and scarce quantitative paleo-constraints on individual forcing factors. Here we examine the catchment-scale link of silicate weathering intensity with various environmental parameters using a global compilation of modern sediment dataset ( n  = 3828). We show the primary control of temperature on silicate weathering given the monotonic increase of feldspar dissolution with it (0–30 °C), while controls of precipitation or topographic-lithological factors are regional and subordinate. We interpret the non-linear forcing of temperature on feldspar dissolution as depletion of more reactive plagioclase (relative to orthoclase) at higher temperature. Our results hint at stronger temperature-weathering feedback at lower surface temperature and support the hypothesis of increased land surface reactivity during the late Cenozoic cooling. How silicate weathering responds to and regulates Earth’s climate remain controversial. This study suggests the primary control of temperature on weathering intensity globally and the temperature-weathering feedback may be stronger in cold Earth.

St. Kitts lies in the northern Lesser Antilles, a subduction-related intraoceanic volcanic arc known for its magmatic diversity and unusually abundant cognate xenoliths. We combine the geochemistry of xenoliths, melt inclusions and lavas with high pressure–temperature experiments to explore magma differentiation processes beneath St. Kitts. Lavas range from basalt to rhyolite, with predominant andesites and basaltic andesites. Xenoliths, dominated by calcic plagioclase and amphibole, typically in reaction relationship with pyroxenes and olivine, can be divided into plutonic and cumulate varieties based on mineral textures and compositions. Cumulate varieties, formed primarily by the accumulation of liquidus phases, comprise ensembles that represent instantaneous solid compositions from one or more magma batches; plutonic varieties have mineralogy and textures consistent with protracted solidification of magmatic mush. Mineral chemistry in lavas and xenoliths is subtly different. For example, plagioclase with unusually high anorthite content (An ≤100 ) occurs in some plutonic xenoliths, whereas the most calcic plagioclase in cumulate xenoliths and lavas are An 97 and An 95 , respectively. Fluid-saturated, equilibrium crystallisation experiments were performed on a St. Kitts basaltic andesite, with three different fluid compositions ( X H 2 O = 1.0, 0.66 and 0.33) at 2.4 kbar, 950–1025 °C, and f O 2  = NNO − 0.6 to NNO + 1.2 log units. Experiments reproduce lava liquid lines of descent and many xenolith assemblages, but fail to match xenolith and lava phenocryst mineral compositions, notably the very An-rich plagioclase. The strong positive correlation between experimentally determined plagioclase-melt Kd Ca–Na and dissolved H 2 O in the melt, together with the occurrence of Al-rich mafic lavas, suggests that parental magmas were water-rich (> 9 wt% H 2 O) basaltic andesites that crystallised over a wide pressure range (1.5–6 kbar). Comparison of experimental and natural (lava, xenolith) mafic mineral composition reveals that whereas olivine in lavas is predominantly primocrysts precipitated at low-pressure, pyroxenes and spinel are predominantly xenocrysts formed by disaggregation of plutonic mushes. Overall, St. Kitts xenoliths and lavas testify to mid-crustal differentiation of low-MgO basalt and basaltic andesite magmas within a trans-crustal, magmatic mush system. Lower crustal ultramafic cumulates that relate parental low-MgO basalts to primary, mantle -derived melts are absent on St. Kitts.