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Arc andesite petrogenesis Olivier Reubi and Jon Blundy present an alternative view of andesite petrogenesis and argue that true liquids of intermediate composition (59 to 66 wt% SiO 2 ) are far less common in the subvolcanic reservoirs of arc volcanoes than is suggested by the abundance of erupted magma within this compositional range. This alternative view resolves several puzzling aspects of arc volcanism and provides important clues to the integration of plutonic and volcanic records. A large proportion of the magmas erupted at continental arc volcanoes are andesites, which are regarded as a major component in the formation of continental crust — consequently, it is important to understand andesite petrogenesis. Here, an alternative view of andesite petrogenesis is presented, based on a review of quenched glassy melt inclusions trapped in phenocrysts, whole-rock chemistry, and high-pressure and high-temperature experiments; this new view resolves several puzzling aspects of arc volcanism. Andesites represent a large proportion of the magmas erupted at continental arc volcanoes and are regarded as a major component in the formation of continental crust 1 . Andesite petrogenesis is therefore fundamental in terms of both volcanic hazard and differentiation of the Earth. Andesites typically contain a significant proportion of crystals showing disequilibrium petrographic characteristics indicative of mixing or mingling between silicic and mafic magmas, which fuels a long-standing debate regarding the significance of these processes in andesite petrogenesis 2 and ultimately questions the abundance of true liquids with andesitic composition. Central to this debate is the distinction between liquids (or melts) and magmas, mixtures of liquids with crystals, which may or may not be co-genetic. With this distinction comes the realization that bulk-rock chemical analyses of petrologically complex andesites can lead to a blurred picture of the fundamental processes behind arc magmatism. Here we present an alternative view of andesite petrogenesis, based on a review of quenched glassy melt inclusions trapped in phenocrysts, whole-rock chemistry, and high-pressure and high-temperature experiments. We argue that true liquids of intermediate composition (59 to 66 wt% SiO 2 ) are far less common in the sub-volcanic reservoirs of arc volcanoes than is suggested by the abundance of erupted magma within this compositional range. Effective mingling within upper crustal magmatic reservoirs obscures a compositional bimodality of melts ascending from the lower crust, and masks the fundamental role of silicic melts (≥66 wt% SiO 2 ) beneath intermediate arc volcanoes. This alternative view resolves several puzzling aspects of arc volcanism and provides important clues to the integration of plutonic and volcanic records.

Piston cylinder experiments are used to investigate the effect of oxygen fugacity (ƒO 2 ) on sulphur speciation and phase relations in arc magmas at 0.5–1.5 GPa and 840–950 °C. The experimental starting composition is a synthetic trachyandesite containing 6.0 wt% H 2 O, 2880 ppm S, 1500 ppm Cl and 3800 ppm C. Redox conditions ranging from 1.7 log units below the Ni–NiO buffer (NNO − 1.7) to NNO + 4.7 were imposed by solid-state buffers: Co–CoO, Ni–NiO, Re–ReO 2 and haematite–magnetite. All experiments are saturated with a COH fluid. Experiments produced crystal-bearing trachydacitic melts (SiO 2 from 60 to 69 wt%) for which major and volatile element concentrations were measured. Experimental results demonstrate a powerful effect of oxidation state on phase relations. For example, plagioclase was stable above NNO, but absent at more reduced conditions. Suppression of plagioclase stability produces higher Al 2 O 3 and CaO melts. The solid sulphur-bearing phases and sulphur speciation in the melt are strong functions of ƒO 2 , as expected, but also of pressure. At 0.5 GPa, the anhydrite stability field is intersected at NNO ≥ +2, but at 1.0 and 1.5 GPa, experiments at the same ƒO 2 produce sulphides and the stability field of sulphate moves towards higher ƒO 2 by ~1 log unit at 1.0 GPa and ~1.5 log units at 1.5 GPa. As a result, models that appeal to high oxidation state as an important control on the mobility of Cu (and other chalcophiles) during crustal differentiation must also consider the enhanced stability of sulphide in deep- to mid-crustal cumulates even for relatively oxidized (NNO + 2) magmas. Experimental glasses reproduce the commonly observed minimum in sulphur solubility between the S 2− and S 6+ stability fields. The solubility minimum is not related to the Fe content (Fe 2+ /Fe 3+ or total) of the melt. Instead, we propose this minimum results from an unidentified, but relatively insoluble, S-species of intermediate oxidation state.

The textures of minerals in volcanic and plutonic rocks testify to a complexity of processes in their formation that is at odds with simple geochemical models of igneous differentiation. Zoning in plagioclase feldspar is a case in point. Very slow diffusion of the major components in plagioclase means that textural evidence for complex magmatic evolution is preserved, almost without modification. Consequently, plagioclase affords considerable insight into the processes by which magmas accumulate in the crust prior to their eventual eruption or solidification. Here, we use the example of the 1980–1986 eruptions of Mount St. Helens to explore the causes of textural complexity in plagioclase and associated trapped melt inclusions. Textures of individual crystals are consistent with multiple heating and cooling events; changes in total pressure ( P ) or volatile pressure ( P H 2 O ) are less easy to assess from textures alone. We show that by allying textural and chemical analyses of plagioclase and melt inclusions, including volatiles (H 2 O, CO 2 ) and slow-diffusing trace elements (Sr, Ba), to published experimental studies of Mount St. Helens magmas, it is possible to disambiguate the roles of pressure and temperature to reconstruct magmatic evolutionary pathways through temperature–pressure–melt fraction ( T – P H 2 O – F ) space. Our modeled crystals indicate that (1) crystallization starts at P H 2 O  > 300 MPa, consistent with prior estimates from melt inclusion volatile contents, (2) crystal cores grow at P H 2 O  = 200–280 MPa at F  = 0.65–0.7, (3) crystals are transferred to P H 2 O  = 100–130 MPa (often accompanied by 10–20 °C of heating), where they grow albitic rims of varying thicknesses, and (4) the last stage of crystallization occurs after minor heating at P H 2 O  ~ 100 MPa to produce characteristic rim compositions of An 50 . We hypothesize that modeled P H 2 O decreases in excess of ~50 MPa most likely represent upward transport through the magmatic system. Small variations in modeled P H 2 O , in contrast, can be effected by fluxing the reservoir with CO 2 -rich vapors that are either released from deeper in the system or transported with the recharge magma. Temperature fluctuations of 20–40 °C, on the other hand, are an inevitable consequence of incremental, or pulsed, assembly of crustal magma bodies wherein each pulse interacts with ancestral, stored magmas. We venture that this “petrological cannibalism” accounts for much of the plagioclase zoning and textural complexity seen not only at Mount St. Helens but also at arc magmas generally. More broadly we suggest that the magma reservoir below Mount St. Helens is dominated by crystal mush and fed by frequent inputs of hotter, but compositionally similar, magma, coupled with episodes of magma ascent from one storage region to another. This view both accords with other independent constraints on the subvolcanic system at Mount St. Helens and supports an emerging view of many active magmatic systems as dominantly super-solidus, rather than subliquidus, bodies.

Oceanic lithosphere carries volatiles, notably water, into the mantle through subduction at convergent plate boundaries. This subducted water exercises control on the production of magma, earthquakes, formation of continental crust and mineral resources. Identifying different potential fluid sources (sediments, crust and mantle lithosphere) and tracing fluids from their release to the surface has proved challenging 1 . Atlantic subduction zones are a valuable endmember when studying this deep water cycle because hydration in Atlantic lithosphere, produced by slow spreading, is expected to be highly non-uniform 2 . Here, as part of a multi-disciplinary project in the Lesser Antilles volcanic arc 3 , we studied boron trace element and isotopic fingerprints of melt inclusions. These reveal that serpentine—that is, hydrated mantle rather than crust or sediments—is a dominant supplier of subducted water to the central arc. This serpentine is most likely to reside in a set of major fracture zones subducted beneath the central arc over approximately the past ten million years. The current dehydration of these fracture zones coincides with the current locations of the highest rates of earthquakes and prominent low shear velocities, whereas the preceding history of dehydration is consistent with the locations of higher volcanic productivity and thicker arc crust. These combined geochemical and geophysical data indicate that the structure and hydration of the subducted plate are directly connected to the evolution of the arc and its associated seismic and volcanic hazards. Serpentine subducted below the Lesser Antilles volcanic arc supplies water to the arc, controlling the location of seismicity, volcanic productivity and thickness of crust.

The Fe–Mg exchange coefficient between olivine (ol) and melt (m), defined as KdFeT-Mg = (Feol/Fem)·(Mgm/Mgol), with all FeT expressed as Fe2+, is one of the most widely used parameters in petrology. We explore the effect of redox conditions on KdFeT-Mg using experimental, olivine-saturated basaltic glasses with variable H2O (≤ 7 wt%) over a wide range of fO2 (iron-wüstite buffer to air), pressure (≤ 1.7 GPa), temperature (1025–1425 °C) and melt composition. The ratio of Fe3+ to total Fe (Fe3+/∑Fe), as determined by Fe K-edge µXANES and/or Synchrotron Mössbauer Source (SMS) spectroscopy, lies in the range 0–0.84. Measured Fe3+/∑Fe is consistent (± 0.05) with published algorithms and appears insensitive to dissolved H2O. Combining our new data with published experimental data having measured glass Fe3+/∑Fe, we show that for Fo65–98 olivine in equilibrium with basaltic and basaltic andesite melts, KdFeT-Mg decreases linearly with Fe3+/∑Fe with a slope and intercept of 0.3135 ± 0.0011. After accounting for non-ideal mixing of forsterite and fayalite in olivine, using a symmetrical regular solution model, the slope and intercept become 0.3642 ± 0.0011. This is the value at Fo50 olivine; at higher and lower Fo the value will be reduced by an amount related to olivine non-ideality. Our approach provides a straightforward means to determine Fe3+/∑Fe in olivine-bearing experimental melts, from which fO2 can be calculated. In contrast to KdFeT-Mg, the Mn–Mg exchange coefficient, KdMn-Mg, is relatively constant over a wide range of P–T–fO2 conditions. We present an expression for KdMn-Mg that incorporates the effects of temperature and olivine composition using the lattice strain model. By applying our experimentally-calibrated expressions for KdFeT-Mg and KdMn-Mg to olivine-hosted melt inclusions analysed by electron microprobe it is possible to correct simultaneously for post-entrapment crystallisation (or dissolution) and calculate melt Fe3+/∑Fe to a precision of ≤ 0.04.

The formation of porphyry Cu-Mo deposits hinges critically on the ability of an exsolving magmatic volatile phases (MVP) to efficiently extract the available Cu and Mo from the silicate melt and transport them to the site of mineralization. There has been substantial debate about the relative importance of the critical parameters likely to control metal partitioning among silicate melts and supercritical fluids, vapors, and brines. To explore the relative contributions of key MVP parameters (XNaCleq, fO2, fS2), we present felsic magmatic Cu-Mo partitioning experiments at both reduced (fO2 = NNO+0.6) and oxidized conditions (fO2 = NNO+2), at high fS2, and over the full range of salinities (XNaCleq) relevant to porphyry deposit formation. The experiments demonstrate that fluid-melt Cu partition coefficients (DCuf/m) have a dominantly second-order exponential relationship with XNaCleq at relevant salinities, consistent with a (Na,K)CuCl2 ion-pair complexation mechanism. We find a strong linear dependence of Cu partitioning on Cl partitioning between coexisting brine and vapor, in good agreement with limited data from unmodified natural fluid inclusions. Whereas H2S can increase Cu partitioning via (Na,K)CuCl(HS) formation, SO2 has no measureable effect on Cu partitioning. These data allow for quantifying the strong partitioning of Cu out of silicate melts at MVP salinities above ∼5 wt%, which will become further enriched into tardo-magmatic brines on phase separation. Our data also highlight that low-salinity (<2-3 wt% NaCleq) oxidized MVPs are not capable of substantially extracting Cu from evolved silicate melts or transporting it to the site of mineralization. We also show that DMof/m is a linear function of XNaCleq, consistent with mono-chloride (e.g., {Na,K}MoO3Cl), Na-K molybdate (e.g., {Na,K}HMoO4), or thio-molybdate ({Na,K}HMoO2S2) complexation mechanisms at modest salinities (>3 wt%) rather than the Mo-oxy-hydroxy [MoO2(OH)2] complexation observed at lower salinities. The fO2 of the magmatic system has a subordinate effect on DMf/om, with enhanced partitioning at higher fO2. We use the combined data set to produce expressions for DCuf/m and DMof/m as functions of XNaCleq, XH2S, and fO2 Df/mCu = 8.0(±1.4) × 104[(XNaCleq)2(XH2O)14±2{1 + 180(±60)(XH2S)}] + 380(±50)(XNaCleq) +0.8(±0.5) Df/mMo(at NNO + 2) = 430(±60)·(XNaCleq) + 1.5(±0.7)·(XH2O). These equations provide Cu-Mo fluid-melt partition coefficients for common arc melt-MVP assemblages as their composition evolves through time and space. Quantitative modeling of the contrasting partitioning behavior of Cu and Mo using these equations will allow for significant improvement in understanding metal extraction and porphyry deposit formation.

The global frequency of volcanic eruptions is inversely proportional to the volume of magma erupted in a single event. Numerical modelling of magma reservoirs evolving in Earth’s crust shows that frequent, small eruptions are triggered by injections of magma into the reservoir, but rare, giant supervolcano eruptions are triggered by magma buoyancy. Super-eruptions are extremely rare events. Indeed, the global frequency of explosive volcanic eruptions is inversely proportional to the volume of magma released in a single event 1 , 2 . The rate of magma supply, mechanical properties of the crust and magma, and tectonic regime are known to play a role in controlling eruption frequency and magnitude 3 , 4 , 5 , 6 , 7 , but their relative contributions have not been quantified. Here we use a thermomechanical numerical model of magma injection into Earth’s crust and Monte Carlo simulations to explore the factors controlling the recurrence rates of eruptions of different magnitudes. We find that the rate of magma supply to the upper crust controls the volume of a single eruption. The time interval between magma injections into the subvolcanic reservoir, at a constant magma-supply rate, determines the duration of the magmatic activity that precedes eruptions. Our simulations reproduce the observed relationship between eruption volume and magma chamber residence times and replicate the observed correlation between erupted volumes and caldera dimensions 8 , 9 . We also find that magma buoyancy is key to triggering super-eruptions, whereas pressurization associated with magma injection is responsible for relatively small and frequent eruptions. Our findings help improve our ability to decipher the long-term activity patterns of volcanic systems.

The Lesser Antilles Volcanic Arc is remarkable for the abundance and variety of erupted plutonic xenoliths. These samples provide a window into the deeper crust and record a more protracted crystallisation history than is observed from lavas alone. We present a detailed petrological and in situ geochemical study of xenoliths from Martinique in order to establish their petrogenesis, pre-eruptive storage conditions and their contribution to construction of the sub-volcanic arc crust. The lavas from Martinique are controlled by crystal–liquid differentiation. Amphibole is rarely present in the erupted lavas, but it is a very common component in plutonic xenoliths, allowing us to directly test the involvement of amphibole in the petrogenesis of arc magmas. The plutonic xenoliths provide both textural and geochemical evidence of open system processes and crystal ‘cargos’. All xenoliths are plagioclase-bearing, with variable proportions of olivine, spinel, clinopyroxene, orthopyroxene and amphibole, commonly with interstitial melt. In Martinique, the sequence of crystallisation varies in sample type and differs from other islands of the Lesser Antilles arc. The compositional offset between plagioclase (~An 90 ) and olivine (~Fo 75 ), suggests crystallisation under high water contents and low pressures from an already fractionated liquid. Texturally, amphibole is either equant (crystallising early in the sequence) or interstitial (crystallising late). Interstitial amphibole is enriched in Ba and LREE compared with early crystallised amphibole and does not follow typical fractionation trends. Modelling of melt compositions indicates that a water-rich, plagioclase-undersaturated reactive melt or fluid percolated through a crystal mush, accompanied by the breakdown of clinopyroxene, and the crystallisation of amphibole. Geothermobarometry estimates and comparisons with experimental studies imply the majority of xenoliths formed in the mid-crust. Martinique cumulate xenoliths are inferred to represent crystal mushes within an open system, through which melt can both percolate and be generated.

The margins of the Caribbean and associated hazards and resources have been shaped by a poorly understood history of subduction. Using new data, we improve teleseismic P -wave imaging of the eastern Caribbean upper mantle and compare identified subducted-plate fragments with trench locations predicted from plate reconstruction. This shows that material at 700–1200 km depth below South America derives from 90–115 Myr old westward subduction, initiated prior to Caribbean Large-Igneous-Province volcanism. At shallower depths, an accumulation of subducted material is attributed to Great Arc of the Caribbean subduction as it evolved over the past 70 Ma. We interpret gaps in these subducted-plate anomalies as: a plate window and tear along the subducted Proto-Caribbean ridge; tearing along subducted fracture zones, and subduction of a volatile-rich boundary between Proto-Caribbean and Atlantic domains. Phases of back-arc spreading and arc jumps correlate with changes in age, and hence buoyancy, of the subducting plate. Seismic imaging of subducted plates offers a way to improve plate tectonic reconstructions. Here, Braszus et al. use new ocean-bottom seismometer data from the Lesser Antilles to locate subducted spreading centres and faults thus providing a new understanding of the evolution of the Caribbean plate.

Arc magmas have higher water contents (2-6 wt.% H 2 O) than magmas generated in other tectonic environments, with a growing body of evidence suggesting that some deep arc magmas may be ‘super-wet’ (>6 wt.% H 2 O). Here, we use thermodynamic modelling to show that the behaviour of zirconium during magmatic differentiation is strongly sensitive to melt water contents. We demonstrate that super-wet magmas crystallise zircon with low, homogeneous titanium concentrations (75 th percentile <10 ppm) due to a decrease in zircon saturation temperatures with increasing melt H 2 O. We find that zircon titanium concentrations record a transition to super-wet magmatism in Central Chile immediately before the formation of the world’s largest porphyry copper deposit cluster at Río Blanco-Los Bronces. Broader analysis shows that low, homogeneous zircon titanium concentrations are present in many magmatic systems. Our study suggests that super-wet magmas are more common than previously envisaged and are fundamental to porphyry copper deposit mineralisation. Titanium concentrations in zircon crystals reveal a link between the world’s largest copper resources and magmas with very high-water contents.