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New geochemical data, including Sr–Nd–Pb isotopes for whole-rock and groundmass samples, are reported for edifice-forming eruptives at Tongariro volcano, New Zealand, which span its ~ 350 ka to late Holocene history. Tongariro eruptives are medium-K basaltic-andesites to dacites (53.0–66.2 wt% SiO2) that evolved via assimilation-fractional crystallisation (AFC) processes partly or mostly in the uppermost 15 km of the crust. When ordered chronologically using a high-resolution 40Ar/39Ar-dated eruptive stratigraphy, the compositional data show systematic 10–130 kyr cycles. Mafic replenishment events inferred from MgO values occurred at ~ 230, ~ 151, ~ 88 and ~ 56 ka and in the late Holocene, with high-MgO flank vents erupting at ~ 160, ~ 117, ~ 35 and ~ 17.5 ka. Cycles in Sm/Nd, 87Sr/86Sr, 143Nd/144Nd and Pb isotopic ratios, which are decoupled from MgO, K2O and Rb/Sr cycles, indicate periods of prolonged crustal residence of magmas from ~ 230 to ~ 100 ka and ~ 95 to ~ 30 ka. AFC modelling shows that intermediate and silicic melt compositions, with r-values between 0.1 and 1, are needed to reproduce Tongariro compositional arrays. AFC models also indicate that ~ 20% of the average Tongariro magma comprises assimilated (meta)sedimentary basement material. Locally, Tongariro and adjacent Ruapehu volcanoes attain their most crust-like 87Sr/86Sr and 143Nd/144Nd compositions at ~ 100 and ~ 30 ka, paralleling with zircon model-age crystallisation modes at the rhyolitic Taupō volcano ~ 50 km to the NNE. These coincidences suggest that the timing and tempo of magma assembly processes at all three volcanoes were contemporaneous and may have been tectonically influenced since at least 200 ka. [ABSTRACT FROM AUTHOR]

The South Aegean Volcanic Arc overlies a slowly subducting, cool slab of oceanic-to-transitional crust, and hosts the hazardous Christiana–Santorini–Kolumbo volcanic field. In order to investigate the primitive melts feeding the volcanic field, we present major and trace element analyses of 130 olivine-hosted melt inclusions from Santorini, integrated with previously published H2O and CO2 data. Following post-entrapment corrections, we identify four endmember primitive melt types preserved in Fo ≥ 80 olivines, ranging from low-K island-arc basalts with La/Yb ~ 1.5 and 1.5–3.0 wt% H2O to andesites with La/Yb ~ 6–10 and 3.0–3.5 wt% H2O. They are consistent with melting at 1.3 to 2.3 GPa and 1350–1440 °C of variably depleted peridotitic mantle fluxed by slab-derived melts and fluids. The chemical signatures of sediment melts dominate, while those of fluids derived from the ocean crust are low compared to global datasets. This is consistent with thick sediment accumulations observed in the Hellenic trench, and with low calculated fluid fluxes from the downgoing slab. The low H2O contents estimated for the primary melts (0.8–1.8 wt%) may imply a component of decompression melting beneath the arc. Coupled with a well-constrained chronostratigraphic context, the melt inclusion archive provides a time series of mantle-derived input into the silicic crustal magmatic system over the last 530 ka. Primitive melts with La/Yb ≤ 5 have been erupted encased in olivines over the last 530 ky, without any evident time variation. Melt inclusions with La/Yb > 5 have, on the other hand, been restricted to two periods: (1) prior to the onset of major explosive volcanism at ~ 360 ka, and (2) the products of the 3.6 ka Late-Bronze-Age eruption and the 22-to-3.6 ka inter-Plinian period immediately preceding it. The observations may be explained by time-varying differential extraction of melts from deep storage zones in the mantle or lower crust, related to lithospheric rifting and caldera collapse events. Temporal variations in the supplies of slab-derived melts and fluids may also play a role. [ABSTRACT FROM AUTHOR]

Exposures of arc crustal sections represent rare opportunities to directly evaluate lower crustal magmatic processes and their link to arc products in the middle and upper crust. Within the southernmost Sierra Nevada batholith, the Bear Valley Intrusive Suite (BVIS) exposes a contemporaneously constructed ~ 30 km thick intrusive suite, and thus is ideal for this type of examination. Here we present detailed petrography and mineral major and trace element data for the BVIS. The deepest exposed portion of the BVIS (8–9 kbars) is composed of heterogeneous mafic igneous intrusions of olivine metagabbro, olivine-hornblende orthopyroxenite, olivine-bearing hornblende norite, hornblende norite, hornblende gabbronorite, hornblendite and hornblende gabbro. Shallower crustal intrusions (3–7 kbars) are comparatively homogeneous and dominated by hypersthene-bearing and hypersthene-free tonalites. Using amphibole-plagioclase geothermometry, we show that the mafic lower crustal intrusions crystallized over a wide temperature range from 850 to 1070 °C, highlighting mafic igneous fractionation during isobaric cooling in the lower crust of the Sierran arc, while tonalitic liquids were emplaced at temperatures < 800 °C in the middle and upper crust. Calculated trace element melt compositions in equilibrium with amphibole in lower crustal gabbros are similar to measured tonalite bulk compositions and support the generation of tonalites through fractionation of the observed gabbros. Further, petrography and mineral chemistry suggest multiple distinct crystallization sequences recorded in the different types of gabbro, requiring the presence of coexisting parental melts with contrasting compositions and H2O contents. Using available experimental data, we develop a model by which mixing of variably fractionated dry and wet magmas with similar viscosities followed by crystallization-differentiation in the deep crust to explain the formation of uniform tonalitic melts at shallower crustal levels in the BVIS. This process also explains the unusual predominance of orthopyroxene in the BVIS, and the limited aluminum enrichment compared to experimental differentiation sequences of hydrous basalts. Considering the similar geochemical characteristics of intermediate and felsic igneous rocks from the Sierra Nevada batholith and the Cascades, mixing magmas of variable H2O contents in the lower crust represents a viable petrological process to produce SiO2-rich liquids that may be more common than previously recognized. [ABSTRACT FROM AUTHOR]

The partitioning of major and trace elements between eclogite and aqueous fluids with variable salinity was studied at 700–800 °C and 4–6 GPa in piston cylinder and multi anvil experiments. Fluid compositions were determined using the diamond trap technique combined with laser ablation ICP-MS measurements in the frozen state. In addition to NaCl, SiO2 is the main solute in the fluids. The fluid/eclogite partition coefficients of the large ion lithophile elements (LILE), such as Rb, Cs, Sr, and Ba as well as those of the light rare earths (LREE), of Pb, and of U increase by up to three orders of magnitude with salinity. These elements will therefore be efficiently transported by saline fluids. On the other hand, typical high field strength elements, such as Ti, Nb, and Ta, are not mobilized even at high salinities. Increasing temperature and pressure gradually increases the partitioning into the fluid. In particular, Th is mobilized by silica-rich fluids at 6 GPa already at low salinities. We show that we can fully reproduce the trace element enrichment pattern of primitive arc basalts by adding a few percent of saline fluid (with 5–10 wt% Cl) released from the basaltic slab to the zone of melting in the mantle wedge. Assuming 2 wt% of rutile in the eclogite equilibrated with the saline fluid produces a negative Nb Ta anomaly that is larger than in most primitive arc basalts. Therefore, we conclude that the rutile fraction in the subducted eclogite below most arcs is likely < 1 wt%. In fact, saline fluids would even produce a noticeable negative Nb Ta anomaly without any rutile in the eclogite residue. Metasomatism by sediment melts alone, on the other hand, is unable to produce the enrichment pattern seen in arc basalts. We, therefore, conclude that at least for primitive arc basalts, the release of hydrous fluids from the basaltic part of the subducted slab is the trigger for melting and the main agent of trace element enrichment. The contribution of sediment melts to the petrogenesis of these magmas is likely negligible. In the supplementary material, we provide a “Subduction Calculator” in Excel format, which allows the calculation of the trace element abundance pattern in primitive arc basalts as function of fluid salinity, the amount of fluid released from the basaltic part of the subducted slab, the fluid fraction added to the source, and the degree of melting. [ABSTRACT FROM AUTHOR]

Under the right circumstances, fluid-fluxed crustal melting occurs, as demonstrated by some amphibolite-facies migmatites, but the relatively low temperatures involved make this mode of formation an unsatisfactory model for most silicic magma genesis in arcs. The concept of silicic arc magma formation through either partial melting of metabasalts or fractionation of hydrous basaltic parent magmas should also be treated with scepticism, as both these processes produce sodic and moderately to strongly peraluminous liquids that are chemically unlike most arc silicic rocks and glasses. Furthermore, if the silicic magmas are formed in the deep crust, the fractionation model predicts evolved liquids with extreme H2O contents. Thus, the answer to the question posed in the title is that neither of these two models, in isolation, seems likely for the formation of the majority of the more silicic magmas in arc environments. Given the totality of the evidence, we favour models in which high-T processes dominate. These could include fluid-absent partial melting of older non-basaltic arc crust, entrainment of source-derived crystal cargos, hybridisation with mantle-derived magmas and, in some cases, crystal fractionation of andesitic magmas at shallow crustal levels. [ABSTRACT FROM AUTHOR]

Amphibole has been discussed to potentially represent an important phase during early chemical evolution of arc magmas, but is not commonly observed in eruptive arc rocks. Here, we present an in-depth study of metastable calcic amphibole megacrysts in basaltic andesites of Merapi volcano, Indonesia. Radiogenic Sr and Nd isotope compositions of the amphibole megacrysts overlap with the host rock range, indicating that they represent antecrysts to the host magmas rather than xenocrysts. Amphibole-based barometry suggests that the megacrysts crystallised at pressures of >500 MPa, i.e., in the mid- to lower crust beneath Merapi. Rare-earth element concentrations, in turn, require the absence of magmatic garnet in the Merapi feeding system and, therefore, place an uppermost limit for the pressure of amphibole crystallisation at ca. 800 MPa. The host magmas of the megacrysts seem to have fractionated significant amounts of amphibole and/or clinopyroxene, because of their low Dy/Yb ratios relative to the estimated compositions of the parent magmas to the megacrysts. The megacrysts' parent magmas at depth may thus have evolved by amphibole fractionation, in line with apparently coupled variations of trace element ratios in the megacrysts, such as e.g., decreasing Zr/Hf with Dy/Yb. Moreover, the Th/U ratios of the amphibole megacrysts decrease with increasing Dy/Yb and are lower than Th/U ratios in the basaltic andesite host rocks. Uranium in the megacrysts' parent magmas, therefore, may have occurred predominantly in the tetravalent state, suggesting that magmatic fO in the Merapi plumbing system increased from below the FMQ buffer in the mid-to-lower crust to 0.6-2.2 log units above it in the near surface environment. In addition, some of the amphibole megacrysts experienced dehydrogenation (H loss) and/or dehydration (HO loss), as recorded by their variable HO contents and D/ H and Fe/Fe ratios, and the release of these volatile species into the shallow plumbing system may facilitate Merapi's often erratic eruptive behaviour. [ABSTRACT FROM AUTHOR]

In volatile-saturated magmas, degassing and crystallisation are interrelated processes which influence the eruption style. Melt inclusions provide critical information on volatile and melt evolution, but this information can be compromised significantly by post-entrapment modification of the inclusions. We assess the reliability and significance of pyroxene-hosted melt inclusion analyses to document the volatile contents (particularly HO) and evolution of intermediate arc magmas at Volcán de Colima, Mexico. The melt inclusions have maximal HO contents (≤4 wt%) consistent with petrological estimates and the constraint that the magmas crystallised outside the amphibole stability field, demonstrating that pyroxene-hosted melt inclusions can preserve HO contents close to their entrapment values even in effusive eruptions with low effusion rates (0.6 m s). The absence of noticeable HO loss in some of the inclusions requires post-entrapment diffusion coefficients (≤1 × 10 m s) at least several order of magnitude smaller than experimentally determined H diffusion coefficient in pyroxenes. The HO content distribution is, however, not uniform, and several peaks in the data, interpreted to result from diffusive HO reequilibration, are observed around 1 and 0.2 wt%. HO diffusive loss is also consistent with the manifest lack of correlations between HO and CO or S contents. The absence of textural evidence supporting post-entrapment HO loss suggests that diffusion most likely occurred via melt channels prior to sealing of the inclusions, rather than through the host crystals. Good correlation between the melt inclusion sealing and volcano-tectonic seismic swarm depths further indicate that, taken as a whole, the melt inclusion population accurately records the pre-eruptive conditions of the magmatic system. Our data demonstrate that HO diffusive loss is a second-order process and that pyroxene-hosted melt inclusions can effectively record the volatile contents and decompression-induced crystallisation paths of vapour-saturated magmas. [ABSTRACT FROM AUTHOR]

The solubility of U and Th in aqueous solutions at P-T-conditions relevant for subduction zones was studied by trapping uraninite or thorite saturated fluids as synthetic fluid inclusions in quartz and analyzing their composition by Laser Ablation-ICPMS. Uranium is virtually insoluble in aqueous fluids at Fe-FeO buffer conditions, whereas its solubility increases both with oxygen fugacity and with salinity to 960 ppm at 26.1 kbar, Re-ReO buffer conditions and 14.1 wt% NaCl in the fluid. At 26.1 kbar and 800°C, uranium solubility can be reproduced by the equation: $$ \log {\text{U}} = 2.681 + 0.1433\log f{\text{O}}_{2} + 0.594{\text{Cl,}} $$ where fO is the oxygen fugacity, and Cl is the chlorine content of the fluid in molality. In contrast, Th solubility is generally low (<10 ppm) and independent of oxygen fugacity or fluid salinity. The solubility of U and Th in clinopyroxene in equilibrium with uraninite and thorite was found to be in the order of 10 ppm. Calculated fluid/cpx partition coefficients of Th are close to unity for all conditions. In contrast, D for uranium increases strongly both with oxygen fugacity and with salinity. We show that reducing or NaCl-free fluids cannot produce primitive arc magmas with U/Th ratio higher than MORB. However, the dissolution of several wt% of oxidized, saline fluids in arc melts can produce U/Th ratios several times higher than in MORB. We suggest that observed U/Th ratios in arc magmas provide tight constraints on both the salinity and the oxidation state of subduction zone fluids. [ABSTRACT FROM AUTHOR]

The stability field and composition of immiscible sulfate melts in equilibrium with silicate magmas has been determined using experiments over a range of crustal pressures, allowing an assessment of their possible role in transporting sulfur into the sub-volcanic arc and porphyry copper deposit systems. Experimental starting materials were based on natural trachyandesite and trachydacite compositions, with 3.5–7 wt% H2O and 3.5–5.5 wt% sulfur added to produce large, analyzable amounts of sulfate phases. Conditions ranged over 800–1200 °C, 0.2-1GPa and ƒO2 > NNO + 2.5. Sulfate melts formed at temperatures above 1000 °C at 0.75 and 1 GPa and above 900 °C at 0.2 GPa, suggesting some pressure dependence on their stability. At temperatures below 1100 °C sulfate melts and anhydrite crystals commonly coexist. Sulfate melts quenched to an intergrowth that was difficult to prepare for analysis. However, the composition was approximated by EPMA and further constrained by mass balance calculations. Sulfate melts were dominated by CaO and SO3, but also contained, in order of decreasing abundance, Na2O, K2O, MgO, FeO, Cl and P2O5. Chlorine showed a particular preference for the sulfate melt relative to the coexisting silicate melt, and calculated partition coefficients for sulfate/silicate melts were 5–13 at 1200 °C, 0.75–1 GPa. Experimental data show that, in the absence of an exsolved, hydrous fluid phase, sulfate melts can form in natural arc magmas at near-liquidus temperatures ≥ 1000 °C, assuming that magmas are oxidized and contain sufficient sulfur (> 2000–3000 ppm S). These results suggest that sulfate melt could be an important component in transporting sulfur as well as chlorine to shallow levels in the crust for hydrous magmas under a specific range of conditions. Both the non-quenchable and water-soluble nature of sulfate melts (and anhydrite) make them difficult to identify, unless trapped as mineral inclusions similar to the "wormy anhydrite" trapped in high-temperature amphiboles from Yanacocha, Peru. [ABSTRACT FROM AUTHOR]