Subduction zone magmas are characterized by high concentrations of pre-eruptive H2O, presumably as a result of an H2O flux originating from the dehydrating, subducting slab. The extent of mantle melting increases as a function of increasing water content beneath back-arc basins and is predicted to increase in a similar manner beneath arc volcanoes. Here, we present new data for olivine-hosted, basaltic melt inclusions from the Mariana arc that reveal pre-eruptive H2O contents of similar to 1 center dot 5-6 center dot 0 wt %, which are up to three times higher than concentrations reported for the Mariana Trough back-arc basin. Major element systematics of arc and back-arc basin basalts indicate that the back-arc basin melting regime does not simply mix with wet, arc-derived melts to produce the observed range of back-arc magmatic H2O concentrations. Simple melting models reveal that the trend of increasing extents of melting with increasing H2O concentrations of the mantle source identified in the Mariana Trough generally extends beneath the Mariana volcanic front to higher mantle water contents and higher extents of melting. In detail, however, each Mariana volcano may define a distinct relationship between extent of melting and the H2O content of the mantle source. We develop a revised parameterization of hydrous melting, incorporating terms for variable pressure and mantle fertility, to describe the distinct relationships shown by each arc volcano. This model is used in combination with thermobarometry constraints to show that hydrous melts equilibrate at greater depths (34-87 km) and temperatures (> 1300 degrees C) beneath the Mariana arc than beneath the back-arc basin (21-37 km), although both magma types can form from a mantle of similar potential temperature (similar to 1350 degrees C). The difference lies in where the melts form and equilibrate. Arc melts are dominated by those that equilibrate within the hot core of the mantle wedge, whereas back-arc melts are dominated by those that equilibrate within the shallow zone of decompression melting beneath the spreading center. Despite higher absolute melting temperatures (> 1300 degrees C), Mariana arc melts reflect lower melt productivity as a result of wet melting conditions and a more refractory mantle source.

We report the first known occurrence of high-Ca boninites within an active submarine island arc, at Volcano A within the Tonga Arc. Both the whole rock and a population of melt inclusions (in Fo(86-92) olivines) from a dredged satellite cone have compositions classified as high-Ca boninite. All samples from Volcano A, however, may be related to parental boninites, given the similarity in their rare earth element patterns and their coherency along a similar liquid line of descent. The primary high-Ca boninite liquids were generated in the mantle wedge by high cumulative degrees of melting (>similar to 24%) at typical mantle wedge temperatures (<1300 degrees C) driven by an influx of slab-derived fluid (>4 wt % H2O in primary liquids). We propose a two-stage model for generating primary boninite liquids at Volcano A: (1) melting of fertile peridotite within the Lau back-arc basin, followed by (2) remelting of this residual peridotite with slab-derived fluid beneath the Tonga Arc. The occurrence of high-Ca boninites at Volcano A is related to the relative location and duration of back-arc spreading. Here, the Eastern Lau Spreading Center has been processing mantle for similar to 1 Ma, and corner flow circulation brings mantle from the back-arc melting regime into the arc melting regime at a rate that is a significant fraction (>30%) of the convergence rate. On the basis of Si-6.0 and Ti-6.0 relationships, we argue that a significant portion of the central Tonga Arc near Volcano A, as well as several other arc volcanoes with active back-arc basins, are also erupting basaltic andesites with boninite parentage.

Light noble gas (He-Ne-Ar) solubility has been experimentally determined in a range of materials with six-member, tetrahedral ring structures: beryl, cordierite, tourmaline, antigorite, muscovite, F-phlogopite, actinolite, and pargasite. Helium solubility in these materials is relatively high, 4 x 10(-10) to 3 x 10(-7) mol g(-1) bar(-1), which is similar to 100 to 100,000 x greater than He solubility in olivine, pyroxene, or spinel. Helium solubility broadly correlates with the topology of ring structures within different minerals. Distinctive He-Ne-Ar solubility patterns are associated with the different ring structure topologies. Combined, these observations suggest ring structures have a strong influence on noble gas solubility in materials and could facilitate the recycling of noble gases, along with other volatiles (i.e., water, chlorine, and fluorine), into the mantle. Measurements of Ne and Ar solubility in antigorite, however, are highly variable and correlated with each other, suggesting multiple factors contribute the solubility of noble gases in serpentine-rich materials. (C) 2015 Elsevier Ltd. All rights reserved.

The diffusion kinetics of He and Ne in four amphibole specimens have been experimentally determined using stepwise degassing analysis of samples previously irradiated with energetic protons, and Arrhenius relationships have been fit to these data. The primary finding is that He and Ne diffusivities are systematically lower in amphiboles that have higher concentrations of unoccupied ring sites, suggesting that unoccupied ring sites act as traps for migrating noble gases. Ring site influence of noble gas diffusivity in amphiboles has substantial implications for 40Ar/39Ar thermochronology applied to these phases and the efficiency of noble gas recycling in subduction zones. These findings are consistent with the correlation between noble gas solubility and the concentration of unoccupied ring sites in amphibole (Jackson et al., 2013a, 2015) but are inconsistent with the ionic porosity model for noble gas diffusion (Fortier and Giletti, 1989; Dahl, 1996). Rather, these findings suggest that the topology of ionic porosity and absolute volume of ionic porosity compete in determining the rate at which noble gases diffuse. (C) 2015 Elsevier Ltd. All rights reserved.

Measurements of Xe isotope ratios in ocean island basalts (OIB) suggest that Earth's mantle accreted heterogeneously, and that compositional remnants of accretion are sampled by modern, high-He-3/He-4 OIB associated with the Icelandic and Samoan plumes. If so, the high-He-3/4(H)e source may also have a distinct oxygen isotopic composition from the rest of the mantle. Here, we test if the major elements of the high-He-3/He-4 source preserve any evidence of heterogeneous accretion using measurements of three oxygen isotopes on olivine from a variety of high-He-3/(4) He OIB locations. To high precision, the Delta O-17 value of high-He-3/He-4 olivines from Hawaii, Pitcairn, Baffin Island and Samoa, are indistinguishable from bulk mantle olivine (Delta O-17(Bulk Mantle) - Delta O-17(High3He/4He) olivine = -0.002 +/- 0.004 (2 x SEM)parts per thousand). Thus, there is no resolvable oxygen isotope evidence for heterogeneous accretion in the high-He-3/(4) He source. Modelling of mixing processes indicates that if an early-forming, oxygen-isotope distinct mantle did exist, either the anomaly was extremely small, or the anomaly was homogenised away by later mantle convection.

Visible to near-infrared (V-NIR) remote sensing observations have identified spinel in various locations and lithologies on the Moon. Experimental studies have quantified the FeO content of these spinels (Jackson et al. 2014), however the chromite component is not well constrained. Here we present compositional and spectral analyses of spinel synthesized with varying chromium contents at lunar-like oxygen fugacity (f(o2)). Reflectance spectra of the chromium-bearing synthetic spinels (Cr# 1-29) have a narrow (similar to 130 nm wide) absorption feature centered at similar to 550 nm. The 550 nm feature, attributed to octahedral Cr3+, is present over a wide range in iron content (Fe# 8-30) and its strength positively correlates with spine chromium content [ln(reflectance(min)) = -0.0295 Cr# -0.3708]. Our results provide laboratory characterization for the V-NIR and mid-infrared (mid-IR) spectral properties of spinel synthesized at lunar-like f(o2). The experimentally determined calibration constrains the Cr# of spinels in the lunar pink spinel anorthosites to low values, potentially Cr# < 1. Furthermore, the results suggest the absence of a 550 nm feature in remote spectra of the Dark Mantle Deposits at Sinus Aestuum precludes the presence of a significant chromite component. Combined, the observation of low chromium spinels across the lunar surface argues for large contributions of anorthositic materials in both plutonic and volcanic rocks on the Moon.

We report C, N, Si, and Al-Mg isotope data for 39 presolar X silicon carbide (SiC) and four silicon nitride grains-a group of presolar grains that condensed in the remnants of core-collapse Type II supernovae (CCSNe)-isolated from the Murchison meteorite. Energy dispersive X-ray data were used to determine the Mg and Al contents of the X SiC grains for comparison with the Mg/Al ratios determined by secondary ion mass spectroscopy (SIMS). Previous SIMS studies have used O-rich standards in the absence of alternatives. In this study, the correlated isotopic and elemental data of the X SiC grains enabled accurate determination of the initial 26Al/27Al ratios for the grains. Our new grain data suggest that (i) the literature data for X grains are affected to varying degrees by asteroidal/terrestrial contamination, and (ii) the Al/Mg ratios in SiC are a factor of 2 (with +/- 6% 1 sigma uncertainties) lower than estimated based on the SIMS analyses that used O-rich standards. The lowered Al/Mg ratios result in proportionally higher inferred initial 26Al/27Al ratios for presolar SiC grains. In addition, the suppression of asteroidal/terrestrial contamination in this study leads to the observation of negative trends for 12C/13C-30Si/28Si and 26Al/27Al-30Si/28Si among our CCSN grains. We discuss these isotope trends in the light of explosive CCSN nucleosynthesis models, based on which we provide new insights into several nontraditional CCSN nucleosynthesis processes, including explosive H burning, the existence of a C/Si zone in the outer regions of CCSNe, and neutrino-nucleus reactions in deep CCSN regions.