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An analytical protocol was developed for correcting Kr baseline-induced bias and Rb isobaric overlap factors to analyse Sr isotope ratios for small glass samples using excimer laser ablation (LA) with an Aridus II desolvating nebuliser dual-intake system and multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The combined use of a low-oxide interface setup, along with high-gain Faraday amplifiers with a 1012Ω resistor, enabled precise determination of Sr isotope ratios from 50–100 μm diameter craters at 10 Hz laser repetition rate. Residual analytical biases of 84Sr/86Sr and 87Sr/86Sr isotope ratios, obtained from Kr baseline suppressions (Kimura et al., 2013, Journal of Analytical Atomic Spectrometry, 28, 945–957), were found to be nonlinear, but the correction method was applicable to 50–200 μm/10 Hz craters. We also found that the 85Rb/87Rb overlap correction factor changed with time with a change in the surface condition of the sampler–skimmer cones. The correction factor of 85Rb/87Rb was thus determined at least once per five unknown measurements using the Aridus solution intake line. We determined 87Sr/86Sr isotope ratios from MkAn anorthite (Sr = 305 ppm, Rb = 0.07 ppm), BHVO-2G, KL2-G, ML3B-G (Sr = 312–396 ppm, Rb = 5.8–9.2 ppm), and BCR-2G (Sr = 337 ppm, Rb = 48.5 ppm) basalt glasses using a 50–100 μm/10 Hz crater. The results agree well with their reference values, determined by thermal ionisation mass spectrometry, even with the high Rb/Sr ratio (0.14) in the BCR-2G glass. The internal/intermediate precisions were ±0.0002 (two-standard deviation: 2SD) for 100 μm craters and ±0.0005 for 50 μm craters. The new instrument settings and analytical protocol improved the precision by a factor of two compared to the previous report using LA-(sector field)-ICP-MS and enables the analysis of sample volumes that are ten times smaller than those used in previous LA-MC-ICP-MS analyses with equal precision. [ABSTRACT FROM AUTHOR]

The Mikabu and Sorachi-Yezo belts comprise Jurassic ophiolitic complexes in Japan, where abundant basaltic to picritic rocks occur as lavas and hyaloclastite blocks. In the studied northern Hamamatsu and Dodaira areas of the Mikabu belt, these rocks are divided into two geochemical types, namely depleted (D-) and enriched (E-) types. In addition, highly enriched (HE-) type has been reported from other areas in literature. The D-type picrites contain highly magnesian relic olivine phenocrysts up to Fo, and their Fo-NiO trend indicates fractional crystallization from a high-MgO primary magma. The MgO content is calculated as high as 25 wt%, indicating mantle melting at unusually high potential temperature ( T) up to 1,650 °C. The E-type rocks represent the enrichment in Fe and LREE and the depletion in Mg, Al and HREE relative to the D-type rocks. These chemical characteristics are in good accordance with those of melts from garnet pyroxenite melting. Volcanics in the Sorachi-Yezo belts can be divided into the same types as the Mikabu belt, and the D-type picrites with magnesian olivines also show lines of evidence for production from high T mantle. Evidence for the high T mantle and geochemical similarities with high-Mg picrites and komatiites from oceanic and continental large igneous provinces (LIPs) indicate that the Mikabu and Sorachi-Yezo belts are accreted oceanic LIPs that were formed from hot large mantle plumes in the Late Jurassic Pacific Ocean. The E- and D-type rocks were formed as magmas generated by garnet pyroxenite melting at an early stage of LIP magmatism and by depleted peridotite melting at the later stage, respectively. The Mikabu belt characteristically bears abundant ultramafic cumulates, which could have been formed by crystal accumulation from a primary magma generated from Fe-rich peridotite mantle source, and the HE-type magma were produced by low degrees partial melting of garnet pyroxenite source. They should have been formed later and in lower temperatures than the E- and D-type rocks. The Mikabu and Sorachi Plateaus were formed in a low-latitude region of the Late Jurassic Pacific Ocean possibly near a subduction zone, partially experienced high P/ T metamorphism during subduction, and then uplifted in association with (or without, in case of Mikabu) the supra-subduction zone ophiolite. The Mikabu and Sorachi Plateaus may be the Late Jurassic oceanic LIPs that could have been formed in brotherhood with the Shatsky Rise. [ABSTRACT FROM AUTHOR]

The conditions under which rear-arc magmas are generated were estimated using primary basalts from the Sannome-gata volcano, located in the rear of the NE Japan arc. Scoriae from the volcano occur with abundant crustal and mantle xenoliths, suggesting that the magma ascended rapidly from the upper mantle. The scoriae show significant variations in their whole-rock compositions (7.9-11.1 wt% MgO). High-MgO scoriae (MgO > ~9.5 wt%) have mostly homogeneous Sr/Sr ratios (0.70318-0.70320), whereas low-MgO scoriae (MgO < ~9 wt%) have higher Sr/Sr ratios (>0.70327); ratios tend to increase with decreasing MgO content. The high-MgO scoriae are aphyric, containing ~5 vol% olivine microphenocrysts with Mg# [100 × Mg/(Mg + Fe)] of up to 90. In contrast, the low-MgO scoriae have crustal xenocrysts of plagioclase, alkali feldspar, and quartz, and the mineralogic modes correlate negatively with whole-rock MgO content. On the basis of these observations, it is inferred that the high-MgO scoriae represent primary or near-primary melts, while the low-MgO scoriae underwent considerable interaction with the crust. Using thermodynamic analysis of the observed petrological features of the high-MgO scoriae, the eruption temperature of the magmas was constrained to 1,160-1,220 °C. Given that the source mantle was depleted MORB-source mantle, the primary magma was plausibly generated by ~7 % melting of a garnet-bearing spinel peridotite; taking this into consideration, and considering the constraints of multi-component thermodynamics, we estimated that the primary Sannome-gata magma was generated in the source mantle with 0.5-0.6 wt% HO at 1,220-1,230 °C and at ~1.8 GPa, and that the HO content of the primary magma was 6-7 wt%. The rear-arc Sannome-gata magma was generated by a lower degree of melting of the mantle at greater depths and lower temperatures than the frontal-arc magma from the Iwate volcano, which was also estimated to be generated by ~15 % melting of the source mantle with 0.6-0.7 wt% HO at ~1,250 °C and at ~1.3 GPa. [ABSTRACT FROM AUTHOR]

The water content of low-K tholeiitic basalt magma from Iwate volcano, which is located on the volcanic front of the NE Japan arc, was estimated using multi-component thermodynamic models. The Iwate lavas are moderately porphyritic, consisting of ~8 vol.% olivine and ~20 vol.% plagioclase phenocrysts. The olivine and plagioclase phenocrysts show significant compositional variations, and the Mg# of olivine phenocrysts (Mg#78-85) correlates positively with the An content of coexisting plagioclase phenocrysts (An). The olivine phenocrysts with Mg# > ~82 do not form crystal aggregates with plagioclase phenocrysts. It is inferred from these observations that the phenocrysts with variable compositions were primarily derived from mushy boundary layers along the walls of a magma chamber. By using thermodynamic calculations with the observed petrological features of the lavas, the water content of the Iwate magma was estimated to be 4-5 wt.%. The high water content of the magma supports the recent consensus that frontal-arc magmas are remarkably hydrous. Using the estimated water content of the Iwate magma, the water content and temperature of the source mantle were estimated. Given that the Iwate magma was derived from a primary magma solely by olivine fractionation, the water content and temperature were estimated to be ~0.7 wt.% and ~1,310 °C, respectively. Differentiation mechanisms of low-K frontal-arc basalt magmas were also examined by application of a thermodynamics-based mass balance model to the Iwate magma. It is suggested that magmatic differentiation proceeds primarily through fractionation of crystals from the main molten part of a magma chamber when it is located at <~200 MPa, whereas magma evolves through a convective melt exchange between the main magma and mushy boundary layers when the magma body is located at >~200 MPa. [ABSTRACT FROM AUTHOR]

Pagan is one of the largest volcanoes along the Mariana arc volcanic front. It has a maximum elevation of 570 m (Mt. Pagan), but its submarine flanks descend to 2000–3000 m below sea level, and are unexplored. Bathymetric mapping and ROV Hyper-Dolphin dives (HPD1147 and HPD1148) on the submarine NE and SW flanks of Pagan were carried out during cruise NT10-12 of R.V. Natsushima in July 2010. There are no systematic compositional differences between subaerial lavas reported in the literature and differentiated submarine lavas collected in HPD1148, with <7 wt % MgO, suggesting they are derived from the same magmatic system. However, these differentiated lavas show complexities including magma mixing; thus we concentrate on magnesian submarine lavas (>7 wt % MgO). Twenty least-fractionated basalts (48·5–50 wt % SiO2) collected during HPD1147 extend to higher MgO (10–11 wt %) and Mg# (66–70) than the subaerial lavas. Olivine (up to Fo94) and spinel (Cr# up to 0·8) compositions suggest that these Pagan primitive magmas formed from high degrees of mantle melting. Two basalt types can be distinguished based on their geochemistry at similar (10–11 wt %) MgO; these erupted recently, 500 m apart. Both contain clinopyroxene and olivine phenocrysts and are referred to as COB1 and COB2. Lower TiO2, FeO, Na2O, K2O, incompatible trace element abundances, and Nb/Yb suggest that COB1 formed from higher degrees of mantle melting. In addition, light rare earth element (LREE) enrichment and higher Th/Nb in COB2 contrast with LREE depletion and lower Th/Nb in COB1. Higher Ba/Th and Ba/Nb and lower Th/Nb indicate that the main subduction addition in COB1 was dominated by hydrous fluid, whereas that in COB2 was dominated by sediment melt. Sr–Nd–Pb–Hf isotopes are also consistent with this interpretation. These observations suggest that the subduction component responsible for the greater degree of melting of the COB1 source was mostly hydrous fluid. The origin of such different metasomatic agents resulted in different primary magmas forming in the same volcano. Both hydrous fluid and sediment melt components may have unmixed from an originally homogeneous supercritical fluid in or above the subducting slab below the volcanic front. These may have been added separately to the mantle wedge peridotite (mantle diapir) and resulted in two neighboring but completely different primary magmas from the same diapir. Moreover, these primitive lavas suggest that even for intra-oceanic arcs assimilation–fractional crystallization is inevitable when these magmas evolve in the crust and, in addition, that phlogopite is present in their mantle residue and thus played an important role in their genesis. [ABSTRACT FROM PUBLISHER]

The crystallization sequence of a basaltic andesite from Bezymianny Volcano, Kamchatka, Russia, was simulated experimentally at 100 and 700 MPa at various water activities ( aHO) to investigate the compositional evolution of residual liquids. The temperature ( T) range of the experiments was 950-1,150 °C, aHO varied between 0.1 and 1, and the log of oxygen fugacity ( fO) varied between quartz-fayalite-magnetite (QFM) and QFM + 4.1. The comparison of the experimentally produced liquids and natural samples was used to constrain the pressure (P)- T- aHO- fO conditions of the Bezymianny parental magma in the intra-crustal magma plumbing system. The phase equilibria constraints suggest that parental basaltic andesite magmas should contain ~2-2.5 wt% HO; they can be stored in upper crustal levels at a depth of ~15 km, and at this depth they start to crystallize at ~1,110 °C. The subsequent chemical evolution of this parental magma most probably proceeded as decompressional crystallization occurred during magma ascent. The final depths at which crystallization products accumulated prior to eruption are not well constrained experimentally but should not be shallower than 3-4 km because amphibole is present in natural magmas (>150 MPa). Thus, the major volume of Bezymianny andesites was produced in a mid-crustal magma chamber as a result of decompressional crystallization of parental basaltic andesites, accompanied by mixing with silicic products from the earlier stages of magma fractionation. In addition, these processes are complicated by the release of volatiles due to magma degassing, which occurs at various stages during magma ascent. [ABSTRACT FROM AUTHOR]

We present a new dataset for whole-rock major, trace, isotopic, and phenocryst compositions indicating a genetic link between andesites of the Holocene eruptions of the Bezymianny stratovolcano (the Bezymianny stage), the andesitic to dacitic Late Pleistocene lava dome complex (the pre-Bezymianny stage), and the magnesian to high-alumina basalts of the adjacent Kliuchevskoi Volcano. We demonstrate that volcanic products from the Bezymianny stage of volcano evolution are most likely the products of magma mixing between silicic products of the earliest stages of magma fractionation and the less evolved basaltic andesite parental melts periodically injected into the magma reservoir. In contrast, the intermediate and silicic magmas of the pre-Bezymianny stage together with basalts from Kliuchevskoi much more closely resemble the liquid line of descent and may represent a unique prolonged and continuous calc-alkaline trend of magma evolution from high-magnesian basalt to dacite. As a result of the geothermobarometry, we recognize variable conditions of magma fractionation and magma storage beneath Bezymianny for different magma types during its evolution since the Late Pleistocene: (1) 1100–1150°C, 500–640MPa, 1–2.5wt.% H2O for parental basaltic andesite; (2) 1130–1050°C, 700–600MPa, 2.5–5wt.% H2O for two-pyroxene andesites; (3) 1040–990°C, 560–470MPa, 5–6.5wt.% H2O for orthopyroxene-bearing andesites; (4) 950–1000°C, 450–150MPa, 3.5–5.5wt.% H2O for hornblende-bearing andesites; and (5) 950–900°C, 410–250MPa, 6–7wt.% H2O for dacites. Repeated basalt injections and magma fractionation combined with internal mixing in the magma chamber are the main processes responsible for both the complex petrography and the geochemical trends observed in the lavas of Bezymianny Volcano. [ABSTRACT FROM AUTHOR]

Initial subduction-related boninitic magmatism occurred between 48 and 44 Ma in the Izu–Bonin–Mariana (IBM) arc. High-Mg adakites and low-Ca boninites have been dredged from the Bonin Ridge fore-arc seamount. Whole-rock 40Ar/ 39Ar ages suggest that the boninite (44·0 ± 1·4 Ma) and adakite (43·1 ± 1·0 and 40·8 ± 0·8 Ma) magmatism overlapped, or that the adakite magmatism occurred slightly later than the boninite magmatism. The low-Ca boninites are high-Mg andesites and exhibit U-shaped rare earth element (REE) patterns with an elevated average Mg# of 0·78 [Mg# = Mg/(Mg + Fe) molar ratio] and Ni content of 667 ppm. The high-Mg adakites are andesitic to dacitic in composition; they exhibit markedly high Sr contents and low Y contents and are highly enriched in light REE but depleted in heavy REE, with an average Mg# of 0·79 and Ni content of 433 ppm. A geochemical mass-balance model (Arc Basalt Simulator Version 3) indicates that both magma types could be generated by partial melting of a depleted mantle source fluxed by water-rich slab-derived melts in a hot subduction environment, comparable with the present-day South Chile (ridge subduction) or Southwest Japan (young slab subduction) arcs. An extremely high slab melt flux of 22% is required for the formation of the high-Mg adakite, whereas a low flux of 3% is sufficient for the low-Ca boninite. The low-Ca boninite requires a high-temperature shallow slab (854°C, 2·7 GPa on average), consisting of altered oceanic crust of the Pacific plate and volcaniclastic sediments from HIMU seamounts, and high-temperature shallow mantle melting (1216°C, 0·8 GPa) of depleted Indian mid-ocean ridge basalt (MORB)-type mantle. These modelled conditions are consistent with the occurrence of hot shallow mantle wedge melting in the initial subduction zone at the boundary between Pacific- and Indian-type mantle domains, as suggested by previous studies. In contrast, high-Mg adakite requires a higher temperature and deeper slab (929°C, 4·1 GPa), with the same slab components and slightly deeper but less hot melting (1130°C, 1·1 GPa) of HIMU-type depleted mantle, to satisfy the low Hf isotope ratios. This may occur because of the subsequent cooling of the mantle wedge by the establishment of the subduction system after the boninite magmatism and involvement of a small volume of an isotopically enriched mantle source embedded in the Indian-type mantle. The petrogenetic conditions provide constraints for reconstructing the tectonic settings of the early IBM arc. The hot subduction model would be consistent with the tectonic models with regard to the initiation of subduction associated with fore-arc spreading; this allowed the upwelling of the asthenospheric mantle to generate slab melts from the old Pacific plate slab and hot shallow mantle melting by slab melt fluxing for both boninite and adakite activities. [ABSTRACT FROM AUTHOR]

Abstract: The origin of historic potassic basalts (from the 1719 to 1721AD eruption) from the Wudalianchi volcanic field, northeast China, is investigated. The samples examined in this study are aphyric, and contain ~5vol.% olivine microphenocrysts. Geochemical analyses of the samples, along with published data for material from the 1719 to 1721AD eruption, show that the basalts exhibit linear compositional trends in whole-rock variation diagrams, and are characterized by K2O enrichment (>4wt.%) and an EM1-like isotopic signature. Using thermodynamic calculations and the observed petrological features of the basalts, the temperature of the magmas shortly before eruption is estimated to have been ~1250°C, and the water content of the magma at depth is estimated to have been >~1.1wt.%. Because the estimated temperatures are significantly higher than the projected maximum temperature of the lithospheric mantle beneath the Wudalianchi volcanic field, the magmas were likely derived from the asthenospheric mantle. We suggest that both the potassic- and EM1-like natures of the basalts originated from the mantle transition zone, metasomatized by K-rich sediment fluids ~1.5Ga ago through a stagnation of an ancient slab. The linear whole-rock compositional trends of the basalts primarily reflect the geochemical heterogeneity in the mantle transition zone, with variable but coupled contributions of ~1.5Ga sediments in the ancient stagnant slab, and recent sediments and peridotites in the stagnant Pacific slab. We infer that the Wudalianchi magmatism was caused by an upwelling of a hydrous mantle plume from the mantle transition zone, which was hydrated through the stagnation of the ancient subducted slab and the recent Pacific slab. [Copyright &y& Elsevier]