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Primary andesitic magmas could be an important component of arc magma genesis and might have played a key role in the advent of continents. Recent studies hypothesized that primary andesitic magmas occur in the oceanic arc, where the crust is thin. The Kermadec arc has the thinnest crust among all the studied oceanic arcs (<15 km in thickness); however, there are no studies that corroborate the formation of primary andesitic magmas in the arc. The aim of this study is to develop a better understanding of primary andesites in oceanic arcs through the petrology of the Kermadec arc. Here, we present the petrology of volcanic rocks dredged from the Kibblewhite Volcano in the Kermadec arc during the R/V SONNE SO-255 expedition in 2017. Magma types range from andesite to rhyolite at the Kibblewhite Volcano, but basalts dominate at the neighboring cones. This study focuses on magnesian andesites from the northeastern flank of this volcano. The magnesian andesites are nearly aphyric and plagioclase free but contain microphenocrysts of olivine (Fo84–86) and clinopyroxene (Mg# = 81–87). Using olivine addition models, the primary magmas were estimated to contain 55–56 wt % SiO2 and 10–12 wt % MgO, similar to the high-Mg andesites observed in other convergent plate margins, indicating the generation of primary andesitic magma beneath the Kibblewhite Volcano. The trace element and isotopic characteristics of the magnesian andesites are typical of volcanic rocks from the Kermadec arc. This indicates that the subduction of a young plate or melting of a pyroxenitic source is not necessary to produce magnesian andesites. Instead, we propose that the magnesian andesites were produced by the direct melting of the uppermost mantle of the Kermadec arc. The thin crust of the Kermadec arc should yield low-pressure conditions in the uppermost mantle, allowing the sub-arc mantle to generate primary andesitic melts. This study supports the hypothesis that primary andesitic magmas generate in the arc where the crust is thin and provides a new insight into the magma genesis of the Kermadec arc. [ABSTRACT FROM AUTHOR]

Intra-plate basalts of ~ 35–0 Ma in East Eurasia formed in a broad backarc region above the stagnant Pacific Plate slab in the mantle transition zone. These basalts show regional-scale variations in Nd-Hf isotopes. The basalts with the most radiogenic Nd-Hf center on the Shandong Peninsula with intermediate Nd-Hf at Hainan and Datong. The least radiogenic basalts occur in the perimeters underlain by the thick continental lithosphere. Shandong basalts possess isotopic signatures of the young igneous oceanic crust of the subducted Pacific Plate. Hainan and Datong basalts have isotopic signatures of recycled subduction materials with billions of years of storage in the mantle. The perimeter basalts have isotopic signatures similar to pyroxenite xenoliths from the subcontinental lithospheric mantle beneath East Eurasia. Hainan basalts exhibit the highest mantle potential temperature ( T p ), while the Shandong basalts have the lowest T p . We infer that a deep high- T p plume interacted with the subducted Pacific Plate slab in the mantle transition zone to form a local low- T p plume by entraining colder igneous oceanic lithosphere. We infer that the subducted Izanagi Plate slab, once a part of the Pacific Plate mosaic, broke off from the Pacific Plate slab at ~ 35 Ma to sink into the lower mantle. The sinking Izanagi slab triggered the plume that interacted with the stagnant Pacific slab and caused subcontinental lithospheric melting. This coincided with formation of the western Pacific backarc marginal basins due to Pacific Plate slab rollback and stagnation. [ABSTRACT FROM AUTHOR]

We report a new, rapid method for the high-precision determination of chlorine isotope ratios in halite and AgCl pellets formed from seawater and igneous rock samples. Use of 266 nm ultra violet-femtosecond laser ablation (UV-FsLA) allowed quantitative sampling of halite and AgCl and enabled precise determination of 37Cl/35Cl isotope ratios (δ37Cl) when coupled with a multiple Faraday collector-inductively coupled plasma mass spectrometer (MFC-ICPMS). We used 36Ar+/38Ar+ as an external standard for the mass bias corrections between 39K+–41K+, 36Ar1H+–38Ar1H+–40Ar1H+, and 35Cl+–37Cl+ with isobaric overlap corrections between K+, ArH+, and Cl+ ions. Overlap of the sulfur (36S+) isobar with 36Ar+ was indirectly monitored and corrected by baseline modelling using the 36Ar+/38Ar+ measurement. These procedures collectively helped accomplish an accurate and high-precision measurement of 37Cl+/35Cl+ ratios. Using the proposed analytical method, δ37Cl ratios in natural halite samples were analysed by direct laser ablation. δ37Cl in JB-1a and JB-3 igneous rocks were analysed as AgCl powder pellets produced by pyrohydrolytic separation and co-precipitation of the separated Cl with silver. The external reproducibility of the δ37Cl measurements was ±0.2‰ 2SD (2 standard deviations) for halite and ±0.4‰ 2SD for AgCl precipitate rivalling that of gas source isotope ratio mass spectrometry. The new analytical protocol enabled a precise and rapid δ37Cl analysis of igneous rock samples as AgCl with as little as 4 μg chlorine from the ∼500 μg chlorine separated. This is the first time that high-precision in situ determination of δ37Cl from halite using 4 μg of chlorine has been reported. [ABSTRACT FROM AUTHOR]

The Miocene Kaikomagatake pluton is one of the Neogene granitoid plutons exposed in the Izu Collision Zone, which is where the juvenile Izu-Bonin oceanic arc is colliding against the mature Honshu arc. The pluton intrudes into the Cretaceous to Paleogene Shimanto accretionary complex of the Honshu arc along the Itoigawa-Shizuoka Tectonic Line, which is the collisional boundary between the two arcs. The pluton consists of hornblende-biotite granodiorite and biotite monzogranite, and has SiO contents of 68-75 wt%. It has high-K series compositions, and its incompatible element abundances are comparable to the average upper continental crust. Major and trace element compositions of the pluton show well-defined chemical trends. The trends can be interpreted with a crystal fractionation model involving the removal of plagioclase, biotite, hornblende, quartz, apatite, and zircon from a potential parent magma with a composition of ~68 wt% SiO. The Sr isotopic compositions, together with the partial melting modeling results, suggest that the parent magma is derived by ~53% melting of a hybrid lower crustal source comprising ~30% Shimanto metasedimentary rocks of the Honshu arc and ~70% K-enriched basaltic rocks of the Izu-Bonin rear-arc region. Together with previous studies on the Izu Collision Zone granitoid plutons, the results of this study suggest that the chemical diversity within the parental magmas of the granitoid plutons reflects the chemical variation of basaltic sources (i.e., across-arc chemical variation in the Izu-Bonin arc), as well as a variable contribution of the metasedimentary component in the lower crustal source regions. In addition, the petrogenetic models of the Izu Collision Zone granitoid plutons collectively suggest that the contribution of the metasedimentary component is required to produce granitoid magma with compositions comparable to the average upper continental crust. The Izu Collision Zone plutons provide an exceptional example of the transformation of a juvenile oceanic arc into mature continental crust. [ABSTRACT FROM AUTHOR]

New geochemical, isotopic, and geochronological data and interpretations are presented for late Neoproterozoic intrusive carbonates and related rocks of southern Sinai, Egypt (northernmost Arabian–Nubian Shield). The Tarr carbonates are coarsely crystalline and related to explosive emplacement of hypabyssal and volcanic albitite at 605 ± 13 Ma. The carbonates associated with the albitites are divisible into two types: primary dolomitite and secondary breunneritite (Fe-rich magnesite). The dolomitite was clearly intrusive but differs from classic igneous carbonatites, containing much lower abundances of incompatible elements, such as REE, U, Th, Rb, Nb, Y, P, Sr, Zr, Ba, and total alkalies. The breunneritite is a secondary replacement of dolomitite, probably marking the roots of a vigorous hydrothermal system. Albitites show pristine abundances of major and trace elements and were not subjected to a major metamorphic overprint. They are relatively more fractionated, alkaline and related to within-plate A-type magmas, were emplaced in an extensional or non-compressive tectonic regime in the cupola of high-level A-type granite. Tarr albitites may represent residual magma remaining after near-total crystallization of an A-type granite pluton at depth, forcibly emplaced into the roof above the cooling pluton. The intrusive dolomitite exsolved from highly differentiated albitite melt, in the apical regions of a still-buried alkaline “A-type” granite pluton that was rich in CO2; these volatiles migrated upwards and towards the cooler margins of the magma body. Late NNE-SSW extension allowed a shallow-level cupola to form, into which albitite melts and carbonate fluids migrated, culminating in explosive emplacement of albitite breccia and intrusive carbonate. Isotopic compositions of Tarr dolomitite and albitite indicate these are consanguineous and ultimately of mantle origin. Magmatic volatiles fenitized the wall rock, while submarine hydrothermal activity transformed some of the dolomitite into breunneritite. Recognition of Tarr-type should encourage similar hypabyssal complex intrusions to be sought for in association with A-type granitic plutons elsewhere. [ABSTRACT FROM AUTHOR]

Analysis of middle and upper Eocene rocks from the IBM forearc, including the Ogasawara and Mariana Islands, help illuminate early arc volcanism of the proto-IBM arc. Dredged volcanic rocks from the forearc are two-pyroxene basalt to andesite, and may be divided into two groups, tholeiite and calc-alkaline, on the basis of mineralogy, petrography, and bulk chemistry. Tholeiites are characterized by high HFSE contents, high crystallization temperatures, and low water contents. In contrast, the calc-alkaline rocks are characterized by low HFSE contents, low crystallization temperatures, and higher water contents. These characteristics indicate that magma genesis for the two series differed. The tholeiites resulted from high degrees of partial melting of slightly depleted mantle under anhydrous conditions, whereas the calc-alkaline rocks were generated by low degrees of melting of depleted mantle under hydrous conditions. We believe that differences in mantle depletion arose from compositional layering and fluid zonations caused by MORB volcanism and slab dehydration, respectively. [ABSTRACT FROM AUTHOR]

Abstract: The 1.75 Ma Chayano–Ebisutoge Pyroclastic Deposits (PD) is a large (300 km3 volume) eruptive unit consisting of ignimbrite, coignimbrite ash fall and pumice fall located in the central portion of the North Japanese Alps. The Chayano–Ebisutoge PD covered all central Japan, including the Osaka, Kyoto, Nagoya, Tokyo, and Niigata areas, distributed over a radius of >300 km. The tephra was erupted after a dacitic ignimbrite Nyukawa Pyroclastic Flow Deposit (PFD) at 1.76 Ma, and is regarded as the effusive phase of the Takidani Granodiorite, which is now exposed at the surface due to extremely rapid uplift in the area. The Chayano–Ebisutoge rhyolitic tephras have high K2O characteristics, and feature iron enrichment relative to magnesium, combined with low oxygen fugacity. The major element composition of the Chayano–Ebisutoge PD is similar to silicate melts produced experimentally from pelitic gneiss and granites at upper crustal pressures, suggesting a crustal melt origin. Neodymium and Sr isotopic compositions of the Chayano–Ebisutoge PD match those of the basement granitoids and gneiss. Assimilation-fractional crystallization of contemporaneous Ueno Basalts magma in the same area cannot account for the compositions of the Chayano–Ebisutoge PD. However, mixing between Chayano–Ebisutoge rhyolite magma and evolved Ueno basalt can generate the dacitic magma responsible for the Nyukawa Pyroclastic PFD and the Takidani Granodiorite, judging from major, trace, and isotope compositions. All the geochemical evidence above strongly supports a crustal melt origin for the Chayano–Ebisutoge PD. Accumulation of a voluminous basalt magma caused extensive melting of the crust, thus producing the huge volume of the Chayano–Ebisutoge PD. [Copyright &y& Elsevier]

Abstract: Adakitic plutons dominate the Early Cretaceous igneous suite of the Kitakami Mountains of northeast Japan. The zoned plutons typically consist of adakitic granite in their centers (central facies) surrounded by adakitic to non-adakitic granites in their margins (marginal facies). The exposure of the adakitic plutons is divided into two zones: from the eastern part of the north Kitakami belt to the eastern margin of the south Kitakami belt (E zone: Hashikami, Tanohata, Miyako, and Kinkasan plutons), and the inner part of the South Kitakami belt (W zone: Tono, Senmaya, and Hitokabe plutons). The central facies granites are characterized by low Y and high Sr concentrations and fractionated LREE/HREE patterns, characteristics common to Archean TTG and modern adakite. Chemical compositions of the central facies granites can be explained by the “slab melting” model. In addition, from the trace element and isotopic characteristics, the contribution from sediments to slab melting is estimated to be 7 to 20%. The marginal facies granites are characterized by slightly lower Sr/Y ratios, less fractionated REE patterns, and weak negative Eu anomalies. The marginal facies magma is considered to be derived from the reaction of slab melts with mantle peridotite and lower crustal amphibolite. Central facies granites in the W zone may have slightly interacted with mantle wedge peridotite during their ascent, while those in the E zone show little or no evidence for mantle contamination. The difference in the degrees of mantle contamination in the adakitic granites may be related to the crust–mantle structure beneath the accretionary complex (E zone) and the microcontinental block (W zone). [Copyright &y& Elsevier]

Abstract: A remarkable record of early arc volcanism in the Izu–Bonin–Mariana (IBM) arc is exposed in and around the Bonin Islands, an uplifted segment of the IBM forearc. New 40Ar/39Ar dating results imply that the boninitic volcanism on Chichijima Island occurred in a brief period during Eocene time, between 46–48 Ma. A slightly younger volcanic succession is identified along the Bonin Ridge, including 44.74±0.23 Ma high-Mg andesite from the Mikazukiyama Formation, the youngest volcanic sequence on Chichijima, 44.0±0.3 Ma tholeiitic to calcalkaline andesite from Hahajima Island, and 3 samples of andesite collected by the submersible SHINKAI 6500 from the Bonin Ridge Escarpment (BRE) that range in age from 41.84±0.14 to 43.88±0.21 Ma. Four SHINKAI 6500 dives (YK 04–05) on the BRE mapped an elongated constructional volcanic ridge atop the escarpment; we observed steeply west-dipping volcaniclastic debris flows shed from the summit of this ridge into the Ogasawara Trough to the west. These dives recovered fresh andesitic clasts from debris flows along the northern segment of the ridge, and high-Mg andesite lava blocks and Nummulitic limestone of middle Eocene age from the escarpment northwest of Chichijima. Our results also confirm previous inferences that melting of depleted mantle at shallow levels beneath the length of the arc with the aid of hydrous fluids from newly subducted slab to produce boninitic volcanism occurred nearly simultaneously along the entire length of the IBM arc system during the earliest stage of arc evolution. BRE–Mikazukiyama Formation–Hahajima andesites represent a transitional stage from forearc spreading (represented by ODP site 786-Chichijima boninites) and the stable, mature arc that developed in the Oligocene. These OPX-bearing high-Mg or tholeiitic to calcalkaline andesites were erupted along the BRE, as the arc magmatic axis localized and retreated from the trench. [Copyright &y& Elsevier]

Tonalitic rocks dredged from the Komahashi-Daini Seamount, northern Kyushu-Palau Ridge are classified as biotite–hornblende tonalites and hornblende tonalites. These rocks have radiometric ages of 37–38 Ma, indicating that felsic plutonic activity occurred during the early stages of Izu-Ogasawara (Bonin)-Mariana (IBM) arc volcanism. Therefore, this tonalite complex has great importance for understanding the initial processes of island arc and continental crust formation. These tonalitic rocks exhibit the following petrological and geochemical characteristics: (1) common lamellar twins and oscillatory zoning patterns in plagioclase phenocrysts throughout the compositional range; (2) hornblende tonalite shows parallel REE patterns and increasing total REE content with increasing SiO2, except for an increasingly strong negative Eu anomaly at higher SiO2 levels; and (3) isotopic composition remains constant over a wide silica variation. We compare this tonalite with younger tonalities of the same arc from the Tanzawa Complex (10–5 Ma), central Japan, considered to represent the lower–middle crust of the IBM arc, and find the following differences: (1) cumulate textures found in Tanzawa tonalites are not observed in samples from the Komahashi-Daini Seamount; and (2) Komahashi-Daini Seamount tonalites, unlike those from Tanzawa, exhibit linear variations of Zr and REEs vs. SiO2 plots. These data and other observations support the interpretation that tonalite in the Komahashi-Daini Seamount was produced by crystal fractionation from basaltic magma. We suggest that fractional crystallization operated during the early stage of oceanic island arc formation to produce tonalite, whereas tonalities in later stages formed largely by partial melting of basaltic lower crust, as represented by the tonalites in the Tanzawa Complex. [ABSTRACT FROM AUTHOR]

Abstract: Lower Cretaceous and Eocene igneous rocks in the Kitakami Mountains, northeast Japan, attract special interest because of the occurrence of adakitic rocks. In addition, high-Mg andesites (HMAs) were discovered from the Lower Cretaceous dike rocks and from the Eocene Jodogahama rhyolitic rocks in the Kitakami Mountains. Lower Cretaceous HMAs occur as several small dikes (less than 5 m thick, rarely up to 70 m) from Ubaishi, Gongen, Shizugawa, Numazu, and Oshika, southern Kitakami Mountains, intrude slightly older than the Lower Cretaceous plutonic rocks. Among these HMAs, the HMA dike from Gongen, characteristically contains ultramafic inclusions. Eocene HMA, a small intrusive mass (observed maximum diameter is 300 m) consisting of aphyric andesite and olivine andesite, occurs only in Matsuhashi, Iwaizumi, northern Kitakami Mountains. The Eocene HMA is associated with hornblende rhyolite belonging to the Eocene Jodogahama rhyolitic rocks. The Lower Cretaceous and Eocene HMAs show similar petrochemical characteristics to those of Cenozoic adakite (high LREE/HREE ratios and Sr contents, low Y and HREE contents) except higher Cr, Ni, and Mg contents. In addition, the Eocene HMAs and the ultramafic inclusions in the Lower Cretaceous HMAs show less radiogenic Nd–Sr isotopic characteristics than those from the adakitic granites in Kitakami, and similar to those of the Cenozoic adakite. On the other hand, the Miocene Setouchi HMAs in southwestern Japan are characterized by lower Sr/Y ratios, less fractionated REE patterns, weak negative Eu anomalies, and more radiogenic Nd–Sr isotopic characteristics than those of Cenozoic adakite. The ultramafic inclusions from Gongen have clinopyroxene characterized by high Sr concentrations and high LREE/HREE ratios. These clinopyroxenes are considered to be crystallized during interaction of slab derived adakitic melt with overlying mantle peridotite. The olivine phenocrysts of the Eocene HMA show extremely high NiO contents (maximum 0.58 wt.%), which may be an evidence for slab melt/mantle reaction. Concludingly, the petrochemical features of HMA magmas in Kitakami can be explained by reaction of slab derived adakitic melt with overlying mantle peridotite to equilibrate with mantle olivine. The difference between the chemical compositions of the Kitakami and Setouchi HMAs can be explained by the difference in compositions of initial slab melt resulted from the difference in depth of slab melting. [Copyright &y& Elsevier]

Despite the widespread use of Re–Os isotopes in various geoscience fields, relatively little is known about the behavior of Re and Os during fluid-mediated hydrothermal alteration of igneous rocks in the (sub)seafloor, especially in an intraoceanic arc setting. Here, we provide a depth profile of Re–Os geochemistry at Site U1527, located on the NW caldera rim of the Brothers volcano hydrothermal field in the Kermadec arc and drilled during International Ocean Discovery Program (IODP) Expedition 376 in 2018. Volcaniclastic rocks from Hole U1527C that had experienced various degrees of high- and low-temperature hydrothermal alteration were analyzed for bulk chemical composition and Re–Os isotopes. The concentration of Re varied from 0.172 to 18.7 ppb, and that of Os from 9.7 to 147.1 ppt. Hydrothermal alteration usually resulted in Re uptake by rocks, but a part of Re was released into the ocean by later oxidative weathering. Compared with Re, Os mobility resulting from hydrothermal alteration was limited. The relatively homogenous 187Os/188Os values (0.13–0.14) of the original volcaniclastic rocks were altered, becoming slightly more radiogenic (up to 0.212), by the addition of seawater-derived radiogenic Os by (1) Os precipitation induced by seawater–hydrothermal fluid mixing, which was accompanied by enrichment with Ba (barite precipitation) as well as by the highest Re and Os concentrations in the profile, and (2) adsorption of seawater-derived radiogenic Os onto Fe hydroxide without any corresponding enrichment of Ba, Re, or Os. Compared with high temperature-altered zones of mid-ocean ridge basalt, the altered samples in this study showed intense Re enrichment, presumably reflecting abundant pyrite precipitation in a Re-rich magmatic–hydrothermal environment unique to the arc setting. The results at Hole U1527 may suggest that subseafloor high-temperature alteration can be a sink for Re degassed from magma. The altered rock may also be a source of Re in the ocean as a result of subsequent low-temperature alteration. • High-temperature (sub)seafloor hydrothermal alteration resulted in pyrite precipitation and magmatic Re uptake. • Part of precipitated Re was released into the ocean by later oxidative weathering. • Os mobility during hydrothermal alteration was limited compared with Re. • 187Os/188Os as a sensitive tracer for subseafloor adsorption/precipitation of seawater Os. [ABSTRACT FROM AUTHOR]

Abstract: Cretaceous peraluminous granitic intrusions occur in Kyushu in the southwest Japan arc as batholiths or stocks accompanied by metaluminous tonalite to granodiorite. The peraluminous intrusions are classed as adakitic based on Sr/Y–Y (ppm) relationships, but they contain abundant K-feldspar, a phase not common in typical adakites. To consider their significance in the arc system, we investigated the Tsutsugatake stock as representative of the intrusions exposed in central Kyushu. The Tsutsugatake rocks are mainly garnet-bearing two-mica leuco-granitoids. Their Sr/Y ratios and rare earth element concentrations are appropriate for adakite, but K2O/Na2O ratios (0.53–1.43), and Al2O3 (13.7–15.0 wt.%), MgO (0.20–0.55 wt.%), Sr (207–462 ppm), Cr (≤12 ppm), and Ni (≤8 ppm) contents differ from those of typical adakite. Geochemical modeling shows that the petrogenesis of the Tsutsugatake stock can be explained by partial melting of arc-type tonalite or adakitic granodiorite under middle to lower crustal conditions, leaving amphibole and plagioclase as residual phases. The differences between the Tsutsugatake stock and classical adakites lie in their source rocks and melting conditions. The Tsutsugatake source was not basaltic, and the melting pressure was insufficient to stabilize garnet. The Tsutsugatake rocks are here designated “pseudo adakites”. Such “pseudo adakites” may be found in both modern and ancient arcs, and would play an important role in crustal evolution as a member of the recycled constituent. [Copyright &y& Elsevier]