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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]

The relationship between plutonic and volcanic components of magmatic plumbing systems continues to be a question of intense debate. The Oki-Dōzen Islands, Sea of Japan, preserve outcrops of temporally associated plutonic, hypabyssal, and volcanic rocks. Post-intrusion uplift juxtaposed Miocene syenites in inferred faulted contact with volcanic trachytes that are cut by rhyolite hypabyssal dikes. This provides a window deep into the timing and origins of magma storage architecture and dynamics. Zircon is ubiquitous in all samples; our aim is to determine what its age and composition can reveal about the plutonic-volcanic connection. Here we show magma source characteristics are recorded in zircon Hf isotopes; source composition and assimilation of heterogeneous hydrothermally altered crust in zircon O isotopes; and extensive fractional crystallization in zircon trace elements. Combined with new U-Th-Pb SHRIMP zircon ages, 6.4–5.7 Ma, compositional data show pluton formation was by protracted amalgamation of discrete magma pulses. The rhyolite dike preserves an evolved fraction segregated from these discrete magmas. Synchronous with plutonism was a volcanic eruption of trachyte magma derived from the same source, which may have stalled at a relatively shallow depth prior to eruption. Stalling occurred at least above the amphibole stability zone because amphibole-compatible Sc and Ti were not depleted in the trachyte melt resulting in elevated values of these in volcanic compared to plutonic zircon. Identifying smaller episodic magma pulses in a larger magmatic complex places constraints on potential magma fluxes and eruptible volumes. High-flux, large volume, plume-related ocean island magmatic systems may have extensive vertically distributed multi-stage magmatic reservoirs and subduction-related systems transcrustal magma reservoirs. By contrast, Oki-Dōzen was a low-flux system with incremental pluton growth and small- to moderate-scale eruptions. [ABSTRACT FROM AUTHOR]

IODP Expedition 350 was the first to be drilled in the rear part of the Izu-Bonin, although several sites had been drilled in the arc axis to fore-arc region; the scientific objective was to understand the evolution of the Izu rear arc, by drilling a deep-water volcaniclastic section with a long temporal record (Site U1437). The Izu rear arc is dominated by a series of basaltic to dacitic seamount chains up to ~100-km long roughly perpendicular to the arc front. Dredge samples from these are geochemically distinct from arc front rocks, and drilling was undertaken to understand this arc asymmetry. Site U1437 lies in an ~20-km-wide basin between two rear arc seamount chains, ~90-km west of the arc front, and was drilled to 1804 m below the sea floor (mbsf) with excellent recovery. We expected to drill a volcaniclastic apron, but the section is much more mud-rich than expected (~60%), and the remaining fraction of the section is much finer-grained than predicted from its position within the Izu arc, composed half of ashes/tuffs, and half of lapilli tuffs of fine grain size (clasts <3 cm). Volcanic blocks (>6.4 cm) are only sparsely scattered through the lowermost 25% of the section, and only one igneous unit was encountered, a rhyolite peperite intrusion at ~1390 mbsf. The lowest biostratigaphic datum is at 867 mbsf (~6.5 Ma), the lowest palaeomagnetic datum is at ~1300 mbsf (~9 Ma), and the rhyolite peperite at ~1390 mbsf has yielded a U–Pb zircon concordia intercept age of (13.6 + 1.6/−1.7) Ma. Both arc front and rear arc sources contributed to the fine-grained (distal) tephras of the upper 1320 m, but the coarse-grained (proximal) volcaniclastics in the lowest 25% of the section are geochemically similar to the arc front, suggesting arc asymmetry is not recorded in rocks older than ~13 Ma. [ABSTRACT FROM AUTHOR]

West Rota Volcano (WRV) is a recently discovered extinct submarine volcano in the southern Mariana Arc. It is large (25 km diameter base), shallow (up to 300 m below sealevel), and contains a large caldera (6 × 10 km, with up to 1 km relief). The WRV lies near the northern termination of a major NNE-trending normal fault. This and a second, parallel fault just west of the volcano separate uplifted, thick frontal arc crust to the east from subsiding, thin back-arc basin crust to the west. The WRV is distinct from other Mariana Arc volcanoes: (i) it consists of a lower, predominantly andesite section overlain by a bimodal rhyolite-basalt layered sequence; (ii) andesitic rocks are locally intensely altered and mineralized; (iii) it has a large caldera; and (iv) WRV is built on a major fault. Submarine felsic calderas are common in the Izu and Kermadec Arcs but are otherwise unknown from the Marianas and other primitive, intraoceanic arcs. 40Ar–39Ar dating indicates that andesitic volcanism comprising the lower volcanic section occurred 0.33–0.55 my ago, whereas eruption of the upper rhyolites and basalts occurred 37–51 thousand years ago. Four sequences of rhyolite pyroclastics each are 20–75 m thick, unwelded and show reverse grading, indicating submarine eruption. The youngest unit consists of 1–2 m diameter spheroids of rhyolite pumice, interpreted as magmatic balloons, formed by relatively quiet effusion and inflation of rhyolite into the overlying seawater. Geochemical studies indicate that felsic magmas were generated by anatexis of amphibolite-facies meta-andesites, perhaps in the middle arc crust. The presence of a large felsic volcano and caldera in the southern Marianas might indicate interaction of large normal faults with a mid-crustal magma body at depth, providing a way for viscous felsic melts to reach the surface. [ABSTRACT FROM AUTHOR]

The Izu–Bonin volcanic arc is an excellent example of an intra-oceanic convergent margin. A total of 1011 chemical analyses of 17 Quaternary volcanoes of the arc are reviewed to estimate relative proportions of magmas erupted. Basalt and basic andesite (SiO2 < 57 wt %) are the predominant eruptive products of the Izu–Bonin arc, and rhyolite (SiO2 > 70 wt %) forms another peak in volume. Such rhyolites possess compositions identical to those of partial melts produced by dehydration-melting of calc-alkaline andesites at low pressure (<7 kbar). Meanwhile, the major element variation of the Shirahama Group Mio-Pliocene volcanic arc suite, Izu Peninsula, completely overlaps that of the Quaternary Izu–Bonin arc volcanoes, and groundmasses of Shirahama Group calc-alkaline andesites have compositions similar to those of Izu–Bonin rhyolites. Moreover, phenocryst assemblages of calc-alkaline andesites of the Shirahama Group resemble restite phase assemblages of dehydration-melting of calc-alkaline andesite. These lines of evidence suggest that the rhyolite magmas may have been produced by dehydration-melting of calc-alkaline andesite in the upper to middle crust. If so, then the presence of large amounts of calc-alkaline andesite (3–5 times more abundant than the rhyolites) within the oceanic arc crust would be expected, which is consistent with a recently proposed structural model across the Izu–Bonin arc. The calc-alkaline andesite magmas may be water saturated, and would crystallize extensively and solidify within the crust. The model proposed here suggests that rhyolite eruptions could be triggered by an influx of hot basalt magma from depth, reheating and partially melting the calc-alkaline andesite component of the crust. [ABSTRACT FROM PUBLISHER]

Domes of the Sumisu volcanic complex (western Pacific), having summits at ocean depths of 1100, 600, 245, and 95 m, are mantled with compositionally identical rhyolitic pumice that has similar vesicularity, but that varies systematically in distribution, size, and surface texture-suggesting that facies and morphology can serve as useful indicators of eruption depth. At depths >500 m, the pumice formed a thick carapace on dense rhyolite and disintegrated by quench fracture and mechanical failure into a jumble of giant (meters to tens of meters) polyhedral blocks with smooth curviplanar surfaces that display a single quenched margin. Vesiculation was arrested on eruption in seawater in all but the interior of the thickest carapace. At <500 m depths, the pumice occurs as an apron of blocky giant and smaller rough-textured clasts enclosed by quenched margins and pockmarked by coarse (cm) vesicles. No carapace pumice occurs, and the summit is composed of craggy dense dome rock. These shallower water pumice clasts resemble those spalled from historic submarine dome-forming eruptions that buoyed to the sea surface. We interpret spalling to result from vent-derived, weak volatile-driven explosions that take place at water depths <500 m. Our study shows that an increase in hydrostatic pressures over a range of 12 MPa reduces volatile-driven explosivity for subaqueous, rhyolitic, dome-forming eruptions, but does not affect vesicularity. We conclude that meter-size, highly vesicular pumice is diagnostic of subaqueous dome eruptions in water depths of at least 1300 m, and its morphology can be used to distinguish between explosive and effusive origins. [ABSTRACT FROM AUTHOR]