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The southern Mariana Arc-Trench system is rapidly deforming, resulting in unusual interactions between arc and back-arc basin (BAB) magmatic systems. New geochemical data for volcanoes in this region are presented and explored. Tracey Seamount, an extinct submarine volcano about 30 km northwest of Guam, is the southernmost stratovolcano of the Mariana Arc. Tracey is built about 125 km above the subducted slab and has erupted a bimodal suite of typical arc mafic and felsic lavas as recently as 0.527 ± 0.023 Ma (40Ar/39Ar age). An unusual cluster of small basaltic volcanoes, informally termed the Alphabet Seamount Volcanic Province (ASVP), is found about where the next arc volcano to the southwest of Tracey Seamount should have grown. Samples from six of these volcanoes were studied here. At least two ASVP volcanoes were recently active, as shown by hydrothermal activity. The lack of magmatic focusing to build a single stratovolcano where the ASVP is situated reflects strong extension in the BAB. Construction of northern ASVP volcanoes is controlled by east-west extension accompanying opening of the Mariana Trough. In contrast, southern ASVP volcanoes are affected by north-south extension due to rapid rollback of a narrow slab of Pacific seafloor that is subducted along the east-west trending Challenger Deep segment of the Mariana Trench to the south. ASVP lava compositions are distinct from Tracey Seamount and other Mariana Arc lavas, instead showing affinities with Mariana Trough BAB basalt (BABBs): they are mafic, tholeiitic, low-K2O and LREE-depleted, with low 87Sr/86Sr, but show a subtle gradient from somewhat more arc-like lavas closer to the trench to BABBs farther west. The unusual tectonic setting of ASVP provides a unique perspective on how different arc magma batches reflect melting of mantle with strong compositional gradients which are mixed together beneath long-lived arc volcanoes but here rise to form scattered small ASVP volcanoes. [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]

During German R/V SONNE cruise (SO-255) mafic basaltic to magnesian andesitic lavaswere recovered from the Kermadec arc front volcano Kibblewhite (Hirai et al., EGU2018).Notably, these primitive lavas contain forsteritic olivines and olivine-hosted melt inclusions,which were re-homogenized using a heating stage. Here, we present the chemical variationsof the melt inclusions and their host olivines and discuss the interaction between mafic meltsand subarc lithospheric mantle. The olivines are divided into two groups based on their occurrence in different hostrock and NiO contents. Olivines (Fo90.3−92.4) found as phenocrysts in ankaramites(olivine-clinopyroxene rich basalts; 50.4–50.6 wt.% SiO2; 13.1–13.5 wt.% MgO; Mg# =71.8–72.1) show unusually low Ni contents (912–1462 ppm). Olivines (Fo91.0−92.2)occurring as xenocrysts in magnesian andesites (57.5–57.6 wt.% SiO2; 5.5–5.6 wt.%MgO; Mg# = 56.8–57.3) show higher Ni contents (1108–3057 ppm) than olivines inankaramites. All re-homogenized melt inclusions have primitive basalt compositions (49.6–52.0 wt.%SiO2; 12.7–15.9 wt.% MgO, 0.36–0.58 wt.% TiO2, 1.2–2.0 wt.% Na2O+K2O, 0.06–0.37wt.% SO3, and 0.05–0.20 wt.% Cl), which could represent parental melts for the ankaramites.However, melt inclusions hosted by olivines in ankaramites have lower Al2O3 andhigher CaO contents than those hosted by olivine xenocrysts in magnesian andesites;CaO/Al2O3 values are 1.2–1.6 and 0.9–1.3 respectively, indicating that there is a negativecorrelation between Ni contents in host olivines and CaO/Al2O3 values in their meltinclusions. The variable Ni and constant Fo contents in the host olivines could be explained by aninteraction of primary mafic melts with lithospheric mantle, which causes assimilation ofpyroxenes in the lithosphere and simultaneous fractional crystallization of olivines (AFC;Kelemen, 1990; Tamura et al., 2018). Moreover, the high CaO/Al2O3 values in the meltinclusions hosted by the ankaramite olivines cannot be produced by partial melting oflherzolite (e.g. as compilation by Médard et al., 2004). These melts have most likelyextensively interacted with the subarc lithospheric mantle. In addition, a negative correlationbetween Ni content in the host olivine and CaO/Al2O3 ratio in their melt inclusionssupports the melt-mantle interaction. Conversely, xenocrystic olivines in the andesiteswith high Ni contents must have been crystallized from more primitive basalticmelts within lithospheric mantle prior to significant melt-mantle interaction. Theseolivines were then trapped by primary magnesian andesite magmas as xenocrysts. [ABSTRACT FROM AUTHOR]

Nishinoshima, a submarine volcano in the Ogasawara Arc, approximately 1 000 km south of Tokyo, Japan, suddenly erupted in November 2013, after 40 years of dormancy. Olivine‐bearing phenocryst‐poor andesites found in older submarine lavas from the flanks of the volcano have been used to develop a model for the genesis of andesitic lavas from Nishinoshima. In this model, primary andesite magmas originate directly from the mantle as a result of shallow and hydrous melting of plagioclase peridotites. Thus, it only operates beneath Nishinoshima and submarine volcanoes in the Ogasawara Arc and other oceanic arcs, where the crust is thin. The primary magma compositions have changed from basalt, produced at considerable depth, to andesite, produced beneath the existing thinner crust at this location in the arc. This reflects the thermal and mechanical evolution of the mantle wedge and the overlying lithosphere. It is suggested that continental crust‐like andesitic magma builds up beneath submarine volcanoes on thin arc lithosphere today, and has built up beneath such volcanoes in the past. Andesites produced by this shallow and hydrous melting of the mantle could accumulate through collisions of plates to generate continental crust. Nishinoshima, a submarine volcano in the Ogasawara Arc, ~ 1 000 km south of Tokyo, Japan, suddenly erupted in November 2013, after 40 years of dormancy. Primary andesite magmas originate directly from the mantle as a result of shallow and hydrous melting of plagioclase peridotites. It is suggested that continental crust‐like andesitic magma builds up beneath submarine volcanoes on thin arc lithosphere today, and has built up beneath such volcanoes in the past. [ABSTRACT FROM AUTHOR]