This paper reports new geochemical data for submarine volcanic pumice sampled from two volcanoes (SM‐1 and SV‐1) which are part of the Andaman subduction system. SM‐1 volcano is located closer to the Andaman back‐arc spreading axis while SV‐1 volcano is part of a linear chain of arc volcanoes in the southern Andaman Basin. The pumice are products of phreatomagmatic eruptions and are highly vesicular in nature; however, immediately after eruption of these lava froth, sea water entering into the vesicles made them sink to the sea floor. A sheet of dull grey to white‐coloured pumice observed on top of the SM‐1 volcano indicates a nonexplosive eruption referred to as Tangoroan type. Pumice from these two spatially apart volcanoes share uniform physical properties but display minor variations in mineralogical compositions and distinct characteristics of selected major and trace elements. Pumice from SM‐1 comprised plagioclase, hornblende, and spinel while those from SV‐1 are composed of plagioclase and quartz as main crystalline phases. Geochemically, the pumice samples from SM‐1 volcano are classified as medium‐K calc‐alkaline rhyolites, while those from SV‐1 volcano show low‐K tholeiitic compositions. Trace and REE compositions for these rhyolitic pumice collectively reflect LILE‐LREE‐enriched, HFSE‐depleted signatures of subduction zone magmatism and implicate an oceanic arc tectonic affinity. Selective enrichment in LILE and LREE with relative HFSE depletion is attributed to infiltration of fluid‐mobile LILE and LREE into the mantle via fluids released from slab dehydration with retention of fluid‐immobile HFSE in the subducted slab. Elevated abundances of LILE/HFSE, LREE/HFSE, and LREE/HREE for the medium‐K calc‐alkaline rhyolitic pumice relative to their low‐K tholeiitic counterparts invoke increased fluid flux into the mantle with progressive maturation of oceanic slab subduction. The precursor magmas parental to the SM‐1 and SV‐1 volcanic pumice were derived by partial melting of a mantle wedge metasomatized by variable slab–mantle interactions, and influx of slab‐dehydrated fluids and sediments. Pronounced REE fractionation trends suggest evolution of resultant melts in a cold, hydrous, oxidizing regime of intraoceanic arc setting that eventually reheated, remelted, remobilized, and assimilated lower oceanic arc crust and erupted as rhyolites. The volcanic pumice from Andaman are geochemically and tectonically analogous to those from Mariana arc and Okinawa Trough of Pacific Ocean.

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<p>Composite volcanoes can progressively weaken through hydrothermal alteration, which may lead to volcano collapse, forming far-reaching debris avalanches. Hydrothermal minerals can also contribute to flank instability as they play a critical role in moderating volcanic degassing by changing the porosity and permeability of the rock and thereby changing the local pore-pressure distribution. Therefore, a robust model and understanding of hydrothermal alteration within a volcanic edifice is important to improve hazard assessment efforts. This study investigates the type and extent of hydrothermal alteration on Mt Ruapehu, New Zealand, using a combination of mineralogical, hyperspectral imaging, and aero-magnetic studies.</p> <p>Mt Ruapehu shows a diverse suite of surface weathering and hydrothermal alteration minerals, which are distributed heterogeneously on the surface. The surface weathering has abundant goethite, hematite and phyllosilicate mineral associations, while the hydrothermal alteration is characterised by phyllosilicates, Fe-oxides, pyrite, jarosite, alunite, gypsum anhydrite, and native sulphur minerals. Although surficial evidence of alteration on Mt Ruapehu is limited, aero-magnetic data and inversion modelling indicate deep-seated (≤500 m) alteration of demagnetized rocks. The decrease of magnetic susceptibility can be linked to the dissolution of (Ti-) magnetite phases, as well as the deposition of brecciated horizons between lava flows and intercalated glacial till and volcaniclastics. Surface outcrops mapped by airborne hyperspectral imaging combined with Scanning Electron Microscopy (SEM-EDS), Short wavelength Infrared (SWIR) Spectroscopy, X-Ray Diffraction and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) data of ground samples reveal a complex alteration history developed around older vent/crater systems of Mt. Ruapehu in last 250 ky. This study provides a simplified geological model to capture the hydrothermal processes on Mt Ruapehu, aiding future studies on delineating areas prone to mass movements.</p>

The prevalence of antecrysts in arc volcanic rocks is widely accepted, yet the origin of their carrier melts remains debated. Crystal cargo in lava flows from Taranaki volcano, New Zealand, is dominated by plagioclase, clinopyroxene, and amphibole. Except for some crystal rims, mineral phases are in disequilibrium with the melt they are entrained in. Major element chemistry reveals an almost complete compositional overlap between the crystals in the lava and those in xenoliths. The large volume fraction of crystals (35–55 vol%) exerts a strong control on whole-rock compositions, reducing silica by 5–11 wt% compared to the carrier melt. Yet there is no clear relationship between mineral proportion and bulk rock compositions. Our data are inconsistent with extensive fractional crystallization, commonly invoked as a driver of magma evolution towards silica-rich compositions. Instead, high-temperature, aphyric carrier melts with varied compositions (55–68 SiO 2 wt%) entrain crystal cargo while ascending through colder, low-silica rocks. Thus, some primary melts at Taranaki volcano are significantly more silica-rich than arc basalts commonly invoked as parental magmas. Further, thermometric and hygrometric constraints preclude a deep crustal hot zone for the source of these melts, which we argue are of subcrustal origin. Supplementary material: https://doi.org/10.6084/m9.figshare.c.6406813

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