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We report Sr, Nd, and Pb isotope compositions for 17 bulk-rock samples from the submarine Hana Ridge, Haleakala volcano, Hawaii, collected by three dives by ROV Kaiko during a joint Japan–US Hawaiian cruise in 2001. The Sr, Nd, and Pb isotope ratios for the submarine Hana Ridge lavas are similar to those of Kilauea lavas. This contrasts with the isotope ratios from the subaerial Honomanu lavas of the Haleakala shield, which are similar to Mauna Loa lavas or intermediate between the Kilauea and Mauna Loa fields. The observation that both the Kea and Loa components coexist in individual shields is inconsistent with the interpretation that the location of volcanoes within the Hawaiian chain controls the geographical distribution of the Loa and Kea trend geochemical characteristics. Isotopic and trace element ratios in Haleakala shield lavas suggest that a recycled oceanic crustal gabbroic component is present in the mantle source. The geochemical characteristics of the lavas combined with petrological modeling calculations using trace element inversion and pMELTS suggest that the melting depth progressively decreases in the mantle source during shield growth, and that the proportion of the recycled oceanic gabbroic component sampled by the melt is higher in the later stages of Hawaiian shields as the volcanoes migrate away from the central axis of the plume. [ABSTRACT FROM AUTHOR]

The Hawaiian–Emperor volcanic island and seamount chain is usually attributed to a hot mantle plume, located beneath the Pacific lithosphere, that delivers material sourced from deep in the mantle to the surface. The shield volcanoes of the Hawaiian islands are distributed in two curvilinear, parallel trends (termed ‘Kea’ and ‘Loa’), whose rocks are characterized by general geochemical differences. This has led to the proposition that Hawaiian volcanoes sample compositionally distinct, concentrically zoned, regions of the underlying mantle plume. Melt inclusions, or samples of local magma ‘frozen’ in olivine phenocrysts during crystallization, may record complexities of mantle sources, thereby providing better insight into the chemical structure of plumes. Here we report the discovery of both Kea- and Loa-like major and trace element compositions in olivine-hosted melt inclusions in individual, shield-stage Hawaiian volcanoes—even within single rock samples. We infer from these data that one mantle source component may dominate a single lava flow, but that the two mantle source components are consistently represented to some extent in all lavas, regardless of the specific geographic location of the volcano. We therefore suggest that the Hawaiian mantle plume is unlikely to be compositionally concentrically zoned. Instead, the observed chemical variation is probably controlled by the thermal structure of the plume. [ABSTRACT FROM AUTHOR]

To constrain the preeruptive physical and chemical conditions (pressure, temperature, H2O content, and fO2) of the most recent and largest eruption (Aso‐4) of Aso volcano, Japan, we compared phase assemblages and chemical compositions of erupted material with those of experimentally determined run charges of the silicic end‐member magma of Aso‐4 (the KJ5665 pumice and MY5774 welded lens from the Aso‐4 pyroclastic flow deposit; SiO2 = 70.5 wt %). Experimental conditions were as follows: starting material = KJ5665 pumice, pressure = 100–700 MPa, temperature range = 810–950 °C, and injected H2O content = 2.9–6.7 wt %. The starting material comprised mainly rhyolitic glass with plagioclase, orthopyroxene, magnetite, and ilmenite phenocrysts. Hornblende phenocrysts are not found on cross sections of the KJ5665 pumice. Core compositions of these phenocrysts are unimodal. Using the magnetite‐ilmenite thermometer, the preeruptive magmatic temperature and fO2 of KJ5665 were estimated to be ~900 °C and Nickel‐Nickel oxide (NNO) +1.1 log units, respectively. Run products contain plagioclase (An20–48), orthopyroxene (Mg# = 53–72), magnetite, ilmenite, biotite, and K‐feldspar. Hornblende was not observed. Preeruptive conditions of the silicic end‐member magma of the Aso‐4 eruption are estimated to be ~900 °C, ≤ 300 MPa, and ≤ 3 wt % H2O. Key Points: Preeruptive magma conditions of the Aso‐4 silicic end‐member magma were determinedPreeruptive conditions agree with geophysical constraints on magma chamber depth [ABSTRACT FROM AUTHOR]

Abstract: Hydrogen in nominally anhydrous minerals (NAMs) in volcanic rocks can be used as a proxy for dissolved H2O in melt prior to eruption. Plagioclase is a NAM that accommodates hydrogen in concentrations of up to hundreds of wt. ppm H2O. The species of hydrogen in volcanic plagioclase is structural OH. We report the analytical results of OH concentrations in Ca-rich plagioclase from the 1986–1987 summit eruption of Izu-Oshima volcano, a frontal-arc volcano in Izu arc. We demonstrate that the island arc low-K tholeiitic basalt magmas erupting from the frontal-arc volcanoes are H2O-saturated and undergo polybaric degassing during the magma ascent. The analyzed OH concentrations in plagioclase range from 20 to 300 wt. ppm H2O, and three levels of OH (20–80 wt. ppm H2O, 100–180 wt. ppm H2O, and 220–300 wt. ppm H2O) are found. These variations in OH indicate that crystallized plagioclase is equilibrated with H2O-saturated melt at three depths beneath the Izu-Oshima volcano prior to eruption: near the surface level (≈1wt.% H2O in melt), at a 4-km-deep magma chamber (≈3wt.% H2O in melt), and at a 8–10-km-deep magma chamber (≈5wt.% H2O in melt). It is proposed that deep-seated island arc low-K tholeiitic basalt magmas erupting from frontal-arc volcanoes are richer in H2O than previously thought, containing approximately 1wt.% H2O based on analyses of “leaked” melt inclusions and phase equilibrium studies at “low-pressure conditions”. [Copyright &y& Elsevier]

Abstract: In order to evaluate the influence of weathering/alteration on K–Ar dating for Hawaiian tholeiites, unspiked K–Ar ages were determined for 21 samples from four lava flows with varying degrees of weathering or alteration collected from the Makapuu Head section of Koolau volcano, Hawaii. The samples were classified based on freshness of olivine phenocrysts and the groundmass olivine, and the presence of secondary minerals in vesicles. The results indicate that the ages for samples with fresh groundmass olivine are reliable, even though olivine phenocrysts may be slightly altered (thin reaction rims) or secondary minerals may have crystallized in the vesicles. The ages for the lowermost lava flow in the Makapuu Head section and the lava flow approximately 120m above it are 2.58±0.13 and 2.36±0.09Ma, respectively. The accumulation rate of this section is calculated to be 0.04–0.11cm/year. We also report K–Ar ages for lava samples collected from the submarine flank of the Koolau volcano and the Nuuanu landslide blocks. The age for an early Makapuu-stage lava collected from the submarine flank of Koolau volcano is 2.5Ma, similar to the age from the lower part of the subaerial Makapuu Head section. Another lava sample collected from the submarine flank of the Koolau volcano has an age of 3.3Ma, older than any subaerial part of Koolau volcano. These results suggest that the onset of Koolau''s shield-stage volcanism was no later than ~3.3Ma, and the duration of the shield stage was at least 1.2m.y. [Copyright &y& Elsevier]