The alteration mineralogy, major and trace element chemistry, and Sr-Nd-Pb-Hf isotopic compositions of post-shield lavas from Mauna Kea, Kohala, and Hualalai on the island of Hawaii in the Pacific Ocean are used to constrain the origin and distribution of heterogeneities in the Hawaiian mantle plume. Ocean island basalts contain a variety of secondary minerals that must be removed by acid-leaching to achieve high-precision Pb isotopic compositions, a powerful geochemical tracer of variation in plume source composition. Post-shield lavas range from transitional/alkalic basalt to trachyte and are enriched in incompatible trace elements (e.g. LaN/YbN=6.0-16.2) relative to shield stage tholeiites. Post-shield lavas are characterized by a limited range of Sr-Nd-Hf isotopic compositions(⁸⁷Sr/⁸⁶Sr=0.70343-0.70365; ¹⁴³Nd/¹⁴⁴Nd = 0.51292-0.51301;¹⁷⁶Hf/¹⁷⁷Hf= 0.28311-0.28314) and have Pb isotopic compositions(²⁰⁶Pb/²⁰⁴Pb = 17.89-18.44; ²⁰⁷Pb/²⁰⁴ 15.44-15.49;²⁰⁸Pb/²⁰⁴Pb= 37.68-38.01) that belong to their respective Kea or Loa side of the Pb-Pb boundary. Mauna Kea lavas show a systematic shift to less radiogenic Pb isotopic compositions from the shield to post-shield stage and trend to low ⁸⁷Sr/⁸⁶Sr towards compositions characteristic of rejuvenated stage lavas. Hualalai post shield lavas lie distinctly above the Hf-Nd Hawaiian array (ƐHf = +12 to +13; ƐNd = +5.5 to +6.5) and have some of the least radiogenic Pb isotopic compositions (e.g.²⁰⁶/²⁰⁴pb= 17.89-18.01) of recent Hawaiian volcanoes. In contrast, comparison of Kohala with the adjacent Mahukona shows that lavas from these volcanoes become more radiogenic in Pb during the late stages of volcanism. The Sr-Nd-Pb-Hf isotope systematics of the post shield lavas cannot be explained by mixing between the Kea and Koolau end-members or by assimilation of Pacific lithosphere and are consistent with the presence of ancient recycled lower oceanic crust and sediments in their source. More than one depleted component is sampled by the post-shield lavas and these components are long-lived features of the Hawaiian plume that are present in both the Kea and Loa source regions. The geochemistry of the post-shield lavas provide evidence for a bilaterally zoned plume, where the compositional boundary between the Kea and Loa sources is complex and vertical components of heterogeneity are also significant.

The alteration mineralogy, major and trace element chemistry, and Sr-Nd-Pb-Hf isotopic compositions of post-shield lavas from Mauna Kea, Kohala, and Hualalai on the island of Hawaii in the Pacific Ocean are used to constrain the origin and distribution of heterogeneities in the Hawaiian mantle plume. Ocean island basalts contain a variety of secondary minerals that must be removed by acid-leaching to achieve high-precision Pb isotopic compositions, a powerful geochemical tracer of variation in plume source composition. Post-shield lavas range from transitional/alkalic basalt to trachyte and are enriched in incompatible trace elements (e.g. LaN/YbN=6.0-16.2) relative to shield stage tholeiites. Post-shield lavas are characterized by a limited range of Sr-Nd-Hf isotopic compositions(⁸⁷Sr/⁸⁶Sr=0.70343-0.70365; ¹⁴³Nd/¹⁴⁴Nd = 0.51292-0.51301;¹⁷⁶Hf/¹⁷⁷Hf= 0.28311-0.28314) and have Pb isotopic compositions(²⁰⁶Pb/²⁰⁴Pb = 17.89-18.44; ²⁰⁷Pb/²⁰⁴ 15.44-15.49;²⁰⁸Pb/²⁰⁴Pb= 37.68-38.01) that belong to their respective Kea or Loa side of the Pb-Pb boundary. Mauna Kea lavas show a systematic shift to less radiogenic Pb isotopic compositions from the shield to post-shield stage and trend to low ⁸⁷Sr/⁸⁶Sr towards compositions characteristic of rejuvenated stage lavas. Hualalai post shield lavas lie distinctly above the Hf-Nd Hawaiian array (ƐHf = +12 to +13; ƐNd = +5.5 to +6.5) and have some of the least radiogenic Pb isotopic compositions (e.g.²⁰⁶/²⁰⁴pb= 17.89-18.01) of recent Hawaiian volcanoes. In contrast, comparison of Kohala with the adjacent Mahukona shows that lavas from these volcanoes become more radiogenic in Pb during the late stages of volcanism. The Sr-Nd-Pb-Hf isotope systematics of the post shield lavas cannot be explained by mixing between the Kea and Koolau end-members or by assimilation of Pacific lithosphere and are consistent with the presence of ancient recycled lower oceanic crust and sediments in their source. More than one depleted component is sampled by the post-shield lavas and these components are long-lived features of the Hawaiian plume that are present in both the Kea and Loa source regions. The geochemistry of the post-shield lavas provide evidence for a bilaterally zoned plume, where the compositional boundary between the Kea and Loa sources is complex and vertical components of heterogeneity are also significant.

The alteration mineralogy, major and trace element chemistry, and Sr-Nd-Pb-Hf isotopic compositions of post-shield lavas from Mauna Kea, Kohala, and Hualalai on the island of Hawaii in the Pacific Ocean are used to constrain the origin and distribution of heterogeneities in the Hawaiian mantle plume. Ocean island basalts contain a variety of secondary minerals that must be removed by acid-leaching to achieve high-precision Pb isotopic compositions, a powerful geochemical tracer of variation in plume source composition. Post-shield lavas range from transitional/alkalic basalt to trachyte and are enriched in incompatible trace elements (e.g. LaN/YbN=6.0-16.2) relative to shield stage tholeiites. Post-shield lavas are characterized by a limited range of Sr-Nd-Hf isotopic compositions(⁸⁷Sr/⁸⁶Sr=0.70343-0.70365; ¹⁴³Nd/¹⁴⁴Nd = 0.51292-0.51301;¹⁷⁶Hf/¹⁷⁷Hf= 0.28311-0.28314) and have Pb isotopic compositions(²⁰⁶Pb/²⁰⁴Pb = 17.89-18.44; ²⁰⁷Pb/²⁰⁴ 15.44-15.49;²⁰⁸Pb/²⁰⁴Pb= 37.68-38.01) that belong to their respective Kea or Loa side of the Pb-Pb boundary. Mauna Kea lavas show a systematic shift to less radiogenic Pb isotopic compositions from the shield to post-shield stage and trend to low ⁸⁷Sr/⁸⁶Sr towards compositions characteristic of rejuvenated stage lavas. Hualalai post shield lavas lie distinctly above the Hf-Nd Hawaiian array (ƐHf = +12 to +13; ƐNd = +5.5 to +6.5) and have some of the least radiogenic Pb isotopic compositions (e.g.²⁰⁶/²⁰⁴pb= 17.89-18.01) of recent Hawaiian volcanoes. In contrast, comparison of Kohala with the adjacent Mahukona shows that lavas from these volcanoes become more radiogenic in Pb during the late stages of volcanism. The Sr-Nd-Pb-Hf isotope systematics of the post shield lavas cannot be explained by mixing between the Kea and Koolau end-members or by assimilation of Pacific lithosphere and are consistent with the presence of ancient recycled lower oceanic crust and sediments in their source. More than one depleted component is sampled by the post-shield lavas and these components are long-lived features of the Hawaiian plume that are present in both the Kea and Loa source regions. The geochemistry of the post-shield lavas provide evidence for a bilaterally zoned plume, where the compositional boundary between the Kea and Loa sources is complex and vertical components of heterogeneity are also significant. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

No description available.

The physical properties and dynamic processes that control effusions of rhyolitic lavas are poorly constrained because of a paucity of direct observations. To assess the pre-eruptive storage conditions, eruptive ascent, and subaerial emplacement for a suite of volumetrically diverse rhyolitic lavas, I studied 10 obsidian lavas from Yellowstone Caldera, Wyoming and Mono Craters, California. Storage, ascent, and emplacement of those lavas were quantitatively constrained using phenocryst compositions, high temperature experiments, microlite textures, and compositional gradients surrounding spherulites. Compositions of phenocrysts and quartz-hosted glass inclusions indicate the magmas at Yellowstone were stored at 750±25 °C in the shallow crust (<7 km), in agreement with phase equilibria experiments. Following the initiation of an eruption, magma leaves the chamber and ascends in a conduit. Microlite number density can be used to quantify eruptive ascent rates. To generate the observed microlite number densities (10⁸·¹¹±⁰·⁰³) to 10⁹·⁴⁵±⁰·¹⁵ cm⁻³), the magmas decompressed at ~1 MPa hour⁻¹, equivalent to ascent rates of ~10 mm s⁻¹. Upon subaerial emplacement, microlites act as rigid particles in a deforming fluid (lava), and hence their 3D orientations could indicate flow direction and how strain accumulates in the fluid during flow. Microlites are strongly aligned in samples from all flows, but variations in alignment were found to be independent of flow volume or distance travelled. Together, those observations suggest that strains accumulated during subaerial transport must be small (<2). Instead, microlites most likely aligned in response to strain in the conduit, which can be generated by collapse and flattening. Upon reaching the surface, the cooling history and longevity of rhyolitic lavas are critical for developing models of emplacement and hazard assessment. Compositional gradients surrounding spherulites provide one method to assess such temporal characteristics. Spherulites, crystalline spheres of radiating quartz and feldspar, form by crystallization of obsidian glass in response to cooling. An advection-diffusion model was developed to simulate the growth of spherulites and compositional gradients that develop in the surrounding glass during spherulite growth. Observed gradients are consistent with spherulites growing between ~700 and ~400 °C, and cooling at rates of 10⁻⁵·²±⁰·³) °C s⁻¹. / text

No description available.

<p> Chemical variations between the matrix and variole fractions of variolitic lavas are quite distinct. Analyses for major. and trace elements and trace gold content was carried out on separated fractions of matrix and varioles from several handspecimens. </p> <p> Discussion of the results (including the origin of variolitic textures) was aided by thin section analysis and field relationships. </p> / Thesis / Bachelor of Science (BSc)

Volcanicity in six stages is described from tee Plio/Pleistocene volcanoes on the north coast of Viti Levu, Fiji; the larger one at Vutukoula shown eruption in five stages of rocks of the alkaline olivine basalt — trachyte association and the smaller one at Vatia, six miles away.

The Little Drum Mountains represent a deeply eroded Eocene-Oligocene volcano, consisting of a vent complex which erupted mafic flows and flow breccias, accompanied by lahars. Flows are dominated by members of the shoshonite suite and contain up to 3.95 percent K2O, mainly occult in K-rich glass, with K2O/Na2O ratios greater than 1.0. In a few interbedded flows, apparently of the calc-alkaline series, pyroxene with varying amounts of plagioclase in a fine-grained groundmass of plagioclase, mafic minerals, and interstitial glass. An ash-flow tuff of the Oligocene Needles Range Formation unconformably overlies the volcanic sequence. Contemporaneous eruptions of calc-alkaline and shoshonitic lavas are possibly related to different depths of magna derivation corresponding to two mid-Cenozoic imbricate subduction zones beneath the western United States.

No description available.