Geochemistry of post-shield lavas from Kea- and Loa-trend Hawaiian volcanoes : constraints on the origin and distribution of heterogeneities in the Hawaiian mantle plume

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 ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat

Geochemistry of the Baie Charrier Basaltic Section, Courbet Peninsula, Kerguelen Archipelago: Implications for the Composition of the Kerguelen Mantle Plume.

The Baie Charrier basaltic section is located on the northern Courbet Peninsula on the Kerguelen Archipelago. The archipelago represents the emergent part of the Northern Kerguelen Plateau, part of the Kerguelen large igneous province in the southern Indian Ocean. The archipelago formed through volcanism (40 Ma to recent) related to the Kerguelen mantle plume. This study presents a detailed petrographic and geochemical analysis of basalts from the Baie Charrier section. The results of this study will compliment the 1000 m Mt. Crozier section located in the center of the Courbet Peninsula. Mineral compositions of olivine and clinopyroxene phenocrysts by electron microprobe analysis were obtained for phenocryst zoning profiles and mineral-melt equilibria. Major and trace element concentrations by XRF and high-resolution ICP-MS, as well as Sr, Nd, and Pb isotopic compositions by TIMS and multi-collector ICP-MS define the chemical and isotopic characteristics of the Baie Charrier section. Based on petrographic and geochemical criteria, the Baie Charrier section is subdivided into four distinct units. The Baie Charrier basalts are primarily olivine-phyric, and sometimes contain phenocrysts of plagioclase or clinopyroxene. Mineral chemistry reveals normally zoned phenocrysts, with olivine core compositions ranging from Fo86-70 and clinopyroxene core Mg# ranging from 0.88 to 0.79. The Baie Charrier basalts are mildly alkalic and possess relatively high MgO contents; Units A and C are comprised of high-MgO (8-10 wt.%) basalts, and sample 240 of unit D is a picrite (>16 wt.%). CaO depletion at ~6 wt.% MgO, coupled with a continuously increasing Al2O3 content, reflect fractionation driven by significant clinopyroxene crystallization. Trace element depletion in Ni, Cr, and Sc with decreasing MgO reflect the relative compatibility of these elements in olivine and clinopyroxene phenocrysts. Incompatible element abundance relationships are used to define a relative incompatibility index in order of increasing compatibility: Th<Ce, Nb, Rb<Ba<Zr<Sr. The Baie Charrier basalts are enriched in highly incompatible elements and light rare earth elements. The units of the Baie Charrier section are also isotopically distinct; Unit C possesses some of the highest 143Nd/144Nd, lowest 87Sr/86Sr, and lowest Pb isotopic ratios, while Unit D has the highest Pb isotopic ratios of the section. The small-scale petrographic and geochemical variation observed in the Baie Charrier section is interpreted to reflect temporal changes in volcanism derived from a heterogeneous source region. Mineral-melt equilibria constrain the maximum MgO content for magmas without accumulated phenocrysts to be between 8-10 wt.% MgO. The similarities between the trace element and isotopic compositions of the Baie Charrier section and the Mt. Crozier section suggests that both are derived from the same source, which supports the interpretation of the Courbet Peninsula as a single volcanic unit. However, the abundance of olivine-phyric basalts at Baie Charrier, and their absence at Mt. Crozier, suggest that the eruptive center of this volcano may not coincide with the present geographic center of the Courbet Peninsula. Lastly, the Baie Charrier basalts, with their highly radiogenic Pb compositions, contain a strong signature of an enriched plume-derived component, and provide additional constraints on the source composition of the Kerguelen mantle plume.Science, Faculty ofEarth and Ocean Sciences, Department ofUnreviewedUndergraduat

Horizontal and vertical zoning of heterogeneities in the Hawaiian mantle plume from the geochemistry of consecutive postshield volcano pairs: Kohala-Mahukona and Mauna Kea-Hualalai.

Sr-Nd-Pb-Hf isotopic compositions of postshield lavas from two pairs of Hawaiian volcanoes, Mauna Kea and Kohala (Kea trend) and Hualalai and Mahukona (Loa trend), allow for identification of small-scale (tens of kilometers) heterogeneities in the Hawaiian mantle plume and provide constraints on their distribution. The postshield lavas range from transitional/alkalic basalt to trachyte and are enriched in incompatible trace elements (e.g., LaN/YbN = 6.0–16.2). These 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, and 176Hf/177Hf = 0.28311–0.28314) and have distinct Pb isotopic compositions (206Pb/204Pb = 17.89–18.44, 207Pb/204Pb = 15.44–15.49, and 208Pb/204Pb = 37.68–38.01) that correspond to their respective Kea or Loa side of the Pb-Pb isotopic boundary. Mauna Kea lavas show a systematic shift to less radiogenic Pb isotopic compositions from the shield to postshield stage and they trend to low 87Sr/86Sr toward, but not as extreme as, compositions characteristic of rejuvenated stage lavas. Hualalai postshield lavas lie distinctly above the Hf-Nd Hawaiian array and have much lower Pb isotopic ratios than shield lavas, including some of the least radiogenic values (e.g., 206Pb/204Pb = 17.89–18.01) of recent Hawaiian volcanoes. In contrast, comparison of Kohala with the adjacent Mahukona volcano shows that these older postshield lavas become more radiogenic in Pb during the late stages of volcanism. The isotope systematics of the postshield lavas cannot be explained by mixing between Hawaiian plume end-members (e.g., Kea, Koolau, and Loihi) or by assimilation of Pacific lithosphere and are consistent with the presence of ancient recycled lower oceanic crust (±sediments) in their source. More than one depleted component is sampled by the postshield lavas and these components are long-lived features of the Hawaiian plume that are present in both the Kea and Loa source regions. The depleted components in the postshield lavas, particularly as sampled at Hualalai, are different from the much more homogeneous component present in rejuvenated lavas. The geochemistry of the postshield lavas provides evidence for a bilateral symmetry to the plume where the compositional boundary between the Kea and Loa sources is complex and vertical components of heterogeneity are significant. An edited version of this paper was published by AGU. Copyright 2010 American Geophysical Union.Science, Faculty ofEarth and Ocean Sciences, Department ofReviewedFacult

Widespread secondary volcanism near northern Hawaiian Islands.

Hot spot theory provides a key framework for understanding the motion of the tectonic plates, mantle convection and composition, and magma genesis. The age-progressive volcanism that constructs many chains of islands throughout the world's ocean basins is essential to hot spot theory. In contrast, secondary volcanism, which follows the main edifice building stage of volcanism in many chains including the Hawaii, Samoa, Canary, Mauritius, and Kerguelen islands, is not predicted by hot spot theory. Hawaiian secondary volcanism occurs hundreds of kilometers away from, and more than 1 million years after, the end of the main shield volcanism, which has generated more than 99% of the volume of the volcano's mass [Macdonald et al., 1983; Ozawa et al., 2005]. Diamond Head, in Honolulu, is the first and classic example of secondary volcanism. An edited version of this paper was published by AGU. Copyright 2008 American Geophysical Union.Science, Faculty ofEarth and Ocean Sciences, Department ofReviewedFacult

Hf isotope compositions of U.S. Geological Survey reference materials.

A systematic multi-isotopic and trace element characterization of U.S. Geological Survey reference materials has been carried out at the Pacific Centre for Isotopic and Geochemical Research, University of British Columbia. Values of 176Hf/177Hf are recommended for the following reference materials (mean ±2 SD): G-2: 0.282523 ± 6; G-3: 0.282518 ± 1; GSP-2: 0.281949 ± 8; RGM-1: 0.283017 ± 13; STM-1: 0.283019 ± 12; STM-2: 0.283021 ± 5; BCR-1: 0.282875 ± 8; BCR-2: 0.282870 ± 8; BHVO-1: 0.283106 ± 12; BHVO-2: 0.283105 ± 11; AGV-1: 0.282979 ± 6; and AGV-2: 0.282984 ± 9. Reproducibility is better than 50 ppm for the granitoid compositions and better than 40 ppm for the basaltic/andesitic compositions. For the isotopic analyses acquired early in this project on glass columns, Hf isotopic analyses from several of the reference materials were significantly less reproducible than Nd and Sr isotopic analyses determined from the same sample dissolution. The 176Hf/177Hf ratios for relatively radiogenic compositions (BCR-1, 2; BHVO-1, 2; RGM-1) were shifted systematically toward lower values by 100–150 ppm when a borosilicate primary column was used. Although systematic, the shift for felsic compositions was generally within analytical error, except for GSP-2, which has a very low Hf isotopic ratio, where the shift was to higher 176Hf/177Hf. Trace element and isotopic characterization of the borosilicate glass column, borosilicate frits, and quartz columns reveals extremely variable levels of trace elements. The 176Hf/177Hf ratios for these materials are very unradiogenic (borosilicate glass <0.28220; frit = 0.28193 ± 4). The borosilicate frit material appears to be the most variable in elemental concentration and isotopic composition. The quartz material has very low levels (<ppm) of all trace elements. Low 176Hf/177Hf and high Hf concentrations of the borosilicate glass column (16 ppm) and frit material (22 ppm) indicate that only small amounts of such unradiogenic material could cause significant contamination of small samples. For the basaltic (BCR-1, 2; BHVO-1, 2) and rhyolitic (RGM-1) samples, approximately 3 ng of Hf from the column or frit would be enough to produce the observed 100–150 ppm shift. Accurate, high-precision 176Hf/177Hf data can only be acquired if samples are processed using all PTFE Teflon® labware, or quartz and polypropylene. An edited version of this paper was published by AGU. Copyright 2007 American Geophysical Union.Science, Faculty ofEarth and Ocean Sciences, Department ofReviewedFacult

Hf isotope compositions of U.S: Geological Survey reference materials. Geochemistry

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Comprehensive and systematic MC-ICP-MS/TIMS isotopic characterization of USGS Standards

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High-précision Sr, Nd, Pb and Hf isotopic charactérization of USGS reference materials by MC-ICP-MS and TIMS

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Widespread secondary volcanism near northern Hawaiian Islands

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