Diane Hanano

High‐precision isotopic characterization of USGS reference materials by TIMS and MC‐ICP‐MS

The Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia has undertaken a systematic analysis of the isotopic (Sr, Nd, and Pb) compositions and concentrations of a broad compositional range of U.S. Geological Survey (USGS) reference materials, including basalt (BCR-1, 2; BHVO-1, 2), andesite (AGV-1, 2), rhyolite (RGM-1, 2), syenite (STM-1, 2), granodiorite (GSP-2), and granite (G-2, 3). USGS rock reference materials are geochemically well characterized, but there is neither a systematic methodology nor a database for radiogenic isotopic compositions, even for the widely used BCR-1. This investigation represents the first comprehensive, systematic analysis of the isotopic composition and concentration of USGS reference materials and provides an important database for the isotopic community. In addition, the range of equipment at the PCIGR, including a Nu Instruments Plasma MC-ICP-MS, a Thermo Finnigan Triton TIMS, and a Thermo Finnigan Element2 HR-ICP-MS, permits an assessment and comparison of the precision and accuracy of isotopic analyses determined by both the TIMS and MC-ICP-MS methods (e.g., Nd isotopic compositions). For each of the reference materials, 5 to 10 complete replicate analyses provide coherent isotopic results, all with external precision below 30 ppm (2 SD) for Sr and Nd isotopic compositions (27 and 24 ppm for TIMS and MC-ICP-MS, respectively). Our results also show that the first- and second-generation USGS reference materials have homogeneous Sr and Nd isotopic compositions. Nd isotopic compositions by MC-ICP-MS and TIMS agree to within 15 ppm for all reference materials. Interlaboratory MC-ICP-MS comparisons show excellent agreement for Pb isotopic compositions; however, the reproducibility is not as good as for Sr and Nd. A careful, sequential leaching experiment of three first- and second-generation reference materials (BCR, BHVO, AGV) indicates that the heterogeneity in Pb isotopic compositions, and concentrations, could be directly related to contamination by the steel (mortar/pestle) used to process the materials. Contamination also accounts for the high concentrations of certain other trace elements (e.g., Li, Mo, Cd, Sn, Sb, W) in various USGS reference materials.

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

[1] 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 smallscale (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 ( 87 Sr/ 86 Sr = 0.70343–0.70365, 143 Nd/ 144 Nd = 0.51292–0.51301, and 176 Hf/ 177 Hf = 0.28311–0.28314) and have distinct Pb isotopic compositions ( 206 Pb/ 204 Pb = 17.89–18.44, 207 Pb/ 204 Pb = 15.44–15.49, and 208 Pb/ 204 Pb = 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 87 Sr/ 86 Sr 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., 206 Pb/ 204 Pb = 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.

Alteration mineralogy and the effect of acid-leaching on the Pb-isotope systematics of ocean-island basalts

Abstract The effect of alteration phases on the Pb-isotope systematics of weakly altered basalts from two major ocean islands (Hawaii and Kerguelen) was investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), and acid-leaching experiments. Alteration within vesicles and as replacement products of olivine and glass consists mainly of phyllosilicate mixtures (most commonly smectite ± talc, serpentine, chlorite, celadonite), goethite, and zeolite with minor pyrolusite, barite, apophyllite, dolomite, pyrite, and chalcopyrite. The presence of distinct alteration assemblages within the Hawaiian and Kerguelen basalts can be related to differences in their eruption environment, age, and sampling method. In particular, the Hawaiian basalts have been contaminated by highly radiogenic components (seawater and drilling mud), demonstrating the importance of acid-leaching prior to Pb-isotopic analysis even for young basalts that appear to be relatively unaltered. However, for some basalts, leaching may not remove the alteration completely or in a reproducible way, which may reflect variable extents of dissolution of secondary minerals that are not readily soluble in HCl (e.g., celadonite) or inconsistencies in the leaching procedure. The incomplete removal of foreign Pb components, which typically have distinct Pb-isotopic signatures, is a major source of uncertainty and may represent the ultimate limitation on high-precision Pb-isotopic compositions of ocean-island basalts. To achieve the highest precision and accuracy, we recommend that Pb-isotope studies of basalts include an investigation of the alteration mineralogy and an evaluation of the effectiveness of the leaching procedure as part of standard quality control protocols.

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 worlds 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 volcanos mass [Macdonald et al., 1983; Ozawa et al., 2005]. Diamond Head, in Honolulu, is the first and classic example of secondary volcanism.