High-Precision Pb Isotopic Compositions of Basalts From Phase 2B of the Hawaiian Scientific Drilling Project

In this study we report Pb isotopic compositions by MC-ICP-MS of 22 basalts from Mauna Kea volcano sampled from the deepest part of the Hawaiian Scientific Drilling Project (HSDP-2B). The samples correspond to the last 215 m drilled, with ages >550 kyr in the stratigraphic record of Mauna Kea. All samples were subjected to sequential acid leaching before dissolution and analysis. Pb isotopic compositions of these new samples range from 18.429-18.596 for 206Pb/204Pb, 15.473-15.499 for 207Pb/204Pb, and 38.069-38.207 for ^{208}Pb/204Pb. These compositions fall in the more radiogenic part of the HDSP hole, although they are not the most radiogenic. The variation in Pb isotopes presented by these samples is 5x and 8x smaller for 206Pb/204Pb and ^{208}Pb/204Pb, respectively, than the variation observed for samples from the previous drilling phases. This limited variation is also observed in SiO2, MgO, Zr/Nb and Nb/Y. All samples plot on the Kea side in ^{208}Pb-206Pb space (Abouchami et al., 2005), with the exception of one sample, and define a tight trend that lies between the "Kea-mid8" and "Kea-hi8" arrays of Eisele et al. (2003). This trend has a similar slope to the "Kea-mid8" array, but presents distinctly more radiogenic ^{208}Pb/204Pb values. Contrary to the "Kea-mid8" and "Kea-hi8" arrays where the basalts are characterized by relatively low silica, these new samples all have high SiO2 and low MgO, plotting at the higher extremity of silica values for the entire hole. This new Pb array points towards the more radiogenic common component for the Kea side of the Hawaiian mantle plume. It also suggests the existence of several other compositional domains within the plume source that were sampled in variable proportions during partial melting and formation of the Mauna Kea volcano. The more restricted nature of this new Pb trend compared to previous ones indicates the sampling of a more homogeneous source component during the interval of time represented by these basalts. Integration of these results with Sr and Nd isotopic compositions on the same samples will allow us to quantify and model the size of the magma capture zone corresponding to this section of the Mauna Kea volcano.