Geochemical structure of the Hawaiian plume: Sr, Nd, and Os isotopes in the 2.8 km HSDP-2 section of Mauna Kea volcano

Sr, Nd, and Os isotopic measurements were made on 110 Mauna Kea lava and hyaloclastite samples from the drillcore retrieved from the second phase of the Hawaii Scientific Drilling Project (HSDP-2). The samples come from depths of 255 to 3098 meters below sea level, span an age range from 200 to about 550-600 kyr, and represent an ordered record of the lava output from Mauna Kea volcano as it drifted a distance of about 40 km over the magma-producing region of the Hawaiian hot spot. The deepest (oldest) samples represent the time when Mauna Kea was closest to the center of the melting region of the Hawaiian plume. The Sr and Os isotopic ratios in HSDP-2 lavas show only subtle isotopic shifts over the similar to 400 kyr history represented by the core. Neodymium isotopes (epsilon(Nd) values) increase systematically with decreasing age from an average value of nearly +6.5 to an average value of +7.5. This small change corresponds to subtle shifts in Sr-87/Sr-86 and Os-187/Os-188 isotope ratios, with small shifts of epsilon(Hf), a large shift in Pb-208/Pb-204 and Pb-208/Pb-207 values, and with a very large shift in He isotope ratios from R/R-A values of about 7-8 to values as high as 25. When Mauna Kea was closest to the plume core, the magma source did not have primitive characteristics for Nd, Sr, Pb, Hf, and Os isotopes but did have variable amounts of primitive helium. The systematic shifts in Nd, Hf, Pb, and He isotopes are consistent with radial isotopic zoning within the melting region of the plume. The melting region constitutes only the innermost, highest-temperature part of the thermally anomalous plume mantle. The different ranges of values observed for each isotopic system, and comparison of Mauna Kea lavas with those of Mauna Loa, suggest that the axial region of the plume, which has a radius of similar to 20 km, is a mixture of recycled subducted components and primitive lower mantle materials, recently combined during the formational stages of the plume at the base of the mantle. The proportions of recycled and primitive components are not constant, and this requires there be longitudinal (vertical) heterogeneity within the core of the plume. The remainder of the plume, outside this plume core zone, is less heterogeneous but distinct from upper mantle as represented by mid-ocean ridge basalt (MORB). The plume structure may provide a detailed view of mantle isotopic composition near the core-mantle boundary.