Mixing of isotopic heterogeneities in the Mauna Kea plume conduit

We have measured high-precision Hf, Nd, and Pb isotope compositions for 40 samples from the final leg of the Hawaii Scientific Drilling Project (HSDP) core using solution chemistry and MC-ICP-MS. The new isotope data are indistinguishable from those higher up in the Mauna Kea part of the core. The Pb isotope data define three linear trends with a common intersection. Principal component analysis recognizes no more than three geochemical end-members among these trends. When all the Pb isotope data acquired so far for the HSDP core are pooled, two contrasting groups stand out. A first coherent, prevalent group represents the Kea main eruptive sequence. This group combines the Kea-mid8 and Kea-lo8 subgroups of Eisele et al. (Eisele, J., Abouchami, W., Galer, S.J.G., Hofmann, A.W., 2003. The 320 kyr Pb isotope evolution of Mauna Kea lavas recorded in the HSDP-2 drill core. Geochem. Geophys. Geosyst. 4, doi: 10.1029/2002GC000339): although defining two separate trends in Pb–Pb isotope space, they follow each other smoothly in time. A second group of 14 flows below 2000 mbsl is characterized by distinctly higher source Th/U and high 3He/4He. These samples belong to the Kea-hi8 group and are about 0.4 (Hf) and 0.5 (Nd) ε units below the main sequence Kea values. The toggle between these two groups is so sharp and repetitive and the gap between the values so conspicuous that the alternation must reflect eruptions from distinct volcanic centers, those of Mauna Kea and an unknown volcano, the latter of which bears isotopic resemblance to Kilauea for Nd and Hf and to Loihi for He. This ‘lost’ volcano stopped erupting about 550 ka ago, i.e., 200 ka before the oldest known ages for Kilauea, and is presently buried under 2000–3000 m of Kea flows. Periodograms show no wavelengths exceeding the 95% significance level (white spectrum). Except for a few minor wiggles in Hf at shallow depth, the isotopic signals display overall smooth up-core variations with little short-distance variability. Analysis of the flow regime within the plume conduit shows that temperature dependence of viscosity makes the conduit narrow and sharply bound. The hot core is highly mobile with minimum shear rates and a regime of pipe (Poiseuille) flow. The core is rimmed by a sheath of colder, strongly stretched material (filaments). A broadly concentric zoning of the plume is expected. The white spectra suggest, however, that, at any given depth within the plume conduit, lateral mixing results from the rheological variability across the plume conduit.