Rapid helium isotopic variability in Mauna Kea shield lavas from the Hawaiian Scientific Drilling Project

[1] This paper presents new magmatic helium isotopic compositions in a suite of lavas from phase II of the Hawaiian Scientific Drilling Project (HSDP2) core, which sampled Mauna Kea volcano to a maximum depth of 3098 m below sea level. Most of the measurements were performed by in vacuo crushing of olivine phenocrysts, but include submarine pillow glasses from the 2200 to 2500 meter depth interval, and orthopyroxene phenocrysts from an intrusive at 1880 m. The magmatic He-3/He-4 ratios range from 6 to 24.7 times atmospheric (Ra), which significantly extends the range of values for Mauna Kea volcano. The He-3/He-4 ratios are lowest (i.e., close to MORB values of similar to 8 Ra) near the top of the Mauna Kea section and rise slowly, to 10 - 12 Ra, at 1000 m below sea level, consistent with results from the HSDP1 core. At depths greater than 1000 m in the core, primarily in the submarine lavas, there are brief periods when the He-3/He-4 ratios are higher than 14.5 Ra, always returning to a baseline value. Twelve such excursions were identified in the core; all but one are in the submarine section, and most ( 7) are in the deepest section, at depths of 1950 to 3070 m. The baseline He-3/He-4 value rises from 10 - 12 R-a near 1000 m depth to 12-14 R-a at 3000 m. The helium spikes are found only in lavas that are older than 380 Ka in age, based on an age model derived from Ar-Ar data (W. D. Sharp et al., manuscript in preparation, 2003). Excluding the excursions defined by single lava flows ( 3) and intrusive units ( 3), the average spike duration is approximately 15 (+/-9) Ka (n = 6). The high He-3/He-4 spikes are interpreted as pulses of magma from the center of the actively upwelling Hawaiian hot spot. The short duration of the high He-3/He-4 excursions suggests that Mauna Kea was never directly over high the He-3/He-4 component of the plume (during the HSDP2 eruptive period), presumed to be the plume center. Assuming that the Mauna Kea helium spikes result from melting of heterogeneities within the plume, their short duration implies that the length scales of heterogeneities in the solid upwelling mantle are between 60 m and 12 km ( for upwelling rates of 2 to 40 cm/yr). The high He-3/He-4 are associated with high Pb-208/Pb-204, and relatively low Nd-143/Nd-144, Zr/Nb, and SiO2. The correlations with major elements, trace elements and isotopes demonstrate that helium is coupled to the other geochemical variations, and that the Mauna Kea isotopic variability is caused by heterogeneities within the upwelling plume.