Helium isotopic evolution of Mauna Kea Volcano: First results from the 1-km drill core

Helium concentrations and isotopic ratios have been measured in a suite of basaltic olivines from the Hawaii Scientific Drilling Project (HSDP) core at Hilo, Hawaii, which allows a characterization of the temporal helium isotopic evolution of Mauna Kea volcano. Typically more than 85% of the helium within the olivines is released by crushing, which demonstrates that helium is dominantly contained within the melt inclusions, and strongly suggests that helium behaves as an incompatible element during silicate melting. Lavas from the base of the Mauna Loa section of the core (240-270 m depth) have He-3/He-4 ratios between 13.9 and 15.8 times atmospheric (Ra). These values, when combined with Sr and Nd isotopic data, correspond to those previously found in subaerial Mauna Loa lavas that are greater than 39 ka in age, place a minimum age of 30 ka for the top of the Mauna Kea section, and are consistent with radiocarbon chronology. The upper part of the Mauna Kea lava section (290-620 m) has He-3/He-4 ratios of 7.0 to 6.8 Pa similar to most mid-ocean ridge basalts and is thought to represent the normal asthenospheric mantle signature. Between 620 and 670 m depth there is a transition to higher values, with He-3/He-4 ratios between 10.2 and 12.5 Pa at depths greater than 670 m, suggesting a greater contribution of undegassed plume material to the older Mauna Kea tholeiites. The correlations between He, Nd, and Pb isotope ratios demonstrate that the helium isotopic variations are related to mixing of source materials rather than shallow contamination effects (i.e., via addition of radiogenic He-4). The transition from high tp low He-3/He-4 ratios occurs within the tholeiitic shield of Mauna Kea. The highest He-3/He-4 ratios in the Mauna Kea lavas (up to 12.5 Ra) are significantly lower than the values found in the shield tholeiites of the adjacent volcanoes (Loihi Seamount, Mauna Loa, and Kilauea); this relates to a smaller hot spot contribution to the Mauna Kea lavas. The temporal helium isotopic evolution of Hawaiian shield volcanos (with highest He-3/He-4 ratios found in the oldest lavas) and the inter-volcano variations (with the Mauna Kea shield having the lowest He-3/He-4 ratios on the island) are attributed to movement of the Pacific plate over a radially zoned hot spot.