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

Abstract

[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 3 He/ 4 He ratios range from 6 to 24.7 times atmospheric (Ra), which significantly extends the range of values for Mauna Kea volcano. The 3 He/ 4 He ratios are lowest (i.e., close to MORB values of $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 3 He/ 4 He 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) Nd, Zr/Nb, and SiO 2 . 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.

Combined U–Th/He and 40Ar/39Ar geochronology of post-shield lavas from the Mauna Kea and Kohala volcanoes, Hawaii

“…Hawaiian lavas reported in the literature have a large range of trapped helium concentrations, from 2.2 to 1560 x 10 -5 nmol/g (KURZ et al, 2004). Even the lower limit of this range is comparable to the amount of radiogenic helium measured in our samples.…”

Deep Drilling into a Mantle Plume Volcano: The Hawaii Scientific Drilling Project

“…8). The data show that there is radial geochemical zoning of the melting region of the plume in terms of He, Pb, Nd, Sr and Hf isotopes (Blichert-Toft et al, 2003;Eisele et al, 2003;Kurz et al, 2004;Bryce et al, 2005). This geochemical structure represents the hot core of the plume and does not reflect entrainment of ambient lower or upper mantle.…”

Submarine radial vents on Mauna Loa Volcano, Hawai'i

“…Nd isotopes ratios for the submarine lavas show a more restricted range compared to Sr isotopes (Figure 12). [26] Helium isotopes in olivine melt inclusions and matrix glasses were measured by crushing in vacuo at Woods Hole Oceanographic Institution, using methods described by Kurz et al [2004]. Only a handful of the samples have olivine phenocrysts or glassy material suitable for the helium measurements (see Table 7).…”