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

Abstract

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 3He/4He 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 30 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 3He/4He ratios of 7.0 to 6.8 Ra, 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 3He/4He ratios between 10.2 and 12.5 Ra 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 4He). The transition from high to low 3He/4He ratios occurs within the tholeiitic shield of Mauna Kea. The highest 3He/4He 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 3He/4He ratios found in the oldest lavas) and the inter‐volcano variations (with the Mauna Kea shield having the lowest 3He/4He ratios on the island) are attributed to movement of the Pacific plate over a radially zoned hot spot.

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

“…In the worst case, and 10% of the gas remained after crushing (e.g. KURZ et al, 1996), all of the samples would in fact have much younger ages, which would make the U-Th/He ages less consistent with the 40 Ar/ 39 Ar ages.…”

“…HILTON et al, 1999) while undercrushing can result in trapped 4 He remaining, between 0.2 and 10% (e.g. WILLIAMS et al, 2005;KURZ et al, 1996). Samples were crushed using 300 beats in 5 minutes, then sieved to remove the remaining pieces larger than 100 μm, which may have magmatic helium remaining.…”

High-3He/4He, depleted mantle and low-δ18O, recycled oceanic lithosphere in the source of central Iceland magmatism

“…CO 2 is the carrier phase for volatile He and so for any confining pressure (e.g. depth to a magma chamber) a waterrich melt will have lost a greater fraction of its intrinsic helium and is, therefore, more likely to record addition of extraneous (crustal) He [49,54]. [27]; c.f.…”

Geology, geochemistry and earthquake history of Lṑihi Seamount, Hawaìi's youngest volcano

“…This residue could then retain unradiogenic helium isotopic signatures (e.g., Anderson, 1998;Meibom et al, 2003 He ratios. This includes data from Kīlauea (Kurz, 1993), Mauna Loa (Kurz and Kammer, 1991;Kurz et al, 1995), Mauna Kea (Kurz et al, 1996;, Haleakalā (Kurz et al, 1987) and Kaua`i (Mukhopadyay et al, 2003). (Fig.…”