Temporal helium isotopic variations within Hawaiian volcanoes: Basalts from Mauna Loa and Haleakala

Abstract

Helium isotope ratios in basalts spanning the subaerial eruptive history of Mauna Loa and Haleakala vary systematically with eruption age. In both volcanoes, olivine mineral separates from the oldest samples have the highest 3He/4He ratios. The Haleakala samples studied range in age from roughly one million years to historic time, while the Mauna Loa samples are radiocarbon dated flows younger than 30.000 years old. The Honomanu tholeiites are the oldest samples from Haleakala and have 3He/4 ratios that range from 13 to 16.8× atmospheric, while the younger Kula and Hana series alkali basalts all have 3He/4 close to 8×atmospheric. A similar range is observed on Mauna Loa; the oldest samples (roughly 30,000 years) have 3He/4 ratios of 15 to 20 × atmospheric, with a relatively smooth decrease to 8 × atmospheric with decreasing age. The consistent trend of decreasing 3He/4He ratio with time in both volcanoes, coherence between the helium and Sr and Nd isotopes (for Haleakala), and the similarity of 3He/4 in the late stage basalts to depleted mid-ocean ridge basalt (MORB) helium, argue against the decrease being the result of radiogenic ingrowth of 4He. The data strongly suggest an undegassed (i.e., high 3He/(Th + U)) mantle source for the early shield building stages of Hawaiian volcanism. and are consistent with the hotspot/mantle plume model. The data are difficult to reconcile with models for Hawaiian volcanism that require recycled oceanic crust or derivation from a MORB-related upper mantle source. We interpret the decrease in 3He/4 with volcano evolution to result from an increasing involvement of depleted mantle and/or lithosphere during the late stages of Hawaiian volcanism.