Cosmic-ray-produced neon and helium in the summit lavas of Maui

Abstract

A ³He component attributed to cosmic-ray-induced spallation reactions (³He_c) in silicate minerals has recently been identified in lava flows on the summit of Haleakala volcano on Maui^1–3. This³He_c is observed in the helium extracted by fusion of olivine and pyroxene phenocrysts after first removing almost all of the mantle-derived helium by vacuum-crushing, a procedure which releases the volatiles trapped in fluid inclusions within the phenocrysts. Cosmic-ray-produced ³He was found in the high-altitude (∼3,000 m) summit lavas, which are ∼500,000 years old, but was not observed in sub-surface samples which were overlain by younger basalts² or in a 200-yr-old lava flow at sea level on Maui^1,2. We report here the identification of cosmic-ray-produced ²¹Ne_c, in addition to ³He_c, in gases extracted by fusion of olivines and clinopyroxenes after vacuum-crushing. The observed (³He/²¹Ne)_c ratios and the ratio of ²¹Ne_c in olivine to that in clinopyroxene are consistent with an in situ origin of ³He_c and ²¹Ne_c by cosmic-ray spallation reactions. This component could be important for interpreting helium isotopic data in terrestrial reservoirs. Geophysical applications could include determinations of erosion rates and exposure histories of terrestrial rocks.