Helium distributions in ocean island basalt olivines revealed by X-ray computed tomography and single-grain crushing experiments

Abstract

X-ray computed tomography of individual olivine crystals in basalts from Ofu and Olosega islands, American Samoa, reveals that a small fraction of the olivines contain the vast majority of the fluid inclusions. Single-grain crushing experiments demonstrate that He and CO_2 reside primarily in these inclusions. Low CO_2 pressures in most grains, corresponding to depths of less than 1 km, provide evidence of ubiquitous decrepitation and associated pressure reduction in the fluid inclusions. Even so, the olivines with the highest inclusion volumes yielded sufficient He to obtain precise He concentrations and isotopic compositions. Within analytical uncertainty, ^3He/^4He ratios are homogeneous among the olivines from each basalt, but among basalts, the ratios range from 21 to 35 Ra. The total range in C/^3He ratio within the analyzed olivines is from 3.6 × 10^7 to 1.5 × 10^(10), and varies by nearly an order of magnitude within the olivines from each basalt. We postulate that this wide range of C/^3He ratios is caused by grain-scale decoupling of C and ^3He due to extensive He diffusion out of fluid inclusions through the olivine lattice during magma ascent and cooling. If so, primary Ofu-Olosega magmas probably had C/^3He ratios less than 4 × 10^8, which is lower than previous estimates for hotspot magmas.

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