Capture of Helium and other volatiles during the growth of Olivine phenocrysts in Picritic basalts from the Juan Fernandez Islands

Abstract

Olivine crystals in basalt contain helium that can be extracted for isotopic analysis. Helium-bearing olivine phenocrysts in picritic tholeiites from the Juan Fernandez Islands, SE Pacific, crystallized from moderately differentiated liquids. None are xenocrysts of mantle peridotite. The helium occurs in cavities or bubbles in inclusions best seen at high magnification in the olivine. The olivine grew around spinel, sulfide, and bubbles that preferentially nucleated on crystal surfaces. Inclusions within these phases range from large cavities to tiny, faceted pits arranged in rows. Many crystals contain curving trains of inclusions along annealed features. The inclusions formed during mixing between cooler differentiated magma and hotter olivine-charged magma, which accelerated vesiculation. Bubbles nucleated on the olivine as they do on ice when stirred in carbonated water. Mixing also induced thermal stress fracturing, like the cracking of ice dropped into water. Cracking, irregular extinction, and subgrain formation occurred when faceted crystals collided with each other or with conduit walls. Boundary layer melts and bubbles were drawn quickly into the fractures. Thus few inclusions contain equilibrium proportions of minerals and vapor. Mantle-derived helium clearly permeated into shallow storage reservoirs, including rift zones where magmatic differentiation, mixing, and vapor exsolution were fairly extensive.