Determination of total CO2 in melt inclusions with shrinkage bubbles

Abstract

The compositions of volatiles in magmas can be estimated using melt inclusions encapsulated in phenocrysts. Shrinkage bubbles are occasionally observed in natural and rehomogenized melt inclusions. Because of the low solubility of CO2 in melts, CO2 is partitioned in both the melts and shrinkage bubbles in melt inclusions. We developed a method to determine total CO2 in melt inclusions by measuring (1) CO2 concentration in glass with secondary-ion mass spectrometry, (2) CO2 density in bubbles with micro-Raman spectrometry, and (3) bubble/glass volume ratio with micro X-ray computed tomography on rehomogenized melt inclusions. We applied this method to olivine-hosted melt inclusions in high-μ (HIMU)-type ocean island basalts from Raivavae in Austral Islands in French Polynesia.The shape of melt inclusions is predominantly related to their size; larger melt inclusions tend to be more angular than smaller ones. The volume of large melt inclusions may be overestimated by previous approaches in which the dimension of melt inclusions was measured by microscopy and the volume was calculated assuming ellipsoidal shape. CO2 dissolved in glass in rehomogenized melt inclusions ranges up to ~7000 ppm. The glass with a more alkalic composition shows greater CO2 concentration than that with a less alkalic composition in accordance with the composition-dependent CO2 solubility. By contrast, the correlation between the total CO2 content and the chemical composition of the glass is unclear because CO2 in shrinkage bubbles is generally greater than that in the glass. Some melt inclusions demonstrate an extremely high total CO2 content (up to 25,000 ppm). Because these melt inclusions show a large bubble volume proportion in melt inclusions (>12 vol%), they are likely formed by the simultaneous entrapment of melts and fluids during the host olivine growth. Excluding these melt inclusions, the total CO2 content and CO2/Nb ratio in melt inclusions were up to 7000 ppm and 200, respectively. This study offers an accurate determination of CO2 concentration and CO2/Nb of the HIMU basalts from Raivavae.