Isotopic fractionation in Hawaiian volcanic gases

Abstract

A complete mass spectrometer has been constructed around a mass spectrometer tube originally designed by Nier. Modifications of this single collector device have permitted its use as a medium precision instrument in the determination of isotope abundances in carbon dioxide. In addition, a previous gas chromatograph design has been modified to allow a one step separation and collection of carbon dioxide from a volcanic gas matrix, thus eliminating the usual and tedious volumetric method. To qualitatively predict the magnitude and direction of the isotopic fractionation in volcanic gases at higher temperatures (600°K-1500°K), equilibrium constants were calculated for numerous reactions involving carbon or oxygen isotope exchanges, such as 1/2C16O2 + H2180 =1/2C18O2 + H2160. The validity of these calculations was discussed on a basis of high temperature anharmonicity, and the resultant effect on the partition function ratios was estimated. Samples from a Hawaiian volcanic system were collected and analyzed for carbon and oxygen isotope abundances, in the form of carbon dioxide. Samples were taken from several areas, including fumarolic gases from Sulfur Bank in Kilauea Iki Crater, on Hawaii Island, gases from the drill holes in the lava lake of recently erupted Makaopuhi Crater, also on Hawaii, and assorted carbonaceous samples of marine origin. The results of this work have illustrated the need for (1) a further investigation, on a more detailed and precise basis, of the fundamental properties of isotopic species at elevated temperatures, and (2) a further analysis of the Hawaiian volcanic system incorporating more precise isotope measurements.