A mass spectrometric investigation of the volatile content of deep submarine basalts

Abstract

High temperature Knudsen-cell quadrupole mass spectrometry has been used to identify and determine amounts of gases contained in deep submarine basalt glassy rims and glass-vapor inclusions found in associated phenocrysts. Samples analyzed were from three different geologic settings: ocean ridges; Kilauea East Rift Zone, Hawaii; and near Bouvet Island, South Atlantic Basin. Glass-vapor inclusions from ocean ridge and Hawaiian samples released CO2 and SO2 gas above 1020°C, but showed no H2O release. A maximum value of 0.002 wt-% water has been calculated for these inclusions, using the detection limit of the mass spectrometer. The corresponding glass surrounding the phenocrysts averages 0.20 wt-% water in the ocean ridge and 0.62 wt-% water in the Hawaiian samples. Plagioclase xenocrysts from the Bouvet Island area contain two types of glass-vapor inclusions, one which releases H20, CO2 and S02 between 1020 0C and l250°C and another which releases only CO2 and S02 above l250°C. The glass from the associated pillow rims contains 0.68 wt-% water. The results from the ocean ridge and Hawaiian samples imply that the magma from which the phenocrysts grew was depleted in water and that water entered the magma sometime after phenocryst formation. The Bouvet Island plagioclase appears to have sampled magma at two stages, before and after the addition of water to the melt. Infrared studies were made on selected ocean ridge and Hawaiian samples in an effort to determine the mechanism of volatile retention in the glassy basalt rims. The concentrations of water and carbon dioxide in these samples were too low to allow a definite assignment of a particular . retention mechanism. Volatile release temperatures for glassy rim samples from ocean ridges and Hawaii are very consistent. Hawaiian samples release water from 650 to 950°C. Nearly all ocean ridge samples show a bimodal release of water with peak maxima at 700 and 900°C. These include samples from both Atlantic and Pacific ocean ridges. Grinding the samples to less than 64 microns alters the release patterns. The bimodal release from the ocean ridge samples "collapses" to a single peak which has a maximum at 600°C. Mass 28 peaks from N2 and CO appear at about 700 and 900°C, apparently due to chemisorption from the air during grinding. Water and carbon dioxide also adsorb on the sample, altering the volatile abundances. Extreme care is needed in sample handling in order for results to reflect original sample volatile concentrations and retention mechanisms.