Infrared spectroscopic measurements of CO 2 and H 2O in Juan de Fuca Ridge basaltic glasses

Abstract

Dissolved H 2O and CO 2 contents in basaltic glasses from the Juan de Fuca Ridge and neighboring seamounts were determined by infrared spectroscopy. CO 2 contents range from about 45 to 360 ppm by weight, with carbonate ion complexes the only detectable form of dissolved carbon. Samples erupted at a given depth exhibit a large range in dissolved CO 2 contents that we interpret to be the result of variable amounts of degassing. The lowest CO 2 contents at each depth are in reasonable agreement with the experimentally determined CO 2 solubility curve for basalt at low pressures. All glasses with CO 2 values higher than the experimentally determined solubility at the eruption depth are oversaturated because of incomplete degassing. The highest CO 2 contents are spatially associated with the local topographic highs for each ridge segment. Lavas from relatively deep areas may have had greater opportunity to degas during ascent from a magma chamber or during lateral flow in dikes or seafloor lava flows. The highest observed CO 2 concentrations are from the axial seamount and lead to an estimate of a minimum depth to the magma chamber of 2.7 km beneath the ridge axis. H 2O contents vary from 0.07 to 0.48 wt.%, with hydroxyl groups the only detectable form of dissolved water. Water contents correlate positively with FeO */MgO and the highest water contents are found in the incompatible element-enriched Endeavour segment lavas. Variations in ratios of water to other incompatible elements suggest that water has a bulk partition coefficient similar to La during partial melting ( D ∼ 0.01).