Infrared Spectroscopic Measurements of COsub2/sub and Hsub2/subO in Juan de Fuca Ridge Basaltic Glasses

Dissolved H ₂ O 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 ₂ O 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 ).