C-O-H ratios of silicate melt inclusions in basalts from the Galapagos Spreading Center near 95°W: A laser decrepitation mass spectrometry study

Abstract

Volatile ratios (primarily of H 2O and CO 2) in individual silicate melt (glass) inclusions in minerals have been analyzed using laser volatilization and mass spectrometry. A Nd-glass laser was used to produce 50-micrometer diameter pits in silicate melt inclusions. Released volatiles were analyzed directly with a computer-controlled quadrupole mass spectrometer. The detection limits for CO 2 and H 2O were on the order of 3 × 10 −14 and 3 × 10 −13 moles, respectively. The reproducibility for CO 2 H 2O was better than ±9%. The total range of volatile ratios from vitreous silicate glass inclusions contained in a suite of Galapagos lavas were: 0.018 to 1.193 for CO 2 H 2O ; 0.002 to 0.758 for CO H 2O ; 0 to 0.454 for CH 4 H 2O ; and 0 to 0.432 for Ar H 2O . The mean CO 2 H 2O from the propagating rift (0.245 ± 0.068) silicate glass inclusions is significantly lower than that of the actively failing rift (0.641 ± 0.241); this difference probably reflects different degrees of degassing during contrasting magmatic histories for the two regions. Relatively undifferentiated failing rift magmas must have relatively short crustal residence times prior to eruption and, therefore, have not undergone significant degassing of CO 2, as would appear to be the case for the more highly fractionated propagating rift magmas. The laser-mass spectrometric system described herein has the ability to act as a point-source probing device that can differentiate between the various volatile sites in minerals and rocks (as well as synthetic materials) on a micrometer scale.