Volatiles in basalts and andesites from the Galapagos Spreading Center, 85° to 86°W

Abstract

Glasses from submarine lavas recovered by the ALVIN submersible from the Galapagos Spreading Center (GSC) near 86°W have been analyzed by electron microprobe for major elements and by high-temperature mass spectrometry for volatiles. The samples studied range in composition from basalt to andesite and are more evolved than typical MORBs. Previous studies indicate that they are related to normal MORB by extensive crystal fractionation in small, isolated magma chambers. The H 2O, Cl and F contents of these lavas are substantially higher than any previously reported for MORBs. H 2O, Cl and F abundances increase linearly with P 2O 5 content, which is used as an indicator of the extent of crystal fractionation. The Fe 2O 3 (FeO + Fe 2O 3) ratios measured in the andesite glasses progressively decrease with increasing P 2O 5 content and are probably related to fractionation of Fe-Tioxides. Reduced carbon gas species, principally CH 4 and CO, were discovered in these glasses. The presence of reduced carbon species in GSC glasses may be indicative of a more reduced oxidation state of the upper mantle than is commonly assumed.