Sources, degassing, and contamination of CO 2, H 2O, He, Ne, and Ar in basaltic glasses from Kolbeinsey Ridge, North Atlantic

Abstract

New volatile data (CO 2, H 2O, He, Ne, and Ar) are presented for 24 submarine basaltic glasses from the Kolbeinsey Ridge, Tjörnes Fracture Zone and Mohns Ridge, North Atlantic. Low CO 2 and He contents indicate that magmas were strongly outgassed with the extent of degassing increasing toward the south, as expected from shallower ridge depths. Ne and Ar are significantly more abundant in the southernmost glasses than predicted for degassed melt. The strong atmospheric isotopic signal associated with this excess Ne and Ar suggests syn- or posteruptive contamination by air. Degassing, by itself, cannot generate the large variations in δ 13C values of dissolved CO 2 or coupled CO 2-Ar variations. This suggests that δ 13C values were also affected by some other processes, most probably melt-crust interaction. Modelling indicates that degassing had a negligible influence on water owing to its higher solubility in basaltic melt than the other volatiles. Low H 2O contents in the glasses reflect melting of a mantle source that is not water-rich relative to the source of N-MORB. Before eruption, Kolbeinsey Ridge melts contained ∼400 ppm CO 2 with δ 13C of −6‰, 0.1 to 0.35 wt.% H 2O, 3He/ 4He ∼11 R A, and CO 2/ 3He of ∼2 × 10 9. We model restored volatile characteristics and find homogeneous compositions in the source of Kolbeinsey Ridge magmas. Relative to the MORB-source, He and Ne are mildly fractionated while the 40Ar/ 36Ar may be low. The 3He/ 4He ratios in Tjörnes Fracture Zone glasses are slightly higher (13.6 R A) than on Kolbeinsey Ridge, suggesting a greater contribution of Icelandic mantle from the south, but the lack of 3He/ 4He variation along the Kolbeinsey Ridge is inconsistent with active dispersal of Icelandic mantle beyond the Tjörnes Fracture Zone.