Volatiles in submarine volcanic rocks from the spreading axis of the East Scotia Sea back-arc basin

Abstract

The volatile content of glassy pillow rims from East Scotia Sea back-arc basin (BAB) lavas are unlike those of mid-ocean ridge (MOR) pillow-rim glasses, although non-volatile compositions of the two rock groups overlap. The East Scotia Sea samples have three to ten times greater water contents and nearly twice the average CO 2 and Cl contents of MOR samples; F contents are similar. S contents are only one-third those from MOR samples. H 2O and CO 2 contents of glassy pillow rims from Mariana island arc andesites are similar to those in the BAB lavas studied. Nevertheless, volatiles in the East Scotia Sea BAB magmas are probably not directly derived from the subducted slab, because there is no seismic evidence that the slab extends within 200 km of the spreading axis of the East Scotia Sea. Available data do not preclude the possibility that the magmas were contaminated by seawater prior to eruption or that the mantle under the East Scotia Sea spreading center is volatile-rich. The volatiles may have been added to the mantle during an earlier period of subduction, perhaps during the initial formation of the East Scotia Sea basin.