SURFACE NOBLE GAS RECYCLING TO THE TERRESTRIAL MANTLE

Abstract

In a recent paper, Ballentine and Barfod [Earth Planet Sci. Lett. 180 (2000) 39–48] emphasize the general existence of an atmospheric noble gas component closely resembling modern air in glass samples of Mid-Ocean Ridge Basalt (MORB) and Ocean Island Basalt (OIB). They show that this atmospheric component cannot be due to interaction of melt with seawater. They propose that air is added after sample recovery, perhaps in the laboratory, and trapped in microfractures that affect glass in response to cooling, where these microfractures should reseal after air trapping. In this paper, I show that this air component could also be of deep, mantle origin via recycling. This alternative concept explains virtually all of the puzzling observations and paradoxes related to the atmospheric component, and helps understanding noble gas systematics as generally seen in MORB and OIB samples. In particular, it explains why the air component is mainly borne by the largest vesicles. If this model is correct, popping rocks have in their vesicles a full volatile record of the melting process that affected the two-component or marble-cake upper mantle.