Origin of air-like noble gases in oceanic basalts

Abstract

Noble gas studies of oceanic basalts show the ubiquitous presence of an air-like component, which is usually attributed to a simple atmospheric contamination. However, simultaneous nitrogen and argon isotopic study shows the presence of heavy nitrogen with an isotopic composition (δ¹⁵N) ~ +18%, accompanied by ⁴⁰Ar/³⁶Ar values much lower than the mantle value. Such a component can be attributed to recycled materials from subduction zones, which are incorporated into these basaltic magmas during their ascent through a shallow enriched region in the mantle. This suggests that a part of the air-like noble gases in oceanic basalts is due to recycled atmospheric gases. Thus the air-like noble gases in the oceanic basalts, hitherto considered a hindrance, provide useful clues for understanding the geochemical evolution of these samples.