Light noble gases in basalt glasses from Mariana Trough

Abstract

We have measured elemental and isotopic compositions of light noble gases (He, Ne and Ar) of eight glasses in submarine pillow basalts dredged from the northern Mariana Trough. The 3He 4He ratios are quite uniform with the value of (9.4–10,3) × 10 −6 despite large variations in 4He contents. The 4He 20Ne ratios vary significantly from 4.6 to 2.1 × 10 5 and the variation is well explained by mixing between mantle-derived and atmospheric noble gases. Based on the 1/ 20Ne- 4He/ 20Ne diagram, the 4He content in pristine glasses was estimated to be 9.2 × 10 −6 ccSTP/g. A similar correlation is found between the 36Ar content and the 40Ar 36Ar ratio, which leads to the estimation of the 40Ar content. The 4He 40Ar ratio is comparable to that in the vesiculated basalt and considerably higher than the radiogenic production ratio in the primary magma. The observed 3He 4He ratios are somewhat lower than that of average MORB and are higher than those of volcanic gases in subduction zones, suggesting the isotopic signature of He in the back-arc basin is distinct from mid-ocean ridges and island arcs.