Noble gas and carbon isotopes in Mariana Trough basalt glasses

Noble gas elemental and isotopic compositions have been measured as well as the abundance of C and its isotopic ratios in 11 glasses from submarine pillow basalts collected from the Mariana Trough. The 3He/4He ratios of 8.22 and 8.51 Ratm of samples dredged from the central Mariana Trough (not, vert, similar18°N) agree well with that of the Mid-Ocean Ridge Basalt (MORB) glasses (8.4±0.3 Ratm), whereas a mean ratio of 8.06±0.35 Ratm in samples from the northern Mariana Trough (not, vert, similar20°N) is slightly lower than those of MORB. One sample shows apparent excess of 20Ne and 21Ne relative to atmospheric Ne, suggesting incorporation of solar-type Ne in the magma source. There is a positive correlation between 3He/4He and 40Ar/36Ar ratios, which may be explained by mixing between MORB-type and atmospheric noble gases. Excess 129Xe is observed in the sample which also shows 20Ne and 21Ne excesses. Observed δ13C values of not, vert, similar20°N samples vary from −3.76‰ to −2.80‰, and appear higher than those of MORB, and the corresponding CO2/3He ratios are higher than those of MARA samples at not, vert, similar18°N, suggesting C contribution from the subducted slab.