Nitrogen and noble gas isotopes in mafic and ultramafic inclusions in the alkali basalts from Kutch and Reunion - implications for their mantle sources

Abstract

Olivine phenocrysts and ultramafic xenoliths from the alkali basalts of Kutch and Reunion have been analyzed simultaneously for nitrogen and light noble gases. The samples have low amounts of neon (²²Ne=1.4-5.2×10^-10 ccSTPg^-1) and argon (³⁶Ar=0.7-12.6×10^-10 ccSTPg^-1). The variations in ²⁰Ne/²²Ne and ²¹Ne/²²Ne ratios indicate the presence of two trapped components: air and mantle (OIB and MORB), consistent with the range of ⁴⁰Ar/³⁶Ar ratios observed (2166-9268 in the xenoliths, ~550 in the olivine phenocryst from Kutch, and 350 in the Reunion sample). The noble gas data indicate an OIB type mantle source for the phenocrysts from Kutch and Reunion, but a MORB type mantle source for the Kutch xenoliths, and are consistent with the plume origin of Deccan. Nitrogen concentrations in these samples vary from 1 to 2.5 ppm. Its isotopic composition, which varies from -15 to +15% in the stepped temperature data, is consistent with the presence of at least two components mantle (δ¹⁵N~-15%) and recycled nitrogen (δ¹⁵N~+15%), often in the same sample. The simultaneous presence of the mantle and recycled components in these samples, which is also consistent with the accompanying argon isotopic data, requires a millimeter-scale heterogeneity that can be explained by the presence of recycled materials in the form of a global enriched region at the lithosphere/asthenosphere boundary beneath Kutch and Reunion.