Noble gas and nitrogen isotopic components in Oceanic Island Basalts

Abstract

We present helium, argon, xenon and nitrogen isotopic data from Oceanic Island Basalts (OIBs), obtained from stepped heating and vacuum crushing experiments. Signatures of 3He/ 4He in the present samples, except for those from the Canary Islands, are significantly higher (up to 20 × the air ratio) than the MORB signature (8.5 × air ratio). The argon ( 40Ar/ 36Ar) and xenon ( 129Xe/ 132Xe and 136Xe/ 132Xe) isotopic data are consistent with a two-component mixture of gases from the mantle (either MORB or OIB source) and air contamination. In the stepwise heating data, δ 15N varies from − 20‰ to + 15‰ in the different heating steps. The vacuum crushing data show a narrower range (− 5‰ to + 8‰) of δ 15N, which, except for one Réunion sample, is somewhat heavier than the totals from the stepped heating experiments. Simultaneous argon and nitrogen isotopic systematics of the present data suggest contributions from mantle (somewhat different from MORB), recycled materials and air contamination. This is also consistent with the 36Ar/ 132Xe elemental ratio. The present stepped heating data are consistent with a nitrogen isotopic component with δ 15N ~ − 25‰ in the OIB mantle. In line with the oxygen isotopic composition of the bulk Earth, this would be consistent with a contribution from enstatite chondrite-like volatiles to the planet's accretion.