Nitrogen isotopic composition of the MORB mantle: a reevaluation

Abstract

Nitrogen in the mantle source of mid oceanic ridge basalts (MORBs) is commonly believed to have δ¹⁵N~−5‰. This is based on a (selective) statistical average drawn from nitrogen isotopic data obtained simultaneously with argon from MORBs. However, on critical evaluation, using the accompanying ⁴⁰Ar/³⁶Ar data, we show this approach to be somewhat ambiguous. Considering the fact that MORBs can have a number of geochemical components in them, as has been shown by several recent studies, the approach of statistical averaging may actually point to a mixture rather than the true mantle end-member. A more reasonable approach, also used for deriving the mantle noble gas signatures, suggests nitrogen in the MORB mantle to be much lighter (~−15‰) in its isotopic composition. Such a signature is consistent with nitrogen data from MORBs as well as from other mantle derived samples that are believed to have sampled volatiles from the MORB mantle.