THE NITROGEN RECORD FOR CRUST-MANTLE INTERACTION AND MANTLE CONVECTION FROM ARCHEAN TO PRESENT

Abstract

Mantle fluids sampled by mid-ocean ridge basalts and by diamonds are depleted in the heavy isotope of nitrogen, 15N, by about 3–5 parts per mil relative to atmosphere, suggesting a common nitrogen end-member since at least the Archean. In contrast, deep mantle material sampled by mantle plumes (Kola Devonian magmatic province, Iceland, Loihi Seamount, Hawaii, Society Islands) is enriched in 15N by up to 8 parts per mil, as are post-Archean sediments. Several independent lines of evidence strongly suggest that mantle nitrogen is mostly recycled. Notably, the ratio between nitrogen and an incompatible lithophile element like potassium is nearly constant between the surface of the Earth and the different mantle reservoirs, whereas the ratio between nitrogen and a noble gas like 36Ar varies over two orders of magnitude between these reservoirs. We propose that the large-scale N isotope heterogeneity of the mantle results from secular variation of the nitrogen isotope composition of recycled sediments, which is governed by specific metabolic paths having changed with the advent of oxygenated ocean. If this is the case, the contrast between mantle reservoirs reflects a profound change in the mantle convection regime through time. In the Archean, recycled material was stored in a mantle domain sampled by diamonds and by mid-ocean ridges. Starting from the Proterozoic, slabs reached deep, volatile-rich regions of the mantle which are now sampled by mantle plumes.