Nitrogen isotopic compositions and concentrations in MARID xenoliths

• We studied nitrogen isotope ratios of mantle phlogopites.
• Nitrogen in the upper mantle is present as ammonium.
• Sedimentary organic nitrogen is present near lithosphere–asthenosphere boundary.
• Sedimentary nitrogen in the upper mantle is not effectively cycled to the surface.

Nitrogen isotopic compositions and concentrations in micas from kimberlite-hosted MARID (mica-amphibole-rutile-ilmenite-diopside) suite of xenoliths from the Kimberley area of the Kaapvaal Craton, South Africa, were measured using continuous-flow isotope-ratio mass spectrometry and Fourier transform infrared spectroscopy. Fourier transform infrared spectroscopy data suggest that the nitrogen in MARID phlogopite is from ammonium in the mica structure. The replacement of potassium by ammonium is the major mechanism for the storage of nitrogen within these phlogopites. The δ15N values of micas from MARID xenoliths range from − 11 to + 9‰, and nitrogen contents range between 113 and 272 ppm. The 15N enrichment of most samples relative to the average upper mantle δ15N value of − 5‰ suggests that the nitrogen reflects a recycled reservoir from subducted material within their source region. The presence of nitrogen from a recycled reservoir indicates that sedimentary organic nitrogen in the sub-continental lithosphere is not effectively cycled to the surface resulting in a geochemically heterogeneous upper mantle.