Carbon and nitrogen systematics in nitrogen-rich, ultradeep diamonds from Sao Luiz, Brazil

Abstract

Three diamonds from Sao Luiz, Brazil carrying nano- and micro-inclusions of molecular δ-N2 that exsolved at the base of the transition zone were studied for their C and N isotopic composition and the concentration of N utilizing SIMS. The diamonds are individually uniform in their C isotopic composition and most spot analyses yield δ13C values of −3.2 ± 0.1‰ (ON-SLZ-390) and − 4.7 ± 0.1‰ (ON-SLZ-391 and 392). Only a few analyses deviate from these tight ranges and all fall within the main mantle range of −5 ± 3‰. Most of the N isotope analyses also have typical mantle δ15N values (−6.6 ± 0.4‰, −3.6 ± 0.5‰ and − 4.1 ± 0.6‰ for ON-SLZ-390, 391 and 392, respectively) and are associated with high N concentrations of 800–1250 atomic ppm. However, some N isotopic ratios, associated with low N concentrations (<400 ppm) and narrow zones with bright luminescence are distinctly above the average, reaching positive δ15N values. These sharp fluctuations cannot be attributed to fractionation. They may reflect arrival of new small pulses of melt or fluid that evolved under different conditions. Alternatively, they may result from fractionation between different growth directions, so that distinct δ15N values and N concentrations may form during diamond growth from a single melt/fluid. Other more continuous variations, in the core of ON-SLZ-390 or the rim of ON-SLZ-392 may be the result of Rayleigh fractionation or mixing.