Formation of unusual yellow Orapa diamonds

Abstract

Twenty eclogitic diamonds from Orapa Mine (Botswana) with an unusual yellow colour are characterised for their growth structure, N systematics, and C isotope composition, and the major element composition of their silicate inclusions. The diamonds show complex luminescence with green, blue and non-luminescent zones and occasional sector zonation. All parts of the diamonds have low total N concentrations (<50 at.ppm, with one exception of <125 at.ppm) and a limited range in C isotope composition (−5.7 to −10.6‰). Fourier Transform Infrared spectra show bands at 1334, 1332, 1282, and 1240 cm−1 typical for Ib-IaA diamonds. Relict unaggregated N defects (Nso and Ns+) are present and the preservation is likely caused by the low N concentrations and possible low mantle residence temperatures rather than young diamond formation (inclusion ages of 140, 1096, 1699 Ma; Timmerman et al. Earth Planet Sc Lett 463:178–188, 2017). Garnet and clinopyroxene inclusions extracted from 14 diamonds have an eclogitic composition with relatively low Ca contents and based on all characteristics, these diamonds form a distinct population from Orapa.