Depth-related carbon isotope and nitrogen concentration variability in the mantle below the Orapa kimberlite, Botswana, Africa

Abstract

Cubic diamonds from the Orapa kimberlite have a very restricted δ 13 C range with a mean of −5.98 ± 0.57‰, a relatively high and constant nitrogen content 897 ± 171 ppm and low nitrogen aggregation state. They are thought to have been derived from shallow mantle depth. Diamonds with peridotitic inclusions (P-Type) range in δ 13 C between−4.2 and − 18.9‰ with a major mode between −5 and−7‰. Their nitrogen content is low and state of aggregation high. Pressure-temperature ( P- T) equilibration conditions estimated for their inclusions are close to the wet peridotite solidus. Eclogitic inclusions in Orapa diamonds (E-Type) indicate higher P- T equilibration conditions than P-Type minerals. The δ 13 C values of their hosts vary from −2.6 to−18.0%. with a major mode between −13 and −15‰. The nitrogen content of E-type diamonds is significantly higher than that of P-Type diamonds. Comparison of the mineral and carbon isotopic composition of Orapa diamond eclogites with those of E-Type diamonds indicates that only part of the E-Type diamonds could have been derived from a physical breakup of diamond eclogites. 13C-depleted E-Type diamonds and eclogite xenoliths with low δ 13 C diamonds were formed in a similar and restricted P- T range. An inclusion mineral paragenesis with compositions transitional between those of peridotitic and eclogitic minerals (websteritic, W-Type) has been recognized. It is characterized by a unique combination of a relatively high chrome and Fe content. The δ 13 C values of W-Type diamonds lie, with one exception (−6.9%.), in the range of −15.2 to −22.4‰. The nitrogen content of these diamonds is significantly lower than that of E-Type and is indistinguishable from that of P-Type diamonds. The P- T equilibration conditions estimated for them are similar to those of the peridotitic paragenesis. The collective data indicate a higher frequency of occurrence of 13C-depleted diamonds, a decrease in the N content of diamonds, and an increase in the N aggregation state with increasing mantle depth. The decline in N content is not monotonic however, because P-Type diamonds tend to have lower N contents than E-Type diamonds. The combined high chrome and Fe content of the websteritic diamond inclusions are very difficult to reconcile with a subduction origin of the low δ 13 C host and indicate that 13C-depleted C may be a primary mantle feature.