DEPTH-RELATED CARBON-ISOTOPE AND NITROGEN CONCENTRATION VARIABILITY IN THE MANTLE BELOW THE ORAPA KIMBERLITE, BOTSWANA, AFRICA

Cubic diamonds from the Orapa kimberlite have a very restricted deltaC-13 range with a mean of -5.98 +/- 0.57 parts per thousand, 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 deltaC-13 between -4.2 and -18.9 parts per thousand with a major mode between -5 and -7 parts per thousand. 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 deltaC-13 values of their hosts vary from -2.6 to -18.0 parts per thousand with a major mode between -13 and -15 parts per thousand. 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. C-13-depleted E-Type diamonds and eclogite xenoliths with low deltaC-13 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 deltaC-13 values of W-Type diamonds lie, with one exception (-6.9 parts per thousand), in the range of -15.2 to -22.4 parts per thousand. 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 C-13-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 deltaC-13 host and indicate that C-13-depleted C may be a primary mantle feature.