The isotopic composition of strontium in some South African kimberlites

Strontium isotopic studies of kimberlites reveal no significant differences between the respective whole-rock Sr⁸⁷/Sr⁸⁶ ratios of fissure and pipe kimberlites. Kimberlites from the Swartruggens fissure (calcareous micaceous kimberlite) have Sr⁸⁷/Sr⁸⁶ ratios of from 0.709 to 0.716, whilst those from the Wesselton pipe have Sr⁸⁷/Sr⁸⁶ ratios of from 0.708 to 0.715. Other kimberlites range from 0.706 to 0.715. Samples are considered to be late Cretaceous to early Tertiary and thus the ratios are approximately initial ratios. The Sr⁸⁷/Sr⁸⁶ ratios bear no relation to the Rb or Sr content of individual kimberlite bodies. The high initial ratios are not due to bulk assimilation of granitic material in either a kimberlite or carbonatitic magma. Rb-Sr data for garnet peridotites and eclogite xenoliths in kimberlite are not compatible with production of kimberlite by eclogite fractionation from a melt derived from garnet lherzolite. The Sr isotopic composition of kimberlite is compatible with partial melting of garnet mica peridotite. The isotopic composition of liquids formed by partial melting of this rock can be modified by (i) gross contamination with material of low Sr⁸⁷/Sr⁸⁶ ratio or (ii) selective diffusion of material of high Sr⁸⁷/Sr⁸⁶ ratio into kimberlitic fluids.