Peridotite xenoliths from the Jagersfontein kimberlite pipe: I. Primary and primary-metasomatic mineralogy

Abstract

The geochemistry and textures of peridotite xenoliths from the Jagersfontein kimberlite pipe are reported. The xenoliths have a primary mineralogy of olivine ± orthopyroxene ± clinopyroxene± garnet ± spinel. They are subdivided into coarse and deformed xenoliths corresponding to high- and low-temperature estimates, respectively. Coarse-grained xenoliths are further subdivided into low- and medium-temperature groups. Mineral chemistry of these two groups is distinct, e.g., clinopyroxene 100 Al/(Al + Cr) 24 to 60 and 60 to 70 in the medium- and low-temperature groups, respectively. Low-temperature xenoliths have undergone exsolution of pyroxene, spinel, and garnet in their pyroxenes. Primary modal metasomatism has occurred in the coarse xenoliths with the replacement of orthopyroxene by edenitic amphibole in the low-temperature xenoliths and of clinopyroxene by low-Ti phlogopite in the medium-temperature xenoliths. The amphibole stability limit confines it to shallower depths. Metasomatised xenoliths have been enriched in K, Na, Al, and Ca, and trace incompatible elements. Metasomatism is considered to have occurred at around 1 Ga by the infiltration of, and reaction with, ascending H 2O-rich fluids with Sr and Nd isotopic characteristics similar to group II kimberlites. The widespread chemical equilibrium seen in metasomatised xenoliths suggests that the particularly distinctive features of the low-temperature Jagersfontein xenoliths, namely exsolution and very low equilibration temperatures, may also be a result of the primary metasomatism.