GEOCHEMICAL CHARACTERISTICS OF MANTLE XENOLITHS FROM UDACHNAYA KIMBERLITE PIPE

Abstract

Abundances of trace elements (La, Ce, Nd, Sm, Eu, Dy, Er, Yb, Ti, V, Cr, Sr, Y, and Zr) were determined using a Cameca IMS of ion microprobe in clinopyroxenes and garnets in peridotite xenoliths from Udachnaya kimberlite pipe. Twenty six samples (4 spinel peridolites and 22 garnet peridotites) were analyzed. Unlike the Kaapvaal suite, the Udachnaya suite is complex in correspondence between mineral equilibrium temperatures and chemistry, containing purple garnet-bearing depleted peridotites with equilibration temperatures as high as 1287°C. Although the results display broad geochemical similarities between the Siberian and Kaapvaal lithospheric peridotites, the Udachnaya suite possesses important distinctions with which the observed trace element variations can be placed in a geologic context. Salient features include: (1) the Udachnaya suite has much greater concentration ranges than the Kaapvaal suite, and the low concentration end of the spectrum is represented by extremely depleted garnet harzburgites that have never been reported from Kaapvaal. Some garnets may have had orthopyroxene precursors. (2) Different degrees of reaction between depleted garnet harzburgites and melt in both clinopyroxene and garnet. (3) Clinopyroxene appears to react with melt more rapidly than garnet, resulting in disequilibrium distributions of trace elements. (4) Red garnets are commonly zoned with Ti, Zr, Y, and M-HREE increasing rimward. One such example, UV246/89, has a core which resembles purple garnets with signatures of incipient reaction with melt, and a rim with typical high-temperature red garnet composition. A likely scenario for the observed trace element variations begins with depleted harzburgites, which then reacted with LREE-enriched melts and mineral chemistry and perhaps mineral modes were modified. Mineral phase disequilibria as well as zoned minerals were produced in the process. Based on the diffusion kinetics, this process must have occurred shortly before eruption of the host kimberlite. Based also on the Ti-Zr characteristics, the reactant melt may have had affinity to Group I kimberlite.