MAJOR ELEMENT AND RAMAN SPECTROSCOPY DATA ON GARNET XENOCRYSTS FROM A NEW KIMBERLITE LOCALITY IN TANZANIA

The compositions of mantle-derived garnet xenocrysts are commonly used in the exploration for diamond-bearing kimberlites. In particular, nearly 85% percent of the diamond-bearing kimberlites contain what are known of as G10 garnets, i.e. garnets that have relatively high Cr₂O₃ but low CaO contents and are believed to have been derived from a Ca-undersaturated or harzburgitic source. Here we report on the compositions of garnet xenocrysts from a newly discovered kimberlite locality in the Shinyanga district of Tanzania, where more than 350 kimberlite pipes have been discovered. The new locality is ca. 16 km NW of the Mwadui mine, one of the biggest diamond mines in the world. Twelve samples, each including an average of ~10 individual garnets, form the basis of this preliminary petrographic study. The garnets are reddish purple to reddish orange in color and range from 2 to 20 mm in size. Individual garnet crystals were first screened using a Raman microscope equipped with a 532 nm and 633 nm lasers to determine broad compositional variations and to look for mineral inclusions that could be used to estimate pressures of equilibration (e.g. quartz, apatite), but none were found. Based on these results, eleven garnets were selected for high-precision microprobe analysis (Si, Al, Fe, Mg, Mn, Ti, Na, K, Y, Cr, Ca, P) using a JEOL JXA-8230 Superprobe. The majority of analyzed garnets are pyrope in composition, with Cr₂O₃ ranging from 0.1 to 7.3 wt%; Mg # > 0.75; TiO₂ < 1wt%. They plot within the G9 compositional field, indicating derivation from a lherzolite source rock. Such garnet compositions are typical of kimberlite-hosted garnet xenocrysts worldwide. One garnet plots within the G10 field (i.e. “Ca-undersaturated” or “harzburgitic” source). Although it plots below the graphite – diamond boundary, it also has low MnO (< 0.36 wt%). Low-Mn G10 garnets have been observed in association with diamonds in other kimberlites, indicating the potential for this new locality to be diamond-bearing.