GEOLOGY OF THE ELLIOTT COUNTY KIMBERLITE, KENTUCKY

Several small exposures of kimberlite crop out in Elliott County, Kentucky. These kimberlites intrude into Pennsylvanian sandstones and have been dated at approximately 257-279 Ma (Early Permian). These kimberlites include two types of mantle garnets: a high-Cr, G9 type derived from lherzolite, and low-Cr, G4 type megacrysts. Ilmenite crystals are clearly kimberlitic, with ~13 wt% MgO and ~50 wt% TiO₂. Olivine phenocrysts are abundant and strongly forsteritic (~91 mol% Fo). Other minerals found in the kimberlite include orthopyroxene (~88 mol% En), perovskite (>1 wt% Ce₂O₃, ~0.5 wt% ZrO₂, ~0.8 wt% HfO₂), apatite, and phlogopite. Multivariate analysis of garnet compositions (I. Kryvoshlyk, personal communication) indicates that the diamond potential for these kimberlites is very low (0-6 cpht). This is consistent with geothermobarometric calculations, which suggest equilibration at relatively shallow depths (~26 kbar) and high temperatures (>1100°C), well within the field of graphite stability.