FROM THE BOTTOM TO THE TOP: PRELIMINARY REPORT ON THE ORIGIN AND CONDITIONS OF KIMBERLITE ASCENT AND EMPLACEMENT—FOCUS ON DIAMOND SURVIVABILITY (ELLIOTT COUNTY KIMBERLITE, KY)

The occurrence and preservation of diamond in kimberlite is believed to be the result of the origin of the magma and conditions during ascent. In Elliot County two Cretaceous, off-craton diamond-barren kimberlites intrude the Paleozoic strata of the Appalachian Basin. The westernmost pipe (Ison Creek) contains abundant mantle xenoliths, crustal xenoliths, and megacrysts (discrete single phase nodules of uncertain parentage). Geochemical analyses of clinopyroxene grains within nine separate samples suggest that mantle xenoliths (lherzolite and garnet lherzolite) originated within the diamond stability field in the upper mantle. Maximum equilibration estimates of clinopyroxene are at 1,302º C and 65 kbar. Electron microprobe analyses of discrete garnet nodules within the kimberlite matrix indicate at least one occurrence of low Ca harzburgite-suite garnet (“G10” of Gurney, 1984) disaggregated from its host rock. The presence of G10 garnet is significant because this is the most dominant garnet type found as inclusions in diamonds. Final emplacement conditions of the kimberlite may also have significant implications for diamond survival. Past work has shown that the last event recorded in the Elliott County kimberlite is the equilibration of orthopyroxene megacrysts at 970-1,240°C at 47-50 kb (Garrison and Taylor, 1980). The presence of coked organic material observed in a preliminary investigation of kerogen concentrates derived from organic-rich crustal xenoliths (coal and black shale) indicates a significant level of maturity beyond ambient levels of the coals and black shales of the Appalachian Basin. Measurement of the level of maturity of the organics may be utilized as a paleothermometer to assess late stage ascent/final emplacement temperatures. The presence of Ti-bearing hydrogrossular in shale clasts also suggests final emplacement conditions exceeded ambient basinal temperatures.