Lake Ellen kimberlite, Michigan, U.S.A.

The recently discovered Lake Ellen kimberlite, in northern Michigan, indicates that bedrock sources of diamonds found in glacial deposits in the Great Lakes area could lie within the northern U.S. Magnetic surveys show a main kimberlite 200 m in diameter and an adjacent body 25 x 90 m(?). The kimberlite cuts Proterozoic volcanic rocks that overlie Archean basement, but is post-Ordovician in age based on abundant Ordovician(?) dolomite inclusions. Xenocrysts and megacrysts are ilmenite (abundant, 12.5-19% MgO), pyropealmandine and Cr-pyrope (up to 9.3% Cr2O3), Cr-diopside (up to 4.5% Cr2O3), olivine (Fo 91), enstatite and phlogopite. The kimberlite contains fragments of crustal schist and granulite, as well as disaggregated crystals and rare xenoliths of eclogites, garnet pyroxenites and garnet peridotites from a heterogeneous upper mantle. Eclogites, up to 3 cm size, show granoblastic equant or tabular textures and consist of jadeitic cpx (up to 8.4% Na20, 15.3% Al2O3), pyrope-almandine, ? rutile ? kyanite ? sanidine ? sulfide. Garnet pyroxenite contains pyrope--(0.44% Cr2O3) + cpx (0.85% Na2O, 0.53% Cr2O3) + Mg-Al spinel. Mineral compositions of rare composite xenocrysts of garnet + cpx are distinctively peridotitic, pyroxenitic or eclogitic. Calculated temperatures of equilibration are 920-1060 ?C for the eclogites and 820-910?C for the garnet pyroxenite using the Ellis-Green method. Five peridotite garnet-clinopyroxene composite xenocrysts have calculated temperatures of 980-1120?C using the Lindsley-Dixon 20 kb solvus. Spinel pyroxenite and clinopyroxene-orthopyroxene composites have lower calculated temperatures of 735?C and 820-900?C, respectively. Kyanite-bearing eclogites must have formed at pressures greater than 18-20 kb. Using the present shield geotherm with a heat flow value of 44mW/m 2 for the time of kimberlite emplacement, the eclogite temperatures imply pressures of 35-48 kb (105-140 km) and the garnet pyroxenite temperatures indicate pressures of 24-29 kb (75-90 km). Temperatures of two peridotitic garnet-cpx composite xenocrysts if on a shield geotherm, imply pressures within the diamond stability field.