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The Lake Ellen Kimberlite, Michigan, U.S.A.

DOI: 10.1016/b978-0-444-42273-6.50018-3


Abstract 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 United States. Magnetic surveys show a main kimberlite 200 m in diameter and an adjacent body 25 × 90 m(?). Xenocrysts and megacrysts are ilmenite (abundant, 12.5-19% MgO), pyrope-almandine 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 inclusions as well as xenoliths from a heterogeneous upper mantle. Eclogites, as large as 3 cm, show granoblastic equant or tabular textures and consist of jadeitic cpx (up to 8.4% Na2O, 15.3% Al2O3), pyrope-almandine, ± rutile ± kyanite ± sanidine ± sulfide. Garnet pyroxenite contains pyrope (0.44% Cr2O3) + cpx (0.85% Na2O, 0.53% Cr2O3) + M9Al spinel. 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.

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