87Sr/86Sr in kimberlitic carbonates by ion microprobe: Hydrothermal alteration,crustal contamination and relation to carbonatite |
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Authors: | R. A. Exley A. P. Jones |
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Affiliation: | 1. Department of the Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, 60637, Chicago, Illinois, USA
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Abstract: | Carbonates in a 30 cm wide zoned kimberlite dyke from the De Beers Mine, Kimberley, S. Africa were studied by cathodoluminescence and electron microprobe techniques and their 87Sr/86Sr ratios were measured using an AEI-IM20 ion microprobe. Primary carbonates (including calcite dendrites, rhombohedral calcites in segregation vesicles and mosaic dolomite) have high Sr (0.69–1.35 wt.% SrO) and Ba (0.24–0.44% BaO) and 87Sr/86Sr ratios in the range 0.7046 to 0.7056. Secondary sparry calcite in amygdales and veins is characterised by low Ba (<0.05% BaO) and 87Sr/86Sr near 0.72. Rhombohedral calcite 0.5 cm from a contact with 2,900 my. old biotite-gneiss has minor element chemistry like that of primary carbonate, but an elevated 87Sr/86Sr ratio of 0.7103, possibly indicating crustal contamination in a boundary layer of the kimberlite magma. Amygdale-like segregations of carbonate and/or serpentine originated as gas-cavities and were not formed by liquid immiscibility. They are now filled either by secondary calcite or by minerals precipitated from residual kimberlite liquid. However, dendritic calcite and primary dolomite and calcite with high Sr, Ba and low 87Sr/86Sr demonstrate shared chemical characteristics between these carbonates and carbonatite. The primary kimberlite magma had initial 87Sr/86Sr close to 0.7046. |
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