87Sr/86Sr in kimberlitic carbonates by ion microprobe: Hydrothermal alteration, crustal contamination and relation to carbonatite
✍ Scribed by R. A. Exley; A. P. Jones
- Book ID
- 104745351
- Publisher
- Springer
- Year
- 1983
- Tongue
- English
- Weight
- 562 KB
- Volume
- 83
- Category
- Article
- ISSN
- 0010-7999
No coin nor oath required. For personal study only.
✦ Synopsis
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 S7Sr/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% Bad) and S7Sr/S6Sr ratios in the range 0.7046 to 0.7056. Secondary sparry calcite in amygdales and veins is characterised by low Ba (< 0.05% Bad) and STSr/S6Sr 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 S7Sr/S6Sr 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 S7Sr/S6Sr demonstrate shared chemical characteristics between these carbonates and carbonatite. The primary kimberlite magma had initial S7Sr/S6Sr close to 0.7046.