The recognition of meteoric-hydrothermal and supergene processes in volcanic carbonatites, Nyanza Rift, western Kenya, using carbon and oxygen isotopes
✍ Scribed by I.O Onuonga; A.E Fallick; P Bowden
- Publisher
- Elsevier Science
- Year
- 1997
- Tongue
- English
- Weight
- 1007 KB
- Volume
- 25
- Category
- Article
- ISSN
- 1464-343X
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✦ Synopsis
Carbon and O isotopic compositions were determined for calcite and siderite from the Buru and Kuge carbonatite centres. Wide ranges in the isotopic compositions of these minerals were observed with values of ~zc and ~sO for the Buru calcites ranging from + 1.3 to -3.2%o (PDB) and + 11.3 to + 26.2%o (SMOW), respectively. The 8~3C and ~80 ranges for the Kuge calcites are -3.1 to -8.4%0 (PDB) and + 18.1 to + 25.7%o (SMOW). The Buru siderites plot in a narrow and restricted range at -3.1 to -4.4%o (PDB) and +12.6 to +16.1%o (SMOW). Calcite and siderite from the Buru and Kuge volcanic carbonatite centres do not retain their isotope signatures expected for primary magmatic carbonatites. Most of the variations in C-O isotope composition can be attributed to secondary processes involving low-temperature hydrothermal alteration and isotope exchange between carbonatites and meteoric water. Higher ~leO values (+21.9 to +26.20/o0) with a significant increase in 8~3C values (-1.5 to + 1.3%o) shown by the most oxidised samples from the Buru carbonatite may indicate the involvement of supergene exchange with atmospheric CO 2 at surface temperatures.