Diagenetic stability of the isotopic composition of phosphate-oxygen: palaeoenvironmental implications

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Abstract
Meteoric diagenesis of the Lower Pliocene Varswater Formation phosphate deposit, Cape Province, South Africa, has altered the chemistry of the francolite component. The isotopic composition of the phosphate-oxygen has been changed, as have other chemical and isotopic parameters that are known to be susceptible to meteoric diagenesis. Reduced major axis regressions between δ18OPO4 and Sr/Ca, 87Sr/86Sr and francolite-δ13CCO3, and the correlation of francolite-δ18OCO3 and francolite-δ13CCO3, predict an original isotopic composition of +30.7‰ (SMOW) for francolite-δ18OCO3 and + 22.7‰ (SMOW) for francolite-δ18OPO4. These data fix the temperature of phosphogenesis at 16°C and the δ18O of the phosphatizing fluid as +0.3‰ (SMOW).

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