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Geological Society, London, Special Publications; 1990; v. 52; p. 119-124;
DOI: 10.1144/GSL.SP.1990.052.01.08
© 1990 Geological Society of London

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

J. M. McArthur1 & A. Herczeg2,3

1 Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, UK
2 Research School of Earth Science, Australian National University, ACT 2601, Canberra, Australia
3 Division of Water Resources, CSIRO, Private Bag 2, Glen Osmond, SA 5064, Australia

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 {delta}18OPO4 and Sr/Ca, 87Sr/86Sr and francolite-{delta}13CCO3, and the correlation of francolite-{delta}18OCO3 and francolite-{delta}13CCO3, predict an original isotopic composition of +30.7{per thousand} (SMOW) for francolite-{delta}18OCO3 and + 22.7{per thousand} (SMOW) for francolite-{delta}18OPO4. These data fix the temperature of phosphogenesis at 16°C and the {delta}18O of the phosphatizing fluid as +0.3{per thousand} (SMOW).