Isotope geochemistry of oxygen and carbon in phosphate and carbonate of phosphorite francolite

Oxygen isotope ratios in phosphate (δ¹⁸O_p) were determined in 219 samples of phosphoritic rocks from all major phosphogenic provinces of the world, ranging in age from Lower Proterozoic to Recent. In 58 samples, oxygen was analysed in the two oxygen lattice sites of francolite-both in phosphate and in structural carbonate (δ¹⁸O_c), δ¹³C was also analysed in the latter, and in coexisting calcite for 10 of them. Both δ¹⁸O_p and δ¹⁸O_c decrease sharply from the Recent to the Cretaceous, and decrease with a gentler slope to the Precambrian. The nature of the time variation is elucidated by means of a δ¹⁸O_p vs. δ¹⁸O_c plot, which suggests that: 1. (a) the main cause of the time trends is the increase in temperature between Recent and the Cretaceous; 2. (b) the Pre-Cretaceous trend is overprinted by both low-temperature nonequilibrium exchange between ¹⁸O-depleted water and the rocks, and higher temperature exchange between phosphate and carbonate; 3. (c) the observed trend cannot be explained by ¹⁸O-depleted Cretaceous and Paleozoic oceans. The calcite coexisting with apatite in phosphatic rocks is similar in both δ¹⁸O and δ¹³C to the structural carbonate of the apatite. Even though phosphate may exchange its oxygen with environmental water to a greater extent than we originally claimed (Kolodny et al., 1983), it is probably still a very good isotopic recorder of sedimentary environments.