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

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

Nov 1988

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988gecoa..52.2565s&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 52, Issue 11, pp.2565-2572

Physics

9

Scientific paper

Oxygen isotope ratios in phosphate ( 18 O 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 ( 18 O c ), 13 C was also analysed in the latter, and in coexisting calcite for 10 of them. Both 18 O p and 18 O 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 18 O p vs . 18 O 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 18 O-depleted water and the rocks, and higher temperature exchange between phosphate and carbonate; 3. (c) the observed trend cannot be explained by 18 O-depleted Cretaceous and Paleozoic oceans. The calcite coexisting with apatite in phosphatic rocks is similar in both 18 O and 13 C 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 ( et al ., 1983), it is probably still a very good isotopic recorder of sedimentary environments.

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