Diagenetic trends of fluorine concentration in Negev phosphorites, Israel:implications for carbonate fluorapatite composition during phosphogenesis

An electron probe and chemical study of bulk phosphorite samples and separa ted constituents from various Negev deposits was carried out together with XRD, FTIR spectroscopy and textural analysis. The results allow a better un derstanding of the distribution of fluorine in these Upper Cretaceous phosp horite sequences and shed light on variations in the composition of the car bonate fluorapatite (CFA) phase during phosphogenesis. Two facies are recog nized: (1) a pristine, microbially generated phosphorite facies; (2) a recy cled, peloidal and biodetrital facies. Fluorine distribution in the Negev p hosphorites is facies controlled: F/P2O5 is much lower in the pristine faci es (0.090-0.107) than in the recycled facies (0.107-0.120). In addition, F/ P2O5 varies considerably between the various constituents of the phosphate fraction; F-poor francolites (F/P2O5 as low as 0.080) co-exist with F-rich francolites (F/P2O5 as high as 0.135) in the same phosphorite bulk sample. A lower F/P2O5 in francolite is associated with higher Cd and Zn concentrat ions in the phosphorite, an increase in Fe-rich smectites in the clay fract ion and the presence of structural OH in the francolite. The lower F/P2O5 r atios in the pristine facies are attributed to high organic deposition rate s during the formation of these matted sediments, leading to rapid burial o f the in situ-forming CFA. This is possibly coupled with diffusion of F fro m sea water into bottom sediments being hampered by microbial mat coatings. These conditions resulted in O-2-depleted porefluids, inducing the precipi tation of Cd-rich Zn sulphides and the formation of Fe-rich smectites. F-en richment probably takes place when the earlier formed F-poor 'primary' CFA is relocated close to the sea floor and bathed with interstitial sea water solutions of higher F concentrations. Oxidation and removal of the sulphide -bound Cd and Zn apparently occurred together with enrichment in F of the f rancolite. Combining chemical data with XRD and FTIR results suggests a mul tistage growth for the Negev phosphate constituents in shifting formational sites and porefluids of varying F concentrations. This multiphase growth i s reflected in the patchy distribution of F in the Negev constituents and m ight explain the inverse correlation between mean CO2/F and F/P2O5 ratios o f the analysed phosphorites in the two facies. It also suggests that CFA (o r an amorphous precursor) initially formed with some OH groups in the apati te structure, which were subsequently substituted by F ions in recycled fra ncolite through re-equilibration with porefluids of higher F concentrations .