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Diagenetic trends of fluorine concentration in Negev phosphorites, Israel: implications for carbonate fluorapatite composition during phosphogenesis
Author(s) -
Soudry David,
Nathan Yaacov
Publication year - 2001
Publication title -
sedimentology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.494
H-Index - 108
eISSN - 1365-3091
pISSN - 0037-0746
DOI - 10.1046/j.1365-3091.2001.00392.x
Subject(s) - phosphorite , fluorapatite , facies , geology , diagenesis , carbonate , mineralogy , geochemistry , phosphate , apatite , chemistry , geomorphology , organic chemistry , structural basin
An electron probe and chemical study of bulk phosphorite samples and separated constituents from various Negev deposits was carried out together with XRD, FTIR spectroscopy and textural analysis. The results allow a better understanding of the distribution of fluorine in these Upper Cretaceous phosphorite sequences and shed light on variations in the composition of the carbonate fluorapatite (CFA) phase during phosphogenesis. Two facies are recognized: (1) a pristine, microbially generated phosphorite facies; (2) a recycled, peloidal and biodetrital facies. Fluorine distribution in the Negev phosphorites is facies controlled: F/P 2 O 5 is much lower in the pristine facies (0·090–0·107) than in the recycled facies (0·107–0·120). In addition, F/P 2 O 5 varies considerably between the various constituents of the phosphate fraction; F‐poor francolites (F/P 2 O 5 as low as 0·080) co‐exist with F‐rich francolites (F/P 2 O 5 as high as 0·135) in the same phosphorite bulk sample. A lower F/P 2 O 5 in francolite is associated with higher Cd and Zn concentrations in the phosphorite, an increase in Fe‐rich smectites in the clay fraction and the presence of structural OH in the francolite. The lower F/P 2 O 5 ratios in the pristine facies are attributed to high organic deposition rates during the formation of these matted sediments, leading to rapid burial of the in situ ‐forming CFA. This is possibly coupled with diffusion of F from sea water into bottom sediments being hampered by microbial mat coatings. These conditions resulted in O 2 ‐depleted porefluids, inducing the precipitation of Cd‐rich Zn sulphides and the formation of Fe‐rich smectites. F‐enrichment 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 francolite. Combining chemical data with XRD and FTIR results suggests a multistage growth for the Negev phosphate constituents in shifting formational sites and porefluids of varying F concentrations. This multiphase growth is reflected in the patchy distribution of F in the Negev constituents and might explain the inverse correlation between mean CO 2 /F and F/P 2 O 5 ratios of the analysed phosphorites in the two facies. It also suggests that CFA (or an amorphous precursor) initially formed with some OH groups in the apatite structure, which were subsequently substituted by F ions in recycled francolite through re‐equilibration with porefluids of higher F concentrations.