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- Title
Origin of sedimentary francolite from its sulphur and carbon isotope composition.
- Authors
Benmore, Richard A.; Coleman, Max L.; McArthur, John M.
- Abstract
Francolite, the phosphorus-bearing mineral of nearly all sedimentary phosphorites, has a variable chemical composition which can be represented by (Ca, Mg, Sr, Na)10(PO4, SO4, CO3)6F2-3. It is now well established that this mineral can form authigenically in organic-rich muds1 or by replacement of precursor carbonate2 but the origin of some phosphorite deposits is still in dispute or unknown. Recently the isotopic composition of structural PO4-O, CO3-C+O, and SO4-S has been considered in attempts to resolve this problem3-9. Structural CO3-C has proved particularly useful in discriminating between phosphatized carbonates and authigenic phosphorites, the former preserving the δ13C of its precursor and the latter containing a substantial proportion of lighter carbon generated by anaerobic bacterial degradation of organic matter4,5. From new δ34S data we present here a more refined interpretation of the formational environment. δ34S values heavier than that of sea-water indicate that genesis has occurred within the bacterial sulphate reduction zone whilst δ34S values lighter than that of seawater delimit formation to a zone between sulphate reducing and overlying oxic conditions where isotopically light H2S is re-oxidized.
- Publication
Nature, 1983, Vol 302, Issue 5908, p516
- ISSN
0028-0836
- Publication type
Article
- DOI
10.1038/302516a0