Conditions of formation and diagenetic evolution of Upper Proterozoic phosphate nodules from southern Sweden: evidence from petrology, mineral chemistry and isotopes

Dark-gray to black cryptocrystalline francolite occurs as nodules (∼ 2–8 cm in diameter) which are embedded in organic-matter-rich mudstones of the Visingso Group (Upper Proterozoic, southern Sweden). The δ13CPDB values (−18.4 to + 0.98‰) of structural-CO2, complemented by studies of the petrographic character and strontium isotopic composition of francolite, indicate precipitation in the suboxic and sulfate-reduction zones. Francolite is intergrown with berthierine that has formed in the suboxic Fe-reduction zone. Calcite enriched in Mn and, to a lesser extent Fe, occurs as fracture- and void-fillings, and as replacement of francolite. Mn- and Mg-rich calcian siderites have precipitated as pseudomorphs after bacteria in the zone of microbial methanogenesis. The carbon ( δ 13 C PDB =−0.75 to +7.04‰ ) and oxygen ( δ 18 O PDB =−14.21 to −10.58‰ ) isotopes of calcite suggest derivation of carbonate ions from microbial methanogenesis and from thermal decarboxylation of organic matter at temperatures of 60° to 80°C. Some of the void- and fracture-filling calcites ( δ 13 C = −20.8 to −18.63‰ δ 18 O = −5.46 to −5.21‰ ) have, however, precipitated at lower temperatures (∼ 25–30°C) in the sulfate-reduction zone.