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 Visingsö Group (Upper Proterozoic, southern Sweden). The δ¹³C_PDB 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 (δ¹³C_PDB =-0.75 to +7.04‰) and oxygen (δ¹⁸O_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 (δ¹³C= -20.8 to -18.63‰ δ¹⁸O = -5.46 to -5.21‰) have, however, precipitated at lower temperatures (∼ 25-30°C) in the sulfate-reduction zone.