a1 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ
Zoned dolomite rhombs which occur in the organic-rich Jet Rock Formation (Toarcian) of northeast England have been studied using backscattered scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray microanalysis. The rhombs, which are 5–20 μm in size, are variable in chemical composition, but many consist of a dolomite core surrounded by a zone of ferroan dolomite, ankerite or ferroan calcite. Zoned rhombs occur in early-diagenetic calcite-cemented concretions and layers as well as dispersed throughout the shales. Distributional and textural evidence suggests that they formed mainly by primary precipitation from pore fluids within the zone of bacterial sulphate reduction. The ferroan outer zones probably formed after burial below the sulphate reduction zone when insufficient H2S was available to react with all the Fe2+ in solution.
(Received September 10 1984)
(Accepted November 19 1984)