ABSTRACT

Systematic compositional differences between nonequivalent growth sectors in dolomite rhombs from the Mississippian Burlington Fm., Southeastern Iowa, are documented using various analytical methods. Cathodoluminescence petrography reveals that sectors forming under {1120} faces have a different overall luminescence than adjacent sectors forming under {1014} faces, indicating differences in Mn and/or Fe contents. Microprobe analyses confirm such a difference, with {1120) sectors enriched in Fe and slightly enriched in Mn relative to {1014} sectors. In addition, {1120} sectors are enriched with Mg relative to {1014} sectors; both sector types are rich in calcium, however. Transmission electron microscopy reveals that growth microstructures differ within these different growth sectors.

The sectoral variations can be explained using a model that depicts atomic configurations on different growth surfaces in relation to occupancies of partially formed cation sites called protosites. It is shown that sector zoning requires metastable growth, since growth layers added simultaneously on nonequivalent faces are compositionally different. Both trace and major element distributions for these dolomites, and perhaps many other carbonates, are influenced by factors relating to the mechanisms and perhaps rates of growth on different faces, and do not represent equilibrium. In addition, crystal habit may indirectly influence bulk composition.