Аннотации:
Lower Permian (280 to 300 Ma) evaporite dolomite rocks from the western part of the East European platform were studied by optical microscopy, scanning electron microscopy with microprobe analysis, and X-band electron paramagnetic resonance (EPR). Microscopic studies showed that many dolomite grains consist of an aggregation of submicron dolomite crystals (0.2-2 μm) as a spheroidal nucleus which is overgrown by a larger dolomite rhombohedron (5-20 μm). The EPR spectrum of a raw rock sample is mainly determined by thermally unstable radiation centers and Mn 2+ ions substituted at Ca and Mg sites in the dolomite crystal structure. The presence of dispersed organic matter at low concentrations was revealed from the backscattered electron images with a microprobe analysis of carbon-enriched areas on cleaved surfaces and the registration of the carbon-centered free organic radical signal in EPR spectra of heat-treated samples. A model of two successive growing stages for dolomite grains is suggested: the dolomite (or dolomite precursor) nucleation and aggregation in the colloidal stage to form suspended organo-mineral particles before the gravitational settling, and early diagenetic overgrowth at lower temperature and more stable environment of the marine bottom to form a dolomite rhombohedron. The observation of both growing stages for dolomite grains indicates that there was no recrystallization in the later stages of the rock history. © Springer-Verlag 2009.
