The dolomite problem: evidence from 3D modeling, XRD and geochemical data of Zechstein reefs (Upper Permian, Germany)

Abstract

Three dimensional modeling of the limestone and dolomite distribution in an Upper Permian (Zechstein) stromatolite-bryozoan reef, ca. 500m in diameter and 35m thick (77 drill cores, 172 point data), shows that dolomite occurs as laterally and vertically discontinuous intervals. The prevailing mineral phases are near stoichiometric dolomite and Mg free calcite, respectively (370 XRD and 341 XRF analyses). Both ∂¹³C and ∂¹⁸O (526 analyses) show a spread of ca. 10‰ and covary with the mineralogy, and the heaviest dolomite and calcite ∂¹³C differ by ca. 1,5‰. Diagenetic modifications caused by flowing meteoric fluids could account for the observed “inverted J” trend of stable and the radiogenic signature of ⁸⁷Sr/⁸⁶Sr (23 analyses), but neither vertical nor horizontal gradients occur in the modeled reef. Because dolomite geometries are incompatible with those predicted by fluid flow models, and the limestone-dolomite difference in ∂¹³C overlaps estimates of isotope fractionation associated with Mg content, the studied dolomite was a depositional Very High Mg Calcite recrystallized to dolomite in a semi-closed diagenetic system rather than a Low Mg Calcite transformed by a dolomitization process. The isotope pattern suggests biogenic fractionation and/or loss of heavy stable and light Sr isotopes during diagenesis.