Micromorphology and differential preservation of Upper Permian brachiopod low-Mg calcite

Calcareous shells of Rhynchonelliform brachiopods fromthe Nesen Formation (Late Permian) of Iran were studied by scanning electron microscope, cathodoluminescence and trace element chemistry to define their microstructure and state of preservation. In the Strophomenata shell succession, the secondary layer consists of cross-bladed, laminar calcite, and in a few species there is a tertiary layer of prisms. In the Rhynchonellata shell succession, the secondary layer consists of elongated fibers and the tertiary layer of prisms. All specimens were classified into six micromorphological types based on the fabric of the layer and the morphology of their microstructural units (laminae, fibres and prisms). Shell micromorphology and enclosing whole rock were further analyzed by cathodoluminescence and trace chemistry to assess preservation of brachiopod low-Mg calcite and specifically of their layers and microstructural units. Relative color frequencies were determined for each morphological type and degree of luminescence based on the lithology of the host rocks to evaluate the main factors that affect shell preservation. Results suggest that diagenetic alteration may depend on shell fabric and taxonomy as well as lithology of the host rock, but it appears that the first and the second features are the most important factors in determining the degree of brachiopod low-Mg calcite preservation. In general, the fibrous secondary layer is likely more often and better preserved than its laminar counterpart irrespective of host rock lithology. Furthermore, tertiary layers tend to be better preserved than secondary ones in Rhynchonellata low-Mg calcite shells, whereas no preferential preservation of either layer over the other was observed in the shells of Strophomenata. This information is important when selecting fossil brachiopod low-Mg calcite shells as geochemical proxies for paleoclimatic, paleoecologic, paleoenvironmental and other studies.