TRACE ELEMENT DISTRIBUTION AND CATHODOLUMINESCENT SIGNATURE AS A REFLECTION OF DIAGENESIS IN MIDDLE JURASSIC BELEMNITE ROSTRA FROM UTAH

Belemnite rostra have been regarded typically as unaltered remains, yet that assumption has been questioned in the literature for nearly 30 years. Furthermore, it is well known that the use of d¹⁸O and d¹³C values from presumed unaltered belemnite rostra has provided the original calibration of those values for characterization of paleotemperatures and paleoenvironments. Serial thin sections of fragmental belemnite rostra from northeastern Utah were examined by standard petrographic methods, and cathodoluminescence (CL). Polished sections from the same specimens were also analyzed by electron microprobe (EMPA) for sodium, sulfur, strontium, iron, manganese, and magnesium. Using standard carbonate staining techniques, the rostra are nonferroan calcite and reveal no diagenetic alteration visible by standard petrographic examination . Although the original composition of the belemnite rostrum is still debated in the literature, EMPA indicates that the Utah rostra were originally low-Mg calcite. EMPA exhibited a range of only 0.09-0.54 weight percent for magnesium. Higher concentrations of magnesium at the exterior margins of the rostrum and along some growth lines suggest that even those low values may reflect diagenesis. CL signatures of the Utah specimens, including non-luminescent, intrinsic blue, dully red/orange, and bright orange/yellow, compare favorably with those described for rostra from different horizons and localities. As expected, EMPA traverses demonstrated spiking of manganese at points falling on growth lines with bright orange/yellow CL signatures, although overall concentration of manganese was low (<100 ppm-985 ppm; average 137 ppm). Unexpectedly, magnesium, and to a lesser extent iron, tracked the variation in manganese concentration. Variation in the CL signature and trace element distribution in the Utah belemnite rostra clearly demonstrate a complex diagenetic history.