METHODS

The petrography of dolomite samples was studied using transmitted light microscopy complemented with back-scattered electron (BSE) imaging and microprobe analyses of selected polished thin sections. CL investigation of specimens was performed using a Nuclide ELM-3R luminiscope coupled to a SPEX 1681 spectrophotometer operating at 2.6 Pa with an excitation voltage of 10 kV. CL photomicrographs were taken at very long exposure times of 10-20 min because sample luminosities were extremely low. BSE photomicrographs and energy-dispersive X-ray spectra of dolomite phases were acquired with a Philips XL-30 scanning electron microscope equipped with a Princeton Gamm-Tech thin-window detector. Fe and Mn contents of dolomite samples were collected using a fully automated CAMECA SX-50 microprobe operating in the wavelength-dispersive mode, with the following operating conditions: excitation voltage = 15 kV, beam current = 10 nA, peak count time = 20 s. Data reduction was done with the "PAP" (Z) method (Pouchou and Pichoir, 1985). Chemical formulas were normalized on six anions assuming two carbon atoms per formula unit. Step-scan X-ray powder diffraction data were collected over the range 3°-60°2 with CuK radiation using a Siemens D5000 Bragg-Brentano diffractometer equipped with a diffracted-beam graphite monochronometer crystal, 2-mm (1°) divergence and anti-scatter slits, 0.6-mm receiving slit, and incident-beam Soller slit. A profile-fitting program provided by Siemens using the ICDD PDF-2 database was utilized to identify X-ray peaks. Carbon and oxygen isotope analyses of dolomite samples were performed at Bremen University. Six to thirteen subsamples were taken from each specimen. Values in the text are averages calculated from individual data sets. Carbon and oxygen isotopic results are reported in per mil relative to the Peedee belemnite (PDB) standard.