INTERFACING GEOCHEMICAL AND PETROGRAPHIC DATA FROM CHUAR GROUP DOLOMITES: WAS MID-NEOPROTEROZOIC SEAWATER CHEMISTRY PRESERVED DURING EARLY DIAGENESIS/DOLOMITIZATION?

Isotopic and trace element data from m-scale dolomites beds of the organic-rich mudrock-dominated Chuar Group (1600 m thick), Grand Canyon, may provide insight into mid-Neoproterozoic seawater chemistry. To best interpret the geochemical data, data from traditional petrographic and cathodoluminescence analyses were integrated with the trace element and stable isotope data.

d¹⁸O values from the lower and upper Chuar Group range between -4 to -9‰ (PDB), and corresponding d¹³C values range from +3 to -6‰ (PDB). d¹⁸O values from the middle Chuar Group range between 0 and -5‰ (PDB), and corresponding d¹³C values range from +3 to -3‰ (PDB). Concentrations of Mn and Fe range from 350-5300 and 20-17,000 ppm, respectively. We see no correlation between lower d¹⁸O values and lower d¹³C values, nor between lower d¹⁸O values and high Mn or Fe values. Corresponding petrographic data show that Chuar dolomites range from dominantly finely to coarsely crystalline dolomites. Medium to coarsely crystalline dolomites are more dominant in the lower and upper parts of the succession, while the middle Chuar Group is dominated by finely crystalline dolomites. Primary fabrics are commonly preserved, yet dolomite of all crystal sizes can be massive. Luminescence is typically dull in the finely crystalline dolomites, and moderately bright in the medium to coarsely crystalline dolomites.

There does not appear to be a trend between d¹⁸O and d¹³C values and the textural changes in the succession, nor do Mn and Fe concentrations show any obvious relationship with texture or the d¹⁸O values. The lack of correlations between different data sets suggests that these geochemical parameters were set early in the history of the Chuar dolomites. Higher d¹⁸O values in the middle Chuar Group suggest either evaporative effects or possibly open seawater values, and lower d¹⁸O values in the lower and upper Chuar Group suggest either open seawater values or random exchange with more depleted ¹⁸O waters during early diagenesis. Therefore, it is possible that the original seawater d¹⁸O values (and d¹³C values) were preserved during early diagenesis/dolomitization of the Chuar dolomites.