CONSTRAINING THE ORIGIN AND EVOLUTION OF FLUIDS ON THE EAST KAIBAB MONOCLINE USING THE ISOTOPE GEOCHEMISTRY OF CARBONATES

This study uses geochemistry to understand the relative timing and nature of fluid flow in the East Kaibab monocline (EKM), which comprises predominantly Triassic to Cretaceous sedimentary rocks that have undergone deformation by Laramide tectonic stress. Deformation bands, faults, and joints in the EKM contain mineral precipitates that formed from paleo-groundwater flowing through the rocks during regional deformation. Geochemical analyses of these carbonate precipitates were the focus of this research. Carbonates were collected (n = 31) along five transects from north to south in the EKM. Analyses were performed on each carbonate sample to determine paleo-fluid composition (δ¹⁸O, δ¹³C), carbonate content (thermogravimetric analysis), and mineralogic composition (X-ray diffraction). Additional analyses were completed on a subset of samples to determine fluid source using Sr isotopes.

Thermogravimetric analysis reveals that the samples’ carbonate concentrations range from 0.8 to 13.9% of the total sample mass. XRD analysis indicates that minerals in the EKM carbonates are primarily calcite and dolomite. Carbonate minerals precipitate in isotopic equilibrium with the water in which they form, and δ¹⁸O_carbonate can therefore be used to determine δ¹⁸O_fluid during precipitation if a temperature is assumed. The δ¹⁸O_carbonate values from EKM show a range of –20.6 to +0.9‰ (VPDB), indicating that minerals along the EKM precipitated from a combination of different fluid sources, different temperatures, or a combination of both. Individual carbonate crystals range from –0.6 to –13.3‰; bulk analyses of deformation band carbonates range from –20.3 to –1.5‰; whole-rock carbonates in undeformed matrices range from –17.7 to +0.9‰; and matrix values range from –19.0 to –11.0‰. Matrix values show the least variation (8.0‰), and deformation bands show the largest (18.8‰). δ¹³C_carbonate values range from –7.7 to +2.6‰ (VPDB). These results indicate that carbonate minerals in the EKM likely precipitated from a range of sources and at different temperatures. Sr isotope analysis will further constrain the source and composition of paleo-fluids from which the carbonate minerals precipitated.