Geochemistry of dolomite fluorescence in response to thermal maturity: An example from Upper Ordovician Utica Shale of southern Québec, Canada

Abstract

This study presents results of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) elemental analysis of scattered dolomite crystals in the Upper Ordovician Utica Shale of southern Québec to investigate the geochemical cause(s) of color variation in dolomite fluorescence and its possible relationship to thermal maturity. Utica Shale dolomite crystals show fluorescence zonation under ultraviolet (UV) light, with a shift from higher red/green quotient (R/G) in the crystal core to lower R/G in the crystal rims within dolomite crystal, while the crystal cores and rims R/G consistently increase with increasing thermal maturity.The LA-ICP-MS results show a consistent increase of Fe, Mn, Sr, Pb, and rare earth elements (REE) from crystal cores to rims, while Mg content shows the opposite trend. The decline in Mg content from the core to rim suggests depletion of Mg in the dolomitizing fluids during dolomite growth. Iron and Fe/Mn ratio are the only parameters that varied systematically with Mg content. The negative correlation between the red/green (R/G) quotient and Fe/Mn ratio suggests that temperature likely had an overarching effect on the chemistry of dolomite that ultimately controls the crystal zonation and its optical properties.Our data suggest that dolomite fluorescence can potentially be used as an alternative thermal maturity indicator in mudrocks with low organic content. However, compilation of a larger dataset is required to completely understand the relationship between dolomite R/G quotient and other established thermal maturity indicators.