THE COMPLEXITY OF DOLOMITE AND IMPLICATIONS FOR INTERPRETING STABLE ISOTOPIC SECULAR VARIATION: AN EXAMPLE FROM THE PERMIAN PHOSPHORIA AND PARK CITY FORMATIONS

Geochemical analyses are routinely performed on carbonate rocks to determine paleoenvironmental conditions. Dolomite is often the only carbonate mineral present in ancient carbonate rocks, and is frequently a multiphase mineral that contains well-defined cores that often undergo recrystallization, followed by growth of cement zones. Dolomite phases exhibit distinct isotopic and trace element compositions that represent evolving geochemical environments during burial, and may not represent the original paleoenvironment. Failing to consider compositional complexities when testing whole dolomite crystals for paleoenvironmental conditions could result in compositions that represent multiple diagenetic phases.

We analyzed dolomite from the Permian Phosphoria and Park City formations, Wyoming to explore geochemical contributions from mimetic, replacement, and zoned dolomite cement phases identified under cathodoluminescent petrography (CL). Striking zonation of dolomite crystals visualized in CL reveals four dolomite generations. T-1 forms crystal cores and mimetically replaced peritidal sediments, and is yellow-orange in CL, with low Fe (<200 ppm), moderate Mn (200-800 ppm), and mean ∂¹⁸O and ∂¹³C of 1.1‰, and 1.6‰, respectively. T-2 is a complexly zoned burial cement phase with low Fe (200-500 ppm) and moderate Mn (200-400 ppm). T-3 is volumetrically the most abundant phase, it can be both mimetic and subhedral cement that is red in CL, with high Fe (500-1,600 ppm), high Mn (200-900 ppm), and mean ∂¹⁸O = 0.5‰ and ∂¹³C = 2.2‰. This phase also replaces T-1 and T-2. T-4 is paragenetically the most recent cement phase and is euhedral and Fe-rich (5,000-17,000 ppm), and higher in Mn (400-950 ppm), causing it to be dully luminescent in CL, with mean ∂¹⁸O and ∂¹³C values of -2.0‰ and -1.5‰, respectively. ∂¹⁸O and ∂¹³C values of whole rock and even whole crystal samples represent mixing of cement phases. The values of whole crystals in a given stratigraphic horizon were different, indicating that secular variation in isotopic profiles is at least partially due to phase mixing. These results shed light on the complexity in many dolomites, and suggest that more thorough examination of dolomite crystal growth and diagenesis be considered before paleo-environmental secular change interpretations are made.