Constraining Temperatures of Formation and Composition of Dolomitizing Fluids In the Upper Devonian Nisku Formation (Alberta, Canada) With Clumped Isotopes

Abstract

Abstract Three dolomite phases in the burial dolomitization system of the Nisku Formation in central Alberta (Canada) were measured for carbonate clumped isotopes. Independent temperature constraints using fluid inclusions and traditional oxygen isotope geothermometry allow us to test the application of this method using a new calibration curve extrapolated to higher temperatures. These three phases of dolomite correspond to three diagenetic stages in the burial history. The oldest phase is replacive gray matrix dolomite formed during relatively shallow burial depths at average clumped-isotope temperatures (Δ47T) of +80 ±7°C. Previous temperature estimates from standard oxygen isotope geothermometry ranged from 50 to 80°C. A calculated δ18O(H2O) of 3 ± 1.4‰ VSMOW points to precipitation from Devonian seawater modified by evaporation and/or water–rock interaction. The first phase of saddle dolomite cement formed by pressure-solution during intermediate burial depths yielded average Δ47T of 96 ±4°C (previously calculated at ~80°C) and δ18O(H2O) of 4 ± 1‰ VSMOW. A second phase of saddle dolomite formed in association with thermochemical sulphate reduction during deep burial records average Δ47T of 151 ±5°C, comparable to fluid inclusion temperatures ranging from 125 to 145°C. The average δ18O(H2O) of the fluid that formed this saddle dolomite phase is calculated as 8 ± 1‰ VSMOW, suggesting much more advanced water–rock interaction. Δ47T are consistent with previous temperature estimates from a combination of geological data conventional O-isotope and fluid-inclusion thermometry. Another important finding is that the Δ47 signatures are preserved even after reaching deeper burial and/or during uplift and cooling, which points to a closure temperature in dolomites in excess of 180°C. Thus, dolomite appears to be a better phase for clumped-isotope analysis than calcite for rocks that have been exposed to higher temperatures upon further burial. This study demonstrates the power of the clumped-isotope thermometer to reconstruct complex diagenetic histories in burial diagenetic environments where multiple generations of diagenetic cements can be analyzed individually.