Time-capsule concretions: Unlocking burial diagenetic processes in the Mancos Shale using carbonate clumped isotopes.

Septarian carbonate concretions contain carbonate precipitated during progressive growth of the concretion and subsequent fracture-filling. As such, they have been used to track variations in δ 13C and δ 18O of pore waters during diagenesis and to define diagenetic zones in clastic rocks. However, the δ 18O value of the carbonate is dependent on precipitation temperature and the δ 18O value of the pore fluid from which it precipitated. Interpretations must assume one of these parameters, both of which are highly variable through time in diagenetic settings. Carbonate clumped isotopes of the cement can provide independent estimates of temperature of precipitation, allowing the pore-water δ 18O to be back-calculated. Here, we use this technique on carbonate concretions and fracture fills of the Upper Cretaceous Prairie Canyon Member, Mancos Shale, Colorado. We sampled concretions from two permeable horizons separated by a 5 m shale layer, with one permeable horizon containing concretions with septarian fractures. We show cores precipitated at cooler temperatures (31 °C, ∼660 m burial depth) than the rims (68 °C (∼1980 m burial depth) and relate that to the δ 13Ccarbonate values to suggest the concretion core precipitated in the methanogenic zone, with increasing input from thermogenically produced CO2. The two concretion-bearing horizons have different back-calculated δ 18Oporewater values (mean and 1.13‰ VSMOW) for cements formed at the same temperature and similar δ 13C values, suggesting the shale layer present between the two horizons acted as a barrier to fluid mixing. Additionally, the δ 18Ocarbonate of the septarian fractures ( VPBD) are due to precipitation at high temperatures (102 to 115 °C) from a fluid with a mean δ 18Oporewater of 0.32‰ (VSMOW). Therefore, we can tie in the cementation history of the formation to temporal and spatial variations in δ 18Oporewater. [ABSTRACT FROM AUTHOR]