TOWARDS NEW PHOSPHATE GEOTHERMOMETER USING DAHLLITE CONCRETIONS OF THE THERMOPOLIS SHALE, WYOMING USING MULTI-CHANNEL SPECTROSCOPY

Dahllite concretions commonly occur in the basal part of the Late Cretaceous Thermopolis Shale and are particularly abundant in the Bighorn Basin of Wyoming. These concretions consist primarily of the calcium phosphate dahllite [Ca₅(PO₄)₃(OH)], a mineral that also forms conodonts, a unique type of Paleozoic microfossil. The most likely genetic process responsible for dahllite concretions is abiotic precipitation during diagenesis. Dahllite concretions generally show a concentric zonation of colors and a structure of radially arranged needles. Conodont color varies from light beige, to brown and blue in the 50 to 300ºC range and has been used to evaluate peak burial temperature in Paleozoic formations.

Based on this well-known color variation, we hypothesized that dahllite concretions also change in color as a function of temperature. The importance of these concretions as a potential geothermometer is compounded by the fact that the Thermopolis Shale constitutes a major petroleum source rock in the Rocky Mountains.

We collected hundreds of concretions and selected the best preserved twenty concretions to conduct stepwise heating experiments combined with optical multi-channel spectroscopy (red, green and blue). Each sample was split in half, then polished, and the polished face was imaged with a high-resolution digital camera. In addition to these spectroscopic analysis, the specific gravity of the concretions was determined to assess their degree of lithification.

The specific gravity on average is 2725 kg/m³, showing a limited range of variation. The spectral signature of dahllite concretions depends on the volumetric importance of their central parts, generally made of microcrystalline hematite. Upon baking at 200ºC for 6 hrs, a change in all channels was detected and was most visible in the red channel. A more drastic color change occurred after baking at 600ºC for 2 days.

These results suggest that the color of dahllite concretions in the Thermopolis Shale changes as a function of increasing temperature with clear detectable changes at 200ºC and beyond. Therefore, the color index of dahllite concretions appears to have potential as a crude geothermometer for peak burial temperature in the Thermopolis Shale. Development of this method for oil exploration requires calibration.