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Reports: ND853525-ND8: Quantitative Constraints on Thermal Histories in Carbonates and Marine Shales: Conodont (U-Th)/He Thermochronology

Rebecca Flowers, University of Colorado, Boulder

Description of Problem. Apatite (U-Th)/He (AHe) thermochronology is one of the major tools used to decipher the thermal histories of rocks in the upper kilometers of the Earth’s crust. The method is sensitive to temperatures from ~30-90 °C depending on apatite chemistry, and AHe has been extensively applied in active orogens and basinal settings around the world to constrain thermal histories and associated episodes of burial and unroofing. However, AHe thermochronology is currently limited to rocks in which crystalline apatites of sufficient quality occur. Acquisition of thermochronology data in carbonates and shales has traditionally been elusive owing to the absence of dateable minerals of suitable size for analysis. Development of such a thermochronometer is highly attractive owing to the widespread occurrence of these lithologies, especially in large sections of sedimentary basins where thermal histories are relevant for hydrocarbon exploration. Conodont (U-Th)/He thermochonometry has the potential to fill this niche. A previous conodont He investigation suggested that this method has promise (Peppe and Reiners, 2007).

Conodonts are biomineralized structures composed of microcrystalline hydroxyapatite. They are thought to represent the feeding apparatus of a soft-bodied marine chordate that existed from Cambrian through Triassic time. Conodonts are often used as semi-quantitative indicators of peak temperature because remnant organic matter within the conodont irreversibly changes color as it progresses from ~50 to 600 °C. This color progression is well-documented as the Conodont Alteration Index (CAI) but imposes no temporal constraints.

This project is aimed at exploring the development of the conodont AHe thermochronometer via three distinct approaches. First, we are testing the applicability of conodont AHe thermochronometry to core samples from the Illinois Basin of the midcontinent United States. Second, we are conducting calibration studies on outcrop samples from the southern Rocky Mountains of the western United States. Third, are performing diffusion experiments on conodonts from both locations.

Research Efforts. Our initial conodont He study in the Illinois Basin displays both the potential of this tool and the outstanding questions that remain in its development (Landman et al., 2014a,b). We acquired conodont He data for 7 drillcore samples of Pennsylvanian marine black shale and limestone. The conodonts have CAI values of 1-1.5, indicating maximum burial temperatures of ≤ 90 °C. Several interesting results emerge from this dataset. First, the conodonts yield He dates substantially younger than their depositional age, indicating that the ≤ 90 °C temperatures were sufficient to cause He loss, and therefore implying that the conodont He closure temperature is no higher than the conventional apatite He system (70-80 °C). This result is compatible with conodont laboratory diffusion experiments. Second, most of our conodont dates are < 90 Ma, consistent with regional apatite He data for basement drillcore samples and suggesting that the conodonts and apatites record similar thermal histories. Third, whole platform elements – one of the three different conodont parts – yield the most reproducible results. Finally, several samples contain highly dispersed dates that range from 100 to 250 Ma. The least reproducible samples are limestones that are characterized by wide ranges of conodont [U] and Th/U ratios as well as negative correlations between date and [U]. The patterns suggest that U loss generated the anomalously old conodont dates.

Plans for Next Reporting Period. Our immediate work is aimed at ascertaining whether U mobility in limestones – as implied by the negative date-U correlations in the Illinois Basin conodont limestone data - occurred during diagenesis or was induced by the acid-based separation process for limestones. The greater reproducibility of data for shale samples, for which an acid-based separation procedure is not used, suggests that U loss from conodonts in limestones may be due to the separation process. We are exploring whether Selfrag – an apparatus that disaggregates rocks using high voltage – can 1) release conodonts of sufficient size from limestones for analysis, and 2) whether these conodonts lack the characteristic U loss patterns. If so, we would pursue this avenue for conodont separation from limestone. If not, then we will more strongly focus our conodont efforts on shale samples.

Second, we are performing diffusion experiments to more fully characterize conodont He diffusivity. Experiments on whole conodonts of various morphologies and fragments that isolate tissue types will allow us to better understand the natural variability in conodont He diffusion systematics.

Third, we have established new collaborative efforts with Prof David Schneider and PhD student Jeremy Powell at the University of Ottawa. They are exploring complementary aspects of conodont He thermochronometer development, and are running conodont He analyses in our lab (Powell et al., 2014a,b). We are specifically working with them at present to determine whether our simplified alpha-ejection corrections (that must be applied to account for the ejection of some fraction of He from the crystal margins) compare well with more accurate FT corrections being obtained by Powell and Scheider through detailed geometric characterization using X-ray computed microtomography.

References.

Landman, R.L., Rosenau, N.A., and Flowers, R.M., 2014a, Constraining thermal histories in marine carbonates and black shales: A pilot conodont (U-Th)/He thermochronometry study in the Illinois basin: AAPG, Houston, TX, April 2014.

Landman, R.L., Flowers, R.M., Rosenau, N., Metcalf, J., and Powell, J., 2014b, Constraining thermal histories in carbonates and marine shales: exploring the conodont AHe thermochronometer: Thermo2014, 14thInternational Conference on Thermochronology, Chamonix, France, September 2014.

Peppe, D.J., and Reiners, P.W., 2007, Conodont (U–Th)/He thermochronology: Initial results, potential, and problems: Earth and Planetary Science Letters, v. 258, no. 3-4, p. 569–580.

Powell, J., Schneider, D., Flowers, R., Metcalf, J., Stockli, D., 2014a, Thermal evolution of the Anticosti Basin, Eastern Canada: an empirical calibration of the conodont (U-Th)/He thermochronometer: Thermo2014, 14thInternational Conference on Thermochronology, Chamonix, France, September 2014.

Powell, J., Schneider, D., Flowers, R., Metcalf, J., Stockli, D., 2014b, Thermal evolution of the Anticosti Basin, Eastern Canada: an empirical calibration of the conodont (U-Th)/He thermochronometer: National GSA meeting, Vancouver, Canada, October 2014.