Reports: ND853525-ND8: Quantitative Constraints on Thermal Histories in Carbonates and Marine Shales: Conodont (U-Th)/He Thermochronology
printer friendlyApatite (U-Th)/He (AHe) thermochronology is a 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, 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.
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). 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.
Our second conodont study was performed on a limestone from the Mississippian Terrero Formation in the southern Sangre de Cristo Range. We acquired (U-Th)/He dates for 17 conodonts and 3 fish teeth. The fish teeth contained no He, indicating that they are not He retentive. The 17 conodonts He dates are characterized by an inverse date-eU correlation. Most of the results are anomalously old when compared with nearby AHe data for basement samples, with half being older than the depositional age of the unit. These observations are consistent with the patterns observed in the Illinois Basin limestone samples, again suggesting that U loss generated the anomalously old dates.
Our third effort was aimed at determining the conodont He closure temperature using laboratory diffusion experiments. We carried out two diffusion experiments on conodonts from different samples, from which we estimate closure temperatures of ~60-67 °C. These results are consistent with previous work, implying similar temperature sensitivities of the conodont and conventional AHe systems.
Our subsequent work was aimed at evaluating the sources of dispersion in the limestone conodont He data. First, to determine how much scatter was caused by using our simplified alpha-ejection correction (that must be applied to account for the ejection of some fraction of He from the crystal margins), we collaborated with Prof David Schneider and PhD student Jeremy Powell at the University of Ottawa to obtain accurate FT corrections using detailed geometric characterization using X-ray computed microtomography. The result suggests that our estimated uncertainties based on grains size measurements typically differs by <5% from the more accurate ones obtained by X-ray computed microtomography, and thus is not the dominant cause of excess dispersion in our dataset. We also are working with Schneider and Powell by running their conodont He analyses in our lab (Powell et al., 2014a,b), because they are exploring complementary aspects of conodont He thermochronometer development. Second, we used a Selfrag separation process to ascertain whether U mobility in limestones – as implied by the negative date-U correlations in the limestone datasets - 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 was not used, had suggested to us that U loss from conodonts in limestones may be due to the separation procedure. Selfrag is an apparatus that disaggregates rocks using high voltage, thereby allowing us to separate conodonts without using the acid-based separation process. Our results show that Selfrag separation can indeed release conodonts of sufficient size from limestones for analysis. However, the conodont He dates revealed that although the dispersion was reduced when compared with the data for conodonts separated using the conventional acid-based approach, a subset of the data is still anomalously old. This result implies that at least some conodont U loss occurred in situ and contributed to the anomalously old dates in the two limestone datasets.
The primary aim of our work during the grant extension period is to write up our existing results for publication. We also hope to carry out additional analyses of conodonts from shale samples, because our results thus far for whole platform elements from conodonts obtained from shale samples are relatively reproducible.
