DETERMINATION OF SR-ISOTOPE COMPOSITION OF THE LOWER CAMBRIAN SEKWI FORMATION, NORTHWEST TERRITORIES, CANADA, USING SOLUTION CHEMISTRY AND LASER ABLATION

The Sr-isotope record for the trilobitic Early Cambrian is incomplete, most likely due to the lack of suitable fossil proxies.  Most Sr-isotope studies of Phanerozoic successions rely heavily on brachiopods, which were not exceptionally diverse or abundant in the early Cambrian.  Trilobite carapaces and marine cements are more abundant and can be used as proxies for the Sr-isotopic composition of seawater.

A two-pronged approach was used to determine the ⁸⁷Sr/⁸⁶Sr values of carbonate components from the Early Cambrian Sekwi Formation of the Mackenzie Mountains, Northwest Territories, Canada.  Traditional solution chemistry involving ion chromatography analyzed on a multicollector ICP-MS was compared with laser ablation coupled to a multicollector MC-ICP-MS of brachiopods, marine cements, and trilobite carapaces.  Laser ablation was used to accommodate analysis of small, submillimeter-scale carbonate components, including marine cement and trilobite carapaces that are more abundant within the Sekwi Formation than brachiopods.  Sr-isotope data generated from this study were then compared with previously published data to evaluate a secular Sr-isotope curve for the Early Cambrian. 

The ⁸⁷Sr/⁸⁶Sr values from previous studies of Lower Cambrian successions range from 0.7080 to 0.7088 and show an increase in ⁸⁷Sr/⁸⁶Sr values toward the Early-Middle Cambrian boundary.  Sr-isotope data from the Sekwi Formation span from the base of the Atdabanian to the mid-Toyonian, and values tend to be more radiogenic than other Lower Cambrian successions, indicating a diagenetic influence.  However, Sr-isotope data from the Sekwi Formation show similar trends to previous studies. 

Additionally, we compare the utility of traditional solution chemistry and laser ablation in determining accurate Sr-isotope values.  This study suggests that traditional solution chemistry generates precise and accurate ⁸⁷Sr/⁸⁶Sr values by removing diagenetic-Sr and isobaric interferences.  However, poor spatial resolution inherent in microdrilling, limit the usefulness of traditional solution chemistry.  Small samples (30 to 100 µm thick) and heterogeneous zones can be analyzed in situ using laser ablation, however limitations include a less stable signal and the lack of a suitable laser Sr-standard.