Unraveling the histories of Proterozoic shales through in situ Rb-Sr dating and trace element laser ablation analysis

Authigenic components in marine sediments are important archives for past environment reconstructions. However, defining reliable age constraints and assessing the effects of post depositional overprints in Precambrian sequences are challenging. We demonstrate a new laser-based analytical approach that has the potential to rapidly and accurately evaluate the depositional and alteration histories of Proterozoic shales. Our study employs a novel application of in situ Rb-Sr dating coupled with simultaneous trace-element analysis using reaction-cell laser ablation-inductively coupled plasma-tandem mass spectrometry (LA-ICPMS/MS). We present results from shales sourced from two wells in the Proterozoic McArthur Basin, northern Australia. These rocks have been widely used by previous studies as a key section for ancient biogeochemical and paleo-redox reconstructions. Shales from well UR5 yielded initial Sr-87/Sr-86 ratios, Rb-Sr ages, and rare earth element plus yttrium (REEY) patterns similar to those of a dolerite sampled from the same core. We propose that the UR5 samples chronicle hydrothermal alteration instigated by the dolerite intrusion. In contrast, a correlative shale from well UR6 yielded an age consistent with the expected depositional age (1577 +/- 56 Ma) with REEY and initial Sr-87/Sr-86 ratios similar to ca. 1.5 Ga seawater. We suggest that this sample records the minimum depositional age and early marine diagenetic history for this unit. This new technique can date Proterozoic shales quickly, cheaply, and with minimum sample preparation. Importantly, ages are triaged to differentiate between those recording primary marine versus secondary processes. This novel approach provides a potentially powerful tool for dating and fingerprinting the vast array of ancient marine shales for further studies of Earth systems through deep time.

Automated summary

Authigenic components in marine sediments play a crucial role in reconstructing past environments. However, determining reliable age constraints and assessing post-depositional processes in Precambrian sequences is challenging. This study introduces a new laser-based technique that combines in situ Rb-Sr dating and trace-element analysis to evaluate the depositional and alteration histories of Proterozoic shales. The results demonstrate the potential of this method in efficiently dating and distinguishing primary marine processes from secondary ones, making it a powerful tool for future studies of ancient marine shales.