Abstract
Abstract 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-ICP-MS/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 87Sr/86Sr 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 87Sr/86Sr 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.
Highlights
Analyses of the sedimentary record have been integral in constraining how Earth systems have evolved over geological time
The majority of illite in Proterozoic shales of the McArthur Basin is believed to have formed during early marine diagenesis via reverse weathering processes in equilibrium with paleoseawater, as opposed to being detrital products of continental weathering from soils like those commonly found in Phanerozoic shales (Rafiei and Kennedy, 2019)
The Gibb Member shale from an intrusionabsent borehole was abundant in authigenic illite and yielded a Rb-Sr age consistent with the depositional age for the unit
Summary
Analyses of the sedimentary record have been integral in constraining how Earth systems have evolved over geological time. The radiogenic 87Sr/86Sr isotope record of seawater inferred from marine carbonates reflects the balance between Sr input from continental weathering versus that from submarine hydrothermal systems, reflecting global-scale tectonic changes (Kuznetsov et al, 2014). These archives are meaningful only if one is able to accurately date and assess the primary compositions of the rocks investigated. These prerequisites are further complicated by the lack of a diverse faunal record in the Proterozoic, restricting the use of biostratigraphy to constrain depositional ages. An array of other chronological techniques has been developed to date Precambrian sedimentary rocks
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