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

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-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 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.

LA‐ICP‐MS/MS Single‐Spot Rb‐Sr Dating

“…Induced gases allow the reaction of targeted masses and thus to chemically separate 87 Rb and 87 Sr to avoid the isobaric overlap during analysis (e.g., Moens et al 2001, Bolea-Fernandez et al 2016. Being cost and time efficient, it is possible to collect large 87 Rb/ 86 Sr and 87 Sr/ 86 Sr data sets from multiple grains and mineral species, which makes it an attractive new tool in geochronology (e.g., Gorojovsky and Alard 2020, Olierook et al 2020, Redaa et al 2021, Tillberg et al 2020, van Daele et al 2020, Laureijs et al 2021, Subarkah et al 2022.…”

Constraining the geothermal parameters of in situ Rb–Sr dating on Proterozoic shales and its subsequent applications

“…Age constraints of the Roper Group have been established through several geochronological methods (Ahmad and 135 Munson, 2013;Jackson et al, 1999;Kendall et al, 2009;Nixon et al, 2021;Page et al, 2000;Southgate et al, 2000;Subarkah et al, 2021;Yang et al, 2019;Yang et al, 2018). The beginning of the group's genesis is bracketed by a SHRIMP U-Pb zircon study from a tuff within the unconformably underlying Nathan Group as well as minimum depositional age from an in situ Rb-Sr analysis in the lower Roper Group that yielded ages of 1589 ± 3 Ma and 1577 ± 56 Ma, respectively (Page et al, 2000;Subarkah et al, 2021). The unconformity 140 between the Roper Group and the older Nathan Group is likely related to the Isan Orogeny ca.…”

“…. Thus, the primarily authigenic nature of clay minerals in Proterozoic shales make them ideal targets for in situ Rb-Sr dating (Subarkah et al, 2021).…”

“…In these environments, fluid-induced recrystallisation and alteration can drive element and isotopic exchange at lower effective closure temperatures than those empirically determined for classic thermal volume diffusion reactions (Dodson, 1973;Field and Råheim, 1979;Jenkin et al, 1995;Villa, 1998). Nevertheless, these weaknesses 95 can in turn be used advantageously to date secondary events such as episodes of hydrothermal fluid-flow (Dodson, 1973;Li et al, 2020;Redaa et al, 2021b;Shepherd and Darbyshire, 1981;Subarkah et al, 2021;Tamblyn et al, 2020).…”

“…In this study, we show how in situ Rb-Sr analysis of the Proterozoic Velkerri Formation in the Roper Group, McArthur Basin, northern Australia (Figure 1) dated clay-mineral recrystallisation at temperatures similar to those at 100 which kerogen catagenesis occurs. The Roper Group is a good case study for in situ Rb-Sr shale dating as it has been shown to be dominated by authigenic clays (Rafiei and Kennedy, 2019;Subarkah et al, 2021) and is chronologically well-constrained (Ahmad and Munson, 2013;Bodorkos et al, 2020;Cox et al, 2022;Kendall et al, 2009;Southgate et al, 2000;Subarkah et al, 2021;. Furthermore, the resurgence of interest in the resource potential of the organic-rich Velkerri Formation has also yielded a framework of palaeotemperature data 105 that aim to discern the maturation history of hydrocarbons within the unit (Ahmad and Munson, 2013;Capogreco, 2017;Cox et al, 2016;Crick et al, 1988;George and Ahmed, 2002;Jarrett et al, 2019;Lemiux, 2011;Revie, 2014;Summons et al, 1994;Taylor et al, 1994;Volk et al, 2005).…”