Direct dating of global Cryogenian sections by in situ U–Pb mapping of carbonates

The Cryogenian period represents a critical interval in Earth’s history, characterized by drastic tectonic and environmental changes. Evidence of low-altitude glacial deposits from this time has been recognized globally, alluding to the most extensive icehouse regimes known on our planet. These conditions of successive global freezing and warming during the Neoproterozoic have been dubbed as ‘Snowball Earth’ events. Importantly, the Cryogenian may have played a key role in the accelerated evolution of early life, as microorganisms became more complex and abundant after this period. Consequently, it is important to constrain the absolute timing, duration, and termination of these glacial and interglacial events. Despite their significance, robust, direct dating of Cryogenian sections remains challenging. The most accurate way to constrain these units is through dating of interbedded volcanics. However, they are not present across all sections globally, making correlations difficult to establish.

As such, we present a novel strategy to address this issue by directly dating a broad array of Cryogenian carbonates through an in situ U-Pb mapping approach. Our case study includes inter-glacial and post-glacial carbonates from sections in Australia, Oman, and Greenland. We show that this method allows for the concurrent collection of geochemical, petrographic, and geochronological information at sufficient precision to address key geological questions. Geochemical proxies such as elevated Mn/Sr ratios and Al or Si can be used to filter areas affected by alteration or detrital input, respectively. Secondary phases such as veins and overgrowths can also be petrographically avoided as an advantage of the spatially coherent mapping technique. Regions that yield enrichment in U and best spread in U–Pb ratios can be preferentially selected. Triaging such datasets and spatial information can help identify subdomains within a sample that is most suitable for dating, maximizing the success rate of this approach. The technique is capable of yielding age precision of ±1% depending on the concentration of U, the range in radiogenic isotopes, and the number of pixels that make up an analytical point.