Direct U–Pb dating of carbonates from micron-scale femtosecond laser ablation inductively coupled plasma mass spectrometry images using robust regression

Abstract

Abstract. Uranium–lead (U–Pb) dating of carbonates by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) spot analysis is an increasingly used method in the field of geosciences, as it brings very strong constraints over the geological history of basins, faults or reservoirs. Most ages currently published are based on the measurement of U and Pb ratios on spot ablations, using nanosecond lasers coupled to sector field or multi-collector ICP-MS. Here, we test a new strategy for the U–Pb dating of carbonates from 2D isotopic ratio maps, based on the use of a robust regression approach in the data reduction workflow. The isotopic maps, with a minimum area of 0.65 mm2 (∼ 1000 pixels of 13×25 µm resolution), are obtained using a 257 nm femtosecond laser ablation system at a high repetition rate (500 Hz) coupled to a high-resolution ICP-MS. The maps commonly show significant variations in isotope ratios at the pixel scale, allowing the plotting of pixel U–Pb ratios in concordia or isochron diagrams and the calculation of U–Pb ages. Due to the absence of individual ratio uncertainties, the ages are calculated using MM-robust linear regression rather than the more commonly used York-type regression. The goodness of fit to the data is assessed by the calculation of the residual standard error (RSE) of the regression and by the calculation of a mean square of weight deviates (MSWD) on discretised data. Several examples are provided that compare the ages calculated by robust regression with those obtained by other techniques (e.g. isotope dilution, LA-ICP-MS spot analyses and the pixel-pooling approach). For most samples, characterised by high U concentrations (> 1 ppm), robust regression allows for the calculation of ages and uncertainties similar to those obtained with the other approaches. However, for samples with lower U concentrations (< 0.5 ppm), the ages obtained are up to 10 % too young due to pixels with high U ∕ Pb acting as leverage points for the regression. We conclude that the U–Pb ages calculated by the regression method tested here, although statistically robust, should be critically analysed before validation, especially for samples with low U concentrations.

Depositional age models in lacustrine systems from zircon and carbonate U‐Pb geochronology

“…Although the interplay between these factors remains unclear, carbonates precipitated from lacustrine water, meteoric water and groundwater are particularly amenable to dating (e.g. Rasbury & Cole, 2009; Drost et al ., 2018; Liivamägi et al ., 2018; Frisch et al ., 2019; Parrish et al ., 2019; Woodhead & Petrus, 2019; Kurumada et al ., 2020; Nicholson et al ., 2020; Roberts et al ., 2020a, 2021; Brigaud et al ., 2021; Hoareau et al ., 2021; Montano et al ., 2021; Nuriel et al ., 2021; Rasbury et al ., 2021).…”

“…In spite of this, LAcarb dating may provide ages consistent, within uncertainties, with biostratigraphic and radiometric constraints. This has been shown for meteoric cements in speleothems (Hopley et al ., 2019; Woodhead & Petrus, 2019; Nicholson et al ., 2020) and hardgrounds (Liivamägi et al ., 2018; Scardia et al ., 2019; Kurumada et al ., 2020; Brigaud et al ., 2021; Środoń et al ., 2022), lacustrine tufa, micrites and associated cements (Drost et al ., 2018; Frisch et al ., 2019; Parrish et al ., 2019; Hoareau et al ., 2021; Montano et al ., 2021; Rasbury et al ., 2021), marine cements (Meinhold et al ., 2020; Hoareau et al ., 2021) and bioclasts (Drost et al ., 2018). Despite these remarkable results, no study has attempted to define, solely based on LAcarb, a robust depositional age depth model along an entire stratigraphic section, which was also dated by zircon geochronology.…”

Burial-Deformation History of Folded Rocks Unraveled by Fracture Analysis, Stylolite Paleopiezometry and Vein Cement Geochemistry: A Case Study in the Cingoli Anticline (Umbria-Marche, Northern Apennines)

“…We propose to build on recent methodological advances to better constrain the timing of deformation in fold-and-thrust belts and foreland basins. The development of U-Pb dating on calcite cement enables absolute dating on the tectonic vein filling and synkinematic clay minerals (e.g., [47][48][49]), clumped isotopes can be reliably used to reconstruct the exact temperature of precipitation under 90-100 • C [50], and its combination to fluid inclusion microthermometry allows to reconstruct the temperature-pressure-time path in sedimentary sequence [51]. Moreover, the understanding of how stylolites can be used as stress gauges [52,53] enables reconstruction of the burial experienced by sedimentary rocks (e.g., [7,[54][55][56][57][58][59][60]).…”

How the geochemistry of syn-kinematic calcite cement depicts past fluid flow and assists structural interpretations: a review of concepts and applications in orogenic forelands

“…Alternatively, U-Pb does not suffer that limit and allows dating of cements as old as 10 8 years. The growing size of the community using U-Pb geochronology leads to a variety of data treatment that permits either treating the cement as a bulk or providing a mapping of U-Pb ratio in microstructures (Drost et al 2018;Hoareau et al 2021b), and also an interest in expanding the limits of LA-ICP-MS detection (Kylander-Clark, 2020). Beyond being fast and relatively cheap, the LA-ICP-MS U-Pb geochronology has been successfully compared to other geochronometers (Mottram et al 2020) like K-Ar, a system that has long been used in tectonics, especially through the dating of neoformed illite in fault gouges by K-Ar or Ar-Ar geochronology (Lyons & Snellenburg, 1971;van der Pluijm et al 2001van der Pluijm et al , 2006Zwingmann et al 2004;Haines & van der Pluijm, 2008;Clauer, 2013;Viola et al 2013;Hnat & van der Pluijm, 2014;Scheiber et al 2019).…”