Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb carbonate geochronology: strategies, progress, and limitations

Abstract

Abstract. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb geochronology of carbonate minerals, calcite in particular, is rapidly gaining popularity as an absolute dating method. The high spatial resolution of LA-ICP-MS U–Pb carbonate geochronology has benefits over traditional isotope dilution methods, particularly for diagenetic and hydrothermal calcite, because uranium and lead are heterogeneously distributed on the sub-millimetre scale. At the same time, this can provide limitations to the method, as locating zones of radiogenic lead can be time-consuming and “hit or miss”. Here, we present strategies for dating carbonates with in situ techniques, through imaging and petrographic techniques to data interpretation; our examples are drawn from the dating of fracture-filling calcite, but our discussion is relevant to all carbonate applications. We review several limitations to the method, including open-system behaviour, variable initial-lead compositions, and U–daughter disequilibrium. We also discuss two approaches to data collection: traditional spot analyses guided by petrographic and elemental imaging and image-based dating that utilises LA-ICP-MS elemental and isotopic map data.

Regional-scale paleofluid system across the Tuscan Nappe–Umbria–Marche Apennine Ridge (northern Apennines) as revealed by mesostructural and isotopic analyses of stylolite–vein networks

“…the surface energy at scale typically <1 mm and the elastic energy at scale >1 mm). The stylolite growth model (Koehn et al, 2007;Ebner et al, 2009a;Rolland et al, 2012;Toussaint et al, 2018) predicts that the surface energy-controlled scale returns a steep slope characterized by a roughness exponent (so-called Hurst exponent) of 1.1±0.1, while the elastic energy-controlled scale returns a gentle slope with a roughness exponent of 0.5 to 0.6 ( Fig. 5).…”

“…As the dissolution occurs along a fluidic film (Koehn et al, 2012;Rolland et al, 2012;Toussaint et al, 2018), the stylolite roughness is unaffected by local fluid overpressure until the system is fluidized and hydro-fractures (Vass et al, 2014), meaning it is possible to translate vertical stress magnitude directly into depth if considering an average dry rock density for clastic and carbonate sediments (2400 g m −3 ; Manger, 1963), without any additional assumption on the past thermal gradient or fluid pressure . This technique has already provided meaningful results in various settings (Bertotti et al, 2017;Rolland et al, 2014;Beaudoin et al, 2019Beaudoin et al, , 2020.…”

Regional-scale paleofluid system across the Tuscan Nappe – Umbria Marche Arcuate Ridge (northern Apennines) as revealed by mesostructural and isotopic analyses of stylolite-vein networks

“…Of the 35 samples screened for favorable U-Pb ratios, only 2 were selected for U-Pb dating (FAB5 and FAB6). Noteworthy, all samples from veins, whatever the set they belong to, reveal to have a U/Pb ratio not high enough to return an age, with a too low U content and/or dominated by common lead (see Supplementary material), which seems to be common in tectonic veins (Roberts et al, 2020). Only two samples consisting of calcite fault coating provided suitable material and were further analysed.…”

Syncing fault rock clocks: Direct comparison of U-Pb carbonate and K-Ar illite fault dating methods

“…Carbonate U-Pb geochronology can provide direct timing constraints for geological processes including diagenesis, fluid flow, and tectonic processes (e.g., Rasbury and Cole, 2009). In situ U-Pb dating has been applied to low-U fault-related carbonate (e.g., Roberts and Walker, 2016;Nuriel et al, 2017;Parrish et al, 2018;Roberts et al, 2020). Previous indirect comparison of fault-gouge illite 40 Ar/ 39 Ar and carbonate U-Pb ages suggests concordance between the two methods (Nuriel et al, 2019b), but no direct comparison has been undertaken by applying both techniques to colocated samples from the same fault system.…”