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

geochronology

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