U-Pb dating of brittle deformation

A new method for dating brittle deformation is presented, based on U-Pb dating of syndeformationally grown calcite fibres, or "tectonic carbonates", found e.g. on brittle slickensides and in tension fractures. Brittle structures from various areas in the Alps were sampled from outcrops where the deformation age is well constrained in the literature, to be able to test the results of the new method.
Laser ablation inductively-coupled plasma mass spectrometry (LA-ICPMS) is used for rapid screening of samples with respect to their U, Th, Pb and other trace and major element concentrations and spacial distribution. For data reduction of LA-ICPMS data, an add-on package for the open-source "R" environment was developed that in addition to one-dimensional single line scans allows two-dimensional trace and major elemental mapping of the samples.
Based on this spatial information, regions within the samples with high, yet variable U/Pb ratios are chosen for mechanical subsampling. For subsequent solution-based U-Th-Pb isotopic analysis, an optimized low-Pb blank methodology has been developed to facilitate work with very low-concentration (typ. ≤ 1 ppm U, as low as tens of ppb Pb) calcite samples.
High precision isotope-dilution U-Th-Pb isotope ratio measurements are conducted on a multi-collector (MC-) ICPMS. Alternatively, determination of isotopic ratios directly from LA-ICPMS analyses are conducted in situ, which yields results much faster, but with limited precision due to instrumental limitations.
Only a small fraction of screened samples yielded precise ages. The deformation ages fit respective published models of regional structural evolution. The ages obtained in situ correspond well with the data from solution chemistry, although with very large error margins. Where repeated measurement of the same structures was achieved, the respective ages lie within error of each other.
For faulting in the Swiss Jura, ages of 9.05 ± 0.94 Ma were obtained, constraining maximum age of later folding. Thrusting of the most external Digne Nappe (French Alps) was dated to 9.59 ± 0.05` Ma, in accordance with literature. In the Austrian Gosau Basin, distinct deformation phases at 29 ± 2 and 23 ± 1 Ma could be observed, ages from the LA-ICPMS data of ~42 and ~27 Ma. confirm the first age and add an older observed deformation phase. This correlates with basin formation and Miocene tectonic overprinting in this area. Presumably hydrothermally grown calcite sealing fissures associated with the formation of the Miocene Fohnsdorf basin (Austria), could be dated to 13.44 ± 0.84 Ma. Other samples yielded less conclusive age results, mainly due to geological scatter, aggravated by the extremely low Pb concentrations and U/Pb ratios in some samples.