Laser-cut Rb-Sr microsampling dating of deformational events in the Mont Blanc-Aiguilles Rouges region (European Alps)

• Published in: Terra nova (Oxford, England) Vol. 28; no. 1; pp. 35 - 42
• Main Authors: Egli, Daniel, Müller, Wolfgang, Mancktelow, Neil
• Format: Journal Article
• Language: English
• Published: Oxford Blackwell Publishing Ltd 01.02.2016
• Subjects: Deformation; Geological time; Geophysics
• ISSN: 0954-4879; 1365-3121
• DOI: 10.1111/ter.12184

Spatial control for in situ dating of mineral phases in fine‐grained rocks is a significant challenge in geochronology, and the precision of microsampling is a crucial factor in obtaining accurate results. In this study, a new microsampling approach to Rb–Sr geochronology has been applied to greenschist facies mylonitic shear zones in the Mont Blanc‐Aiguille Rouges region of the western European Alps. Using a laser‐ablation system for microsampling by laser cutting followed by conventional TIMS Rb–Sr isotopic analysis of μg‐sized samples provides an improved workflow for texturally controlled, quasi in situ dating of mineral phases. The automated cutting process minimizes material loss and the risk of handling errors, while facilitating sampling of complex shapes of almost any size, a significant improvement over earlier microscope‐mounted microdrills. The new Rb–Sr white mica–calcite ages of between 27 and 30 Ma indicate Oligocene deformation in Alpine shear zones from two specific areas in the Mont Blanc‐Aiguilles Rouges region.