GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 25-3
Presentation Time: 8:30 AM


ROBERTS, Nick M.W., NERC Isotope Geosciences Laboratory, British Geological Survey, Nottingham, NG12 5GG, United Kingdom, WALKER, Richard J., Department of Geology, University of Leicester, Leicester, LE1 7RH, United Kingdom and RASBURY, Troy, Department of Geosciences, Stony Brook University, Stony Brook, NY 11794-2100,

There is a lack of readily available techniques for providing absolute chronological data on brittle fault zone evolution. Calcite is an abundant fracture-filling mineral in many geological settings, and has the potential to be dated by U-Pb geochronology. Calcite mineralisation can often be linked directly to fault displacement/opening events, or to bracketing such events; it can also provide a record of fluid flow and the associated fluid composition through a fault/fracture system. Thus, the absolute dating of calcite formation has the potential to provide a widely accessible technique to enhance our understanding of fault evolution and upper crustal deformation.

Previous attempts at dating hydrothermal calcite have been unsuccessful due to unfavorable U/Pb ratios. So far no physical criteria have been identified that would help to distinguish the best samples for geochronology. It is clear that the source fluid must be able to transport U in solution, requiring it to be oxygenated, because U is immobilized with reducing fluids. Here we present: 1) a compilation of U and Pb concentrations and ratios of calcite from faults and fractures in a range of geological settings; 2) a method of dating calcite with the U-Pb isotopic system using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS); 3) discuss the benefits and pitfalls of applying this technique to calcite and brittle fault zone evolution; and 4) present a case study of rift fault evolution in the Faroe Islands, NE Atlantic margin.