USING U-PB GEOCHRONOLOGY OF CALCITE TO CONSTRAIN THE TIMING OF FAULT EVOLUTION

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.