Diffusion of osmium in pyrrhotite and pyrite: implications for closure of the Re¯Os isotopic system

A technique for texturally controlled microsampling from rock thick sections and subsequent Rb¯Sr dating of mu g-sized samples has been developed. It allows direct dating of minerals unequivocally related to deformation. The technique has been applied to strictly synkinematically grown white mica prepared from mylonites related to the Defereggen¯Antholz¯Vals (DAV) fault south of the Tauern window (Eastern Alps). Control of deformation ages is provided by field relationships with independently dated plutons and dykes dated here by single mineral Rb¯Sr. The field observations and isotopic ages are fully consistent and demonstrate that this is a reliable method for sub-mm-scale geochronology. The ages obtained establish that sinistral strike slip shearing associated with the DAV fault lasted from at least 33 Ma to 30 Ma, followed by an abrupt change to dextral transpression related to the adjacent Periadriatic fault.