PB STEPWISE LEACHING (PBSL) DATING OF GARNET-ADDRESSING THE INCLUSIONPROBLEM

The problem of monazite and zircon inclusions for garnet geochronology has long been recognized. Both phases occur as (sub)microscopic inclu sions in the majority of metamorphic garnets, and their presence can m assively influence the budgets of U, Th, Pb and other elements and the refore may strongly affect - if not dominate - the apparent age obtain ed for the host mineral. Special care has to be taken if such inclusio ns were formed during earlier events, as they are often highly retenti ve for U,Th and Pb, and bulk garnet dating will give geologically mean ingless mixing ages. To assess this critical issue we have selected th ree garnets from different metamorphic terrains on which we per formed Pb stepwise leaching (PbSL). We demonstrate that PbSL is potentially successful (i) to detect micro-inclusions of monazite and zircon in ga rnet by analyzing the step leaching Pb isotope spectra in the thorogen ic vs uranogenic data array, (ii) to selectively and quantitatively re move monazite-derived Pb from sufficiently fine-grained host garnet sp lits and therefore to obtain monazite-free Pb-Pb isochron ages of the host garnet, (iii) to assess whether or not inclusions were in isotope equilibrium with their host as this crystallized, and subsequently (i v) to constrain the approximate age of the interfering monazite and zi rcon inclusions.