Pb stepwise leaching (PbSL) dating of garnet-addressing the inclusion problem

The problem of monazite and zircon inclusions for garnet geochronology has long been recognized. Both phases occur as (sub)microscopic inclusions in the majority of metamorphic garnets, and their presence can massively influence the budgets of U, Th, Pb and other elements and therefore may strongly affect - if not dominate - the apparent age obtained for the host mineral. Special care has to be taken if such inclusions were formed during earlier events, as they are often highly retentive for U, Th and Pb, and bulk garnet dating will give geologically meaningless mixing ages. To assess this critical issue we have selected three garnets from different metamorphic terrains on which we performed Pb stepwise leaching (PbSL). We demonstrate that PbSL is potentially successful (i) to detect micro-inclusions of monazite and zircon in garnet by analyzing the step leaching Pb isotope spectra in the thorogenic vs uranogenic data array, (ii) to selectively and quantitatively remove monazite-derived Pb from sufficiently fine-grained host garnet splits 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 (iv) to constrain the approximate age of the interfering monazite and zircon inclusions.