The influence of high U-Th inclusions on the U-Th-Pb systematics of almandine-pyrope garnet: results of a combined bulk dissolution, stepwise-leaching, and SEM study

Abstract

The isotope systematics of garnet are increasingly being used to define prograde thermal histories in metamorphic belts. This approach to geochronology has the distinct advantage that time information is directly related to the pressure-temperature-deformation history recorded by the same mineral. One potential problem with the technique, however, is that the inventories of radiogenic isotopes apparently measured in garnet separates may be located in inclusions rather than the garnet lattice itself. Inclusions of old, refractory minerals such as zircon may bias the ages to the extent that they are rendered meaningless in terms of dating garnet growth, establishing PT histories, and constraining tectonic mechanisms. In this contribution we present U-Th-Pb data obtained on bulk dissolutions of garnet from the Himalaya as well as the results of a detailed SEM and U-Th-Pb stepwise-leaching study in order to further clarify these issues. Our experiment has the advantage that the age difference between the garnets and potential inclusions is very large so that the influence of the latter is highlighted. The U-Th-Pb data from the bulk dissolutions are highly complex and yield apparent ages ranging from 1000 Ma to 30 Ma. The stepwise-leaching study demonstrates that this complexity results from small inclusions of allanite and zircon detected with the SEM. The bulk of the radiogenic 206Pb in these samples was released only by treatment at high pressure in a bomb. In addition, the 208Pb/ 206Pb ratio of the radiogenic Pb released in earlier leach steps is inconsistent with the available constraints on the Th/U ratio of garnet lattices as measured by SIMS. In this case, the U-Pb ages of these garnets is dependent purely on the age of the included high U-Th phases and the degree to which they have isotopically re-equilibrated with the whole rock during the subsequent metamorphism that grew garnet. Simple mass-balance arguments show that, in this particular instance, very small amounts of these inclusions are required, amounts that escape detection during conventional sample preparation. These data also have implications for current thinking on the closure temperature for the Sm-Nd and U-Pb systems in garnet.