A Contribution to U (Th)-Pb Dating of Epidote and Allanite, and its Implications for Metamorphic and Hydrothermal Fluid Flow Histories

Abstract

The application of U-(Th)-Pb geochronology to rockforming silicates has gained significant interest, and improvements in the last few years are due to analytical achievements and due to the development of alternative dating methods. One of these methods, Pb stepwise leaching (PbSL), profits from the advantage that, through acid leaching, radiogenic Pb in a mineral can be preferentially released relative to common Pb supposedly incorporated into the mineral's structure at the time of its formation (e.g. Frei et al., 1997). Distinct, selective dissolution of a host silicate phase may be able to account for an increased data spread along the axes of the Pb-Pb isochron diagram. The selectivity regarding the two types of Pb consequently allows for the assessment of Pb-Pb geochronological data from silicates with unfavourable U/Pb ratios (e.g. Frei et al., 1997; Dahl and Frei, 1998). This study aims at assessing the utility of epidote group minerals in U-Pb dating of metamorphism and ancient crustal fluid flow, and to place preliminary constraints on closure temperature to Pb diffusion in both, epidote and allanite. Whereby epidote is commonly characterized by low U/Pb, allanite exhibits usually very high Th and high U concentrations, making this mineral a suitable candidate to UTh-Pb dating (e.g. Barth, 1994; von Blanckenburg, 1992, and others).