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Forward Modeling and Interpretation of (U-Th)/He Ages

Faculty of Earth and Life Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV, Amsterdam, The Netherlands, dunt@geo.vu.nl

The first 20% of the full text of this article appears below.

	INTRODUCTION

 
Soon after the initial studies on He-diffusion in apatite pointed out the use of the (U-Th)/ He system for low temperature geochronology (Zeitler et al. 1987; Wolf et al. 1996; Warnock et al. 1997), the potential value of forward modeling for refined interpretation of (U-Th)/He dating was indicated (Wolf et al. 1998). While the first studies provided evidence for the low closure temperature (T_C, Dodson 1973) of the (U-Th)/He system in apatite for monotonously cooling systems, the latter study investigated the sensitivity of the (U-Th)/He system for a wide range of geologically feasible heating/cooling scenarios. Forward modeling of He-accumulation in minerals expanded the use of the (U-Th)/He geochromometer to geological systems that do not fit one of the main conditions for the meaningful application of the closure temperature concept, i.e. monotonous cooling at a constant rate through T_C. Forward modeling can help to give a meaning to apparent (U-Th)/He-ages from rocks with cooling histories with protracted residence time in the helium partial retention zone (HePRZ; e.g., ~40–85 °C in apatite; Wolf et al. 1998), where the closure temperature is undefined (Fig. 1).

View larger version (14K): [in this window] [in a new window]   Figure 1. Apatite He-age evolution curves (dotted lines) for several representative time-temperature paths (solid lines). Forward modelling was conducted using the Crank-Nicolson method for spheres of 60 µm radius. Note that only for history 2 the closure temperature concept (Dodson 1973) is defined, and the age obtained relates to passing through a point (Tc) on the time-temperature path. For histories that are similar to 3–5, which know a protracted residence time in the HePRZ, there is no direct geological meaning of He-ages. Forward modelling is required to give a geological meaning to the apparent ages obtained from He and U, Th concentrations in samples. . . . [Full Text of this Article]

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