Improved Confidence in (U-Th)/He Thermochronology Using the Laser Microprobe: An Example from a Pleistocene Leucogranite, Nanga Parbat, Pakistan
Copyright 2009 by the American Geophysical Union. DOI: 10.1029/2009GC002497
The newly developed laser microprobe (U-Th)/He thermochronometer permits, for the first time, the ability to generate precise (U-Th)/He cooling ages for even very young (<1>Ma) samples with a spatial resolution on the order of tens of micrometers. This makes it possible to test the reproducibility of independent (U-Th)/He age determinations within individual crystals, further increasing the reliability of the method. As an example, we apply it here to a Pleistocene granite from Nanga Parbat, Pakistan, where previous constraints on the thermal history are consistent with rapid exhumation and cooling. Twenty-one (U-Th)/He dates determined on two monazite crystals from a single granite sample yield a mean of 748,000 years with a ~95% confidence level of ±19,000 years. There is no discernible variation in the distribution of (U-Th)/He ages in the cores of these crystals and therefore no evidence for the development of substantial diffusive-loss 4He zoning over 80% of the interior of the monazite crystals during postcrystallization cooling of the granite. Modeling of these data suggests that cooling at a mean rate of ~300 K/Ma would be necessary to produce the observed ages and the lack of a 4He gradient, which is consistent with preexisting constraints for Nanga Parbat. Increased precision in thermochronology permits more tightly constrained exhumation models, which should aid geologic interpretation.