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 (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.
Copyright 2009 by the American Geophysical Union. DOI: 10.1029/2009GC002497
Boyce, J. W.; Hodges, K. V.; King, D.; Crowley, James L.; Jercinovic, M.; Chatterjee, N.; Bowring, S. A.; and Searle, M.. (2009). "Improved Confidence in (U-Th)/He Thermochronology Using the Laser Microprobe: An Example from a Pleistocene Leucogranite, Nanga Parbat, Pakistan". Geochemistry Geophysics Geosystems, 10Q0AA01. http://dx.doi.org/10.1029/2009GC002497