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Improved confidence in (U‐Th)/He thermochronology using the laser microprobe: An example from a Pleistocene leucogranite, Nanga Parbat, Pakistan
Author(s) -
Boyce J. W.,
Hodges K. V.,
King D.,
Crowley J. L.,
Jercinovic M.,
Chatterjee N.,
Bowring S. A.,
Searle M.
Publication year - 2009
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2009gc002497
Subject(s) - leucogranite , geology , thermochronology , microprobe , geochemistry , mineralogy , zircon , metamorphic rock , gneiss
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 4 He 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 4 He 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.