DETRITAL THERMOCHRONOLOGY AND DOUBLE-DATING OF ZIRCON

The mineral zircon holds a wealth of geochronological information that has been exploited in hundreds of studies. A significant fraction of this work has centered on thermochronologic information retained in single grains shed off orogenic belts. The advantage with this approach is that the thermochronological record is directly related to near-surface processes that drive exhumation of rock in the orogenic belt. As such, the thermochronology of zircons is sensitive to the overall development of an orogen. In many cases, the detrital record is tied to exhumation evolution, and in most exhumation studies the young fraction of grain ages are of particular interest because these cooling ages represent the most rapidly exhuming part of the orogen. One significant issue is that the young population may also contain volcanic zircon, or in more complicated settings it may represent a reset age due to a later thermal event that affected basin strata. Double-dating techniques (FT and ZHe, or ZHe/FT and U/Pb) that are now emerging allow us to determine simultaneously the age of initial crystallization and the timing of subsequent cooling of a single grain. Double-dating using fission tracks involves first doing FT analysis on neutron-irradiated grains with one side of the crystal that has been cut (polished) and etched. This non-destructive technique can be followed by partly destructive (LA-ICPMS) or fully destructive (ZHe) analyses because the neutron irradiation produces neither He nor Pb and other fission products are insignificant. One particularly important issue that double-dating can address is the significance of first-cycle volcanic zircons in sedimentary basins. With single-dating techniques, it is difficult to determine definitively whether a zircon is first cycle and volcanic (or sub-volcanic), or whether it is older and subsequently reheated. Double-dating techniques allow for discrimination of these grains. We highlight the double-dating technique for provenance analysis using several examples from the Pacific Rim.