40Ar/39Ar Thermochronology of Detrital Minerals

Abstract

Clastic sediments and sedimentary rocks provide important records of the erosional history of active and ancient orogenic systems. For example, foreland basin deposits are especially rich sources of information about the character of orogenic hinterlands that have long-since eroded away. A popular approach to extracting such information is the U-Pb dating of detrital zircon (e.g., Ross and Bowring 1990; Gehrels 2000); by matching detrital zircon age populations with age patterns in exposed bedrock regions, it is possible to constrain the source regions for basin fill—provided, of course, that what is left of the hinterland is at least representative of what was there at the time of exhumation! One reason that zircon is used for such studies is that it has a very high closure temperature for Pb diffusion, higher that metamorphic temperatures in all but high-temperature granulite settings (Cherniak and Watson 2000), so that detrital U-Pb zircon dates almost always reflect provenance ages. Unfortunately, the refractory nature of the U-Pb zircon system means that it is less useful for understanding the thermal and erosional evolution of the hinterland than other systems with lower closure temperatures. Other chapters in this volume deal with the application of low-temperature (U-Th)/He and fission-track thermochronometers to detrital samples. Here we review the methodology of detrital 40Ar/39Ar thermochronology and explore how it has been used in a variety of tectonic studies. Additional perspectives are available in Stuart (2002). We presume a basic familiarity with the fundamentals of 40Ar/39Ar geochronology and the laser 40Ar/39Ar microprobe; readers who would like a review should consult Harrison and Zeitler (2005) of this volume, McDougall and Harrison (1998), and Hodges (1998). With the development of laser microanalytical protocols for terrestrial 40Ar/39Ar geochronology in the late …