HR: 10:20h
AN: V32B-01 INVITED    [Abstracts]
TI: Nonmonotonic (reheating) thermal histories from contrasting kinetics of multiple thermochronometers
AU: * Reiners, P W
EM: reiners@u.arizona.edu
AF: University of Arizona, Dept. of Geosciences, Tucson, AZ 85721, United States
AU: Thomson, S N
EM: stuart.thomson@yale.edu
AF: Yale University, Dept. of Geology & Geophysics, New Haven, CT 06511, United States
AU: Min, K K
EM: kmin@ufl.edu
AF: University of Florida, Dept. of Geological Sciences, Gainesville, FL 32611, United States
AB: Reheating events are often difficult to deduce in thermochronology, because the age resetting they cause can usually be modeled by varying the form of a presumably simpler monotonic cooling path (an exception to this is fission-track length modeling). However, reheating and full or partial resetting due to metamorphism, hydrothermal circulation, magmatism, wildfire, or (at least in the case of meteorites) impacts, are likely common in many settings. Such effects may be particularly important for samples that have resided for long periods at or near the surface with old cooling ages, where they are susceptible to brief, high-temperature events. Failure to recognize reheating may lead to erroneous tectonic interpretations. Nonmonotonic thermal histories may be resolved by using multiple thermochronometric systems with appropriately contrasting kinetic properties. At relatively high temperatures and short timescales, systems with different activation energy ( E), frequency factor ( D₀) and domain size (a) display crossovers in diffusion (or annealing) rates that may be used to diagnose reheating episodes of particular intensity and duration. The most diagnostic effect of these kinetic crossovers are apparent "age inversions" in which systems with higher closure temperatures ( T_c) are more strongly reset (resulting in younger ages) than systems with lower T_c (e.g., apatite fission-track and He systems). In cases of complete resetting of the higher- T_c system and partial resetting of the lower- T_c system, reheating may be diagnosed and the intensity and duration of the event partially constrained. When both systems are partially reset, Dt/a² of the reheating event can be calculated and used to estimate the specific form and timing of reheating thermal histories. Examples of high temperature thermochronometers with potentially useful kinetic crossovers include the Rb-Sr system in both biotite and muscovite coupled with many higher temperature systems such as Ar in hornblende and muscovite, Pb in titanite and apatite, Sm/Nd in garnet, and Rb-Sr in Kspar. The anomalously low E of the Rb- Sr system in micas makes these couplings useful for examining reheating events on the order of ~400-700 °C over timescales of 10⁶-10⁸ yr. At lower temperatures, systems with anomalously low E, such as Ar in maskelynite and He in basaltic glass may be coupled with higher E systems such as AFT, and He in apatite or goethite to diagnose reheating events at ~200-400 °C over timescales from seconds to ~10² yr. We applied these approaches to investigating reheating related to wildfire and meteorite impacts. We observe abundant FT-He age inversions in apatite from the outer surfaces of exposed bedrock and hillslope detritus, requiring heating from 200-450 °C over timescales of minutes to hours. Fluvial detrital apatite from the same catchments, however, only rarely carry such signatures, indicating fractionation of apatite from hillslopes to channels. In Martian meteorite ALH84001, Ar in maskelynite has a much lower E than He in phosphates, resulting in a kinetic crossover at roughly 150 °C (for equal fractional resetting of both systems). Taken together, the relatively large He losses from phosphate and low Ar losses from maskelynite require very long residence at extremely low temperatures, but at least one short duration (minutes to hours), high temperature heating event, which if it occurred only once (at 15 Ma) reached temperatures of ~400-450 °C.
DE: 1115 Radioisotope geochronology
DE: 1140 Thermochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting