HR: 14:55h
AN: V22E-06    [PDF]
TI: CA (Chemical Abrasion) - TIMS: High-Resolution U-Pb Zircon Geochronology Combining High-Temperature Annealing of Radiation Damage and Multi-Step Partial Dissolution Analyis
AU: * Mattinson, J M
EM: mattinson@geol.ucsb.edu
AF: Department of Geological Sciences, University of California, Santa Barbara, CA 93106-9630 United States
AB: Recent advances in methods for U-Pb geochronology of zircon demonstrate excellent potential for high-accuracy (sub 0.1% uncertainty) dating of magmatic events, and also shed new light on complexities in isotopic systematics related to intermediate daughter product disequilibrium and decay constant uncertainties. Multi-step analysis of zircon by "PDA" (partial dissolution analysis) has long shown promise. Unfortunately, leaching effects related to alpha-recoil and other forms of radiation damage seriously compromise results for many samples. However, radiation damage can be removed prior to PDA by annealing in the range of 800 to 1100$\deg$C, dry, at 1 Atm, for 48 hours. Subsequent multi-step analysis is largely free from any leaching effects, and represents a progressive stripping (Chemical Abrasion) of zircons, typically from high U-Th rim material to low U-Th core material. For zircons lacking inheritance, initial steps showing Pb-loss are followed by a plateau of 206Pb*/238U ages, commonly yielding plateau ages with uncertainties in the range of 0.05 to 0.1% (2-sigma). Obviously results are more complex when inherited components (especially of multiple ages) are present. However, since early steps remove virtually all Pb-loss effects for most samples, inheritance is easier to recognize and interpret. Multi-step high-precision 207Pb*/206Pb* ages for samples yielding excellent 206Pb*/238U plateau ages shed new light on isotopic complexities related to intermediate daughter product disequilibrium effects. For example, results from zircons from plagiogranites in Jurassic ophiolites indicate that minor but significant 231Pa excess is quite common. Thus high-precision 207Pb*/206Pb* ages are not necessarily high in accuracy, and should be interpreted with some care. In such cases, approximate agreement of the 206Pb*/238U and 207Pb*/206Pb* ages provides a first order indication of "concordance" or "discordance" at best. A high-resolution 206Pb*/238U multi-step plateau age is a superior measure of the true magmatic age.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1055 Organic geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting