HR: 1340h
AN: V23B-1445    [Abstracts]
TI: Isotopic Exchange in Igneous Zircons
AU: * Weaver, K L
EM: karrie@eps.berkeley.edu
AF: University of California, Berkeley, Deptartment of Earth and Planetary Science, Berkeley, CA 94720-4767, United States
AU: DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: University of California, Berkeley, Deptartment of Earth and Planetary Science, Berkeley, CA 94720-4767, United States
AB: Igneous zircons are a powerful tool for determining crystallization age and metamorphic history of rocks spanning Earth's entire geologic history. High concentrations of uranium and exclusion of lead in zircons make the U-Pb system the primary choice for radiogenic dating. This system has the added advantage of revealing potential parent or daughter loss as discordance on a concordia diagram. For a single mineral grain the U-Pb system reveals not only the age, but an estimate of post-crystallization alteration. As the range of isotopic analyses applied to ancient zircons broadens (oxygen isotopes, Lu-Hf), the ability to evaluate mobility in radiogenic systems other than U-Pb becomes more important. Low concentrations of many trace elements in zircon make them sensitive recorders of post-crystallization isotopic exchange, but their behavior is poorly understood. Comparison of the Sr- and Nd-isotopic systematics of zircon and its host rock can be used to evaluate the retentivity of zircon for the isotopes of these elements . We examined zircon populations from granitoids ranging in age from 100 Ma to 3.7 Ga. Zircon populations were dated via SHRIMP and both multigrain zircon separates and whole rock powders were analyzed for trace element concentrations, ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd. Analyses of other mineral separates (feldspar, apatite) allow the construction of whole- rock mineral isochrons. In each sample the zircons display disturbance in both the Rb-Sr and Sm-Nd systems. In fact, when compared to the all the other minerals represented on the isochron, the zircons display the highest amount of discordance. Likely promoted by radiation damage, the degree of open-system behavior of the Rb-Sr and Sm-Nd systems increases with the age of the samples. With greatly increased effective diffusivities and trace element mobility, general acceptance of zircons as impervious isotopic time capsules should be regarded with caution.
DE: 3617 Alteration and weathering processes (1039)
DE: 3618 Magma chamber processes (1036)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 8410 Geochemical modeling (1009, 3610)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting