HR: 14:40h
AN: V22E-05 [PDF]
TI: High-Precision U-Pb Zircon Dates as Benchmarks in Absolute Time
AU: * Schmitz, M D
EM: schmitz@dtm.ciw.edu
AF: Department of Terrestrial Magnestism, Carnegie Institution of Washington, 5241 Broad Branch Road NW,
Washington, DC 20015 United States
AU: Bowring, S A
EM: sbowring@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77
Massachusetts Avenue, Cambridge, MA 02139 United States
AU: Schoene, B
EM: schoene@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77
Massachusetts Avenue, Cambridge, MA 02139 United States
AB:
High-precision IDTIMS U-Pb zircon dates provide the most precise and accurate isotopic benchmarks in absolute time, due to
the concordancy check of the paired U-Pb decay schemes, the precisely measured $^{235}$U and $^{238}$U decay constants, very
high initial parent/daughter ratios, and the robust nature of zircon to loss or gain of U and Pb over geologic time. However,
caveats to the use of such zircon dates include the accurate assessment and minimization of random and systematic errors in
the analytical methods, and decay constant uncertainties. Unfortunately, there exists little consensus within the U-Pb
geochronological community regarding an international zircon standard for the external assessment of interlaboratory
reproducibility, while residual questions remain regarding the potential for systematic error in the single available
high-precision counting experiment of the U decay constants$^{1}$.
Stringent criteria are imposed on candidates for zircon geochronology standards including the absence of inheritance and
Pb-loss at both the single grain scale and the resolution of microbeam techniques. We present an example of the potential and
limitations of a possible zircon standard, AS3, from the Duluth Complex, North American Midcontinent Rift$^{2}$. New data
for 27 single zircons are indistinguishable from prior results, with $^{207}$Pb/$^{206}$Pb and upper intercept dates
identical within error to a U-Pb concordia date of 1099.1$\pm$0.2 Ma ($\pm$1.2 Ma with systematic errors) based on 12
concordant and equivalent analyses. However, we must reiterate that a zircon population exhibiting consistent concordancy
remains elusive, as AS3 and all Paleozoic and older standard candidates so far examined contain grains exhibiting Pb-loss,
although rigorous selection and preparation of zircons through diamagnetic separation and aggressive abrasion can mitigate
this phenomenon. The continued screening of candidate standards by both IDTIMS and SHRIMP techniques should be an organized,
international endeavor involving all high-precision geochronology laboratories.
Under the single assumption that the equivalent data represent the approach to closed system behavior, the correspondence of
the AS3 zircons with the presently defined concordia curve suggests the accuracy of the ratio of the presently accepted decay
constants$^{1}$ to within their 0.1% (2$\sigma$) counting errors. While a proposed revision of the $^{235}$U decay
constant$^{3}$ is apparently unnecessary, additional high-precision, high n, statistically equivalent zircon population
samplings are necessary to further evaluate decay constants and their uncertainties at the per mil level.
When measured on appropriate lithologies, high-precision U-Pb dates also become powerful tools for the intercalibration of
other radioisotope decay rates more difficult to measure through counting or accumulation (e.g. $^{40}$K, $^{176}$Lu,
$^{187}$Re). We will present several new high-precision data sets for quickly cooled Oligocene to Archean extrusive and
intrusive rocks, and assess their usefulness as benchmarks for the intercalibration of radiometric time; a growing number of
high-precision U-Pb zircon and $^{40}$Ar/$^{39}$Ar feldspar, biotite or hornblende pairs spanning more than 3 Ga in absolute
time indicate a consistently younger bias in $^{40}$Ar/$^{39}$Ar dates of between 0.7 and 1%.
$^{1}$Jaffey et al. (1974) Phys Rev C 4:1889-1906; $^{2}$Paces and Miller (1993) J Geophys Res 98:13997-14013;
$^{3}$Mattinson (2000) EOS 81:S444
DE: 1035 Geochronology
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting