HR: 0800h
AN: V51B-0569    [Abstracts]
TI: An Assessment of the Total Uncertainly in Hf Isotope Analyses by LA-MC-ICPMS
AU: * Vervoort, J D
EM: vervoort@wsu.edu
AF: School of Earth & Environmental Sciences, Washington State University, Pullman, WA 99164, United States
AU: DuFrane, S A
EM: dufrane@wsu.edu
AF: School of Earth & Environmental Sciences, Washington State University, Pullman, WA 99164, United States
AU: Hart, G L
EM: ghart@wsu.edu
AF: School of Earth & Environmental Sciences, Washington State University, Pullman, WA 99164, United States
AB: The analysis of Hf isotopes in zircon by LA-MC-ICPMS is a promising new analytical method that is increasingly being used by many laboratories. The total uncertainty of these laser Hf isotopic analyses, however, has several potential components that should be fully accounted for in assigning total analytical uncertainty. Currently, this full respresentation of uncertainty is not widely done by the laser Hf community. The sources of potential uncertainty in a laser Hf isotope analysis include: 1) uncertainty during the Hf analysis (within-run uncertainty); 2) uncertainty in the correction of ¹⁷⁶Yb and ¹⁷⁶Lu on ¹⁷⁶Hf (included in this must be the allowance for the differences in mass bias behavior between Yb/Lu and Hf, which are not equivalent, sensu stricto); 3) matrix effects between zircon grains (this effect has been demonstrated for U-Pb zircon analysis and may be significant for some zircon grains); 4) any instrumental biases in the determination of Hf isotopic composition (most MC- ICPMS analyses of the JMC 475 Hf standard, for example, do not give exactly ¹⁷⁶Hf/¹⁷⁷Hf=0.282160 and require some normalization to a common value, which is difficult to do with laser Hf analyses). Furthermore, the determination of initial Hf isotope values requires both precise and accurate age determinations and accurate ¹⁷⁶Lu/¹⁷⁷Hf ratios in addition to the above factors that contribute uncertainty in present-day Hf isotope ratios. The effect of ¹⁷⁶Lu/¹⁷⁷Hf uncertainty on the initial values is small because of the very low ratio in zircon but the age correction is highly significant because the CHUR reference varies by ~2.3 ε_Hf units/100 my. For zircons with a highly complex growth history (e.g., early Archean and Hadean zircons) linking precise and accurate age determinations to the Hf isotopic analysis is especially challenging and represents a potentially large source of uncertainty for these zircons. Because only in-run uncertainty can be determined during a single analysis of an unknown, most laboratories only report in-run precision of the laser Hf analysis. This clearly underestimates (in some cases drastically) the total uncertainty. One solution to determining overall uncertainty in a laser Hf analysis is to analyze multiple standards with Lu/Hf and Yb/Hf ratios that closely match the unknown zircons being analyzed. The variance of these analyses (estimated by 2σ SD of the population) relative to precisely determined Hf isotopic composition by analysis of purified solutions will provide a fair and reasonable estimate of the overall uncertainty of individual analyses of unknowns.
DE: 1040 Radiogenic isotope geochemistry
DE: 1094 Instruments and techniques
DE: 1115 Radioisotope geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting