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| Titel |
Evaluating 238U/235U in U-bearing accessory minerals: implications for U-Pb geochronology |
| VerfasserIn |
Joe Hiess, Daniel J. Condon, Stephen R. Noble, Noah McLean, Samuel A. Bowring, James M. Mattinson |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250056910
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| Zusammenfassung |
| U-daughter (U-Pb, Pb-Pb, and U-series) geochronology and cosmochronology utilise the
absolute value of the present day 238U/235U ratio to determine U/Pb and Pb/Pb isotope ratios
and compare derived dates. For decades, this value has been assumed to be invariant and
equal to 137.88, but recent experiments indicate that there is potential for per mil level
variation in 238U/235U in natural materials, hypothesized to be the result of redox reactions.
These studies have largely focused on materials formed in low-temperature environments
(e.g. speleothems, corals) and U ore deposits. At present there are no published
high-precision high-accuracy 238U/235U data for U-bearing accessory minerals commonly
used for U-Pb geochronology.
We present accurate and precise 238U/235U determinations (absolute uncertainties of
~200 ppm) for a suite of common U-bearing accessory minerals (zircon, monazite etc.), from
a variety of geological environments and ages. Measurements have been made by
multi-collector thermal ionization mass spectrometry and multi-collector inductively coupled
plasma mass spectrometry, accurately correcting for mass fractionation using the IRMM
3636 233U-236U double spike. These results indicate that accessory mineral 238U/235U ratios
are generally lower than the ‘consensus’ value of 137.88 and record limited but resolvable
variation.
Systematic discordance has been observed in 238U-206Pb and 235U-207Pb dates obtained
for closed-system minerals, and has been used to reassess the relative decay constants of 238U
and 235U (Mattinson, 2000, 2010; Schoene et al., 2006). These studies derive λ235U
relative to λ238U by assuming equivalence between 238U-206Pb and 235U-207Pb dates
and using assumed values (i.e., 137.88 or 137.80) for the present-day 238U/235U
ratio. Our new determination of coupled 238U/206Pb, 235U/207Pb and 238U/235U
measurements on the same closed system zircons permits further refinement of λ238U/λ235U
estimates using parameters whose values and uncertainties are all traceable to SI
units.
Mattinson J.M. 2000. Revising the "gold standard" - the Uranium decay constants of
Jaffey et al., 1971. EOS, AGU Fall meeting Supplement Abstract V61A-02.
Mattinson J.M. 2010. Analysis of the relative decay constants of 235U and 238U by
multi-step CA-TIMS measurements of closed-system natural zircon samples. Chemical
Geology 275: 186-198.
Schoene B., Crowley J.L., Condon D.C., Schmitz M.D. and Bowring S.A. 2006.
Reassessing the uranium decay constants for geochronology using ID-TIMS U-Pb data.
Geochimica et Cosmochimica Acta 70: 426-445.
Research funded by the European Community’s 7th Framework Programme grant
agreement nË 215458. |
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