Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010e%26psl.300..343a&link_type=abstract
Earth and Planetary Science Letters, Volume 300, Issue 3-4, p. 343-350.
Physics
20
Scientific paper
Accurate determination of the absolute ages of the oldest Solar System objects — chondrules and Ca-Al-rich inclusions (CAIs), requires knowledge of their 238U/235U ratios. This ratio was assumed to be invariant in all U-Pb dating of meteorites so far, but the recent discovery of U isotope variations in CAIs (Brennecka et al., 2010a) shows that this assumption is invalid. We present the first combined high-precision U and Pb isotopic data for a CAI, and U isotopic data for chondrules and whole rock fractions of the Allende meteorite. The Pb-Pb isochron age of the CAI SJ101 is 4567.18 ± 0.50 Ma, calculated using the measured 238U/235U = 137.876 ± 0.043 (2σ), reported relative to 238U/235U = 137.837 of the CRM 145 standard. Our best current estimate of the average terrestrial value is: 238U/235U = 137.821 ± 0.014.The error in the age includes uncertainties in the Pb-Pb isochron intercept and in the 238U/235U ratio. Allende bulk rock and chondrules have 238U/235U = 137.747 ± 0.017 (2σ), distinctly lower than the CAI. The difference in the 238U/235U ratio of 0.129 ± 0.046 (2σ) between the CAI and chondrules and bulk meteorite increases the 207Pb-206Pb age difference by ~ 1.4 Ma, and eliminates apparent disagreement between the CAI-chondrule formation time interval determinations with the U-Pb and extinct nuclide (26Al-26Mg and 182Hf-182W) data. We discuss standardisation of 238U/235U measurements for U-Pb geochronology and cosmochronology, elemental and isotopic fractionation induced by intensive acid leaching, ages of CAIs in the context of 238U/235U variability, and possible causes of U isotopic variations in CAIs and meteorites.
Amelin Yuri
Iizuka Tsuyoshi
Ireland Trevor R.
Jacobsen Stein B.
Kaltenbach Angela
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