Amelin, Y. (2002) Lead Isotopic Ages of Chondrules and Calcium-Aluminum-Rich Inclusions. Science, 297 (5587). 1678-1683 doi:10.1126/science.1073950
| Reference Type | Journal (article/letter/editorial) | ||
|---|---|---|---|
| Title | Lead Isotopic Ages of Chondrules and Calcium-Aluminum-Rich Inclusions | ||
| Journal | Science | ||
| Authors | Amelin, Y. | Author | |
| Year | 2002 (September 6) | Volume | 297 |
| Page(s) | 1678-1683 | Issue | 5587 |
| Publisher | American Association for the Advancement of Science (AAAS) | ||
| DOI | doi:10.1126/science.1073950Search in ResearchGate | ||
| Mindat Ref. ID | 2537462 | Long-form Identifier | mindat:1:5:2537462:9 |
| GUID | 63c0d657-ce8b-4bc4-9356-a98e23f7f410 | ||
| Full Reference | Amelin, Y. (2002) Lead Isotopic Ages of Chondrules and Calcium-Aluminum-Rich Inclusions. Science, 297 (5587). 1678-1683 doi:10.1126/science.1073950 | ||
| Plain Text | Amelin, Y. (2002) Lead Isotopic Ages of Chondrules and Calcium-Aluminum-Rich Inclusions. Science, 297 (5587). 1678-1683 doi:10.1126/science.1073950 | ||
| In | (2002, September) Science Vol. 297 (5587) American Association for the Advancement of Science (AAAS) | ||
References Listed
These are the references the publisher has listed as being connected to the article. Please check the article itself for the full list of references which may differ. Not all references are currently linkable within the Digital Library.
| G. J. MacPherson D. A. Wark J. T. Armstrong in Meteorites and the Early Solar System J. F. Kerridge M. S. Matthews Eds. (Univ. of Arizona Press Tucson AZ 1988) pp. 746–808. | |
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 | Mostefaoui, Smail, Kita, Noriko T., Togashi, Shigeko, Tachibana, Shogo, Nagahara, Hiroko, Morishita, Yuichi (2002) The relative formation ages of ferromagnesian chondrules inferred from their initial aluminum-26/aluminum-27 ratios. Meteoritics & Planetary Science, 37 (3) 421-438 doi:10.1111/j.1945-5100.2002.tb00825.x |
| J. N. Goswami H. A. T. Vanhala in Protostars and Planets IV M. Mannings A. P. Boss S. S. Russell Eds. (Univ. of Arizona Press Tucson AZ 2000) pp. 963–995. | |
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| Chaussidon M., Robert F., McKeegan K. D., Lunar Planet. Sci. 33, 1563 (2002). | |
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| The 207 Pb/ 206 Pb isotopic chronometer is based on radioactive decay of two long-lived radionuclides: 235 U and 238 U ( 57 ). 235 U decays about six times faster than 238 U and the 235 U/ 238 U atomic ratio decreases through geological time. Hence the atomic ratio of stable radiogenic nuclides 207 Pb/ 206 Pb produced by decay of 235 U and 238 U also changes with time. This ratio can be used to measure the time since formation of a U-bearing mineral or rock (e.g. chondrule or CAI). The 207 Pb/ 206 Pb chronometer has a number of unique features distinguishing it from other isotopic chronometers. Because the date calculation is based on isotopic composition of one element and does not include U/Pb ratio it is free of analytical uncertainties in the U/Pb ratio. The 207 Pb/ 206 Pb date is also insensitive to recent fractionation of U and Pb in the nature or in laboratory (U/Pb fractionation still affects the determination of concordance of the U-Pb system but not the Pb-Pb age). Exceptionally high precision of the Pb-Pb method is a consequence of high abundance of 235 U in the early solar system and its fast decay which in combination caused fast accumulation of radiogenic 207 Pb. Relative errors 207 Pb/ 206 Pb dates in this age interval are about three times smaller than the relative errors in the 207 Pb/ 206 Pb radiogenic isotope ratios from which the age is calculated. The precision of a date determined from highly radiogenic Pb isotopic ratios is mainly limited by the uncertainty in Pb isotopic fractionation in a mass-spectrometer. This uncertainty is typically between 0.03 and 0.1% (2σ) per mass unit producing the uncertainty of 0.4 to 1.4 My in a single 207 Pb/ 206 Pb date. | |
| Göpel C., Manhés G., Allégre C. J., Meteoritics 26, 338 (1991). | |
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| Two varieties of Pb are present in rocks including meteorites: common (or initial) Pb which was incorporated into the mineral during its formation; and radiogenic Pb which was accumulated by radioactive decay of U during the lifetime of the mineral. If the mineral contains U but no common Pb the Pb present in it is a pure radiogenic component that can be directly used for age calculation. Unfortunately U-bearing minerals free of common Pb are very rare in meteorites. If common Pb is present in a mineral along with radiogenic Pb it has to be subtracted. Two approaches are used for Pb-Pb dating in the presence of common Pb: model date (more often called “model age”) and isochron. In the model date approach an isotopic composition of common Pb is assumed and common Pb is subtracted from total Pb on the basis of the measured isotopic abundance of 204 Pb. In the isochron approach a set of samples (mineral fractions chondrules or even whole meteorites) is assumed to be cogenetic i.e. having the same age and the same initial common Pb. No particular common Pb composition is assumed but the isochron model requires that the common Pb isotopic composition is homogeneous. In 207 Pb/ 206 Pb versus 204 Pb/ 206 Pb isochron plot used in this study (Fig. 1) the y -axis intercept gives the radiogenic 207 Pb/ 206 Pb ratio and therefore the age whereas the slope depends on the common Pb isotopic composition. The exact composition of common Pb cannot be determined from a Pb-Pb isochron but can be estimated from the intercept of the isochron with a model Pb growth curve for the studied reservoir ( 57 ). | |
| Zolensky M. E., Meteoritics 26, 414 (1991). | |
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| A. N. Krot A. Meibom M. K. Weisberg K. Keil Meteorit. Planet. Sci. in press . | |
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| Krot A. N., Aléon J., McKeegan K. D., Lunar Planet. Sci. 33, 1412 (2002). | |
| Marhas K. K., Krot A. N., Goswami J. N., Meteorit. Planet. Sci. 36, A121 (2001). | |
| Krot A. N., Weisberg M. K., Petaev M. I., Keil K., Scott E. R. D., Lunar Planet. Sci. 31, 1470 (2000). | |
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| Krot A. N., Ulyanov A. A., Keil K., Meteorit. Planet. Sci. 35, A93 (2000). | |
| Mineral fractions were spiked with mixed 235 U- 205 Pb (or U-Th-Pb) 149 Sm- 150 Nd and 85 Rb- 84 Sr tracer solutions. Mineral dissolution chemical separations isotopic analyses and data reduction were performed as described ( 42 43 ). Two procedure blanks were measured with each batch of samples. The Pb isotopic composition of the blank that was used for the correction is an average value (±2σ) of 12 procedure blank determinations done previously and during the course of this study. Typical procedure blanks were 1 × 10 −12 to 1.5 × 10 −12 g for Pb U and Th. Because of small chondrule sizes uncertainties in the blank corrections were a dominant source of the total errors. Isotopic fractionation and mass bias of the Daly detectors were determined with National Institute of Standards and Technology standards SRM-982 for Pb U-500 for U and an in-house Th isotope standard. Isochrons and weighted means calculated with Isoplot-Ex version 2.49 are reported at 95% confidence level. | |
 | Amelin, Yuri V., Ritsk, Eugeni Yu., Neymark, Leonid A. (1997) Effects of interaction between ultramafic tectonite and mafic magma on Nd-Pb-Sr isotopic systems in the Neoproterozoic Chaya Massif, Baikal-Muya ophiolite belt. Earth and Planetary Science Letters, 148 (1) 299-316 doi:10.1016/s0012-821x(97)00046-0 |
| Individual chondrules were extracted from the Acfer 059 meteorite by using stainless-steel tools. Fractions consisting of chondrule fragments of uniform appearance were picked from a coarsely crushed portion of the meteorite. Chondrules and fragments were first cleaned by ultrasonic agitation in ethanol to remove adhering matrix minerals weighed and crushed to about <50 μm pieces with an aluminum oxide mortar and pestle. Preparation of the CAI fractions was similar but the starting CAI fragments were coarsely crushed instead of pulverizing. Final cleaning was performed by five or six 10-min cycles of ultrasonic agitation in 0.5 to 2.0 M high-purity HCl in Savillex vials which were subsequently used for digestion. During early analyses (chondrules 1 to 5) acid leachates from several fractions were preserved and analyzed separately. One chondrule and matrix sample were analyzed without acid leaching. During the later stages chondrules were subjected to a more intensive acid leaching before crushing: three to five 30-min cycles of ultrasonic agitation in ethanol plus 6 M HCl 1:1 mixture. This treatment effectively removed all residual matrix from the surface of chondrules and helped to minimize the common Pb content but it also caused etching and in some cases bleaching of chondrules and chondrule fragments. In addition this treatment partially removed chondrule rims. | |
| Amelin Y., Lunar Planet Sci. 32, 1389 (2001). | |
| E. Rotenberg Y. Amelin Eleventh Annual V.M. Goldschmidt Conference Hot Springs VA 20 to 24 May 2001 no. 3626. | |
| Y. Amelin in preparation. | |
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| Primordial Pb is the Pb present in the solar system at the time of its formation. This is the least radiogenic Pb known which is preserved nearly unchanged in the mineral troilite (FeS) in some iron meteorites ( 58 59 ). Modern common Pb is a mixture of primordial Pb and 20 to 50% additions of radiogenic 206 Pb 207 Pb and 208 Pb accumulated since formation of the Earth and meteorite parent bodies. | |
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| “Errorchron” is a term for an isochron with the scatter of the data exceeding analytical uncertainties. The excess scatter implies that the conditions of isochron model were not precisely fulfilled e.g. the samples are not coeval or the common Pb isotopic composition is variable or the U-Pb systems of the samples were disturbed in the past. Thus the errorchron dates may be inaccurate and should be interpreted with caution. | |
| Amelin Y., et al., Lunar Planet Sci. 33, 1151 (2002). | |
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| Mg isotope compositions were measured with PANURGE a modified Cameca IMS-3f ion microprobe at Lawrence Livermore National Laboratory with the operating conditions and procedures as described ( 60 ). The Mg isotope ratios were corrected for both instrumental and intrinsic fractionation assuming the standard ratios of 25 Mg/ 24 Mg = 0.12663 and 26 Mg/ 24 Mg = 0.13932 ( 61 ). The corrected ratios ( 26 Mg/ 24 Mg) C were used to calculate δ 26 Mg = [( 26 Mg/ 24 Mg) C /0.13932 – 1] × 1000. | |
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| The error of 1.2 My in the CAI to chondrule formation interval (a simple sum of individual errors) is the most conservative estimate which corresponds to the case of anticorrelated errors. A more probable estimate for the case of uncorrelated errors is 0.9 My (a quadratic sum of individual errors). | |
| Boss A. P., Lunar Planet. Sci. 31, 1084 (2000). | |
| G. Faure Principles of Isotope Geology (Wiley New York 1986). | |
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| We are grateful to A. Greshake for providing samples of Acfer 059. This work was supported by Canadian Space Agency contract 9F007-010128/001/SR (Y.A. principal investigator) and NASA grants NAG5-10610 (A.N.K. principal investigator) and NAG5-11591 (K. Keil principal investigator). Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract W-7405-ENG-48. |
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