Details
| Original language | English |
|---|---|
| Pages (from-to) | 145-158 |
| Number of pages | 14 |
| Journal | Geochimica et cosmochimica acta |
| Volume | 148 |
| Publication status | Published - 1 Jan 2015 |
Abstract
Recent high-precision mass spectrometric studies of the uranium isotopic composition of terrestrial and meteoritic materials have shown significant variation in the 238U/235U ratio, which was previously assumed to be invariant (=137.88). In this study, we have investigated 27 bulk meteorite samples from different meteorite groups and types, including carbonaceous (CM1 and CV3), enstatite (EH4) and ordinary (H-, L-, and LL-) chondrites, as well as a variety of achondrites (angrites, eucrites, and ungrouped) to constrain the distribution of U isotopic heterogeneities and to determine the average 238U/235U for the Solar System.The investigated bulk meteorites show a range in 238U/235U between 137.711 and 137.891 (1.3‰) with the largest variations among ordinary chondrites (OCs). However, the U isotope compositions of 20 of the 27 meteorites analyzed here overlap within analytical uncertainties with the narrow range defined by terrestrial basalts (137.778-137.803), which are likely the best representatives for the U isotope composition of the bulk silicate Earth. Furthermore, the average 238U/235U of all investigated meteorite groups overlaps with that of terrestrial basalts (137.795±0.013). The bulk meteorite samples studied here do not show a negative correlation of 238U/235U with Nd/U or Th/U (used as proxies for the Cm/U ratio), as would be expected if radiogenic 235U was generated by the decay of extant 247Cm in the early Solar System. Rather, ordinary chondrites show a positive correlation of 238U/235U with Nd/U and with 1/U.The following conclusions can be drawn from this study: (1) The Solar System has a broadly homogeneous U isotope composition, and bulk samples of only a limited number of meteorites display detectable U isotope variations; (2) Bulk planetary differentiation has no significant effect on the 238U/235U ratio since the Earth, achondrites, and chondrites have indistinguishable U isotope compositions in average. (3) The cause of U isotopic variation in Solar System materials remains enigmatic; however, both the decay of 247Cm and isotope fractionation are likely responsible for the U isotopic variations observed in CAIs and ordinary chondrites, respectively.The average 238U/235U of the investigated meteorite groups (including data compiled from the literature) and terrestrial basalts is 137.794±0.027 (at a 95% student's t confidence level, including all propagated uncertainties) and represents the best estimate for the U isotope composition of the Earth and the Solar System. This value may be used for U-Pb and Pb-Pb dating of Solar System materials, provided the precise U isotope composition of the sample is unknown. Compared to Pb-Pb ages that were determined with the previously assumed value for 238U/235U (137.88), this new value results in an age adjustment of -0.9Ma.
ASJC Scopus subject areas
- Earth and Planetary Sciences(all)
- Geochemistry and Petrology
Cite this
- Standard
- Harvard
- Apa
- Vancouver
- BibTeX
- RIS
In: Geochimica et cosmochimica acta, Vol. 148, 01.01.2015, p. 145-158.
Research output: Contribution to journal › Article › Research › peer review
}
TY - JOUR
T1 - The uranium isotopic composition of the Earth and the Solar System
AU - Goldmann, Alexander
AU - Brennecka, Gregory
AU - Noordmann, Janine
AU - Weyer, Stefan
AU - Wadhwa, Meenakshi
N1 - Funding information: We want to thank Jutta Zipfel from the Senckenberg Museum in Frankfurt and Thorsten Kleine from the Westfälische Wilhelms-Universität Münster for providing meteorite samples and the Goethe Universität Frankfurt for the analyses of trace element concentrations. We also thank Steve Romaniello (Arizona State University) for assistance with the U isotope analysis of the Stannern homogenized powder. Fruitful discussions with Jutta Zipfel, Steven Galer and Frank Wombacher helped to interpret the findings of this study. The comments of Frederic Moynier, Herbert Palme and two anonymous reviewers significantly helped to improve the manuscript. Frederic Moynier is also thanked for editorial handling. This project was funded by program SPP 1385 of the Deutsche Forschungsgemeinschaft ( DFG ) and NASA grants NNX11AK56G and NNX11AK75G (M.W.).
PY - 2015/1/1
Y1 - 2015/1/1
N2 - Recent high-precision mass spectrometric studies of the uranium isotopic composition of terrestrial and meteoritic materials have shown significant variation in the 238U/235U ratio, which was previously assumed to be invariant (=137.88). In this study, we have investigated 27 bulk meteorite samples from different meteorite groups and types, including carbonaceous (CM1 and CV3), enstatite (EH4) and ordinary (H-, L-, and LL-) chondrites, as well as a variety of achondrites (angrites, eucrites, and ungrouped) to constrain the distribution of U isotopic heterogeneities and to determine the average 238U/235U for the Solar System.The investigated bulk meteorites show a range in 238U/235U between 137.711 and 137.891 (1.3‰) with the largest variations among ordinary chondrites (OCs). However, the U isotope compositions of 20 of the 27 meteorites analyzed here overlap within analytical uncertainties with the narrow range defined by terrestrial basalts (137.778-137.803), which are likely the best representatives for the U isotope composition of the bulk silicate Earth. Furthermore, the average 238U/235U of all investigated meteorite groups overlaps with that of terrestrial basalts (137.795±0.013). The bulk meteorite samples studied here do not show a negative correlation of 238U/235U with Nd/U or Th/U (used as proxies for the Cm/U ratio), as would be expected if radiogenic 235U was generated by the decay of extant 247Cm in the early Solar System. Rather, ordinary chondrites show a positive correlation of 238U/235U with Nd/U and with 1/U.The following conclusions can be drawn from this study: (1) The Solar System has a broadly homogeneous U isotope composition, and bulk samples of only a limited number of meteorites display detectable U isotope variations; (2) Bulk planetary differentiation has no significant effect on the 238U/235U ratio since the Earth, achondrites, and chondrites have indistinguishable U isotope compositions in average. (3) The cause of U isotopic variation in Solar System materials remains enigmatic; however, both the decay of 247Cm and isotope fractionation are likely responsible for the U isotopic variations observed in CAIs and ordinary chondrites, respectively.The average 238U/235U of the investigated meteorite groups (including data compiled from the literature) and terrestrial basalts is 137.794±0.027 (at a 95% student's t confidence level, including all propagated uncertainties) and represents the best estimate for the U isotope composition of the Earth and the Solar System. This value may be used for U-Pb and Pb-Pb dating of Solar System materials, provided the precise U isotope composition of the sample is unknown. Compared to Pb-Pb ages that were determined with the previously assumed value for 238U/235U (137.88), this new value results in an age adjustment of -0.9Ma.
AB - Recent high-precision mass spectrometric studies of the uranium isotopic composition of terrestrial and meteoritic materials have shown significant variation in the 238U/235U ratio, which was previously assumed to be invariant (=137.88). In this study, we have investigated 27 bulk meteorite samples from different meteorite groups and types, including carbonaceous (CM1 and CV3), enstatite (EH4) and ordinary (H-, L-, and LL-) chondrites, as well as a variety of achondrites (angrites, eucrites, and ungrouped) to constrain the distribution of U isotopic heterogeneities and to determine the average 238U/235U for the Solar System.The investigated bulk meteorites show a range in 238U/235U between 137.711 and 137.891 (1.3‰) with the largest variations among ordinary chondrites (OCs). However, the U isotope compositions of 20 of the 27 meteorites analyzed here overlap within analytical uncertainties with the narrow range defined by terrestrial basalts (137.778-137.803), which are likely the best representatives for the U isotope composition of the bulk silicate Earth. Furthermore, the average 238U/235U of all investigated meteorite groups overlaps with that of terrestrial basalts (137.795±0.013). The bulk meteorite samples studied here do not show a negative correlation of 238U/235U with Nd/U or Th/U (used as proxies for the Cm/U ratio), as would be expected if radiogenic 235U was generated by the decay of extant 247Cm in the early Solar System. Rather, ordinary chondrites show a positive correlation of 238U/235U with Nd/U and with 1/U.The following conclusions can be drawn from this study: (1) The Solar System has a broadly homogeneous U isotope composition, and bulk samples of only a limited number of meteorites display detectable U isotope variations; (2) Bulk planetary differentiation has no significant effect on the 238U/235U ratio since the Earth, achondrites, and chondrites have indistinguishable U isotope compositions in average. (3) The cause of U isotopic variation in Solar System materials remains enigmatic; however, both the decay of 247Cm and isotope fractionation are likely responsible for the U isotopic variations observed in CAIs and ordinary chondrites, respectively.The average 238U/235U of the investigated meteorite groups (including data compiled from the literature) and terrestrial basalts is 137.794±0.027 (at a 95% student's t confidence level, including all propagated uncertainties) and represents the best estimate for the U isotope composition of the Earth and the Solar System. This value may be used for U-Pb and Pb-Pb dating of Solar System materials, provided the precise U isotope composition of the sample is unknown. Compared to Pb-Pb ages that were determined with the previously assumed value for 238U/235U (137.88), this new value results in an age adjustment of -0.9Ma.
UR - http://www.scopus.com/inward/record.url?scp=84918797336&partnerID=8YFLogxK
U2 - 10.1016/j.gca.2014.09.008
DO - 10.1016/j.gca.2014.09.008
M3 - Article
AN - SCOPUS:84918797336
VL - 148
SP - 145
EP - 158
JO - Geochimica et cosmochimica acta
JF - Geochimica et cosmochimica acta
SN - 0016-7037
ER -