[meteorite-list] Reevaluating the age of the Solar System

From: Charles O'Dale <codale0806_at_meteoritecentral.com>
Date: Wed, 27 Jan 2010 13:46:46 -0800 (PST)
Message-ID: <812038.6481.qm_at_web88005.mail.re2.yahoo.com>

YouTube at the end of the e-mail (a must)
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Reevaluating the age of the Solar System Gregory A. BrenneckaSchool of Earth and Space Exploration, Arizona State University
Lead-lead (Pb-Pb) dating is among the most widely used radiometric dating techniques to determine the age of really old things, such as the age of the Earth or the Solar System. However, recent advances in instrumentation now allow scientists to make more precise measurements that promise to revolu?tionize the way the ages of some samples are calculated with this technique.
Radiometric dating can be used to determine the age of a wide range of natural and human-made materials. The comparison between the observed abundance of a naturally occurring radioactive isotope, such as uranium (U), and its decay products can be used to determine the age of a material, using known decay rates. The Pb-Pb dating technique has been used for decades under the assumption that the ratio of the 238U and 235U isotopes, both of which decay to different isotopes of Pb, is constant
?in meteoritic material. Any deviation from this assumed value causes miscal?culation in the determined Pb-Pb age of a sample, meaning that the age of the Solar System could be miscalculated by as much as several million years. Although this is a small fraction of the ~4.57 billion year age of the Solar Sys?tem, it is significant since some of the most important events that shaped the Solar System occurred within the first ~10 million years of its formation. in the Solar System. This assumed value is built into the Pb-Pb age equation.
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According to research published online in the Dec. 31 issue of Science Express and in the Jan. 22 issue of Science magazine by Greg Brennecka, a graduate student in SESE, the 238U/235U ratio can no longer be considered a constant in meteoritic material. Any deviation from this assumed value causes miscal?culation in the determined Pb-Pb age of a sample, meaning that the age of the Solar System could be miscalculated by as much as several million years. Although this is a small fraction of the ~4.57 billion year age of the Solar Sys?tem, it is significant since some of the most important events that shaped the Solar System occurred within the first ~10 million years of its formation.
Brennecka and colleagues measured the 238U/235U ratio in the earliest solids in the Solar System, calcium-aluminum-rich inclu?sions (CAIs). CAIs were the first solids to condense from the cooling protoplanetary disk during the birth of the Solar System. The absolute ages of the CAIs, determined through Pb-Pb dating, are generally considered to date the origin of the Solar System. The high-precision data they obtained from CAIs of the Allende meteor?ite showed that the 238U/235U ratio is not the same in all CAIs.
Brennecka began to think about the idea that the U isotope ratio might not be constant in meteoritic material after learning about work done by Professor Stefan Weyer of the Goethe University of Frankfurt during a sabbatical visit to ASU the previous year. Weyer?s work revealed measurable differences in 238U/235U in different environments on Earth..
At this time, Brennecka was taking a class on meteorites and the origin of the Solar System from Meenakshi Wadhwa, a professor in SESE and director of the ASU Center for Meteorite Studies. For a class assignment, Brennecka developed a research proposal centered on the implications of variable U isotopes in early Solar System materials. Anbar and Wadhwa encouraged him to take the proposal from the classroom to the laboratory.
Brennecka worked with Anbar and Wadhwa to refine the proce?dures at ASU to be able to measure 238U/235U in the extremely small CAIs. Eleven of the thirteen CAIs were from the ASU Center for Meteorite Studies collection; the other two were from the Senckenberg Museum collection in Frankfurt. The project was supported by NASA, including the NASA Origins of Solar Systems Program, and the NASA Astrobiology Institute (NAI). ASU is home to one of 14 research teams from across the country that com?prise the NAI which explores the origin, evolution, distribution, and future of life on Earth and in the universe.
The U isotope ratios in all but two CAIs differed significantly from the standard ?assumed? value. One of the possible mechanisms that could have produced these U isotope variations in meteorites is the decay of extant 247Cm to 235U. 247Cm is created during only certain types of supernovae and has a very short half-life (15.6 million years) compared to the age of the Solar System, so all of the 247Cm that was present originally has since completely decayed away. Brennecka and colleagues performed additional tests to determine if this was the cause of the U isotope variation.
If a correlation existed between the 238U/235U values and the original Cm/U in the CAIs, it would provide evidence that 247Cm was the reason for the 238U/235U variations. Since 235U is from the decay of 247Cm, higher Cm/U ratios mean there is relatively more 235U produced from 247Cm decay. As Cm has no long-lived stable isotope, the initial Cm/U ratio of a sample cannot be direct?ly determined, so geochemical proxies were used. The correlation of these proxies, or elements that behave like Cm, with U isotope ratios in the CAIs provided strong evidence for the presence of ex?tant 247Cm in the early Solar System. The 238U/235U ratios Bren?necka obtained from the Allende meteorite were used to quantify the amount of 247Cm present in the early Solar System.
?Cosmochemists have searched for evidence for live 247Cm in the early Solar System for decades, and this is the first time that its pres?ence has been demonstrated definitively. This work not only impacts precise and accurate dating of the earliest events to occur in our Solar System, but it also has broader implications for the environment and conditions in which our Solar System was born,? explains Wadhwa.
?It?s possible that in the future we?ll be able to use the 247Cm-235U system as a short-lived chronometer,? says Brennecka. ?But most importantly, this will help improve the accuracy of Pb-Pb dating.?
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238U/235U Variations in Meteorites: Extant 247Cm and Implications for Pb-Pb Dating
G. A. Brennecka,1,* S. Weyer,2,M. Wadhwa,1 P. E. Janney,1 J. Zipfel,3 A. D. Anbar1,4
The 238U/235U isotope ratio has long been considered invariantin meteoritic materials (equal to 137.88). This assumption isa cornerstone of the high-precision lead-lead dates that definethe absolute age of the solar system. Calcium-aluminum?richinclusions (CAIs) of the Allende meteorite display variable238U/235U ratios, ranging between 137.409 ? 0.039 and137.885 ? 0.009. This range implies substantial uncertaintiesin the ages that were previously determined by lead-lead datingof CAIs, which may be overestimated by several million years.The correlation of uranium isotope ratios with proxies for curium/uranium(that is, thorium/uranium and neodymium/uranium) provides strongevidence that the observed variations of 238U/235U in CAIs wereproduced by the decay of extant curium-247 to uranium-235 inthe early solar system, with an initial 247Cm/235U ratio ofapproximately 1.1 x 10?4 to 2.4 x 10?4.
1School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287 USA.
2 Institut fur Geowissenschaften, Goethe-Universit?t, Frankfurt, Germany..
3 Senckenberg Forschungsinstitut und Naturmuseum, Frankfurt, Germany.
4 Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287, USA.
Present address: Institut f?r Geology und Mineralogie,Universit?t zu K?ln, Cologne, Germany.
*To whom correspondence should be addressed. E-mail: brennecka at asu.edu
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.... and how the creationists try and distort science:
http://www.youtube.com/watch?v=aFozOBz7eig
Received on Wed 27 Jan 2010 04:46:46 PM PST