Uranium Isotope Compositions of the Basaltic Angrite Meteorites and the Chronological Implications for the Early Solar System
Total Page：16
File Type：pdf, Size：1020Kb
Uranium isotope compositions of the basaltic angrite meteorites and the chronological implications for the early Solar System Gregory A. Brennecka1 and Meenakshi Wadhwa School of Earth and Space Exploration, Arizona State University, P.O. Box 871404, Tempe, AZ 85287-1404 Edited by Frank M. Richter, The University of Chicago, Chicago, IL, and approved April 26, 2012 (received for review August 26, 2011) Events occurring within the first 10 million years of the Solar Sys- assumption that the parent isotope was homogenously distributed tem’s approximately 4.5 billion-year history, such as formation of in the solar nebula. The application of an extinct chronometer the first solids, accretion, and differentiation of protoplanetary then requires that the abundances of the short-lived parent iso- bodies, have determined the evolutionary course of our Solar Sys- tope at the formation times of two different objects be deter- tem and the planetary bodies within it. The application of high-re- mined. With the knowledge of the half-life of the parent isotope, solution chronometers based on short-lived radionuclides is critical it is then possible to obtain relative time constraints between to our understanding of the temporal sequence of these critical these two objects with high precision [uncertainties as low as a events. However, to map the relative ages from such chronometers few tens of thousands of years have been reported for samples onto the absolute time scale, they must be “anchored” to absolute older than approximately 4.5 billion years (2)]. These relative ages of appropriate meteoritic materials using the high-precision ages can be mapped to an absolute time scale only if they can be lead–lead (Pb–Pb) chronometer. Previously reported Pb–Pb dates anchored to an absolute date obtained with the long-lived but of the basaltic angrite meteorites, some of which have been used highly precise Pb–Pb chronometer. This can only be accom- extensively as time anchors, assumed a constant 238U∕235U ratio plished if there is an appropriate anchor, which is a meteoritic (¼137.88). In this work, we report measurements of 238U∕235U material for which a precise and accurate Pb–Pb age as well as ratios in several angrites that are distinct from the previously the abundance of the short-lived parent isotope at that time assumed value, resulting in corrections to the Pb–Pb ages of ≥1 has been determined. The inferred abundance of the short-lived million years. There is no resolvable variation in the 238U∕235U ratio parent isotope in the object to be dated is then compared to that among the angrite bulk samples or mineral separates, suggesting in the anchor, based upon which an age can be calculated for that homogeneity in the U isotopic composition of the angrite parent object relative to the known absolute Pb–Pb age of the anchor. body. Based on these measurements, we recalculated the Pb–Pb Since this calculation does not yield an independent absolute age age for the commonly used anchor, the D’Orbigny angrite, to be for the object, but one that is relative to the age of the anchor and 4563.37 Æ 0.25 Ma. An adjustment to the Pb–Pb age of a time an- is based on the assumptions that (i) the abundance of the parent chor (such as D’Orbigny) requires a corresponding correction to the radioisotope in the anchor is known for the time dated with “model ages” of all materials dated using that anchor and a short- the Pb–Pb chronometer and (ii) both the object and the anchor lived chronometer. This, in turn, has consequences for accurately originated from a reservoir in which the parent radioisotope was defining the absolute timeline of early Solar System events. initially homogeneously distributed, it is referred to hereafter as a “model age” and not an absolute age. anchor ∣ geochronology The Pb–Pb system is the only absolute dating technique able to resolve sub-Ma time differences. Because 238U and 235U decay to he time from the formation of the first solids in the Solar 206Pb and 207Pb, respectively, a known 238U∕235U is required for EARTH, ATMOSPHERIC, TSystem to the accretion and differentiation of protoplanetary age calculation, as shown in Eq. 1. AND PLANETARY SCIENCES embryos is less than approximately 10 million years (Ma) (ref. 1 207 Ã 235 λ t λ t and references therein), and these events have determined the 235 − 1 235 − 1 Pb ¼ Ue ¼ e : [1] evolutionary course of our Solar System and the planetary bodies 206 Ã 238 λ t λ t Pb 238 − 1 238 − 1 within it. Knowledge of the precise timing of events during this Ue e period is critical to a broader understanding of how star systems Recent work has shown that the assumption of an invariant and planetary bodies form and evolve. As such, precise and ac- 238U∕235U ratio in Solar System materials is no longer valid curate geochronology, allowing the resolution of events occurring (3–6); the uranium isotopic compositions must be measured to within this critical approximately 10 Ma interval, is required to obtain accurate Pb–Pb dates. As the high-resolution relative ages understand this earliest sequence of events in the Solar System. of early Solar System events based on extinct chronometers (such A very limited number of chronometers can provide the sub-Ma as 26Al-26Mg) must be anchored by precise and accurate absolute precision necessary to resolve early Solar System events. These – include the long-lived lead–lead (Pb–Pb) chronometer, and the ages from Pb Pb dating, determination of the uranium isotope 26 26 compositions of meteoritic objects that serve as age anchors is short-lived chronometers such as Al- Mg (t1∕2 approx. 53 53 182 182 particularly important, since this has implications for the accuracy 0.72 Ma), Mn- Cr (t1∕2 approx. 3.7 Ma), and Hf- W(t1∕2 approx. 9 Ma). The Pb–Pb chronometer is based on two radio- of the model ages obtained using short-lived chronometers. active isotopes of uranium, utilizing the distinct decay schemes 235 207 238 206 of U → Pb (t1∕2 approx. 704 Ma) and U → Pb (t1∕2 Author contributions: G.A.B. and M.W. designed research; G.A.B. performed research; approx. 4.47 Ga) to calculate an absolute age of the sample. G.A.B. and M.W. analyzed data; and G.A.B. and M.W. wrote the paper. The short-lived or extinct radionuclide chronometers have parent The authors declare no conflict of interest. isotopes with half-lives significantly shorter than the age of the This article is a PNAS Direct Submission. Solar System; even though they were initially present in the solar 1To whom correspondence should be addressed. E-mail: [email protected]. nebula, they have fully decayed to their daughter isotopes at the This article contains supporting information online at www.pnas.org/lookup/suppl/ present time. Chronology based on an extinct system relies on the doi:10.1073/pnas.1114043109/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1114043109 PNAS ∣ June 12, 2012 ∣ vol. 109 ∣ no. 24 ∣ 9299–9303 Downloaded by guest on September 27, 2021 Previous studies have revealed inconsistencies between ages to the SRM950a standard, which has a measured 238U∕235U ratio determined for the same meteoritic object by different high- of 137.837 Æ 0.015 (17) [in good agreement with the value re- resolution chronometers mentioned above (e.g., 1, 7). These ported by (18) for this same standard]. discrepancies may be caused by a variety of reasons, including Uranium isotope ratios in a pyroxene separate and two whole- heterogeneous distribution of the short-lived radionuclides and rock fractions of D’Orbigny, a phosphate separate from Angra resetting of different isotopic systems to different degrees by dos Reis (ADOR), whole-rock fractions of NWA 4590, NWA 238 235 secondary processes. The assumption of an invariant U∕ U 4801, and NWA 6291, as well as an acid leach (0.5 M HNO3) and ratio in Solar System materials could also be responsible for some remaining residue from a second whole-rock fraction of NWA of these discrepancies since it may result in erroneous Pb–Pb ages 6291 were determined during this study. All uncertainties re- of the time anchors. ported here are 2 × standard deviation (2SD). We report a The accuracy of the model ages obtained using short-lived 238U∕235U ratio of 137.778 Æ 0.034 for the D’Orbigny pyroxene chronometers is ultimately dependent on the choice of an appro- separate, 137.790 Æ 0.025 for a whole-rock sample of D’Orbigny priate meteoritic material to serve as an age anchor. An ideal (Bulk D’Orbigny-1), and 137.791 Æ 0.026 for another whole-rock age anchor (i) originated from an isotopically uniform source re- sample of D’Orbigny (Bulk D’Orbigny-2). Whole-rock fractions servoir, (ii) contains measureable quantities of the daughter of NWA 4590, NWA 4801 and NWA 6291 yielded 238U∕235U ra- products of multiple short-lived radioisotope systems (e.g., tios of 137.772 Æ 0.026, 137.778 Æ 0.026, and 137.769 Æ 0.026, 26 26 53 53 182 182 iii Al- Mg, Mn- Cr, and Hf- W), ( ) cooled rapidly respectively. The 0.5 M HNO3 acid leach of a second whole-rock while those systems were still “live,” and (iv) has remained undis- sample of NWA 6291 yielded a 238U∕235Uof137.800 Æ 0.026, turbed by secondary alteration since the initial closure of these with the remaining residue having a 238U∕235Uof137.759 Æ isotopic systems (7, 8).