Primitive components in meteorites contain a detailed record of the conditions and processes in the solar nebula, the cloud of dust and gas surrounding the infant Sun. Determining accurately when the first materials formed requires the lead-lead (Pb-Pb) dating method, a method based on the decay of uranium (U) isotopes to Pb isotopes. The initial ratio of U-238 to U-235 is critical to determining the ages correctly, and many studies have concluded that the ratio is constant for any given age. However, my colleagues at Arizona State University, Institut für Geowissenschaften, Goethe-Universität (Frankfurt, Germany), and the Senckenberg Forschungsinstitut und Naturmuseum (also in Frankfurt) and I have found that some calcium-aluminum-rich inclusions (CAIs) in chondritic meteorites deviate from the conventional value for the U-238/U-235 ratio. This could lead to inaccuracies of up to 5 million years in the age of these objects, if no correction is made. Variations in the concentrations of thorium and neodymium with the U-238/U-235 ratio suggest that the ratio may have been lowered by the decay of curium-247, which decays to U-235 with a half-life of 15.6 million years. Curium-247 is created in certain types of energetic supernovae, so its presence suggests that a supernova added material to the pre-solar interstellar cloud between 110 and 140 million years before the Solar System began to form.
Meteorites and the Uranium Clock
Meteorites provide a wealth of information about the formation and evolution of the Solar System. Found within a certain type of meteorite, called chondrites, calcium-aluminum-rich inclusions (CAIs) are present. These ultra-refractory materials represent the first solids to condense during the birth of the Solar System and, therefore, the ages of CAIs date the origin of the Solar System. Obtaining an absolute age of these types of materials requires the use of the Pb-Pb dating method, a method based on the decay of different isotopes of uranium (U) to stable daughter isotopes of lead (Pb). Uranium-238 decays to lead-206 (206Pb) with a half-life of ~4.5 billion years, and 235U decays to 207Pb with a half-life of ~700 million years. Cosmochemists use the ratio of 206Pb/207Pb as a "clock" to date how old a material is, and have used this chronometer for decades to date rocks on Earth, as well as meteorites. (See, for example, PSRD article: Dating the Earliest Solids in our Solar System.) However, the method relies on a known ratio of parent U isotopes, and up until this point, we have assumed that the modern 238U/235U ratio (137.88) is invariant in meteoritic material. Modern techniques and mass spectrometers now allow us to measure slight variations in isotope systems long thought not to fractionate, and our research suggests that the assumption of an invariant 238U/235U ratio is not valid. Our paper demonstrates deviations from the standard ratio, explores the reasons for the variation in the 238U/235U ratio in CAIs, and discusses the implications that a variable 238U/235U ratio has on the Pb-Pb dating method and the age of the Solar System.
Uranium isotope variations in meteorites may be produced by many mechanisms, ranging from anomalies produced during synthesis of U isotopes in exploding stars, fractionation of U isotopes during chemical reactions (as recently observed on Earth), or from the decay of the short-lived isotope 247Cm to 235U. While any or all of these mechanisms may play some role in 238U/235U variability in early Solar System materials, the existence and effect of 247Cm on the 238U/235U ratio can be studied using geochemical proxies for Cm.
Curium-247 is created exclusively in certain types of supernovae during something called "r-process nucleosynthesis." It decays to 235U with a half-life of 15.6 million years, so has been long extinct in meteoritic material. If 247Cm was present during the formation of the Solar System, it would be detected by apparent excesses in 235U found in ancient meteoritic materials. The largest 235U excesses would occur in materials in which the original Solar System Cm/U ratio was significantly fractionated by processes associated with their formation. The CAIs in chondritic meteorites are likely to be such materials, as many of them experienced elemental fractionation during condensation/evaporation processes involved in their formation.
Quantification of the abundance of extant 247Cm has the potential to provide new constraints on the origin of short-lived radionuclides in the early Solar System. By determining the original amount of 247Cm in the Solar System, and comparing to isotope production models, it should be possible to determine the approximate time interval between the last r-process nucleosynthetic event (supernovae) and the formation of the Solar System. We made high-precision 238U/235U ratio measurements on 13 CAIs from the Allende meteorite [Data link from Meteoritical Bulletin] to quantify the amount of 247Cm present in the early Solar System and to determine the extent of potential offsets in the calculated Pb-Pb ages of early Solar System materials.
We separated the 13 CAIs from different sections of the CV3 Allende meteorite. We crushed the samples and dissolved them utilizing HNO3, HF, and HClO4 acids, and reserved approximately 5% of each sample for trace element measurements (i.e., REE patterns, Th/U and Nd/U ratios). Uranium isotope measurements were performed both at Arizona State University and the University of Frankfurt (Germany) using multi-collector inductively coupled plasma mass spectrometers (MC-ICPMS).
|This research used samples of calcium-aluminum-rich inclusions (CAIs) from the Allende meteorite, which I am holding in my hand in the ASU laboratory. Photo on the right shows the multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS) at ASU.|
These state-of-the-art instruments allow for extremely precise measurement of isotope ratios by measuring multiple isotopes signals at the same time. This collaboration between two labs was important because in many cases during this study, we measured samples and standards independently at the two labs to ensure data accuracy and reproducibility. The 238U/235U ratios of the two bulk meteorites (Allende and Murchison [Data link from Meteoritical Bulletin]) are 137.818±0.012 and 137.862±0.042, respectively (see the graph below). The 13 CAIs show a large range of U isotope compositions, with 238U/235U ratios varying from 137.409±0.039 to 137.885±0.009. All but two CAIs differ outside uncertainties from the standard value of 137.88 and five CAIs have significantly lower 238U/235U values than that of the bulk Allende meteorite. These differences seem small, but they are highly significant!
|238U/235U isotope values for the samples of this study. The box represents the measured value and analytical precision of replicate analyses of 20-100 ppb solutions of a standard named SRM950a. Error bars are calculated as twice the standard deviation (2SD) of multiple runs of each sample, when possible.|
If 247Cm decay is the primary mechanism for 238U/235U variability, then materials with high initial Cm/U would contain a higher relative amount of 235U than those with lower initial Cm/U. However, as Cm has no long-lived stable isotope, the initial Cm/U ratio of a sample cannot be directly determined. Fortunately, thorium (Th) and neodymium (Nd) have similar geochemical behavior as Cm, so Th/U and Nd/U ratios can serve as proxies for the initial Cm/U ratio in the sample. Our sample set spans a large range of Th/U and Nd/U, and both these ratios correlate with the U isotopic composition, as shown in the graphs below.
232Th/238U and 144Nd/238U ratios plotted versus 235U/238U ratios. (We plotted the inverse values of our measured 238U/235U ratios. We did that because geochronological plots usually have the daughter isotope in the numerator of the y-axis, making the line slope up to the right.) The grey dashed lines represent the 2SD errors on the best-fit line (solid black). Errors on the Y-axis data are ± 2SD; X-axis error bars are ± 5% of the determined value of the elemental ratio. The correlation of increasing 235U/238U with increasing Th/U and Nd/U, which we think are proportional to Cm/U, is consistent with excess formation of 235U by decay of 247Cm.
Due to the higher volatility of uranium, substantial fractionation of Cm (and other geochemically similar elements such as Th and Nd) from U is possible in the early solar nebula. A special group of CAIs, called "Group II" CAIs, are distinguished by a unique abundance pattern of the rare earth elements (REEs); they are highly depleted in the most refractory and the most volatile REEs, yet the moderately refractory light REE are present only in chondritic relative abundances. This REE pattern characteristic of Group II CAIs suggests a complex condensation history involving fractional condensation. The four CAIs of this study that have the highest Nd/U and Th/U ratios (as well as the lowest 238U/235U ratios) are all classified as Group II CAIs by their REE patterns. Because U has a lower condensation temperature than do Nd and Th, the fractional condensation history that resulted in the characteristic Group II REE pattern in these objects is likely to have produced the elevated Nd/U and Th/U ratios.
|REE patterns of four Group II CAIs analyzed in this study, normalized to CI chondrites. All other CAI samples studied here (except 3531-D, for which the REE abundances were not measured) display flat REE patterns indicating chondritic relative abundances of these elements (light-grey lines).|
The correlation of both Th/U and Nd/U with U isotope ratios in the CAIs indicates that the 238U/235U variations do not arise from nucleosynthetic anomalies or U isotope fractionation--neither of which easily give rise to such a trend--and instead provides evidence for the presence of extant 247Cm in the early Solar System. The initial 247Cm/235U ratio in the early Solar System is estimated by using the slopes of the best-fit lines on the 235U/238U vs Th/U and Nd/U diagram. Using Th and Nd as proxies for Cm, we estimate the initial Solar System 247Cm/235U ratio to be 2.4±0.6 x 10-4 and 1.1±0.2 x 10-4, respectively. The difference between the estimates may be caused by slight differences in the geochemical behavior of Th and Nd, or possibly by uncertainties in the assumed Solar System Nd/U or Th/U ratios. If 247Cm is inherited from galactic chemical evolution, the range of initial Solar System 247Cm/235U ratios estimated here translates to a time interval between the last r-process supernovae and the formation of the Solar System of approximately 110 to 140 million years.
Our findings of variable 238U/235U in meteoritic materials also have implications for precise dating of early events in the history of the Solar System. Geochronologists have used a standard Pb-Pb age equation for decades to calculate the absolute ages of both meteoritic and terrestrial materials. This equation assumes that 238U/235U is invariant at any given time, and that the present-day value is 137.88. Therefore, any deviation from this assumption would cause miscalculation in the determined Pb-Pb age of a sample. The differences seen in these samples would require correction of up to -5 million years if the Pb-Pb ages of these CAIs were obtained using the previously assumed 238U/235U value.
Age adjustment required for samples found not to have a 238U/235U value of 137.88, as assumed in the Pb-Pb age equation. The shaded region represents the range of U isotope compositions reported in this study, and the dots represent the specific 238U/235U ratios measured in these samples.
From the correlations of Th/U and Nd/U with 238U/235U ratios in Allende CAIs, we infer that 247Cm was present in the early Solar System and that the initial 247Cm/235U ratio was ~1-2 x 10-4. This value constrains the time interval between the last r-process nucleosynthetic event and the formation of the Solar System to approximately 110-140 million years. It is also clear from these samples that the 238U/235U ratio can no longer be assumed to be invariant in Solar System materials. The Pb-Pb dating technique is the only absolute dating technique able to resolve age differences of less than one million years in materials formed in the early Solar System and in order to produce a truly robust Pb-Pb age, it is essential to measure precisely and accurately the 238U/235U ratio in the dated material. At least for the oldest materials in the Solar System the good old days of measuring only lead isotopes are gone. The Pb-Pb dating method will still work fine, but cosmochemists will have to measure uranium isotopes, too.
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