3/24/2023 0 Comments Origin for mac scienceThe slope of the two-point isochron between Curious Marie and the rest of the samples translates into a 247Cm/ 235U of (5.6 ± 0.3) × 10 −5 at the time of Nd/U fractionation in Curious Marie. The scatter in the data (for example, at very low Nd/U ratios) suggests that stable isotopic fractionation during evaporation/condensation also influenced the U isotopic composition of CAIs. The +59‰ δ 235U value observed in the Curious Marie CAI is well outside the range of variations expected from fractionation during condensation (gray rectangle) and is thus interpreted as definitive evidence for live 247Cm in the ESS. Open circles, previous studies ( 11, 17– 19, 33, 34) blue circles, Allende CAIs from this work light-blue square, bulk Allende from this work]. Extensive testing was also done to ensure that no interferences or matrix effects were affecting the measurement by combining the matrix cut from Curious Marie with CRM-112a, purifying the U by column chemistry, and finding that the measured U isotopic composition is correct (that is, identical to pure CRM-112a) after purification. The tests yielded δ 235U values of +52.79 ± 14.91‰ after one purification step with 235U measured on Faraday, +59.12 ± 2.80‰ after two purifications with 235U on an electron multiplier, and +58.97 ± 2.72‰ after three purifications with 235U on an electron multiplier. The measurement was triplicated using different sample purification schemes and various measurement setups. Considerable effort was expended to confirm this result (see the Supplementary Materials). For comparison, the highest 144Nd/ 238U ratio and 235U excess measured in CAIs prior to this work were 794 and +3.43‰ ( 238U/ 235U = 137.37), respectively ( 11). One sample (named Curious Marie) has an extremely high 144Nd/ 238U ratio (~22,640) and a 235U excess of +58.9 ± 1.9‰ (equivalent to an absolute 238U/ 235U ratio of 130.17 ± 0.27 Fig. Although Th has sometimes been used as a proxy for Cm ( 11, 16), coherent behavior for the Th-Cm and Th-Pu pairs is neither expected ( 13) nor observed ( 12), and the light REEs (for example, Nd or Sm) are therefore taken as more reliable proxies of Cm behavior during condensation. This conclusion is supported by the observed coherent behavior of Pu, Nd, and Sm in pyroxene/melt and phosphate/melt partitioning experiments ( 15) and during magmatic differentiation in achondrites ( 12, 14). ![]() On the basis of their near-identical valence states, ionic radii, and volatilities, the light rare earth elements (REEs) are thought to behave similarly to Pu and Cm during nebular processes ( 12– 14). Because Cm has no long-lived isotopes, another element must be used as a proxy for Cm in isochron diagrams. To demonstrate that those variations are due to 247Cm decay, one must show that the δ 235U values correlate with the Cm/U parent-to-daughter ratios. Recently, excesses in 235U of up to 3.5‰ were documented in four fine-grained CAIs ( 11). The two cannot be easily distinguished because U has only two long-lived isotopes and both mechanisms would produce similar correlations between light U isotope enrichments and U concentrations. Unfortunately, its abundance in the early solar system (ESS) has been the subject of some debate that boils down to knowing whether the uranium isotope variations (that is, 235U/ 238U ratio) measured in early-formed nebular condensates are due to the decay of 247Cm or isotopic fractionation during condensation. In contrast, 247Cm, which decays into 235U, has a much shorter half-life ( t 1/2 = 15.6 My) and would be very well suited to address this question. However, 244Pu has a long half-life and its stellar yield is uncertain ( 10), which makes it insensitive to the history of nucleosynthesis before SS formation and whether or not multiple r-process sites contributed to the synthesis of the actinides. The existence of an “actinide” production site is motivated by two main observations: (i) the ages of some old stars inferred from their Th/Eu ratios are negative, meaning that they must have formed with a Th/Eu ratio that was higher than that relevant to the solar system (SS) ( 9), and (ii) the abundance of the short-lived radionuclide (SLR) 244Pu in meteorites is low compared to another nominal r-process radionuclide, 182Hf ( 7). The r-process label may comprise some diversity as it was suggested that up to three r-processes were responsible for producing light r-nuclides (A 140), and actinides (for example, U and Th). Unlike the s- and p-process, which are relatively well understood ( 3, 4), little is known regarding the astrophysical conditions under which r-process nuclides are produced ( 5– 8). ![]() All elements beyond the iron peak (above ~70 atomic mass units) are the products of three main processes of stellar nucleosynthesis: the s- ( slow neutron capture), r- ( rapid neutron capture), and p-process ( proton process) ( 1, 2).
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |