Live107Pd in the Early Solar System and Implications for Planetary Evolution [chapter]

J. H. Chen, G. J. Wasserburg
2013 Earth Processes: Reading the Isotopic Code  
We present a report on the status of 107 Pd (>r = 9.4 x 10 6 y) in the early solar system and the implications of its presence for protoplanet evolution. Over the last two decades we have carried out an extensive search for the evidence of presently extinct 107 Pd in meteorites. From these results we conclude that: 1) 107 Ag* (excess 107 Ag) is present in a wide variety of iron and stony-iron meteorites; 2) 107 Ag* is due to the in situ decay of 107 Pd in these meteorites; 3) Pd-Ag metal-PeS
more » ... metal whole-rock isochrons have been established for a few meteorites; 4) the correlation observed for the total metal isochrons reflects a large variation in normal silver contents ( -1 0 3 ) with much small variations in Pd contents (factors of 10); 5) there is a wide range in apparent 107 Pdf 108 Pd ratios but many samples show a narrow range ( 107 Pdf 108 Pd = 1.5-2.5 x 10-5 ); and 6) there are clear cases of complex or pathologic behavior relating to some sulfides and their associated metal phases. The 107 Pd-107 Ag chronometer reflects the times of major chemical fractionation of Pd and Ag. The scenarios we have postulated to explain the Pdf Ag fractionation include two basically different fractionation processes, namely nebular and planetary. Major fractionation between PdfAg can only be achieved during condensation, early accretion, and metal segregation in the solar nebula. The reaction ofFeNi with H 2 S gas to form PeS and subsequent melting and segregation ofFeNi and PeS provides a mechanism for minor (x2) fractionation. For some volatile-depleted meteorites, the processes of condensation ofFeNi with isolation from later condensates effectively fractionated PdfAg and produced metal with a 108 Pdf 109 Ag -10 4 -10 5 . For other meteorites not so extensively depleted in volatiles, a smaller degree of effective Pdf Ag fractionation is produced due to the presence of later condensates which, upon planetary melting, produced metal with a 108 Pdf 109 Ag-50-100. Some fractionation ofPd and Ag occurred by metal-liquid, metal-crystal fractionation during crystallization in planets similar to the models of Scott [1972], Larimer and Anders [1967], and Wai and Wasson [1977]. If the variation in initial 107 Pdf 108 Pd among meteorites indicates a time difference (AT) in the condensation and melting-segregation of planetesimals, the data indicate a total range of -12 my for many meteorites. This tight cluster includes samples ofthe IIAB, IIIAB, IVA, IVB, and "anomalous" groups, as well as mesosiderites and pallasites. However, some meteorites exhibit no evidence of 1°7Pd. A comparison of 107 Pd and 53 Mn (>r = 5.3 x 10 6 y) chronometers on the same meteorites is possible for a few samples. In one case, 107 Pdf 108 Pd and 53 Mn! 55 Mn from Cape York (group IliA) show values of -2x1o-5 . In other cases, group IIIAB irons show 107 Pdf 108 Pd -2x1o-5 , but a much smaller 53 Mn! 55 Mn -1x1o-6 . In addition, 107 Pd is absent in the metal ofthe Eagle Station pallasite, but has 53 Mn! 55 Mn = 2.3x1o-6 in the Eagle Station silicates. There appear to be major discrepancies between the 107 Pd and 53 Mn chronometers. The 53 Mn-53 Cr system may be affected by the formation or equilibration of the microscopic phases containing Mn and Cr and the FeNi "host" during extended cooling and exsolution reaction. Earth Processes: Reading the Isotopic Code Geophysical Monograph 95
doi:10.1029/gm095p0001 fatcat:g2nuscedtvgkjcpf5wxocgtlce