Live ^(107)Pd in the Early Solar System and Implications for Planetary Evolution

Abstract

We present a report on the status of ^(107)Pd (τ = 9.4 × 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 - FeS and 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 (∼10^3) with much small variations in Pd contents (factors of 10); 5) there is a wide range in apparent ^(107)Pd/^(108)Pd ratios but many samples show a narrow range (^(107)Pd/^(108)Pd = 1.5 − 2.5 × 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 Pd/Ag fractionation include two basically different fractionation processes, namely nebular and planetary. Major fractionation between Pd/Ag can only be achieved during condensation, early accretion, and metal segregation in the solar nebula. The reaction of FeNi with H_2S gas to form FeS and subsequent melting and segregation of FeNi and FeS provides a mechanism for minor (x2) fractionation. For some volatile-depleted meteorites, the processes of condensation of FeNi with isolation from later condensates effectively fractionated Pd/Ag and produced metal with a ^(108)Pd/^(109)Ag ∼ 10^4 – 10^5. For other meteorites not so extensively depleted in volatiles, a smaller degree of effective Pd/Ag fractionation is produced due to the presence of later condensates which, upon planetary melting, produced metal with a ^(108)Pd/^(109)Ag ∼ 50–100. Some fractionation of Pd 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)Pd/^(108)Pd among meteorites indicates a time difference (ΔT) 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 of the IIAB, IIIAB, IVA, IVB, and “anomalous” groups, as well as mesosiderites and pallasites. However, some meteorites exhibit no evidence of ^(107)Pd. A comparison of ^(107)Pd and ^(53)Mn (τ = 5.3 × 10^6 y) chronometers on the same meteorites is possible for a few samples. In one case, ^(107)Pd/^(108)Pd and ^(53)Mn/^(55)Mn from Cape York (group IIIA) show values of ∼2×10^(−5). In other cases, group IIIAB irons show ^(107)Pd/^(108)Pd ∼2×10^(−5), but a much smaller ^(53)Mn/^(55)Mn ∼1×10^(−6). In addition, ^(107)Pd is absent in the metal of the Eagle Station pallasite, but has ^(53)Mn/^(55)Mn = 2.3×10^(−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.