Live¹⁰⁷Pd in the Early Solar System and Implications for Planetary Evolution

Abstract

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 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 ( -1 0 3 ) with much small variations in Pd contents (factors of 10); 5) there is a wide range in apparent 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 108Pd 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

Pd–Ag chronometry of iron meteorites: Correction of neutron capture-effects and application to the cooling history of differentiated protoplanets

“…Although most of the IVBs analyzed for Pd-Ag isotope systematics exhibit radiogenic Ag isotope compositions, they all plot below but approximately parallel to the Pd-Ag isochron of the IVA iron Gibeon (Chen and Wasserburg, 1996). This is surprising, because it would indicate a less radiogenic initial 107 Ag/ 109 Ag for the IVBs, in spite of their more volatile-depleted composition compared to IVA iron meteorites.…”

“…For instance, different metal samples of the IVA iron meteorite Gibeon define an isochron corresponding to an initial 107 Pd/ 108 Pd of (2.40 ± 0.05) Â 10 À5 at the time of Pd-Ag closure, the highest and most precise ratio determined so far (Chen and Wasserburg, 1990). A detailed Pd-Ag study on a large set of magmatic iron meteorites has demonstrated the presence of excess 107 Ag in almost all investigated magmatic iron meteorites, with inferred initial 107 Pd/ 108 Pd ratios of between $1.5 Â 10 À5 and $2.5 Â 10 À5 (Chen and Wasserburg, 1996).…”

“…Furthermore, the Pd/Ag ratios of different samples from a given group of iron meteorites also vary, reflecting Pd/Ag fractionation during fractional crystallization of the metal cores (Chabot and Drake, 1997). Finally, within a magmatic iron meteorite the Pd/Ag ratio is strongly fractionated between metal and sulfide, making it possible to determine precise internal isochrons and, hence, cooling ages of individual iron meteorites (Chen and Wasserburg, 1983, 1996Horan et al, 2012).…”

Short-Lived Radionuclides and Early Solar System Chronology

“…Extrapolation back to the time of CAI formation would yield an initial 107 Pd/ 108 Pd of approximately twice this value for the solar system, though with considerable uncertainty. Further isochrons were determined in many other iron and stony-iron meteorites, showing that there is a wide range of initial 107 Pd/ 108 Pd ratios, but that many samples have ratios in the range (1.5-2.5) Â 10 À5 (Chen and Wasserburg, 1996;Chen et al, 2002). There are a few iron meteorites and pallasites for which 107 Pd-107 Ag systematics have been studied and the 53 Mn-53 Cr systematics have been determined in phosphate inclusions (Chen and Wasserburg, 1996).…”

“…Further isochrons were determined in many other iron and stony-iron meteorites, showing that there is a wide range of initial 107 Pd/ 108 Pd ratios, but that many samples have ratios in the range (1.5-2.5) Â 10 À5 (Chen and Wasserburg, 1996;Chen et al, 2002). There are a few iron meteorites and pallasites for which 107 Pd-107 Ag systematics have been studied and the 53 Mn-53 Cr systematics have been determined in phosphate inclusions (Chen and Wasserburg, 1996). Some are clearly disturbed, but extrapolation of the line through four iron meteorites to the early solar system 53 Mn/ 55 Mn ratio of $ 6.7 Â 10 À6 implies an early solar system 107 Pd/ 108 Pd ratio of $ 5 Â 10 À6 .…”