The isotopic composition of Ag in meteorites and the presence of ^(107)Pd in protoplanets

The isotopic composition of silver in the metal phase of several types of iron meteorites has been determined. The ratios of ^(107)Ag/^(109)Ag are found to range from the terrestrial value of 1.09 to 9.2. These results show the presence of excess ^(107)Ag (^(107)Ag^∗) in almost all iron meteorites studied with a ratio of ^(108)Pd/^(109) Ag greater than ~ 400. ^(107)Ag^∗ has been found in iron meteorites of types IVA-IVB, IIIA-IIIB, IIB, and some anomalous types. The typical ratio found is ^(107)Ag^*/^(108)Pd ≈ 2 X 10^(-5) in general agreement with the original report of Kelly and Wasserburg (1978). An intensive study of Gibeon (IVA) shows that the ^(107)Ag/^(109)Ag correlates with ^(108)Pd/^(109)Ag over a wide range for bulk samples of the metal rich phase and yield ^(107)Ag^*/^(108)Pd = (2.40 ± 0.05) X 10^(-5) and an extrapolated initial (^(107)Ag/^(109)Ag)_0 = 1.11 ± 0.01. This is considered to represent an isochron resulting from the in situ decay of ^(107)Pd (T_(½) = 6.5 X10^6) on the parent planet. The bulk sulfide shows ^(107)Ag^∗ uncorrelated with ^(108)Pd and a highly variable isotopic composition of Ag. This phenomena is not well understood. We interpret the sulfide data as partly due to diffusion transport of Ag and segregation into minor phases. Some of this complexity is due to recrystallization after shock melting of FeS and anthropogenic heat treatment. Several other IVA meteorites were analyzed with a wide range in Pd/Ag and all showed the presence of ^(107)Ag^∗. A sample of Santa Clara (IVB) metal was analyzed, and it was confirmed that this does not lie on the IVA correlation line. Analysis of metal, schreibersite, and sulfide from the unshocked meteorite Derrick Peak (IIB) shows a correlation of ^(107)Ag^∗ with ^(108)Pd and ^(107)Ag^*/^(108)Pd = 1.5 X 10^(-5) and initial(^(107)Ag/^(109)Ag)_0 = 1.092 ± 0.002. These observations demonstrate a correlation of ^(107)Ag^* with ^(108)Pd within samples of the metal phase of a single meteorite and for samples of different meteorites. These results are compatible with the model of decay of ^(107)Pd in the parent planets which initially contained normal Ag. The ^(107)Ag^* in the sulfide is attributed to diffusion, although this is not proven. The cooling rates for the parent bodies must have been much faster than 150°K/ma thus implying a radius less than ~30 km. The presence of 107Pd in the parent bodies implies that the time scale between production of ^(107)Pd and planet formation, melting, and cooling must be less than ~10^7a. A small but distinct variation in ^(107)Ag^* content exists among different meteorites. If this variation is interpreted to represent an age difference, then the present results indicate a maximum difference in age of ~4 × 10^7 a. The nuclide ^(107)Pd is inferred to have been widespread throughout the terrestrial-type protoplanets. While the nucleosynthetic origin of ^(107)Pd and ^(26)Al is distinct, their presence in the early solar system appears to fix the time scales for small planet formation relative to the nucleosynthetic sources that produced them. The observations are not consistent with a model in which protosolar material is mixed with ^(107)Ag^∗ from the previous decay of extinct ^(107)Pd in old interstellar dust grains. However, the presence of ^(26)Al in protoplanets is not yet established, nor is the heat source for the later melting and planetary differentiation established.