The isotopic composition of Ag in meteorites and the presence of 107Pd in protoplanets
Abstract
The isotopic composition of silver in the metal phase of several types of iron meteorites has been determined. The ratios of 107Ag /109Ag are found to range from the terrestrial value of 1.09 to 9.2. These results show the presence of excess 107Ag ( 107Ag ∗) in almost all iron meteorites studied with a ratio of 108Pd /109 Ag greater than ~ 400. 107Ag ∗ has been found in iron meteorites of types IVA-IVB, IIIA-IIIB, IIB, and some anomalous types. The typical ratio found is 107Ag ∗/108Pd ≈ 2 × 10 -5 in general agreement with the original report of KELLY and WASSERBURG (1978). An intensive study of Gibeon (IVA) shows that the 107Ag /109Ag correlates with 108Pd /109Ag over a wide range for bulk samples of the metal rich phase and yield 107Ag ∗/108Pd = (2.40 ± 0.05) × 10 -5 and an extrapolated initial ( 107Ag /109Ag) 0 = 1.11 ± 0.01 . This is considered to represent an isochron resulting from the in situ decay of 107Pd ( τ 1/2 = 6.5 × 10 6y ) on the parent planet. The bulk sulfide shows 107Ag ∗ uncorrelated with 108Pd 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 107Ag ∗. 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 107Ag ∗ with 108Pd and 107Ag ∗/108Pd = ~1.5 × 10 -5 and initial ( 107Ag /109Ag ) 0 = 1.092 ± 0.002 . These observations demonstrate a correlation of 107Ag* with 108Pd 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 107Pd in the parent planets which initially contained normal Ag. The 107Ag* 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 107Pd and planet formation, melting, and cooling must be less than ~10 7a. A small but distinct variation in 107Ag* 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 107Pd is inferred to have been widespread throughout the terrestrial-type protoplanets. While the nucleosynthetic origin of 107Pd and 26Al 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 107Ag ∗ from the previous decay of extinct 107Pd in old interstellar dust grains. However, the presence of 26Al in protoplanets is not yet established, nor is the heat source for the later melting and planetary differentiation established.
- Publication:
-
Geochimica et Cosmochimica Acta
- Pub Date:
- June 1990
- DOI:
- 10.1016/0016-7037(90)90404-9
- Bibcode:
- 1990GeCoA..54.1729C
- Keywords:
-
- Iron Meteorites;
- Meteoritic Composition;
- Palladium Isotopes;
- Protoplanets;
- Silver Isotopes;
- Interstellar Matter;
- Recrystallization;
- METEORITES;
- PROTOPLANETS;
- ISOTOPES;
- COMPOSITION;
- SILVER 107;
- MELTING;
- PALLADIUM 107;
- IRON METEORITES;
- ISOTOPIC RATIOS;
- GIBON;
- PARENT BODIES;
- IVA IRONS;
- SANTA CLARA;
- IVB IRONS;
- DRP METEORITES;
- IIB IRONS;
- SILVER;
- TIMESCALE;
- COOLING;
- NUCLEOSYNTHESIS;
- METAL;
- SULFIDE;
- DECAY;
- HEAT;
- MUNDRABILLA;
- SAMPLES;
- METEORITE;
- LABORATORY STUDIES;
- PROCEDURE;
- CATALOGS;
- DATA;
- OCTAHEDRITES;
- DIAGRAMS;
- DRPA78009;
- FORMATION;
- ORIGIN;
- ISOCHRONS