Evidence for the existence of ^(107)Pd in the early solar system

Abstract

The concentration and isotopic composition of Ag and Pd were measured in the Santa Clara iron meteorite. This meteorite has Pd/Ag of about 10^4. The isotopic composition of Pd is identical to terrestrial Pd within 0.1%. The ^(107)Ag/^(109)Ag ratio was 4% greater than the terrestrial value and suggests the in situ decay of ^(107)Pd(τ_½ = 6.5 × 10^6y) in this meteorite or its parent body. From the observed ^(107)Ag^*/^(110)Pd ratio (^* ≡ excess) we infer an initial (^(107)Pd/^(110)Pd)_⊙ ⩾2 × 10^(−5). These data are incompatible with an interval of ∼ 10^8y between the end of nucleosynthesis and the formation of planetary objects but are compatible with a last-minute injection of material. The inferred existence of ^(107)Pd and ^(26)Al indicates that the late injection included freshly synthesized material of both intermediate and low atomic weight on a similar time scale. We estimate the fraction of freshly synthesized material to be between 10^(−5) to 10^(−4) in the region of Z ∼ 50. The ^(244)Pu and some of the ^(129)I present in the early solar system may be due to earlier contributions. The highly granular nature of stellar nucleosynthesis is made apparent by these considerations. The ^(107)Pd-^(107)Ag chronometer is of particular importance as it can measure the time interval between the last injection of freshly synthesized material and the formation of differentiated planetary-size objects. Our data suggest that the time scale for the differentiation of some planets into large scale metallic and silicate segregations is as short as ∼10^6y.

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