SmNd systematics of a silicate inclusion in the Caddo IAB iron meteorite

Abstract

We report the first SmNd data from an iron meteorite silicate inclusion. Mineral separates from an inclusion in the Caddo IAB iron define a 147Sm 143Nd age of 4.53±0.02 Ga, and an initial 146Sm 144Sm of 0.0086±0.0021. A plagioclase separate from the inclusion yields the lowest 142Nd 144Nd ratio measured to date, which provides an absolute maximum initial solar sysem ϵ 142 of −2.1 ± 0.4. When the 146Sm 142Nd data are combined with the 147Sm 143Nd data, we obtain our preferred initial solar system ϵ 142 value of −3.2 ϵu, which corresponds to an initial 146Sm 144Sm of 0.009. Rare earth element data from the Caddo silicate inclusion suggest that it was derived by partial melting of a chondritic source prior to incorporation into the metal. Therefore, differentiation of the silicate portion of the parent body must have started approximately within the first 30 Ma of solar system history. The data reported here clearly show that IAB meteorites preserve a record of very early planetary differentiation processes. Modeling of element diffusion indicates that, if the silicate material was incorporated into its FeNi host at a time much later than 4.53 Ga, then the mixing must have taken place at shallow levels (i.e., < 50 m) in the parent body, in order to preserve the SmNd systematics. It is more likely that incorporation of the silicate took place at depths greater than 2 km, and that it occurred within the first ∼ 30 Ma of solar system history. This is consistent with estimates of cooling rates determined for IAB irons.