Formation of the Treysa quintet and the main-group pallasites by impact-generated processes in the IIIAB asteroid.

Wasson, J. T.

Meteoritics & Planetary Science. doi: 10.1111/maps.12635


“Treysa and Delegate have compositions closely similar to those of IIIAB irons but plot above the IIIAB field on Ir-Au diagrams; for this reason they are designated anomalous members of IIIAB. All refractory siderophiles share this anomaly. Wasson (1999) interpreted the large spread on IIIAB Ir-Au diagrams to result from melt-trapping and generated solid and liquid fractional crystallization tracks; almost all IIIAB irons fall between the tracks. In contrast, Treysa, Delegate, and three other irons (the Treysa quintet) plot beyond the liquid track. Ideal fractional crystallization cannot account for compositions that plot outside the region between the tracks. Possible explanations for the anomalous compositions of the Treysa quintet are that (1) these meteorites did not form in the IIIAB magma or (2) they formed by the mixing of early crystallized solids with a late liquid. The weight of the evidence including cosmic-ray ages favor the second explanation. Although this explanation can account for positions plotting above the liquid track, it requires special circumstances. The infalling blocks must be assimilated and the resulting melt must crystallize quickly into pockets small enough (<1 m) to allow igneous gradients to be leveled by subsequent diffusion. The Treysa quintet shares the region beyond the liquid track with most main-group pallasites (PMG), which may have also originated in the IIIAB body. It appears that Treysa, its relatives, and the PMG were formed in one or more impact events that mixed olivine and solid metal formed near the core-mantle boundary with nearby magma. It is then necessary to cool the melt rapidly; the best way to achieve rapid cooling is by heat exchange with cooler solids. That the Treysa quintet and the PMG can be explained by the same processes operating on late IIIAB magma supports the conclusion that PMG formed on the IIIAB parent asteroid."