Reconciling fast and slow cooling during planetary formation as recorded in the main group pallasites

Abstract

Pallasite meteorites contain evidence for vastly different cooling timescales: rapid cooling at high temperatures (K/yrs) and slow cooling at lower temperatures (K/Myrs). Pallasite olivine also shows contrasting textures ranging from well-rounded to angular and fragmental, and some samples record chemical zoning. Previous pallasite formation models have required fortuitous changes to the parent body in order to explain these contrasting timescales and textures, including late addition of a megaregolith layer, impact excavation, or parent body break-up and recombination. We investigate the timescales recorded in Main Group Pallasite meteorites with a coupled multiscale thermal diffusion modelling approach, using a 1D model of the parent body and a 3D model of the metal-olivine intrusion region, to see if these large-scale changes to the parent body are necessary. We test a range of intrusion volumes and aspect ratios, metal-to-olivine ratios, and initial temperatures for both the background mantle and the intruded metal. We find that the contrasting timescales, textural heterogeneity, and preservation of chemical zoning can all occur within one simple ellipsoidal segment of an intrusion complex. These conditions are satisfied in 13% of our randomly generated models (2200 model runs), with small intrusion volumes (with a mean radius ≲100 m) and colder background mantle temperatures (≲1200 K) favourable. Large rounded olivine can be explained by a previous intrusion of metal into a hotter mantle, suggesting possible repeated bombardment of the parent body. We speculate that the formation of pallasitic zones within planetesimals may have been a common occurrence in the early Solar System, as our model shows that favourable pallasite conditions can be accommodated in a wide range of intrusion morphologies, across a wide range of planetesimal mantle temperatures, without the need for large-scale changes to the parent body. We suggest that pallasites represent a late stage of repeated injection of metal into a cooling planetesimal mantle, and that heterogeneity observed in micro-scale rounding or chemical zoning preservation in pallasite olivine can be explained by diverse cooling rates in different regions of a small intrusion.