Generating potassium abundance variations in the Solar Nebula

TL;DR

This paper investigates the mechanisms behind potassium abundance variations in chondrules within meteorites, proposing a model involving high-altitude formation and settling that explains observed compositional differences.

Contribution

It introduces a novel explanation for potassium variations in chondrules, emphasizing high-altitude formation and settling processes over traditional models.

Findings

Potassium variations can be explained by high-altitude formation and settling.

Local separation of potassium and chondrules requires long timescales.

Models must account for correlated reservoirs with differing potassium abundances.

Abstract

An intriguing aspect of chondritic meteorites is that they are complementary: while their separate components have wildly varying abundances, bulk chondrites have nearly solar composition. This implies that the nearly-solar reservoirs in which chondrites were born were in turn assembled from sub-reservoirs of differing compositions that birthed the different components. We focus on explaining the potassium abundance variations between chondrules even within a single chondrite, while maintaining the observed CI $^{41}$K to $^{39}$K ratios. This requires physically separating potassium and chondrules while the temperature is high enough for K to be in the gas phase. We examine several mechanisms which could drive the dust through gas and show that to do so locally would have required long (sub-orbital to many orbits) time scales; with shortest potassium depletion time scales occurring in a scenario where chondrules formed high above the midplane and settled out of the evaporated potassium. While orbital time scales are at odds with laboratory chondrule cooling rate estimates, any other model for the origin for the potassium abundance variation has to wrestle with the severe logistical difficulty of generating a plethora of correlated reservoirs which varied strongly in their potassium abundances, but not in their potassium isotope ratios.