Alkali recondensation into chondrules

TL;DR

This paper investigates alkali element behavior in chondrules, revealing that alkalis are lost during heating but recondense at lower temperatures, with isotopic evidence constraining cooling rates and densities in the early solar nebula.

Contribution

It demonstrates alkali recondensation in chondrules and constrains nebular conditions using isotopic fractionation data.

Findings

Alkalis are recondensed during cooling in chondrules.

Isotopic fractionation indicates rapid cooling or quenching.

Nebular densities above 10^{-6} kg/m^3 are inferred.

Abstract

While sub-mm melt droplets should rapidly lose alkali elements in a vacuum at liquidus temperatures, chondrules are only modestly depleted in them (by less than one order of magnitude). The detection of sodium in olivine cores has previously suggested very high saturating partial pressures of gaseous sodium, but we show that alkalis were lost during heating and recondensed at lower temperatures, essentially in the present-day chondrule mesostases. This recondensation was accompanied by mass-dependent enrichment in light isotopes (for multi-isotope alkalis such as K and Rb), but its limited extent indicates a cooling acceleration (or "quenching"). The isotopic fractionation also constrains the ratio of the chondrule density and the cooling rate prior to the quench around $10^{-6}\:\mathrm{kg.m^{-3}.K^{-1}.h}$ suggesting densities above $\sim 10^{-6}\:\mathrm{kg/m^3}$. In a nebular context, this is achievable by radial and vertical concentrations near pressure bumps.