Alkali recondensation into chondrules

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 1 0 − 6 kg.m − 3 .K − 1 .h suggesting densities above ∼ 10 − 6 kg/m 3 . In a nebular context, this is achievable by radial and vertical concentrations near pressure bumps. • Alkalis recondensed into chondrules after olivine crystallization. • Solid density in chondrule-forming regions constrained to 10 -6±1 kg/m 3 . • Chondrules formed in settled dust disk near pressure bumps.