Alkali recondensation into chondrules

TL;DR

This study reexamines the long-standing alkali-retention problem in chondrules by proposing that alkalis are lost during high-temperature heating but recondense as chondrules cool, concentrating in mesostases. By combining thermodynamic and kinetic analyses with isotopic considerations, the authors derive how recondensation depends on chondrule density, temperature history, and ambient gas composition, and show that limited isotopic fractionation can constrain the closure temperature and cooling rate. They infer chondrule-forming densities around $\rho_p \sim 10^{-6}\ \mathrm{kg\,m^{-3}}$, attainable near nebular pressure bumps, thus keeping nebular chondrule formation viable in a dust-enriched disk. The work also integrates glass inclusions, olivine zoning, and alkali-zoned chondrules to present a coherent open-system narrative for alkalis, with implications for disk structure and cooling histories across chondrite groups.

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.

Figures (14)

• Figure 1: Abundance patterns for elements arranged in order of increasing volatility for different types of chondrules in ordinary chondrites (type I and II in panels A and B respectively). The numbers of averaged chondrules were 19 (IA), 17 (IAB), 9 (IB), 7 (IIB), 45 (IIAB), 41 (IIA). Error bars are one standard error of the mean. The profile for amoeboid olivine aggregates Ruzickaetal2012 is also shown.
• Figure 2: Mean Na/Al versus Mg/Si ratios for different chondrule types in ordinary, enstatite and CMO chondrites. Error bars are one standard error of the mean. The numbers of averaged chondrules were the same as Fig. \ref{['pattern']} for OCs, and, for CMO, 70 (IAB), 18 (IB), 8 (IIA), and for EH, 17 (IAB) and 19 (IB).
• Figure 3: Mean K/Al versus Mg/Si ratios for different chondrule types in ordinary, enstatite and CMO chondrites. Error bars are one standard error of the mean. Same numbers of averaged chondrules as Fig. \ref{['Na_vs_Mg']}.
• Figure 4: Minor element concentration ratio between rim and core olivine in ordinary chondrites Alexanderetal2008Hewinsetal2012, as a function of core FeO content.
• Figure 5: Mesostasis/glass Na/Al vs. SiO$_2$ contents for chondrules in different chondrite groups. Colors code the groups and symbol shapes/filling code the petrographic setting. In particular, open circles represent type II chondrules and crosses refer to olivine-hosted glass inclusions ("incl.") from CR Varelaetal2002, C3-an Acfer 094 VarelaKurat2009, CM Fuchsetal1973Desnoyers1980VarelaKurat2009Florentinetal2017, CV Varelaetal2005VarelaKurat2009, LL VarelaKurat2009Hewinsetal2012 chondrites. Values below detection are reported as upper limits at $10^{-3}$.