The effect of Na vapor on the Na content of chondrules

Abstract— Chondrules contain higher concentrations of volatiles (Na) than expected for melt droplets in the solar nebula. Recent studies have proposed that chondrules may have formed under non-canonical nebular conditions such as in particle/gas-rich clumps. Such chondrule formation areas may have contained significant Na vapor. To test the hypothesis of whether a Na-rich vapor would minimize Na volatilization reaction rates in a chondrule analog and maintain the Na value of the melt, experiments were designed where a Na-rich vapor could be maintained around the sample. A starting material with a melting point lower that typical chondrules was required to keep the logistics of working with Na volatilization from NaCl within the realm of feasiblity. The Knippa basalt, a MgO-rich alkali olivine basalt with a melting temperature of 1325°± 5 °C and a Na₂O content of 3.05 wt%, was used as the chondrule analog. Experiments were conducted in a 1 atm, gas-mixing furnace with the fO₂ controlled by a CO/CO₂ gas mixture and fixed at the I-W buffer curve. To determine the extent of Na loss from the sample, initial experiments were conducted at high temperatures (1300 °C–1350 °C) for duration of up to 72 h without a Na-rich vapor present. Almost all (up to 98%) Na was volatilized in runs of 72 h. Subsequent trials were conducted at 1330 °C for 16 h in the presence of a Na-rich vapor, supplied by a NaCl-filled crucible placed in the bottom of the furnace. Succeeding Knudsen cell weight-loss mass-spectrometry analysis of NaCl determined the P_Na for these experimental conditions to be in the 10^?6 atm range. This value is considered high for nebula conditions but is still plausible for non-canonical environments. In these trials the Na₂O content of the glass was maintained or in some cases increased; Na₂O values ranged from 2.62% wt to 4.37% wt. The Na content of chondrules may be controlled by the Na vapor pressure in the chondrule formation region. Most heating events capable of producing chondrules are sufficient to volatilize Na. Sodium volatilization reaction rates will be reduced to varying degrees from melt droplets, depending on the magnitude of the P_Na generated. A combination of Na vapor during, and Na diffusion back into chondrules after, formation could maintain and/or enrich Na concentrations in chondrules.