THE EFFECT OF NA VAPOR ON THE NA CONTENT OF CHONDRULES

Chondrules contain higher concentrations of volatiles (Na) than expect ed for melt droplets in the solar nebula. Recent studies have proposed that chondrules may have formed under non-canonical nebular condition s such as in particle/gas-rich dumps. Such chondrule formation areas m ay have contained significant Na vapor. To test the hypothesis of whet her 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 sa mple. A starting material with a melting point lower that typical chon drules was required to keep the logistics of working with Na volatiliz ation from NaCl within the realm of feasiblity. The Knippa basalt, a M gO-rich alkali olivine basalt with a melting temperature of 1325-degre es +/- 5-degrees-C and a Na2O content of 3.05 wt%, was used as the cho ndrule analog. Experiments were conducted in a 1 atm, gas-mixing furna ce 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, in itial experiments were conducted at high temperatures (1300-degrees-C - 1350-degrees-C) for duration of up to 72 h without a Na-rich vapor p resent. Almost all (up to 98%) Na was volatilized in runs of 72 h. Sub sequent trials were conducted at 1330-degrees-C for 16 h in the presen ce of a Na-rich vapor, supplied by a NaCl-filled crucible placed in th e bottom of the furnace. Succeeding Knudsen cell weight-loss mass-spec trometry analysis of NaCl determined the PN, for these experimental co nditions to be in the 10(-6) atm range. This value is considered high for nebula conditions but is still plausible for non-canonical environ ments. 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 press ure in the chondrule formation region. Most heating events capable of producing chondrules are sufficient to volatilize Na. Sodium volatiliz ation reaction rates will be reduced to varying degrees from melt drop lets, depending on the magnitude of the P(Na) generated. A combination of Na vapor during, and Na diffusion back into chondrules after, form ation could maintain and/or enrich Na concentrations in chondrules.