Origin of Na in glass inclusions hosted in olivine from Allende CV3 and Jbilet Winselwan CM2: Implications for chondrule formation

Glass inclusions trapped in Mg-rich olivines within type I chondrules from the Allende (CV3) and Jbilet Winselwan (CM2) chondrites were analyzed by EPMA (Electron Probe Microanalysis) for major elements and by SIMS (Secondary Ion Mass Spectrometry) for Cl and S (analyzed here for the first time in chondrule-hosted glass inclusions). The inclusions from Jbilet Winselwan are poor in Na2O, whereas those from Allende are Na-rich, displaying up to 8 wt.% Na2O. The source of Na is a central issue in terms of chondrule origins because of the volatility of Na at high temperature. The wide scatter in Na2O contents of olivine-hosted glass inclusions from chondrules has led the community to propose that Na2O came from late interactions of chondrules with a Si/Na-rich gas. To gain new insights into the origins of the Na2O recorded in glass inclusions, heating experiments (up to 1810 degrees C) were performed on Allende inclusions in an effort to constrain the initial composition of the trapped melts. Our results demonstrate that sodium (although volatile) does not escape from inclusions during heating, thus confirming that glass inclusions behave as closed systems. Furthermore, heated olivines still bear inclusions containing up to 7.2 wt.% of Na2O. Olivines are thought to form at temperatures at which Na is volatile. This implies that (1) Na from glass inclusions cannot come from condensation but rather results from trapping in a Na-rich environment, which implies a high pressure, as in a melting planetasimal (2) there may be two distinct origins for the sodium: an indigenous origin for the sodium trapped inside glass inclusions and a gaseous origin for the sodium recorded in mesostasis from chondrules. Consequently, these results are in favor of a planetesimal origin for olivine from chondrules. (C) 2017 Elsevier B.V. All rights reserved.