PHASE RELATIONS IN TWO Na- AND Al-RICH CHONDRULES AND THE FORMATION OF THE CHONDRULE PRECURSOR

Introduction: Chondrules are believed to have formed by a brief but intense heating and melting event in the early solar system. Melting erased most mineralogical information about chondrules’ precursors. A sub-group of chondrules is rich in alkali elements and was classified as Naand Al-rich chondrules [1]. We describe two Na-rich chondrules and discuss their phase relations in order to disentangle primary and secondary processes. Concepts of chondrule precursors, nebular and parent body metasomatism will be discussed. Petrography: We describe two chondrules from UOCs Dar al Gani 369 and 378. Trace elements in these chondrules were previously published [2]. Chondrule DaG369-RF02 is ~600 μm in diameter and consists of forsteritic olivine in a glassy mesostasis of approximately albitic composition. The vitreous structure of the mesostasis was proven by means of micro-Raman spectroscopy. The mesostasis is Narich (~10 wt.%) throughout and notably poor in CaO and MgO (<1 wt.%). The porphyritic olivine exhibits cathodoluminescence and is poor in FeO (0.29 wt.%) and moderately enriched in CaO (0.40 wt.%) [3]. The largest phenocryst contains a melt inclusion with shrinkage bubble (Fig. 1). The inclusion is partly devitrified to nepheline.