CONDENSATION DURING CHONDRULE FORMATION: ELEMENTAL AND MG ISOTOPIC EVIDENCE

Introduction: Recent petrochemical studies have revealed that chondrules were formed in an open system [e.g. 1,2], and models that explain the compositional change by evaporation have been proposed [3,4]. Although volatile loss has been one of the most concerns [5,6], the models include the major elements such as MgO and SiO2. On the contrary, clear evidence for open system behavior of MgO and SiO2 has not yet been found. We have studied highly refractory chondrules in Semarkona and found evidence for condensation of MgO, SiO2, FeO, Na2O, and K2O during chondrule formation under the condition of decreasing temperature. Texture: A polished thin section of Semarkona (USMN 1805-9) was studied with a JEOL 8900L EPMA. Two chondrules were selected for the study, which are characterized by highly refractory compositions, anomalous porphyritic texture, and perfectly rounded shape. Chondrule #5 (CH5) is about 800μm in diameter, and consists of forsterite and glass. Forsterite crystals with ten to several tens of μm size and euhedral to partly hopper shape are rimming the inner surface, which occupy 70-80% of the entire surface. There is only one tabular forsterite crystal with 80x200 μm size in the apparent center. Modal concentration of forsterite is 10-20 vol%. Another portion of the chondrule is clear glass. Chondrule #13 (CH13) is about 500μm in diameter, and consists of forsterite and glass. Rectangular to tabular forsterite crystals grow from the surface, and one of it grows inward to form a 250μm large hopper crystal. About 70% of the chondrule surface is occupied by forsterite, and 30-40 vol% of the entire chondrule is occupied by forsterite. Glass is clear under an optical microscope. CH13 is texturally similar to CH5 except for a little higher forsterite modal concentration. Chemical Composition: Both CH5 and CH13 are highly refractory; forsterite is poor in FeO (<1.0 wt%), and glass is rich in CaO and Al2O3. Forsterite in CH5 shows Al2O3 and CaO decrease and FeO increase from the core to rim. The concentration of those trace elements ranges from 0.05 to 0.46 wt% (Al2O3), 0.26 to 0.4 (CaO), and 0.44 to 1.0 (FeO), which are similar to forsterite in other type IA chondrules [7] or group A1 chondrules [8], but differs in that the CaO content is almost constant. Forsterite in CH13 is zoned with Al2O3and CaO-depletion and FeO-enrichment from the core to rim, and those near the margin show growth zoning in terms of Al2O3. Contrary to fairly constant CaO concentration in CH5 forsterite (0.3-0.4 wt%), those in CH13 (0.4 to 0.65 wt%) is linearly correlated with Al2O3 (0.1 to 0.4 wt). Groundmass glass in CH5 and CH13 is strongly zoned. Al2O3, CaO, MgO and TiO2 decrease from the center to surface (Fig. 1). Local heterogeneity of strong Al2O3 enrichment, weak CaO enrichment, and MgO depletion around forsterite crystals are found, which is attributable to forsterite crystallization. SiO2 decreases monotonously from the surface to the center, whereas Na2O and K2O are enriched only near the surface, of which distribution is scarcely affected by forsterite distribution. Na2O is specifically enriched along curved veins which resembles perlitic texture in rhyolite glass formed by cooling contraction or expansion due to hydration.