Petrochemical study of the Yamato-691 enstatite chondrite (E3) V: Comparison of major element chemistries of chondrules and inclusions in Y-691 with those in ordinary and carbonaceous chondrites

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Major element compositions of chondrules and inclusions in Y-691 are different on the whole from those in ordinary and carbonaceous chondrites; the (CaO+AlO_<1.5>)/(CaO+AlO_<1.5>+SiO_2) and MnO/MgO ratios of chondrules in Y-691 are lower than those in ordinary and carbonaceous chondrites, but the SiO_2/(SiO_2+MgO) and MgO/(MgO+FeO) ratios are higher. These differences suggest that the chondrule precursors of Y-691 were produced by intense fractional condensation in comparison with those of ordinary and carbonaceous chondrites. For Y-691,the Ca/Al ratios of chondrules showing high Na/Al are lower than those of chondrules showing low Na/Al. This indicates that a fraction of Ca component was expelled from the chondrule precursors by reactions with a nebular gas, which also introduced alkalis into the chondrule precursors. In addition to magnesian chondrules, Y-691 includes unusual inclusions and ferroan chondrules. One unusual glassy inclusion, as well as others, are similar in chemical composition to fine-grained CAI's and amoeboid olivine inclusions in carbonaceous chondrites, respectively. The precursors of the unusual inclusions and ferroan barred-Ol-Px chondrules seem to have formed under oxidized conditions, whereas the precursors of magnesian chondrules were produced from a reduced nebular gas. The oxidizing conditions may have been produced by evaporation of chondritic or cometary materials in the solar nebula.
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