Oxygen isotope study of the Asuka-881020 CH chondrite I: Non-porphyritic chondrules

Abstract

We measured oxygen isotope ratios and major elemental compositions of non-porphyritic chondrules and lithic fragments with various textures and chemical compositions in the Asuka-881020 CH chondrite. The oxygen isotope ratios plot along the primitive chondrule mineral line with Δ17O (=δ17O – 0.52 × δ18O) values from ∼−21‰ to +5‰. The Δ17O values increase with decreasing Mg# (=molar [MgO]/[MgO + FeO]%) from 99.6 to 58.5, similarly to the Δ17O-Mg# trends for the chondrules in other carbonaceous chondrites.Most of the measured objects (non-porphyritic chondrules and lithic fragments) including chondrules analyzed in the previous studies are classified into three groups based on the Δ 17O values and chemistry; the −2.3‰ group with FeO-poor compositions (the most abundant group), the +1.4‰ group with FeO-rich compositions, and the −6.3‰ group with FeO-poor compositions. Skeletal olivine and magnesian cryptocrystalline (MgCC) chondrules and MgCC chondrule fragments, which are the −2.3‰ group objects, may have formed via fractional condensation in the isotopically uniform gaseous environment with Δ 17O of −2.3‰. When silica-normative materials condensed from gas at ∼1200 K, 16O-rich refractory solids, similar to Ca-Al-rich inclusions, were incorporated into the environment. The silica-normative materials that condensed onto the 16O-rich refractory solids were reheated at 1743–1968 K and formed cristobalite-bearing chondrules with Δ17O of ∼−6‰. This scenario can explain the absence of silica-bearing chondrules in the −2.3‰ group and refractory element abundances in the cristobalite-bearing chondrules as high as those in the MgCC chondrules.Refractory element abundances of the +1.4‰ group objects decrease from FeO-Al-rich and ferroan CC (FeCC) chondrules to FeCC chondrule fragments to FeNi metal-bearing to silica-bearing chondrules. This suggests the formation via fractional condensation in the isotopically uniform gaseous environment. The Δ17O values and FeO-rich compositions of this group could be explained by an addition of 16O-poor water ice as an oxidant to the relatively 16O-rich solids with Δ17O of −2.3‰, which may also explain existence of some MgCC chondrules and fragments with intermediate Δ17O values between −2.3‰ and +1.4‰. The immiscibility textures in the silica-bearing chondrules suggest a reheating event at a temperature of >1968 K after condensation of silica-normative materials. Thus, the non-porphyritic chondrules and fragments in CH and CB chondrites, which are classified into three distinct Δ17O groups, require multiple chondrule-forming environments and heating events. Energy source for the heating events could be either impact plume and/or other dynamical processes in the protoplanetary disk, though a single heating event would not fully explain observed chemical and isotope signatures in these non-porphyritic chondrules.