Oxygen isotope study of the Asuka-881020 CH chondrite I: Non-porphyritic chondrules
Daisuke Nakashima, Makoto Kimura, Kouichi Yamada, Takaaki Noguchi, Takayuki Ushikubo, Noriko Kita
Geochimica et Cosmochimica Acta
In Press, Journal Pre-proof, Available online 16 September 2020
“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 Δ17O 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 Δ17O 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.”