Silica‐rich objects in the Acfer 182 CH chondrite: A new view

Five silica-rich objects (SRO) from Acfer 182 were studied. They have cryptocrystalline textures characterized by micro-emulsion and amoeboid patterns that point toward the coexistence of pyroxene- and silica-normative liquids that were quenched. Both objects have variable contents of refractory lithophile elements. Their positive Yb versus La correlation around primordial values suggests a cosmochemical process (e.g., a gas/liquid condensation) as responsible for SRO formation. The bulk trace element abundances of amoeboid- and emulsion-type SRO as well as their fractionation do not support an origin through high temperature processes. Conversely, their formation might have taken place while cooling of the nebular gas in two different chondrule-forming regions characterized by having different evolution paths. Cooling of these dust-enriched regions might lead to the condensation of pyroxene-rich liquids first, followed by formation of Mg-rich and SiO2-rich liquids, provided irradiation and annealing were active in these regions. Irradiation could be the process involved both in the formation of cristobalite (with annealing similar to 1200 K) and in triggering a spinoidal decomposition causing unmixing of the enstatite liquid into two coexisting phases, such as Mg-rich and SiO2-rich liquids, the precursors of the SRO in Acfer 182. Formation of emulsion- and amoeboid-type objects may be the result of exposing those chondrule-forming regions to different degrees of radiation.