Evidence for successive episodes of condensation at high temperature in a part of the solar nebula

A centimeter-sized CaAl-rich inclusion, CAI 3643, in the Allende meteorite has preserved a record of its multiple stages of growth by condensation at high temperature. It consists of three distinct, concentric layers—core, mantle and crust. The open-textured core is composed of hibonite laths, interstitial melilite and voids, grains of alumina, and OsIr metal nuggets in the inner core that are complementary in composition to RuPt-rich nuggets in the outer core. The core formed out of rapidly-aggregated crystals from two episodes of condensation in the same gaseous reservoir. Mo and W depletions in the nuggets indicate condensation from a relatively oxidizing gas, with 60 times higherH/₂OH₂ than solar gas. The compact mantle of coarse melilite contains perovskite inclusions and rare NiFe metal and sulfide grains. The compact, fine-grained crust consists of melilite, hibonite, spinel, perovskite, and opaques similar to those in the mantle. Neither mantle nor crust has been molten; each grew by condensation of solids directly onto the surface of the CAI. The formed CAI experienced some metamorphic recrystallization of melilite, and volatile alteration producing grossular, feldspathoids and hercynite. Trace element analyses of perovskites and a bulk sample show the mantle and crust to be depleted in refractory elements (i.e. Group II) and complementary to the ultra-refractory core. Despite differences in texture and composition, the core, mantle and crust display a continuity in their modal mineralogical compositions, their extreme Al₂O₃ enrichments (53–69% cf. 25–45% for other CAI's) and their trace element complementarity. This implies that all the layers of 3643 formed from the same cooling gaseous reservoir within a limited interval of time and space, consistent with an origin in a local, transient heating event in the CV chondrite formation region of the nebula.