Elemental and isotopic fractionations produced through evaporation of the Allende CV chondrilte: Implications for the origin of HAL-type hibonite inclusions

Abstract

Through evaporation of samples from the Allende carbonaceous chondrite we have produced a series of residues that show correlated variations in mineralogy, chemistry, and isotopic compositions. Major and minor elements are evaporated in the order (Fe, Mn, Cr) → (Mg, Si) → (Ca, Ti) → (Al) and their loss is reflected in the mineralogy of the remaining samples. Residues of low to moderate degrees of evaporation consist of increasingly Mg-rich olivine and silicate glass. After complete evaporation of Fe, Mg, and Si at approximately 96% mass loss, the residues consist of very fine-grained Ca aluminates. Evaporation at higher temperatures produced three residues that contain hibonite and a less refractory CaAl glass. Magnesium, Si, Ca, Ti, and O isotopes show mass-dependent fractionations that are consistent with Rayleigh-type distillation. The rare earth elements and other refractory trace elements are enriched in the residues up to ∼100 × CI, although several elements (V, Ba, Ce) are depleted due to their increased volatilities under oxidizing conditions. The most refractory residues also exhibit depletions in Eu, an element that is volatile under reducing conditions, but is as refractory as the other light rare earth elements under oxidizing conditions. The apparently contradictory presence of both Ce and Eu depletions in the residues is a result of changing evaporation dynamics in the course of the experiments: the release of large amounts of O during evaporation of the major element oxides creates “locally oxidizing” conditions in the samples; later, after most major elements have been vaporized, the local sample environments become more “reducing.” The three hibonite-bearing residues share many chemical and isotopic characteristics with five HAL-type hibonite inclusions for which an origin as distillation residues has been proposed; our data show that many of the unique features of these inclusions can be produced in a single evaporation event. Strong similarities between the hibonite-bearing residues and the hibonite inclusions HAL and DH-H1 suggest that the evaporation histories of these inclusions may be roughly comparable to those of our residues.