MODELING THE EVAPORATION OF CAI-LIKE MELTS, AND CONSTRAINING THE ORIGIN OF CH-CB CAIs

1Marina A.Ivanova,2,3Ruslan A.Mendybaev,1Sergei I.Shornikov,1Cyril A.Lorenz,4Glenn J.MacPherson
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.12.023]
1Vernadsky Institute of Geochemistry of the Russian Academy of Sciences, Moscow, Russia
2Department of the Geophysical Sciences, University of Chicago, Chicago, IL, United States
3Chicago Center for Cosmochemistry, University of Chicago, Chicago, IL, United States
4National Museum of Natural History, Smithsonian Institution, Washington DC, USA
Copyright Elsevier

To address the bulk compositions of CAIs from CH-CB chondrites we have used a new thermodynamic method to model the evaporation of CAI-like melts. The model calculations agree closely with the results of evaporation experiments on individual bulk compositions, and thus could provide a general means of predicting the evaporation trajectory of any CAI bulk composition melt. The model calculations and evaporation experiments show that the initial stages of CAI melt evaporation are controlled by the relative evaporation rates of MgO and SiO2, whereas the late stages are dominated by the initial CaO/Al2O3 ratio of the melt. Application of the model to the puzzling bulk compositions of very refractory CAIs from CH-CB chondrites, many of which are grossite-, hibonite-, and spinel-rich, shows that such compositions can be derived via evaporation of precursors unusually enriched in Al2O3 with CaO/Al2O3 ratios (weight %) < 0.3. This rules out most silicate-rich CAI varieties. Only spinel- and spinel-hibonite-rich fine-grained inclusions with group II REE patterns (common in CV3 chondrites), which may have been present in the region where CH CAIs formed, could be a precursor for the grossite- and hibonite-rich igneous CAIs.

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