Temperatures of canonical condensation: Constraints from CAIs

Abstract

The analogy between Al_8/3O₄ – MgAl₂O₄ spinels and AgSbS₂ − PbS galenas is explored. Although the spinel observed in calcium-aluminum-rich inclusions in carbonaceous chondrites (CAIs) is nearly pure MgAl₂O₄ and the galena found in hydrothermal ore deposits is nearly pure PbS, in both occurrences a very strong case may sometimes be made that these phases originally crystallized as solid solutions with substantially higher abundance of these minor components, Al_8/3O₄ in CAI spinel and AgSbS₂ in hydrothermal galena. It is shown that a few retrograde net-transport reactions accompanying Al_8/3O₄ breakdown in Al_8/3O₄ − MgAl₂O₄ spinels can account for several important features of spinel-rich CAIs including reverse zoning of åkermanite−gehlenite melilites, the production of Al-rich diopside and the depletion of anorthite in reaction rims, and the scarcity of grossite despite its previous presence as a high temperature condensate. AgSbS₂ in galena is analogous, because galenas in ore deposits today have an undetectable amount of AgSbS₂ (for example, Knowles,1983), yet, for example, the galena-rich ore deposits from the Coeur d9Alene mining district (ID) were responsible for the largest recorded number of ounces of Ag produced in the world until recently. Accordingly, it is appropriate that the primary Al_8/3O₄ contents of spinels in CAIs are reconstructed following methods analogous to those employed by Sack and others (2002,2005) to reconstruct the primary AgSbS₂ contents of galenas from the Coeur d9Alene ores, if CAIs spinels are to be used to constrain temperatures, and hence pressures, of canonical condensation. Several examples are given to demonstrate the thermal constraints which may be obtained from reconstructed primary spinel Al_8/3O₄ concentrations starting with the Al_8/3O₄ -rich CAI spinels reported by Yurimoto and others (2021) and El Goresy and others (1984). The compositions of spinels which form rims around corundum and corundum-hibonite aggregates in CAIs from the Murchison and Murray chondrites (Makide and others,2013) are then used to prove that Al_8/3O₄ in Al_8/3O₄ − MgAl₂O₄ spinels from CAIs are completely analogous to AgSbS₂ in AgSbS₂ − PbS galenas from hydrothermal ore deposits.