Temperatures of canonical condensation: Constraints from CAIs

Abstract

The analogy between Al8/3O4 – 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, Al8/3O4 in CAI spinel and AgSbS2 in hydrothermal galena. It is shown that a few retrograde net-transport reactions accompanying Al8/3O4 breakdown in Al8/3O4 − 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 d'Alene 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 Al8/3O4 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 d'Alene 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 Al8/3O4 concentrations starting with the Al8/3O4 -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 Al8/3O4 in Al8/3O4 − MgAl2O4 spinels from CAIs are completely analogous to AgSbS2 in AgSbS2 − PbS galenas from hydrothermal ore deposits.