Dry, hydrous, and CO2-bearing liquidus phase relationships in the CMAS system at 28 KB, and their bearing on the origin of alkali basalts

Dry, hydrous, and CO₂-bearing liquidus phase relationships were studied within the CMAS system (CaO-MgO-Al₂O₃-SiO₂) at 28 kb. In the dry system garnet, diopside, enstatite, and forsterite were found to coexist with a liquid of the approximate composition 50% SiO₂, 16% Al₂O₃, 25% MgO, 9% CaO. With 20 wt % H₂O added, this liquid is relatively more CaAl₂O₄-rich and less magnesian than it is under dry conditions. Addition of CO₂ results in a large expansion of the garnet and enstatite liquidus fields. This causes CO₂-rich liquids in equilibrium with garnet lherzolite to be SiO₂-poor and have high CaO/Al₂O₃. The simple-system liquid trends can be compared with the compositional trends of primitive alkali basalts from southeastern Australia, central Europe, Hawaii, and the Canary Islands. Variation in pressure during partial melting of garnet lherzolite also becomes a possible mechanism for producing the variations found in primitive alkali basalt composition. Both variation in pressure and variation in CO₂ concentrations during partial melting of garnet lherzolite appear to be capable of producing the compositional range of the natural alkali basalts. The data presently available do not enable a clear choice between these two possibilities. -from Author