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 {aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}50%\ SiO₂,\ 16%\ A1₂O₃,\ 25%\ MgO,\ 9%\ CaO.\ With\ 20\ wt\ %\ H₂O{document} 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/Ai₂O₃. 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. Although the two sets of data cannot be directly superimposed, the primitive alkali basalts show the same sense of variation as do liquids produced in the simple system by partial melting of garnet lherzolite in the presence of variable concentrations of CO₂. In the presence of the additional components TiO₂, FeO, P₂O₅, alkalis and H₂O (found in natural systems), the peridotite-melting trends become more closely matched with the compositional range of the natural basalts. Variation in pressure during partial melting of garnet lherzolite also becomes a possible mechanism for producing the variations found in primitive alkali basalt compositions. 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.