Effect of carbon dioxide on dehydration melting reactions and melt compositions in the lower crust and the origin of alkaline rocks

Abstract

Dehydration melting experiments of alkali basalt associated with the Kenya Rift were performed at 0·7 and 1·0 GPa, 850-1100°C, 3-5 wt % H2O, and f(O2) near nickel-nickel oxide. Carbon dioxide [X(CO2) = molar CO2/(H2O + CO2) = 0·2-0·9] was added to experiments at 1025 and 1050°C. Dehydration melting in the system alkali basalt-H2O produces quartz and corundum-normative trachyandesite (6-7·5 wt % total alkalis) at 1000 and 1025°C by the incongruent melting of amphibole (pargasite-magnesiohastingsite). Dehydration melting in the system alkali basalt-H2O-CO2 produces nepheline-normative tephriphonolite, trachyandesite, and trachyte (10·5-12 wt % total alkalis). In the latter case, the solidus is raised relative to the hydrous system, less melt is produced, and the incongruent melting reaction involves kaersutite. The role of carbon dioxide in alkaline magma genesis is well documented for mantle systems. This study shows that carbon dioxide is also important to the petrogenesis of alkaline magmas at the lower pressures of crustal systems. Select suites of continental alkaline rocks, including those containing phonolite, may be derived by low-pressure dehydration melting of an alkali basalt-carbon dioxide crustal system.