A high-pressure experimental study on the evolution of the silicic magmatism of the Main Ethiopian Rift

In this contribution we report the results of an experimental study that investigated equilibrium and fractional crystallization of hydrous, transitional alkaline basalt at low oxygen fugacity, under lower to middle crustal conditions to constrain the generation of subaluminous and peralkaline differentiation products that typically occur in rift systems. The experiments reveal that liquids produced by equilibrium crystallization in the range 0.7–1 GPa cannot cross the subaluminous/peralkaline compositional divide. In contrast, fractional crystallization experiments under isobaric and polybaric conditions approach closer the naturally observed trend from subaluminous to evolved peralkaline products suggesting that polybaric differentiation starting at elevated pressures can indeed lead to the transition from subaluminous to peralkaline derivative liquids. The presence of water in the parental magmas of silicic derivative products is of prime importance for the fractionation equilibria as well as for the mobility of such magmas toward shallow crustal levels. We suggest that peralkaline magmas in rift environments are indicative for differentiation under relatively low oxygen fugacity conditions in an extensional environment characterized by a high degree of crustal fracturing that allows rapid upward migration of mafic parental magmas and formation of shallow magma reservoirs. Crystallization–differentiation of parental, hydrous transitional alkaline basalt in such reservoirs is controlled by low pressure phase equilibria that typically evolve through early saturation of anorthite-rich plagioclase and suppressed amphibole crystallization resulting in ‘low-alumina’, peralkaline derivative liquids.