The genesis of alkaline magmas with emphasis on their source regions: inferences from experimental studies

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Abstract
Summary: Experimental studies bearing on the genesis of alkaline magmas are reviewed, with emphasis on alkaline mafic-ultramafic magmas and the nature of their mantle sources. In contrast with previous experiments on simplified systems mainly at low pressures, more recent experiments under mantle pressures have resolved some of the problems of the earlier work. Inadequacies of petrogenetic schemes based on the model pyrolite mantle source for the generation of alkaline magma are also partly resolved by recent studies which suggest that with increasing alkalinity the possibility of deriving mafic-ultramafic magmas from a dry lherzolitic mantle source, or one with H 2 O as the only volatile, becomes increasingly difficult. The addition of CO 2 and H 2 O to the source regions allows generation of magmas such as olivine melilitites, but few magmas of the olivine leucitite kindred can be generated from lherzolitic sources. Such magmas appear to be partial melts of clinopyroxenite or orthopyroxenite sources containing both volatiles. Preliminary experimental modelling of mantle metasomatism suggest that this process may readily occur. Experiments on felsic alkaline rocks with emphasis on liquid immiscibility and halogens and experiments aimed at solving specific problems are briefly considered.