One-atmosphere melting behaviour and nomenclature of terrestrial lavas

On a plot of MgO against one-atmosphere liquidus almost all published data for lavas delineate a single trend, which is fairly well-defined above 1160° C. Segments of this trend form a convenient set of major classificatory subdivisions of lavas. It appears to be hard to justify the use of the term basalt for lavas with less than 6 percent MgO. Even with this restriction, basaltic aphanites occupy a substantial thermal (1150 to 1250° C one-atmosphere liquidi) and chemical (e.g. 6 to 11 percent MgO) range, which might profitably be subdivided. Finally, it is shown that the one-atmosphere liquidus thermal range occupied by the strongly alkalic sodic and potassic lavas is as wide as that covered by the entire set of basaltic volcanic suites from picrites and ankaramites to rhyolites, trachytes and phonolites.Presented at the symposium Recent Advances in the Studies of Rocks and Minerals at High Pressures and Temperatures held in Montreal, 1972. Jointly sponsored by the International Mineralogical Association and the Commission on Experimental Petrology.