DAWSON OLDOINYO-LENGAI CALCIOCARBONATITE - A MAGMATIC SOVITE OR AN EXTREMELY ALTERED NATROCARBONATITE

In 1962 Dawson described a calcite carbonatite (specimen BD83) from th e Tanzanian volcano Oldoinyo Lengai as a sovite, thus implying that at an earlier stage in its evolution this volcano had crystallized magma tic calciocarbonatites as well as the highly alkalic natrocarbonatite lava that has been erupted in more recent times. This proposition is d ifficult to reconcile with the currently fashionable hypothesis whereb y the natrocarbonatite lava separated immiscibly from a type of nephel inite magma, most recently thought to be a wollastonite nephelinite. I n 1993 Dawson sought to discredit the magmatic origin of this sovite s pecimen by arguing that it was derived from natrocarbonatite lava thro ugh extreme alteration (calcitization) in which process the original n yerereite was replaced by calcite in near-perfect pseudomorphs. We sug gest that the arguments advanced in support of this concept are unconv incing and that the specimen is exactly what it was originally describ ed as, namely a magmatic sovite in which the calcite crystallized from a magma rather than having replaced nyerereite. We do not seek to dis credit the liquid immiscibility hypothesis but do believe that whateve r process is responsible for the Oldoinyo Lengai natrocarbonatites and silicate rocks must also allow for the crystallization of calciocarbo natite.