Silicate--Carbonate Immiscibility at Oldoinyo Lengai

Abstract

For approximately the last 50 years eruptions at Oldoinyo Lengai have produced passive natrocarbonatite lavas interspersed with mixed silicate-natrocarbonatite events approximately every 15–25 years. In 1993 an unusual blocky lava erupted and preserved detailed mixed silicate-natrocarbonatite textures clearly indicating an immiscible origin. The 1993 blocky flow consists of natrocarbonatite with small silicate crystal aggregates which constitute ∼2–5% of the rock. These inclusions are composed of nepheline, melanite, clinopyroxene and wollastonite occurring both as isolated crystals and ijolite micro-xenoliths. Most significantly, these ijolitic inclusions are surrounded by ‘globules’ of a fine-grained intergrowth of nepheline, wollastonite and gregoryite, interpreted as quenched melt. Petrographic textures are characteristic of liquid immiscibility between coexisting natrocarbonatite and silicate melts. The presence of gregoryite within the silicate melt globules is particularly important as it represents the common liquidus phase between the silicate and natrocarbonatite melts theoretically required to demonstrate immiscibility between two conjugate liquids. This is the first time that liquid immiscibility has been so clearly demonstrated in natural rock samples from Oldoinyo Lengai and agrees very closely with recent experimental work. Our detailed model for the petrogenesis of the natrocarbonatites at Oldoinyo Lengai involves extensive fractionation of a carbonate-rich alkaline silicate magma followed by immiscible separation of natrocarbonatite at low pressures.