Silicate–natrocarbonatite liquid immiscibility in 1917 eruption combeite–wollastonite nephelinite, Oldoinyo Lengai Volcano, Tanzania: Melt inclusion study

Abstract

Primary silicate–melt and carbonate–salt inclusions occur in the phenocrysts (nepheline, fluorapatite, wollastonite, clinopyroxene) in the 1917 eruption combeite–wollastonite nephelinite at Oldoinyo Lengai. Silicate–melt inclusions in nepheline clearly show liquid immiscibility phenomena expressed in the presence of carbonate globules in silicate glass. The coexistence of inclusions with markedly different proportions of silicate glass+vapor-carbonate globule in the core of nepheline phenocrysts, the presence of carbonate–salt inclusions in fluorapatite and our heating experiments strongly suggest that their entrapment began at temperatures higher than 1130°C in an intermediate chamber when initial carbonated nephelinite melt was heterogeneous and represented a mixture of immiscible liquids. Silicate–natrocarbonatite melt immiscibility took place at high temperature and immiscible nephelinite and carbonatite liquids coexisted over a wide temperature range from ≥1130°C to 600°C. Homogenization of a carbonate globule (dissolution of the gas bubble in carbonate melt) at 900–940°C indicates that after separation from silicate magma the natrocarbonatite represented homogeneous liquid in the 900–1130°C temperature range, whereas below these temperatures immiscible melts of different composition and fluid phase have separated from it. The bulk composition of homogeneous natrocarbonatite melt may be estimated as ≈20% CaF2, 40–60% (Na,K)2CO3 and 20–40% CaCO3 based on the coexistence of nyerereite, calcite and fluorite and the rapid phase transition (carbonate aggregate→carbonate liquid) at 550–570°C observed in vapor-carbonate globules of nepheline-hosted silicate–melt inclusions and on the Na2CO3–CaCO3–CaF2 phase diagram. Silicate glasses of nepheline-hosted immiscible inclusions drastically differ from host nephelinite in the abundance of major and trace elements. They are high peralkaline ((Na+K)/Al — up to 9.5) and virtually free of water (H2O<0.6wt.%). Their very high Zr/Hf and Nb/Ta ratios and Li contents indicate that these silicate glasses represent the most evolved compositions at Oldoinyo Lengai. The peralkaline character of nephelinite melt is expressed in the composition of the daughter mineral assemblage within silicate–melt inclusions in nepheline (delhayelite, leucite, mica, clinopyroxene). These minerals show strong deficiency in Al and enrichment in Fe3+ that is also common to the groundmass of the Oldoinyo Lengai combeite–wollastonite nephelinites.