Lavas erupted from Oldoinyo Lengai in November, 1988, carry phenocrysts of the alkali carbonates nyerereite and gregoryite, with inclusions of apatite. They are set in a matrix of microphenocrysts of nyerereite and gregoryite wih tiny grains of (Mn, Fe)S, MnFe spinel, sodic sylvite, Na- and Si-rich apatite, spurrite, and intergrowths of sylvite, fluorite and a phase similar to nyerereite. The matrix also contains an unidentified complex carbonate? of Ca, Ba, Sr, K and Na. All phases represent solid solutions (attributed to the high-temperature crystallization of the chemically complex dominantly carbonate liquid) that are very rare or not previously reported.Chemically, the lavas are very similar to those erupted in 1960, containing high amounts of Na2O, K2O, CaO, CO2 and lesser, but nonetheless significant, amounts of BaO, SrO, Cl, F, P5O5 and SO3. Trace element concentrations and REE patterns indicate that the carbonatite is highly fractionated. Compositional variations arise mainly between phenocryst-rich and aphyric varieties, with the aphyric variety being richer in K, Ba, Cl, F and Rb. More subtle variations in Fe, Mn, Pb and Zn may reflect groundmass sulphide sedimentation. The isotope ratios of carbon, oxygen and sulphur are similar to those in other primary carbonatites. Lead and thorium isotope data, combined with previously published Sr and Nd isotope data, indicate that the carbonate magma originates from an upper mantle source with the same isotopic characteristics as mantle sources for ocean island basalts. The isotopic data provide no evidence for interaction between crustal material and the carbonatite or its parental magma during ascent from the mantle source.
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- Petrology and Geochemistry of Oldoinyo Lengai Lavas Extruded in November 1988: Magma Source, Ascent and Crystallization
J. B. Dawson
G. E. Norton
D. M. Pyle
A. E. Fallick
- Springer Berlin Heidelberg