Isotopic Composition of Sr, Nd, and Pb in Pirssonite, Shortite and Calcite Carbonatites from Oldoinyo Lengai Volcano, Tanzania111

摘要

The carbonatite-nephelinite-phonolite volcano Oldoinyo Lengai, located in the Gregory rift, which is part of the East-African rift system, is an important locality at which to study the origin and evolution of alkali-car-bonatite magmas [1].rnThis is due mainly to the unusual chemical, and resulting mineralogical, compositions of the silicate rocks and carbonatites that occur at the volcano. A particular feature of the volcanic rocks at Oldoinyo Lengai is their enrichment in the alkali elements Na and K; which attain the maximum content that has been established in carbonatites-39.0-41.5 wt % Na_2O + K_2O [2]. It is noteworthy that high alkalinity is typical even for olivine melilitites at Oldoinyo Lengai, which contain 5.3-7.1 wt % Na_2O + K_2O, and particularly, melil-itite glassy groundmass where the total content of sodium and potassium oxides reaches 21.5 wt % [3]. The natrocarbonatites of Oldoinyo Lengai are fundamentally different in terms of chemistry and mineralogy from any other carbonatites worldwide. The natrocarbonatites consist of nyerereite Na_2Ca(CO_3)_2 and gregoryite Na_2CO_3, which occur as phenocrysts and groundmass phases; the groundmass also contains flu-orite, sylvine, alabandite, magnetite and fluorapatite.