Petrology and genesis of natrocarbonatite

Abstract

Microprobe analyses of phenocrysts and groundmass, and crystal-size distributions of phenocrysts of pahoehoe natrocarbonatite lavas of the 1963 eruption of Oldoinyo Lengai have been determined. Nyerereite phenocrysts are homogeneous, with average composition Nc41Kc9Cc50 (neglecting F, Cl, P2O5, and SO3) where Nc=Na2CO3, Kc=K2CO3, and Cc= (Ca,Sr)CO3. Gregoryite phenocrysts have turbid, pale brown, oscillatorily zoned cores (average composition Nc77Kc5Cc18) with 0–30% oriented inclusions of exsolved nyerereite. Overgrowths on gregoryites (30 μm wide) are relatively sodic (Nc81Kc4Cc15) and are free of inclusions. Cores and rims are rich in SO3 (4%) and P2O5 (2%). Blebs of pyrite-alabandite mixtures (≤100 μm) occur in the groundmass. The groundmass has the simplified composition Nc65Kc15Cc20, less calcic than the composition of the 1-kbar nyerereite+gregoryite +liquid cotectic in the ternary system Nc-Kc-Cc. Groundmass quench growth of alkali halides + carbonate was followed by slower growth of coarse-grained and irregular gregoryite +KCl+BaCO3. Crystal size distributions of gregoryite and nyerereite in one sample are linear, implying little loss or gain of phenocrysts by crystal settling. AverageGτ is 0.15 mm, compared toGτ=0.03 mm for combeite phenocrysts from consanguineous nephelinite. Assuming an equal residence time (τ) for both lavas, the apparent crystal growth rate (G) in carbonate melt is 5 times greater than in peralkaline undersaturated silicate melt. Data from experiments with natrocarbonatite and related synthetic systems indicate that Na−K−Ca carbonatite magmas which crystallize calcite cannot fractionate to nyerereite+gregoryite +liquid assemblages. Natrocarbonatites plot in the liquidus field of nyerereite, and minor fractionation of nyerereite to produce the erupted lavas is indicated. The term natrocarbonatite has been inappropriately applied to other eruptive rocks with calcite phenocrysts, and the only known occurrence of gregoryite-bearing natrocarbonatite is Oldoinyo Lengai. Natrocarbonatite probably originates by liquid immiscibility from strongly peralkaline nephelinites, which have also been erupted at Oldoinyo Lengai.