Carbonatite lava and tephra are now well known. The only modern eruptive carbonatites, from Oldoinyo Lengai, Tanzania, are of alkali carbonatite, whereas all of the pre-modern examples are of calcite or dolomite. Chemical and stable isotope analyses were made of separate phases of Pliocene carbonatite tuffs of the Laetolil Beds in Tanzania and of Miocene carbonatite tuffs of the Kaiserstuhl in Germany in order to understand the reasons for this major difference. The Laetolil Beds contain numerous carbonatite and melilitite-carbonatite tuffs. It is proposed that the carbonatite ash was originally of alkali carbonate composition and that the alkali component was dissolved, leaving a residuum of calcium carbonate. The least recrystallized melilitite-carbonatite tuff contains early-deposited calcite cement and calcite pseudomorphs after nyerereite (?) that have contents of strontium and barium and ??18O and ??13C values suggestive of incomplete chemical and isotopic exchange during alteration and replacement of alkali carbonatite ash. Carbonatite tuffs of the Kaiserstuhl contain globules composed of calcite phenocrysts and microphenocrysts in a groundmass of calcite with a small amount of clay, apatite, and magnetite. The SrO contents of phenocrysts, microphenocrysts, and groundmass calcite average 0.90, 1.42, and 0.59 percent, respectively. The average ??18O and ??13C values of globules (+14.3 and -9.0, respectively) fall between those of coarse-grained intrusive Kaiserstuhl carbonatite (avg. +6.6, -5.8) and those of low-temperature calcite cement in the carbonatite tuffs (+21.8, -14.9). The phenocrysts and microphenocrysts are primary magmatic calcite, but several features indicate that the groundmass has been recrystallized and altered in contact with meteoric water, resulting in weathering of silicate to clay, leaching of strontium, and isotopic exchange. The weight of evidence favors an original high content of alkali carbonatite in the groundmass, with recrystallization following leaching of the alkalies. ?? 1983 Springer-Verlag.
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Carbonatite tuffs in the Laetolil Beds of Tanzania and the Kaiserstuhl in Germany