Carbonatite-alkaline silicate rock associations in the Gregory Rift, East Africa

• Applicants: Professor Dr. Gregor Markl; Privatdozent Dr. Michael Marks
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 499962419

This project concerns the magmatic evolution of carbonatite-nephelinite-phonolite-associations in the Gregory branch of the East African Rift, where carbonatites show highly variable enrichment of alkalis and halogens. The project will characterize the carbonatite-silicate rock associations of Kerimasi, Mosonik, and Shombole volcanoes and compare them to each other, to the extreme example of Oldoinyo Lengai (OL) with its peralkaline nephelinites, phonolites and natrocarbonatites and to further regional nephelinitic volcanoes (Sadiman, Burko, Essimongor). As OL has been extensively studied in the last decades, we will not gather new data on it, but will use literature data for comparison. Such an integrated approach that compares carbonatites and associated silicate rocks from several spatially associated volcanoes in detail has not been done before - although it may explain the reasons for the observed variations of alkali- and halogen-enrichment in carbonatites and the role of the associated silicate rocks on a regional scale. We specifically note our project will put the unique volcano OL into a much broader, regional context. Our working hypotheses are: • Nephelinites and carbonatites from the three volcanoes derive from a common mantle source and evolved from a common parental magma via fractional crystallization followed by liquid-liquid immiscibility. • Nephelinite-phonolite associations of the investigated volcanoes reflect variable amounts of assimilation during fractional crystallization (AFC processes).• The variable Na/Ca-ratios of the carbonatites are caused by different "timing" of carbonatite unmixing during the evolution of the parental CO2-bearing melts. Testing them involves a systematic textural and petrological approach using isotopic age-dating, whole-rock data, mineral chemistry (SEM, EPMA) and phase equilibrium calculations (QUILF, PerpleX and others). Selected key samples identified with these methods will be further investigated by means of halogen whole-rock and mineral data (CIC), spatially resolved radiogenic isotope data (LA ICP-MS) and especially melt inclusion studies which will be co-supervised by melt inclusion expert Victor Sharygin (Novosibirsk). A major focus will be to correlate compositional with isotopic zonations and melt inclusion data of zoned phenocrysts, antecrysts and xenocrysts of the rocks. In doing so, various processes that may occur during their growth (magma mixing/replenishment, fluid exsolution/degassing, liquid-liquid immiscibility and assimilation) can be distinguished. Combining the information from the three newly studied volcanoes with literature data from Oldoinyo Lengai and further examples (see above) will allow to draw conclusions on the potential dependence between the level of enrichment of alkalis and halogens in carbonatites and the petrological and geochemical evolution of melt compositions parental to nephelinites, phonolites and carbonatites.

• DFG Programme: Research Grants
• International Connection: Russia
• Cooperation Partners: Victor Sharygin, Ph.D., until 3/2022; Professor Dr. Anatoly N. Zaitsev, until 3/2022