Carbonatite and highly peralkaline nephelinite melts from Oldoinyo Lengai Volcano, Tanzania: The role of natrite-normative fluid degassing

Márta Berkesi,Enikő Bali,Robert J Bodnar,Ábel Szabó,Tibor Guzmics

doi:10.1016/j.gr.2020.03.013

Márta Berkesi, Enikő Bali + Show 3 more

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https://doi.org/10.1016/j.gr.2020.03.013

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Abstract

Oldoinyo Lengai, located in the Gregory Rift in Tanzania, is a world-famous volcano owing to its uniqueness in producing natrocarbonatite melts and because of its extremely high CO2 flux. The volcano is constructed of highly peralkaline [PI = molar (Na2O + K2O)/Al2O3 > 2–3] nephelinite and phonolites, both of which likely coexisted with carbonate melt and a CO2-rich fluid before eruption. Results of a detailed melt inclusion study of the Oldoinyo Lengai nephelinite provide insights into the important role of degassing of CO2-rich vapor in the formation of natrocarbonatite and highly peralkaline nephelinites. Nepheline phenocrysts trapped primary melt inclusions at 750–800 °C, representing an evolved state of the magmas beneath Oldoinyo Lengai. Raman spectroscopy, heating-quenching experiments, low current EDS and EPMA analyses of quenched melt inclusions suggest that at this temperature, a dominantly natritess-normative, F-rich (7–14 wt%) carbonate melt and an extremely peralkaline (PI = 3.2–7.9), iron-rich nephelinite melt coexisted following degassing of a CO2 + H2O-vapor. We furthermore hypothesize that the degassing led to re-equilibration between the melt and liquid phases that remained and involved 1/ mixing between the residual (after degassing) alkali carbonate liquid and an F-rich carbonate melt and 2/ enrichment of the coexisting nephelinite melt in alkalis. We suggest that in the geological past similar processes were responsible for generating highly peralkaline silicate melts in continental rift tectonic settings worldwide.

Highlights

Oldoinyo Lengai, located in Gregory Rift (East African Rift System) in Tanzania, is one of the best-known volcanoes in the world, and is the only active volcano that periodically produces peralkaline nephelinite and phonolite lavas together with small amounts of natrocarbonatite melts (Donaldson et al, 1987; Dawson, 1998)
That degassing drives the immiscible carbonate melt and the degassed, residual natritess-normative liquid to form a mixed alkali-rich carbonate melt phase (Fig. 3), which is immiscible with the peralkaline nephelinite melt (Fig. 2)
Based on results presented above, we suggest that extreme peralkalinity of the nephelinite melt is the result of reequilibration (Eq 1) between the outgassed, residual alkali carbonate liquid and the silicate melt

Summary

Introduction

Oldoinyo Lengai, located in Gregory Rift (East African Rift System) in Tanzania, is one of the best-known volcanoes in the world, and is the only active volcano that periodically produces peralkaline nephelinite and phonolite lavas together with small amounts of natrocarbonatite melts (Donaldson et al, 1987; Dawson, 1998). Oldoinyo Lengai is dominantly composed of pyroclastic rocks (Dawson, 1962), suggesting intense volatile degassing (de Moor et al, 2013) during eruption. During the explosive eruptive activity at Oldoinyo Lengai, the CO2-flux is extremely high (Brantley and Koepenick, 1995), making this volcano one of the most significant CO2 emitters on Earth. The CO2 emissions are accompanied by the release of H2O, which represents 24–34 mol% of the degassing volatile phase (Koepenick et al, 1996). All of these observations strongly suggest that at the subvolcanic level a CO2-bearing fluid is associated with immiscible carbonate-silicate melts

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