Early carbonatite magmatism at Oldoinyo Lengai volcano (Tanzania): carbonatite–silicate melt immiscibility in Lengai I melt inclusions

Lydéric France,Florian Brouillet,Sarah Lang

doi:10.5802/crgeos.99

Abstract

Carbonatites are unusual C-rich alkaline magmas that have been reported throughout the geological record. Nevertheless, there is only one currently active carbonatite system on Earth: Oldoinyo Lengai stratovolcano in northern Tanzania (God’s mountain in Maasai culture). Present-day Lengai carbonatites are natrocarbonatites, peculiar Na-rich carbonatites that, under atmospheric conditions, alter and leach to compositions similar to the more common Ca-carbonatites within weeks, preventing any long-term geological record of such Na-rich magmas. It follows that the oldest report of natrocarbonatites at Oldoinyo Lengai dates to the 19th century. Here, by using samples from the Lengai I cone (>11 ka), we show that immiscible silicate–carbonatite melts were already present at reservoir conditions at that time. Measurements of three-phase (carbonatite + silicate + gas) melt inclusions from Lengai I highlight that their chemical compositions were similar to those of immiscible melts recently present in the reservoir. Alkaline carbonatites in melt inclusions from both Lengai I and historical explosive eruptions are enriched in Ca relative to those historically effused at the surface and likely record higher equilibrium temperatures (>1100 °C). We also report chemical maps that qualitatively document elemental partitioning between immiscible silicate–carbonatite melts. We show that at the melt inclusions’ entrapment conditions Si, Fe, K, Na, and Cl are compatible with the silicate phase when C, Ca, P, Sr, Ba, and F are compatible with the carbonate phase.

Highlights

Carbonatites are relatively rare magmas that contain >50% carbonate minerals [Le Bas et al, 1972] and, along with associated alkaline silicate magmas, constitute the main rare earth element (REE) de-ISSN : 1778-7025 https://comptes- rendus.academie- sciences.fr/geoscience/posits worldwide [e.g., Ling et al, 2013, Verplanck et al, 2014]
The results presented clearly document the presence of carbonatite–silicate melt immiscibility in nepheline-hosted melt inclusions (MIs) from Lengai I products
Mas predating 11 ka at Oldoinyo Lengai. These newly identified carbonatites are enriched in Ca and depleted in Na compared to the modern natrocarbonatites historically emitted at the summit

Summary

Introduction

Carbonatites are relatively rare magmas that contain >50% carbonate minerals [Le Bas et al, 1972] and, along with associated alkaline silicate magmas, constitute the main rare earth element (REE) de-ISSN (electronic) : 1778-7025 https://comptes- rendus.academie- sciences.fr/geoscience/posits worldwide [e.g., Ling et al, 2013, Verplanck et al, 2014]. The rare available data directly documenting carbonatite melt compositions (i.e., from melt inclusions that shelter carbonatites from surface weathering) indicate the alkaline character of parental carbonatite magmas in various regions [Kogarko et al, 1991, Mitchell, 2009, Sharygin et al, 2012, Chen et al, 2013, Guzmics et al, 2015, Mollex, 2017, Weidendorfer et al, 2017] This raises the question of whether Ca-carbonatites effectively represent the magmatic composition or rather represent the leaching products of initially alkaline carbonatites [e.g., Dawson, 1962b, Hay, 1983, Keller and Zaitsev, 2006, Chen et al, 2013]

Methods

Discussion

Conclusion