2020 | Experimental study on the genesis and early evolution of alkali-rich silicate melts and the formation of carbonatites in the Oldoinyo Lengai volcanic system (Tanzania)

Oldoinyo Lengai (OL) is an active terrestrial volcano situated in the East African Rift of NW Tanzania [e.g. 1, 2]. Its geochemistry is highly unusual among other terrestrial volcanoes, due to the eruption of unique Na-carbonatitic and high-alkaline silicate lavas of nephenelitic and phonolitic compositions with (Na+K)/Al = 1.6-7.2 [e.g. 2, 3, 4, 5]. The geochemical evolution of OL is suggested to be a complex process of fractional differentiation starting with a low degree partial melting of olivine melilitites [2, 3] or olivine nephelinites [1] at 1.5 GPa, 1300°C. The evidence for the early formation of the peralkaline trend at depth of the garnet stability field was reported [3]. The water content in the silicate melt is estimated to be 0.1-0.4 wt% H2O [3], whereas the concentrations of CO2 in the melt at this depth remain unconstrained. The subsequent evolution of these magmas, as they mi-grate through the crust, is also controversial: it is proposed that magmas either remained dry with increasing CO2 content up to 6 wt% [3] or that magmas became enriched in both volatile compo-nents with H2O contents of the melt up to 10 wt% and CO2 up to 9 wt% [5]. High ƒCO2 and CO2 content of melts are likely to be a result of an open magmatic differentiation and CO2-flushing, which is a common feature of volcanic systems. The final formation of immiscible Ca-, Na-carbonate liquid globules next to the dry silicate melt is estimated to take place under sub-shallow conditions at 0.2 GPa [3].

Open question that remain unclear:

(i) How does the early formation, accumulation and migration of silicate melts occur at depth? If alkali-rich silicate melts are produced by very low degree of mantle melting, how do these minuscule amounts of melt achieve their mobility and how effectively can they migrate to shallower crustal levels?

(ii) What is the role of carbonate-silicate immiscibility in the differentiation of alkali- and vola-tile-rich magmas and how do silicate melt compositions evolve in terms of major ele-ments and volatiles during magma ascent from mantle depths to the surface?

To tackle these questions, we aim to experimentally investigate alkali-rich magmas of the Oldoinyo Lengai system. High-pressure experiments at 1.5-2.0 GPa will give a quantitative estimate for the melt fraction, melt composition (e.g. Na, K, Ca, P-enrichment) and CO2-solubility in the first melts. Intermediate experiments between 1.0 and 0.6 GPa will provide further insights into the evolution of CO2- and alkali-rich silicate melts, and hopefully, give more details about the separation of carbon-atite at crustal levels, as previously suggested [3].

References [1] Dawson, J. B., and P. G. Hill (1998) Mineral chemistry of a peralkaline combeitelamprophyllite nephelinite from Oldoinyo Lengai, Tanzania. Mineralogical Magazine 62.2: 179-196. [2] Keller, Jörg, Anatoly N. Zaitsev, and Daniel Wiedenmann (2006) Primary magmas at Oldoinyo Lengai: the role of olivine melilitites. Lithos 91.1-4: 150-172. [3] Baudouin, Céline, Fleurice Parat, and Thierry Michel (2018) CO2-rich phonolitic melt and carbonatite immiscibility in early stage of rifting: Melt inclusions from Hanang volcano (Tanzania). Journal of Volcanology and Geothermal Research 358: 261-272. [4] Weidendorfer, Daniel, Max W. Schmidt, and Hannes B. Mattsson (2017) A common origin of carbonatite magmas. Geology 45.6: 507-510. [5] de Moor JM et al. (2013) Volatile-rich silicate melts from Oldoinyo Lengai volcano (Tanzania): Implications for carbonatite genesis and eruptive behaviour. Earth Planet Sci Lett 361:379-390.