Cool carbonate volcano in Africa, Ol Doinyo Lengai… No really, it’s cool!

Figure 1: The Arusha province of northern Tanzania is outlined in pink. White line through center is the border of Tanzania and Kenya. Ol Doinyo Lengai marked with yellow marker. Image courtesy of Google LandSat imagery (2018).

Ol Doinyo Lengai, or “Mountain of God”, is an active stratovolcano located along the East African Rift (EAR) in the Arusha volcanic province of northern Tanzania (Fig. 1 & 2). This volcano is unique in a few different ways: 1) it erupts carbonate material, rather than the usual silica rich material,  2) typical eruptions are extremely effusive, resembling black water more than lava flows; 3) the lavas are very cool compared to other lavas, at ~ 510℃ compared to 1171℃ at Kilauea; and 4) solidified lava weathers rapidly and turns white (Fig. 3).

Figure 2: Ol Doinyo Lengai volcano. Water stream in foreground. (By Clem23 - Own work, CC BY-SA 3.0)

For the past century Ol Doinyo Lengai has been erupting lavas that are uniquely poor in silicate minerals yet enriched in sodium, potassium, and rare earth elements (REE) such as rubidium, strontium, zirconium, niobium, thorium, and uranium. The most common type of lava on Earth today is basalt. Basalts typically contain 45 - 55% silica (SiO2). The carbonatite eruptions of Ol Doinyo Lengai contain <20% SiO2. So where is this cool, silica poor, REE-enriched lava coming from? This is a question scientists have been trying to answer since the discovery of this volcano. Mollex et al. (2018) may have new answers in their paper “Tracing helium isotope compositions from mantle source to fumaroles at Oldoinyo Lengai volcano, Tanzania.”

Figure 3: Satellite imagery of Ol Doinyo Lengai. The white is not snow. It is weathered carbonate material from previous eruptions. (Image courtesy of Google Maps satellite imagery.)

Helium has two stable isotopes: 3He and 4He. 3He was made during the Big Bang, and as the Earth formed it became incorporated into the mantle. 4He is a product of alpha decay of Uranium and Thorium. By studying the relative abundances of 3He and 4He in lavas, scientists can infer where the magma originates. In the Earth’s atmosphere  the ratio of 3He / 4He is 1.384 x 10-6, while crustral ratios are 0.4 to 1 x 10-8. The 3He / 4He ratios are typically reported relative to the atmospheric level as R/RA. In these units crustal rocks typically have 0.01 to 0.1 R/RA, and mantle rocks have 5 - 50 R/RA (Emsley, 2001) (Fig. 4). Mollex et al. (2018) sampled rocks, fumaroles, and cognate xenoliths1 in order to find the 3He/4He ratios of Old Doinyo Lengai.They found that the helium isotope compositions from cognate xenoliths at the summit of the volcano ranged from 5.79 ± 0.82 RA  to 7.24 ± 0.44 RA (Fig. 5). These ratios closely match those of more traditional silicic volcanoes of the surrounding Arusha volcanic province.

Figure 4: 3He/4He ratios found on Earth and in the solar system. Around 10-5 are Ocean Island Basalts (OIB), Mid-Ocean Ridge Basalts (MORB), and Island Arcs (Arcs). Figure 12.1 from White (2015).

Figure 5: Helium isotope ratios from previous studies (left) and the current study of Mollex et al., 2018 (right). RC / RA are helium isotope ratios corrected for atmospheric contamination. SCLM is Subcontinental Lithospheric Mantle (shallow mantle), and MORB is Mid-Ocean Ridge Basalt. The typical ratios of Typical 3He/ 4He ratios for SCLM are shown in orange (Gautheron & Moreira, 2002). And typical 3He/ 4He for MORB are shown in yellow (Graham, 2002). Gray circles denote uncorrected 3He/ 4He ratios. (R / RA). Figure 4 from Mollex et al. (2018).

Based on the helium isotopic ratios found in rocks and summit fumaroles on Ol Doinyo Lengai as well as other volcanoes in the Arusha volcanic province, Mollex et al. (2018) believe that the magma source may be the same for both. They propose that the magma source is old enriched sub-continental lithospheric mantle (SCLM) that has undergone compositional changes from fluids or a mid-ocean ridge basalt (MORB) type mantle magma that mixes with the Ol Doinyo Lengai magma chamber.

The research of Mollex et al. (2018) sheds more light on this enigmatic volcano but more work is required in order to fully understand what is happening in the complex magma system beneath Ol Doinyo Lengai.

References

Emsley, J., 2011, Natures building blocks: an A-Z guide to the elements: Oxford, Oxford University Press.

Mollex, G., Füri, E., Burnard, P., Zimmermann, L., Chazot, G., Kazimoto, E.O., Marty, B., and France, L., 2018, Tracing helium isotope compositions from mantle source to fumaroles at Oldoinyo Lengai volcano, Tanzania: Chemical Geology, v. 480, p. 66–74, doi:10.1016/j.chemgeo.2017.08.015.

White. W.M., 2015, Chapter 12 Noble Gas Isotope Geochemistry, in Isotope Geochemsitry, New York, Wiley-Blackwell, p. 418-452.