Author: Sreemoyee Chakraborty, email
Let’s begin the discussion with a short introduction to a Mantle Plume. A Mantle Plume is a column of hot buoyant material rising through the mantle trying to reach the surface. In popular belief, it has a distinct head and a tail. Most mantle plumes are likely to be originated from the core-mantle boundary (See the figure below). According to alternative hypothesis, a shallower origin of mantle plumes also exists. However the concept of Mantle Plume itself is controversial, then the real question of this article is – if mantle plumes exist and from where do they originate. During 1980’s in this context, geochemists broke through new developments using helium isotope studies and started to advocate the origin of mantle plumes uing isotope tools. Before going into that science first we should know little about helium isotopes. Present day in atmosphere helium gas is relatively rare, mostly comes from radioactive decay of Uranium (U) and Thorium (Th). During radioactive decay they (U, Th) produce alpha particle which is nothing but 4He. On the other hand 3He (one of the isotopes of helium) is a primordial gas (early formed along with the solar system or universe) so that can only be present if the gas has a primary magma source deep inside the earth’s mantle. This means it can be used as a good geochemical tracer, though 3He can be created by some other processes but those are not significant. Using 3He/4He ratio we can identify the different mantle reservoirs mainly the pristine types where the helium ratios are high. As we have already discussed 3He is primordial so the deep/lower mantle (either partly or entirely) which are more pristine in nature will be more enriched in 3He than 4He that concludes high 3He/4He in lower mantle. In that case if we find any mantle plume with high 3He/4He then we can say that plume is coming from lower mantle because there is high possibility that mantle plume bring these gases to the surface. Like, a widely accepted high 3/4He mantle plume is present beneath the main Ethiopian Rift and the Kenyan Rift. Once the presence of mantle plumes and its lower mantle origin have been established, the next question is whether or not Superplumes exist! As the name suggests, Superplume is nothing but a gigantic mantle plume. Many geoscientists hypothesize superplumes as clusters of plumes. Schubert et al., (2003) suggested that the superplume under Africa has been formed by atleast 2 or 3 smaller plumes; thus in short, it is not a superplume. In order to find out the truth behind the nature of the mantle plume under Africa, Halldórsson and others collected samples of rocks and crystals during eruptions in volcanoes from different locations along the East African Rift System (EARS). These rocks and mineral crystals such as olivine, trap the volcanic gases. These gases were later studied in the laboratory to find out their origin. They also looked for 22Ne – another primordial gas. Comparing 3He/22Ne ratios (gives a unique signature to a mantle plume) from different locations which were found to be very close to each other, workers argued that the Kenya Dome and the Ethiopia Dome are fed by different heads of a common mantle plume source. This work (Halldórsson et. al., 2014) not only took a step towards proving the existence of mantle plumes but also indicated that a giant plume or Superplume is ripping the African continent apart.
So it may seem that helium isotopic study solved one great enigma behind mantle plume origin. But still there are many complexities buried under our feet.
References: Halldórsson, Sæmundur A., et al. "A common mantle plume source beneath the entire East African Rift System revealed by coupled helium‐neon systematics." Geophysical Research Letters 41.7 (2014): 2304-2311. Schubert, G., et al. "Superplumes or plume clusters?." Physics of the Earth and Planetary Interiors 146.1 (2004): 147-162.
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