Wet transition zone marked in Komatiites
Primeval mantle was very different from present-day; it was much hotter. Despite its hotness, it contained copious amount of water, just like modern mantle. It is well known that magmatism is a key mechanism in order to convey water onto the Earth’s surface, but the influence of modern magmatism is restricted to shallow depth. Still, the water reservoir and degassing process of the primitive mantle remains enigmatic. This year, Sobolev and his colleagues1 provided a new insight on how the Archaean mantle kept and released water onto the surface.
Billions of years ago, extremely hot mantle produced ‘komatiite’, an olivine-rich ultramafic lava. Olivine crystals often trap ambient melts as they grow, which is called ‘melt inclusion’. Generally, water diffuses out of the lava when it is erupted to the surface, so these ‘olivine-hosted melt inclusions’ are a good target to study the initial condition of the magma. Consequently, source of the water in the komatiites can be envisioned by scrutinizing the chemistry of these inclusions.
Numerous studies have ascertained these ancient lavas involved water.2 Among those studies, several possible sources of water have been proposed: seawater or brines3, diffused water after crystallization4, or fluid released from crust entering into the mantle3. All of these options imply that water reservoir located at the surface, is directly linked to the water in magma.
Sobolev et al.1 conducted a set of geochemical analysis on olivine-hosted melt inclusions and their host olivine from 2.7 billion-years-old Canadian komatiite lavas. Those inclusions are enriched in water, but their chlorine contents are not as much elevated. This high H2O to Cl ratio suggests that komatiite was not altered by seawater of brines, since chlorine is major constituent of sodium chloride, or salt. An alternative explanation would be, diffused water into the melt inclusion through olivine crystals, which is also unlikely due to inconsistency with relatively low crystallization temperatures estimated for olivine. With the points made above, the source of water in komatiite melt is unlikely to be crustal. The last option among previous thoughts is the water derived from subducting crust, which releases water as pressure increases5. Magma produced by subduction fluid, however, is characterized by specific trace-element patterns and highly oxidized state, which shows discrepancy with the Canadian komatiite. This implies that the water is not from the subducting slab.
Then where on the earth did the water in komatiites come from? Sobolev et al.1 suggests that mantle transition zone, which lies between upper and lower mantle, as a possible answer. Recent studies6,7 have revealed that high-pressure minerals in the transition zone can accommodate significant amount of water. Extremely hot ancient mantle might have been able to partially melt at this depth, and generate hydrous magma which could be transferred to the surface with plume – an ascending flow from the deep mantle. In contrast, the cooled modern transition zone cannot produce sufficient amount of melt.
Given the research by Sobolev et al.1 used only Canadian komatiites as a sample, similar chemical analyses on various types of komatiites will verify whether the interaction between the transition zone and hot plume was a general process responsible for hydrous komatiites. Yet at least now we have one more option to say: Archean transition zone was juvenile, hot, and wet, after all.
Figure 1. Cartoon1 illustrating a hot Archaean plume passing through the mantle transition zone, where volatiles (H2O, F and Cl) are mainly contained. Melt generated at around 410 km depth carries the excess volatiles and is erupted as hydrous komatiite lava.
References
1. Alexander V. Sobolev, et al. Komatiites reveal a hydrous Archaean deep-mantle reservoir. Nature 531, 628–632 (2016)
2. Arndt, N. et al. Were komatiites wet? Geology 26, 739-742 (1998)
3. Shimizu, K., Shimizu, N., Komiya, T., Suzuki, K., Maruyama, S., and Tatsumi, Y. CO2-rich komatiitic melt inclusions in Cr-spinels within beach sand from Gorgona Island, Colombia. Earth Planet. Sci. Lett. 288, 33-43 (2009)
4. Portnyagin, M., Almeev, R., Matveev, S. & Holts, F. Experimental evidence for rapid water exchange between melt inclusions in olivine and host magma. Earth Planet. Sci. Lett. 272, 541-552 (2008)
5. Vadim S. Kamenetsky, et al. Composition and temperature of komatiite melts from Gorgona Island, Colombia, constrained from olivine-hosted melt inclusions. Geology 38, 1003-1006 (2010)
6. Bercovici, D. & Karato, S. Whole-mantle convection and the transition-zone water filter. Nature 425, 39-44 (2003)
7. Mibe, K., Orihashi, Y., Nakai, S. i. & Fujii, T., Element partitioning between transition-zone minerals and ultramafic melt under hydrous conditions. Geophys. Res. Lett. 33, L16307 (2006)
