New solidus for the mantle: evidence for layers only existed in the Archean mantle

The early Earth was very hot. Therefore, the Archean mantle was more ductile than now and accordingly the Archean mantle dynamics should be different from the modern plate tectonics. Present study suggests that the molten layer existed in the Archean mantle and explains how it makes unique dynamics.

By refining new solidus for the mantle, Dr. Andrault and co-workers propose that the new layer was in the mantle under the Archean environment. Briefly, the solidus is the melting point of solid. Previous studies determined the solidus by quench method, which is that rapidly cools the samples under various pressure-temperature condition (P-T condition) and finds the glass which is quenched melt. However, it is hard to define the ‘exact’ melting condition (solidus) because the quenched glass is very little and hard to find at the start of melting. Andrault brought two sensitive methods to determine the solidus of mantle rock and consequently defined the lower solidus for the mantle.

First method exploits the electrical conductivity (EC) gap between solid and liquid. Because liquid is more conductive phase, we can determine the solidus by sudden increasing of EC when the sample starts melting. Second method uses X-ray spectrum. X-ray shows the spectrum going through minerals. The intensity of the spectrum lowers if the mineral starts melting. While previous studies determined the solidus using existence of visible melt patch, the new two methods utilize sharp change of chemical and physical property between solid and liquid during continuous heating. So, these methods are easier to detect the onset of melting. As a result, the new solidus in the deep mantle (>100km) is lower than that is proposed previously whereas it is similar in the shallow mantle (<100km) (figure).

In the figure, if the temperature of the mantle is above the solidus, the mantle melts. At about 200km depth, the Archean mantle is hotter (Tp=1800-1850K) than the solidus. So the molten layer exists at about 200km depth. Because of its low viscosity, it induces mechanical weakening between rigid lid and upper mantle. Thus, the Archean upper mantle is mechanically separated from the lid which consequently doesn’t break up and be merged like the modern plate tectonics.

This molten layer may progressively have disappeared by the secular cooling of the mantle. With extinction of the molten layer, the viscosity of the mantle progressively increases. That affects the mechanical hardening, which favors mechanical coupling between the lid (crust in present) and the upper mantle. After this transition, the modern plate tectonics has become mature.

Present study defines new solidus for mantle and suggests new molten layer in the Archean upper mantle. Perhaps it is maybe simple. But by this discovery, we can understand the Archean dynamics and evolution of the modern plate tectonics.

Figure Liquidus is the melting point that the mantle melting ends. Tp is the potential temperature that represents the temperature of the mantle if it were adiabatically decompressed to the surface of the Earth. Archean Tp is 1800K-1850K.