Viscosity of peridotite melt suggests a layered mantle in the early Earth

The Earth's lower mantle may have heterogeneities, indicated by a viscosity peak at ~1000 km depth inferred from geoid analysis [Rudolph et al. 2015], two large low-shear velocity provinces (LLSVP) detected by seismic tomography [e.g. McNamara et al. 2019] and the persistence of primitive reservoirs evidenced by isotopic geochemistry [e.g. Touboul et al. 2012]. Whether these heterogeneities were produced by early accretion processes or resulted from later mantle dynamics remains an open question. The solidification of the terrestrial magma ocean (MO), which formed by the Moon-forming impact, led to the primordial mantle structure from which the present-day mantle may have evolved. The solidification type (fractional or equilibrium) of the MO is therefore vital to understand the evolution of the mantle. Geodynamic investigations so far made were not able to determine the solidification type due to insufficient constraint of physical parameters, especially viscosity [Solomatov et al. 2007].

For these reasons, we directly measured the viscosity of a peridotite melt at pressures of 7 to 24.5 GPa and temperatures of 2173 to 2773 K by means of the falling sphere method. The obtained viscosity ranges from 0.017 (±0.001) to 0.038 (±0.002) Pa s. The present measurement has provided consistent but much more precise data than the previous study at pressures of 7 and 11 GPa, because of the use of an ultrafast camera and small Re spheres. Our results suggest more than 5 times lower MO viscosity in the lower mantle than a previous estimation, which was made by linear combination of logarithmic viscosity of endmember melts. The low viscosity favors the fractional solidification and resulted chemical layering of the early mantle. Therefore, the heterogeneities in lower mantle maybe a legacy from this initially layered mantle.