Direct Viscosity Measurement of Peridotite Melt to Lower‐Mantle Conditions: A Further Support for a Fractional Magma‐Ocean Solidification at the Top of the Lower Mantle

Abstract

Identifying the magma-ocean (MO) solidification type is vital to understand the evolution of the Earth's mantle leading to the present-day isotopic heterogeneity. The Earth is believed to have experienced largescale melting owing to massive energy released during the accretion and differentiation, which formed MOs with various depths (e.g., Elkins-Tanton, 2012 and reference therein). The most important MO was caused by a giant Moon-forming impact (e.g., Tonks & Melosh, 1993), which may have reached the lower mantle's bottom. The solidification of this MO produced the initial mantle structure that evolved to the present-day mantle. If the solidification was fractional, the initial structure should already have had substantial chemical heterogeneity. If equilibrium solidification occurred, the present heterogeneity (e.g., Hofmann, 1997), which isotope geochemistry suggests, should have been produced later. Thus, an assumption of the solidification type leads to an entirely different scenario of mantle evolution. Based on phase relations and elemental partitioning at pressures less than 26 GPa, geochemical models showed that even ∼10 wt% of bridgmanite segregation can substantially deviates the ratios of minor and trace elements from the observed ranges, suggesting a maximum fractionation of ∼10 wt% (Ito et al., 2004;Liebske, Corgne, et al., 2005;Walter et al., 2004). However, no conclusion was made for the solidification type. Although the solidification type has been investigated by geodynamic modeling, poorly constrained physical parameters prevent the modeling from giving a definitive conclusion (Solomatov, 2007).

Insights into magma ocean dynamics from the transport properties of basaltic melt

“…The viscosity of such a magma ocean or its constituent silicate melt is crucial for determining the thermal and chemical evolution of the planet. This is because the melt viscosity directly influences the cooling time of the magma ocean, the nature of its crystallization, melt percolation, and the rate at which the metal droplets could sink in the magma ocean [7][8][9][10] . Silicate melt viscosity also affects magma transport in the modern mantle as silicate melts are likely to be present in various tectonic settings across the entire mantle including the base of the lower mantle [11][12][13][14][15] .…”

“…Despite its significance, experiments on the viscosity of silicate melts are mostly available for pure end member compositions [26][27][28][29][30] , and experimental constraints are often limited to 25 GPa owing to technical challenges at the high pressures relevant for the magma ocean 10,[31][32][33][34] . The pressure dependence of viscosity is sensitive to the melt composition.…”

“…The pressure dependence of viscosity is sensitive to the melt composition. Yet, there are disagreements between studies even for the same melt compositions 10,26,31,35,36 . Extrapolations of viscosity from experiments conducted at relatively low-pressures to higher pressures often add significant uncertainties due to pressure-induced changes to the atomistic scale structure of the melt and resulting changes in properties.…”

“…Since the timescale of freezing of the magma ocean depends largely on the viscosity of the magma ocean, it is important to have better constraints on the viscosity of its constituent silicate melt 3,47,48 . Even though the studies based on isotope geochemistry show evidence of heterogeneity in the Earth's lower mantle, the sources of heterogeneity are poorly constrained because of limited information on the crystallization of the magma ocean 10,30 . Fractional solidification of the magma ocean would suggest that observed chemical heterogeneity in the present-day Earth's mantle was preserved from early magma ocean solidification 49,50 .…”

“…In contrast, equilibrium solidification would imply that the chemical heterogeneity was added at a later stage of the Earth's history 51 . Melt viscosity is likely to have a significant influence on how the magma ocean solidified 10,30 . Therefore, a better constraint on the viscosity of the magma ocean is crucial for understanding present-day mantle heterogeneity.…”