LONG-TERM THERMO-CHEMICAL EVOLUTION OF EARTH’S MANTLE
Seminars
Semester 1
The study of mantle heterogeneity can inform about the evolution and dynamics of terrestrial planets. Heterogeneity is formed due to fractionation during mantle melting and crystallization. Already in their earliest stage, terrestrial planets are thought to evolve through a large-scale melting episode. On Earth, a deep magma ocean has been formed due to the massive energy release during planetary accretion and differentiation. During crystallization of the deep magma ocean at high pressures, bridgmanite can be fractionated to form ancient heterogeneity in the mantle.
After magma-ocean crystallization, partial melting in the upper-mantle creates heterogeneity between basaltic (magma) and harzburgitic (residue) end-members. These lithologies are formed near the surface and mixed through the mantle over billions of years. Using geodynamic modeling, we show that the distribution of rock heterogeneity in the present-day Earth’s mantle strongly depends on their physical properties. For example, while basaltic and harzburgitic materials are stirred by mantle convection, they segregate from each other due to their different densities, particularly in regions of relatively low mantle viscosity. This leads to an enrichment of basalt near the core-mantle boundary, as well as near 660 km depth. In turn, ancient heterogeneity enhanced in bridgmanite can avoid complete mixing due to its high viscosity. Intrinsically strong material may survive as blobs in the convecting mantle for long time scales. We also find that bridgmanitic-blob preservation is promoted by the accumulation of basaltic piles near the core-mantle boundary.
The uneven distribution of basaltic material, as well as the survival of ancient materials in the lower mantle, has implications for our interpretation of geochemical and geophysical observations. For example, the materials sampled by mid-ocean ridge or hotspot melting may not be representative of that of the average mantle (i.e., bulk silicate Earth). Seismic data related to acoustic-wave reflections from phase transitions and sharp compositional contrasts provide additional constraints on the distribution of mantle heterogeneity. The district propagation of P-waves vs S-waves through the mantle can also inform about the preservation of bridgmanitic blobs, and thus the Mg/Si ratio of the lower mantle.
Additional information: Prof. Guochun ZHAO, gzhao@hku.hk