Emerging Constraints on the Chemical Composition of the Entire Earth

John Hernlund, Vice Director of the Earth-Life Science Institute at Tokyo Institute of Technology

The chemical composition of the Earth’s inaccessible interior holds the key to numerous big questions regarding the formation and evolution of the Earth as a habitable planet. Humankind has only directly sampled the Earth’s crust and shallowest mantle, and inferences regarding the composition of the entire Earth must be indirectly obtained from mantle-derived lavas and cosmochemical constraints, and subsequently compared to experimental/theoretical predictions and geophysical observations. The chemical composition of the lower mantle and core (comprising the vast majority of Earth’s mass) is particularly challenging, and numerous degrees of freedom exist in interpretations based on classic approaches such as comparison of seismological profiles to high pressure experiments and theory. We presently need new ideas and approaches to break through to the next level and unlocking the secrets of the deep interior of our planet.

In this talk I will discuss ongoing research efforts into the chemical composition of the lower mantle and core, and share our most promising ideas and results thus far. Our thinking regarding the lower mantle has expanded with the recognition that its rheology may be far more complex than previously thought, in addition to being highly sensitive to chemical composition. The latest ideas and models allow for the sequestration and partial isolation of large compositionally distinct domains in the deep interior, including the mid-mantle as well as the slow seismic velocity features beneath the Pacific and Africa. In the outer core a key observation is the presence of stably stratified layers at both the top and bottom, which must be lighter and heavier (respectively) than the bulk of the outer core. However, both of these regions betray evidence of a seismic velocity decrease, in spite of their opposite density anomalies, motivating detailed examination of core-mantle reactions, “snow,” and other processes. Discovery that Earth’s core has a significantly higher conductivity than previously thought has also motivated a search for alternative buoyancy sources for outer core convection and the geodynamo, such as exsolution/crystallization of alloy-rich phases. Some of these ideas are already being applied to exoplanets, showing their importance for understanding the general behavior of terrestrial planets and moons. In this talk I think there will be something interesting for every member of the department, and I very much welcome further dialogue with all of you on this and other topics in the future.

Host: Doug Wiens


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