Polymorphic Nature of Iron and Degree of Lattice Preferred Orientation Beneath the Earth's Inner Core Boundary

Abstract

Deciphering the polymorphic nature and the degree of iron lattice‐preferred orientation in the Earth's inner core holds a key to understanding the present status and evolution of the inner core. A multiphase lattice‐preferred orientation pattern is obtained for the top 350 km of the inner core by means of the ab initio based Candy Wrapper Velocity Model coupled to a Monte Carlo phase discrimination scheme. The achieved geographic distribution of lattice alignment is characterized by two regions of freezing, namely within South America and the Western Central Pacific, that exhibit an uncommon high degree of lattice orientation. In contrast, widespread regions of melting of relatively weak lattice ordering permeate the rest of the inner core. The obtained multiphase lattice‐preferred orientation pattern is in line with mantle‐constrained geodynamo simulations and allows to setup an ad hoc mineral physics scenario for the complex Earth's inner core. It is found that the cubic phase of iron is the dominating iron polymorph in the outermost part of the inner core.

A New Probe Into the Innermost Inner Core Anisotropy via the Global Coda‐Correlation Wavefield

“…The preferred orientation of different iron phases (mostly hexagonal‐closed‐packed (hcp) and body‐centered‐cubic (bcc) iron crystals) have been proposed to explain the origin of anisotropy in the IC. However, the difficulty in computing the elastic properties of these iron phases at core pressure and temperature conditions results in debates for which iron phase best matches the seismological observations (e.g., Belonoshko et al., 2008; Jackson et al., 2000; Li et al., 2018; Mattesini et al., 2018; Ritterbex & Tsuchiya, 2020; Romanowicz et al., 2016; Stixrude & Cohen, 1995).…”