Experiments reveal how the Earth’s core was formed
The aim of the DECORE (Deep Earth chemistry at the core) project was to investigate the primordial differentiation of the Earth during accretion and formation of its core. It studied how accretionary material comprising rocks and iron-rich metal melted and separated out the metallic portion from the rocky portion to form the core and the silicate mantle (magma ocean) respectively. Researchers followed the evolution of siderophile trace elements and light atomic elements incorporation into the core, and how they were linked. This was because siderophile trace-element concentrations in the mantle and light-element concentration in the core can be measured using geochemistry and geophysics to provide major constraints on core formation. Scientists used high-pressure, high-temperature apparatus involving large-volume presses and laser heated anvil cells to reproduce the conditions found deep in the Earth. This allowed to cover for the first time the entire depth (pressure and temperature) range at which the core formed. Results indicated that core formation in a FeO-rich magma ocean was the most viable solution. Oxygen is the major light element in the core, with smaller amounts of silicon. The scenario of core formation in such an “oxidised” magma ocean that becomes reduced over time provided a viable mechanism for explaining the reduction of the magma ocean through the incorporation of oxygen in the core. It also provided a better explanation of the partitioning of the elements chromium (Cr) and vanadium (V). From a geophysical point of view, calculations showed that oxygen was the only element that is always required in the outer core, to match its seismological velocity and density profiles. Therefore, geophysical requirement of an oxygen-rich outer core supported the hypothesis of accretion and core formation in an oxidised environment. DECORE scientists also noted that this oxidised environment is akin to the oxidation state of asteroids like 4 Vesta, which is typical of the type of object from which they propose the Earth may have accreted.
Keywords
Earth, core, DECORE, accretion, silicate, mantle, magma ocean, siderophile, large-volume presses, anvil cell, 4 Vesta
