The current status of speculations on the composition of the core of the Earth

Abstract

The earth's core, at a pressure of between 1.3 and 3.7 Mbar and a temperature between 3000° and 5000°C, consists primarily of Fe‐Ni in the inner core and molten Fe alloyed with from 8 to 20% of a light element, most likely S or Si, giving an average atomic number for the elements in the outer core of about 23. If the core accreted before the mantle, then the apparent disequilibrium of some elements with respect to core and mantle is explained, but it is difficult to explain the presence of appreciable quantities of a light element that are required in the core. Alternatively, the core formed by melting, coalescence, and subsequent collapse of metallic particles in a protoearth that was relatively homogeneous initially. Such collapse would raise the temperature of the earth as much as 2300°K. Silicon must be present in the core if (Fe + Mg)/Si> 1 in the mantle and if that ratio for the bulk earth lies within the limits set by the sun and chondrites. If Si is present, the apparent chemical disequilibrium between metallic Si in the core and Fe 2+ and Fe 3+ in the mantle must be explained. Arguments for S in the core based on the abundance of S in rocks of crustal and mantle origin appear to have little foundation; however, there are strong geochemical and cosmochemical arguments justifying the presence of S in the core if the earth accreted at relatively low temperatures. The intriguing speculation that the bulk of the earth's potassium is present in the outer core, thus providing energy for core and mantle, remains to be tested by more partitioning experiments. A number of explanations exist to explain the apparent disequilibrium of some elements between core and mantle; however, none is satisfactory. The core formed soon after the earth accreted; in fact, the age generally taken as the age of the earth probably represents the time of core formation.