Mineralogists and geochemists explore highly siderophile
elements, which strongly prefer metal, and chalcophile elements, which strongly prefer sulfide, relative to silicate or oxide phases, at very high temperatures.
Most researchers think the highly siderophile
elements joined with iron and sank into the core as it formed in the first tens of millions of years of Earth's history.
Examination of the samples determined that the highly siderophile
elements present in the diogenite meteorites were present during formation of the rocks, which could only occur if late addition or 'accretion' of these elements after core formation had taken place.
elements tend not to ally themselves with the oxygen- and silicon-based compounds that form the bulk of Earth's crust.
Thus, density varies in relation to the presence of siderophile
Why are certain siderophile
elements like molybdenum, which have a natural affinity for iron, even rarer in lunar material than in the Earth's mantle?
and chalcophile elements such as Au and the Platinum Group elements (PGE's) are more strongly partitioned into immiscible sulfide liquid than Ni.
This demonstrates a common source for these siderophile
Creaser titled "A review of the Rhenium-Osmium (Re-Os) isotope system with application to organic-rich sedimentary rocks" emphasizes the importance of the Re-Os isotope system to the direct dating of sedimentary rocks, particularly black shales in which these broadly chalcophile and siderophile
elements are concentrated by reductive extraction from seawater.
Leading models of the kind known as bulk silicate Earth (BSE) assume that the mantle and crust contain only lithophiles ("rock-loving" elements) and the core contains only siderophiles
(elements that "like to be with iron").
(1995) indicated presence of three levels of chocolate brown coloured clay layers, less than 1 cm thick and separated from each other by 25 - 30 cm, heavily enriched with Ir, Os and other siderophiles
together with chalcophiles (Se, Sb, Ag, As and Zn) and depleted in lithophiles (Sc, Hf and Al) within the intertrappean bed of Anjar, in the western periphery of the Deccan volcanic province (Fig.1).
The bulk of siderophiles
do in fact reside in the core, yet even the lesser amounts in the mantle strike researchers as too high.