Characterizing Siderophile Elements in the Bulk Mantle

Siderophile elements are those chemical elements that prefer to partition into metal compared to silicate. The highly siderophile elements (HSE: including Re, Os, Ir, Ru, Pt, Rh, Pd and Au) are characterized by 1 atmosphere metal/silicate distribution coefficients (concentration ratios) that are typically >10,000. Consequently, progressive planetary core formation may have stripped >95% of these elements from the silicate portions of the Earth, Mars and other differentiated bodies. Despite this removal of the HSE from the silicate portion of the Earth, the estimated abundances of these elements in Earth’s upper mantle are higher than expected from metal-silicate equilibria, with only an estimated ~98% of the HSE presently contained in the Earth’s core (see figure below). The moderately siderophile elements (MSE: including Co, Ni, Mo and W), are less strongly attracted to metal, with metal/silicate distribution coefficients of typically 2-100. Depletions of MSE in the silicate Earth, relative to the bulk Earth, are much less than for HSE, and more variable (see figure below).

The abundances of the highly siderophile elements (orange symbols) estimated for the Bulk Silicate Earth (BSE) are about 200 times lower than primitive chondritic meteorites, and occur in approximately chondritic relative abundances (CI chondrites). The abundances of the moderately siderophile elements (green symbols) are generally depleted relative to primitive chondritic meteorites, but the depletions appear most consistent with high pressure-temperature metal-silicate partitioning during progressive core formation. Absolute abundance estimates are summarized in Walker (2016).

Both the HSE and MSE have been used to great advantage to provide important insights to the origin and early chemical evolution of Earth’s mantle. Our work, and the work of others, has broadly characterized the abundances of the HSE in the mantle as being in essentially chondritic relative abundances, although the elements Ru and Pd appear to be in slightly higher relative abundances (e.g., see Becker et al., 2006).

For Earth, the abundances of highly siderophile elements in mantle that was isolated from convective mixing 1-3 billion years ago can be directly measured in lithospheric mantle xenoliths that are brought to the surface by recent volcanic activity (left). Photo shows former UMd Ph.D. student Jingao Liu hefting a large peridotite xenolith from the Hannuoba locale, China. (right) Abundances of the highly siderophile elements in the Bulk Silicate Earth (BSE) can be estimated via projection of data for variably melt-depleted mantle peridotites to an undepleted, primitive BSE composition for Al2O3, shown as a box (from Becker et al., 2006). Note that most terrestrial peridotites, regardless of MgO have higher Ru/Ir than chondritic meteorites.

The absolute and relative abundances of the HSE in the Earth’s mantle were, therefore, most likely established via a combination of core segregation (stripping the mantle of HSE), possibly including precipitation of a Hadean Matte (see O’Neill, 1991) consisting of sulfides, that removed additional HSE from the mantle, followed by planetary late accretion (re-enriching the mantle in HSE). This process of late accretion may also have dominated the HSE signature of other bodies in the inner solar system including the Moon (Walker et al., 2004; Day and Walker, 2015), Mars (Brandon et al., 2012), the angrite parent body (Riches et al., 2012) and the asteroid Vesta (Day et al., 2012).

In contrast to the HSE, the abundances of the MSE in the mantle were likely primarily established by metal-silicate partitioning at the high temperature and pressure conditions that may have ensued at the bases of periodically-formed magma oceans during Earth accretion (see figure below).

(Left) Schematic diagram of a terrestrial magma ocean, wherein metal-silicate equilibration occurs at the high pressures and temperatures present at the base of a transient magma ocean (figure from Wade and Wood, 2005). Under these conditions, some of the MSE are considerably less siderophile than at 1 atmosphere of pressure. Many current models to account for MSE abundances in the mantle assume such a process.
(Right) Mineral assemblages that may have resulted from the crystallization of a 2,000-km deep terrestrial magma ocean (figure from Elkins-Tanton, 2008).

For more information about our contributions to this topic, please refer to the following papers:

Last Updated January 2023