So what have Au-Pt-Pd-Rh-Re-Os and Ir got in common? They're all highly siderophile elements!
Siderophile elements: An element with a weak affinity for oxygen and sulfur and that is readily soluble in molten iron
http://encyclopedia2.thefreedictionary.com/Siderophile element
Something very unusual (or exotic) has happened to assimilate the metals into the Tres-Ema gabbros.
SIDEROPHILE ELEMENTS IN THE TERRESTRIAL 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 atm. metal/silicate distribution coefficients (concentration ratios) that are typically >10,000.
Consequently, progressive planetary core formation may have stripped >99% 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) in 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 (stony meteorites) relative abundances, although the elements Ru and Pd appear to be in slightly higher relative abundances (e.g., see Becker et al., 2006).
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), 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) 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).
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). 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.
(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).
Walker R. J., Horan M.F., Shearer C.K. and Papike J.J. (2004) Depletion of highly siderophile elements in the lunar mantle: evidence for prolonged late accretion. Earth Planet. Sci. Lett. 224, 399-413.
Becker H., Horan M.F., Walker R.J., Gao S., Lorand J.-P. and Rudnick R.L. (2006) Highly siderophile element composition of the Earth’s primitive upper mantle: Constraints from new data on peridotite massifs and xenoliths. Geochim. Cosmochim. Acta 70, 4528-4550.
Walker R.J. (2009) Highly siderophile elements in the Earth, Moon and Mars: Update and implications for planetary accretion and differentiation. Chemie der Erde 69, 101-125.
Bottke W.F., Walker R.J., Day J.M.D., Nesvorny D. and Elkins-Tanton L. (2010) Stochastic late accretion to Earth, the Moon and Mars. Science 330, 1527-1530.
Brandon A.D., Puchtel I.S., Walker R.J., Day J.M.D., Irving A.J. and Taylor L.A. (2012) Evolution of the martian mantle inferred from the 187Re-187Os isotope and highly siderophile element systematics of shergottites meteorites. Geochim. Cosmochim. Acta 76, 206-235.
Day J.M.D., Walker R.J., Qin L. and Rumble D. (2012) Early timing of late accretion in the solar system. Nature Geoscience 5, 614-617.
Walker R.J. (2014) Siderophile element constraints on the origin of the Moon. Phil. Trans. Roy. Soc. A 372, 20130258, DOI:10.1098/rsta.2013.0258.
Day J.M.D. and Walker R.J. (2015) Highly siderophile element depletion in the Moon. Earth Planet. Sci. Lett. 423, 114-124.
Walker R.J. (2016) Siderophile elements in tracing planetary formation and evolution. Geochemical Perspectives 5-1, 1-143.
Last Updated June 2017.
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Abstract
Osmium, Ru, Ir, Pt, Pd and Re abundances and 187Os/188Os data on peridotites were determined using improved analytical techniques in order to precisely constrain the highly siderophile element (HSE) composition of fertile lherzolites and to provide an updated estimate of HSE composition of the primitive upper mantle (PUM). The new data are used to better constrain the origin of the HSE excess in Earth’s mantle. Samples include lherzolite and harzburgite xenoliths from Archean and post-Archean continental lithosphere, peridotites from ultramafic massifs, ophiolites and other samples of oceanic mantle such as abyssal peridotites. Osmium, Ru and Ir abundances in the peridotite data set do not correlate with moderately incompatible melt extraction indicators such as Al2O3. Os/Ir is chondritic in most samples, while Ru/Ir, with few exceptions, is ca. 30% higher than in chondrites. Both ratios are constant over a wide range of Al2O3 contents, but show stronger scatter in depleted harzburgites. Platinum, Pd and Re abundances, their ratios with Ir, Os and Ru, and the 187Os/188Os ratio (a proxy for Re/Os) show positive correlations with Al2O3, indicating incompatible behavior of Pt, Pd and Re during mantle melting.
The combination of chondritic and modestly suprachondritic HSE ratios of PUM cannot be explained by any single planetary fractionation process. Comparison with HSE patterns of chondrites shows that no known chondrite group perfectly matches the PUM composition. Similar HSE patterns, however, were found in Apollo 17 impact melt rocks from the Serenitatis impact basin [Norman M.D., Bennett V.C., Ryder G., 2002. Targeting the impactors: siderophile element signatures of lunar impact melts from Serenitatis. Earth Planet. Sci. Lett, 217–228.], which represent mixtures of chondritic material, and a component that may be either of meteoritic or indigenous origin. The similarities between the HSE composition of PUM and the bulk composition of lunar breccias establish a connection between the late accretion history of the lunar surface and the HSE composition of the Earth’s mantle. Although late accretion following core formation is still the most viable explanation for the HSE abundances in the Earth’s mantle, the “late veneer” hypothesis may require some modification in light of the unique PUM composition.
https://www.sciencedirect.com/science/article/pii/S0016703706002821
