Chondrites highly siderophile elements

Holzheid A., Sylvester P., O Neill H., St C., Ruble D. C., and Palme H. (1998) Late chondritic veneer as source of the highly siderophile elements in the Earth s mantle insights from high pressure-high temperature metal-silicate partition behavior of Pd. Nature 406, 396-399. 

Alard O., Griffin W. L., Lorand J. P., Jackson S. E., and O Reilly S. Y. (2000) Non-chondritic distribution of the highly siderophile elements in mantle sulphides. Nature 407, 891-894. 

Two different kinds of metals are found in chondrites. Small nuggets composed of highly refractory siderophile elements (iridium, osmium, ruthenium, molybdenum, tungsten, rhenium) occur within CAIs. These refractory alloys are predicted to condense at temperatures above 1600 from a gas of solar composition. Except for tungsten, they are also the expected residues of CAI oxidation. 

The elemental abundance of the lunar mare rocks as compared to that of carbonaceous chondrites vary up to 6 orders of magnitude (Fig. 3a). The strongly siderophile elements and the very volatile elements are highly depleted, while the refractory elements Al, Ca, Ti, REE, Th, U. etc. are enriched. Hence, it is rather difficult to explain the fractionation of the lunar mare basalts by... 

Siderophile elements Alkaline elements Elements highly depleted in normal chondrites Refractory elements Other elements... 

For the siderophile elements, the metal particles contribute between 50 and 90% to the bulk composition of the soil samples. As carbonaceous chondrites do not contain metal, reduction and equilibration is required to explain the high contribution of the metal particles. 

In addition to making comparisons with chondrites, the bulk composition of the Earth also has been defined in terms of a model mixture of highly reduced, refractory material combined with a much smaller proportion of a more oxidized volatile-rich component (Wanke, 1981). These models follow on from the ideas behind earlier heterogeneous accretion models. According to these models, the Earth was formed from two components. Component A was highly reduced and free of all elements with equal or higher volatility than sodium. All other elements were in Cl relative abundance. The iron and siderophile elements were in metallic form, as was part of the silicon. Component B was oxidized and contained all elements, including those more volatile than sodium in Cl relative abundance. Iron and all siderophile and lithophile elements were mainly in the form of oxides. 

The accretion of a late veneer. Whether or not a late veneer was added to the Earth towards the end of accretion is not clear. The principal evidence comes from the elevated siderophile element chemistry of the mantle. If there was a late veneer, it had to happen after core formation. At present the evidence from the trace element chemistry is ambiguous, because these data can also be explained by the formation of the core at high pressures and temperatures in a magma ocean, or by continuous core formation with decreasing metal input. Currently, the best evidence for a late veneer comes from Os-isotope evidence, where there is a clear mismatch between the composition of the PUM and chondrite. However, even this is uncertain, as is discussed in the next chapter (Chapter 3, Section 3.2.3.4), for... 

A cosmochemical periodic table, illustrating the behavior of elements in chondritic meteorites. Cosmic abundances are indicated by symbol sizes. Volatilities of elements reflect the temperatures at which 50°/o of each element would condense into a solid phase from a gas of solar composition. As in Figure 1.2, the chemical affinities of each element, lithophile for silicates and oxides, siderophile for metals, and chalcophile for sulfides, are indicated. Some of the most highly volatile phases may have remained uncondensed in the nebula. Stable, radioactive, and radiogenic isotopes used in cosmochemistry are indicated by bold outlines, as in Figure 1.2. Abundances and 50% condensation temperatures are from tabulations by Lodders and Fegley (1998). 

Differentiated achondrites. They are achondrites that exhibit igneous textures or igneous textures modified by impact and/or thermal metamorphism, and that have compositions of lithophUe, siderophile, chalcophile, and atmophile elements that are highly fractionated from the ranges of chondritic materials. 

The issue of niobium in the core is of particular interest for the chondritic model of the bulk Earth. Niobium has always been thought to be refractory and hthophile, yet it is depleted in the upper mantle relative to other refractory and lithophile elements. Failure to locate hidden niobium-rich reservoirs in the lower mantle or the core would lead to serious problems for the well-established chondritic model. Recently, Wade and Wood (2001) studied partitioning of niobium between hquid metal and liquid sihcate under high pressure and temperature. They found that niobium becomes more siderophile with increasing pressure, hence opening up the possibility of storing niobium in the core. 

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