Chondrites refractory siderophile elements

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. 

Figure 24 Concentration profiles of siderophile elements in a radially zoned Fe,Ni grain in the CBb chondrite, QUE 94411 (a) electron microprobe data (b) and (c) trace element data from laser ablation ICPMS (Campbell et ai, 2001). The nickel, cobalt, and chromium profiles can be matched by nonequilibrium nebular condensation assuming an enhanced dust-gas ratio of —36 X solar, partial condensation of chromium into silicates, and isolation of 4% of condensates per degree of cooling (Petaev etal, 2001). Concentrations of the refractory siderophile elements, osmium, iridium, platinum, ruthenium, and rhodium, are enriched at the center of the grain by factors of 2.5-3 relative to edge concentrations, which are near Cl levels after normalization to iron (reproduced by permission of University of Arizona on behalf of The Meteoritical Society from Meteorit. Planet. ScL, 2002, 37, pp. 1451-1490).

Itqiy is distinct from chondritic meteorites in bulk composition. Aluminum, FREE, europium, sodium, potassium, vanadium, chromium, and manganese are aU depleted. Itqiy has La/Yb of 0. lOxCI, and Eu/Sm of 0.16 X Cl. Refractory siderophile elements are enriched —2-3 X Cl, while moderately volatile siderophile elements are at roughly Cl abundances. The bulk rock Mg/Si and Fe/Si ratios are greater than those of EH or EL chondrites. 

The refractory component comprises the elements with the highest condensation temperatures. There are two groups of refractory elements the refractory lithophile elements (RLEs)—aluminum, calcium, titanium, beryllium, scandium, vanadium, strontium, yttrium, zirconium, niobium, barium, REE, hafnium, tantalum, thorium, uranium, plutonium—and the refractory siderophile elements (RSEs)—molybdenum, ruthenium, rhodium, tungsten, rhenium, iridium, platinum, osmium. The refractory component accounts for —5% of the total condensible matter. Variations in refractory element abundances of bulk meteorites reflect the incorporation of variable fractions of a refractory aluminum, calcium-rich component. Ratios among refractory lithophile elements are constant in all types of chondritic meteorites, at least to within —5%. 

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... 

Silicon-bearing kamacite is a minor component of most aubrites, and contains 3.7-6.8 wt.% Ni and 0.1-2.4 wt.% Si. Metal nodules from several aubrites contain approximately chondritic abundances of refractory to moderately volatile siderophile elements (Casanova et al., 1993). 

Few comprehensive bulk compositional analyses are available for silicate inclusions from HE irons, and many of them are of small samples. Netschaevo silicates have magnesium-normalized abundances of refractory, moderately volatile, and volatile lithophile elements within the ranges of ordinary chondrites. The nickel-normalized abundances of refractory and moderately volatile siderophile elements are also similar to those of ordinary chondrites. The silicates have siderophi-le/Mg ratios of (1.9-2.2)xCI chondrites, however. The silicate inclusion in Watson has Cl-normalized element/Mg ratios of —0.86 for most refractory and moderately volatile lithophile elements (Figure 2). Siderophile elements are depleted, and show increasing abundance with increasing volatility (Olsen et al., 1994) Os/Mg = 0.028 X Cl and Sb/Mg = 0.066 X Cl. 

Figure 6 Volatile/refractory element ratio-ratio plots for chondrites and the silicate Earth. The correlations for carbonaceous chondrites can be used to define the composition of the Earth, the Rb/Sr ratio of which is well known, because the strontium isotopic composition of the BSE represents the time-integrated Rb/Sr. The BSE inventories of volatile siderophile elements carbon, sulfur, and lead are depleted by more than one order of magnitude because of core formation. The values for Theia are time-integrated compositions, assuming time-integrated Rb/Sr deduced from the strontium isotopic composition of the Moon (Figure 8) can be used to calculate other chemical compositions from the correlations in carbonaceous chondrites (Halliday and Porcelli, 2001). Other data are from Newsom (1995).

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 two elements calcium and aluminum are RLEs. The assumption is usually made that aU RLEs are present in the primitive mantle of the Earth in chondritic proportions. Chondritic (undifferentiated) meteorites show significant variations in the absolute abundances of refractory elements but have, with few exceptions discussed below, the same relative abundances of lithophile and siderophile refractory elements. By analogy, the Earth s mantle abundances of refractory lithophile elements are assumed to occur in chondritic relative proportions in the primitive mantle, which is thus characterized by a single RLE/Mg ratio. This ratio is often normalized to the Cl-chondrite ratio and the resulting ratio, written as (RLE/Mg)N, is a measure of the concentration level of the refractory component in the Earth. A single factor of (RLE/Mg) valid for all RLEs is a basic assumption in this procedure and will be calculated from mass balance considerations. 

A chondritic model for the Earth s mantle is usually based upon the composition of Cl chondrites, adjusted for the loss of volatile elements and for the separation of the siderophile elements into the core. This leads to a mantle composition which is enriched in refractory lithophile elements by about 1.5 times the Cl chondrite value. There are however difficulties with the chondritic model because Cl chondrites and the Earth have different Mg/Si ratios (Fig. 3.9), as was discussed in Chapter 2 (Section 2.4.4). Some authors believe that this difference is original and dates from processes within the solar nebula indicating different evolutionary histories between the two. If this is true then Cl chondrites are not an appropriate starting composition for the composition of the bulk Earth and alternative models have to be considered. 

The U (uranium)-Th (thorium)-Pb (lead) isotopic system represents three independent decay schemes and is a powerful but complex tool with which to unravel the history of the Earth s mantle (Text box 3.2). During planetary accretion U and Th are refractory, lithophile elements and will reside in the mantle. Pb on the other hand is a volatile and chalcophile/ siderophile element and may in part, be stored in the core. Initial U and Th concentrations are derived from chondritic meteorites, and initial Pb isotope compositions are taken from the iron-sulfide troilite phase in the Canyon Diablo meteorite. The initial bulk Earth U/Th ratio was 4.0 0.2 (Rocholl Jochum, 1993). 

The decay of Hf to is well suited to date core formation in planetary objects mainly for three reasons. First, owing to the Hf half-life of 9 Myr, detectable W isotope variations can only be produced in the first -60 Myr of the solar system. This timescale is appropriate for the formation of the Earth and Moon in particular and to planetary accretion and differentiation in general. Second, both Hf and W are refractory elements such that there is only limited fractionation of Hf and W in the solar nebula or among different planetary bodies (see above). The HfrW ratio of the bulk Earth therefore can be assumed to be chondritic and hence can be measured today. Third, Hf is a lithophile and W is a siderophile element such that the chondritic HfrW ratio of the Earth is fractionated internally by core formation. If core formation took place during the effective lifetime of Hf, the metal core (HfrW-O) will develop a deficit in the abundance of whereas the silicate mantle, owing to its enhanced Hf/W, will develop an excess of (7-P). 

Updated solar photospheric abundances are compared with meteoritic abundances. It is shown that only one group of chondritic meteorites, the Cl chondrites, matches solar abundances in refractory lithophile, siderophile, and volatile elements. All other chondritic meteorites differ from Cl chondrites. The agreement between solar and Cl abundances for all elements heavier than oxygen and excluding rare gases has constantly improved since Goldschmidt (1938) published his first comprehensive table of cosmic abundances. 

Figure 6 Correlation of Sm versus Sn in partial melts and residues of mantle melting. The constant Sm/Sn ratio suggests that this ratio is representative of the PM. Sn is depleted relative to Cl-chondrites by a factor of 35. The knowledge of the abundance of the refractory element Sm in PM allows the calculation of the Sn PM abundance. Many of the PM abundances of siderophile and chalcophile elements are calculated from similar correlations (after Jochum et al, 1993).

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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