Siderophile elements core formation

Iron meteorites offer the unique opportunity to examine metallic cores from deep within differentiated bodies. Most of these samples were exposed and dislodged when asteroids collided and fragmented. Although irons constitute only about 6% of meteorite falls, they are well represented in museum collections. Most iron meteorites show wide variations in siderophile-element abundances, which can be explained by processes like fractional crystallization in cores that mimic those in achondrites. However, some show perplexing chemical trends that may be inconsistent with their formation as asteroid cores. 

Righter K. and Drake M. J. (1999) Effect of water on metal-silicate partitioning of siderophile elements a higji pressure and temperature magma ocean and core formation. Earth Planet Sci. Lett. 171, 383—399. 

Figure 1 The estimated composition of the silicate portion of the Earth as a function of condensation temperature normalized to Cl values in Anders and Grevesse (1989). Open circles lithophile elements shaded squares chalcophile elements shaded triangles moderately siderophile elements solid diamonds highly siderophile elements. The spread in concentration for a given temperature is thought to be due to core formation. The highly siderophile element abundances may reflect a volatile depleted late veneer. Condensation temperatures are from Newsom (1995).

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).

The major problem presented by the Earth s chemical composition and core formation models is providing mechanisms that predict correctly the siderophile element abundances in the Earth s upper mantle. It long has been recognized that siderophile elements are more abundant in the mantle than expected if the sihcate Earth and the core were segregated under low-pressure and moderate-temperature equilibrium conditions (Chou, 1978 Jagoutz et al, 1979). Several explanations for this siderophile excess have been proposed, including ... 

Newsom H. E. (1990) Accretion and core formation in the Earth evidence from siderophile elements. In Origin of the Earth (eds. H. E. Newsom and J. H. Jones). Oxford University Press, Oxford, pp. 273-288. 

Walter M. J., Newsom H. E., Ertel W., and Holzheid A. (2000) Siderophile elements in the Earth and Moon Metal/silicate partitioning and implications for core formation. In Origin of the Earth and Moon (eds. R. M. Canup and K. Righter). University of Arizona Press, Tucson, pp. 265-289. 

Abundances of nonrefractory incompatible lithophile elements (potassium, rubidium, caesium, etc.) or partly siderophile/chalcophile elements (tungsten, antimony, tin, etc.) are calculated from correlations with RLE of similar compatibility. This approach was first used by Wanke et al. (1973) to estimate abundances of volatile and siderophile elements such as potassium or tungsten in the moon. The potassium abundance was used to calculate the depletion of volatile elements in the bulk moon, whereas the conditions of core formation and the size of the lunar core may be estimated from the tungsten abundance, as described by Rammensee and Wanke (1977). This powerful method has been subsequently applied to Earth, Mars, Vesta, and the parent body of HED meteorites. The procedure is, however, only applicable if an incompatible refractory element and a volatile or siderophile element have the same degree of incompatibility, i.e., do not fractionate from each other during igneous processes. In other words, a good correlation of the two elements over a wide... 

Figure 15(a) plots Cl and magnesium-normalized abundances of lithophile moderately volatile elements against their condensation temperatures. The trend of decreasing abundance with increasing volatility is clearly visible. As mentioned above only few elements can be used to define this trend most of the moderately volatile elements are siderophile or chalcophile and their abundance in the Earth s mantle is affected by core formation. 

Figure 15 Abundances of moderately volatile elements in the Earth s mantle versus condensation temperatures (a) lithophile elements define the volatility trend (b) siderophile elements have variable depletions reflecting the process of core formation and (c) chalcophile elements. The difference between siderophile and chalcophile elements is not well defined, except for S and Se. The large depletions of S, Se, and Te are noteworthy (see text) (after...

Models for core formation in the Earth and other terrestrial planets are based on the distribution of siderophile elements between core and mantle. Interpretations of these data have focused on several characteristics of siderophile elements in... 

Righter K. (2003) Metal/silicate partitioning of siderophile elements and core formation in the early Earth. Ann. Rev. Earth Planet. Sci. 31, 135—174. 

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