Siderophile elements mantle

Chondrite-normalized abundances of siderophile elements in the Earth s mantle. The measured concentrations do not match those expected from low-pressure metal-silicate partition coefficients determined by experiments. Modified from Tolstikhin and Kramers (2008). 

Energy released from impacts, together with heat from the decay of radioactive isotopes, led to differentiation in planetary embryos, once these objects became partially molten (Tonks and Melosh, 1992). Iron and siderophile elements (e.g., platinum, palladium, and gold) preferentially sank to the center to form a core, while the lighter silicates and lithophile elements formed a mantle. Differentiation was probably a continuous process rather than a single event, so that large planets like Earth accreted from embryos that were already partially or wholly differentiated. 

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 ... 

Arculus R. J. and Delano J. W. (1981) Siderophile element abundances in the upper mantle evidence for a sulfide signature and equilibrium with the core. Geochim. Cosmochim. Acta 45, 1331-1344. 

Azbel 1. Y., Tolstikhin 1. N., Kramers J. D., Pechernikova G. V., and Vityazev A. V. (1993) Core growth and siderophile element depletion of the mantle during homogeneous Earth accretion. Geochim. Cosmochim. Acta 57, 2889-2898. 

Chou C. L. (1978) Fractionation of siderophile elements in the Earth s upper mantle. Proc. 9th Lunar Sci. Conf. 219-230. 

Murthy V. R. (1991) Early differentiation of the Earth and the problem of mantle siderophile elements a new approach. Science 253, 303-306. 

Righter K., Walker R. J., and Warren P. W. (2000) The origin and significance of higjily siderophile elements in the lunar and terrestrial mantles. In Origin of the Earth and Moon (eds. R. M. Canup and K. Rigjiter). University of Arizona Press, Tucson, pp. 291—322. 

Mare basalts indicate that the Moon s pattern of crust-mantle siderophile element depletions is roughly similar to that of the Earth (Figure 7). Unfortunately, for four siderophile elements, rubidium, rhodium, palladium, and platinum, no reliable data have been reported for lunar basalts. Neal et al. (1999, 2001) reported ICP-MS data, but their analyses show obvious artificial contamination, based on comparison with iridium data and palladium upper limits reported by Wolf et al. (1979). The most noble of the siderophile elements in Figure 7 are osmium and iridium. For noble siderophile elements, scatter among individual rocks is very great (e.g.. Figure 8), but the factor-of-4 disparity between average mare and terrestrial basalt for osmium and iridium... 

Morgan J. W., Walker R. J., Brandon A. D., and Horan M. F. (2001) Siderophile elements in Earth s upper mantle and lunar breccias data synthesis suggests manifestations of the same late influx. Meteorit. Planet. Sci. 36, 1257—1275. 

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...

Garuti G., Gorgoni C., and Sighinolli G. P. (1984) Sulfide mineralogy and chalcophile and siderophile element abundances in the Ivrea-Verbano zone mantle peridotites (Western Italian Alps). Earth Planet. Sci. Lett. 70, 69-87. 

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. 

Schmidt G., Palme H., Kratz K.-L., and Kurat G. (2000) Are highly siderophile elements (PGE, Re and Au) fractionated in the upper mantle New results on peridotites from Zabargad. Chem. Geol 163, 167-188. 

Spettel B., Palme H., and Wanke H. (1990) Siderophile elements in the primitive upper mantle. Lunar Planet. Sci. XXI. Lunar and Planetary Institute, Houston, pp. 1184-1185. 

Jochum K. P. and Hofmann A. W. (1994) Antimony in mantle-derived rocks constraints on Earth evolution from moderately siderophile elements. Min. Mag. 58A, 452—453. 

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. 

Pearson D. G., Irvine G. J., Ionov D. A., Boyd F. R., and Dreibus G. E. (2004) Re-Os isotope systematics and platinum group element fractionation during mantle melt extraction a study of massif and xenolith peridotite suites. Chem. Geol. (special volume) Highley Siderophile Elements (in press). 

Differentiation of terrestrial planets includes separation of a metallic core and possible later fractionation of mineral phases within either a solid or molten mantle (Figure 1). Lithophile and siderophile elements can be used to understand these two different physical processes, and ascertain whether they operated in the early Earth. The distribution of elements in planets can be understood by measuring the partition coefficient, D (ratio of concentrations of an element in different phases (minerals, metals, or melts)). 

The behavior of some siderophile elements is controlled by the composition of the metal, and, in particular, the Fe/Ni ratio. Because the activity coefficient of a siderophile element, M, may be different in a nickel-rich metal than an iron-rich metal, an understanding of this effect is necessary before partition coefficients can be successfully applied to a natural system. A good example is tin, which has a low activity coefficient in iron-rich metal, and a high activity coefficient in nickel-rich metal (Figure 8 Capobianco et al., 1999). Metal composition can change as a function of/o, and this can then change the solubility in the silicate melt. In addition, the Fe/Ni ratio in metal is sometimes used to impose a specific/o on a system. These effects are linked and must be unraveled first in order to understand tin in planetary mantles (Righter and Drake, 2000). 

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... 

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