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Platinum-Group Elements: A New Set of Key Tracers for the Earth's Interior

¹ Unité de Recherche <<Minéralogie>> Muséum National
d'Histoire Naturelle and CNRS (UMR 7160)
61 Rue Buffon, 75005, Paris, France
E-mail: jplorand{at}mnhn.fr
² Steimann Institüt-Endogene Prozesse
Universität Bonn Poppelsdorfer Schloss, 53115 Bonn, Germany
E-mail: ambre.luguet{at}uni-bonn.de
³ Géosciences Montpellier (UMR 5243), Université de Montpellier 2
-CNRS, Pl. E. Bataillon, 34095 Montpellier, France
E-mail: oalard{at}gm.univ-montp2.fr

View larger version (34K): [in this window] [in a new window]   FIGURE 1 Elemental mantle abundances relative to carbonaceous chondrite. The stair-step depletion pattern is a function of the affinity of each group of elements for iron; thus, lithophile elements, which have no affinity for iron, occur at the chondrite concentration level in the terrestrial mantle, whereas the PGE, Re, and Au, which are "iron-loving elements," are depleted by two orders of magnitude relative to chondritic meteorites. The uniform depletion of these highly siderophile elements cannot be modelled by metallic melt-silicate melt partition coefficients determined at 1 bar pressure. The use of high-pressure metallic melt-silicate melt partition coefficients can reconcile the abundance of Pd (Righter et al. 2007) but not that of Pt (Ertel et al. 2006). FIGURE ADAPTED FROM DRAKE AND RIGHTER (2002) AND COTTRELL ANDWALKER (2006)

View larger version (26K): [in this window] [in a new window]   FIGURE 2 Chondrite-normalized PGE abundances in residual mantle rocks (dashed lines) and mantle-derived magmas (colored lines). Due to extraction of the low-melting-temperature Cu-Ni-sulfide melt, which concentrates Pt and Pd, the PGE patterns of residual mantle rocks are depleted in Pt and Pd. The depletion factor increases with the degree of melting (10 to 40%), and therefore with the amount of magma extracted from the mantle column, due to the concentration of the PGE in monosulfide solid solution (mss) and to the fact that an increase in the degree of melting decreases the amount of mss remaining in the residual mantle. DATA SOURCES: HANDLER AND BENNETT 1999;PEARSON ET AL. 2004; LUGUET ET AL. 2007). Mantle-derived magmas show the opposite behavior: mid-ocean ridge basalts are IPGE-depleted (IPGE = Ir, Os, Ru) relative to the mantle composition because their mantle source still contains base-metal sulfides. In contrast, the very high (30-50%) degree of partial melting needed to generate Archean komatiitic magma, a now-extinct type of highly magnesian magma, consumed all the base-metal sulfides in the mantle, generating PGE abundances (absolute and relative) close to those of the mantle. DATA SOURCES FOR ARCHEAN KOMATIITES: REHKÄMPER ET AL. (1999b); PUCHTEL ET AL. (2004); AND FOR MORB: REHKÄMPER ET AL. (1999b); BEZOS ET AL. (2005)

View larger version (136K): [in this window] [in a new window]   FIGURE 3 Backscattered-electron images, elemental (Ni and Cu) X-ray maps, and in situ analyses of the two populations of base-metal sulfides from mantle-derived rocks. (A) "Residual" Fe-Ni-monosulfide inclusion in olivine. (B) "Basaltic" intergranular sulfide bleb. The color intensity indicates the relative abundance of the element. Mss: monosulfide solid solution; Pn: pentlandite; Cp/Icb: chalcopyrite-isocubanite. The chondrite-normalized PGE patterns are from a combination of different peridotites. Note that a combination of "residual" mss and an appropriate amount of "basaltic" intergranular sulfide produces a primitive mantle (PM)-like PGE pattern. DATA SOURCES: ALARD ET AL. (2000); LORAND AND ALARD (2001)

View larger version (241K): [in this window] [in a new window]   FIGURE 4 Lherzolite samples from the upper mantle. (A) Yellow-green nodules ("xenoliths") in a basalt flow (Kerguelen Islands, Indian Ocean). COURTESY OF MICHEL GRÉGOIRE. (B) A kilometer-sized massif composed of orogenic lherzolite in the Pyrenean range (Lherz, southern France)

View larger version (29K): [in this window] [in a new window]   FIGURE 5 Ir- and chondrite-normalized PGE composition of the primitive mantle (Becker et al. 2006) compared with the main groups of chondritic meteorites (A) and with impact rocks of lunar maria (B). Note that the primitive mantle estimate does not correspond to any known chondrite composition; only the strongly brecciated rocks in the heights around lunar maria, and contemporaneous with the 3.9-3.8 Ga late heavy bombardment recorded by the Moon's surface, provide a reasonably good match with the Becker et al. (2006) primitive mantle estimate.

View larger version (37K): [in this window] [in a new window]   FIGURE 6 Chondrite-normalized PGE abundances measured in basalt-hosted mantle-lherzolite xenoliths collected from five continents. Note that very few analyses match the Becker et al. (2006) primitive mantle estimate. Most xenoliths are significantly depleted in PGE, especially Os. DATA SOURCES: REHKÄMPER ET AL. (1997); HANDLER ANDBENNETT (1999); LORAND AND ALARD (2001); PEARSON ET AL. (2004); BECKER ET AL. (2006)