Noble Metals in the Primitive Mantle and their Behaviour During Various Mantle Processes

G. Hartmann Geochemical Institute, University Göttingen, 37077 Göttingen, Germany

In order to assess the absolute concentrations of the siderophile Elements PGE (IPGE= Ir, Ru, PPGE = Rh, Pt, Pd) and Au in the primitive mantle and to characterize their behaviour during partial melting and mantle metasomatism, a wide variety of ultramafic and mafic rocks from orogenic tectonites, xenolith localities, MORB-glasses and alkali basalts were investigated. In the mainly unaltered asthenospheric lherzolite massifs of Balmuccia and Baldissero (Ivrea Zone) (Hartmann and Wedepohl, 1993), the fertile spinel xenoliths from the Hebei Province (China) and Kilbourne Hole (New Mexico) an unfractionated PGE distribution pattern was found, with a ratio (PPGE/IPGE)PM of about 1. In these peridotites, the overall positive correlations of all PGEs and Au with S, Cu, Al2O3 and the clinopyroxene contents explain a negative correlation between the percentage of melt extracted and the noble metal concentrations of depleted peridotites because of a progressive release of PGEs and Au during partial melting of sulfur-bearing peridotites. According to Hartmann (1996) the above mentioned asthenospheric MORB-source peridotites, with about log fO2 = -1 below the FMQ buffer, are more reduced than the subcontinental lithospheric mantle, with log fO2 = +1. Oxidizing metasomatic fluids that have conditioned the lithospheric upper mantle (Hartmann and Wedepohl, 1990) for the formation of sulfate-bearing alkali basaltic magmas (Wedepohl et al., 1994) have also oxidized and mobilized the PGE-bearing sulfides in the source mantle. This oxidation of mantle sulfides by metasomatic processes caused a redistribution of PGEs between the residual mantle minerals. On account of their higher solubility in silicate melts (Borisov et al., 1994) PPGEs along with sulfur were favourably partitioned into the partial melt, while the IPGEs are retained in the residue, being either incorporated in recrystallized spinels or precipitated as IPGE-alloys. Therefore depleted peridotites, affected by various types and degrees of
mantle metasomatism, display fractionated PGE distribution patterns, (PPGE/ IPGE)PM < 1. The type of mantle metasomatism investigated has not caused an enrichment of PGEs in the upper mantle. Nevertheless, oxidizing mantle metasomatism combined with partial melting is an essential process controlling the distribution of the noble metals between peridotitic residue and mafic magma, which
apparently has the result of a S and PPGE depletion and a sporadical enrichment of Au and Cu in the residue.

Only fertile sulfur-bearing peridotites, in which PGEs and Au are correlated with Al2O3, are used to assess the abundances of PGEs and Au in the primitive mantle. We favoured the extrapolation to the Al2O3-concentration of the primitive mantle instead of the pyrolite model. In comparison to the chondritic approach (0.008 times C-I chondrite concentrations, mainly depending on certain chondritic concentrations) our reversed partial melting model yields PGEs and Au contents which are broadly in agreement with the results of the chondritic approach, with the exception of a minor Ru and Pd enrichment in our mantle.


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