The Late Veneer Hypothesis for the Origin of Highly Siderophile Elements in the Mantle of the Earth

Gerhard Schmidt Institut für Kernchemie, Johannes Gutenberg-Universität Mainz,

Fritz-Straßmann-Weg 2, D-55099 Mainz, Germany

Herbert Palme Mineralogisch-Petrographisches Institut, Universität zu Köln,

Zülpicher Straße 49b, D-50674 Köln, Germany

It has been known since some time that the highly siderophile elements (HSE), including the platinum group elements (PGE: Os, Ir, Ru, Rh, Pd, Pt) and Au and Re, have comparatively high abundances in the upper mantle of the Earth. Given equilibrium between metal and silicate these elements will strongly partition into the metal phase. Metal/silicate partition coefficients (Dmet/sil) for some of these elements have recently been determined at one bar. Values as high as 1012 were found for Ir at oxygen fugacities appropriate for core formation (Borisov and Palme, 1995). The Dmet/sil of Pd and Au are several orders of magnitude lower. The large differences among partition coefficients of HSE and the extremely high values make metal/silicate equilibrium models, including equilibrium at very high temperatures unlikely (Borisov and Palme, 1995; Borisov et al., 1994). A more plausible hypothesis is the addition of a late chondritic veneer to the Earth after completion of core formation suggested as early as 1974 (Kimura et al., 1974).

Our knowledge of the abundances of PGEs in the upper mantle is primarily derived from analyses of ultramafic rocks believed to be representative of the Earth mantle. These are primarily spinel and garnet peridotites, either as xenoliths in alkali basalts and kimberlites or samples of peridotite massifs, such as Zabargad, Ronda etc. Both, xenoliths and peridotitic massifs are very similar in absolute and relative abundances of PGEs. The elements Au and Re appear to be more variable. Rhenium and to some extent Pd concentrations depend on the degree of fertility, i.e., the Al2O3 content of the analysed rocks (Reisberg and Lorand, 1995; Pattou et al., 1996), although Os-isotopes suggest a basically chondritic Re/Os ratio for the primitive mantle. There is a remarkable similarity in the absolute concentrations and in the PGE patterns from xenoliths and peridotite massifs.

The approximately ten times higher S-content in samples from peridotite massifs compared to xenoliths indicates that S and the PGE are decoupled. It has been suggested that S was lost as SO2 from the xenoliths during pressure release on their way to the surface of the Earth. However, the low Cu content in the xenoliths (Mitchell and Keays, 1981) and thus the approximately similar S/Cu ratios in both types of rocks, xenoliths and peridotite massif samples does not support this possibility, except if loss of volatile Cu is assumed, an unlikely possibility.

Recent data from Pyrenaen lherzolites (Pattou et al., 1996) and from Zabargad peridotites (Schmidt et al., 1996) suggest some deviation of the PGE patterns from CI-chondritic patterns. Higher than CI-chondritic ratios of Pd/Ir were found in both suites of samples. Deviations from chondritic ratios were also found in Rh/Ir and Ru/Ir ratios (Pattou et al., 1996; Schmidt et al., 1996) Recent data obtained in Mainz on samples from Ronda and Lanzo (see also Lorand et al., 1993) as well as xenoliths from Mongolia show similar features, in particular a significantly higher than CI-ratio of Pd/Ir. Earlier data on xenoliths from Western Australia (Mitchell and Keays, 1981) and xenoliths data obtained by Morgan et al. (1981) did not show this Pd enhancement. The significance of these non-chondritic ratios is not clear at present. There may be residual components in the mantle (perhaps inhomogeneously distributed) that contributed to the added late chondritic veneer, or the late veneer was compositionally different from CI-chondrites or the late veneer hypothesis is not valid and the upper mantle PGEs were somehow derived from the core. It is, therefore
important to establish the areal extent of the non-chondritic PGE-ratios before further conclusions can be drawn.


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