The abundances of Ir, Ru, Rh, Pt, Pd, Au and Cu have been determined by ICP-MS in sixteen spinel peridotite xenoliths collected in alkali volcanics from the Massif Central area (France). Eight samples are protogranular peridotites (mainly Montbriançon, Devès) which sampled the lithospheric mantle stabilized since the Proterozoïc. LREE-enriched, amphibole-bearing granular peridotite xenoliths (Le Fau en Chambon, Vivarais), lherzolites with atypical coarse-granular textures (Plan de Céléssou, Languedoc) and porphyroclastic lherzolites (Montferrier; Languedoc) were also included in this study. The protogranular lherzolites display the roughly flat C1 chondrite-normalized PGE patterns that are believed to result from addition of a
late chondritic component to the Earth's primitive mantle (PM). Their mean PGE contents are : Ir = 3±0.7 ppb;
Ru = 5.3±1.4 ppb; Rh = 0.92±0.26 ppb; Pt = 5±1.2 ppb;
Pd = 3.5±1.6 ppb; Au = 0.6±0.4 ppb. The PGE ratios are within ten percents of chondritic values (Ru/Ir =1.76; Rh/Ir = 0.306; Pd/Ir =1.17. Gold is depleted relative to the PGEs (Au/Pd= 0.17). The contents of S and Cu range between
0 and 140 ppm (average 50 ppm) and 3.8 and 20 ppm (average 13 ppm), respectively. The highest contents generally correspond to the most primitive lherzolites that have Pd/Ti and Cu/Ti ratios akin to PM estimates derived from Archean komatiites (3.105 and 2.5 102, respectively). There are positive correlations between compatible (e.g. Ir) and incompatible (e.g. Pd) PGE which probably illustrates the strong partitioning of both categories of PGE into the same discrete (sulfide) microphases. A lot of protogranular samples are depleted in S by factors of ten to fifty compared to PM estimates because of a late-stage sulfur loss. The three harzburgites analysed give the following average composition : Ir = 2.7±0.6 ppb ; Ru= 5.1±0.9 ppb ; Rh = 0.74±0.2 ppb; Pt = 3.6±1 ppb; Pd = 1.5±0.5 ppb; Au = 0.3±0.1 ppb. The depletion in incompatible PGEs (i.e. Pd, Pt and to a lower extent Rh) and Au produce lower than chondritic Rh/Ir (0.27), Pd/Ir (0.53) and Pd/Pt (0.39) ratios. These elements were removed along with a sulfide melt during the silicate melt extraction process that produced the harzburgites, as S and Cu contents also decrease relative to the lherzolites (<18 and <12 ppm, respectively).
The amphibole-rich granular peridotite analysed lacks
in S and Cu enrichment. Its PGE contents (Ir = 2.1 ppb;
Ru = 4.8 ppb; Rh = 0.8 ppb; Pt = 3.6 ppb; Pd = 3.7 ppb;
Au = 0.3 ppb) differ from the protogranular lherzolites only by a depletion in the most siderophile PGEs (Ir and Pt) relative to the less siderophile PGEs (Ru, Rh and Pd). This signature could reflect metasomatism by deep-seated fluids equilibrated with Ir-Pt-Fe alloys. At Plan de Céléssou and Montferrier, the PGE abundances record a strong effect of sulfide modal contents, although some sulfur was lost at a late-stage as in protogranular lherzolites. The Plan de Céléssou lherzolites are strongly impoverished in all of the PGEs and Au with respect to the protogranular lherzolites
(Ir =0.8±0.3ppb; Ru = 1.7±0.7ppb; Rh =0.21±0.13;
Pt =1.2±0.4 ppb; Pd = 0.95±0.5 ppb; Au =0.2±0.1ppb). However, the PGE and Au still occur in nearly chondritic relative abundances (Rh/Ir =0.28; Pd/Ir =1.2). It seems that PGE-bearing sulfide melts were able to separate out from the silicate matrix during the high temperature (1200°C) melt-assisted recrystallization process that affected the xenoliths of this occurrence, just before their uplift to the surface. The Montferrier lherzolites are enriched in all of the PGEs and Au (Ir =3.7±1.3 ppb; Ru =6.4±1.5ppb; Rh =1.27±0.4 ppb;
Pt =6.9±2 ppb; Pd = 5.6±1.4 ppb; Au = 1.7±0.63 ppb) as well as in Cu (36±3 ppm) compared to the protogranular lherzolites. However, these enrichments are more pronounced for Rh, Pd and Au as shown by the higher than chondritic Rh/Ir (0.34), Pd/Ir (1.5) and Au/Pd ratios (0.3). A similar high Pd mantle has been sampled by orogenic lherzolites massifs, which like the Montferrier xenoliths, were uplifted by lithospheric shear zones (Lorand et al., 1993; Pattou et al., 1996; Schmidt et al., 1996).
Lorand, J.P., Keays, R.R. & Bodinier, J.L., J. Petrology, 34, 1111-1140 (1993).
Pattou, L., Lorand, J.P. & Gros.M., Nature (in press) (1996).
Schmidt, G., Palme, H., Kratz, K.L. & Kurat, G., Chem Geol. (in press) (1996).