Re and Os in Peridotite Massifs: Implications for Models of Continental Lithospheric Mantle Evolution

O. M. Burnham Dept. Earth Science, Open University, Walton Hall, Milton Keynes, MK7 6AA, UK

N. W. Rogers Dept. Earth Science, Open University, Walton Hall, Milton Keynes, MK7 6AA, UK

The Re-Os isotope system differs from virtually all other isotope systems in that Os is highly compatible in the mantle residue during partial melting whilst Re is incompatible and partitions into the melt. Residues after melt extraction therefore possess Re/Os ratios which are usually much lower than those which existed prior to melting. The most highly depleted residues effectively retain the Os isotopic composition of the mantle at the time of melting and remain comparatively unaffected by subsequent metasomatic processes. Re and Os are highly chalcophile and siderophile in magmatic systems, but the processes responsible for Re/Os fractionation are only poorly understood. Recent studies of orogenic peridotites from the Ronda ultramafic complex (Reisberg et al., 1991) and the Eastern Pyrenean ultramafic massifs (complex (Reisberg et al., 1991; Reisberg and Lorand, 1995) have demonstrated correlations between 187Os/188Os and major element compositions (e.g. Al2O3). Although these confirmed that fractionation of the Re-Os isotope system in the massive peridotites is associated with major element depletion, either coincident with or prior to the incorporation of the peridotites into the mantle lithosphere, no relationship was discernible between the Re-Os isotopic compositions of the peridotites and their chalcophile element abundances. In order to investigate the fractionation of Re from Os during mantle melting, samples of massive peridotite were collected from five of the Eastern Pyrenean ultramafic bodies and their Re-Os isotopic compositions determined in conjunction with their whole-rock major and chalcophile element contents.

The Os contents of the peridotites are limited in range (3.13 - 4.74 ppb) and are unrelated to either their major or trace element compositions. In contrast, their Re contents vary widely (65 - 497 ppt) and correlate with both the degree of major element depletion (e.g. MgO and Al2O3) and the S content of the peridotites. Whilst the Re - S trend passes through the origin, the Re - Al2O3 trend intersects the Al2O3 axis at ~ 0.6 wt% Al2O3. The 187Os/188Os ratios of the peridotites range from 0.1157 to 0.1309. Although the 187Os/188Os and 187Re/188Os ratios of the peridotites appear to be positively correlated, forming a diffuse array on an isochron plot, far stronger correlations are observed between 187Os/188Os and both S and Al2O3. mirroring the trends shown by Re. Whilst the determined 187Os/188Os - Al2O3 trend overlaps that determined previously for the massifs (Reisberg and Lorand, 1995), a clear link between the Re-Os isotope system and the chalcophile element composition of mantle peridotites has not been previously established.

Although decoupling of the Os abundances of the peridotites from their lithophile element compositions is consistent with the partitioning of the element into the mantle sulphide phase, the linear decrease in Re as a function of melt extraction cannot be accounted for using a simple melting model. However, the linear decrease in Re may be adequately modelled by a process involving non-modal melting of the mantle base-metal sulphide phase. A sulphide - silicate melt partition coefficient (DResul/sil) of ~ 325 may be calculated for Re, indicating that the compatibility of Re in the sulphide phase during mantle melting may be similar to those of the moderately chalcophile elements, such as Cu and Ni (Dsul/sil × 245) (Rajamani and Naldrett, 1978).

Because the Os contents of residual mantle peridotites are relatively constant, their Re/Os ratios will vary in proportion to their Re contents, which exhibit a linear correlation with major element depletion. The strong correlation of 187Os/188Os with Al2O3 and S therefore may be interpreted as isochron analogues, whose gradients reflect the time since isotopic homogenisation. Although the y-intercept of the 187Os/188Os - Al2O3 trend has no age significance, owing to the positive intercept of the Re - Al2O3 trend on the Al2O3 axis, the y-intercept of the 187Os/188Os - S array indicates an initial 187Os/188Os of 0.1158. A model age of 2.06 ± 0.13 Ga may be calculated for the isotopic homogenisation of the Eastern Pyrenean ultramafic massifs and the stabilisation of the underlying mantle lithosphere. This age is significantly younger than that determined by Reisberg and Lorand (1995), however it is in agreement with early Proterozoic crustal ages obtained from both the U-Pb analysis of zircons from granulites on the Iberian continental margin (Guerrot et al., 1989) and Nd-model ages for Hercynian basement and volcanic rocks in the Pyrénées (Ben Othman et al., 1984; Gilbert et al., 1994).


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