The petrogenesis of the eastern Pyrenean peridotites: an integrated study of their whole-rock geochemistry and Re-Os isotope composition

Abstract

The Re-Os isotopic compositions of sixteen peridotites from the eastern Pyrenean ultramafic massifs have been determined in conjunction with their whole-rock major, trace, and chalcophile element contents. The data suggest that the peridotites represent residues after the extraction of up to 25% partial melt from a fertile mantle source at relatively high pressures (>20 kbar). Linear correlations between the incompatible-element contents of the peridotites and indices of major element depletion require that the initial melt extraction must have been followed by metasomatism by an incompatible-element enriched component. The S contents of the majority of the peridotites show a linear correlation with major element depletion, consistent with the extraction of a S-saturated basaltic melt from a fertile peridotite source. Low Cu/S ratios in a number of samples, however, suggest that parts of some of the massifs may have been infiltrated by an exotic sulphide component. The range of Os abundances in the peridotites is relatively limited (3.13–4.74 ppb) and unrelated to either the major or trace element contents of the peridotites. In contrast, the Re abundances vary widely (65–497 ppt) and correlate with their degree of major element depletion. Rhenium, sulfur, and Al 2O 3 are all positively correlated, suggesting all are controled by melt extraction. The 187Os/ 188Os ratios of the peridotites range from 0.1157 to 0.1294 and show good correlations with whole-rock Re, S, and Al 2O 3 abundances. From the y-intercept of the 187Os/ 188Os-S trend an initial 187Os/ 188Os ratio of ∼0.1159 is calculated for the peridotites, yielding a model age of 1.9 ± 0.3 Ga for the isotopic homogenisation of the mantle lithosphere that underlies the massifs. This model age is similar to the age of major crustal growth in Europe and implies a broadly synchronous stabilisation of both crust and lithospheric mantle at this time. The linear decrease of Re with degree of melt extraction is consistent with nonmodal melting of a Re-bearing base-metal sulphide phase. A sulphide-silicate melt partition coefficient (D sul/sil Re) of ∼325 is calculated for Re, indicating that the chalcophile behaviour of Re during melting may be similar to that of the moderately chalcophile elements, such as Cu rather than the highly chalcophile Pt-group elements.