Sulphur isotope composition of orogenic spinel lherzolite massifs from Ariege (North-Eastern Pyrenees, France): An ion microprobe study

Abstract

The orogenic spinel Iherzolite massifs from Ariege (Northeastern Pyrenees, France), which represent tectonically emplaced fragments of the sub-continental upper mantle, are composed mainly of variously depleted peridotites. These rocks are crosscut by two generations of pyroxenites. The first is made up of layered pyroxenites, which are interpreted either as crystal segregates from Triassic continental tholeiites or as subducted parts of the oceanic crust re-injected within the upper mantle. The second consists of amphibole-rich dikes separated from Cretaceous alkali basalts. Forty sulphide grains, occurring either as inclusions within silicates or as interstitial grains, were investigated by ion microprobe for their sulphur isotopic compositions. The δ 34S values range between −5.7 ± 1.5 and +4.5 ± 1.5%. with an average of −0.7%., being very similar to the value of 0 ± 2%. commonly assumed for the upper mantle. However, significant differences in δ 34S values are observed between the different rock types. Negative δ 34S values (average = −3.2%. for the massive peridotites) are discovered for the first time in mantle peridotites. In contrast, amphibole pyroxenites have positive δ 34S values (mean = +3.3%.), while layered pyroxenites show a wide range of variations from −3.1 ± 1.5 to +4.5 ± 1.5%.. Peridotites adjacent to layered pyroxenite display δ 34S values ranging from −5.2 ± 1.5%. to +2.8 ± 1.5%., which reflect partial re-equilibration with the tholeiitic melt. Comparison between sulphide inclusions in silicates and interstitial sulphide grains strongly suggests that serpentinization and pyrenean metamorphism had no significant effect on the δ 34S values. Likewise, these values are broadly independent of the degree of partial melting. The negative δ 34S values of the massive peridotites could represent an ancient depletion event in the upper mantle. By contrast, the positive δ 34S values observed in the layered pyroxenites and the amphibole-rich dikes indicate that the two parent magmas had in common a mantle source variously enriched in 34S. Therefore, the present study reveals two extreme reservoirs characterized by different δ 34S values in the upper mantle (≈ −3 and ≈ +3%.), respectively. This range of variations can explain most δ 34S values found in MORB, continental tholeiites, and alkali basalts.