Sulfide Petrology of Spinel and Garnet Pyroxenite Layers from Mantle-derived Spinel Lherzolite Massifs of Arieg`, Northeastern Pyrenees, France
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Pyroxenite layers in the orogenic spinel lherzolite massifs of Ariège are porphyroclastic textured and range in composition from spinel websterite to garnet clinopyroxenite. Each pyroxenite type forms individual layers or occurs as part of composite layers in which the Opx/Cpx and Sp/Gt ratios decrease from margins to core. They are interpreted as crystalline segregations separated by flow crystallization from continental tholeiites en route to the surface. The primary magmatic phases consist of Al-rich pyroxenes, together with a minor amount of spinel, which started to crystallize at 1400°C and 20–22 kb pressure; the pyroxenes have locally survived plastic strains and subsolidus rccrystallizations and now occur in the form of clinopyroxene and orthopyroxene megacrysts displaying unmixing features. Although the differentiated silicate liquid was fully expelled during the flow crystallization process, the layered pyroxenites have concentrated the highly incompatible elements S and Cu and locally display significant chalcophile platinum-group element enrichment (Pd, Pt). Cu and S behave coherently over the whole range of pyroxenite composition; their highest concentrations are found in the thinnest websterite layers or at the margin of composite layers. Microscopic investigation of 214 polished thin sections shows these elements to be present as accessory Cu-Fe-Ni sulfides interstitial among the silicate phase or forming discrete bodies included in the relic pyroxene megacrysts. All these features indicate the presence of a sulfide liquid, immiscible with the silicate magma, during the crystallization of the layered pyroxenites. Sulfide liquid immiscibility probably occurred in response to thermal contrast between the pyroxenites and the cooler surrounding peridotites. It is proposed that the megacryst-hosted sulfide inclusions were trapped as linear arrays arranged on host megacryst growth planes. Due to the slow cooling and complex unmixing history of the megacrysts, these arrays have been transformed into coarse, isolated sulfide inclusions by subsolidus migration and spheroidization processes. They started to crystallize at T = 1200°C as monosulfide solid solution (MSS), probably coexisting with a minor amount of Ni- and Cu-rich sulfide liquid down to r=900°C. The reconstruction of the bulk chemistry of each individual inclusion reveals significant between-inclusion variations of Cu/Ni+ Fe and Ni/Fe ratios, which would result from strain-induced immobilization of these liquids. On cooling, the high-temperature MSS has decomposed below 230°C into Ni-rich pyrrhotite, nickeliferous pentlandite, chalcopyrite and minor pyrite. The post-magmatic history of the interstitial sulfide grains was not unlike that of the inclusions, except at near-surface temperatures where the primary sulfides resulting from unmixing of MSS have been partly altered into secondary sulfides by serpentinizing aqueous fluids. In spite of these post-magmatic alterations, the inclusions and the interstitial sulfide phases are remarkably homogeneous as regards their bulk Ni/Cu ratio, which is close to 3. This value is characteristic of sulfide separated from primary rather than partially differentiated tholeiitic melts. It is thus concluded that the continental tholeiite parent to the layered pyroxenites was saturated with sulfides when it left its mantle source regioa In this aspect, it would not be different from MORBs which contain similar sulfide compositions. In both cases, sulfide fractionation cannot be ignored in models for chalcophile trace element fractionation during initial evolution of these magmas.Keywords:
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Fractional crystallization (geology)
Platinum group
Solid solubility
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Olivine is one of the earliest crystallization minerals from basaltic magma, and its composition can directly give the metallogenesis information on parent magma composition, processes of magmatic fractional crystallization, and sulfide segregation degree. In this study, the authors selected the Tudun mafic-ultramafic complex to conduct electron microprobe analysis of olivine minerals. According to the Fo and Ni values of olivine, he Mg O content of parent magma, which contains the highest Fo olivine,was estimated to be 12.95%, indicating that the parent magma of the Tudun complex is a high-Mg O basalt. The modeling results show that sulfide segregation and fractional crystallization almost took place at the same time. A slight sulfide segregation(about0.2%) occurred during the early stage(0- 2%) of olivine fractional crystallization process, and less sulfide segregation(about0.01%) occurred during the later stage(6%-7%), which suggests that the metallogenic potential of the Tudun complex is limited. In addition, some data implying the negative relationship between Fo and Ni indicate that Fe-Ni exchange reaction happened between Fe-rich olivine and trapped sulfide liquid, which caused remarkable differences in olivine composition.
Fractional crystallization (geology)
Magma chamber
Ultramafic rock
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Platinum group
Fractional crystallization (geology)
Nickel sulfide
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