Fractionation of highly siderophile and chalcogen elements in the lower oceanic crust: Insights from the troctolites of the Alpine-Apennine Jurassic ophiolites

The present study aims at investigating the role of reactive porous flow in the fractionation of highly siderophile (HSE: Os, Ir, Ru, Rh, Pt, Pd, Au and Re) and chalcogen (S, Se, Te) elements during construction of the lower oceanic crust in (ultra-)slow spreading ridges. At this purpose, we analyzed the whole-rock HSE and chalcogen elements, and Re-Os isotopes in olivine-rich troctolites embedded within large-scale gabbroic sequences from the Alpine-Apennine Jurassic ophiolites. Leucotroctolites and chromitites associated with the olivine-rich troctolites were also investigated. The olivine-rich troctolites have initial γOs ranging from +0.2 to +5.9. Their primitive mantle-normalized patterns are characterized by a slight enrichment of Os over Ir, and an increase from Ir to Ru, Rh, Pt, Pd and Au, nearly flat Au-Te-Se-S, and weak Re depletion with respect to S. This HSE and chalcogen element, and Os isotopic signature was acquired in response to sulfide segregation, which was most likely triggered by reaction between an olivine-rich matrix and migrating MORB-type melts. The chromitite layers occurring within the olivine-rich troctolites exhibit high concentrations of platinum group elements (PGE: Os, Ir, Ru, Rh, Pt and Pd). Formation of the chromitites might produce melts depleted in PGE that reactively migrated within the olivine-rich matrix, and ultimately locally led to olivine-rich troctolites characterized by a slight increase of the PGE/chalcogen element fractionation. The leucotroctolites differ from associated olivine-rich troctolites in the lower concentrations of HSE and chalcogen elements, with subparallel primitive mantle-normalized patterns but slightly higher Se/Te values. The crystallization of the olivine-rich troctolites might release HSE and chalcogen elements-depleted melts that reacted with an olivine + plagioclase crystal mush to form the leucotroctolites. Alternatively, formation of the leucotroctolites involved a process of reacting melt flow characterized by a low melt/crystal matrix ratio, which led to a relatively low proportion of trapped melt and related sulfides. At (ultra-)slow spreading ridges, the crystallization of sulfides in high-Mg# lower crustal rocks may exert a significant control in shaping the HSE and chalcogen signature of erupted basalts.