Origin of highly siderophile element fractionation and excess 187Os in rocks from the CM-1 core of the Oman Drilling Project (Oman ophiolite)

The southern massifs of the Oman ophiolite were often considered as a model for the structure and composition of crust at fast-spreading ocean ridges. However, the influence of late-stage convergent plate margin magmatism on the evolution of lower crustal and upper mantle rocks in the ophiolite is still poorly understood. Here, we report new constraints from highly siderophile and other chalcophile element abundances and 187Os/188Os on samples from the CM-1 drill core of the Oman Drilling Project. Main results are: (1) Abundances of most elements in this study were not affected by low-temperature alteration. (2) In harzburgites and dunites from the top of the mantle section, chalcophile elements predominantly are controlled by traces of sulfides, precipitated from sulfide-saturated, incompatible element-depleted magmas. (3) The overlying dunite-rich crust-mantle transition zone (CMTZ) is extremely depleted in Pd, Se and Re, indicating that parent magmas were sulfide-undersaturated. (4) The mafic unit on top shows local sulfide saturation. (5) Ru/Ir and Os/Ir in CMTZ dunite are suprachondritic, whereas values in the overlying mafic unit are subchondritic. Hence, rocks of the mafic unit are products of magmas that initially crystallized cumulate olivine and a laurite-ehrlichmannite phase in CMTZ. (6) Suprachondritic initial 187Os/188Os of CMTZ, the mafic unit and other lower crustal gabbros suggest that most of the lower crust was affected by magmas that contained excess radiogenic 187Os, presumably a result of slab-derived inputs during the short-lived subduction zone stage.