Abstract

The Earth's major heat and mass fluxes are output from the mantle to surface via mid-ocean-ridge-basalt (MORB) magmatism. The variations of MORBs are determined both by the heterogeneity of mantle source and the melt migration processes from mantle source to seafloor. However, before the MORB magmas enter crust, the role of the major melt migration passageways within the mantle (dunite channel systems) in regulating the compositions of MORBs has been rarely evaluated, especially from the perspective of Re-Os isotopes. Here, we report new Re-Os isotopic compositions of base-metal sulfides (BMS), chromites and dunites from dunite lenses with low spinel Cr# [Cr³⁺/(Cr³⁺+Al³⁺) ≤0.66] (products of interaction between MORB-like melts and upper-mantle harzburgites) from the Zedang ophiolite (South Tibet). Rhenium-Os isotopic compositions of low-Cr# dunites from the Samail and Wadi Tayin massifs (typical MOR segments) of the Oman ophiolite are also shown for comparison to reveal the melt plumbing processes at mantle depths beneath mid-ocean ridges. Mineralogical evidence suggests that the Zedang sulfides and desulfurized alloy portions were originally precipitated as monosulfide solid solutions. The highly variable ¹⁸⁷Os/¹⁸⁸Os initial ratios (0.1191-0.1702) and low ¹⁸⁷Re/¹⁸⁸Os (<0.22) of the sulfides suggest that the chromite acted as a sink for Os-bearing sulfides, aggregating discrete Os components with heterogeneous isotopic signatures from asthenospheric or lithospheric mantle into dunite channels. The Zedang chromites and dunites show ¹⁸⁷Os/¹⁸⁸Os ratios similar to the primitive upper mantle (PUM), except for two dunites with sub-PUM ratios, reflecting the contribution of Os balanced by smaller volumes of Os-rich, unradiogenic sulfides (likely nucleating on Os nanoparticles) and larger volumes of Os-poor radiogenic BMS. Such isotopic heterogeneity, despite with less variation, has been observed in dunite channels from the Samail and Wadi Tayin massifs of the Oman ophiolite and present-day mid-ocean ridges. Formation of dunite channels in the upper mantle thus can aggregate Os-bearing sulfides with chromite, leaving high Re/Os components into the residual MOR melts. Once such channel systems were built up at the Moho transition zone, the newly incoming MOR magmas would preferentially dissolve the volumetrically abundant radiogenic BMS and retain Os-rich alloys/sulfides in the channels, further amplifying the Os-isotope mismatch observed between global oceanic crust (more radiogenic) and lithospheric mantle (less radiogenic).

This study reveals that the long-distance reactive migration of melts from mantle source to at least Moho depths can result in strong Re-Os fractionation and isotopic mismatch between primary MORBs and mantle residues. It also shows that, unlike other lithophile tracers (e.g., Nd-Hf isotopes) whose signatures are almost completely transferred from mantle source to crust, the sub-crustal melt plumbing processes beneath mid-ocean ridges can be finely constrained by the multistage evolution of chalcophile and siderophile elements (e.g., Re-Os) and their isotopes (e.g., ¹⁸⁷Re-¹⁸⁷Os) with sulfides and chromites. Our study supports that the MORB compositions are actually regulated by their reactive migration processes occurred between melt generation at depth and eruption on seafloor.