The Lesvos ophiolite, Greece, was recently identified as an analog to the External Liguride Unit formed at an ocean-continent-transition (OCT) setting. To further study the evolutionary history of this body, we analyzed highly siderophile element abundances and Re-Os isotopic compositions of 14 Lesvos peridotites, in combination with petrology, bulk-rock major, and trace element geochemistry. Almost all the Lesvos peridotites fall within the field of global OCT and mid-ocean-ridge (MOR) peridotites, indicating that these rocks have not experienced subduction-related processes. The near-horizontal 187Os/188Os versus Al2O3 trend over a wide range of Al2O3 (0.45–3.66 wt. %) may have been caused by recent melt depletion during rifting and thinning of the Lesvos lithosphere. Combining data from available global ophiolitic peridotites, we find that a large proportion of the more Al-depleted supra-subduction-zone (SSZ) peridotites show lower Os/Ir, higher Pd/Ir and remarkably elevated radiogenic Os relative to other tectonic environments (OCT and MOR). By linking this kind of geochemical evolution to the Wilson Cycle, a complete picture emerges: (a) In the OCT to MOR stages, the extensional rifting environment may lead to mild to moderate melt depletion, followed by, or associated with, infiltration of S-saturated (high fS2) basaltic magmas; (b) when progressing into the SSZ stage, more extreme degrees of mantle melt depletion may be driven by aqueous fluids in the sub-arc mantle. During this stage, high-fO2 slab-derived fluids and/or S-undersaturated (low fS2) boninitic magmas may infiltrate the sub-arc mantle, followed by subsequent S-saturated forearc basaltic magma infiltration.
Plain Language Summary
Establishing the relationship between the geochemical evolution and geodynamic mechanism of the Earth's upper mantle is an important objective of the study of ophiolites. There are two major aims of this study: (a) Using highly siderophile elemental (HSE) and Re-Os isotopic geochemical tools to evaluate the tectonic setting for the Greek Lesvos peridotites; (b) on the basis of geochemical and isotopic comparisons among available global ophiolitic peridotites from various tectonic settings to reconstruct the Earth's upper mantle evolution. First, we find that although the Lesvos peridotites underwent S-saturated melt overprinting after primary melt extraction that was related to the Mesozoic Pangea breakup, they did not experience subduction-related processes. Second, we summarize that supra-subduction-zone (SSZ) peridotites in other ophiolites have not only recorded extremely high degrees of melt depletion but also show significantly higher Pd/Ir ratios and more radiogenic 187Os/188Os, compared with ocean-continent-transition (OCT) and mid-ocean-ridge (MOR) peridotites that formed beneath spreading centers. We believe that episodic infiltrations by S-saturated and/or undersaturated melts/fluids may be an important driving force for these mantle compositional changes during the Wilson Cycle, which can also cause HSE-bearing sulfides/alloys to be abnormally enriched, and possibly lead to formation of platinum group elements (PGE)-rich chromite ore deposits.
Article link: https://doi.org/10.1029/2021JB022445