Modification Of Lithospheric Mantle By Melts/Fluids With Different Sulfur Fugacities During The Wilson Cycle: Insights From Lesvos And Global Ophiolitic Peridotites

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 Os-187/Os-188 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 fS(2)) 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-fO(2) slab-derived fluids and/or S-undersaturated (low fS(2)) boninitic magmas may infiltrate the sub-arc mantle, followed by subsequent S-saturated forearc basaltic magma infiltration.