gems, peridot, olivine, Egypt, Zabargad, St. John’s Island

Mantle peridotites exposed on Zabargad Island in the Red Sea exhibit features traditionally linked to recent rifting tied to the proto-oceanic basin’s formation; however, trace element and Li isotope data suggest some characteristics instead stem from ancient subduction processes during the Pan-African orogeny (700–600 Ma). Exceptionally fresh spinel-lherzolites display high Li and trace element signatures indicating hydrous fluid metasomatism, yet their δ⁷Li values (~+4.9‰) align with MORB and OIB mantle ranges, implying an upper mantle–like Li isotope composition for both the lherzolites and metasomatic agents. In contrast, intense modal metasomatism forming up to 20% pargasitic amphibole correlates with heavier δ⁷Li values, resembling those from altered oceanic crust (AOC), suggesting the metasomatic Li originated from dehydrating subducted slabs—consistent with a Pan-African subduction setting. Cross-cutting pyroxenite veins and associated meta-basalts share these heavy AOC Li signals and trace element patterns typical of slab-derived fluids in modern island arc basalts (IAB), though one pyroxenite displays a negative δ⁷Li (−4.1‰), likely from a distinct, later-stage fluid source depleted in ⁷Li. This geochemical evidence indicates preserved AOC-derived signatures in a paleo-subduction mantle wedge—an expected but seldom-observed phenomenon in modern arc basalts.