Large (−0.6‰ to +2.3‰) δ97/95Mo variations occur in molybdenum-enriched iron oxide veins located in anticlines adjacent to the Dead Sea transform, and are thought to reflect the solutions from which the veins formed because of efficient scavenging of Mo by iron oxides. The wide range of δ97/95Mo values occurs in a paleohydrologic setting in which dense, concentrated, evaporitic marine brines in the Dead Sea Rift valley (δ97/95Mo ~1.6‰) penetrated and interacted with sedimentary rocks of the rift flanks, which included Cretaceous bituminous chalks (δ97/95Mo 0.8‰–1.2‰), before intermixing with Fe-bearing groundwater containing molybdenum of igneous and clastic origin. The δ97/95Mo values higher than seawater (1.6‰–2.3‰) are attributed to Rayleigh isotopic fractionation along the path of the brine migration. Molybdenum isotopes thus reveal a powerful potential to trace the sources and processes governing subsurface metalliferous fluid migration.

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