Tectonic environments of sapphire and ruby revealed by a global oxygen isotope compilation

Many sapphire and ruby occurrences are spatially linked with orogenic belts such as the Pan-African Orogen, the Himalayas, and regions of active or former subduction along the western margin of the Pacific Ocean. These gemstones have oxygen isotope compositions (δ¹⁸O) that span >45‰, reflecting the wide range of environments and conditions involved in corundum (Al₂O₃) formation. We compiled a global data base of sapphire and ruby δ¹⁸O, from which the following major groups of gemstones emerge: a dominant population of sapphires with δ¹⁸O centred around 5.5‰ (the mantle value) that is spatially related to regions of former subduction; a lesser population of sapphires and rubies with slightly higher δ¹⁸O that are associated with skarn and pegmatite; rubies with relatively low δ¹⁸O of 0‰–7‰ that occur in hydrothermally altered ultramafic metamorphic rocks in collision zones; and rubies with high δ¹⁸O of 14‰–25‰ that are found, almost exclusively, in Himalayan marble. The spatial distribution of the δ¹⁸O groups relative to plate boundaries provides insight into the two major periods of continental collision involved in sapphire and ruby formation: the Ediacaran collision of East and West Gondwana (the East African Orogeny) and the Cenozoic collision of India and Asia.