LAUSR

ABSTRACT 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.