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.

全球范围内多处蓝宝石与红宝石矿点在空间分布上与造山带（orogenic belts）紧密关联，涵盖泛非造山带（Pan-African Orogen）、喜马拉雅造山带（Himalayas），以及太平洋西缘的现今活动或古俯冲带（subduction zone）区域。这类宝石的氧同位素组成（δ¹⁸O，oxygen isotope composition）跨度超过45‰，这一特征反映出刚玉（corundum，Al₂O₃）形成过程中所涉及的地质环境与形成条件具有显著多样性。本研究构建了一套全球蓝宝石与红宝石氧同位素（δ¹⁸O）数据库，基于该数据库可划分出以下四类主要宝石群组：第一类为优势群组的蓝宝石，其δ¹⁸O值集中于5.5‰左右（即地幔典型值（mantle value）），空间分布与古俯冲带区域密切相关；第二类为占比较小的蓝宝石与红宝石群组，其δ¹⁸O值略高，与夕卡岩（skarn）和伟晶岩（pegmatite）成因关联；第三类为δ¹⁸O值相对偏低（0‰~7‰）的红宝石，赋存于碰撞带内经热液蚀变（hydrothermally altered）的超镁铁质变质岩（ultramafic metamorphic rocks）中；第四类为δ¹⁸O值偏高（14‰~25‰）的红宝石，几乎仅发现于喜马拉雅造山带的大理岩（marble）中。不同δ¹⁸O群组的空间分布与板块边界的对应关系，为揭示蓝宝石与红宝石形成相关的两次主要大陆碰撞事件提供了关键依据：一是新元古代埃迪卡拉纪东冈瓦纳与西冈瓦纳的碰撞事件（即东非洲造山运动（East African Orogeny）），二是新生代（Cenozoic）印度与亚洲大陆的碰撞事件。