Petrogenesis of alkaline basalt-hosted sapphire megacrysts. Petrological and geochemical investigations of in situ sapphire occurrences from the Siebengebirge Volcanic Field, Germany

Megacrystic sapphires are frequently associated with alkaline basalts, most notably in Asia and Australia, although basalt is not generally normative in corundum. Most of these sapphire occurrences are located in alluvial or eluvial deposits, making it difficult to study the enigmatic relationship between the sapphires and their host rocks. Here, we present detailed petrological and geochemical investigations of in situ megacrystic sapphires within alkaline basalts from the Cenozoic Siebengebirge Volcanic Field (SVF) in Germany. Markedly, the sapphires show several micrometer thick spinel coronas at the contact with the host basalt, indicating chemical disequilibrium between the sapphire and the basaltic melt, supporting a xenogenetic relationship. However, in situ U-Pb dating of a Columbite Group inclusion within one Siebengebirge sapphire using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) indicates a close genetic relationship between sapphire crystallization and alkaline mafic volcanism in the SVF. The syngenetic mineral inclusion suite including carbonates, members of the Pyrochlore, Betafite and Columbite Groupe minerals, as well as a high abundance of HFSE and of gaseous low-density CO2 inclusions support a parentage of a highly evolved, MgO and FeO deficient carbonatitic melt. We identified CO2 to be the link between alkaline basaltic volcanism and the xenocrystic sapphires. Only alkaline volcanic suites can build up enough CO2 in this magma chamber upon fractionation so that at high degrees of fractionation a carbonatitic melt exsolves which in turn can crystallize sapphires.