Petrogenesis of Basalt-hosted sapphires from the Siebengebirge Volcanic Field (SVF) in western Germany

Abstract

Megacysts of blue sapphires are hosted by alkaline mafic rocks from the Cenozoic Siebengebirge Volcanic Field (SVF) in western Germany. Similar occurrences of sapphires related to intra-continental alkaline mafic volcanism have been reported from many other localities around the world, predominantly from Asia and Australia (e.g. Graham et al., 2008). Markedly, the sapphires are rimmed by a several micrometer thick spinel rim, indicating them to be in disequilibrium with the basaltic melt. Consequently, the sapphires are regarded to be xenocrysts rather than phenocrysts. Yet, in-situ U-Pb dating of a columbite inclusion within one Siebengebirge sapphire using Laser Ablation – Inductively Coupled Plasma – Mass Spectrometry (LA-ICP-MS) revealed that the time of sapphire crystallization (24.73 ± 0.35 Ma) is within the errors the same as the age of alkaline mafic volcanism in the SVF (Przybyla, 2013), indicating a close genetical relationship between sapphire crystallization and alkaline mafic volcanism in the SVF. The sapphires mineral inclusion suite that is comprised amongst others of carbonates, pyrochlore, betafite and columbite, as well as the strong enrichment in the HFSE compared to the primitive mantle and the abundant occurrence of gaseous low-density CO₂ inclusions, indicates that a carbonatitic melt played a major role in the sapphire formation. Glass inclusions of nephelinitic and of carbonated silicate composition probably represent the quenched products of two immiscible liquids in the silicate – carbonatite melt system, indicating that the crystallization of the sapphires was the result of complex magmatic processes in highly evolved SiO₂ undersaturated, CO₂-rich magmatic system, including extensive magma differentiation, the exsolution of two immiscible melts and the assimilation of the sapphire bearing host rocks by ascending fresh pulses of basaltic magma.<br /> Experiments that were conducted in the carbonatite – silicate melt system revealed that carbonatites are indeed eligible to precipitate corundum, given that the carbonatitic melts are saturated in Al₂O₃ and low in FeO and MgO. Yet, another set of experiments demonstrated that due to much better wetting propertied in a silicate melt, corundum that precipitated from a carbonatitic melt will always migrate into silicate melt if given the chance.<br /> Results from this study infer that the Siebengebirge sapphires probably crystallized from a highly evolved FeO- and MgO-deficient carbonatite that exsolved from a highly evolved phonolite as a consequence of increasing CO₂ partial pressure accompanying melt fractionation. CO₂-rich fluid inclusions have densities up to 0.9 g/cm³, infering that the sapphires must have formed at a minimum pressure of 0.6 GPa, corresponding to middle crustal levels.<br /> The sapphires were later trapped by fresh pulses of ascending alkaline mafic melts and subsequently transported to the surface. The process of sapphire trapping and transportation to the surface was very rapid on geological time-scales and lasted no longer than a few weeks to months, as indicated by the thickness of the spinel-rims. This petrogenetic scenario implies CO₂ to be the confining variable connecting the genesis of the sapphire megacrysts and the alkaline mafic volcanism in intra-continental volcanic areas.