Researchers from the Heidelberg University, Germany, unveil how sapphires are formed in volcanoes, according to a study published in the journal Contributions to Mineralogy and Petrology.
The Eiffel is a volcanic region in the center of Europe, where magma has been penetrating the crust for nearly 70,000 years. The melted rocks are low in silicon dioxide and rich in sodium and potassium. In these conditions, it’s common to find a rare type of sapphire, but how they’re formed is still a mystery. “One explanation is that sapphire in the Earth’s crust originates from previously clayey sediments at very high temperatures and pressure, and the ascending magmas simply form the elevator to the surface for the crystals,” explained Prof. Dr. Axel Schmitt from the Institute of Earth Sciences at Heidelberg University.
The researchers analysed over 200 sapphires from the Eifel region to test this possibility. The team found some of these millimeter-sized crystals in rock samples collected from volcanic deposits, but most sapphires come from river sediments. “Like gold, sapphire is very weathering-resistant compared to other minerals. Over protracted time periods, the grains are washed out of the rock and deposited in rivers. Because of their high density, they are easy to separate from lighter sediment components using a gold pan,” explains Sebastian Schmidt from Heidelberg University.
The team determined the age of the sapphires using uranium-lead method on mineral inclusions combined with a secondary ion mass spectrometer to identify the composition of oxygen isotopes. The different relative abundances of the isotopes O-16 and O-18 provide information on the origin of the crystals like a fingerprint.
The age analysis showed that sapphires in the Eifel formed at the same time as the volcano. They inherited the isotopic signature of the deep mantle melts (low O-18) but were partially melted at a depth of about five to seven kilometers. “In the Eifel, both magmatic and metamorphic processes, in which temperature changed the original rock, played a role in the crystallization of sapphire,” concluded Sebastian Schmidt.
Schmidt, S., Hertwig, A., Cionoiu, K. et al. Petrologically controlled oxygen isotopic classification of cogenetic magmatic and metamorphic sapphire from Quaternary volcanic fields in the Eifel, Germany. Contrib Mineral Petrol 179, 55 (2024). https://doi.org/10.1007/s00410-024-02136-x