Sapphires, highly valued gems, are primarily composed of chemically “contaminated” aluminum oxide, or corundum. These distinctive blue crystals are typically found in association with silicon-poor volcanic rocks around the world. Geoscientists at Heidelberg University have discovered that millimeter-sized sapphire grains found in the Eifel (Germany) formed in connection with volcanism.
The Eifel, a volcanic region in central Europe, has been experiencing magma penetration from the Earth’s mantle for almost 700,000 years. The unique melts in the Eifel, low in silicon dioxide but rich in sodium and potassium, explain the prevalence of sapphire in the region. This groundbreaking research sheds light on the previously mysterious association of sapphires with this type of volcanic deposit.
“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,” explains Prof. Dr Axel Schmitt, a researcher at Curtin University in Perth (Australia) who is investigating isotope geology and petrology as an honorary professor at the Institute of Earth Sciences at Heidelberg University – his former home institution.
To validate this hypothesis, the team analyzed 223 sapphires from the Eifel region. While some of these tiny crystals were discovered in rock samples from volcanic deposits in local quarries, the majority were sourced 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, who conducted the studies as part of his master’s degree at Heidelberg University.
Researchers determined the age of the sapphires from the Eifel using the uranium-lead method for mineral inclusions in the sapphire. They utilized a secondary ion mass spectrometer capable of identifying the composition of oxygen isotopes. The varying relative abundances of the light isotope O-16 and the heavy isotope O-18 offer insights into the origin of the crystals, akin to a fingerprint. Deep crustal rocks contain more O-18 than melts from the Earth’s mantle.
The age determinations reveal that the sapphires in the Eifel formed concurrently with the volcanism. They, in part, inherited the isotopic signature of the mantle melts, which had been contaminated by heated and partially melted crustal rock at a depth of approximately five to seven kilometers. Sapphires were formed through contact with subterranean melts, which permeated the surrounding rock and initiated the formation of sapphires.
“In the Eifel, both magmatic and metamorphic processes, in which temperature changed the original rock, played a role in the crystallization of sapphire,” states Sebastian Schmidt.
Journal reference:
- Sebastian Schmidt, Andreas Hertwig, Katharina Cionoiu, Christof Schäfer & Axel K. Schmitt. Petrologically controlled oxygen isotopic classification of cogenetic magmatic and metamorphic sapphire from Quaternary volcanic fields in the Eifel, Germany. Contributions to Mineralogy and Petrology, 2024; DOI: 10.1007/s00410-024-02136-x