A blue miracle in the Eifel: how sapphires were formed in volcanoes

 

Credit: A sapphire from the sediments of the Kyll, a river in the Western Eifel. The crystal measures about 0.9 mm in diameter. Credit: Sebastian Schmidt

Researchers at the University of Heidelberg are studying the formation of this characteristic blue crystal in volcanic melts.

Sapphires are among the most valuable gemstones, but they are made entirely of chemically “contaminated” aluminum oxide, or corundum. Worldwide, these distinctive blue crystals are found primarily in association with silicon-poor volcanic rocks. It is generally assumed that this association is because sapphires originate from deep crustal rocks and accidentally find their way to the Earth’s surface when magma rises.

Geoscientists from the University of Heidelberg have demonstrated through geochemical analyses that the millimeter-sized sapphire grains found in the Eifel (Germany) were formed in association with volcanism. The work is published in the journal Contributions to mineralogy and petrology.

The Eifel is a volcanic region in central Europe, where magma from the Earth’s mantle has been penetrating the Earth’s crust for almost 700,000 years. The flows are poor in silicon dioxide but rich in sodium and potassium. Magmas of similar composition worldwide are known for their abundance of sapphire. Why this extremely rare variant of corundum is frequently found in this type of volcanic deposit has remained a mystery until now.

“One possible explanation is that the sapphire in the Earth’s crust originates from previously clayey sediments at very high temperatures and pressures, and the rising magmas simply provide the elevator to the surface for the crystals,” says Professor Axel Schmitt, a researcher at Curtin University in Perth, Australia, who studies isotope geology and petrology as an honorary professor at the Institute of Earth Sciences at Heidelberg University, his former home institution.

To test this hypothesis, the researchers examined 223 sapphires from the Eifel. They found some of these millimeter-sized crystals in rock samples from volcanic deposits in the region’s many quarries. Most of the sapphires, however, come from river sediments.

“Like gold, sapphire is very resistant to weathering compared to other minerals. Over time, the grains are washed out of the rock and deposited in rivers. Due to their high density, they are easy to separate from the lighter components of the sediment using a gold pan,” explains Sebastian Schmidt, who conducted the studies as part of his master’s degree at Heidelberg University.

The researchers determined the age of the Eifel sapphires using the uranium-lead method on mineral inclusions in the sapphire using a secondary ion mass spectrometer that also allowed the oxygen isotope composition to be identified. The different relative abundances of the light isotope O-16 and the heavy isotope O-18 provide information about the origin of the crystals like a fingerprint. Deep crustal rocks contain more O-18 than molten rocks in the Earth’s mantle.

Age determinations show that the Eifel sapphires were formed simultaneously with volcanism. They partly inherited the isotopic signature of the mantle melts, which were contaminated by heated and partially melted crustal rocks at a depth of about five to seven kilometres. Other sapphires were formed in contact with the underground melts, which impregnated the adjacent rocks and thus triggered the formation of sapphires.

“In the Eifel, magmatic and metamorphic processes, in which temperature changed the original rock, played a role in the crystallization of sapphire,” explains Schmidt.

More information:

Sebastian Schmidt et al., Petrologically controlled oxygen isotope classification of cogenetic magmatic and metamorphic sapphire from the Quaternary volcanic fields of the Eifel, Germany, Contributions to mineralogy and petrology (2024). DOI: 10.1007/s00410-024-02136-x

Provided by Heidelberg University

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