Sapphire grains discovered in the Eifel region of Germany: how they are formed
Sapphires, renowned for their preciousness, are composed of aluminum oxide known as corundum. Typically displaying a captivating blue hue, these crystals are predominantly found in association with silicon-deficient volcanic rocks. It is commonly believed that sapphires originate from deep crustal rocks and inadvertently make their way to the Earth's surface when magma ascends. Geochemical analyses conducted by geoscientists at Heidelberg University have shed light on the formation of millimeter-sized sapphire grains discovered in the Eifel region of Germany, providing insights into their link with volcanism, reports ScienceDaily.
Situated in central Europe, the Eifel region has witnessed the intrusion of magma from the Earth's mantle into the Earth's crust for nearly 700,000 years. These magmas, characterized by low silicon dioxide and high sodium and potassium concentrations, have been known across the globe for their abundance of sapphires. The mystery surrounding the occurrence of this exceptionally rare form of corundum in volcanic deposits of this specific type has now been unraveled. "One explanation is that the sapphires in the Earth's crust originally come from clayey sediments that have been subjected to very high temperatures and pressure. The ascending magmas then act as a conduit, transporting the crystals to the surface," explains Prof. Dr. Axel Schmitt, who conducts research on isotope geology and petrology as an honorary professor at the Institute of Earth Sciences at Heidelberg University.
To test this hypothesis, the researchers examined 223 sapphires from the Eifel. Some of these millimeter-sized crystals were found in rock samples extracted from volcanic deposits in various quarries scattered across the region, whereas the majority were sourced from river sediments. "Similar to gold, sapphire exhibits remarkable resistance to weathering compared to other minerals. Over extensive periods of time, the grains are eroded from the rocks and deposited in rivers. Due to their high density, they can be easily separated from lighter sediments using a gold pan," explains Sebastian Schmidt, who conducted these studies during his master's degree at Heidelberg University.
To determine the age of the sapphires in the Eifel, the researchers utilized the uranium-lead method on mineral inclusions within the sapphire crystals, alongside a secondary ion mass spectrometer capable of identifying the composition of oxygen isotopes. The varying ratios of the lighter O-16 isotope and the heavier O-18 isotope provided crucial clues regarding the origin of these crystals, acting as a fingerprint. Deep crustal rocks contain a higher proportion of O-18 compared to melts from the Earth's mantle.
As the age determinations indicate, the sapphires in the Eifel formed concurrently with the volcanic activity. Some sapphires inherited the isotopic signature of the mantle melts, which had undergone contamination from heated and partially molten crustal rock at a depth of around five to seven kilometers. Others originated from direct contact with intruding melts, with the melts permeating the surrounding rock and triggering sapphire formation. "In the Eifel, both magmatic and metamorphic processes, influenced by temperature alterations in the original rock, played a role in the crystallization of sapphire," reveals Sebastian Schmidt.
The research findings, published in the journal "Contributions to Mineralogy and Petrology," were supported by the Dr. Eduard Gübelin Association for Research and Identification of Precious Stones in Switzerland, as well as the German Research Foundation.
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