The shimmering blue sapphires are sought-after gemstones, but how and from what they were formed is unclear. New insights into their origins are now being provided by analyses of sapphires from the Eifel region – gemstones just a few millimetres in size that are often found there in volcanic rock, but also as washed-out grains. The mineralogical and isotopic analyses show that the Eifel sapphires are apparently linked to two different geological mechanisms. Some of these corundum minerals are therefore derived from magmatic melts, while others are derived from metamorphic rocks that were swept along and transformed by the rising volcanic material. The findings thus confirm that sapphires can be formed in different ways.
Sapphire is usually the bluish colored variant of the mineral corundum (Al2O3). They are formed when iron, titanium or cobalt ions are embedded in the crystal lattice of this mineral, changing its light refraction. The sought-after blue gemstones are usually found in silicon-poor volcanic rocks, but how these minerals came about is often disputed in individual cases. In this context, the widespread assumption that sapphire comes from deep crustal rocks and accidentally reached the earth’s surface with rising magma. “One explanation is that sapphire was formed in the earth’s crust at very high pressure and very high temperatures from former clayey sediments and that the rising magma only acts as an elevator for the crystals to the surface,” says senior author Axel Schmitt from the University of Heidelberg. But there are also assumptions that at least some sapphires could come from deeper magma reservoirs in the earth’s upper mantle or lower crust.
Sapphires from the Eifel
To provide more clarity, researchers led by Sebastian Schmidt from the University of Heidelberg have now examined 223 small sapphires from the Eifel in more detail. This volcanic region is part of the intracontinental Central European Volcanic Province, which also includes the Massif Central in France, the Eger Graben in Bohemia and the Western Carpathians. For around 700,000 years, magma has been penetrating the earth’s crust from the earth’s mantle here. The molten rock produced in this way is poor in silicates, but rich in sodium and potassium and often contains sapphires. Unlike in other areas of this volcanic province, however, the last major eruption of Eifel volcanism was not long ago by geological standards: it occurred a good 12,000 years ago with the Laacher See eruption. This means that the rock surrounding the sapphires is easily accessible.
In addition, tiny sapphire grains are also found in the sediment of the Eifel rivers. “Similar to gold, sapphire is very resistant to weathering compared to other minerals. Over long periods of time, the grains are washed out of the rock and deposited in rivers,” explains Schmidt. “There, due to their high density, they can be separated from lighter sediment components using a gold pan.” For their study, the researchers collected 223 sapphire samples from rock and sediment and dated them using the uranium-lead decay series. To obtain clues about the type of formation of the gemstones, they analyzed the composition of the corundum and the surrounding rocks attached to it and examined the ratio of the stored oxygen isotopes using a secondary ion mass spectrometer. The different relative abundances of the light isotope O-16 and the heavier isotope O-18 can reveal at what depths the sapphires were formed, because crustal rocks contain proportionally more O-18 than melts from the Earth’s mantle.
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The analyses showed that the majority of the sapphires in the Eifel only grew at the same time as the volcanism. According to their age and composition, they were formed in melts from the earth’s mantle that were contaminated by heated and partially melted crustal rock at a depth of around five to seven kilometers, as Schmidt and his colleagues report. However, some sapphires show different characteristics that indicate a different origin. Accordingly, these gemstone grains formed in the contact area of the underground melt accumulations, with the melts penetrating the surrounding rock and triggering the formation of sapphire. “In the Eifel, both magmatic and metamorphic processes, in which the temperature changed the original rock, for example, played a role in the crystallization of sapphire,” says Schmidt.
Source: Sebastian Schmidt (Heidelberg University) et al., Contributions to Mineralogy and Petrology, doi: 10.1007/s00410-024-02136-x