How did volcanic rage form sapphires?

Throughout the world, these distinctive blue crystals are found mainly in association with silicon-poor volcanic rocks.

It is widely assumed that this association is due to rubies originating in deep crustal rocks and accidentally ending up on the Earth’s surface with ascending magma.

Through geochemical analyses, geologists at the University of Heidelberg have shown that the millimetre-sized ruby ​​grains found in the Eifel (Germany) were formed in conjunction with volcanic activity.

The Eifel is a volcanic region in central Europe where molten rock from the Earth’s mantle has been seeping into the overlying crust for about 700,000 years.

The molten rocks consist of a small percentage of silicon dioxide but are rich in sodium and potassium.

Molten rocks of similar composition are known throughout the world for their abundance of rubies, but the presence of this extremely rare type of corundum in this type of volcanic deposit has remained a mystery until now.

“One explanation is that rubies in the Earth’s crust originate from earlier clay deposits at very high temperatures and pressures and that the rising molten rock simply creates a lift to the surface for the crystals,” explains Professor Dr Axel Schmidt, a researcher at Curtin University in Perth (Australia) who studies isotope geology and petrology as an honorary professor at the Institute of Geosciences at Heidelberg University.

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

“Like gold, rubies are very resistant to weathering compared to other minerals,” says Sebastian Schmidt, who conducted the studies as part of his master’s degree at the University of Heidelberg. “Over long periods of time, the grains are washed out of the rocks and deposited in rivers. Because of their high density, they are easy to separate from the lighter sediment components using the same method as separating gold from pebbles in a bowl.”

The origin of the ruby ​​was traced using oxygen isotope and trace element analysis, revealing that it was formed at the same time as volcanic activity began in the area.

The oxygen isotope composition within the ruby ​​suggests that it did not remain at high temperatures above 900 °C (1,652 °F) for long, contrary to the established theory that it formed within the mantle or lower crust.

Instead, some rubies get their isotopic signature from mantle melting that mixes with hot, partially melted crustal rocks at a depth of about 5 to 7 kilometers (3.1 to 4.3 miles).

Other rubies were formed when molten rock underground came into contact with surrounding rock walls, causing those rocks to produce rubies.

“In the Eifel region, both magmatic and metamorphic processes (where temperature changes the parent rocks) played a role in the crystallization of rubies,” Schmidt says.

The study was published in the journal Contributions to Mineralogy and Petrology.

Source: phys.org