“A Blue Miracle: How Sapphires Formed in Volcanoes” – A Study for the Gem Labs to Review
Often times, an article like this might float across the Roskin gem desk and never make it to the Newsletter or online Magazine. It’s just a tad bit too much science for us. However, this time, we not only found this article in our usual search space, it was also sent to us from Stone Group Laboratories. So, we thought best to include it here, hoping that the labs will find it useful.
Petrologically controlled oxygen isotopic classification of cogenetic magmatic and metamorphic sapphire from Quaternary volcanic fields in the Eifel, Germany.
Heidelberg University
Sebastian Schmidt, Andreas Hertwig, Katharina Cionoiu, Christof Schäfer & Axel K. Schmitt
Sapphires are among the most precious gems, yet they consist solely of chemically “contaminated” aluminum oxide, or corundum. Worldwide, these characteristically blue-colored crystals are mainly found in association with silicon-poor volcanic rocks. This connection is widely assumed to result from sapphires originating in deep crustal rocks and accidentally ending up on the Earth’s surface as magma ascended. Through geochemical analyses, geoscientists at Heidelberg University have shown that the millimeter-sized sapphire grains found in the Eifel (Germany) formed in association with volcanism.
The Eifel is a volcanic region in the center of Europe where magma from the Earth’s mantle has been penetrating the overlying crust for nearly 700,000 years. The melts are poor in silicon dioxide but rich in sodium and potassium. Magmas similar in 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 been a mystery until now. “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 test this assumption, the researchers examined a total of 223 sapphires from the Eifel. They found a portion of these millimeter-sized crystals in rock samples collected from volcanic deposits in the numerous quarries in the region. Most of the sapphires, however, come 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.
The researchers determined the age of the sapphires from the Eifel using the uranium-lead method on mineral inclusions in the sapphire using a secondary ion mass spectrometer that could also identify the composition of oxygen isotopes. The different relative abundances of the light isotope O-16 and the heavy isotope O-18 provide information on the origin of the crystals like a fingerprint. Deep crustal rocks have more O-18 than melts from the Earth’s mantle. As the age determinations show, the sapphires in the Eifel formed at the same time as the volcanism. In part, they inherited the isotopic signature of the mantle melts, which were contaminated by heated and partially melted crustal rock at a depth of about five to seven kilometers. Other sapphires originated in contact with the subterranean melts, whereby melts permeated the adjacent rock and thus triggered sapphire formation. “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.
The research results were published in the journal “Contributions to Mineralogy and Petrology.” Support for the work came from the Dr. Eduard Gübelin Association for Research and Identification of Precious Stones in Switzerland as well as the German Research Foundation.
Image from https://link.springer.com/
Conclusions
Sapphire from the Quaternary Eifel intracontinental volcanic field occurs in xenoliths present in pyroclastic deposits and lava, and in detrital grains in modern river sediment. Rutile and zircon inclusion ages indicate sapphire formation coeval with Quaternary volcanic activity in the WEVF and EEVF, with one detrital sapphire possibly formed in association with Paleogene Hocheifel volcanism. Three sapphire sub-types are classified based on δ18O compositions as (1) magmatic with δ18O between ~4 and 6‰ that match bulk phonolite compositions from ~6 to 8‰. This type also displays low Cr/Ga and high Fe/Mg, and they are best represented by sapphire in syenites with the paragenesis sanidine + hercynite ± nepheline; (2) metamorphic with δ18O > 10‰ and high Cr/Ga and low Fe/Mg. This type includes mica schist and metapsammite hosts with the paragenesis sanidine + oligoclase + sillimanite + corundum + spinel + biotite; high δ18O detrital sapphire from the Brohlbach river also belong to this group although their origin is uncertain; and (3) intermediate with δ18O (6–10‰) values and trace element abundances which indicate hybrid origins in metasomatized wall rock surrounding the contact aureole of low-level crustal (5–7 km) magma reservoirs (i.e. hornfels comprising sanidine + sillimanite + corundum + hercynitic spinel ± biotite ± andalusite).
The coexistence of magmatic, metamorphic, and hybrid sapphire suggests a continuum of processes where evolved phonolite assimilated metasedimentary wall rocks which conversely became metasomatized and/or heated in a contact metamorphic aureole. Exsolution of a CO2-and alkali-rich immiscible melt or mixing with peraluminous partial melts aided in shifting contaminated evolved phonolite melt into the corundum stability field. In contrast to previous studies that advocated corundum formation in the upper mantle or lower crust, the available barometric constraints for the Eifel including fluid inclusions, the presence of andalusite, isotopic trends that are consistent with mid-upper crustal metasedimentary contaminants, and the preservation of sharp intra-grain O-isotopic zonation all favor sapphire formation and residence in low-level crustal intrusions where mantle-derived magmas differentiated to volatile-rich alkaline compositions (Fig. 10). Although the wide range of O-isotopic compositions even within a single volcanic field limits the power of δ18O as a geographical provenance indicator, this study showcases the valuable genetic information that can be extracted from δ18O analysis of corundum.
Tap here to find our original discovery – in ScienceDaily magazine.
Tap here to go to the journal “Contributions to Mineralogy and Petrology” for the complete report!
