Geodigest

Abstract

Calcium silicate perovskite (CaSiO3) is arguably the most geochemically important phase in Earth’s lower mantle, because it concentrates elements that are incompatible in the upper mantle. No one has ever successfully retrieved this high-pressure compound from the lower mantle before. This is because CaSiO3perovskite is ‘unquenchable’, meaning that it cannot retain its structure after being removed from its highpressure environment. In a new study, US geologists have finally found the first calcium silicate perovskite from Earth’s lower mantle in a diamond from the Orapa kimberlite pipe in Botswana (sci-news.com, 17 November 2021). ‘Calcium silicate perovskite is among the most geochemically important minerals in the lower mantle, largely because it concentrates elements that are incompatible in the upper mantle, including rareearth elements and radioactive isotopes that make an important contribution to the heat of Earth’s mantle’, said lead author Oliver Tschauner from the Department of Geoscience at the University of Nevada, Las Vegas, and his colleagues. ‘Although theorized for decades, to date, no one has ever successfully retrieved a high-pressure phase silicate from Earth’s lower mantle, largely because they cannot retain their mineralogical structure after being removed from a high-pressure, high-temperature environment. The only other highpressure phase silicate mineral confirmed in nature, bidgmanite, was found inside a highly shocked meteorite.’ In the new study, the researchers identified and characterized an inclusion of the high-pressure CaSiO3-perovskite within a deep-earth diamond using synchrotron X-ray diffraction. The unique diamond was unearthed from Botswana’s Orapa mine—the world’s largest diamond mine by area—in the 1980s (Fig. 1). A gem dealer sold the diamond in 1987 to a mineralogist at the California Institute of Technology. ‘For jewellers and buyers, the size, colour, and clarity of a diamond all matter, and inclusions—those black specks that annoy the jeweller—for us, they're a gift. I think we were very surprised. We didn't expect this’, Tschauner said. The crystalline compound the researchers found was named davemaoite in honor of the prominent experimental high-pressure geophysicist Ho-kwang (Dave) Mao and confirmed as a new mineral by the Commission of New Minerals, Nomenclature, and Classification of the International Mineralogical association. ‘This honour is a fitting tribute given the profound impact Dave’s work has had throughout the geosciences,’ said Richard Carlson, director of Earth and Planets Laboratory at the Carnegie Institution for Science. ‘His contributions have shaped our understanding of our world and now a piece of the world will forever bear his name’. The structural and chemical analysis of davemaoite showed that it is able to host a wide variety of elements in its structure, including potassium, thorium and uranium—three of the major heat-producing elements. The findings support the existence of compositional heterogeneity within the lower mantle and, given the mineral’s overall abundance, suggest that davemaoite likely influences heat generation in the deep mantle. ‘We believe davemaoite originated between 660 and 901 km (410–560 miles) below Earth’s surface’, Tschauner said. ‘The discovery of davemaoite inspires hope for finding other difficult high-pressure mineral phases in nature’, said Yingwei Fei, a researcher in