“Diamonds are forever,” and so are the tiny mineral inclusions encapsulated by diamonds (pictured left). Only a small number of diamonds are inclusion-free, while the vast majority of them contain various minerals valuable to petrologists who study Earth’s mantle. One such inclusion is majoritic garnet – a high-silica garnet formed only at depths of around 300 – 700 km. These inclusions provide a unique window into the mantle transition zone (410-660 km) and allow researchers to speculate about the composition of deep Earth.
In a study published in Geochemical Perspectives Letters, DCO early career scientist Kate Kiseeva (University of Oxford, UK) and colleagues compared major and minor element compositions of approximately 100 majoritic garnet inclusions in diamonds . They used these analyses to extrapolate the bulk composition of the parental rock from which these garnets, and potentially diamonds, crystallised. Their work supports the idea that most of these majoritic garnet inclusions are derived from pyroxenitic, or intermediate in composition rocks between peridotite and basalt. More than 50% of the recovered majoritic garnets could represent the bulk rock pyroxenite, while the other half requires an additional phase, i.e. clinopyroxene to be in equilibrium with the majoritic garnet.
The authors also conducted high-pressure high-temperature sandwich experiments, the results of which suggest that zoned garnet pyroxenites can form as a result of a wall-rock interaction between silica under-saturated peridotite and silica saturated carbonated eclogite introduced into the deep mantle by subduction.
Thus, the authors establish a genetic connection between pyroxenite and diamond, and the abundance of pyroxenitic inclusions in diamonds reflects this connection.
Image: Cluster of deep purple peridotitic garnet inclusions (ca. 0.1 mm in size) with imposed octahedral morphology and stepped crystal faces, strongly suggestive of syngenetic growth. Credit: Anetta Banas