Diamondiferous Mantle Eclogite: Diamond Surface Features Reveal a Multistage Geologic History
More than 99% of diamonds originally formed in the lithospheric mantle, the cold, rigid mantle that underlies continents, generally <300 km deep (T. Stachel and J.W. Harris, “The origin of cratonic diamonds – Constraints from mineral inclusions,” Ore Geology Reviews, Vol. 34, No. 1-2, 2008, pp. 5–32). In the lithospheric mantle, the two major rock types—and host rocks for diamond—are peridotite (>95 vol.%), composed predominantly of olivine, and eclogite (<5 vol.%), composed of iron-magnesium-calcium garnet and sodium-rich clinopyroxene. Eclogite is a high-pressure and high-temperature metamorphic rock that can form when dense oceanic crust subducts into the mantle beneath continents at convergent margins.
We cannot directly study the deep portion of the earth where diamonds form. Instead, geologists study diamonds themselves, as well as pieces of mantle rock that are transported to the surface as xenoliths by volcanic magmas such as kimberlite or lamproite. The authors have studied one such mantle eclogite xenolith, part of the GIA Museum collection (figure 1). This sample was recovered from a kimberlite in Russia and contained a partially exposed octahedral diamond. The exposed portions of the diamond had visible surface features, including trigons (figure 2).
Trigons are inferred to be etch features that form after diamond crystallization, through interaction with fluids or melts either in the mantle source region or during transport to the surface (J.W. Harris et al., “Morphology of monocrystalline diamond and its inclusions,” Reviews in Mineralogy and Geochemistry, Vol. 88, No. 1, 2022, pp. 119–166). Trigons form exclusively on the octahedral faces of diamond and can have several different orientations. Positive trigons are oriented in the same direction as the octahedral face, and negative trigons have opposing orientation (figure 2). Positive trigons are very rare and may relate to interaction with carbon-oxygen-hydrogen-rich fluids at temperatures between ~800° and 1000°C at near-surface pressures (Z. Li et al., “Positively oriented trigons on diamonds from the Snap Lake kimberlite dike, Canada: Implications for fluids and kimberlite cooling rates,” American Mineralogist, Vol. 103, No. 10, 2018, pp. 1634–1648). Trigons with negative orientation are much more common. It has been shown experimentally that increasing water content in melts could be one cause for the change of a trigon from positive to negative (e.g., A.F. Khokhryakov and Y.N. Pal’yanov, “The dissolution forms of diamond crystals in CaCO3 melt at 7 GPa,” Russian Geology and Geophysics, Vol. 41, No. 5, 2000, pp. 682–687). Diamonds with coexisting positive and negative trigons are extremely rare (e.g., Harris et al., 2022) and may indicate that the diamond experienced a complex multistage history related to thermal or fluid changes after its formation.
Using the acquired infrared absorption spectrum, the authors identified the diamond as type IaAB, containing nitrogen in the A- and B-aggregated forms. When the entire xenolith was exposed to ultraviolet light, the diamond fluoresced blue due to N3 defects (three nitrogen atoms surrounding a lattice vacancy), consistent with the presence of nitrogen impurities.
The diamond also contained an unidentified mineral inclusion with visible iridescence (figure 2). Minerals included fully within diamond (which itself is an impermeable time capsule) represent pieces of the mantle source region present at the time of diamond formation. For this sample, the mineral was too deep within the diamond to conclusively identify it nondestructively. However, if the mineral inclusion could be extracted, its chemical composition might be very useful in reconstructing the geologic history of the host diamond, as well as that of the mantle rocks in which the diamond formed.
