Diamond-inclusion system recording old deep lithosphere conditions at Udachnaya (Siberia)

Abstract

Diamonds and their inclusions are unique fragments of deep Earth, which provide rare samples from inaccessible portions of our planet. Inclusion-free diamonds cannot provide information on depth of formation, which could be crucial to understand how the carbon cycle operated in the past. Inclusions in diamonds, which remain uncorrupted over geological times, may instead provide direct records of deep Earth’s evolution. Here, we applied elastic geothermobarometry to a diamond-magnesiochromite (mchr) host-inclusion pair from the Udachnaya kimberlite (Siberia, Russia), one of the most important sources of natural diamonds. By combining X-ray diffraction and Fourier-transform infrared spectroscopy data with a new elastic model, we obtained entrapment conditions, P trap = 6.5(2) GPa and T trap = 1125(32)–1140(33) °C, for the mchr inclusion. These conditions fall on a ca. 35 mW/m 2 geotherm and are colder than the great majority of mantle xenoliths from similar depth in the same kimberlite. Our results indicate that cold cratonic conditions persisted for billions of years to at least 200 km in the local lithosphere. The composition of the mchr also indicates that at this depth the lithosphere was, at least locally, ultra-depleted at the time of diamond formation, as opposed to the melt-metasomatized, enriched composition of most xenoliths.

Mineral Inclusions in Lithospheric Diamonds

“…For preexisting minerals, equilibration is a diffusion-controlled process and for centimeter-sized crystals in coarse peridotites may take over a billion years at the temperatures of diamond stable lithospheric mantle. For protogenetic inclusions in diamond (see above), which are typically recovered in the 0.1-0.2 mm range, diffusive equilibration during a fluid or melt infiltration event will, however, be rapid, e.g., ~20,000 years for REE in a 0.1 mm defectfree garnet residing at 1150 °C (Nestola et al 2019b). Faster diffusion along dislocations could decrease the required time significantly.…”

Thermobarometry of Diamond Inclusions: Evidence Mantle Evolution beneath Siberian Craton and Archean Cratons Worldwide.

“…1-7) which is the mechanical layer caused by magmatic fracturing produced by protokimberlite and kimberlite magmas and it seems that most spinels are formed during the latest magmatic event in the SCLM and often trace the inflexion of the convective geotherm. Such conditions 1140-1200 -6 GPa were determined for chromites by the elastic barometry [192].…”

Effect of Thermoelastic Properties of the Pyrope-Almandine Solid Solutions on the Entrapment Pressure of Garnet-Related Elastic Geobarometer

“…Diamond is the only direct sample obtained from inaccessible portions of Earth (Nestola et al, 2019) and the subduction zone metamorphism controls many global geochemical cycles and the lithosphere (Ashley et al, 2016). Estimating the pressure and temperature of these minerals or rocks provides fundamental data for studying many such geological processes, but their determination remains extremely controversial (Bebout, 2007;Tajčmanová et al, 2021).…”