Thermal State, Thickness, and Composition of the Lithospheric Mantle beneath the Upper Muna Kimberlite Field (Siberian Craton) Constrained by Clinopyroxene Xenocrysts and Comparison with Daldyn and Mirny Fields

Abstract

To gain better insight into the thermal state and composition of the lithospheric mantle beneath the Upper Muna kimberlite field (Siberian craton), a suite of 323 clinopyroxene xenocrysts and 10 mantle xenoliths from the Komsomolskaya-Magnitnaya (KM) pipe have been studied. We selected 188 clinopyroxene grains suitable for precise pressure (P)-temperature (T) estimation using single-clinopyroxene thermobarometry. The majority of P-T points lie along a narrow, elongated field in P-T space with a cluster of high-T and high-P points above 1300 °C, which deviates from the main P-T trend. The latter points may record a thermal event associated with kimberlite magmatism (a “stepped” or “kinked” geotherm). In order to eliminate these factors, the steady-state mantle paleogeotherm for the KM pipe at the time of initiation of kimberlite magmatism (Late Devonian–Early Carboniferous) was constrained by numerical fitting of P-T points below T = 1200 °C. The obtained mantle paleogeotherm is similar to the one from the nearby Novinka pipe, corresponding to a ~34–35 mW/m2 surface heat flux, 225–230 km lithospheric thickness, and 110–120 thick “diamond window” for the Upper Muna field. Coarse peridotite xenoliths are consistent in their P-T estimates with the steady-state mantle paleogeotherm derived from clinopyroxene xenocrysts, whereas porphyroclastic ones plot within the cluster of high-T and high-P clinopyroxene xenocrysts. Discrimination using Cr2O3 demonstrates that peridotitic clinopyroxene xenocrysts are prevalent (89%) among all studied 323 xenocrysts, suggesting that the Upper Muna mantle is predominantly composed of peridotites. Clinopyroxene-poor or -free peridotitic rocks such as harzburgites and dunites may be evident at depths of 140–180 km in the Upper Muna mantle. Judging solely from the thermal considerations and the thickness of the lithosphere, the KM and Novinka pipes should have excellent diamond potential. However, all pipes in the Upper Muna field have low diamond grades (<0.9, in carats/ton), although the lithosphere thickness is almost similar to the values obtained for the high-grade Udachnaya and Mir pipes from the Daldyn and Mirny fields, respectively. Therefore, other factors have affected the diamond grade of the Upper Muna kimberlite field.

Oxidation State of the Lithospheric Mantle Beneath Komsomolskaya–Magnitnaya Kimberlite Pipe, Upper Muna Field, Siberian Craton

“…The samples exhibit a P-T path consistent with the cratonic geotherm with surface heat flux = 35 mW/m 2 , recently obtained for the lithospheric mantle beneath the KM pipe [65] using the FITPLOT program [66] and for the nearby Novinka pipe [67] (Figure 7).…”

“…The uncertainty reported in calculating the f O 2 calculated with this method ranges from 0.6-0.9 log units (Figure 8). [65]. Lines running parallel to the paleogeotherm show the calculated uncertainty.…”

“…Plot of P NG85 against T T98/NG10 for the mantle xenolith from the KM pipe. The thick black line represents mantle paleogeotherm beneath the KM pipe[65]. Lines running parallel to the paleogeotherm show the calculated uncertainty.…”

Composition of the Sub‐Cratonic Mantle of the Guiana Shield Inferred From Diamond‐Hosted Inclusions

“…Paleo‐geotherms for the Siberian craton, Kaapvaal craton, and Guiana Shield are after Dymshits et al. (2020), Stachel et al. (2003), and Schulz et al.…”

Editorial for Special Issue “Minerals of Kimberlites: An Insight into Petrogenesis and the Diamond Potential of Deep Mantle Magmas”

“…Dymshits et al [5] constrained mantle paleogeotherm beneath the Komsomolskaya-Magnitnaya kimberlite pipe (Upper Muna field, Siberian craton) using clinopyroxene xenocrysts. This allowed the authors to estimate the lithosphere thickness beneath the Upper Muna field at the time of kimberlite eruption (Late Devonian-Early Carboniferous).…”