Abstract
Minerals from mantle xenoliths in the Zapolyarnaya pipe in the Upper Muna field, Russia and from mineral separates from other large diamondiferous kimberlite pipes in this field (Deimos, Novinka and Komsomolskaya-Magnitnaya) were studied with EPMA and LA-ICP-MS. All pipes contain very high proportions of sub-calcic garnets. Zapolyarnaya contains mainly dunitic xenoliths with veinlets of garnets, phlogopites and Fe-rich pyroxenes similar in composition to those from sheared peridotites. PT estimates for the clinopyroxenes trace the convective inflection of the geotherm (40–45 mW·m−2) to 8 GPa, inflected at 6 GPa and overlapping with PT estimates for ilmenites derived from protokimberlites. The Upper Muna mantle lithosphere includes dunite channels from 8 to 2 GPa, which were favorable for melt movement. The primary layering deduced from the fluctuations of CaO in garnets was smoothed by the refertilization events, which formed additional pyroxenes. Clinopyroxenes from the Novinka and Komsomolskaya-Magnitnaya pipes show a more linear geotherm and three branches in the P-Fe# plot from the lithosphere base to the Moho, suggesting several episodes of pervasive melt percolation. Clinopyroxenes from Zapolyarnaya are divided into four groups according to thermobarometry and trace element patterns, which show a stepwise increase of REE and incompatible elements. Lower pressure groups including dunitic garnets have elevated REE with peaks in Rb, Th, Nb, Sr, Zr, and U, suggesting mixing of the parental protokimberlitic melts with partially melted metasomatic veins of ancient subduction origin. At least two stages of melt percolation formed the inclined PT paths: (1) an ancient garnet semi-advective geotherm (35–45 mW·m−2) formed by volatile-rich melts during the major late Archean event of lithosphere growth; and (2) a hotter megacrystic PT path (Cpx-Ilm) formed by feeding systems for kimberlite eruptions (40–45 mW·m−2). Ilmenite PT estimates trace three separate PT trajectories, suggesting a multistage process associated with metasomatism and formation of the Cpx-Phl veinlets in dunites. Heating associated with intrusions of protokimberlite caused reactivation of the mantle metasomatites rich in H2O and alkali metals and possibly favored the growth of large megacrystalline diamonds.
Highlights
The Upper Muna kimberlite field in Yakutia (Figure 1) has been involved in diamond exploration since 2018
We present compositions of clinopyroxenes (Cpx), garnets (Gar), chromites (Chr), ilmenites (Ilm), amphiboles (Amph), and phlogopites (Phl) and use them to reconstruct the mantle structures beneath the Upper Muna field, using both mantle xenoliths and concentrates from four kimberlite pipes
We suggest that protokimberlite magmas were responsible for the heating and refertilization of the mantle column and possibly were responsible for the growth of large diamonds
Summary
The Upper Muna kimberlite field in Yakutia (Figure 1) has been involved in diamond exploration since 2018. Other large kimberlite pipes in the central part of Yakutian kimberlite province (YKP) [1] were exhausted and excavation in open quarries stopped except for Yubileinaya [2], Zarnitsa, Komsomolskaya and Zarya pipes [3] in the Alakit field. The largest pipes are explored by underground mining in the Daldyn, Alakit and Malo-Botuobinsky regions [4]. Minerals 2019, 9, x FOR PEER REVIEW commercial kimberlites that may be exploited. Laptev sea Anabar shield Upper Muna field Ol e ne k Norilsk ui Vily Yakutsk Am ga na Le ey
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