Research ArticleDissolution of mantle orthopyroxene in kimberlitic melts: Petrographic, geochemical and melt inclusion constraints from an orthopyroxenite xenolith from the Udachnaya-East kimberlite (Siberian Craton, Russia)
Introduction
Kimberlites are rare, volumetrically small igneous rocks that are derived from great mantle depths (>150–200 km; Mitchell, 1986; Giuliani and Pearson, 2019). Over the past few decades, kimberlites have received an almost disproportionate amount of attention in geosciences due to their association with diamonds and mantle xenoliths, which present unparalleled insights into the nature of the subcontinental lithospheric mantle (SCLM) and asthenosphere.
One of the most important yet enigmatic conundrums in the study of kimberlites is our understanding of the composition and evolution of initial (i.e. primary/parental) kimberlite melts. This is largely hampered by the hybrid nature of kimberlite rocks, as the magmas that give rise to kimberlites commonly entrain large quantities of mantle and crustal xenoliths, as well as xenocrysts (e.g., megacrysts and diamond), and their composition can be extensively and invariably modified by volatile (e.g., CO2, H2O) exsolution and wall-rock interaction, magma differentiation and syn−/post-magmatic alteration (see review by Giuliani et al., 2014). Without taking all these factors fully into account, earlier studies employing whole-rock geochemistry to reconstruct parental melt compositions were led to the notion that kimberlites originated from volatile-rich (CO2 + H2O) ultramafic/ultrabasic melts (Becker and le Roex, 2006; le Roex et al., 2003). However, this approach has been shown to be challenging, as these putative melt compositions are likely too Mg-rich to have been in equilibrium with mantle lithologies (Kopylova et al., 2007) and the primary magmatic versus xenocrystic, as well as secondary nature, of kimberlite mineralogy was not fully considered. In contrast, a growing list of studies on melt inclusions in magmatic and xenocrystic kimberlite minerals (Abersteiner et al., 2017, Abersteiner et al., 2018a; Golovin et al., 2018; Kamenetsky et al., 2004, Kamenetsky et al., 2014b) and megacrysts (see Bussweiler (2019) and references therein), as well as geochemical (Brett et al., 2015; Giuliani et al., 2020; Kopylova et al., 2007; Soltys et al., 2018b) and experimental works (Kamenetsky and Yaxley, 2015; Russell et al., 2012; Sharygin et al., 2015; Shatskiy et al., 2020) have provided insights into an initial kimberlite melt composition that contained less SiO2, MgO and H2O, and more CaO, and have high abundances of CO2 (e.g., ‘carbonatitic’ or ‘transitional silicate‑carbonate’).
One of the main processes driving kimberlite melt modification is thought to be the dissolution mantle-derived silicates (e.g., Hunter and Taylor, 1982; Kamenetsky et al., 2009a; Sharygin et al., 2017; Soltys et al., 2016). The rarity of orthopyroxene xenocrysts (i.e. the most Si-rich mantle mineral) relative to other mantle xenocrysts in kimberlites (Mitchell, 1973) is viewed as an indication for disequilibrium conditions within kimberlite melts and is supported by numerous experimental or theoretical studies (Kamenetsky et al., 2009a; Kamenetsky and Yaxley, 2015; Kopylova et al., 2007; Luth, 2009; Russell et al., 2012; Sharygin et al., 2015; Sharygin et al., 2017; Stone and Luth, 2016). Consequently, there are few examples of preserved orthopyroxene in kimberlite rocks that can present us with in-situ geochemical and petrographic insights into the potential reactions that may occur between orthopyroxene and the host kimberlite melt. Orthopyroxene in kimberlites is typically confined to mantle-derived xenoliths or as inclusions in other xenocrystic minerals where it is typically shielded from significant melt interaction, or occurs in the groundmass of kimberlites as highly resorbed crystals (Kamenetsky et al., 2009a; Russell et al., 2012).
In this study, we examined a well-preserved spinel-bearing orthopyroxenite xenolith (UV-50/05; see also Sharygin et al., 2007), which was derived from the serpentine-free units of the Udachnaya-East kimberlite (Russia). The xenolith is dominated by orthopyroxene (~90 vol%), along with lesser amounts of olivine, Cr-rich clinopyroxene and rare Ti-poor aluminous magnesian chromite. The margins of individual orthopyroxene grains composing this xenolith are partially replaced by complex assemblages composed of clinopyroxene, olivine and phlogopite, along with lesser amounts of FeNi sulphides (including djerfisherite (K6(Fe,Ni,Cu)25S26Cl)) and Cr-spinel. We present textural, geochemical and melt inclusion data to show that orthopyroxene in this xenolith reacted with a carbonate-rich and Na-K-Cl-S bearing melt, and present evidence of a type of decarbonation reaction that occurred during the petrogenesis of the Udachnaya-East kimberlite.
Section snippets
Geological setting and Host Kimberlite
The ~365 Ma (perovskite UPb dating by Kinny et al., 1997; phlogopite RbSr dating by Kamenetsky et al., 2009c) diamondiferous Udachnaya kimberlite is located in the Daldyn-Alakit kimberlite field (Siberian Craton, Russia). The Udachnaya pipe has a composite structure, where it consists of an older eastern and younger western body, which formed due to multiple kimberlitic magmatic events (Kharkiv et al., 1991; Zinchuk et al., 1993). The pipe intruded thick Ordovician and Devonian sedimentary
Analytical techniques
Sample UV-50/05 was prepared as a polished thin section. Initial mineralogical and textural investigations were undertaken using a petrographic microscope (Nikon Eclipse 50i POL) at the University of Tasmania. Detailed studies of minerals and inclusions were performed using a Hitachi SU-70 field emission (FE) scanning electron microscope (SEM) equipped with an Oxford AZtec XMax 80 detector at the Central Science Laboratory, University of Tasmania. A beam accelerating voltage of 15 kV was used
Sample description and petrography
Sample UV-50/05 is a mantle xenolith that was collected by A.G. (2005) from the mine dumps of the diamondiferous Udachnaya-East kimberlite. The sample is a semicircle-shaped (~3 × 3 cm) offcut, which is set in unserpentinised kimberlite rock. The petrography and mineralogy was examined by optical microscope and scanning electron microscope (SEM; see section 3 for methodology). The kimberlite surrounding the xenolith is macrocrystic in texture, which is defined by abundant (~50 vol%) fresh (i.e.
Xenolith rock-forming minerals
The major and trace element compositions of minerals in sample UV-50/05 were examined by electron microprobe and LA-ICPMS (see section 3 for methodology).
Geothermobarometry
Estimation of the pressure (P) and temperature (T) of equilibration of orthopyroxenite xenolith sample UV-50/05 was performed using the orthopyroxene-clinopyroxene thermometer by Brey and Köhler (1990). Since pressures cannot be reliable estimated (e.g., Brey and Köhler, 1990), spinel peridotites are assumed to equilibrate at 2.5 PGa (Ionov et al., 2010), or just below the lowest P value where garnet peridotite is observed. This pressure coincides with the maximum pressure estimate where
Melt inclusions
Xenolith-forming olivine, orthopyroxene and clinopyroxene are host to round-to-irregular shaped micro melt inclusions trails (Figs. 2e, 9a - e) that are likely secondary in origin. The compositions of inclusions were examined by SEM and LA-ICPMS (see section 3 for methodology). These micro melt inclusions range in size from 1 to 10 μm (rarely up to 20 μm) and are commonly polymineralic, where they contain up to four daughter minerals. The majority of melt inclusions are composed of halite,
Discussion
Sample UV-50/05 is a unique example of a well-preserved orthopyroxenite xenolith that was recovered from the ‘SFUE’ kimberlite, which was interpreted to have been unaffected by post-magmatic hydrous alteration (Abersteiner et al., 2018b). In this xenolith, orthopyroxene was partially replaced along the grain margins by a secondary assemblage dominated by clinopyroxene (i.e. Cpx-I and -II), olivine (i.e. Ol-I) and phlogopite. These mineral assemblages replacing orthopyroxene in the xenolith can
Conclusions
This study presents detailed petrographic and geochemical data on a unique example of a well-preserved (i.e. unserpentinised) mantle-derived orthopyroxenite xenolith from the Udachnaya-East kimberlite (Siberian Craton, Russia) in order to constrain the nature of the parental kimberlite melt composition. In our study, we show:
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Textural and geochemical features of the studied xenolith show that orthopyroxene was partially replaced by compositionally unique assemblages of clinopyroxene (Cpx-I and
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We would like to thank Maya Kamenetsky (University of Tasmania) for her assistance with sample preparation, as well as James Tolley and Maxwell Morissette (University of Tasmania) for their assistance with LA-ICPMS analyses and useful discussion with Christoph Beier (University of Helsinki). In addition, we would like to thank Greg Shellnut for the efficient editorial handling of this manuscript and the constructive reviews by Yannick Bussweiler and an anonymous reviewer. This work was
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