A Reply to the Comment by Kostrovitsky, S. and Yakovlev, D. on ‘Was Crustal Contamination Involved in the Formation of the Serpentine-free Udachnaya-East Kimberlite? New Insights into Parental Melts, Liquidus Assemblage and Effects of Alteration’ by Abersteiner et al. (J. Petrology, 59, 1467–1492, 2018)

The Comment by Kostrovitsky & Yakovlev aims to demonstrate that Abersteiner et al. (2018) ‘presented erroneous statements regarding the genesis’ of the mineralogically and geochemically unique Serpentine-Free Udachnaya-East (SFUE) kimberlite (Russia) as it contradicts the views presented by Kostrovitsky et al. (2013). Furthermore, these authors reassert that ‘the supposition that surfaces salts and sulphates represent the main source for Na–Cl–S mineralization of kimberlites with unaltered olivine (Kostrovitsky et al., 2013) is still valid’ and ‘the mantle origin of salts is doubtful’. Numerous hypotheses are presented in both Kostrovitsky & Yakovlev (Comment) and Kostrovitsky et al. (2013) advocating a crustal brine origin for salts in the SFUE kimberlite. However, we find these interpretations to be poorly substantiated by empirical evidence and often contradictory. It is, therefore, important to revisit the earlier publication by Kostrovitsky et al. (2013) that is used to support the Comment by Kostrovitsky & Yakovlev. Here we address some of the erroneous and contradictory points presented by Kostrovitsky et al. (2013), which render their interpretations tenuous and misleading: Presence of evaporites in country rocks: Kostrovitsky et al. (2013) contradict themselves by suggesting that evaporites in the country rocks are both present and absent at the Udachnaya-East kimberlite. These authors stated that the Udachnaya-East kimberlite ‘does not intersect massive evaporites’ (p. 77), but later claimed that ‘...the highest (8 vol %) abundance of halite is restricted to depth levels of southern diatremes (Mir, Udachnaya, and International’naya) where they cut through halite-rich evaporite’, (p. 84) and ‘The absence of all textural types of serpentine in SFUE kimberlite is easily explained by the model of the evaporite country rock contamination’ (p. 88). The published and unpublished reports for parametric and geotechnical drill holes [figures 1 and 2 of Kamenetsky et al. (2014)] showed no evidence of sedimentary evaporite beds in or around the Udachnaya-East kimberlite. The composition of the putative kimberlite melt: The composition of the melt that is supposedly parental to Udachnaya-East was suggested by Kostrovitsky et al. (2013) to have resulted from ‘assimilation of evaporite xenoliths at relatively high, magmatic temperatures producing hybrid melt with elevated contents of Na, K, Cl, and S’, (p.88) and that ‘the hybrid residual kimberlite that digested evaporite xenoliths had lower H2O activity due to increased halogen and alkali abundances’ (p. 88). However, these authors also stated that ‘the contents of H2O, Na2O, and, by inference, the mode of halite are thus controlled only by the spatial position of the kimberlite specimen, rather than by the composition of the kimberlite melt’. Again, Kostrovitsky et al. (2013) did not present a consistent view as to whether salt was controlled by assimilation of alleged ‘evaporites’ or by the spatial position of the kimberlite rock. Primary versus secondary enrichment in Na and Cl: (Kostrovitsky et al., 2013) again presented conflicting views on the origin of Na and Cl mineralisation. On the one hand, these authors considered Na and Cl to be primary magmatic: ‘Na-rich kimberlite compositions are not solely restricted to unserpentinized kimberlites and that groundmass serpentine does not replace primary alkali- and chlorine-bearing minerals’. (p. 84), and ‘...alkali-, sulfur-, and chlorine-rich minerals may have crystallized from this late hybrid melt and may be "comagmatic" with kimberlite’ (p.88), but on the other hand they pushed for their secondary origin: ‘the strongest evidence for the secondary origin of Na-, Cl-, and S-rich minerals in the Udachnaya-East kimberlite... is the regional correlation between the geology and hydrogeology of the local country rocks and the mineralogy of Yakutian kimberlites’ (p.86). With this preface, we discuss below the Comment by Kostrovitsky & Yakovlev, and emphasise that previous data and interpretations by Abersteiner et al. (2018b) and other works are still valid and concordant with a magmatic origin of alkalis and halogens in the SFUE kimberlite.