Chloride-carbonate nodules in kimberlites from the Udachnaya pipe: Alternative approach to the evolution of kimberlite magmas

Abstract

Group-I kimberlites are the crystallization products of low-degree partial melts derived from the convective mantle [1, 2]. Since kimberlites are the main economic source of diamonds, the evolution of primary kimberlite melts is of great importance for deciphering mantle compositions, melting conditions, and diamond genesis. Most kimberlites consist of a brecciated mixture of olivine phenocrysts and xenogenic mantle and crustal material embedded in a fine- or coarse-grained groundmass, which is dominated by serpentine, calcite, and, occasionally, dolomite [3-5]. Thus, the evolutionary trend of kimberlite magmas is controlled by Mg silicates (high contents of MgO and SiO2) and carbonates (high contents of CaO and CO2). However, this trend can be intensely affected by interactions during ascent, the assimilation of mantle and crustal xenoliths, and the syn- and postmagmatic alteration of most kimberlite rocks. Almost all studied kimberlites around the world, even aphanitic [6, 7] or "uncontaminated" or "fresh" ones, were variably affected by these processes [7-10]. In the studying kimberlites, special attention was given to one aspect of the evolution of kimberlite magmas in mantle and crustal conditions: the entrapment of country rocks and interaction with them [3, 11]. Nodules in kimberlites (peridotites, eclogites, metamorphic and sedimentary rocks, as well as diamondiferous rocks) are usually used to estimate physicochemical and structural conditions in the subcontinental lithosphere, the properties of kimberlite magma, and the crystallization conditions of diamonds. It is convenient to study these processes in the Udachnaya kimberlite pipe in Yakutia because of a large number, diversity, and remarkable freshness of its nodules [11, 12]. Our study was focused on a new (carbonate-chloride) type of nodules recently found in kimberlites at deep levels (>350 km) of the open-pit mine at the Udachnaya-Vostochnaya pipes. Host kimberlites are also unique in chemical and mineral composition. The groundmass is predominantly olivine-carbonate-chloride in composition and shows almost no evidence of replacement [13, 14]. The presence of such nodules in unaltered kimberlites seems not to be coincidental and provides new insight into the evolution of kimberlite magmas.