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

Abstract

The petrologically unique Udachnaya-East kimberlite (Siberia, Russia) is characterised by unserpentinised and H 2 O-poor volcaniclastic and coherent units that contain fresh olivine, along with abundant alkali-rich carbonates, chlorides, sulphides and sulphates in the groundmass. These mineralogical and geochemical characteristics have led to two divergent models that advocate different origins. It has been suggested that the unserpentinised units from Udachnaya-East are representative of pristine unaltered kimberlite. Conversely, the alkali-chlorine-sulphur enrichment has been attributed to interactions with crustal materials and, or, post-emplacement contamination by brines. The mineralogical and geochemical features and the compositions of melt inclusions in unserpentinised and serpentinised Udachnaya-East kimberlite varieties are compared in this study. Both varieties of kimberlite have similar major, compatible and incompatible trace element concentrations and primitive mantle normalised trace element patterns, groundmass textures and silicate, oxide and sulphide mineral compositions. However, these two kimberlite varieties are distinguished by:(i) the presence of unaltered olivine, abundant Na-K-Cl-S-rich minerals (i.e. chlorides, Sbearing alkali-carbonates, sodalite) and the absence of H 2 O-rich phases (i.e. serpentine, iowaite (Mg 4 Fe 3+ (OH) 8 OCl•3(H 2 O)) in unserpentinised samples, and (ii) the absence of alkali-and chlorine-enriched phases in the groundmass and characteristic olivine alteration (i.e. replacement by serpentine and, or, iowaite) in serpentinised samples. In addition, melt inclusions hosted in olivine, monticellite, spinel and perovskite from unserpentinised and serpentinised kimberlite contain identical daughter phase assemblages that are dominated by alkali-carbonates, chlorides and sulphates/sulphides. This enrichment in alkalis, chlorine and sulphur in melt inclusions demonstrates that these elements were an intrinsic part of the parental magma. The paucity of alkali-carbonates and chlorides in the groundmass of serpentinised Udachnaya-East kimberlite is attributed to their instability and removal during

A Plethora of Epigenetic Minerals Reveals a Multistage Metasomatic Overprint of a Mantle Orthopyroxenite from the Udachnaya Kimberlite

“…Based on the experimental and natural evidence, we posit that in our case, the rock-forming orthopyroxene reacted with the melt parental for the Udachnaya pipe via similar to the described above but more complicated reactions, since natural melts have more complex multicomponent compositions with respect to mixtures used in experiments. The widespread development of mica, a suite of K-Na amphiboles (see Table 1) and calcite in the sample suggests that the reacting melt was probably alkaline-rich carbonate melt as shown previously for the Udachnaya pipe [10,11,13,20,82,112]. Since carbonates are in subordinate amounts relative to other minerals inside the CMI, we assume that at pressure values <2.5 GPa in the system "orthopyroxene-infiltrating kimberlite melt" a decarbonization reaction occurred with the formation of a free carbon dioxide fluid phase (see Reactions (1)-(3), (6), (7)).…”

“…Djerfisherite is a common mineral crystallizing during late stages of kimberlite melt evolution; it has also been identified in kimberlite-borne xenoliths (see review in [58]). In particular, djerfisherite was repeatedly found in the Udachnaya-East pipe among the groundmass minerals of various units of kimberlites [12,[77][78][79] and as a daughter mineral inside primary and secondary melt inclusions in the groundmass kimberlite minerals [13,[80][81][82]. It also occurs in the intergranular space of various mantle xenoliths [15,16] and as a daughter mineral inside the secondary melt inclusions in olivine from sheared peridotite xenoliths [16,20,22].…”

“…We consider the lizardite studied here to have crystallized with a participation of an external hydrothermal fluid based on the following evidence. In partially altered (serpentinized) kimberlites of the Udachnaya-East mine, the association serpentine + iowaite + calcite ± barite has been repeatedly established both within the kimberlite groundmass and among the hydrothermal associations in cavities and veins (e.g., [13,64]). However, in non-serpentinized ("fresh") kimberlites and mantle xenoliths from such kimberlites the association serpentine + talc + goethite + kuliginite + iowaite + barite has not been identified [10,11,16].…”

“…However, in non-serpentinized ("fresh") kimberlites and mantle xenoliths from such kimberlites the association serpentine + talc + goethite + kuliginite + iowaite + barite has not been identified [10,11,16]. Moreover, all the minerals from this association are absent from daughter assemblages in melt inclusions (interpreted to represent the composition of evolving kimberlite melts during ascent) in rock-forming kimberlite minerals of the Udachnaya pipe [13,19,[80][81][82], as well as among daughter minerals in crystallized secondary melt inclusions (interpreted to reflect the composition of primitive kimberlite fluids) in olivine from the deepest xenoliths of sheared peridotites [19,20,22].…”

Metasomatic Evolution of Coesite-Bearing Diamondiferous Eclogite from the Udachnaya Kimberlite

“…As indicated above, the Udachnaya primary kimberlite melt and/or kimberlite magma had a low-H 2 O and Cl-rich alkali-carbonatitic composition with high bulk Zr/Y of 6-9 early during its ascent [55]. Moreover, some exceptionally well-preserved varieties of non-serpentinized kimberlite are likewise poor in water but rich in Cl, S and alkalis, in the bulk composition as well as within melt inclusions in rock-forming minerals [54,59,79,80,[132][133][134]. As is true in other cratons, such as the Kaapvaal [123,126] and Slave [63], primary kimberlite melt from the source region apparently percolated through the mantle before the formation of the Udachnaya kimberlite.…”