INTRODUCTION

Melt inclusions in minerals of ultramafic alkaline rocks can be used to obtain information on the compositions of parental mantle melts and their evolution. For example, the study of melt inclusions in olivine, minerals of the spinel group, perovskite, monticellite, ilmenite, phlogopite, and other minerals from kimberlites allowed us to estimate the composition of the primary kimberlite melt and to trace its evolution (see review in [1]). The latter indicates that kimberlite melts were generated and further evolved mainly within the Na2O‒K2O‒CaO‒MgO‒СO2‒Cl-system; i.e., they represented alkali-enriched carbonatite/carbonate–chloride liquids. Recently, this approach to deciphering the composition and evolution of melts has been used to study the origin of ultramafic alkaline rocks related to kimberlites—aillikites and related carbonatites.

Aillikites are ultramafic alkaline lamprophyres, which are composed of olivine and phlogopite macrocrysts and the groundmass including primary carbonate, phlogopite, spinel, ilmenite, rutile, perovskite, Ti-rich garnet, clinopyroxene, and apatite [2]. In contrast to kimberlites, aillikites can vary widely in the contents of phlogopite and carbonates, forming transitional varieties to carbonatites [3]. In addition, the K–Na component could have accumulated in ultramafic alkaline lamprophyres leading to crystallization of feldspathoid and/or feldspars in the groundmass and the formation of damtjernite [2]. In this case, damtjernite melts can be considered as transitional from ultramafic alkaline lamprophyres to alkaline lamprophyres and to melts forming nepheline-bearing ultramafic rocks of the large alkaline massifs.

The first results of studying the composition of melt inclusions in minerals from aillikites from alkaline-carbonatite complexes [4–8] show that the composition of daughter phases of silicate–carbonate melt inclusions in olivine is identical to the mineral composition of the groundmass.

The, data points of daughter phases of inclusions often lie at the beginning of the evolution of rock-forming minerals, indicating the juvenile character of the composition of the trapped melt.

The subsequent evolution of alkaline melts recorded in captured inclusions in minerals of the lamprophyre groundmass indicates the presence of the separated fraction of predominantly alkaline–carbonate composition. The latter, subsequently, led to the formation of saline fluids/melts of sulfate–phosphate–chloride–carbonate composition.

The study of melt inclusions in the minerals from damtjernites has received almost no attention. However, their study can have a significant fundamental contribution to the petrology of ultramafic alkaline rocks, in particular, to the issues related to the accumulation of the K–Na alkaline component in the melts, which induced the beginning of nepheline crystallization. The latter issue is important in terms of the transition of ultramafic alkaline rocks to alkaline lamprophyres and nepheline-bearing ultramafic rocks.

To decipher the evolution of the ultramafic alkaline damtjernite melt and its relationship with the kimberlite and aillikite melts, polyphase melt inclusions in olivine macrocrysts, magnesian Cr-spinel, and monticellite from the sample of monticellite–nepheline damtjernite of Victoria pipe, Anabar diamondiferous province were studied.

SUBJECTS OF RESEARCH

The Anabar diamondiferous province is located in the northern part of the Yakutian diamondiferous province within the Archean–Proterozoic Khapchan terrane, on the eastern slope of the Anabar Anteclise [9, 10]. In this area, there are several fields, including pipes and dike bodies of ultramafic alkaline rocks, mostly of Triassic (231–215 Ma old) and Jurassic (171–156 Ma old) age [11, 12].

The Victoria pipe is located in the Starorechensk field of the Anabar diamondiferous province, which is dominated by pipes and dike bodies of Triassic ultramafic alkaline rocks [12]. Rocks of the Starorechensk field show a wide variation in the composition of silicate (olivine, phlogopite, monticellite, clinopyroxene, nepheline, etc.) and carbonate phases. The study of the compositions of olivine and phlogopite, as well as the presence of nepheline, made it possible to diagnose previously rocks of the field as ultramafic alkaline lamprophyres of the aillikite–damtjernite series [13].

Rocks of the Victoria pipe are represented by two pyroclastic varieties with a high proportion of olivine macrocrysts [13]: (1) moderately carbonatized aillikite and (2) weakly serpentinized monticellite–nepheline damtjernite (Sample VK-2147). In the latter case, the rocks are characterized by a high degree of preservation of olivine macrocrysts and minerals of the groundmass and the matrix, which allowed us to classify the rocks as damtjernites and to study melt inclusions in rock-forming minerals. Olivine macrocrysts, up to 2 mm in size, from sample BK-2147 are represented by hypidiomorphic, less often irregularly shaped grains. They usually form magmaclasts (Fig. 1a). Olivine is replaced by monticellite at the rims (Fig. 1b).

Fig. 1.
figure 1

Pyroclastic monticellite–nepheline damtjernite, Sample VK-2147: (a) transmitted light image, parallel nicols: olivine macrocrysts (Ol) in the center of a magmaclast; (b‒d) BSE images: (b) olivine macrocrysts (Ol), overgrown by monticellite rim (Mtc) in the groundmass composed of monticellite grains (Mtc), perovskite (Prv), spinel (Spl) (magnesian Cr-spinel), cryptocrystalline feldspathoid aggregate (Fsp) (mainly nepheline) and apatite; (c‒d) enlarged fragments of the magmaclast groundmass (c) and the matrix (d), relationship of minerals.

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Magmaclasts are usually isometric and microporphyritic in shape. Small olivine phenocrysts (up to 0.02 mm) are enclosed in the groundmass of grains less than 0.01 mm in size. The latter consists of small monticellite grains (about 35 vol %), perovskite, ore minerals (<5 vol %), apatite, single phlogopite flakes (<5 vol %), xenomorphic nepheline grains, and cryptocrystalline intergrowths of apatite and feldspathoids (Figs. 1c–1d).

Compared to the groundmass of magmaclasts, the fraction of ore minerals decreases and the contents of feldspathoids and apatite increase in the damtjernite matrix.

RESEARCH METHODS

The polyphaser melt inclusions in minerals from damtjernites of the Victoria pipe were studied at the Center for Collective Use for Multielemental and Isotope Studies, Siberian Branch, Russian Academy of Sciences (Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk), and the University of Tasmania (Australia).

Raman spectra for crystalline phases of non-opened inclusions were recorded using a LabRam HR800 spectrometer, Horiba Jobin Yvon (IGM, Novosibirsk) equipped with an Olympus BX 41 optical microscope. The Ar+-laser line 514.5 nm was used to excite the spectra. The RRUFF database (http://rruff.info) was used to identify the mineral spectra. The compositions of the opened inclusions were determined on a TESCAN MIRA 3 LMU JSM-6510LV scanning electron microscope equipped with an Oxford Instruments Energy-Prex X-Max EDS detector (IGM, Novosibirsk). Analysis conditions: electron beam energy 20 keV, electron probe current 1.5 nA. Pure Co metal was used for quantitative optimization of EDS analysis. In addition, the compositions of the opened inclusions were determined on a Hitachi SU-70 scanning electron microscope equipped with an Oxford INCA EnergyXMax 80 analyzer in the Central Science Laboratory (Australia).

RESULTS

Melt inclusions of 10–15 µm in size in olivine macrocrystals (Fo81–89) were found (Fig. 2). In most cases, these are single inclusions or groups of inclusions located irregularly within the olivine grains; i.e., they are considered primary (Figs. 2a, 2b). The Raman spectroscopy of melt inclusions of this type allowed us to establish nepheline, phlogopite, kalsilite, monticellite, F-apatite, and calcite, which are daughter phases (Figs. 2a, 2b, 2d). Chains of secondary inclusions mainly of carbonate composition are also observed in olivine. The latter contain magnetite and calcite (Fig. 2c).

Fig. 2.
figure 2

Diagnostic results of daughter crystalline phases of melt inclusions (MI) in olivine macrocrysts of the Victoria pipe. Images of silicate–carbonate (a, b) and essentially carbonate (c) inclusions in transmitted light; KР-spectra of crystalline phases (d): kalsilite (Kls), monticellite (Mtc), nepheline (Nph), F-apatite (Ap), phlogopite (Phl), and calcite (Cal).

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Monticellite, nepheline, phlogopite (with varying BaO content, the first wt %), F-apatite, as well as perovskite, spinelids, pyrrhotite, and a Zr‒Ti-phase, probably calzirtite, were identified in opened melt inclusions by scanning electron microscopy (Figs. 3a‒3c).

Fig. 3.
figure 3

Diagnostic results of crystalline phases in opened melt, mineral, and cryptocrystalline inclusions in olivine (Fo), BSE images (a‒c), spinel (MUM) (d‒g), and monticellite (Mtc), (h) BSE images. Designations of minerals: Ap – F-apatite (Sr-Ap—Sr-apatite); Bdl—baddeleyite; Foid—feldspathoids; sulf—sulfates; carb—carbonates; phosph—phosphates; chlorides—chlorides; MUM—magnesian ulvospinel-magnetite; Nph—nepheline; Phl—phlogopite; Prv—perovskite.

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In many opened inclusions in olivine, the replacement of primary phases by secondary phases is observed. Figure 3b shows apatite of two generations in the inclusion; Figure 3c, late apatite. Serpentine, often forming fine-grained aggregates with apatite of the second generation, can also be referred to secondary minerals. The hydrothermal alteration probably occurred during opening of the inclusions.

The polyphase melt and polycrystalline inclusions in spinelids and monticellite from the pyroclastic groundmass vary in shape from rounded, elongated, and amoeba-like to angular and have a negative crystal shape; their size as a rule does not exceed 10 µm (Figs. 3g‒3k). The opened inclusions are represented by crystalline phases highly variable in composition. We have identified feldspathoids, kalsilite, phlogopite, apatite (including F- and Sr-bearing varieties), perovskite, pyrrhotite, single baddeleyite grains, and rarely diagnosed Na‒Ca-phosphates, alkaline carbonates (K‒Na‒Ca), and K‒Na-salts as sulfates and chlorides, rarely Ba and Sr sulfates (Figs. 3d‒3k).

DISCUSSION

The study of melt inclusions in olivine macrocrysts and the groundmass minerals allowed us to follow the evolution of the composition of the ultramafic alkaline melt that formed damtjernites of the Victoria pipe and to discuss the formation of monticellite, which is nontypical rock-forming mineral of damtjernites [2].

Features of evolution of the damtjernite melt. The primary melt inclusions in olivine macrocrysts from damtjernites of Victoria pipe, which may reflect the early evolutionary stages of the initial melt, show that damtjernite melts have a K–Na carbonate–silicate composition. It is important to note that K and Na enter the composition of exclusively silicate daughter phases, such as phlogopite, kalsilite, and nepheline. The presence of feldspathoids in inclusions is quite consistent with the mineral composition of the groundmass of damtjernite. Due to this, inclusions in olivines from damtjernites are distinctively different from melt inclusions in olivines from aillikites (Table 1). These inclusions also contain Zr-phases, which were first identified in damtjernites and were established in rare-metal alkaline–carbonatite complexes (for example, ore-bearing carbonatites and phoscorites of the Arbarastakh complex [5].

Table 1. The composition of melt inclusions in ultramafic alkaline and carbonatite complexes of the Siberian craton
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Na and K not only enter silicate daughter phases of inclusions in spinelites and monticellites from the pyroclastic groundmass but also form alkaline phosphates, carbonates, sulfates, and halides. The studied inclusions are similar in composition to ocellis from aillikites and damtjernites of the Chadobets complex (south of the Siberian Craton) consisting of Na-aluminosilicates enriched in Cl, F, S, H2O (natrolite, sodalite, scapolite), carbonate minerals, and feldspars [14]. These inclusions may also reflect the evolution of the primary ultramafic alkaline damtjernites melt toward the formation of a saline orthomagmatic fluid of specific alkaline–sulfate–phosphate–chloride–carbonate composition. There was an increase in the carbonate proportion in the melt at later stages. This can be evidenced by secondary carbonate inclusions in the form of polycrystalline inclusions with calcite in olivine macrocrysts.

Correlation with the data on melt inclusions in minerals from aillikites and kimberlites. The parental K–Na carbonate–silicate composition of primary melt inclusions in olivine macrocrysts is similar to that of melt inclusions in olivines from aillikites of the Neoproterozoic ultramafic alkaline complex with Arbarastakh carbonatites [5], the Devonian Ilbokich complex [7], and the Permian–Triassic Chadobets alkaline–carbonatite complex [4, 15], and different kimberlite manifestations [1]. However, their main distinctive feature is the presence of feldspathoids in the form of daughter phases, which concentrate the main volume of K and Na. The presence of feldspathoids emphasizes the more alkaline character of melts as compared to aillikites and kimberlites. This suggests the occurrence of a separate damtjernitic melt during the formation of olivine macrocrysts during the formation of the Victoria pipe excluding their generation due to fractionation of aillikite melts.

In addition, K and Na exclusively enter silicate phases in olivine macrocrysts. Due to this, the inclusions studied are different from those in olivines from kimberlites, where the K–Na component enters both silicate and alkaline–carbonate phases [1].

The melt inclusions in spinelids and monticellite from damtjernites of the Victoria pipe, reflecting the evolution of the melt towards the saline orthomagmatic alkaline–sulfate–phosphate–chloride–carbonate fluid/melt are similar in composition to inclusions of alkaline–chloride–sulfate–carbonate composition, studied in olivine and minerals of the matrix of aillikites of the Siberian craton (Table 1), the Chadobets complex [4, 6, 8] and the Arbarastakh complex [5], as well as kimberlites of various occurrences, for example, Udachnaya–Vostochnaya pipe [1].

Such similarity in the composition of inclusions suggests a unified mechanism of evolution of ultramafic alkaline melts, including ultramafic lamprophyres and kimberlites towards alkaline carbonate and salt sulfate–chloride–carbonate liquids.

The loss of the fluid component and the formation of monticellite. The main petrographic feature of the studied samples of damtjernite from the Victoria pipe is a higher content of monticellite, which develops in both the pyroclastic groundmass and the breccia matrix and replaces olivine macrocrysts on the edges (Fig. 1b). In addition to its wide distribution in kimberlites, monticellite is typical for lamprophyres, mainly for melilite-bearing varieties [16, 17]. In ultramafic alkaline lamprophyres, such as aillikites, it occurs sporadically [2], but in some occurrences it can amount to 20 vol % (for example, Tikiusaaq aillikites of West Greenland) [3]. In these samples, monticellite also replaces olivine macrocrysts and intensively develops in the groundmass together with carbonate minerals and phlogopite. However, monticellite is not described as the rock-forming mineral in damtjernites [2]; therefore, such a wide distribution of monticellite in the studied samples from the Victoria pipe is a unique phenomenon and expands the possible mineral composition of damtjernite.

Usually, the formation of monticellite is associated with olivine substitution. Monticellite can form during the reaction of olivine with a predominantly silicate alkaline melt, resulting in the formation of the monticellite–phlogopite association [18] or during the reaction of olivine with a predominantly carbonate alkaline melt, resulting in the development of decarbonatization processes and separation of CO2 [19]. The first mechanism is typical of melilite-bearing ultramafic lamprophyres, first of all alnöites [16, 20]; the second one is typical of kimberlites [19].

The study of melt inclusion shows that the interaction of olivine with derivatives of alkaline–carbonate and saline sulfate–phosphate–chloride–carbonate liquids during subsequent evolution could cause a reaction with the formation of monticellite and degassing processes. Such a scenario is consistent with the almost complete absence of carbonate minerals in damtjernites of the Victoria pipe [13].

CONCLUSIONS

The study of primary and secondary melt inclusions in olivine macrocrysts and groundmass minerals from damtjernite of the Victoria pipe (Anabar diamondiferous region) allowed us to draw the following conclusions about the evolution of the ultramafic alkaline melt:

(1) The primary damtjernite melt has a K–Na carbonate–silicate composition. K and Na in the primary melt inclusions within olivine macrocrysts enter the composition of essentially silicate daughter phases. According to this, they are different in composition from similar inclusions in olivine from aillikites and kimberlites and emphasize the more alkaline character of the damtjernitic parental melts.

(2) At the subsequent stages of melt evolution during pipe formation, Na and K in the studied inclusions not only enter silicate daughter phases but they can also form alkaline phosphates, carbonates, sulfates, and halides. This leads to the formation of alkaline carbonate and saline sulfate–phosphate–chloride–carbonate liquids. According to this, the evolution of the damtjernitic melt is similar to that of aillikite and carbonatite melts and can serve as a uniform mechanism of evolution of ultramafic alkaline melts.

(3) During the subsequent evolution, the interaction of olivine with derivatives of alkaline–carbonate and saline sulfate–phosphate—chloride–carbonate liquids could cause a reaction with the formation of monticellite and degassing processes, which led to the formation of monticellite as a rock-forming mineral of damtjernites. The example of damtjernite from the Victoria pipe allows us to consider monticellite not only as a common mineral for kimberlites and alnöites, but also for ultramafic lamprophyres.

The results of studying melt inclusions in damtjernites of the Victoria pipe are consistent with the data on inclusions in aillikites and kimberlites and determine the specifics and uniqueness of the composition of parental melts and its evolution for feldspar ultramafic alkaline lamprophyres of the Siberian craton and analogical alkaline complexes worldwide.