Average, the interval of variation in the composition of Mg-ilmenite from different diamondiferous fields of the Yakutian province (in parentheses - the number of analyses).
The main regularities of the saturation of kimberlite rocks with the accessory mineral Mg-ilmenite (Ilm), the peculiarities of the distribution of Ilm compositions in individual pipes, in different clusters of pipes, in diamondiferous kimberlite fields, are considered as the example of studies carried out within the Yakutian kimberlite province (Siberian Craton). Interpretation of different crystallization trends in MgO-Cr2O3 coordinates (conventionally named “Haggerty’s parabola”, “Steplike”, “Hockey stick”, as well as the peculiarities of heterogeneity of individual zonal and polygranular Ilm macrocrysts made it possible to propose a three-stage model of crystallization Ilm: (1) Mg-Cr poor ilmenite crystallizing from a primitive asthenospheric melt; (2) Continuing crystallization in the lithospheric contaminated melt by MgO and Cr2O3; (3) Ilmenite subsequently underwent sub-solidus recrystallization in the presence of an evolved kimberlite melt under increasing oxygen fugacity (ƒO2) conditions.
- magnesian ilmenite
- kimberlite field
- kimberlite cluster
- mantle xenoliths
Ilmenite proper, corresponding to the chemical formula FeTiO3, often forms a series of solid solutions with isostructural minerals - heikilite MgTiO3, pyrophanite - MnTiO3, hematite - Fe2O3. Along with Mg2+, Mn2+, Fe3+, ilmenite can contain isomorphic impurities of Al, Cr, Nb, V, etc. There is a geochemical specialization of impurity elements in ilmenite, depending on the type of rocks. For example, ilmenite from basic rocks is characterized by the presence of V, Cr, Co, Ni. In ilmenites from kimberlites, there is an increased content of Cr, Al, Nb, Zr. A typical impurity for ilmenites is Mg, while the MgO content can reach up to 20 wt.% . Ilm with MgO content >6 wt% is commonly referred to as Mg-ilmenite. The existence of a continuous series FeTiO3-MgTiO3 is assumed. Isomorphic impurity Fe2O3 occurs according to the scheme of heterovalent isomorphism Fe3+ + Ti4+ ↔2F3+.
Mg-ilmenite (Ilm) is an important kimberlite indicator mineral, which is widely used in diamond exploration to identify primary deposits. In kimberlites, Ilm forms discrete monomineralic grains (i.e., megacrysts, macrocrysts, and micro-phenocrysts), whose content varies widely (from 0.1 to 2–3 wt.%]. Less frequently, Ilm occurs in mantle xenoliths [2, 3, 4] and in Ilm-Prx intergrowths [5, 6]. Due to the fact that syngenetic mineral inclusions of olivine, clinopyroxene, and garnet in Ilm macrocrysts are extremely rare, it is difficult to elucidate the genesis of Ilm. That is why the issues of the occurrence of ilmenite, its mantle sources, and its genetic connection with kimberlite melt continue to be discussed. Potential origins of Ilm macrocrysts and megacrysts include: (I) the disaggregation of Ilm-bearing lithospheric mantle lithologies [1, 7, 8]; (II) crystallization within the asthenosphere [9, 10, 11, 12]; (III) crystallization from an asthenospheric melt within the lithosphere associated with kimberlite magmatism [4, 13, 14, 15, 16, 17]; a modern take on this previous model is (IV) formation in a “metasomatic aureole” surrounding the (proto-) kimberlite melt and or previous pulses of failed (proto-)kimberlite melt, alongside other megacryst suite minerals and sheared xenoliths [18, 19, 20, 21, 22].
This section of the book is a compilation of two published articles [23, 24], written on the basis of a study of the representative collections of Ilm collected by the author. Before proceeding to the presentation of our model of the origin of Mg-ilmenite , let us consider different trends of crystallization Ilm in MgO-Cr2O3 coordinates, which are characteristic of individual pipes, pipe clusters, and diamond-bearing fields, as well as the peculiarities of the heterogeneity of the composition of individual zonal and polygranular Ilm megacrysts.
2. The composition of Mg-ilmenite
Mineralogical assessment of most kimberlite pipes in the four diamond fields of the Yakutian kimberlite province (YaKP) (Figure 1) provided the author with an opportunity to study the compositions of Ilm macrocrysts. A representative number of both the studied pipes (94) and the Ilm macrocrysts (11,003) were studied. Microprobe analyzes were carried out at the Central Analytical Laboratory of the Botuobinskaya Geological Survey of ALROSA on a Superprobe JXA 8800R. Repeated attempts by researchers [1, 25, 26] to reveal the compositional features of Ilm from individual fields were unsuccessful. The reason for the failure lies in the fact that the researchers focused on the comparison of statistical parameters of the distribution of the composition. Consideration of the trends in the variability of the Ilm composition is much more informative. Here we predominantly focus on bivariate plots MgO-Cr2O3, since these coordinates are the most informative for demonstrating differences between Ilm from various fields, clusters, and pipes [4, 7, 27]. It is important to note that the average composition of Ilm and its MgO-Cr2O3 distribution does not vary with sampling depth, or with the textural type (i.e., unit) of kimberlite within a single pipe. Therefore, the composition of Ilm is an invariant characteristic unique to a given kimberlite . A comparison of the Ilm compositions from different fields indicates that their common feature is (Table 1) their fairly consistent homogeneous composition. With wide variations in the content of the main Ilm oxides from different fields, with the exception of the Mirninsky field, they are characterized by a very similar average composition. The Mirninsky field kimberlites contain higher-Fe Ilm, with a higher content of the hematite (Fe2O3) component.
|Mirninsky (1600)||Daldynsky (4171)||Alakit-Marhinsky (4634)||Verhnemunsky (598)|
Despite the closeness of the average oxide contents (in three fields), Ilm from each field shows completely different distributions of the composition points in the MgO-Cr2O3 plots (Figure 2). Below we provide a brief description of them.
Mirninsky field, despite the small number of pipes (only 9), is one of the most productive - 5 pipes (Mir, Internatsionalnaya, 23 KPSS, Dachnaya, Taezhna) belong to the diamond deposits. The distribution of the composition points of the Ilm composition on the MgO-Cr2O3 graph (Figure 2a) resembles the type of distribution, which is conventionally named “Haggerty’s parabola “ after the name of the researcher who first discovered it . The clearest and most numerous group of composition points belongs to low-Cr Ilm (<0.5 wt% Cr2O3) with a variable MgO content, covering almost the entire range of its variation. Other groups of points of composition, corresponding to low-Mg and high-Mg Ilm with variable content of Cr2O3, form two branches of the parabola on the graph and generally demonstrate a scattered type of distribution.
A feature of the MgO-Cr2O3 plot for the Daldynsky field (Figure 2b) is the presence of three distinct groups of composition points that show no or weak correlation between oxides. The presence of three Ilm groups in terms of Cr2O3 content is found for most of the Daldynsky field pipes, but not for all. For example, Ilm from pipes of the Dalnya cluster are characterized by a unimodal distribution of Cr2O3 content.
The Alakit-Marhinsky field also consists of more than 60 pipe and dike bodies. But the Ilm compositions (707 analyzes) were studied from only 12 pipes, which is due to the higher-Mg composition of kimberlites in this field and, accordingly, the limited number of pipes containing Ilm. The overall plot for the entire field in MgO-Cr2O3 coordinates (Figure 2c) reflects the overlap of different distribution types, which are demonstrated by the plots for different pipes (Figure 3a–e). In general, the MgO-Cr2O3 plot for Alakit-Marhinsky field is peculiar in the form of individual clusters of points of composition and certainly differs from the corresponding graphs from other diamondiferous fields.
There are 16 known kimberlite pipes in the Verhnemunsky field. The database characterizes the compositions of Ilm from most of the pipes in this field and includes 513 analyzes. A distinctive feature of the field (Figure 2d) is the presence in each pipe of the low-Mg group Ilm (6.5–8 wt% MgO), which demonstrates the change in the MgO content at constant Cr2O3. Ilm with a MgO content of more than 8 wt% are characterized by a scattered type of distribution in the coordinates MgO-Cr2O3, reflecting wide variations in the composition of the mineral.
The Daldynsky field, in which about 60 kimberlite pipes were discovered, was studied for most of the pipes, and therefore the author classified it as an etalon field, namely, thanks to the studies of this field, the most important conclusions about the origin of Ilm were made. Table 2 reports the most representative average compositions of Ilm grains, in terms of the oxides TiO2, MgO, Cr2O3, and FeOtotal, from pipes in the Daldynsky field (4171 analyses of Ilm). It is evident that Ilm from different pipes of the same cluster yield similar values, while Ilm from different clusters have a significantly different composition. For example, Ilm from the pipes of the Malyutka and Zarnitsa clusters have closely similar MgO abundances, though they differ markedly in Cr2O3 content. Ilmenite from the Dalnya, Leningradska, and Dolgozhdana clusters display similar MgO contents, but are different in Cr2O3 and so on. Some clusters of pipes demonstrate local heterogeneities in Ilm composition. For example, in the Yakutska cluster, closely located pipes (Akademicheska and Aeros’emochna on the one hand, and Yakutska and Ilmenitova, on the other hand), exhibit very similar Ilm compositions. Figure 3 presents a plot of the average contents of MgO and Cr2O3, showing the proximity of compositions of Ilm from different pipes of the cluster and the differences in Ilm compositions between different clusters. As a rule, the points of average Ilm composition from pipes of one cluster are grouped near each other.
|Cluster of pipes||N||Pipe||Number analyzes||Average composition (in wt.%)|
|Bukovinska||11||Jila-75||lmenites are absent in all high-Mg kimberlites pipes of the Bukovinska cluster|
It is evident that the Daldynsky field is characterized by regional heterogeneity along with a clustered distribution of Ilm compositions. The highest Mg content and low Cr2O3 content are found in Ilm from pipes in the southern part of the Daldyn field (Dalnya, Leningradska, Yakutska clusters, Figure 4), while the northern part of the field predominantly contains clusters of pipes (Zarnitsa, Letnya, and Malyutka) with low MgO and high Cr2O3 Ilm (Table 2). By combining the MgO–Cr2O3 plots with histograms of Cr2O3 content (Figures 5 and 6), we can clearly identify significant differences in the distribution of Ilm compositions between different pipes. The histograms of Ilm composition in the Daldynsky field show different types of distribution: (1) unimodal, e.g. pipes of the Dalnya (Figure 6) and Leningradska clusters; (2) bimodal, e.g. pipes of the Yakutska and Rot-Front clusters, according to MgO content; (3) tri-modal, with distinct minima dividing the analyzed Ilm grains into three separate groups, e.g. pipes of the Zarnitsa (Figure 5) and Malyutka clusters. While the first type of distribution is dominant for Ilm from the southern part of the Daldyn field, the third type is essentially characteristic of pipes from the northern part of the field.
3. The heterogeneous composition of Mg-ilmenite macrocrysts
Most Ilm grains are heterogeneous, with rims enriched in MgO. The Ilm zonal megacrysts are illustrated in Figures 7 and 8, in which the rims are enriched with MgO. The gradual change in the primary composition of Ilm is especially clearly observed in polygranular megacrysts from the Mir pipe (Figure 9). Individual granules ranging in size from 100 μm to >1 mm, separated by microcracks, demonstrate a compositional change towards an increase in MgO content from the center to the edge (Figure 9a,b,e
|Figures||Figure 7||Figure 8||Figure 9A|
|Figures||Figure 9a||Figure 9b|
|Figures||Figure 9c||Figure 9d||Figure 9e|
4. Ilm-bearing sheared peridotites
Compositional features of minerals from Ilm-bearing deformed peridotites provide important arguments for developing a model of Ilm genesis in kimberlites. The similarity of the compositions of olivine, garnet, and Mg-ilmenite from deformed peridotites and the corresponding megacrysts from kimberlite rocks [28, 29, 30] indicates a genetic relationship between them. We present a brief description of the petrographic and geochemical features of Ilm-bearing deformed lherzolite (sample 00–83) from the Udachnaya-Eastern kimberlite, described earlier in the article by Solov’eva et al. .
Sample 00–83 is an Ol-Phl-Ilm sheared peridotite xenolith with a fluidal porphyroclastic texture. This sample contains ∼30 vol% olivine (Ol), ∼40 vol% phlogopite (Phl), and ∼ 30 vol% Ilm. Ol predominantly occurs as small (0.05–0.2 mm) euhedral neoblasts, but rare anhedral porphyroclasts (< 1 mm) are present. Laminar Phl porphyroclasts (< 2 mm) are deformed. Ilm occurs as thin lenses (≤0.5 cm wide and ≤ 4 cm long; Figure 11), which have polygranular textures (Figure 9a). In a similar manner to the polygranular Ilm macrocrysts, titaniferous-magnetite has precipitated in the interstitial space between Ilm granules and at the grain margins of Ilm lenses. Ilm from this xenolith is characterized by wide variations in MgO contents (8.6–12.5 wt%) with relatively constant Cr2O3 contents (i.e., 2.6–2.9 wt% –Table 3). Ar40/Ar39 dating of Phl from this sample yields an age of 367.1 ± 1.4 Ma , which overlaps the age of host kimberlite (i.e., 367 Ma, ).
5. Arguments for model of Mg-ilmenite crystallization
5.1 An asthenospheric and lithospheric source for kimberlites, and their megacryst suite
The similarity of Rb-Sr, Sm-Nd, and Lu-Hf isotope systematics, the same age of formation [11, 15, 16, 18, 28, 34] for kimberlites and low-Cr megacryst association of minerals (to which Ilm belongs) testify to a single primary asthenospheric source for them. The similar or almost identical compositions of Ilm in different pipes of one cluster can be accounted for by the existence of a common magmatic supply channel. Various clusters of pipes were fed via different channels of ascending kimberlitic melt, which therefore disintegrated and assimilated different mantle rocks. In the Orapa A/K-1 pipe, the Cr2O3 content of Ilm has been shown to be independent of the variation of other oxide components . Two groups of Ilm are recognized in this pipe, with average Cr2O3 contents of 1.91 and 3.62 wt%, whereas the content of MgO remains virtually constant. The Ilm nodules from the same pipe, although showing discrete zoning in MgO and Fe2O3, are found to have homogeneous Cr2O3 contents. Ilm from the Monastery pipe (South Africa) can be divided into three groups  based on Cr2O3 and Nb contents, while they demonstrate the same trend in terms of major components. Thus, this feature of the behavior of Cr2O3 in Ilm is common for different kimberlite pipes. Moore et al.  suggested that there was a mixing of magmas or assimilation of host rocks in the magma chamber during Ilm crystallization. We suggest that the assimilation of lithospheric mantle rocks by the kimberlite melt might have occurred in the supply channel of kimberlite pipes. It appears that this peculiarity did not originate in the asthenosphere, but rather in the different channels and modes of ascent of the kimberlite melt, which led to the formation of the various clusters of pipes.
5.2 The presence and formation of Ilm/oxide melts
The presence of large Ilm megacrysts (up to 4 cm), their abundance (up to 3% of the total rock volume), sometimes found in pipes, and, finally, the existence of veinlets, Ilm lenses in deformed lherzolites (Figure 11)–all this indicates the existence of a melt Ilm composition. A number of researchers refer to the presence of such melts [18, 36, 37, 38, 39]. The appearance of Ilm melts, judging by the veinlets in deformed lherzolites, is recorded at depths corresponding to the boundary between the asthenosphere and lithosphere. The liquidation of a high-Ti melt corresponding in composition to Ilm in the initial asthenospheric melt was caused by deformation processes and a change in the PT parameters during its ascent. We assume that the latter initiated the formation of deformed lherzolites and the ascent of the asthenospheric melt. Ilm crystallization from the kimberlite melt continued to the later stages of ascent and possibly during and after kimberlite emplacement into the upper crust, as indicated by the presence of small groundmass Ilm [40, 41].
5.3 The model of Ilm crystallization
It is commonly argued that fractional crystallization is the primary mechanism responsible for the formation of composition trends in minerals of the Cr-poor megacryst suite [3, 4, 27, 41, 42]. Geochemical data, as well as petrographic constraints (e.g., the abundance of Cpx inclusions in Ilm macrocrysts and Ilm-Cpx intergrowths), indicates that Ilm and Cpx were the final phases of Cr-poor megacryst suite to crystallize [1, 38]. However, the Ilm composition distributions considered above using the example of MgO-Cr2O3 plots showed that they cannot be readily explained by a process of fractional crystallization.
The features of the composition distribution of Ilm macrocrysts considered above, the heterogeneity of the composition of both individual macrocrysts (Figures 2–6) and polygranular megacrysts (Figure 9) were the basis for distinguishing three stages of Ilm crystallization, which occurred at the level of (1) asthenosphere (in the primary asthenospheric melt); (2) the lithosphere (in the melt, which changed its composition as a result of the capture and partial assimilation of rocks by the mantle lithosphere) and (3) the lithosphere and crust (as a result of changes in P–T–O crystallization parameters during ascent through the lithosphere and crust.
In the first stage, crystallization of minerals of the mega crystal low-Cr association of minerals took place, including Ilm. It is assumed that the leading crystallization mechanism was fractional crystallization. At the same time, Ilm and Cpx crystallized last, after Grt, Ol, and Opx.
The second stage of crystallization of Ilm occurred in a melt enriched in MgO and Cr2O3 (as a result of the assimilation of rocks of the lithospheric mantle), which was reflected in the corresponding graphs by the formation of the left branch of the Haggerty parabola (Figure 3a).
During the third stage, recrystallization of macrocrysts occurred as a result of an increase in fO2 of the kimberlite melt as it ascended through the upper horizons of the lithosphere. This stage is reflected in the formation of heterogeneity in the composition of individual grains. Recrystallization of Ilm led to a decrease in the content of FeO and MnO with a corresponding increase in the content of MgO. Since the content of Cr2O3 remains unchanged, these changes in the composition are reflected in the plot of MgO and Cr2O3 by the formation of the right branch of the “Haggerty parabola”. All three stages of Ilm crystallization occurred in different pipes (pipe clusters) in different ways, which is primarily due to a different section of the lithospheric mantle, with a different set of trapped and assimilated rocks of the lithospheric mantle. The formation of other Ilm compositional distribution patterns (e.g., “Steplike”, and “Hockey stick”) is attributed to different compositions of the entrained and partially assimilated lithospheric mantle material, and different ascent dynamics in each of the different kimberlite conduits (which were different for each different kimberlite cluster). Similar Ilm compositional distributions are also typical of other kimberlite provinces worldwide, and we infer that Ilm’s three-stage crystallization model is responsible for these compositional distributions in all cases [4, 7, 8, 9, 27, 37]. These compositional features are attributed to the existence of a single magmatic conduit feeding all pipes of a given cluster, and different conduits feeding different clusters. Proto-kimberlite melt compositions evolved separately in each cluster (conduit) by the incorporation and partial assimilation of trapped fragments and minerals of the lithospheric mantle rocks.
Summing up, we come to the conclusion that the differences in Ilm compositions in individual pipes, pipe clusters are due to a different set of trapped and partially assimilated mantle xenoliths, or local heterogeneity of the lithospheric mantle. And thus, the similarity of Ilm compositions in the pipes of a particular kimberlite field can serve as a key to deciphering its structure (that is, identifying pipe clusters).
The author thanks the geological management of AK ALROSA for creating favorable conditions during the fieldwork, for financial assistance. The author is grateful to A.S. Ivanov and L.F. Suvorova for the microprobe analyses.
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