Mg-Ilmenite from Kimberlites, Its Origin

*Sergey I. Kostrovitsky*

## **Abstract**

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.

**Keywords:** magnesian ilmenite, kimberlite, kimberlite field, kimberlite cluster, macrocrysts, mantle xenoliths, asthenosphere, lithosphere

## **1. Introduction**

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.% [1]. 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 microphenocrysts), whose content varies widely (from 0.1 to 2–3 wt.%]. Less frequently, Ilm occurs in mantle xenoliths [2–4] 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 [9–12]; (III) crystallization from an asthenospheric melt within the lithosphere associated with kimberlite magmatism [4, 13–17]; 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 [18–22].

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 [24], 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 [22]. 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.

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

*Mg-Ilmenite from Kimberlites, Its Origin DOI: http://dx.doi.org/10.5772/intechopen.102676*

**Figure 1.**

*Map showing the location of diamondiferous kimberlite fields in the Yakutian province.*

conventionally named "Haggerty's parabola " after the name of the researcher who first discovered it [8]. 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


### **Table 1.**

*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).*

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.

*Mg-Ilmenite from Kimberlites, Its Origin DOI: http://dx.doi.org/10.5772/intechopen.102676*

**Figure 2.** *MgO vs. Cr2O3 bivariate plots showing the distribution of ilmenite compositions from different diamond-bearing kimberlite fields of the Yakutian province: (a) Mirninsky; (b) Daldynsky; (c) Alakit-Marhinsky; (d) Verhnemunsky.*

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.

### **Figure 3.**

*Different types of distribution of Ilm composition from different pipes from the Alakit-Markhinsky field: (a) "Haggerty parabola" (pipe Yubileynaya); (b) "hockey stick-like" (3 pipes of Iskorka cluster: Iskorka, Svetlaya, Kollektivnaya); (c) "step-scattered" (pipes Komsomolskaya, Sitikanskaya); (d) "stepped" (pipe Druzhba); (e) "inclined", with an inverse correlation between MgO and Cr2O3 (3 pipes of NIIGA cluster, NIIGA, Marshrutnaya,Talisman).*
