Compositional Variations of Spinels from Ultramafic Lamprophyres of the Chadobets Complex (Siberian Craton, Russia)

Ultramafic lamprophyres (UMLs) are mantle rocks that provide important information about the composition of specific carbonate–silicate alkaline melts in the mantle as well as the processes contributing to their origin. Minerals of the spinel group typically occur in UMLs and have a unique “genetic memory.” Investigations of the spinel minerals from the UMLs of the Chadobets complex show the physicochemical and thermodynamic features of the alkaline rocks’ crystallization. The spinels of these UMLs have four stages of crystallization. The first spinel xenocrysts were found only in damtjernite pipes, formed from mantle peridotite, and were captured during the rising of the primary melt to the surface. The next stages of the spinel composition evolution are related to the high-chromium spinel crystallization, which changed to a high-alumina composition. The composition then changed to magnesian ulvöspinel–magnetites with strong decreases in the Al and Cr amounts caused by the release of carbon dioxide, rapid temperature changes, and crystallization of the main primary groundmass minerals such as phlogopite and carbonates. Melt inclusion analyses showed the predominance of aluminosilicate (phlogopite, clinopyroxene, and/or albite) and carbonate (calcite and dolomite) daughter phases in the inclusions that are consistent with the chemical evolution of the Cr-spinel trend. The further evolution of the spinels from magnesian ulvöspinel–magnetite to Ti-magnetite is accompanied by the formation of atoll structures caused by resorption of the spinel minerals.

The UMLs (aillikites and damtjernites) of the Chadobets complex, located in the southern part of the Siberian Craton, were formed during the Permian-Triassic period of magmatic activity on the craton, accompanied by the formation of one of the largest trap basalt provinces, meimechites, kimberlites, and carbonatites. General information on the spinels of the Chadobets complex is presented in several publications [29][30][31][32], where the authors consider the similarity of their compositions to Group II kimberlites (orangeites).
This study presents the macro-and microcomponent compositions of minerals of the spinel group and their inclusions from the groundmass spinels of UMLs (aillikites and damtjernites) of the Chadobets complex to determine the main factors responsible for variations in the composition of spinels and the characteristics of the crystallization process of these rocks.

Geology and Petrography
The Chadobets alkaline complex of UMLs and carbonatites is located in the southern part of the Siberian Craton ( Figure 1). Tectonically, it is confined to the large positive structure of the platform that is represented by the Chadobets dome-shaped uplift. The core of the uplift forms two protrusions, the northern Terina complex and southern Chuktukon complex, and is composed of carbonate-terrigenous Precambrian and early Cambrian sediments [13,30,[33][34][35][36]. The main intrusive rocks of the Chadobets complex are alkaline-ultramafic rocks (pyroxenites, aillikites, and damtjernites) and carbonatites [30]. The first phase of alkalineultramafic rocks forms stocks, dikes, and sills ( Figure 1b). Carbonatites cut the first phase of alkaline rocks. The carbonatites form dikes and sills. Damtjernites consist of explosion tubes, have crosscutting contacts with the earlier alkaline phases, and contain xenoliths of these alkaline phases as well as fragments of sedimentary rocks [30]. The U-Pb ages of the aillikites (perovskite) and damtjernites (zircon) give values of 252 ± 12 and 256.7 ± 1.1 Ma, respectively [29,38,39]. The Ar-Ar and Rb-Sr dates of the aillikites are 243 ± 3 and 241 ± 1 Ma [31].
The UMLs of the first phase of intrusion are aillikites and mela-aillikites and have a porphyritic structure; the proportion of phenocrysts varies from 20% to 50%. Macrocrysts are represented by idiomorphic grains of olivine (up to 20%), completely or partially replaced by serpentine and calcite. In olivines, the #Mg varies within grains, individual crystals, and their microcomponent compositions [40]. Phlogopite phenocrysts (up to 15-20 vol.%) have a zonal structure; often magnesian cores of #Mg 0.78-0.73 are overgrown with a ferrous rim of #Mg 0.47-0.12. The mineral composition of mela-aillikites differs from the aillikites in the presence of hypidiomorphic elongated clinopyroxene grains: diopside with an aegirine minal (up to 10%) [40]). The groundmass of UMLs contains predominant mineral phases of calcite, dolomite, and phlogopite, as well as disseminated micrograins of spinels, ilmenite, Ti-magnetite, apatite, and rare sulfides (chalcopyrite, pentlandite, etc.).
Damtjernites of the third phase of intrusion of the Chadobets complex have porphyritic and usually brecciated structures and differ by the presence of hypidiomorphic feldspar grains of potassium feldspar and albite (up to 10%) in the groundmass, as well as in the pelletal lapilli (more than 60%). The pelletal lapilli of damtjernites contain macrocrysts of olivine, which are completely replaced by serpentine and calcite, and include magnetite, biotite, and a fine-grained aggregate of chlorite, serpentine, calcite, and quartz. The groundmass of the damtjernites contains phlogopite, dolomite, clinopyroxene, potassium feldspar, albite, microphenocrysts of Cr-spinels with a rim of Ti-magnetite, perovskite, and apatite [29,40].

Materials and Methods
The analytical works were performed using the facilities of the Analytical Center for Multi-Elemental and Isotope Research at the V.S. Sobolev Institute of Geology and Mineralogy (Novosibirsk, Russia). The analyses were performed on spinel grains from UMLs. These grains were collected from the exploratory drill cores of the Chuktukon complex and outcrop samples from the Terina riverbanks.
Polished rock samples were prepared for study in reflected light under a polarizing microscope (Olympus BX51) with a photo camera device. The polished rock samples were used to determine the rock textures and mineral assemblage compositions using energydispersive spectrometry in combination with back-scattered electron imaging (BSE) using the TESCAN MIRA 3 LMU JSM-6510LV scanning electron microscope with the energy prefix from X-Max Oxford Instruments for microprobe analysis. Pure cobalt was used as a calibration standard. All samples for inclusion study were hand polished in dry conditions with diamond pastes, using benzene or alcohol to preserve water-soluble phases in the inclusions.
Basic element oxides (wt%) were converted to the number of cations per atom per formula unit (apfu), in accordance with the recommendations in the appendix to Deer et al. (2013) [16]. The proportion of Fe 2 + and Fe 3+ was calculated assuming spinel stoichiometry [41]. In zonal spinels of different composition, the zones were distinguished using contrast during visualization of BSE images, elemental X-ray images, and linear profiles.

Terina Complex Spinels
In aillikites, zonal idiomorphic and subidiomorphic crystals with sizes of 20-100 µm (Figure 2) represent spinels. Cr-spinels are overgrown with Cr-magnetite or Cr-magnetite with magnesian ulvöspinel-magnetite (Mum), and Mum themselves are covered with a rim of Ti-magnetite (Figure 2i-p). Magnetites are also found in the form of separate subidiomorphic grains, 100-300 µm in size, and do not overgrow other types of spinels. Some samples contain atoll-textured spinels. The lagoon between the core and the rim is filled with chlorite, and the rim is composed of Ti-magnetite (Figure 3).  The main components of magnetites are Fe 3+ , Fe 2+ , and Ti (up to 0.2 ph.u.); the contents of Cr, Al, and Mg do not exceed 0.010 ph.u. In terms of the ratio of Fe, Ti, Cr, Al, and Mg, the spinels belong to the Ti-magnetite trend (Trend 2 according to Mitchell (1995)), which is characteristic of Type II kimberlites (orangeites) and UMLs ( Figure 5). The spinel analyses are plotted on a graph showing Fe 3+ /(Fe 3+ + Al + Cr) vs. Fe 2+ /(Fe 2+ + Mg) and on an Al-Fe 3+ -Cr triangular diagram ( Figure 6). In the aillikites, there is a continuous change in composition from Cr-spinel to Ti-magnetite (Trend 3), with a decrease in the amount of Al and Cr and an increase in Fe. In Figure 7, it can be seen that from Cr-spinel to Ti-magnetite, there is a positive Mg-Cr correlation.  In aillikite spinels, the Mn content increases from 0.20 weight percent (wt%) in Crspinel to 0.90 wt% in Ti-magnetite. Variations in Zn (0.02-0.14 wt%) depend little on the contents of the main components (Cr, Al, Ti, Fe 3+ ). The V content is similar (0.10-0.40 wt%) between different types of spinel, but within the groups shows a significant change in content. In Cr-magnetites and Mum, the V content decreases from the center to the periphery of the grain, while the opposite trend is observed in Ti-magnetites. The content of Mn in Cr-spinel remains at the same level (0.25 wt%), while in Cr-magnetite it begins to increase and reaches a maximum in Ti-magnetite (0.52 wt%). Cr and Ni are closely related and behave in a similar manner.
In damtjernites, as in aillikites, the same types of spinels were identified: It is difficult to trace the change in the composition of Cr-magnetite from the center to the edge of the crystal. In contrast to the grains of Cr-magnetites found in aillikites, a thin rim surrounding Cr-spinels represents Cr-magnetites in damtjernites. Based on X-ray elemental maps, it can be concluded that the contents of Cr and Al decrease and Fe 3+ and Ti increase in Cr-magnetites relative to Cr-spinels.

Chuktukon Complex Spinels
In the aillikites, we have identified the following types of spinels: In Cr-spinels, from the center to the edge of the grain, there is generally a decrease in the content of Cr and Fe 2+ with an increase in the content of Al, Ti, Fe 3+ , and Mg. The Cr/(Cr + Al) ratio varies from 0.63 to 0.71.
In BSE images and in X-ray elemental maps, no pronounced boundary between Crspinel and Al-spinel was found. In pleonaste spinel, the contents of Al, Mg were higher than in Cr-spinel.
In Mum spinels surrounding Al-spinels, the rim size is 10-20 µm. The composition of Mum spinels shows an increase in Fe 3+ and Ti and a decrease in Al and Cr relative to the above-described spinels (Figure 8). Most analyzed spinels fall within the Ti-magnetite trend (Trend 2 according to Mitchell (1995)), which is characteristic of Type 2 kimberlites (orangeites) and UMLs ( Figure 5). However, some analyses gave results within the ulvöspinel trend in the evolution of the compositions of spinels from type I kimberlites (see Figure 5). On the Al-Fe 3+ -Cr diagram (Figure 6b), the spinel compositions form a trend somewhat different from that of minerals from the aillikite-damtjernites of the Terina complex since their evolution starts from higher-alumina and high-chromium spinels and ends in the Mum region, not reaching Mag values.
Several types of spinels are present in damtjernites: ( The spinel crystals from damtjernites are zonal. The Cr-spinel core is surrounded by Cr-magnetite. Atoll spinels are common. The atoll lagoon is filled with dolomite, but potassium feldspar lagoons also occur. The core of the atolls is represented by Crspinel, and the rim is Ti-magnetite (Figure 9). Al-spinels have idiomorphic crystals and are edged with Cr-spinels. Al content varies within the range 0.800-0.900 ph.u., Mg within the range 0.620-0.708 ph.u., and Fe 2+ within the range 0.428-0.431 ph.u.
Cr-magnetites are represented by a thin rim surrounding Cr-spinels. Element maps show that Cr-magnetites are enriched in Fe and Ti and depleted in Cr and Al relative to Cr-spinels. In Ti-magnetite relative to Cr-magnetite, Cr and Mn are absent, and the content of Fe and V increases.
On the Al-Fe 3+ -Cr diagram (Figure 6), the spinel compositions form a trend similar to that of the Chuktukon aillikites, differing in the presence of single grains of spinel xenocrystals from peridotites (Xen) and discrete compositional changes from Cr-spinel to Ti-magnetite. The same discrete quality is seen in the diagrams of Ti/(Ti + Cr + Al) vs. Fe 2+ /(Fe 2+ + Mg) and Fe 3+ /(Fe 3+ + Al + Cr) vs. Fe 2+ /(Fe 2+ + Mg) (see Figures 5 and 6).
In damtjernite spinels, the Mn concentration of 0.20 wt% remains almost unchanged from Cr-spinel to Ti-magnetite. Variations in Zn (0.02-0.14 wt%) in the studied minerals depend little on the content of the main components (Cr, Al, Ti, and Fe 3+ ). The V concentration increases from Cr-spinel to Ti-magnetite (0.20 → 0.70 wt%).

Melt Inclusions Study
Primary melt inclusions were found in several grains of Cr-spinels from damtjernites of the Chuktukon complex ( Figure 10). The inclusions are as large as 30 µm and are distributed without visible regularity within the grains. The melt inclusions have a complex morphology and, less often, a negative spinel shape ( Figure 10). They are completely crystallized. Typical daughter phases are phlogopite (Mg # 83-91, BaO 0.7 wt%, TiO 2 2.1-5.5 wt%, F 1.1-2.3 wt%), diopside (Mg # 68-77, TiO 2 3-4 wt%), calcite or dolomite (Mg # 55-77), and apatite. Albite was diagnosed in several exposed inclusions in damtjernites. No ore phases were found in the opened inclusions, most likely due to the crystallization of Crspinel (host mineral) on the walls of the inclusions and, as a consequence, the consumption of Cr, Fe, and Ti from the melt.

Discussion
Primary Cr-spinels in UMLs of the Chadobets complex have a rather high Cr/(Cr + Al) ratio, reaching values of 0.75, bringing them closer to the composition of those in kimberlites (0.75-0.95) and UMLs (0.60-0.95) [27,43,44]. On the Al-Fe 3+ -Cr diagram (Figure 6b), we can observe the main stages of spinel growth (Xen, Chr, Mum, Mag), and the main trends (1 -3 ) of spinel zoning found in aillikites and damtjernites. Most of the spinels of the present study are characterized by significant changes in composition even within a single grain, which suggests rapidly changing conditions at the Chr, Mum, and Ti-Mag stages with insignificant homogenization of the spinel.
The earliest spinel grains, xenocrysts (Xen) from mantle peridotite, were found in the explosion pipes of damtjernites of the Chuktukon complex and were most likely captured during the rising of the primary melt to the surface. Reaction zones around xenocrysts formed because of interactions with the primary melt of UMLs are not pronounced and are not always diagnosed in BSE. The spinels of the aillikites and damtjernites of the Terina complex are characterized by the absence of xenogenic spinels, which could be due to the relatively slower crystallization at higher temperatures compared to the Chuktukon damtjernites ( Figure 6). This could promote homogenization and/or dissolution of the xenogeneic spinels. It is worth noting that the observed variations in the composition of spinels of the Chuktukon damtjernites do not show a reactionary trend in the interaction of xenogenic spinel from peridotite with kimberlite melt (Trend 4) described by [27] (Figure 6). However, some of the spinel analyses fall within the primary chromite (Chr) fields typical for kimberlites [27].
The composition of high-chromium spinels successively changed to a high-alumina composition, and the obtained values fit into alumina Trend 3, identified by Roedder and Schulze (2008) for kimberlites and UMLs ( Figure 6). Roedder and Schulze (2008) argue that Trend 3 is parallel to the Irvine olivine and spinel isopotential curves [46] and is the result of rapid temperature changes and diffusion-controlled crystallization. Petrographic studies and the results of studying inclusions in olivine from UMLs of the Chuktukon and Terina complexes confirm this possibility [40].
Further crystallization of high-alumina Cr-spinels leads to three similar trends (Trends 1 -3 ) that show a significant decrease in the amounts of Al and Cr with a change in composition from Chr to Mum ( Figure 6). This change could be the result of a combination of various factors such as the degassing of magma with the release of carbon dioxide, rapid temperature changes, and crystallization of other minerals in the groundmass.
First, the crystallization of phlogopite in the UMLs of the Terina and Chuktukon complexes could be the reason for the decrease in Al in the melt. A similar tendency with crystallization of early phlogopite is observed in Group II kimberlites [27,42,47]. No less important is the presence of early carbonates in the aillikites and damtjernites of Chuktukon and Terina, which were formed during the crystallization of UMLs. In this case, carbon dioxide, which is an important component in the evolution of melts, could lead to an explosive character during the rapid rise of melts to the surface, among other things. This fact is confirmed by the presence of the damtjernite explosive tube bodies. Therefore, a sharp change in the composition from Chr to Mum (Trends 1 -3 , Figure 6) could be initiated additionally by degassing, which leads to rapid changes in the temperature and composition of the melt.
The mineralogical data on the composition of UMLs of the Chadobets complex are in full agreement with data from studies of melt inclusions in the Cr-spinels of the UMLs. This study of the chemical composition of the crystalline primary melt inclusions showed the predominance of carbonate (calcite and dolomite) and aluminosilicate daughter phases (phlogopite, clinopyroxene, and/or albite). [27] argue that in carbonate-bearing kimberlites formed under relatively oxidizing conditions, magnesium activity remained high during spinel crystallization. This fact, in their opinion, led to an almost constant and relatively low Fe 2+ /(Fe 2+ + Mg) ratio in the spinel of Trend 1 (see Figure 6). In the spinels studied here, with the evolution of their composition from Chr to Mum, the amount of Mg and the Fe 2+ /(Fe 2+ + Mg) ratio change insignificantly (Figure 6), which also confirms the important role of carbonates. Similar examples, where spinel contains an increased amount of magnesioferrite (MgFe 3+ 2 O 4 ) component, include carbonate-containing UMLs in Greenland and Canada [43,44].
Further evolution from Mum to Ti-Mag is accompanied by the formation of atoll structures. There are several points of view on the origin of the atoll texture: (1) the structure of the atoll in spinel is a sign of resorption and is formed when the previously formed spinel dissolves [18,[48][49][50]; (2) the structure of the atoll could develop with supersaturation and rapid cruciform growth of spinel in the corners of the crystal, and the crystal faces did not have enough components to complete their growth [51]; and (3) spinel did not crystallize in the atoll lagoon due to the lack of necessary chemical components and/or the immiscibility of the spinel [27]. We suggest that the atoll texture in the present study is due to spinel resorption. The middle part of the atoll spinel may have an unstable composition as a result of rapid uplift and significant changes in fluid/melt composition at later stages. As a result, upon dissolution and replacement of Mum spinels, lagoons form, followed by the formation of the Mag rim. The mechanism of metasomatic replacement observed in atoll spinel is described in numerous works [52][53][54][55][56][57]. We suggest that replacement occurs at later stages because of fluid seepage in the intergranular space, where gas bubbles are an effective agent for transporting the reacting fluid and elements.

1.
Chemical investigations of the spinel minerals of the UMLs from the Chadobets complex show the following main stages of the spinels' evolution: Xen → Cr-Spl →Mum → Ti-Mag.

2.
The spinel xenocrysts (Xen) were found only in the damtjernite pipes and formed from mantle peridotite. They were captured during the rapid rising of the primary UML melt to the surface. 3.
The composition of high-chromium (Cr) spinels successively changed to a highalumina composition caused by early olivine and spinel co-crystallization. The highalumina spinels were then transformed to magnesian ulvöspinel-magnetites with a strong decrease in the Al and Cr amounts caused by the release of carbon dioxide, rapid temperature changes, and crystallization of the main magmatic minerals in the groundmass, such as phlogopite and carbonates.

4.
Melt inclusion investigation showed the predominance of carbonate (calcite and dolomite) and aluminosilicate daughter phases (phlogopite, clinopyroxene, and/or albite) that are in full agreement with the chemical evolutional Cr-spinel crystallization trend.

5.
Further evolution of the spinels from Mum to Ti-Mag is accompanied by the formation of atoll structures caused by spinel resorption.