Eclogite Varieties and Their Positions in the Cratonic Mantle Lithosphere beneath Siberian Craton and Archean Cratons Worldwide

Abstract

The pressure-temperature (PT) conditions and position of different groups of eclogites in the sub-cratonic lithospheric mantle (SCLM) worldwide were established using clinopyroxene Jd-Di and garnet thermobarometry. Beneath Siberia, Fe-eclogites found within the 3.0–4.0 GPa formed in Early Archean times. In the Middle and Late Archean, eclogites were melted during and after subduction. High-Mg eclogites (partial melts or arc cumulates) are related to low-T (LT) geotherms. Melt-metasomatized eclogites trace a high-temperature (HT) geotherm. Eclogitic diamond inclusions from Siberia mostly belong to the middle SCLM (MSCLM) part. Ca-rich eclogites from Precambrian Indian kimberlites are located in the MSCLM. In Phanerozoic time, they were located in the lithosphere base. In Proterozoic South Africa, Ca-rich eclogites and grospydites occur within 4.0–5.0 GPa and HT eclogite and diamond inclusions from the Premier pipe trace a HT geotherm at depths of 7.0–4.0 GPa, showing an increase in Fe upwards in the mantle section. Similar trends are common for eclogites worldwide. In the Wyoming craton, kimberlites captured eclogite xenoliths from the 4.0–2.5 GPa interval. Mantle eclogites have clinopyroxenes and garnet trace element patterns with high (La/Yb)_n determined by KDs with melts and are magmatic. Flatter and bell-like REE patterns with Eu anomalies, HFSE troughs, and U and Pb peaks, are common for clinopyroxenes from MORB-type “basaltic” eclogites. High-Mg eclogites show less fractionated incompatible element branch in patterns. LILE-enrichments and HFSE troughs are typical for kyanite-bearing eclogites. Clinopyroxenes from diamond-bearing eclogites show lower REE, troughs in Nb and Zr, and peaks in Pb and U concentrations, compared to barren eclogites with round smooth trace element patterns and small depressions in Pb and Ba.

1. Introduction

1.1. Eclogite Mantle Xenoliths

Mantle eclogites are extremely important objects from the viewpoint of geodynamics [1,2,3,4,5,6,7,8,9,10,11,12,13,14] and petrology [15,16,17,18]. Different eclogite varieties have been found in orogenic collision zones [19,20], in ancient circum-cratonic belts [21,22], and as xenoliths in kimberlites and other deep-seated magmas [23,24,25,26,27,28,29,30,31,32,33,34,35,36].

Most of eclogite mantle xenoliths are bi-mineralic with the addition of the various mineral like orthopyroxene, rutiles, apaptites, ilmenites, sulfides, etc. and opposite to the orogenic eclogites they mostly display magmatic structures.

The geochemistry and isotopic features of the eclogitic xenoliths in kimberlites allow us to decipher the properties of Archean rocks and to reconstruct the geochronology of ancient magmatic events [13,14]. Most of them are suggested to be subducted MORB oceanic crust [2,3,10,15]. Eclogite xenoliths have been suggested to be derivates of tonalite–trondhjemites [30,31] or Mg-tholeiites [32,33] or metapelites [34,35]. Several eclogite types show hybrid signatures [36,37] suggesting interaction of melted eclogites with peridotites [38,39,40].

1.2. Eclogite Thermobarometry

To determine the PT eclogite conditions of origin or re-equilibration is very important for petrology. For the relatively low pressures to 2.5 GPa, this can be solved by the “Thermocalc software” [41] and some polymineral barometers [42,43,44,45] and monomineral barometers based on the NaAl- -Ca-Mg exchange with the corrections for other components Fe, Ti, Cr, K, Si, Fe³⁺ [46,47].

Re-calibration of the garnet–clinopyroxene barometer [45] gave satisfactory results for about 50 experimental runs in the eclogite system but revealed rather high dispersion in PT estimates for the Dharwar craton eclogite xenoliths [48] and depends on the quality of the analyses of the SiO₂ in clinopyroxenes. Introducing the Fe³⁺ equation (Supplementary Material File S1), we recalibrated the Jd-Di thermobarometer [47] using >710 experimental runs including >500 for Al-high clinopyroxenes (>5 wt.% Al₂O₃). It allows working in peridotitic, pyroxenitic and different eclogitic systems (Figure 1). The garnet eclogite barometry gave estimates close to those determined from garnet and omphacites together [45]. The equations and correlations of methods (Figure 1) (Supplementary File S1), used experimental runs (Supplementary File S2) and PT program (Supplementary File S3) give the information about methods. Using the enhanced version of barometers, we obtained more detailed PTX diagrams using the data for eclogitic xenoliths and eclogitic diamond inclusions, adding new data for Udachnaya [49,50,51] and previous publications [1,2,3,52,53,54,55,56,57,58,59,60], and published data for xenoliths from the Mir [61,62,63,64,65,66], Komsomolskaya [67,68,69,70,71,72], and Sytykanskaya [73,74,75] kimberlite pipes from Siberia [76,77]. We also revised the PT conditions using data for eclogites from the Wyoming craton (USA) [78,79], Slave craton [4,5,6,7,80,81,82], the eastern part of the Dharwar craton in India [69,83,84], Angola [85,86,87], and several localities from South Africa [18,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110], to estimate the position of different types of eclogites from different groups varying in K, CaO, Na₂O, Al₂O₃ and FeO content to recognize the layering of the sub-cratonic lithospheric mantle (SCLM) beneath different cratons and investigate the primary origin of eclogites.

We also analyzed new data from concentrates and used published analyses from Roberts Victor [4,95,96,97,98,99] and added new published data from Premier [85,90,100,101], Orapa [15,106], and other pipes.

2. Analytical Methods and Data Sets

In this study, we used original data derived from analysis of epoxy mounts by electron microprobe microanalysis (EPMA) in the Analytic center of the IGM SD RAS using CamebaxMicro, JSX8100, and Jeol 8320 spectrometers in the Analytic Center of Sobolev V.S. Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia (standard conditions) using 15 kV acceleration voltage and 15 nA beam current. Minerals and rocks were analyzed according to the standard procedure [114]. The relative standard deviation did not exceed 1.5%; the precision was close to 0.02–0.01 wt.% for minor elements. More precise analyses (to 0.005 wt.%) were made in thin sections in Vienna University using the Cameca 100SX and methodology described in previous publications [58].

Trace elements for eclogites from Udachnaya, Sloan, KL4 (Wajrakarur), and Obnazhennaya (see Supplementary File S4) were analyzed by the LA-ICP-MS method using a Finnigan Element mass spectrometer and laser ablation system Nd YAG: UV NewWave in the Analytic Center of IGM SB RAS.

The LA-ICP-MS analyses for the samples from the Sytykanskaya and Komsomolskaya pipes were analyzed in Nikolaev’s IIC SB RAS using an ICAP Q (Thermo Scientific) mass spectrometer with the Nd UV laser NWR 213 (New Wave Research) (analyst N.S. Medvedev). The standards were NIST 610-612 SRF. We introduced for the secondary standard garnets and clinopyroxenes, sample 313-73, analyzed by solution ICP MS in MRAC Belgium [115].

In addition, we included the data sets from previous publications for the Udachnaya pipe [1,2,3,49,50,51,52,53,54,55,56,57,58,59,60], Mir pipe [61,62,63,64,65,66], Komsomolskaya [67,68,69,70,71,72], and Sytykanskaya pipe [73,74,75].

3. Subdivisions of Mantle Eclogites

The eclogites were divided into three (four) large groups instead of three groups (A,B,C,) based on MgO–Na₂O variations in clinopyroxene [110]. As was done in previous studies [68,75,76,77], we add the high Ca–Al group D that often contains kyanites. Several more subgroups can be identified according to their positions on the PTX diagrams and trace element signatures.

1. The high-Mg eclogites (Fe# = 0.07 – 0.19) (Group A) consist of several sub-groups: A1) a Cr-bearing group formed after crystallization of partial melts in mantle produced by volatile fluxes or heating [4,6]; they are close to garnet websterites. It is close to a group formed by hybridization of subduction-related melts and fluids with mantle peridotites [80]. A2) a low-Cr group, which could be restites [116] or deep cumulates from tonalite–trondhjemite magmas or Mg-rich arc magmas [30]. A3) a group derived from the protokimberlite melts [62,85] hybridized with the mantle peridotites at the lithosphere base.

2. The largest group (Group B) with Fe# (= Fe/(Fe + Mg) atomic units) ~0.20–0.30, moderate Al₂O₃ and Na₂O values, are suggested to be subduction-related eclogites; they commonly reveal Eu anomalies. The most abundant rocks from Group B1 are interpreted as subducted basalts and their modifications as well as eclogitized metagabbro (with the relics of layering and structures), close to MORB [2,3] or essentially reacted with oceanic water [117]. The high temperature (pressure varieties from the lower SCLM part) are “metabasaltic” of re-melted eclogites by hybridization with the protokimberlite and other plume melts [83]. Group B2—enriched type of group B2 eclogites—are thought to be products of the differentiation and crystallization of eclogite partial melts during their migration upward in the SCLM. Group B3 eclogites may be interpreted as shallow mantle Gar–Cpx cumulates derived from plume or ancient arc and basaltic magmas in cratonic margins [116]; the eclogites located near Moho. They may be also eclogitized lower crustal cumulates.

3. The high-Fe-Na Group C eclogites (Fe# > 0.27) may be subducted Fe-basalts or Na-rich spillites (Group CI); Ca-enriched varieties may be meta-tonalites or trondhjemites (Group C2) [31] and those which are very rich in Al could be metasediments (greywackes and metapellites) [19] (Group C3).

4. High-Ca-Al Group eclogites were divided into: Group D1 rocks high in Ca and low-Fe varieties, commonly Al-rich and kyanite-bearing (sometimes with coesite) (grosspydites), which may be originally carbonate metasomatites [99] or metapellites [75]. Group D2 eclogites are high-Ca and moderate-Fe and may be ancient Mg-granites [88,92]. The D3 groups are rare AlCa-rich eclogites found in the USCLM.

Many investigators consider most mantle eclogites to be of subduction origin [2,13,14,100,101,102,103,104,105,106,107]. Moreover, they have concluded that most eclogites are close to MORB compositions. In fact, subducted slabs are layered and consist of peridotites, lower cumulative pyroxenites, gabbro and upper ophitic gabbro, which are variable in bulk rock compositions especially in Al₂O₃, CaO, Na₂O, and MgO. Authors have reconstructed bulk compositions and trace element patterns of the common bi-mineralic eclogites often receive nearly flat REE patterns, and the spider diagrams are interpreted as primary tholeitic [17,18,76,89]. Nevertheless, experimental results evidence that eclogites should be melted and essentially transformed during subduction and upwelling [118,119]. Eclogites commonly showing the range of the Fe# ~0.1–0.40 for clinopyroxenes were used for the classifications show clustering in diagrams and should be of different origin. The Cpx with Fe# 0.08–0.15 (GrA) should crystallize from partial mantle melts. The Cpx in their Fe# are close to that of bulk rock of eclogites, the variation due to temperature distributions between Cpx and Gar [112] are not high.

The melts equilibrated with Archean olivines having Mg 0.05–0.08 [120] have Fe# ~0.18–0.27 using the melt–solid partition coefficient ~0.33 for Fe [109] and corresponds to the komatiites, picrites, and Mg-basalts or andesites, and other primitive mantle magmas. Consequently, they may not be analogous of the Paleozoic MORB basalts.

Only Cpx, with Fe# ~0.35–45, are close to the Phanerozoic MORB magmas (GrB2).

There is suggestion that eclogites with Al-high omphacites and Fe# 0.15–0.25 could be cumulates or restites of arc Archean magmas [30]. The higher values could already refer to subducted basalts of different types or Mg-rich crustal rocks.

We put compositions of clinopyroxenes from the high pressure experimental work, which we used for calibration of clinopyroxene and garnet universal thermobarometers [47] (Figure 2A), and the compositions of studied clinopyroxenes and worldwide eclogites on variation diagrams (Figure 2A,B). In addition, the compositions of clinopyroxenes obtained in experimental works used for the calibration are shown in the Supplementary File S2. Group B dominates the middle part of the diagrams (Figure 2). The Ca-rich varieties are represented by eclogites from India, which are low-Fe and Al-rich.

4. Distributions of Different Eclogite Groups in the Lithospheric Mantle Beneath Siberia

4.1. Udachnaya Pipe

The distribution of eclogites in the SCLM beneath Udachnaya (Figure 3A) is typical for xenoliths from other Upper Devonian pipes in Yakutia. We separated eclogites from pyroxenites, which commonly contain relatively low-Al and Na-augites or Cr-diopsides.

The PT plot of Udachnaya based on our and published analyses [1,2,3,49,50,51,52,53,54,55,56,57,58,59,60] of the SCLM shows very wide ranges in pressure and temperature divided into several steps in pressure formed by intercalations of hotter and colder parts in the mantle section (Figure 3A) [47,69,70,71,72,73]. Eclogites lie on both hot and cold geotherm branches. The Mg-rich eclogites of GrA1 (with Mg pyroxenites) reflect mainly low temperature conditions. The GrA1 in the lower SCLM partly coincides with the precipitates from protokimberlites. These are Ti-rich associations close in high PT conditions for ilmenites and sheared peridotites [58,121].

The Mg-rich GrA2 pyroxenites also mostly belong to the middle SCLM level including diamond-bearing varieties, which lie on the relatively cold branch of the geotherm, marked by pyrope inclusions in diamonds. This group divides into two trends according to the CaO and Al₂O₃ concentrations.

Group B eclogites, together with the Ti-bearing pyroxenites, follow the heated geotherm branch in the upper SCLM (USCLM) part (GrB1). Many diamond-bearing eclogites in the latter group show signs of reaction with protokimberlites [32,54] in the LSCLM near its base. They form typical magmatic trends of ascending Fe# with decreasing pressure trend (AFDP). The Fe-eclogites (GrB2) belong to the 3.0–3.5 GPa interval and corresponds in pressure to the pyroxenite layer.

Kyanite grospydites (GrD) belong to the 3.0–6.0 GPa pressure interval where they are intercalated with pyroxenites in a pyroxenite layer [101], but they are abundant also in the 5.5–6.5 GPa interval. The relatively low-pressure estimates correspond to the GrD2.

There is rather sharp boundary at 5.0–5.5 GPa, which is shown by abundant low-T Mg-rich eclogites, located just above the hottest sheared peridotite ilmenites, and ilmenite eclogites (pyroxenites) with Fe# = 0.11–0.515 marking the ascent path of differentiating protokimberlite melts.

4.2. Mir Pipe

The geotherm (Figure 3B) determined by the Opx–Gar method [125] and using garnet [47] thermobarometry for eclogites from the Mir pipe is relatively low temperature (LT) near the lithosphere base, e.g., showing 1000 °C at 6.0 GPa. It cuts all conductive geotherms in the upper part and marks the advective PT path starting from the pyroxenite layer located at 3.5–4.0 GPa.

Most diamond-bearing eclogites [65,66] of GrA–GrB–GrC trace the diamond-graphite boundary [95] and reflect rather high temperature (HT) conditions. The location of a rather large group above this may be explained by data after [96], which gave this transition ~2–5 GPa higher; it is shown on the diagram approximately by the magenta line at 1.5 GPA above the line [123] determined by Mg-rich clinopyroxenes. The GrB1 eclogites form a dense cluster (5–6 GPa) near the lithosphere base and are slightly more heated than xenoliths and garnet geotherm. The low-Cr varieties form the cloud in the P–Fe# diagram in the 3.5–4.5 GPa interval. For GrB–GrC–GrD eclogites, there are several AFDP trends which characterize the paths of rising and differentiating melts. The eclogites of Ca-rich GrD are found within a rather wide pressure interval but mainly near the lithosphere asthenosphere boundary (LAB) and in the middle part of the mantle section. The points for the Fe- and Al-rich GrD form a cloud within the 3–5 GPa interval and in the level near 6 GPa. Omphacites in GrC and GrD eclogites occur on the relatively hot branches >45 mW/m², suggesting the influence of protokimberlites. Several low-Cr Gar and Cpx from GrA1 and GrB1 eclogites with relatively low Fe# ~0.15 found in the lithosphere base may have crystallized from protokimberlites.

4.3. Sytykanskaya Pipe

In the SCLM beneath this pipe (Figure 4A) [68,69,70,71,72], common basaltic eclogites GrB are found in the upper part of the SCLM (USCLM) (GrB1,2). Eclogites belonging to the GrD are found near the lithosphere base. As is common, the high-Fe diamond inclusions are related to the hottest part of the geotherm at 50 mW/m².

The diamond-bearing eclogites often contain kyanite and even diamond inclusions and belong to the Ca-Al-rich types GrD. They closely trace the diamond–graphite boundary.

The diamond inclusions of the Mg-rich Cr-bearing type GrA1 are found close to 3 and 5 GPa and plot close to the conductive 35 mW/m² geotherm at pressures of 3–4 GPa. The pyroxenes from GrA2-A4 eclogites reflect hotter conditions (40–45 mW/m²) in the middle SCLM 3–5 GPa near the pyroxenite layer.

The GrC eclogites are rare and refer to the middle part of the SCLM (MSCLM).

4.4. Komsomolskaya Pipe

In the SCLM beneath the Komsomolskaya pipe (Figure 4B) [67,68,69,70,71,72], are rare diamond inclusions of the Mg-rich Cr-bearing type that are found close to 3 and 5 GPa and plot close to the conductive 35 and 40 mW/m² geotherms. The metabasaltic melt-metasomatized eclogites GrB1 are found in the lithosphere base and mark the 40–45 mW/m² geotherm branches. Some of the 5.5–6.5 GPa are diamond-bearing eclogites. The Ca-rich garnets (GrD) from the concentrate frequently occur in the wide pressure interval 3–6.5 GPa but they are absent in among the xenoliths. The Fe-rich group (GrC) is rarely found in MSCLM.

4.5. Obnazhennaya Pipe

The mantle section beneath this Jurassic pipe has a high eclogite population that belong to GrA1 [126,127,128], (Figure 4C), although GrB and C are also present [126,127,128]. Most of them belong to the high-Mg Gr1a and represent the middle or upper part of the SCLM. Hybrid low-Cr varieties (transitional between GrA2 and GrB2) are in the 4.0–2.5 GPa interval.

The new data set [126] allowed us to find eclogites belonging to the melt (GrB2) near 5 GPa. As usual, the high-Fe group GrC was located at the base of the pyroxenite layer in the middle part of the SCLM at the geotherm 45–50 mW/m².

4.6. Bumerang and Khardakh Pipes

They both belong to the Jurassic Ary Mastkh pipe [129] at the western margin of the Anabar shield. They contain mainly pyroxenites located at the M-USCLM but a few omphacites are found in the middle part of the mantle, section 3–2.5 GPa. The garnets of low-Cr type (with Eu anomalies) should belong to eclogites. Some Mg pyroxenes are mainly Cr-bearing. The boundary with the GrB is found at Fe# 0.17. The GrC Fe, Na-garnets plot within the 2.5–3 GPa interval and those from the GrD are located near 5 GPa (Figure 5A).

The Kharamai field [129,130] pipes in general repeat the features of the previously described (Figure 5B) field but the GrB extends to 5.2 GPa. The GrC is represented mainly by pyroxenes and the GrD was detected. The eclogitic garnets and Cpx formed in the P-Fe# diagrams make up several branches of the AFDP trends on the diagram with the different inclinations joining at the 4.5 and 6 GPa levels; possibly they reflect the melting and fractionation or interaction of the partial eclogite melts at several levels in the SCLM.

5. Characteristics of Distributions of Different Eclogite Groups in the Lithospheric Mantle Worldwide

5.1. Colorado, Wyoming Craton Montana

Compared to Yakutia, eclogites from the Sloan kimberlites [78,79] in Colorado, North America are mostly from GrB. Those from the GrA are Cr bearing though are higher in Na and Al pyroxenes. These omphacites refer to LT conditions close to the garnet geotherm. Metabasaltic eclogites GrB1 from the interval 7–3 GPa are typical subduction-related varieties.

Eclogites from the lower part of the mantle section are of metabasaltic GrB1 affinity and reflect heated conditions (plume remelted) with AFDP trend, while the others from GrB1-2 trace the low-T conductive 35 mW/m² geotherm. In the USCLM beneath the Sloan pipe, the GrB2 eclogites are related mainly to the heated geotherm in the 3.5–1.0 GPa interval. The Fe, Na-eclogites (GrC) from the 4.0–3.0 GPa intervals reflect the 45 mW/m² geotherm. High-Ca varieties GrD close to 3.0 GPa show higher temperature conditions (Figure 6A).

5.2. Lac de Gras Cluster, Slave Craton

We have calculated the PT parameters for eclogites from the Slave craton (Canada) from the Jericho pipe [4,6,28] and the Lac de Gras kimberlite field [80,81,82] (Figure 6B). The Mg-rich eclogites GrA from the Jericho kimberlite were captured mainly from the LSCLM. Eclogites are abundant in the kimberlite pipes of the Lac de Gras cluster [80] (Figure 6B) Group A (GrA1,2) or the GrA Mg-rich = Cr-poor varieties in Lac de Gras that belong to the middle mantle section at 4.0–5.0 GPa. Metabasaltic GrB varieties form two groups marking a boundary at 3.0 and 4.0 GPa intervals. The Fe-rich clinopyroxenes form a trend with the increasing Fe# and decreasing pressure (AFDP) from 2.0 to 4.0 (3.0) GPa.

5.3. KL-4 pipe, Dharwar Craton, India

The Proterozoic (1.1 Ga) KL-4 pipe in the Dharwar craton of India [48,69,83] contains mainly kyanite-bearing eclogites or grosspydites (Figure 7A). The Mg-rich GrA is very wide in Al₂O₃ and CaO and distributed from the LAB to Moho. The subduction-related eclogites of GrB [48] with pyroxenes also rich in Al are distributed from 6 to 1.5 GPa. Most xenoliths belong to the basaltic group GrB and lie on the hot 90–60 mW/m² geotherm in the 1.5–3.0 GPa interval. The Fe-eclogites (GrC) were not detected. The kyanite-bearing GrD1 eclogites are found at the lithosphere base GrD1 where they are intercalated with the high Ca-Al-pyropes containing Cr₂O₃. Mainly, they belong to the 3.5–4.5 GPa interval (GrD2) and to the “pyroxenite layer”, which is suggested also by geophysics [84], and in the uppermost part near Moho GrD2a. Eclogites sometime contain several generations of pyroxenes and some of them reveal low pressure conditions.

5.4. Catoca pipe, Congo Kasai Craton, Africa

The majority of pipes are located within the SW–NE “Diamond corridor” including the diamond-bearing Catoca pipe in Angola [86,87,88] with abundant eclogitic xenoliths [88] (Figure 7B). They show a wide range of compositions and two AFDP trends. The high-Mg low-Cr eclogites (GrA3) are probably ancient arc cumulates and are concentrated within the 3.0–4.0 GPa interval. The high-T trend starting from Fe# ~0.12 to 0.18 relates to protokimberlites (GrB1), whereas the second one is low-T and starts from Fe# ~0.17 and probably was formed by earlier Fe-rich plume-derived magma. Both Ca-Al-rich (GrD) and Fe-rich (GrC) eclogites are found in the 4.0–5.0 GPa interval. A wide range of compositions of basaltic origin is found in the USCLM and probably represents the intrusive bodies derived from different magmas.

Protokimberlite-derived pyroxenites (GrB3), which also includes diamond-bearing varieties, are distributed from the SCLM base to 4.0 GPa. The Ca-rich eclogites Gr4 in SCLM beneath the Jericho pipe are found in the lower SCLM from 5.0 to 7.0 GPa. Metabasaltic varieties (GrB2) dominate in the USCLM together with high-Fe eclogites (GrD), which reflect higher pressure. The latter are found in the lower part of SCLM also from 5.0 to 6.5 GPa. Melt-metasomatized eclogites (GrB4) are found in the 7.5–5.0 GPa interval and show an increase in Fe# with decreasing pressures as in the Jericho peridotites (Figure 8B). The high-Ca varieties (Gr4) are widely distributed in USCLM but also occur near the LAB.

5.5. Roberts Victor pipe, Kaapvaal Craton, Africa

This pipe, the most eclogite-rich in the Kaapvaal craton, is described in numerous publications (Figure 8A) [95,96,97,98,99]. We also made >380 analyses of minerals from concentrate mostly from eclogitic type. The reconstructed mantle section is similar to that of the Udachnaya pipe with the broad AFDP trend. The Gr1 are mostly found in the lower part pf the mantle section and are both of Cr-bearing and Cr-free protokimberlitic type, often diamond-bearing. The GrB formed the inclined complex trend with increasing of Fe# with decreasing pressure in general. The Fe-group C is found from 3 to 6 GPa. The kyanite GrD is very wide and located also in the LSCLM forming a gentle trend of pressure and Ca–Al joint decrease.

5.6. Orapa Pipe, Limpopo Belt, Africa

The PTX diagram for this pipe is mainly constructed using diamond inclusions from the Orapa pipe [106] and separate nodules [15] (Figure 8B). The Mg-rich group mostly Cr-free and Cr-bearing are from the MSCLM. The Cpx from the GrB are lower in pressure than garnets, opposite to Premier pipe. They also form the AFDP trend. The Fe-group is in the 3–5 GPa interval. The Gr4 eclogites are widely distributed in the LSCLM and garnets are higher in pressure and lower in temperature than pyroxenes.

6. Geochemistry of Trace Elements of Studied Eclogites

Trace element patterns for minerals in eclogites are rather specific for each group and very often there is a dominant type in some of the investigated pipes like Obnazhennaya [126,127,128] or KL4 [69] in the Dharwar craton. Compared to the crustal eclogites [131,132], which often reveal very inflected patterns with decreasing LREE for both clinopyroxenes and garnet [20,22], all mantle eclogites show the opposite inclination of the Cpx and garnet patterns regulated by the partition coefficients between Cpx, garnet, and melt. This supports the data that all mantle eclogites were melted or were subjected to intense fluid influence.

6.1. Udachnaya Eclogites, Trace Element Distributions for Pyroxenes and Garnets

The most variable in trace element patterns of the studied samples are those from the Udachnaya pipe (Figure 9). During earlier work, a limited number of eclogites were analyzed, which did not cover all types [1,52,53,54]; the recent works are more systematic [49,50,51]. Clinopyroxene from carbonated basaltic eclogites (GrB) reveals bell-like asymmetric REE patterns with very low incompatible trace elements, a strong Sr and small U peaks, and a trough in Zr; these are probably strongly metasomatized subducted MORB (Figure 10) [23,33]. They differ from the diamond-bearing eclogites (Figure 11). Low-Cr type A to B [9] eclogites also show humped REE patterns that are divided into two groups. Those with lower concentrations have less LREE and show troughs in Nb and Zr, a high peak in Pb, and a smaller peak in U. The others have round smooth trace element patterns with small troughs in Pb and Ba.

Clinopyroxenes from diamond-free eclogites show inclined REE patterns with rather high LREE and other incompatible trace elements and a trough in Ba. Those with lower concentrations also show a Zr–Hf trough. Clinopyroxene from the corundum-bearing eclogite shows REE pattern with an inflection in Eu, and a general increase in incompatible elements, with a high peak in LILE, a small one in Sr, and a trough in Th.

Garnets from Udachnaya eclogites show HREE-enriched patterns with spoon-like LREE patterns, typically resulting from metasomatic processes. Some of them show troughs in both HFSE and Th-Ba, others even have peaks in Zr–Hf and Ta that are controlled by rutiles [12]. Some garnets have depressions in HFSE and incompatible elements except for Sr and could be derived from (or have reacted with) rather primitive mantle melts like undifferentiated protokimberlites.

6.2. Sytykanskaya Eclogites, Trace Element Distributions for Pyroxenes and Garnets

Trace element patterns for the different types of eclogites from the Sytykanskaya pipe (Figure 10) are mostly similar to peridotitic trace element diagrams. Clinopyroxenes with the lower REE ~10/CI reveal flatten LREE part and show minima in HFSE as well as those with the highest LREE. Only one garnet with the highest HREE show minima in Eu. The samples with the lowest REE display infections in Eu and minima in HFSE. The other bi-minerals and Cr-less samples display typical peridotitic features with the semi-round and relatively fertile trace element patterns and may be pyroxenites occurred after the melting of eclogites. They have Ce minima undepleted in both U and Th, typical for subduction and plume sources.

6.3. Komsomolskaya Eclogites

We studied all three (A,B,C,) types of eclogite samples from the Komsomolskaya pipe (Figure 11). Minerals from group A eclogites show the highest inclination (La/Yb)_n ratios. The clinopyroxenes sometimes are very enriched in LREE and display Pb peaks. The HFSE minima exist but are variable. Some of the TRE patterns of minerals are close to peridotitic and seem to reveal hybrid peridotite–eclogite features. Group B eclogites reveal division into three sub-groups differing in their REE concentrations. They all have minima, or inflections, on Eu and Ce, and HFSE minima depth is correlated with the depletion in LREE. They display mostly high U and Th concentration due to influence of typical subduction and carbonatitic plume components. The highest HREE and relatively flat REE patterns for the clinopyroxenes and high concentrations of the remaining incompatible elements are from the group C eclogites.

6.4. Obnazhennaya Eclogites

Most eclogites from Obnazhennaya are related to the relatively magnesian Cr-bearing group 1 with a minor amount of GrB transitional eclogites described by [103,104]. They reveal similar asymmetric bell-like REE patterns for clinopyroxenes, which are common for partial melts (Figure 12); however, the left part of the trace element pattern is mildly depleted or flat. Varieties with lower REE concentrations show Zr troughs typical for H₂O-bearing melts and a trough at Pb because of separation of sulfides which are abundant in eclogites. Omphacite from a corundum-bearing eclogite (Figure 12, marked by rhombs) reveals a pattern with an inflection in Eu and lower HREE relative to a similar sample from Udachnaya. It also reveals high peaks in Ba and Sr, but Hf–Zr troughs and higher enrichment in LREE.

Most garnets have rounded trace element patterns with high HREE, low LREE, a peak typically at Pb and a Sr trough. The incompatible elements in general repeat the tendencies of pyroxenes. Garnets from corundum eclogites display greater REE enrichment.

6.5. Ary Mastakh Field

The only Mg-rich eclogite from this locality shows an inclined W-shaped REE pattern consisting of two branches, divided by the Eu minimum. The spider diagram shows a smooth pattern, with increasing content of incompatible elements, a peak in Pb, and a small trough at Nb (Figure 13). The Fe-rich but low-Na “pyroxenitic” eclogites also show inclined sometimes W-shaped REE patterns typical of plagioclase, however, the spider diagram displays rather strange enrichment in all HFS elements (Figure 12).

6.6. Dharwar Craton, Wajrakarur Field, Kl-4 Pipe

Trace elements for eclogitic Cpx from India reveal mostly similar REE patterns and spider diagrams, with high (La/Yb)_n ratios, Eu peaks, and flattened HREE. This proves that the kyanite-bearing eclogites and grospydites are cogenetic. The Cr-bearing Ca-rich eclogites indicate either interaction of peridotitic mantle with carbonate fluids or that carbonatite melt reacted with enriched material of former eclogites (Figure 14). The most Mg- and Cr-rich varieties have lower trace element concentrations, a smaller Eu trough and a Ba peak. Clinopyroxene from the grospydite has the most enriched REE pattern but is depleted in general in incompatible elements except for a very strong peak in Ba. Garnet REE are not equilibrated and highly inclined. Spider diagrams for garnets show a U peak but are low in Sr. They show depletion in HFSE (Ta > Nb); the most depleted sample displays a deep Y trough. For the other Cpx, peaks in both Ba and Sr are typical.

6.7. Wyoming Craton, Sloan Pipe

One type of omphacite from the Sloan pipe displays a high Eu peak and has high concentrations of LILE and U, Sr, and Pb, and deep troughs at Th and Nb, which suggest an origin from a silicic plagioclase-bearing protolith (Figure 15). One strange clinopyroxene with a flattened spoon-like REE pattern has a spider diagram very similar to previous types.

Some eclogites display common rounded patterns for garnets and clinopyroxene without Eu anomalies. Spider diagrams for clinopyroxene are also rounded with common negative anomalies in HFSE, deeper for Nb, Ta, and Pb. Spider diagrams for garnets show lower HFSE troughs and a Pb peak (Figure 15).

7. Discussion

7.1. General Regularities of Eclogite Distribution in the SCLM

There are common features for the location of different types of eclogites in the studied mantle sections. The most Fe-rich eclogites (GrC1) are commonly located in the middle of the mantle section beneath the Daldyn field [23,24,36,56,71], the Alakit field [69,70,71] in Yakutia, and in the Slave craton in Canada [4,80,81,82]. In the Congo-Kasai craton in Angola, Fe-rich eclogites (GrD2) also belong to the middle part of mantle section [86]. The same situation is found for the Orapa [106] and Venetia pipes in the periphery of this craton. The Fe-rich eclogitic pyroxenes from the Roberts Victor mine are also found near 4.0 GPa. Similar trends are seen in the Kaapvaal craton with the maximum Fe-enrichment at 4.0 GPa beneath Finsch [85,94,101], De Beers Pool [108], Kimberley and Letlkhakane pipes [91,109]. Beneath the Jagersfontain pipe [107], two levels of Fe-rich eclogite concentration occur near the lithosphere base and in the middle part of the SCLM. In the Arkhangelsk region, Fe-rich eclogites are found beneath the Arkhangelskaya and Grib pipes [17] in the middle part of the mantle section. Therefore, we conclude that the presence of Fe-rich eclogites in the middle part of the mantle section is a common attribute of the SCLM.

The position of the most Fe-rich varieties is slightly different and varies from 3.0 GPa, typical of the mantle sections beneath the NE and circum-Anabar territories [129] in Yakutia, to 4.0–4.5 GPa, which is more common for the Daldyn–Alakit region. It is likely that the lower pressure conditions of the Fe-rich varieties reflect more ancient events. We suggest that the Fe-rich varieties belonging to the pyroxenite layer and were formed after re-melting of ancient Archean tonalities subducted before as eclogites.

Another group of eclogites Ca-Al-rich including grospydites (GrD) are commonly low-Fe, although there are some varieties enriched in Fe. Beneath the Daldyn field, they mark the interval at 3.5–4.5 GPa and they also occur in the middle part beneath Sytykanskaya and in the lithosphere base beneath Komsomolskaya. However, beneath the Mir pipe, they occur together with the Fe-rich associations in the middle SCLM and close to the lithosphere base. In the Dharwar craton of India, abundant Ca-kyanite eclogites [48,83] of low-Fe type (GrD1) are found in the 4.0–5.0 GPa interval. However, the position of the kyanite eclogites beneath Lac De Gras [4] (Canada) is unusual; they are found in two intervals, in the lower and upper parts of the SCLM.

Many so-called Mg-rich and Cr-poor eclogites (GrA) are diamondiferous [4,6] and are distributed in the wide interval of the LSCLM. They are of different types: Cr-bearing varieties are commonly found in the middle SCLM because of crystallization of carbonatite-silicate melts near the diamond–graphite transition [123]. Nevertheless, a great number of such eclogites were found in the USCLM beneath Obnazhennaya.

The common type B eclogites (GrB) are very abundant in most mantle sections. Common subduction type GrB1 is widely distributed in both the middle and lower parts of the SCLM. The metasomatic and reactional groups GrB2 [107,134], which formed during subduction in ancient hot mantle, belong to the middle part of the mantle section. Nevertheless, reactional products of rising mantle melts GrB3 are widely distributed in the USCLM. They are common in marginal cratonic settings, for example in Colorado, and in the regions subjected to intense plume influence like the Slave craton. Eclogites of GrB2 from the LSCLM beneath Proterozoic kimberlites commonly form linear ascending trends AFDP with rising Fe# and trace high-T geotherms. They are common also in mantle sections beneath the Yakutian kimberlites. The trend of increasing Mg+ with increasing pressures may appear to relate to hydrous melting of eclogites. The rapid changes in Fe# and Mg+ suggest continuous fractionation but interaction of the Mg-rich plume melts with Fe-eclogites is more probable.

The division into oceanic and continental terranes [135] applied to the Siberian craton allows us to identify the Magan and probably the Birekte terranes as continental type, while the Daldyn is close to the oceanic type and contains mainly GrB1,2,3 eclogites. In contrast, the East Daldyn terrane (Alakit field) contains highly depleted mantle, which is closer to the arc type and contains a high amount of GrA1 diamondiferous eclogites.

7.2. Role of Eclogites in the Reconstruction of Mantle Layering

Growth of the Archean cratonic lithosphere was accompanied by peridotite melting [118] as well as melting of eclogites [119]. The number of eclogites in the mantle keel essentially decreases because the resulting melts were hybridized with peridotites and possibly even intruded the low crust. This is the main reason for their relative rarity among mantle xenoliths. Nevertheless, some kimberlite pipes, such as Roberts Victor [95,99], contain abundant eclogitic xenoliths of different types, which suggests that they are captured from the varying substrate [81]. Sometimes they keep the signatures of the ancient rocks of oceanic crust like ophitic gabbro [97] and so were not completely remelted, however, mostly bi-mineralic eclogites are cumulated of eclogite-derived melts and could form the ascending channels in lithospheric mantle later traced by kimberlites.

Without geochemical features it is not easy to determine the origin of eclogites [3]. Their position in the PTX diagrams and distinct trends could be an additional diagnostic tool. Relics of ancient oceanic crust [27,31,120,135] which have clinopyroxenes with Fe# = 15–25 were all re-melted. Their isotopic ages show quite different and sometime rather young and multistage isotopic ages and signs of metasomatism [14,52,54,61,88,93,96,97,98,107,136,137,138]. They form age-groups and reflect the peaks of the different plume events.

But the Re–Os ages are often more ancient and are mainly older 2.7 Ga [14] and even 3.5 Ga when the craton keels were formed. Eclogite garnets and clinopyroxene inclusions give younger isochrone ages of 990 ± 50 for Orapa (Botswana) [15] and 1580 ± 50 Ma for Finsch [101] and 1443 ± 166 Ma and 1657 ± 77 Ma [138]. Clinopyroxene inclusions in diamonds from the Premier mine, South Africa, yielded apparent 40Ar/39Ar ages of 1185 ± 94 Ma and 1198 ± 28 Ma [90].

Available ages for Fe-eclogites from Angola are ~1.2 Ga [87] and reflect the reactivation processes by later plumes because these ages group near the dates of the highest plume activities. As was shown for mantle eclogites from Kaapvaal, they correspond to the stages ~1.1 GA according to Nd–Sm and Lu–Hf isotopic systems close to the superplume event produce Bushveld and phlogopite metasomatism in the mantle [93]. The influence of the thermal impact of a plume may reset these and other systems [99].

Most eclogite ages determined by sulfides for Yakutian eclogites are Early and Middle Archean [2,13,14], though Late Proterozoic (1050 Ma) and quite younger (420 Ma) [52] were also determined for rutiles from the Udachnaya pipe eclogites from Yakutia. Rutile diamond inclusions from the northern placer including the Ebelyakh field also reveal Paleozoic ages [136]. Our unpublished data for eclogites from Alakit are within the 660–490 Ma interval. Therefore, the Devonian plume caused a vast perturbation in the mantle lithosphere beneath northern part of the Siberian Craton. The Lower Paleozoic ages 462 ± 86 Ma and 472 ± 28 M were determined for the eclogite clinopyroxenes from Ural diamonds [137].

Many hybrid eclogite–peridotite rocks [25] were formed by interaction of plume melts with more ancient eclogites and mantle metasomatic rocks [26,61,72,96,97,98,107]. The common presence of Fe-rich material in the middle part of the mantle section could be explained by their formation as remelted ancient tonalite–trondhjemites [30,31] in Early Archean times when the lithosphere was not greater than 130 km in thickness [139,140]. The high PT gradients at that time could not allow subducted material to pass to lower mantle levels without melting [140]. The presence of pyroxenites and eclogites in the middle part of the mantle section was detected not only by petrologic methods [77,121,129,141] but also by geophysical models [84] beneath the Dharwar craton and in other regions.

The age of the grospydites and the ancient oceanic crust beneath Udachnaya are suggested to be Early Archean 4.5–4.1 Ga [1,13] to Proterozoic 1.2 Ga [14], while the other subducted eclogites yielded depleted mantle ages from 2.76 to 3.5 Ga [13,14] or 3.1–2.7 Ga [23].

The Ca-Al-rich material in the middle level could be either Ca-rich sediments or ancient anorthosite crust or metasomatites accreted due to reaction of carbonate-bearing sediments with mantle. Association of Fe-eclogites with grospydites and Mg-rich low-Cr eclogites may be explained by the subduction of sediments as a part of the ancient island arcs together with cumulative eclogites which may simply crystallize in magmatic sources starting from 1.2 GPa. It possible that eroded island arc crust with relatively dense basement could be easily subducted (Figure 16).

Modern-style subduction on Earth is often thought to have started in the Mesoarchean–Neoarchean at 3.2–2.7 Ga with the appearance of water in the crust and mantle and fast convection was followed by the formation of the thick SCLM [139,140,142,143,144] with rather low geothermal gradient [91] and pyroxenites layer in the middle part [141]. Primary subduction material should mark the boundaries of the subduction units consisting of dunite harzburgite eclogite layers. Such units may be detected in the SCLM of the Udachnaya pipe. Though, in many cases such materials were remelted under the influence of the superplumes passing through the mantle lithosphere. Consequently, mostly B-type eclogites were remelted and some of them may be Gar–Cpx cumulates of basaltic melts in the USCLM [143,144].

Many eclogites in the SCLM show AFDP trends that are typical of ascending and differentiating magmas. Such “basaltic eclogites” (GrB1) may show typical features of their magmatic origin. They may create channels within the peridotitic lithosphere starting from the deep subduction stages [119]. These irregularities, formed during subduction stages or under the influence of later plumes, could explain rather irregular distribution of eclogites in kimberlite pipes and the abundance in some of them such as Roberts Victor [89,90,91] and practical absence in others.

Lack of evident layering in the USCLM, for example beneath the Wyoming or Slave cratons, possibly may be attributed to mantle intrusions derived from subducted material. This is probably a common feature for mantle sections close to subduction zones. Another type of eclogite-like rock in the USCLM are the Gar–Cpx basaltic cumulates, which are typical for areas of plume basaltic magmatism such as Bushveld [90] or central Yakutia [103]. The estimation of the cumulates and eclogites volume in different regions beneath the Siberian platform using average seismic velocities show that highest Vp >8.3 possibly corresponding to eclogites are common for the middle part of Yakutia where the Late Devonian kimberlites with the highest diamond grade are located. Nevertheless, calculation of the average mantle density [145] does not show positive gravity anomalies in the mantle beneath the kimberlitic fields. The area beneath Anabar and the Central Yakutian kimberlite province is lighter due to higher Mg+ of mantle peridotites.

7.3. Geochemical Application of Eclogites for the Reconstruction of Geodynamic Processes

Trace element patterns for most bulk rock eclogite compositions and constituent garnets and clinopyroxenes show positive Eu, Sr anomalies which are an attribute of the precursor plagioclase. The presence of eclogitic material in the lithospheric mantle may be marked not only by the eclogites themselves but also by the presence of Eu anomalies in minerals from pyroxenites and even in mantle peridotites. There are some specific high-Na Al pyroxenes in mantle peridotites of normal and increased Fe# that could result from hybridization with eclogites or their partial melts. Elevated LREE and Ba, U, Sr, Pb contents, which are typical subduction components [18,19], are found in many eclogite minerals [21] and their parental melts (Figure 16 and Figure 17), meaning that melting may have started during subduction, yet many peridotitic pyroxenes and especially garnets also have such anomalies [59]. This probably could explain the rarity of eclogites in many kimberlites.

Some omphacites have very high LILE contents that may be explained not only by the composition of their protolith but also by fluid influence [80,146,147,148]. Indian and many other eclogites contain micas which probably reflect primary enrichment of the protolith in LIL elements. Even K-bearing omphacites and abundant alkali K-feldspars are found in Indian Kl-4 grospydites. Therefore, it also possible to suggest the presence of subducted sanukitoid material [92] and disintegrated sediments [18] judging by the patterns for India eclogites and some from the Sloan pipe; the Ba, Sr anomalies typical for such rocks may seriously influence the geochemical features of surrounding peridotites that have been subjected to melt fluid fluxes. However, there are kyanite varieties which are thought to be eclogitized without changes in bulk rock composition [3]. Some of them show presence of two types of garnets, coarse-grained and in symplectites [51], suggesting reactions.

The LREE-enrichment of eclogites by volatile-rich melts is a common feature and this is one of the arguments for wide scale melt – fluid percolation which should totally transform the structure and compositions of the SCLM [80,147].

Reconstruction of the primary melts, which were in equilibrium with the eclogitic minerals using partition coefficients [149,150], show that they rarely correspond to common melts.

Many eclogites contain rutiles or ilmenites. Many so-called eclogites among mantle xenoliths contain rather low-Al and Na pyroxenes and should be identified as pyroxenites. Some of them may result from reaction with Ca-rich melts and fluids related to protokimberlites and trace high temperature geotherms. The low-T varieties possibly were formed during ancient plume events. Subdivision of basaltic eclogites into Ti-bearing and low-Ti series [151,152] is not used in this paper. Many “basaltic” eclogites contain rutile, which suggests that they were derived from alkaline plume-related magmas. Such eclogites should be more abundant in the deep mantle according to geochemical mass balance calculations [12].

The influence of rutiles is possibly the reason for decoupling of Nb and Ta in clinopyroxenes and micas from eclogites from Angolan kimberlites [86] and is also seen in trace element patterns of clinopyroxenes from Udachnaya and other pipes. As rutiles have differences in their Nb–Ta partition coefficients, this demonstrates that rutile could precipitate before garnet and clinopyroxenes. The abundance of TiO₂ suggests formation of eclogites not from primitive MORB sources, but from enriched plume melts, or is evidence for metasomatism by plume melts. It is possible also to suggest a cumulative nature of the protolith.

The role of metasomatic processes in the formation of kimberlite eclogite xenoliths is also a major question [91,92,93,94,95,96,97,98,107,148], especially for samples with elevated volatile-dependent components which are increasing the LREE content. Subduction was accompanied by essential volatile fluxes that accompanied eclogite formation [19]. Nonetheless, for the more ancient diamond-bearing peridotites in the Undachnaya pipe, LREE enrichment is not notable [9].

Cumulates from protokimberlites reveal rather smooth inclined rounded trace element patterns [101], but the more evolved melt compositions close to frammesities [146] reveal a carbonatitic signature of the eclogitic rocks derived from protokimberlites. Cumulates of garnet-clinopyroxene-ilmenite were formed at the high temperature stages [153] and do not show the very deep HFSE anomalies that characterize the late stage of the protokimberlite process.

The trace element patterns of eclogites and their minerals together with the isotopic signatures of eclogites are clues for reconstruction of the early history of the Earth. Nevertheless, this story is still far from finished because, during subduction and further history, the eclogites were subjected to the influence of a fluid system that slightly changed the trace element patterns and fractionated isotopes [19,146,153].

The highest concentrations of the REE and TRE are found for the eclogites from Komasomolskaya and the Sytykanskaya pipe where the mantle keel is nearly completely metasomatized and contains micas. They have also the most variable shape of the REE and TRRE patterns.

We have calculated the parental melts that may have been in equilibrium with clinopyroxenes from eclogites, suggesting that all of them were remelted (Figure 16 and Figure 17). The difference for the Udachnaya diamond-bearing and common eclogites is in the higher inclination of the REE pattern, which means higher numbers of garnets, and the coexisting solid and lower degree of melting as well as in abundance of LREE. In contrast, calculated melts parental for the eclogites of Obnazhennaya [126,127] show a jagged trace element pattern, typical for adakites originating from eclogite melting (Figure 16). The patterns of melts that produced the Kharamai eclogites may be regarded as primitive arc cumulates, however they practically lack HFSE anomalies, which suggests a rather low oxidation state. Only Zr–Hf show some fractionation with elevated Zr. Many other mantle pyroxenes from the northern field, like Khardakh from Ary Mastakh, show the rather complex patterns with the dips in Zr, Nb, and Ta, but rather high Th and U and often Hf. The very strange magmas enriched in HFSE may be the products of contamination by some metasomatite minerals.

For the Sloan pipe, abundant eclogites probably are reactional with the protokimberlite melts though the left part of the spider diagram is not uniform as for most coarse-grained and megacrystalline Cr-diopsides [78]. The others, which are highly enriched, probably are subducted silicic material remelted in the mantle wedge. Probably such an enriched pattern is a mistake. Transformation of plagioclase into omphacite without melting is more realistic. The most elevated patterns that were determined for the India eclogites showing high peaks of Ba, Sr, Eu, Zr and a lower peak for U may be attributed to a volatile-rich melt which had a high contribution of the acid crust (Figure 17).

7.4. Influence of Protokimberlites on Eclogite Compositions

Significant numbers of kimberlitic xenoliths may be cumulates of protokimberlites judging by elevated PT conditions and major and trace element geochemistry [51,154]. Many eclogites trace the advective branch of the geotherm, which is formed by the rising protokimberlites. The range of Fe# (0.11–0.15) for such Gar–Cpx–Ilm xenoliths coincides with the values for olivines that crystallized from protokimberlite melts [155]).

Eclogitic minerals show zonation not only in major but also in trace elements [51,101]. Even diamond crystallization is regarded as a specific metasomatic process associated with the protokimberlite stage [154].

Some eclogites reveal evidence of de-eclogitization reactions when omphacitic pyroxenes, which are substituted by diopsides because of reactions with Ca-carbonatite, melts according to the suggested reactions [146]. When the melt became Na-silicate carbonatite and its solidus temperature significantly decreases. Diamond formation could accompany such processes as is evident from the position of diamonds tracing channels in the eclogite structure. The largest diamond-bearing xenolith [26] shows the presence of intense interaction with melts close to protokimberlites and diamond metasomatism.

8. Conclusions

• Eclogite thermobarometry for clinopyroxenes and garnets allows us to determine the position of eclogites in the mantle sections beneath kimberlite pipes in some detail, showing high-T conditions for protokimberlite-related and melt-metasomatized eclogites and low-T for subducted eclogites.
• Most eclogites have been remelted during subduction and later plume events.
• The division of the sub-cratonic mantle lithosphere into an upper and lower part is probably marked by the presence of Fe-rich eclogites.
• Ca-rich kyanite eclogites are common in the middle part of the SCLM in the Precambrian kimberlites and became more frequent in the LSCLM beneath Phanerozoic kimberlites.
• Different groups of eclogites commonly have individual positions and geothermal gradients in the mantle section beneath kimberlites from different cratons and terranes.
• Trace elements for Fe-rich and high-Ca eclogites commonly demonstrate their derivation from a silicic LILE-enriched substrate. The MORB basalt-like protoliths show derivation from a HFSE- and LILE-depleted source. Common type B (metabasaltic) protoliths show the strong influence of volatile-rich melts and fluid produced by different types of magmatic differentiation. The MgO-rich eclogites show Nb anomalies and participation of rutile in the source rocks.