Journal of Earth Science  2019, Vol. 30 Issue (3): 431-450   PDF    
Heterogeneity of Mantle Peridotites from the Polar Urals (Russia): Evidence from New LA-ICP-MS Data
Vladimir R. Shmelev 1, Shoji Arai 2, Akihiro Tamura 2     
1. Institute of Geology and Geochemistry, Ural Branch, Russian Academy of Sciences, Yekaterinburg 620075, Russia;
2. Earth Science Department, Kanazawa University, Kanazawa 920-1192, Japan
ABSTRACT: To discuss the nature of the compositional heterogeneity of the peridotite massifs of the Polar Urals (Russia), the geochemical study by LA-ICP-MS of pyroxenes and amphiboles from these mantle formations was performed. The trace element compositions in clinopyroxenes indicate the existence of the mantle protolith of two types. The first protolith type, represented by lherzolites and diopside harzburgites, was originated from the partial melting (5%-10%) under the spinel facies conditions, while the second one, represented by diopside harzburgites, was formed under the polybaric partial melting (17%-19%) under garnet and spinel facies conditions. Subsequently, the mantle peridotite protolith was subject to fluidinduced partial melting in the suprasubduction setting that was resulted in the formation of harzburgites. Being affected by penetrating melts and fluids peridotites experienced the refertilization (LREE enrichment of clinopyroxenes) and high-temperature hydratation with subsequent development of pargasite and Mg amphibole. The high-T fluid-induced metamorphism at the subduction zone was accompanied by the formation of metaperidotites with clinochlore and REE-depleted tremolite.
KEY WORDS: pyroxene    amphibole    peridotite    LA-ICP-MS    Polar Urals    Russia    

The compositional and structural-textural features of peridotites of ophiolite associations of folded belts (Urals, Alpine, etc.) and their oceanic analogs reflect the complex tectonomagmatic processes, occurring in the upper mantle. According to the petrological studies, peridotites are usually considered to be polygenic formations, which recorded the evidence of partial melting and depletion of the mantle protolith and subsequent its subsequent transformation by penetrating melts (fluids) (Warren, 2016; Savelieva et al., 2008; Seyler et al., 2006; Niu, 2004; Asimow and Stolper, 1999; Kelemen et al., 1997). Our suggestion that the peridotites recorded more complex history of the mantle recycling follows from the fact that ultramafic rocks and chromitites contain high-pressure (diamond, etc.) minerals (Arai and Miura, 2016; Yang et al., 2015). All this suggests a different degree of heterogeneity of the mantle substrate in the peridotite massifs.

The mantle peridotites of the Urals, comprising large deposits of chromitites, asbestos and some other mineral resources, are quite well studied in terms of structural and compositional features. The problem of the genesis of these formations, however, is still debatable. Savelieva (1987) was the first to provide a significant contribution to the knowledge of the nature of the peridotite complexes considering the structure of the Uralian ophiolites, their composition and the evolutionary history. In this work, the first attempt was made to consider the sequence of processes in the zone of the partial melting of the mantle protolith. Later, the redox state of ultramafic rocks of the Urals was studied (Chashchukhin et al., 2007). The Mossbauer spectroscopy was used to determine a iron oxidation degree in spinel, which showed that oxygen fugacity in ultramafic rocks of the Urals is nonconstant, varying from -2 to +6 units logfO2 relative to FMQ buffer. This fact was interpreted as evidence of their formation in various geodynamic settings (from oceanic to subcontinental).

The beginning of studying the REE distribution in minerals has marked a definite step in understanding the nature of mantle peridotites of the Urals. For the first time, the Mindyak and Nurali peridotite massifs in the Southern Urals have been studied. As a result, peridotites were subdivided into three groups (Spadea et al., 2003). Based on the trace element signatures of pyroxene, Spadea et al. established a varying degree of peridotite depletion and signs of their impregnation by MORB-like and alkali-rich melts or fluids. At this, the main problem of the classification of these peridotites into derivatives of the oceanic or subcontinental lithospheric mantle has not been resolved. The study results of the Voikar massif (Polar Urals) showed that the formation of vein pyroxenites in peridotites was associated with boninite-type melts with progressive melting of the mantle wedge in the suprasubduction zone (Belousov et al., 2009). It was subsequently established that the clinopyroxene from spinel harzburgites of this massif bears signs of partial melting in the garnet (6%) and spinel (8%-10%) fields of stability (Batanova et al., 2011). It was shown that this substrate was subject to the intense deformation and was affected by high-Ca boninite melts that was resulted in the dunitization at the initial stages of subduction. The study of the geochemistry of clinopyroxene has also confirmed the suprasubduction origin of the Voikar igneous complexes. The parental melts recorded in igneous rocks show the geochemical evolution of a mantle source from slightly depleted, relative to those of MORB, to strongly depleted with high Sr/Zr ratios (Pertsev et al., 2003).

The study of the geochemistry of the mantle peridotites of the Polar Urals, which has been started in works (Shmelev et al., 2018; Shmelev, 2011), enables us to characterize in general the processes of depletion, fluid-magmatic transformation and metamorphism in the history of the formation of these rocks. This work presents the main results of geochemical (LA-ICP-MS) study of minerals (pyroxenes and amphibole) from all peridotite massifs of the Polar Urals. The new data obtained enable us to discuss in more details the nature of the compositional heterogeneity of these mantle formations.

1 GEOLOGICAL BACKGROUND 1.1 Tectonic Setting

Peridotite massifs are a part of the extended (~400 km) Polar Ural ophiolite belt, which is constrained by the main Uralian fault (MUF) (Fig. 1). This belt comprises the following allochthonous peridotite massifs: Syum-Key in the north, and the Rai-Iz and Voikar in the south, which overthrust the Paleozoic sedimentary or Precambrian metamorphic formations of the paleocontinental sector of the Urals. At the base of these massifs are fragments of polymictic serpentinite mélange zone, as well as, zones of garnet amphibolites and glaucophane schists (Dobretsov et al., 1977). Mantle peridotites are overlain (rarely underlain) by rocks of the dunite-clinopyroxenite-gabbro and dyke ophiolite complex. To the east (south-east), ophiolites are intruded by a large gabbro-diorite-tonalite intrusive massif (Sob Complex) of the Devonian age (Shmelev and Meng, 2013; Yazeva and Bochkarev, 1984).

Figure 1. Tectonic settings of the Polar Urals ophiolite complexes. The map inset (top left) shows the position of peridotite massifs in the structure of the Polar Urals. The rectangle shows the study area in the Voikar massif (Batanova et al., 2011). Ⅰ, Ⅱ. Kharbey and Kharamatolou blocks; MUF. main Uralian fault.

Based on U-Pb (zircon) age dates of gabbroids and plagiogranites, ophiolite association of the Polar Urals have Late Ordovician age (450-460 Ma) (Remizov et al., 2010). At this, chromitites from peridotites yield a wide range of Precambrian ages (from 600 Ma to 1.8-2.1 Ga) (Savelieva et al., 2013). Thus, one can suggest the existence of a prolonged time gap in the period of formation of mantle and crustal parts of the ophiolite sequence of the Polar Urals.

1.2 Geological Structure

Structural and compositional features of the peridotite massifs in the Polar Urals are considered in many works (Chashchukhin et al., 2007; Shmelev, 1991; Savelieva, 1987; Makeev et al., 1985; Dobretsov et al., 1977). We have used the unified structural scheme (Fig. 1), according to which the following rock units are distinguished in these massifs: lherzolite-harzburgite (LH), dunite-harzburgite (DH), and dunite-clinopyroxenite- gabbro (DCG). In addition, the metaperidotites unit is distinguished within the Rai-Iz massif (Shmelev, 2011). In general, the peridotite massifs have the complex zonal structure represented by the occurrence of zones and areas of the DH-unit, separating formations of the LH-unit (Fig. 1).

The lherzolite-harzburgite unit corresponds to the mantle protolith, which is represented by spinel lherzolites (Rai-Iz and Syum-Keu massifs), diopside and rare typical harzburgites (Figs. 2a-2b). Rocks have massive homogenous or schlieren-banded structure, underlined by indistinct orientation of mineral flatness (foliation) and enclose veins and small bodies of pyroxenites, websterites, and dunites (Figs. 2c-2f). The peridotites of this type were subject to high-temperature deformations that led to the formation of gently dipping synform and antiform folded megastructures (Shmelev, 2011).

Figure 2. Photographs of peridotite outcrops of the Polar Urals. (a) Lherzolites (nonfoliated), Rai-Iz massif, sample 1110; (b) diopside harzburgites of schlieren structure, Voikar massif, sample 2117; (c) harzburgite with a parallel system of dunite bands, Voikar massif, sample 2085; (d) harzburgite with a parallel system of pyroxenite veins (bands), Syum-Keu massif, sample 2341; (e) olivine-enstatite rock (metaperidotite), Rai-Iz massif, sample 2327; (f) wehrlite-clinopyroxenite zonal vein in harzburgite, Voikar massif, sample 2015. Lhz. Lherzolite; Hz. harzburgite; Du. dunite; Cpxt. clinopyroxenite; Werl. wehrlite.

The dunite-harzburgite unit is widespread in the peridotite massifs and is characterized by a high proportion (≥30%-70%) of the dunite component. The matrix of the DH-unit is represented by banded and massive harzburgites usually with distinct deformational mineral foliation. The dunites occur as a stockwork of veins and concordant bands in harzburgites, as well as large (up to 1-3 km2) sub-isometric and lens-shaped bodies. The dunites enclose the occurrences of massive, nodular, and impregnated chromite ores (Perevozchikov et al., 2005). The central chromite deposit, discovered within the Rai-Iz massif, has been developed since 1994. The association of ultrahigh-pressure minerals, including diamond, has been identified in chromitites of this deposit for the first time in the Urals (Yang et al., 2015).

The metaperidotite unit is distributed within the latitudinal metamorphic zone of the Rai-Iz massif (Fig. 1). The matrix of the MP-unit is represented by the secondary harzburgites comprising bodies of massive olivine-enstatite and enstatite rocks (sagvandites) (Fig. 2e) without traces of superimposed serpentinization. The bodies of the metabasites (amphibolites, garnet amphibolites) are closely associated with these harzburgites.

The dunite-clinopyroxenite-gabbro complex is usually separated by the dunite zone from the mantle peridotites. The latter is changed abruptly or gradually by the alternation of wehrlite, clinopyroxenite, and gabbro. In terms of structural and compositional features, they correspond to layered formations (cumulates) (Pertsev et al., 2003; Savelieva, 1987).

1.3 Samples Description

In total, about 30 rock samples from the peridotite massifs of the Polar Urals (Fig. 1) have been studied. Among them are lherzolites, diopside- and normal harzburgites, metaperidotites and pyroxenites (Fig. 3, Table 1).

Figure 3. Microphotographs and BSE image showing the microstructural features of the Polar Urals peridotites. (a) Lherzolite, protogranular in texture, Syum-Keu massif, sample 972; (b) diopside harzburgite, containing large tabular enstatite grains, Voikar massif, sample 2129; (c) harzburgite, protogranular in texture, Syum-Keu massif, sample 1093; (d) enstatite-olivine rocks with nematogranoblastic texture, containing dolomite grains and clinochlore laths, Rai-Iz massif, sample 157; (е) olivine websterite, mesogranular in texture, Voikar massif, sample 2014; (f) BSE image showing a large diopside grain with thin enstatite-spinel exsolution lamella and pargasite grains in lherzolite, sample 972. Ol. Olivine; Sp. spinel; En. enstatite; Di. diopside; Hbl. hornblende; Chl. clinochlore; Dol. dolomite (magnesite)
Table 1 Characteristics of the studied peridotite samples

Lherzolites (Figs. 3а, 3f), protogranular in texture, are composed of olivine (65%-75%), enstatite (19%-25%), diopside (5%-10%), as well as spinel (1%-3%) and amphibole (up to 1%-5%). The olivine matrix is represented by large (2-5 mm) xenomorphic grains, including tabular enstatite (1-3 mm) and smaller diopside grains (0.2-1.5 mm). Spinel is represented by large olive to brownish irregular (holly-leaf) grains, often occurring in clusters with pyroxenes. Colorless amphibole forms small (up to 0.5 mm) interstitial grains, ingrowths in diopside, and rims around pyroxene grains.

Diopside harzburgites are similar in texture to lherzolites, but are different in lower proportion of pyroxenes (including clinopyroxene), as well as in more chromium and less aluminous composition of spinel (Table 1). In samples, diopside occurs as small (0.5-1.0 mm) xenomorphic grains, while enstatite as large (up to 3-5 mm) tabular grains (Fig. 3b).

Amphibole harzburgites represent a relatively rare rock variety, developed after lherzolites and diopside harzburgites (Table 1, samples 768, 1049). They are characterized by an absence of clinopyroxene. Instead of the latter, pargasite amphibole, equilibrated with enstatite, is widespread.

Harzburgites are composed of olivine (70%-80%), orthopyroxene (15%-25%) and chromium spinel (1%-3%); amphibole and clinopyroxene occur in smaller amounts (1%-3%) (Table 1). In terms of textural features, harzburgites vary from proto- and mesogranular to porphyroclastic (Fig. 3c). Olivine and orthopyroxene occur as large xenomorphic grains (porphyroclastes) and small neoblasts. Clinopyroxene is represented by small (≤0.5 mm) grains, while amphibole occurs as subisometric grains and rims. Spinel develops along boundaries or inside olivine and pyroxene grains.

Metaperidotites, nematogranoblastic and porphyroblastic in texture (Fig. 3d), are composed of olivine, enstatite, tremolite, clinochlore, magnesite (dolomite), and Cr-magnetite. Enstatite occurs as large porphyroblasts, containing inclusions of other minerals of the matrix.

Dunites and olivine clinopyroxenites (websterites) (Table 1, Fig. 3e) from veins, bodies, and layers, composed of variable amounts of olivine, clinopyroxene, orthopyroxene, and spinel. In terms of texture, rocks vary from granoblastic to hypidiomorphic and poikilitic. Mineral crystals show no zoning. Secondary alterations are manifested as serpentinization and the development of tremolite and magnetite.


The peridotites were preliminary studied in thin sections and polished thin sections under polarization (petrographic) and scanning electron (JSM-6390) microscopes.

The chemical compositions of rock-forming minerals (olivine, pyroxenes, amphibole, and spinel) were studied on a Cameca SX-100 electron microprobe (Institute of Geology and Geochemistry UB RAS) and additionally on a JEOL JXA 8800 electron microprobe (Kanazawa University, Japan) with a 20 kV accelerating voltage, a 20 nА beam current, and 1-3 μm beam diameter. Spinel, olivine, diopside, and garnet were used as standards under measurements.

Concentrations of trace elements were measured in pyroxenes and amphiboles using laser ablation (193 nm ArF excimer: MicroLas GeoLas Q-Plus) inductively coupled plasma mass spectrometry (Agilent 7500s, Yokogawa Analytical Systems) at the Kanazawa University (Morishita et al., 2005; Ishida et al., 2004) under the following conditions: laser beam diameter, 50-110 μm, frequency, 10 Hz, an energy density of 6 J/cm2 per pulse. The signal integration times were 40 s for a background interval and 30 s for an ablation interval. The NIST SRM 612 standard was used for a primary calibration. Concentrations of elements for NIST SRM 612 were taken from (Pearce et al., 1997). The data reduction was facilitated using 29Si as internal standards for pyroxenes, based on SiO2 contents, following a protocol essentially identical as that given in (Longerich et al., 1996).


This section presents the data on chemical and trace element compositions of rock-forming minerals of peridotites (Tables 2-4, Tables S1-S3, Figs. 4-7), which essentially supplements the data given in earlier works (Savelieva et al., 2016; Batanova et al., 2011; Chashchukhin et al., 2007; Pertsev et al., 2003; Savelieva, 1987).

Figure 4. Diagrams showing the variations in the chemical composition of minerals from the Polar Urals peridotites (a)-(d). (a) NiO (wt.%) vs. Mg# in olivine; (b) Cr# vs. Mg# in spinel; (c) Al2O3 (wt.%) vs. Mg# in clino- and orthopyroxenes; (d) Mg#-Si (a.p.f.u.) diagram (Leake et al., 1997) for amphiboles. Cpx, Opx. clino- and orthopyroxene; Tr. tremolite; Ed. edenite; Mhbl. magnesiohornblende; Prg. pargasite. Mg#=Mg2+/(Mg2++Fe2+); Cr#=Cr/(Cr+Al). The lines connect composition points, belonging to the same samples. Composition fields of spinel in abyssal after Dick and Byllen (1984) and fore-arc peridotites after Iahii et al. (1992). Composition fields of olivine (Ⅰ) and spinel (Ⅱ) in rocks of the Voikar massifs are outlined by dashed line after Batanova et al. (2011).
Figure 5. The chondrite-normalized REE and trace element patterns of clinopyroxenes (a)-(j) and orthopyroxenes (k)-(l) from the Polar Urals peridotites. Normalizing values after Sun and McDonough (1989). Clinopyroxenes: (a)-(b) in lherzolites; (c)-(d) in diopside harzburgites; (e)-(f) in harzburgites of the LH-unit; (g)-(h) in harzburgites of the DH-unit; (i)-(j) in clinopyroxenites and websterites. Orthopyroxenes: (k)-(l) in peridotites; Lzh. lherzolites; DiHz. diopside harzburgites; Hz. harzburgites; Mp. metaperidotites; Px. pyroxenites. The gray area corresponds to the composition in peridotites and pyroxenites (Batanova et al., 2011). The arrowhead line shows the variation in pyroxene composition from early to late generation (Fig. 5a). Sample numbers correspond to those in Table 1.
Figure 6. The chondrite-normalized REE and trace element patterns of amphiboles from the Polar Urals peridotites. Normalizing values after Sun and McDonough (1989). (a)–(b) in lherzolites; (c)–(d) in diopside harzburgite; (e)–(f) in harzburgites; (g)–(h) in metaperidotites. Tr. tremolite; Ed. edenite; Mhbl. magnesiohornblende; Prg. pargasite.
Figure 7. The chondrite-normalized REE patterns in rocks and minerals from representative peridotite samples. Normalizing values after Sun and McDonough (1989). Cpx. Clinopyroxene; Opx. orthopyroxene; Amph. amphibole; Rock. rock composition.
Table 2 Representative trace element compositions (ppm) of clinopyroxene from the Polar Urals peridotites
Table 3 Representative trace element compositions (ppm) of orthopyroxene from the Polar Urals peridotites
Table 4 Representative trace element compositions (ppm) of amphibole from the Polar Urals peridotites
3.1 Major Elements

Olivine from peridotites is represented by forsterite with Mg# (Mg2+/(Mg2++Fe2+)) range of 0.89-0.96 (Fig. 4а, Table S1). It has NiO content (0.2 wt.%-0.5 wt.%) and extremely low CaO content (less 0.05 wt.%) typical for mantle rocks. The Mg# of olivine becomes noticeably higher in dunites and, especially, in metaperidotites. On the contrary, clinopyroxenites (websterites) exhibit a low Mg# (0.83-0.87) and NiO content (0.1 wt.%-0.25 wt.%) (Fig. 4а).

Spinel shows significant variations in Cr, Al, and Mg contents (Table S1). The increase in Cr, Fe, and Ti contents is recorded in spinel from lherzolites to harzburgites and dunites (Fig. 4b, Table S1). Metaperidotites are dominated by Cr- magnetite and magnetite. Spinel in pyroxenites has moderate Cr and Fe contents or occurs as magnetite.

Pyroxenes, represented by diopside and enstatite, are characterized by varying Mg# (correspondingly 0.93-0.95 vs. 0.90-0.92) at comparable level of Al2O3 contents (1 wt.%-4 wt.%) (Fig. 4с). The contents of Al2O3, TiO2, and Cr2O3 in pyroxenes decrease from lherzolites to harzburgites (Fig. 4c, Table S2). The relic diopside with high (5.4 wt.%) contents of Al2O3 and Cr2O3 (1.1 wt.%) was found in lherzolites from the Syum-Key massif (sample 972). Metaperidotites contain abundant Mg enstatite with extremely low Al2O3 (0.1 wt.%-0.5 wt.%) and CaO (< 0.02 wt.%) contents. Diopside from pyroxenites is characterized by variable Mg# and low Al2O3 content (0.9 wt.%-2.2 wt.%); enstatite has low Mg# (0.84-0.86) (Fig. 4с).

Amphiboles form a series from pargasite (edenite) to Mg amphibole and tremolite (Fig. 4d, Table S3). Lherzolites and diopside-bearing harzburgites contain aluminous (9 wt.%-14 wt.%, Al2O3) pargasite and edenite, while harzburgites are dominated by magnesiohornblende with low Al2O3 content (4.5 wt.%-9 wt.%). Metaperidotites contains only tremolite with low Al2O3 content (0.5 wt.%-3.0 wt.%).

3.2 Trace Elements 3.2.1 Clinopyroxene

Diopsides from lherzolites show gently arched HREE and MREE distribution spectra with divergent LREE behavior (Fig. 5а, Table 1). The HREE distribution curve shows a gradual decrease in samples 744, 1116 or slight increase in the LREE concentrations (spoon-shaped distribution curve) in sample 972. The MREE distribution curve shows, on the contrary, the well-defined decrease (ten times) in concentrations of the most of LREE (Rai-Iz massif, sample 1110). It should be noted that aluminous and chromium (primary?) diopside from the Syum- Keu massif (sample 972) has the highest REE concentrations. At this, small diopside grains occurring as intergrowths with spinel (sample 744/2) have lower REE concentrations, as compared with large diopside grains (sample 744/1).

In the Rai-Iz massif, on the contrary, we deal with another situation: the REE concentrations (especially, La and Ce) increase in small diopside grains (sample 1110/2). As a result, we have revealed two REE distribution trends-REE depletion and enrichment in later pyroxene generations of lherzolites. As seen on multi-elemental spectra (Fig. 5b), diopside has distinct Pb and Zr anomalies, as well as positive Ba and Sr (in part) anomalies. There is no Pb anomaly in the extremely depleted diopside from sample 1110/1.

Clinopyroxene from diopside harzburgites also demonstrates two REE distribution patterns (Figs. 5c, 5d, Table 1). The first pattern, similar to that in lherzolites, demonstrates a gradual, at first, and then sharp decrease in concentrations from MREE to LREE (samples 763, 2034, 2115). As in lherzolites, the tendency of slight decrease in REE concentrations in diopside (from core to rim) and an increase in REE concentrations in diopside neoblasts was established (Fig. 5, sample 763/2). In turn, the second REE distribution pattern is characterized ultimately by steeper slope of HREE distribution curves (samples 2120, 2129). This pattern was previously established as a result of studying pyroxenes of the Voikar massif (Batanova et al., 2011). The distribution pattern of incompatible elements in pyroxene (Fig. 5d) is comparable with the above-mentioned REE distribution pattern in lherzolites.

Diopside from harzburgites has the complex REE distribution pattern, especially for MREE and LREE (Figs. 5e-5h, Table 1). The harzburgites of the early LH-unit (Fig. 5e) have arched and linear REE distribution spectra with increase (sample 760) or decrease (samples 395, 1093) in LREE concentrations. The pyroxenes of the similar unusual type were found out earlier in harzburgites from the peridotite massifs in the Southern Urals (Spadea et al., 2003). There is the only case (sample 301), when the REE distribution spectrum is similar to that in diopside harzburgites of the Voikar massif (Fig. 5с, sample 2120) with lower MREE and HREE concentrations. The diopsides from harzburgites of the late DH-unit are characterized by the same-type positive linear REE distribution spectra and a gradual decrease from HREE to LREE (Fig. 5g). This series of diopside compositions shows sharp negative Pb and Zr-Hf and insignificant Ti-Y anomalies (Fig. 5f). The clinopyroxenes of this type dominate in the Syum-Key and Voikar massifs.

Diopside from clinopyroxenites and websterites shows a rather simple REE distribution pattern, which is characterized by an increase from LREE to MREE and subsequent "flattening" at the MREE and HREE levels that is resulted in the occurrence of "N-MORB-like" REE distribution trends (Figs. 5i, 5j). As seen in the multi-element variation diagram, pyroxenes show negative Pb and Zr-Hf anomalies and positive Sr anomaly (Fig. 5j). The geochemical features revealed are close to those in the Voikar massif (Batanova et al., 2011). In our case, however, the LREE concentration level is relatively higher.

3.2.2 Orthopyroxene

Enstatite, coexisting with clinopyroxene, has extremely low concentrations of trace elements. Among them, only HREE, rarely Ce, Sm, Eu, Gd can be determined with confidence in the most of minerals grains (Fig. 5k, Table 3). Enstatites are characterized by positive slope of HREE distribution spectra, much steeper than that in clinopyroxenes. Orthopyroxene from lherzolites are characterized in general by higher REE concentrations in comparison with harzburgites. Orthopyroxene from metaperidotites depleted in Al and Ca (sample 157) has low HREE concentrations. In general, despite the depletion in REE, enstatite has significant Zr, Ti, Sr, Pb, Nb, and Ba contents (Fig. 5l).

3.2.3 Amphibole

Amphibole, common mineral in most peridotite samples, is characterized by, as a rule, REE distribution spectra similar to those in clinopyroxene (Fig. 6, Table 4).

Pargasite from lherzolites shows two REE distribution patterns with similar concentrations of MREE and HREE. The first pattern (samples 972 and 744) is "spoon-shaped" due to the behavior of LREE (Fig. 6a). The latter (less aluminous) edenite, coexisting with pargasite, shows a significant decrease (2-3 times) in REE concentrations (sample 744/1). The second REE distribution pattern is characterized by a distinct LREE decrease (sample 1110). It is characteristic that pargasite from olivine-enstatite-amphibole rock has the similar REE distribution (Fig. 6а, sample 1049). As in diopside, the REE distribution spectra of amphiboles show negative Zr, Pb, Ti and positive Ba and Sr anomalies (Fig. 6b).

Pargasite (edenite) from diopside harzburgites is also characterized by the divergent REE distribution spectra similar to diopside. In the first case, there are the REE distribution curves with gentle decrease in concentrations of MREE and HREE (Fig. 6c). At this, magnesiohornblende coexisting with pargasite is characterized by lower REE distribution level (sample 763/1). In the second case, amphibole (samples 2129, 301) demonstrates steeper slope of MREE and HREE distribution curves. As seen on the multi-elemental spectra, amphibole has distinct negative Pb, Zr, Hf anomalies and Ba and Sr positive anomalies (Fig. 6d).

Magnesiohornblende from harzburgites is characterized by predominantly linear REE distribution spectra (Fig. 6e, samples 812, 1099, and 2028). Similar to diopside, Cr-edenite has unusual arched REE distribution pattern (sample 395). The REE variations in harzburgites show spectra with distinct negative Pb, Zr-Hf anomalies (Fig. 6f).

Tremolite from metaperidotites is characterized by low REE contents and inclined REE distribution spectra. The extremely depleted composition was established only for magnesian (Ti- and Na-free) tremolite (Fig. 6g, sample 280). Tremolite from metadunite(sample 325) is different only by somewhat higher REE contents. The multi-elemental spectra of tremolites are characterized by positive Sr and negative Pb, Zr, Hf anomalies (Fig. 6h).

3.2.4 Correlation of REE compositions in minerals and rocks

Comparison of REE distribution spectra in minerals and peridotites indicates the occurrence of three types of relationships (Fig. 7). The first type, from monomineral rocks (pyroxenites), is characterized by practically similar level of REE concentrations in clinopyroxene and the rock (Fig. 7a). The second type, characteristic of some lherzolites, shows a complete similarity of REE spectra in clinopyroxene, amphibole, and the rock (Fig. 7b). It is evident that both cases indicate to chemical equilibrium in the rock-mineral system. The third case, typical of harzburgites, shows a partial similarity of composition spectra, predominantly at the level of HREE and MREE (Figs. 7c-7f). It is important that REE spectra in rocks, in comparison with those in minerals, show no conjugate decrease in LREE and MREE contents. Instead, they even demonstrate a relative increase. A similar pattern was noted earlier in oceanic peridotites (Niu, 2004), affected by melts and fluids on peridotites. It is evident that the occurrence of Eu negative anomaly in rocks is associated with this process (Figs. 7d, 7f).

4 DISCUSSION 4.1 Spinel-Clinopyroxene Systematics of Peridotites

A comparison of the Yb and Cr concentrations in clinopyroxene and spinel, respectively, which are indicators of a degree of partial melting of the mantle substance, gives the general idea on the causes of variations in the composition of peridotites (Hellebrand et al., 2002, 2001; Arai, 1994). The Polar Urals mantle peridotites in the diagram (Fig. 8) are generally controlled by the abyssal peridotite field, located along the model trend of partial melting of a mantle MORB source in the spinel facies field. They form an almost continuous (a degree of melting, 5%-17%) lherzolite-harzburgite series of rocks.

Figure 8. The Cr# of spinel vs. Yb (ppm) of clinopyroxenes in the Polar Urals peridotites. Melting trend (thick arrowhead line) with a degree of melting in the spinel stability field defined by F=10·ln(Cr#Sp)+24 (Hellebrand et al., 2001). The filled area (AP) corresponds to the composition of abyssal peridotites (Hellebrand et al., 2002, 2001; Johnson et al., 1990). Trends of variations in the clinopyroxene composition under the influence of aqueous fluids and melts are shown with a vertical arrowhead line. Compositions of clinopyroxenes from spinel and plagioclase lherzolites of the Southern Urals (Spadea et al., 2003) are marked with a cross; those from harzburgites and dunites of the Voikar massif are marked with a star (Batanova et al., 2011).

Along with this dependence, the diagram shows a distinct "shift" of the composition points relative to the melting trend towards the field of low and high Yb values. The decrease in REE (and Yb) concentrations in clinopyroxenes from lherzolites and harzburgites in part is probably associated with high-temperature hydration under the influence of fluids and the redistribution of components into coexisting amphibole, for which the same or larger crystal/liquid partition coefficients values were established (Tiepolo et al., 2007). An increase in the ytterbium concentration in the clinopyroxene from harzburgites apparently reflects the effect of melts on peridotites. The dunites of the Voikar massif (Batanova et al., 2011) and the plagioclase lherzolites of the Southern Urals with extremely high ytterbium concentrations in clinopyroxene are an example of such effect of melts (fluids).

4.2 Systematics of Peridotites on Trace Element Compo- sition of Clinopyroxenes

A comparison of the REE concentrations in clinopyroxenes with model compositions that correspond to the conditions of fractional (non-modal) melting of the mantle in the spinel and garnet species fields can provide a more definite idea on the formation mechanisms of the peridotites (Jean et al., 2010; Tamura et al., 2008; Sano and Kimura, 2007; Bizimis et al., 2000). In our case, there exists the following situation (Figs. 9a-9c).

Figure 9. REE patterns for clinopyroxene from the Polar Urals peridotites relative to the model clinopyroxene compositions at the partial melting of a mantle source. Normalizing values after Sun and McDonough (1989). The model trends of REE distribution corresponding to the conditions of non-modal fractional melting of the MORB source in the spinel stability field are shown by gray dashed lines (Sp melting). The model trends of REE distribution corresponding to the conditions of partial melting in the garnet stability field (2%Grt melting) and spinel stability field after 5% melting in the garnet field are shown by blue dashed line. The calculation method and parameters used in this model, following Johnson et al. (1990), are from Sano and Kimura (2007). A MORB source is from Salters and Stracke (2004). The gray field (Fig. 11с) outlines "primary" compositions of clinopyroxenes from the Voikar massif formed corresponding to the conditions of partial melting in the garnet (6%) and spinel (8%-10%) stability fields (Batanova et al., 2011). Arabic numerals, numbers of samples.

Peridotites in the Syum-Keu massif differ markedly from each other in the composition of clinopyroxene (Fig. 9a). Clinopyroxenes from lherzolites and diopside harzburgites demonstrate the REE distribution spectra, which is the most closely related to the model one corresponding to the degree (6%-8%) of partial melting of the MORB source. At the same time, an increase in LREE (in lherzolites) clearly indicates the subsequent modification. Clinopyroxenes from harzburgites demonstrate a general discordancy of spectra with respect to model REE trends, except for two samples (808, 812). It can be inferred that clinopyroxenes from harzburgites are the result of a higher degree (13%-16%) of partial melting under a strong influence of fluid-magmatic processes.

In the Rai-Iz massif, clinopyroxene from lherzolites show two types of REE distribution (Fig. 9b). The REE distribution in the first-type clinopyroxene is nearly completely similar to the model distribution trend with partial (~10%) melting of a mantle source (sample 1110/1). The second-type clinopyroxene (sample 1116) shows the REE distribution similar to the model one (a degree of melting, 12%) only in the MREE-HREE level, while for light REEs, more than tenfold increase in concentrations was established. Such a behavior is definitely a consequence of the refertilization. In general, given that amphibole coexists with clinopyroxene, causing a decrease in Yb content (Fig. 8), the "true" degree of partial melting was at the level of about 7%-9%.

Clinopyroxene from harzburgites has various REE distribution spectra (Fig. 9b). In the first case, the REE distribution is close to the model MREE-HREE distribution, corresponding to an 18% degree of partial melting (sample 301). The LREE enrichment of clinopyroxene and the occurrence of the gently sloping distribution spectrum are connected with refertilization. Taking into account the occurrence of amphibole in the sample, one can assume that the "true" degree of partial melting is about 15%. In the second case, clinopyroxene demonstrates a hump-shaped distribution pattern with a negative slope of the Sm-Lu part of the spectrum, which is sharply different from the model distribution in the spinel species field (sample 395). Judging by the high Cr# of coexisting spinel (Fig. 8) and the Yb-Lu content, clinopyroxene was formed at the 16%-17% degree of partial melting in the spinel field, followed by modification of the composition during refertilization. Alternatively, the hump-shaped configuration of the spectrum is formally close to the model trend of partial melting (2%) in the garnet stability field.

In the Voikar massif clinopyroxenes from peridotites yield two groups of compositions with fundamentally different distribution patterns (Fig. 9c).

The first group is represented by clinopyroxenes from diopside and normal harzburgites of the western part of the massif. Clinopyroxenes from diopside harzburgites (samples 2034, 2115) retain the MREE-HREE distribution close to the model one, which corresponds to partial melting (10%-12%) under the spinel facies conditions, while in harzburgite (samples 2014, 2028) to the 15% degree of melting, respectively. In both cases, the deviation of the LREE compositions from the model distribution towards the enrichment indicates the development of the refertilization process.

The second group is represented by clinopyroxenes from diopside harzburgites of the eastern part of the massif (samples 2120, 2129) with distinctly steeper MREE-HREE composition spectra. The latter is a distinctive feature of peridotites that were formed initially under the garnet stability filed conditions (Hellebrand et al., 2001). Such a spectrum of compositions corresponds to the model distribution at partial melting, first in garnet (5%) and then in spinel (12%-14%) facies (Fig. 11c), which corresponds approximately to the 17%-19% degree of partial melting. Judging by the LREE composition, clinopyroxene from these samples experienced a varying degree of refertilization reaching the maximum value in sample 2120.

Figure 11. The REE distribution variations in amphibole-clinopyroxene pairs from the Polar Urals peridotites. Lhz. lherzolites; DiHz. diopside harzburgites; Hz. harzburgites. Arabic numerals, numbers of samples.

The data obtained are consistent with the previously obtained results (Batanova et al., 2011), according to which clinopyroxenes from the harzburgites of the eastern part of the Voikar massif were formed at the fractional melting in the garnet (6%) and melting in the spinel (8%-10%) stability fields (14%-16% degree of partial melting in total) (Fig. 9c). To some extent, the data confirm the validity of the original ideas that the Voikar massif harzburgites were formed due to partial melting of the mantle protolith in the garnet stability field (Sharma et al., 1995).

As shown above, clinopyroxene in most samples of the Polar Urals peridotites has a significantly higher LREE and even MREE concentrations under dry partial melting in comparison with model compositions. In general, this is resulted from the fluid-magmatic effect on peridotites (Muntener et al., 2010; Ulrich et al., 2010; Warren and Shimuzu, 2010).

As seen on Nd/Yb-Yb diagram (Bizimis et al., 2000), which reflects the evolution of clinopyroxenes under "dry and wet" conditions, only some part of compositions of lherzolites and diopside harzburgites studied are controlled by the trend of dry partial melting. Another significant part of harzburgite composition points lies along the aqueous fluid-induced melting and refertilization with the maximum concentration of elements in the fluid (Fig. 10a, max). Consequently, after the initial partial melting (5%-10%), rocks were probably subject to the fluid-induced melting and refertilization with enrichment in LREE. In this context, the similarity of compositions of harzburgites and clinopyroxenites can reflect their formation in the course of the unified (?) fluid-magmatic process. The shift of a number of clinopyroxene composition points towards higher Yb values (from peridotites of the Voikar massif and plagioclase lherzolites of the Southern Urals) (Fig. 10a) was probably due to penetration of melts. The similar REE distribution patterns were established for trace elements (Ti, Zr, etc.) in clinopyroxenes from peridotites (Fig. 10b).

Figure 10. The Nd/Yb vs. Yb (ppm) and Ti vs. Zr (ppm) diagrams for clinopyroxenes from the Polar Urals peridotites (a)-(b). Dry melting—a compositional trend of clinopyroxene at the dry melting (incremental batch melting at 0.1% increments) of the MORB source (numerals denote a degree of melting (%)); hydrous melting—a compositional trend of clinopyroxenes at the fluid-induced melting and refertilization with minimum (min) and maximum (max) concentrations of components in the fluid; melt interaction—a compositional trend of clinopyroxenes under the influence of penetrating melts. Source composition, mineralogy and melting mode from Kelemen et al. (1993), Johnson and Dick (1992), Johnson et al. (1990), the calculation method from Bizimis et al. (2000). AP, SSZ are composition fields of clinopyroxenes from abyssal peridotites (Johnson and Dick, 1992; Johnson et al., 1990) and suprasubduction peridotites (Bizimis et al., 2000; Parkinson et al., 1992), respectively. Other designations are shown in Fig. 8.

As seen in the diagram (Fig. 10b), the composition points of pyroxenes from lherzolites and diopside harzburgites are localized mainly along the trend line of dry partial melting in the spinel facies stability field, controlled by the field of abyssal peridotites. By contrast, the composition points of pyroxenes from harzburgites are shifted to the field bounded by fluid- induced trends of melting, Ti- and Zr-depleted clinopyroxenes from suprasubduction peridotites. The relative enrichment of clinopyroxenes from harzburgite with Zr (> 10 ppm) is probably the result of their reactionary interaction with the melt.

4.3 Systematics of Peridotites Based on Trace Element Composition of Amphiboles

Amphibole is represented by a series of compositions from pargasite to tremolite in metaperidotites (Fig. 4) that records the evolution of the mantle protolith in the changing conditions. The amphiboles appear in the lherzolites and harzburgites due to the process of mantle metasomatism accompanied by the redistribution of REE and some rare elements from pyroxenes. The reequilibrium of compositions resulted in the observable similarity of REE distribution spectra in clinopyroxenes and amphiboles (Figs. 5-7). The ratios of elemental concentrations calculated for amphibole-clinopyroxene pairs demonstrate the subhorizontal spectra with high (1-8 units) KDAmph/Cpx ratio values (Fig. 11). The constant slope of the distribution trends at varying KD values reflects the equilibrium between minerals (Muntener et al., 2010), which was established at relatively different temperatures.

The crystallization of amphibole is also controlled by the geodynamic settings (Coltorti et al., 2007). In terms of Ti, Zr, and Nb contents, most amphiboles belong to the suprachondritic class, trending towards the field of suprasubduction (S-Amph) formations (Fig. 12). Nb-rich amphibole varieties typical of oceanic and intraplate (I-Amph) formations are absent in peridotites. Tremolites from metaperidotites are characterized by the minimum concentrations of these elements that is consistent with their metamorphic (with the participation of an aqueous fluid) nature.

Figure 12. Ti, Zr vs. Nb (ppm) diagrams for amphiboles from the Polar Urals peridotites. MORB and primitive mantle (PM) values are given after McDonough and Sun (1995). S-Amph. suprasubduction amphibole area; I-Amph. intraplate amphibole area (Coltorti et al., 2007). The composition of amphibole from peridotites of the Voikar massif (Batanova et al., 2011) is marked with a star.
4.4 Petrological-Geodynamical Model of Formation of the Polar Urals Peridotites

The compositional heterogeneity of the Polar Urals mantle peridotites resulted from the complex processes, occurring in the upper mantle and the continental crust in the changing geodynamic environments. The results of analyzing the data obtained provide a basis to assume that the formation history of peridotites was more complex, as it was previously supposed (Shmelev et al., 2018; Savelieva et al., 2016; Batanova et al., 2011; Shmelev, 2011), and included polystage partial melting under different depth conditions, mantle metasomatism and refertilization, depyroxenization (dunitization) and metamorphism. The following stages can be distinguished in the evolution of peridotites.

(Ⅰ) Formation of weakly and moderately depleted peridotites of the early LH-unit predominantly as a result of partial melting in the oceanic spreading environment at different-depth levels of the mantle lithosphere (Fig. 13a). At this stage the following events occurred: (a) Partial polybaric melting in deep "root" (as deep as 80 km and more) spreading zones, first in the garnet facies field, and then in the spinel one (a general degree of melting, 17%-19%), which was resulted in the formation of diopside harzburgites of the eastern part of the Voikar massif and, probably, the southern part of the Rai-Iz massif. This process is recorded in high-angle ("garnet") slope of REE distribution curves in clinopyroxene (Fig. 9с), noted earlier in Batanova et al. (2011). (b) Conjugate (?) partial melting of the mantle protolith at the level of the spinel facies depth with formation of lherzolites and diopside harzburgites (a degree of melting, 5%-10%) in all peridotite massifs of the Polar Urals. This process resulted in progressive depletion in LREE, which are typical of the spinel facies and are close to the model ones (Fig. 9b, sample 1110). (c) Refertilization of peridotites, induced by penetrating primitive MORB-melts with insignificant modification of compositions, including enrichment of rims of clinopyroxene grains and neoblasts with REE, as well as with trace elements such as Pb, Sr, Zr, Ti, and Y (samples 1110/2, 763/2).

Figure 13. The model of the formation of the Polar Ural peridotites in different geodynamic environments: (a) oceanic spreading stage; (b) subduction stage. The mantle protolith types, formed as a result of the partial melting in the garnet-spinel and spinel stability fields are shown by red and blue rectangles, correspondingly; a green rectangle is the mantle protolith formed as a result of the fluid-induced melting. Low- and moderate-temperature (Ant+Liz) and high-temperature (En+Tr+Chl) stability fields of metaperidotites are given after Khedr et al. (2010). Stages Ⅰ, Ⅱ and Ⅲ indicate the stages of peridotite formation (see text). Dashed arrowhead lines show the direction of the substance flow in the mantle. Blue arrows indicate the direction of the rise of fluids/ melts. Thermal pattern according to Peacock and Wang (1999); MUF controlling the locality of peridotite massifs.

(Ⅱ) Formation of harzburgites and dunites of late DH-unit on the mantle protolith (Ⅰ) in the suprasubduction geodynamic environment (Fig. 13b). This stage is characterized by complex transformation of the substrate, including the following: (a) fluid- induced partial melting (13%-17%) of peridotites, resulted in the formation of depleted harzburgites with linear or more complex REE distribution spectra in clinopyroxene. (b) Refertilization of peridotites with enrichment of clinopyroxenes with LREE and MREE relative to the model REE distribution and a decrease in Ti content that is typical of suprasubduction compositions (Figs. 9, 10). As evidenced from geochemical differences between rocks and minerals (pyroxenes, amphiboles) (Fig. 7) it is probable that peridotites were additionally enriched with REE-bearing minerals (zircon, etc.) in course of this process. (c) Crystallization of amphibole (pargasite-magnesiohornblende series) during the fluid infiltration. In some cases, the process was accompanied by the disappearance of clinopyroxene and the development of specific olivine-enstatite-pargasite rocks (samples 768, 1049). (d) Depyroxenization of harzburgites due to penetrating boninite melts above the subduction zone (Batanova et al., 2011; Pearce et al., 2000) in areas of ductile-brittle deformations that was resulted in the formation of dunite veins (bodies) and chromite mineralization (Shmelev, 2011).

The transformations of peridotites of the stage Ⅱ occurred under oxidized conditions (QFM≥+0.5÷2.0) (Shmelev, 2011) that is characteristic of the suprasubduction setting conditions.

(Ⅲ) Formation of metaperidotites in tectonic shear zones during the subduction and the subsequent obduction of peridotites and ophiolites. The high-temperature fluid-induced metamorphism of the rocks (probably at the slab-mantle wedge boundary) led to the formation of olivine-enstatite-amphibole (clinochlore) rocks (Rai-Iz massif) (Shmelev, 2011). Newly- formed tremolites in these rocks are REE-depleted and are characterized by the similar sloping REE distribution spectra (Fig. 6). The processes of moderate- to low-temperature metamorphism, resulted in the formation of olivine-antigorite (amphibole, talc) rocks can be associated with the obduction stage.

When developing the model of the formation and evolution of the Polar Urals peridotites, no attention has been given to the nature of the paragenesis of UHP minerals found out in the chromitites of the Rai-Iz massif. Present interpretations suggest the formation of the diamond-bearing mineral paragenesis at great depths in the mantle transition zone (Arai and Miura, 2016; Yang et al., 2015) and its transport to the level of the shallow upper mantle. There is still no clear answer whether this process was in reality or not. Our studies also provide no answer to this question. However, the discovery of peridotites bearing the geochemical features of both spinel and deeper garnet facies of stability enables us to suggests that the occurrence of the diamond-bearing mineral paragenesis is far from being accidental.


Ophiolite massifs of the Polar Urals are an excellent example of coexistence of heterogeneous mantle peridotites. Based on new LA-ICP-MS data, two types of primary mantle peridotites are distinguished for the first time. The first type is represented by products of the partial melting of the mantle protolith under the spinel facies conditions, while the second one is represented by lherzolites (diopside harzburgites) formed under spinel facies conditions. The formation of ophiolite association occurred in the oceanic spreading environment. The subsequent transformation of the mantle protolith already occurred in the suprasubduction environment. The fluid-induced melting of the mantle protolith was resulted in the formation of harzburgites, experienced the refertilization with LREE enrichment of clinopyroxenes. At this, depyroxenization of peridotites was resulted in the formation of veins (bodies) of dunites and chromitites. During the processes of subduction and obduction, peridotites experienced different- grade metamorphism, and then they were exhumed to the upper crustal horizons.


We are grateful to Dr. Mohamed Zaki Khedr (Kanazawa University, Japan) for assistance in performing analytical studies, as well as Vladimir G. Kotelnikov (Federal State Institution « VSEGEI» , St. Petersburg, Russia) and Dmitriy V. Kuznetsov (Institute of Geology and Geochemistry UB RAS, Yekaterinburg, Russia) for their assistance in carrying out field works in the Polar Urals. We are very grateful to the reviewers for their constructive comments that have been very helpful in improving the manuscript. This study was carried out within the framework of the Project IGCP-649 and the IGG UB RAS (No. АААА-А18- 118052590029-6). The final publication is available at Springer via

Electronic Supplementary Materials: Supplementary materials (Tables 1-3) are available in the online version of this article at

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