GD2.4

The persistence of Archaean cratons for >2.5Ga was aided by thick, mechanically strong, and cool lithospheric mantle keels up to 250km deep. It is widely accepted that the cratonic mantle, dominated by depleted harzburgite, lherzolite and dunite, was formed by extensive melt extraction from originally fertile mantle peridotite. Models seeking to explain the formation of deep cratonic mantle in the garnet and diamond stability fields, initially sought to answer how such rocks could form in-situ at high temperatures and pressures and envisaged large-scale thermochemical plume upwellings. More recently, mineralogical and geochemical observations, namely the high Cr content of garnet and low whole rock HREE concentrations in cratonic harzburgites, have led to the conclusion that the deep cratonic mantle couldn’t have originally melted in the garnet stability field.  Mechanical stacking of shallowly depleted oceanic lithosphere was therefore proposed to have thickened the depleted lithosphere cratonic roots. In this process, the spinel facies minerals are envisaged to transform into the garnet stability field.

Here we present the first results of combined thermodynamic and geochemical modelling at temperatures high enough to reconcile the very refractory residues. We found that the requirement for initially shallow melting is no longer supported. Deep (150-250km), ultra-hot (>1800°C), incremental melting can produce the mineralogical and geochemical signatures of depleted cratonic harzburgites. The modelling also implies a link between areas of extreme depletion in the deep lithospheric mantle and the genesis of Earth’s hottest lavas (Al-enriched komatiite) by re-melting depleted harzburgite. Diamond inclusion minerals have a well-documented skew to the most refractory compositions found in cratonic peridotite. We propose that these ultra-depleted, highly reducing regions of the lithospheric root possess the highest diamond formation and preservation potential.

How to cite: Walsh, C., Kamber, B., and Tomlinson, E.: Deep ultra-hot melting in cratonic mantle roots, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6723, https://doi.org/10.5194/egusphere-egu22-6723, 2022.

The system CaCO₃-MgCO₃ has been used since the '60s for reconstructing the petrogenesis of carbonated lithologies, notably of carbonatite magmas possibly generated in the Earth's mantle. Yet, experimental results at high temperatures and pressures remain contradictory, and a thermodynamic model for the carbonate liquid in this binary is still lacking.

We experimentally investigated the melting of aragonite and magnesite to pressures of 12 GPa, and of calcite-magnesite mixtures at 3 and 4.5 GPa, and at variable Mg/(Mg+Ca) (X_Mg). Results show that the melting of aragonite, and of magnesite have similar slopes, magnesite melting ≈ 30 °C higher than aragonite. The minimum on the liquidus surface is at X_Mg ≈ 0.35-0.40, 1200 °C at 3 GPa, and 1275 °C at 4.5 GPa, which, when combined with data from Byrnes and Wyllie (1981) and Müller et al. (2017), imply that minimum liquid composition remains approximately constant with pressure increase. We present the first thermodynamic model for CaCO₃-MgCO₃ liquids, retrieved from the experimental data available. Although carbonate liquids should be relatively simple molten salts, they display large non-ideality and a three-component (including a dolomite component), pressure dependent, asymmetric solution model is required to model the liquidus surface. Attempts to use an end-member two-component model fail, invariably generating a very wide magnesite-liquid loop, contrary to the experimental evidence.

The liquid model is used to evaluate results of experimentally determined phase relationships for carbonated peridotites modelled in CaO-MgO-SiO₂-CO₂ (CMS-CO₂), and CaO-MgO-Al₂O₃-SiO₂-CO₂ (CMAS- CO₂). Computations highlight that the liquid composition in the CMS-CO₂ and CMAS-CO₂ and in more complex systems do not represent "minimum melts" but are significantly more magnesian at high pressure, and that the pressure-temperature position of the solidus, as well as its dP/dT slope, depend on the bulk composition selected, unless truly invariant assemblages occur. Calculated phase relationships are somewhat dependent on the model selected for clinopyroxene, and to a lesser extent of garnet.

Byrnes A.P. and Wyllie P.J. (1981) Subsolidus and melting relations for the join CaCO₃-MgCO₃ at 10 kbar. Geochim. Cosmochim. Acta 45, 321-328

Müller I.A., Müller M. K., Rhede D., Wilke F.D.H. and Wirth R. (2017) Melting relations in the system CaCO₃-MgCO₃ at 6 GPa. Am. Mineral. 102, 2440-2449.

How to cite: Poli, S., Zhao, S., and Schmidt, M. W.: An experimental determination of the liquidus in the system CaCO3-MgCO3 and a thermodynamic analysis of the melting of carbonated mantle melting, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1531, https://doi.org/10.5194/egusphere-egu22-1531, 2022.

The deep structure of orogenic belts and cratons has become an important part to track evolution and innovation of tectonics. The extremely thick crust and overlying deposition bring obstacles to the deep structure of the orogenic belt and ancient block. Deep seismic reflection profile is globally regarded as an advanced technology to perspective the fine structure of the crust and the top of the upper mantle, especially using large-size dynamite shots. In the 1990s, international scholars used deep seismic reflection profiles to find inclined reflections penetrating from the lower crust to the upper mantle (Calvert et al., 1995; Cook et al., 1999). They believe that these reflections are related to ancient subduction events(or fossil subduction). At the beginning of this century, Chinese scholars began to carry out similar experiments in the Tibet Plateau, Sichuan Basin and Songliao basin. Using big-size dynamite shots, they also found the Moho under the extremely thick crust of the Tibet Plateau and the mantle reflection under the ancient block (Gao et al., 2013, 2016; Zhang et al., 2015). In 2016, with the support of China Geological Survey Project,we arranged a seismic reflection profile around the Scientific Deep Drilling SK-2 Well in the middle of Songliao basin. According to the data processing results of all five big-size dynamite shots and four medium-size dynamite shots of the profile, we obtained a 127.3km long single-fold reflection profile, revealing the reflection characteristics of the lower crust, Moho and its upper mantle in the study area. The Moho structure distributed nearly horizontally at a depth of 33km (estimated by the average crustal velocity of 6km/s) is clearly obtained, and the mantle reflection extending obliquely from Moho to 80km-depth is found. We believe that this dipping mantle reflection represents an ancient subduction relic under the Songnen block.

Calvert, A. J., Sawyer, E. W., Davis, W. J., & Ludden, J. N.  Archaean subduction inferred from seismic images of a mantle suture in the Superior Province. Nature,1995, 375(6533), 670–674.

Cook,F. A., van der Velden, A. J., Hall, K. W., Roberts, B. J.Frozen subduction in Canada’s Northwest Territories: lithoprobe deep lithospheric reflection profiling of the western Canadian Shield. Tectonics 1999,18, 1–24.

Gao R, Chen C, Lu Z W, et al.New constraints on crustal structure an d Moho topography in Central Tibet revealed by SinoProbe deep seismic reflection profiling. Tectonophysics, 2013, 606:160 - 170.

Gao, R., Chen, C., Wang, H. Y., Lu, Z. W., et al.Sinoprobe deep reflection profile reveals a neo-Proterozoic subduction zone be neath Sichuan basin. Earth & Planetary Science Letters, 2016,454(18):86-91

Zhang, X. Z.,Zheng, Z.,Gao, R., et al. Deep reflection seismic section evidence of subduction collision between Jiamusi block and Songnen block. Journal of Geophysics, 2015,58 (12): 4415-4424

How to cite: Li, M., Gao, R., Zhou, J., Wilde, S. A., Hou, H., Tan, X., and Zhu, Y.: Deep seismic reflection profile with big-size dynamite shots reveals Moho and mantle reflection: tracking continental evolution, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3326, https://doi.org/10.5194/egusphere-egu22-3326, 2022.

Most researchers believe that large igneous provinces (LIPs) are formed by adiabatic melting of heads of ascending mantle plumes. Because the LIPs have existed throughout the geological history of the Earth (Ernst, 2014), their rocks can be used to probe the plume composition and to decipher the evolution of deep-seated processes in the Earth’s interior.

The early stages of the LIPs evolution are discussed by the example of the eastern Fennoscandian Shield, where three major LIP types successively changed each other during the early Precambrian: (1) Archean LIP composed mainly of komatiite-basaltic series, (2) Early Paleoproterozoic LIP made up mainly of siliceous high-Mg series, and (3) Mid-Paleoproterozoic LIP composed of picrites and basalts similar to the Phanerozoic LIPs (Sharkov, Bogina, 2009). The two former types of LIPs derived from high-Mg depleted ultramafic material practically were extinct after the Mid-Paleoproterozoic, whereas the third type is survived till now without essential change. The magmas of this LIP sharply differed in composition. Like in Phanerozoic LIPs, they were close to E-MORB and OIB and characterized by the elevated and high contents of Fe, Ti, P, alkalis, LREE, and other incompatible elements (Zr, Ba, Nb, Ta, etc.), which are typical of geochemically enriched plume sources.

According to modern paradigm (Maruyama, 1994; Dobretsov, 2010; French, Romanowiсz, 2015, etc.), formation of such LIPs is related to the ascending thermochemical mantle plumes, generated at the mantle-liquid core boundary due to the percolation of the core’s fluids into overlying mantle. Thus, these plumes contain two types of material, which provide two-stage melting of the plume’s heads: adiabatic and fluid-assisted incongruent melting of peridotites of upper cooled margins (Sharkov et al., 2017).

These data indicate that the modern setting in the Earth’s interior has existed since the Mid Paleoproterozoic (~2.3 Ga) and was sharply different at the early stages of the Earth’s evolution. What was happened in the Mid Paleoproterozoic? Why thermochemical plumes appeared only at the middle stages of the Earth’s evolution? It is not clear yet. We suggest that this could be caused by the involvement of primordial core material in the terrestrial tectonomagmatic processes.  This core survived from the Earth’s heterogeneous accretion owing to its gradual centripetal warming accompanied by cooling of outer shells (Sharkov, Bogatikov, 2010).

References

Dobretsov, N.L. (2008). Geological implications of the thermochemical plume model. Russian Geology and Geophysics, 49 (7), 441-454.

Ernst, R.E. (2014). Large Igneous Provinces. Cambridge Univ. Press, Cambridge, 653 p.

French, S.W., Romanowicz, B. (2015). Broad plumes rooted at the base of the Earth’s mantle beneath major hotspots. Nature, 525, 95-99.

Maruyama, S. (1994). Plume tectonics. Journal of Geological Society of Japan, 100, 24-49.

Sharkov, E.V., Bogina, M.M. (2009). Mafic-ultramafic magmatism of the Early Precambrian (from the Archean to Paleoproterozoic). Stratigraphy and Geological Correlation, 17, 117-136.

Sharkov, E.V., Bogatikov, O.A. (2010). Tectonomagmatic evolution of the Earth and Moon // Geotectonics 44(2), 83-101.

Sharkov, E., Bogina, M., Chistyakov, A. (2017). Magmatic systems of large continental igneous provinces. Geoscience Frontiers 8(4), 621-640

How to cite: Sharkov, E.: The Late Cenozoic global activation of tectonomagmatic processes as a result of physico-chemical processes in the solidifying Earth’s core?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-968, https://doi.org/10.5194/egusphere-egu22-968, 2022.

The results of studying the granulite belts of the Earth show the presence of two types of granulite metamorphism in them: high-pressure and high-temperature ones.

     High-pressure granulites are characterized by P-T trends in the form of clockwise curves. According to widespread opinion,  the granulite metamorphism with such trends characterizes the areas that were formed as a result of the tectonic thickening of the crust due to continent-continent collisions that correspond to the model of the Himalayan type.

     High-temperature granulites are characterized by counterclockwise trends. For the formation of such granulites, researchers involve the mechanism of mantle underplating or the introduction of large volumes of intrusions under stretching. This model requires a mantle plume, which transports hot mantle material to the base of the crust.

  Thus, granulites with contrasting P-T trends, "orogenic" and "anorogenic" may be present inside the same belt. High-temperature granulites are superimposed on the dominant high-pressure ones. The time interval between these discrete events is not clearly defined and can be estimated in several tens of millions of years.

      Let's consider these two types of metamorphism against the background of the events of the supercontinental cycle (SC). Its structure consists of two stages: proper-continental (one continent-one ocean) and intercontinental (several continents-several oceans). In turn, the stages divide into phases. The first agglomeration phase of the proper-continental stage is characterized by compaction of already collected continental fragments. After the supercontinental culmination, the next, destruction phase begins, which precedes and prepares the break-up of the supercontinent. Its main content is continental rifting and the formation of the basic intrusions. The content of the first phase of the second stage consists of the break-up of the supercontinent, the formation of spreading zones and passive margins of young oceans. The next convergent phase of this stage is the assembly of the new supercontinent, the formation of subduction zones and the closure of young oceans as a result of numerous collisions.

     Based on the collision model of high-pressure granulite metamorphism, it is obvious that its formation will occur in this convergent phase of the SC, when, as a result of continent-continent collisions, a new supercontinent is assembled.

     Conditions for high-temperature granulite metamorphism in a tension environment arise in the phases of destruction and break-up of this supercontinent when plume processes are actively manifested as a result of the heat blanket effect.

      The analysis of the modern world factual material on supercontinental cyclicity for 3 billion years of the Earth history, conducted by the author, generally confirms the above correlation of the evolution of metamorphism during the development of granulite belts with events of SC.

Thus, these two types of granulite metamorphism, which fit into the structure of the super continental cycle, are indicators of geodynamic conditions of the corresponding stages and phases of the SC and show a complex interaction in the course of their manifestation of two geodynamic styles - the tectonics of lithospheric plates and mantle plumes.

How to cite: Bozhko, N.: On the manifestation of two types of granulite metamorphism during supercontinental cyclicity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-362, https://doi.org/10.5194/egusphere-egu22-362, 2022.

Eastern Anatolia (Eastern Turkey) resides in the Alpine-Himalayan orogenic belt and hosts the Eastern Anatolian Volcanic Province (EAVP), one of the volumetrically most important volcanic provinces within the circum-Mediterranean region. Previous studies have revealed that the predominant portion of EAVP is composed of the products of the sub-continental lithospheric mantle (SCLM) metasomatized during subduction of the Neo-Tethyan slab. The wide distribution of the lithospheric signatures in EAVP lavas has led to the availability of a large number of geochemical information regarding the regional SCLM in eastern Anatolia. In contrast, the nature of the asthenospheric mantle of eastern Anatolia remains poorly constrained due to scarcity of the asthenosphere-derived melts and lack of detailed information on the source components it comprises. Hence, this study aims primarily to put constraints on the chemical nature of asthenosphere beneath eastern Anatolia by a detailed characterization of its end-members.  

In this study, we provide new trace element and Sr-Nd-Hf-Pb isotope data from Quaternary Elazığ volcanism. This volcanism, entirely represented by mafic alkaline basaltic rocks, is one of the most recent members of EAVP, and its chemistry provides compelling evidence for a predominate asthenosphere origin. Modellings suggest that these mafic volcanics are largely free of crustal assimilation; their geochemical signatures, hence, closely reflect their source regions. Their trace element and Sr-Nd-Hf-Pb isotope systematics are consistent with derivation from an asthenospheric mantle source domain containing approximately 70% recycled oceanic lithologies with the characteristics of the C-like mantle component. However, minor contributions from depleted component (DM; ca. 20%) and an enriched component representing metasomatically modified SCLM (ca. 10%) are also needed to explain their total range of isotope data. With these findings, we propose that the C-like material is dispersed within the asthenosphere, and has mixed with the depleted mantle matrix beneath eastern Anatolia. The SCLM domains, on the other hand, occur as detached pods, following the lithospheric delamination in the region. Having triggered by the extensional dynamics during Quaternary, upwelling of the hot asthenosphere resulted in the melting of the C-DM and SCLM domains. Subsequently, the C-DM melts interacted with the SCLM-type melts, eventually generating the Elazığ volcanism.

How to cite: Aktağ, A., Sayit, K., Peters, B. J., Furman, T., and Rickli, J.: A relatively pristine C-like component in the eastern Anatolian asthenosphere, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-516, https://doi.org/10.5194/egusphere-egu22-516, 2022.

Basalts from high flux intra-plate volcanism (Iceland, Hawaii, Samoa) are characterised by ³He/⁴He that are significantly higher than those from the upper mantle sampled at mid-ocean ridges.  The prevailing paradigm requires that a largely undegassed deep Earth is enriched in primordial noble gases (³He, ²⁰Ne) relative to degassed convecting upper mantle.  However, the He concentration and ³He/²⁰Ne ratio of high ³He/⁴He oceanic basalts are generally lower than mid-ocean ridge basalts (MORB). This so called ‘He paradox’ has gained infamy and is used to argue against the conventional model of Earth structure and the existence of mantle plumes.  While the paradox can be resolved by disequilibrium degassing of magmas it highlights the difficulty in reconstructing the primordial volatile inventory of the deep Earth from partially degassed oceanic basalts.

Basalts from 26 to 11°N on the Red Sea spreading axis reveals a systematic southward increase in ³He/⁴He that tops out at 15 Ra in the Gulf of Tadjoura (GoT). The GoT ³He/⁴He overlaps the highest values of sub-aerial basalts from Afar and Main Ethiopian Rift and is arguably located over modern Afar plume.  The along-rift ³He/⁴He variation is mirrored by a systematic change in incompatible trace element (ITE) ratios that appear to define two-component mixing between E-MORB and HIMU.  Despite some complexity, hyperbolic mixing relationships are apparent in ³He/⁴He-K/Th-Rb/La space.  Using established trace element concentrations in these mantle components we can calculate the concentration of He in the Afar plume mantle.  Surprisingly it appears that the upwelling plume mantle has 5-20 times less He than the convecting asthenospheric mantle despite the high ³He/⁴He (and primordial Ne isotope composition). This contradicts the prevailing orthodoxy but can simply be explained if the Afar mantle plume is itself a mixture of primordial He-rich, high ³He/⁴He (55 Ra) deep mantle with a proportionally dominant mass of He-poor low ³He/⁴He HIMU mantle. This is consistent with the narrow range of Sr-Nd-Os isotopes and ITE ratios of the highest ³He/⁴He Afar plume basalts, and is in marked contrast to high ³He/⁴He plumes (e.g. Iceland) that do not have unique geochemical composition. The HIMU signature of the Afar plume basalts implies origin in recycled altered oceanic crust (RAOC). Assuming that no He is recycled and using established RAOC U and Th concentrations, the low He concentration (< 5 x 10¹³ atoms/g He) of the He-poor mantle implies that the slab was subducted no earlier than 70 Ma and reached no more than 700 km before being incorporated into the upwelling Afar plume. We suggest that the Afar plume acquired its chemical and isotopic fingerprint during large scale mixing at the 670 km transition zone with the Tethyan slab, not at the core-mantle boundary.

This study implies that large domains of essentially He-poor mantle exist within the deep Earth, likely associated with the HIMU mantle compositions. Further, it implies that moderately high-³He/⁴He (< 30 Ra) mantle plumes (e.g. Reunion) need not contain a significant contribution of deep mantle, thus cannot be used a priori to define primitive Earth composition.

How to cite: Balci, U., Stuart, F. M., Barrat, J.-A., and van der Zwan, F. M.: Low He content of the high 3He/4He Afar mantle plume: Origin and implications of the He-poor mantle, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4994, https://doi.org/10.5194/egusphere-egu22-4994, 2022.

[1] Boven et al., 1998, J. Afr. Earth Sci., 26, 463-476.

[2] Holmes and Harwood, 1932, Quarterly J. Geol. Soc., 88, 370-442.

[3] Le Maitre, 2002, Cambridge University Press.

[4] Rosenthal et al., 2009, Earth Planet. Sci. Lett., 284, 236-248.

[5] Link et al., 2008, 9^th Int. Kimb. Conf., 1-3.

[6] Foley, 1992, Lithos, 28, 435-453.

[7] Agostini et al., 2021, Sci. Rep., https://doi.org/10.1038/s41598-021-90275-7.

How to cite: Innocenzi, F., Ronca, S., Foley, S. F., Agostini, S., and Lustrino, M.: Exotic magmatism from the western branch of the East African Rift: insights on the lithospheric mantle source., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5450, https://doi.org/10.5194/egusphere-egu22-5450, 2022.

Large Igneous Provinces (LIPs) represent exceptionally brief (<1 Ma) voluminous magmatic events that punctuate Earth history, frequently leading to continental break-up, global climate changes and, eventually, mass extinctions. Most LIPs emplaced in continental settings are located near cratons, begging the question of a potential control of thick lithosphere on mantle melting dynamics. In this study we discuss the case of the Central Atlantic Magmatic Province (CAMP), emplaced in the vicinity of the thick lithospheric keels of the Precambrian cratons forming the central portion of Pangea prior to the opening of the Central Atlantic Ocean. In particular, we focus on CAMP magmas of the Prevalent group, ubiquitous all over the province, and of the Tiourjdal and High-Ti groups, emplaced (respectively) at the edges of the Reguibat and Leo-Man shields in north-western Africa, and the Amazonian and São Luis cratons in South America. As imaged by recent tomographic studies, there is a strong spatial correlation between most CAMP outcrops at surface and the edges of the thick cratonic keels. Geochemical modelling of trace element and isotopic compositions of CAMP basalts suggests a derivation by partial melting of a Depleted MORB Mantle (DMM) source enriched by recycled continental crust (1-4%) beneath a lithosphere of ca. 80 km. Melting under a significantly thicker lithosphere (>110 km) cannot produce magmas with chemical compositions similar to those of CAMP basalts. Therefore, our results suggest that CAMP magmatism was produced by asthenospheric upwelling along the deep cratonic keels and subsequent decompression-induced partial melting in correspondence with thinner lithosphere. Afterwards, lateral transport of magma along dykes or sills led to the formation of shallow intrusions and lava flows at considerable distances from the source region, possibly straddling the edges of the cratonic lithosphere at depth. Overall, the variations of the lithospheric thickness (i.e., the presence of stable thick cratonic keels juxtaposed to relatively thinner lithosphere) appear to play a primary role for localizing mantle upwelling and partial melting during large-scale magmatic events like the CAMP.

How to cite: Boscaini, A., Marzoli, A., Bertrand, H., Chiaradia, M., Jourdan, F., Faccenda, M., Meyzen, C., Callegaro, S., and Serrano Durán, L.: The architecture of the lithospheric mantle controlled the emplacement of the Central Atlantic Magmatic Province, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9813, https://doi.org/10.5194/egusphere-egu22-9813, 2022.

A study of chrome-spinels and PGE mineralization (PGM) from the podiform chromitites has been carried out on the area of four locations of the Ospa-Kitoy ophiolite massif (northern and southern branches East Sayan ophiolite). It has been established that different PGM assemblages formed at different stages of formation of the Ospa-Kitoy ophiolite massif, at various temperature and fluid regimes, are present at four sites. The chromite pods show both disseminated and massive structures. There are veins of massive chromitites, 0.01-0.5 m thick and 1-10 m long, rarely disseminated, schlieren, and rhythmically banded ores, which are discordant to the host ultramafic rocks. (Os-Ir-Ru) alloys occur as inclusions in the Cr-spinel or intergrowth with them (fig 3a). In addition, FePt3 alloys are found in the PGM assemblage. In such grains, decomposition structures of solid solutions represented by osmium lamellas can be observed. Polyphase PGM assemblage: (Os, Ir, Ru), (Ni, Fe, Ir),  (Ir, Ru, Pt)AsS, CuIr2S4, (Os, Ru)As2, Rh-Sb,  PtCu, and Pd5Sb2 are localized in serpentine, in close association with sulfides, sulfoarsenides, arsenides of nickel.

Figure 1. Chromitite bodies and PGE mineralization in Ospa-Kitoy ophiolite massif: 1 – Harh mountain (north branch of the ophiolites); 2 –  lake Sekretnoye (apically Zun-Ospa river); 3 – stream Zmeevikovyi (south branch of the ophiolites); 4 – Harh-Ilchir site (south flank Harh mountain).

Figure 2. Composition of  Os-Ir-Ru alloys: 1 – Harh mountain, 2 – lake Sekretnoye site, 3 – stream Zmeevikovyi.

Based on chemical and microtextural features of the PGM´s and assemblage with magmatic and hydrothermal minerals in the chromitites, it is established that each studied location of chromitites at different stages of PGM formation are exhibited. High-temperature magmatic Os-Ir-Ru alloys are widely exhibited in the Harh and Zmeevikovyichromitites. In the Harh-Ilchir site, there is no magmatic PGM and are established sulfoarsenides and arsenides Ru, Ir, which are formed from the residual fluid phase in the late magmatic stage. Chromitites in the lake Sekretnoye MPG are contained high-temperature magmatic (Os-Ir-Ru) alloys, and there are signs of PGE remobilization with Os⁰ , Ru⁰ , (Ir-Ru) alloys. Remobilization processes during serpentinization and fluid interaction of peridotites and chromitites.

In addition, it should note that the PGM assemblage of the Zmeevikovyi and Harh-Ilchir locations has been undergone by influence metamorphogenic fluids with increased activity of O₂, As, Sb. and these minerals can be formed directly in hypergenic environments. PGM҆'s such as (Ru, Rh, Pt)Sb, Rh-Sb were created at this stage.

Figure 3. BSE images of primary and secondary PGM: Harh location: а) individual grain of magmatic (Os-Ir-Ru) with microinclusion native Os; b) remobilized polyphase aggregate native Os, (Ir-Ru) (CuIr₂S₄); location Sekretnoye lake: с) inclusion magmatic (Os-Ir-Ru) in the chromite grain; d) remobilized polyphased aggregate (Ir-Ru), (Rh-Sb); location stream Zmeevikovyi: e) idiomorphic magmatic grain (Os-Ir) replaced by (Ir,Ru)AsS, with separation remobilized (Os,Ir);  Harh-Ilchir site: f) inclusion of Pd₅Sb₂ in the heazlewoodite (Hzl).

Analytics  made in Analytical Centre SB RAS. Supported by RFBR  19-05-00764а and  Russian Ministry of Education and Science.

How to cite: Kiseleva, O., Ayriants, E., Belyanin, D., and Zhmodik, S.: Manifestation of various stages PGE mineralization in the different locations Ospa-Kitoy ophiolite massif (East Sayan, Russia)., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1260, https://doi.org/10.5194/egusphere-egu22-1260, 2022.

Spinel peridotite xenoliths have been found in Cenozoic basalts from the Nuomin and Keluo areas in the northern Daxinganling. The Mg content of olivine in the mantleperidotite indicates that the upper mantle in the study area is partially refractory. According to the olivine content and Fo diagram, a part of peridotite xenoliths fell in the Archean and Proterozoic mantle regions, which reveals that there are remnants of ancient lithospheric mantle in the lithospheric mantle of the study area. In the study area, harzburgite and lherzolite show high oxygen fugacity values (FMQ + 1.95-3.15), which is in sharp contrast to the low oxygen fugacity values of the relatively reduced ancient lithospheric mantle. It is possible that the Paleozoic paleo Asian Ocean and Mesozoic paleo Pacific subducted successively under the Xingmeng orogenic belt, resulting in the oxidation of the lithospheric mantle at that time. K ₂O (1% ～ 6%) is found in the reaction edge of mantle xenoliths. It is considered that the mantle in the study area has experienced multiple periods of K-rich meltactivity, and the source of K-rich melt may be related to the crust source material recycled by subduction.

How to cite: Liu, J. and Li, H.: Oxygen fugacity characteristics of lithospheric mantle peridotite in northern Xingmeng orogenic belt, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5941, https://doi.org/10.5194/egusphere-egu22-5941, 2022.

The Tuva-Mongolian microcontinent and Khamardaban terrane are known as major tectonic units accreted to the Siberian paleocontinent. We report ²⁰⁷Pb/²⁰⁶Pb ages of 2.44–2.22 Ga for sources of Late Cenozoic volcanic rocks from the Tunka volcanic zone and of 1.63–1.31 Ga for those from the Khamardaban zone. The new ages are consistent with Precambrian geological events that are characteristic of the area and contradict the existing opinion about the Early Paleozoic collisional connection between these tectonic units inferred from dating of syn-collisional granites.

On the one hand, we constrain ore-forming processes in the Gargan block of the Tuva-Mongolian microcontinent and in the south of the Siberian paleocontinent between 2.45 and 1.4 Ga and between 1.3 and 0.25 Ga, respectively [Rasskazov et al., 2010]. The latest Pb-separating event in the Gargan block was followed by the generation of restite ultrabasic Ilchir belt that bounds the block from the south [Kiseleva et al., 2020]. So, we trace the boundary between the Gargan block and Ilchir belt to magma sources of the Tunka and Khamardaban zones that reasonably denote the root part of the Khamardaban terrane, accreted to the Tuva-Mongolian microcontinent and Siberian paleocontinent 1.63–1.31 Ga ago (Figure). On the other hand, we emphasize the importance of ore-forming events in the Gargan block, launched about 2.45 Ga, simultaneously with source generation in the Tunka zone. Basalts of this zone include xenoliths of fassaitic clinopyroxenites that show wide variations in the oxidation–reduction state. We suggest that fassaite (diopside) mineralization was due to interaction between orthopyroxene and calcite: (Mg, Fe)₂Si₂O₆ + CaCO₃ → (Mg, Ca)₂Si₂O₆ + CO₂ + FeO. Orthopyroxene of high-Mg spinel harzburgite xenoliths from Khobok River lavas (Tunka basin) shows SiO₂ content as high as 58.7 wt. %, while fassaite from pyroxenite xenoliths has SiO₂ content as low as 49 wt. %. Fassaitization of orthopyroxenites and harzburgites, obviously, releases both iron and silica. These components are found as amorphous Fe–Si phases in metasomatite xenoliths with low Mg/Si and Al/Si ratios [Ailow et al., 2021]. From data obtained, we speculate that fassaitization was an effective crust-mantle process of 2.4–2.2 Ga that could provide both the deep-seated Fe–Si mineralization and the generation of ferruginous quartzites displayed in the Great Oxidation Event.

Ailow Y. et al. // Lithosphere. 2021. V. 21, No. 4. P. 517–545.

Kiseleva O.N. et al. // Minerals. 2020. V. 10. P. 1077.

Rasskazov S.V. Brandt S.B., Brandt I.S. Radiogenic isotopes in geologic processes. Springer, 2010. 306 p.

How to cite: Rasskazov, S., Chuvashova, I., Saranina, E., Yasnygina, T., and Ailow, Y.: Crustal versus mantle events of 2.44–2.22 and 1.63–1.31 Ga at the junction between Khamardaban terrane, Tuva-Mongolian microcontinent, and Siberian paleocontinent: Petrogenetic consequences, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6686, https://doi.org/10.5194/egusphere-egu22-6686, 2022.

In terms of Pb isotope ratios, melting anomalies of Central and East Asia show no high μ (HIMU, high ²³⁸U/²⁰⁴Pb) signature that was generated on the Earth about 2 Ga ago and was caused by sulfide sequestration of Pb from the mantle to the core [Hart and Gaetany, 2006]. In such particular environment, we use Pb isotope data on Late Phanerozoic volcanic rocks to develop general systematics of their sources through definition of initial viscous protomantle reservoirs with low μ and elevated μ signatures (LOMUVIPMAR and ELMUVIPMAR, respectively) that imply a solidification time of the mantle in the Hadean magma ocean between 4.54 and 4.44 Ga ago. We suggest that the protomantle reservoirs retained specific Pb isotope signatures in the early, middle, and late epochs of the Earth's evolution (4.54–3.6, 2.9–1.8, and  <0.7 Ga ago, respectively) [Rasskazov et al., 2020]. In this presentation, we report the first representative Pb isotope data on the ELMU signature of Late Cenozoic rocks from the Dariganga volcanic field, Southeast Mongolia. Pb isotope secondary-isochron patterns of volcanic rocks show protomantle material that was not differentiated between 4.474 and 4.444 Ga (i.e. directly ascended from a deep mantle reservoir in the Cenozoic). In addition, the material was also differentiated in the deep mantle at about 3.69, 2.16, and 1.74 Ga. Pb isotope data on volcanic fields of North China are indicative for lateral change from the ELMU to LOMU signature (Figure). We infer that sources of volcanic rocks from Southeast Mongolia and North China display the primary inhomogeneity of the deep mantle that was generated in the Hadean magma ocean from its initial solidification as early as 4.54 Ga to its final respond of 4.44 Ga.   

Hart, S.R. &  Gaetani, G.A. (2006). Mantle paradoxes: the sulfide solution. Contrib. Mineral. Petrol., 152, 295–308.

Rasskazov, S., Chuvashova, I., Yasnygina, T., & Saranina, E. (2020). Mantle evolution of Asia inferred from Pb isotopic signatures of sources for Late Phanerozoic volcanic rocks. Minerals, 10 (9), 739. 

How to cite: Chuvashova, I., Rasskazov, S., Sun, Y., Yasnygina, T., and Saranina, E.: Lateral change of  ELMU–LOMU sources for Cenozoic volcanic rocks from Southeast Mongolia and North China: Tracing zonation of solidified Hadean magma ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6724, https://doi.org/10.5194/egusphere-egu22-6724, 2022.

First ⁴⁰Ar/³⁹Ar isotopic age data for gold hydrothermal veinlet-vein mineralization of the late Mesozoic Ketkap-Yuna igneous province (KYuIP) of the Aldan shield (AS) confirm the geological relation of this type of mineralization with the early Cretaceous sub-alkali magmatism. The combination of geological characteristics and U-Pb dating of magmatites indirectly enabled us to determine the age and highly productive bi-metasomatic «massif-skarn» type of mineralization associated with sub-alkali magmatogenic formations of the province.

Isotopic datings of magmatites and gold mineralization of the KYuIP and other late Mesozoic igneous provinces of the Aldan shield show age conformity of ore-bearing magmatites and ores accompanying them (fig. 1, 2). A relative, in comparison to provinces of the tectonic-magmatic activation (TMA) of the Western and Central Aldan, delay in time of occurrences of the KYuIP late Mesozoic magmatism and gold mineralization related to it, and the difference in volume ratios of formational types of magmatic formations in different provinces can be explained by the characteristics of tectonic structure of the region.

We have distinguished two large areas of the late Mesozoic TMA of the AS differing in the timing of polyformational magmatism and concomitant mineralization of different types, and in dominating formational type of magmatites: Western–Central-Aldan on the one hand, and Eastern-Aldan on the other (fig. 1-3). The first is characterized by a long-term development of magmatic activity from the Berriasian to the early Albian (≈ 30 Ma), and prevalence of leucitite–alkali(foid)-syenite formation; the second is characterized by occurrences of magmatism in a period twice as smaller (≈ 15 Ma), and domination of subalkaline diorite-granodiorite-granite formation.

The termination of the late Mesozoic magmatism in both areas was sub synchronous. The “set” of magmatogenic formations within them is also similar: leucitite–alkali(foid)-syenite with alkali granites, monzonite(subalkaline shonkinite)-syenite and subalkaline diorite-granodiorite-granite. A typical feature of the Eastern-Aldan area of the TMA consists in Coniacian-Santonian burst of alkali volcanoplutonism, which manifested in the KYuIP after a long (about 30 Ma) period of amagmatism.

How to cite: Polin, V., Zvereva, N., Travin, A., and Ponomarchuk, A.: Ketkap-Yuna igneous province gold mineralization age, ore-bearing complexes formational types, and different occurrence time of the late Mesozoic magmatism in different parts of the Aldan shield, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-163, https://doi.org/10.5194/egusphere-egu22-163, 2022.

The article considers the geological position of the Zhidoy massif and its age. The scheme of magmatism of the massif has been developed. The graphs of paired correlations of petrogenic elements in massif rocks which had a consistent trend in composition are given for validation purposes. The present article provides graphs of REE spectra and the spider diagram of rare elements concentration in the massif rocks. Pyroxenites are the early rocks of the massif, which are the ores for titanium. Titanium is concentrated in three minerals: titanium magnetite, ilmenite and perovskite. The main type of titanium ores, perovskite, is known only in the Zhidoy massif. A conclusion about the mantle sources of the primary magma of the massif
is drawn based on the geochemistry of the isotopes Sr and Nd.

Fig. 1 Spectra of rare-earth elements in rocks of the Zhidoy massif (chondrite-normalized).
Symbols: 1−pyroxenite, 2−ijolite, 3−syenite, 4−phenite

Fig. 2 Spider-diagram of the Zhidoy massif rocks

Conclusions
1. Three varieties of ore pyroxenites have been defined−titanium-magnetite, ilmenite and perovskite ore.
2. The petrochemical diagrams show a common trend in the composition of rock- forming elements, indicating the homomorphism of the rocks and their crystallization from a single primary magma.
3. Geochemical data also confirm the genetic relation of the Zhidoy massif.
4. Mantle source, the depurated mantle for the primary magma of the Zhidoy massif, has been determined on the basis of isotope data.

How to cite: Sotnikova, I., Vladykin, N., and Alymova, N.: Zhidoy Alkali-Ultramafic Rock and Carbonatite Massif: GeochemicalFeatures, Its Sources And Ore-Bearing, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7771, https://doi.org/10.5194/egusphere-egu22-7771, 2022.

Mantle xenoliths provide a clear evidence of interaction with low-degree mantle melts, however, this evidence is mostly geochemical, manifested by incompatible element enrichment, or mineralogical, manifested by already crystallised phases (e.g. amphibole, phlogopite) as a result of this interaction.

Despite decades of research, the composition of low-degree melts generated in lithospheric mantle are still not very well-known. In situ characterisation of such melts is hampered due to their modification during the ascent as well as rapid alteration and weathering at the surface, while experiments are hampered by difficulties to produce and analyse very low-degrees (<2-3%) melts.

In this study we report a rare sample of well-preserved low-degree melts within a peridotite xenolith GGU473178 from Majuagaa kimberlite in West Greenland. We report alkali-carbonatitic-chloride melt pools and veins that may represent primary low-degree partial melts and products of their in situ crystallisation.

Melt pools are largely composed of carbonate (predominantly dolomite) and contain spinel, apatite, phlogopite as well as minor amounts of Fe-Ni sulphides, barite and halite.

Euhedral crystals of spinel present in these melt pools contain large usually round aggregates of mineral inclusions, which we explain as former melt pools captures by spinel. Mineral assemblage found in these spinel inclusions is consistently composed of ferropericlase, dolomite, alkali-rich carbonate and apatite, which is indicative of a strongly silicate-undersaturated alkali-carbonatitic melt that contains chlorine and phosphorous. Due to the almost complete absence of SiO₂, ferropericlase (instead of olivine) crystallises in equilibrium with dolomite and alkali-rich carbonate, implying incredibly low degrees of melting, when essentially only carbonated component is melted, or carbonate-silicate liquid immiscibility, previously reported for spinel lherzolite and garnet wehrlite xenoliths (Frezzotti et al., 2002; Soltys et al., 2016).

References

Frezzotti, M. L., Touret, J. L. R., and Neumann, E. R., 2002, Ephemeral carbonate melts in the upper mantle: carbonate-silicate immiscibility in microveins and inclusions within spinel peridotite xenoliths, La Gomera, Canary Islands: European Journal of Mineralogy, v. 14, no. 5, p. 891-904.

Soltys, A., Giuliani, A., Phillips, D., Kamenetsky, V. S., Maas, R., Woodhead, J., and Rodemann, T., 2016, In-situ assimilation of mantle minerals by kimberlitic magmas — Direct evidence from a garnet wehrlite xenolith entrained in the Bultfontein kimberlite (Kimberley, South Africa): Lithos, v. 256-257, p. 182-196.

How to cite: Kiseeva, E. S., Kamenetsky, V. S., and Nielsen, T. F. D.: In situ low-degree melts in peridotite xenolith from Majuagaa kimberlite, West Greenland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1932, https://doi.org/10.5194/egusphere-egu22-1932, 2022.

The method of cluster (R and G methods) analysis of the allocation of the cluster (CG) and chemical-generic (CGG) groups of average values of the compositions of diamond indicator minerals (DIM) (Ivanov, 2017) was used, supplemented with data on the frequency of occurrence (FO) of habitus forms, twins and clusters of diamond crystals of three kimberlite pipes of Angola – Catoca, Luele and Txiuzo (Ganga et al., 2021). Clustering of MSDS by their chemical composition was carried out on the basis of chemical-genetic classifications of Dowson J. and Soboleva N.V. for garnets (Dowson et al., 1975; Sobolev, 1973) and Garanin V. K. for Cr-diopsides, of ilmenite and chromite (Garanin et al., 1991).

It is found that FO CG/СGG of МSD are indirect and inverse significant correlation with FO habitus forms, twins and adhesion of diamond crystals of these kimberlite pipes. This is demonstrated by histograms of the joint distribution of CG DIM and habitus forms, twins and splices of diamond crystals from geological samples of kimberlites at their deposits (Fig. 1).

The fractions of octahedra (O) and transition habit crystals (OD) decrease in parallel with a decrease in the proportions of CG G9, G10 pyropes and an increase in G1 and G2a, an increase in the proportions of CGG 2b and 4b picroilmenites. The shares of rhombododecahedron, including dodecahedrons (RD), grow with the growth of the shares of CG pyropes G1a, G2a, as well as CGG picroilmenites 2b and CGG Cr-diopsides S2 and S5. The shares of twins (Tw), splices (Agr) and polycrystalline bead (PC) decrease in the studied tubes with a decrease in the shares of CG pyropes G10, G10a and an increase in the shares of CGG picroilmenites 2b and 4b and CGG Cr-diopsides S2 and S5 (Fig. 1). The presence of Ti and Fe compounds, which are part of DIM in elevated concentrations, in the process/medium of diamond crystal formation contributes to the formation of habitus forms OD and RD (D - dodecahedrons) during dissolution associated with low-chromium pyropes CG G1 and G2. Medium-high chromium pyropes CG G10 and G10a are associated with octahedral habitus (O) diamond crystals and their spinel counterparts (TwSp), whose shares they control.

Petrogenetic affiliation of CG/CGG MSD to various associations of deep mantle rocks allows us to identify the most favourable conditions and environments for the origin and growth of diamonds (high FO of O+TwSp+Tw) and environments (conditions) of their dissolution (high FO of OD+RD+Th+C). Interesting that the diamond grade calculated diamond deposits (Ct /T) is positively correlated with FO (Ar g+Tw), SGG S6 picroilmenites and SG G10 garnets, but the FO (RD+OD) has a negative effect on diamond grade, which allows determining the degree of the fertility of the mantle sources by DIM diamond.

How to cite: Zinchenko, V. and Ivanov, A.: Correlation of habitus forms, twins and aggregates of diamond crystals with the composition of its indicator minerals from kimberlite pipes of Angola, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6809, https://doi.org/10.5194/egusphere-egu22-6809, 2022.

The nickel content in pyropes interested researchers due to the possibility to create an algorithm for calculating the temperature boundaries of its joint crystallization with diamonds. Griffin proposed such a calculation algorithm, which was named the diamond-bearing corridor with his name [1]. Determination of nickel content in pyropes on microanalysts is difficult for several reasons. The first reason is the limit of detection of nickel in pyropes, which is very high for the determination of this element on electron microscopes (at least 15 ppm). And then, such an analysis is possible at a quantitative level with a probe beam current of 300nA and an analysis time of 3 minutes. The study of the correlation ratios of nickel with other elements in pyropes allowed us to determine two elements that have a significant correlation with the nickel content in pyropes - these are titanium and manganese and their content in pyropes is acceptable for quantitative determination. On the ion microanalyzer, more than two hundred analyses were performed for pyropes from the kimberlite pipes Botuobinskaya and Nyurbinskaya, the remaining determinations were made from kimberlites tr. Jubilee and tr. Victory with the use of a new technique for the determination of nickel in pyropes in the microanalyzer JXA-8230. In total, 443 definitions of nickel in pyropes were performed at the quantitative level. Such definitions made it possible to calculate the functional dependence of nickel contents on titanium and manganese contents. The STATISTICS program is used for such calculations (Fig. 1).

Fig. 1. Map of level lines (nickel manganese titanium for 443 definitions) with the calculation of functional dependence

The calculation of nickel contents in pyropes makes it possible to fully use the Griffin geothermometer to determine the number of pyrope grains from the diamond-bearing corridor area.

• Griffin W.L., Ryan C.G. Trace elements in indicator minerals: Area selection and target evaluation in diamond exploration. J. Geochem. Explor., 1995. Vol. 53., pp, 311-357

How to cite: Ivanov, A.: Precise calculations of Nickel content in pyropes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3014, https://doi.org/10.5194/egusphere-egu22-3014, 2022.

Plateaus and isochronous and integral ages of ³⁹Ar/⁴⁰Ar xenocrysts and phlogopite grains from kimberlite xenoliths can be used to determine the ages of mantle processes (Hopp et al., 2008) and decipher the genesis of diamond-forming processes. Dating of deep xenoliths of kimberlites of the Siberian craton reveals a significant spread (Pokhilenko et al., 2012; Ashchepkov et al., 2015) from the Archean to the time close to the age of the host kimberlites, mainly Devonian. The most ancient ages for Udachnaya Daldyn fields for phlogopites from xenoliths of spinel harzburgites of the highest level belong to the late Archean (2.6-2.0) - early Proterozoic 1.7 -1.5 Ga. In the Alakite field, all ages are younger from 1,87 – 1,05- 0,928 - 0,87 Ga and belong to the metasomatic history of the Rodinia continent mantle. Close dates are set for xenoliths from the Obnazhennaya pipe (Kalashnikova et al. 2017).

Fig.1 PT  Udachnaya pipe. Symbols: Op: ToC(Brey, Kohler, 1990)-P(GPa)(McGregor, 1974). Cpx: 2.ToC-P(GPa)(Nimis, Taylor, 2000); 3.ToC (Nimis, Taylor, 2000 with ed. author)–P(GPa)(Ashchepkov et al., 2011); 4. eclogites ; 5. inclusions in diamond; Gar: 6.ToC (O'Neill, Wood, 1979) -P(GPa) (Ashchepkov et al., 2010Gar), 7. For eclogite garnets Chromite: 8,  inclusions in diamond; 9. chromite ToC (O'Neill, Well, 1987)-P(GPa) (Ashchepkov et al., 2010Gar), 7. For eclogite garnets Chromite: 8, inclusions in diamond; 9. chromite ToC (O'Neill, Well, 1987)-P(GPa) (Ashchepkov et al ., 2010Chr); 10 the same for inclusions in diamond; 11. Film Tom (Taylor et al., 1998)- P(GPa) (Ashchepkov et al., 2010 ilm)

Our data on micas by the ³⁹Ar/⁴⁰Ar method often reveal complex configurations of spectra. The micas of the xenocrysts of the Alakite field give several peaks, ranging from the most high-temperature and ancient, which corresponded to the upper Proterozoic - Vendian and Paleozoic, and only the lowest temperature peaks with a high Ca/K ratio corresponded to the ages of kimberlite introduction. Some peaks may be associated with the thermal effects of the Vilyusky plume (Kuzmin et al., 2012). The lowest temperature peaks, which are close in age to the time of kimberlite formation, which is confirmed by the high ³⁸Ar/³⁹Ar ratios of the gas released at the low-temperature stage, can be used very approximately for dating kimberlites, however, the release of other gases at low-temperature stages significantly increases the measurement error. All of them correspond to the interval 440 -320 Ma. The pipes Mir, Internationalnaya, Ukrainianskaya - 420, Yubileynaya -342, Botuobinskaya -352 Ma). Some definitions practically coincide with Rb/Sr ages (Griffin et al., 1999, Agashev et al., 2005, Kostrovitsky et al., 2008; Zaitsev, Smelov, 2010) and probably represent mixing lines. For many xenocrysts (Feinsteinovskaya, Ukrainskaya, Yubileynaya, Krasnopresnenskaya tr.), the interval from 600 to 500 Ma is manifested, which corresponds to the stage of the Laurasia supercontinent breakdown. The presence of relatively low-temperature plateaus with ancient ages, and high-temperature young ones implies that some stages can be correlated with the mantle history of the mineral.  RFBR grant 19-05-00788.  Supported by Ministry of Science and Higher Education.

Fig.2 PT  Sytykanskaya pipe

How to cite: Iudin, D., Ashchepkov, I., and Travin, A.: Ages of micas from xenoliths and xenocrysts of kimberlites of the Siberian Craton determined by 39Ar/40Ar method, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2177, https://doi.org/10.5194/egusphere-egu22-2177, 2022.

The NE of the Siberian platform in Udzha and Anabar river locate the richest alluvial placers of diamonds Since the discovery of placers > 700 were found, but no industrial bodies. A study of kimberlite magmatism has established that there are kimberlites of three ages on the territory of the North-East of the Siberian platform – middle Paleozoic (single), lower Triassic (few) and Jurassic-Cretaceous (prevailing). The latter are almost non-diamond-bearing.

The nearest kimberlite fields of Kuranakh and Tomtor are poor in diamonds. Some placers in the basin of the Udzha river, the right tributary of the Anabar, contain Cr-rich (≤14 wt.% Cr2O3) sub-calcic pyrope garnet associated with diamond. Comparison of kimberlite indicator minerals (KIMs) from the basins of Udzha and Chemara (its right tributary) shows similarity and a large diversity of pyropes, mostly of lherzolitic type. Cr- diopsides found in the Devonian collector suggest a close kimberlite source.

Mainly eclogitic placer diamonds are abundant in the upper reaches of the Chimara river in the northeastern part of the region. They occur in Permian, Jurassic, and Neogene rocks and in Quaternary alluvium where they coexist with pyrope and ilmenite. The diamonds in this region have mostly eclogitic features (Shatsky et al., 2015).

Reconstructions using monomineral thermobarometry (Ashchepkov et al., 2010) for the sources of pyrope and diamond show that the areas of the Anabar and Udzha placers share the similarity in the structure of mantle roots since 7.5 GPa, with a convective branch at the base.

The P-Fe trend for the Jurassic is slightly inclined, which is typical of the Kuranakh field. For the Devonian kimberlites, non-inclined trends are typical. The subcontinental lithospheric mantle (SCLM) beneath the Udzha basin is rich in pyroxenitic garnets as typical for the Anabar region.

There are 3 intermediate collectors of pyropes and associated diamonds: Permian, Jurassic and Neogenic and alluvium.  A study of the chemistry and thermobarometry of the kimberlite indicator minerals show some variations which possibly indicate different kimberlite sources ( Fig.1).

The detailed trace element geochemistry of the KIM from Udzha and Chemyra rivers show high variations and systematic differences.

Fig.1. PTX diagrams for kimberlite indicator minerals (KIM) from three correctors in the Udzha basin.

Fig.2 TRE distributions for KIM from Udzha alluvium

Fig.3 TRE distributions for KIM from Udzha alluvium

Supported by  RBRF grant 19-05-00788.

How to cite: Vavilov, M., Afanasiev, V., Ashchepkov, I., Baranov, L., and Vera, E.: Possible sources of the alluvial diamonds Udzha basin, northern Anabar region near kimberlite Tomtor field, Yakutia., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3368, https://doi.org/10.5194/egusphere-egu22-3368, 2022.

For the first time, such an indicator as the Ti content in garnets was used as a criterion for the study of heterogeneity of the lithospheric mantle (LM) beneath the Yakutian kimberlite province (YaKP). Comparison of the compositions of garnet from pipes of most fields (18 out of 21) of YaKP was carried out. The study was based on representative collections of garnets from kimberlite concentrates, as well as literature and own data on the composition of garnets from mantle xenoliths from the Upper Muna pipes and northern fields adjacent to the Anabar shield, as well as from the Udachnaya, Dal’nyaya and Obnajennaya pipes. Three groups of YaKP fields with different Ti content (Fig. 1) and Mg# values ​​in garnets have been identified - 1) southern diamondiferous fields - high TiO₂ content (0.26-0.50 wt%) and high Mg# value (80.6-82.6%); an exception is the Mirninsky field (0.13 wt.% TiO₂); 2) the dominant number of northern fields (10 in total) is a low TiO₂ content (0.06-0.26 wt%) (Fig. 2) and a relatively high value of Mg# (78.8-81.7%, middle - 80.2%); 3) three northern fields (Chomurdakh, Ogoner-Yuryakh and Toluopka) - high TiO₂ content (0.53-0.78 wt.%) (Fig. 3) and low Mg# (76.9-78.3%). The trace element composition of garnets from the third group testifies to their mainly equilibrium magmatic crystallization (Fig. 4). It is assumed that the garnet-bearing rocks, due to the relatively low lithospheric mantle (LM) thickness in the marginal part of the Siberian Craton, were subjected to almost complete metasomatic processing by melt-fluids of the asthenospheric mantle. The obtained data on the composition of garnets allowed the authors to clarify the reason for the different compositions of kimberlites in the southern and northern fields of YaKP. The authors believe that the predominantly high-Ti composition of the kimberlites of the northern fields, despite the low-Ti composition of the LM rocks, reflects the primary composition of the kimberlite melt-fluid of asthenospheric origin. The relatively small thickness of the LM beneath the northern fields limited the degree of assimilation by kimberlite melt of high-Mg rocks of LM and initiated an increase in asthenosphere activity, which led to the formation of high-Ti kimberlites, high-Ti alkaline basalts, and alkaline-carbonatite massifs here. Supported by RBRF grant 19-05-00788

How to cite: Kostrovitsky, S., Yakovlev, D., Ashchepkov, I., and Tappe, S.: Inhomogeneity of the composition of lithospheric mantle beneath the Yakutian kimberlite province, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3753, https://doi.org/10.5194/egusphere-egu22-3753, 2022.

The problem of the lithospheric mantle structure under ancient cratons and their evolution attracts researchers in connection with the question of the diamond genesis. The petrological way is based on the mineral composition studying in xenoliths from the mantle depths. The Mirny kimberlite field belongs to the diamond-bearing kimberlite fields in the center of the Siberian craton. The authors studied a collection of mantle xenoliths from the Mir pipe (57 samples). The samples were classified as peridotites (Grt lherzolites) and pyroxenites (Grt websterites, Grt clinopyroxenites and eclogites).

Lherzolites from the Mir pipe are characterized by a high degree of alteration; olivine and orthopyroxene are replaced by serpentine in many samples (up to 50–70%). Websterite rocks are different by the presence of orthopyroxene and clinopyroxene, while clinopyroxene may contain lamellae of exsollution structures. Garnet websterites are distinguished by orange-reddish color of garnet, dark green color of pyroxene and dominanting medium-large-grained hypidiomorphic-granular textures; porphyroblastic and granoblastic textures (up to mosaic) are also observed. In garnet clinopyroxenites rutile is usually present in the form of thin (5–20 µm) needles in garnet and clinopyroxene. Eclogites are characterized by orangish or pinkish garnet color and granoblastic structure.

Garnets from lherzolites and websterites are also characterized by a relatively high Mg# content (75–83) and low TiO₂ contents (up to 0.2 wt %). It belongs to the lherzolite paragenesis by content CaO (3.68 - 5.35 wt.%) and Cr₂O₃ (0.07-3.7 wt.%). Eclogites are characterized by high-calcium (3.78 - 9.46 wt.%) and high-iron (7.77 - 17.20 wt.%) composition of garnet getting into the ​​wehrlite paragenesis area. None of the garnet studied compositions belongs to the high-chromium dunite - harzburgite paragenesis. Also garnets from the lithospheric mantle under the Mirny kimberlite field are characterized by a low-Ti garnet composition (up to 0.7 wt.%). Thus, the lithospheric mantle under the Mirny kimberlite field differs from the lithospheric mantle under other diamondiferous fields (for example, Udachnaya kimberlite pipe). The Mirny mantle xenoliths are characterized by the pyroxenites widespread development (up to 50%), the low-Ti composition and deformed lherzolites absence. These features indicate the minimal silicate metasomatic alteration in the lithospheric mantle under the Mirny field (in contrast to the center of the Siberian craton). The isotopic oxygen composition in garnet and clinopyroxene was also determined. The δ¹⁸O value varies in Cpx from 5.7-5.8‰ in clinopyroxenites and 6.1-6.1‰ in eclogites. On the whole, minerals from pyroxenites demonstrate δ¹⁸O values exceeding mantle values, which suggests a wide development of melting processes in the lithospheric mantle in the south of the Siberian craton Craton and the formation of megacrystalline pyroxene cumulates. In some cases, metamorphic recrystallization leads to oxygen isotope equilibrium between garnet and clinopyroxene. For minerals from eclogites higher values ​​of δ¹⁸O are noted, which may indicate the origin of eclogites from subducted oceanic crust, the presence of a subduction component in the process of formation of the lithospheric mantle.

The research was supported by Russian Science Foundation grant №20-77-00074.

How to cite: Kalashnikova, T., Kostrovitsky, S., Solovieva, L., Sinitsyn, K., and Yudintseva, E.: Garnets from xenolith in Mir kimberlite pipe: chemical composition and genesis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11936, https://doi.org/10.5194/egusphere-egu22-11936, 2022.

Unravelling the processes taking place during the genesis of kimberlites, their ascent through the sub-cratonic mantle and their emplacement in the crust is challenging, as kimberlites are mixtures of mantle-derived and magmatic components, rarely preserving pristine evidence of their original nature. Furthermore, their intense state of alteration makes it difficult to access the textural-compositional record of information engraved in the phase constituents. In this study, fresh samples of kimberlites and related mantle-derived xenocrysts-xenoliths from the Udachnaya pipe (Siberia) were investigated to reconstruct the pressure-temperature-time-composition (P-T-t-X) framework of the sub-cratonic lithosphere at the time of kimberlite emplacement. Routine and high-precision electron microprobe analyses of olivine, phlogopite and spinel from different facies of the Udachnaya pipe (intrusive coherent, hypabyssal and pyroclastic, sensu Scott Smith et al., 2013) showed that specific phase assemblages are associated with each evolutionary stage of the kimberlite. Olivine composition, in particular, is extremely variable, ranging from high-Fo and high-Ni (Fo93; NiO = 0.45 wt%) to low-Fo and low-Ni (Fo85; NiO = 0.10 wt%), but also to high-Fo and low-Ni (Fo>93; NiO <0.05 wt%) terms, often encompassing the whole compositional spectrum in a single sample and/or showing marked zoning within the individual crystals. 
A comparison between the main constituents of the Udachnaya kimberlite and those of the mantle xenoliths sampled during ascent, complemented by detailed major-trace element profiles on olivine crystals, was put forward to: (i) discriminate between the mantle-derived xenocryst cargo and the magmatic assemblage; (ii) model the P-T-fO2 path of kimberlites; (iii) speculate about their ascent rate; (iv) model the interactions between kimberlite-related fluid/melts and the Siberian sub-cratonic lithosphere.

REFERENCES
Scott Smith, B.H., Nowicki, T.E., Russell, J.K., Webb, K.J., Mitchell, R.H., Hetman, C.M., ... & Robey, J.A. (2013). Kimberlite terminology and classification. In Proceedings of 10th international kimberlite conference (pp. 1-17). Springer, New Delhi.

How to cite: Casetta, F., Abart, R., Ntaflos, T., Ashchepkov, I., and Coltorti, M.: Mantle-derived cargo vs. magmatic growth: ascent path, dynamics of the Udachnaya kimberlite and interactions with the Siberian sub-cratonic lithosphere, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9214, https://doi.org/10.5194/egusphere-egu22-9214, 2022.