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.