GD3.2

We study the reaction of garnet lherzolite with carbonatitic melt rich in molecular CO₂ and/or H₂O in experiments at 5.5 GPa and 1200-1450°C. The experimental results show that carbonation of olivine with formation of orthopyroxene and magnesite can buffer the CO₂ contents in the melt, which impedes immediate separation of CO₂ fluid from melt equilibrated with the peridotite source. The solubility of molecular CO₂ in melt decreases from 20-25 wt.% at 4.5-6.8 wt.% SiO₂ typical of carbonatite to 7-12 wt.% in more silicic kimberlite-like melts with 26-32 wt.% SiO₂. Interaction of garnet lherzolite with carbonatitic melt (2:1) in the presence of 2-3 wt.% H₂O and 9-13 wt.% molecular CO₂ at 1200-1450°С yields low SiO₂ (<10 wt.%) alkali‐carbonatite melts, which shows multiphase saturation with magnesite-bearing garnet harzburgite. Thus, carbonatitic melts rich in volatiles can originate in a harzburgite source at moderate temperatures common to continental lithospheric mantle (CLM).

Having separated from the source, carbonatitic magma enriched in molecular CO₂ and H₂O can rapidly acquire a kimberlitic composition with >25 wt.% SiO₂ by dissolution and carbonation of entrapped peridotite. Furthermore, interaction of garnet lherzolite with carbonatitic melt rich in K, CO₂, and H₂O at 1350°С produces immiscible kimberlite-like carbonate-silicate and K-rich silicate melts. Quenched silicate melt develops lamelli of foam-like vesicular glass. Differentiation of immiscible melts early during ascent may equalize the compositions of kimberlite magmas generated in different CLM sources. The fluid phase can release explosively from ascending magma at lower pressures as a result of SiO₂ increase which reduces the solubility of CO₂ due to decarbonation reaction of magnesite and orthopyroxene.

The research was performed by a grant of the Russian Science Foundation (19-77-10023).

How to cite: Kruk, A. and Sokol, A.: Lherzolite - carbonatite melt interaction in the presence of additive CO2 and H2O: Experimental data at 5.5 GPa and 1200-1450°C, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9325, https://doi.org/10.5194/egusphere-egu21-9325, 2021.

Some studies on basaltic and more primitive rocks suggest that their parental magmas have assimilated more than 50 wt.% (relative to the initial uncontaminated magma) of crustal silicate wallrock. But what are the thermodynamic limits for assimilation by primitive magmas? This question has been considered for over a century, first by N.L. Bowen and many others since then. Here we pursue this question quantitatively using a freely available thermodynamic tool for phase equilibria modeling of open magmatic systems — the Magma Chamber Simulator (MCS; https://mcs.geol.ucsb.edu).

In the models, komatiitic, picritic, and basaltic magmas of various ages and from different tectonic settings assimilate progressive partial melts of average lower, middle, and upper crust. In order to pursue the maximum limits of assimilation constrained by phase equilibria and energetics, the mass of wallrock in the simulations was set at twice that of the initially pristine primitive magmas. In addition, the initial temperature of wallrock was set close to its solidus at a given pressure. Such conditions would approximate a rift setting with tabular chambers and high magma input causing concomitant crustal heating and steep geotherms.

Our results indicate that it is difficult for any primitive magma to assimilate more than 20−30 wt.% of upper crust before evolving to intermediate/felsic compositions. However, if assimilant is lower crust, typical komatiitic magmas can assimilate more than their own weight (range of 59−102 wt.%) and retain a basaltic composition. Even picritic magmas, more relevant to modern intraplate settings, have a thermodynamic potential to assimilate 28−49 wt.% of lower crust before evolving into intermediate/felsic compositions.

These findings have important implications for petrogenesis of magmas. The parental melt composition and the assimilant heavily influence both how much assimilation is energetically possible in primitive magmas and the final magma composition. The fact that primitive mantle melts have potential to partially melt and assimilate significant fractions of (lower) crust may have fundamental importance for how trans-Moho magmatic systems evolve and how crustal growth is accomplished. Examples include generation of siliceous high-magnesium basalts in the Precambrian and anorogenic anorthosite-mangerite-charnockite-granite complexes with geochemical evidence of considerable geochemical overprint from (lower) crustal sources.

How to cite: Heinonen, J. S., Spera, F. J., and Bohrson, W. A.: Thermodynamic constraints on assimilation of silicic crust by primitive magmas, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1251, https://doi.org/10.5194/egusphere-egu21-1251, 2021.

The paper presents a non-stationary model of heat-mass transfer of heterophase media in application to the study of the intrusion processes of magmatic melts in permeable zones of the lithospheric mantle and crust. Special emphasis is given to the study of the change in rheological properties of the fluido-magmatic mixture in the process of magmatic channel formation. The increased compressibility of the fluid phase is taken into account in the model by setting the Van der Waals equation of state. The calculated values of thermodynamic parameters of the fluid-magmatic system such as pressure, temperature, volumetric phase content, are the basis for the analysis of metasomatic changes in mantle matter. The Numerical model is based on the Runge-Kutta-TVD method. Verification of the numerical model on standard tests shows good accuracy of the program code and the possibility of using it for investigations of the currents of fluid-magmatic flows. The study of variation in interphase interaction parameters during melt movement in permeable zone, including change in interphase viscous friction, demonstrates a significant change in temperature distribution in the section of fluid-magmatic system. The work was financially supported by the Russian Foundation for Basic Research, grants No. 19-05-00788.

How to cite: Vasilev, G. and Perepechko, Y.: Heat-mass transfer simulation of fluid media in the magma channel, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15256, https://doi.org/10.5194/egusphere-egu21-15256, 2021.

The paper presents a non-isothermal model of hydrodynamic heating of lithospheric rocks above magma chambers in application to the seismic focal zone of the Kamchatka region and associated volcanic arcs. The effect of convective heating of mantle and crustal rocks on dynamics of metasomatic changes and convective melting was studied. In the existing models of ore-forming systems, fluid mass transfer is determined mainly by the retrograde boiling of magmas in meso-abyssal intrusive chambers. Analysis of the manifestations of deposits of the porphyry formation of the Pacific Ocean active margins shows the decisive participation in their formation of mantle-crust ore-igneous systems. The model of convective heat-mass transfer in fluid mantle-crust systems coupled with magma chambers is designed with the consideration of effects of interphase interaction in rocks of permeable zones above igneous fluid sources. Numerical simulation of the dynamics of fluid systems under the volcanoes of the frontal zone of Kamchatka shows altered ultramafic rocks in metasomatic zoning and the presence of facial changes in the mineral composition of wehrlitized rocks. In the mantle wedge of the northwestern margin of the Pacific Ocean, over which epicontinental volcanic arcs developed in the post-Miocene stage, there is possible combination of the products of different-time and different-level igneous systems in the same permeable "earth's crust-lithospheric mantle" transition zones. Assuming that the "cratonization" of volcanic sections of the continental Earth's crust follows the "metasomatic granitization" pattern, the initial element of which is the wehrlitization of mantle wedge ultramafic rocks, the processes of metasomatic fertilization of mantle wedge rocks were investigated using a flow-through multiple-reservoir reactor. In the seismically active regions of the Pacific transition lithosphere, specific conditions for heating of areas of increased permeability above mantle fluid sources should be recorded. Metasomatic columns in such fluid systems can describe the formation of at least three levels of convective melting of metasomatized mantle wedge substrates, as well as the formation of a region of high-temperature fluid change of mafic intrusion rocks in the Earth's crust. The work was financially supported by the Russian Foundation for Basic Research, grants No. 19-05-00788.

How to cite: Perepechko, Y., Sorokin, K., Mikheeva, A., Sharapov, V., and Imomnazarov, S.: Modeling of convective heat-mass transfer in permeable parts of seismic focal zones of the Kamchatka region and associated volcanic arcs, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14200, https://doi.org/10.5194/egusphere-egu21-14200, 2021.

Lamproites of the Aldan Shield were found (Vladykin 1985) at the beginning of 80-es (for the first time in the USSR), being mainly the intrusive varieties of lamproites, though there occur among them some dyke and volcanic varieties. The general geological and geochemical features of lamproites of the Aldan shield were reported at the VI International Kimberlite Conference at Novosibirsk in 1995 (Vladykin 19971).

In  Aldan Shield there are known 14 locations of lamproites mostly referred to the Mesozoic rifting. This zone stretches out over all Aldan Shield, from the Murun massif in the Western part of the shield up to the Konder massif in the Eastern part of the shield. These occurrences of lamproites are of Jurassic age (120-150 m.a.). Only lamproites of Khani massif in the SW part of the Aldan Shield are more ancient. At first (according to the data of V.V.Arkhangelskaya) the Khani massif was considered to be Paleozoic, then using K-Ar method (VSEGEI) it was established Proterozoic age of biotite pyroxenites of the massif 1800 m.a. We found the dyke of olivine lamproites of the massif that crosses the biotite pyroxenites. We obtained even more ancient age – 2700 m.a. by zircons from these lamproites with a device SHRIMP (VSEGEI) (Vladykin, Lepekhina - 2005).

New data on Sr-Nd – systematization of the lamproites of the Aldan Shield have been obtained. The ratios 87Sr/86Sr in lamproites of Aldan vary from 0.703 to 0.708, whereas έ Nd – from -6 to -25. The source of Aldan Shield lamproites is enriched mantle ЕМ-1 (рис.1), that is consistent with their geological position (Vladykin -1997). They are situated between the Aldan Shield and the Siberian platform, where did not occur subduction. The North American lamproites (Leucite Hills, Smoky Bewt, Prery Creak ecc) have a similar position between the Canadian shield and the North-American platform and the same mantle source.

Compared to the Australian lamproites, the lamproites of the Aldan Shield have lower concentrations of rare-earth elements. The TR spectra for the Aldan lamproites (fig. 2) are rather uniform. A slight slope of the spectrum curves and slight Eu-anomaly are typical. For the earlier olivine lamproites lower TR concentrations are typical as compared with more differentiated leucite and sanidine lamproites.

The lamproites of the Aldan Shield originated from the enriched mantle source ЕМ-1, the age of that, according to Pb isotopic data, obtained for the rocks of the Murun massif (Vladykin 19972) is estimated as 3200 m.a. The dykes of the olivine lamproites of the Khani massif are the oldest lamproites in the world (2700 m.a.). The TR spectrum of the same type is indicative of similar genesis of the lamproites from various massifs of the Aldan Shield. In spite of the deep mantle source of the Aldan lamproites, they don’t bear diamonds actually, since the diamonds were likely burnt during their crystallization (at t- 1200-1000o C).

RFBR 09-05-00116, 08-05-9000.

References:

Vladykin N.V. First occurence of lamproites in the USSR.//Doklady Academii Nauk SSSR, 1985, Vol..208, N 3, p.718-722. (in Russia).

How to cite: Sotnikova, I. and Vladykin, N.: Trace element geochemistry and isotopy (Sm, Nd) of lamproites of the Aldan Shield, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8725, https://doi.org/10.5194/egusphere-egu21-8725, 2021.

In the Eastern margin of the  Anabar shield the Proterozoic alkaline pipes, dikes, and stocks are widespread. The largest Talakhtakh diatreme cut Middle-Upper Riphean dolomites (MP3-NP2) at the left bank of Kuonamka River (Fig. 1). The Talakhtakh diatreme includes alkaline basaltoids, ultra-K-trachytes, lamproites, olivine leucitites, tephrites, (Vishnevsky et al., 1986). TRE distribution indicates the proximity of sanidine trachytes to lamproites.

 The ⁴⁰Ar/³⁹Ar laser dating show two age maxima 1476 ± 17 (N = 14) and 1321 ± 17 (N = 9) Ma. If 10 points form a linear regression, with an age value of 1497 ± 40 million years, an initial ratio of 189 ± 100, then most of the remaining points are located along the abscissa axis. This arrangement may be associated with different degrees of rejuvenation of the K/Ar isotope system within the dating sites. The weighted average of 1476 ± 17, as more accurate, corresponds to the age of sample formation.

Ernst et al (2016) identified a new Kuonamka Large Igneous Province (LIP), based on U-Pb dating of baddeleyites from dolerite dikes and sills of the Anabar shield. The age of Kuonamka LIP is ~ 1501 ± 3 Ma. The resulting 40Ar/39Ar is the age of leucite trachytes (lamproites) The Talakhtakh diatreme fully corresponds to the time of occurrence of the Kuonamka LIP and indicates the formation of high-K effusions of the Talakhtakh complex during this period.

This work supported by RFBR grants: No. 18-05-70109 and the Russian Ministry of Education and Science

Gusev N.I., et al. State Geological Map of the Russian Federation. Sheet R-49-Olenek. An explanatory Memorandum. Saint Petersburg: Map factory VSEGEI. . 2016. 448 с.

Vishnevskiy S.A., Dolgov Yu. A., Sobolev N. V.  Geology&Geophysics, 1986. № 8. P. 17-27.

Ernst R. E., Okrugin A.V. et al.  Russian Geology&Geophysics. 2016. № 5. P. 653,-671.

How to cite: Zhmodik, S., Ivanov, P., Travin, A., Vishnevskiy, S., Yudin, D., and Lazareva, E.: 40Ar/39Ar-age of the Talakhtakh diatreme rocks (Arctic Siberia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8395, https://doi.org/10.5194/egusphere-egu21-8395, 2021.

The bulk rock and geochemistry of the Kayla and Khatastyr lamproites is similar to other Aldan lamproites and lamprophyres.  The ultramafic varieties are close to cratonic Ol- lamproites and alkaline Al, Si-rich varieties are closer to orogenic type.

Trace element bulk rock trace element (TRE) spider diagrams show inclined patterns with the LILE, Sr, Pb, U, peaks and Ta, Nb minima suggesting melting of originally depleted metasomatized Phl peridotites and mixed ancient (EMII, Nd, Sr isotopes) source (low crust)  and later olivine and clinopyroxene fractionation. They are dated 132-134 Ma (Late Cretaceous plume) similar to Chompolo lamprophyres and many alkaline complexes.

Thermobarometry for the deep-seated xenocrysts gives the low temperature and Sp-Gar and Gar facies for Cr- diopsides and chromites. Low - Cr- clinopyroxenes derived from lamproites give hot 90 mw/m² advective branches. 

The REE patterns for Cr-diopsides are more inclined for deeper varieties and reveal Ba, Th, U, Sr peaks and minima Ta, Nd and smaller in Zr-Hf. The `low Cr diopsides show flatter REE and HFSE minima TRE patterns of parental melts are lamproitic. Salites reveal hot crust conditions.

Lamproites melted from Phl peridotite eclogites mixture in the lithosphere base and interacted with mantle beneath Moho.

The work was supported by the Ministry of Science and Higher Education of the Russian Federation RBRF grants 19-05-00788a, 18-05-00073a;  Government tasks for Institute of Geochemistry SB RAS and Institute of Geology and Mineralogy SB RAS and the governmental assignment in terms of Project IX. 129.1.4

How to cite: Vladykin, N., Ashchepkov, I., Sotnikova, I., and Mevedev, N.: Lamprophyres of Kayla pipe and their mantle xenocrysts, SE Yakutia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3746, https://doi.org/10.5194/egusphere-egu21-3746, 2021.

The Yllymakh massif is one of the Mesozoic ring intrusions of Central Aldan, Yakutia. Geological relations between rocks in this massif are enough complicated to call it multiphase. Therefore, the idea about one or different magma sources is still the topic of modern discussions. According to the previous works, there are a lot of different rocks in the Yllymakh massif. And our petrological investigation [Vasyukova et al, 2020] accepted three groups of rock that differ a lot from each other. They have not great differences in mineral composition (aegirine in all rocks, feldspars in syenites). But some critical points in their geochemical features and ages. Foid syenites containing nepheline and pseudoleucite belong to the first group. They are 140±1.9Ma old. Second group includes alkali syenites (131±2.4Ma old). And the third group of rocks are alkaline granites mostly consist of alkali pyroxene and quartz (125±1.9Ma old).

All studied rocks are divided into three groups according to the silica content and contents of the most of other elements. Points marking the composition of syenites from different groups form multidirectional trends. The alkali granite’s characteristics make an independent cluster. The REE-plots also vary. Rocks of the first group has U-shape plot and wide variations in absolute contents. Rocks of the second group have high contents of REE and gentle slope. The granites from the third group have also U-shape plot but the lowest contents.

In this work we use the LA-ICP MS to determine the contents of RE elements in minerals. There were two minerals, that have chosen – apatite and pyroxene. Usually, apatite is the main concentrator of noncoherent elements that control the form of REE-spectra and the level of REE-contents in rocks. But in the Yllymakh massif, all apatite have a similar spectra form of normalized contents. The plots of normalized REE contents have a sharp negative slope and are characterized by very insignificant Eu anomalies. Such graphs are typical for the apatite of alkaline complexes. At the same time, the REE-plots of pyroxenes are quite equal to the form of REE-plots of the corresponding rock. Pyroxenes from foid syenites and alkali granites have U-shape plot and pyroxenes from feldspar syenites have a regular negative gentle slope plot. The only difference is that the REE content in the granite pyroxenes is as high as in the syenites.

The results of the research suggest that the formation of the rock spectrum of the Yllymakh massif occurred by reactivation of geochemically similar sources in a different time in addition to others. The contents of REE in rocks were controlled by REE-contents in pyroxene and its ratio with other rock-forming minerals. Supported by RFBR grant 19-05-00788

How to cite: Vasyukova, E. and Medvedev, N.: Geochemistry of minerals of  Yllymakh massif - Mesozoic alkaline ring intrusions of Central Aldan, Yakutia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15592, https://doi.org/10.5194/egusphere-egu21-15592, 2021.

The BelayaZima (645-622 Ma) (Ashchepkov et al., 2020) and Tomtor 700 and ~400 Ma) (Vladykin et al., 2016) ultrabasic alkaline rocks (AR) and carbonatites  (CA)  massifs (UMARC) are located on the SW and NW borders of the Siberian platform, respectively.

Mass-spectrometer FINNIGAN MAT-253 with Gas-Bench II flow (pure He) were used to determine δ13С and δ18O of carbonates from the AR and CA in  BelayaZima and Tomtor massifs (Fig. 1,). Values of δ18O of calcite and δ13С AR (27 samples) is 7.1 to 10.7 ‰ and 6,0 up to-4.0‰, respectively, and the calcite of the CA (116 samples) from 7.0 to 12‰ and from -6,5 to -4.1‰, with most values located in the field of primary magmatic carbonates (Fig. 3). Comparison of δ18O and δ13C in AR and CA from Aillik Bay, Labrador (Tappe et al., 2006), and BelayaZima shows nearly coincidence. In AR, average (av) δ18O_av = 9.11±0.14 (σ) and δ13C_av = -5.13±0.14(σ). In CA, δ18O_av = 8.19±0.086 (σ) and δ13C_av = -5.73±0.012 (σ). Given the greater stability of carbon isotopes compared to oxygen isotopes under the influence of post-magmatic processes (Santos and Claeton, 1995). The δ13C difference between AR and CA , ~0.6%, is an attribute of the initial sources.

On the δ18O -, δ13C-diagram (Fig.1) the population of the CA is partially aligned with the field PIC " Taylor Box" and almost completely with the field PIC by Demeny (1998). δ13С and δ18O values for AR different – the " Taylor Box" is a single value, while the bulk of the points were in the field of primary magmatic carbonates (PIC) (Demeni 1998) points on the δ18O-δ13С diagram for AR, and, especially, CA located along the δ18O axis, marking the horizontal trend is typical for many CA of UMARC  in Africa, Brazil, Canada etc. The prevalence of this trend indicates the global nature of the factors leading to its appearance, primarily the impact on carbonates of post-magmatic carbonless fluids.

The prevalence of this trend indicates the global nature of the factors leading to its appearance, mainly the post-magmatic carbonless fluids influence. A striking example is the δ18O of calcites from Tomtor deposit, unchanged by supergenic processes, along which a thick (100-150 m) weathering crust is developed. It is possible that AR and CA of the BelayaZima massif were formed under higher PT conditions. Due to their longer cooling time, the fractionation of oxygen isotopes possibly was controlled by the Soret effect, when heavy isotopes are concentrated in low-temperature areas under thermogradient conditions (Bindeman et al., 2013; Li, Liu, 2015). Possibly, the same effect makes an additional contribution to the isotopic heterogenization of carbon and oxygen in magmatic melts.

Support: RFBR 19-05-00788, RSF 18-17-00120, Russian Ministry of Education and Science

Fig.1 Aillikites:

Carbonatites:

Fig.2

How to cite: Ponomarchuk, V., Zhmodik, S., Ashchepkov, I., Belyanin, D., Kiseleva, O., and Pyryaev, A.: The C and O isotopes in calcites from aillikites and carbonatites of the Beloziminsky and Tomtor massifs (Siberia, Russia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12681, https://doi.org/10.5194/egusphere-egu21-12681, 2021.

The article provides new Sm-Nd and Nd-Sr isotope-geochronological data on rocks of the Paleoproterozoic Kandalaksha-Kolvitsa gabbro-anorthosite complex.

The Sm-Nd and Rb-Sr studies have provided data on isotope compositions of neodymium and strontium in rocks of both massifs. The isotope compositions of neodymium (eNd) range from -0.02 in norites of the Kandalaksha massif to -5.53 in lens bodies of gneiss granites of the Kolvitsa massif

Weakly radiogenic values of eNd = -1.0 – -1.2 dominate, which complies with characteristic values of Paleoproterozoic layered intrusions in Fennoscandia. Isotope compositions of strontium ranging from 0.7013 to 0.7025 also reflect typical values of a Paleoproterozoic igneous province [.

New data suggest that the Kandalaksha-Kolvitsa gabbro-anorthosite complex is confined to the East-Scandinavian Large Igneous Province with a protracted evolution at the turn of 2.53-2.39 Ga. According to geochronological and isotope Nd-Sr data, rocks of the Kandalaksha-Kolvitsa complex seem to have the same anomalous mantle source with Paleoproterozoic layered intrusions in the Baltic Shield (Fig. 3). The latter include Cu-Ni-Co-Cr+PGE deposits in the Monchegorsk ore area and Pechenga, Cr ores in the Pados massif, Fe-Ti-V Kolvitsa deposit, PGE and Cu-Ni Fedorovo-Pana layered complex  and Burakovsky intrusion, Cu-Ni-Co+PGE deposits in Finland, i.e. Kemi, Penikat, Akanvaara, Kontelainen, Tornio and many other. These deposits formed at two episodes, 2.53-2.39 Ga and 2.0-1.8 Ga, that refer to the beginning of rifting and the late rifting stage of the Fennoscandian Shield evolution, respectively.

Rocks of these intrusions referred to the pyroxenite-gabbronorite-anorthosite formation have similar isotope-geochemical features:

1) according to U-Pb and Sm-Nd geochronological data, the formation time span is 2530 to 2380 Ma;

2) the mantle reservoir feeding magmas that formed the massifs is rich in lithophile elements;  I_Sr values vary from 0.702 to 0.706, ε_Nd(T) varies from +2 to -6;

3) the model Sm-Nd ages of T_DM protoliths are 2.8-3.3 Ga.

The scientific research has been carried out in the framework of the State Research Contract of GI KSС RAS No. 0226-2019-0053, RFBR grant No. 18-05-70082 «Arctic’s Resources» and Presidium RAS Program No. 8.

How to cite: Steshenko, E., Serov, P., Kunakkuzin, E., Ekimova, N., Elizarov, D., and Bayanova, T.: New Nd-Sr isotope-geochronological evidence of its affinity to the East-Scandinavian Large Igneous Province (The Kandalaksha-Kolvitsa gabbro-anorthosite complex, Russia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5342, https://doi.org/10.5194/egusphere-egu21-5342, 2021.

The studied area is in the southeastern region of Eastern Sayan. Several tectonically dissected ophiolite complexes were exposed along the margin of the Gargan block and tectonically thrust over this block. Placer nuggets of PGE alloys from the Kitoy river were examined using a scanning electron microscope. Platinum-group minerals (PGM's) in placer deposits provide vital information about the types of their primary source rocks and ores as well as the conditions of formation and alteration. The primary PGM's are Os-Ir-Ru alloys, (Os, Ru)S₂, and (Os, Ir, Ru)AsS. (Os, Ru)S₂ form overgrowth around the Os-Ir-Ru alloys. The secondary, remobilized PGM's are native osmium, (Ir-Ru) alloys, garutite (Ir, Ni, Fe), zaccarinite (RhNiAs), selenides, tellurides (Os, Ir, Ru), and non-stoichiometric (Pd, Pt, Fe, Te, Bi) phases (Fig.1). Secondary PGM's (garutite and RhNiAs) form rims around Os-Ir-Ru alloys, intergrowth with them, or form polyphase aggregates. Such PGM's (identical in composition and microstructure) are also found in chromitites from Neoproterozoic ophiolite massifs of Eastern Sayan (Kiseleva et al., 2014; 2020). Platinum-metal minerals, exotic for ophiolites, are found among secondary PGM's such as selenides and tellurides (Os, Ir, Ru), (Pt, Pd)₃Fe, Pd₃(Te, Bi), (Au, Ag), and non-stoichiometric (Pd, Pt, Fe, Te, Bi) phases. They occur as inclusions in the Os-Ir-Ru alloys or fill cracks in crushed grains of primary PGM's. PGM's in placer deposits of the Kitoy river are similar to the mineral composition of PGE in chromitites of the Ospa-Kitoy ophiolitic massif, which contain Pt-Pd minerals and Pt impurities in Os-Ir-Ru alloys (Kiseleva et al., 2014). Selenides (Os-Ir-Ru) are rare within PGM's from ophiolite chromitites (Barkov et al., 2017; Airiyants et al., 2020) and also occur in chromitites of the Dunzhugur ophiolite massif (Kiseleva et al., 2016). Features of selenides and tellurides (Os, Ir, Ru) indicate their late formation as a result of the influence of magmatic and metamorphic fluids on primary PGE alloys. The filling of cracks in crushed (Os-Ir-Ru) alloys indicates that selenides and tellurides formed during tectonic deformation processes. The source of platinum-group minerals from the Kitoy river placer is the Ospa-Kitoy ophiolite massif, and primarily chromitites.

Figure 1. BSE microphotographs of PGM from from alluvial placers of the Kitoy river

Mineral chemistry was determined at the Analytical Centre for multi-elemental and isotope research SB RAS. This work supported by RFBR grants: No. 16-05-00737a,  19-05-00764а, 19-05-00464a and the Russian Ministry of Education and Science

References

Airiyants E.V., Belyanin D.K., Zhmodik S.M., Agafonov L.V., Romashkin P.A.  // Ore Geology Reviews. 2020. V. 120. P.  103453

Barkov A.Y., Nikiforov A.A., Tolstykh N.D., Shvedov G.I., Korolyuk V.N. // European J. Mineralogy. 2017. V.29(9). P.613-621.

Kiseleva O.N., Zhmodik S.M., Damdinov B.B., Agafonov L.V., Belyanin D.K. // Russian Geology and Geophysics. 2014. V. 55. P. 259-272.

Kiseleva O.N., Airiyants E.V., Belyanin D.K., Zhmodik S.M., Ashchepkov I.V., Kovalev S.A. // Minerals. 2020. V. 10. N 141. P. 1-30.

Kiseleva O.N., Airiyants E.V., Zhmodik S.M., Belyanin D.K / Russian and international conference proceedings “The problems of geology and exploitation of platinum metal deposits” – St.Petersburg: Publishing house of St.Petersburg State University. 2016. 184 P.

How to cite: Kiseleva, O., Ochirov, Y., Zhmodik, S., and Nharara, B.: Platinum-group minerals from alluvial placers of the Kitoy river (south-east part of East Sayan, Russia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14020, https://doi.org/10.5194/egusphere-egu21-14020, 2021.

Integrating petrography and mineral chemistry data with the determination of volatiles concentration and isotopic fingerprint in fluid inclusions (FI) in ultramafic xenoliths opens a new window on the study of the Sub-Continental Lithospheric Mantle (SCLM). This frontier approach is crucial for understanding nature, evolution and volatiles recycling within the lithosphere, being particularly important in active or dormant volcanic areas, where the signature of the surface gaseous emissions can be compared to that of the deep mantle domains.

Five distinct populations of ultramafic xenoliths brought to the surface in West Eifel (~0.5-0.01 Ma) and Siebengebirge (~30-6 Ma) volcanic fields (Germany) were investigated by combining petrographic and mineral chemistry analyses with noble gases + CO₂ determinations in olivine-, orthopyroxene- and clinopyroxene-hosted FI. Xenoliths from West Eifel are modally and compositionally heterogeneous, as testified by the large forsterite range of olivine, the Cr# range of spinel and the variable Al and Ti contents of pyroxene. Siebengebirge rocks, on the other hand, are quite homogeneous, having mostly refractory composition and reflecting high extents (up to 30%) of melt extraction. Equilibration temperatures vary from 900 to 1180 °C in West Eifel and from 880 to 1060°C in Siebengebirge xenoliths, at comparable oxygen fugacity values. In all xenoliths populations, FI composition is dominated by CO₂, with olivines being the most gas-poor phases and reflecting a residual mantle that experienced one or more melt extraction episodes. The ³He/⁴He ratio corrected for air contamination (Rc/Ra values) in all phases varies from 6.8 Ra in harzburgites to 5.5 Ra in lherzolites and cumulates rocks, suggesting a progressive modification of an original MORB-like mantle signature via interaction with crustal-related components with ³He/⁴He and ⁴He/⁴⁰Ar* signature similar to magmatic gaseous emissions. The mineral phase major element distribution, together with the systematic variations in FI composition, the positive correlation between Al-enrichment in pyroxene and equilibration temperatures, and the concomitant Rc/Ra decrease at increasing temperature, suggest that the SCLM beneath Siebengebirge represented the German lithosphere prior to the massive infiltration of melts/fluids belonging to the Quaternary Eifel volcanism. On the other hand, West Eifel xenoliths bear witness of multiple heterogeneous metasomatism/refertilization events that took place in the German SCLM between ~6 and ~0.5 Ma. According to Ne and Ar isotope systematics, the FI composition in the studied xenoliths can be explained by mixing between recycled air and a MORB-like mantle, being irreconcilable with the presence of a lower mantle plume beneath the Central European Volcanic Province.

How to cite: Faccini, B., Rizzo, A. L., Casetta, F., Faccincani, L., Ntaflos, T., Italiano, F., and Coltorti, M.: Combining volatiles measurements in fluid inclusions with petrology of ultramafic xenoliths: new insights on the evolution of the West Eifel and Siebengebirge (Germany) Sub-Continental Lithospheric Mantle, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13422, https://doi.org/10.5194/egusphere-egu21-13422, 2021.

The investigation of noble gases (He, Ne, Ar) and CO₂ in fluid inclusions (FI) of mantle-derived rocks from the Sub Continental Lithospheric Mantle (SCLM) is crucial for constraining its geochemical features and evolution as well as the volatiles cycle, and for better evaluating the information arising from the study and monitoring of volcanic and geothermal gases. Eastern Transylvanian Basin in Romania is one of the places in Central-Eastern Europe where mantle xenoliths are brought to the surface by alkaline magmatism, offering the opportunity for applying the above-mentioned approach. Moreover, this locality is one of the few places on Earth where alkaline eruptions occurred contemporaneously with calc-alkaline activity, thus being a promising area for the investigation of subduction influence on the magma sources and volatiles composition.

In this work, we studied petrography, mineral chemistry and noble gases in FI of mantle xenoliths found in Perşani Mts. alkaline volcanic products. Our findings reveal that the local mantle recorded two main events. The first was a pervasive, complete re-fertilization of a previously depleted mantle by a calc-alkaline subduction-related melt, causing the formation of very fertile, amphibole-bearing lithotypes. Fluids involved in this process and trapped in olivine, opx and cpx, show ⁴He/⁴⁰Ar* ratios up to 1.2 and among the most radiogenic ³He/⁴He values of the European mantle (5.8 ± 0.2 Ra), reflecting the recycling of crustal material in the local lithosphere. The second event is related to a later interaction with an alkaline metasomatic agent similar to the host basalts, that caused slight LREE enrichment in pyroxenes and crystallization of disseminated amphiboles, with FI showing ⁴He/⁴⁰Ar* and ³He/⁴He values up to 2.5 and 6.6 Ra, respectively, more typical of magmatic fluids.

Although volcanic activity in the Perşani Mts. is now extinct, strong CO₂ degassing (8.7 × 10³ t/y) in the neighbouring Ciomadul volcanic area may indicate that magma is still present at depth (Kis et al., 2017; Laumonier et al., 2019). The gas manifestations present from Ciomadul area are the closest to the outcrops containing mantle xenoliths for comparison of the noble gas composition in FI. ³He/⁴He values from Stinky Cave (Puturosul), Doboşeni and Balvanyos are up to 3.2, 4.4 and 4.5 Ra, respectively, indicating the presence of a cooling magma (Vaselli et al., 2002 and references therein). In the same area and more recently, Kis et al. (2019) measured ³He/⁴He ratios up to 3.1 Ra, arguing that these values indicate a mantle lithosphere strongly contaminated by subduction-related fluids and post-metasomatic ingrowth of radiogenic ⁴He. Our findings consider more likely that magmatic gases from Ciomadul volcano are not representative of the local mantle but are being released from a cooling and aging magma that resides within the crust. Alternatively, crustal fluids contaminate magmatic gases while they are rising to the surface.

Kis et al. (2017). Journal of Volcanology and Geothermal Research 341, 119–130.

Kis et al. (2019) Geochem. Geophys. Geosyst. 20, 3019-3043.

Laumonier et al. (2019) Earth and Planetary Science Letters, 521, 79-90.

Vaselli et al. (2002) Chemical Geology 182, 637–654.

How to cite: Rizzo, A. L., Faccini, B., Bonadiman, C., Ntaflos, T., Seghedi, I., Grégoire, M., Ferretti, G., and Coltorti, M.: Fluids composition in the mantle beneath the Eastern Transylvanian Basin inferred from mineral chemistry and noble gases in fluid inclusions of ultramafic xenoliths, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9473, https://doi.org/10.5194/egusphere-egu21-9473, 2021.

We present results of detail geochemical study of 18–12 Ma volcanic rocks from the Kamar-Stanovoy Zone of Hot Transtension (KSZHT), located in the central Baikal Rift System (BRS), and older pseudotachylytes from the Main Sayan Fault (MSF). These rocks designate geochemically distinguished from the OIB sources that are referred to the Slyudyanka zone of paleocollision occurred between the Khamardaban terrane and Siberian paleocontinent about 488 Ma ago. We define crustal and mantle signatures (with and without garnet, respectively) for the KSZHT volcanic rocks and crustal ones for the MSF basic pseudotachylytes. The signatures are indicative for tracing complementary relations between layers of the crust–mantle transition (CMT). We infer that the KSZHT volcanic activities accompanied rifting of a paleocontinental margin but got quiescent after a structural separation of the South Baikal Basin from the Tunka Valley, when the former had been sufficiently extended and subsided in contrast to the latter, which had been notably compressed and uplifted. From geological evidence and a detail seismic tomography model, we suggest that the KSZHT crust–mantle magmatic processes were due to delamination of a thickened root part of the South Baikal Orogen that preceded rifting in the South Baikal Basin area in the Late Cretaceous and Paleogene. Volcanic rocks of the past 17 Ma from the southwestern BRS denoted similar CMT sources and delamination processes beneath the East Hangay orogen and adjacent Orkhon-Selenga saddle. In the central and southwestern BRS, the CMT sources marked mutually overlapping deformational fields related to Indo-Asian convergence and pool-to-axis forces of the Japan-Baikal geodynamic corridor.

This work is supported by the RSF grant 18-77-10027.

How to cite: Chuvashova, I., Rasskazov, S., Yasnygina, T., Ailow, Y., Saranina, E., and Rodionova, V.: Volcanic rocks and pseudotachylytes from sources of crust-mantle complementary layers: Insight into geodynamics of the Baikal Rift System, Southern Siberia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3665, https://doi.org/10.5194/egusphere-egu21-3665, 2021.

The Nb/U~47 and Th/U~4 ratios are considered as indicative for the OIB source referred by some authors to lower mantle plumes that in fact have no specific geochemical signatures but HIMU component. The Th/U ratio may vary because of the different garnet–melt and/or clinopyroxene–melt partition coefficients of U and Th. Anomalously high or low Th/U values in rocks can also be related to the input or removal of U, the migration of which is controlled by its mobility under oxidizing conditions owing to the formation of water-soluble uranyl  compounds with hexavalent U. These variations definitely distinguish non-plume magmatic sources. The Th/U ratio decreases to 2.5 in the MORB source and increases to 6 in the continental lower crust one. We describe anomalous behavior of uranium in sources of Cenozoic basalts and basaltic andesites from Primorye, Lesser Khingan, Tunka Valley, as well as similar Cretaceous-Paleogene rocks from Tien Shan. Significant deviations of the Th/U and Nb/U ratios from the OIB values are characteristics mostly of garnet-free sources. The U-depleted and U-enriched signatures are used as sensitive indicators for deciphering crust–mantle transitional processes.

This work is supported by the RSF grant 18-77-10027.

How to cite: Rasskazov, S., Chuvashova, I., Yasnygina, T., and Saranina, E.: U-depleted versus U-enriched signatures in complementary crust-mantle sources of volcanic rocks from Asia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3681, https://doi.org/10.5194/egusphere-egu21-3681, 2021.

Potassic rocks from the Wudalianchi field are considered by some authors as derivatives of the mantle transition layer. However, this opinion is contradicted by the contrasting component composition of melts erupted in different volcanoes. From data of lead isotope compositions, the initial eruptions of lava flows 2.5–2.0 Ma ago were derived from the lithospheric Laoshantou and Gelaqiu model sources of about 1.88 Ga, while subsequent eruptions were derived from the Wohu source of about 0.15 Ga and the recent Molabu source (Rasskazov et al., 2020). Detailed sampling of volcanoes yielded geochemical evidence on both the individualization of sources beneath volcanoes and mixing of melts from contrasting sources. We present evidence on mixing of melts from the Gelaqiu, Wuhu, and Molabu sources beneath the Jiaodebushan Volcano and partial similarity of rock components from this volcano to material erupted in the Xiaogushan Volcano.

This work is supported by the RSF grant 18-77-10027.

Rasskazov S., Sun Y-M., Chuvashova I., Yasnygina T., Yang C., Xie Z., Saranina E., Gerasimov N., Vladimirova T. Trace-element and Pb isotope evidence on extracting sulfides from potassic melts beneath Longmenshan and Molabushan volcanoes, Wudalianchi, Northeast China. Minerals. 2020. V. 10: 319; doi:10.3390/min10040319

How to cite: Yasnygina, T., Sun, Y., Rasskazov, S., Chuvashova, I., Yang, C., Xie, Z., and Ivanova, V.: The geochemical consequences of mixing melts from contrast sources beneath the Jiaodebushan and Xiaogushan volcanoes, Wudalianchi volcanic field, Northeast China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3803, https://doi.org/10.5194/egusphere-egu21-3803, 2021.

The Western Limb of the Bushveld Complex hosts a vast, recently documented swarm of orangeite dykes that are significantly younger (177-132 Ma; Hughes et al., in prep.) than the c. 2.06 Ga Bushveld lithologies they intrude. Orangeite dykes are hybrid igneous rocks that form from very low-degree partial melting deep within the sub-cratonic lithospheric mantle (SCLM) and upon ascent entrain foreign material (primarily mantle xenocrysts). Thus, they can be used to probe the composition of and processes within the ancient lithospheric mantle. Whereas similar orangeite dyke swarms in South Africa typically span < 10 km, the considerable size of this swarm (> 50 km along strike and ~10 km wide) and number of closely-spaced dykes offers a unique opportunity to investigate the Kaapvaal SCLM on an unprecedented spatial scale. In this contribution we present the whole rock major and trace element abundances, and the radiogenic isotope compositions of the dykes.

The Bushveld orangeites are mafic-ultramafic (whole rock Mg# of 65 to 88) and have overlapping major element abundances to other Kaapvaal orangeites, with significant similarity to the coeval Swartruggens orangeite dyke swarm (Coe et al., 2008). Trace element abundances of the Bushveld dykes are less consistent with Kaapvaal orangeite variability, displaying greater ranges in concentrations of certain elements (e.g. La, Th, Ba) despite being generally relatively depleted in these elements.

Radiogenic isotope compositions of the orangeites typically confine to the global orangeite variability, with radiogenic Sr (⁸⁷Sr/⁸⁶Sr_iof 0.70642 to 0.70787) and unradiogenic Hf compositions (ɛHf_i of -18.3 to -8.3). Initial Nd compositions are generally unradiogenic (ɛNd_i of -11.6 to -9.0), conforming to values of global orangeites, however three samples display elevated initial Nd (ɛNd_i of -5.4 to -0.4) and plot in a similar Sr-Nd compositional space to Kaapvaal transitional kimberlites.

Using the trace element variations and radiogenic isotope compositions we aim to investigate the geochemistry of the mantle source regions tapped by the orangeites and whether we can identify changes in source characteristics on a swarm scale.

References:

Coe, N. et al. (2008) Cont. Min. Pet. 156(5). 627-652.

Hughes, H.S.R. et al. (in prep).

How to cite: Compton-Jones, C., Hughes, H., McDonald, I., Bybee, G., Kinnaird, J., and Andersen, J.: Bulk rock geochemistry of a swarm of orangeite dykes intersecting the Western Limb of the Bushveld Complex, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12414, https://doi.org/10.5194/egusphere-egu21-12414, 2021.

We show new evidence that natural micro-diamonds can be formed in decompression-cracks by C:O:H bearing volatiles in a bimineralic eclogite. The investigated rock sample is a heterogeneous kyanite- bearing and bimineralic eclogite from the Roberts Victor mine, South Africa. Kyanite reacts out in the kyanite bearing part of the sample, but metastable relics are still present within the bimineralic part of the rock. The presence of these metastable kyanite relicts, suggest very low fH2O during the phase transition from the kyanite- bearing into the bimineralic eclogite. High-spatial-resolution synchrotron based FT-IR and RAMAN spectroscopy have been used to detect C:O:H-bearing volatiles around micro diamonds in planar defect structures in garnet in the bimineralic parts of the sample and N concentration has been analyzed within the micro-diamonds. In micro-diamond-bearing planar defect structures, a correlation between C:O:H-bearing volatiles can be identified whereas in micro-diamond -ree planar defect structures no correlation of the different C:O:H containing volatiles was detected. We suggest that the micro-diamond forming reaction was triggered by water released by the breakdown of water-bearing kyanite. We propose that the C:O:H bearing volatiles acted as a catalyst, changing in composition with changing P-T conditions in the rock during metamorphism. This catalytic process leads to permanent modification of C:O:H ratios and under favourable thermodynamic, stoichiometric and kinetic conditions micro-diamonds can be formed. Nitrogen concentrations in the analyzed micro-diamonds suggest that the formation of the micro-diamonds took place shortly before the uplift of the eclogite from the Earth mantle to the surface. The conclusions from our study proves that C:O:H-bearing volatiles, and their distribution pattern around the investigated micro-cracks, are indicative of the formation mechanisms of micro-diamonds controlled by C:O:H bearing fluids rather than by the solid-solid transformation from graphite into diamond.

How to cite: Sommer, H., Jacob, D., and Regenauer-Lieb, K.: The Formation of Micro-Diamonds in Decompression-Cracks During the Uplift of the Kimberlite Controlled by the C:O:H Ratio in NAMS, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12279, https://doi.org/10.5194/egusphere-egu21-12279, 2021.

For the first time, snapshots of crystallized melts in olivine of sheared garnet peridotite xenoliths from the Bultfontein kimberlite pipe have been studied. This type of xenoliths represents the deepest mantle rocks derived from the base of lithosphere (at depths from 110 to 230 km for various ancient cratons). According to different models, such type of inclusions (secondary) in mantle minerals can be interpreted as relics of the most primitive (i.e., close-to-primary) kimberlite melt that infiltrated into sheared garnet peridotites. In general, these secondary inclusions are directly related to kimberlite magmatism that finally formed the Bultfontein diamond deposits. The primary/primitive composition of kimberlite melt is poorly constrained because kimberlites are ubiquitously contaminated by xenogenic material and altered by syn/post-emplacement hydrothermal processes. Thus, the study of these inclusions helps to significantly advance in solving numerous problems related to the kimberlite petrogenesis.

The unexposed melt inclusions were studied by using a confocal Raman spectroscopy. In total, fifteen daughter minerals within the inclusions were identified by this method. Several more phases give distinct Raman spectra, but their determination is difficult due to the lack of similar spectra in the databases. Various carbonates and carbonates with additional anions, alkali sulphates, phosphates and silicates were determined among daughter minerals in the melt inclusions: calcite CaCO₃, magnesite MgCO₃, dolomite CaMg(CO₃)₂, eitelite Na₂Mg(CO₃)₂, nyerereite (Na,K)₂Ca(CO₃)₂, gregoryite (Na,K,Ca)₂CO₃, K-Na-Ca-carbonate (K,Na)₂Ca(CO₃)₂, northupite Na₃Mg(CO₃)₂Cl, bradleyite Na₃Mg(PO₄)(CO₃), burkeite Na₆(CO₃)(SO₄)₂, glauberite Na₂Ca(SO₄)₂, thenardite Na₂SO₄, aphthitalite K₃Na(SO₄)₂, apatite Ca₅(PO₄)₃(OH,Cl,F) and tetraferriphlogopite KMg₃FeSi₃O₁₀(F,Cl,OH). Note that carbonates are predominant among the daughter minerals in the melt inclusions. Moreover, there are quite a lot of alkali-rich daughter minerals within the inclusions as well. During the last decade, some research groups using different approaches proposed a model of carbonate/alkali‑carbonate composition of kimberlite melts in their source regions. This model contradicts to the generally accepted ultramafic silicate nature of parental kimberlite liquids. This study is a direct support of a new model of carbonatitic composition of kimberlite melts and also shows that alkali contents in kimberlite petrogenesis are usually underestimated.

This work was supported by the Russian Foundation for Basic Research (grant No. 20-35-70058).

How to cite: Tarasov, A., Sharygin, I., Golovin, A., Dymshits, A., and Rezvukhin, D.: Mineral assemblage within secondary crystallized melt inclusions in olivine of mantle xenoliths from the Bultfontein kimberlite pipe (Kaapvaal craton, South Africa), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4486, https://doi.org/10.5194/egusphere-egu21-4486, 2021.

Thermobarometric calculations for diamond inclusions allowed systematically compare the pressure-temperature, fO₂ conditions in the mantle beneath different cratons worldwide. Beneath Siberia, Kaapvaal, and other cratons, the cold geotherm branch, reconstructed using sub-Ca garnets and eclogitic diamond inclusions relates to a major event of continental growth. Colder geotherms (32 mWm^-2) are related to early subduction. High-temperature plume-related geotherms are common for inclusions in Proterozoic kimberlites beneath Africa. In mobile belts such as Magondi, Ural and Limpopo belts, the amount of pyroxenitic and eclogitic garnets is greater than in the central cores of cratons where dunitic pyropes prevail. Beneath the Khapchan accretionary terrane in Siberia, eclogites are highly diamondiferous. In the mantle beneath Archean cratons, peridotite pyropes differ in CaO content. Depleted peridotitic and Fe-eclogitic diamond inclusions are abundant beneath the Zimbabwe craton, whereas beneath the Congo and West Africa, diamond inclusions yield higher temperatures. Beneath North American cratons, diamond-bearing eclogites are mainly Mg-type. In the Superior craton, Archean diamond inclusions from Wawa are Fe-, Ca-rich pyropes. The diamond inclusions of the Slave and Superior cratons give complex high-temperature plume-related geotherms. Beneath the Amazonian craton, peridotite garnets indicate complex layering at the base of the lithosphere and a pyroxene-rich layer in the middle. Fe-Mg eclogites from a high-temperature trend in which FeO increases with decreasing pressure. Diamond inclusions from the Kimberley craton of Australia show the greatest variations in temperature and composition. The  Eastern Europe craton and the Urals have greater amounts of eclogitic diamond inclusions and advective geotherms. Estimated pressure conditions lower than diamond stability field is due to exceeding pressures around magmatic system transferred by hydraulic forces from depth. 

Support: RFBR 19-05-00788, Russian Ministry of Education and Science

How to cite: Ashchepkov, I., Logvinova, A., Spetsius, Z., and Downes, H.: Mantle evolution beneath Siberian and Archean cratons worldwide: evidence from thermobarometry of diamond inclusions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1550, https://doi.org/10.5194/egusphere-egu21-1550, 2021.

The PT conditions and position of different groups of eclogites in the subcratonic lithospheric mantle (SCLM) worldwide has been established using clinopyroxene Jd-Di thermobarometry for different cratons and kimberlite localities. Beneath Siberia, Fe-eclogites found within the 3.0-4.0 GPa  and  were probably formed in Early Archean times forming the base of the lithosphere. In the Middle and Late Archean, eclogites were melted during subduction creating restite and cumulates from partial melts traced ascending channels.

High-Mg eclogites (partial melts or arc cumulates) are related to low-T geotherms. Melt-metasomatized eclogites trace a high-T geotherm and are often close to the middle part of the mantle lithosphere. Abundant eclogitic diamond inclusions from Siberia also mostly belong to the middle part of the lithosphere. 

Ca-rich eclogites from Precambrian kimberlites of India are located in the middle lithospheric mantle whereas those entrained in Phanerozoic magmas are derived from the lithosphere base. In the Wyoming craton, kimberlites carry eclogite xenoliths captured from the 4.0-2.5 GPa interval.  In mantle lithosphere sampled by Proterozoic kimberlites, Ca-rich eclogites and grospydites occur in the 4.0-5.0 GPa interval. South Africa HT eclogite and diamond inclusions from the Proterozoic Premier kimberlites are derived from the deeper part of the mantle lithosphere and trace a high-T geotherm at depths of 7.0-4.0 GPa showing an increase in Fe upwards in the mantle section. Similar trends are common beneath the Catoca cluster kimberlites in Angola.

Mantle eclogites have clinopyroxenes and garnet trace element patterns with opposite inclinations determined by KDs with melts. Flatter and bell-like REE patterns with Eu anomalies? HFSE troughs and U, Pb peaks are common for MORB-type basaltic eclogites. High-Mg eclogites show less fractionated incompatible element patterns.  LILE-enrichments and HFSE troughs are typical for kyanite-bearing eclogites. Clinopyroxenes from diamond-bearing eclogites show lower REE and troughs in Nb and Zr, peaks in Pb and U concentrations compared to barren eclogites with round smooth trace element patterns and small depressions in Pb and Ba.

Support: RFBR 19-05-00788,  Russian Ministry of Education and Science

How to cite: Ashchepkov, I., Logvinova, A., Spetsius, Z., Ntaflos, T., Downes, H., Vladykin, N., and Ivanov, A.: Varieties of eclogites and their positions in the cratonic mantle lithosphere revealed by Jd-Di thermobarometry and trace element geochemistry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1749, https://doi.org/10.5194/egusphere-egu21-1749, 2021.

Detail study kimberlites and mantle xenoliths from Nakyn field pipes has revealed their unusual, interesting and important mineralogical features. Absence of Megacrystic picroilmenites of is compensated by presence of large orange-red titanium pyropes of "megacryst" type, underlining the reduced character asthenospheric melts influences on the mantle lithosphere in Nakyn. Picroilmenite in Nakyn kimberlites present only in xenoliths eclogites, garnet peridotites and clinopyroxenites with directive structures attributed to zones of melt fluid interaction. The clinopyroxene composition referred to Cr-omfacite, c (instead of Cr-diopside) suggest the Na-Al oceanic spilitic metasomatism at subduction stage or later interaction of the mantle material with the subducted pelitic sediments which is in accord with the presence of Al-rich eclogites wide distribution of the wehrlitic associations may suggest carbonatitic metasomatism. Cr- diopsides occurred in the peridotites with primary magmatic textures.

Absence of picroilmenite megacrysts in Nakyn kimberlites is filled with presence of large orange-red titanious-pyropes of "megacryst" associations, underlining the reduced character astenospheric influences on the mantle substratum of area

Picroilmenites in Nakyn kimberlites are present only in xenoliths of eclogites, and garnet peridotites and clinopyroxenites with, directive structures related to the zones of the metasomatism or melt interaction. The picroilmenite compositions from these rock inclusions sharply differs from composition of picroilmenite typical "megacryst" associations the raised contents of the titanium and the lowest share hematite component. In the same types mantle rocks is unusual also the composition of clinopyroxene: omphacite, chrome-omphacite (but not chrome-diopside) suggesting the high activity of the Na-Al metasomatism probably related to the oceanic spilitic metasomatism. The important participation in their formation of subduction processes allows to assume the specific features of a structure, mineral composition and composition of minerals of these rock inclusions.

Th ALCREMITE and MARID associations probably refer to the interaction of the lamprophyric Al2O3, H2O rich melts with peridotites or interaction of mica bearing Al, alkali rich sediments with peridotites. . The Botuobinskaya and Mayskaya kimberlite pipes contain essential amount of color a green garnets of different shades and compositions, that are very rare in worldwide kimberlites. It specifies on intensive influence of processes "calcium" (“chrome-calcium” and the “titanium–chrome-calcium”) metasomatism in mantle lithosphere

How to cite: Sablukov, S., Sablukova, L., and Stegnitsky, Y.: Unusual mineralogical features and origin of the mantle substratum Nakyn kimberlite fields (Yakutia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6477, https://doi.org/10.5194/egusphere-egu21-6477, 2021.

The lithospheric mantle structure and evolution is one of the fundamental problems of the Earth's history. Eclogites and clinopyroxenite xenoliths are characterized by a similar two-mineral composition (garnet and clinopyroxene), but differ in mineralogical and petrographic features (Gonzaga et al., 2010). Questions of their origin and relationship with peridotites remain controversial. There are several classifications of eclogites based on various attributes: structural and textural features (Mercier & Nicolas, 1975; MacGregor & Carter, 1970), chemical composition of garnet (Coleman, 1965), clinopyroxene (Taylor & Neal, 1989), as well as the whole rock composition (Aulbach et al., 2016 and other), the given classifications may not coincide. The geochemical properties of eclogite xenoliths from kimberlite pipes suggest two main points of view for genesis: implication of subduction processes or cumulates of high-pressure melting in lithosphere mantle (Condie, 1993; Jacob et al., 1994). The "classical" cratonic eclogites represent an ancient oceanic crust subsequently subducted and altered possible further metasomatic processes. These rocks are characterized by significant variations in the composition of minerals, a relatively high content of Al₂O₃ (14-20 wt%) and a low MgO content (10-15 wt%), depletion of elements of the LREE and an Eu anomaly (Gonzaga et al., 2010). In addition, eclogites have a wide range of oxygen isotopic composition in garnet δ¹⁸O 4.51 - 8.69 (much higher than mantle values ​​5.3 ± 0.3) (9). Garnet pyroxenites are characterized by a more magnesian garnet - pyrope and bulk composition (MgO - 15-20 wt.%). The oxygen isotope composition of Grt from clinopyroxenites is close to that of the mantle - δ¹⁸O 5.2 - 5.8. It is assumed that these rocks are a consequence of the polybaric partial melting at high temperatures and pressures (Gonzaga et al., 2010). The mantle xenoliths from upper-Jurassic Obnajennaya kimberlite pipe (Kuoika field, Yakutia) were studied. Eclogites and clinopyroxenites occupy about 10-15% population among xenoliths. Garnet in the eclogites differs from that in the clinopyroxenites by a higher content of CaO and FeO (Prp_55-62 Alm_22-30Grs_8-18 in clinopyroxenites and Prp_40-45Alm_13-29Grs_15-30 in eclogites). Clinopyroxenes are distinguished by reduced magnesia content (Mg# 91-84), as well as low calcium content (16-18 wt.%). The high contents of jadeite components in the clinopyroxene (NaAl[Si₂O₆] - 25-32%) classify this group of rocks as eclogites. The high δ¹⁸O varies in eclogite Cpx (more than 6.0), positive Eu anomaly is assumed that the formation of the protolith of the xenolith group occurred as melts in the subduction zone during accretion of the Birekte block to the Siberian craton (Rosen, 2003). However, the presence of garnet clinopyroxenites with narrow variations in mineral composition and relatively low δ¹⁸O suggests melting processes in the lithospheric mantle and the formation of megacrystalline pyroxene cumulates.

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

How to cite: Kalashnikova, T., Solov'eva, L., Kostrovitsky, S., Sinitsyn, K., and Yudintseva, E.: Eclogite and garnet pyroxenite xenoliths from kimberlite pipes of north Siberian craton - evidences of subduction processes and cumulate origin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6911, https://doi.org/10.5194/egusphere-egu21-6911, 2021.

Here we report the first finding of the high-pressure polymorph of calcium carbonate (aragonite) in the interstitial space of a sheared lherzolite xenolith from kimberlites of the Udachanaya diamond deposit (Siberian craton, Russia). Xenoliths with a sheared texture are the deepest mantle rocks sampled by kimberlite magma from 180-230 km depth. According to experimental data, aragonite is the high-pressure polymorph of calcium carbonate, which is stable at upper mantle pressure and temperature. Thereby aragonite is used as a reliable geobarometer in studies of magmatic and ultrahigh-pressure metamorphic rocks.
Aragonite was determined by Raman spectroscopy study. The Raman bands at 208 cm^-1, 702-706 cm^-1 and 1462 cm^-1 are the identification features of aragonite. Chemical analyses of aragonite were obtained by scanning electron microscope with an energy dispersive system. Some analyses were verified by electron microprobe as well. The concentration of SrO in aragonite ranges from 0.5 to 8.8 wt.%. Aragonite has a Na₂O concentration of 0.1-1.1 wt.%.
Aragonite (up to 100 µm) is the most common subordinate mineral from the interstitial space of this xenolith. It occupies on average 70 vol.% of the interstitial space. Aragonite grains consist of three chemically distinct zones. The first zone (core) is characterized by a low content of SrO (<1.5 wt.%) and low Mg# (~15). The second zone has roughly the same SrO but noticeably higher Mg# (~50). The third zone (rim) contains much higher concentration of SrO (up to 8.81 wt.%) and high Mg# (~50).
Sheared peridotite are located in the lithospheric mantle significantly below the aragonite-calcite equilibrium line. In particular, the investigated peridotite equilibrated at 1350°С and 69 kbar (~215 km). The presence of zoned aragonite from this peridotite means that this rock has been infiltrated by metasomatic agent. Numerical calculations reveals that such zoning can be preserved for 1 year at 1300°С (~equilibrium temperature of sheared peridotites) and for 10 years at 1000°С (~temperature of kimberlite magma at subsurface conditions). The short preservation time of zoning in aragonite (1-10 years) proves that aragonite could be formed immediately prior to kimberlite magmatism or after the capturing of the xenolith by kimberlite magma. Using adiabats of kimberlite magma and P-T parameters of aragonite stability in the upper mantle, aragonite in the studied sample was formed at the depth range of 80-215 km.
As the preservation time of zoning in aragonite is noticeably short (taking into account high temperatures), the best candidate for the role of an agent, which infiltrated the xenolith, is a primitive kimberlite melt of the Udachnaya pipe. The high percentage (70%) of aragonite in the interstitial space of the studied sheared lherzolite xenolith proves that such primitive kimberlite melt had carbonatitic composition. Our results show that not only different silicate-rich melts, but also carbonate or cabonated silicate melts might play a key role in mantle modifications. Carbonate melts are very suitable diamond-forming media and may support the idea of a genetic link between some diamonds and kimberlite magmatism.
This study was supported by the Russian Science Foundation (grant No 18-77-10062).

How to cite: Solovev, K., Golovin, A., Sharygin, I., Rezvukhin, D., and Tarasov, A.: Epigenetic aragonite in a sheared lherzolite xenolith from the Udachnaya kimberlite pipe, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4610, https://doi.org/10.5194/egusphere-egu21-4610, 2021.

Microprobe analysis (JSM 6510 LA/JET-2200) of eclogite pyroxenes from xenoliths (Nikitina et al., 2014) and pyroxenes of Сatoca pipe (reveal the absence of grain zonation ) (Fig.1). Two major groups are in Na₂O–Al₂O₃ and Cr₂O₃–Al₂O₃ diagrams (Sobolev, 1974): high alumina (Hi-Al₂O₃), low magnesian (LMgO), high magnesian (Hi-MgO) (Nikitina et al., 2014). Most sodium-rich pyroxenes are Hi-Al₂O₃, and chrome-Hi-MgO eclogites. Pyroxene grains from magnesian eclogites are enriched with sodium and depleted in chromium in center.  Pyroxene grains from Hi-Al2O3 eclogites are not zonal. Pyroxenes from kimberlites are more diverse in composition, lower in Al₂O₃ higher in Cr₂O₃. In the classification diagrams, the fields of their compositions corresponding to eclogite pyroxenes are well identified, while the field of Hi-Al₂O₃ eclogites is absent. To determine the genetic affiliation of pyroxenes from kimberlites of facies and 3 allows the method of identifying chemical-genetic groups (Garanin et al., 1991) on the basis of cluster analysis (Ivanov, 2017). Only 21% of pyroxenes from kimberlites belong to weakly diamondiferous eclogites, the most numerous – 52% – pyroxenes of weakly diamondiferous lherzolites, 10% – weakly diamondiferous ilmenite peridotites and pyroxenites, 15% – weakly diamondiferous lherzolites and websterites, 2% – intergrowths with diamonds.  This indicates multistage diamond generation in eclogites of  Catoca  kimberlites  (Korolev et al., 2013; Nikitina et al., 2014), and in peridotites, pyroxenites, lherzolites, and websterites (Fig.2)

a.

b.                                                                                                     

c.     

  d.

a.

 b.

c.   

d.

How to cite: Zinchenko, V., Ivanov, A., and Nikitina, L.: Composition of  pyroxenes  from kimberlite and eclogite xenoliths of Catoca kimberlite pipe (Angola), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1774, https://doi.org/10.5194/egusphere-egu21-1774, 2021.

To determine diamond grade Ti, Mn, and Na in  pyropes from kimberlites of the Angola diamond-bearing sub-province, the triangular diagrams of their ratios is proposed. The JX-8230 microprobe, allows determining the composition of minerals by WDS and EDS spectrometers simultaneously.

The diagrams Fig.1 shows the compositions of these oxides in pyropes with their breakdown into cluster groups (CG) of Dawson J.B., Stephens W.E. classifications [3]. It complements the generally accepted diagrams [1,2,3] and creates an opportunity to determine the degree of diamond content of kimberlites and their belonging to the same field or cluster of kimberlite pipes. The diagrams shows the ratio of oxides of the main trace elements in pyropes of Angola kimberlites with diamonds and dots – Mn, Ti and Na in the diamondiferous kimberlites  (Luele, Chyuzu) and in empty ones (Shandongu, Lx 150).

The Na₂O content for the compositions of low-chromium pyropes is the main sign of their crystallization with diamonds, which is reflected in the Na₂O-TiO₂ diagram by J. Gurney [2].

The TiO₂ is undoubtedly an important and significant impurity oxide that determines diamond content and tthe CG of pyropes according to Dawson J.B., Stephens W.E. [3]: its content in G3, G10, G9 is low, <0,3; in G1 – medium, 0,3-0,6 and G2 – high,> 0,6 wt.%.

The MnO content in kimberlite pyropes, as a rule, does not exceed 0,6 wt%. Changes in the contents of this oxide can occur in the process of metasomatic transformations of pyropes, which affects the diamond content in kimberlites [4].

From the presented diagrams (Fig. 1) it can be seen that 97,5% of the compositions of pyrope grains from the highly productive kimberlites of the Luele pipe lie in the diamond-bearing contour, while high-magnesian-chromium pyropes CG G10, whose share is 46%, together with pyropes CG G9 – 21%, evenly distributed over this area and prevail over the rest of the CG. Medium-high titanium CG G1-G4 and G-11 are compactly concentrated in the lower area of ​​the diamond-bearing contour, next to the low-titanium G3.

In low diamondiferous kimberlites of the Chyuzu pipe, about 65% of pyrope grains fall into the diamondiferous contour, while the compositions of CG G10 and G9 are represented by less than 10% of grains, 90% of grains are high-medium titanium CG G1, G2 and G11, and the compositions of single pyropes CG G3 shifted to the upper region of the diamondiferous contour.

The non-diamond pipes Shandongu and Lx-150 are also characterized by the displacement of CG G10 (16% and 7%, respectively) and G9 to the upper part of the diagram, with a predominance of the proportion of pyropes G9, with an outflow of diamond content up to 30-50% of the grain compositions. The proportion of high-titanium CG G1, G2, and G11 (up to 25% in the Lx-150 pipe) is quite large here, most of the compositions of which go beyond the diamond-bearing contour of kimberlites.

Conclusions

New JX-8230 microprobe allows quantitative determination of trace elements in kimberlite pyropes. Diagram MnO, Na2O, TiO2 give additional criteria for kimberlite diamond grade

How to cite: Ivanov, A. and Zinchenko, V.: The  Ti, Mn, and Na oxide distribution in kimberlite pyropes of Angola as criterion of diamond grade, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12891, https://doi.org/10.5194/egusphere-egu21-12891, 2021.

For industrial deposits that contain particularly large and expensive diamonds, a diagram of the compositions of KIM (kimberlite indicator minerals) for Cr, Al, Fe, Mg, Mn is proposed. The proximity or rather convergence of the compositions of KIM is also complemented by a high correlation of the frequency of occurrence of their cluster groups (Ivanov, 2017), such as pyropes, chromites, picroilmenites and pyroxenes. The increase in oxygen fugacity  in  KIM is correlating with Fe and Ti , this fact can also explain the intensity of metasomatism of kimberlites, shown in the proposed graph by an increase in ilmenite content. This type of diagram was proposed by Mitchell (1986) for chromite compositions, which the authors supplemented with compositions of  picroilmenites, pyropes and pyroxenes. The manganese  concentration is shown by the size of the figurative point  – a bubble diagram. For better perception, the drawing is supplemented with different colors of the composition groups of  KIM. Below are diagrams for two industrial deposits that contain expensive and large diamonds. The diagrams show the approximate regions of diamond-bearing associations for chromite and pyrope compositions with red asterisks. The blue lines show two main trends in chromite compositions, the horizontal one is picrate trend and the vertical one is kimberlite. For picroilmenite compositions, the diagram shows two main trend lines: the red line for paramagnetic compositions and the black line for ferrimagnetic compositions. For pyrope compositions, trend lines are shown in red for a number of cluster groups G10 by (Dawson, Stephens, 1975) and for groups G11 – in black. The diamond content in kimberlites of the Aykhal pipe is several times higher than in the Komsomolskaya pipe, but the cost of diamond crystals from the latter is several times more expensive than in the Aykhal pipe. A distinctive feature of the compositions of the Komsomolskaya pipe KIM, as well as the above-proposed kimberlite pipes (Griba and Karowe ) – is the presence of diamond-bearing websterite parageneses of chromites and pyropes, which corresponding in the diagram, to areas with elevated Cr, Fe  values . The kimberlite of the Aykhal pipe is characterized by a higher degree of metasomatism, which is recorded in the diagram by the trend of more ferruginous picroilmenite compositions, which affects the quality of its diamonds.

CONCLUSIONS. The proposed method is based on a comprehensive assessment of KIM compositions. The diagram allows to assess the presence of expensive and large diamonds in kimberlite pipes at the  exploration  stage, as well as to reconstruct the composition of deep mantle rocks, based on  the KIM graphically presenting analyses of their compositions for five elements.

1. Ivanov A. S. Statistical analysis of indicator minerals of kimberlites. Proceedings of the XIII All-Russian (with international participation) Fersman session. KSC RAS. G. Apatity. 2017. C. 172-181.

2. Mitchell R.H. Kimberlites: mineralogy, geochemistry and petrology. New York, Plenum Press. 1986. 442 P.

3. Dawson J.B., Stephens W.E. Statistical classification of garnets from kimberlites and xenoliths. J. Geol. 1975. 83, 589-607.

How to cite: Ivanov, A. and Zinchenko, V.: Method of complex analyses of the compositions of kimberlite indicator minerals to assess the presence of large diamonds, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1907, https://doi.org/10.5194/egusphere-egu21-1907, 2021.