GD2.4

EDI
Geochemical and geodynamic perspectives on the origin and evolution of deep-seated mantle melts and their interaction with the lithosphere

The origin and evolution of the continental lithosphere is closely linked to changes in mantle dynamics through time, from its formation through melt depletion to multistage reworking and reorganisation related to interaction with melts formed both beneath and within it. Understanding this history is critical to constraining terrestrial dynamics, element cycles and metallogeny. We welcome contributions dealing with: (1) Reconstructions of the structure and composition of the lithospheric mantle, and the influence of plumes and subduction zones on root construction; (2) Interactions of plume- and subduction-derived melts and fluids with the continental lithosphere, and the nature and development of metasomatic agents; (3) Source rocks, formation conditions (P-T-fO2) and evolution of mantle melts originating below or in the mantle lithosphere; (4) Deep source regions, melting processes and phase transformation in mantle plumes and their fluids; (5) Modes of melt migration and ascent, as constrained from numerical modelling and microstructures of natural mantle samples; (6) Role of mantle melts and fluids in the generation of hybrid and acid magmas.These topics can be illuminated using the geochemistry and fabric of mantle xenoliths and orogenic peridotites, mantle-derived melts and experimental simulations.

Public information:

Dear collegues Dear  all.

Thank you very much for all presentations! The quality of most was very high. 

Aspecially  we shou be grateful  to Sonja Aulbach  who was a a wonderful presenter of the session.  Hope next  year the situation will be better and  more participants will be participated personally in Vienna.  There is one opened spacial issue in Minerals https://www.mdpi.com/journal/minerals/special_issues/Deep_Seated_Melts. I'll try to find the possibility for another one not so expensive.

Best wishes Igor Ashchepkov and conveners team.   

Co-organized by GMPV2/TS10
Convener: Igor Ashchepkov | Co-conveners: Sonja Aulbach, Kate Kiseeva, Evgenii Sharkov
Presentations
| Fri, 27 May, 10:20–11:40 (CEST), 13:20–16:38 (CEST)
 
Room -2.91
Public information:

Dear collegues Dear  all.

Thank you very much for all presentations! The quality of most was very high. 

Aspecially  we shou be grateful  to Sonja Aulbach  who was a a wonderful presenter of the session.  Hope next  year the situation will be better and  more participants will be participated personally in Vienna.  There is one opened spacial issue in Minerals https://www.mdpi.com/journal/minerals/special_issues/Deep_Seated_Melts. I'll try to find the possibility for another one not so expensive.

Best wishes Igor Ashchepkov and conveners team.   

Presentations: Fri, 27 May | Room -2.91

Chairpersons: Kate Kiseeva, Evgenii Sharkov
10:20–10:30
10:30–10:36
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EGU22-6723
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ECS
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On-site presentation
Carl Walsh, Balz Kamber, and Emma Tomlinson

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.

10:36–10:42
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EGU22-1531
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Highlight
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On-site presentation
Stefano Poli, Sutao Zhao, and Max W. Schmidt

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) (XMg). 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 XMg ≈ 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.

10:42–10:48
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EGU22-3326
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Presentation form not yet defined
Mingrui Li, Rui Gao, Jianbo Zhou, Simon A Wilde, Hesheng Hou, Xiaomiao Tan, and Yanlin Zhu

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.

10:48–10:54
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EGU22-3284
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ECS
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Virtual presentation
Jae Hoon Kim and Jin-Han Ree

Most of earthquakes occur below 10-km depth in the Korean Peninsula. For example, the focal depth of the Mw 5.5 Gyeongju Earthquake in 2016, the largest instrumental earthquake in South Korea since scientific earthquake monitoring started in 1978, is about 14 km with hypocentral basement rocks of granitoid and temperature of 370°C (thus, brittle-plastic transition condition). A study on ancient granitoid shear zones with the similar temperature condition will aid in understanding the seismogenesis in the brittle- plastic transition regime. The Yecheon shear zone is an NE- to NNE-striking right-lateral shear zone cross-cutting Mesozoic granitoid belt in South Korea. The deformation temperature of the main shear zone was estimated to be about 350 ℃. In the southeastern margin of the shear zone, protomylonites change gradually into mylonites and then abruptly into ultramylonites toward southeast. Quartz and feldspar grains both of protomylonite and mylonite deform by dislocation creep and brittle fracturing, respectively. Greenish ultramylonite consists of quartz-, feldspar-, muscovite- and epidote-rich layers within matrix of quartz, muscovite and epidote. The protomylonite commonly displays a composite S-C foliation. The deflecting S-foliation of mylonite toward ultramylonite is sharply truncated by the boundary between mylonite and ultramylonite. Thin (several mm to several cm) greenish layers occur in protomylonite subparallel to mylonitic foliation or cross-cutting the foliation at a low angle. They also show injection structure with flow banding and cataclastic deformation along the protomylonite boundary. The greenish layer consists of fragments of protomylonite and matrix of very fine-grained quartz, feldspar, muscovite and epidote. Epidote grains of ultramylonite and greenish layers replace phengitic mica, biotite and plagioclase and show graphic texture. Together with epidote formation, chloritization of biotite and albitization of K-feldspar are prominent in the greenish layers. The growth of hydrothermal minerals including epidote and chlorite within the greenish layers and shear band along the C-foliation indicates fluid circulation in the layers. We interpret the greenish layers were generated during seismic events in fluid-rich conditions and thus seismic event may be caused by pore pressure build up. Once the greenish layers develop, deformation was localized along the layers due to much reduced grain size in interseismic periods, and the greenish layers became ultramylonite with further grain-size reduction.

How to cite: Kim, J. H. and Ree, J.-H.: Seismogenesis in granite under brittle-plastic transition condition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3284, https://doi.org/10.5194/egusphere-egu22-3284, 2022.

10:54–11:04
11:04–11:10
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EGU22-968
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Presentation form not yet defined
Evgenii Sharkov

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.

11:10–11:16
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EGU22-362
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Presentation form not yet defined
Nikolai Bozhko

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.

11:16–11:22
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EGU22-449
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Presentation form not yet defined
Internal structure of zircon from gneisses of the Ingozero massif of the Achaean TTG complex as an evidence of its more than 3.1 Ga formation age and complicated history of superimposed processes (Kola region, Russia).
(withdrawn)
Elena Nitkina and Tatiana Kaulina
Mantle beneath large igneous provinces
11:22–11:28
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EGU22-516
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ECS
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Highlight
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Virtual presentation
Alican Aktağ, Kaan Sayit, Bradley J. Peters, Tanya Furman, and Jörg Rickli

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.

11:28–11:34
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EGU22-4994
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ECS
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Virtual presentation
Ugur Balci, Finlay M. Stuart, Jean-Alix Barrat, and Froukje M. van der Zwan

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.

11:34–11:40
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EGU22-5450
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ECS
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On-site presentation
Francesca Innocenzi, Sara Ronca, Stephen F. Foley, Samuele Agostini, and Michele Lustrino

The northernmost sector of the western branch of the East African Rift (EAR) includes the young (~40-50 ka [1]) volcanic province of Toro Ankole, characterized by the presence of exotic volcanic products such as carbonatites, melilitites, kamafugites and foidites [2]. Among these, the occurrence of kamafugites (kalsilite-bearing volcanic rocks [3]) is noteworthy, as Toro Ankole represents the type locality for these compositions, found in only two other localities worldwide. The Toro Ankole volcanic province developed along the margin of the Archean Tanzanian craton, and its magmatic products show the influence of metasomatic processes and phases developed in the thick continental lithosphere. Indeed, MARID-like metasomatism is proposed in literature, with the formation of a veined mantle [4].

A multidisciplinary approach, based on a detailed petrographic, mineral chemical, geochemical and isotopic (Sr, Nd, Pb and B) study, has been carried out on 53 samples, which include not only lavas and tuffs, but also holocrystalline and wall rock xenoliths. Two types of lava may be identified: the first is represented by carbonatites and silico-carbonatites, characterized by low SiO2 (4.89-21.78 wt%) and low alkali (0.44-2.03 wt%) and high CaO (25.17-47.57 wt%), while the second most peculiar lithotypes is represented by kamafugites; katungites (melilite-rich kalsilite-olivine-bearing volcanic rocks), mafurites (kalsilite-rich melilite-olivine-bearing) and ugandites (olivine-rich kalsilite-melilite-bearing). The kamafugites are strongly SiO2-undersaturated and moderately ultrabasic, potassic to ultrapotassic volcanic rocks, with high MgO (6.08-22.20 wt%) and CaO (up to 15.46 wt%). They consist of phenocrysts of clinopyroxene and olivine set in a hypo-holocrystalline fine-grained groundmass made up of microliths of clinopyroxene, olivine, perovskite, kalsilite, nepheline, leucite, melilite, phlogopite, carbonates and opaques.

The xenolith cargo shows wide range of compositions, varying from clinopyroxenite to glimmerite, with low modal abundance of opaques and perovskite in agreement with the literature data that generally report a lack of olivine and orthopyroxene in the mineral assemblage [5]. The common presence of phlogopite, abundant clinopyroxene and carbonate-rich veins indicate the presence of veined lithosphere [6]. This is consistent with the isotopic data for lavas and xenoliths (87Sr/86Sr = 0.70480-0.70563 and 143Nd/144Nd = 0.512515-0.512575), which outlines an enriched and complex mantle source. 206Pb/204Pb is extremely variable, with values from the holocrystalline xenolith (19.99-19.27) being slightly higher than lava samples (19.28-19.63). The d11B values for lavas and xenoliths, show a wide range, varying from DMM-like values (-6 and -8‰) to more variable OIB-like values (down to -12 and up to -3‰; [7]), through to positive values (up to +6.6‰ in the lavas). These latter also exhibit the highest Sr isotopic ratios of the dataset, pointing to the possible occurrence of old and altered oceanic crust and/or serpentinite in the mantle source.

Bibliography

[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, 9th 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.

Lunch break
Chairpersons: Evgenii Sharkov, Igor Ashchepkov
13:20–13:30
13:30–13:36
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EGU22-9813
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ECS
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On-site presentation
Andrea Boscaini, Andrea Marzoli, Hervé Bertrand, Massimo Chiaradia, Fred Jourdan, Manuele Faccenda, Christine Meyzen, Sara Callegaro, and Lina Serrano Durán

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.

13:36–13:42
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EGU22-13248
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Virtual presentation
Jana Kotkova, Renata Čopjaková, and Radek Škoda

Orogenic garnet peridotites exhumed in ultrahigh-pressure-ultrahigh-temperature terranes represent windows into material transfer in deep subduction zones. Multiphase solid inclusions (MSI) trapped in garnet proved to be important tracers of metasomatism by crustal-derived fluids. Our study of the MSI from the Saxothuringian basement in the Bohemian Massif, European Variscan Belt, allowed identifying the source and evolution of the liquids metasomatized the mantle rocks. As the MSI could not be re-homogenized due to a high content of volatiles, their bulk composition was estimated considering the proportions, phase densities and chemical composition of the constituent minerals.

The MSI occur in an annulus at garnet rim of garnet lherzolite and harzburgite, and throughout garnet in garnet pyroxenite. The major phases of the MSI include amphibole, barian mica and carbonate (dolomite, magnesite). Minor phases are clinopyroxene, orthopyroxene, garnet II, spinel, apatite, monazite, thorianite, graphite, pentlandite, scheelite and sulphides. The proportion of hornblende systematically decreases from pyroxenite and close harzburgite and lherzolite to more distal mantle rocks, where clinopyroxene and garnet II occur instead. By contrast, the amount of barium-bearing phases (barian mica, Ba-Mg carbonate norsethite, barian feldspar) and carbonates increases in the same direction.

Major element composition of garnet pyroxenite, including enrichment in alkalies and barium, approaches carbonate-silicate melts similar to kimberlites.  Trace element signatures indicate that it is a rare example of low-degree supercritical liquid derived from a mixed crust-mantle source frozen in the mantle. The MSI hosted by garnet in pyroxenite represent a residual solute-rich liquid after high-pressure fractional crystallization of the parental melt, enriched in alkalies (Na, K), highly incompatible elements (LILE – Ba, Sr; Th, U), LREE, Ti, W and volatiles (CO2, Cl, F, P). The MSI in peridotites allow tracing the changes of this metasomatizing liquid during its reactive infiltration into peridotite through silicate crystallization as well as interaction with mantle minerals distinct in lherzolite and harzburgite (garnet±clinopyroxene). The liquid evolved from more silicic, solute-rich to more diluted carbonate-rich, with gradual enrichment in LILE (K, Ba) and volatiles (CO2, Cl) and LREE fractionation, similar to evolution of kimberlitic to carbonatitic melts through differentiation by fractional crystallization.  

Here we demonstrate that the MSI trapped in garnet can be used as a unique tool for tracing chemical evolution of the liquids metasomatizing the mantle wedge. Importantly, these results are valid even in the case of the interaction of the trapped material (MSI) with the host garnet, as this potential contamination mainly concerns Al, Si and Cr while majority of the other elements used for petrogenetic implications remained unaffected

How to cite: Kotkova, J., Čopjaková, R., and Škoda, R.: Source and evolution of metasomatizing liquids in orogenic peridotites: evidence from multiphase solid inclusions , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13248, https://doi.org/10.5194/egusphere-egu22-13248, 2022.

13:42–13:48
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EGU22-9186
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Presentation form not yet defined
Scott A. Whattam

International Ocean Discovery Program (IODP) Expedition 357 drilled 17 shallow sites spanning ~10 km in the spreading direction (from west to east) across the Atlantis Massif oceanic core complex (OCC, Mid-Atlantic Ridge, 30°N). Exposed mantle in the footwall of the Atlantis Massif OCC is predominantly nearly wholly serpentinized harzburgite and subordinate dunite. Altered peridotites are subdivided into: (I) serpentinites, (II) melt-impregnated serpentinites, and (III) metasomatized serpentinites. Type I serpentinites show no evidence of melt-impregnation or metasomatism apart from serpentinization and local oxidation. Type II serpentinites have been intruded by gabbroic melts and are distinguishable in some cases based on macroscopic and microscopic observations, e.g., mm-cm scale mafic-melt veinlets, rare plagioclase (˂0.5 modal % in one sample) or by the local presence of secondary (replacive) olivine after orthopyroxene; in other cases, ‘cryptic’ melt-impregnation is inferred on the basis of incompatible element enrichment. Type III serpentinites are characterized by silica metasomatism manifest by alteration of orthopyroxene to talc and amphibole, anomalously high anhydrous SiO2, and low MgO/SiO2. Two fundamental features of the mantle serpentinites are identified: (1) A pattern of increasing melt-impregnation from west to east; and (2) a link between melt-impregnation and metamorphism. In regard to (1), whereas a dominant fluid- rock alteration (mostly serpentinization) is distinguished in the western serpentinites, a dominant mechanism of melt-impregnation is recognized in the central and eastern serpentinites. Melt-impregnation in the central and eastern sites is characterized by enrichment of incompatilble elements, Cr-spinel with anomalously high TiO2 (up to 0.7 wt.%) and olivine forsterite (Fo) compositions that range to a minimum of Fo86.5.  With respect to (2), in contrast to unmetamorphosed Cr-spinel of western site Type I serpentinized peridotites, spinel of the melt-dominated central and eastern peridotites record metamorphism, which ranges from sub-greenschist (<500°C) to lower amphibolite (>600°C) facies. Low grade, sub-greenschist facies metamorphism resulted in Mg and Fe2+ exchange between Cr-spinel and olivine resulting in Cr-spinel with anomalously low Mg# (cationic Mg/(Mg+Fe2+)). Higher grade amphibolite facies metamorphism resulted in Al-Cr exchange and the production of Fe-chromite and Cr-magnetite. Heat associated with magma injection and subsequent melt-impregnation resulted in localized contact metamorphism. High degrees of melt extraction are evident in low whole-rock Al2O3/SiO2 and low concentrations of Al2O3, CaO, and incompatible elements. Estimates of the degree of melt extraction based on Cr# (cationic Cr/Cr+Al, up to ~0.4) of unaltered Cr-spinel and modeled whole rock REE patterns, suggest a maximum of ~18-20% non-modal fractional melting. As some serpentinite samples are ex-situ rubble, the magmatic histories at each site are consistent with derivation from a local source (the fault zone) rather than rafted rubble that would be expected to show more heterogeneity and no spatial pattern. In this case, the studied sites may provide a record of enhanced melt-rock interactions with time, consistent with proposed geological models for OCC formation.  

How to cite: Whattam, S. A.: Spatial patterns of fluid- and melt-rock processes and link between melt-impregnation and metamorphism of Atlantis Massif peridotites (IODP Expedition 357), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9186, https://doi.org/10.5194/egusphere-egu22-9186, 2022.

13:48–13:54
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EGU22-1260
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Highlight
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Presentation form not yet defined
Olga Kiseleva, Evgeniya Ayriants, Dmitriy Belyanin, and Sergey Zhmodik

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.