To shed light on the evolution of the Earth’s mantle in different geodynamic settings and investigate the nature and distribution of fluids and melts at various depths in the lithosphere, it is necessary to adopt multi-parametric and multi-disciplinary approaches, combining petrology and geochemistry of mantle-derived rocks to field studies, modeling and theoretical approaches about the rheology of the lithosphere-asthenosphere system. Such integrated studies are better suited not only to image and trace melts and fluids in various geological environments, but also to identify specific seismicity patterns and chemical signatures in order to mitigate natural and anthropogenic hazards.
This session welcomes contributions from a broad range of topics, including: i) petrology and geochemistry of minerals and melt/fluid inclusions in mantle rocks and mantle-derived melts; ii) gaseous emissions and thermodynamic modelling; iii) seismic monitoring and tomography. Contributions from Early Career Scientists are specifically encouraged.
The cratonic ‘roots’ of Earth's major continents extend to depths of over 160 km and have remained stable for more than 2.5 billion years due to buoyant, refractory harzburgites formed by Archean mantle melting. However, kimberlite-hosted harzburgite mantle xenoliths from some global cratons (e.g., Kaapvaal, Siberia, Slave, Rae and Tanzania) show unusual orthopyroxene and silica enrichment, alongside titanium depletion, which cannot be explained by simple melting processes (Boyd, 1989). These are abundant in the xenolith suites and were typically entrained at depths shallower than ~150 km. Many have escaped the pervasive carbonated silica undersaturated melt metasomatism that dominates the base of global cratonic lithosphere. The origins of the orthopyroxene-rich harzburgites have long been debated: hypotheses include high-pressure melting residues (Aulbach et al., 2011), komatiite melt interaction (Tomlinson & Kamber, 2021), or subduction-related silicic melts and fluids (Bell et al., 2005).
To further investigate the origin of the widespread excess silica in ancient mantle we analysed volatile (H₂O, F, Cl) contents by Secondary Ion Mass Spectrometry in well-characterised peridotites from the Kaapvaal craton. The orthopyroxene-rich harzburgites, including a diamond-bearing sample, show elevated volatile contents and depletions in Ti. The results of mass balance calculations suggest that the orthopyroxene-rich harzburgites formed by reactive infiltration of supercritical C-O-H fluids -- rich in silica, potassium, fluorine and chlorine but depleted in Ti -- fluxed from subducted oceanic lithosphere (carbonated pelites, eclogites and serpentinites). These findings highlight the role of C-O-H-F-Cl bearing fluids in shaping cratonic lithosphere globally and offer a new framework for understanding craton evolution, mantle metasomatism and diamond genesis in early Earth (Gibson et al., 2025).
Figure 1. Schematic illustration of the formation of excess orthopyroxene in Archean cratons.
Aulbach, S., Stachel, T., Heaman, L. M., Creaser, R. A. & Shirey, S. B. (2011). Formation of cratonic subcontinental lithospheric mantle and complementary komatiite from hybrid plume sources. Contributions to Mineralogy and Petrology 161, 947–960.
Bell, D. R. et al. (2005). Silica and volatile-element metasomatism of Archean mantle: a xenolith-scale example from the Kaapvaal Craton. Contributions to Mineralogy and Petrology 150, 251–267.
Boyd, F. R. (1989). Compositional distinction between oceanic and cratonic lithosphere. Earth and Planetary Science Letters 96, 15–26.
Gibson, S.A., Jackson, C.J., Crosby, J.C. & Day, J.A.F. (2025). The role of C-O-H-F-Cl fluids in the making of Earth’s continental roots. Nat Commun doi:10.1038/s41467-025-62888-3
Tomlinson, E. L. & Kamber, B. S. (2021). Depth-dependent peridotite-melt interaction and the origin of variable silica in the cratonic mantle. Nat Commun 12, 1082.
How to cite: Gibson, S., Jackson, C., Crosby, J., and Day, J.: The role of C-O-H-F-Cl fluids in the making of Earth’s continental roots, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7735, https://doi.org/10.5194/egusphere-egu26-7735, 2026.
Harzburgite, amphibole-harzburgite, pyroxenite and dunite collected from the borehole 639 provide insights into the complex metasomatic modifications in the paleo-oceanic lithosphere of the Kempirsai ophiolite, Kazakhstan, hosting the world’s largest podiform chromite deposits. Three distinct metasomatic events have been identified in ultramafic rocks of the Kempirsai ophiolite, based on detailed petrographical observations and geochemical data. In the first stage, the mantle melting residues were percolated by SiO2-rich melts, likely derived from the decompression melting of asthenosphere, forming the orthopyroxene-rich harzburgites. Orthopyroxene1 is observed to corrode and crosscut coarse olivine grains (olivine1), enclose them in crystallographic continuity with surrounding olivines, or occur along olivine grain boundaries. These various mineral relationships imply the crystallization of orthopyroxene1 through the consumption of olivine1. As the melt/rock ratio decreased with melts migrating upward, less coarse orthopyroxene1 was precipitated, but more irregular counterparts were observed to scatter along olivine1 boundaries in the shallow-level harzburgites. In the second stage, orthopyroxene1, particularly the orthopyroxene porphyroclasts, reacted with water-poor, SiO2-unsaturated melts to crystallize clinopyroxene, orthopyroxene2 and olivine2, with occasional presence of amphibole. The rare earth element depleted patterns of clinopyroxene in both harzburgites and Amp-harzburgites (La/Yb: 0.01-0.16), together with the insignificant enrichment in fluid mobile elements, attest to the water-poor nature of the metasomatic melts. Subsequently, water-rich liquid-rock interaction at the third stage triggered the dissolution of previous metasomatic products (orthopyroxene1 and clinopyroxene) and the precipitation of mineral assemblages of amphibole+orthopyroxene2+olivine2±spinel±phlogopite. They either form veinlets that crosscut the coarse orthopyroxene1, mantle the orthopyroxene porphyroclasts, or form fine-grained polymineralic domains running parallel to the lineation of samples. This interaction is likely a syn-deformation metasomatism (869-991 ℃), as witnessed by the decrease in orthopyroxene1 and clinopyroxene modal contents as well as the increase in the proportion of amphibole with depth. The metasomatic agent is suggested to be alkaline- and water-rich liquids, which infiltrated the mantle peridotites as a reactive porous flow and enriched other silicate minerals in light rare earth elements and fluid-mobile elements, especially for those in the deep amphibole-harzburgites. We propose that the water-rich liquids derived from the dehydration of amphibolite of the metamorphic sole rocks, based on syn-deformation metasomatism and enrichment of fluid mobile elements. The liquids are also of Cl-rich nature, which could gradually enrich the fluids with Cr2O3, as suggested by the high Cr2O3 contents in amphibole in the mantle peridotites (up to 3.10 wt.%). The fluids could migrate upward to metasomatize mantle peridotites or trigger its partial melting, forming podiform chromitites containing similar Cr2O3-rich amphibole (up to 4.02 wt.%).
How to cite: Liu, X., Melcher, F., Xiao, Y., Su, B.-X., Erlandsson, V. B., and Merseburger, S.: Multiple mantle metasomatism scenarios in Kempirsai ophiolite, Kazakhstan: Evidence from the borehole peridotites, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9064, https://doi.org/10.5194/egusphere-egu26-9064, 2026.
Volcanic-borne magmatic megacrysts (e.g., clinopyroxene and amphibole) are commonly deep cumulate phases bearing critical insights into magmatic thermochemical conditions (e.g., P-T-fO₂-H₂O). The post-110 Ma diffuse basaltic province covering the central-eastern Asian continent share similar geochemical characteristics but the entrained clinopyroxene megacrysts exclusively occur in the east with unknown controlling factors. This study focuses on the Langshan-Chaoge basaltic field far west from the eastern coast, where megacrysts of clinopyroxene (Mg# = 60–70) and amphibole (Mg# = 70–80) exist in ~95 Ma and ~89 Ma basalts, respectively. They display comparable trace-element patterns, Sr-isotope compositions and crystallization P-T conditions (clinopyroxene: 0.8–1.2 GPa, 1061–1143°C; amphibole: 1.1–1.2 GPa, 1068–1129°C) and likely represent fragmented lower crustal cumulates from compositionally similar parental magmas with different degrees of fractionation. Given the limited water storage capacity of asthenosphere, the estimated magmatic water contents (H₂O = 2.1–2.8 wt.% for clinopyroxene vs. H₂O = 3.6–4.7 wt.% for amphibole) should reflect the contamination by low-extent melts from variably hydrated lithospheric lower boundary. Together with the established cooling trend of regional lithosphere during 110–80 Ma, the basalt-borne megacryst transition from clinopyroxene to amphibole likely mirrors the stability of amphibole in the cooling deep lithospheric mantle. This recognition has general relevance to the exclusive occurrences of clinopyroxene megacrysts in eastern Asian continent with hot lithosphere and emphasizes the potential of volcanic-borne megacryst species as an independent reflection of province-scale lithospheric mantle thermal states.
How to cite: Hu, L., Dai*, H.-K., Xiong, Q., and Zheng, J.-P.: Mantle thermal state controls the post-110 Ma basalt-borne megacryst species across central-eastern Asian continent, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10393, 2026.
The evolution of the lithospheric mantle at the transition between supra-subduction and cratonic settings is poorly constrained in terms of thermal and chemical history.
In Canada, the transition between the hot and thin lithospheric mantle beneath the Canadian Cordillera to the west and the adjacent thick and cold cratonic lithosphere, together with the slow cooling rate of the Cordillera mantle, is the perfect study case for constraining the evolution of the lithospheric mantle between subduction fronts and cratonic settings.
The Canadian Cordillera is the westernmost section of the North American Plate and formed as a result of a subduction active over the past ~50Mys, followed by the shortening in the past 10Mys by the formation of the Queen Charlotte fault (Canil et al., 2021). This tectonic transition subsequently led to the formation of extended hot back arc basins across the Canadian Cordillera (Hyndman, 2010), which made the lithospheric mantle beneath the Canadian Cordillera hot and thin (Canil et al., 2021), until the abrupt transition to the Laurentian craton to the East (Canil and Russell, 2022).
To reconstruct the thermo/chemical evolution of the lithospheric mantle beneath the Cordillera, n. 40 ultramafic xenoliths from Lightning Peak and Mt. Timothy (British Columbia) were studied. Based on their modal composition, the Lightning Peak xenoliths are dominantly lherzolites (Ol 43-73%, Opx15-33%, Cpx6-30%), with a single occurrence of an Ol-websterite (Ol~27%, Opx~9%, Cpx~60%). The samples in this group are relatively coarse-grained and display protogranular texture. Notable petrographical features in the lherzolites include sieved-texture rims of clinopyroxene and the rare occurrence of plagioclase and spinel as Al-bearing phases. The former may suggest the occurrence of metasomatic processes, whereas the latter is indicative of shallow depth of last equilibrium.
Mt timothy xenoliths display a wider compositional range, varying from lherzolites to harzburgites (Ol 44-78%, Opx19-32%, Cpx3-24%), with coarse-grained and protogranular texture similarly to Lightning Peak xenoliths. They show less (or not at all) sign of metasomatic modification, and the common exsolution of spinel from primary orthopyroxene. Corroborated by mineral chemistry and thermobarometric models, our study sheds light on the melting and metasomatic processes experienced by the lithospheric mantle beneath the Canadian Cordillera, revealing the thermal history and the architecture of the lithosphere-asthenosphere boundary at the transition between subduction and cratonic settings.
Acknowledgments
This research was supported by Grant (83985) from the Research Committee of the University of Patras via the “C. CARATHEODORI” program.
References
Canil, D. and Russell, J. K.: Xenoliths reveal a hot Moho and thin lithosphere at the Cordillera-craton boundary of western Canada, Geology, 50, 1135–1139, https://doi.org/10.1130/g50151.1, 2022.
Canil, D., Russell, J. K., and Fode, D.: A test of models for recent lithosphere foundering or replacement in the Canadian Cordillera using peridotite xenolith geothermometry, Lithos, 398-399, 106329, https://doi.org/10.1016/j.lithos.2021.106329, 2021.
Hyndman, R. D.: The consequences of Canadian Cordillera thermal regime in recent tectonics and elevation: a review, Canadian Journal of Earth Sciences, 47, 621–632, https://doi.org/10.1139/e10-016, 2010.
How to cite: Kalantzi, L., Casetta, F., Koutsovitis, P., Ntaflos, T., Nardini, N., Russell, K. J., and Coltorti, M.: Nature and evolution of the lithospheric mantle beneath the Canadian Cordillera: clues from ultramafic xenoliths from Lightning Peak and Mt Timothy (British Columbia, Canada), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5026, https://doi.org/10.5194/egusphere-egu26-5026, 2026.
Refractory lithospheric deep roots are the cornerstone for the prolonged stability of cratons and mantle xenoliths are normally the key targets for study on the evolution of such deep roots. In regions with few mantle xenoliths discovered, the basalts enriched in radiogenic isotopic compositions due to marked lithospheric mantle contribution are crucial to unmask the lithospheric mantle evolution based on comprehensive study involving petrology, geochemistry and thermodynamic modelling. Here, the Early Cretaceous basaltic suites from the northwest North China Craton are taken as an example to show the significance of enriched basalts on the study of lithospheric mantle. These basalts with few mantle xenoliths are characterized by high silica and alkali contents (SiO2 = 45.8 – 59.8 wt.%, K2O+Na2O = 4.81 – 9.88 wt.%), arc-type trace-element patterns and enriched radiogenic isotope compositions (e.g., εNd = -2.64 – -12.88, 87Sr/86Sr = 0.7063 – 0.7093). The TiO2 and Fe2O3 (total) contents are higher than those of natural and experimental melts from refractory mantle peridotite but comparable to those of partial melts of fertile mantle rocks. The high contents of fluid-loving elements are suggestive of source metasomatism by aqueous fluids. Combined with thermodynamic modelling and regional tectonic history, these enriched basalts likely record concurrent melting of the asthenosphere and hydrated fertile lithospheric lower boundary. The inferred lithospheric mantle contrasts with the coeval thick and refractory one supporting the eastern NCC, and highlights that the craton destruction, especially the loss of its ancient refractory mantle root, should take place in a diachronous manner related to the craton-girded subduction episodes. Our study illustrates the potential of enriched basalts to recover the nature and evolution of mantle lithosphere beneath craton margins and associated tectonic histories.
Related papers:
1. Dai, H.-K., Zheng, J.-P., Xiong, Q., Hu, L.-L, & Zhou, X. (2024). Insight of enriched basalts into the nature and evolution of mantle lithosphere beneath craton margins. Science China Earth Sciences, 67, 3128–3142.
2. Dai, H.-K., Zheng, J.-P., Xiong, Q., Griffin, W. L., & O’Reilly, S. Y. (2023). Continental thermal blanketing explains the compositional dichotomy of the diffuse basaltic province across central-eastern Asia. Geophysical Research Letters, 50, e2023GL104951.
3. Dai, H.-K., Oliveira, B., Zheng, J.-P., Griffin, W. L., Afonso, J. C., Xiong, Q., & O'Reilly, S. Y. (2021). Melting dynamics of Late Cretaceous lamprophyres in central Asia suggest a mechanism to explain many continental intraplate basaltic suite magmatic provinces. Journal of Geophysical Research: Solid Earth, 126, e2021JB021663.
4. Dai, H.-K., Zheng, J.-P., Xiong, Q., O'Reilly, S. Y., & Griffin, W. L. (2021). Deep lithosphere of the North China Craton archives the fate of the Paleo-Asian Ocean. Earth-Science Reviews, 215, 103554.
How to cite: Dai*, H.-K., Zheng, J.-P., Xiong, Q., Hu, L.-L., and Zhou, X.: Insight of enriched basalts into the nature and evolution of mantle lithosphere beneath craton margins, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15691, 2026.
Ultramafic xenoliths brought to the surface by primitive magmas in the Macaronesian archipelagos (Canary, Cape Verde, Madeira and Azores islands) provide important constraints on the structure and chemical composition of the oceanic lithospheric mantle, its record of geodynamic processes, and the evolution of deeper portions of plumbing systems beneath active volcanic areas (Munha et al., 1990).
Here we present a detailed study of n. 16 ultramafic xenoliths collected from basaltic to basanitic magmas in the NW part of Madeira Island (Portugal). The xenoliths are mostly protogranular to porphyroclastic spinel harzburgites and dunites, with subordinate cumulate-textured wehrlites and olivine clinopyroxenites.
Two groups are recognized based on the mineralogy and major and trace element compositions. The first group is represented by harzburgites, which are composed of highly forsteritic (90-92) olivine, high Mg# (90-93) pyroxene and high Cr# (0.3-0.7) spinel, suggesting their derivation from a refractory mantle which experienced a high degree (20-25%) of melt extraction. The second group is represented by dunites, wehrlites and olivine clinopyroxenites, with low forsterite (82-86) olivine, low Mg# (82-86) pyroxene and low Cr# (0.2-0.4) spinel, suggesting formation either as cumulates of basaltic/basanitic magmas (Munha et al., 1990) or by extensive mantle metasomatism. The equilibration temperatures obtained from the two-pyroxene geothermometer and the spinel-olivine geothermometer are 1009-1073°C for the harzburgites and 896-1056°C for the cumulates, assuming a pressure of 11 kbar. The oxygen fugacity varies between +0.94 and +1.90 ΔlogƒO2 [FMQ].
The chondrite-normalised REE patterns of clinopyroxenes from harzburgite xenoliths are broadly consistent with those of residua after 10-18% partial melting of primitive mantle under spinel-facies conditions, with LREE typically ranging from ~0.2 to 0.8 × chondrite and Ce from ~0.5 to 1.5 × chondrite. The REE patterns of clinopyroxenes from dunite, wehrlite and olivine clinopyroxenite xenoliths exhibit strongly elevated LREE–MREE abundances, with La and Ce typically ranging from ~2 to 20 × chondritic and Nd from ~4 to 15 × chondritic, corroborating the evidence that these xenoliths crystallized from, or re-equilibrated with, infiltrating basaltic to basanitic melts.
References
Munhá, J., Palácios, T., MacRae, N. D., & Mata, J. (1990). Petrology of ultramafic xenoliths from Madeira Island. Geological Magazine, 127(6), 543-566.
How to cite: Zheng, X., Casetta, F., Ntaflos, T., Abart, R., Aulbach, S., and Nardini, N.: Oceanic Lithospheric Mantle evolution and plumbing system roots beneath Madeira Island (Portugal): evidence from petrology and geochemistry of ultramafic xenoliths, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11567, 2026.
Rodingites are reported for the first time in the Shergol ophiolitic melange in the northwest part of the Ladakh ophiolitic belt, which is associated with the Himalayan orogeny. These are embedded within serpentinites recording metasomatic processes driven by fluid-rock interactions and element transfer between mafic protoliths and the host ultramafic ophiolitic rocks. The Shergol rodingites are distinguished into two distinct types. Type I paragenesis includes clinopyroxene + garnet ± chlorite ± zeolite, whereas Type II comprises garnet + clinopyroxene + vesuvianite ± quartz. Mineralogical and geochemical data suggest that these two types correspond to a two-stage metasomatic evolution: initially, a main rodingitization phase during which mafic protoliths transformed into rodingites (Type I), followed by a derodingitization stage, during which calcium-rich minerals were replaced by Mg-rich minerals, forming chlorite-dominated blackwall zones at the rims. These rodingites are regarded as metasomatic derivatives of mafic protoliths that include gabbro and basaltic dolerite. Rodingitization resulted in significant depletion of Si, Na, K, along with Ca and in cases Mg enrichment. It is also associated with significant mobilization of large-ion lithophile elements (LILE). High Mg#, low k, Rb, U, Th, and Pb concentrations, with low 87Sr/86Sr Isotopic signatures, suggest a depleted mantle source for the protoliths. Pressure-temperature conditions indicate that rodingitization occurred under low- to moderate-grade metamorphic conditions (250-325°C and ~3 kbar, respectively).
Keywords: Shergol ophiolitic belt, Rodingite, metasomatism, MORB, Ladakh Himalaya
How to cite: Kumar, A., Barman, P., koutsovitis, P., Sahoo, I., Singh, A., Singh, A., Sheikh, J., Patel, S., and Verma, A.: Rodingites in the Shergol ophiolitic melange in the Ladakh ophiolitic belt: Insights into rodingitization processes and geodynamic implications, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4215, https://doi.org/10.5194/egusphere-egu26-4215, 2026.
Subduction is a fundamental process driving mantle evolution and material recycling, yet distinguishing the contributions of oceanic versus continental subduction to mantle heterogeneity remains a challenge. This study examines Mo isotopes, combined with elemental and Sr-Nd-Pb isotopic data, of Early Cretaceous basalts in Sihetun (western Liaoning) and Feixian (western Shandong) in the North China Craton (NCC), regions influenced by distinct subduction processes. Sihetun basalts, exhibit light Mo isotope (δ98/95Mo = -0.97 ~ -0.60‰), low Mo contents (0.32~0.41 ppm), high Ce/Mo ratios (292~426), and EMI-like Sr-Nd isotopic compositions. These characteristics suggest a mantle source containing dehydrated oceanic crust and pelagic sediments, possibly related to the oceanic subduction of the Paleo-Pacific plate. In contrast, Feixian basalts show heavier Mo isotope signatures (δ98/95Mo = -0.09 ~ 0.07‰), higher Mo content (1.1~1.4 ppm), lower Ce/Mo ratios (137~197), and EMII-like Sr-Nd isotopic compositions. These features point to the contribution of recycled continental crust and terrigenous sediments in their mantle sources, suggesting that their source were effected by both the deep subduction of the Yangtze continent and oceanic subduction of the Paleo-Pacific plate. These distinct Mo isotopic variations across the NCC reveal that oceanic subduction typically leads to lighter Mo signatures, while continental subduction introduces heavier Mo isotopic signature to the overlying lithospheric mantle. This study highlights Mo isotopes as a powerful tracer for subduction-related processes and provides new insights into how oceanic and continental subduction regimes drive mantle enrichment and global geochemical cycles.
How to cite: Guo, J., Ma, Q., Ma, L., Xu, Y.-G., Zheng, J.-P., Zhang, C., and Hu, H.: Contrasting Mo Isotopic Signatures in Basalts linked to Distinct Subduction Processes in the North China Craton, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8043, https://doi.org/10.5194/egusphere-egu26-8043, 2026.
At slow- and ultraslow-spreading ridges, the presence of mantle rocks outcropping at the seafloor indicates that plate spreading is mainly accommodated by tectonic processes, with little or no magmatism. The Southwest Indian Ridge (SWIR) between 42 and 46°E is one of these magma-starved segments, with large exposures of mantle rocks (54%) and basaltic rocks (40%) on the seafloor. Gabbroic rocks constitute only 1% of the recovered material. Mantle rocks sampled at 42-46°E include some of the freshest peridotites ever sampled in an oceanic context, which provide the unique opportunity to identify deep deformation mechanisms not overprinted by low temperature hydration alteration.
These samples show variable degrees of deformation ranging from weakly deformed (protogranular/porphyroclastic) to strongly deformed (i.e., mylonites). In both samples, olivine microstructure combined to its crystallographic preferred orientation (CPO) suggest deformation in presence of melt, at high temperature and low strain conditions, close to the solidus. In porphyroclastic samples, melt circulation, evidenced by polymineralic film-like trails along olivine grain and subgrain boundaries, seems controlled by olivine crystallographic network. In mylonites, hydrated phases replace similar interstitial polymineralic assemblages along olivine grain boundaries, revealing the involvment of hydrous fluid at lower temperature conditions (T < 800°C). By combining chemical maps and EBSD data, we show that a Si-rich melt was involved during high-temperature deformation, forming these film-like trails; we propose that these melt reaction zones at near-solidus conditions, are the initial stage of strain localization in the mantle lithosphere, leading to further grain size reduction and to the formation of mylonites through further strain focusing and fluid/melt channelization.
This work is supported by PRIN2017KY5ZX8. This project also received funding from the European Union’s Horizon 2020 research and innovation program (EXCITE) under grant agreement No 101005611 for Transnational Access conducted at the EBSD CNRS-INSU national facility at Géosciences Montpellier (CNRS & Université de Montpellier).
How to cite: Bickert, M., Brunelli, D., Tommasi, A., and Koepke, J.: Melt-assisted deformation at subsolidus conditions in mantle-derived peridotites from the Southwest Indian Ridge (42-46°E)., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13021, 2026.
The Develidağ volcanic complex is a part of the Middle Miocene-Quaternary Cappadocian Volcanic Province (CVP), which is located on the eastern side of the Plio-Quaternary Sultansazlığı pull-apart basin along the Ecemiş left-lateral strike-slip fault in central Anatolia. Volcanic rocks are classified as basalt, basaltic andesite, and minor amounts of andesites.
Primitive mantle-normalized multi-element patterns reveal that basaltic rocks are depleted in LIL and HFS elements compared to an OIB signature except for Ba and Pb. In contrast, andesites are represented by high LIL element content and differ from an OIB source magma by Nb, Ta, P, Ti depletion, and Pb enrichments.
High Zr/Ba (0.49-1.42), Zr/Hf (42.19–47.46), and Th/U (3.13–4.69) ratios are attributed to contributions from an asthenospheric source component. The 87Sr/86Sr ratios of basaltic samples range between 0.703656 and 0.703940. Oxygen isotope compositions are characterized by δ18O values ranging from +4.8 to +6.1‰ and Δ¹⁷O values of -57 to -51 ppm, consistent with a mantle signature. These isotopic insights imply that basaltic rocks are related to mantle-derived processes and are consistent with a spinel peridotite source without much crustal contamination. Recent studies widely suggest that the geodynamic development of the region may be linked to processes such as slab break-off, asthenospheric upwelling, and lithospheric mantle drip. Within this framework, δ18O values and their combined usage with Sr isotope data will provide significant insight for the evolution of the basaltic rocks and for the mantle dynamics in terms of interpreting the contamination processes in the CVP.
How to cite: Çağ, T. A., Kürkcüoğlu, B., Pack, A., Willbold, M., and Di Rocco, T.: The development of mantle-derived basaltic rocks in the Develidağ Volcanic Complex: New Sr and triple O isotope evidence, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15658, 2026.
During the deepest stages of subduction, melts and fluids released from the slab are responsible for crust-mantle interaction and the transfer of elements to the mantle. Primary melt inclusions of crustal origin now trapped in eclogite lenses within peridotite bodies of the Granulitgebirge, Bohemian Massif (Germany), represent witnesses of this process. The scenario proposed to explain the origin of these eclogites was that a granitic crustal melt interacted with a mafic/ultramafic layer already present in the peridotite to produce the eclogites at around 2.2 GPa and 1000°C. However, the nature of the protolith is still unknown because of the lack of mineralogical relics.
To fill this gap and better constrain the role of crustal melt-mantle interaction in generating garnet-bearing rocks, we performed melt-rock reaction experiments at the conditions at which metasomatism took place in the Granulitgebirge. Piston cylinder experiments were performed at 1.5, 2.2 and 2.9 GPa and 1100°C, after an initial stage at 1300°C for 1 hour. As starting materials, we used a homogeneous synthetic glass with the same composition as the granitic melt measured in the Granulitgebirge eclogites and two mantle protoliths: a fertile lherzolite pre-synthesized starting from a gel, and a natural spinel clinopyroxenite (bulk XMg = 0.74). Two initial melt:rock weight proportions were chosen: 1:9 and 3:7, which simulate a rock-dominated metasomatic reaction.
The lherzolite-granitic melt reaction generally produces orthopyroxene-rich ± garnet websteritic assemblages. At 2.2 GPa, reaction products are orthopyroxene along garnet and rare phlogopite. At 1.5 GPa, the same reaction does not produce garnet, and pargasitic amphibole is present instead.
Independent of the pressure and melt:rock ratio, the spinel clinopyroxenite-granitic melt reactions completely consumed spinel and produced garnet and new clinopyroxene coexisting with an andesitic residual melt. New clinopyroxene has higher XMg (0.84-0.89) and Na at lower Ti contents. At 1.5 GPa, a coarse poikilitic grain (around 1 mm in size) of garnet developed, suggesting a very fast crystallisation rate likely promoted by a high amount of Al-rich reacted melt. The modal abundance of garnet, as well as its grossular content, increases with pressure and decreases with temperature, according to the reaction: spinel + cpx1 + granitic glass = garnet + cpx2 + andesitic reacted glass. Interestingly, garnets resulting from this last set of reaction experiments have a composition similar to those in the Granulitgebirge eclogites. On the contrary, clinopyroxenes crystallized by reacted melt are Al-rich diopside with low jadeite contents (Jd < 0.14), much lower than clinopyroxenes in the natural eclogites.
Our study shows that the best fit between nature (Granulitgebirge case study) and experiments is visible when the reacting rock is a spinel clinopyroxenite rather than a peridotite. However, the differences between nature and experiments still need to be evaluated.
How to cite: Borghini, A., Borghini, G., Ferrero, S., and Vescera, F.: Granitic melt-mantle reaction experiments: insights into crust-mantle interaction products during the subduction of the continental crust, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17130, 2026.
Mafic enclaves are usually considered so as to indirectly investigate primitive melts in subduction settings. Spinel, among the earliest crystallizing phases in primitive magmas, provides key constraints on melt composition and, in turn, on mantle source characteristics. Despite being one of the oldest active subduction systems, the Aegean subduction zone remains highly complex, with its petrogenetic processes and evolutionary history still often under debate. Mafic enclaves are relatively common along the present–day Hellenic Volcanic Arc, particularly at its margins, with representative occurrences on Nisyros Island and the Methana Peninsula at the eawstern and western margins, respectively. In contrast, basaltic lavas are generally scarce, with the most primitive ones identified in the submarine Pausanias Volcanic Field at the western margin of the arc. This study focuses on the western margin of the Hellenic Volcanic Arc and presents a comparative investigation of spinel major element chemistry, obtained by EPMA, between mafic enclaves from the Methana Peninsula as well as mafic enclaves and lavas from both Methana Peninsula and its adjacent submarine Pausanias Volcanic Field. Mafic enclaves examined in this study reveal a previously unrecognized spinel population at Methana, characterized by distinct mineral chemistry. These spinels are notably Al–rich and Cr–poor (Cr# < 60) compared to previously reported spinels from Methana enclaves and lavas (Cr# > 60), and closely resemble spinels from Pausanias lavas. Variations in spinel chemistry can reflect either differences in primary melt compositions, resulting from mantle source heterogeneity or related to variable degrees of partial melting of a common mantle source, or else the evolution of a primary melt through subsequent petrogenetic processes. However, the compositional similarity between spinels from Methana mafic enclaves and Pausanias lavas suggests that the submarine Pausanias Volcanic Field may represent eccentric volcanism related to the Methana Volcanic Field rather than an independent system. This interpretation is supported by the close spatial association of the two volcanic fields and their near–contemporaneous development. These findings highlight the need for further integrated studies to better constrain mantle source characteristics, primitive melt, and petrogenetic processes in the western margin of the Hellenic Volcanic Arc that will shed new light and enhance our understanding about the mechanisms and the dynamics of the Aegean subduction zone.
Acknowledgments
This work is part of the first author's Ph.D. research, which is financially supported by the «Andreas Mentzelopoulos Foundation».
How to cite: Giamas, V., Kalantzi, L., Koutsovitis, P., Petrounias, P., and Ntaflos, T.: Spinel insights into mantle source characteristics and petrogenetic processes in the western Hellenic Volcanic Arc, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5025, https://doi.org/10.5194/egusphere-egu26-5025, 2026.
In the island of Saint Martin, selected lava samples reveal a diverse volcanic suite that comprises of tholeiitic and calc-alkaline basalts, basaltic andesites, andesites and dacites. Basalts and basaltic andesites exhibit variable MgO (3.6-7.9 wt.%), CaO (6.9-11.8 wt.%) and TiO2 (0.5-1.0 wt.%) contents. Andesites and dacites generally show lower MgO (1.2-5.5 wt.%) and Al2O3 contents (11.7-18.0 wt.%). Interestingly, a dacite sample exhibits relatively enhanced MgO (5.5 wt.%), comparable to the recently reported melatonalites in St. Martin[1]. Alteration was rather moderate, as revealed from their LOI (0.5-2.6 wt.%) and the presence of prehnite, chlorite and sericite. The rock forming minerals in the basalts and basaltic andesites include compositionally broad plagioclase (Ab5-74An26-94Or0-4), diopside and augite (Wo42-52En29-45Fs9-21), magnesiohornblende (MgO=13.0-19.8 wt.%) and phlogopite (FeOt/MgO≈0.57, Al2O3=14.4-14.7 wt.%). Andesites and dacites contain plagioclase (Ab10-76An1-90Or0-86), diopside and augite (Wo45-48En40-42Fs10-15), enstatite (Wo1-4En55-71Fs29-43), magnesiohornblende (MgO=13.2-17.5 wt.%), biotite (FeOt/MgO≈1.5, Al2O3=12.1-12.9 wt.%) and phlogopite (FeOt/MgO≈0.5, Al2O3=13.4-14.0 wt.%). LREE in basalts and basaltic andesites are either slightly depleted or variably enriched [(La/Yb)CN=0.6-4.7], with enhanced HREE (10.2-21.2xCN) and negative Eu anomalies (EuCN/Eu*=0.7-0.9). Andesites and dacites display comparable LREE patterns [(La/Yb)CN=0.9-3.8], followed by differentiated HREE (11.7-24.8xCN) and pronounced negative Eu anomalies (EuCN/Eu*=0.6-0.9). Petrogenetic modelling calculations reveal that the primary hydrous basaltic magma was generated at pressure and temperature ~1.6 GPa and ~1280 oC respectively, after partial melting ~14% of a depleted DMM source. The least differentiated basalts evolved after ~50% fractional crystallization of olivine, plagioclase, clinopyroxene and amphibole. Subsequent additional removal of plagioclase and Fe-Ti oxides further differentiated the residual melts toward andesitic and dacitic compositions.
Acknowledgments: Georgia Kolovadi was financially supported by the «Andreas Mentzelopoulos Foundation».
Reference: [1] Koutsovitis, P. et al., 2025. Granitoids from St. Martin/Maarten Island, Caribbean: Insights on the role of mantle processes in the Lesser Antilles arc. Lithos, 494-495, 107926.
How to cite: Kolovadi, G., Koutsovitis, P., van der Meulen, M. J., Tsikos, H., Petrounias, P., Ntaflos, T., Mason, P., and Gregoire, M.: A Petrological and Mineralogical approach to the Saint Martin Volcanics, Caribbean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11932, 2026.
