PICO
PICO: Fri, 8 May, 08:30–10:15 | PICO spot 2
Mantle/melt partitioning of trace elements is governed by both melt composition and the chemistry of peridotite-forming minerals (olivine, orthopyroxene, clinopyroxene and garnet/spinel), which in turn are controlled by the pressure-temperature conditions in the melting column. Although several sets of mineral/melt partition coefficients are available for various mantle lithologies and P-T conditions, none constrains the partitioning behavior for realistic CO2-H2O-bearing silicate melts saturated with the four mantle minerals along the mantle adiabat, conditions that will determine the geochemical signatures of melts released from asthenosphere upwellings. To thus performed “forced multiple saturation experiments” on a highly Si-undersaturated primitive ocean island basanite composition from Cape Verde in which the melt is forced into equilibrium with four-phase garnet lherzolite at adiabatic temperatures (1380-1420 oC) at 3-7 GPa. This yields mineral and melt compositions in the melting column of a mantle upwelling from the incipient redox melts forming at 7 GPa to the oceanic lithosphere-asthenosphere boundary (LAB). In-situ analyses by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) were conducted to determine the mineral/melt partitioning of high field strength elements (HFSE: Nb, Ta, Zr, Hf), Ba, Sr, Th, U, REEs, Y, moderately siderophile elements (e.g., W, Mo), alkalis (K2O, Na2O) and other minor elements (TiO2, P2O5) at each pressure step. These pressure-dependent partition coefficients and our melting reaction stoichiometries are then employed to model the geochemical signatures of CO2-bearing silicate melt rising through the asthenosphere. The modeled results are then compared to primitive alkaline magmas erupted in both continental and oceanic settings to test whether peridotite/melt trace element partitioning to varying depths effectively encompasses the geochemical spectrum of intraplate magmatism.
How to cite: Schettino, E. and Schmidt, M. W.: Peridotite/melt partitioning experiments constraining the geochemical signature of CO2-bearing alkaline magmas from redox melting to the source of ocean island basalts, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22348, https://doi.org/10.5194/egusphere-egu26-22348, 2026.
The mantle transition zone (MTZ) is widely considered to be water rich, whereas the upper mantle has a much lower water storage capacity. Materials from the Earth’s surface are transported to the bottom of mantle transition zone and topmost lower mantle by slab subduction, resulting in the global upwelling of the mantle transition zone materials to shallow regions as counterflows. Since minerals in the mantle transition zone are considered to be water-rich, dehydration melting could occur near the 410-km discontinuity when the water-rich materials are transported to the low-water-storage-capacity upper mantle. However, the amount of the melt produced by the dehydration melting process remains poorly constrained. Here, we performed high-pressure phase equilibrium experiments under the conditions just above the mantle transition zone (at 13 GPa and 1800 K) using peridotite + 1 wt.% H2O, which represents the bulk compositions of a water-rich mantle transition zone. Our results show a very high melt fraction ~ 10 wt.% (equivalent to 10.21 vol.%) produced by the dehydration melting process near the 410-km discontinuity, far exceeding the minimum melt fraction required to significantly reduce seismic velocities. Because of the low density and low viscosity, most melts formed near the 410-km discontinuity should migrate upwards rapidly to shallow regions. They may accumulate near the lithosphere-asthenosphere boundary, causing the rheological weakening of the asthenosphere.
How to cite: Chen, J., Yu, H., Fu, S., Xu, F., Zhang, B., and Fei, H.: The melt fraction induced by the dehydration melting at the base of upper mantle, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13795, https://doi.org/10.5194/egusphere-egu26-13795, 2026.
We previously estimated geologically current rates of hotspot motion of 2 to 4 mm/yr from Monte Carlo inversion of the trend of 56 young tracks of hotspots. Plate motions were constrained to consistency with the MORVEL set of plate relative angular velocities. To determine the average rate of motion, each realization randomly assigns a motion direction to each hotspot and a globally uniform rate of motion is imposed ranging from 0 to 15 mm/yr. We require the misfit for the solution set to lie in a range that is neither too small nor too large given objectively estimated uncertainties of observed hotspot trends. From one million realizations, only 21,749 (≈2%) gave an acceptable fit.
The set of successful solutions also contains information about what directions of hotspot motion produce misfits to the observed trends that are significantly better than those obtained assuming fixed hotspots. For each hotspot we generate a Rose diagram showing the distribution of the direction of motion for the successful realizations.
We test the directions of motion for each hotspot using the Rayleigh test of uniformity. Six of the 53 hotspots have a value of p > 0.05, which is not significantly different from a uniform distribution. The other 47 hotspots tend to move perpendicular to the plate-motion direction (p=5.1 × 10–11 for the Rayleigh test applied to the set of hotspot-motion directions relative to the local plate motion direction).
Exceptions to this pattern occur for hotspots on ultra-slow-moving lithosphere. Because they are sited on ultra-slow-moving lithosphere, the tracks of these hotspots may record the direction of motion of individual hotspots relative to the mean hotspot reference frame. Examples of hotspot tracks on the Eurasian, Antarctic, and part of the Nubian plate, all sites of ultra-slow-moving lithosphere, will be examined and discussed.
How to cite: Gordon, R., Gaastra, K., and Mifflin, G.: Geologically Current Directions of Motion of 53 Hotspots Estimated from Monte Carlo Inversion, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15215, https://doi.org/10.5194/egusphere-egu26-15215, 2026.
Cratons host diverse metal deposits, including Cu-Ni-PGE deposit systems, and their lithospheric mantle roots have recently been proposed to contain sulfide-hosted metal reservoirs that can provide a potential metal source for ore-forming systems. The base of cratonic mantle lithosphere (cratonic roots) have been suggested to be metasomatized by carbonate-rich magmas episodically over long periods of time. However, whether carbonated cratonic roots are ubiquitously enriched in Platinum-group elements (PGEs) and related metal elements remains debated. The Aillik Bay intrusive suite in Labrador, Canada, preserves magmatic rocks formed by the melting of cratonic roots in two stages: carbonate-poor lamproites in the Mesoproterozoic (~1.37 Ga) and carbonate-rich ultramafic lamprophyres (aillikites) in the Neoproterozoic (~590-555 Ma). These were succeeded by nephelinites during the Early Cretaceous (~142 Ma) by melting at shallower levels after the craton had been split. These samples constitute an ideal natural archive to test the hypothesis of whether carbonated melts drive PGE enrichment in cratonic roots. Here we present a systematic petrographic, whole-rock PGE, and Re-Os isotopic study of these alkaline silicate rocks and associated carbonatites, aiming to evaluate the temporal evolution of PGE budgets within cratonic roots. Rocks from all three periods contain well-preserved magmatic sulfides with negligible alteration, indicating that the observed PGE signatures are controlled by magmatic processes rather than post-emplacement overprinting or secondary alteration. Geochemical constraints further suggest that these magmas were generated under sulfide-saturated (or near-saturated) conditions in their source regions, establishing a basis for assessing sulfide control on PGE behavior. The lamproites formed in reduced, metal-bearing rocks and display MORB-like PGE patterns with depletion of IPGEs and enrichment of PPGEs, with IPGE contents slightly higher than MORBs. In contrast, the aillikites show significant IPGE enrichment, markedly different from MORB patterns. The lamproites and aillikites yield low and primitive mantle-like initial 187Os/188Os ratios (0.078 and 0.130), respectively. The Cretaceous nephelinites originated from melting of mantle source metasomatized by aillikite magmas and show MORB-like PGE patterns and initial 187Os/188Os ratios typical for metasomatized mantle sources. These observations point to a key control of CO2 concentrations in magmas on PGE signatures. Therefore, we suggest that carbonatite metasomatism can enrich cratonic roots in IPGE.
How to cite: yu, S., chen, C., Foley, S., Liu, J., He, D., Jiang, W., and Liu, Y.: Carbonatite metasomatism drives PGE enrichment in cratonic roots, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15570, https://doi.org/10.5194/egusphere-egu26-15570, 2026.
The oxygen fugacity (fO2) of the mantle governs the behaviors of multivalent elements (e.g., Fe, V) and the speciation of C–O–H fluids, influencing mantle melting, magmatic evolution and volatile distribution across tectonic settings. However, the estimation of mantle fO2 is limited by challenges in measuring Fe3+ in minerals like clinopyroxene (Cpx) due to analytical constraints and inconsistencies between oxybarometer methods. Here, we applied machine learning (ML) to predict Cpx Fe3+ content and equilibrium pressure-temperature and fO2 conditions. We employed a nested cross-validation approach to minimize coincidental perfomance biases. Our models outperformed previous ferric iron and thermobarometer models on both metrics and generalization. The ML-based oxybarometer shows adequate generalization with R2 peaking at 0.74, average MAE is 0.95, and average RMSE is 1.45. We compiled an application dataset comprising of 9,832 global mantle xenolith samples. For the subset with Fe3+ measurements (n≈600), ML-predicted fO2 closely matches thermodynamic estimates, supporting the robustness and global applicability of our approach. Applying the model to the rest samples lacking Fe3+ analyses expands geographic coverage to data-sparse provinces (e.g., South America, India, and Eastern Europe), and reveals coherent global redox gradients. Xenoliths from cratonic mantle domains show no temporal fO2 trends since Mesoproterozoic. Comparative analysis across cratonic mantle xenoliths, abyssal peridotites, and oceanic intraplate xenoliths indicates that mantle residues are initially oxidized by short-term metasomatism, but eventually equilibrate to stable redox conditions through interactions with neutral or reducing agents.
How to cite: Ye, C., Liu, C., and Zhang, Z.: Calibrating Mantle Redox Conditions Using Ferric Iron in Clinopyroxene Xenoliths: A Machine Learning Approach, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9001, https://doi.org/10.5194/egusphere-egu26-9001, 2026.
IR spectra of garnets from mantle xenoliths, diamond inclusions and UHP metamorphic rocks indicate that this mineral can be a principal participant in a water balance of the mantle and subduction zones. This conclusion is consistent with experiments showing that H2O content in garnet increases with pressure and could reach up to >2000 ppm H2O at the transition zone conditions (Liu et al., 2024; Chen et al., 2025). Although being controversial (dependent on starting materials, i.e. crystalline natural garnets vs. synthesized ones), available experimental data allow determination regularities of the H2O solubility in garnet with respect to P, T, fO2 and garnet composition. Present study shows an attempt to parametrize these regularities.
The parametrization is not possible for the experiments with starting crystalline garnets (Lu, Keppler, 1997; Zhang et al., 2022; Zhang, Yang, 2025). These data are not consistent between each other, and the reason for the inconsistency is not clear. The data on garnets synthesized from oxide mixtures are better self-consistent. 54 data points from (Geiger et al., 1991; Khomenko et al., 1994; Withers et al., 1998; Mookherjee, Karato, 2010; Fan et al., 2017; Bolfan-Casanova et al., 2000; Katayama et al., 2003; Thomas et al., 2015; Panero et al., 2020; Liu et al., 2021, 2024) represent intervals 2 – 25 GPa and 900 - 2000°C for a wide range of garnet composition including majoritic ones. The H2O content (the Bell et al., 1995 calibration) in this set varies from 130 to 1620 ppm (the above mentioned data on the H2O content >2000 ppm in garnet were excluded). The data were approximated with an equation DH - TDS + (p-1)DV – nRTlnfH2O + RTlnCH2O + WAl*XAl2 + WSi*XSi2 + WCa*XCa3 = 0, where CH2O is the H2O content in garnet, DH = 0 kJ/mol, DS = -92.96(±3.94) J/mol/K, DV = 0.475(±0.022) J/mol/bar are thermodynamic effects of the reaction Grt + nH2O = Grt*nH2O, fH2O is a H2O fugacity (Pitzer, Sterner, 1995), n = 0.5, WAl = -30554.3(±4515) J/mol, WSi = -81777.4(±29415) J/mol, WCa = -498078.1(±131842) J/mol, XAl = [Al]/2 and XSi = ([Si] - 3])/2 – Al and Si mole fractions in the VI site and XCa = [Ca]/3 – Ca mole fraction in the VIII site ([Al], [Si], [Ca] – a.p.f.u. per 12 О). The equation reproduces the H2O content in garnet from 54 data points with a mean accuracy ±280 ppm.
Showing an increase of the H2O solubility in garnet with pressure and a decrease with temperature, the equation predicts a solubility maximum, which is dependent on temperature (for pyrope, it is 2400 ppm at 18.5 GPa for 1000°C and 1270 ppm at 22 GPa for 1500°C). The H2O solubility decreases with an increase of the majorite component in garnet. Following to these effects, the H2O content in garnet in the upper mantle is expected to be about 600-800 ppm along the sub-cratonic geotherm.
The study is fulfilled under support of the RSCF project 23-17-00066.
How to cite: Safonov, O.: Water content in garnet: review of available experimental data and parameterization with respect to temperature, pressure and composition , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-173, https://doi.org/10.5194/egusphere-egu26-173, 2026.
Minerals from heavy concentrates from two phases of the Aykhal kimberlite pipe, Yakutia, were analyzed with the EPMA, SEM, and ICP-MS. They were used to reconstruct the mantle sections and their evolution, and to determine the features of the protokimberlite melts and melt/fluid metasomatic agents responsible for the geochemistry. A high amount of garnets belong to the dunitic type. The clinopyroxenes, as well as amphiboles, are Mg-rich and highly vary in Al, Cr, Ti, Na. Micas are Ti-biotites derived from protokimberlites. The ilmenites and chromites show domination of Mg- and Cr-rich compositions.
The mantle section of subcratonic lithospheric mantle (SCLM) for autholitic kimberlite breccia (AKB) reveals a long range of PT estimates for garnets from 8 GPa to Moho heated at the deeper part, showing in the P-Fe# plot sharp layering of 6 thick layers (subdivided to 2 sub-layers) visible by high Mg deviations and Ca fluctuations for garnets and grouping of PT points for other minerals. The lithosphere asthenosphere boundary (LAB) is marked by the ilmenite trend going from LAB to middle layer (4.5-3.5) GPa, traced by the Ti-augite and pyrope megacrysts. The minerals from tuffisitic kimberlitic breccia (TKB), show a similar division of the mantle section but amount of low-pressure pyrope and eclogite garnets is much higher.
The geochemistry of lherzolitic garnets show rounded curves of depletion in light rare earth elements (LREE) allows to subdivide them into the enriched, depleted, and common lherzolitic types. The megacrystic and low-crust garnets show higher HREE levels. The dunitic garnets reveal S-shaped, harzburgitic depressions in HMREE and curved patterns. All peridotitic garnets demonstrate U, Nb, Zr enrichment in multicomponent spider diagram (MSD). The Cr-diopsides show small U enrichment and pyroxenites with higher Th peaks Pb, Ba depressions. Ilmenites display very high Ta-Nb and Zr-Hf peaks and very low REE level except for two samples. The Cr-spinels demonstrate Ta peaks on the MSD. The phlogopites reveal Eu peaks and W-shaped REE distributions and high LILE in MSD. Diamonds show low REE levels and Pb peaks. The differences in TKB and AKB geochemistry of garnets and diopsides are in the higher level of the Th-Nb and Zr-Hf levels, showing the influence of the carbonate and H2O-bearing melts that accompanied the interactions with the protokimberlite melts.
Reconstructed with partition coefficients, parental melts reveal highly inclined lines up to 1000/PM (primitive mantle). Peridotites show U-Ba- enrichment typical for subduction related melts and high Nb also – due to super plume melts influence. Cr-diopsides and pyroxenites show dominating Th enrichment due to interaction with the carbonatite melt.
The high diamond grade of the Aykhal pipe is determined by mixing of subduction-related Na-Mn-U and peridotitic high Mg-Cr with Ti-Nb-Th plume components and hybrid melt interaction with peridotite eclogitic material with the mixing of all components.
Work was done on state assignments of IGM SB RAS FWZN-2026-0007. Russian Science Foundation Grant (24-27-00411).
How to cite: Ashchepkov, I., Logvinova, A., Ivanov, A., Sotnikova, I., and Medvedev, A.: Mineralogy and geochemistry of the xenocrysts from Aykhal kimberlite pipe, Yakutia: comparison of phases, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3137, https://doi.org/10.5194/egusphere-egu26-3137, 2026.
Geochemistry and thermobarometry of mantle xenocrysts and xenoliths from Mir kimberlite pipe were studied using new EPMA, SEM and LA ICP MS analyses
The PTX plot for the Mir pipe (Malo-Botuobinsky field) (Ashchepkov et al., 2010; 2014; 2019; 2022; 2023) shows the large interval from 8 to 1.1 GPa. The garnets show rathe narrow PT and P-Fe# but very wide P-Ca plots starting from the middle pyroxenitic layer to LAB. The Cr-Cpx and Cr-Sp are coinciding in Fe# in general. But the eclogites show very wide range of compositions trend. Diamond inclusions (DIA) (Sobolev, et al., 1976; 1997; Bulanova et al., 2002; Logvinova et al., 2004) the DIA pyropes have an opposite trend. In the P (GPa)-CaO plot largest variations in CaO are in the lower part of the mantle section. The most magnesian dunite varieties form an interval from 6.5 to 5 GPa, and then above them, the harzburgitic garnets again appear in the middle part of SCLM. There is high proportion of peridotite Cr-bearing varieties of ortho-and clinopyroxenes in the middle SCLM, which suggests that pyroxenites originated from peridotite partial melts. Omphacites together with garnets form an ascending P-Fe# plot. The geothermal conditions traced by DIA also form two branches. Even Cr-garnets partly trace the convective branch, although this is not evident in the middle part. The Cr-garnets are found at higher temperature conditions at deeper part of the SCLM. However, most of them plot between the 35–40 mWm−2 geotherms. The Cr-pyroxenites and Cr-diopsides form the colder branches to 35 mWm−2 geotherms or even lower. In the P-fO2 diagram, the less oxidized conditions correspond to the eclogitic clinopyroxenes in middle SCLM. At high pressures, the Cr-rich garnets give the lowest fO2 conditions.
The REE patterns of the pyropes show wide range of compositions from S-shaped dunitic to semi – rounded lherzolitic and flattened HREE harzburgitic and LREE enriched pyroxenitic. In multicomponent diagram they show peaks in Th-U and Pb and troughs in Sr and highly synchronously varying HFSE.
The Cpx form Gar lherzolites are showing several groups commonly inclined La/Ybn ~100 (normalization to primitive mantle (McDonough and Sun, 1995) with the hump at Ce- to Nd. In MCD they show very wide variations even in LILE from peak in Ba Rb to deep troughs and the same for U, Th. Pb, Sr. The HFSE are mostly moderately depleted (Zr <Hf) or deep minima Nb-Ta. Some low -Gar pyroxenites display less inclined patterns. There are more flattened patterns and REE -low for Sp lherzolites.
The ilmenites in REE show different inclinations from positive (Lan ~100 and La/Ybn 70-100) to negative (Lan ~1-2 and La/Ybn ~0.1). They all show very high Ta-Nd peaks to 1000/PM and a bit less Zr-Hf (150-1000/PM).
Work was done on state assignments of IGM SB RAS FWZN-2026-0007. Russian Science Foundation Grant (24-27-00411).
How to cite: Vavilov, M., Ashchepkov, I., Medvedev, A., and Logvinova, A.: Geochemistry and thermobarometry of mantle xenocrysts and xenoliths from the Mir pipe, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5851, https://doi.org/10.5194/egusphere-egu26-5851, 2026.
Plateau, isochron and integral ages of 40Ar/39Ar xenocrysts and phlogopite grains from kimberlite xenoliths can be used to determine the ages of mantle processes (Hopp et al., 2008) and decipher the genesis of diamond-forming processes. Datings of deep xenoliths of kimberlites of the Siberian Craton reveal a significant spread (Pokhilenko et al., 2012; Solovieva et al., 2017b; Ashchepkov et al., 2015) from the Archean to a time close to the age of the host kimberlites, mainly Devonian. The oldest ages for the Udachnaya tr of the Daldyn field for phlogopites from xenoliths of spinel harzburgites of the uppermost level belong to the late Archean-early Proterozoic 2.1-1.5 Ga. In the Alakit field, all ages are younger than 1.6–1.05 and 0.928–0.87 Ga and belong to the metasomatic history of the Rodini mantle. Similar dates have been established for xenoliths from the Obnazhennaya trench (Kalashnikova et al., 2017).
Our 39Ar/40Ar data on micas often reveal complex spectral configurations. Micas from the Alakit field xenocrysts yield a series of peaks, beginning with the highest-temperature and oldest, which correspond to the Upper Proterozoic, Vendian, and Paleozoic, and only the lowest-temperature peaks with high Ca/K ratios correspond to kimberlite emplacement ages. Some peaks are possibly related to the thermal influence of the Vilyui plume (Kuzmin et al., 2012). The lowest temperature peaks are close in age to the time of kimberlite formation, which is confirmed by high 38Ar/39Ar ratios of gas released at the low-temperature stage, and can be used for dating kimberlites very approximately; however, the release of other gases at the low-temperature stages significantly increases the measurement error. All of them correspond to the interval 440-320 Mir, Internatsionalnaya, Ukrainskaya - 420, Yubileynaya - 342, and Botuobinskaya - 352); some determinations practically coincide with Rb/Sr ages (Griffin et al., 1999, Agashev et al., 2005, Kostrovitsky et al., 2008) and probably represent mixing lines. For many xenocrysts (Fainshteynovskaya, Ukrainskaya, Yubileynaya, and Krasnopresnenskaya pipes), the interval from 600 to 500 million years is manifested, which corresponds to the stage of breakup of Laurasia. The presence of relatively low-temperature plateaus with ancient ages and high-temperature young ones implies that some stages can be correlated with the mantle history of minerals.
How to cite: Iudin, D., Ashchepkov, I., and Travin, A.: Ages of micas from xenoliths and xenocrysts of kimberlites of the Siberian Craton determined by the 39Ar/40Ar method, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4754, https://doi.org/10.5194/egusphere-egu26-4754, 2026.
Potassic to ultrapotassic, volatile-rich alkaline magmas such as lamprophyres originate from deep, metasomatized mantle domains and occur in specialized geodynamic settings. Their enrichment in REE–HFSE and ability to sample the sub-lithospheric mantle make them key indicators of mantle metasomatism, enrichment–depletion cycles, and regional tectonomagmatic evolution. In the early Paleogene, Indo–Asian collision produced widespread post-collisional magmatism across the Trans-Himalaya; however, lamprophyres, lamproites, shoshonites, and alkaline basalts remain largely restricted to regions north of the Indus–Tsangpo Suture Zone (ITSZ). The occurrence of lamprophyres within the Ladakh Batholith is therefore significant in revealing the nature of the sub-Indian lithospheric mantle. These lamprophyres display alkaline to ultrapotassic affinities and yield Permian–Triassic U-Pb apatite ages (212 ± 17 Ma and 329 ± 71 Ma), substantially older than the Cenozoic (80-50 Ma) Ladakh arc magmatism. Whereas the TDM age calculated from the isotopic Nd systematics imparts a late Proterozoic (1.0 Ga to 0.7 Ga) ages for the origin of source for investigated lamprophyres. The results demonstrate that they are pre-batholith basement intrusions, representing pre-collisional magmatism along the Asian continental margin. Their ages correspond to regional Permo-Triassic alkaline and lamprophyric magmatic events, including those of the Panjal Trap-related province of the NW Himalaya-Karakoram region, indicating a broader mantle metasomatism event predating Himalayan orogenesis. Trace element ratios like Th/La (0.4-0.5) and Nb/Y (1.3-1.9) are closer to the bulk continental crust implementing crustal input during their genesis. Moreover, the lamprophyres also exhibit fractionated REE patterns with pronounced Nb–Hf–Ti anomalies, reflecting crustal involvement or subduction modification during magma generation, while non-radiogenic initial Sr ratios and near-CHUR to negative εNd(t) values (+0.2 to –5.9) point to a subduction-modified, metasomatized mantle source. Carbonate metasomatism-related modification is also evident in these lamprophyres. Collectively, these features imply a major geodynamic episode during the waning stages of Gondwana breakup, involving mantle metasomatism beneath the Indian plate and interaction with remnants of the Tethyan oceanic lithosphere. The Late Paleozoic lamprophyres of the Ladakh region thus record an important pre-Himalayan mantle signature, preserving a magmatic history fundamentally distinct from the Cenozoic Indo-Asian collisional system.
How to cite: Pandey, R., kumar, A., naskar, S., baumann, N. B., dev, J. A., rao, N. V. C., and jk, T.: Pre-Collisional Permo-Triassic Lamprophyres in Ladakh Reveal a Metasomatized Lithospheric Mantle Predating the Indo–Asian Collision , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-685, https://doi.org/10.5194/egusphere-egu26-685, 2026.
Trachytes of the Deccan Traps from the Manori–Gorai area of Mumbai host numerous mafic enclaves which record magma chamber processes in continental flood basalt (CFB) settings. In this study, we undertook a comprehensive study, including petrography, mineral chemistry, whole-rock Sr-Nd isotope, in-situ trace elements and Sr isotopic analysis of the host trachyte (SiO2 = 65-72 wt.%) and the mafic enclaves (SiO2 = 45-52 wt.%) to understand magma chamber processes. A sharp-to-transitional hybrid mixed zone is evident between the host trachyte and enclaves, indicating mixing and mingling of two different magmas. Plagioclase and clinopyroxene are the major phenocrysts residing within a glassy groundmass. Plagioclase occurs as euhedral to anhedral grains, as inclusions, and within the groundmass across different zones. Clinopyroxene is predominantly augitic in composition throughout these zones. The wide compositional range from bytownite to sanidine indicates fractional crystallization coupled with heterogeneous magma mixing. Light rare earth element (LREE) enriched patterns (LaN/SmN = 3.4–5.4; SmN/YbN= 4.2–5.3), incompatible trace element enrichment, and whole-rock Sr–Nd isotopic compositions of both the enclaves (87Sr/86Sri = 0.70524–0.70536; εNdi = +1.8 to +2.3) and trachyte (87Sr/86Sri = 0.70506–0.70511; εNdi = +0.5 to +0.6) suggest derivation from a common parental magma, with minor crustal contamination recorded in the trachyte. In-situ trace element analyses and Sr isotopic ratios in feldspar (87Sr/86Sri = 0.7039–0.7056) further support a shared source affected by heterogeneous mixing. The observed geochemical trends in both the mafic enclaves and trachyte indicate recharge of mafic melt into an evolved, fractionated magma chamber, followed by buoyancy-driven ascent forming mafic enclaves at the interface.
How to cite: Halder, M., Mohan, M. R., Upadhyay, D., Shankar, R., and Bhunia, S.: A geochemical perspective of mafic enclaves in the Deccan Traps Continental Flood Basalts reveals magma chamber dynamics, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-678, https://doi.org/10.5194/egusphere-egu26-678, 2026.
The Paleoproterozoic volcanic sequences of the Singhbhum Craton, Eastern India, encompass a broad compositional range, from ultramafic to felsic lithologies, and preserve important records of early subduction-related magmatism. This study investigates the petrogenesis of a newly identified Nb-enriched basalt (NEB) from the Kanjipani-Telkoi (KT) region, associated with the Malangtoli volcanics, offering key insights into mantle heterogeneity and slab melt-mantle interaction during the Paleo-Proterozoic era. The KT NEB is characterized by high niobium (Nb) levels, ranging from 7 to 29 ppm, along with elevated ratios of (Nb/Th)PM (0.40-1.86), (Nb/La)PM (0.29-0.82), and Nb/U (6.99-22.43). These geochemical features suggest that the NEB originated from the partial melting of a metasomatized mantle wedge that had interacted with subducting slab melts. Petrogenetic modeling suggests that the NEB compositions can be generated by ~5–20% partial melting of a metasomatized mantle wedge modified by interaction with high-silica, adakitic slab melts produced by ~15% partial melting of subducting oceanic crust. Furthermore, the chemistry of clinopyroxene in the NEB suggests crystallization at high temperatures, around 1016 to 1141 °C, at shallow to intermediate depths (1.6-7.6 kbar), consistent with conditions typical of a hot subduction environment. Collectively, these results provide robust evidence for Neoarchean–Paleoproterozoic arc magmatism in the Singhbhum Craton and underscore the critical role of slab-melt metasomatism of the mantle wedge in generating Nb-enriched magmas and promoting early continental crustal growth.
Keywords: Singhbhum craton, Nb-enriched basalt, clinopyroxene, slab melts, metasomatized mantle wedge, Partial melting.
How to cite: Sahoo, T. K. and Das, D. P.: Slab melt metasomatisation of a Paleo-Proterozoic mantle wedge: An insight from the geochemistry of rare Nb-enriched basalt of Singhbhum craton, India, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8209, https://doi.org/10.5194/egusphere-egu26-8209, 2026.
A recent expedition aboard R/V Nathaniel B. Palmer (NBP25-01, 2025) sampled submarine seamounts across six regions of the Terror Rift, western Ross Sea, Antarctica. Volcanism in this area is dominated by explosive mafic alkaline magmas forming monogenetic and polygenetic seamounts (Tominaga et al., 2025). Of the 50 dredges collected, nearly half recovered mantle xenoliths, offering a rare opportunity to constrain the composition, thermal structure and evolution of the mantle beneath an active Antarctic rift.
The xenolith suite is dominated by peridotites (dunite to lherzolite), with subordinate pyroxenite and hornblendite. We present mineral chemistry and thermobarometric data from these lithologies to constrain their P-T-fluid history, and potential spatial heterogeneities.
Preliminary results from peridotite xenoliths sampled at Squid Ridge, a seamount located close to the rift axis, reveal evidence for two distinct melt–peridotite interaction events. The first event occurred at high temperature and is marked by the formation of interstitial diopside and Cr-rich spinel. Melt percolation was coeval with viscous deformation, recorded by olivine subgrain development and dynamic recrystallization of orthopyroxene when present. Pyroxene thermobarometry yields equilibrium conditions of 1065 ± 5 °C and 1.0 ± 0.2 GPa, corresponding to depths of ~30 km. The second event is characterized by brittle fracturing of the peridotites and the formation of alkali-rich glass, amphibole, augite and Mg-poor olivine in fractures. It is interpreted to reflect xenolith entrainment during magma ascent.
These results indicate a deep lithospheric mantle origin for the studied xenoliths, consistent with previous estimates from Franklin Island peridotite xenoliths located farther from the rift axis (Martin et al., 2023). Together, these observations suggest that rift-related fault systems efficiently channel deep-sourced melts to the surface and support the presence of a relatively cold geotherm beneath the Terror Rift, consistent with an idealized dynamic rift.
Martin et al. (2023). A review of mantle xenoliths in volcanic rocks from southern Victoria Land, Antarctica.
Tominaga, M. et al. (2025). Subglacial explosive volcanism in the Ross Sea of Antarctica. Communications Earth & Environment, 6(1), 921.
How to cite: Prigent, C., Walter, M., Panter, K. S., Berthod, C., Tominaga, M., and Cross, A.: Preliminary results on the composition of the Antarctic mantle below the Terror Rift, Western Ross Sea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17162, https://doi.org/10.5194/egusphere-egu26-17162, 2026.
It remains a persistent point of contention among igneous petrologists as to how alkaline magmas generated by small degrees of melting within the upper-most convecting mantle can traverse through relatively cool continental lithosphere without freezing and being trapped at depth. Melting experiments demonstrate that volatile-rich, silica-undersaturated liquid react with peridotite under P-T conditions equivalent to the base of lithosphere and can form hydrous cumulates consisting of clinopyroxene, amphibole and phlogopite1-3. Furthermore, this metasomatic process enriches the mantle in incompatible elements in amounts similar to basalt (i.e. nephelinite, basanite) and may be a melt source for alkaline lavas4. The experiments and theoretical models provide important clues as to the cause and source of alkaline volcanism that occur within plates but demand evidence from natural settings. Here we present major and trace elements and 40Ar/39Ar ages from Pliocene-Pleistocene, olivine-phyric alkaline basalt erupted through extended continental lithosphere within and bordering the Terror Rift, southwestern Ross Sea, Antarctica. Subaerial and submarine basaltic tephra and lava from this region contain mantle xenoliths that include hydrous-phases that also display melt-solid reaction textures5,6. We compare basalt erupted across the central portion of the rift with basalt erupted at the rift shoulder along the base of the Transantarctic Mountains7,8. Our comparison shows that silica-undersaturation (i.e. nepheline-normative content) and highly incompatible trace element concentrations decrease with decreasing degree east longitude within the rift and on average are at their lowest on the rift shoulder. Variable degrees of partial melting of a common mantle source are modelled to match the trace element trends but require an unrealistic range of values: F = <3% beneath rift and as much as 15% beneath the rift shoulder. The models are also not consistent given the greater depth to the lithosphere-asthenosphere boundary (LAB) beneath the rift shoulder (>95 km) relative to the central rift (<85 km)9. We propose that the compositional variability may be explained by interaction-reaction of asthenospheric melt with mantle lithosphere manifest to a greater degree beneath the rift shoulder. But also, that that portions of the continental lithosphere that have been metasomatized by low-degree, volatile-rich, silica-undersaturated melt, evident in mantle xenoliths hosted by the basalt, are likely to be a contributing source for alkaline volcanism in this region.
1Foley 1992, Lithos 28; 2Pilet et al., 2008, Science 320; 3Pilet et al., 2010, Contrib. Mineral. Petrol. 159; 4Pilet et al., 2011, Jour. Petrol. 52; 5Martin et al., 2021, Geol. Soc. Lond. Mem. 55; 6Panter et al., 2025, AGU Fall Meet. Abst. 2025, OS51F-0475; 7Tominaga et al., 2025, Comm. Earth Environ. 6:921; 8Panter et al., unpubl.; 9An et al., 2015, Jour. Geophys. Res. 120:12.
How to cite: Panter, K., O'Conke, R., Tominaga, M., Prigent, C., Berthod, C., Pilet, S., and Konrad, K.: Lithosphere-melt interactions, evidence from basalt and xenolith cargo, Terror Rift, Ross Sea, Antarctica, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21679, https://doi.org/10.5194/egusphere-egu26-21679, 2026.
Magmatic activity associated with active transform faults in the Eastern Mediterranean region is widely observed around Toprakkale (Osmaniye), Ceyhan (Adana), and Hatay. This magmatic activity is associated with the Toprakkale Fault (East Anatolian Fault Zone) to the west, the Amanos Segment (East Anatolian Fault Zone) and Yesemek Segment (Dead Sea Fault Zone), which border the Karasu Valley to the east.
Lavas from the western sector (Toprakkale region) are represented by predominantly alkaline mafic compositions, plotted within the basanite and basalt fields in total alkaline-silica (TAS) diagram and displaying SiO₂ and MgO contents of 43.70–48.77 wt% and 5.98–10.46 wt%, respectively. Furthermore, in the eastern sector (Karasu Valley) of the study area, mafic lavas similarly show alkaline affinities and mainly represented by basalts and trachybasalts with SiO₂ and MgO values ranging from 44.89–51.01 wt% and 4.53–9.21 wt% respectively.
Primitive mantle–normalized [1] multi-element patterns of basaltic rocks display LIL element enrichment relative to HFS elements and have broadly OIB-like affinities, but these rocks differ from OIB signature by the depletion in LIL element contents. In contrast, samples from the Karasu Valley are represented by enrichment in LIL and depletion of HFS elements, and are distinct from the OIB signature by enrichment in Cs, Ba, and Pb, along with depletion in Sm, Zr, and Hf. Incompatible element ratios of the mafic lavas show systematic similarities between the western (Toprakkale) and eastern (Karasu Valley) parts of the study area. Ba/La ratios from Toprakkale region range 7.29-9.41 whereas lavas from the Karasu Valley are characterized by higher values that range between 9.76-18.70. Similarly, both sectors are represented by elevated Th/U (3.02–9.16) and consistently high Dy/Yb ratios (>2) [2].
These geochemical features may indicate that the basaltic rocks were derived from a garnet-bearing mantle source. Decompression process appears to be related to the transform fault activities, and the upwelling of the asthenosphere is capable of producing alkaline magmatism within both sectors of the fault zones.
1. Sun, S., McDonough, W.F., 1989. In Magmatism in the Ocean Basins Geological Society London Special Publications, pp. 313–345.
2. Peters, et al., 2008. Lithos, 102(1-2), 295–315.
ACKNOWLEDGEMENTS
This research has been founded by TUBITAK COST project 125Y257
This research is financially supported by TUBITAK 2224-A
How to cite: Önder, D. A., Kurkcuoglu, B., Kahraman, B., Doğan Külahcı, G. D., Yürür, M. T., and Yüce, G.: Source melting and origin of the basaltic rocks situated along the active East Anatolian and Dead Sea Transform Faults, Southeastern Türkiye, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10459, https://doi.org/10.5194/egusphere-egu26-10459, 2026.
Cratons are ancient continental crust formed primarily in the Archean-Mesoproterozoic ages. Their perceived stability has been challenged over the past decades. Investigations reveal that cratonic lithosphere contains weak layers/zones at various scales and can undergo destabilization, leading to large-scale delamination under specific tectonic perturbations. Mantle plumes represent a key mechanism for such cratonic destruction. The Tarim Craton, amalgamated from Archean crystalline basements in the Neoproterozoic, hosts a Permian large igneous province potentially linked to plume activity, making it a natural laboratory for studying plume-craton interaction. This study systematically compiles geochemical data from Permian magmatic rocks in the Tarim Craton. Focusing on mafic-ultramafic and alkaline rocks with MgO >8 wt%, we employ an experimentally calibrated whole-rock thermobarometer to estimate the pressure-temperature conditions of melt generation, thereby constraining the paleo-lithospheric thickness. Integrating these results with seismic evidence for a mid-lithosphere low-velocity zone (the mid-lithospheric discontinuity, MLD) beneath Tarim, we propose a novel model: By the late Carboniferous, an MLD had developed at ~100 km depth in the craton lithosphere. The initial arrival of a mantle plume at the lithospheric root generated minor kimberlitic and carbonatitic melts. The thick (~200 km) lithosphere initially impeded the plume's ascent until delamination of the root below the MLD occurred. This removal enabled more efficient heating and melting of the upper lithosphere, producing voluminous flood basalts. Subsequent upwelling and melting of the plume itself formed the mafic-ultramafic rocks. Concurrently, interaction between the plume and the metasomatized MLD generated a portion of the alkaline melts. This process induced a local thickening of the MLD to ~130 km, consistent with its present-day depth. Our findings indicate that the mantle plume first thinned and subsequently thickened the cratonic lithosphere, with the MLD playing a crucial role in this evolution. This mechanism of cratonic destruction followed by "healing" may have operated not only in the Paleozoic but also during the Proterozoic, suggesting it could be a vital process for the episodic destabilization of cratons throughout geological time.
How to cite: Pan, Z., Cai, K., and Cheng, Q.: Modifying the Cratonic Lithosphere: The Role of Mantle Plumes Revealed by the Permian Tarim Large Igneous Province, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8766, https://doi.org/10.5194/egusphere-egu26-8766, 2026.
The ages for zircons from xenoliths of Cenozoic volcanics from TransBaikalia and Mongolia were determined by the same (LA-ICP-MS) aniCAP Q mass spectrometer (Thermo Scientific) and a NWR 213 in IGM SB RAS
The isotopic Pb/U and Pb/Pb ages of the 6 zircon grains from the tuffs of the Bartoy volcano. They are plotting practically on the207Pb/235U and 206Pb/238U concordia line (Figure 9). For the calculations ages the polynomial equations were used, obtained from the works of Khubanov et al., (2016-2024).
They may be divided into three groups. The ages of the zircons from tuffs 1160-1250 Ma mainly correspond to the meta-terrigenous rocks of the Shubutuiskaya Formation in the Khamar Daban zone (Gordienko et al., 2006). The Th/U ratio of 0.8-0.6 of these zircons corresponds to the common magmatic rocks, mainly of the basic type (Hawkesworth et al., 1997).
The ages near 800-860 Ma are determined for the suit from the metamorphic in Central Khamar-Daban (Shkol’nik et al., 2016). The collision events at the boundaries of the Paleoasian ocean occurred earlier in the Vend-Cambrian (Byzov, Sankov, 2024; Donskaya et al., 2013). Though some plutons with the model ages 1100-800 Ma were suggested by some authors to be referred to as collision. And elevated Th/U ratios of two zircons, 1.6-1.1, commonly correspond to the granites with the admixture of material of island arc environment with 3-5 Th/U ratios.
The age of granitic zircon, 300 Ma, just corresponds to the beginning of the Angaro-Vitim Batholith (AVP) formation (Tsygankov et al., 2010-2025). It may be connected to the movement of the superplume-formed kimberlites in Yakutia at the interval 420-340 Ma and later created the Biryusa and Tumanshet lamproites (Kostrovitsky et al., 2025) and later the Ingashi lamproites in Eastern Sayan ~306-309 Ma (Gladkochub et al., 2013). Further movement through Khamar-Daban and interaction with the lower crust and granulites brings to the creation of alkaline granitoids of AVP. But the Th/U ratio is rather low, 0.07, which commonly corresponds to the metamorphic type [90]; thus, they should be from granulites possibly remelted by a plume.
The ages of the granulite xenoliths from the Vitim picrobasalts (Ashchepkov et al., 2011) correspond to the initial stage of AVB. And the next one, 873 may be correlated with the basic magmatism in Baikal uplift (Gladkochub et al., 2010).
In Shavaryn-Tsaram volcano two ages of zircons corresponds to Carboniferous stage 322 Ma of rifting in Mongolia (Kozlovsky et al., 2005) or close to last stage AVP. The next one refers to the initial stage of Miocene plume magmatism (Ashchepkov et al., 2026).
The trace elements for two zircons determined in the granulites differ significantly. The inclined enriched in HREE pattern looks similar to carbonatitic zircons (Hardman, et al., 2025). The next acid sample with La/Ybn <2 with Eu minimum and without Ce anomaly. In MSD it shows the same peaks but without Y, Ta anomalies.
Work was done on state assignments of IGM SB RAS FWZN-2026-0007 and IG SB RAS. Russian Science Foundation Grant (23-17-00030).
How to cite: Tsygankov, A., Ashchepkov, I., and Burmakina, G.: The ages for zircons from xenoliths of Cenozoic volcanics from TransBaikalia and Mongolia, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4803, https://doi.org/10.5194/egusphere-egu26-4803, 2026.
