Ultramafic xenoliths provide valuable insights into the physico-chemical, compositional and thermal characteristics of Earth’s mantle along with its heterogeneities. Integrating petrography and mineral chemistry data with the determination of volatile concentrations and isotopic fingerprints in fluid inclusions (FI) in these xenoliths has become the state-of-the-art approach, as it provides not only important information on the nature and evolution of the lithospheric mantle, in terms of melting and metasomatic processes, but also illustrates the storage and migration pathways of volatiles throughout the lithosphere. This tandem approach makes the Comoros Archipelago (Mozambique Channel, Western Indian Ocean) ideal candidates to explore, because the characteristics of the local lithosphere are intimately tied to the complex regional geodynamic setting. Indeed, the origin of the Comoros magmatism remains enigmatic and controversial despite extensive documentation in the literature, as it has been attributed to either a plume-related hot spot or to a passive response to lithospheric break-up.

In this study, we investigate a unique suite of ultramafic xenoliths from Mayotte island by combining petrographic observation, mineral phase major and trace element analysis with the geochemistry of noble gases (He, Ar, Ne) and CO₂ hosted in olivine (ol), orthopyroxene (opx) and clinopyroxene (cpx) FI. Mineral major elements results show refractory compositions in terms of MgO (Mg#_Ol > 90.5, Mg#_Opx > 91 and Mg#_Cpx > 91.5) and Al₂O₃ contents (ranging from about 1.60 to 3.00 wt.% for opx and from 2.50 to 3.60 wt.% for cpx, respectively), with Cr# of spinel falling between about 0.4 and 0.55. Overall, these features indicate that the local lithosphere experienced relatively high degrees of melting, from ~20% to 25%. This is also supported by highly depleted chondrite-normalized rare earth element (REE) patterns for cpx, where HREE are roughly (1.5)_N.

Volatile concentrations and isotopic fingerprints in FI hosted in Mayotte xenoliths are variable, with CO₂ standing out as the most abundant gas species. The air-corrected ³He/⁴He isotopic ratios (5.6 to 6.8 Ra) are intermediate between the typical signatures of MORB (8±1 Ra) and the SCLM (6.1±2.1 Ra) mantle, as measured in local subaerial (Liuzzo et al., 2021) and submarine (Fani Maoré Seamount, Mastin et al., 2023) gas emissions. The relationships between ³He/⁴He and the extrapolated air-free mantle ²¹Ne/²²Ne ratios, together with ⁴⁰Ar/³⁶Ar versus ³He/³⁶Ar systematics, suggest a dominating MORB-like component in the upper mantle below the Comoros archipelago, mixed with recycled crustal and atmospheric components, in agreement with recent data of ultramafic xenoliths from Grande Comore island (Ventura Bordenca et al., 2023).

References

Bordenca, C.V., Faccini, B., Caracausi, A., et al., 2023. Geochemical evidence for a lithospheric origin of the Comoros Archipelago (Indian Ocean) as revealed by ultramafic mantle xenoliths from La Grille volcano. Lithos, 462, 107406.

Liuzzo, M., Di Muro, A., Rizzo, A.L., et al., 2021. Gas geochemistry at Grande Comore and Mayotte volcanic islands (Comoros archipelago), Indian Ocean. Geochemistry, Geophysics, Geosystems, 22, e2021GC009870.

Mastin, M., Cathalot, C., Fandino, et al., 2023. Strong geochemical anomalies following active submarine eruption offshore Mayotte. Chemical Geology, 640, 121739.

How to cite: Faccini, B., Faccincani, L., Rizzo, A. L., Casetta, F., Nardini, N., Liuzzo, M., Di Muro, A., and Coltorti, M.: Petrology and fluid inclusions geochemistry of ultramafic xenoliths from Mayotte and the origin of the Comoros Archipelago, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20036, https://doi.org/10.5194/egusphere-egu24-20036, 2024.

The Marais de Limagne (MdL) site is located at the Devès volcanic field in the French Massif Central. MdL is known for abundant peridotite xenoliths, which come from the “southern domain” of lithospheric mantle underlying Variscan crust of the Massif Central. The MdL xenolith suite consists of spinel lherzolites and harzburgites which contain from 0 to 21 vol. % of amphibole (Touron et al. 2008). We studied in detail 13 xenoliths in order to understand better the mechanisms of amphibole formation by modal metasomatism.

The suite studied by us contains from 0 to ca. 20 vol. % of amphibole, the textural context and volume of which varies from thin rims of the mineral on spinel to rocks containing thick mantles of amphibole on subordinate or relict spinel. Exceptionally (one xenolith), amphibole occurs as interstitial grains independent of spinel. Amphibole is pargasite sensu IMA 2012 classification. Peridotites consist of olivine Fo 89.4-90.5, orthopyroxene Al 0.11-0.19 atoms per formula unit (apfu), clinopyroxene Al 0.13-0.28 apfu and spinel Cr# 0.09-0.39. The exception is harzburgite with olivine Fo 87.0 (orthopyroxene Al 0.11, clinopyroxene Al 0.15 apfu, spinel Cr# 0.15). Clinopyroxene REE patterns vary from LREE-depleted to LREE-enriched. The abundance of amphibole in many xenoliths and equal participation of peridotites with LREE-depleted and LREE-enriched clinopyroxene distinguishes the MdL suite from other xenolith suites from the Devès volcanic field, dominated by lherzolites which contain no or only traces of amphibole and mostly contain aluminous clinopyroxene with LREE-depleted REE patterns.

Clinopyroxene occurring in rocks containing no or little amphibole has < 100 ppm of Sr, whereas that coexisting with abundant amphibole has 230-370 ppm of Sr. The content of Ba in amphibole varies from 2.6 ppm in rocks with only thin rims of mineral on spinel to 467.0 ppm in those which contain high volumes of amphibole. The REE patterns of amphibole are similar to those of coexisting clinopyroxene. Because both Sr and Ba are fluid-mobile elements, we assume that amphibole has originated by reaction of hydrous fluid with spinel, in which clinopyroxene has also participated as a source of elements not present in a fluid or spinel. By analogy with a similar mechanism described by Puziewicz et al. (2023), we speculate that hydrous fluid migration through the peridotite host might have induced its recrystallization.

Puziewicz et al. 2023, Journal of Petrology 64: https://doi.org./10.1093/petrology/egad049

Touron et al. (2008), In: Metasomatism in Oceanic and Continental Lithospheric Mantle, Geological Society Special Publ. 293, 177-196.

How to cite: Puziewicz, J., Aulbach, S., Ntaflos, T., and Matusiak-Małek, M.: Amphibole origin in the mantle lithosphere peridotites beneath continents: An example from Marais de Limagne (Massif Central, France) xenoliths, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3978, https://doi.org/10.5194/egusphere-egu24-3978, 2024.

Studies on mantle xenoliths evidence heterogeneities occurring in scale from single localities to large geological units. Variations of chemical composition (exceeding analytical uncertainty) occur also within single crystals, as well-defined zoning or as patchy zones. Whereas mantle-derived melts average out such micro-heterogeneities, in-situ study of mantle xenoliths allows to probe their extent and thereby unveil processes affecting the mineralogical and compositional evolution of the mantle lithosphere. Here, we present different types of micro-heterogeneities recorded by clinopyroxene and associated phases from xenolithic mantle peridotites from Sweden (Scania) and the French Massif Central (Mt. Briançon).

Micro-heterogeneities in clinopyroxene occur in only one out of five types of peridotites from Scania and are mirrored by contents of major and trace elements which in general vary between cores and rims of grains, however the composition of cores may also be not homogenous. Cores of clinopyroxene crystals containing oriented lamellae of spinel are enriched in Cr (0.033-0.035 vs. 0.028-0.02 afpu in Cr-poor parts of cores) in 10-100 µm thick layers parallel to the elongation of the spinel crystals. Variations in Cr content are not reflected in trace element compositions. Furthermore, margins and areas along cracks in some clinopyroxene grains (both, lamellae-bearing and lamellae-free), are enriched in Al (0.195-0.206 vs. 0.159-0.196 afpu in unaffected rims), Cr (0.026-0.032 vs. 0.018-0.031 afpu in unaffected rims) as well as in LREE, Zr, Hf, and Ti, the latter recorded also in orthopyroxene.

In Mt. Briançon mantle lherzolites, chemically heterogeneous clinopyroxene occurs in ~14% of the studied xenoliths. In some samples, the Mg# of pyroxenes and olivine Fo change gradually across a petrographic section from 0.87 to 0.89 and from 86.5 to 89.0%, respectively. The REE patterns of clinopyroxene are homogeneous at the grain scale and vary among the grains from spoon-shaped to LREE-enriched, but with no correlation to Mg# values. Another type of heterogeneity is evidenced by higher (by 0.01-0.03) values of Mg# (0.89-0.91) and contents of Al (0.245-0.252 vs 0.226-0.232 apfu in rims) and Cr (0.037-0.038 vs 0.028-0.033 apfu in rims) in cores than in rims of the same clinopyroxene grain. In those samples, clinopyroxene rims are LREE-enriched, whereas REE patterns in cores vary from spoon-shaped to LREE-enriched.

The heterogeneities in cores of lamellae-bearing crystals of clinopyroxene from Sweden result from incomplete exsolution of Cr-rich spinel from the structure of clinopyroxene, while the enrichment in Al, Cr and trace elements evidences infiltration of silicate melts along margins and cracks in grains. The section-scale heterogeneities in xenoliths from Mt. Briançon document chromatographic effects of alkaline silicate metasomatism that modifies spoon-shaped REE clinopyroxene patterns to LREE-enriched ones. The LREE-enrichment of clinopyroxene rims is a record of the early stage of this process. The data unveil the dynamic nature of lithospheric mantle in terms of chemical composition and texture (e.g., exsolution) due to varying degrees of metasomatism and emphasize that detailed in-situ studies of mantle phases are necessary to fully understand Earth’s evolution.  

Funded by Polish National Science Centre grants no. UMO-2016/23/B/ST10/01905, UMO-2018/29/N/ST10/00259 and 2021/41/B/ST10/00900.

How to cite: Matusiak-Małek, M., Ziobro-Mikrut, M., Mikrut, J., Buczko, D., Puziewicz, J., Ntaflos, T., Aulbach, S., Grégoire, M., and Johansson, L.: Micro-heterogeneities in lithospheric mantle evidenced by clinopyroxene forming peridotitic xenoliths from S Sweden and French Massif Central, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17699, https://doi.org/10.5194/egusphere-egu24-17699, 2024.

Mafic volcanic rocks occurring in Devès volcanic field (3.5 – 0.5 Ma) are rich in peridotitic xenoliths sampling the southern domain of the French Massif Central (FMC). The origin of the relative fertile composition of the continental lithospheric mantle (CLM) beneath FMC is widely discussed in literature and two competing major mechanisms are proposed: 1) extraction of small amounts of partial melt(s), 2) refertilization by upwelling asthenospheric melts [1, 2]. Data from xenoliths at individual localities contribute to this discussion, providing insight into the local conditions of CLM and documenting its regional variability. In this study, we present major and trace elements chemistry of peridotites from Mt. Coupet xenolith suite.

Mt. Coupet peridotites (n = 24) are represented by porphyroclastic to fine–granular lherzolites, which are characterized by within-sample chemical homogeneity. Between samples, olivine Fo varies from 88.63 to 90.85. The Mg# and Al content in orthopyroxene (Opx) are: 0.90 – 0.92 and 0.12 – 0.20 a pfu, respectively. In clinopyroxene (Cpx), Mg# is 0.89 – 0.94 and Al content is 0.11 – 0.32 a pfu. In spinel, Cr# and Mg# are: 0.04 – 0.39 and 0.62 – 0.80, respectively. The REE patterns in Cpx vary, but are always above PM values (in 10 samples analyzed). Based on these patterns, three groups are recognized: (1) LREE–depleted, (2) LREE–enriched, and (3) spoon–shaped (i.e. LREE-enriched with inflection from La to Sm). Other trace elements contents, e.g. U, Th, Nb, Ta, Ti and Sr also differ between samples. Only 10 of the 24 samples are in major-element chemical equilibrium based on Opx–Cpx thermometry [3]. Thus, calculated temperatures are: 900 – 1070 ℃, suggesting sampling of a considerable depth interval if equilibrated to a conductive geotherm. No relationship between textures, geochemistry and thermometry were observed. By analogy to literature data [2, 4] we assume that the Mt. Coupet xenolith suite represents lithospheric mantle typical for the southern domain in FMC. Studies of REE and trace elements on greater number of samples from Mt. Coupet are, however, required to provide more profound interpretation which could contribute to the discussion on the structure and thermochemical evolution of CLM beneath FMC.

Funding. We gratefully acknowledge funding by the project of Polish National Centre of Research 2021/41/B/ST10/00900 to JP.

[1] Lenoir et al. (2000). EPSL 181, 359-375.

[2] Puziewicz et al. (2020). Lithos 362–363, 105467.

[3] Brey and Köhler (1990). JoP 31, 1353-1378.

[4] Uenver-Thiele et al. JoP 58, No. 3, 395–422.

How to cite: Mazurek, H., Matusiak-Małek, M., Puziewicz, J., Aulbach, S., and Ntaflos, T.: Insight into lithospheric mantle beneath Devès volcanic field recorded by xenoliths from Mt. Coupet (Massif Central, France): preliminary data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8658, https://doi.org/10.5194/egusphere-egu24-8658, 2024.

The Sinsanri region of Jeju Island, South Korea, is renowned for its abundant xenoliths, making it one of the prime locations for their occurrence. Jeju Island itself is a Cenozoic intraplate volcano situated in East Asia. A variety of xenolith types such as Spinel Lherzolite, Spinel Harzburgite to Wehrlite are entrapped in Quaternary intraplate alkali basalts on Sinsanri, Jeju Island, South Korea, during the first and second eruption episodes of the Quaternary Volcanic Activity.  Here we report the major element composition of the xenoliths, including the first Raman spectroscopy analysis of the melt inclusion inside olivine to constrain the post-entrapment depth and the latest recrystallizing depth of the melt inclusion itself.  The prominent xenoliths are spinel lherzolite, showing transitions between proto-granular textures and porphyroclastic textures. Mg# ranges from 73.4 to 93.8 in the Sinsanri xenolith’s samples. The spinel lherzolite xenoliths originated at pressure of 15kb with an equilibrium temperature ranging from 922°C.  The size of the CO₂ rich melt inclusion entrapped inside the mantle peridotite xenoliths ranges from 20 to 40 µm.  The densities of CO₂ melt inclusion were measured to be between 302.8516 kg/m³ and 972.4025 kg/m³.  Most of the melt inclusions inside the xenoliths may have formed while they are ascending to the surface together with the host basalt.  The post-entrapment depth is calculated using the equilibrium temperature of the olivine mineral, which caused the most melt inclusion.  It can be estimated that the post-entrapment depth of the CO₂ melt inclusion of the Sinsanri is 86 MPa. The Sinsanri xenoliths are predominantly formed at relatively lower temperatures and pressures.

Acknowledgement 
This research was funded by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (grant numbers 2019R1I1A2A01057002 and 2019R1A6A1A03033167); and Korea Ministry of Environment as “The SS (Surface Soil conservation and management) projects; 2019002820004”.

How to cite: Khant, N. A., Ryu, H.-S., Moon, J., and Kim*, H.: Geochemistry and Melt Inclusion study of Peridotite Xenoliths from Sinsanri, Jeju, South Korea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13712, https://doi.org/10.5194/egusphere-egu24-13712, 2024.

Neoproterozoic ophiolites are widespread in the Arabian-Nubian Shield (ANS) and mostly occur along or within suture zones that extend roughly from north to south. These ANS ophiolites represent fragments of forearc-derived oceanic lithosphere generated during a stage of subduction initiation in the Mozambique Ocean. One of these ophiolites is the Jabal Ess ophiolite (JEO) which is the most complete of the Arabian ophiolites. The ~780 Ma JEO is situated in the far north of Saudi Arabia, where it crops out in an area more than 30 km east-west by 5 km north-south and comprises a segment of the Yanbu suture zone separating the Hijaz terrane from the Midyan terrane. The JEO comprises an assemblage of mantle peridotite, isotropic and layered gabbro, a dike complex, pillow basalt, boninite and pelagic sediments and the focus of this study is the mineralogy and geochemistry of the peridotite. Our study identifies predominantly serpentinized harzburgite in the NE and lesser serpentinite to the SW. Whole rock Al₂O₃/SiO₂ of 0.01–0.02 confirm a high degree of melt extraction for the serpentinized harzburgite; the two serpentinites yield Al₂O₃/SiO₂ of 0.04 and 0.08 with the higher value possibly due to silica-melt addition. Harzburgites exhibit U-shaped chondrite-normalized REE patterns with low concentration (∑REE 7.46–10.79 µg/g), typical of melts derived from a depleted mantle. These signatures probably represent residual mantle after boninite extraction. Alternatively, the two serpentinites have wider ranges of ∑REE=4.35–7.10 µg/g with one showing a U-shaped pattern akin to the harzburgite and one showing a LREE depletion (La/Yb_CN = 0.51) relative to the MREE and HREE. Mineral assemblages from the serpentinized harzburgite and serpentinite record greenschist facies metamorphism. The serpentinized harzburgite unit comprises Mg-rich olivine (Fo_92-90), bastite after orthopyroxene (En_90-86), minor clinopyroxene (Wo_52-50, En_47-46, Fs_4-1), and euhedral to anhedral spinel. Some enstatite shows crystallographic banding, indication of high temperature sub-solidus deformation within the plastic mantle. Serpentinites are characterized by abundant high-pressure antigorite and altered anhedral spinel, e.g., spinels which exhibit noticeable chemical zonation of cores rich in Al (a.p.f.u=0.47–0.49) and Cr (a.p.f.u=0.50–0.99) with Fe-enrichment (a.p.f.u=0.72–0.98) towards the rim. Based on spinel Cr# (0.56–0.72) the serpentinized harzburgites and serpentinites have undergone high degrees of partial melting of ~18–20% consistent with the low whole rock Al₂O₃/SiO₂.  Moreover, based on Cr# vs Mg# (0.48-0.61) the serpentinized harzburgite and serpentinite plot within the field of forearc peridotite. Results of oxygen fugacity and thermobarometry calculations will be discussed in the context of the compositional evolution of Jabal Ess harzburgite and serpentinite in the framework of formation during subduction initiation.

How to cite: Chattopadhyay, S., Sulistyohariyanto, F., Whattam, S., Osman, M., and Iskandar, O.: Petrogenesis of mantle peridotite of the Jabal Ess ophiolite, NW Saudi Arabia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4509, https://doi.org/10.5194/egusphere-egu24-4509, 2024.

Upper mantle rocks from tectonically emplaced fragments of the oceanic lithosphere are generally considered to represent variably depleted residues of the oceanic upper mantle remaining after mantle melting and crust-mantle segregation. Geochemical data of these mantle rocks and their compositions of mineral phases are pondered as a powerful petrogenetic indicator and their chemical compositions are dependent on the conditions and degree of partial melting and melt-rock interactions and also contribute to our understanding of the original tectonic setting of lithosphere generation. The Neo-Tethyan ophiolites of the Indo-Myanmar Orogenic Belt (IMOB), northeast India which lie along the southern extension of the Indus-Tsangpo Suture Zone (ITSZ) are investigated through the mantle-derived peridotite sequence. The lithology of the IMOB comprises ultramafic tectonic (dunite–harzburgite–lherzolite), ultramafic–mafic cumulates (pyroxenite–gabbro), mafic intrusives, volcanic, and volcano-clastics dominated by basalt, spilite, and marine sediments. A wide range of chemical compositions is observed in the mantle sequence of the IMOB ophiolites. Lherzolites display low Cr# (0.12-0.26) and TiO₂ (<0.11) associated with high Mg# [Mg/(Mg+Fet] (0.69-0.76) in the Cr-spinels present in them. They represent the residual product of a fertile mantle that underwent low-degree partial melting (2-10%) in a divergent mid-ocean ridge (MOR) tectonic setting. Conversely, the harzburgites and dunites have high Cr# (0.84–0.90) and low TiO₂ (< 0.06 wt%) Cr-spinels and exhibit slightly U-shaped REE distributions indicating their derivation from a highly depleted mantle source. The dunite is composed of a very refractory olivine-spinel assemblage (Fo: 92.1-93.6; Cr#: 71-83), corroborating a boninitic parentage, with influence from melt-rock interactions. The NMO also hosts both refractory grade high-Al chromitites (0.46 < Cr# < 0.53) and metallurgical grade high-Cr chromitites (0.71 < Cr# < 0.79). The high-Al chromitites originated from MORB-like melts whereas high-Cr chromitites were crystallized from a boninitic melt. The available isotopic ages reveal two episodes of the IMOB ophiolites formation at 148 Ma (K–Ar ages) and 118-117 Ma (U–Pb age). Upper mantle rocks show lower concentration of PGE (Rh < 2 ppb; Pd < 25 ppb; Re < 16 ppb; Pt = < 10 ppb; Au < 28 ppb; Os < 9 ppb; Ir < 3 ppb; Ru = 5-11 ppb). And the total PGE content (60-190 ppb) of the high-Al chromitites is less as compared with the total PGE content (118-2341 ppb) in high-Cr chromitites. The occurrence of both MOR and SSZ types of melting regimes indicates that the peridotites along with chromitites in the IMOB ophiolites formed at different stages of the pre-subduction period and subduction, respectively. Thus, we argue that the upper mantle of the NMO of the IMOB has been modified by a substantial amount of supra-subduction zone components after initially being formed in a mid-ocean ridge tectonic environment supporting multistage melting and melt-rock reaction processes.

How to cite: Chaubey, M. and Singh, A. K.: Geochemical Insights into Neo-Tethyan Ophiolites of Indo-Myanmar Orogenic Belt, Northeast India: Multistage Melting and Supra-Subduction Zone Modification of Upper Mantle Rocks., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15695, https://doi.org/10.5194/egusphere-egu24-15695, 2024.

We present a petrological and geochemical study of the Hongguleleng ophiolite in Western Junggar of Xinjiang (Southwestern Central Asian Orogenic Belt), which contains the classic Penrose-type ophiolite stratigraphic units. The ophiolite actually includes mantle spinel harzburgites, olivine-rich troctolites within the harzburgites, layered troctolites, plagioclase-bearing wehrlites, olivine gabbros, gabbros and a minor basalt proportion. Compared to other ophiolites from the accretionary orogenic belt, such as those from Tianshan and Inner Mongolia, the lower crust sequence of the Hongguleleng ophiolite is well preserved. The Hongguleleng spinel harzburgite is highly depleted, for instance as documented by the low whole-rock Al₂O₃ and CaO contents, the low REE concentrations of clinopyroxene, and the high Cr# [molar Cr/(Cr+Al)] of spinel. Overall, the harzburgite chemical compositions document that these mantle peridotites underwent a high degree of partial melting, similar to mantle peridotites from supra-subduction zones. Remarkably, the trace element signature of clinopyroxene is similar in harzburgites and included olivine-rich troctolites, thereby providing evidence for formation of the olivine-rich troctolites by melt-peridotite reaction. On the other hand, the trace element compositions of clinopyroxene from other lower crustal rocks, including the plagioclase-bearing wehrlites, may be reconciled with crystallization from MORB-type melts. However, these clinopyroxenes locally display a significant enrichment in La, Ce and Pb, which could reflect either contamination of the MORB-type melts by continental crust material, or late infiltration of a melt enriched in La-Ce-Pb through the lower ophiolitic crust. To conclude, the Hongguleleng ophiolite shares close similarities to oceanic lithospheres formed at supra-subduction zones. Along with the coeval ophiolites from the Junggar and Tianshan area, the Hongguleleng ophiolites might represent the remnants of the Junggar Ocean during the initiation of subduction.

How to cite: Yang, Z., Tribuzio, R., Gong, X., Luo, Q., and Xu, J.: The origin and tectonic evolution of Hongguleleng ophiolite, Western Junggar, southwestern Central Asian Orogenic Belt, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21219, https://doi.org/10.5194/egusphere-egu24-21219, 2024.

The Naga Hills Ophiolite (NHO) exposed on the northeastern margin of India, represents a thrusted section of Neo-Tethyan oceanic lithosphere comprising distinct crustal and upper mantle lithologies accreted onto continental margins at Indo-Myanmar ranges during late Cretaceous-Eocene collision between Indian and Eurasian Plates. This study focuses on the whole-rock geochemistry and Re-Os isotopic compositions of the mantle peridotites from NHO to address the implications on the upper mantle heterogeneity and thermo-tectonic evolution of Neo-Tethyan oceanic lithosphere through time. The chondrite normalized REE patterns, and low Re/Os, suggests that, these are mantle residues resulting from ~5–20% of melt extraction from a spinel peridotite source. The refractory characters of the studied mantle peridotites from NHO are further complemented by relatively enriched concentrations of transition elements like Cr, Ni and Co with respect to primitive mantle composition. PGE modeling from melt fractionation indices [(Pt/Ir)_N and (Pd/Ir)_N] and depletion index (Al₂O₃) corroborates discernible melt depletion trends. Isotopically, these mantle peridotites are subchondritic with ¹⁸⁷Os/¹⁸⁸Os (0.1218-0.1266) and γ_Os (gamma osmium) values of -3.73 to -0.09 comparable with depleted MORB mantle source.  LILE-LREE enrichment, HFSE depletion, and U-shaped chondrite-normalized REE patterns, distinct S-undersaturated trend and Re addition suggest multistage petrogenetic processes including refertilization of pre-existing depleted, refractory mantle wedge by subduction-derived fluids and instantaneous fractional melts operative in an intraoceanic fore-arc environment. Re-Os isotopic plots reflect the role of ancient depleted Subcontinental lithospheric mantle (SCLM) and the large variations in ɣ_Os values attest to involvement of both SCLM and depleted MORB component thereby contributing to pronounced Os isotopic heterogeneities in the upper mantle that in turn manifest inputs of discrete depleted and enriched mantle components during opening and closure of ocean basins synchronized with assembly and dispersal of continental blocks. The model ages (T_MA) for the mantle peridotites reveals a wide spectrum of ages  ranging from 1142 Ma – 145 Ma suggesting multiple episodes of melt extraction events prior to the opening of Neo-Tethys (~250Ma) and encapsulation of ancient SCLM fragments correlatable with Gondwana Supercontinent amalgamation and disintegration coupled with closure and opening of the Tethyan ocean basins.

How to cite: Verencar, A., Saha, A., Ganguly, S., Satyanarayanan, M., and Ram Mohan, M.: Multiple melt extraction in Neo-Tethyan mantle and the Supercontinent heritage: insights from geochemical signatures of mantle peridotites from Naga Hills ophiolite, Indo-Myanmar ranges, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-124, https://doi.org/10.5194/egusphere-egu24-124, 2024.

In this study, we present a new analysis of 12,515 pre-analysed garnet xenocrysts recovered from kimberlites from the Barkly West kimberlite cluster. These kimberlites were emplaced in the western Kaapvaal Craton, South Africa in two pulses. These analyses are used to constrain the chemical and thermal structure of the subcontinental lithospheric mantle (SCLM) beneath the Kaapvaal Craton. In South Africa, kimberlites are distinguished based on geochemistry and petrology: (1) the hydrous, K-rich, and generally older Group II (age: ~200-114Ma; aka Kaapvaal lamproites) and (2) the younger, carbonate-rich and magnesian Group I kimberlites (age: ~112-86Ma). Upon ascent, kimberlites entrain mantle xenoliths, xenocrysts (e.g., garnet) and sometimes diamonds. Historically, mantle xenoliths have been employed to investigate the SCLM, but the large abundance of xenocrysts, available from a larger array of kimberlites, allows a more spatially robust characterisation of the SCLM. We investigate peridotitic garnets, i.e., G10 (CaO-poor) and G9 (CaO-rich) garnets, to allow single-mineral thermobarometry. Generally, G10 garnets have low CaO and high Cr2O3 contents and high Mg-numbers relative to G9 garnets. G9 garnets are typically more enriched in incompatible elements (i.e., Zr, Y, Ti) and heavy REE. Profiles of mantle composition obtained from garnets exhumed by both Group I and Group II kimberlites provide evidence for (likely metasomatic) enrichment of the SCLM at depths greater than 90-150 km, with the enrichment revealed by garnets from group I kimberlites being generally greater in magnitude and extending to shallower depths. Distinctions between Group II and Group I derived garnets suggest modification of the SCLM by two distinct metasomatic agents, and that Group I-related metasomatism was much more pervasive. For each kimberlite group, two paleogeotherms were calculated (one each for G10 and G9 garnets) using garnet thermobarometry. For Group II kimberlites, the G10- and G9-based paleogeotherms are very distinct from each other, cooler and warmer paleogeotherms, respectively. However, for Group I kimberlites, the paleogeotherms obtained from G10 & G9 are consistent with each other and are similar to the G9-based paleogeotherm obtained from Group II kimberlite xenocrysts. We infer that the Group II G10-based paleogeotherm, which has the lowest thermal gradient, is consistent with a steady state cratonic geotherm, devoid of transient thermal perturbations. However, geotherms with higher gradients obtained from Group II G9 and all Group I peridotitic garnet xenocrysts are the products of transient thermal disturbances, likely related to the effect of hot infiltrating melts. Recent seismological studies have discovered sharp velocity changes within the SCLM known as mid-lithospheric discontinuities (MLDs). Globally, negative MLDs observed in cratonic areas have been associated with metasomatism. However, beneath the Kaapvaal Craton we note positive MLDs, which coincide with the observed layer of metasomatism which has resulted in increased clinopyroxene and garnet modal abundance. Is clinopyroxene and garnet generation responsible for positive MLDs beneath the Kaapvaal Craton?

How to cite: Hashibi, S., Janney, P., Sloan, A., and Quiros, D.: Using kimberlite indicator mineral geochemistry to better constrain the thermal and chemical structure of the lithospheric mantle beneath the Kaapvaal craton: correlations with S-to-P receiver functions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1107, https://doi.org/10.5194/egusphere-egu24-1107, 2024.

Understanding the temperature and pressure within Earth's lithospheric mantle is crucial for comprehending the dynamics of Earth's interior, its geochemical and geophysical properties, as well as their influence on magma formation and the stability of cratons. While traditional mineral-based thermobarometers have provided valuable insights in estimating temperature and pressure, their reliance on specific mineral pairs and reactions limits their global applicability. Furthermore, as a type of high-dimensional data, mineral compositions have complex relationships that are difficult to capture by conventional methods, and these conventional methods are prone to overfitting, which can produce inaccurate results in certain cases. Our work introduces a new method using XGBoost to develop machine learning-based thermometers and barometers. These models are trained on a comprehensive dataset from 985 high-temperature and high-pressure experiments. This approach is designed to fully leverage the potential of high-dimensional data, offering more precise and widely applicable estimations while also preventing overfitting inherent in traditional methods. Comparison between machine learning models and classic thermobarometer reveals that the machine learning models significantly improve the accuracy in predicting temperature and pressure. This improvement can be attributed to the capability of machine learning models in processing the high-dimensional data of mineral pairs. The global application of these machine learning models has enabled a re-evaluation of the mantle's thermal conditions across diverse cratons. Most notably, the depth estimations to the lithosphere-asthenosphere boundary using machine learning thermobarometry typically exceed seismic measurements by approximately 40 km. This discrepancy suggests the presence of melt-bearing zones at the lithosphere-asthenosphere boundary.

How to cite: Ye, C., Qin, B., Liu, J., Huang, S., Wang, S., and Zhangzhou, J.: Mapping Global Lithospheric Mantle Pressure-Temperature Conditions by Machine Learning-Based Thermobarometry, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7019, https://doi.org/10.5194/egusphere-egu24-7019, 2024.

Extensional processes related to subduction initiation stages often result in the formation of back-arc basins. In central Anatolia and the northern-central Aegean, the main lithotypes geochemically correspond to high-K calk-alkaline rocks that formed during the Early Miocene, while volcanism in the Middle Miocene was predominantly Na-rich. The latter lithotypes were in most cases significantly affected by differentiation processes. On the island of Chios, volcanism is comprised of lithotypes that vary significantly from alkali basalts to rhyolites. In particular, the Pyrgi lavas are fine-grained with typical porphyritic and seriate textures, which are classified as alkali basalts according to the AFM ternary plot. These lavas also exhibit positive anomalies in PM-normalized P, Sr, and Pb elements and significant enrichments in LILE (i.e., Cs, Rb, Ba), presenting a probable interaction with subduction melts. The negative correlation between SiO₂ contents and the Nb/Yb ratio indicates that crustal contamination did not play a significant role during differentiation. Crystallization appears to be under extremely dry conditions due to the absence of hydrous phases (e.g., amphibole). The presence of MORB signature spinel crystals suggests the implication of a MORB-type source. In the case of Chios, the high Nb tholeiites can be associated with petrogenetic processes in which OIB-related melts interacted with the previously metasomatized mantle wedge, confirming the back-arc extension of the Northern Aegean during the Miocene and possibly to mantle delamination in the western Anatolia region.

Acknowledgments

This work is part of the first author's MSc. research, which is financially supported by the «Andreas Mentzelopoulos Foundation».

How to cite: Kalantzi, L., Giamas, V., Koutsovitis, P., Petrounias, P., Hauzenberger, C., and Ntaflos, T.: Miocene basaltic lavas from Chios Island: Petrogenetic implications, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6324, https://doi.org/10.5194/egusphere-egu24-6324, 2024.

In St. Martin, the Oligocene granitoids comprise granodiorites, leucotonalites, melatonalites and Qz-monzodiorites. Tonalites are low-K, whereas granodiorites and Qz-monzodiorites are related with calc-alkaline suites. Mineralogical, geochemical and Sr-Nd isotopic data denote that most rocks are I-type calc-alkaline, except for the melatonalites that seemingly resemble peraluminous S-type granitoids. The melatonalites display the lowest Al₂O₃/TiO₂ and highest CaO/Na₂O ratios, pointing to high temperature conditions. Various geothermometry applications, which include Ti-in-zircon thermometry reveal high generated temperatures for the melatonalites, exceeding by ~100 °C those calculated for the other granitoids. Regarding the granodiorites (Type-I low REE; Type-II high REE), Type-II are associated with higher temperature conditions by ~70 °C. Zircon saturation thermometry also show higher crystallization temperatures for the melatonalites and Type-II granodiorites. Thermobarometric results elucidated from mineral chemistry and bulk-rock geochemical point to  higher temperature and pressure crystallization conditions for the melatonalites compared to the leucotonalites and granodiorites. The granitoids were affected by extensive differentiation processes; plagioclase preferably fractionated in the Type-I granodiorites; Type-II mainly involved K-feldspar removal. Fluctuation of hydrous and slab-derived fluid fluxes  contributed to magma differentiation as inferred by the Th/Nb and Ba/La ratios, with hydrous-saturated conditions favouring formation of granodiorites rather than leucotonalites.

Melatonalites and Type-II granodiorites likely formed at proto-arc settings, with melting of a fertile mantle during subduction initiation. Melatonalites may have involved magma mixing via interaction of a hotspot plume within the forearc mantle, as denoted by geochemical and geothermometry results. The geochemical features of the Type-II granodiorites likely reflect formation at the early subduction stages, associated with a fertile source.

Reference: Koutsovitis et al. 2024. Granitoids from St. Martin/Maarten Island, Caribbean: Insights on the role of Mantle processes in the Lesser Antilles Arc. Lithos (Under Review).

How to cite: Koutsovitis, P., van der Meulen, M. J., van Daalen, T., Tyrologou, P., Koukouzas, N., Sideridis, A., Karkalis, C., Grégoire, M., Petrounias, P., Ntaflos, T., and Lentas, K.: A petrogenetic approach on the St. Martin/Maarten granitoids (Lesser Antilles Arc) and associated mantle processes  , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11601, https://doi.org/10.5194/egusphere-egu24-11601, 2024.

The presence of mafic enclaves within volcanic rocks, which present an evolved geochemical character, tends to be related to magma mixing. In the Hellenic Volcanic Arc, mafic enclaves are regularly encountered within the volcanics. There are a few reports regarding the islands of Santorini and Nisyros as well as in the peninsula of Methana. The petrographic and mineral chemistry data we present here concern enclaves only from the Methana peninsula. Enclaves in Methana vary in size (from a few cm up to hand-sized samples) and color (reddish to black) depending on their host lava. The typical mineral assemblage is amphibole + clinopyroxene + olivine + plagioclase ± spinel ± mica. Mineral phases of amphibole, olivine, and clinopyroxene form the major phenocrysts that are embedded within a matrix mostly of fine-grained plagioclase laths and fine-grained needle-shaped amphibole. Textural features of plagioclase reveal the predominance of wet conditions during their crystallization since their cores and/or intermediate growth zones are usually dissolved suggesting a hydrous and volatile enriched magma related to the mafic enclaves. The aforementioned mineral assemblage along with the latter petrographic features match with lamprophyric rocks and their typical porphyritic and panidiomorphic textures. This counterpart is further ascertained by the mineral chemistry of Mg-rich amphibole varieties, and primary forsterite-rich olivine as well as by the lamprophyre-related trends of clinopyroxene and mica. These preliminary results provide new insights regarding magma mixing processes that occurred at the western margin of the Hellenic 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.: Following the tracers of magma mixing; mafic enclaves within volcanic rocks at the Methana peninsula, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6225, https://doi.org/10.5194/egusphere-egu24-6225, 2024.

The Spongtang Ophiolite positioned along the Indus Suture Zone (ISZ) in Ladakh, NW Himalaya, stands as a key geological site for unraveling the origin and tectonic evolution of serpentinized ultramafic rocks. The petrographic study of highly serpentinized ultramafic rocks is characterized by presence of relict olivines and orthopyroxenes exhibiting porphyroclastic texture. Orthopyroxene grains exhibit bastite texture pseudomorphing the primary relict grains and are altered along their rims to form lizardite. Olivine grains have been mostly altered to lizardite having mesh and hourglass texture and show characteristic peaks at 230 cm^-1, 383 cm^-1 and 692 cm^-1 under Raman spectroscopy. Chrysotiles are observed to have fibrous vein cross-cutting relict orthopyroxene and olivine grains and show peaks at around 135 cm^-1, 232 cm^-1, 385 cm^-1 and 694 cm-¹. Antigorites show interlocking to interpenetrating planar texture peaking at 229 cm^-1, 377 cm^-1 and 687 cm^-1. All these major phases are associated with minor brucite, indicating initial stage of serpentinization, and showing characteristic peak at 276 cm^-1 and 442 cm^-1, 721 cm^-1 and 1086 cm^-1. Magnetites are formed as alteration products along the rims of chromites and within the serpentine veins indicating mature stage of serpentinization. Lizardite and chrysotile commonly occur during sea floor serpentinization at temperature condition ranging between 50-400˚C. Antigorite mostly occurs in a subduction setting and formed at temperature between 300-600˚C. Mineral chemistry data of serpentines exhibit wide variation in MgO (8.66 wt. % to 27.20 wt. %) and FeO (4.10 wt. % to 12.26 wt. %). However, SiO₂ (43 wt. % to 40.82 wt. %) and Al₂O₃ (0.16 wt. % to 1.07 wt. %) content in serpentines varies within a small range indicating that Si and Al are relatively immobile compared to Mg and Fe during serpentinization. The crosscutting brucite veins have high MgO and FeO composition of 50.53 wt. % and 16.56 wt. % respectively. Mg# of serpentines vary between 0.80 to 0.85, which is much lower than olivines (Mg# = 0.91-0.92) and pyroxenes (Mg# = 0.91-0.97) indicates that magnesium loss has been taken place during serpentinization from the primary minerals. High CaO (22.84 wt. % to 24.73 wt. %) content in diopside grains than the surrounding serpentine grains (0.19 wt. % to 0.55 wt. %) implies that Ca has been removed from the serpentines during serpentinization. The base metal sulfide (BMS) mineral assemblages in serpentinized Spongtang ultramafics are dominated by presence of pyrrhotite, pentlandite, awaruite, magnetite and a few Cu-Fe-Ni alloys. Pyrrhotite, pentlandite and awaruite occur together in serpentine mesh centers and indicate presence of reducing condition at low water-rock ratio during their mineralization. Magnetites formed within serpentinized veins suggest a high water-rock condition leading to a more oxidizing environment.

How to cite: Nayak, B. R. and Ovung, T. N.: Geochemical effects of hydrothermal alteration in the Neotethyan oceanic lithosphere: inferences from the serpentinized mantle rocks of the Spongtang ophiolite, Ladakh, NW Himalaya, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7131, https://doi.org/10.5194/egusphere-egu24-7131, 2024.

The Nidar Ophiolite Suite (NOS) is well exposed in the Indus Suture Zone that separates the Indian plate in the south from Asia to the north, where the ophiolite sequence is best exposed in the Nidar Valley and the outcrop is elongated NW-SE along the regional tectonic trend. The Nidar ophiolite contains both mantle and crustal sections. The mantle section includes ultramafic rocks, namely lherzolite, harzburgite, dunite and serpentinized peridotite with chromite. In the present study, our focus is on chromium-spinel (chromite), which is a common mineral in ophiolitic rocks, and the study of this mineral from the mantle sections of NOS ophiolites can shed light on their petrogenetic origin and tectonic setting.
NOS contains disseminated chromite grains in mantle harzburgites and podiform chromitites associated with dunites and serpentinized peridotites. Due to alteration, most chromite grains display compositional zoning, but the fresh cores preserve primary igneous compositions. Podiform chromitites in the NOS dunites and serpentinized peridotites are compositionally similar to typical ophiolitic chromitites elsewhere. NOS chromite samples exhibit two clusters based on Mg and Cr numbers, indicating two different chromite formation stages.
They are in the initial chromite precipitating stage and later form chromite pods due to the accumulation of chromite crystal precipitation. Chromite from can be texturally and chemically classified into two main types: primary high-Al (spinel Cr# < 0.67) and high-Cr (spinel Cr# > 0.75) chromite. High Cr/Al ratios of the investigated spinel cores (Cr# 0.7– 0.81) point to a higher degree of partial melting of the depleted mantle source. Low (Cr# 0.27-0.4) indicates a lower degree of partial melting. MORB-like tholeiitic melt generated during proto-forearc spreading at the onset of subduction leads to the generation of high-Al chromite. In contrast, the latter was formed from boninitic melts resulting from the high degree melting of the sub-arc depleted mantle in slab-derived fluids at a mature-arc stage.
However, chromite grains in the peridotites show mixed MORB and arc signatures. Thus, the mineralogy and geochemistry of the NOS peridotites suggest that the chromite in the NOS formed in a forearc tectonic setting during a reaction between boninitic melts and MORB-type harzburgite in a supra-subduction zone (SSZ) mantle wedge.

How to cite: Kumar, A., Barman, K., Verma, A., Barman, P., Malviya, V. P., and Koutsovitis, P.: High-Cr and High Al chromite from the Nidar Ophiolite Suite, Ladakh, India: implications for its petrogenesis and tectonic evolution, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8393, https://doi.org/10.5194/egusphere-egu24-8393, 2024.