Displays

A zoned reaction rim (kelyphite) around garnet of xenolith of fresh sheared lherzolite from the Udachnaya-East kimberlite pipe, Russia, has been investigated. The aim of the study is a detailed characterization of bulk major and trace element compositions of the kelyphite zones, kelyphite-forming minerals and theirs relationships with each other and with rock-forming minerals of the lherzolite. 
There are three point of possible origin of the kelyphite: 1) a solid-solid reaction (between garnets and rock-forming minerals) during transporting to the surface and modifying by a kimberlite melt (introduction of Na, K, Ca and H₂O into the kelyphite) after reaction, 2) a reaction between garnets and a kimberlite melt, 3) mantle metasomatism.
Scanning electron microscopy coupled with energy dispersive spectrometry was used for phase determination and chemical analyses. Chemical composition of large grains (>6 μm) was also examined with wave-length-dispersive spectrometry on electron probe micro-analyzer. Raman spectroscopy was used for phase verification. Bulk trace element composition of reaction rim was studied by laser ablation‐inductively coupled plasma‐mass spectrometry.
Garnet forms rounded grains up to 4 mm in size, which are surrounded by the kelyphitic rim. The kelyphite has a concentric structure forming three distinct textural and chemical zones, which are extremely fine-grained aggregates of Cr- and Al-rich orthopyroxene, spinel with a wide range of Cr#, Cr and Al-rich clinopyroxene, amphibole, phlogopite, sodalite and olivine. Veinlets, which traverse the reaction rim and the garnet, are composed of the kelyphite-like mineral aggregate.
The kelyphite formation took place after the lherzolite was entrapped by the kimberlite magma during ascent and emplacement. Orthopyroxene, clinopyroxene and spinel were primarily formed (hereafter the first association). Known limits of pressure-temperature stability of sodalite, phlogopite and amphibole suggest their low-pressure crystallization in the kelyphite (hereafter the second association). The kimberlite melt participated in the formation of both the first mineral association and the second mineral association of the kelyphite. Olivine is believed to be result from a reaction between the kimberlite melt and the kelyphite after forming of the first association but before forming of the second association. On the basis of bulk chemical composition for each zone of the kelyphite and chemical composition of the precursor garnet, a material transfer into the kelyphite during the formation was quantitatively evaluated. Introduction of Mg, Fe, Ti and Ca in the kelyphite occured before formation of the second mineral association and introduction of Na, K, Ca, Cl, F and H₂O due to formation of the second mineral association. Therefore, we can expect that the kimberlite melt was a diffusion agent during formation of the first mineral association (the garnet and rock-forming minerals are considered as reactants) and was a reactant during formation of the second mineral association.
This study was supported by the Russian Science Foundation (grant No 18-77-10062).

How to cite: Solovev, K., Sharygin, I., and Golovin, A.: Composition and origin of kelyphitic rims around garnets in fresh sheared lherzolite from the Udachnaya-East kimberlite pipe, the Siberian Craton, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-820, https://doi.org/10.5194/egusphere-egu2020-820, 2020.

Orogenic peridotites are expected to provide direct information with high spatial resolution for a better understanding of the processes taking place in the lithosphere and asthenosphere boundary zones (LABZ), where the transfer mechanisms of heat, material, and momentum from the Earth’s interior to the surface drastically change. Plagioclase peridotite or olivine-plagioclase assemblage sensu lato has been reported from some orogenic peridotites. The olivine-plagioclase assemblage in fertile systems is in principle not stable even at the depth of the upper most subcontinental lithospheric mantle (SCLM) because (1) the common crustal thickness in normal non-cratonic SCLM is ~35km, (2) the Moho temperature for the mean steady-state continental geotherm is much lower than 600°C, (3) the upper stability limit of plagioclase (plagioclase to spinel facies transition) becomes shallower with decrease in temperature, and (4) kinetic barrier for subsolidus reactions in the peridotite system becomes enormous at temperatures below 600°C. The occurrence of olivine-plagioclase assemblage in some orogenic peridotite bodies, therefore, implies transient and dynamic high-temperature (>800°C) processing at depth shallower than 20km (plagioclase-spinel facies boundary at ~800°C), i.e., high-temperature decompression of LABZ up to the depth closer to the Moho. Adiabatic decompression of high-temperature LABZ leading to decompressional melting with inefficient melt segregation may give rise to plagioclase peridotite. Decompression along moderately high temperature adiabatic path or heating to allow subsolidus reactions leading to transformation of either spinel peridotites or garnet peridotites may give rise to plagioclase peridotite. However, decompression of LABZ associated with efficient cooling does not produce any olivine-plagioclase assemblage. Plagioclase peridotites thus could provide precious information on the dynamics of shallowing LABZ and underlying asthenosphere.

We have examined several orogenic peridotite complexes, Ronda, Pyrenees, Lanzo, and Horoman, to clarify the extent of shallow thermal processing based on olivine-plagioclase assemblage. The key approach of this study is searching olivine-plagioclase assemblage not only in various lithologies but also in microstructures, whose scale and mode of occurrence provide extent and strength of thermal processing in the shallow upper mantle. The wide-spread occurrence of plagioclase peridotites and localized partial melting in Lanzo suggest exhumation along high temperature adiabatic paths from the thermally structured LABZ in the Seiland subfacies; the predominance of plagioclase peridotites and its localized partial melting in Horoman suggest exhumation along variously heated paths from the garnet stability field; the moderate development of plagioclase peridotites without partial melting in Ronda suggest exhumation along variously but weekly heated paths from the spinel-garnet stability field, and the occurrence of minor plagioclase peridotites in Pyrenees suggests exhumation along cold path from the garnet-spinel facies boundaries. We propose that the extent of shallower thermal processing decreases, and thus lithosphere thinning becomes less extensive in this order.

How to cite: Ozawa, K., Garrido, C., Hidas, K., Bodinier, J.-L., Aoki, T., and Boudier, F.: Plagioclase peridotite or olivine-plagioclase assemblage in orogenic peridotites: its implications on high-temperature decompression of the subcontinental lithosphere-asthenosphere boundary zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12256, https://doi.org/10.5194/egusphere-egu2020-12256, 2020.

Permo-Carboniferous mafic alkaline volcanism in Scotland sampled deep lithosphere beneath Lower Paleozoic basement. In the eastern part of Midland Valley terrane (central Scotland, Fife peninsula) volcanic and volcanoclastic rocks carry (among others) ultramafic xenoliths. The peridotite xenoliths give an insight into structure and composition of lithospheric mantle at the time of volcanic activity.

We studied spinel lherzolites and wehrlites, occurring as usually <10 cm in diameter (up to 25 cm) xenoliths. The lherzolites have either protogranular or porphyroclastic texture, while wehrlites are equigranular. Clinopyroxene in porphyroclastic lherzolites and wehrlites is texturally secondary growing at the expense of orthopyroxene.

Chemical composition of minerals is related to texture of the rock. Olivine forming protogranular lherzolites has Fo=88.35-88.80 and is Ca-poor (<900 ppm) which together with #Cr in spinel varying from 0.08 to 0.21 plot those rocks within the Olivine-Spinel Mantle Array (OSMA, Arai, 1994). Orthopyroxene in this group is chemically homogenous within a sample (Mg#=0.89-0.90, Al=0.15-0.19 a.p.f.u.); clinopyroxene in some samples is heterogeneous, but its composition varies in a narrow range (Mg#=0.89-0.92 and Al=0.22-0.34 a.p.fu.). The REE pattern of clinopyroxene from protogranular lherzolites varies from LREE-depleted to LREE-enriched one; it is always enriched in Th and U and depleted in Nb and Ta.

Composition of minerals forming porphyroclastic lherzolites and wehrlites is strongly heterogeneous and varies in the same ranges. Olivine and spinel of some rare porphyroclastic lherzolites plot within OSMA (Ol_{Fo=88.50-88.80}, Spl_{#Cr=0.12-0.20}), but in majority of samples olivine composition grades toward lower forsterite contents (Fo=78.33-89.78) while Cr# is higher in spinel (Cr#=0.72-0.53) which locate them outside OSMA. Olivine has Ca content up to 2000 ppm. Orthopyroxene and clinopyroxene are chemically heterogeneous in terms of Mg# (0.82-0.89 and 0.81-0.93, respectively) and Al content (0.06-0.17 and 0.10-0.33 a.p.f.u., respectively). Clinopyroxene in porphyroclastic lherzolites and wehrlites has REE-enriched pattern and is enriched in Th and U and depleted in Nb and Ta.

The clinopyroxene-orthopyroxene equilibrium temperatures for the protogranular lherzolites are usually ~980°C, only single sample gave temperature ~900°C (Brey and Köhler, 1991, JoP). Pyroxenes in porphyroclastic lherzolites and wehrlites are not equilibrated, but the elevated Ca content in olivine suggests that those rocks were affected by heating.

The protogranular lherzolites from Fife are restites after relatively low (1-7%) degrees of partial melting and were further affected by cryptic metasomatism. Bonadiman et al. (2008, Geol. Soc.) suggested that the enrichment in Th and U may result from reaction with subduction-related melt(s), possibly related in central Scotland with Caledonian closure of Iapetus ocean. Composition of clinopyroxene from the most orthopyroxene-rich porphyroclastic lherzolites resembles that of clinopyroxene from protogranular rocks, but as the modal composition grades toward wehrlites, the clinopyroxene becomes more variable in composition but also LREE-enriched. We suggest, that the chemical and textural paths from protogranular to equigranular samples  record different stages of wehrlitization process triggered by infiltration of LREE-enriched mafic melt(s).

This study was possible thanks to project NCN UMO-2016/23/B/ST10/01905 from the Polish National Centre for Science and Polish-Austrian project WTZ PL 08/2018 .

How to cite: Matusiak-Malek, M., Upton, B. J. G., Matczuk, P., Puziewicz, J., Ntaflos, T., Grégoire, M., and Aulbach, S.: Wehrlitization of lithospheric mantle beneath Fife, Scotland., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13626, https://doi.org/10.5194/egusphere-egu2020-13626, 2020.

As clinopyroxene is the main host of most lithophile elements in the lithospheric mantle, the trace element and radiogenic isotope systematics of this mineral have frequently been used to characterise mantle metasomatic processes. To further our understanding of mantle metasomatism, both solution-mode Sr-Nd-Hf-Pb and in situ trace element and Sr isotopic data have been acquired for clinopyroxene grains from a suite of peridotite (lherzolites and wehrlites), MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside), and PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks from the Kimberley kimberlites (South Africa). The studied mantle samples can be divided into two groups on the basis of their clinopyroxene trace element compositions, and this subdivision is reinforced by their isotopic ratios. Type 1 clinopyroxene, which comprises PIC, wehrlite, and some sheared lherzolite samples, is characterised by low Sr (~100–200 ppm) and LREE concentrations, moderate HFSE contents (e.g., ~40–75 ppm Zr; La/Zr < 0.04), and restricted isotopic compositions (e.g., ⁸⁷Sr/⁸⁶Sr_i = 0.70369–0.70383; εNd_i = +3.1 to +3.6) resembling those of their host kimberlite magmas. Available trace element partition coefficients can be used to show that Type 1 clinopyroxenes are close to equilibrium with kimberlite melt compositions, supporting a genetic link between kimberlites and these metasomatised lithologies. Thermobarometric estimates for Type 1 samples indicate equilibration depths of 135–155 km within the lithosphere, thus showing that kimberlite melt metasomatism is prevalent in the deeper part of the lithosphere beneath Kimberley. In contrast, Type 2 clinopyroxenes occur in MARID rocks and coarse granular lherzolites, which derive from shallower depths (<130 km), and have higher Sr (~350–1000 ppm) and LREE contents, corresponding to higher La/Zr of >~0.05. The isotopic compositions of Type 2 clinopyroxenes are more variable and extend from compositions resembling the “enriched mantle” towards those of Type 1 rocks (e.g., εNd_i = -12.7 to -4.4). To constrain the source of these variations, in situ Sr isotope analyses of clinopyroxene were undertaken, including zoned grains in Type 2 samples. MARID and lherzolite clinopyroxene cores display generally radiogenic but variable ⁸⁷Sr/⁸⁶Sr_i values (0.70526–0.71177), which might be explained by the interaction between peridotite and melts from different enriched sources with the lithospheric mantle. In contrast, the rims of these Type 2 clinopyroxenes trend towards compositions similar to those of the host kimberlite and Type 1 clinopyroxene from PIC and wehrlites. These results are interpreted to represent clinopyroxene overgrowth during late-stage (shortly before/during entrainment) metasomatism by kimberlite magmas. Our study shows that an early, pervasive, alkaline metasomatic event caused MARID and lherzolite genesis in the lithospheric mantle beneath the Kimberley area, which was followed by kimberlite metasomatism during Cretaceous magmatism. This latter event is the time at which discrete PIC, wehrlite, and sheared lherzolite lithologies were formed, and MARID and granular lherzolites were partly modified.

How to cite: Fitzpayne, A., Giuliani, A., Hergt, J., Woodhead, J., and Maas, R.: Isotopic analyses of clinopyroxene in mantle xenoliths demonstrate the effects of kimberlite melt metasomatism upon the lithospheric mantle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5331, https://doi.org/10.5194/egusphere-egu2020-5331, 2020.

Garnet from the lithospheric mantle underneath cratons can contain oriented lamellar inclusions of pyroxene and oxides like rutile as a result of exsolution of majoritic and titaniferous components due to cooling and/or decreasing pressure. We investigated ten new such microstructure-bearing samples of pyroxenite and eclogite from six peridotite bodies in SW Norway, which were once located in the E Greenland mantle lithosphere. The lamellar inclusions occur in porphyroclastic garnet and vary – dependent on their size – systematically in shape, (acicular to short-prismatic), width (~50 μm to sub-micron size), spacing (several 100 to ~10 μm), and phase (pyroxene to pyroxene + Ti-oxides to Ti-oxides). Smaller lamellae can fill the space between larger lamellae, which support consecutive generations. The larger (early formed) lamellae are more poorly preserved and more difficult to locate in the suite of samples than the smaller (lately formed) exsolutes. A younger generation of lamellar and other inclusions occur lined-up along healed cracks cutting across cores but not rims of garnet. These inclusions comprise oxides, silicates, carbonates (aragonite, calcite, magnesite) and fluid inclusions (N₂, CO₂, H₂O). Their origin either relates to the Precambrian rock history and/or to a hydrous environment as typical for mantle wedge metasomatism prior to Scandian recrystallisation. Mineral chemistry suggests that the lamellae-bearing garnet grains equilibrated at two discrete depth levels, corresponding to ~3.7 GPa (850 °C) and ~3.0 GPa (710 °C), at a cratonic geotherm corresponding to 38 mW/m² surface heat flow. Five samples contain porphyroclastic orthopyroxenes with Al₂O₃ concentration showing W-shaped profiles and/or very low Al₂O₃ content (0.18–0.23 wt%) in cores of large (>200 µm) recrystallised grains. Both characteristics typify short intracrystalline diffusion lengths and are consistent with an early prograde metamorphic evolution into the diamond stability field. This evolution is related to subduction during the Scandian orogeny. Porphyroclastic orthopyroxenes in other samples show U-shaped Al₂O₃ concentration profiles and long diffusion lengths of several 100 μm, i.e. longer than the grain radius of the recrystallised grains. Their cores contain high Al₂O₃ contents (0.65–1.16 wt%) consistent with a diffusional overprint that followed partial rock recrystallisation and obliterated pro- and peak metamorphic records. The presence of systematic exsolution microstructures in all samples demonstrates a similar early evolution of pyroxenite and eclogite in all six peridotite bodies. The wide distribution of our samples across the Western Gneiss Region indicates that (1) majoritic and titaniferous garnet occurred widespread in the E Greenland lithospheric mantle and (2) rock bodies of Scandian ultra-high pressure metamorphism can be found in nearly the entire area between Nordfjord and Storfjord and from the coast towards ~100 km in the hinterland, i.e. in a region much larger than previously anticipated.

How to cite: Spengler, D., Alifirova, T. A., van Roermund, H. L. M., and Massonne, H.-J.: Petrology of sub-cratonic pyroxenite and eclogite containing lamellae-bearing garnet, Western Gneiss Region, Norway, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19040, https://doi.org/10.5194/egusphere-egu2020-19040, 2020.

The appearance of alkali- and volatile-rich melts often marks the opening of major magmatic cycles, always reflecting the partial melting of heterogeneously enriched mantle domains. In these cases the study of highly alkaline, H₂O-CO₂-rich magmatic pulses provide important insights on the composition and behavior of the sub-continental lithospheric mantle (SCLM) prior to rift initiation. The camptonitic dykes cropping out at Predazzo (Dolomitic Area, NE Italy) are among the oldest examples of lamprophyric rocks in Italy, and were historically related to the orogenic-like Middle Triassic magmatism of the Southern Alps. A detailed petrological, geochemical and geochronological characterization of these rocks was developed to frame them inside the articulated geodynamic evolution of the Southern Alps domain during Triassic. Whole-rock and mineral phase geochemistry, together with ⁴⁰Ar/³⁹Ar data showed that Predazzo lamprophyres represent an alkaline-carbonatitic magmatic event temporally isolated (~220 Ma) from the major Ladinian orogenic-like magmatism of the Southern Alps (~238 Ma). Lamprophyres can thus be attributed to the volumetrically limited alkaline magmatic phase that infiltrated several portions of the Southern Alps lithosphere between 225 and 190 Ma. Partial melting models and Sr-Nd isotopes demonstrate that Predazzo lamprophyres were produced by low partial melting degree of a garnet-amphibole-bearing mantle source interacting with a significant asthenospheric contribution. In the light of these new findings, they are interpreted as the geochemical/geochronological bridge between the orogenic-like Ladinian magmatism and the rifting phase related to the opening of the Alpine Tethys. This study highlights the paramount importance of alkaline magmas for tracking the volatiles cycle in the SCLM and the potential lithosphere-asthenosphere interactions during large-scale geodynamic processes.

How to cite: Casetta, F., Ickert, R. B., Mark, D. F., Bonadiman, C., Giacomoni, P. P., Ntaflos, T., and Coltorti, M.: Volatile-rich melts as markers of the asthenospheric influx prior to rifting events: the case of the alkaline-carbonatitic lamprophyres of the Dolomitic Area (Southern Alps, Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8899, https://doi.org/10.5194/egusphere-egu2020-8899, 2020.

Triassic volcanism developed during the rifting stage of Gondwana, with subsequent formation and development of the Tethyan oceanic basin as the Pangaea (Apulia promontory) and Pelagonia continents spread apart. Volcanic rocks formed from this activity outcrop over all mainland Greece, comprising of trachybasalts and basaltic trachyandesites. Relatively immobile to the effects of alteration processes major and trace element abundances classify the volcanics into OIB and E-MORB lavas. They have been distinguished based upon their: i) LREE contents, ii) silica-saturation index, iii) Zr/Nb and Nb/Y ratio values; iv) Th, U, and Ta contents v) geotectonic discrimination diagrams. Their geochemistry indicates that most rocks were affected by moderate to extensive differentiation processes, mostly expressed by clinopyroxene fractionation. Some of the OIB and E-MORB volcanics are considered as being primitive undersaturated, displaying relatively low SiO₂ and S.I. index values and also high Mg# and CaO/Al₂O₃ ratios.

Calculated average mantle potential temperatures are comparable (1410 ˚C OIB; 1370 ˚C E-MORB), with melt fractions estimated at 3-5% for primary OIB magmas and 6-8%for primary E-MORB magmas. An asthenospheric origin is inferred for the OIB lavas, with melting in the garnet stability field (75-95 km; 2.5-3.0 GPa), whereas E-MORB parent magmas were generated with shallower melting processes within the garnet/spinel (transitional) stability field (55-70 km; 1.8-2.2 GPa). Lithospheric attenuation and extension, followed by subsequent asthenospheric upwelling of the mantle was enhanced due to lithospheric thinning as rifting progressed. The rather high calculated partial melting degrees and the observed relatively thick lava formations account for fast-spreading rift settings, consistent with the opening of the Tethys during the Triassic. Temperature results indicate that the Hellenic Triassic rift-related magmas were generated from mantle at ambient temperature, precluding a mantle plume-based scenario or of thermal anomalies.

How to cite: Koutsovitis, P., Magganas, A., Ntaflos, T., Koukouzas, N., and Rassios, A. E.: Constraints on the formation and nature of the Hellenic Triassic rift-related lavas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4249, https://doi.org/10.5194/egusphere-egu2020-4249, 2020.

Orogenic peridotites associated with high-grade felsic rocks record mass exchange between crust and mantle reservoirs at convergent plate margins. In this geodynamic framework, fluids released by submerging slabs can mobilize redox-sensitive elements such as carbon (C) and sulfur (S) and percolate the mantle wedge, eventually forming hydrate minerals associated with carbonate and sulfide phases at appropriate T, P and f O₂ conditions. The introduction of sulfur into the sub-continental lithospheric mantle (SCLM) wedge and its mobilization at grain-scale can be investigated by means of in situ δ³⁴S analyses of mantle wedge sulfides, which may have inherited the composition of the fluid sources. To date, the impact of the S transfer through the SCLM wedge is poorly known and limited in situ S isotope values of sulfides from mantle wedge peridotite are available in literature. Our study focuses on the Ulten Zone (UZ) orogenic-garnet peridotites, which provide an ideal case to investigate the S mobilization through the SCLM wedge and the effects of crustal fluids on the sulfide δ³⁴S signature, especially during the exhumation stage. We therefore integrate a well-constrained paragenesis with mineral chemistry and in situ S isotope signature of sulfides. The UZ peridotites were involved in a collisional setting during the Variscan orogenesis, recording HP-eclogite-facies conditions and exhumation after their incorporation in a mélange with the associated garnet-kyanite gneisses. A suite of coarse to fine-grained peridotites was investigated in order to cover all the metasomatic stages preserved in these rocks, considering the grade of serpentinization and the occurrence of carbonates. Microstructural observations and major element compositions indicate that pentlandite (± chalcopyrite ± chalcocite ± sphalerite) is the ubiquitous primary sulfide, which is commonly replaced by secondary heazlewoodite and millerite in medium to highly serpentinized peridotite. Pentlandite occurs in different textural positions related to several metasomatic stages: (i) polycrystalline aggregates (pentlandite + Cl-apatite + phlogopite + ilmenite + calcite-brucite intergrowths) included in spinel (in garnet); (ii) interstitial in matrix; (iii) in carbonate and serpentine veins. Overall, the S isotope signature of pentlandite exhibits a relatively narrow range between -1.62 and +3.76 ‰. The relatively low S isotope values require a mantle-like source for the metasomatizing fluids enriched in sulfur, with possible contamination with fluids of other different sources. These new results show that sulfur was introduced into the lithospheric mantle and mobilized by influxes of late metasomatic fluids, in part related to the serpentinization, and provide additional constraints on the S isotope composition of the SCLM wedge.

How to cite: Consuma, G., Braga, R., Fiorentini, M. L., Martin, L., Tropper, P., and Aulbach, S.: New constraints on the Sulfur isotope signature of the sub-continental lithospheric mantle wedge: in situ δ34S analyses of pentlandite from the exhumed orogenic garnet-bearing peridotite of the Ulten Zone, Eastern Italian Alps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1077, https://doi.org/10.5194/egusphere-egu2020-1077, 2020.

Grand Comore is the youngest island of the Comoros volcanic chain and it is composed of two alkali shield volcanoes, Karthala and La Grille. Karthala is one of the most active volcanoes of the Indian Ocean (together with Piton de la Fournaise at La Reunion Island) with last volcanic activity recorded in January 2007, while there are no available historic eruptions from La Grille. However, contrary to those of Karthala, La Grille lavas often enclose xenolithic nodules of ultramafic rocks resulting from phreatomagmatic maar-like eruptions. Here we report the first ever analyses of light noble gases (He, Ne and Ar) in fluid inclusions coupled with radiogenic isotopes (Sr, Nd and Pb) of olivine, clinopyroxene and orthopyroxene (hereafter Ol, Cpx and Opx) mineral separates from ultramafic peridotite xenoliths collected at La Grille volcano during 2017-2018 field campaigns with the aim of constraining the mantle source beneath Grand Comore Island. Xenoliths are lherzolites, harzburgites, dunites and wehrlites with a protogranular to porphyroclastic texture, overprinted by Type A, B and C metasomatic reactions (Coltorti et al. 1999). Previous investigations of Grand Comore lithotypes were focused on bulk samples and mineral separates from lavas (i.e., Class et al. 1998; Class et al. 2005), while major and trace element data from clinopyroxenes and glasses from La Grille mantle xenoliths were reported in the literature by Coltorti et al. (1999). The ³He/⁴He isotopic signature in fluid inclusions (up to 7.3Ra) in Ol, Cpx and Opx is in good agreement with that from Class et al. (2005) and falls in a range that overlaps the SCLM (Sub Continental Lithospheric Mantle) and the MORB mantle signature. These values are systematically higher than those measured on gases from crater fumaroles (Istituto Nazionale di Geofisica e Vulcanologia and Institute de Physique du Globe de Paris dataset) and fluid inclusions in olivine phenocrysts from Karthala lavas (Class et al. 2005), indicating that Karthala volcano is still degassing volatiles with a He isotopic signature similar to those in volcanic products of the last eruption. The ²⁰Ne/²²Ne, ²¹Ne/²²Ne and ⁴⁰Ar/³⁶Ar isotope ratios in fluid inclusions are indistinguishable from those of volatiles in typical MORB-type reservoirs. Sr-Nd-Pb systematics in Opx and Cpx from La Grille xenoliths displays higher variability than La Grille bulk lavas (Class and Goldstein 1997; Class et al. 1998). Sr-Nd isotopic ratios of these mantle minerals fall along a mixing line between Depleted MORB and Enriched Mantle reservoirs, but for two samples whose higher Sr isotope signatures point towards an EM2 source. They show isotopic similarities with carbonatite rocks from the East African Rift System and central-northern Madagascar Cenozoic alkaline rocks. These results contribute to highlight the geochemical features of Gran Comore volcanic system (La Grille-Karthala) and its relationships with the underlying mantle, providing useful tools for future geochemical monitoring of an active, dangerous and very poorly explored natural system.

References

Coltorti et al. (1999) – J. Petr., vol. 40

Class & Goldstein (1997) – EPSL 150

Class et al. (1998) - J. Petr., vol. 39

Class et al. (2005) – EPSL 233

How to cite: Ventura-Bordenca, C., Caracausi, A., Di Muro, A., Boudoire, G., Coltorti, M., Faccini, B., Liuzzo, M., Rizzo, A. L., Pik, R., and Aiuppa, A.: Geochemistry of noble gases and radiogenic isotopes of ultramafic mantle xenoliths from La Grille volcano (Grand Comore Island, Indian Ocean), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16145, https://doi.org/10.5194/egusphere-egu2020-16145, 2020.

Phosphorus(P)-rich zones in olivine may reflect incorporation of P in excess of equilibrium partitioning during rapid growth (e.g. Milman-Barris et al. 2008). We investigated (by optical microscopy and electron microprobe) a composite mantle xenolith from the Middle Atlas Mountains (Morocco) containing two lithologies, wehrlite and harzburgite, in direct contact. The host alkali basalt (El Messbahi et al. 2015) is present on the margins of the hand sample but not included in our thin section. Both lithologies display porphyroclastic texture and contain interstitial devitrified glass. Large primary matrix olivine in both wehrlite and harzburgite has P₂O₅ concentrations ≤0.09 wt.% and nearly constant composition, Fo₉₀, except for Fe-rich reaction rims in contact with the interstitial devitrified glass. The P-rich interstitial spaces between these primary matrix olivines consist of devitrified glass, secondary olivine, clinopyroxene, spinel, and apatite. The secondary olivine ranges between Fo_86-93 and is obviously enriched in P₂O₅, with concentrations from 0.36-1.98 wt.%. Whereas matrix clinopyroxene in the wehrlite forms isolated subhedral to euhedral crystals, the interstitial regions contain elongated and dendritic clinopyroxene up to 10 μm long as well as replacive clinopyroxene rims on matrix minerals. Spinel occurs as tiny discrete grains associated with the devitrified glass. Apatite is found only as very small crystals embedded in devitrified glass.

High-resolution X-ray mapping of P in olivine reveals both alternating P-rich bands parallel to crystal elongation and patchy zoning. P⁵⁺ correlates negatively with Si⁴⁺ (R = –0.90) and positively with Na+ (R = +0.73). Correlation with total divalent cations (Mg²⁺+Mn²⁺+Fe²⁺+Ca²⁺+Ni²⁺) is weakly negative (R = –0.44). Although correlation of P⁵⁺ and Al³⁺ is weak (R = -0.42), the combination P⁵⁺+Al³⁺ displays a better anticorrelation with Si⁴⁺ (R = –0.92). Overall, the observed correlations suggest the predominant substitution mechanism is 2 ^IVSi⁴⁺ <=> ^IVP⁵⁺ + ^IVR³⁺, with some additional accommodation by ^IVSi⁴⁺ +^VIM²⁺ <=> ^IVP⁵⁺ + ^VINa⁺.

Because no glass was observed, the apparent olivine/melt partition coefficient could not be directly measured. However, using the maximum P₂O₅ contents (1.05, 1.18 and 2.31 wt%) measured in glass in melt veins from other xenoliths from the a nearby Moroccan volcanic flow (Baziotis et al. 2019) and the P-rich olivines from the present study, we infer a D^P_ol/melt range 0.85-1.88. The most probable value is greater than unity, despite P being incompatible in olivine during equilibrium growth. Such an apparent partitioning suggests that olivine crystallization was rapid enough, ~1-10 K/hour, to develop a P-rich diffusive boundary layer from which the growing olivine incorporated P in excess of equilibrium partitioning with the bulk melt pocket (Grant & Kohn, 2013).

We consider several scenarios for the formation of the interstitial pockets, including partial melting of the xenolith, intrusion of a metasomatic melt in an event earlier than eruption, and reaction with the host lava during ascent.

References

El Messbahi et al. 2015. Tectonophysics 650, 34-52.

Grant, T. B. & Kohn, S. C. 2013. American Mineralogist 98, 1860-1869.

Milman-Barris et al. 2008. Contributions to Mineralogy and Petrology 155, 739-765.

Baziotis et al. 2019. Geochimica et Cosmochimica Acta, 266, 307-331.

How to cite: Baziotis, I., Xydous, S., Asimow, P., Mavrogonatos, C., Flemetakis, S., Papoutsa, A., Klemme, S., and Berndt, J.: Phosphorus-rich olivines in a composite xenolith from Morocco: implications for growth processes , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7207, https://doi.org/10.5194/egusphere-egu2020-7207, 2020.

Peridotite xenoliths from kimberlites provide important information about the composition, structure and thermal regime of the lithospheric mantle of ancient cratons. In this paper, we present the results of mineralogical studies of peridotite xenoliths from kimberlites of the Upper Muna field. The Middle Paleozoic (D3-C1) high diamondiferous kimberlite pipe Komsomolskaya-Magnitnaya was chosen as the object of research.

We studied a collection of 180 peridotite xenoliths of the Komsomolskaya-Magnitnaya pipe, of which 104 belong to dunite-harzburgite paragenesis, 74 to lherzolite and 4 websterites.

The chemical composition of basic minerals from xenoliths was determined using JEOL JXA-8100 electron microprobe. Chemical analysis of xenolith garnet compositions was also performed using the Agilent 7700cs LAM-ICPMS method.

Based on a study of the collection of deep xenoliths, we found that the lithospheric mantle under the Upper Muna kimberlite field is composed mainly of garnet-bearing and chromite-bearing dunites and harzburgites, as well as coarse grained garnet lherzolites.

The olivine Mg# varies from 88.4 to 94.12%, while the magnitude of the majority (60%) of the studied olivines does not exceed 92% and 30% of olivines have Mg#> 93%. We identified 2 groups according Mg # olivine from xenoliths. Group 1 with “typical” mantle values Mg # 88.39-90.70mol%, it is characteristic for fertile peridotites. And group 2 with highly depleted compositions Mg # 91.20-94.12mol%. A high proportion (~ 30%) of peridotites with high magnesian olivines (Mg #> 93 mol%) indicates the presence of a block of highly depleted rocks in the lithospheric mantle beneath the Upper Muna kimberilte field.

According to the distribution of calcium and chromium in garnets, 10 out of 35 studied garnets from xenoliths belong to diamondiferous harzburgite-dunite paragenesis. According to the distribution of rare-earth elements, we distinguish two groups of garnets. Group 1 includes garnets with typical rare earth element distribution spectra typical for fertile garnets, and group 2 garnets with S-shaped spectra that are characteristic of garnet mineral inclusions in diamonds. We noted a high proportion of garnets with S-shaped REE distribution spectra (~ 66%), as well as garnets belonging to the harzburgite-dunite paragenesis, it indicate a moderate role of metasomatic changes associated with silicate melts, as well as interaction with carbonatite melts enriched in LREE.

Using clinopyroxene monomineral thermobarometry, we found that the “diamond” window in the lithosphere mantle beneath the Upper Muna field, at the time of kimberlite magmatism (~ 360 Ma) was significant (about 95 km) and was located at a depth of 125 to 220 km.

The study was supported by the Russian Science Foundation (grant No. 18-17-00249).

How to cite: Iakovlev, I., Malkovets, V., and Gibsher, A.: Mantle xenoliths from the Komsomolskaya-magnitnaya kimberlite pipe (Upper Muna kimberlite field, Siberian Craton) Evidences of the composition of the SCLM, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12666, https://doi.org/10.5194/egusphere-egu2020-12666, 2020.

Metasomatism is omnipresent in subcontinental lithospheric mantle (SCLM). Whatever a distribution scale in the SCLM, it has a strong link to changes in oxygen fugacity (ƒO₂) [1]. It is also known that ƒO₂ of Earth’s interior controls speciation within the C–H–O–S–N system and stability of C-bearing phases (diamond, graphite, carbonates, carbides, volatile-bearing fluid and melt; [2]). Our new geochemical, Mössbauer and Raman spectroscopic results suggest no less than one episode of mantle metasomatism related to the formation and preservation of elemental carbon minerals within Siberian SCLM, while later events are connected to interaction of rocks with hydrous and carbon dioxide components.

We studied a graphite-bearing mantle xenolith from the diamond-free Obnazhennaya kimberlite pipe, Republic of Sakha (Yakutia), Russia, that represents a garnet websterite consisting of garnet (Grt), clinopyroxene (Cpx), orthopyroxene (Opx), graphite, rutile, ilmenite, pyrrhotite, pentlandite, secondary serpentine, phlogopite and carbonates (calcite). Cpx hosts Opx and Grt lamellae. Grt cores contain scarce but oriented mineral inclusions (silicates and Ti-oxides) that we interpret to be exsolved from a Si- and Ti-rich precursor. Linearly distributed melt and fluid inclusions in silicates are thought to postdate the exsolutions. Both major and trace elements of rock-forming silicates match that of peridotites and pyroxenites with exsolutions in Grt and pyroxenes from Obnazhennaya and worldwide. Pressure and temperature estimates (T 910 °C, P 3.5 GPa) also fall into the range in which alike rocks have been equilibrated in the SCLM.

Microstructural and chemical data allowed to propose crystallization of the garnet websterite from high-T Mg-rich magmas similar to komatiite [3], which forms in deep Grt-bearing depleted mantle sources with low oxygen fugacity [4]. Subsequent metasomatism of the reduced websterite by oxidising C-O-H fluids caused graphite precipitation through redox freezing [5], and such reactions constitute an important part of Earth’s hidden carbon cycle. Infiltration of hydrous and CO₂-rich fluids likely postdated this episode.

The work was supported by the Russian Science Foundation (grant No 16-77-10062).

[1] Frost D.J. & McCammon C.A. (2008) Annu Rev Earth Planet Sci 36: 389-420. [2] Stagno V. et al. (2013) Nature 493:84-88. [3] Spengler D. & Alifirova, T.A. (2019) Lithos 326: 384-396. [4] Berry A.J. et al. (2008) Nature 455: 960-963. [5] Rohrbach A. & Schmidt M.W. (2011) Nature 472: 209-212.

How to cite: Alifirova, T. A., Aulbach, S., Korolev, N. M., Golovin, A. V., and Oleinikov, O. B.: Redox events in cratonic mantle underneath Obnazhennaya kimberlite, Yakutia – chemical records in pyroxenites, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18688, https://doi.org/10.5194/egusphere-egu2020-18688, 2020.

Cameroon Volcanic Line (CVL) is a ca. 1600 km long Cenozoic volcanic chain which crosses the boundary between ocean and continent in West Africa. Its origin, as well as  the nature and age of the underlying continental lithospheric mantle (CLM), is still a matter of debate. Some of the CVL lavas contain peridotite xenoliths that can provide data elucidating the role of the CLM in the sustained magma generation along the line. In this abstract we describe xenolith suite from the Befang pyroclastic cone (< 1Ma) in the Oku Massif in the continental part of CVL, consisting of 14 spinel lherzolites, one spinel harzburgite and one websterite. The xenoliths are between 3 and 21 cm in diameter and have porphyroclastic to serial or equigranular texture, with porphyroclasts of olivine or orthopyroxene up to 9 mm in diameter. Some are weakly foliated. Olivine is Fo 88.6-90.4, contains 0.36 to 0.42 wt.% NiO and 180-750 ppm of Ca. Orthopyroxene (Mg# 0.89-0.91) contains 0.14 – 0.19 atoms of Al pfu, and clinopyroxene (Mg# 0.90-0.92) contains 0.24 – 0.31 atoms of Al pfu. The Cr# of lherzolite spinel is 0.09-0.15, in the harzburgitic one it is 0.18-0.19. Pyroxenes in all studied peridotites record a temperature range of 910 – 1010°C (Brey and Köhler 1990). Clinopyroxenes’ REE patterns are flat at HREE-MREE and make a spectrum from slightly LREE-depleted to slightly LREE-enriched (La_N/Lu_N from 0.08 to 2.65). The trace-element patterns are flat except well-defined negative Nb-Ta and positive Th-U anomalies. Orthopyroxenes’ REE patterns are variably depleted from HREE to LREE (LaN/LuN from 0.001 to 0.037). The REE pattern of clinopyroxene occurring in websterite exhibits enrichment from HREE towards LREE with hump in Sm/Nd, typical of silicate melt crystallization. The REE pattern of clinopyroxene The Befang lherzolites represent CLM metasomatised by melts produced by various, but generally low degrees of melting of DMM-like (Depleted MORB Mantle) source. Conversely, the harzburgite was formed by low degrees (few percent) of melting of DMM.

Acknowledgements. The study was funded by Polish National Centre for Science project UMO-2017/27/B/ST10/00365 to JP. EPMA analyses were done thanks to the Polish-Austrian project WTZ PL 08/2018.

References:

Brey, G.P. & Köhler, T. (1990). Geothermobarometry in four-phase lherzolites II. New thermobarometers and practical assessment of existing thermobarometers. Journal of Petrology 31, 1353-1378.

How to cite: Tedonkenfack, S. S. T., Puziewicz, J., Ntaflos, T., Aulbach, S., Kukula, A., Matusiak-Małek, M., Ziobro, M., and Mazurek, H.: Mantle xenoliths from Befang (Oku Massif) in the Cameroon Volcanic Line, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4235, https://doi.org/10.5194/egusphere-egu2020-4235, 2020.

Migration of strategic metals through the lithospheric mantle can be tracked by sulfides in mantle xenoliths. Cenozoic mafic volcanic rocks from the SW Poland (Lower Silesia, Bohemian Massif) host a variety of subcontinental lithospheric mantle (SCLM) xenoliths. To understand metal migration in the SCLM we studied metal budget of peridotites from the Wilcza Góra basanite and their metasomatic history.

The Wilcza Góra xenoliths are especially appropriate to study metasomatic processes as they consist of 1) peridotites with Ol_Fo=89.1-91.5 representing depleted mantle (group A); 2) peridotites with Ol_Fo=84.2-89.2 representing melt-metasomatized mantle (group B), as well as 3) hornblende-clinopyroxenites and websterites with Ol_Fo=77.2-82.5 representing former melt  channels (group C; Matusiak-Małek et al., 2017). The inherent sulfides are either interstitial or enclosed in the silicates. High-temperature exsolutions of pyrrhotite (Po), pentlandite (Pn) and chalcopyrite (Ccp) indicate magmatic origin of the sulfides.

The three peridotitic groups differ by sulfide mode and composition. The sulfide modes are enhanced in group C (0.022-0.963 vol.‰) and group B (<0.028 vol. ‰) with respect to group A (<0.002 vol.‰). The sulfides of group C are Ni-poor and Fe-Cu-rich as reflected in their mineral composition (Po_55-74Ccp_1-2Pn_24-44 in group A, Po_67-85Ccp_1-6Pn_14-33, in group B and Po_80-97Ccp_1-7Pn_2-20 in group C) and major element chemical composition. Ni/(Ni+Fe) of pentlandite is the lowest in group C (~0.25) and the highest in group A (0.54-0.61). Cu/(Cu+Fe) of chalcopyrite is 0.32-0.49 in group C contrasting to~0.50 in groups A and B. 

The sulfide-rich xenoliths of group C indicate an important role of pyroxenitic veins in transporting Fe-Cu-S-rich melts from the upper mantle to the crust. However, the moderately enhanced sulfide modes in melt-mantle reaction zones represented by xenoliths of group B demonstrate that the upper continental mantle is refertilized with these melts during their ascent. Hence, significant portion of S and metals remains in the mantle never reaching the crust, as has been previously observed in the oceanic lithosphere (Ciazela et al., 2018).

Acknowledgments: This study was supported by the NCN project no. UMO-2014/15/B/ST10/00095. The EPMA analyses were funded from the Polish-Austrian project WTZ PL 08/2018.

References:

Ciazela, J., Koepke, J., Dick, H. J. B., Botcharnikov, R., Muszynski, A., Lazarov, M., Schuth, S., Pieterek, B. & Kuhn, T. (2018). Sulfide enrichment at an oceanic crust-mantle transition zone: Kane Megamullion (23 N, MAR). Geochimica et Cosmochimica Acta, 230, 155-189

Matusiak-Małek, M., Puziewicz, J., Ntaflos, T., Grégoire, M., Kukuła, A. & Wojtulek P.   M. (2017). Origin and evolution of rare amphibole-bearing mantle peridotites from Wilcza Góra (SW Poland), Central Europe. Lithos 286–287, 302–323.

How to cite: Mazurek, H., Ciążela, J., Matusiak-Małek, M., Puziewicz, J., and Ntaflos, T.: Fe-Cu-S rich melts in the subcontinental lithospheric mantle: insight from the Lower Silesian (SW Poland) xenoliths, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18544, https://doi.org/10.5194/egusphere-egu2020-18544, 2020.

The Cenozoic volcanic field of Vogelsberg (part of CEVP in Central Germany) is located at the northern extension of the Upper Rhine Graben. Three Variscan basement units underlie Vogelsberg from NW to SE: the Rheno-Hercynian Zone, the Northern Phyllite Zone and the Mid-German Crystalline High. Xenoliths from the Breitenborn basanite sample lithospheric mantle (LM) beneath the Mid-German Crystalline High.

The Breitenborn suite comprises xenoliths of 3-7.5 cm in diameter: clinopyroxene-poor spinel lherzolites, spinel harzburgites and clinopyroxenites. Peridotites exhibit different degrees of deformation: porphyroclastic textures, foliation development and grain size reduction. Mineral components are chemically homogenous at the grain and xenolith scale. Forsterite content (Fo) in olivine ranges between 89.8 and 91.5% with exception of Fo ~89.0% in one xenolith. Orthopyroxene (opx) is characterized by Mg# of 0.900-0.923 and 0.06-0.18 atoms of Al pfu, whereas clinopyroxene (cpx) by Mg# of 0.894-0.931 and 0.11-0.23 atoms of Al pfu. Spinel Cr# ranges from 0.18 to 0.45. Clinopyroxenites exhibit protogranular textures with no deformation. They are significantly less magnesian (cpx Mg# 0.834-0.863) and more aluminous (0.25-0.31 atoms of Al pfu) than peridotites.

Peridotite cpx REE patterns show different degree of enrichment in LREE, except two xenoliths being strongly depleted in LREE. Opx from those two xenoliths exhibits patterns steeply depleted from HREE to LREE. The remaining opx shows mild depletion in LREE relative to HREE or slight LREE enrichment.

Temperatures calculated using REE content (T_REE) [1] range between 1030 and 1130°C for most of the xenoliths and show that pyroxenes are in REE equilibrium. Exceptions are LREE-depleted xenoliths which have 940-975°C and exhibit no LREE equilibrium. Temperatures calculated on the basis of pyroxene major element contents (T_BKN) [2] are ~40-140°C lower than T_REE.

During Cenozoic rifting which formed the Upper Rhine Graben, a diversity of melts interacted with the LM beneath Vogelsberg. LREE-enriched cpx and opx patterns suggest metasomatic alteration of LM by alkaline melts, which is typical of other studied sites in the area. A calculated hypothetical melt in equilibrium with clinopyroxenite cpx patterns resembles those of basanites and alkaline basalts occurring in Vogelsberg, which were possibly involved in the alkaline metasomatism of the LM. Varying discrepancy between T_REE and T_BKN indicate that the xenoliths experienced cooling after melt metasomatism of the LM, which was not followed by recrystallisation. Different degrees of LREE enrichment and gradual changes in major element compositions of peridotite minerals indicate the chromatographic character of the alkaline metasomatism. Strongly LREE-depleted cpx and opx patterns probably are effects of metasomatism by melts derived from depleted MORB mantle, which are typical products of advanced melting in continental rifting environments.

The study was funded by Polish National Science Centre to MZ (project UMO-2018/29/N/ST10/00259). EPMA analyses were done within the frame of the Polish-Austrian project WTZ PL 08/2018. MZ acknowledges the DAAD fellowship at the Goethe University in Frankfurt.

References

[1] Liang Y. et al. (2013). GeochimCosmochimAc 102, 246–260.

[2] Brey G. & Köhler T. (1990). JPetrol 31, 1353–1378.

How to cite: Ziobro, M., Puziewicz, J., Aulbach, S., Ntaflos, T., and Matusiak-Małek, M.: Lithospheric mantle beneath the Mid-German Crystalline High Variscan unit: Breitenborn (Vogelsberg, Central Germany) case study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7902, https://doi.org/10.5194/egusphere-egu2020-7902, 2020.

Mirdita Ophiolite in northern Albania is a part of 30-40 km wide ophiolitic Pindos Zone in Dinaride-Hellenide part of the Alpine orogenic system (e.g. Dilek & Furnes 2009, Lithos). Mantle and crustal sections in the eastern part of this zone have Supra-Subduction Zone geochemical affinities. The goal of our study is to examine chemical diversity of rocks within Kukesi Massif and to decipher its evolution.

The Kukesi Massif is composed mostly of coarse- to medium-grained spinel harzburgites and dunite with chromite layers (e.g. Morishita et al. 2011, Lithos), locally  cross-cut by orthopyroxenite veins. Uppermost part of the sequence consist of cumulate pyroxenites and peridotites (composed of olivine, orthopyroxene, clinopyroxene and spinel). Most of the rocks are pervasively serpentinised, but degree of serpentinisation varies within the massive. Samples of peridotites and pyroxenites from over a dozen localities within the massif were collected.

Olivine occurring in the lower sections of the ophiolite has composition of Fo_89.5-91.2 (NiO 0.28-0.52 wt.%) in peridotites and Fo_90.6-92 (NiO 0.38-0.52 wt.%) in orthopyroxenite veins. Olivine forming cumulates has Fo_82.4-83.3 and NiO content=0.12-0.23 wt. %. Orthopyroxene (enstatite) in mantle peridotites is Al-poor (0.05-0.08 Al a.p.f.u.) and has Mg# 90.5-91.5. Orthopyroxene from peridotite cut by orthopyroxenite veins is even poorer in Al (0.03-0.04 a.pfu) and has lower Mg# 91.1-91.7 and is chemically indistinguishable from pyroxenitic orthopyroxene. Orthopyroxene forming cumulates has Mg#=82.3-84.0 and the highest Al content among all the lithologies (0.12-0.14 a.p.f.u.). Peridotitic clinopyroxene (diopside) has Al=0.02-0.08 a.p.f.u. which corresponds well to this in orthopyroxene, but Mg# is higher – 92.5-95.4. Clinopyroxene in cumulate rocks has Al content=0.13-0.16 a.p.f.u. and Mg#=87-88. Spinel in mantle peridotites has Cr#=0.47-0.80 and is negatively correlated with Mg# (0.38 to 0.56). The cumulative spinel has lower Cr# (0.18-0.27), but the  Mg# is similar to that forming peridotite (0.38-0.45). 

The orthopyroxene equilibration temperatures calculated with Witt-Eickschen & Seck (1991, CMP) algorithm, yield wide range of temperatures (800-950˚C in mantle peridotites and 950-1020˚C in cumulate peridotites suggesting its magmatic origin). Low Al content in orthopyroxene suggest that peridotites suffered from high degree of melt extraction.

Chemical composition of minerals forming rocks of Kukesi Massif is typical  for mantle sections of SSZ ophiolites (e.g. Troodos ophiolite, Batanova & Sobolev 2000, Geology). Our preliminary mineral chemical data for Kukesi ultramafics have a wider range than those previously obtained by Morishita et al. (2011, Lithos). The chemical composition of ultramafic rocks within this massif varies, which may result from variable geochemical history, but further studies are required to fully characterize the composition of Kukesi ultramafics and to reconstruct its geochemical and tectonic evolution.

This study was financed from scientific funds for years 2018-2022 as a scientific project within program “Diamond Grant” (DI 024748).

How to cite: Mikrut, J., Matusiak-Małek, M., Puziewicz, J., and Onuzi, K.: Mineralogy and petrology of ultramafic section of Kukesi Massif, Mirdita Ophiolite (Albania) – preliminary results, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13813, https://doi.org/10.5194/egusphere-egu2020-13813, 2020.

The northernmost part of Scotland – the Hebridean Terrane – is formed of Archean rocks originally being part of the Laurentian North Atlantic Craton. The geological history of the terrane is well recognised, however details of its internal structure remain unknown. The Eocene (Faithfull et al. 2012, JGS) Loch Roag monchiquite (Lewis Island) sampled deep-seated lithologies, providing insight on evolution and geological structure of the deeper lithosphere of the Hebridean terrane. The monchiquite comprises abundant xenoliths of ultramafic, mafic and felsic rocks. The peridotitic xenoliths represent pieces of Archean mantle underlying marginal parts of the North Atlantic Craton, whereas the origin of non-peridotitic lithologies is uncertain.

The studied suite of samples comprises two groups: 1) “xenoliths” of diorites (plagioclase, clinopyroxene, orthopyroxene, apatite, opaques) and biotite clinopyroxenites (+apatite), 2) “megacrysts” of clinopyroxene and K-feldspar, both with inclusions of clinopyroxene, biotite and apatite. Megacrysts of alkali-rich feldspar associated with corundum and HFSE-bearing minerals, and composite xenoliths formed of pyroxenite and K-feldspar-rich lithology have also been described from this locality (Menzies et al., 1986, Geol. Soc. Australia Spec. Pub.; Upton et al., 2009, Mineral. Mag.).

We interpret the “xenoliths” as products of crystallization of fractionated mafic melt(s). The primary character of Sr isotopic ratios in plagioclase (⁸⁷Sr/⁸⁶Sr <0.702) suggests that parental melt of those lithologies originated from melting of depleted lithospheric mantle sources. The “megacrysts” represent fragments of disintegrated alkaline pegmatite(s) formed from melt of plausible mantle origin, possibly enriched (⁸⁷Sr/⁸⁶Sr in feldspar >0.704).

Trace element composition, similar Sr isotopic ratios of minerals and textural features of “xenoliths” and “megacrysts” groups suggest their close genetic relationship. This geochemical resemblance may reflect crystallisation from primarily similar melt(s) and source regions affected by similar metasomatism. Petrographic features observed in rocks described by Upton et al., (2009) imply that the parental magma of megacrysts might have intruded the rocks forming the xenoliths group. Moreover, the Rb-Sr ages of xenoliths (Der-Chuen et al., 1993, GCA) indicate crystallisation during (or shortly after) Caledonian orogeny. Preliminary age relationship between groups will be determined by on-going Rb-Sr dating of megacrysts.

Xenoliths similar to diorites from Loch Roag were reported by Badenszki et al. (2019, JoP) from the Midland Valley terrane (“metadiorites” of protolith ages ca. 415 Ma). They were interpreted as products of alkaline syn-/post-collisional Caledonian magmatism. Our study shows that non-peridotitic xenoliths from Loch Roag dyke might represent a record of similar (or the same) magmatism in the northernmost, “Laurentian” part of Scotland. This study presents the first report of such Caledonian magmatism record within the Hebridean Terrane.

Founded by Polish National Science Centre grant no. UMO-2016/23/B/ST10/01905, part of the data was obtained thanks to the Polish-Austrian project no. WTZ PL 08/2018.

How to cite: Buczko, D., Matusiak-Małek, M., Upton, B. J. G., Ntaflos, T., Aulbach, S., Grégoire, M., and Puziewicz, J.: Caledonian magmatism record within Hebridean Terrane? Loch Roag dyke (Lewis Island, northern Scotland) non-peridotitic xenoliths and megacrysts as messengers from deep lithosphere. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14255, https://doi.org/10.5194/egusphere-egu2020-14255, 2020.

EN-101, a rare albitite [Pl +Fe-Ti oxide +Ap +Zrn] xenolith from Elie Ness, Scottish Midland Valley, is hosted by a c. 290 Ma old alkali basaltic diatreme [1, 2].  EN-101 is considered to belong to the Scottish “anorthoclasite suite” comprising xenoliths and megacrysts of various compositions which are interpreted as samples from the upper mantle – lower crust where they form (syenitic) vein or dyke-like bodies e.g., [3, 4, 5]. The “anorthoclasite suite” has been found in all Scottish terranes suggesting that the presumed dyke system must be extensive.

Xenoliths of the “anorthoclasite suite” primarily consist of Na-rich and Ca-poor feldspar megacrysts, with generally high Na/K ratios [3] that are typically accompanied by accessory zircon, apatite, biotite, magnetite and Fe-rich pyroxene whereas garnet and corundum with Nb-rich oxides are only occasionally present [3, 4, 5]. Upton et al. [4, 5] argued that the parental melt of the “anorthoclasite suite” formed though small–fraction melting of metasomatized mantle and subsequent melt–solid phase reaction was also involved.  Upton et al. [5] proposed that crystallization of the anorthoclasite suite samples occurred shortly prior to- or contemporaneously with their entrainment. However so far no in-situ dating has been carried out on these samples.

Early attempts to date the anorthoclasite suite using zircon and feldspar megacrysts from Elie Ness suggested at least a two-stage formation mechanism, where zircon megacrysts yielded a U-Pb age of c. 318 Ma, while euhedral feldspar xenocrysts are significantly younger and roughly coeval with the host volcanism yielding a K-Ar whole-rock age of c. 294 Ma [6].  In this study we present the first in situ U-Pb dating of zircon, which yielded a concordia age of 328 ± 2 Ma (MSWD=0.19; n=12) for EN-101. Zircons εHf₃₂₈ values range from +5.2 to +7.5 consistent with a mildly depleted source refreshed by metasomatism. These results may indicate that the proposed extensive syenitic veining within the Scottish upper mantle not only has a complex source [5], but is possibly the result of repeated episodes of magma intrusion.

References:

1. Gernon, T.M. et al. 2013 Bulletin of Volcanology. 75:1-20.
2. Gernon, T.M. et al. 2016 Lithos. 264:70-85.
3. Aspen, P. et al. 1990 European Journal of Mineralogy 2:503-17.
4. Upton, B.G.J. et al. 1990 Journal of Petrology.40:935-56.
5. Upton, B.G.J. et al. 2009 Mineral Mag. 73:943-56.
6. Macintyre, R.M. et al. 1981 Transactions of the Royal Society of Edinburgh: Earth Sciences. 72:1-7.

How to cite: Badenszki, E., Daly, J. S., Whitehouse, M. J., and Upton, B. G. J.: Age constraints for rare felsic mantle xenoliths from Elie Ness, Scottish Midland Valley, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20848, https://doi.org/10.5194/egusphere-egu2020-20848, 2020.

Field relationships, mineralogy, petrology, geochemistry, geochronology, and Nd-Hf-O isotopes of the mafic-ultramafic rocks from the east part of the Qilian block are studied in the present work. The Aganzhen intrusive body only exposed in the Zhigoumen, Shiguanzi, Xianggoumen outcrops and includes Hornblende peridotite, wehrlite, olivine-bearing pyroxenite, hornblende-bearing pyroxenite, websterite, clinopyroxenite, hornblendite, olivine-bearing gabbro. The gabbroic rocks are also layered or massive cumulates with rock types varying continuously from noritic gabbro through hornblende gabbro to dioritic norite. Contact metamorphic zones are well developed between the Aganzhen intrusive body and the country rock. Major element contents of Aganzhen ultramafic-mafic rocks show subalkalic series and are characterized by low SiO₂ contents (38.09-54.96 %), low TiO₂ contents (0.09-0.72 %), low P₂O₅ contents (0.00-0.36 %) and alkali contents (Na₂O+K₂O 0.01-5.35 %), but high MgO contents (9.68-33.06 %), Ni contents (116-1505 ppm), Cr contents (713-2808 ppm). Similar LREE-rich pattern ((Ce/Yb)_N =0.95-3.80 except two Samples) and tiny Eu anomaly (Eu/Eu* =0.6-1.2) indicate the Aganzhen ultramafic-mafic rocks have the same magma source. Trace elements are enriched in LILE (Rb, Th, U, K), relatively depleted in HFSE (Nb and Ta), and the La/Yb, Ce/Yb, Th/Yb, Nb/La, La/Sm values suggest the limited crustal contamination during the rise of the magma. The ε_Nd (430 Ma) values are −6.9–+2.5 and T_DM values are 3.6–1.4 Ga. The SHRIMP ages are 433±2 Ma for the Zhigoumen websterite(101-2101A), 434±3 Ma for Shiguanzi hornblendite(101-2104A) and 412±3 Ma for the Xianggoumen serpentinite(101-2107A). In situ zircon O-Hf isotope, the δ¹⁸O compositions of vary from +9.03 to +9.50 (except three points +11.33, +12.38, +12.44) and ε_Hf(t) value is +0.29 to +4.13 for the Zhigoumen pyroxenite(101-2101A), the δ¹⁸O compositions of vary from +6.39 to +7.12 and ε_Hf(t) value is +7.76 to +13.26 for Shiguanzi gabbro(101-2104A), and the δ¹⁸O compositions of vary from +4.68 to +5.31 and ε_Hf(t) value of +0.28 to +2.79 for the Xianggoumen serpentinite(101-2107A). According to the above datum, we suggest that middle Paleozoic magmatisms last ~20 m.y. (434-412 Ma) on the northern margin of the Qilian Block was related to the Early Paleozoic continental collision between the Qilian and Alax blocks, and to subsequent subduction and thermal underplating.

How to cite: Tung, K.-A., Yang, H.-Y., Yang, H.-J., Zhang, J., Liu, D., and Li, X.: Geochronology and geochemistry of late Silurian-early Devonian mafic-ultramafic complexes in the eastern section of Qilian block, NW China: Implications for late early Paleozoic tectonic evolution of the Qilian orogeny belt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10602, https://doi.org/10.5194/egusphere-egu2020-10602, 2020.

Neoproterozoic ophiolites in the Eastern Desert (ED) of Egypt are pervasively carbonated and listvenitized. Two types of carbonation are recognized: 1) intergrown magnesite (and to lesser extent dolomite) with serpentine and talc that in cases form pure carbonate veins, and 2) cryptocrystalline magnesite veins filling the fractures crosscutting other ophiolitic host rocks. Few studies address the conditions of carbonate alteration of ultramafic rocks, especially the temperature of altering fluids. We employ clumped isotope thermometry on natural dolomite and magnesite from 17 variably carbonated ophiolitic rocks and veins in the ED. Five samples of antigorite-bearing serpentinite, talc-carbonate, and associated carbonate veins yield wide range temperatures of magnesite and dolomite between 213 to 426°C (285±73°C). These temperatures are comparable with previous fluid inclusion thermometry carried out on some of the vein samples (homogenization temperature between 225 to 383°C; Boskabadi et al. 2017). Ten samples of fully quartz-carbonate altered peridotites (i.e. listvenites) record even a wider range of clumped isotope carbonation temperatures between 90 and 452°C (227±112°C). In contrast, two samples of late-stage veins of cryptocrystalline magnesite record lower temperatures of 19 and 28°C. While the constraints on the pressure of carbonation are lacking, the wide range of temperatures for the carbonates in antigorite-bearing serpentinite, talc-carbonate, and listvenite lithologies suggest that carbonation probably occurred at variable depths, whereas the low temperature of cryptocrystalline magnesite veins points to conditions nearer the surface most likely associated with post-obduction processes. Therefore, different sources of carbon and CO₂-bearing fluids should have been responsible for the formation of high- and low-temperature carbonates in the region.

  Boskabadi et al. 2017. International Geology Review 59, 391–419.

How to cite: Boskabadi, A., Kluge, T., Pitcairn, I., Ali, R., Azer, M., Maurice, A., Stern, R., Bakhit, B., Shahien, M., and Zoheir, B.: Temperatures of Neoproterozoic Regional Carbonate Alteration in the Eastern Desert of Egypt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18336, https://doi.org/10.5194/egusphere-egu2020-18336, 2020.

3. Ducea, et al.,2013, GEOLOGY, Vol:41, 413-417

This study   is financially supported by Hacettepe University, BAB project no: FHD-2018-17283

How to cite: Kurkcuoglu, B. and Yurur, T.: New insights for the mantle source components of the most primitive recent basaltic rocks from central and western Anatolia: Evidences for the involvement of pyroxenite and the peridotite source domains , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8074, https://doi.org/10.5194/egusphere-egu2020-8074, 2020.

The current flux of nitrogen into the mantle in subduction zones is about three times its amount outgassing at mid-ocean ridges, arc and intraplate volcanoes, i.e., some efficient nitrogen hosts and carriers should exist in slabs. The K⁺ → (NH₄⁺) substitution in silicate minerals is possible only within limited redox-favorable parts of slabs. Whether nitrogen can be transported and immobilized in the mantle as part of solids by some redox-independent mechanisms? The experimental study of the muscovite-NH₃-N₂-H₂O and eclogite+muscovite-NH₃-N₂-H₂O systems at 6.3-7.8 GPa and 1000 to 1200°C shows that NH₃- and N₂-rich K-cymrite can be stable in metapelite and act as a redox insensitive carrier of nitrogen to mantle depths >200 km in downgoing slabs. This ability is related to its unique clathrate structure that can accommodate three species of nitrogen: N₂ and NH₃ molecules in cages and (NH₄)⁺ substituting for K⁺, while imprisoned N₂ and NH₃ were first discovered in cages of ultra-high pressure minerals. The storage capacity K-cymrite with respect to nitrogen increases from 2.9 to 6.3 wt.% with increase of fO₂ from ~IW to ~NNO, at the N₂/(NH₃+N₂) ratio in fluid from 0.1 to 0.9. Comparison of equilibrated muscovite and K-cymrite synthesized at 7.8 GPa, 1070°C, and fO₂ ~IW demonstrates that the clathrate mechanism of nitrogen entrapment by aluminosilicates (in the form of N₂ and NH₃ molecules) is much more efficient than the K⁺ ® (NH₄⁺) substitution even in strongly reduced conditions. The presence of an N-bearing fluid in the studied systems stabilizes the K-cymrite structure. Muscovite does not convert to K-cymrite in the absence of NH₃-N₂-bearing fluid within 7.8 GPa and 1070-1120°C. Our estimates of normalized volume per non-hydrogen atom show that N₂-bearing cymrite is the densest in the series of K-cymrite with cages filled to different degrees: K-Cym_NH3 > K-Cym_H2O > K-Cym_N2 and is thus the most stable among cymrite-type compounds under high pressure.

The research was performed by a grant of the Russian Science Foundation (16-17-10041).

How to cite: Sokol, A., Kupriyanov, I., Seryotkin, Y., and Sokol, E.: K-cymrite as Redox Insensitive Transporter of Nitrogen in the Mantle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3317, https://doi.org/10.5194/egusphere-egu2020-3317, 2020.