The Hessian Depression in Germany represents the northern continuation of the Upper Rhine Graben and is known for Cenozoic alkaline basaltic lavas. Many of these carry peridotite xenoliths of mantle origin, which were studied mainly in the  80-ies of the XX century (Hartmann & Wedepohl 1990 and references therein). These studies documented mantle lithosphere melting followed by metasomatism. Here we describe the xenoliths from the Stöpfling quarry near Homberg upon Efze. The quarry has been recultivated and sampling is not possible now, our samples come from the archival collection of the Department of Geochemistry of the University of Göttingen. In this abstract, we give an overview of newly collected major- and trace-element mineral-chemical data from 11 xenoliths.

The xenoliths from Stöpfling are spinel-facies lherzolites and harzburgites. They consist of aggregates of few coarse (typically 4-6 mm across) grains of olivine and orthopyroxene embedded in fine-grained matrix of olivine, ortho- and clinopyroxene and spinel. Coarse-grained aggregates represent fragments of protogranular texture and are volumetrically prevailing. Spinel is commonly interstitial and has amaeboidal morphology. Locally, centimetre-thick layers of websterites cross-cut the peridotites. Hartmann & Wedepohl (1990) report traces  (<1 vol. %) of phlogopite in 2 of 12 lherzolites they studied.

The major element composition of olivine is strikingly homogeneous in all studied rocks (91±0.5 % forsterite and 0.40 wt. %. NiO), Ca content is < 500 ppm. Orthopyroxene is mildly aluminous (0.10-0.17 atoms of Al per formula unit, [pfu]) as is clinopyroxene (0.12-0.25 atoms Al pfu). Spinel Cr# [= Cr/(Cr+Al)] varies from 0.18 to 0.46. Clinopyroxene coexisting with spinel of low Cr# is Al-rich and contains 1600-2200 ppm Ti, whereas that coexisting with spinel of higher Cr# is less aluminous and contains 600-1200 ppm Ti. Clinopyroxene coexisting with spinel of Cr# 0.46 is extremely impoverished in Ti (50 ppm). The REE patterns of clinopyroxene in most samples are above the primitive-mantle (PM) level, are LREE-enriched and flat at MREE-HREE. Those extremely depleted in Ti show a decrease from HREE towards MREE, the contents of which are below PM level, and are strongly LREE-enriched.

Peridotites from Stöpfling consist of olivine which chemical homogeneity  across the xenolith suite supposedly records melt depletion. The variable content of Al in orthopyroxene from different samples probably is due to subsequent refertilization event(s) involving silicate melt, whereas the REE characteristics of clinopyroxene suggests that it was additionally cryptically  metasomatized. The unaffected olivine composition indicates low ratio of metasomatic agent to protolith.

Acknowledgements. JP is grateful to G. Wörner for enabling access to the xenoliths from Stöpfling that were originally collected by H. Wedepohl and are now archived at the Geochemistry and Isotope Geology Division of the Geoscience Center at University Göttingen (GZG).

Funding. This study originated thanks to the project of Polish National Centre of Research 2021/41/B/ST10/00900 to JP.

References:

Hartmann G., Wedepohl. K.H. (1990): Metasomatically altered peridotite xenoliths from the Hessian Depression (Nortwest Germany). Geochim. Cosmochim. Acta 54: 71-86.

How to cite: Puziewicz, J., Aulbach, S., Matusiak-Małek, M., Ntaflos, T., and Ziobro-Mikrut, M.: Peridotite xenoliths from Stöpfling in Hessian Depression (Germany) revisited, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2878, https://doi.org/10.5194/egusphere-egu23-2878, 2023.

The 3.5-0.5 Ma Devès volcanic field is located in the “southern” mantle domain of the French Massif Central (MC), which originated by partial melting, likely followed by refertilization by melts from the upwelling asthenosphere [1, 2]. However, the extent of melting versus degree of refertilization remains unclear. In order to obtain new insights into this fundamental question, we studied a large mantle xenolith population (n – 21) from a cinder cone in the NW of Devès, the Mt. Briançon nepheline basanite. Extensive use of EMPA and LA-ICP-MS allowed us to gather a comprehensive and representative dataset. Here, we present preliminary interpretations. Ongoing EBSD analyses will provide further data to confirm or correct our hypothesis.

The lithospheric mantle (LM) beneath the Devès is heterogeneous. It contains lherzolite with clinopyroxene (cpx) exhibiting REE patterns with relatively flat Lu-Eu and variable LREE-depletion. The coexisting spinel (spl) is highly aluminous (Cr# 0.09-0.15). By analogy with prior work [2], we suggest that cpx and spl were added to the rock by a MORB-type melt [2]. Those lherzolites probably represent refertilized LM similar to the Lherz massif [3], which obscures the original degree of depletion.

A distinct mantle region below the Devès is represented by harzburgites and cpx-poor lherzolites containing cpx with REE patterns that show moderately increasing Lu-Sm and steeply increasing towards La. The coexisting spl has medium to high Cr# (0.17-0.28). We suggest that this lithology was not refertilized by MORB-like melts, but records some other metasomatic event(s).

A single harzburgite xenolith contains LREE-enriched cpx similar to those described above, but of significantly lower element abundances. This harzburgite is the most magnesian in the entire suite, with olivine Fo ~91.2% and Mg# in pyroxenes ~0.92 (vs Fo 88.5-90.4% and Mg# 0.88-0.91 for other peridotites). Pyroxenes have the lowest Al, Fe, Ti, Na contents in the whole suite and spinel is the most chromian (Cr# ~0.43). This rock resembles harzburgites from the northern domain of the MC, interpreted as a relatively depleted residue of partial melting [4].

This study was funded by Polish National Science Centre to MZM (UMO-2018/29/N/ST10/00259).

References:

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

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

[3] Le Roux et al. (2007). EPSL 259, 599–612.

[4] Downes et al. (2003). Chem Geol 200, 71–87.

How to cite: Ziobro-Mikrut, M., Puziewicz, J., Aulbach, S., and Ntaflos, T.: The lithospheric mantle beneath Devès volcanic field – case study of mantle xenoliths from Mt. Briançon (Massif Central, France), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14795, https://doi.org/10.5194/egusphere-egu23-14795, 2023.

The Mirdita Ophiolite (N Albania) consists of two meridional belts of different geochemical affinities: supra-subduction zone for the Eastern belt and mid-ocean ridge (MOR) for the western belt. Puke Massif described in this study is a mantle dome belonging to the MOR belt.

Structurally, the Puka Massif is interpreted as an Oceanic Core Complex formed of harzburgites cross-cut by dunitic channels grading to mylonitized plagioclase and amphibole bearing lherzolites with minor dunites and chromitites at the top of the section. The massif experienced an intense magmatic activity evidenced by gabbroic and pyroxenitic dykes. Field and petrographic evidences revealed that plagioclase, clinopyroxene and amphibole in lherzolitic mylonites crystallized from impregnating melts (Nicolas et al. 1999, 2017). Scientific question behind our study is whether this conclusion is confirmed by geochemical data.

Clinopyroxene from magmatic veins cross-cutting mylonites, has trace elements (TE) composition identical to that from the host peridotite. In general, 3 types of TE patterns can be identified in the veins and mylonites: 1. Strongly depleted (Yb=0.3-0.6x primitive mantle, PM, McDonough & Sun 1995); 2. Intermediate (Yb=1.1-4xPM); 3. Enriched (Yb=5-11xPM). The group 1 comprises only pyroxenites. Two relatively undeformed harzburgites occurring in the lowermost section of the mantle dome contain TE-poor clinopyroxene. One, which is amphibole-bearing, exhibits TE pattern resembling that in group 1, while the other one shows even more depleted signature, with Yb=0.8-1.3xPM and La <0.001xPM. Intrusive rocks from groups 2 and 3 are widespread in the whole massif while the occurrences of the depleted group are restricted to the lowermost sections. Rocks from different groups may occur within a single outcrop.

The TiO₂ content in clinopyroxene mimics the TE-based division of the rocks. Clinopyroxene in the group 1 and harzburgites has TiO₂<0.1 wt.%, whereas that from group 2 and 3 has 0.1<TiO₂<0.5 wt.% and TiO₂>0.5 wt.%, respectively. Similar relationships are observed in the composition of spinel, which has TiO₂<0.1 wt.% in group 1 rocks, 0.1 - 0.25 wt.% in group 2 and between 0.1 and 2.0 wt.% in the group 3 rocks.

As magmatic rocks and deformed peridotites share common clinopyroxene TE trends, as well as similar Ti variations in clinopyroxene and spinel, geochemical data support impregnating origin of mylonites. Impregnating melts, differing in enrichment level, were active within whole massif; only the most depleted seem to be restricted to some of its parts. Only internal or easternmost harzburgites could have escape magmatic impregnations; these samples are relatively undeformed and have depleted melting-like TE trends. These findings are in agreement with melt impregnation origin of mylonites. Presence of the depleted lithologies supports primarily harzburgitic origin of the massif, later followed by mylonitization of some of its part. 

This study was financed as a project within program “Diamond Grant” (DI 024748).

How to cite: Mikrut, J., Matusiak-Małek, M., Gregoire, M., Ceuleneer, G., Onuzi, K., and Puziewicz, J.: The impact of melt impregnation on the genesis of mantle peridotites from Puke Massif (Mirdita Ophiolite, Albania) revealed by geochemical data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13627, https://doi.org/10.5194/egusphere-egu23-13627, 2023.

Large, single crystals (> 1cm) are a familiar component of mantle xenolith suites of many kimberlites.  Confusion between different suites exists in the literature, however, which affects petrogenetic models, and some clarification is warranted.  Megacrysts of the Cr-poor suite[1] are most common.  Cr-poor silicates (garnet, clinopyroxene, orthopyroxene, olivine) are characterized by lower Mg/(Mg+Fe) and Cr₂O₃ and higher TiO₂ values than typical mantle peridotite minerals.  Strong geochemical trends in most occurrences of Cr-poor megacryst suites (e.g., concomitant decrease in Mg/(Mg+Fe) and Cr₂O₃) are interpreted by most authors as the result of fractional crystallization of a kimberlite, or kimberlite-like, magma.   

The Cr-rich megacryst suite, comprising garnet, clinopyroxene, orthopyroxene and olivine, but not ilmenite, was described from the Sloan/Nix kimberlites in northern Colorado[1].  Constituent minerals, all four of which are essential to the definition of the suite, are characterized, in part, by high and restricted values of Mg/(Mg+Fe) and wt% Cr₂O₃ (e.g., 0.791 to 0.837 and 6.1 to 13.0, respectively, in garnet [2]).  Elsewhere, large crystals with Mg/(Mg+Fe) and Cr₂O₃ values higher than Cr-poor suite minerals do occur, but none correspond to the Sloan-Nix Cr-rich suite in paragenesis, size and/or composition[2].  For example, almost no garnet megacrysts described as “Cr-rich” or “high-Cr” from other localities (e.g., refs 3-6) contain >6 wt% Cr₂O₃ and even garnets with <2 wt% Cr₂O₃ are termed “Cr-rich” or “high-Cr”.  Most, or all, of these so-called “Cr-rich garnet megacrysts” are simply xenocrysts from coarse-grained peridotite. 

The “Granny Smith” suite, first described from Kimberley and Jagersfontein [7], is dominated by Cr-clinopyroxene associated with phlogopite (and ilmenite at Kimberley), with uncommon olivine or rutile.  Garnet and orthopyroxene do not occur in this suite, which is neither equivalent to nor a subset of the Cr-rich megacryst suite.  Other suites dominated by Cr-clinopyroxene, also not shown to coexist with garnet and orthopyroxene, have been described from Orapa and Bobbejaan [6] and Grib [8], though analogies have been drawn with the Cr-rich megacryst suite despite compositional and paragenetic differences.  A similar megacrystalline assemblage (Cr-cpx, ilmenite, phlogopite, olivine) has been described from Attawapiskat [9] and at Balmoral megacrysts of Cr-cpx occur with ilmenite, Nb-Cr rutile and zircon [10].

All of these suites of Cr-cpx +/- ilmenite, rutile, phlogopite, olivine, zircon (lacking garnet/opx), though varied, have more in common with each other than with the Cr-rich megacryst suite.  All might be best termed “Granny Smith”, and may have common origins.  The only feature they share with the Sloan-Nix Cr-rich megacryst suite is the presence of large chromian clinopyroxene.  Use of such populations as equivalents of the Sloan-Nix Cr-rich megacryst suite in mantle petrogenetic schemes can lead to faulty conclusions. 

References:  1) Eggler et al. (1979) The Mantle Sample, 2) Schulze (2022) Goldschmidt Conf. Abstr., 3) Hunter and Taylor (1984) Am. Min., 4) Kopylova et al. (2009) Lithos, 5) Bussweiller et al. (2018) Min. Pet., 6) Nkere et al. (2021) Lithos, 7) Boyd et al. (1984) GCA, 8) Kargin et al. (2017) Lithos, 9) Hetman (1996) MSc., 10) Schulze, unpub. data. 

How to cite: Schulze, D.: Megacryst suites in kimberlite, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16528, https://doi.org/10.5194/egusphere-egu23-16528, 2023.

The Edessa ophiolite complex represents remnants of oceanic lithosphere which was thrust out of one or more ocean basins during Upper Jurassic to Lower Cretaceous time. Petrographic, geological and geochemical evidences indicate that this ophiolite complex consists of both mantle and crustal suites. It includes lherzolites, serpentinised harzburgites with high degree of serpentinisation, diorites, gabbros, diabase dolerites and basalts. We present here new data on mineral compositions and geochemistry in mafic rocks. The basalt displays N-MORB composition, having enhanced TiO₂ (1.9-2.4 wt.%) and flat REE patterns, whereas the gabbros show E-MORB affinities, having moderate to high Ti content (TiO₂ = 1.1-1.2 wt.%) with strong LREE-HREE fractionations. Such geochemical enrichment from N-MORB to E-MORB composition indicates mixing of melts derived from a depleted mantle and fertile mantle source at the spreading centre. On the other hand, diorites and partially diabase dolerites display SSZ-type composition with low Ti content (TiO₂ = 0.1-0.7 wt.%) and depleted LREE pattern with respect to HREE. They also display high Ba/Zr, Ba/Nb and Ba/Th ratios relative to primitive mantle, which strongly represents the melt composition generated by partial melting of depleted lithospheric mantle wedge influenced by hydrous fluids derived from subducting oceanic lithosphere in a forearc setting. Based on these geochemical evidence, we suggest that mid ocean ridge (MOR) type mafic rocks (basalts and gabbros) from the Edessa ophiolite represent the section of older oceanic crust which was generated during the opening of the Axios Ocean. Conversely, the diorites and diabase dolerites represent the younger oceanic crust which was formed at the forearc region by partial melting of the depleted mantle wedge modified by hydrous fluids released from the subducting oceanic slab.

How to cite: Rogkala, A., Petrounias, P., Koutsovitis, P., Giannakopoulou, P. P., Pomonis, P., and Hatzipanagiotou, K.: Geochemical characteristics of mafic rocks from the Edessa ophiolite (North Greece): Implications for their petrogenesis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10026, https://doi.org/10.5194/egusphere-egu23-10026, 2023.

The Khrami crystalline massif is located in the northern part of the Lesser Caucasus, in the Black Sea - Transcaucasian terrane. The massif outcrops Middle Paleozoic metabasites, which crosscut the Precambrian gneiss-migmatite complex and, in turn, are cut by Late Variscan granites. These metabasites have not experienced Precambrian prograde HT/LP (720-770°C, P<1.5 kbar) regional metamorphism, although retrograde LT/LP (T≈430-510⁰C, P≈0.6-1 kbar) metamorphism, associated with the Sudetian orogeny has been recorded. According to the presented geological data, the age of metabasites is within the Cambrian and Upper Paleozoic. Considering the analogy between the metabasites spread in the Dzirula crystalline massif, which is exposed in the same terrane, and the metabasites of the Khrami massif, the age of the latter is most likely Middle Paleozoic (Shengelia et al., 2022). The metabasites of the Khrami massif are represented by veins (1-60 m) and stock-shaped bodies (80-800 m) of fine-grained ophitic gabbro, gabbro-diabases and diabases of various thicknesses. They are cut by numerous granite veins and penetrated by thin quartz-feldspar injections. The paragenesis of the high-temperature magmatic stage - Cpx+Pl_78-84 has been preserved in metabasites in some places; Further, under the conditions of greenschist facies, the paragenesis Ab+Act(Tr)+Chl+Ep±Qz develops. According to the petrogenic diagrams Na₂O+K₂O – SiO₂, the metabasites of the Khrami massif belong to the formations of the subalkaline series (Irvine and Baragar, 1971), correspond to basalts and andesite-basalts (Le bas et al., 1986) and basalts and picrites (Cox et al., 1979). This is confirmed by the data of diagrams Zr/Ti-Nb/Y (Pearce, 1996) and Zr/TiO₂ – Yb/Y (Winchester, Floyd, 1977). According to the Na₂O+K₂O–FeO*-MgO (Irvine and Baragar, 1971), a great part of the metabasites is of tholeiitic composition, and only a small part is of calc-alkaline composition. On the diagram Fe*-SiO₂ (Frost et al., 2008) the dots denoting metabasites are completely disposed in the magnesian field. According to the TiO₂ - Zr/(P₂O₅*10⁴) diagram (Winchester, Floyd, 1976), the metabasites correspond to tholeiite basalts. According to the diagram V-Ti/1000 (Shervias, 1982), the metabasites belong to the MORB genetic formation, and according to the diagram Cr-Y (Pearce, 1982), they belong mainly to the VAB, and also to the MORB. According to the ratio MnO-TiO₂/10-P₂O₅ (Mullen, 1983), dots of mafic rocks are located in the island-arc tholeiitic field. Thus, the Middle Paleozoic metabasites of the Khrami crystalline massif are represented by shallow subvolcanic magmatites predominantly of andesite-basalt and tholeiite-basalt groups of the tholeiitic series. They correspond to the MORB and VAB genetic groups.

How to cite: Shengelia, D., Tsutsunava, T., Beridze, G., and Javakhishvili, I.: Geology, geochemical typification and petrogenic model of formation of Middle Paleozoic metabasites of the Khrami crystalline massif (Georgia), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2760, https://doi.org/10.5194/egusphere-egu23-2760, 2023.

Peridotite xenoliths provide valuable insight into lithospheric mantle conditions, composition, and evolution. The origin of amphibole in the lithospheric mantle and whether amphibole melts to produce alkaline intraplate magmas is a highly debated topic. Large areas of the lithospheric mantle forming Earth’s youngest continent, Zealandia, have chemical compositions comparable to Archean mantle lithosphere but Re-Os isotope and bulk rock data indicate that lithosphere stabilisation occurred in the Mesozoic. Some areas of this refractory lithospheric mantle have been metasomatized, with one of the clearest occurrences being MARID-like veins in xenoliths in alkaline intraplate magmas in the Southern Alps of New Zealand. These xenoliths contain abundant veinlets composed of amphibole, phlogopite, clinopyroxene and apatite in rocks that have average olivine compositions exceeding Mg# 92 and spinel Cr# 70. The latter indicates that these peridotites have undergone >25% partial melting prior to metasomatism.

Using a combination of quantitative scanning electron beam methods, trace element and in-situ laser ablation inductively coupled mass spectrometry (LA-ICP-MS) analysis, and conventional ⁸⁷Sr/⁸⁶Sr isotope analysis by solution, we seek to establish the origin of hydrous phases in this mantle lithosphere. The benefit of inspecting formerly highly depleted peridotites is that the chemistry of the metasomatic agent, which is typically enriched in incompatible elements, is less diluted than in the cases where melts infiltrate fertile lithosphere. Although minor Fe-diffusion has occurred within the studied host rock, the bulk compositions of the veins are picro-basaltic. The mica separates, measured by solution chemistry, are today more radiogenic than the in-situ diopside and amphibole analyses, however, we find that the age-corrected ~25 Ma, ⁸⁷Sr/⁸⁶Sr initials fall in a tight cluster of very depleted mantle-like ratios from 0.7027 to 0.7056. Although the fluids appear to have sub-alkaline bulk compositions, the amphibole trace elements are enriched in HFSE and lack depleted Nb components.

The data suggests that these basaltic veins are not arc-related and do not derive from melting of subducted sediment, but also have no direct genetic link to the host alkaline melts. If this latter interpretation is correct, then the injection of hydrous veins was not part of a continuous process that resulted in alkaline magmatism, although they may have been subsequently melted to give rise to alkaline magmas with depleted mantle-like isotopic characters. 

How to cite: Cooper, N., Scott, J., Brenna, M., Palmer, M., Reid, M., Stirling, C., and le Roux, P.: The origin of hydrous amphibole in the subcontinental lithospheric mantle beneath the Southern Alps of New Zealand, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4569, https://doi.org/10.5194/egusphere-egu23-4569, 2023.

Earth’s mantle is highly heterogeneous, with mantle-derived rocks sampling depleted and enriched domains both in intraplate settings and along spreading ridges. The most notorious isotopic anomaly is the DUPAL anomaly, where an overall strong recycled isotopic signature occurs. Studies on Tethyan and Paleo-Tethyan ophiolites have shown the persistence of “DUPAL signature” in those oceans, which paleogeographic reconstructions place on approximately the same position as the present-day Indian Ocean and thus argue for a long-lived “DUPAL signature” in the mantle. The origin of the DUPAL anomaly is controversial, with many studies pointing to it being a primordial feature. More recently, however, it has been shown that plume products in the African Mantle Domain (AMD), of which the DUPAL anomaly region is a part of, generally bear a more enriched signal than plume-related rocks in the Pacific Mantle Domain. This observation has been hypothesized to be related to the formation of the Pangea supercontinent above the present-day AMD, and therefore offering a geodynamic scenario capable of explaining the origin of the enriched isotopic signature of the AMD. However, present-day ocean crust record is limited in time, extending to 200 Ma at maximum, younger than the formation of Pangea at ca. 320 Ma. To investigate the oceanic record of mantle enrichment further back in time and test the influence of supercontinent cycle on the composition of the AMD, it is necessary to utilise preserved oceanic terranes in orogenic belts. In this study we compiled isotopic data from preserved oceanic terranes related both to the formation of the AMD, starting from the assembly of Gondwana till the duration of Pangea, including that of the Mozambique, Adamastor, Goias-Pharusian, Iapetus, Rheic, Qilian-Shangdan, Paleo-Tethys, Meso-Tethys and Neo-Tethys paleo-oceans. Neodymium isotopic data is the most widely available for these ophiolites. The Nd isotopic data indicates a progressively more depleted signal before Gondwana formation until it reaches a maximum and stays relatively stable until shortly after Pangea break-up, where noticeable decrease in depletion occurs. Lead isotopic data is less readily available, existing data nevertheless allow to observe an increase in Th/U ratio during Gondwana formation. Taken together, these observations indicate an increase in recycled continental components in the mantle source of the AMD ophiolites. We envisage this to be evidence for mantle enrichment during the formation of Gondwana and Pangea within the AMD. New isotopic analyses are still needed to paint a clearer picture of the interplay between the supercontinent cycle and mantle geochemistry.

How to cite: Azevedo Berquo de Sampaio, P., Li, Z.-X., Doucet, L. S., and Gamaleldien, H.: Evolution of the African Mantle Domain and its enriched signal: perspective from pre-200 Ma ophiolites, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4686, https://doi.org/10.5194/egusphere-egu23-4686, 2023.

Primitive mantle rocks usually contain rare earth elements (REE) in very low concentrations. Here we report an occurrence of monazite associated with REE-rich apatites in a carbonate-bearing wehrlite xenolith from Pleiku, central Vietnam. The sampled xenolith displays an equigranular matrix of rounded olivine grains. Texturally primary orthopyroxene, clinopyroxene and spinel are notably absent. Scattered within the olivine matrix two types of domains are present: domain-I contains numerous blocky clinopyroxene grains within a matrix of quenched silicate melt and is associated with a second generation of olivine, small euhedral spinel and rare grains of carbonates. Both apatite and monazite may be present. Domain-II typically contains abundant irregularly shaped patches of carbonate associated with quenched silicate melt, secondary olivine, spinel, and clinopyroxene. No phosphate phases are observed within type-II domains. Monazite occurs in different generations: monazite I is found as very small rounded to elongate grains included in primary olivine, partly crosscut by fine melt veinlets, monazite II as large grains up to 300 x 200 µm in size with embayed grain boundaries and monazite III as very small euhedral and needle-like crystals in silicate melt pools. For apatite two textural types occur: apatite I forms lath-shaped to rounded crystals up to 200 x 50 µm in size, apatite II is present within silicate melt pools of type-I domains where it forms euhedral needle-like to equant grains. Some of the apatite II crystals may have cores of monazite III. Monazites show compositional variation mainly with respect to ∑REE₂O₃ (63-69 wt%) and ThO₂ (1.1-5.3 wt%) and only minor variations in P₂O₅ (29-32 wt%) SiO₂ (<0.05-0.4 wt%) and CaO (0.2-0.4 wt%) Apatites are characterized by strongly variable and high REE₂O₃ and SiO₂ contents (4-27 wt% ∑REE₂O₃,0.6-6.8 wt% SiO₂) as well as with significant Na₂O (0.3-1.5 wt%), FeO (0.1-1.8 wt%), MgO (0.2-0.6 wt%) and SrO (0.2-0.9 wt%) contents. F and Cl contents are in the range 1.9-3.0 wt% and 0.2-0.8 wt%, respectively. Based on textural evidence and chemical composition of the metasomatized mineral phases an initial stage of metasomatism is proposed which was triggered by a P-REE-CO₂-rich agent with low a_H2O resulting in the co-precipitation of carbonates as patches and along micro-veins and of phosphates in a peridotite assemblage. A subsequent second stage is characterized by pervasive infiltration of an alkali-rich basaltic melt into the carbonate + phosphate-bearing assemblage. The presence of monazite prior to silicate melt infiltration is indicated by narrow melt veins crosscutting monazite I grains. Reactions of the silicate melt with the pre-existing phases led to the formation of domains-I and -II and changed the composition of the infiltrating melt towards phonolitic-trachytic composition. The second stage led to partial breakdown and recrystallization of monazite and apatite.

How to cite: Hauzenberger, C., Konzett, J., Joachim-Mrosko, B., and Nguyen, H.: A monazite- and REE-rich apatite-bearing mantle xenolith from Pleiku, central Vietnam, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10058, https://doi.org/10.5194/egusphere-egu23-10058, 2023.