The nature of Earth’s lithospheric mantle is largely constrained from the petrological and geochemical studies of xenoliths. They are complemented by studies of orogenic peridotites and ophiolites, which show the space relationships among various mantle rock kinds, missing in xenoliths. Mantle xenoliths from cratonic regions are distinctly different from those occurring in younger non-cratonic areas. Percolation of melts and fluids through the lithospheric mantle significantly modifies its petrological and geochemical features, which is recorded in mantle xenoliths brought to the surface by oceanic and continental volcanism. Basalts and other mantle-derived magmas provide us another opportunity to study the chemical and physical properties the mantle. These various kinds of information, when assembled together and coupled with experiments and geophysical data, enable the understanding of upper mantle dynamics.
This session’s research focus lies on mineralogical, petrological and geochemical studies of mantle xenoliths, orogenic and ophiolitic peridotites and other mantle derived rocks. We strongly encourage the contributions on petrology and geochemistry of mantle xenoliths and other mantle rocks, experimental studies, the examples and models of mantle processes and its evolution in space and time.
vPICO presentations: Mon, 26 Apr
Assessing the nature and evolution of the Sub-Continental Lithospheric Mantle (SCLM) is crucial to understand the dynamics of Earth’s interior and the global scale tectono-magmatic processes. The study of ultramafic xenoliths brought to the surface in specific context, such as northern Victoria Land (Antarctica), is a key to investigate how the SCLM bear witness of large-scale geodynamic episodes. Indeed, the Antarctica lithosphere was involved into three main tectono-magmatic episodes since Paleozoic, i.e. the 550-110 Ma Ross subduction, the Jurassic (~182 Ma) Ferrar magmatism and the Cenozoic alkaline magmatism responsible for the opening of the West Antarctic Rift System (WARS).
In this study, a review of the petrological and geochemical features of >200 mantle-derived and cumulate xenoliths brought to the surface at Baker Rocks, Greene Point, Handler Ridge, Harrow Peaks, Browning Pass and Mount Overlord enabled us to reconstruct the main depletion and enrichment processes that took place in the Antarctica SCLM. Strong depletion is recorded by Greene Point lherzolites and harzburgites (18-21%), which likely began melting in the garnet facies and terminated in the spinel facies (Perinelli et al. 2006), whereas mild melt extraction in the spinel stability field was hypothesized at Baker Rocks and Handler Ridge (12-16% and 7-13% melting, respectively). The onset of the Jurassic Ferrar large magmatic event is testified by both the refertilisation in Greene Point-Baker Rocks peridotites and the appearance of cumulate orthopyroxenites/olivine-websterites at Harrow Peaks and Baker Rocks. Late enrichment process/es took place in concomitance with the Cenozoic alkaline magmatism of the WARS, resulting in both cryptic and modal metasomatism and overprinting earlier chemical modifications. This metasomatism was particularly effective at Baker Rocks, as shown by the increase of clinopyroxene abundance, its trace element enrichment and the formation of amphibole disseminated and in veins. Clinopyroxene composition in Cenozoic cumulate rocks matches the enrichment path observed in the peridotites, supporting the link between the last metasomatic process and the recent alkaline magmatism.
Among mantle xenoliths populations, Greene Point record the highest T-P (870-1059 °C; 0.8-1.6 GPa) and the least oxidized conditions (fO2 down to -2/-3 ΔFMQ). Cumulate rocks yield the highest fO2 (up to +1.5 ΔFMQ), at T varying between 900 and 1150°C, approximating the conditions of crystallizing melts. No discrepancies in fO2 emerged between amphibole-bearing and amphibole-free peridotites, ruling out a strict correlation between amphibole stability, H2O activity and fO2. Nevertheless, the alkaline metasomatic event, which led to amphibole formation, caused a remarkable increase in the H2O content of the system. In fact, anhydrous peridotites preserve bulk H2O contents ≤128 ppm, while lherzolites with disseminated amphibole and hornblendites have H2O contents as up to 354-1120 ppm and 1.42 wt%, respectively.
Perinelli, C., et al. 2006. Geochemical and O-isotope constraints on the evolution of lithospheric mantle in the Ross Sea rift area (Antarctica). Contributions to Mineralogy and Petrology, 151(3), 245-266.
How to cite: Coltorti, M., Bonadiman, C., Casetta, F., Faccini, B., Giacomoni, P. P., Pelorosso, B., and Perinelli, C.: Depletion, metasomatism and refertilisation in the Sub-Continental Lithospheric Mantle beneath northern Victoria Land (Antarctica): a review, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8450, https://doi.org/10.5194/egusphere-egu21-8450, 2021.
The North Tanzanian Divergence (NTD) is the prolongation of the eastern branch of the East African Rift and is a place of intense volcanism. Numerous volcanoes erupted deep subalkaline to highly alkaline magmas, including the particular active natrocarbonatite Oldoinyo Lengai. On the North-South axis (Natron to Manyara basins), three highly alkaline volcanoes, Pello Hills, Lashaine and Labait, erupted melilite magmas that originated from low degree of partial melting of asthenospheric mantle (depth > 120 km). The particularity of these volcanoes is that they sampled numerous mantle xenoliths during ascent. This represents a unique opportunity to study the composition and the rheology of lithospheric mantle. Mantle xenoliths are deep garnet-bearing peridotites (120 km depth), amphibole and phlogopite peridotites and phlogopitites. They contain abundant hydrous minerals as isolated crystals or veins that attest to an important metasomatism beneath the NTD. Previous geochemical and petrological studies have highlighted interactions of alkaline magmas and the thick cratonic lithosphere as metasomatic agent. However, the presence and composition of magmas, the degree of metasomatism, and the role of metasomatism on mantle rheology below the NTD is still debated.
To characterize these previous parameters, in this study we performed geochemical and petrophysical analyses on metasomatized, fertile and refractory mantle xenoliths from Labait (on-craton volcano) and Pello Hills (in-rift volcano). Using mineral compositions and thermobarometer calibrations, we estimated the depth of mantle xenoliths between 40 and 140 km (14 to 47 kbar) and temperatures from 930 to 1340°C. EBSD analysis on thin sections indicate that peridotites and amphibole/phlogopite-bearing mantle xenoliths display a moderate to strong deformation induced crystal preferred orientation. In contrast, weak mineral orientations have been observed in phlogopite-amphibole-clinopyroxene-bearing veins. Calculation of seismic properties using MTEX program show that peridotites are seismically anisotropic, up to 12.4% for P-wave velocity (Vp) and 6.8% for S-wave velocity (Vs). The Vp and Vs in hydrous veins are lower than in peridotites (Vp: 7.5-7.9 and 8.3-9.6 km/s; Vs: 4.4-4.6 and 5.0-5.3 km/s respectively) and therefore the Vp and Vs velocities decrease with the increasing proportion of metasomatic minerals. We estimate that a peridotite with 20 vol.% metasomatic vein has a velocity decrease of 3.5% for Vp and 2.9% for Vs, compared to a fertile peridotite.
These geochemical and petrophysical approaches are important to understand P- and S-wave propagation in the lithospheric mantle beneath the NTD and more specifically in metasomatized lithospheric mantle. The new in situ data and models from mantle xenoliths will be compared to tomographic acquisition and discussed in term of temperature, presence of melt or metasomatism processes. Both petrophysical and geophysical data will help to precisely determine the structure and rheology of the lithospheric mantle, which may control the propagation of the rift at early stage rifting between the Tanzanian craton and the mobile Proterozoic belts.
How to cite: Clutier, A., Parat, F., Gregoire, M., Gibert, B., Gautier, S., and Tiberi, C.: Geochemical and petrophysical characterization of mantle metasomatism beneath the North Tanzanian Divergence, East African rift., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10603, https://doi.org/10.5194/egusphere-egu21-10603, 2021.
Cameroon Volcanic Line (CVL) is located in the western part of equatorial Africa and consists of volcanoes which were active from Eocene to recent, stretching ca. 1700 km from the Atlantic in the SW into the African continent in the NE. The continental part of the CVL is located on the Neoproterozoic Central African Orogenic Belt and is situated between the Congo craton and Sahara/Western Africa craton. Mantle peridotite xenoliths which occur locally in lavas of the CVL come from the spinel facies only, suggesting a relatively shallow lithosphere-asthenosphere boundary (LAB). This is supported by seismic studies, showing the LAB at 90-100 km.
In order to understand better the evolution of the lithospheric mantle beneath the CVL, we studied xenolith suite (16 xenoliths) from Befang in the Oku Massif (Tedonkenfack et al., submitted). The Befang xenoliths are almost entirely lherzolites which have cataclastic to weakly porphyroclastic texture. Harzburgites and websterites occur subordinately. Spinel is interstitial and has amoeboidal shape. The studied peridotites (14 lherzolites, 1 harzburgite) consist of minerals with almost constant composition (olivine Fo88.7-90.3, orthopyroxene Al 0.17-0.19 atoms per formula unit (a pfu), clinopyroxene Al 0.28-0.30 a pfu, spinel Cr# dominantly 0.09-0.11). Spinel of Cr# 0.15 occurs in one of the lherzolites, whereas that occurring in harzburgite has Cr# 0.19. Clinopyroxene REE patterns are similar to those of Depleted MORB Mantle (DMM) except LREEs, which vary from depleted to enriched. The A-type olivine fabric occurs in the EBSD-studied subset of 8 samples (one harzburgite and 7 lherzolites). Orthopyroxene shows deformation consistent with olivine. The fabric of LREE-enriched clinopyroxene is equivalent to those of orthopyroxene and olivine, whereas spinel and LREE-depleted clinopyroxene are oriented independently of the fabric of host rock.
These data, thermometry, phase relationships and phase equilibria diagrams suggest that the Befang mantle section was refertilised by MORB-like melt at pressures 1.0-1.4 GPa and temperatures slightly above 1200 – 1275 ºC. The olivine-orthopyroxene framework and LREE-enriched clinopyroxene preserve the fabric of protolith. On the other hand, the LREE-depleted clinopyroxene shows discordant orientation relative to olivine-orthopyroxene protolith framework, and amoeboidal spinel crystallized from the melt. The major element and REEs composition of pyroxenes occurring in the Befang peridotites indicate chemical reequilibration at temperatures 930 – 1000 ºC. Trace element modeling shows that websterites can be linked to Cenozoic volcanism. We speculate that they form veins in the lithospheric mantle. Our study therefore supports the origin of fertile SCLM via refertilization rather than by extraction of small melt fractions, and further emphasizes the involvement of depleted melts in this process, which contrasts with the incompatible element-enriched melts typically invoked in within-plate settings.
This study originated thanks to the project of Polish National Centre of Research NCN 2017/27/B/ST10/00365 to JP. The bilateral Austrian-Polish project WTZ PL 08/2018 enabled extensive microprobe work.
Tedonkenfack SST, Puziewicz J, Aulbach S, Ntaflos T., Kaczmarek M-A, Matusiak-Małek M, Kukuła A, Ziobro M: Lithospheric mantle refertilization by DMM-derived melts beneath the Cameroon Volcanic Line – a case study of the Befang xenolith suite (Oku Volcanic Group, Cameroon). Submitted.
How to cite: Puziewicz, J., Tedonkenfack, S. S. T., Aulbach, S., Ntaflos, T., Kaczmarek, M.-A., Kukula, A., Matusiak-Małek, M., and Ziobro, M.: Evolution of lithospheric mantle beneath mobile belt between two cratons: An example from the Oku Massif, Cameroon Volcanic Line (W Africa), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1670, https://doi.org/10.5194/egusphere-egu21-1670, 2021.
Vogelsberg is a Cenozoic volcanic field situated at the northern tip of the Upper Rhine Graben. It stretches over two major Variscan basement units: the Rheno-Hercynian Zone in the NW and the Saxo-Thuringian Zone in the SE. We studied peridotite xenoliths from Breitenborn, Nidda and Dreihausen (SE, central and NW part of Vogelsberg, respectively) in order to reveal the evolution of the subcontinental lithospheric mantle (SCLM) rejuvenated during a Cenozoic rifting episode.
The Vogelsberg xenoliths are spinel harzburgites and clinopyroxene-poor spinel lherzolites. Most samples show grain size reduction leading to serial or porphyroclastic texture, or slight to well-defined foliation. All studied sites have similar major elements chemistry: olivine Fo 89.3-91.7%; orthopyroxene (opx) Mg# 0.89-0.92 and 0.06-0.25 atoms of Al pfu (per formula unit); clinopyroxene (cpx) Mg# 0.89-0.93 and 0.10-0.33 atoms of Al pfu. Spinel Cr# is highly variable: 0.18-0.45 for Breitenborn, 0.14-0.57 for Nidda and 0.11-0.61 for Dreihausen.
Vogelsberg peridotites exhibit a diversity of REE patterns:
(1) opx with a sinusoidal pattern, no cpx (Nidda, Dreihausen);
(2) cpx with flat patterns; coexisting opx with strong LREE-depletion, (La/Lu)N ~0.02 (Nidda, Dreihausen)
(3) cpx with flat, spoon-shaped patterns with La-Ce-enrichment (La/Pr)N ~4.3; opx similar to (2) but partly spoon-like, (Nd/Lu)N ~0.02 (Nidda, Breitenborn)
(4) cpx with different degree of LREE-enrichment, (La/Lu)N of 4-21.4; coexisting opx with mild LREE-depletion, (La/Lu)N of 0.1-0.3 (Breitenborn, Nidda, Dreihausen)
(5) cpx with flat HREE pattern and strongly LREE-depleted, (La/Eu)N ~0.03; coexisting opx similar to (2) but with (Ce/Lu)N ~0.001 (Breitenborn)
Temperatures calculated using REE content (TREE)  for the Breitenborn peridotites exhibit two ranges: 930-990°C and 1050-1130°C, for the Nidda ones: 880-930°C, 1000-1050°C and 1110-1150°C and for Dreihausen ones: 1140-1190°C. Temperatures calculated on the basis of pyroxene major element contents (TBKN)  are 40-90°C lower than TREE in Breitenborn and Nidda and lower by 10-55°C in Dreihausen.
The most common pyroxene REE patterns (type 4) are products of two-phase metasomatism: by Vogelsberg alkali basalt followed by a highly LREE-rich melt that further increased LREE contents in cpx, up to observed abundances. Strongly LREE-depleted opx (types 2, 3, 5) and cpx (type 5) patterns could be residues after partial melting of a fertile protolith, or products of metasomatism by melts derived from depleted MORB mantle. Cpx patterns of type 2 and 3 might have been once similar to type 5 but were later affected by the second phase of metasomatism: highly LREE-rich melt that increased chromatographically their LREE contents to variable degrees. The diversity of REE patterns and calculated temperatures shows that the SCLM beneath Vogelsberg is highly heterogeneous, probably due to spatial variability of deformation and percolation of hot melts connected with Cenozoic rifting.
This study was funded by Polish National Science Centre to MZ (UMO-2018/29/N/ST10/00259) and JP (UMO-2014/15/B/ST10/00095). EPMA analyses were done thanks to the Polish-Austrian projects WTZ PL/16 and WTZ PL 08/2018. MZ acknowledges the DAAD fellowship at Goethe University Frankfurt.
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 Brey G. & Köhler T. (1990). JPetrol 31, 1353–1378.
How to cite: Ziobro, M., Puziewicz, J., Aulbach, S., Ntaflos, T., Matusiak-Małek, M., and Grégoire, M.: Lithospheric mantle beneath the Vogelsberg volcanic field (Central Germany) , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5234, https://doi.org/10.5194/egusphere-egu21-5234, 2021.
Migration of metals such as gold, silver and copper through the subcontinental lithospheric mantle (SCLM) can be tracked by the investigation of sulfides in mantle xenoliths. Therefore, to understand relations between the metal migration and metasomatism of silicate phases in the SCLM beneath SW Poland we studied sulfides in a set of mantle ultramafic xenoliths with variable metasomatic history. The xenoliths occur in the Cenozoic alkaline mafic volcanic rocks from the SW Poland (N Bohemian Massif).
The studied sulfides occur in mantle rocks of variable history: 1) strongly depleted (group A0) to weakly metasomatized peridotites (Group A1); 2) strongly melt-metasomatized peridotites (Group B); 3) pyroxenites (Group C; for details of group definition see Matusiak-Małek et al., 2014, JoP). The metasomatism was of mixed silicate/carbonatite nature. The sulfides are either interstitial or enclosed in the silicates and form mostly globular monosulfide solid solution-chalcopyrite (mss-Ccp) assemblages typical of igneous sulfides separated and crystallized from mafic magmas, with mss partially re-equilibrated to exsolutions of pentlandite (Pn) and pyrrhotite (Po) when temperature dropped to <600°C (e.g., Craig and Kullerud, 1969, Econ. Geol. Monogr.).
The sulfide abundances increase from Group A (≤ 0.008 vol.‰) through Group B (up to 0.060 vol. ‰) to Group C (up to 0.963 vol.‰) xenoliths. The sulfides of Groups C (Po15–99Pn0–20Ccp0–70)and B (Po0–85Pn14–100Ccp0–27) are generally poorer in Ni compared to Group A (Po0–74Pn24–100Ccp0–35). Consequently, Ni/(Ni+Fe) in the Group C pentlandites (0.41–0.52) is lower than in those in Group A (0.45–0.69). Moreover, the sulfide grains of Group B are enriched in chalcophile elements (e.g., the median content of Zn is 90 ppm) compared to sulfides from Groups C (52 ppm Zn) and A (51 ppm of Zn). The same relations occur in PGE contents, e.g., Pt in Group B is 1.6 ppm, while in Groups C and A it is 0.1 and 1.3 ppm, respectively. Observed differences in δ56Fe between the Groups are probably due to modal composition of bulk sulfide grains between Groups A (Ni-rich), B and C (Fe-Cu-rich). As no difference is observed between the grains of the same composition, any fractionation of Fe isotopes in sulfide melt seems to be possible only upon its differentiation from Ni-rich to Fe-Cu-rich.
The host peridotites were affected by strong depletion as the degree of partial melting was possibly ~30%. Thus, the observed enhanced sulfide modes in the metasomatized peridotites (Groups A1 and B) are most likely brought by the metasomatic melt. This is also evidenced by their Fe-Cu-rich composition, similar to that of the sulfides from the pyroxenites. In this view, melt metasomatism likely affects the chalcophile and highly-siderophile metal budget of the continental lithosphere.
The measurements of Fe isotopic ratios were financed from funds for years 2020-2024 within program “Diamond Grant” (DI2019 0093 49), the LAICPMS measurements were financed from 2016/23/N/ST10/00288 to J.C., and the EPMA analyses were done within the frame of the Polish-Austrian project WTZ PL/16 and WTZ PL 08/2018.
How to cite: Mazurek, H., Ciazela, J., Matusiak-Małek, M., Pieterek, B., Puziewicz, J., Lazarov, M., Horn, I., and Ntaflos, T.: Metal enrichment as a result of SCLM metasomatism? Insight from ultramafic xenoliths from SW Poland., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15992, https://doi.org/10.5194/egusphere-egu21-15992, 2021.
Studying the isotopic composition of fluids trapped in mantle xenoliths opens avenues to understanding the origin and cycling of volatiles in the Earth’s upper mantle. Here, we present the first isotopic results for noble gases and CO2 in fluid inclusions (FI) trapped in mantle xenoliths from El Hierro the youngest island of the Canarian archipelago. Our results are based on 6 mantle xenolith samples (3 Spinel-lherzolites and 3 Spinel-harzburgites) collected from the El Julan cliff valley (Oglialoro et al., 2017), from which we hand-picked crystals of Ol, Opx, and Cpx. Isotopic determinations were performed at the INGV (Sezione di Palermo) noble gas and stable isotopes laboratories, following the preparation methods and analytical procedures described in Rizzo et al. (2018 and references therein).
The Ne-Ar isotopic compositions reveal the presence of an atmospheric component in the FI. Most of the samples exhibit 4He/20Ne ratios > 60, 20Ne/22Ne ratios between 9.84 and 10.49, 21Ne/22Ne ratios from 0.0295 to 0.0330, and 40Ar/36Ar > 800, suggesting mixing between MORB-like mantle fluids and an air-derived component. We argue this latter may (at least in part) derive from upper mantle recycling of atmospheric fluids via paleo-subduction event(s). Excluding samples possibly affected by diffusive fractionation processes, the average Rc/Ra ratio (3He/4He ratio corrected for atmospheric contamination) measured in El Hierro xenoliths is ~7.45 + 0.26 Ra, within the MORB range (8 + 1 Ra; Graham, 2002). The He homogeneous signature of these xenoliths agrees well with the 3He/4He compositions previously reported in lava phenocrysts and cumulates (Day and Hilton, 2011) and is slightly below the maximum ratios measured in groundwater samples during the 2012 volcanic unrest (~8.2 Ra; Padron et al., 2013). All these pieces of evidence argue against a primordial source involved in the local lithospheric mantle. Putting these data in the context of previous literature results for FI and surface gases in the Canary Islands (La Palma, La Gomera, Tenerife, Gran Canaria, and Lanzarote), we identify an eastward 3He/4He decreasing trend that parallels a corresponding increase of the oceanic crust thickness. In addition to the mantle heterogeneity, we propose that part of the 3He/4He east-to-west variation along the archipelago is caused by the variable thickness of the oceanic crust (and hence, different interactions with 4He-rich crustal fluids during emplacement).
The FI δ13C(CO2) isotopic composition ranges from -2.38 to -1.23‰ in pyroxenes and -0.2 to +2.0‰ in olivine. These unusually positive δ13C compositions support the existence of a recycled crustal carbon component in the local source mantle, likely pointing to mantle metasomatism (Oglialoro et al., 2017) from fluids carrying carbon from subducted sediments and/or altered oceanic crust (AOC).
How to cite: Sandoval Velasquez, A., Rizzo, A. L., Aiuppa, A., Frezzotti, M. L., Remigi, S., Padrón, E., and Pérez, N.: First assessment of the noble gas and CO2 isotopic composition of fluid inclusions hosted in mantle xenoliths from El Hierro (Canary Islands) , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12817, https://doi.org/10.5194/egusphere-egu21-12817, 2021.
Late Carboniferous/early Permian mafic volcanic rocks occurring in Scotland carry a broad spectrum of peridotitic and pyroxenitic xenoliths. The latter provide evidence of magmatic processes in the lower crust and the lithospheric mantle. In this study we present textural and compositional data on twenty-eight pyroxenitic xenoliths from six localities from southern Scotland (Midland Valley and Southern Uplands Terranes).
Most are interpreted as adcumulates (varying in grain size from fine to coarse) although some others are mesocumulates. They include both clinopyroxenites and websterites with variable amounts of olivine; phlogopite is present in only one sample. Cores of greenish clinopyroxene in three of the olivine clinopyroxenites are enveloped by brownish clinopyroxene, while one composite xenolith comprising coarse-grained olivine clinopyroxenite in sharp contact with harzburgite. Five groups, based on textural and mineralogical features were distinguished. Representatives of more than one group can be present in a single locality.
Most of the samples from the same textural group share similar chemical composition. In general, the clinopyroxenes are Ti,Al-diopside/augite with Mg#=0.74-0.86; where clinopyroxenes are zoned the rims have lower Mg# and higher Al content. The orthopyroxene is an Al (±Cr)-enstatite with Mg#=0.78-0.89, olivine (Fo76-77) is relatively NiO-rich (0.16-0.29 wt.%). In clinopyroxenites the pyroxenes are LREE-enriched (LaN/LuN=1.31-3.17) with convex-upward REE patterns (SmN/LuN=2.48-7.37).
The temperatures and pressures of clinopyroxene crystallization in most of the clinopyroxenites are 1220-1300°C and 1.08–1.30 GPa (Putirka, 2008), respectively. Only the composite xenolith and the coarse-grained clinopyroxenites recorded higher pressures (1.42 and 1.65-2.03 GPa, respectively). As the Moho beneath S Scotland is located at ~35 km (corresponding to ~1 GPa; Davis et al., 2012), most of the clinopyroxenites are considered to come from the uppermost portions of lithospheric mantle or lowermost continental crust; the coarse-grained clinopyroxenites and the composite xenolith sample lithospheric mantle.
Clinopyroxenites from the southern Scotland crystallized from alkaline basaltic magmas similar to those that entrained them. Whilst Downes et al. (2007, 2001) had previously suggested this for clinopyroxenites from Midland Valley localities, our studies show that crystallization of mafic melts was more widespread. Strong chemical and textural variations in the pyroxenites together with relatively constant PT conditions of crystallization suggest that they formed either from melts of slightly different composition, perhaps in response to magma chamber processes such as magma replenishment and/ or mixing. While, the presence of mafic cumulates points to possible crustal underplating beneath S Scotland, the presence of a high-pressure clinopyroxenites and composite clinopyroxenitic-peridotitic xenolith imply that some of the pyroxenites originated in the lithospheric mantle.
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Downes et al., (2007). J. Geol. Soc., 164, 1217-1231.
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This study was funded by Polish National Science Centre to MMM no. DEC-2016/23/B/ST10/01905. EPMA analyses were done within the frame of the Polish-Austrian project WTZ PL/16 and WTZ PL 08/2018.
How to cite: Matusiak-Małek, M., Matczuk, P., Upton, B. G. J., Ntaflos, T., Aulbach, S., Puziewicz, J., and Kukuła, A.: Pyroxenitic xenoliths from southern Scotland and what they tell us., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8810, https://doi.org/10.5194/egusphere-egu21-8810, 2021.
The presence of Pacific-type and Indian-type mid-ocean ridge (MORB) isotopic source domains in the upper mantle is a clear manifestation of global-scale mantle compositional heterogeneities. The Indian-type mantle domain is a long-lived feature that can be traced back to, at least, the Palaeozoic Tethyan mantle domain. Little temporal constraints currently exist, however, regarding the longevity of Pacific-type mantle domain. The extinct Paleo-Asia Ocean (PAO), a subsidiary ocean of the Panthalassic Ocean that formed during the breakup of the Rodinia Supercontinent in Mesoproterozoic to Neoproterozoic, can provide a solution to this dilemma. Here, we report the first complete geochemical and Sr, Nd and high-precision Pb isotopic data set for representative mafic rock samples from ophiolites representing remnants of the PAO basement ranging in age from 275 to 624Ma to constrain the composition of their mantle provenance. Data suggest that the sub-PAO mantle has a similar long time-integrated, high Sm/Nd ratio as the global depleted upper mantle, but also shows typical Pacific MORB-like Pb isotopic compositions with lower 207Pb/204Pb(t) and 208Pb/204Pb(t) for given 206Pb/204Pb(t) ratios, and low radiogenic 208Pb*/206Pb*, indicating a long time-integrated, low Th/U ratios. Thus, the Pacific-type mantle domain, like the Indian-type mantle domain, is a long-lived secular mantle domain that can be traced back to early Paleozoic or even to the Neoproterozoic. Data further indicate that the Nd and Pb isotopic distinction between such two large-scale and long-term mantle domains is due to the different evolutionary and tectonic histories of the circum-Pacific (PAO, Paleo- and modern Pacific) and sub-Tethys-Indian oceanic mantle realms. The Panthalassic-Pacific ocean realm had remarkable permanency existing as a big ocean at lease throughout the Phanerozoic, that implies that continental materials were limit to recycle into underlying mantle, thus the underlying mantle was relative free of the continental material contamination and then produce the low time-integrated Th/U Pacific-type mantle domain. In contrast, the break-up of the Gondwana supercontinent makes the Tethys realms to experience repeated opening and closures, which transferred large volume of continental materials into the underlying mantle and then produce the high Th/U Indian-type mantle domain. Our results indicate that the high Sm/Nd and low Th/U ratio of Pacific-type mantle domain most likely are an inherited, long-standing intrinsic feature of the depleted upper mantle derived from the Earth's primordial mantle with less contamination of continental materials. In contrast, the large-scale and long-lived Indian-type mantle heterogeneity is produced by plate tectonic-driven continental material circulation in the upper mantle. Such a genetic link between plate tectonics and mantle chemical geodynamics is crucial to our understanding of how the Earth system works.
This study was financially supported by the National Natural Science Foundation of China (92055208，41772059) and the CAS “Light of West China” Program (2018-XBYJRC-003).
How to cite: Liu, X., Zhang, Z., Liu, P., Song, Y., and Xiao, Y.: Long-lived low Th/U Pacific-type isotopic mantle domain, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10496, https://doi.org/10.5194/egusphere-egu21-10496, 2021.
Melt inclusions of very unusual nature occur in garnets of eclogites of the Granulitgebirge, Bohemian Massif. This is one of the first direct characterization of a preserved metasomatic melt responsible for the formation of eclogites enclosed in garnet peridotites. The inclusions are micrometric, from glassy to fully crystalized as nanogranitoids and randomly distributed in the garnet core. Nanogranitoids contain kumdykolite/albite, phlogopite, osumilite and kokchetavite with a variable amount of quartz, pyroxene, carbonate and rare white mica. The melt has a granitic composition rather than basaltic or tonalitic/trondhjemitic as would be expected from the partial melting of ultramafic or mafic rocks and it is as well hydrous and peraluminous. The trace elements composition is also unusual for melts in mantle rocks with elements typical of continental crust (Cs, Li, B, Pb and Rb) and subduction zone (Th and U). Similar signatures, i.e. continental crust and subduction, are visible also in the whole rock trace elements in the form of high amounts of LILE and U. The eclogite major elements composition is similar to a Ca- and Fe - rich mafic rock akin more to the crust than to the mantle.
The peculiar melt composition and the lack of a clear residue of a melting reaction in the eclogites suggest that this melt is external, i.e. metasomatic. It infiltered the peridotites during subduction of the continental crust at mantle depth and aided the transformation of basic layers, already in the peridotite, to eclogite. In addition, similar trace elements patterns to the melt reported here can be found in the so-called durbachite -ultrapotassic melanosyenite present in the high-grade Variscan basement- and in the garnet peridotites and garnet pyroxenites of the T-7 borehole. In both case metasomatism was suggested but the agent was just inferred based on the geochemical signature. All these occurrences suggest that mantle contaminated by melts from deeply subducted continental crust is widespread beneath the Bohemian Massif.
How to cite: Borghini, A., Ferrero, S., O'Brien, P. J., Wunder, B., and Laurent, O.: Pristine metasomatic melt preserved in mantle rocks of the Bohemian Massif, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14751, https://doi.org/10.5194/egusphere-egu21-14751, 2021.
South Armorican mantle peridotites represent a great diversity of protoliths from supra-subduction zone to arc-fore arc ophiolites. In this study, we investigate the serpentinization of these protoliths. Numerous samples were collected in five different units, which represent ophiolitic dismembered pieces (Ty-Lan Peridotites (TLP) from the Audierne Complex, and Pont de Barel Peridotites (PBP), Folies Siffait Peridotites (FSP), l’Orgerais Peridotites (LOP) and Drain Peridotites (DP) from the Champtoceaux Complex). Field and microscopic observations together with Raman spectroscopy and electronic microprobe analysis (EMPA) allowed to identify several stages of serpentinization. All samples display a high rate of serpentinization, up to 80-90 %. Primary assemblage is represented by spinel (TLP, PBP, DP and LOP), olivine (TLP and FSP) and Ti-poor or Cr-rich pargasite (TLP and PBP). In all the samples, lizardite from olivine and bastites from pyroxene and amphibole characterize the first stage of serpentinization. It is associated with magnetite crystallization. No Al-rich lizardite meshe is identified by EMPA suggesting a low temperature (< 340°C) event. This serpentinization is followed by two generations of veins (V1 and V2). The V1 are Al-poor lizardite shear veins and crack-seal chrysotile veins characterize the V2. In PBP, microprobe mapping shows that V2 displays heterogeneous chemical chrysotile composition with significant variations of Al, Fe and Mg contents, suggesting metasomatism and/or variation of fluid composition during serpentinization. All these observations are closely similar to those of oceanic serpentinized peridotites. In the TLP, we identified a second stage of serpentinization characterized by antigorite after lizardite suggesting a high temperature event. In the OP, antigorite after lizardite was also identified. However, compared to the TLP ones, LOP antigorite is related to ductile (i.e., ultramylonite) deformations. This clearly indicates a high temperature stage of serpentinization (up to 500 °C). Furthermore, LOP ultramylonitized samples display one more chrysotile veins generation (V3) characterized by three distinct vein networks. The first one (V3a) is a crack-seal type vein network opened parallel to the main foliation. The second one (V3b) is perpendicular to the first one, whereas the third one (V3c) corresponds to tension gashes connected to C’ plans. This latter is perpendicular to V3a and V3b networks. The mylonitic foliation of LOP is similar to the surrounding micaschists schistosity, suggesting an orogenic high temperature stage of serpentinization. In the FSP, σ-type polycrystalline structures were identified. Lizardite meshes are progressively transposed and recrystallized into the foliation plan. This stage is associated with the crystallization of chlorite after tremolite, suggesting a retrograde stage of serpentinization during serpentinites exhumation. Finally, despite a great diversity of mantle-derived protoliths, our study shows that South-Armorican peridotites recorded a similar first low temperature oceanic stage of serpentinization. According to the Variscan history, it could have started during the Cambro-Ordovician for TLP, and during the Late Devonian for PBP, DP, LOP, FSP. Furthermore, some of these peridotites also recorded an orogenic serpentinization (LOP and FLP). Such observations provide new constraints that could be useful to a better understanding of the tectonometamorphic evolution of the South Armorican suture zones during the Variscan orogeny.
How to cite: Aertgeerts, G., Lahondère, D., Monnier, C., and Lorand, J.-P.: Multiple stages of serpentinization in mantle derived peridotites of the South Armorican Variscan suture zone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11060, https://doi.org/10.5194/egusphere-egu21-11060, 2021.
In Central Evia island (Aegean-Greece) serpentinized ultramafic rocks appear as elongated thrust sheets or in the form of olistostromes incorporated within Maestrichtian-Paleocene flysch. These are crosscut by well-developed rodingite dykes that were derived from four main protoliths that include i) Boninites, ii) Island-arc Tholeiitic Basalts and Gabbros, iii) Alkaline basalts and iv) Calc-alkaline basalts. They mainly comprise of minerals that include (hydro)garnet + chlorite + clinopyroxene ± vesuvianite. Accessory minerals include spinel ± calcite ± prehnite ± amphibole ± orthopyroxene ± olivine ± quartz ± opaque Fe-Ti oxides. Rodingites that were formed at the expense of boninites and island-arc tholeiitic rocks were likely formed within a single rodingitization stage, since garnet is mainly grossular-rich and relict primary clinopyroxene has been preserved. The rodingitization of the alkaline and calc-alkaline basalts seems to have occurred as a multi-stage metasomatic process that occurred during the exhumation of the mafic-ultramafic mantle wedge complex. This resulted in the development of late-stage andradite, vesuvianite and in some cases of chlorite during derodingitization. In this case, successive reaction zones with variability in the participating mineral phases were developed. Geochemical results reveal remarkable rare earth element (REE) enrichments, especially in the inner zones, likely being the result of successive diffusion and element transfer. Few rodingites are characterized as calcite-bearing, whose stable 13C-18O isotopic data points to the restricted involvement of late-stage mixed hydrothermal and seawater-related carbonation processes.
How to cite: Karkalis, C., Magganas, A., Koutsovitis, P., and Ntaflos, T.: Rodingitization of mafic rocks from Central Evia (Greece) associated with serpentinite exhumation: Evidence from Petrological, Geochemical and Isotopic data , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3319, https://doi.org/10.5194/egusphere-egu21-3319, 2021.
Extensive magmatic activities were developed in Central and Western Anatolia, since middle miocene to quaternary times, the most primitive lavas are situated in eastern end of Central (Sivas) and also western (Kula) Anatolia, besides Kula basalts are one of the most recent basaltic rocks together with basalts from south-central Anatolia. Although the magmatism is generally observed at several different locations, the recent basaltic rocks in both of the regions seem to be derived from the melting of the peridotite and pyroxenite source domains and the latter one was ignored in previous studies as source component.
The previous studies indicate that many of the basaltic rocks from Central and Western Anatolia are related with spinel-garnet transition, but typical Tb/Yb(N) (>1.8; ) and Zn/Fe (separates peridotite-derived (Zn/Fe <12; ) and pyroxenite-derived (Zn/Fe 13-20);  melts) Co/Fe ratios of the basaltic rocks from several volcanic centers from Central and Western Anatolia reveal that melting from the single source component are not solely capable of the producing basaltic rocks.
Sr-Nd and Pb isotopic compositions clearly display the distinction of samples which are linked to asthenospheric source. The lead isotopic systematic shows no siginificant differences among the Central and Western Anatolian basalts, of all the samples are above the NHRL line and close to EM II mantle component, Sr- Nd isotopes also display similar compositions as well, the majority of the samples are in and close to mantle array, but the Sr isotopic composition of Miocene aged Gediz and Simav lavas have high radiogenic values.
Tb/Yb(N), Zn/Fe ratios and as well as the Pb isotopic compositions and REE-based melting model reveal that Sivas, Erciyes Hasandağ, and Develidağ samples in central Anatolia, and Kula, Gediz basalt in western Anatolia seem to be derived from the amalgamated melting of pyroxenite and peridotite sources, besides, the sources melting is capable of the producing elemental variations in basaltic rocks related with either lithospheric delamination or lithospheric unstability
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This study is financially supported by Hacettepe University, BAB project no: FHD-2018-17283
How to cite: Kurkcuoglu, B. and Yürür, T.: Sr-Nd-Pb isotopic significance of mantle source components from Central and Western Anatolia: Melting evidences from peridotite and pyroxenite source domains, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12162, https://doi.org/10.5194/egusphere-egu21-12162, 2021.
Four OH stretching bands in the frequency range 3300 – 3700 cm-1 dominate the infrared spectra of lithospheric pyroxenes. Depending on their metasomatic history or geodynamic origin, they have characteristic OH signatures. Pyroxenes from continental lithosphere that undergone “wet” metasomatism have distinct signature of those having undergone “dry” metasomatism. Pyroxenes from oceanic lithosphere have yet a third type of signature. Our most recent analyses of xenoliths and a critical review of the literature show that the phenomenon is widely distributed among continents and oceans. The phenomenon affects simultaneously opx and cpx from the same rock and various lithologies: peridotites, pyroxenites and granulites. In continental lithosphere, pyroxenes affected by “wet” metasomatism are dominated by OH bands at 3600 and 3415 cm-1 for opx and 3635 and 3445 cm-1 for cpx . Whereas pyroxenes affected by “dry” metasomatism are dominated by OH bands at 3570 and 3515 cm-1 for opx and 3595 and 3515 cm-1 for cpx. Opxs from oceanic lithosphere have OH spectra dominated by the band at 3415 cm-1, and with a smaller by bands at 3520 and 3570 cm-1 (Fig. 1).
In all these observations it was not possible to correlate the signatures with a specific major, minor or trace element. Therefore, the exact nature of the observed signatures remains unidentified. Notwithstanding, these OH signatures are representative of specific lithospheric events and offer a potential new benchmark for the study of lithospheric processes.
Fig.1 : Schematic diagram showing the 3 types of signatures for opx. Spectra from opxs in oceanic lithosphere are from Gose, J., Schmadicke, E. and Beran A.: Geology, 37, 543-546, 2009. Drawing of subduction is from WangZ-Z., Liu,J., Xia, Q-K., Hao Y_T. and Wang Q-Y.: Lithos, 360-361, 2020.
How to cite: Ingrin, J.: A new tool to probe lithosphere evolution: OH signatures of pyroxenes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14209, https://doi.org/10.5194/egusphere-egu21-14209, 2021.
The Xigaze ophiolite (Tibet), which occurs in the central segment of the Yarlung Zangbo Suture Zone, exposes a complete portion of a mantle sequence that consists essentially of fresh as well as serpentinized peridotites. We studied a sequence beneath the crustal section that exposes fresh, Cpx-bearing harzburgites and dunites that are underlain by serpentinized Cpx-bearing harzburgites and dunites. The rocks at the bottom are crosscut by dykes that have undergone different degrees of rodingitization. The modal compositions of peridotite from both fresh and serpentinized sections plot in abyssal upper mantle fields, with clinopyroxene modes less than 5 vol. %. Although harzburgites and dunites indicate that melt has been lost relative to primitive mantle compositions, the trace element patterns carry signatures of enrichment in incompatible elements, such as (i) “bowl-shaped” patterns of trace elements in silicate-Earth normalized spider diagrams, (ii) positive anomalies in highly incompatible trace elements such as Rb, Th, U, Ta, and (iii) enrichment of LREE in the clinopyroxenes from dunites and harzburgites. These features are indicative of complex melt transfer processes and cannot be produced by simple melt extraction. Petrographic studies reveal that harzburgite and dunite contain interstitial polyphase aggregates of olivine + Cpx + spinel + Opx and olivine + Cpx + Spinel, respectively. Experimental studies (e.g. Morgan and Liang, 2003) suggest that these aggregates represent frozen melt-rich components, indicating that fertile melt was percolating through the depleted harzburgite – dunite matrix. Presence of such “melt pods” would explain the trace element enrichment patterns of the bulk rock, as well as features such as reverse zoning (core: Cr, Fe2+ rich, rim: Al, Mg rich) of spinels in polyphase aggregates in fresh dunites. These results show that melt extraction from the mantle is not a single stage process, and that evidence of multiple melt pulses that propagated through a rock are preserved in the petrographic features as well as in the form of chemical signatures that indicate refertilization of initially depleted rocks.
How to cite: Zhao, L., Chakraborty, S., and Schertl, H.-P.: Ophiolitic peridotites in Xigaze (Tibet): Constraints on modes of melt transport in the mantle, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8372, https://doi.org/10.5194/egusphere-egu21-8372, 2021.
Pyroxenites constitute the major form of heterogeneity in the upper mantle. Their occurrence in supra-subduction zone settings is mostly testified by veins and layers in refractory ophiolitic peridotites, where they represent a crucial witness of melt migration in the forearc/subarc environment [1,2]. The New Caledonia ophiolite hosts one of the largest forearc mantle section worldwide, providing a unique perspective into upper mantle processes. The sequence is dominated by ultra-depleted harzburgites , locally overlain by mafic-ultramafic cumulates [4,5,6]. The harzburgites are highly refractory residues that register a multi-phase evolution, including fluid-assisted melting in a forearc environment and contamination by fluid- and melt inputs triggered by Eocene subduction . Pyroxenitic rocks intruding the harzburgites are only known in the Bogota peninsula shear zone, which records HT deformation along a paleotransform fault . In this contribution, we report a comprehensive petrological and geochemical characterization on a new set of pyroxenites from this locality. The pyroxenites (~5-15 cm-thick) generally cut the peridotite foliation at variable angles, but concordant, locally boudinaged, layers also occur. Pyroxenite textures range from cumulitic to porphyroclastic or granoblastic-polygonal. The studied samples mostly consist of amphibole-bearing (5-44 vol.%) websterites, with variable amounts of orthopyroxene (27-67 vol.%) and almost constant clinopyroxene contents (~ 25-29 vol.%). Minor olivine-bearing orthopyroxenites are also present. Accessory phases include high-Ca (An= 82-86 mol%) plagioclase, Cr-rich spinel (Cr# = 50-61), sulfides and, occasionally, apatite. Pyroxenes displays high Mg# (Mg# Opx= 91-92; Mg# Cpx= 84-93), coupled with low Al2O3 contents (0.97-1.92 wt% and 1-2.42 wt% for orthopyroxene and clinopyroxene, respectively). Amphibole is high Mg# edenite. Application of conventional pyroxene thermometry yield equilibration temperatures ranging between 930-1040°C, comparable to the enclosing harzburgites (~ 950°C), whereas amphibole-plagioclase geothermometer provides lower temperatures (~ 800°C). Bulk rock composition of the websterites show variable Mg# (82-91) and REE concentrations ranging between 1 to 10 times chondritic values. They are characterized by flat to LREE-depleted (LaN/SmN 0.28-0.92) patterns, coupled to weak MREE-HREE fractionation (GdN/YbN = 1.73-1.92) and Eu negative anomalies. By contrast, orthopyroxenites display notably lower concentrations (0.1≤REE≤1 chondrite abundances). As a whole, clinopyroxene REE patterns of the websterites mirror bulk rocks at higher absolute values. Putative melts in equilibrium with clinopyroxene indicate strongly enriched compositions (up to 300 times chondritic values) coupled to variable LREE-HREE fractionation (LaN/LuN = 3-19) and flat to fractionated HREE (GdN/LuN 1-2). Such enriched liquids, which show some analogies with pre-obduction adakite-like dikes , have never been recorded in the MTZ cumulitic sequence of the New Caledonia ophiolite and shed new light on the magmatic activity in the early stage of subduction.
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How to cite: Ferrari, E., Secchiari, A., Montanini, A., and Cluzel, D.: Supra-subduction mantle pyroxenites in an infant subduction system: the New Caledonia ophiolite record., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15299, https://doi.org/10.5194/egusphere-egu21-15299, 2021.
Kukesi massif is located in the eastern part of the Mirdita Ophiolite (northern Albania), which marks suture after Neo-Thetyan ocean closure. It is formed of well-preserved mantle and crustal sections which exhibit Supra-Subduction Zone affinity (e.g. Dilek and Furnes 2009, Lithos). Lower part of the mantle section of the Kukesi massif consist mainly of harzburgites, whereas dunites are located close to Moho. Crustal section records transition from lower part formed by peridotites and pyroxenites (so called intermediate zone after Hoxha and Boullier 1995, Tectonophysics) to gabbros. In this study we focus on composition and origin of pyroxenites occurring in the mantle and lower crustal parts of the Kukesi massif.
In this study we studied 9 samples. They have composition of olivine websterite, clinopyroxenite, orthopyroxenite, hornblende-clinopyroxenite and websterite. Five of the analyzed samples have mantle origin (M): we studied (M)-olivine websterites and (M)-clinopyroxenite from harzburgitic part, as well as two (M)-orthopyroxenitic veins (one with clinopyroxenitic central part - composite vein) with minor amphibole cross-cutting dunites from one locality. From intermediate zone in crustal (C) part we collected (C)-hornblende-clinopyroxenites and (C)-websterite.
Clinopyroxene composition is homogeneous in (M)-olivine-websterites (Mg#=84.5-87 and 88.8-90.5; Al=0.07-0.1 and 0.05-0.07, respectively), (M)-clinopyroxenite (Mg#=84-86, Al=0.04-0.08), (C)-hornblende-clinopyroxenites (Mg#=88.5-91, Al=0.08-0.12a.p.f.u.) and (C)-websterite (Mg#=87-88; Al=0.13-0.16a.p.f.u.). It differs widely between (M)-orthopyroxenitic veins: from Mg#=85-94 and Al=0.02-0.08 a.p.f.u in clinopyroxenitic part of composite vein to Mg#=93.6-95 and Al=0.01-0.03 in the purely orthopyroxenitic one. Orthopyroxene from two samples of (M)-olivine websterites have either Mg#=83 and Al~0.07 a.p.f.u (Foolivine=81.5) or Mg#=87 and Al~0.04 a.p.f.u (Foolivine=86). Orthopyroxene composition in composite(M)-vein varies in wide ranges (Mg#=83-89; Al=0.04-0.08 a.p.f.u.); the other vein is homogeneous (Mg#=90-91, Al=0.02-0.03 a.p.f.u, Foolivine=86.8-90); in (C)-websterite orthopyroxene has Mg#=82.4-84 and Al=0.12-0.14 a.p.f.u. Amphibole has composition of tremolite-actinolite. Spinel, where present, is highly chromian (Cr#=0.59-0.80).
Clinopyroxene is LREE-depleted in most of the samples, the (La/Lu)N=0.03-0.08. It is also LREE-depleted in (M)-clinopyroxenite ((La/Lu)N=0.05-0.23), but the contents of trace elements are higher than in other samples (eg. LuN=0.79-2.75 vs. 0.40-0.85). In (M)-veins the LREE contents are approximately at primitive mantle level ((La/Lu)N=0.28-1.66). Clinopyroxene in all samples has positive Th-U, Pb and Sr anomalies and negative Ta and Zr anomalies, but concentrations of trace elements is significantly higher in (M) clinopyroxenite and veins.
The presence of tremolite and actinolite points to a retrogressive metamorphism which affected the rocks. The LREE-depleted nature of clinopyroxene forming all the pyroxenites and presence of orthopyroxene point to crystallization of the rocks from tholeiitic melt, but variations in Mg# and REE content in clinopyroxene may reflect formation either from different generations of melts or from melts fractionated due to reactive percolation. Variations in composition of the parental melts is visible even in a scale of one outcrop, which is demonstrated by (M)-orthopyroxenite veins with various modal composition and mineral major and trace elements compositions.
This study was financed from scientific funds for years 2018-2022 as a project within program “Diamond Grant” (DI 024748).
How to cite: Mikrut, J., Matusiak-Małek, M., Puziewicz, J., and Onuzi, K.: Pyroxenites of Kukesi Massif, Mirdita Ophiolite – geological record for magmatic system in SSZ environment – preliminary results, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15136, https://doi.org/10.5194/egusphere-egu21-15136, 2021.
The Masirah ophiolite is one of the few true ocean ridge ophiolites that have been preserved (Rollinson, 2017) and lacks any indication that it formed in a subduction environment. The Masirah ophiolite in south-eastern Oman is a different and older ophiolite from the more famous northern Oman ophiolite. Chromite and copper ores comprise large deposits in the Samail ophiolite, northern Oman. In comparison, chromite and copper deposits have not been described in previous reports or previous exploration in Masirah ophiolite. Rollinson (2017) has proposed that the apparent absence of chromitites in the mantle section of Masirah ophiolite is an important discriminant between subduction related and ocean ridge ophiolites. However, during recent studies on the Batain ophiolite mélange, and Masirah ophiolite, several chromitite pods have been discovered. The chromitites occur as separated small concordant, lenticular pods (3–10 m in thickness), which have been extensively altered and deformed, with the host pyroxenite serpentinites serpentinized harzburgites and dunites. The largest chromitite pods found within the pyroxenite and dunite of Masirah are up to 10 m across. Unusual minerals and mineral inclusions (orthopyroxene, clinopyroxene, amphibole, phlogopite, serpentine, native Fe, FeO, alloy, sulfide, calcite, laurite, celestine and halite) within chromite have been observed in the chromitites from the Masirah ophiolites. The existence of hydrous silicate inclusions in the chromite calls for a role of hydration during chromite genesis. Both phlogopite and hornblende were possibly formed from alkali-rich hydrous fluids/melts trapped within the chromite during the chromitite formation. High-T green hornblende and phlogopite included in the chromites is evidence of the introduction of water in the magma at the end of the chromite crystallization. Such paragenesis points to the presence of hydrous fluids during the activity of the shear bands. The chromitites parental magmas are rich in K, Na, LREE, B, Cs, Pb, Sr, Li, Rb and U relative to HREE, reflecting the alkalic fluids/melts that prevailed during the chromitites genesis.
The mineral inclusions in association with host peridotites may have been brought by the uprising asthenosphere at mid-oceanic ridges due to the mantle convection. It appears that this chromite has been formed through reaction between amid-ocean-ridge basalt-melt with depleted harzburgite in the uppermost mantle. The chromitite deposits have similar cr# (55-62% Al-chromitites), mg# Al2O3 and TiO2 contents to spinels found in MORB, and have been interpreted as having formed in amid-ocean ridge setting. This suggests that this chromitites is residual from lower degree, partial melting of peridotite, which produced low-Cr# chromitites at the Moho transition zone, possibly in a mid-ocean-ridge setting. The chemistry of both mineral inclusions and chromite suggests MORB-related tectonic setting for the chromitites that were crystallized at 1000 °C–1300 °C under pressures <3 GPa . The host peridotites were generated during the proto-Indian Ocean MORB extension and emplaced as a result of the obduction of the ophiolite over the Oman Continental margin during Late Cretaceous-Early Paleocene.
Rollinson, H., 2017. Geoscience Frontiers, 8: 1253–1262.
How to cite: Nasir, S.: New podiform chromitites Occurrence from the Masirah Ophiolite, Oman, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-72, https://doi.org/10.5194/egusphere-egu21-72, 2021.
Within the Samail Ophiolite, Oman, there are intervals of listvenite outcrops between layers of serpentinite zones above the basal thrust zone, atop the metamorphic sole. Near the base of the ophiolite mantle section, some peridotites underwent 100% carbonation from metasomatic introduction of CO2-bearing fluids <200°C to form listvenites during the time of emplacement (97 ± 29 Ma, Falk and Kelemen, 2015). The carbonate rocks comprise mostly magnesite and/or dolomite, quartz, Cr-spinel, and Fe-(hydr)oxides; with carbonates as the sole Mg-minerals and quartz as the only silicate phase. The aim of this study is to chemically and petrographically investigate the Fe-bearing minerals within the fluid-altered mantle rocks in drill core samples from hole BT1B of the ICDP Oman Drilling Project. Sequential chemical extractions are useful for recognizing iron pools based on the minerology. We investigated the quantities of Fe-oxide/hydroxide phases through a series of chemical extractions (Poulton and Canfield, 2005) via atomic absorption spectroscopy in addition to optical microscope, SEM/EDS, EPMA/WDS and ICP analysis. Extractions performed at room temperature and one at 50°C included: carbonate-associated Fe (sodium acetate) targeting siderite, HCl-extractable Fe(II), reducible oxides (citrate-dithionite) targeting hematite and possible goethite, and magnetite (oxalate). Carbonate-based Fe in the listvenites from a sodium acetate extraction ranges from 12-28 mg/g, while the same extraction performed at 50°C for twice as long resulted in higher proportions of carbonate-associated Fe (15-35 mg/g). Easily reducible iron quantities from the diluted HCl solution extraction display the lowest overall Fe fractions (0.75-5.5 mg/g) following the room temperature acetate and 0.63-1.7 mg/g after the 50°C acetate extraction. Fe in reducible oxides extracted by dithionite ranged from 1.4-15 mg/g with similar result after both a room-temperature acetate and a 50°C acetate step. Oxalate extraction succeeding the room-temperature acetate yielded magnetite concentrations of 1.9-8.0 mg/g, while the increased temperature and time in the first step (acetate extraction) were followed by significantly lower amounts of Fe extracted by oxalate (0.47- 3.6 mg/g). Additionally, the same extractions were performed on a pure siderite sample from Greenland. For siderite samples crushed a week prior to analysis, the carbonate-associated Fe in sodium acetate extract was 165±17 mg/g; the sidenote yielded 42 wt% of overall extracted Fe (392±33 mg/g). This is only slightly lower than the expected 48.2 wt% of Fe for a pure siderite sample. Dilute HCl extractions display results of 126±5.4 mg/g, dithionite solution extracted 25±0.5 mg/g and oxalate proportions were 76±9 mg/g. Due to possible oxidation of siderite to magnetite occurring during the time between powdering the samples and analysis, the full dissolution of siderite may not be fully represented in only the acetate. Microprobe data shows a total amount of FeO in carbonates as 1.3-10.8 wt%. This is more than or similar to the acetate and HCl proportions of Fe which represent carbonate associated minerals in the listvenites. Data obtained from EMPA and ICP will additionally be discussed in relation to the Fe-oxide phases with relation to the listvenites minerology.
How to cite: Severin, Z., Till, J. L., and Phase 1 Science Party, O. D. P.: Sequential geochemical extractions and mineralogy of Fe-bearing minerals in carbonatized mantle rocks in the Samail Ophiolite, Oman , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15840, https://doi.org/10.5194/egusphere-egu21-15840, 2021.
During the Upper Jurassic-Lower Cretaceous times the western margin of Gondwana in southern Patagonia experienced extreme lithospheric extension and generation of rift and marginal back-arc basins. The ophiolitic complexes of the Rocas Verdes basin comprises incomplete ophiolite pseudostratigraphy lacking ultramafic rocks. The Tortuga Ophiolitic Complex, the southernmost seafloor remnant of the Rocas Verdes basin, record the most advanced evolutionary stage of the back-arc basin evolution in a mid-ocean ridge-type setting. The base of the Tortuga Complex consists of massive and layered gabbros, most of which are two pyroxene and olivine gabbros, leucogabbros, and clinopyroxene troctolites intruded by dikes of basalt and diabase with chilled margins. We present new major and trace element composition of clinopyroxene from the gabbros and sheeted dikes complexes to assess the geochemical affinity of parental basaltic magmas. Clinopyroxene in gabbros is mostly augite and have Al contents of 0.06-0.14 a.p.f.u. and Mg# of 80-92. Clinopyroxene in dolerites in the sheeted dike unit (augite and diopside) have Al content of 0.11-0.12 a.p.f.u. and Mg# of 85-92. Some immobile trace elements (e.g. Zr, Ti, Y) are sensitive to the degree of partial melting and mantle source composition, and can be used as a proxy for distinguishing tectonic environments. The Ti+Cr vs. Ca diagram, coupled with moderate-high TiO2 content of clinopyroxene (0.4-1.4 wt.%) suggests their generation in mid-oceanic ridge-type environment (cf. Beccaluva et al., 1989). The high Ti/Zr ratios (of ~4-11) coupled with low Zr contents (~0.2-1.1) are expected for higher degrees of partial melting or for melting of more depleted mantle sources. Conversely, low Zr/Y ratios (0.05-0.13) plot between the range of arc basalts. Chondrite-normalized REE patterns in clinopyroxene display a strong depletion of LREE compared to HREE and have an almost flat pattern in the MREE to HREE with a positive Eu (Eu*= 0.9-1.1) anomaly, indicating that clinopyroxene crystallized from a strongly depleted mid-ocean-ridge-type basalt, formed by extensive fractional melting of the mantle source and/or fractional crystallization and accumulation of anhydrous phases. The general trend of the incompatible trace elements patterns exhibit depletion in LILEs, minor HFSEs depletion, positive anomaly of Rb and negative anomalies in Ba, Zr, Ti and Nb, consistent with their generation from a refractory mantle source barely influenced by subduction components derived from the oceanic slab. This agrees with basalt generation in a back-arc basin located far away from the convergent margin. This study was supported by the Fondecyt grant 1161818 and the Anillo Project ACT-105.
How to cite: Torres Garcia, F., Calderón, M., Fadel Cury, L., Theye, T., Opitz, J., Rojo, D., Hopfenblatt, J., and Fuentes, F.: Trace element composition of clinopyroxene in gabbros and dolerites of the Tortuga Ophiolitic Complex, southernmost Patagonia., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14475, https://doi.org/10.5194/egusphere-egu21-14475, 2021.
AD Kotze and RJ Roberts
Department of Geology, University of Pretoria, Hatfield, Pretoria, South Africa; email@example.com
The Karoo Large Igneous Province (KLIP) in South Africa consists of both a spatially limited extrusive basalt suite (Drakensberg Group) and a spatially extensive dolerite suite, both generally considered to be remarkable homogenous and of a “low-Ti” character (Luttinen, 2018). The homogeneity of the rocks requires that statistical analysis is necessary to look for spatial and geochemical trends in the data, which may yield clues to the mantle processes producing the 60 000 km2 expanse of basaltic magma. In this project, data derived from several locations are used as proxies to check for lateral variability in the Karoo dolerites. A principal component analysis (PCA) on trace element data using a covariance matrix was performed, and comparisons based on variables that are 1) common to the Karoo dolerites and Lesotho basalts and, 2) responsible for the most amount of variation to the data set are made. Trace element modelling is then used to test different mantle melting scenarios possibly responsible for the variation seen in the dolerites.
Principal component analyses revealed several trace elements are responsible for most of the variability in the dolerites. Cr and Ni has the strongest positive loading on Component 1 whereas Cr and Ba has the strongest positive loading on Component 2. Ba has a strong negative loading on Component 1. Cu, Sr, V and Zr do impart an appreciable amount of variation to the data, but all four variables have weak negative loadings on both components. Interestingly, the activity of Cu and V seems to be the inverse of that of Cr and Ni.
Due to the nature of a PCA, this work is afforded an opportunity to place the geochemistry of the Karoo dolerites within a larger geodynamic context without bias. From the observed variation, the activity of Ba and Cr is interpreted as an assimilation-oxidation process, whereas the Ni signature reflects the mantle origin of the magmas. Further modelling of these processes will allow the testing of suggested mechanisms for the formation of the KLIP, especially whether the magmatism is plume-related or related to the foundering of crustal blocks.
Luttinen, A., 2018. Bilateral geochemical asymmetry in the Karoo large igneous province. Scientific Reports, 8(5223).
How to cite: Kotze, A. and Roberts, R. J.: Spatial variability in Karoo dolerites, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5975, https://doi.org/10.5194/egusphere-egu21-5975, 2021.
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