Combining volatiles measurements in fluid inclusions with petrology of ultramafic xenoliths: new insights on the evolution of the West Eifel and Siebengebirge (Germany) Sub-Continental Lithospheric Mantle

<p>Integrating petrography and mineral chemistry data with the determination of volatiles concentration and isotopic fingerprint in fluid inclusions (FI) in ultramafic xenoliths opens a new window on the study of the Sub-Continental Lithospheric Mantle (SCLM). This frontier approach is crucial for understanding nature, evolution and volatiles recycling within the lithosphere, being particularly important in active or dormant volcanic areas, where the signature of the surface gaseous emissions can be compared to that of the deep mantle domains.</p><p>Five distinct populations of ultramafic xenoliths brought to the surface in West Eifel (~0.5-0.01 Ma) and Siebengebirge (~30-6 Ma) volcanic fields (Germany) were investigated by combining petrographic and mineral chemistry analyses with noble gases + CO<sub>2</sub> determinations in olivine-, orthopyroxene- and clinopyroxene-hosted FI. Xenoliths from West Eifel are modally and compositionally heterogeneous, as testified by the large forsterite range of olivine, the Cr# range of spinel and the variable Al and Ti contents of pyroxene. Siebengebirge rocks, on the other hand, are quite homogeneous, having mostly refractory composition and reflecting high extents (up to 30%) of melt extraction. Equilibration temperatures vary from 900 to 1180 °C in West Eifel and from 880 to 1060°C in Siebengebirge xenoliths, at comparable oxygen fugacity values. In all xenoliths populations, FI composition is dominated by CO<sub>2</sub>, with olivines being the most gas-poor phases and reflecting a residual mantle that experienced one or more melt extraction episodes. The <sup>3</sup>He/<sup>4</sup>He ratio corrected for air contamination (Rc/Ra values) in all phases varies from 6.8 Ra in harzburgites to 5.5 Ra in lherzolites and cumulates rocks, suggesting a progressive modification of an original MORB-like mantle signature via interaction with crustal-related components with <sup>3</sup>He/<sup>4</sup>He and <sup>4</sup>He/<sup>40</sup>Ar* signature similar to magmatic gaseous emissions. The mineral phase major element distribution, together with the systematic variations in FI composition, the positive correlation between Al-enrichment in pyroxene and equilibration temperatures, and the concomitant Rc/Ra decrease at increasing temperature, suggest that the SCLM beneath Siebengebirge represented the German lithosphere prior to the massive infiltration of melts/fluids belonging to the Quaternary Eifel volcanism. On the other hand, West Eifel xenoliths bear witness of multiple heterogeneous metasomatism/refertilization events that took place in the German SCLM between ~6 and ~0.5 Ma. According to Ne and Ar isotope systematics, the FI composition in the studied xenoliths can be explained by mixing between recycled air and a MORB-like mantle, being irreconcilable with the presence of a lower mantle plume beneath the Central European Volcanic Province.</p>

Metal enrichment as a result of SCLM metasomatism? Insight from ultramafic xenoliths from SW Poland.

<p>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).  </p><p>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.).</p><p>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 (Po<sub>15–99</sub>Pn<sub>0–20</sub>Ccp<sub>0–70</sub>)<sub></sub>and B (Po<sub>0–85</sub>Pn<sub>14–100</sub>Ccp<sub>0–27</sub>) are generally poorer in Ni compared to Group A (Po<sub>0–74</sub>Pn<sub>24–100</sub>Ccp<sub>0–35</sub>). 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 δ<sup>56</sup>Fe 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.</p><p>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.</p><p> </p><p>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.</p>

Nature and evolution of the northern Victoria Land lithospheric mantle (Antarctica) as revealed by ultramafic xenoliths

A review of northern Victoria Land ultramafic xenoliths, collected and studied over more than 30 years, was carried out. More than 200 samples were gathered and characterized in a coherent and comparative manner, both for mantle-derived and cumulate xenoliths. Almost 2000 analyses of major elements and more than 300 analyses of trace elements of in situ and separated olivine, pyroxenes, amphibole, spinel and glass were taken into consideration. Particular attention was devoted to mantle lithologies in order to emphasize the composition and the evolution of this portion of the subcontinental lithosphere. The three main localities in northern Victoria Land where mantle xenoliths were found (i.e. Mount Melbourne (Baker Rocks), Greene Point and Handler Ridge), over a >200 km distance, were described and compared with ultramafic xenoliths in three other localities (Harrow Peaks, Browning Pass and Mount Overlord) that are mainly cumulate in nature. Altogether, these data enabled us to reconstruct a long evolutionary history, from old depletion to most recent refertilization and metasomatic events, for this large sector of the northern Victoria Land subcontinental lithospheric mantle.

Petrofabric and geochemical features of ultramafic rocks on the example of restite metamorphites of the Kuznetsk Alatau (Western Siberia), olivine cumulates of the Yoko-Dovyren layered massif (Northern Cisbaikalia) and their analogues from ultrabasic xenoliths of the Canary Islands (Spain)

The article presents the results of petrostructural and mineralogical studies of olivine grains from ultrabasic and basic rocks of different genesis. In particular, they correspond to cumulative dunites of the Yoko-Dovyren layered massif (Northern Cisbaikalia), restite hyperbasites of the Velvet massif (Kuznetskiy Alatau), and xenoliths of peridotites from basalts of the Canary Islands (Spain). The relationship between the petrostructural and mineralogical features of ultrabasic rocks is shown, which makes it possible to identify their cumulative and restite varieties. An important element of the petrostructure of ultrabasites is the orientation of olivine, which reflects either the conditions for the formation of primary crystals in the magmatic melt, or the conditions for its recrystallization as a result of plastic deformations during exhumation to the Earth's surface. The mineral composition of rocks is an additional feature that reflects the real conditions of formation. In the presence of basic plagioclase, it is already quite difficult to speak about the restitic nature of these ultrabasic rocks. On the other hand, plastic deformations of olivine are characteristic of restitic hyperbasites, in which plagioclase is absent. The results of mineralogical studies in ultrabasic xenoliths of the Canary Islands showed the presence of basic plagioclase (labradorite), as well as chrysolite-type olivine (12-16 Fa), which corresponds to the attributes of ultrabasic layered intrusions such as the Yoko-Dovyren dunite-troctolite-gabbro pluton. For restite hyperbasites (by the example of the Kuznetsk Alatau), the iron content of olivine does not exceed 9-10% of the fayalite end, while plagioclase and clinopyroxene are absent. A characteristic feature of the xenoliths of the Canary Islands is the presence of endiopside, which is present in layered intrusions, but is absent in the restrained fragments of the upper and lower mantle. It is assumed that the xenoliths in the basalts of the Canary Islands are not of a mantle nature, but are fragments of a deep magma chamber. The studies of the optical orientation of olivine in xenoliths of the rocks of Lanzarote Island confirm these conclusions. The geochemical parameters of ultramafic xenoliths on Tenerife Island may well correspond to deeper formations.