Abstract
Knowledge of the products originating from the subcontinental lithospheric mantle (SCLM) is crucial for constraining the geochemical features and evolution of the mantle. This study investigated the chemistry and isotope composition (noble gases and CO2) of fluid inclusions (FI) from selected mantle xenoliths originating from Wilcza Gora (Lower Silesia, southwest Poland), with the aim of integrating their petrography and mineral chemistry. Mantle xenoliths are mostly harzburgites and sometimes bear amphiboles, and are brought to the surface by intraplate alkaline basalts that erupted outside the north-easternmost part of the Eger (Ohře) Rift in Lower Silesia. Olivine (Ol) is classified into two groups based on its forsterite content: (1) Fo88.9–91.5, which accounts for a fertile-to-residual mantle, and (2) Fo85.5–88.1, which indicates large interactions with circulating (basic) melts. This dichotomy is also related to orthopyroxene (Opx) and clinopyroxene (Cpx), which show two ranges of Mg# values (87–90 and 91–93, respectively) and clear evidence of recrystallization. CO2 predominates within FI, followed by N2. The δ13C of mantle CO2 varies between –4.7‰ and –3.1‰, which mostly spans the MORB range (−8‰ < δ13C < −4‰). The 3He/4He ratio is 6.7–6.9 Ra in Cpx, 6.3–6.8 Ra in Opx, and 5.9–6.2 Ra in Ol. These values are within the range proposed for European SCLM (6.3±0.3 Ra). The decrease in 3He/4He from Cpx to Ol is decoupled from the He concentration, and excludes any diffusive fractionation from FI. The chemistry of FI entrapped in Ol indicates that the mantle is depleted by variable extents of partial melting, while that of Opx and Cpx suggests the overprinting of at least one metasomatic event. According to Matusiak-Malek et al. (2017), Cpx, Opx, and amphiboles were added to the original harzburgite by carbonated hydrous silicate melt related to Cenozoic volcanism. This process resulted in entrapment of CO2-rich inclusions whose chemical and isotope composition resembles that of metasomatizing fluids. We argue that FI data reflect a mixing between two endmembers: (1) the residual mantle, resulting from partial melting of European SCLM, and (2) the metasomatic agent, which is strongly He-depleted and characterized by MORB-like features.
Highlights
During the last 60–70 Myr, Europe, and the circum-Mediterranean area have experienced extensive volcanic activity related to their complex geodynamic evolution
This study investigated the chemistry and isotope composition of fluid inclusions (FI) from selected mantle xenoliths originating from Wilcza Góra (Lower Silesia, southwest Poland), with the aim of integrating their petrography and mineral chemistry
Considering that most of the studied Ol samples (WG1II, WG2II, WG3II, WG6II, WG8II, and WG9II) represent the residual mantle, while Cpx and some of the Opx samples (WG4II, WG7II, and WG10II) are partially or completely modified by the metasomatic process, we argue that FI from Ol and Px represent a mixing of two endmembers: (1) a residuum after partial melting, characterized by narrow variability of 3He/4He (∼6 Ra) and δ13C (∼ −3.8‰), and (2) a metasomatic agent, characterized by more-primitive and He-depleted fluids originating from an asthenosphere melt, originally with a MORB-like 3He/4He ratio (∼8 Ra) and δ13C ∼ −4.2‰ (Figures 7B, 10)
Summary
During the last 60–70 Myr, Europe, and the circum-Mediterranean area have experienced extensive volcanic activity related to their complex geodynamic evolution. Most cases of anorogenic magmatism started up to 30 Myr later than orogenic magmatism, and brought mantle-derived ultramafic xenoliths to the surface These xenoliths represent an essential source of information about (1) the nature and evolution of the European lithospheric mantle, (2) the genesis of basic magmas, and (3) magma–mantle geochemical interactions at mantle depth or at the Earth’s surface (Downes et al, 2001; Gautheron and Moreira, 2002; Coltorti et al, 2009, 2010; Bonadiman and Coltorti, 2011; Day et al, 2015). The northeastern prolongation of this rift is located in Lower Silesia (southwest Poland) at the margin of the Variscan orogenic belt, where some SiO2undersaturated basalts carry mantle xenoliths (Cwiek et al, 2018 and references therein) Most of these xenoliths are anhydrous spinel-bearing harzburgites and dunites, but there are some lherzolites. Those two factors resulted in the formation of strong heterogeneities that are visible in the modal and chemical compositions of the mantle
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