Geochemistry of Noble Gases and CO2 in Fluid Inclusions From Lithospheric Mantle Beneath Wilcza Góra (Lower Silesia, Southwest Poland)

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 CO 2 ) 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. 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) Fo 88.9−91.5 , which accounts for a fertile-to-residual mantle, and (2) Fo 85.5−88.1 , 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. CO 2 predominates within FI, followed by N 2 . The δ 13 C of mantle CO 2 varies between −4.7‰ and −3.1‰, which mostly spans the MORB range (−8‰ < δ 13 C < −4‰). The 3 He/ 4 He 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 3 He/ 4 He 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-Małek 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 CO 2 -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.

Geochemistry of Fluid Inclusions in Travertines From Western and Northern Turkey: Inferences on the Role of Active Faults in Fluids Circulation

“…We highlight that the addition of secondary He occurs in the crystal matrix as observed in FI study. In addition, reticular helium is normally released via rock/mineral melting or multistep crushing, while we used single‐step crushing, which is a conservative method in order to minimize the release of helium from the crystal matrix (e.g., Correale et al, ; Kurz, ; Rizzo et al, ). However, other studies that used our approach found that in a sufficient time span a certain percentage of helium might diffuse from the crystal matrix into the FIs (Barry et al, ; Day et al, ).…”

“…The analytical uncertainty (1 σ ) of the He isotopic ratio is between 1.5% and 15% and that of 20 Ne is <0.4%. 20 Ne was corrected for isobaric interference at m / z 20 ( 40 Ar 2+ ) in accordance with INGV‐Palermo analytical protocol (e.g., Rizzo et al, ). 40 Ar was analyzed by a multicollector mass spectrometer (GVI Argus), with an analytical uncertainty (1 σ ) generally <0.4%.…”

“…H 2 O and CO 2 in FIs were extracted by single‐step crushing and quantified in terms of mol/g following the protocol described in Gennaro et al () and Rizzo et al (). These major gas species retained in FIs provide an important contribution for the understanding of the type of circulating fluids during the rock formation, irrespective of the origin of minerals where FIs are trapped.…”

“…In order to define the elemental and isotope composition of 3 He, 4 He, 20 Ne, and 40 Ar as well as the atmospheric content (N 2 + O 2 + Ar) in the FIs hosted in the travertines, the samples were carefully fragmented in order to select the most homogeneous part, thus handpicked under a binocular microscope from the >1‐mm sieved fraction. The selected crystals were cleaned in an ultrasonic bath following internal protocol (e.g., Rizzo et al, , ), and then each group of samples (0.2–5.2 g) was placed inside a stainless steel crusher capable of loading up to six samples simultaneously. FIs were released by in‐vacuum single‐step crushing at maximum 200 bars.…”

“…Typical blanks for He, Ne, and Ar were < 10 −15 , <10 −15 , and <10 −13 mol, respectively. Further details about the sample preparation and analytical procedures are available in Rizzo et al (2015Rizzo et al ( , 2018 and Correale et al (2019).…”