⁴⁰Ar ∕ ³⁹Ar age constraints on the formation of fluid-rich quartz veins from the NW Rhenohercynian zone (Rursee area, Germany)

A substantial part of the subsurface geology in northwestern and central Europe is defined by the late Palaeozoic Variscan Orogeny (∼ 350 Ma). Our focus is mainly on veining in anchimetamorphic sedimentary rocks affected by this orogeny. Mineral veins serve as repositories for documenting the origin of subsurface fluid flows and dynamics, and dating them may provide crucial insight into the timing of orogenic and possible reactivation events. The Rursee area (Rhenish Massif, Germany), part of the Variscan foreland zone on the Avalonia microcontinent, represents a key locality for studying Variscan quartz vein formation. Based on structural grounds, two groups/types of Rursee quartz veins have been linked with the early stages of Variscan, but their absolute ages are still unknown.

The aim of this study is to date these quartz veins using the ⁴⁰Ar $/$ ³⁹Ar stepwise crushing method based on the radioactive decay of ⁴⁰K dissolved in high-salinity fluid inclusions. We obtained Jurassic to Cretaceous ages, and isotopic analysis of the argon gases revealed that the fluid-rich quartz fractions release ³⁹Ar in two distinct phases. Regardless of the salinity of fluid inclusions in quartz veins, stepwise crushing provides apparent K $/$ Cl > 1. Electron probe micro-analyser data confirm the presence of K (³⁹Ar) in the K-bearing mineral inclusions (e.g. sericite, white mica, and chlorite) and microcracks and possibly in the crystal lattice of quartz.

Secondary fluid inclusions or K-bearing mineral inclusions and/or the crystal lattice of quartz, which formed subsequently in the Variscan vein fractures, provide a plausible explanation for the young apparent isotopic ages. Deformation-induced quartz sub-grains may suggest that obtained maximum apparent ages are likely to reflect post-Variscan fluid-assisted reactivation–recrystallization due to tectonic activity or its cooling moment during the Jurassic–Cretaceous period rather than the original Variscan vein formation.

1 Introduction

Quartz veins are abundant in metamorphosed terranes and sedimentary basins filled with siliciclastic sediments, witnessing significant fluid movement during diagenesis and metamorphism (Yardley, 1983; Mullis et al., 1994; Cartwright and Buick, 2000; Oliver and Bons, 2001). An increase in both temperature and pressure during burial diagenesis, orogenesis, and deformation causes sedimentary and volcanic rocks to lose their volatile components and to release warm fluids, which cumulate minerals in fractures and faults (Baumgartner and Ferry, 1991; Yardley and Bottrell, 1993; Oliver and Bons, 2001; Cox, 2007). These often saline fluids contain, among others, KCl _(aq) or K₂CO_3 (aq) (Rauchenstein-Martinek et al., 2014), which are partly precipitated during the crystallization of minerals in veins or as inclusions in these minerals (Sterner et al., 1988). One of the isotopes of potassium, ⁴⁰K, is radioactive and can be used for K–Ar or its derivative, ⁴⁰Ar $/$ ³⁹Ar dating. Progressive crushing techniques enable the liberation of gases from fluid inclusions, mineral inclusions, and/or crystal lattice for the age determination of geological events provided that K concentrations are high enough (Qiu and Dai, 1989; Turner and Bannon, 1992; Turner and Wang, 1992; Qiu, 1996; Kendrick et al., 2001; Qiu and Wijbrans, 2006; Kendrick et al., 2006; Qiu and Wijbrans, 2008; Qiu and Jiang, 2007; Jiang et al., 2012; Bai et al., 2013; Liu et al., 2015). This method not only defines an age but also quantifies the ratio of noble gases (e.g. ³⁹Ar_K $/$ ³⁷Ar_Ca, ³⁹Ar_K $/$ ³⁸Ar_Cl) derived from Ca, K, and Cl, respectively, that have been formed during neutron radiation before analysis. The ³⁹Ar_K $/$ ³⁸Ar_Cl provides crucial information on the composition of parental fluids and their sources (Sumino et al., 2011; Cartwright et al., 2013). Beyond studies on fluid composition and provenance (Kelley et al., 1986; Turner and Bannon, 1992; Kendrick et al., 2001, 2006), the initial ⁴⁰Ar $/$ ³⁶Ar values of fluid inclusions in quartz can considerably vary and may be used to differentiate between meteoric-sourced water (∼ 298.6) (Ballentine et al., 2002; Ozima and Podosek, 2002) and deeper crustal or mantle-derived fluids (> 10 000; MORB > 40 000) (Burnard et al., 1997). Additionally, hydrothermal waters can present sub-atmospheric ⁴⁰Ar $/$ ³⁹Ar ratios, as can be deduced from inverse isochrons of fluid-altered rocks (e.g. 280–290; Baksi, 2007).

To accurately determine the age of fluid inclusions in quartz veins using the ⁴⁰Ar $/$ ³⁹Ar stepwise crushing method or the source of the fluid based on ⁴⁰Ar $/$ ³⁶Ar ratios, it is necessary to consider three distinct components of ⁴⁰Ar, namely (1) radiogenic ⁴⁰Ar_R or ⁴⁰Ar*, which is produced in the sample itself through the radioactive decay of ⁴⁰K, and (2) ⁴⁰Ar that was initially trapped in the fluid inclusion, either as (2a) atmospheric ⁴⁰Ar_A or (2b) excess ⁴⁰Ar_E. The presence of ⁴⁰Ar_E in fluid inclusions could be a challenge in determining accurate vein formation ages using the K–Ar dating technique (Rama et al., 1965). More recently, isochron diagrams using ⁴⁰Ar $/$ ³⁹Ar geochronology helped to overcome this issue (McKee et al., 1993; Qiu, 1996; Qiu et al., 2002). In addition to ⁴⁰Ar_E, the origin of ³⁹Ar_K (or K content) has been a topic of debate, with the possibility that the ³⁹Ar_K (and thus K) may come from the dissolved salts in fluid inclusions, leaking from the crystal lattice during crushing (Kendrick et al., 2011), and/or from any K-bearing mineral inclusions trapped inside the crystals (Qiu and Wijbrans, 2006; Kendrick, 2007; Qiu and Wijbrans, 2009; Kendrick and Phillips, 2009).

This study aims (i) to determine the absolute age of quartz vein formation by analysing fluid inclusions using the stepwise crushing ⁴⁰Ar $/$ ³⁹Ar dating method, (ii) to elucidate the location of K in the vein minerals (e.g. fluid inclusions, mineral inclusions, and/or crystal lattice), and (iii) to identify when different K sources release their ³⁹Ar_K through the examination of released argon gases during the crushing process and geochemical analysis of quartz mineral samples using an electron probe micro-analyser (EPMA).

Quartz samples were obtained from an outcrop near the Rursee in the upper reaches of the Rur River in the North Eifel region of western Germany. Detailed structural investigations of this area have been previously conducted by Van Noten et al. (2007), who differentiated quartz veins into two groups. The older generation of quartz veins, the so-called bedding-normal veins (BNVs), is assigned to the early stages of the Variscan Orogeny, and the second group, comprising bedding-parallel veins (BPVs), is linked to the main stage of the Variscan Orogeny. Absolute ⁴⁰Ar $/$ ³⁹Ar ages of fluid inclusions representing the age of quartz vein formation would allow us to better constrain the structural evolution and subsurface fluid flow during the Variscan Orogeny in northwestern Europe. Reliable ⁴⁰Ar $/$ ³⁹Ar age constraints of quartz vein formation would provide the opportunity to understand the timing and evolution of mountain building in analogue fold-and-thrust belts.

Geological setting

The Rhenohercynian fold-and-thrust belt, part of the Variscan, is primarily located in the Rhenish Massif in Germany and extends westward into the Ardennes, southwest England, and eastward to the Harz Mountains (Kötonik et al., 2018). The Ardennes allochthon (Fig. 1a), the western part of the Rhenish Massif, structurally comprises three main components: the Dinant fold-and-thrust belt, the lower Palaeozoic inliers, and the High-Ardenne Slate Belt (HASB). The HASB primarily consists of Lower Devonian metasediments, including the Rurberg (upper Pragian) and Heimbach (upper Pragian to lower Emsian) units.

Figure 1 (a) Geological map with the Variscan frontal zone in the Ardennes–Eiffel region (study area marked with a red star). (b) Geological map of the North Eifel region (modified after Ribbert et al., 1992; Van Noten et al., 2011). The Lower Devonian layers overlay metamorphic deposits of the lower Palaeozoic Stavelot–Venn inlier. These layers have been locally distorted in the Monschau Shear Zone (MSZ), as documented by Fielitz (1992). Triassic sediments overlay the Lower Devonian layers in the eastern region. The sample location, indicated by a green star, is situated next to the Rursee reservoir, which is near the Schwammenauel dam. Below, the cross-section illustrates the continuous northwest–southeast-trending overturned folds that characterize the North Eifel zone.

For this study, quartz vein samples were collected near the Schwammenauel dam in the Rursee area of the North Eifel region, Germany (Fig. 1b). The Rurberg and Heimbach units feature alternating layers of siltstones and fine- to coarse-grained sandstones (Goemaere and Dejonghe, 2005), deposited in shallow marine to deltaic environments in the northern Rhenohercynian Ocean (Oncken et al., 1999). The Early Devonian strata have accumulated to a total thickness of up to 7 km due to rapid subsidence and deposition (von Winterfeld, 1994), forming the Eifel syncline (Fig. 1b). These strata are overlain by a ∼ 3 km thick sequence of lower Lochkovian to Pragian deposits.

The late Carboniferous deformation of the Variscan foreland led to initial burial metamorphism (Mansy et al., 1999), with prehnite–pumpellyite facies similar to the anchizone conditions in the North Eifel area (Fielitz, 1995), where temperatures reached up to 220 °C (Littke et al., 2012). There is also evidence of the upward migration of warm fluids into the northern Variscan front in the Ardennes, driven by Variscan thrusting (Muchez et al., 2000; Schroyen and Muchez, 2000; Lünenschloss et al., 2008).

Following the Variscan period, the Rhenish Massif has been affected by transpressional and transtensional deformation that resulted in the formation of complex fault networks that host vein mineralization (Franzke and Anderle, 1995; Ziegler and Dèzes, 2005). During the Jurassic–Cretaceous period, the southern Rhenish Massif was periodically affected by hydrothermal activities (Kirnbauer et al., 2012), as indicated by geochronological data for post-Variscan vein mineralization (Bonhomme et al., 1983; Mertz et al., 1986; Bähr, 1987; Jakobus, 1992; Hein and Behr, 1994; Klügel, 1997; Schneider and Haack, 1997; Glasmacher et al., 1998; Schneider et al., 1999; Chatziliadou and Kramm, 2009).

The ⁴⁰Ar $/$ ³⁹Ar study targets the BNVs and BPVs (Fig. 2), which formed in low-grade metamorphosed (prehnite–pumpellyite facies) conditions as a result of the precipitates from warm fluids in fractures (Van Noten et al., 2008). The structural cross-cutting relationships between these quartz vein generations suggest that they originated from different geological events (Van Noten et al., 2008), revealing that BPVs are younger than BNVs. BNVs are mostly found within the competent psammite and hardly occur in incompetent pelitic layers. This positioning suggests that BNVs formed during the early stages of the Variscan Orogeny, associated with the final burial phases of the Ardennes–Eifel basin (Sintubin et al., 2000; Urai et al., 2001; Van Noten et al., 2008, 2009).

Figure 2 Images of studied outcrops from the Rursee area. The image in (a) presents the bedding-normal veins (red lines), while panel (b) shows the bedding-parallel veins (red lines). Yellow lines indicate the bedding in both images.

In contrast, BPVs follow the strata between the psammatic and pelitic layers due to bedding-parallel slip caused by flexural folding during the Variscan Orogeny (Van Noten et al., 2008).

2 Material and methods

2.1 Quartz and inclusions in quartz minerals

In total, seven samples of different veins (three BNVs and four BPVs) were collected from the Rursee outcrop for ⁴⁰Ar $/$ ³⁹Ar analysis (Table 1). Both vein types mainly consist of elongated-fibrous milky quartz grains that exhibit syntaxial growth, whereby the growth starts from the wall of the veins towards the central part of the veins (Ramsay, 1986). The pelitic host rock consists of sericite, illite, white mica, and chlorite. Chlorite is abundant within the vein fractures and between the host rock and the vein wall.

Table 1 Summary of ⁴⁰Ar $/$ ³⁹Ar age spectra, including inverse isochron data of all analysed quartz samples. The maximum apparent ages of the late converging section and inverse isochron selected, as discussed in the text, are highlighted in bold, with an asterisk marking those used for geological interpretation.

Both quartz vein generations lack primary fluid inclusions in the crystal growth zones and contain pseudo-secondary and secondary fluid inclusions (FIs) (< 10 µm) (Van Noten et al., 2011) in the sealed microcracks being perpendicular to crystal elongation (Fig. 3). The Rursee quartz vein samples yield average fluid inclusions homogenization temperatures (minimum trapping temperature, T_h) of ∼ 135 ± 25 and ∼ 160 ± 20 °C for BPV and BNV, respectively, with salinities of 3.5–8 eq. wt % NaCl. The T_h values of pseudo-secondary and secondary fluid inclusions cover an equally broad range of 110–180 °C (Van Noten et al., 2011).

Figure 3 Fluid inclusions in quartz veins under optical microscopy. (a) Image of BNVs under crossed polarizer light microscopy. Both (a.1.1) and (a.2.1) are the zoomed-in view of the images in (a.1) and (a.2), respectively, indicating pseudo-secondary fluid inclusions. (b) Crossed polarizer images of the BPV sample under microscopy. The images in (b.1.1) and (b.2.1) are areas focused on secondary and pseudo-secondary fluid inclusion (respectively), which are zoomed-in views of the images in (b.1) and (b.2), respectively. The white arrows represent the fluid inclusion assemblages (FIAs). Both generations of quartz veins bear FIAs in sealed microcracks rather than in crystal growth zones.

2.2 Mineral separation

Before ⁴⁰Ar $/$ ³⁹Ar analysis, mineral separation was conducted at Vrije Universiteit Amsterdam (VU; The Netherlands). The bulk vein samples were crushed, washed, and cleaned in an ultrasonic bath for at least 1 h to remove the adhering host rock contaminants from quartz grains. The samples were sieved into 250 and 500 µm fractions and dried in an oven at 60 °C. The samples were further separated by a custom-made system using an overflow centrifuge with conventional heavy liquids based on IJlst (1973) and Frantz magnetic separation (Porat, 2006). We used heavy liquids with a density of 2.62 and 2.64 g cm⁻³ to obtain a fluid inclusion-rich fraction of quartz grains (ρ= 2.62–2.64 g cm⁻³). The fraction was rinsed with acetone, dried, and further sieved to separate the 400–500 µm grain size range. From this fraction, only the purest quartz grains were handpicked under a binocular microscope for ⁴⁰Ar $/$ ³⁹Ar dating.

2.3 ⁴⁰Ar $/$ ³⁹Ar stepwise crushing

Fluid-rich quartz grains (400–500 µm; 2.62–2.64 g cm⁻³) were carefully selected under a binocular zoom microscope, and a quantity of 200–270 mg of material was packed in aluminium foil and placed in 20 mm i.d.–22 mm o.d. aluminium cups. Drachenfels (DRA-2) sanidine standard was loaded after each three samples to monitor the neutron flux. The samples were irradiated at Oregon State University (USA) using the CLICIT (Cadmium-Lined In-Core Irradiation Tube) facility for 12 h (batch VU123). After irradiation, standards were placed in 2 mm copper planchet holes for single grain fusion analysis and pre-baked in a vacuum at 250 °C. The samples were then placed in an ultra-high vacuum system; baked at 120 °C; and connected with hot NP10 and ST172 getters, a Ti getter sponge at 400 °C, and a cold trap at −70 °C. The standards were fused with a Synrad 48-5 CO₂ continuous-wave laser fusion system.

The samples were crushed in an in-house developed and built crusher consisting of a stainless-steel tube (height: 18 cm, outer diameter: 1.8 cm) that has a spherical curve on its interior base and a magnetic stainless-steel pestle (height: 5 cm, diameter: 1.6 cm, weight: ∼ 69.5 g) with rounded tips with a slightly narrower outer radius. These geometries allow optimization of the impact on the sample while crushing. Once a split of the sample (∼ 30 mg of quartz grains) was loaded into the crusher tube, the pestle was carefully relocated to the bottom of the tube to avoid crushing the sample. The crush tube, the pestle, and the sample were baked overnight at 250 °C. The pestle was dropped into a free-fall state using an external electromagnet with a frequency of 1 Hz controlled by an adjustable power supply and pulse generator to crush the sample. The pestle was dropped from a height of ∼ 3, ∼ 4, or ∼ 5 cm in vacuo. Subsequently, the gases emitted from fluid inclusions in the fragmented quartz sample were analysed. To obtain a sufficient amount of argon in the mass spectrometer, the number of pestle drops per extraction step and drop height were systematically increased during the experiment, with a maximum of 999 drops per analysis (in total, ∼ 40 000 cumulated pestle drops per experiment).

The gas released from the samples and standards was analysed isotopically using a ThermoFisher Scientific Helix MC+ mass spectrometer. The Helix MC+ mass spectrometer is a 5-collector channel instrument, equipped with a total of 10 collectors, a Faraday (Far) collector optionally fitted with a 10¹² or 10¹³ Ohm resistor amplifier, and a compact discrete dynode secondary electron multiplier (CDD-SEM) collector on each collector channel. Five collectors can be used at the same time to simultaneously collect the beam intensity signals of the five isotopes of argon. The H2 Faraday collector is employed to detect ⁴⁰Ar using a 10¹³ Ohm amplifier. Similarly, the H1 CDD collector is used for the measurement of ³⁹Ar (H1 Faraday was used for the runs on the DRA-2 sanidine standard because of the higher ³⁹Ar signal), the AX CDD collector for ³⁸Ar, the L1-CDD collector for ³⁷Ar, and the L2 CDD collector for ³⁶Ar.

Line blanks were measured after every three to four unknowns and subtracted from the succeeding sample data. Gain calibration is done by correcting for gain relative to the beam intensity measured on the AX CDD, using measurements of ∼ 50 fA (⁴⁰Ar-measured beam intensities) pipettes of air on each cup, and mass discrimination corrections are made by measuring a series of ∼ 400 fA (⁴⁰Ar-measured beam intensities) air pipettes roughly every 12 h. Raw data were processed using the ArArCALC software (Koppers, 2002). Ages are calculated relative to Drachenfels (DRA-2) sanidine of 25.552 ± 0.078 Ma (Wijbrans et al., 1995), which was recalibrated against Fish Canyon Tuff sanidine of 28.201 ± 0.023 Ma (Kuiper et al., 2008). The decay constants of Min et al. (2000) are used. The atmospheric ⁴⁰Ar $/$ ³⁶Ar ratio of 298.56 ± 0.31 is based on Lee et al. (2006). The correction factors for neutron interference reactions are (2.64 ± 0.02) × 10⁻⁴ for (³⁶Ar $/$ ³⁷Ar)_Ca, (6.73 ± 0.04) $\times {\mathrm{10}}^{-\mathrm{4}}$ for (³⁹Ar $/$ ³⁷Ar)_Ca, (1.21 ± 0.003) $\times {\mathrm{10}}^{-\mathrm{2}}$ for (³⁸Ar $/$ ³⁹Ar)_K, and (8.6 ± 0.7) $\times {\mathrm{10}}^{-\mathrm{4}}$ for (⁴⁰Ar $/$ ³⁹Ar)_K. Gain correction factors and their standard errors (±1 SE) are 1.00162 ± 0.00028 for H2 Far, 0.97963 ± 0.00021 for H1 CDD, 0.99921 ± 0.00027 for L1 CDD, and 0.96163 ± 0.00064 for L2 CDD for data measured in 2022 (R2.1) and 1.00465 ± 0.00031 for H2 Far, 0.97033 ± 0.00027 for H1 CDD, 0.99824 ± 0.00033 for L1 CDD, and 0.96309 ± 0.00070 for L2 CDD for data measured in 2023 (R1–R6). The K $/$ Cl ratios are calculated by K $/$ Cl $=\mathit{\beta }{\times }^{\mathrm{39}}$Ar $/$ ³⁸Ar, with β= 0.06 derived from K $/$ Cl = ∼ 18.7 in GA1550 and ³⁹Ar_K $/$ ³⁸Ar_Cl= ∼ 316 for a 12 h irradiation at the OSU Triga CLICIT facility. All errors are quoted at the 2σ level and include all analytical uncertainties (Table 1).

Please note that it is impossible to directly correct the crushing blank because we cannot perform the exact experiment without crushing sample material. We tested the blanks for each tube without sample material, following the identical procedures as for real experiments. With this approach, we have direct metal-to-metal contact during pestle drops, which might not be representative of a real sample. We did observe a substantial increase in background, with a higher number of drops and a higher drop level. Notably, the composition of this blank is similar to that of atmospheric argon. Therefore, we follow the approach that the ⁴⁰Ar signal derived from the line blank (measured every three to four unknowns where we mimic the sample experiment but without the crushing/pestle drops) is subtracted from the measured ⁴⁰Ar intensity. The real blank has an atmospheric ⁴⁰Ar $/$ ³⁶Ar ratio and is incorporated in the air corrections, leading to a lower radiogenic ⁴⁰Ar* if the real blanks are relatively high.

2.4 Electron probe microanalysis

Quartz grains of sub-samples that were analysed for ⁴⁰Ar $/$ ³⁹Ar were mounted in epoxy resin and carbon-coated for the JEOL JXA-8530F Hyperprobe field emission electron probe micro-analyser (EPMA) at Utrecht Universiteit (UU; the Netherlands) to define the elemental compositions of (1) the host quartz; (2) minerals that are present in fluid inclusions, filled cavities, or fractures; and (3) mineral inclusions in the quartz. For this analysis, an accelerating voltage of 15 kV and a beam current of 8 nA for host rock (quartz) and 7 nA for mineral inclusions are used with beam sizes of 10 and 1 µm, respectively. The elements analysed are Si, Ti, Al, Fe, Mn, Ca, Na, K, P, Cl, F, Ba, and Zr. The data are calibrated using Icelandic rhyolite glass (ATHO-G) and basalt glass (KL2-G) standards that were measured with a beam size of 10 µm multiple times both before and after measurements of the samples.

3 Results

The age spectra of the in vacuo stepwise crushing of the quartz samples are plotted in Fig. 4. All samples show typical release patterns with unrealistically old apparent ages (> 6 Ga) in the initial 10 % of ³⁹Ar_K released. Note that samples Rursee 1a BNV and Rursee 1b BNV are measured in two different experiments on subsets from the same irradiated sample, yielding different results. A lighter pestle (68 g) has been used for the sample Rursee 1a BNV than for sample Rursee 1b BNV (69.5 g) and all other samples.

Figure 4 The apparent late converging section age of all quartz vein experiments. The red boxes focus on the last part of the age spectra, where apparent ages are more or less stable.

The apparent ages of the spectra in samples Rursee 1b BNV, Rursee 2 BPV, Rursee 2.1 BNV, and Rursee 4 BPV exhibit a gradual decrease in age over the next 10 %–40 % of ³⁹Ar_K released, eventually stabilizing at a more or less consistent maximum apparent age from ∼ 80 % ³⁹Ar_K to ∼ 100 % ³⁹Ar_K. Rursee 3 BPV, Rursee 5 BNV, and Rursee 6 BPV show comparable behaviour with, after the initial old apparent ages, a decrease in a maximum apparent age to an “early converging section” from ∼ 15 % to ∼ 40 % ³⁹Ar_K released, followed by a gradual decrease in apparent age and a more or less uniform apparent age in the > 80 % released ³⁹Ar_K part of the spectrum. For these early converging sections, we arrive at averaged maximum apparent ages of ∼ 84 Ma for Rursee 1b BNV, ∼ 97 Ma for Rursee 2 BPV, ∼ 117 Ma for Rursee 4 BPV, ∼ 216 Ma for Rursee 2.1 BNV, ∼ 190–200 Ma for Rursee 5 BNV and Rursee 6 BPV, and ∼ 560 Ma for Rursee 3 BPV. The maximum apparent ages of Rursee 2.1 BNV and Rursee 4 BPV correspond to the inverse isochron maximum apparent ages; however, due to significant uncertainty, the maximum apparent ages of other samples were obtained from the average “late converging section” age (Table 1).

The inverse isochrons (Fig. 5) confirm that the first part of all experiments is heavily affected by excess argon (³⁶Ar $/$ ⁴⁰Ar ratios are much lower than atmospheric composition), followed by an increase in ³⁶Ar $/$ ⁴⁰Ar and ³⁹Ar $/$ ⁴⁰Ar ratios and clustering of data points on the reference line. We derived maximum apparent ages from the data points that cluster along the reference line in the isochrons in the final part of the age spectra. There is no systematic maximum apparent age difference between BNV and BPV.

Figure 5 Inverse isochrons of all quartz vein samples. The black line corresponds to a regression where the ⁴⁰Ar $/$ ³⁶Ar intercept with the vertical is fixed at the ratio of atmospheric argon, and hence its radiogenic intercept corresponds to the age obtained from a regression of the corresponding steps in the age spectrum, whereas the pink line represents a regression without this restriction, providing us with information on the actual ⁴⁰Ar $/$ ³⁶Ar ratio of non-radiogenic components in the fluid mixture.

All quartz samples release argon during in vacuo stepwise crushing with different isotopes of argon contributing to the gas release at different stages of the experiment. Figure 6 shows, for each step, the percentage (relative to the total amount) of a specific isotope released through the experiment. All quartz samples are characterized by a release of most of the ³⁶Ar_air in the first 20 steps. ⁴⁰Ar* and ³⁸Ar_Cl follow the pattern of ³⁶Ar_air. The ³⁹Ar_K generally increases after the first 20 analysing steps (∼ 790 pestle drops from 3 cm height). At steps 30–35, we observe fluctuations in the data. These shifts are artefacts caused by increasing the drop height (from 3 to 4 cm at ∼ step 30 and from 4 to 5 cm at ∼ step 35) and adjusting the number of pestle drops. To prevent high signals, we started with a relatively low number of pestle drops at a higher drop height, yielding low signals, as observed as two troughs at ∼ step 30 and ∼ step 35 in all experiments. All quartz samples are low in ³⁶Ar_air, ³⁸Ar_Cl, and ⁴⁰Ar* at the end of analysis compared to their total release. For ⁴⁰Ar*, we still measure a small, reliable signal, but this is obscured in Fig. 6 due to the high signals in the first steps since we plot percentages of the total released ⁴⁰Ar per experiment. Note that huge amounts of excess ⁴⁰Ar (as part of the ⁴⁰Ar* signal) are released in the initial steps of the experiment and dominate the total percentage.

Figure 6 Released argon isotopes per analysing step relative to their total release. Note that the data are expressed against the analysing steps instead of the crushing steps, and that the upper x-axis scaling (cumulative pestle drops) is neither linear nor logarithmic (non-continuous scaling).

4 Discussion

During in vacuo stepwise crushing, the release of argon isotopes from the samples follows systematic patterns. The challenge is to link this release of argon from the samples to the different potential reservoirs of K and, as a next step, the geological meaning of the age and elemental ratios of K $/$ Cl and Ca $/$ Cl. Here, we first discuss potential issues related to the analytical quality of the data. Next, we discuss potential reservoirs of K and subsequently ⁴⁰Ar* that need to be considered, evaluating the maximum apparent ages and their wider implications.

4.1 Data quality

4.1.1 Rursee 1a and 1b BNV

We speculate that for the Rursee 1a BNV experiment, we sampled a smaller part of the argon reservoirs in the quartz minerals comparable to the first 10 % of the spectrum of Rursee 1b BNV. This is corroborated by the fact that for Rursee 1a BNV, 46 mg of quartz released 12.7 fA ³⁹Ar_K (0.3 fA mg⁻¹ quartz), while for Rursee 1b BNV, 89.1 fA was released from 25 mg of quartz (3.6 fA mg⁻¹ of quartz). We therefore do not further discuss the results of Rursee 1a BNV but note that sample heterogeneity might also have contributed to this difference.

4.1.2 Impact of blank correction

The blank correction procedure likely does not impact the weighted mean age computation; however, it does influence the ⁴⁰Ar $/$ ³⁶Ar intercept of the inverse isochron. This is only the case when the regression line has a non-radiogenic intercept that differs from the atmospheric ³⁶Ar $/$ ⁴⁰Ar. When the intercept is within the error overlapping with the atmospheric ratio, the blank correction only causes the point to move along the regression line, which comes out of the discussion below. We described our blank correction procedure in methods (Supplement file S1). The fact that we cannot mimic the dropping of the pestle when a sample is present in the tube limits how well we can determine the blank during the experiments. The blank tends to increase with a higher number of pestle drops, but the composition of this blank is atmospheric. For the test of the blank, we used quartz glass fragments to mimic zero-age minerals, as a blank determination using metal-on-metal impacts was considered to be an unrealistic scenario. As a next test, we artificially increase the ⁴⁰Ar blank (and thus the ³⁶Ar blank) assuming atmospheric composition. If the data are located on the mixing line between radiogenic and atmospheric argon, this should not affect the isochron apparent age (pink part – final stage in Figs. 5 and 11). We tested this for sample Rursee 1b BNV with an apparent age of ∼ 88 Ma. The ⁴⁰Ar $/$ ³⁶Ar intercepts increase with increasing blank values, and the weighted mean late converging section ages change with a maximum of 2.5 Ma in the chosen example. We therefore conclude that the isotopic ages remain largely unaffected by varying the amounts of atmospheric argon of the blanks. Note that if the isochron is not a mixing line between radiogenic and atmospheric argon (e.g. blue and green parts in Figs. 5 and 11), this assumption is incorrect. The ⁴⁰Ar $/$ ³⁶Ar intercept is then pulled away from the real ⁴⁰Ar $/$ ³⁶Ar composition in the direction of the atmospheric ⁴⁰Ar $/$ ³⁶Ar intercept. Consequently, the intercept with the inverse isochrons' x axis (and thus age) will also be affected.

4.1.3 Recoil artefacts

Recoil artefacts occur when ³⁷Ar and ³⁹Ar, which are formed from K and Ca isotopes, form with kinetic energy. As a result, they can travel from their original sites to other sites, potentially even into the adjacent phase (Turner and Cadogan, 1974; Foland, 1983; Lo and Onstott, 1989; Féraud and Courtillot, 1994; Baksi, 1994; Onstott et al., 1995; Villa, 1997). However, this phenomenon is assumed to have a smaller impact than the blank correction.

4.2 Potential reservoirs of K

To date, three main hypotheses have been debated about the origin of the released argon in a stepwise crushing experiment. The first group (Qiu and Wijbrans, 2006, 2008; Bai et al., 2019) suggests that progressive crushing releases gases mainly from fluid inclusions and therefore represents fluid inclusions maximum apparent ages. Additionally, there is a possibility of argon releasing within K minerals by prolonged crushing when the grain sizes are reduced to tens of nanometres (Bai et al., 2019).

The second group (e.g. Kendrick and Phillips, 2009) discusses the possibility of K-bearing mineral inclusions within the inclusion cavity and/or in microcracks serving as argon reservoirs in the later crushing stages. Obtained maximum apparent ages therefore represent mineral closure ages or a mixture of fluid inclusions and mineral inclusion ages. Accordingly, the gas release sequence under sufficient crushing progresses from microcracks to secondary fluid inclusions, followed by primary fluid inclusions, and finally to micro- to nanometre-sized minerals (Bai et al., 2022).

In addition, the third potential source of potassium in the minerals might be the presence of K⁺ in the crystal lattice, which was postulated for zeolites (Kendrick et al., 2011) but could also work for feldspars but may be less of an issue in nominally potassium-free minerals such as quartz (or garnet), which is representative of the formation age of veins. Hydrothermal quartz veins, characterized by their substitution in the crystal structure, have been studied by Weil (1984) and Götze et al. (2021). These studies indicate that Si⁴⁺ derived from hydrothermal quartz veins can be substituted by other ions such as Al³⁺, Ga³⁺, Fe³⁺, B³⁺, Ge⁴⁺, Ti⁴⁺, and P⁴⁺. Al³⁺ usually replaces Si⁴⁺ since it is found in significant amounts (∼ 300–700 ppm) in quartz, based on EPMA data. Additionally, small numbers of monovalent ions such as K⁺ may fill empty spaces in the crystal structure, serving as charge balancers for trivalent substitutional ions such as Al³⁺ (Bambauer, 1961; Kats, 1962; Perny et al., 1992; Stalder et al., 2017; Potrafke et al., 2019). However, Jourdan et al. (2009) postulated that substitutions of these components may be so minor that they are even undetectable using a secondary ion mass spectrometer (SIMS). Furthermore, it is important to note that not all hydrothermal sources or quartz minerals have this particular form of substitution (Jourdan et al., 2009).

Apart from these potential ³⁹Ar_K reservoirs above, detrital minerals (e.g. mica present in the surrounding pelitic rock) that might be trapped by the quartz veins during the growth may also contribute to the obtained maximum apparent ages.

4.2.1 Identification of different K reservoirs in the Rursee quartz samples

During in vacuo stepwise crushing, the release of argon isotopes from the samples follows systematic patterns. In this study, we aim to connect this release to the sequential contributions of different reservoirs of K and, consequently, argon from the Rursee samples. The release patterns of ³⁶Ar_air, ³⁸Ar_Cl, ³⁹Ar_K, and ⁴⁰Ar* (Fig. 6) for all quartz vein samples may originate from multiple existing argon reservoirs.

Depending on the size (< 10 µm), location, and generation of fluid inclusions, they may contribute successively to the argon release patterns in the early or middle stage of stepwise crushing. Figure 6 reveals that the concentration of ³⁹Ar_K increases throughout the process of in vacuo stepwise crushing, while the concentration of other argon isotopes decreases. This suggests that K-containing reservoirs were not opened in the first part of the experiment. The release patterns of ³⁹Ar_K can be categorized into two distinct groups during stepwise crushing.

The first group of samples shows a small initial release in the early stages. This is followed by a drop around the 10th step, then an increase from the ∼ 10th to ∼ 35th step, and finally a gradual decrease (Rursee 3 BPV, Rursee 5 BNV, and Rursee 6 BPV).

The second group does not have the ³⁹Ar_K release in the steps 1–10 but after that it shows a similar pattern to the first group, gradually increasing until about the 35th step and then gradually decreasing (Rursee 1b BNV, Rursee 2 BPV, Rursee 2.1 BNV, and Rursee 4 BPV).

The continuous rise in ³⁹Ar_K levels after ∼ 10 steps in both sample groups suggests that the gas release process can be divided into at least two phases. Initially, during the first ∼ 10 steps, ³⁹Ar_K is emitted from fluid inclusions in microcracks (secondary fluid inclusions). From steps ∼ 10 to ∼ 70, the release happens as a result of the mixing of potential pseudo-secondary fluid inclusions (∼ 10th to ∼ 15th steps), mineral inclusions, and/or the crystal lattice of quartz veins. This interpretation is supported by the K $/$ Cl correlation plots (Fig. 7), which show a consistently lower K $/$ Cl ratio until the ∼ 10th step.

Figure 7 K $/$ Cl ratios plotted against analysis steps for all quartz veins.

From the 10th to the 15th K $/$ Cl ratio, it reaches ∼ 1 with a steep rise for all quartz samples, and later (from the ∼ 20th step) this ratio continues to increase steeply for the second group of samples, while it shows a less pronounced increase for the first group of samples.

The lower K $/$ Cl ratio may be attributed to the presence of Cl and a lack of or limited amounts of K in combination with relatively constant low salinity levels (3.5–8 eq. wt % NaCl) inside the fluid inclusions, which are likely to be opened in the early phase. After most fluid inclusions have been mechanically opened, the subsequent rapid increase in K (reflected by the ³⁹Ar_K) and the steady decline in Cl (reflected by the ³⁸Ar_Cl) occur throughout successive crushing steps and are reflected in the K $/$ Cl ratio. Therefore, this increase is likely due to the exhaustion of the Cl-rich fluid inclusions along with the presence of minerals containing potassium and/or potassium from the crystal lattice of quartz that release their argon in the later crushing steps.

This approach to distinguish between fluid inclusions and other K reservoirs was first suggested by Kendrick et al. (2006, 2011): K $/$ Cl ratios ≤ 1 are representative of fluid inclusions and K $/$ Cl ratios > 1 for other sources. If K $/$ Cl ≤ 1, the apparent age we measure is the oldest possible age for the fluid inclusions. In case K $/$ Cl > 1, the obtained apparent age corresponds to the maximum apparent age of the trapped K-bearing mineral and/or K from the crystal lattice (Kendrick et al., 2006, 2011). In our samples, the K $/$ Cl is greater than 1 after the first ∼ 15 ± 3 steps in all quartz vein samples, indicating the presence of major K-related reservoir(s) other than fluid inclusions. It is worth noting that this is based on the assumption that there are no other K-bearing phases, such as KNO₃, K₂SO₄, or K₂CO₃, rather than KCl dissolved in aqueous fluid inclusions. This assumption may be verified by Raman analysis (see Fig. A1), which does not show significant peaks for these alternative K-bearing phases. However, the low peaks in ∼ 1080 cm⁻¹ may be related either to K₂CO₃ or to interferences in relation to the epoxy background. Therefore, K $/$ Cl > 1 suggests that K is related to the salinity of the fluid inclusions along with different K components (i.e. KCl and K₂CO₃). Alternatively, if there is interference from the epoxy background, it suggests that there should be at least one significant additional source present, which could include the crystal lattice of quartz and/or mineral inclusions within the quartz crystals and/or even microcracks.

4.2.2 K-bearing mineral inclusions

EPMA data (Table 2) from cleaned, hand-picked fluid-rich separated quartz grains indicate the presence of sericite, chlorite–sericite, and illite–sericite in the microfractures and cavities of fluid inclusion. The presence of such minerals (or mixtures) in the inclusion cavity and microfractures is also invisible under a binocular or petrographic microscope during the mineral separation and was confirmed captured using electron-backscattered imaging (Fig. 8). In thin sections of quartz, veins with the associated host rock, illite–sericite, and white mica are abundant in the surrounding pelitic layer of the Rursee formation (Fig. 9). These minerals that contain a significant amount of K₂O are also detected by an EPMA, in the separated quartz samples, especially in Rursee 2 BPV (see EPMA data, Table 2). High K concentrations (∼ 8.8 wt % K₂O) are likely related to intergrowth with sericite or a closely related mineral.

Table 2 Elemental analysis of quartz grain, microcracks, and mineral inclusions in quartz vein samples under an EPMA.

^* bdl – below detection limit.

Figure 8 Backscattered electron images of mineral inclusions obtained by SEM. Secondary minerals (e.g. chlorite, sericite, and white mica as determined using an EPMA) occur in cavities and microfractures (white arrow) in fluid-rich quartz.

Figure 9 Microscopic image of the quartz vein host rock matrix from the Rursee formation. (a) Crossed polarizer and (b) plane polarizer images of the pelitic host rock (Rursee 2 BPV). White arrows (image a) indicate the presence of the white mica and sericite in the host pelitic rock. (c) Crossed polarizer and (d) plane polarizer images of the quartz veins' matrix (Rursee 1 BNV). The white arrow (image c) shows the presence of the quartz sub-grains. Quartz sub-grains in the vein are due to the local tectonic activity, indicating that this period corresponds to tectonic activity.

Petrographic analysis of thin sections of whole rock samples representing both vein generations (BPV and BNV) reveals a significant presence of chlorite located between the vein wall and host rock, as well as in fractures (Fig. 10). Although chlorite typically lacks K in its crystal structure, previous studies have reported trace amounts of K in chlorites (Pacey et al., 2020; Li et al., 2022).

Figure 10 Chloritization distribution in the vein wall and fractures for both generations of quartz veins. (a) Plane (a.1) and crossed (a.2) polarizer of bedding-parallel veins: chloritization mainly between vein wall and host rock and fractures. (b) Plane (b.1) and crossed (b.2) polarizer of bedding-normal veins: chloritization in fractures.

4.2.3 K from crystal lattice and detrital minerals

EPMA analyses of the quartz matrix indicate that K concentrations in the crystal lattice are below the detection limit of ∼ 100 ppm. A maximum K concentration of ∼ 100 ppm K (for example, 100 ppm K in Rursee 2.1 BNV) and a maximum apparent age of 144 Ma would result in ∼ 16000 fA ⁴⁰Ar* when measured on our Helix MC+ mass spectrometer, which is a comparable amount of total ⁴⁰Ar* released from K-bearing mineral inclusion. Given the large sample amount (∼ 30 mg), this would translate into a significant contribution of K from the crystal lattice of quartz. We therefore suggest that K in the crystal lattice may contribute to the observed ⁴⁰Ar* signals (see calculation in the Supplement file S2).

In this study, argon molecules might also be derived from secondary minerals in cracks and embedded detrital minerals (e.g. mica from host rock). This interpretation aligns with the observation that the homogenization temperatures of fluid inclusions within the quartz veins are below the closure temperature for argon in detrital minerals. Under such conditions, the expected maximum apparent ages from K-bearing detrital minerals would correspond to pre-Variscan periods, reflecting the maximum apparent age of the deposits hosting the quartz veins, although the obtained maximum apparent ages are significantly younger in this study. Therefore, we infer that detrital minerals do not significantly contribute to the ⁴⁰Ar* signals.

To summarize, during the first stages (until the ∼ 20th analysis steps) of the stepwise vacuo crushing, gases are likely released only from fluid inclusions (secondary and pseudo-secondary, as is also observed for fluid inclusion in garnets (Qiu and Wijbrans, 2006, 2008)). Huseynov et al. (2024) demonstrated that a significant amount of fluid inclusion water can be extracted from these samples by a single crushing step using a spindle crusher. In this study, throughout the crushing process, the total amount of argon released steadily increases (Fig. 6). In the latter stages of the experiment (from the 20th analysing steps), the substantial release of ³⁹Ar_K isotopes may originate as follows:

• a.

  The gas release only from the small-sized fluid inclusions (i.e. < 5 µm pseudo-secondary) until the last stage of the experiment (∼ 40–50th analysing step) and then in the last stage (after the 50th step) from solid-phase K-bearing minerals and/or crystal lattice, which corresponds to the end of the early converging section in some samples. As K $/$ Cl > 1 after (the 20th analysing step), the low peak (∼ 1080 cm⁻¹) in Raman spectroscopy may correspond to a K-related component (e.g. K₂CO₃) from fluid inclusions.

• b.

  The significant release of ³⁹Ar_K isotopes in the middle to later stages of the experiment may be related to the presence of non-crushed, small-sized fluid inclusions (< 5 µm) together with K-bearing mineral inclusions in the samples and/or ⁴⁰Ar* from the crystal lattice under the condition that the low peak in ∼ 1080 cm⁻¹ from Raman spectroscopy belongs to the epoxy background of the quartz grain. The contribution of crushing-induced degassing K-bearing mineral inclusions is also corroborated by EPMA data, and the presence of K in the lattice cannot be ruled out for the Rursee samples.

4.3 Age spectra and isochrons

As mentioned previously, the distribution of argon isotopes (Fig. 6) indicates that ³⁹Ar_K is derived from distinct sources, likely mineral inclusions and/or eventually crystal lattice rather than fluid inclusions in particular in the later phase of the experiment, which was used for the age determinations. These potential sources of K, including fluid and mineral inclusions and/or crystal lattice, may all contribute to the variability observed in the age spectra derived from the different samples. Due to the presence of ⁴⁰Ar_E from the fluid inclusions, the initial analytical stages of the analyses yield anomalously high apparent ages in the first part of their age spectra (Fig. 4). Some samples (Rursee 3 BPV, Rursee 5 BNV, and Rursee 6 BPV) show an early converging section in the first part of the experiment. The early converging section effect occurs between the 20th and 30th analysing steps, which may be associated with sudden changes in K $/$ Cl ratios (Fig. 7). These sudden changes may be due to the sharp transition from fluid-state reservoirs (e.g. small-sized fluid inclusions) to solid-state reservoirs (e.g. K-bearing mineral inclusions). However, it does not occur in the second group of quartz samples (Rursee 1b BNV, Rursee 2 BPV, Rursee 4 BPV), revealing smooth transitions from fluid to solid states of ³⁹Ar_K reservoirs. The transition for the Rursee 2.1 BNV is neither abrupt as for the first group samples nor smooth as for the second group samples; hence, the impact of the early converging section is minimal.

The transition from fluid-state reservoirs to solid-state reservoirs can be supported by grain size distribution (see Supplement file S4), indicating that fluid-state reservoirs may remain unreleased beyond around 800 crushes (around the 20th analysis step). However, the presence of small particles at the bottom of the crusher (non-recoverable size) after 800 crushes may result in the measured results not accurately representing the whole grain size distribution. As the grain size distribution depends on many factors (i.e. crushing efficiency, presence of microcracks), there may even be a factor of difference for two groups.

The impact of ⁴⁰Ar_E results in inverse isochrons (Fig. 5) during the initial stage. The relationship between the ³⁶Ar $/$ ⁴⁰Ar and ³⁹Ar $/$ ⁴⁰Ar for all samples resulted in a decrease in the ³⁶Ar $/$ ⁴⁰Ar ratio and an increase in the ³⁹Ar $/$ ⁴⁰Ar ratio (initial stage in Fig. 11). The presence of an elevated concentration of ³⁶Ar at the beginning of the experiment could be due to the atmospheric argon gas that is either trapped in the stainless steel crusher and/or the original fragment surfaces or perhaps released during the initial stage of crushing. Following the opening of fluid inclusions, the ratio of ³⁶Ar $/$ ⁴⁰Ar increases linearly with the ratio of ³⁹Ar $/$ ⁴⁰Ar. This is probably due to a decrease in excess argon throughout the crushing and an increase in ³⁹Ar_K associated with K-bearing minerals and/or crystal lattice (intermediate stage in Fig. 11). In the last phase of ⁴⁰Ar $/$ ³⁹Ar analysis, the concentration of ³⁹Ar_K decreases (final stage in Fig. 11). This last part is particularly important for determining the age of quartz vein samples.

Figure 11 Inverse isochron representation of quartz veins (ex: Rursee 2.1 BNV): three stages: (1) initial stage with the opening of fluid inclusions, (2) intermediate stage where argon is released from mineral inclusions or microfractures and/or crystal lattice, and (3) final stage of argon release from mineral inclusions and neglectable excess argon in samples.

Inverse isochrons may assist in determining the maximum apparent age of fluid inclusions by linear regression of the data related to fluid inclusions. However, the high amounts of excess argon in the system obscure geologically meaningful ages.

4.4 Implications

Unlike studies that obtained consistent maximum apparent ages from high-salinity (> 20 eq. wt % NaCl) primary fluid inclusions of garnet (in eclogite) and wolframite (Qiu and Wijbrans, 2006; Qiu et al., 2011; Bai et al., 2013, 2019), we were unable to date pseudo-secondary and secondary fluid inclusions in recrystallized Rursee quartz samples, likely due to high ⁴⁰Ar_E concentrations and/or low salinity (3.5–8 eq. wt % NaCl). The reduced K concentration in the pseudo-secondary and secondary fluid inclusions, due to the loss of the primary brine and its replacement by a lower-salinity, lower-K fluid, likely led to inaccurate maximum apparent age determination. While no precise age was determined for the fluid inclusions, ⁴⁰Ar $/$ ³⁶Ar ratios (above atmospheric but below 4000) indicate a mixed metamorphic–meteoric fluid source (Ballentine et al., 2002; Ozima and Podosek, 2002). Later, during the crushing experiment, the K-bearing mineral inclusions may provide geologically meaningful ages, although the argon closure temperatures in quartz remain uncertain. For reference, the closure temperature of smaller-size sericite grains (∼ 20 µm) corresponds to temperatures (∼ 300–350 °C) (Glasmacher et al., 2001; Watson and Cherniak, 2003), while the vitrinite reflectance from psammatic and pelitic layers indicates maximum burial temperatures (220 °C) near the Carboniferous–Permian boundary, with gradual cooling thereafter (Littke et al., 2012).

Three ⁴⁰Ar $/$ ³⁹Ar maximum apparent ages ranging from 117 to 84 Ma differ from the interpretation based on structural analyses, which posit that veining occurred during the early Variscan Orogeny (Van Noten et al., 2007), possibly due to argon loss during cooling and/or recrystallization. The obtained maximum apparent ages may be influenced to some extent by the presence of neo-crystallized quartz sub-grains, although their volume appears relatively small (Fig. 9c). However, since the maximum apparent ages primarily reflect solid-phase reservoirs (i.e. K-bearing mineral inclusions) rather than fluid-phase components, likely, K-bearing solid-phase reservoirs intergrow simultaneously with the recrystallization process.

Post-Variscan tectonic activity is known for the southern Rhenish Massif due to late- and post-orogenic fault movements and coeval reactivation of Variscan structures leading to the fluids' (re)activity (Herbst and Muller, 1969; Schwab, 1987; Korsch and Schäfer, 1991; Hein and Behr, 1994; Moe, 2000; Kirnbauer et al., 2012).

Given that reactivation of existing veins could have occurred without forming new fractures (Virgo et al., 2013), this reactivation is usually associated with the infiltration of high-salinity (> 20 eq. wt % NaCl) fluids in central Europe and the Rhenish Massif (Behr et al., 1987; Redecke, 1992; Hein and Behr, 1994; Germann and Friedrich, 1999; Heijlen et al., 2001; Kuèera et al., 2010).

This saline fluid activity is at odds with the low-salinity fluid inclusions (3.5–8 eq. wt % NaCl) in the Rursee quartz veins (Van Noten et al., 2011). However, they agree with low-salinity fluid inclusions in quartz veins of the Rhenish Massif, which are attributed to upward migration of Variscan fluid remnants during Jurassic–Cretaceous reactivation (Kirnbauer et al., 2012).

Near Rursee (Stavelot inlier), low-salinity (0.2–7.2 eq. wt % NaCl) and high-temperature fluid activity (∼ 250 °C) along the Variscan front reflects warm meteoric fluid circulation (Schroyen and Muchez, 2000). Such warm, low-salinity fluids may have also contributed to the chloritization of veins in the Rursee outcrops. We propose that tectonic activity and quartz vein reactivation–recrystallization could explain the observed ⁴⁰Ar $/$ ³⁹Ar maximum apparent ages, as low-salinity Variscan fluids perhaps moved along the reactivated fractures, forming new quartz minerals within the Variscan-related veins during Jurassic–Cretaceous tectonic activity (i.e. opening of North Atlantic).

5 Conclusions

• The analysis of argon isotope patterns and their interpretations (including K $/$ Cl and inverse isochrons) indicates that the main reservoir ³⁹Ar_K for geologically meaningful ages originated from the K-bearing minerals (illite–sericite and possibly chlorite) in quartz vein microcracks and/or cavities of fluid inclusions and/or crystal lattice of quartz.

• The determination of a primary crystallization age, i.e. Variscan age, for the quartz veins, is not feasible, owing to the low amount of K in fluid inclusions and the high amount of excess argon within the FIAs, resulting in anomalously old apparent ages in the first ∼ 20 crushing steps.

• The reduced K concentration, due to the loss of primary fluid from inclusions and replacement by a lower-salinity, lower K fluid, led to bias in the age determination of fluid inclusions. The obtained ages potentially correspond either to the presences of a secondary generation of low-salinity fluids or to the contribution of radiogenic argon reservoirs hosted in solid phases related to intergrowth mineral inclusions during the recrystallization of quartz veins.

• The maximum apparent ages obtained from the quartz samples span the Jurassic–Cretaceous period. The presence of neo-crystallized quartz sub-grains in the veins is due to the local tectonic activity, indicating that this period corresponds to the tectonic activity of the Rhenish Massif.

Appendix A

Figure A1 Raman spectroscopy of fluid inclusion from Rursee quartz veins. (a) Microscopic image of an epoxied and polished fluid-rich quartz fraction. The fluid inclusion that underwent Raman analysis is represented by the red box. (b) The Raman plot is presented with the wavelength on the x axis and intensity on the y axis. The Raman spectra show a stretching band in the wavelength range of 3000 to 3700 cm⁻¹, which indicates the presence of an aqueous system.

Appendix B

Figure B1 Normal isochron plots of all quartz vein samples.

Appendix C

Table C1 Rursee quartz vein samples' J values and mass-dependent fractionation (MDF) with analytical error.