Results and discussion

Mixed-layer illite-smectite (Ilt-Sme) accompanied by minor amounts of illite, kaolinite, tridymite, quartz, feldspar and gypsum make up the bulk mineralogy of the analysed tephra (Fig. 2a,b). The prominent 15–35°2θ hump in the XRD traces (Fig. 2a,b) is attributed to the presence of volcanic glass and possibly amorphous silica (i.e. opal-A). The XRD traces of the bulk samples and the clay fractions of both samples show a predominance of highly crystalline Ilt-Sme. This is somewhat less pronounced in the case of the Glavice sample, which encompasses multiple Ilt-Sme generations of lower crystallinity. The relative positions of the Ilt-Sme 002/003 peaks (15.78 vs 15.69°2θ) combined with the 001/002 and 002/003 Δ°2θ values (5.33 vs 5.31Δ°2θ; Moore & Reynolds, Reference Moore and Reynolds1997) suggest a Sme-rich (i.e. >90%) Ilt-Sme in both clay fractions, with the Glavice sample probably having a slightly greater Sme content.

Fig. 2. XRD traces of (a,b) bulk and (c,d) clay fractions (<2 μm). The insets indicate FTIR spectra of the clay fractions. The red and blue lines in the XRD traces of the clay fractions correspond to air-dried and ethylene glycol-solvated samples, respectively. The y-axis values were square-root transformed to assist with the correlation. (e,f) SEM images of the Tušnica and Glavice samples, respectively. Ilt-Sme = illite-smectite; Ilt = illite; Kln = kaolinite; Trd = tridymite; Qz = quartz; Gp = gypsum; Pl = plagioclase.

The IR-ATR spectra in both samples reveal Si–O and Al–Al–OH bands at ~990 and 914 cm^–1, respectively (Fig. 2c,d). The Tušnica sample features an 843 cm^–1 Al–Mg–OH band, while the Glavice sample displays an Al–Fe–OH band at 875 cm^–1 (Fig. 2c,d; Madejová et al., Reference Madejová, Gates, Petit, Gates, Kloprogge, Madejová and Bergaya2017).

The SEM-EDS analyses reveal glass shards of essentially constant chemical composition that seem to be coated with Ilt-Sme of variable morphology (Table 1). Ilt-Sme outgrowths of similar phase chemistry emerge readily from the glassy substrate (Fig. 2e).

Table 1. EDS and LA-ICP-MS analyses of the studied tuff. Major oxides and trace elements are expressed in wt.% and ppm, respectively.

bdl = below detection limit.

In the case of the Tušnica clays, the Ilt-Sme crystals are up to ~0.3–2.0 μm long (Fig. 2e & Table 1). Conversely, the size of Ilt-Sme in the Glavice clays is smaller, being within the sub-micron range. These clays form aggregates with a typical honeycomb texture (Christidis, Reference Christidis and Christidis2010) found as glass vesicle infills (Fig. 2f). In comparison with volcanic glass, the observed clay minerals are depleted in SiO₂ and alkalis while being enriched in all other oxides. The FeO and TiO₂ in the clay minerals may be related to the decomposition of igneous phases such as biotite, while the increased MgO and CaO are attributed to the availability of these cations in karstic lake waters (Sironić et al., Reference Sironić, Barešić, Horvatinčić, Brozinčević, Vurnek and Kapelj2017). With reference to the Tušnica clays, the SiO₂/Al₂O₃ ratio of Ilt-Sme in the Glavice clays is greater (3.8 vs 3.5; Table 1), which is consistent with the XRD data. The K and Na contents of the Glavice clays are greater than those of the Tušnica clays (1.8 vs 0.8 wt.% and 1.0 vs 0.6 wt.%, respectively; Table 1), while the octahedral Mg content seems to be halved (Fig. 2c & Table 1).

Chondrite-normalized LA-ICP-MS analyses of the volcanic glass revealed an evolved igneous character (LREE enrichment (La_N/Yb_{n (Tušnica)} = 3.5, La_N/Yb_{n (Glavice)} = 8.6), Eu anomaly (Eu*_(Tušnica) = 0.3, Eu*_(Glavice) = 0.4; Fig. 3a,b & Table 1).

Fig. 3. (a,b) Chondrite-normalized plots (Boynton, Reference Boynton and Henderson1984) and (c,d) element mobility plots of the analysed samples.

The same normalization applied to the REE of the clay fractions resulted in greater LREE/HREE ratios (La_N/Yb_{n (Tušnica)} = 12.8, La_n/Yb_{n (Glavice)} = 20.5; Fig. 3a,b & Table 1) and Eu anomalies of reduced intensities (Eu*_(Tušnica) = 0.4, Eu*_(Glavice) = 0.5). To inspect trace-element mobility during the argillization of volcanic glass, we first normalized the clay and glass Y and REE contents to the respective Al₂O₃ concentrations of the samples. The two normalized samples were then compared against each other using the equation of Nesbitt (Reference Nesbitt1979) (Fig. 3c,d). Both samples feature similar rates of Y and HREE depletion (~70%) and positive Eu anomalies (Fig. 3c,d). The LREE depletion rate of the Glavice clays remains flat at ~40%, while in the Tušnica clays the LREE tends to deplete systematically (~7 to ~40% loss; Fig. 3d).

The high rates of Y and HREE depletion in the Ilt-Sme studied herein may be attributed to carbonate complexation of those elements and limited adsorption of these complexes on the clay mineral surfaces (Byrne & Kim, Reference Byrne and Kim1990; Sholkovitz et al., Reference Sholkovitz, Landing and Lewis1994). Various carbonate species presumably were abundant in karstic lakes such as the DLS (Cantrell & Byrne, Reference Cantrell and Byrne1987; Möller & Bau, Reference Möller and Bau1993; Christidis, Reference Christidis1998). In such environments, the REE distribution is controlled by sorption (Chen et al., Reference Chen, Algeo, Zhao, Chen, Cao, Zhang and Li2015), and a positive Eu anomaly as depicted in the mobility diagrams (Fig. 3c,d) commonly is related to plagioclase weathering (Weill & Drake, Reference Weill and Drake1973; Christidis, Reference Christidis1998). However, the DLS tephra is poor in plagioclase (Šegvić et al., Reference Šegvić, Mileusnić, Aljinović, Vranjković, Mandic and Pavelić2014); therefore, the Eu anomaly reflects a preferential mobilization of divalent Eu in anoxic porewaters rich in organic matter (Bau, Reference Bau1991; Hong et al., Reference Hong, Algeo, Fang, Zhao, Ji and Yin2019). The latter probably was derived from the neighbouring coal layer in the Tušnica clays (Badurina et al., Reference Badurina, Šegvić, Mandic and Slovenec2021). Particles of Ilt-Sme are dominated by their basal siloxane surfaces, which are the least reactive surfaces in clay minerals (Schoonheydt & Johnston, Reference Schoonheydt, Johnston, Bergaya and Lagaly2013). However, in the case of isomorphism and particularly octahedral substitution of Al by Mg, the charge deficit is delocalized over the surface oxygen atoms of siloxane planes (Sposito et al., Reference Sposito, Skipper, Sutton, Park, Soper and Greathouse1999). This represents a favourable adsorption domain for solvated cations such as LREE, which are suggested to vacate the glass in the form eight- or nine-fold hydrated outer-sphere complexes (Borst et al., Reference Borst, Smith, Finch, Estrade, Villanova-de-Benavent and Nason2020). A relatively great Mg content in the Tušnica clays, as is corroborated by the FTIR (Fig. 2c) and EDS spectra (Table 1), may therefore explain the pronounced LREE retention potential of these clays.

Conversely, the smaller particle size of the Glavice clays (Fig. 2f), which leads to a greater density of edge/defect charges, probably played a secondary role in LREE adsorption. Finally, the octahedral sheet of the Ilt-Sme in the Glavice clays seems to host Fe, which, in addition to the substitution of Mg by Al (Table 1), is further supported by the noticeable FTIR Al–Fe–OH band (inset in Fig. 2d). Under the oxidizing conditions that prevailed in Lake Sinj (Glavice locality; Vranjković, Reference Vranjković2011), structural Fe³⁺ did not contribute to permanent structural charge and therefore had no impact on the surface geochemistry of the Ilt-Sme.