*3.2. Rock Classification and Geochemistry*

Volcanic rocks from the region of Volos correspond to small scattered outcrops with an age range from 0.5 to 3.4 Ma [21,29]. Their formation was attributed to Pleistocene back-arc extension in the Aegean Sea [19,30,31]. Based upon the total alkali–silica (TAS) diagram (Figure 3a), the extensional-related volcanic rocks from the Volos plot formed within the basaltic trachyandesite and trachyandesite fields. Pleistocene basalts from the adjacent regions of Kamena Vourla, Lichades islands, Psathoura, and Achilleio also plot in the same compositional fields (Figure 3a). Chondrite-normalised REE patterns of the Volos basaltic rocks (Figure 3b) are highly enriched in LREE (La/YbCN = 0.34–0.44) and also present notable negative Eu anomalies (EuCN/Eu\* = 0.73–0.80), with the later implying plagioclase fractionation. These are additionally characterised from trace element ratios that account for a clear OIB (Ocean Island Basalt)-signature: Zr/Nb = 4.66–19.82, La/Nb = 0.75–10.38, and Ba/Th = 39.4–100.95 [32]. Basalts from the aforementioned adjacent regions exhibit lower LREE enrichments (Figure 3b), indicating higher degrees of partial melting and/or differentiation processes.

The Pleistocene extensional-related basaltic rocks from Volos and the adjacent regions differ from other recent age (Pliocene–present) volcanic rocks in Greece. The latter are subduction-related volcanics from the South Aegean (Methana [26,33], Nisyros [34], and Santorini [2,35,36]), associated with the subduction of the African plate beneath Eurasia [37–39]. These compositionally correspond to subalkaline basalts and andesites (Figure 3a), which possess significantly lower LREE and also higher HREE contents (Figure 3b). From this comparison, it is evident that the basaltic rocks from Volos are among the very few alkaline basaltic rocks of recent age that are compositionally suitable for considering mineral carbonation of CO2.

**Figure 3.** (**a**) Total alkali–silica (TAS), Na2O + K2O vs. SiO2 [40], and (**b**) chondrite-normalised REE patterns [41,42] of volcanic rocks from Volos, Kamena Vourla, Psathoura, Achilleio, Lichades [21], Methana [26,33], Nisyros island [34], and Santorini island [2,35,36].

#### *3.3. Water Chemistry and Temperature Data*

Geothermal data from the Almyros–Microthives basin [28] indicate that the north part of the basin is characterised by Pleistocene volcanic activity. Deep groundwater (sample GTES-038; >250 m depth) exhibits a temperature of 30.2 ◦C and a pH of 7.30, whereas shallow groundwater (sample GTES-040; probably 170–180 m depth) presents a temperature of 23.0 ◦C and a pH of 7.40. The elevated water temperatures appear in the vicinity of the basaltic dominated areas. Based on the Castany classification [43], the analysed groundwaters belong to hypothermal, neutral-to-alkaline types. Their total dissolved solids (TDS) content is 660 mg/L (Table 2). TDS calculation was based on the cations and anions sum, including HCO3 <sup>−</sup> (0.49 ×(HCO3 −)) and B. The total hardness values are 309 and 363 mg/L for the deep and shallow groundwaters, respectively. Non-carbonated hardness values are 22 and 0 mg/L for the deep and shallow groundwater samples, respectively.

**Table 2.** Hydrochemical analyses of groundwater samples from Microthives locality [28]. *T* ( ◦C); conductivity (μS/cm); concentrations (mg/L); total dissolved solids (TDS) (mg/L).


From the Hem [44] and Sawyer and McCarty [45] classifications, the analysed groundwater samples are classified as very hard. Deep and shallow groundwater samples belong to the Ca-Mg-Na-HCO3-Cl and the Mg-HCO3 hydrochemical types, respectively (Figure 4).

Hydrogeochemical comparisons between the two water samples from Microthives and Aegean seawater [47], are discussed below (see Discussion paragraph).

**Figure 4.** Piper diagram [46] for the water samples of Microthives and Aegean regions.

## **4. Discussion**
