**1. Introduction**

The use of fossil fuels (coal and oil) in the industries has increased the CO2 emissions in the atmosphere. Anthropogenic CO2 is a major greenhouse gas that contributes to the change of climate [1,2]. To mitigate the problem of global warming, several technologies have been developed. CO2 Capture and Storage (CCS) is one of the most advanced technologies that mediates the increase of the CO2 contents in the atmosphere [3]. Basaltic rocks exhibit appropriate physicochemical properties for the implementation of carbonate mineral precipitation, through interaction of the Ca-Mg-Fe rich minerals with carbonic acid, derived from the dissolution of the injected CO2 in water [4]. The newly formed minerals mostly consist of calcite, magnesite and siderite [5,6], which provide the potential for long-term and safe storage. Selection of the appropriate type of basalt and region for implementing CO2 storage techniques via mineral carbonation requires detailed mineralogical and petrophysical (porosity, permeability) studies. The nature of the injected CO2 affects the integrity and trapping potential of

the rock material [7]. CO2 is present in the supercritical form (sCO2) at pressure and temperature conditions that correspond to depths greater than 1 km. In such environments, sCO2 can give rise to various geochemical reactions, causing the dissolution/precipitation of primary and secondary minerals, as well as changes in porosity and permeability properties [8,9]. Successful CCS pilot injection projects have been implemented, including the sites Ferrybridge (UK), Aberthaw Pilot Plant (UK), Puertollano (Spain), Ketzin (Germany), and Hvergerdi (Iceland; CarbFix project) [10].

Alkali basaltic rocks with the potential of CO2 storage are relatively restricted but widespread throughout mainland Greece [10]. Main areas of basalt appearance are located in the regions of Pindos (NW Greece; [11]), Central and Southern Aegean islands [12–14], Koziakas [15], Othris [16], Evia island [17], and Argolis [18]. The present study focuses on studying the Porphyrio and Microthives alkali basaltic outcrops for their mineralogical potential of CO2 sequestration. The study areas are located 8 and 12 km south-southwest of the industrial city of Volos, respectively (Figure 1). These volcanic rocks formed along with other adjacent scattered volcanic centers that were active during the Late Pleistocene–Quaternary period, including the islands of Lichades, Achilleio, and Agios Ioannis between the gulfs of Pagasitikos and North Evoikos. Their formation is attributed to back-arc extensional volcanism and affected by the activity of the Northern Anatolian fault [19–21]. They comprise massive lavas and pyroclastic rocks that include basaltic rock fragments and pumice. These volcanic rock formations are located in the Pelagonian Zone and the Eohellenic tectonic nappe [22,23]. The Pelagonian Zone is part of the Internal Hellinides, and it can be distinguished into two metamorphic and non-metamorphic groups, respectively [24,25]. It was over-thrusted by the Eohellenic nappe during the Late Jurassic to Early Cretaceous period [24,26]. In the studied regions, the Pelagonian Zone mostly consists of clastic sedimentary rocks, limestones, and ophiolitic occurrences [22,23]. The Eohellenic tectonic nappe consists mainly of metamorphosed sedimentary rocks, serpentinites, and ophicalcites [22,23], as well as gneissic formations of the Volos Massif [21], composed of gneiss, muscovite, and mica-chlorite schists.

**Figure 1.** Geological map of Microthives locality and calculated water temperatures, EGSA87.

This study presents new mineralogical, mineral chemistry, and petrographic data of the volcanic rocks from the localities of Microthives and Porphyrio, coupled with hydrochemical data of groundwater samples from irrigation wells, to estimate their potential for the development of geological carbon capture and storage (CCS) [27]. The present study focuses on examining the physicochemical features necessary for applying CCS technologies focusing on: (i) degree of alteration, (ii) nature and geochemistry of the basalts, (iii) presence of Ca-bearing minerals, (iv) porosity (v), indications of enhanced heat calculated in groundwater samples from irrigation wells, and (vi) locality advantages.

#### **2. Materials and Methods**

This study includes the investigation of rocks that have been collected from the region of Volos (Central Greece; SE Thessaly), focusing on the volcanic occurrences of the Porphyrio and Microthives localities. Sampling was carried out to select the most appropriate rocks regarding their porosity and mineralogical assemblage. Modal composition of pores was calculated by applying ~500 counts on each thin section. Calculations were cross-correlated with image analysis techniques performed on the same thin sections. More than 50 rock samples were examined through detailed petrographic observations upon polished thin sections with the use of a Zeiss Axioskop-40 (Zeiss, Oberkochen, Germany), equipped with a Jenoptik ProgRes CF Scan microscope camera at the Laboratories of the Center for Research and Technology, Hellas (CERTH). Mineral chemistry analyses were conducted at CERTH using a SEM–EDS JEOL JSM-5600 scanning electron microscope (Jeol, Tokyo, Japan), equipped with an automated energy dispersive analysis system ISIS 300 OXFORD (Oxford Instruments, Abington, UK), with the following operating conditions: 20 kV accelerating voltage, 0.5 nA beam current, 20 s time of measurement, and 5 μm beam diameter. SEM-EDS facility was calibrated to obtain accurate quantitative results using standard reference materials. In order to perform standardised quantitative analyses, thin sections were flat, polished, and carbon coated. XRD analyses were conducted at CERTH using a Philips X'Pert Panalytical X-ray diffractometer (Malvern Panalytical, Malvern, UK), operating with Cu radiation at 40 kV, 30 mA, 0.020 step size, and 1.0 sec step time. For the evaluation of the XRD patterns, DIFFRAC plus EVA software v.11 was deployed (Bruker, MA, USA) based on the ICDD Powder Diffraction File (2006). Physicochemical data (from the Hellenic Survey of Geology and Mineral Exploration (HSGME)) [28], including temperature and pH values, are also provided for two groundwater samples from local irrigation wells (sample GTES-038; 250 m depth, sample GTES-040; 180 m depth).

#### **3. Results**

#### *3.1. Petrography and Mineral Chemistry*

Basalts from Microthives and Porphyrio localities exhibit porphyritic, vesicular textures. The groundmass is fine-grained holocrystalline, being either trachytic or aphanitic (Figure 2a–f) and often enriched in oxide minerals (ilmenite and magnetite). The porosity, after the examination of an extended number of thin sections of basaltic rock samples (*n* > 50), varies highly from 5 to 40% in the more massive and porous samples, respectively. The vast majority, though, were determined to have porosity that ranges from 15% to 23% (Avg. 18%). Vesicles are in cases filled with secondary calcite.

Their mineralogical assemblage is predominantly composed by prismatic subhedral and rarely euhedral clinopyroxene (15–30%) and olivine (10–20%) phenocrysts (450–700 μm diameter), enclosed within a clinopyroxene and plagioclase-rich groundmass (50–60%). Plagioclase is mostly restricted in the groundmass, appearing in the form of needle- to lath-shaped crystals that compositionally are either bytownite and labradorite (An68.9–71.6). Accessory minerals (<5%) include alkali-feldspar, quartz, calcite, amphibole, orthopyroxene, apatite, opaque minerals (ilmenite, titanomagnetite, and magnetite), and pyrite.

**Figure 2.** (**a**) Olivine and clinopyroxene phenocrystals in a hypo-crystalline trachytic groundmass mostly consisting of lath-shaped plagioclase but also K-felspar (Sample M3). (**b**,**c**) Vesicular basaltic lava samples M1 and M8, mainly consisting of clinopyroxene and also olivine phenocrystals, exhibiting glomeroporphyritic textures. It includes a hypo-crystalline trachytic groundmass, as well as vesicular textures. (**d**) Vesicular basaltic lava sample M2, with clinopyroxene and olivine phenocrystals in a trachytic groundmass. Vesicles are occasionally filled with secondary calcite-forming amygdaloidal textures. (**e**) Vesicular basaltic lava sample M5, within a microcrystalline vesicular groundmass, filled with secondary calcite. (**f**) Pyroclastic tuff with a high percentage of vesicles. Groundmass locally aphanitic with rare feldspar phenocrysts. (**g**–**i**) BSE (Back Scattered Electron) images with olivine and clinopyroxene phenocrysts.

Clinopyroxene is mainly classified as augite and less often as diopside, displaying highly variable TiO2 and Al2O3 contents (0.55–2.94 wt.% and 2.22–7.69 wt.%, respectively). SiO2 contents range between 44.52 and 51.34 wt.%. Representative compositions of olivine are presented in Table 1. They contain 38.10–40.55 wt.% SiO2 and variable FeO and MgO contents (10.30–24.90 wt.% and 36.58–48.50 wt.%, respectively). Mg# ranges between 72.78 and 89.36 wt.%.

The mineralogical composition of the studied basaltic rocks was also investigated by powder X-ray diffraction (XRD). In accordance with petrographic observations and mineral chemistry results, the main mineral phases were confirmed with XRD patters, based upon the DIFFRACplus EVA software (version11, Bruker, MA, USA) recommendations. In particular, the peaks at ~51.0◦ 2θ correspond to the olivine porphyroblasts, whereas clinopyroxene corresponds to peaks at 29.80–30.80◦ 2θ. The presence of magnetite in small amounts is characterised by small peaks at ~30◦ 2θ, ~52◦ 2θ and 62.20–62.80◦ 2θ. The plagioclase crystals of the basaltic groundmass were recognised by the peaks at ~28.0◦ 2θ, ~22.0◦ 2θ, and ~24.30◦ 2θ.


**Table 1.** Representative mineral chemistry analyses. (Abbreviations: Ol: olivine, Cpx: clinopyroxene, Plg: plagioclase, K-fs: K-feldspar, Amph: amphibole, Opx: orthopyroxene, Spl: spinel, n: number of analysis, Mg# = 100 × molar MgO/(MgO + FeOt), Cr# = 100 × molar Cr2O3/(Cr2O3 + Al2O3).)
