امکان ذخیره سازی CO2 از طریق تشکیل کلسیت در سنگ های آتشفشانی (مطالعه موردی بازالت های مجدر واقع در جنوب شرق اردبیل)

Abstract
The ever-increasing global demand for energy is associated with the more consumption of fossil fuels, which has caused the widespread release of CO2 gas into the earth's atmosphere. The process of carbonation in geological formations is one of the safest and most promising approaches for CO2 storage. Majdar area is located in the northwest of Iran on the Alborz-Azerbaijan magmatic belt. Eocene volcanic units consisting of andesite, basaltic andesite, basalt, agglomerate and tuff form the thickest and widest rock units in the study area. Basalt and basaltic andesite rocks have been studied in this area. Petrographic studies showed that plagioclase, pyroxene and some olivine minerals are the main phenocrysts of host rock placed in a glassy to microcrystalline matrix. The XRF results showed that Majder basalt samples contain adequate amounts of Ca oxides (10.29–12.27 wt%), Fe (9.83–12.66 wt%) and Mg (5.75–7.45 wt% ) that can react with fluids containing CO2 to form stable carbonate minerals. According to the XRD results, three carbonate minerals including calcite, manganocalcite and ankrite are formed along with secondary alteration minerals clinochlore, clinoptilolite, illite, montmorillonite, glauconite and quartz. The study of microscopic sections showed that the alteration of calcium and magnesium minerals such as augite and anorthite by CO2-rich fluids has caused the release of cations needed for the formation of carbonate minerals. The value of δ18O isotopes in the studied samples ranges from -10.14 to -12.54. The calculations based on the results of δ18Ocalcite show the temperature range of 71.35–88.35 °C for the formation of calcite, which corresponds to a depth of 2.4-3 km. According to the measured average porosity of rocks, the possibility of storing 154,000 tons of CO2 in a block measuring 1000 meters long, 1000 meters wide and 70 meters high in the studied area can be estimated.
Introduction
The ever-increasing global demand for energy is associated with the more consumption of fossil fuels, which has caused the widespread release of CO2 gas into the earth's atmosphere. The significant effect of CO2 as a greenhouse gas on global warming and climate change has caused researchers to look for ways to control its release into the earth's atmosphere. CO2 storage through geological processes requires the presence of suitable host rock capable of CO2 fixation and minimal post-storage monitoring. The process of carbonation in geological formations is one of the safest and most promising approaches for CO2 storage, because CO2 reacts with divalent elements Ca, Mg and Fe and is stabilized in the form of stable carbonate minerals such as calcite, magnesite and ankerite. The requirement of this process is the presence of silicate minerals rich in the mentioned elements such as olivine, pyroxene and plagioclase. Because these minerals containing these elements can provide the required compounds for the formation of carbonate minerals to fluids containing CO2 and in this way provide the possibility of stabilizing and storing CO2 without the need for long-term monitoring. Volcanic rocks such as basalts are among the available options for absorbing and stabilizing CO2 due to their spread on the surface of the earth and being rich in elements that form carbonate minerals.
In recent decades, Iran has faced a rapid rate of urbanization due to political-social changes and industrialization, and this has caused a significant increase in energy consumption, which is mainly extracted from natural sources. For this reason, Iran is among the top ten countries in the world in terms of CO2 emissions.
Methodology
In this study, the mineralogical and geochemical characteristics of Mejdar basalts located in the Alborz-Azerbaijan magmatic belt in the northwest of Iran were investigated based on field and laboratory studies, and their potential for the formation of carbonate minerals was evaluated. For this research, petrographic studies of rocks have been done using microscopic thin sections by polarizing microscope. Also, XRF analysis methods have been used to determine the chemical composition of the whole rock, XRD analysis to determine the type of rock-forming minerals, and oxygen stable isotope analysis to determine the temperature and depth of formation of carbonate minerals.
During the field observation and the study of microscopic sections, it was found that carbonate minerals are formed in the form of veins and open-space filling in the volcanic host rock. Petrographic studies showed that plagioclase, pyroxene and some olivine minerals are the main phenocrysts of host rock placed in a glassy to microcrystalline matrix. Cox et al. (1979) diagram was used to determine the type of rocks in the area. In this classification, which is one of the most common classification methods for volcanic rocks, the sum of alkaline oxides (Na2O + K2O) is used against silica (SiO2). The chemical composition of the host rock based on thid diagram is mainly located in the range of basaltic andesite and basalt. The XRF results showed that Majder basalt samples contain adequate amounts of Ca oxides (10.29–12.27 wt%), Fe (9.83–12.66 wt%) and Mg (5.75–7.45 wt%) that can react with fluids containing CO2 to form stable carbonate minerals. According to the XRD results, three carbonate minerals including calcite, manganocalcite and ankrite are formed along with secondary alteration minerals clinochlore, clinoptilolite, illite, montmorillonite, glauconite and quartz whithin the basaltic rocks.
The dissolution of CO2 in water has a significant effect on the fluid's reactivity with alkaline rocks due to the release of H+ ions and the creation of acidic conditions. Such fluid causes the release of Ca, Mg and Fe cations from some reactive minerals. These cations react with carbonic acid resulting from the dissolution of CO2 in water and form different carbonate minerals such as calcite (CaCO3), magnesite (MgCO3), siderite (FeCO3) or ankrite (Ca(Fe,Mg)(CO3)2). Studies have shown that calcium ions released from augite and anorthite minerals in the basalt rocks of the region react with carbon dioxide-rich fluids to form calcite.
Due to the significant effect of temperature on the values of oxygen isotopes, it can be used to obtain the relative temperature of the formation of carbonate minerals. By having the relative temperature of the formation of carbonate minerals, the approximate depth of the carbonation process can be obtained. The value of δ18O isotopes in the carbnate samples ranges from -10.14 to -12.54. The calculations based on the results of δ18Ocalcite show the temperature range of 71.35–88.35 °C for the formation of calcite, which corresponds to a depth of 2.4–3 km.
Conclusion
Field studies and laboratory investigations showed that the volcanic rocks of the Mejdar area have a high potential to react with CO2-rich fluids and form various carbonate minerals. These fluids have caused the release of cations required for the formation of carbonate minerals by reacting with primary silicate minerals and altering them. According to the measured average porosity of rocks, the possibility of storing 154,000 tons of CO2 in a block measuring 1000 meters long, 1000 meters wide and 70 meters high in the studied area can be estimated.