## 4 Results
### Geochemical Modelling of Experiments with Cumulative Gabbro  
#### 4.3.1 Outlet Solution Composition  
The results of the experimental and simulated variations and tendencies in the output concentration over time are shown in Figure 7. The measured and simulated pH in brine supersaturated with CO\({}_{2}\) are presented as a function of the reactive surface area (Figure 7a) and compared to the variation of dissolved CO\({}_{2}\) (Figure 7b).  
Initially, the fluid pH at the onset of the modelling was 4.0 and increased rapidly to 4.5, and then gradually to 7.0, in the range of the measured values. Clearly, over the simulated time (0 to 200 days), pH ranged from 4.0 to 5.1 for the low SSA, increasing up to 7 for high SSA (170-220 cm\({}^{2}\).g\({}^{-1}\)). The interaction between the rock and acidified fluid is a function of the dissolved CO\({}_{2}\) and the reactive surface area, since the decrease in HCO\({}_{3}\)\({}^{-}\) is inversely correlated to the increase in pH. Hence, the increase in pH above 5.5 was correlated to the consumption of the dissolved CO\({}_{2}\). As a result, at t \(<\) 50 days for SSA \(<\) 75 cm\({}^{2}\).g\({}^{-1}\), and t \(<\) 10 days, for SSA = 170-220 cm\({}^{2}\).g\({}^{-1}\), the pH remains lower than 5.0 and the dissolved CO\({}_{2}\) concentration remained constant. The constant concentration of HCO\({}_{3}\)\({}^{-}\) at a low pH denotes a dominant dissolution phase, where the concentration of the primary aqueous species (Ca, Mg, Fe, SiO\({}_{2}\)) increases with time (Figure 8). However, at moderate and high pH values, CO\({}_{2}\) consumption is inferred from the decline in HCO\({}_{3}\)\({}^{-}\) concentration (below 0.48 mol.kgw\({}^{-1}\)), triggering CO\({}_{2}\) mineralization into carbonates.  
Simulated aqueous silica, Ca, Mg and Fe were observed to initially increase and then level off or decrease over increasing time. For instance, a minor increase in Ca concentration with respect to the initial concentration (\(\Delta\)C\({}_{\rm Ca}\) = 0.14 mol\(\cdot\)kgw\({}^{-1}\)) was observed for t \(<\) 40 days before a continuous decrease began. The pattern of increase and decrease match perfectly with the evolution of dissolved CO\({}_{2}\), and are in agreement with the observations in previous studies [14; 25; 29; 55].  
Figure 7: (**a**) Simulated pH variations of the outlet fluid compared with the measured pH; (**b**) simulated dissolved CO\({}_{2}\) (as HCO\({}_{3}\)\({}^{-}\)) variations as a function of time computed with different specific surfaces. The measured HCO\({}_{3}\)\({}^{-}\) values are obtained after opening the chamber and are not relevant for comparison.  
The simulated magnesium concentration doubled before levelling off (after 100 days). Moreover, the SiO\({}_{2}\)(aq) concentration stabilized shortly before a sharp and progressive decrease, indicating quantitatively important secondary silicate phases' precipitation. A steady increase in iron concentration was observed; the simulation indicates levelling off occurred later, after 150 days, and was attributed to fayalite dissolution. Aluminum values, initially \(1.0\times 10^{-4}\) mol/kg, dropped sharply by five orders of magnitude, following the albite, Ca zeolite and kaolinite precipitation observed in the first simulation.  
Because of the assumptions made in the initial mineralogy, the simulation could not reproduce the potassium and SO4- behaviors.