## 1 Introduction  
The International Energy Agency, in its flagship report \"Energy Technology Perspectives\", has demonstrated that CO\({}_{2}\) capture, utilization and storage (CCUS) is a key technology for CO\({}_{2}\) emissions reduction, essential to achieve the targets set in the Paris Agreement [1]. In the CCUS chain of technologies, CO2 is captured in large stationary sources and transported to a utilization or permanent storage in deep geological formations [2; 3]. Adequate geological environments for CO2 storage are provided by depleted hydrocarbon reservoirs, uneconomic coal seams, deep saline aquifers or mafic and ultramafic rocks [4; 5; 6; 7; 8]. The latter relies on the effectiveness of mineral carbonation, in which the CO2 reacts with the enriched calcium (Ca), magnesium (Mg) and iron (Fe) minerals, to precipitate as carbonate minerals, thus ensuring safe and permanent sequestration of the CO2 in solid phases. Most subsurface carbon storage projects to date have injected CO2 into sedimentary formations, either deep saline aquifers or hydrocarbon fields, but the exciting results obtained at two pilot sites where CO2 is injected in basalts, the Carbfix and Wallula projects [9; 10], have raised the profile of mafic and ultramafic rocks as suitable candidates for in situ mineral carbonation. Although at a less developed research stage than other CO2 storage environments, the possibility of using mafic and ultramafic rock massits should be considered when they occur near major CO2 emission sources. That is the case of the main industrial clusters in Sines and Setubal. As described in an accompanying article [11], mafic and ultramafic rock massits in southern Portugal may present a valid alternative for mineral carbonation (be it in situ, ex situ, or even enhanced weathering) of CO2 captured in that cluster. The interested reader is directed to that paper for details on the rationale for selecting the rock massits and their characterization. CO2-brine-rock interaction experiments are a well-established method to understand and explore the mechanisms and processes of geological storage [12; 13]. However, unlike most previous experiments, this article focuses on mafic plutonic rocks. These have seldom been considered for in situ mineral carbonation, given the low porosity and permeability, but previous experiments on ex situ or enhanced weathering applications have been published (e.g., [14; 15]). This study deals with mineral carbonation experiments, in the laboratory, to assess the rate of reaction between CO2 and rock samples from the Sines Massif, a subvolcanic massif mainly composed of gabbros, diorites and subordinated spenites. The laboratory experiments were designed to replicate the early stages of interaction between the mineral phases and a brine supersaturated in CO2, as one would expect around an injection well. Indeed, we attempted to understand the initial dissolution of the rock minerals that should provide the cations to react with the dissolved CO2. The possible mineralogical-textural changes of the rock after interaction with brine-CO2 was studied by a multi-analytical approach (optical microscopy (OM); scanning electron microscopy with X-ray detector (SEM-EDS); X-ray diffraction (XRD); and infrared Fourier transform spectroscopy (FTIR)) and the chemical compositional evolution of the brine and whole rock by comparative analyses (inductively coupled plasma optical emission spectrometry (ICPM-OES) and X-ray fluorescence (XRF)) before and after the experiment. Finally, numerical geochemical computation (CrunchFlow code) was used to interpret, replicate and predict the system's behavior for periods of time longer than 64 days, the maximum duration of the experiments. The article is organized as follows: a brief background on mineral carbonation experiments and tests is presented, followed by a description of the methodology applied to characterize the rock samples and conduct the laboratory experiments.