Control of CO2 on flow and reaction paths in olivine-dominated basements: An experimental study

Abstract

The objective of this paper is to quantify the mass transfers involved in the hydrothermal alteration of olivine-rich peridotites in the presence of CO2-enriched waters, and to determine their effects on the rock hydrodynamic properties. Three flow-through experiments were performed at a temperature of 185 °C and a total pressure of 22.5 ± 2.5 MPa. They consisted in injecting a hydrothermal fluid with different concentrations of carbon dioxide (CO2 = 6.26, 62.6 and 659.7 mmol·L−1 i.e. pCO2 = 0.1, 1 and 10 MPa, respectively) into cylinders of sintered San Carlos (Arizona, USA) olivine grains. The results show that for low pCO2 conditions (from 0.1 to 1 MPa), olivine is mainly altered into hematite and Mg(Fe)-rich phyllosilicates. Such iddingsitic-type assemblages may clog most of the rock flow paths, resulting in a strong decrease in permeability. Rare Ca-Fe-carbonate minerals also precipitated under these conditions despite the initial Mg-rich system. For higher pCO2 conditions (∼10 MPa), olivine is more efficiently altered. A greater amount of poorly crystallized Fe(Mg)-rich phyllosilicates and magnesite is produced, and the carbonation rate of olivine is 3–11 times higher than when the pCO2 is 10–100 times lower. Interestingly, the changes in porosity caused by the formation of carbonated and hydrous minerals are small while a strong decrease in permeability is measured during the experiments. The formation of reduced carbon is also observed. It is located preferentially at the inlet, where pH is the lowest. This testifies to a competition between reduction (probably associated with the oxidation of ferrous iron) and carbonation; two processes involved in the fixation of CO2 in a mineral form. One may speculate that the formation of reduced carbon can also be a significant mechanism of CO2 sequestration in olivine-dominated basements.