Uncertain how the climate-alarmists will get around this one - 'upsets the basalt ecosystem' - 'we need more science!' but the Icelanders seem to have solved the carbon dioxide sequestration problem according to this paper by Matter et al. in the journal Science. The big issue with sequestration deep underground is that carbon dioxide can ultimately still escape to the surface but what if the gas could be truly sequestered as in solid? As part of the Icelandic CarbFix project 175 tonnes of carbon dioxide generated by the fiendishly named Hellisheidi geothermal power plant was injected into plain basalt rock. Not sure what one tonne of carbon dioxide looks like. According to this site it is equivalent to the energy use of an average house for one month. Anyway, the gas was dissolved in water and pumped to a depth of between 400 and 800 meters. The concentration of about 1 mol per litre works out to 22 cubic meters per tonne.
The gas had been spiked with carbon-14 enriched carbon dioxide and also with sulfur hexafluoride. Seventy meters downstream from the injection well at a depth of 400 meters was a monitoring well. There the hexafluoride was found to emerge again after day 50. This is expected. The initial peak of isotope enriched carbon dioxide was also registered at 50 days. From concentration measurements it was then conservatively concluded that 95% had "probably mineralized". Not that there was any need for caution, after 300 days the pump in the monitoring well broke down being clogged up with calcite. The chemistry facilitating all of this: in the alkaline water conditions calcium ions are released from the basalt rock, calcium reacts with carbon dioxide to calcite. Carbon dioxide rock-solidly sequestered for eternity.