Chemical weathering, carbonate alkalinity and geologic carbon cycle

The chemical weathering of ultramafic rocks by carbon dioxide–bearing fluids contributes to the geologic carbon cycle through carbonate alkalinity production. Experiments were conducted on a fresh feldspathic lherzolite from Tamarack, Minnesota, USA, and a serpentinised harzburgite from the Semail ophiolite, Oman, to investigate the dependence of chemical weathering and carbonate alkalinity on the (i) initial serpentinisation extent, (ii) rock microstructural characteristics and (iii) initial rock/water ratio. The experiments on the granulated Tamarack feldspathic lherzolite with a large rock/water ratio showed the (i) rock weathering increased pH to 9·5, which stabilised at 8·5 as fluid residence time increased, (ii) rock chemical weathering produced carbonate alkalinity by developing favourable pH conditions and (iii) dissolution rate decreased following cycles of carbon-bearing fluid injection. Analysis of feldspathic lherzolite weathering products after some 2500 h of reaction showed (i) the carbonate and hydrated carbonate minerals were not detectable for experiments with both large and small rock/water ratios and (ii) phyllosilicate minerals (talc and chrysotile) formed during the experiment with a large rock/water ratio. Comparison with experiments on serpentinised harzburgite from Oman (intact) and feldspathic lherzolite from the USA (intact and granulated), all with smaller available reactive surface areas, suggests that a decrease in the rock/water ratio and an increase in serpentinisation extent reduced reaction rates.