Sequestration of Martian CO2 by mineral carbonation

Abstract

Carbonation is the water-mediated replacement of silicate minerals, such as olivine, by carbonate, and is commonplace in the Earth's crust. This reaction can remove significant quantities of CO2 from the atmosphere and store it over geological timescales. Here we present the first direct evidence for CO2 sequestration and storage on Mars by mineral carbonation. Electron beam imaging and analysis show that olivine and a plagioclase feldspar-rich mesostasis in the Lafayette meteorite have been replaced by carbonate. The susceptibility of olivine to replacement was enhanced by the presence of smectite veins along which CO2-rich fluids gained access to grain interiors. Lafayette was partially carbonated during the Amazonian, when liquid water was available intermittently and atmospheric CO2 concentrations were close to their present-day values. Earlier in Mars' history, when the planet had a much thicker atmosphere and an active hydrosphere, carbonation is likely to have been an effective mechanism for sequestration of CO2.

Figure 1. BSE images of Lafayette thin section USNM 1505-5.

(a) An olivine (O) grain that contains secondary mineral veins (dark grey) and one curving open fracture. The narrow veins extend from the interface with augite (Aug) and are discontinuous. The lower left hand side inset highlights a discontinuous vein. Wider veins cross-cut the entire grain and have coarsely serrated walls. They contain an axial strip of ferrous saponite that is flanked by Fe-rich smectite. The two dotted black squares indicate where veins cross-cut the open fracture. The inset EBSD pole figure (top right) shows that the veins lie parallel to the trace of (001)_ol. Image width = 242 μm. (b) An olivine-hosted secondary mineral vein that cross-cuts a vertical open fracture (arrows). The axis of the vein comprises ferrous saponite (dark grey). In the centre of the field of view is a notch that contains Fe-rich smectite (FS) and siderite (S). The inset EBSD pole figure (top left) shows that the axis of the vein is close to the trace of (001)_ol and the walls of the notches lie parallel to {111}_ol. Image width = 49 μm. (c) Two olivine-hosted secondary mineral veins. Both veins contain axial ferrous saponite (FSa) that has a Fe-rich rim (white) and is flanked by bands of Fe-rich smectite (FS). Between the two veins is a ~10-μm wide selvage of olivine (O) plus siderite (S). The siderite contains a spray of phyllosilicate fibres. Image width = 52 μm. (d) A patch of siderite (S) and Fe-rich smectite (FS) that is enclosed by augite (Aug), orthopyroxene (Opx) and titanomagnetite (T). Fe-rich smectite occurs around the margins of the patch and also cross-cuts the siderite. The inset shows an unaltered patch of mesostasis (M) that is comprised mainly of plagioclase feldspar with some titanomagnetite crystals (white). The insert image width = 198 μm. The size, shape and petrographic context of the mesostasis is very similar to the patch of siderite plus Fe-rich smectite. Image width = 202 μm.

Figure 2. The sequence of replacement reactions within an olivine-hosted vein.

(a–c) are cartoons made by editing the BSE image in d. (a) Propagation of a ferrous saponite (FSa) vein through an olivine (O) grain parallel to (001). (b) Ferrous saponite inclusions have merged to make a continuous vein, but it has stopped short of the right hand side of the olivine grain. (c) Crystallographically controlled replacement of the olivine vein walls by siderite (S). (d) Replacement of siderite by Fe-rich smectite (FS), working inwards from its interface with ferrous saponite, is the final event that is recorded. Grain separated from NHM 1959 755. Scale bar=10μm.

Figure 3. Mars’ H₂O and CO₂ reservoirs and Amazonian secondary mineral formation.

(a) Summary of the relative concentrations of atmospheric CO₂ over Mars’ history, and the relative abundances of surface and groundwater. (b) Diagram showing the distribution of primary and secondary minerals in Lafayette. The olivine grain is shown cut parallel to (100), and the orientation of various planes within the grain is shown in the lower right hand side. The image (b) width is ~250 μm.