Progressive veining during peridotite carbonation: insights from listvenites in Hole BT1B, Samail ophiolite (Oman)

The reaction of serpentinized peridotites with CO₂-bearing fluids to listvenite (quartz-carbonate rocks) requires massive fluid flux and significant permeability despite increase in solid volume. Understanding the mechanic-hydraulic interplay and the conditions, mechanisms and structures that enhance or hamper progress of this reaction is key to estimate the scale of long-term carbon fluxes and reservoirs in mantle rocks and their potential for industrial CO₂ removal by mineral carbonation. Here we present a detailed microstructural analysis of listvenite and serpentinite samples from Hole BT1B of the Oman Drilling Project, which helps to understand the mechanisms and feedbacks during vein formation in this process [1]. The samples contain abundant magnesite veins in closely spaced, parallel sets and younger quartz-rich veins. These veins constitute large volumes of the listvenites, showing that fracturing and related advective fluid flow were integral to carbonation progress. Cross-cutting relationships suggest that antitaxial, zoned carbonate veins with elongated grains growing from a median zone towards the wall rock are among the earliest structures to form during carbonation of serpentinite. They show a bisymmetric chemical zoning of variable Ca and Fe contents with a systematic distribution of SiO₂ and Fe-oxide inclusions; this and cross-cutting relations with Fe-oxides and Cr-spinel indicate that they record progress of reaction fronts during replacement of serpentine by carbonate in addition to dilatant vein growth. Euhedral terminations and growth textures of carbonate vein fill together with local dolomite precipitation and voids along the vein – wall rock interface suggest that these antitaxial veins acted as preferred fluid pathways allowing infiltration of CO₂-rich fluids necessary for carbonation to progress. Fluid flow was probably further enabled by external tectonic stress, as indicated by the close spacing and subparallel alignment of these carbonate veins. As carbonation progressed, permeability was reduced during subsequent quartz veining and silica replacement of the matrix, but the scarcity of remnant serpentine in listvenite horizons indicates that penetration of CO₂-rich fluid through the vein and matrix permeability network was sufficient for carbonation to proceed to completion.

[1] Menzel, et al., Solid Earth Discussions [preprint], https://doi.org/10.5194/se-2021-152, in review, 2022.

M.D.M. and J.L.U. acknowledge funding of DFG grants UR 64/20-1, UR 64/17-1.