Closely-spaced carbonate replacement veins: the influence of external stress on focused fluid flow during carbonation of peridotite

The reaction of serpentinized peridotites with CO2-bearing fluids to listvenite (quartz-carbonate rocks) requires massive fluid flux and maintained permeability despite volume increase. Here we investigate listvenites and serpentinites samples from Hole BT1B of the Oman Drilling Project to improve our understanding of the mechanisms and feedbacks of fracturing and vein formation during peridotite carbonation. The samples are characterized by a high abundance of magnesite veins which are often bundled into closely-spaced, parallel sets. Relative cross-cutting relationships suggest that these veins are among the earliest structures related to carbonation of serpentinite. These veins often show some features that are typical for antitaxial veins such as growth from a median line outwards. Their bisymmetric chemical zonation of variable Ca and Fe contents, a systematic distribution of SiO2 and Fe-oxide inclusions in these zones, and cross-cutting relations with Fe-oxides and Cr-spinel suggest that they are micro-scale reaction fronts recording the replacement of serpentine by carbonate. Local dolomite precipitation and voids along the vein – wall rock interface suggest that the veins acted as a preferred fluid pathway also after the first fracturing formed the central parts of the zoned magnesite veins. The close spacing and (sub)parallel alignment of the veins points to preferential fracturing of the weaker wall rock, in line with the interpretation that the veins formed in a serpentine matrix. The zoned magnesite veins therefore record an early stage of fluid infiltration during listvenite formation, at which focused fluid flow was controlled to large parts by external tectonic stress.