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

Abstract

Abstract. The reaction of serpentinized peridotite with CO2-bearing fluids to form listvenite (quartz–carbonate rock) requires massive fluid flux and significant permeability despite an increase in solid volume. Listvenite and serpentinite samples from Hole BT1B of the Oman Drilling Project help to understand mechanisms and feedbacks during vein formation in this process. Samples analyzed in this study contain abundant magnesite veins in closely spaced, parallel sets and younger quartz-rich veins. Cross-cutting relationships suggest that antitaxial, zoned magnesite veins with elongated grains growing from a median zone towards the wall rock are among the earliest structures to form during carbonation of serpentinite. Their bisymmetric chemical zoning 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 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 magnesite vein fill, together with local dolomite precipitation and voids along the vein–wall rock interface, suggest that these veins acted as preferred fluid pathways allowing infiltration of CO2-rich fluids necessary for carbonation to progress. Fracturing and fluid flow were probably further enabled by external tectonic stress, as indicated by closely spaced sets of subparallel carbonate veins. Despite widespread subsequent quartz mineralization in the rock matrix and veins, which most likely caused a reduction in the permeability network, carbonation proceeded to completion within listvenite horizons.

Pervasive carbonation of peridotite to listvenite (Semail Ophiolite, Sultanate of Oman): clues from iron partitioning and chemical zoning

“…The conversion of serpentinites to listvenites in the Fanja region produced a wealth of vein microstructures and matrix replacement textures including zoned magnesites of highly variable compositions. The abundant veins characterizing our samples indicate efficient cracking and faulting that favored, in turn, the recurrent redistribution of fluid pathways and the infiltration of highly reactive fluids throughout the identified sequences of carbonation (e.g., Kelemen et al, 2022;Menzel et al, 2022) despite the increase in rock volume associated with the formation of listvenites (e.g., Falk and Kelemen, 2015;Godard et al, 2021;Okazaki et al, 2021). Veins form a multi-scale discrete network with matrix replacement textures occurring as small (up to 1 mm) mag- nesite, or more rarely dolomite, overgrowths to aggregates around the finest veins (e.g., Figs.…”

“…6 and S6f-j). All carbonation textures are crosscut by cryptic quartz veins indicative of local silica remobilization (Menzel et al, 2022).…”

“…Based on mineral replacement textures and cross-cutting vein relationships, we identified three main groups of carbonates corresponding to different stages of carbonation during the conversion of serpentinized peridotite to listvenite (Table 1 in the Supplement; this study; Kelemen et al, 2020;Beinlich et al, 2020;Menzel et al, 2022).…”

“…Fuchsite is observed only in listvenites from the lherzolitic protolith domain. Listvenites and carbonated serpentinites are cut by numerous generations of different carbonate and quartz veins (Kelemen et al, 2020;Menzel et al, 2022) and cataclasites (Menzel et al, 2020). From Hole BT1B, 15 listvenites, 5 fuchsite-bearing listvenites and 9 carbonated serpentinites were selected for this study (Fig.…”

“…The earliest occurrences of carbonates are pseudomorphic replacements of serpentine mesh and crack-seal veins by magnesite and, in rare samples, by dolomite (Menzel et al, 2022). Pseudomorphic magnesite has XMg > 0.85 (XMg: molar Mg / (Mg + Fe)) except when growing near to or at the expense of magnetite along the mesh (XMg < 0.85) (Fig.…”