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

. The reaction of serpentinized peridotite with CO 2 -bearing fluids to form listvenite (quartz-carbonate rock) requires massive fluid flux and significant permeability despite 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 15 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 SiO 2 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 20 preferred fluid pathways allowing infiltration of CO 2 -rich fluids necessary for carbonation to progress. Fracturing and fluid flow was probably further enabled by external tectonic stress, as indicated by closely spaced sets of subparallel carbonate veins. Despite

This was followed by normal faulting, folding and low-angle detachments in the Eocene and strike-slip faulting in the Oligocene, related to the exhumation of the Jebel Akhdar and Saih Hatat anticlinoria and tectonic and erosional thinning of the ophiolite (e.g., Grobe et al., 2019;Mattern and Scharf, 2018, and references therein). 90

Listvenites in the Samail Ophiolite
Listvenites occur along or in close proximity to the basal thrust of the Samail ophiolite at the western contact of the Aswad massif (United Arab Emirates and North-Western Oman) and in the Northern Samail massif (Fanjah region, Oman) (Glennie et al., 1974;Stanger, 1985;Wilde et al., 2002) (Fig. 1a). In the area surrounding OmanDP Site BT1 in the Northern Samail massif, listvenites crop out as 10s-of-meter thick bands along and parallel to the contact between banded peridotites and the 95 underlying metamorphic sole, which in turn is underlain by multiply deformed, allochthonous metasediments of the Hawasina nappes (Fig. 1b). Directly north of Site BT1, these units form a broad anticline (Falk and Kelemen, 2015). The geometry of the listvenite outcrops and evidence for ductile deformation synchronous with carbonation (Menzel et al., 2021) indicates that the listvenites formed along a shallow-dipping fault zone at the interface between ophiolite, metamorphic sole and underlying metasediments (Kelemen et al., 2021). Together with an imprecise internal Rb-Sr isochron age of 97±29 Ma (2) for Cr-100 muscovite-bearing listvenite close to Site BT1 (Falk and Kelemen, 2015), this points to listvenite formation during subduction/underthrusting of the Arabian continental margin below the obducting ophiolite, consistent with a deep source of CO2-bearing fluids as inferred from Sr and C stable isotope geochemistry (De Obeso et al., 2021a). Earlier models that proposed listvenite formation related to sub-meteoric fluids and normal faulting during extensional tectonics after ophiolite emplacement (Nasir et al., 2007;Stanger, 1985) are inconsistent with these findings. CO2-fluid flux and carbonation concurrent 105 with an early reactivation of the basal ophiolite thrust as an extensional decollement would also be consistent with the outcrop geometry, and extensional top-to-the-NE shearing in the authochtonous carbonates below the ophiolite (64±4 Ma) (Hansman et al., 2018) falls just within the 2margin of the Rb-Sr isochron of Falk & Kelemen (2015). However, while possible, based on the currently available data this is less likely than a subduction-obduction setting (Kelemen et al., 2021). The listvenites contain abundant veins of various generations ( Fig. 1 c & d) and, together with adjacent units, have been overprinted by 110 cataclastic and sharp normal to strike-slip faults that obscure the original structures, showing that multiple brittle deformation phases occurred after listvenite formed (Menzel et al., 2020).

Serpentinites and listvenites of Hole BT1B
Hole BT1B consists in its upper part of listvenite intercalated with two serpentinite layers, separated by a fault at 200 m downhole depth from underlying greenschist-facies metamafic rocks of the metamorphic sole ( Fig. 1e) (Kelemen et al., 2020b). 115 At Site BT1, magnesite predominates, while dolomite and calcite are common in listvenites further north in the Fanjah area.
Clumped isotope thermometry, the presence of quartz-antigorite (± talc) intergrowths, and recrystallization microstructures of quartz after opal point to listvenite formation temperatures of 80 -150 °C in this area (Falk and Kelemen, 2015). Estimates of vein and matrix carbonate precipitation in serpentinite and listvenite of core BT1B range from 45±5 to 247±52 °C based on clumped isotope thermometry (Beinlich et al. 2020). The pressure and depth of listvenite formation are less well constrained. 120 Based on data from underlying carbonate sediments (Grobe et al., 2019) and a plausible ophiolite thickness of 8 -10 km, pressure was at least ~0.3 GPa, while the peak conditions recorded by the metamorphic sole set an upper bound of < 1.2 GPa https://doi.org/10.5194/se-2021-152 Preprint. Discussion started: 5 January 2022 c Author(s) 2022. CC BY 4.0 License. (Kotowski et al., 2021). Except for some dolomite-enriched intervals, especially close to the basal fault, most BT1B listvenites are composed of magnesite, quartz, minor Cr-spinel, and locally Fe-(hydr)oxides or Cr-bearing muscovite (fuchsite). The bulk chemistry and proportions of magnesite and quartz can vary significantly on a small scale, but at the meter scale they are 125 consistent with overall isochemical replacement of peridotite and minor addition of fluid-mobile elements (Godard et al., 2021;Kelemen et al., 2020b). Massive listvenite domains show two main types of pervasive microstructures: (i) zoned, ellipsoidal to spheroidal magnesite particles with euhedral to dendritic habit in a finer grained quartz matrix (Beinlich et al., 2020b;Menzel et al., 2021), and (ii) variably large quartz (±fuchsite) aggregates with microstructures resembling those of orthopyroxene or bastite, surrounded by a matrix of vermicular, mesh-like magnesite-quartz intergrowths (Kelemen et al., 130 2020b;Menzel et al., 2021). Trace element geochemistry suggests that the protolith of the BT1B listvenites was part of the banded peridotite unit commonly found at the base of the Samail ophiolite, with compositions of fuchsite-bearing listvenite overlapping with amphibole-bearing basal lherzolite and fuchsite-free listvenite similar to the composition of refractory peridotite (Godard et al., 2021). Sr and C isotope geochemistry points to deep-sourced metamorphic fluids derived from metasediments similar to the underlying Hawasina Formation as the CO2 source (De Obeso et al., 2021a). 135 Visual core logging by the OmanDP science team showed that veins are abundant in serpentinite and listvenite, with densities of > 200 veins per meter of core for veins < 1 mm wide, and 50 -200 veins/m for veins > 1 mm (Kelemen et al., 2020b). In serpentinites, the vein logging team distinguished between four main vein types, with a narrow (< 0.1 mm) serpentine vein network defining a mesh texture as the earliest generation. The serpentine mesh is cut by multiple generations of serpentine veins, early carbonate oxide veins characterized by a Fe-oxide bearing median zone and antitaxial growth habit, and younger 140 carbonate veins with rare quartz (Kelemen et al., 2020b). In listvenites, the vein logging team identified narrow (< 0.1 mm) carbonate-oxide veins with antitaxial habit and a median line as the earliest vein generation, followed by discontinuous carbonate veins, comparatively wider carbonatechalcedony/quartz veins (> 1 mm to > 1 cm), and late, partially open dolomite and/or magnesite veins (Kelemen et al., 2020b).
Based on deformation and overgrowth microstructures of folded and ductile transposed veins, Menzel et al. (2021) concluded 145 that the early carbonate-oxide veins in listvenites formed during the incipient stage of the carbonation reaction, while most of the rock was still composed of serpentine, confirming similar inferences from previous studies (Beinlich et al., 2020b;Kelemen et al., 2020b). In this study, we refine the preliminary vein classification of the core logging (c.f. Tables 1 & 2) and investigate in detail those vein generations that were directly involved in the carbonation reaction progress in order to understand the mechanisms controlling focused fluid flux, permeability and reactivity during carbonation. 150

Samples
The highly variable range of (micro)structures in serpentinite and listvenite of Hole BT1B was sampled during the Oman drilling Phase 1 core logging onboard R/V Chikyu in September 2017. Additional samples from the area north and east of Hole BT1B were obtained during a field campaign in January 2020. Thin sections produced from core samples are oriented 155 with respect to the core reference frame (CRF), an arbitrary orientation along which contiguous core sections were split. Due to the inclination of 75° of Hole BT1B and discontinuities across which the orientation of core sections could not be reconstructed, structural measurements and sample orientations are not easily comparable between different parts of the core.
Thin sections produced from field samples were either oriented in relation to structural elements, i.e. perpendicular to foliation, or, when no foliation was visible, in the geographical reference frame. For this study, vein microstructures and cross-cutting 160 relationships were inspected in 115 thin sections, of which a subset lacking late cataclastic overprint was investigated in more detail. Thin sections and samples from BT1B are named here with an abbreviated form of the ICDP convention, following the scheme "Hole"_"Core"-"Section"_"top"-"bottom", where top/bottom denote the distance (in cm) from the top of the section. https://doi.org/10.5194/se-2021-152 Preprint. Discussion started: 5 January 2022 c Author(s) 2022. CC BY 4.0 License.

Optical and scanning electron microscopy
A PetroScan Virtual Microscope (RWTH Aachen University) was used to obtain high-resolution scans of full thin sections in 165 plane-polarized light, reflected light, and at 10 different crossed polarizer orientations with a 10x objective. A high precision automated stage allows the interpolation of the extinction behavior of each pixel to visualize extinction at all polarization angles. A selection of these digitized thin sections are available for download as Supplementary material of the Oman Drilling Project (Kelemen et al., 2020b) at http://publications.iodp.org/other/Oman/SUPP_MAT/index.html#SUPP_MAT_Z. Of these, samples of special interest regarding vein microstructures are: Core [77][78][79][80][12][13][14][30][31][32][9][10][11][36][37][38][39][40] Back-scattered electron (BSE) and energy-dispersive X-ray spectroscopy (EDX) maps were acquired for phase identification and imaging of chemical zoning using a Zeiss Gemini SUPRA 55 field-emission scanning electron microscope (FE-SEM) at the Institute of Tectonics and Geodynamics of RWTH Aachen University. Whole thin sections and specific areas of interest were mapped at an acceleration voltage of 15 kV, 8.5 mm working distance and dwell times of 0.2 -1.5 ms/point. Samples 175 were coated with a 6 -8 nm thick layer of tungsten for conductivity.
We used a Zeiss Axio Scope optical microscope equipped with a "cold" cathode luminoscope CL8200 MK5-2 to obtain optical-CL panorama images of large thin section areas. Single images were taken at operating conditions of 15 kV, 320 -350 µA with a 10x objective and exposure times of 5 -10 s.
In selected samples and areas of specific interest, panchromatic and blue-filtered SEM-CL images were obtained using a Zeiss 180 Sigma High Vacuum field emission FE-SEM equipped with a Gatan MonoCL4 system at the University of Texas at Austin.
Following the guidelines of Ukar and Laubach (2016), carbon-coated thin sections were imaged at 5 kV accelerating voltage, 120 µm aperture, 125 µs dwell time, and 2048 x 2048 pixel resolution at magnifications up to 2500x.

Serpentine mesh, magnetite-serpentine veins and serpentine crack-seal veins
Networks of serpentine mesh veins with the typical polygonal hourglass texture of serpentinite (Wicks and Whittaker, 1977) are ubiquitous in non-foliated serpentinites (Fig. 2a). In these veins, serpentine is often brownish in transmitted light and magnetite is abundant. The serpentine mesh is cut by various generations of magnetite-serpentine veins and serpentine crackseal veins (Table 1; Fig. 2 b & c). Similar to the serpentine mesh, magnetite-serpentine veins contain a median zone rich in 190 magnetite with flaky to fibrous serpentine towards the vein walls. Serpentine is clear in transmitted light and the veins are discrete and continuous, occasionally forming parallel sets. Serpentine crack-seal veins are characterized by uniform to cloudy extinction under crossed polarizers with similar fiber orientations over the entire length of the vein. Commonly these veins show vein-parallel banding with oscillatory extinction patterns that are typical of serpentine crack-seal veins (Andreani et al., 2004), likely due to alternating precipitation of chrysotile and lizardite during crack-seal cycles (Tarling et al., 2021). 195 Clusters of parallel, en-echelon and/or branched serpentine veins occur in foliated serpentinites of the area north of site BT1.
The veins are parallel to the penetrative serpentinite foliation, which is defined by flattened mesh cells delineated by magnetite aggregates (Fig. 2d). Serpentine in these cleavage-parallel veins has the same extinction direction with the lambda plate, indicating a strong and consistent shape and crystallographic preferred orientation, which is different from the crystallographic preferred orientation of matrix serpentine (Fig. 2b). 200 https://doi.org/10.5194/se-2021-152 Preprint. Discussion started: 5 January 2022 c Author(s) 2022. CC BY 4.0 License.

Pseudomorphic carbonate
In carbonate-bearing serpentinite, small magnesite aggregates locally occur along the serpentine mesh, tracing its polygonal outlines (Fig. 2 e). Magnetite, partly transformed to Fe-magnesite, is commonly present as inclusions within these magnesite aggregates. Pseudomorphic magnesite has core-rim zoning of variable Fe contents, and magnesite rims in the vein network commonly have euhedral crystal facets towards contacts with serpentine, or a dendritic habit. In places, the pseudomorphic 205 magnesite vein network has a preferred orientation tracing flattened serpentine mesh cells.
Pseudomorphic replacement of serpentine by carbonate also occurs within and along the walls of serpentine crack-seal veins ( Fig. 2f). Here, vein-perpendicular magnesite columns locally replace serpentine along serpentine fibers.

Carbonate veins
Most carbonate in serpentinites of Hole BT1B is in veins, and only occurs in minor amounts as dispersed grains within the 210 serpentine matrix (Fig. 3 Fig. 3 c-e). During core logging, these have been named carbonate-oxide or antitaxial carb-oxy veins (Kelemen et al., 2020b). Because an Fe-oxide median line and antitaxial texture is not always present, we hereafter refer to this vein group as zoned carbonate veins (sc2).
These are characterized by a planar morphology with rather constant vein width (typically 50 -200 µm), and a vein-parallel, often bisymmetrical chemical zoning from a median zone towards the vein walls (Fig. 4). The veins are mostly composed of 220 magnesite of variable composition but dolomite can also be present along the vein walls, and minor vein segments locally consist of (or are replaced by) dolomite and rare quartz. The type of zoning and width of different zones varies between different samples, and, occasionally, within the same thin section. Where present, the median line (2 -10 µm wide) consists of Fe-oxide, and/or soft Fe-hydroxides (possibly goethite) that are rarely well preserved during thin section preparation. The median zone is commonly composed of Ca-and Fe-bearing magnesite in the center, followed by Fe-rich magnesite with 225 systematic variations in the amount of SiO2-inclusions, and Fe-poor magnesite or dolomite towards the vein walls ( Fig. 4  In some serpentinite core intervals, sc2 veins form closely spaced, parallel sets (clusters) ( Fig. 3; Fig. 4a). Locally, they occur as two conjugate anastomosing sets that resemble an s-c fabric of scaly fractured serpentinite (Fig. 3 b, e). Where they intersect elongated Cr-spinel grains, zoned carbonate veins branch into numerous narrow veinlets, locally fragmenting the Cr-spinel ( Fig. 4b). Elongated/flattened Cr-spinel is commonly aligned, probably showing the orientation of a remnant high-temperature 235 plastic deformation fabric of former olivine. All carbonate veins in Hole BT1B are oblique to that early fabric. No systematic relationship between sc1-sc2 vein orientations and the serpentinite mesh texture is apparent. Cross-cutting relationships between sc2 and sc1 veins are usually ambiguous because zoned carbonate veins are locally deflected along serpentine cleavages. However, sc2 veins locally branch into narrow veinlets where they transect sc1 veins, indicating that at least some sc2 are relatively younger. 240 Cross-cutting relationships between sc2 veins and mesh-pseudomorphic magnesite aggregates in the serpentinite matrix are also often complex and ambiguous. For example, Figure 4g shows that only the median zone of the sc2 vein cuts the matrix https://doi.org/10.5194/se-2021-152 Preprint. Discussion started: 5 January 2022 c Author(s) 2022. CC BY 4.0 License. magnesite aggregate, while the Fe-enriched magnesite zone (II) does not. Facetted crystal terminations of carbonate towards matrix serpentine are locally present along the walls of some sc2 veins (yellow arrow in Fig. 4g). Sc2 veins commonly pinch out in narrow vein tips, but abrupt, partially corroded, wide vein terminations are also present. In places, magnesite veins show 245 narrow talc vein terminations (Fig. 4h), Fe-oxides veinlets ( Fig. 4i), or feathery quartz veins (sq1, see below) that emanate from carbonate vein tips (Fig. 5a).

Quartz veins
Quartz veins are surprisingly common in the serpentinites of Hole BT1B, unlike typical serpentinites and peridotites of the Samail ophiolite. Quartz-serpentine intergrowths have previously been observed in samples near Hole BT1B (Falk and 250 Kelemen, 2015) and a few quartz veins were logged during shipboard core description (Kelemen et al., 2020b). We note that their abundance in BT1B was underestimated during logging because they are usually narrow and easily overlooked (Fig. 5a).
Two types are common: "feathery" quartz vein aggregates intergrown with serpentine at the micro-scale (sq1 ; Table 1) and wider, poly-granular to blocky quartz / quartz-magnesite veins (sq2; Table 1). Cross cutting relationships between both types of quartz veins are ambiguous. Sq1 veins are strongly branched, locally emanating from the tips of carbonate veins (Fig. 5a). 255 They commonly cut and may offset sc2 carbonate veins (Fig. 5b). Some wide sq1 veins (> 20 µm) show an antitaxial habit, with a median zone composed of pure quartz and margins enriched in nm-to µm-sized serpentine and/or carbonate inclusions Sq2 quartz / quartz-magnesite veins locally contain euhedral magnesite and are commonly oriented parallel to sc2 veins ( Fig.   3d; Fig. 4e), suggesting that they formed due to preferential fracturing along the walls of zoned carbonate veins. 260

Late carbonate veins
Late, partially open or brecciated carbonate veins cut serpentinites and all previous vein generations (Table 1). These are unrelated to the formation of listvenite, and possibly linked to young magnesite and dolomite precipitation in open joints from groundwater or hyperalkaline serpentinization fluids. Similar carbonate veins and travertine are common in the weathering horizon of the Samail ophiolite peridotites (e.g., Chavagnac et al., 2013;Giampouras et al., 2020;Noël et al., 2018). Therefore, 265 we do not consider them further, but we note that they can locally obscure structures of veins synchronous with listvenite formation.

Veins in listvenite
Some listvenite intervals of Hole BT1B are highly veined, such that locally > 50 % of the listvenite volume consists of veins. 270 This is the result of pseudomorphic replacement of previous serpentine veins, and the superposition of veins formed at different time steps (Table 2).

Pseudomorphic veins after serpentine
Based on their strong microstructural resemblance with common serpentine veins in the serpentinites, we identify two types of pseudomorphic veins: pseudomorphic magnesite (and/or quartz) vein networks after serpentine mesh (lss0), and magnesite-275 quartz veins after serpentine crack-seal veins (lss2). Incipient stages of both pseudomorphic vein replacement microstructures are also present in carbonate-bearing serpentinite. Pseudomorphic magnesite-quartz veins after serpentine crack-seal veins (lss2) are characterized by irregular magnesite along vein walls, and columnar to fibrous magnesite extending from the walls into the vein center ( Fig. 6b; c.f. Fig. 2f). Magnesite 285 fibers are highly variable in length, ranging from small fractions of the vein aperture to fully bridging the vein. Quartz forms a polycrystalline, non-fibrous aggregate in between the magnesite fibers. This vein microstructure is uncommon for classical antitaxial, syntaxial or stretching veins (Bons et al. 2001), which supports the interpretation that they result from pseudomorphic replacement.

Early, zoned carbonate veins 290
Zoned carbonate veins (lc1, Table 2) are the most abundant in many listvenite core intervals ( Fig. 6 c -g) and occur in nearly all studied listvenite samples. As in serpentinites, they form closely spaced parallel or anastomosing branched to crosscutting sets of fibrous to blocky veins. They show a well-defined median zone that is brown in plane-polarized light or contains Feoxides or hydroxides and variably strong chemical zoning from the median zone towards the vein walls, indicating antitaxial growth. In most investigated thin sections, lc1 veins are the earliest generation based on cross-cutting relationships ( Fig. 6 e -295 g). In some samples, zoned carbonate veins with a wide-blocky habit form an early generation of this vein type. In some cases, wide-blocky veins are cut by fibrous carbonate veins, whereas in others fibrous segments alternate with wide-blocky sections within a single vein. Owing to these variations in texture that commonly occur together we group them into one vein type (lc1).
In listvenite samples that have a foliated matrix defined by aligned magnesite ellipsoids or dendritic magnesite-quartz 300 intergrowths (Menzel et al., 2021), subparallel clusters of lc1 carbonate veins are oriented at various angles with respect to the matrix foliation. In samples where the vein orientations are at a high angle to the matrix foliation, the veins are locally folded and/or transposed (Menzel et al., 2021).
Lc1 veins are mostly composed of Fe-to Ca-bearing magnesite (Fig. 7). No preserved serpentine inclusions have been observed in this type of veins, but quartz inclusions are common. In some core intervals, where these veins form anastomosing sets, vein 305 segments composed of dolomite alternate with magnesite. The chemical zoning is similar to that of zoned carbonate veins in serpentinite (sc2) and typically bi-symmetric. Lc1 veins commonly have an antitaxial habit, with elongated to fibrous crystals oriented with their long axes perpendicular to the median zone. The median zone usually shows high Fe contents (XFe = Fe/[Fe + Mg] up to 0.30 in listvenites that contain little or no hematite) near the vein center and becomes progressively Fe-poor towards the vein walls (Fig. 7b). In some veins, a zone of Ca-and Fe-bearing magnesite (XFe = 0.10 -0.15) with rare quartz 310 inclusions occurs along the center of the Fe-rich median zone. In listvenites that contain significant hematite, Fe-contents in zoned carbonate veins are comparatively low and show less variability, although systematic chemical variations are still apparent owing to variations in minor or trace Ca contents and silica (nano-) inclusions (Fig. 7f). Fe-enriched domains are commonly brown to red in the core and in plane-polarized light due to Fe-oxides or -hydroxides along the median zone. Crosscutting relationships show that in some cases the presence of Fe-oxides or -hydroxides is due to oxidation of Fe-magnesite 315 after formation of the lc1 veins (red zones in Fig. 6d; lc1* in Fig. 6g).
SEM-CL images reveal elongated, vein-perpendicular, Fe-rich magnesite in the vein centers (dark-luminescent) and lighterluminescent magnesite (Fe-poor) overgrowths towards the vein walls in crystallographic continuity (Fig. 7d). Many magnesites show euhedral terminations with concentric growth zoning away from the vein center, confirming the antitaxial nature of these veins (Fig. 7d, e). Vein boundaries are irregular with dendritic embayments that extend into the listvenite matrix. A bright-320 luminescent SiO2 overgrowth rim separates the dendritic embayments from the quartz-rich listvenite matrix (Fig. 7d). This irregular magnesite rim with quartz overgrowth has similar microstructure, luminescence and composition as the outermost, typically dendritic rims around matrix magnesite ellipsoids as described by Menzel et al. (2021). https://doi.org/10.5194/se-2021-152 Preprint. Discussion started: 5 January 2022 c Author(s) 2022. CC BY 4.0 License. Cross-cutting relationships between different carbonate veins and passive markers in the form of oxides inclusions within the veins show that carbonate vein formation did not only occur by dilatancy but was accompanied by replacement of the 325 serpentinite matrix (Fig. 8 a & b). Measurement of the extent of replacement versus opening is possible in samples where different generations of zoned carbonate veins crosscut each other, because the dilatant vein aperture is recorded by the displacement of the previous vein generation (Fig. 8b). The cumulative vein width is typically more than twice the opening aperture, showing that epitaxial carbonate growth by replacement of serpentine accounts for much of the vein volume.
Moreover, Fe-oxides within Fe 3+ -rich listvenite samples and systematic variations in the content of SiO2 may act as passive 330 markers that document vein growth by replacement (Fig. 8 ce). While hematite may have co-precipitated during serpentine replacement, magnetite (c.f. yellow arrow in Fig. 8d), is a remnant of the prior serpentinization stage and thus a passive marker.
Fe-oxides are only cut by dolomite-and quartz-bearing median lines and oxide aggregates are preferentially aligned oblique to the lc1 carbonate veins, recording a previous fabric that appears mostly unaffected by veining. Similarly, many magnetite aggregates were passively overgrown during expansion of carbonate veins. Notably, magnesite ellipsoids in the listvenite 335 matrix have the same, albeit concentric, patterns of silica zoning and similar crosscutting relationships with Fe-oxides ( Fig.   8c), indicating that this stage of carbonate growth proceeded similarly and simultaneously in ellipsoidal matrix grains and along vein rims.

Cryptic quartz veins
Cryptic quartz veins are one of the earliest quartz generations in listvenite. They are usually indistinguishable from matrix quartz grains in plane-and crossed-polarized light but become visible by CL due to their dull luminescence compared to brighter matrix quartz (Fig. 9 a, b). Cryptic quartz veins commonly have a vermicular, highly irregular and discontinuous geometry. Many show several stages of growth as revealed by cross-cutting or reactivated zones of different luminescence 350 (Fig. 9b). Locally, CL reveals the presence of a thin, dark-luminescent zone (< 10 µm) with constant thickness over short length scales that could indicate a median zone or refracturing. Notably, most of these veins do not cut zoned magnesite ellipsoids of the listvenite matrix. Instead, they have highly variable thickness and deflect around magnesite ellipsoids. The cryptic quartz veins usually abut against zoned carbonate veins or exploit their wallhost rock interface. The abundance of this vein type is difficult to estimate due their cryptic nature and because matrix quartz in places shows a similarly dull 355 luminescence, but overall they are less abundant and younger than zoned carbonate veins.

Microcrystalline quartz veins
The most enigmatic vein type in listvenites of Hole BT1B are microcrystalline quartz veins (Fig. 9 ce). They consist of micro-crystalline, equigranular quartz with a strong crystallographic preferred orientation over long distances, with small variations of the preferred orientation locally producing striped or chess-board patterns under crossed-polarized light. Similar 360 micro-crystalline quartz occurs as variably sized patches in the listvenite matrix suggesting that it may be a replacement microstructure instead of a classical vein infill. SEM-CL imaging shows that the microcrystalline quartz is composed of spheroidal to equant, dull luminescent quartz grains (3 -8 µm) surrounded by fibrous, bright-luminescent SiO2 matrix. https://doi.org/10.5194/se-2021-152 Preprint. Discussion started: 5 January 2022 c Author(s) 2022. CC BY 4.0 License.
Magnesite spheroids are not cut by these veins, but in places occur within them. Inner parts of zoned carbonate veins appear to cut microcrystalline quartz veins and patches. However, botryoidal, euhedral and dendritic carbonate vein rims are 365 undisturbed by the microcrystalline quartz ( Fig. 9 d, e), suggesting that at least some of the microcrystalline quartz formed after zoned carbonate veins.

Quartz/chalcedony-magnesite and quartz-dolomite veins
Bi-mineralic quartz/chalcedony-magnesite and quartz-dolomite veins (lc4 and lc5 in Table 2) cut earlier quartz and carbonate veins ( Fig. 9 c,d). These veins are mostly syntaxial, with sharp, straight vein walls and abundant host rock inclusions. Wide-370 blocky carbonate and quartz can be present in the vein center while crystals at the vein walls are smaller and commonly have euhedral terminations towards the vein center. Irregular domains of radial chalcedony growth are common, in places also nucleating on the wide-blocky carbonate and quartz crystals along the vein center. Cross-cutting relationships indicate that these veins cut listvenite host rock and are thus younger than carbonation of serpentinite. They are usually older than cataclasites (although in some cases also younger than cataclasis), and are cut by late, open or brecciated carbonate veins 375 (Menzel et al., 2020).

Sequence of reactions and vein formation
Vein microstructures in carbonated peridotites are key for understanding the coupled feedbacks between deformation, fluid flow and carbonation, and may provide valuable insights for industrial carbon storage by mineral carbonation (van Noort et 380 al., 2013). In the BT1B listvenite, veins may have formed due to (i) precipitation from supersaturated fluids along fluid pathways in serpentinite during an incipient stage of carbonation; (ii) precipitation of the reaction products magnesite and/or quartz during in-situ dissolution and replacement of the host serpentine; and (iii) precipitation along fractures in listvenite after termination of the actual carbonation reaction. Microstructural evidence and cross-cutting relationships presented in this study indicate that pseudomorphic veins, zoned carbonate veins and cryptic and microcrystalline quartz veins are coeval with 385 different stages of the carbonation reaction sequence that consumes serpentine (Fig. 10). Some of the textures and cross-cutting relationships are ambiguous, but in general terms a first stage of carbonate veining preceded extensive crystallization of quartz in veins and the listvenite matrix. We distinguish the following stages:

I
Early, high temperature (T > 700 °C) deformation of the banded peridotite protolith, producing a fabric with elongated and aligned Cr-spinel and orthopyroxene. The protolith was partly refertilized through high-T metasomatism (Godard 390 et al., 2021) that is typical of the basal peridotites in the Samail ophiolite (Prigent et al., 2018).

II
Serpentinization of olivine and pyroxene to mesh and bastite serpentine, respectively, likely at T < 250 °C, with formation of magnetite and, in dunitic protolith compositions, brucite. Deformation after and possibly also during serpentinization caused ductile shear zones with aligned lizardite and flattened magnetite mesh structures (Menzel et al., 2021). In places, serpentinization may have been accompanied by cataclasis, similar to partially serpentinized 395 peridotite of the Wadi Tayin massif (Aupart et al., 2021).

IV
Incipient carbonate precipitation as ellipsoidal/spheroidal grains in the serpentine matrix (Beinlich et al., 2020b), along 400 the outlines of polygonal mesh cells ( Fig. 2; Fig. 10) and in early carbonate veins (sc1 and the median zone of sc2 V Locally (about 10 -15 % of core BT1B): ductile deformation of the reacting, serpentine-bearing assemblage, leading to folding of early carbonate veins and development of a penetrative foliation by oriented growth of ellipsoidal 405 magnesite in the matrix (Menzel et al., 2021).

VI
Concentric growth of matrix magnesite grains and widening of zoned carbonate veins by replacement of the serpentine matrix and/or opening, in places with precipitation of some talc or quartz (Fig. 4). This is consistent with the microstructures of overgrowths on folded magnesite veins in ductily deformed listvenites from Hole BT1B, which indicate carbonate vein opening and deformation occurred before listvenite formation was completed (Menzel et al., 410 2021). This stage may have been accompanied by silica loss on a local scale.
VII Incipient precipitation of quartz in the remaining serpentine matrix and formation of early, syn-carbonation quartz veins (sq1 in Table 1; lq1 and lq2 in Table 2). In places, opal may have precipitated initially and later recrystallized to quartz or chalcedony (Kelemen et al., 2021).
VIII Dendritic growth of magnesite on ellipsoidal matrix grains and along the walls of early carbonate veins ( Fig. 7; Fig. 9 415 e) and precipitation of cryptic and/or microcrystalline quartz in veins and the matrix. Complete replacement of remnant matrix serpentine by quartz and minor carbonate concluded the carbonation reaction.

IX
Syntaxial to blocky quartz/chalcedony-carbonate veins that cut listvenite and, rarely, serpentinite (lq4, lc4; Fig. 9 f, g; Fig. 10). Quartz proportions in these veins commonly are higher than carbonate, pointing to silica influx or redistribution during this stage. It is possible that the formation of bimineralic quartz-carbonate veins (lq4, lc4) 420 occurred in listvenite while carbonation proceeded at the advancing reaction front along the serpentinite-listvenite contact. Alternatively, they may have formed due to fracturing during a first deformational overprint following carbonation.

X
Cataclasis, faults and late carbonate veins overprinting listvenite and serpentinite ( Fig. 10; sc4, lc5, lc6 in Table 1 & 2), in parts related to local Ca gain and Mg loss in listvenite (Menzel et al., 2020). 425 Because the reaction of serpentine to magnesite and quartz consumes CO2 while releasing H2O, the fluid evolves to more aqueous compositions with reaction progress. Thus, steps (4) -(8) may have occurred at the same time as serpentinization, along different advancing reaction fronts, which correspond to the contacts between partially hydrated peridotite, serpentinite, carbonate-bearing serpentinite and listvenite.

Influence of pre-existing serpentine structures on veining 430
Pseudomorphic carbonate after mesh and crack-seal serpentine veins (Fig. 2 e & f; Fig. 6 a & b) demonstrate that the microstructure of the precursor serpentinite determined the location and structure of vein networks to a great extent. The local presence of brucite and/or variations in Si, Al and Fe contents of serpentine may have caused preferential carbonation at specific microstructural sites where carbonation reaction affinity is higher. Brucite, which shows very fast carbonation reaction kinetics in low-temperature experiments (Harrison et al., 2013;Hövelmann et al., 2012), is commonly observed together with 435 magnetite along serpentine mesh veins in serpentinites (Schwarzenbach et al., 2015). The preferential replacement of previous brucite by magnesite may thus explain the polygonal, mesh-pseudomorphic carbonate vein network in some serpentinites and listvenites of Hole BT1B ( Fig. 2e; Fig. 6a). However, brucite is typically only abundant in serpentinized dunite, because its stability requires high Mg/Si of the bulk rock. As large parts of the listvenites of Hole BT1B are inferred to have had a serpentinized lherzolite protolith, based on major and trace element geochemistry (Godard et al., 2021), we infer that brucite was only common in minor dunitic intervals. On the other hand, different parts of mesh microstructures and different veins in serpentinite can be composed of a variety of serpentine polytypes with different crystal structure. Acid-leaching experiments have shown that dissolution rates can differ greatly between these polytypes, with much higher Mg extraction rates for chrysotile, nano-tubular chrysotile and poorly-ordered lizardite compared to Al-bearing lizardite, polygonal serpentine, and antigorite (Lacinska et al., 2016). It is therefore likely that different serpentine polytypes also show variable dissolution rates 445 during reaction with moderately acidic, CO2-bearing aqueous fluids. This may explain why specific microstructural sites are preferentially replaced by carbonate, producing pseudomorphic textures. Besides variable dissolution rates, serpentine polytypes also have different crystal habits with differing strength and surface area. Thus, we propose that the heterogeneous microstructures of different serpentine polytypes form micro-environments with different inter-and intra-granular nano-porous matrix permeability and micon-scale permeability along fractures. These heterogeneities create complex relationships between 450 diffusive and advective solute transfer, fluid-flow rates and kinetics that control different levels of pseudomorphic inheritance.
Banded serpentine crack-seal veins appear to have a particularly strong impact on local fracture formation and small-scale porosity morphology. Such serpentine veins typically consist of chrysotile fibers alternating with lizardite or polygonal serpentine, recording repeated crack-seal cycles (Andreani et al., 2004;Tarling et al., 2021). Due to the high tensile strength of chrysotile parallel to fiber orientations, fracturing and associated permeability is expected to occur preferentially along the 455 veinhost rock interface, which is what we observed in pseudomorphic replacement microstructures ( Fig. 2f; Fig. 6b).

Vein growth mechanismsopening versus replacement
Opening of dilatant fractures can increase permeability and provide pathways for fluid infiltration that would allow carbonation to proceed. What type and how much permeability is created, however, depends on how soon after opening and in which direction the vein becomes filled. When crystals precipitate at the vein walls and grow inwards towards the vein center 460 (syntaxial veins), fluid replenishment and, potentially, crack-seal events occur along the vein center (Bons et al., 2012). This potentially results in a loss of connectivity between the vein and matrix permeability network because of mineralization along the vein-matrix interface. In contrast, if crystal growth proceeds from a median zone towards the vein walls (antitaxial veins), fluid flow is focused along the vein-host rock interface creating a connected permeability network between the fracture and rock matrix. We found examples of both types of vein growth in the BT1B serpentinites and listvenites (Tables 1 & 2). In 465 general terms, early serpentine and carbonate veins (e.g., Fig. 2, Fig. 4) as well as some early quartz veins (Fig. 5b) tend to show antitaxial textures, whereas younger quartz-carbonate veins (lq4, lc4; Table 2) tend to be syntaxial. If the process was entirely mechanical, this would suggest a reduction in the connectivity of the permeability network over time. Owing to chemical-mineralogical replacements that occur during carbonation, however, the mechanism is more complex during listvenite formation. 470 Current models of vein formation treat the host rock as a non-reactive substrate with vein formation due to precipitation from aqueous solution in fluid-filled fractures (Ankit et al., 2015;Hilgers et al., 2001;Hubert et al., 2009;Spruženiece et al., 2021a;Spruženiece et al., 2021b). In the case of carbonate veining during listvenite formation, however, mechanical opening was accompanied by replacement of the host serpentinite (Fig. 8) so that the morphology of the fracture wall is controlled by dissolution and replacement in addition to dilatancy. Therefore, the wall rock changes its morphology by dissolution, and vein 475 volume is accommodated by replacement in addition to dilatancy. In addition to evidence from cross-cutting relationships and overgrown passive markers within veins and the listvenite matrix (Fig. 8), the high abundance of SiO2 nano-inclusions within zoned magnesite veins in listvenite of Hole BT1 as confirmed by transmission electron microscope (TEM) (Beinlich et al., 2020b;Menzel et al., 2021) indicates that silica saturation and quartz nucleation rate were high during carbonate vein growth, which provides further evidence for simultaneous serpentine dissolution. 480 Microstructures indicative of growth zoning during antitaxial carbonate vein growth from a median zone into the serpentine matrix (Fig. 7d) suggest that there was a reactive fluid film and significant permeability along the veinhost rock interface https://doi.org/10.5194/se-2021-152 Preprint. Discussion started: 5 January 2022 c Author(s) 2022. CC BY 4.0 License. (Fig. 11). Compared to the fracture permeability created initially by dilatant opening of the vein, which may easily clog due to mineral precipitation, this interface permeability was maintained by vein growth and coupled dissolution of serpentine.
Facetted carbonate crystal terminations, partial talc infills and secondary exploitation by quartz veins (Fig. 4) suggest that the 485 vein-serpentinite interface was a preferential site of focused, advective fluid flow and, in places, new fracture formation. This interface permeability thus promoted continued vein growth by serpentine dissolution, in addition to supplying CO2-bearing fluid to the nano-porous matrix of the non-veined host serpentine through diffusive solute transfer, facilitating progressive carbonation of serpentinite. Subsequent syntaxial quartz-carbonate veins most likely lacked such a reaction front, with fluid pathways concentrated along the center of the vein. 490

Formation of closely spaced carbonate vein sets
Because early, zoned carbonate veins are extremely abundant in serpentinite and listvenite of Hole BT1B, and because of their likely role in acting as main fluid pathways early in the carbonation process, understanding their formation mechanism is integral to deciphering the factors controlling carbonation reaction progress. A key feature of these veins is that they commonly form closely spaced, subparallel sets. Repeated fracturing parallel to existing veins requires that the veins and the vein-host 495 rock interface are stronger than the host rock (Virgo et al., 2014). However, the reaction front at the veinserpentinite interface ( Fig. 11) speaks against a strong veinhost rock interface during this stage of reaction. Furthermore, the zoning patterns, documenting changing fluid compositions and/or redox conditions, are consistent within different veins of the same set. Hence, a sequential process of repeated parallel fracturing and sealing by zoned carbonate growth is unlikely, because it would require similar, cyclic variations of fluid composition and redox conditions to be repeated for each vein. 500 A more feasible explanation is that the zoned parts of the carbonate veins formed along a preexisting fracture or vein set. If the vein material had a higher strength than the host serpentinite, closely spaced vein sets may form. This may have happened if the initial vein fill had a higher permeability or higher carbonation reaction affinity than serpentine of the host rock, so that the veins preferentially became replaced by carbonate. Zoned carbonate growth may then have proceeded from the narrow vein set into the serpentine matrix in a later step. A precursor vein fill of fibrous chrysotile may be a suitable candidate because 505 chrysotile has the same or higher tensile strength compared to matrix serpentine, facilitating fracturing parallel to existing veins. Chrysotile veins may also show higher carbonation reaction rates than lizardite due to the larger surface area of fibrous aggregates, especially if they have a nano-tubular crystal morphology (Lacinska et al., 2016), in line with the observation of other pseudomorphic carbonate veins (Fig. 2 c-f). Subparallel serpentine vein sets occur in serpentinites in the vicinity of listvenites in the area (Fig. 2b) and are common in other serpentinized peridotites of the Samail ophiolite (Kelemen et al., 510 2020a, c). Similar parallel, closely spaced serpentine vein sets are also known from oceanic peridotites (Andreani et al., 2007).

Reaction-induced fracturing?
Listvenites are inferred to form, among other settings, at the base of obducted ophiolites and in the shallow mantle wedge of 515 subduction zones (e.g. Kelemen & Manning, 2015). At these conditions, all principal stresses will be compressive. Thus, fracture by tensile or shear failure typically requires a reduction of effective stress by fluid overpressure (Hilgers et al., 2006;Sibson, 2017). Experiments and numerical models of volume-expanding hydration reactions have shown that crystallization pressure may locally create gradients in differential stress, which can also facilitate fracture formation, increasing permeability and reactive surface area (Malthe-Sørenssen et al., 2006;Rudge et al., 2010;Shimizu and Okamoto, 2016). In combination 520 with elevated fluid pressure, these local stress gradients caused by "force of crystallization" could lead to dilatant opening and propagation of existing veins, formation of new fractures, or enhanced pressure solution of the rock matrix (Fletcher and Merino, 2001). Kelemen and Hirth (2012) propose that crystallization pressure during peridotite carbonation can be large https://doi.org/10.5194/se-2021-152 Preprint. Discussion started: 5 January 2022 c Author(s) 2022. CC BY 4.0 License. enough to exceed the stress required for frictional failure, creating a positive feedback for reaction progress via fracturing.
While this process has been shown to be efficient for reactions where volume changes are very large, such as during hydration 525 of periclase (MgO) to brucite (Zheng et al., 2019;Zheng et al., 2018) and important during serpentinization of olivine (Evans et al., 2020;Plümper et al., 2012;Yoshida et al., 2020), the extent to which crystallization pressure influences listvenite formation is less certain (van Noort et al., 2017). Full hydration and carbonation of olivine increases the solid volume by 33% and > 40% (Kelemen et al., 2011), respectively, while the conversion of serpentine to magnesite and quartz is predicted to cause a solid volume expansion of 18 -22 %. Reaction-induced fracturing due to crystallization pressure and volume expansion 530 may thus have occurred at the advancing serpentinization and carbonation reaction fronts that formed the serpentinites and listvenites at site BT1.
Zoned magnesite veins may theoretically have opened through crystallization pressure to some extent, because, unlike in syntaxial veins, carbonate growth occurred from the center outwards. However, chemical evidence and the open fluid conduit we infer existed at the vein-matrix interface (Fig. 11) speak against this mechanism dominating during early carbonation. 535 Moreover, most zoned carbonate veins in serpentinites and listvenites contain a much smaller proportion of SiO2 (mostly as inclusions in magnesite) than expected for isochemical replacement of serpentine (Fig. 4, Fig. 7 Fig. 8), indicating that silica was leached. On the other hand, Mg isotope geochemistry and bulk chemistry mass balance calculations suggest that Mg in listvenite magnesite is derived from local dissolution of the peridotite protolith (de Obeso et al., 2021b;Godard et al., 2021).
Combined influx of CO2 and local leaching of silica would thus have resulted in a solid volume decrease at the vein-serpentine 540 interface because magnesite has a higher density than serpentine. The leached silica may have precipitated synchronously in different microstructural sites in the rock matrix, forming quartz-rich domains, or as cryptic, microcrystalline or syntaxial quartz veins (Fig. 8) further downstream along the reaction front or in listvenite. Abundant SiO2 nano-inclusions in magnesite point to widespread quartz oversaturation and high nucleation rates, suggesting a non-trivial coupling between the surface properties, porosity and dissolution rate of serpentine and the interface geometry, solute transport and precipitation kinetics 545 during vein growth, possibly with some local silica mobilization in the form of suspended silica nano-aggregates at high fluidflow rates. The occurrence of centimeter-scale bulk chemical variations in the BT1B listvenites suggests that similar local mass transfer was commonplace during listvenite formation (Godard et al., 2021).
Numerical models suggest that volume-increasing reactions with fast reaction kinetics induce polygonal and hierarchical fracture patterns (e.g., Okamoto and Shimizu, 2015;Ulven et al., 2014), in agreement with the typical mesh textures in 550 serpentinites. In contrast, the BT1B zoned carbonate vein sets have parallel or anastomosing patterns that indicate a strong influence of tectonic stress during initial fracture formation.
Taken together, these observations suggest that the zoned magnesite veins did not primarily grow through force of crystallization, although crystallization pressure may have contributed to the external stress responsible for the initial, dilatant fractures along which the carbonate veins developed. A similar conclusion can be drawn from the microstructures of 555 pseudomorphic carbonate (sc0) and feathery, cleavage-parallel carbonate veins (sc1) in carbonate-bearing serpentinite (Table   1): quartz is rare or absent in their vicinity, indicating that their formation did not require volume expansion if Mg was sourced locally from dissolving serpentine. Dendritic microstructures support this, pointing to early carbonate growth primarily through replacement. Reaction-induced fracturing was however likely prevalent during the preceding, highly volume-expanding serpentinization, which created mesh and vein textures with heterogeneous permeability and carbonation affinity. Sets of parallel to anastomosing carbonate veins point to an important role of tectonic stress during early carbonation, likely complemented by deviatoric stress generated by volume expansion at the serpentinization front advancing ahead of the carbonation reaction front, whereas crystallization pressure from magnesite precipitation was most likely not significant during veining. As carbonation progressed, permeability was probably reduced during subsequent quartz veining and further silica 575 replacement of the matrix, but a lack of remnant serpentinite 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.

Sample and data availability
Archive halves and samples of core BT1B are available through the Oman Drilling Project

Author contribution
MDM and JLU designed the study, conducted field work and studied the microstructures and petrography; MDM and TD 585 refined the vein classification; MDM performed SEM imaging, EDX mapping, optical CL analysis and image processing, and drafted the figures; EU conducted SEM and SEM-CL analysis. All authors discussed and interpreted the results. MDM led writing of the manuscript, to which all authors contributed.

Competing interests
The authors declare that they have no conflict of interest. 590

Acknowledgments
We would like to thank Michael Kettermann and Yumiko Harigane for sampling onboard Chikyu, and Peter Kelemen for providing an invaluable set of additional thin sections. Werner Kraus and Jonatan Schmidt are thanked for thin section preparation and technical assistance, and Sara Elliott for assistance with SEM-CL imaging and post-processing. We are grateful to the Oman Public Authority of Mining for support to conduct field work and sample export. This study has benefitted from 595 fruitful and inspiring discussions with Peter Kelemen, Romain Lafay, Juan Carlos de Obeso, Craig Manning and others over the past years.         Tables   Table 1: Vein classification in Hole BT1B serpentinites (ordered from relatively older to younger) Mineral abbreviations: Mgsmagnesite, Doldolomite, Qtzquartz, Tlctalc, Serpserpentine, Lzlizardite, Chrchrysotile, Brcbrucite, Magmagnetite, Hemhematite, CrSp -Cr-spinel