# Legacy of mantle fabrics preserved within heavily serpentinized peridotites in Hole BA3A cores of the Oman Drilling Project

Katsuyoshi Michibayashi, Yuki Kakihata, Itsuki Natsume, Takeo Okuwaki, Marguerite Godard, Peter Kelemen

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## To cite this version:

Katsuyoshi Michibayashi, Yuki Kakihata, Itsuki Natsume, Takeo Okuwaki, Marguerite Godard, et al.. Legacy of mantle fabrics preserved within heavily serpentinized peridotites in Hole BA3A cores of the Oman Drilling Project. Lithos, 2025, 496-497, pp.107970. 10.1016/j.lithos.2025.107970. hal-05026644

HAL Id: hal-05026644

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Submitted on 9 Apr 2025

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Research Article

# Legacy of mantle fabrics preserved within heavily serpentinized peridotites in Hole BA3A cores of the Oman Drilling Project

Katsuyoshi Michibayashi<sup>a,b,*</sup>, Yuki Kakihata<sup>a</sup>, Itsuki Natsume<sup>a,c</sup>, Takeo Okuwaki<sup>a</sup>, Marguerite Godard<sup>d</sup>, Peter Kelemen<sup>e</sup>, The Oman Drilling Project Science Team

## ARTICLE INFO

**Keywords:**

Olivine

Mantle

Crystal fabric

CPOs

Oman Ophiolite

ICDP OmanDP

## ABSTRACT

Crystallographic preferred orientations (CPO) of olivine within natural peridotites are commonly expressed by pole figures for the [100], [010], and [001] axes, and they can be categorized into six well-known crystal fabric types: A, B, C, D, E, and AG. In this paper, we have studied olivine crystal fabrics preserved within serpentinized peridotites in Hole BA3A cores from the Wadi Tayin mantle section of the Samail ophiolite in the Sultanate of Oman, drilled by the ICDP Oman Drilling Project (OmanDP) as part of the Multi-Borehole Observatory (MBO). Hole BA3A cores mostly consist of serpentinized harzburgites with small proportions of thin serpentinized dunitic and mafic layers. Quarter slabs were obtained from Hole BA3A cores, from parts of the core in which the degree of serpentinization was slightly lower than the range of the average through the whole core (70–96 %). The mineral phases and crystal orientations in each polished thin section were analyzed using a scanning electron microscope (SEM) equipped with an electron backscatter diffraction (EBSD) system. EBSD mapping and data processing were then performed to reveal the original CPO of mantle olivine, based on the orientation of relict grains of olivine in surrounding serpentine that retain the legacy of mantle fabrics. As a result, we found that olivine CPOs in Hole BA3A cores were mostly characterized by A to D type fabrics typical of steeply oriented, late, relatively low temperature mantle shear zones in the Samail ophiolite, as opposed to A to AG type fabrics typical of high-T asthenospheric mantle textures. However, Hole BA3A cores preserve foliations subparallel to the crust-mantle boundary like some of the high temperature fabrics. This implies that the olivine fabric transition could have occurred during continued deformation at progressively lower temperature until mantle flow ceased within the lithospheric mantle.

## 1. Introduction

The mantle section of the Samail ophiolite is the largest piece of the uppermost oceanic lithosphere exposed on the Earth's surface. Extensive structural mapping of these rocks has been conducted throughout the Oman mountain range to unravel mantle processes associated with the generation of oceanic lithosphere (e.g., Boudier et al., 1988; Nicolas et al., 2000).

Although the presence of mantle diapirs has been a highly emphasized result of structural mapping in the Oman ophiolite in the

1980–90s, the most common structure in the mantle section is a homogeneous flow in planes subparallel to the crust-mantle boundary (i.e. Moho), with the flow direction perpendicular to the paleo-spreading axis (Fig. 1; e.g., Ceuleneer et al., 1988; Dijkstra et al., 2002; Ildefonse et al., 1995; Kelemen et al., 2021; Lippard et al., 1986; Michibayashi et al., 2000; Michibayashi and Mainprice, 2004; Nicolas et al., 2000; Nicolas and Violette, 1982). These structures are consistent with mantle flow inferred from seismic anisotropy in the mantle lithosphere underlying Pacific oceanic crust (e.g., Christensen, 1984; Hess, 1964; Kodaira et al., 2014; Kourim et al., 2019; Nicolas and Christensen, 1987).

<footnote><sup>a</sup> Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan</footnote>

<footnote><sup>b</sup> Volcanoes and Earth's Interior Research Center, Research Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan</footnote>

<footnote><sup>c</sup> Kanagawa Prefectural Museum of Natural History, Odawara 250-0031, Japan</footnote>

<footnote><sup>d</sup> Géoscience Montpellier, CNRS, Université Montpellier, Montpellier 34095, France</footnote>

<footnote><sup>e</sup> Lamont Doherty Earth Observatory, Columbia University, Palisades, NY, USA</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: michibayashi@nagoya-u.jp (K. Michibayashi).</footnote>
<figure><img src="image_4.png" /><figcaption>Fig. 1. Schematic section across a spreading ridge showing forced mantle flow possibly driven by active mantle diapirism underneath the ridge axis, which is faster than the motion of the overlying lithospheric plate (after Nicolas et al., 1994; Ildefonse et al., 1995; Michibayashi et al., 2000; Dijkstra et al., 2002).</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 2. Geological map of the southeastern massifs of the Samail ophiolite after Nicolas et al. (2000) showing the OmanDP drill site locations and their relative stratigraphic positions (Kelemen et al., 2020a). Site BA3 is in the mantle section of the Wadi Tayin massif. The location of Fig. 3 is shown in the box.</figcaption></figure>

There are numerous studies of olivine crystal-fabrics (crystallographic preferred orientations) since 1980s in the Samail ophiolite (Benn et al., 1988; Boudier et al., 1988; Boudier et al., 2010; Boudier and Coleman, 1981; Boudier and Nicolas, 1995; Ceuleneer et al., 1988; Dijkstra et al., 2002; Ismail and Mainprice, 1998; Jousselin and Mainprice, 1998; Linckens et al., 2011; Lippard et al., 1986; Michibayashi et al., 2006; Michibayashi and Mainprice, 2004; Michibayashi and Oohara, 2013; Prigent et al., 2018a, 2018b; Smewing et al., 1984; Tommasi et al., 2000; Tommasi and Vauchez, 2015). However, there is a lack of systematic analyses that reveal characteristics of olivine crystal-fabrics associated with the formation of the lithospheric mantle due to cooling of the asthenospheric mantle flow.

In this study, we present a set of olivine crystal-fabrics within serpentinized peridotites of Hole BA3A drilled by the ICDP Oman Drilling

Project (OmanDP) as part of the Multi-Borehole Observatory (MBO) conducted in the southern part of the Samail ophiolite in 2016–2018 (Kelemen et al., 2020a, Kelemen et al., 2021). Hole BA3A cores provide an excellent opportunity to study mantle rheology in a 300 m thick mantle section. We show downhole variations in olivine crystal fabrics within the 300 m-deep borehole, with sub-parallel to the crust-mantle boundary, where asthenospheric mantle flow continues in progressively cooling peridotite as it becomes part of the lithospheric mantle.

## 2. Geological setting

In this section, we summarize the geological setting of Site BA3 and Hole BA3A core according to Kelemen et al. (2020a), Kelemen et al. (2020b), and Kelemen et al. (2021), in which a full description is
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** (A) Geological map in the vicinity of Site BA after Nicolas et al. (2000). The cross section in (B) is drawn along A-B red line. (B) A-B cross section in the vicinity of Site BA after Kelemen et al. (2021), where the relative positions of Site BA1–4 are shown. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

available. Site BA3 is situated in the southern end of Wadi Lawayni, hosted in heavily serpentinized peridotites of the Wadi Tayin mantle section (Figs. 2 and 3). The lithology of this site is predominantly serpentinized harzburgite, with minor dunite bodies, pyroxenite, and gabbroic dikes.

The 300 m-deep, vertical Hole BA3A recovered serpentinized mantle peridotite from the shallow mantle lithosphere with ~100 % recovery (Fig. 4A). The main lithology logged in Hole BA3A is harzburgite (97.6 %, 289.2 m in total), with minor abundances of dunite (2.0 %, 6.0 m) and crosscutting gabbroic to clinopyroxenitic dikes (Fig. 4A). The remaining 0.5 % consists of gabbro, olivine gabbro, and clinopyroxenite, mostly occurring as thin (millimeters to a few centimeters thick) and strongly altered dikes (Fig. 4A).

In hand samples, the harzburgites are protogranular to porphyroclastic. The modal abundance of primary olivine ranges from 65 % to 85 %, with average grain sizes of 1.5–3.5 mm (Kelemen et al., 2020b). The orthopyroxene mode ranged from 15 % to 20 % with average grain sizes of 2.3–4 mm. Harzburgites are commonly foliated dipping to 30° (Fig. 4B), subparallel to the lithological boundary to the lower gabbro shown in Fig. 3, with a plane defined by the shape-preferred orientation (SPO) of orthopyroxenes. Compositional banding is generally weak.

Extensive alteration and serpentinization in Hole BA3A are ubiquitous and increase further where the rocks are fractured (Katayama et al., 2021; Kelemen et al., 2020a; Kelemen et al., 2021). Olivine and orthopyroxene are extensively replaced by serpentine, brucite and magnetite. Clinopyroxene appeared to be the most resistant to alteration.

### 3. Methods

Thin sections were prepared from 31 quarter cores of harzburgite from Hole BA3A core (Figs. 4A, D and 5; the complete sample list is present in Table S1). It is noted that we tried to select our samples from less serpentinized parts of the core during sampling party on board, although Hole BA3A core is heavily serpentinized in the whole section. Thin sections were made on the cut surface of the working halves, where the horizontal and vertical axes were set as the $X_0$ and $Z_0$ axes,

respectively, according to the core reference frame defined by the OmanDP ChikyuOman core description campaign (Fig. 4D; see also Fig. F5 in Kelemen et al., 2020a). All thin sections were polished using 1 µm diamond paste and colloidal silica for >5 h for textural analysis.

The crystallographic orientations of the major constituent minerals occurred in the mantle peridotite protolith, such as olivine, orthopyroxene, clinopyroxene, chromite, and amphibole, were measured using a scanning electron microscope (SEM) equipped with an electron backscatter diffraction (EBSD) system (Hitachi S-3400 N Type II with Oxford Instruments INCASynergy AZTEC 4.0) at the Rock and Mineral Laboratory, Nagoya University, Japan (e.g., Asano et al., 2021; Endo et al., 2024; Harano and Michibayashi, 2024; Igami and Michibayashi, 2021; Kakihata et al., 2022; Matsuyama and Michibayashi, 2023; Matsuyama and Michibayashi, 2024a, 2024b; Uhmb and Michibayashi, 2022, 2023). The operating conditions were 20 kV acceleration voltage, working distance of 28 mm, and low-vacuum mode (30 Pa).

We measured the crystallographic orientations of the minerals throughout the entire thin section using a mapping mode, so that mineral-phase maps were obtained using step sizes of either 30 or 50 µm, as shown in Fig. 5 and Table S1. We used MATLAB MTEX Toolbox software (version 5.8.2; http://mtex-toolbox.github.io/; Bachmann et al., 2010, 2011) to process the map data by ignoring single isolated spikes and extrapolating the orientation of non-indexed pixels from the average orientation of neighboring pixels. We then determined grain boundaries in the mineral-phase maps, where the critical grain boundary misorientation angle was set to 10°.

The crystal-preferred orientations (CPO) of the olivine grains in each thin section were projected onto a lower-hemisphere equal-area stereonet at one point per grain. We used the *J* index to quantify the intensity of a given CPO (Mainprice and Silver, 1993). The *J* index has a value of one for randomly oriented crystals and infinity for a single crystal. Similarly, the *pfJ* index has a value of unity for a random distribution and a maximum value of ~60 for olivine in the present case for a single olivine crystal (Michibayashi and Mainprice, 2004). Fabric strength was also determined using the *M* index technique (Skemer et al., 2005). *M* = 0 represents a random fabric and *M* = 1 represents a
<figure><img src="image_7.png" /><figcaption>Fig. 4. (A) The stratigraphy of Hole BA3A showing lithology and sample depths. Each circle corresponds to one sample (see Table S1). (B) Upper panel: Dip angle of pyroxene foliation in harzburgite foliation (Kelemen et al., 2020a). Lower panel: Rose diagram for dip angle of pyroxene. Most dip angles of pyroxenes are ~30°. (C) Crystal-plastic intensity: 0 = undeformed-protogranular, 1 = weakly foliated (porphyroclastic), 2 = moderately foliated (porphyroclastic), 3 = protomylonite, 4 = mylonites (Kelemen et al., 2020a). (D) Scan image of the working half of a core section and a sample size (quarter core), where the horizontal and vertical axes were set as the X₀ and Z₀ axes, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

single crystal. We calculated the *J*, *pfJ*, and *M* index values using MATLAB MTEX Toolbox (version 5.8.2).

The olivine CPOs are commonly categorized into six fabric types: A, B, C, D, E, and AG (Fig. S1; Mainprice, 2007; Karato et al., 2008; Michibayashi et al., 2016). The fabric types are generally attributed to the predominant activity of a particular olivine slip system: A with (010) [100], B with (010)[001], C with (001)[001], D with {0kl}[100], and E with (010)[100] (Karato et al., 2008). AG type was named by Mainprice (2007), which is otherwise known as the axial-[010] or [010]-fiber pattern (e.g., Tommasi et al., 2000; Tommasi and Vauchez, 2015).

The six olivine fabric types can be mapped quantitatively onto P-wave velocities (*Vp*) (Fig. S1; Michibayashi, 2015; Michibayashi et al., 2016). Here, we calculate three principal P-wave velocities (*V*₁, *V*₂, *V*₃) based on olivine CPO using MATLAB MTEX Toolbox (version 5.8.2; Mainprice et al., 2011, 2015), where *V*₁ is the maximum *Vp*, *V*₃ is the minimum *Vp*, and *V*₂ is the intermediate *Vp* in the direction perpendicular to both *V*₁ and *V*₃ (Fig. S1). To define the field of the six olivine fabric types, we should consider the orientations of the three principal axes in the structural framework (*x*-y-z axes) defined by a strain ellipsoid (e.g., Bouchez and Nicolas, 2021), where the foliation is the *x*-y plane, and the lineation is parallel to the *x*-axis (Fig. S1). Thus, *V*ₓ is the velocity parallel to the *x*-axis, *V*ᵧ is parallel to the *y*-axis, and *V*z is parallel to the

*z*-axis (Fig. S1). Thereby, we translated the suite [*V*₁, *V*₂, *V*₃] to [*V*ₓ, *V*ᵧ, *V*z], where the magnitude of each velocity corresponds to *V*₁, *V*₂, or *V*₃, depending on the attitude of foliation and lineation in each rock. Finally, we used three P-wave velocities (*V*ₓ, *V*ᵧ, *V*z) in a Flinn-type diagram with *V*ᵧ/*V*z for the horizontal axis and *V*ₓ/*V*ᵧ for the vertical axis, and its origin as (1,1), which characterizes the fabric type of each CPO with its intensity (Fig. S1; Michibayashi et al., 2016; Soustelle and Manthilake, 2017; Kakihata et al., 2022; Matsuyama and Michibayashi, 2023, 2024a, 2024b).

In case of Hole BA3A core, thin section planes were not perpendicular to foliation and parallel to lineation because they were made from the cut surfaces of the working halves of the drilled cores (Figs. 4 and 5). Therefore, *V*₁/*V*₂ and *V*₂/*V*₃ are set to be the vertical axis and horizontal axis in the Flinn-type diagram, respectively, and (1,1) is the origin. Thus, the *Fabric-Index-Angle* (FIA) for an olivine fabric type can be defined by the angle of inclination between the origin and its CPO point in the diagram: 0° for the AG type, 63° for single maxima types such as the A type, and 90° for the D type (Harano and Michibayashi, 2024; Harigane et al., 2016; Kim et al., 2022; Kourim et al., 2019; Michibayashi, 2015; Oya et al., 2022).
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Thin section image, Raw EBSD map, restored texture of Sample Hole BA3A 81Z3 16-21 cm. Thin section is parallel to the cut surface of the working half, where the horizontal and vertical axes were set as the X₀ and Z₀ axes, respectively.</figcaption></figure>

# 4. Results

## 4.1. Restored textures

Hole BA3A core samples are heavily serpentinized harzburgites, so their primary microstructures are mostly obscured by the serpentine, brucite and magnetite that have replaced the mantle minerals, as shown in raw data from the EBSD mapping (Figs. 5 and S2). The indexing (mineral identification) rate of original mantle minerals using EBSD mapping is in the range of 4%–30% (Table S1). However, we managed to restore the original textures of peridotite using scattered crystallographic orientation data to define the grains of the main constituent minerals (Figs. 5 and 6). As a result, microstructures consisting of the restored grains appeared to be relatively coarse (> 1 mm) textures with a proportion of smaller grains (Figs. 6 and S2). However, it is important to note that these restored textures were made based on exaggerated grain sizes and shapes because of the method used two neighboring indexed pixels, even when those pixels are relatively far apart.

## 4.2. Crystallographic preferred orientations

Whereas the vertical dimension of drill core is consistently close to the geographical vertical direction, the horizontal orientation varies with respect to the true geographical azimuth. This arises because drill core was recovered as fractured rock segments, some of which had rotated with respect to other parts of the core. Therefore, their stereographic projections are not fixed to the structural framework (i.e., foliation and lineation). Nonetheless, we successfully extracted approximate olivine CPOs based on the restored textures (Figs. 7 and S3), where the red, green, and blue great circles in each pole figure show three planes normal to the direction of [100], [010], and [001] maximum densities, respectively, for each sample. In general, most CPOs are identified as [100]{0kl} patterns (i.e., D type) (Figs. 7 and S3).

There are a few variations in the CPOs, which may be described with respect to the three great circles. The [100] axes tend to align their maxima in the horizontal direction, although the thin section planes are parallel to cut surfaces of the working halves of the drilled cores (Figs. 7 and S3) unlike the XZ planes. Both the [010] and [001] axes show girdle patterns perpendicular to the [100] maxima but their maxima align either horizontally or vertically, as shown by the green (normal to the

[010] maxima) and blue (normal to the [001] maxima) great circles (Figs. 7 and S3).

P-wave anisotropies were in the range of 7%–12% (Figs. 7 and S3; Table S2). The Fabric-Index-Angle (FIA) is in the range of 48°–86° (Figs. 7 and S2). The Vp-Flinn plot shows that olivine CPOs are mostly A to D types (open green circles in Fig. 8), although their structural attitudes (i. e., foliation and lineation) are not well defined.

## 4.3. Vertical profiles of olivine CPOs

In vertical profiles of Hole BA3A core section, the amount of non-index pixels (i.e., zero solution) of EBSD mapping is in the range of 70%–96% (Fig. 9a). The olivine CPOs show variations of fabric types from A (~25 m) to D to A (~100 m) to D to AG–A (~280 m) from the top to the bottom (Fig. 9b). Fabric strengths vary from 2.11 up to 4.99 in J index and from 0.072 to 0.166 in M index, respectively, regardless of the type of olivine fabric (Fig. 9c, d). The pole figure strengths show that the magnitude of the [100] pfJ index tended to be the highest among the three principal axes (Fig. 9e).

# 5. Interpretation and discussion

Site BA3 was selected to investigate the in situ serpentinization process (Kelemen et al., 2020b; Kelemen et al., 2021). Indeed, our samples are heavily serpentinized peridotites (Fig. 6). The indexing (mineral identification) rate of EBSD mapping was in the range of 4%–30% (Table S1), so that we tentatively estimate the degree of serpentinization as 70%–96%, as shown in Fig. 9a, where the amount of non-index pixels (i.e., zero solution) may be roughly equal to the degree of serpentinization. This estimate is consistent with measured H₂O contents of 9.7–16.9 wt%, averaging 13.3 ± 1.0 wt% (1σ) in 65 serpentinized harzburgite and dunite core samples from Hole BA3A (Kelemen et al., 2020b; see also Katayama et al., 2021).

We successfully restored the former peridotite textures based on EBSD maps, because many relics of primary minerals such as olivine and pyroxene occurred relatively equally in each thin section (Figs. 5 and 6). This suggests that the texture modification during and after serpentinization may have been sufficiently weak to preserve its original texture. At least, because many restored grain sizes are coarser than one millimeter (Figs. 5 and 6), we could analyze enough fresh relics to have
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Five examples of restored Mineral Phase maps (the same colour scales as Fig. 5) with their thin section micrographs (Plane polarized light). All Mineral Phase maps are shown in Fig. S2. Ol: olivine, Opx: orthopyroxene, Cpx: clinopyroxene, Ch: chromite.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Five examples of olivine CPOs and their P-wave velocities in Hole BA3A cores, plotted on equal-area, lower-hemisphere projections with the X₀-Z₀ reference frame. Contours are multiples of uniform density. Red, green, and blue great circles in each crystal-axis pole figure show planes normal to the direction of [100], [010], and [001] maximum densities (solid circle in each pole figure), respectively. N: number of grains, *J*: *J*-index, *M*: *M*-index, Max: maximum density (%), Min: minimum density (%), *pfJ*: *pfJ*-index, *Vp*: P-wave velocity, FIA: Fabric-index-angle. Three velocities below each *Vp* pole figure are the maximum, intermediate, and minimum *Vp*, respectively. All data are shown in Fig. S3. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Vp Flinn plot for olivine CPOs after Michibayashi et al. (2016) (see also Fig. S1). OmanDP BA3A: BA3A core CPOs in this study, MM04 HT: high-T CPOs within the Hilti mantle section (Michibayashi and Mainprice, 2004), BN95 MTZ: high-T CPOs within the Moho Transition Zone (Boudier and Nicolas, 1995), J98 MQD: high-T CPOs within the Maqsad Diapir (Jousselin and Mainprice, 1998), MM04 LT: low-T CPOs within the shear zone in the Hilti mantle section (Michibayashi and Mainprice, 2004), MIK06 SZ: intermediate-T CPOs within the shear zone in the Fizh mantle section (Michibayashi et al., 2006), MO13 SZ: low-T CPOs within the hydrous shear zone in the Fizh mantle section (Michibayashi and Oohara, 2013).</figcaption></figure>

the crystallographic orientations of the minerals. Thus, we discuss the characteristics of olivine CPOs in the core from Hole BA3A.

**5.1. Comparison of olivine CPOs between high-T and low-T textures in the Oman ophiolite**

First, we compared our Hole BA3A results with previously published data for the Samail ophiolite in the Vp Flinn plot (Fig. 8). The olivine CPOs in the Samail and Hilti mantle sections have coarse granular textures (Boudier and Nicolas, 1995; Jousselin and Mainprice, 1998; Michibayashi and Mainprice, 2004), interpreted as retaining primary fabrics developed in asthenospheric mantle flow (Fig. 10; Nicolas and Poirier, 1976; Bouchez and Nicolas, 2021).

The “Maqsad diapir” in the Samail mantle section is the most famous in the Samail ophiolite. It is an irregularly shaped, somewhat circular feature several kilometers in diameter, that preserves steep lineations in orthopyroxene and spinel shape fabrics (nearly perpendicular to the crust-mantle transition zone and the inferred, paleo-seafloor; Fig. 1); Surrounding the “Maqsad diapir” to the NW, SW and SE, mantle foliations are approximately horizontal, parallel to the crust-mantle boundary and the inferred seafloor (Boudier and Nicolas, 1995; Ceuleneer et al., 1988; Godard et al., 2000; Jousselin et al., 1998; Jousselin et al., 2021; Jousselin and Mainprice, 1998; Jousselin and Nicolas, 2000). The olivine CPOs in the Maqsad mantle diapir are dominated by the AG-A type (solid red diamonds for Jousselin and Mainprice, 1998 and solid squares for Boudier and Nicolas, 1995 in Fig. 8) (Fig. 10a). The Hilti mantle section preserves textures consistent with horizontal mantle flow away from the paleo-spreading axis (Fig. 1; Ceuleneer et al., 1988; Ceuleneer and Rabinowicz, 1992; Nicolas et al., 1994; Ildefonse et al., 1995; Michibayashi et al., 2000; Michibayashi and Mainprice, 2004;

Dijkstra et al., 2002). The olivine CPOs in the horizontal mantle flow are dominated by the AG type (solid red stars in Fig. 8) (Fig. 10b). Thus, in general, olivine CPOs in high-T mantle fabrics of the Samail ophiolite are dominated by the AG type with few occurrences of A and D types (red symbols in Fig. 8) (Fig. 10).

In contrast, olivine CPOs in lower-T shear zones in the Samail ophiolite represent secondary fabrics modified later by shear deformation, presumably mostly related to the obduction process of the Oman ophiolite. Boudier et al. (1988) mapped three large-scale shear zones developed in the Fizh and Hilti mantle section of the northern Samail ophiolite. Michibayashi et al. (2006) studied the olivine CPOs within the northernmost shear zone outcropped in the Fizh mantle section, where the olivine CPOs varied from A to D types as the textures varied from coarse granular to porphyroclastic (blue solid circles in Fig. 8) (Fig. 10b). Michibayashi and Oohara (2013) studied olivine CPOs within the same shear zone but within the Moho Transition Zone, where the olivine CPOs varied from A to D to E to C types as the textures varied from coarse granular to mylonitic (open blue diamonds in Fig. 8) (Fig. 10b). Michibayashi and Mainprice (2004) studied olivine CPOs within the southernmost shear zone in the Hilti mantle section, where the olivine CPOs were of the E type, as the textures varied from coarse elongated to porphyroclastic to mylonitic (open star symbols in Fig. 8) (Fig. 10b). In summary, the olivine CPOs in the secondary, lower temperature mantle fabrics widely range from D to E to C types (blue symbols in Fig. 8), depending on the deformation conditions, such as temperature, creep mechanism, water, strain, and mechanical anisotropy (Fig. 10b; Boudier et al., 1988; Michibayashi and Mainprice, 2004; Michibayashi et al., 2006; Linckens et al., 2011; Michibayashi and Oohara, 2013, Prigent et al., 2018a).

Consequently, there is a distinct difference in the olivine CPOs
<figure><img src="image_12.png" /><figcaption>Fig. 9. Vertical profiles of olivine CPOs (all data is available in Tables S1 and S2). (a) Serpentine rate (%) indicated by zero solution rate in the EBSD maps. (b) Olivine fabric types determined by the Fabric-Index-Angle (FIA). (c) Fabric strength indicated by *J* index values. (d) Fabric strength indicated by *M* index values. (e) Pole figure strength of three principal axes in olivine indicated by *pfJ* index values.</figcaption></figure>

between the high-T and low-T mantle textures: the high-T olivine CPOs are dominated by the AG type (red symbols in Fig. 8), whereas the low-T olivine CPOs vary from the A to D to E to C types (blue symbols in Fig. 8) (Fig. 10).

In this study, the olivine CPOs in Hole BA3A core are characterized by A to D types (open green circles in Fig. 8 and Fig. 9b), different from the AG types in high temperature structures, and similar to olivine CPOs in later, lower temperature structures (blue symbols in Fig. 8). In more detail, the variation in olivine CPOs in Hole BA3A core section (open green circles in Fig. 8) is similar to that in the northernmost, intermediate-temperature shear zone in the Fizh mantle section (solid blue circles in Fig. 8).

## 5.2. Olivine fabric transition from high-T asthenospheric to low-T lithospheric mantle flow

Although the restored textures in Hole BA3A cores were made based on exaggerated grain sizes and shapes by the limited indexing rate due to heavily serpentinized peridotites, they were more or less dominated by

coarse-grained rather than porphyroclastic textures (Fig. 5; see also Fig. S2). It suggests that the later, lower temperature strain in Hole BA3A cores may not have been large enough to modify their entire, high temperature textures. However, unlike the shear zones developed in the northern Samail ophiolite (Fig. 10b), Site BA3 is located where mantle foliations are subparallel to the crust-mantle boundary (Figs. 3 and 4; Nicolas et al., 2000; Kelemen et al., 2020b; Kelemen et al., 2021). Thus, the mantle structure in the vicinity of Site BA3 is similar to that of the Hilti mantle section, which is also characterized by a subhorizontal structure subparallel to the crust-mantle transition zone (Fig. 10b; Ceuleneer et al., 1988; Ildefonse et al., 1995; Michibayashi et al., 2000; Dijkstra et al., 2002; Michibayashi and Mainprice, 2004).

However, Site BA3 core samples differ from the analyzed samples of the Hilti mantle section in terms of the characteristics of their olivine CPOs: Hole BA3A cores are dominated by A to D types (open green circles in Fig. 8), whereas the Hilti mantle section is dominated by the AG type (solid red stars in Fig. 8). Although Hole BA3A cores are heavily serpentinized peridotites, EBSD maps revealed that they consisted of coarse-grained (>1 mm) olivine and pyroxenes (see Fig. 6). Note that
<figure><img src="image_13.png" /><figcaption>**Fig. 10.** Summary of olivine crystal-fabric types in the Samail ophiolite. BN95: Boudier and Nicolas (1995); J98: Jousselin and Mainprice (1998); MM04: Michibayashi and Mainprice (2004); MIK06: Michibayashi et al. (2006).</figcaption></figure>

although there are smaller (<1 mm) olivine grains among the coarse grains in the restored phase maps in Fig. 6, it is difficult to distinguish between recrystallized grains and relics grains due to heavy serpentinization.

In vertical profiles of Hole BA3A core section, it is important to note that [100] *pfJ* increased toward higher FIA, whereas both [010] and [001] *pfJ* appeared to be relatively constant regardless of FIA (Fig. 9e). In Fig. 11a, we compare the pole figure intensities (*pfJ*) of Hole BA3A (solid circles) with previously published olivine CPOs in terms of the Fabric-Index-Angle (FIA) such as those in the high-T horizontal structures in the Hilti mantle section (star symbols; Michibayashi and Mainprice, 2004) and in the intermediate-T shear zone in the Fizh mantle section (square symbols; Michibayashi et al., 2006). This comparison shows that the *pfJ* index values of the [010] axis are highest in the AG type and decrease through A type to their lowest values in the D types (green symbols with two green arrows in Fig. 11a). In contrast, the *pfJ* index values of the [100] axis decrease from the AG to A type and then increase toward the D type (red symbols with two red arrows in Fig. 11a). The *pfJ* index values of the [001] axis tend to be lower in all fabric types than those of the other two principal axes (blue symbols in Fig. 11a). This suggests that the D type olivine CPO could have developed from the A type along with increasing the intensity of the [100] concentration (Fig. 11a). Thus, the olivine CPOs in Hole BA3A cores appear to have been translated from AG-A to D types, while the large-scale subhorizontal structure was maintained during continued deformation (Fig. 11b).

Consequently, the olivine fabric transition could have occurred during continued deformation at progressively lower temperature until mantle flow ceased within the lithospheric mantle (Fig. 11). In the Samail ophiolite, this may have occurred during flow away from a spreading center (Fig. 10a) and/or during the initial stage of the

obduction process (e.g., Boudier et al., 1988). In either scenario, it is possible that the olivine CPOs preserved in Hole BA3A cores were partly influenced by relatively low temperature deformation, resulting in the A to D types (Fig. 11b).

## 6. Conclusions

Hole BA3A cores in the Wadi Tayin mantle section obtained by the ICDP Oman Drilling Project have been studied in terms of mantle rheology. EBSD maps revealed original mantle fabrics within heavily serpentinized peridotites, which appear to be relatively coarse (> 1 mm) textures with a proportion of smaller grains. The Vp Flinn plot shows that olivine CPOs in Hole BA3A cores are mostly A to D types, which could be modified during progressive retrogression. Comparing with the previously known olivine CPOs in the high-T mantle flow, the olivine fabric transition could have occurred from the high-T AG-A type fabrics to the low-T A-D type fabrics in Hole BA3A core, until the mantle flow ceased to be a part of the lithospheric mantle. This might occur during flow away from a spreading center and/or during the initial stage of the obduction process.

## CRediT authorship contribution statement

**Katsuyoshi Michibayashi:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.
**Yuki Kakihata:** Visualization, Data curation.
**Itsuki Natsume:** Formal analysis, Data curation.
**Takeo Okuwaki:** Data curation.
**Marguerite Godard:** Project administration, Investigation.
**Peter Kelemen:** Writing – review & editing, Resources, Project administration, Funding
<figure><img src="image_14.png" /><figcaption>**Fig. 11.** Comparison of the relationship between the *Fabric-Index-Angle* (FIA) and the Pole Figure Strength (pfJ) in the olivine CPOs of the Oman ophiolite. BA3A: this study, MM04: Michibayashi and Mainprice (2004), M06: Michibayashi et al. (2006).</figcaption></figure>
acquisition, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank D. A. H. Teagle, J. M. Matter, J. A. Coggon, and all members during description campaign onboard D/V Chikyu as well as all members of the science party of the Oman Drilling Project and D. A. H. Teagle and M. Cannat (the editors), W. Hoover, and an anonymous reviewer for their thoughtful comments. This research was supported by the Japan Society for the Promotion of Science (JP16H06347 and 20H02005). This study used data from the Oman Drilling Project, which has been possible through co-mingled funds from the International Continental Scientific Drilling Project (ICDP; Kelemen, Matter, Teagle Lead PIs), the Sloan Foundation-Deep Carbon Observatory (Grant 2014-3-01, Kelemen PI), the National Science Foundation (NSF-EAR-1516300, Kelemen Lead PI), NASA-Astrobiology Institute (NNA15BB02A, Templeton PI), the German Research Foundation (DFG: KO 1723/21-1, Koepe PI), the Japanese Society for the Promotion of Science (JSPS KAKENHI 16H06347, Michibayashi PI; and 16H02742, Takazawa PI), the European Research Council (Adv: no. 669972, Jam-veit PI), the Swiss National Science Foundation (SNF: 20FI21_163,073, Früh-Green PI), JAMSTEC, the TAMU-JR Science Operator, and contri-butions from the Sultanate of Oman Ministry of Regional Municipalities and Water Resources, the Oman Public Authority of Mining, Sultan Qaboos University, CRNS-Univ. Montpellier II, Columbia University of New York, and the University of Southampton.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.lithos.2025.107970.

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