# Geophysical Research Letters®

## RESEARCH LETTER

10.1029/2025GL116304

**Key Points:**

* We measure seismic velocities on different types of serpentinites that underwent various pressure-temperature conditions related to the subduction zone

* The results show a clear Vp/Vs ratio trend that allows identifying different types of serpentinites in high-pressure subduction conditions

* The new velocity data sets can calibrate seismic data to interpret the serpentinization and geological structures in the subduction zone

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

H.-M. Wang, heming.wang.geo@gmail.com

**Citation:**

Wang, H.-M., Doan, M.-L., Auzende, A.-L., Schwartz, S., Chapman, S., Fortin, J., & Tang, X.-M. (2025). Seismic velocities of subducted serpentinites through the lizardite-antigorite transition.

*Geophysical Research Letters*, 52, e2025GL116304. https://doi.org/10.1029/2025GL116304

Received 29 APR 2025

Accepted 22 JUN 2025

**Author Contributions:**

**Conceptualization:** He-Ming Wang, Mai-Linh Doan, Anne-Line Auzende, Stéphane Schwartz

**Data curation:** Samuel Chapman

**Formal analysis:** He-Ming Wang

**Investigation:** He-Ming Wang, Anne-Line Auzende, Stéphane Schwartz, Samuel Chapman

**Methodology:** He-Ming Wang, Mai-Linh Doan, Anne-Line Auzende, Stéphane Schwartz, Samuel Chapman, Jérôme Fortin

**Resources:** Mai-Linh Doan, Anne-Line Auzende, Stéphane Schwartz, Jérôme Fortin, Xiao-Ming Tang

© 2025. The Author(s). This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

<figure><img src="image_1.png" /></figure>

## Seismic Velocities of Subducted Serpentinites Through the Lizardite-Antigorite Transition

He-Ming Wang<fnref n="1" />,<fnref n="2" /> ID, Mai-Linh Doan<fnref n="1" /> ID, Anne-Line Auzende<fnref n="1" /> ID, Stéphane Schwartz<fnref n="1" /> ID, Samuel Chapman³ ID, Jérôme Fortin³ ID, and Xiao-Ming Tang<fnref n="2" /> ID

**Abstract** The mineralogical transition (lizardite to antigorite) occurring in serpentinites dragged down in subduction zones depends on the metamorphic pressure and temperature conditions. The seismic Vp/Vs ratio is useful for distinguishing serpentinites from other mantle rocks. To understand the effects of the metamorphic conditions and the associated microstructural evolution on the serpentinite velocity, we collected seven natural serpentinite rocks with different metamorphic imprints in the Western Alps, from blueschists to eclogite facies conditions. We measured the ultrasonic velocity of the samples at confining pressures up to 70 MPa. The measured seismic Vp and Vs increase with the increasing metamorphic conditions, while the Vp/Vs ratio decreases. The petrological results are used to explain the velocity anisotropy and the weak pressure-dependency of velocity. The new Vp/Vs transition trend provides new constraints for tracking the lizardite/antigorite transition with increasing metamorphic condition from field seismic data and thus studying geodynamic evolution in the subduction environment.

**Plain Language Summary** The formation of the various types of serpentinites depends mainly on the metamorphic pressure and temperature conditions related to geodynamic contexts. The Vp/Vs ratio is a widely used tool to discriminate serpentinite rocks from other mantle rocks and to quantify the amount of serpentinization. However, the effect of serpentine mineral transition from low- to high-metamorphic grade (lizardite/antigorite transition) on seismic velocity is not taken into account. In the internal zone of the Western Alps, we sampled several blueschist and eclogitic serpentinite rocks which record the lizardite/antigorite transition. The ultrasonic velocity of the samples was measured at ambient and confining pressure (up to 70 MPa) environments. The measured velocity increases and the Vp/Vs ratio decreases with the increasing metamorphic condition. The 2D thin sections and 3D X-ray tomography images provide the petrological interpretation for the velocity anisotropy variation and insignificant pressure dependency of velocity. The new data sets provide a more complete Vp-Vs crossplot by filling the gap between the low- and high-grade metamorphic serpentinites, allowing for discrimination of the different types of serpentinites. Our data can calibrate the field seismic velocity data to study the serpentine mineralogical transition and its associated subduction dynamics.

### 1. Introduction

Serpentinites result from the hydration of ultramafic rocks (e.g., peridotite and pyroxenite), during which the primary mineralogical assemblages (olivine-orthopyroxene) transform into different serpentine species (lizardite-antigorite). The type of serpentinization depends on the pressure-temperature (P-T) conditions (high T dependence with low P dependence) related to the various geodynamic contexts (Auzende et al., 2006; Mével, 2003): the lizardite (low-temperature (LT) serpentine) forms in the LT oceanic environments; the transition from lizardite to antigorite (high-temperature (HT) serpentine) will result from increasing temperature and pressure associated with the low thermal gradient (6–10°C/km) of subduction zone (Guillot et al., 2015).

Serpentinites, due to their low density, weak strength and unique rheological behavior, play a key role in tectonic processes (e.g., Escartin et al., 2001; Hilairet et al., 2007; Hirth & Guillot, 2013; Wada et al., 2008) and seismic activities (e.g., Reynard, 2013; Seno, 2005). In addition, the breakdown of antigorite (antigorite/olivine transition) may trigger deep seismicity (Chollet et al., 2011; Dobson, 2002; Ferrand et al., 2017; Jung et al., 2004). The physical properties of serpentinite depend on the type of serpentine minerals present in the rock (Reynard, 2013). Therefore, identifying the various types of serpentinites and quantifying the serpentinization extent (i.e., the

<footnote n="1">¹Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, Univ. Gustave Eiffel, ISTerre, 38000, Grenoble, France,</footnote>

<footnote n="2">²School of Geosciences, China University of Petroleum, Qingdao, China, ³Laboratoire de Géologie, Ecole Normale Supérieure/CNRS UMR 8538, PSL Research University, Paris, France</footnote>
**Supervision:** Mai-Linh Doan, Anne-Line Auzende, Stéphane Schwartz, Xiao-Ming Tang

**Validation:** He-Ming Wang, Mai-Linh Doan, Anne-Line Auzende, Stéphane Schwartz, Samuel Chapman, Jérôme Fortin

**Visualization:** He-Ming Wang, Mai-Linh Doan, Anne-Line Auzende, Stéphane Schwartz

**Writing – original draft:** He-Ming Wang

**Writing – review & editing:** He-Ming Wang, Mai-Linh Doan, Anne-Line Auzende, Stéphane Schwartz, Samuel Chapman, Jérôme Fortin

proportion of serpentinized lithosphere) from oceanic environment at LT conditions to subduction zones at HT conditions, are crucial for understanding mantle rock hydration and the geodynamic processes.

The Vp/Vs ratio is a useful tool in discriminating serpentinites from other mafic and ultramafic rocks by field seismic data, thus investigating the serpentinization extent (e.g., Kamiya & Kobayashi, 2000; Hyndman & Peacock, 2003; Matsubara et al., 2009; Grevemeyer, Hayman, et al., 2018; Grevemeyer, Ranero, & Ivandic, 2018; Li et al., 2021, 2022). Most laboratory velocity data sets used for calibrating the field data are based on the LT lizardite- and chrysotile-bearing serpentinite (e.g., Christensen, 1966, 1972, 1978; Watanabe et al., 2007; Hatakeyama & Katayama, 2020). Several laboratory velocity data sets of the HT antigorite-bearing serpentinite (Bezacier et al., 2010; David et al., 2018; Ji et al., 2013; Shao et al., 2014; Wang et al., 2019) and antigorite-bearing serpentinized peridotites (Birch, 1960; Christensen, 1978; Kern et al., 1997; Watanabe et al., 2007) show that the antigorite has a higher velocity and lower Vp/Vs ratio than lizardite. The seismic velocity of serpentinites depends on the type, the relative volume and the microstructural evolution of serpentine species (i.e., lizardite/antigorite transition) present in the rock, however, these topics remain poorly understood.

This study has two objectives: (a) providing new velocity data sets related to the change of mineralogy in different serpentinites which recorded increasing metamorphic grade and (b) linking the velocity variation to the microstructural evolution of the different types of serpentinite. We collected seven high-pressure fully serpentinized samples from the internal zone of the Western Alps which underwent low- to high-grade metamorphic conditions. Then the seismic wave velocities of the samples are measured under ambient and confining pressures up to 70 MPa using the ultrasonic transmission method. The petrology and microstructures of the samples were also characterized using both 2D petrological thin sections and 3D X-ray tomography, allowing us to interpret the velocity-pressure behavior and velocity anisotropy. Our findings show that the serpentinite velocity increases with the metamorphic grade, while the Vp/Vs ratio decreases. The new data sets fill the gap between the existing laboratory velocities of low- and high-grade metamorphic serpentinites, thus providing a more complete Vp-Vs crossplot for calibrating the field seismic data to track the lizardite/antigorite transition. Moreover, our new results provide useful insights for interpreting the serpentinization and associated geodynamic evolution (e.g., deep tectonic structure, water migration and serpentinite exhumation) in the subduction environment, especially in the Western Alps region.

## 2. Materials and Methods

### 2.1. Geological Setting and Sample Description

Seven serpentinite samples are collected in the Piedmont zone (the internal zone of the Western Alps shown in Figures 1a and 1b), which corresponds to an exhumed oceanic lithosphere that has experienced subduction and collision and presently outcrops (Schwartz et al., 2009). The studied area is thus a natural laboratory to study the different parts of a paleo-subduction zone, with increasing metamorphic (temperature and pressure) degrees from west to east and from north to south (Tricart & Schwartz, 2006). Depending on the type of serpentine mineral found in the serpentinites, three different blueschist domains of the Piedmont zone can be distinguished (Figure 1c) and correspond to progressive sampling at increasing depths of the accretionary wedge of the subduction zone (Figure 2); while the Monviso eclogitic unit (pink area of Figure 1c) represents the subduction channel (Figure 2). The 7 serpentinite rocks (fully serpentinized) are sampled in all these sub-units, the sampling locations being shown by the black stars in Figure 1c.

The mineral assemblages in the samples are determined from Raman spectroscopy (Auzende et al., 2004). The metamorphic temperature condition experienced by the samples is estimated by the method of Raman spectroscopy of carbonaceous material, while the metamorphic pressure conditions are constrained by the thermodynamic equilibrium between different mineral assemblages (see Tricart and Schwartz (2006) and Schwartz et al. (2013) for extensive descriptions). The collected lizardite-bearing serpentinite samples (Cris-01 and Cris-02) record the LT blueschist facies conditions (320 ± 20°C with 9 ± 1 kbar). The lizardite-antigorite mixed serpentinite rock (RB-01 Sample) represents the medium-temperature blueschist facies conditions (365 ± 25°C with 11 ± 1 kbar). The antigorite-bearing serpentinite samples (Guil-01, Guil-02, and Guil-03) correspond to the HT blueschist facies conditions (435 ± 45°C with 13.5 ± 1.5 kbar). In the Monviso eclogitic units (>480°C with >20 kbar), we collected 1 antigorite-bearing serpentinite sample with a small amount (<1%) of secondary olivine (Viso-01) caused by the destabilization of antigorite. The above samples include the successive lizardite/antigorite and antigorite/olivine transitions in the alpine subduction environment. Each sample's mineral
<figure><img src="image_2.png" /><figcaption>**Figure 1.** (a) Geographical map of the Western Alps with the location of the internal Piedmont Zone (square). (b) Simplified structural and geological map of the Western Alps between France and Italy, with the detailed location of the studied area (square). (c) Structural sketch map of the metamorphic units of the Piedmont zone, with the location of the seven collected serpentinite samples (modified after Schwartz et al., 2020). (d) Photomicrographs of characteristic textures of collected serpentinite samples from each metamorphic condition: Lz: lizardite; Atg: antigorite; Chr: chrysotile, Mag: magnetite; Px: pyroxene. Sample Cris-01 (from LT-blueschist domain) is dominated by lizardite with mesh texture underlined by magnetite. Secondary veins are infilled by chrysotile. A small amount of secondary antigorite is crystallized at the boundary of the lizardite minerals. In Sample RB-01 (from MT-blueschist domain), antigorite occurs in the patches scattered in a lizardite matrix and at grain boundaries. In sample Guil-01 (from HT-blueschist domain), antigorite is the sole serpentine variety, and the antigorite blades of a few hundred microns starts to obliterate the mesh texture with the interpenetrative texture. There are a few inherited crystals of clinopyroxene. In Sample Viso-01 (eclogitic facies), antigorite is the sole serpentine variety with magnetite. A small amount of olivine (<1%) is evidenced by X-ray diffraction. The antigorite blades of the Viso-01 are elongated along the foliation.</figcaption></figure>

assemblages, facies, and metamorphic conditions are summarized in Table S1 in Supporting Information S1. The chemical element compositions (Table S2 in Supporting Information S1) are acquired by Inductively Coupled Plasma Emission Spectrometry and Agilent 7700X quadrupole ICP-MS (Lafay et al., 2013), the results indicate that the studied serpentinites have relatively homogeneous major element compositions and have a common oceanic-lithosphere origin.

To relate seismic velocity to the mineralogy (mineral type and proportion) and the microstructures (e.g., foliation), thin sections and 3D X-ray computed tomography (μCT) of the studied samples are acquired. Figure 1d displays photomicrographs of characteristic textures of the studied serpentinites which recorded different metamorphic conditions, the corresponding μCT images are in Figures S1–S6 in Supporting Information S1: Cris (lizardite-serpentinite), RB (matrix of lizardite dotted with antigorite patches), and Guil (antigorite-serpentinite)
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Schematic illustration of serpentinite occurrences in alpine subduction zone with sampling locations. The collected serpentinites sample a range of depths in the accretionary wedge and subduction channel of the paleo-subduction zone, covering different metamorphic conditions (modified after Lafay et al. (2013)).</figcaption></figure>

samples show an isotropic texture mostly characterized by meshes (former olivine grains) and bastite (former orthopyroxene grains) inherited from the oceanic serpentinization. In these samples, meshes are underlined by magnetite grains and rare relict orthopyroxenes are scattered in the matrix. The deformation features are insignificant in all observation directions of Cris, RB, and Guil samples. The Viso-01 sample (antigorite with minor (<1%) metamorphic olivine), which underwent the highest metamorphic conditions, has a foliation along which antigorite grains recrystallized. Moreover, no open cracks were observed in all the samples.

## 2.2. Ultrasonic Velocity Measurements

We cored and polished the different serpentinite samples into cubes and cylinders, the samples' sizes are given in Table S3 in Supporting Information S1. To understand how the seismic properties of serpentinite evolve with the increasing metamorphic degree in the subduction environment, the Vp and Vs of samples are measured using the ultrasonic (~MHz) transmission method. Note that, as all the studied serpentinite samples have very low porosity (<1%), the velocity experiments were performed on the dry samples.

Velocity measurements are performed initially at ambient condition using the acoustic equipment of ISTerre at the University of Grenoble Alpes. Measurements are performed on both the cubic (along the three orthogonal directions) and cylindrical samples (along the long axis of the cylinder). The testing frequency is 1 MHz. The schematics of ultrasonic measurement directions on studied samples are shown in Figures S1–S6 in Supporting Information S1.

The pressure-dependent velocity measurements are performed only on the cylindrical samples. The Vp and Vs velocities are measured up to a confining pressure of 70 MPa in a triaxial cell installed at the Laboratoire de Géologie at École Normale Supérieure de Paris (Borgomano et al., 2020). The testing frequency is again 1 MHz, and the wave propagates along the long axis of the cylindrical samples.
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Cubic samples' ultrasonic results: the variation of (a) Vp, (b) Vs, and (c) Vp/Vs ratio of studied four types of serpentinites with increasing metamorphic grade, where the velocities are measured in three orthogonal directions of the cubic samples at ambient pressure. Cylindrical samples' ultrasonic results: (d) Vp, (e) Vs, and (f) Vp/Vs of the dry cylindrical samples as a function of the confining pressure.</figcaption></figure>

To determine the first arrival of the ultrasonic signals, we used the classical STA/LTA method (Short-Term Average/Long-Term Average) (Withers et al., 1998). The principle of the STA/LTA method is shown in Text S2 in Supporting Information S1.

## 3. Results

### 3.1. Seismic Velocity Variation Through Lizardite-Antigorite Transition

We first investigate how the velocities in the different serpentinite samples vary with the degree of metamorphism. Vp and Vs were measured in the three orthogonal directions on the cubic samples at ambient pressure conditions. Raw laboratory data (circle) and mean values (diamond) are reported in Figures 3a–3c, the error bars represent the variation of raw data due to the anisotropy: Vp (Figure 3a) and Vs (Figure 3b) increase with the metamorphic grade, the mean Vp increases from 5,486 m/s for lizardite-bearing serpentinite to 6,485 m/s for the slightly dehydrated antigorite-bearing serpentinite; the mean Vs increases from 2,666 m/s for lizardite to 3,686 m/s for the slightly dehydrated sample. In contrast, the Vp/Vs ratio (Figure 3c) shows a clear decreasing trend with the metamorphic grade: the Vp/Vs ratio is 2.06 for lizardite, 1.95 for the mixture of lizardite and antigorite, 1.81 for antigorite, and 1.76 for the antigorite with minor olivine.

The different serpentinite samples also vary in terms of their Vp and Vs anisotropy (see Figure S8 in Supporting Information S1 for the Vp and Vs in three orthogonal directions of all cubic samples). The anisotropy degree of Vp and Vs (i.e., A(p) and A(s)) is defined by $(V_{max} - V_{min})/V_{mean}$ (Birch, 1961), where the $V_{max}$, $V_{min}$, and $V_{mean}$ are the maximum, minimum, and mean values of three orthogonal measuring directions. The A(p) and A(s) of each sample are shown in Table S4 in Supporting Information S1: the velocity anisotropy degrees of the Viso-01 sample (deformed antigorite-serpentinite sample) are A(p) = 20% and A(s) = 16%, which are higher than A(p): 3%–7% and A(s): 1%–3% of Cris, RB, and Guil samples (undeformed samples). These velocity anisotropies can be interpreted by rock microstructures (see X-ray images, the associated ultrasonic measurement direction, and sample photos in Figures S1–S6 in Supporting Information S1). In Cris, RB, and Guil samples, the relatively isotropic mesh textures cause slight velocity anisotropy degrees; in the deformed Viso-01 sample, a foliation plane underlined by elongated blades of antigorite showing a strong foliation plane parallel to the x and y directions of ultrasonic measurement, causing the higher velocities at x and y directions in Figure S8 in Supporting Information S1.

Moreover, the uncertainty of picked first arrival of the ultrasonic wave depends on the pre-set threshold of the STA/LTA method. Figure S8 in Supporting Information S1 shows that the picked first-arrival and calculated velocities are robust.
## 3.2. Velocity–Pressure Relationship

To extend our measured data set to natural conditions, the pressure effect on the serpentinite velocity is investigated. The pressure-dependencies of Vp, Vs, and Vp/Vs ratio are reported in Figures 3d–3f. Overall, Vp and Vs increase slightly during pressure loading: Vp and Vs increase by 5.2% and 4.8% for lizardite-dominated samples and by 1.9% and 1.7% for antigorite-dominated samples from ambient pressure to 70 MPa, due to the closure of a tiny amount of cracks. The Vp/Vs ratio (Figure 3f) is almost invariant with pressure loading. The weak crack-closure effect on the velocity of the studied (fully serpentinized) samples is consistent with petrological observations.

# 4. Discussion and Conclusion

## 4.1. Applicability of the Results at In Situ Conditions

The correlation between the P-T conditions at which the serpentine species formed and their Vp/Vs ratio makes them an important marker for studying and estimating the serpentinization extent in oceanic, ophiolitic and subduction environments. In order to calibrate field seismic data with the measured laboratory velocities, the in situ P-T effects on the velocity need to be considered. To discuss the pressure effect on the velocity data, Figure 4 compares the new results (solid markers) of this study with the existing classical laboratory data sets measured at higher pressures. For LT lizardite-serpentinite, the Vp/Vs = 2.13 of this study measured above 60 MPa is closely aligned with the Vp/Vs = 2.12 (green star) measured at 1,000 MPa by Christensen (1996). For HT antigorite-serpentinite, the Vp/Vs = 1.81 of this study measured above 60 MPa is consistent with the Vp/Vs = 1.81 (green points with error bars) of laboratory antigorite-serpentinite velocities by Christensen (1978) at 1,000 MPa; Kern et al. (1997) at 600 MPa; Ji et al. (2013) and Shao et al. (2014) at 600 MPa, David et al. (2018) at 150 MPa, as well as Watanabe et al. (2007) at 200 MPa. Furthermore, the elastic moduli converted from the velocities of this study (Table S5 in Supporting Information S1) are within the range of the elastic moduli obtained by existing research and are in good agreement with those from ultrasonic measurements. One of the main effects of increasing pressure is closing the open cracks in the rock (e.g., Hatakeyama & Katayama, 2020; Ji et al., 2013; Shao et al., 2014), thus increasing the seismic velocity. The consistency between the measured new velocities and the literature data measured at pressures above the crack-closure pressure of ~150 MPa (Ji et al., 2013) suggests the crack impact is slight in our studied samples. The increasing temperature generally decreases the velocity due to thermal effects (e.g., thermal dilatation of mineral lattices and microcracking) (Ji et al., 2013). In the P-T range of serpentine stability, the compensating effects of pressure and temperature should maintain velocity variations within the statistical variability of seismological data (Bezacier et al., 2013). Therefore, the measured Vp/Vs ratio above 60 MPa in this study can be reliably used to identify different serpentine species from field seismic data. It's important to note that the serpentinite velocity can vary significantly when the mineralogy and microstructure (e.g., microcracks) are modified by metamorphic alteration controlled by P-T conditions (e.g., Moar-efvand, 2022). Thus, it's essential to first evaluate the P-T structure when interpreting the in situ seismic data, so as to choose the appropriate Vp/Vs ratio of the corresponding serpentine species and evaluate the crack-closure effect (Ji et al., 2013; Shao et al., 2014; Wang et al., 2019).

Grevemeyer, Hayman, et al. (2018) summarized the widely-used laboratory velocities for LT lizardite- and chrysotile-bearing samples by Christensen (1966, 1972, 1978), as given in dashed and dotted lines of Figure 4. In contrast, the new results are from a wide range of serpentine species from LT oceanic to HT subducted environments. Thus, Figure 4 provides a more complete Vp-Vs crossplot for identifying the transition of various serpentinite types: Vp/Vs ratio ~ 2.1 for lizardite; ~1.9 for lizardite/antigorite transition; ~1.8 for antigorite. As argued by Ji et al. (2013), Shao et al. (2014), and Wang et al. (2019), the serpentinization extent estimated by the HT serpentinite velocity constraint can be twice the estimation by the LT serpentinite velocity constraint. Figure 4 further fills the gap between the low- and high-grade metamorphic serpentinites, the clear Vp/Vs decreasing trend is valuable in calibrating the field seismic data for tracking the serpentine mineralogical transition (lizardite/ antigorite transition) in all geological settings, and further investigating serpentinization at different metamorphic conditions.

The deformed Viso-01 sample with clear foliation has a higher seismic velocity anisotropy, indicating that the high seismic anisotropy is a useful proxy for identifying the deformed serpentinite in the deformed plate in the subduction environment (e.g., Katayama et al., 2009; Shao et al., 2014). Moreover, the seismic anisotropies of this study are weaker than those from single-crystal serpentine data (e.g., Bezacier et al., 2010, 2013; Deng et al., 2022),
<figure><img src="image_5.png" /><figcaption>**Figure 4.** The Vp-Vs crossplot for identifying different serpentinite types. The Vp-Vs data of this study (markers) are obtained from different types of serpentine species, the markers' edge colors represent different samples. Green star represents the lizardite-serpentinite velocities by Christensen (1996). Green circle and error bars show the velocities of antigorite-serpentinite by Christensen (1978), Kern et al. (1997), Ji et al. (2013), Shao et al. (2014), David et al. (2018), and Watanabe et al. (2007). The dashed line and dotted line (Grevenmeyer, Hayman, et al., 2018) are based on the data sets of Christensen (1966, 1972, 1978) on the low-temperature serpentinite samples.</figcaption></figure>

mainly because (a) most of the studied natural samples are characterized by various serpentine crystals with no preferred orientation; (b) natural samples have other randomly distributed minerals (e.g., magnetite, relict pyroxene) which contribute to lower velocity anisotropy.

## 4.2. Geodynamic Implications

The serpentinite in the oceanic and subducted environments controls the tectonic processes and seismic activities of these areas. The new results can provide useful petrophysical insights for interpreting the geodynamic evolution (e.g., deep tectonic structure, water migration and serpentinite exhumation) associated with various serpentine species at different P-T conditions when interpreting the field seismic data.

Figure 5 reported the variation of Vp (Figure 5a), Vs (Figure 5b), and Vp/Vs (Figure 5c) of different serpentinites and peridotites with their stable P-T conditions (modified after Malusà et al. (2021)). In Figure 5, the ellipse size is proportional to the maximum and minimum velocities, and the ellipse location corresponds to the P-T stable field of each lithology (see more details in caption of Figure 5). The Vp and Vs values increase, while the Vp/Vs ratio decreases with the pressure/depth and temperature (i.e., metamorphic grade). Compared with the green ellipses of
<figure><img src="image_6.png" /><figcaption>**Figure 5.** The variation of (a) Vp, (b) Vs, and (c) Vp/Vs ratio of serpentinite and peridotite with increasing pressure and temperature (modified after Malusà et al. (2021)). The green curves are the P-T boundaries of the Lz/Atg and Atg/peridotite transition (data sources: Evans (2004) and Schwartz et al. (2013)); the green ellipses are the velocities of low-temperature and high-temperature serpentinites (data sources: Christensen (1978); Kern et al. (1997); Watanabe et al. (2007); Ji et al. (2013); Shao et al. (2014); Bezacier et al. (2013)) and peridotite (data sources: Khazanehdari et al. (2000); Pera et al. (2003)); the dashed-curve ellipses represent our new results which include four types of serpentinites with increasing metamorphic condition.</figcaption></figure>

LT and HT serpentinites, our new results cover more P-T conditions including the Lz/Atg transition and the slightly dehydrated antigorite (P-T boundaries (II) and (III) in Figure 5). Thus, Figure 5 provides more laboratory evidence for interpreting the subduction structure and the serpentinite exhumation path, especially in the Alpine geodynamic context.

**Data Availability Statement**

The data presented in this study are available in the following repository via Wang et al. (2025).

**Acknowledgments**

This research was supported by ISTerre BQR funding and the China Scholarship Council. We sincerely thank Daoyuan Sun, Ikuo Katayama, and two anonymous reviewers for their insightful comments and constructive suggestions, which greatly improved the quality of this manuscript.

**References**

Auzende, A. L., Daniel, I., Reynard, B., Lemaire, C., & Guyot, F. (2004). High-pressure behaviour of serpentine minerals: A Raman spectroscopic study. *Physics and Chemistry of Minerals*, 31(5), 269–277. https://doi.org/10.1007/s00269-004-0384-0

Auzende, A. L., Guillot, S., Devouard, B., & Baronnet, A. (2006). Serpentinites in an Alpine convergent setting: Effects of metamorphic grade and deformation on microstructures. *European Journal of Mineralogy*, 18(1), 21–33. https://doi.org/10.1127/0935-1221/2006/0018-0021

Bezacier, L., Reynard, B., Bass, J. D., Cardon, H., & Montagnac, G. (2013). High-pressure elasticity of serpentine and seismic properties of the hydrated mantle wedge. *Journal of Geophysical Research: Solid Earth*, 118(2), 527–535. https://doi.org/10.1002/jgrb.50076
Bezacier, L., Reynard, B., Bass, J. D., Sanchez-Valle, C., & Van de Moortèle, B. (2010). Elasticity of antigorite, seismic detection of serpentinites, and anisotropy in subduction zones. *Earth and Planetary Science Letters*, 289(1–2), 198–208. https://doi.org/10.1016/j.epsl.2009.11.009

Birch, F. (1960). The velocity of compressional waves in rocks to 10 kilobar, Part 1. *Journal of Geophysical Research*, 65(4), 1083–1102. https://doi.org/10.1029/JZ065i004p01083

Birch, F. (1961). The velocity of compressional waves in rocks to 10 kilobars, Part 2. *Journal of Geophysical Research*, 66(7), 2199–2224. https://doi.org/10.1029/JZ066i007p02199

Borgomano, J. V., Gallagher, A., Sun, C., & Fortin, J. (2020). An apparatus to measure elastic dispersion and attenuation using hydrostatic-and axial-stress oscillations under undrained conditions. *Review of Scientific Instruments*, 91(3). https://doi.org/10.1063/1.5136329

Chollet, M., Daniel, I., Koga, K. T., Morard, G., & van de Moortèle, B. (2011). Kinetics and mechanism of antigorite dehydration: Implications for subduction zone seismicity. *Journal of Geophysical Research*, 116(B4), B04203. https://doi.org/10.1029/2010JB007739

Christensen, N. I. (1966). Elasticity of ultrabasic rocks. *Journal of Geophysical Research*, 71(24), 5921–5931. https://doi.org/10.1029/JZ071i024p05921

Christensen, N. I. (1972). The abundance of serpentinites in the oceanic crust. *The Journal of Geology*, 80(6), 709–719. https://doi.org/10.1086/627796

Christensen, N. I. (1978). Ophiolites, seismic velocities and oceanic crustal structure. *Tectonophysics*, 47(1–2), 131–157. https://doi.org/10.1016/0040-1951(78)90155-5

Christensen, N. I. (1996). Poisson's ratio and crustal seismology. *Journal of Geophysical Research*, 101(B2), 3139–3156. https://doi.org/10.1029/95JB03446

David, E. C., Brantut, N., Hansen, L. N., & Mitchell, T. M. (2018). Absence of stress-induced anisotropy during brittle deformation in antigorite serpentinite. *Journal of Geophysical Research: Solid Earth*, 123(12), 10616–10644. https://doi.org/10.1029/2018JB016255

Deng, X., Luo, C., Wentzcovitch, R. M., Abers, G. A., & Wu, Z. (2022). Elastic anisotropy of lizardite at subduction zone conditions. *Geophysical Research Letters*, 49(18), e2022GL099712. https://doi.org/10.1029/2022GL099712

Dobson, D. P. (2002). Self-diffusion in liquid Fe at high pressure. *Physics of the Earth and Planetary Interiors*, 130(3–4), 271–284. https://doi.org/10.1016/S0031-9201(02)00001-10

Escartin, J., Hirth, G., & Evans, B. (2001). Strength of slightly serpentinized peridotites: Implications for the tectonics of oceanic lithosphere. *Geology*, 29(11), 1023–1026. https://doi.org/10.1130/0091-7613(2001)029<1023:SOSSI>2.0.CO;2

Evans, B. W. (2004). The serpentinite multisystem revisited: Chrysotile is metastable. *International Geology Review*, 46(6), 479–506. https://doi.org/10.2747/0020-6814.46.6.479

Ferrand, T. P., Hilairet, N., Incel, S., Deldicque, D., Labrousse, L., Gasc, J., et al. (2017). Dehydration-driven stress transfer triggers intermediate-depth earthquakes. *Nature Communications*, 8(1), 15247. https://doi.org/10.1038/ncomms15247

Grevetmeyer, I., Hayman, N. W., Peirce, C., Schwardt, M., Van Avendonk, H. J., Dannowski, A., & Papenberg, C. (2018). Episodic magmatism and serpentinized mantle exhumation at an ultraslow-spreading centre. *Nature Geoscience*, 11(6), 444–448. https://doi.org/10.1038/s41561-018-0124-6

Grevetmeyer, I., Ranero, C. R., & Ivandic, M. (2018). Structure of oceanic crust and serpentinization at subduction trenches. *Geosphere*, 14(2), 395–418. https://doi.org/10.1130/GES01537.1

Guillot, S., Schwartz, S., Reynard, B., Agard, P., & Prigent, C. (2015). Tectonic significance of serpentinites. *Tectonophysics*, 646, 1–19. https://doi.org/10.1016/j.tecto.2015.01.020

Hatakeyama, K., & Katayama, I. (2020). Pore fluid effects on elastic wave velocities of serpentinite and implications for estimates of serpentinization in oceanic lithosphere. *Tectonophysics*, 775, 228309. https://doi.org/10.1016/j.tecto.2019.228309

Hilairet, N., Reynard, B., Wang, Y., Daniel, I., Merkel, S., Nishiyama, N., & Petitgirard, S. (2007). High-pressure creep of serpentine, interseismic deformation, and initiation of subduction. *Science*, 318(5858), 1910–1913. https://doi.org/10.1016/j.tecto.2013.08.027

Hirth, G., & Guillot, S. (2013). Rheology and tectonic significance of serpentinite. *Elements*, 9(2), 107–113. https://doi.org/10.2113/gselements.9.2.107

Hyndman, R. D., & Peacock, S. M. (2003). Serpentinization of the forearc mantle. *Earth and Planetary Science Letters*, 212(3–4), 417–432. https://doi.org/10.1016/S0012-821X(03)00263-2

Ji, S., Li, A., Wang, Q., Long, C., Wang, H., Marcotte, D., & Salisbury, M. (2013). Seismic velocities, anisotropy, and shear-wave splitting of antigorite serpentinites and tectonic implications for subduction zones. *Journal of Geophysical Research: Solid Earth*, 118(3), 1015–1037. https://doi.org/10.1002/jgrb.50110

Jung, H., Greenli, H. W., & Dobrzhinetskaya, L. F. (2004). Intermediate-depth earthquake faulting by dehydration embrittlement with negative volume change. *Nature*, 428(6982), 545–549. https://doi.org/10.1038/nature02412

Kamiya, S. I., & Kobayashi, Y. (2000). Seismological evidence for the existence of serpentinized wedge mantle. *Geophysical Research Letters*, 27(6), 819–822. https://doi.org/10.1029/1999GL011080

Katayama, I., Hirauchi, K. I., Michibayashi, K., & Ando, J. I. (2009). Trench-parallel anisotropy produced by serpentine deformation in the hydrated mantle wedge. *Nature*, 461(7267), 1114–1117. https://doi.org/10.1038/nature08513

Kern, H., Liu, B., & Popp, T. (1997). Relationship between anisotropy of P and S wave velocities and anisotropy of attenuation in serpentinite and amphibolite. *Journal of Geophysical Research*, 102(B2), 3051–3065. https://doi.org/10.1029/96JB03392

Khzanehdari, J., Rutter, E. H., & Brodie, K. H. (2000). High-pressure-high-temperature seismic velocity structure of the midcrustal and lower crustal rocks of the Ivrea-Verbano zone and Serie dei Laghi, NW Italy. *Journal of Geophysical Research*, 105(B6), 13843–13858. https://doi.org/10.1029/2000JB0900025

Lafay, R., Deschamps, F., Schwartz, S., Guillot, S., Godard, M., Debret, B., & Nicollet, C. (2013). High-pressure serpentinites, a trap-and-release system controlled by metamorphic conditions: Example from the Piedmont zone of the western Alps. *Chemical Geology*, 343, 38–54. https://doi.org/10.1016/j.chemgeo.2013.02.008

Li, Y., Grevetmeyer, I., Huang, H., Qiu, X., & Murray-Bergquist, L. (2022). Crustal compositional variations from continental to oceanic domain: A Vp/Vs ratio study across the Zhongsha Block, South China Sea. *Journal of Geophysical Research: Solid Earth*, 127(5), e2021JB023470. https://doi.org/10.1029/2021JB023470

Li, Y., Grevetmeyer, I., Huang, H., Qiu, X., & Xu, Z. (2021). Seismic constraint from Vp/Vs ratios on the structure and composition across the continent-ocean transition zone, South China Sea. *Geophysical Research Letters*, 48(16), e2021GL094656. https://doi.org/10.1029/2021GL094656

Malusà, M. G., Guillot, S., Zhao, L., Paul, A., Solarino, S., Dumont, T., et al. (2021). The deep structure of the Alps based on the CIFALPS seismic experiment: A synthesis. *Geochemistry, Geophysics, Geosystems*, 22(3), e2020GC009466. https://doi.org/10.1029/2020GC009466

Matsubara, M., Obara, K., & Kasahara, K. (2009). High-VP/VS zone accompanying non-volcanic tremors and slow-slip events beneath southwestern Japan. *Tectonophysics*, 472(1–4), 6–17. https://doi.org/10.1016/j.tecto.2008.06.013
Mével, C. (2003). Serpentinization of abyssal peridotites at mid-ocean ridges. *Comptes Rendus Geoscience*, **335**(10–11), 825–852. https://doi.org/10.1016/j.crte.2003.08.006

Moarefvand, A. (2022). *Active acoustic monitoring of phase transformations under lithospheric pressure and temperature conditions (Doctoral dissertation)*. Université Paris sciences et lettres. Retrieved from https://theses.hal.science/tel-04989692

Pera, E., Mainprice, D., & Burlini, L. (2003). Anisotropic seismic properties of the upper mantle beneath the Torre Alfina area (Northern Apennines, Central Italy). *Tectonophysics*, **370**(1–4), 11–30. https://doi.org/10.1016/S0040-1951(03)00175-6

Reynard, B. (2013). Serpentine in active subduction zones. *Lithos*, **178**, 171–185. https://doi.org/10.1016/j.lithos.2012.10.012

Schwartz, S., Gautheron, C., Ketcham, R. A., Brunet, F., Corre, M., Agranier, A., et al. (2020). Unraveling the exhumation history of high-pressure ophiolites using magnetite (U-Th-Sm)/He thermochronometry. *Earth and Planetary Science Letters*, **543**, 116359. https://doi.org/10.1016/j.epsl.2020.116359

Schwartz, S., Guillot, S., Reynard, B., Lafay, R., Debret, B., Nicollet, C., et al. (2013). Pressure-temperature estimates of the lizardite/antigorite transition in high pressure serpentinites. *Lithos*, **178**, 197–210. https://doi.org/10.1016/j.lithos.2012.11.023

Schwartz, S., Tricart, P., Lardeaux, J. M., Guillot, S., & Vidal, O. (2009). Late tectonic and metamorphic evolution of the Piedmont accretionary wedge (Queyras Schistes lustrés, Western Alps): Evidences for tilting during Alpine collision. *Geological Society of America Bulletin*, **121**(3/4), 502–518. https://doi.org/10.1130/B26223.1

Seno, T. (2005). Variation of downdip limit of the seismogenic zone near the Japanese islands: Implications for the serpentinization mechanism of the forearc mantle wedge. *Earth and Planetary Science Letters*, **231**(3–4), 249–262. https://doi.org/10.1016/j.epsl.2004.12.027

Shao, T., Ji, S., Kondo, Y., Michibayashi, K., Wang, Q., Xu, Z., et al. (2014). Antigorite-induced seismic anisotropy and implications for deformation in subduction zones and the Tibetan Plateau. *Journal of Geophysical Research: Solid Earth*, **119**(3), 2068–2099. https://doi.org/10.1002/2013JB010661

Tricart, P., & Schwartz, S. (2006). A north-south section across the Queyras Schistes lustrés (Piedmont zone, Western Alps): Syn-collision refolding of a subduction wedge. *Eclogae Geologicae Helvetiae*, **99**(3), 429–442. https://doi.org/10.1007/s00015-006-1197-6

Wada, I., Wang, K., He, J., & Hyndman, R. D. (2008). Weakening of the subduction interface and its effects on surface heat flow, slab dehydration, and mantle wedge serpentinization. *Journal of Geophysical Research*, **113**(B4), B04402. https://doi.org/10.1029/2007JB005190

Wang, D., Liu, T., Chen, T., Qi, X., & Li, B. (2019). Anomalous sound velocities of antigorite at high pressure and implications for detecting serpentinization at mantle wedges. *Geophysical Research Letters*, **46**(10), 5153–5160. https://doi.org/10.1029/2019GL082287

Wang, H. M., Doan, M. L., Auzende, A. L., Schwartz, S., Chapman, S., Fortin, J., & Tang, X. M. (2025). Data of Seismic properties of subducted serpentinites [Dataset]. Figshare. https://doi.org/10.6084/m9 figshare.27859482

Watanabe, T., Kasami, H., & Ohshima, S. (2007). Compressional and shear wave velocities of serpentinized peridotites up to 200 MPa. *Earth Planets and Space*, **59**(4), 233–244. https://doi.org/10.1186/BF03353100

Withers, M., Aster, R., Young, C., Beiriger, J., Harris, M., Moore, S., & Trujillo, J. (1998). A comparison of select trigger algorithms for automated global seismic phase and event detection. *Bulletin of the Seismological Society of America*, **88**(1), 95–106. https://doi.org/10.1785/BSSA0880010095

## References From the Supporting Information

Han, L., Wong, J., & Bancroft, J. C. (2009). Time picking and random noise reduction on microseismic data. *CREWES Research Report*, **21**, 1–13. https://www.crewes.org/Documents/ResearchReports/2009/CRR200930.pdf