Estimating excess bound water content due to serpentinisation in mature slow-spreading oceanic crust using Vp/Vs

Domain: earth_science
Published: 2025-07-23
Editorial outcome: minor_revision

Abstract
Mature oceanic crust carries chemically bound water which may be released in subduction zones or delivered to the deep mantle. Estimating water content in slow-spreading crust is challenging due to its complex lithology, requiring both P- and S-wave seismic velocity (Vp and Vs), the latter of which has been limited. Here we show 2D high-resolution Vp, Vs and excess bound water models due to serpentinisation of mature Atlantic crust near the Lesser Antilles. The ridge-parallel line crosses eight seafloor-spreading segments with equal numbers of magma-robust and magma-poor. Hydration is highly variable and mainly accommodated in strongly serpentinised peridotites, dominantly in magma-poor segments, which are not preferentially located near fracture zones. Serpentinised peridotites (17% of the crust) host four times more water than normal magmatic crust, increasing Atlantic subduction bound water budget by ~50%. This has implications back in geological time such as during supercontinent breakup when slow-spreading crust subduction was more common.

Introduction
The subduction of hydrated oceanic lithosphere is a key driver of the solid Earth water cycle, transferring water from Earth’s surface to the deep interior1. Water in the oceanic lithosphere can be stored in pores and cracks (free) or in hydrous minerals (chemically bound) within mafic crustal rocks, resulting from hydrothermal alteration2,3. Further hydration of the ultramafic upper mantle occurs during plate bending at the outer rise by the hydration of olivine to serpentine4,5. Any free water (particularly from the sediment component) is thought to be largely expelled at shallow depths, into the accretionary complex and overriding-plate crust6,7. The chemically bound water is released by mineral precipitation within pores and cracks7, and is thought to increase within fracture zones due to additional faulting at the ridge-transform axis15. This model of hydration, hereafter referred to as the 'Penrose hydration model', dominates calculations of subduction fluxes and models of slab dehydration6,7,16,17. While the Penrose hydration model is appropriate for the fast-spread lithosphere in the Pacific, in the Atlantic, slow-spreading ridges produce a more diverse crustal lithology as they alternate (in space and time) between magma-robust (magmatic) and magma-poor (tectonic) accretion modes. During magma-poor accretion, mantle (ultramafic) rocks can be exhumed along detachment faults18–21 and form oceanic core complexes (OCCs). Once exposed to seawater, the serpentinisation of olivine within the ultramafics can bind up to 13 wt% water, about three times more than what can be stored in altered mafics7,22,23. Thus, OCCs, which are estimated to occupy about a quarter of the slow-spreading seafloor and more common at segment ends21,24, could provide an important contribution to the subducting-plate water budget24–27. However, how much excess bound water, beyond the Penrose hydration model, is delivered from these OCCs has not yet been estimated. Whilst slow-spreading lithosphere is not dominant in modern-day subduction systems, it was likely more prevalent in the past and hence is an important endmember for understanding the solid Earth water cycle25.

A key challenge in determining the hydration state of slow-spreading oceanic lithosphere, with its heterogeneous crustal composition, is that experimentally determined P-wave velocities (Vp) of mafic and hydrated ultramafic rocks overlap28,29. However, if the ratio of P- to S-wave seismic velocities (Vp/Vs) can be measured, it significantly improves our ability to discriminate between the degree of hydration and lithological variations30 and other factors that may reduce Vp, such as fractures31. However, high-quality active-source marine S-wave studies are uncommon, as typically S-waves are recorded on just 20 to 30% of ocean bottom seismometers (OBS)32 because there needs to be a suitable boundary to convert the source-generated P-waves. Such a boundary need to have a high impedance contrast33, e.g. the seafloor, the sediment-basement interface and/or the seismic layer 2A/2B interface in the young crust34,35. For mature crust, the sediment-basement interface is generally the most efficient conversion interface as the Vp above the boundary is close to the Vs below32,34,36,37. Phases from other boundaries are either low-amplitude or obscured under the long 'ringing' coda of the S-waves converted from the sediment-basement interface36. The second requirement to record S-waves is either good coupling of the geophones with the seafloor and/or good conversion back from S- to P-waves so that they are present on hydrophone components. In this study, we acquired wide-angle data in a region with thick sediments, which is favourable for both phase conversion at the basement and geophone coupling. The data were collected with a dense receiver spacing, and ~85% of the OBS returned high-quality S-wave data suitable for travel time tomography.

Our study region lies in the western central Atlantic over crust that is 60–75 Ma old and was formed with an average half-spreading rate of ~16 mm/yr (Fig. 1)38,39. The seismic line (VoiLA line 2/3) was acquired as part of the VoiLA Project (Volatile Recycling in the Lesser Antilles)40. It is oriented approximately north-south and parallel to the isochrons. The line spans 384 km, from ~50 km north of the Fifteen-Twenty Fracture Zone (1520FZ) to ~60 km south of Marathon Fracture Zone (MFZ), crossing the Barracuda Trough, Barracuda Ridge, Tiburon Basin and Demerara Abyssal Plain (Fig. 1a). The Barracuda Ridge has been proposed to be originated as a transverse ridge flanking 1520FZ, similar to those along Vema, St. Paul and Romanche41 formed by vertical tectonism42 and it has been further deformed as a result of past compression across the North-American-South American plate boundary rather than volcanic construction43,44. The seismic line is ~250 km east of the deformation front of the Lesser Antilles subduction zone (LASZ), ideal for assessing the hydration of incoming mature Atlantic oceanic lithosphere before bending into the subduction zone. Bending-related hydration was studied using data from VoiLA Line 1 (Fig. 1a)45.

Results
The final inverted Vp and Vs models are shown in Fig. 2a, b. The travel time tomography procedure followed to generate these models is detailed in the 'Methods' section and Supplementary Figs. 1—16.

The Vp and Vs models display strong lateral heterogeneities. Four regions with Vp inversions/reductions with depth are seen (at model distances 67, 140, 217 and 240 km, labelled as VI-1 to 4 respectively). These are considered candidates for OCCs where lower crustal and upper mantle materials are displaced to shallow depths. The most homogeneous velocity structure is found below the Demerara Abyssal Plain (south of model distance 280 km) and the Barracuda Trough (north of model distance 50 km). The average velocity and vertical velocity gradients are systematically higher within a given depth range in these two regions than in the rest of the line. Beneath the resolved Moho reflector, Vp and Vs is up to 7% lower below the Barracuda Ridge and northern Tiburon Basin (model distances 100 to 180 km) compared to elsewhere, where both Vp and Vs increase to normal mantle velocity within ~1 km below the Moho reflector.

Given the complexity of the velocity models, we applied a K-means cluster analysis for P- and S-wave velocity-depth attributes to aid the interpretation (see 'Methods' section and Supplementary Figs. 17—23). We found that the velocity-depth profiles can be grouped into four clusters with exclusive characters (shown in different colours in Fig. 3). The cluster in red (Fig. 3b, c) has a prominent two-layer seismic structure representing Layer 2 (the upper crust) and Layer 3 (the lower crust). It is the most dominant along the line, especially within Segments A, G and H (Fig. 3a). We termed it two-layer, and it has the least V-Z variations of all the clusters. The cluster in cyan (Fig. 3b) has prominent velocity inversions within ~2 km below the top basement and relatively high velocity at a given depth compared to other clusters. It is termed Velocity-inversion and includes the four areas VI-1 to VI-4 identified previously, and has the most V-Z variability. The cluster in blue (Fig. 3b) consistently has the lowest velocity and a rather linear velocity gradient over the whole crustal column. It is termed Linear-gradient with a high presence in Segments B, C and F. The cluster in black (Fig. 3b, c) has a character intermediate between the two-layer and linear-gradient; therefore, it is termed Median-gradient. It is the most dominant cluster at segment ends and forms nearly 50% of Segments C and D. At NTOs, velocity-depth profiles are dominantly characterised by linear- and median-gradient clusters. At 1520FZ and MFZ, a mix of median and two-layer clusters is present, suggesting relatively higher velocity-depth gradients compared to those of NTOs. Note that although the cluster analysis was applied independently on Vp and Vs the results were broadly consistent with only minor discrepancies (Fig. 3a and Supplementary Fig. 24). Overall, we note there is not a strong contrast between segment ends and segment centres, but rather the variation seem to be on a segment-by-segment basis.

Having assigned clusters, the next step was to categorise the seafloor spreading mode based on the frequency of occurrence of each cluster within each segment (Supplementary Fig. 25). Segments A, G and H are considered magma-robust (magmatically accreted) because of the dominance of two-layer structure (Fig. 3c and Supplementary Figs. 25, 26a, f). Segments B, C, E and F are classified as magma-poor (tectonically accreted), characterised mainly by linear- and median-gradient clusters, and more importantly, velocity-inversion clusters (Fig. 3b and Supplementary Figs. 25, 26b, d, e). Note that in the previous visual classification46, segment D was classified as magma-poor. Here, we interpret it as a magma-robust segment because only two-layer and median-gradient clusters are found within it.

In general, the average crustal thickness in the region is about 5.2 km, and the crust of magma-robust segments is consistently thicker than that of magma-poor segments (Fig. 3a). Within magma-robust segments, the P Moho reflector is interpreted as a petrological boundary between the crust and mantle46. Segments G and H have the thickest crust of ~6.4 km along the line. Segment D is exceptional with a thin crustal thickness of 4.4 km, similar to the surrounding magma-poor segments. The thin crust of this short segment could be due to its formation being at the beginning or end of a magma-robust supply cycle. Within magma-poor segments, crustal thickness is on average of ~4.4 km, estimated from only intermittently resolved Moho reflector. This is interpreted as an alteration Moho, which might be associated with the hydrothermal alteration of mantle peridotites, mafics or both46. Segment C has the thinnest crust with a thickness of only ~4.0 km, but PmP is hardly observed below about half of this segment. The Moho reflector resolved from SmS phases is generally ~0.7 to 1 km deeper than that from PmP phases (Fig. 3a) although they are present at similar offset ranges (Supplementary Fig. 6). Such a difference might be related to the influence of the properties and lithologies of the crust and/or Moho transition zone on P- and S-wave propagation. The depth uncertainty for the P Moho reflector could introduce an uncertainty of the crustal thickness of up to ±0.5 km (Supplementary Fig. 16a).

With the benefit of Vp/Vs ratios, we can now re-examine the interpretation of potential oceanic core complexes (OCCs) made from Vp alone (VI-1-to-4). The velocity reduction could be most reasonably explained by serpentinisation28,29 and/or increased porosity due to water-filled fractures31,49. To help discriminate between these two possibilities, we used gravity modelling as an independent diagnostic, given that the serpentinisation of ultramafics significantly reduces bulk density50,51. In contrast, although fractures can significantly reduce seismic properties, the total volume of fractures (shown by porosity) is small and so has little effect on bulk density.

We converted our Vp model to density using various standard empirical velocity-density relationships for dry rocks (see ‘Methods’ section; Fig. 4b). We then gave areas with Vp/Vs >1.9 a reduced density. In Fig. 4d, the first density model, without any anomalous zones, gives an overall misfit of 8 mGal, with large local misfits up to +20 mGal (indicated by yellow arrows). After adding negative density anomalies, the areas with the largest misfit are significantly reduced, with best results achieved with a drop of −0.15 g/cm3 relative to the background density, leading to an average density of 2.52 g/cm3 for these bodies.

The body with the highest Vp/Vs ratio (>2.0) is VI-1, which is ~10 km wide and 2 to 3 km thick. It is located less than 1 km below the top basement and the basement above forms a dome-like structure which outcrops at the seabed (Fig. 4a). In the gravity, the reduced density model fits the free-air anomaly better than the standard model in this region (Fig. 4c, d). The Vp/Vs geometry suggests two circular features, and we label these OCC-3 and OCC-4, which may be associated with 1520FZ and NTO-1, respectively.

Moving south, VI-2 and VI-3 do not correspond with either a Vp/Vs or a gravity anomaly. These regions might therefore contain mafics from the lower crust or ultramafics from the upper mantle exhumed to the crustal level, but not significantly hydrated or with more fractures. Alternatively, perhaps the 2D seismic line only cuts through the margin of their 3D structure. Note that VI-3 was previously interpreted as OCC-1 from Vp-information alone by ref. 46. This example highlights the need for Vs and density for a more definite interpretation.

Finally, VI-4 is similar to VI-1 in terms of Vp/Vs and gravity characteristics, and we label it OCC-2. The Vp/Vs model highlights a fifth region at model distances 290 to 320 km, where Vp/Vs >1.9 is present at mid-crustal depth but does not show similar P-wave velocity-depth inversions as the others. Unfortunately, it is too small to allow its characterisation with gravity. The body could be the eastern toe of a ~20×20 km large basement outcrop sitting west side of the seismic line (Fig. 4a). We refer to it as another potential oceanic core complex, OCC-5.

The P- and S-wave velocity-depth profiles of the identified OCCs fall into velocity envelopes of known OCCs at slow- and ultra-slow-spreading ridges (Fig. 5), validating our interpretations. They have structural variations which might be linked to lithology that can be clearly distinguished from their seismic properties. Overall, S-wave constraints from known OCCs are much more limited than those for P-waves. However, the P- and S-wave velocity-depth profiles of OCC-4 align with the lower range of known velocity envelopes, while OCC-2 and OCC-3 align with the upper range. The eastern rim of offline OCC-5 has a velocity-depth profile that sits between those of OCC-4 and OCC-2/OCC-3, exhibiting a two-layer structure characteristic of a magmatically robust crust.

The most prominent variation within the OCCs is the higher initial Vp gradient with a lower Vs gradient found in OCC-2 and 3 compared to OCC-4, resulting in the highest Vp/Vs ratio found on the line (Fig. 2c). Their high Vp within the top 1 km aligns with velocities of gabbroic cores from Atlantis OCC (Fig. 5). We interpret the inflexion at 1 km depth as a detachment fault zone49, below which Vp and Vs remain high, consistent with a mixed composition of gabbroic and/or exhumed mantle observations30,49,52,53. As mantle exhumation by oceanic detachment faults is not fully amagmatic18–20,54, above the inflexion, high-velocity materials likely represents shallow magmatic intrusives in the hanging wall, while the footwall may contain exhumed mantle with gabbroic plutons24,55. OCC-4, in contrast, has the lowest Vp and Vs and linear gradient in the top 2 to 3 km with no velocity inversions, following that of the drilled serpentinised peridotite from OCCs on MAR at 13°N and Atlantis Massif (Fig. 5). If ascribing velocity variation to lithology, OCC-4 may be predominantly composed of highly serpentinised peridotite with little or no involvement of mafic rocks.

The eastern rim of the potential offline OCC-5 stands out as a buried feature with a high Vp/Vs ratio in the centre of a magma-robust segment. This high Vp/Vs ratio is likely due to the presence of olivine-rich components in mafics to form low-velocity serpentine and/or a large proportion of other high-water hydrous phases like chlorite13. The proximity of the crust to the topographic high suggests it was accreted during a transition from magma-poor (west of the seismic line) to magma-robust (east of the seismic line) modes. The mafics in the magma-robust segment may have been altered by fluids from nearby outcrops, facilitated by high porosity in the basaltic layer56. For simplicity, we ascribe the high Vp/Vs ratio of OCC-5 to a similar hydration mechanism as other identified OCCs and thus include it in the water calculations.

In order to generate a systematic geological interpretation of the seismic line, especially for regions with Vp/Vs >1.9, which may contain excess bound water due to serpentinisation, i.e. in addition to the typical Penrose hydration model, we evaluate the effect of serpentinisation and fracturing on Vp/Vs. First, we establish a Vp/Vs versus Vp template for each process and then project velocity measurements from our model (Fig. 2c) onto them.

The lithology/serpentinisation template was constructed from laboratory measurements of oceanic rock samples (Fig. 6a and 'Methods'). The template separates mafic and dry ultramafic rocks (dark red, mustard yellow and olive green) from serpentinised peridotite (blue and purple). The serpentinisation degree, i.e. serpentine-bound water content, increases with the increase of Vp/Vs.

The water-filled fracture template was constructed using differential effective medium theory (Fig. 6b, Supplementary Fig. 31 and 'Methods'). To simulate fracturing within the interpreted OCCs, we add randomly oriented, water-filled fractures, with aspect ratios of 0.0001–0.05, to a peridotite and gabbro medium and assess the effects on physical properties with increasing porosity. We found that for Vp/Vs >1.9 and Vp of ~7 km/s, fractured peridotite with thinner cracks (aspect ratios of 0.0001–0.002) are required while thicker cracks (aspect ratio >0.005) can explain Vp/Vs >1.9 and Vp of ~5 km/s. An increase in fracture water directly links to porosity, reducing the bulk modulus but not the shear modulus of the rock, and so primarily affects Vp. In contrast, an increase in serpentine-bound water, altering rock’s mineralogy, more strongly lowers shear modulus than bulk modulus and so primarily drops Vs but less for Vp, and thus increases Vp/Vs.

Here, it is also noted that in a 65 Ma lithosphere, the high Vp/Vs anomalies present underneath over 5 km water column and sediments, ~1.5 to 2 km below top basement and extend to bottom of the crust, where fracture/cracks might be largely sealed due to sedimentary loading, increasing lithostatic pressure and precipitations of hydrothermal mineral as lithosphere ages11. Moreover, maintaining free water within fractured peridotite without alteration is likely unrealistic, especially given the reactivity of seawater with olivine and the widespread occurrence of serpentinite in the oceans57–59. Therefore, we think reality is much closer to alteration than the pure fracturing model. Moreover, density variations allow us to further differentiate between the two hydration mechanisms (Fig. 5c). We rule out the pure fracture water model as the fracture porosities required to explain Vp/Vs >1.9 anomalies only yield a negligible density reduction of 0.1 g/cm3, inconsistent with our gravity results. In contrast, serpentinisation at the inferred hydration levels readily accounts for the observed density deficit of ~0.5 g/cm3, supporting a serpentine-bound water-dominated system.

As previously noted, the serpentinisation model is our preferred interpretation based on the geometry and internal structure of the anomalous Vp/Vs bodies, their proximity to elevated or exposed basement and gravity signature (Figs. 4–6). Using the lithology/serpentinisation template (Fig. 6a), each distance/depth (x, z) point from the Vp/Vs model (Fig. 2c) was categorised to give the lithological and excess bound water model due to serpentinisation shown in Fig. 7. It should be noted that this is an upper bound estimate of the excess water content. We present two alternate models, one with a combination of serpentinisation and fracturing and one with pure fracturing in the Supplementary Figs. 31, 32. Note that water stored in other forms of hydrous minerals due to alteration and/or pore-clogging, especially in the upper mafic crust, is not included in the calculation (see 'Methods' section). Instead, we assume the Penrose hydration model applies to magma-robust and other portions of magma-poor segments (white regions of the upper panels in Fig. 7b).

Figure 7 shows that in magma-robust segments, basalts overlying dolerite/gabbros in the upper ~2 km are estimated to have negligible excess hydration with the exception of the eastern rim of OCC-5. In other words, according to our classification, the hydration levels of these segments do not surpass what would be expected for Penrose-style oceanic crust of this age. In magma-poor segments, the boundary between basaltic and gabbroic sections is laterally heterogeneous and interrupted by regions with higher excess bound water content (at model distances 50 to 110 km and 230 to 270 km). These hydrated pockets overall give a bound water content estimate of 4 wt% averaged across the crustal column.

Below the Moho, the mantle is effectively dry throughout the line. The seismic properties are nearly identical to those on our orthogonal line before bending45 and so anisotropy does not seem significant. In magma-robust segments, the dry mantle consistently has Vp/Vs over 1.70, perhaps representing a more depleted mantle. However, Vp/Vs is notably below 1.70 in magma-poor segments, suggesting a more primitive mantle due to limited melting. There are a few potentially interesting anomalies in the magma-poor segment C below Barracuda Ridge (model distances 90 to 170 km), with dry mafics displaced within the mantle portion. However, as they are not central to the water estimation due to serpentinisation in the crustal column, we will not discuss them further in this paper.

Methods
Seismic data acquisition and preprocessing
The wide-angle seismic data were acquired by 86 4-component ocean bottom seismometers (OBS) spaced ~4.5 km apart, with 45 OBS deployed in two groups (hence line 2/3). Instruments were shot at 60 s (~150 to 190 m), and the northern set was additionally shot at 20 s (50 m) during a multi-channel seismic line acquisition. Eleven instruments in the centre of the line recorded both the northern and southern shots. Seismic data acquisition parameters are detailed in ref. 40. Each OBS record was band-pass filtered (zero-phase) with corner frequencies of 3, 5, 20 and 30 Hz. For the P-wave inversion, a similar strategy to ref. 46 was adopted, with the advance that shallow sedimentary and upper crustal velocities were improved using downward continued (DC) OBS records56. For the S-wave inversion, first, a hodogram analysis of the direct wave arrivals was employed to determine the OBS orientation of horizontal (X and Y) geophone components in relation to the shot line (Supplementary Fig. 1). These components were then rotated to give radial and transverse directions36, with the radial direction providing the highest signal-to-noise ratio for S-wave picking.

Identifying converted S- and P-wave phases
Multiple phases on OBS records were identified by preliminary forward modelling with the RayInvr software70 and TOMO2D codes71 (Supplementary Figs. 2, 3). For S-wave phase identification, an initial Vp/Vs ratio of 1.8 in the crust and upper mantle was assumed. Mid-crustal P-wave refractions outside the water wave cone are nearly subhorizontal, with an apparent velocity of ~8 km/s (Supplementary Fig. 3a, c), while converted S-wave refractions have an apparent velocity of ~4.5 km/s (Supplementary Fig. 3b, d). There is a family of S-wave arrivals, as conversion can happen at the sediment-basement interface on the way down, on the way up or both. The first group, termed Sg, Sn, and SmS (conversion on way down), are only observed on the geophone components (vertical, radial and transverse) but not on the hydrophone component (Supplementary Fig. 2). Sg was picked for offsets below 40 km, Sn from 40 to 160 km and SmS from 20 to 50 km (Supplementary Fig. 3c, d). The second group, PPS phases (PPgS and PPnS) travel as P-wave in the crust and uppermost mantle but convert to S-wave at the sediment-basement interface underneath OBS on the upward propagating leg. They have travel time-distance branches that are parallel to Pg/Pn with delayed arrival times in geophone components (Supplementary Fig. 2b). They were picked to estimate the delay time of the S-wave relative to the P-wave travelling in the sediments below each instrument. This delay was subtracted from Sg/Sn/SmS arrivals to produce 'effective PSP arrivals'36 i.e. as if they had travelled through the sediments at both source and receiver sides as P-waves but through the crust/mantle as S-waves in order to give symmetry for tomographic inversion using Vp in the sediments (Supplementary Fig. 4). The final group of S-waves, PSP phases, including PSgP, PSnP and PSmSP, convert on both the way down and up, so they only travel below basement as S-waves. These double-mode converted phases are only intermittently observed on the hydrophone and vertical components (Supplementary Fig. 2b) and were not used in this analysis.

S-wave data quality was categorised as good (45% of OBS records), moderate (38%) or poor (17%) based on signal-to-noise ratio and continuity of arrivals for travel time picking (Supplementary Fig. 5). Picking uncertainties for the S-wave arrivals were assigned based on their offset, with a starting uncertainty of 0.75, 1.0, and 1.25T for good, moderate and poor arrival quality respectively (Supplementary Table 1). T is 200 ms, which is the average period of the S-waves for all offset ranges.

For P-waves, crustal refractions (Pg2 and Pg3) in the upper and lower crust (Supplementary Fig. 3a, c) are seen up to 40 km. They change from low to high apparent velocity with variable amplitude. Pg2 turning within the top 2 km below the basement inside the water wave is shown and confidently pickable on DC OBS records56. Mantle refractions (Pn) show low amplitude with an apparent velocity above 7.5 km/s at offsets up to ~150 km. Moho reflections (PmP) are spatially limited to offsets of 20 to 40 km and observed on ~80% of OBS records. Picking uncertainties were assigned based on offset for Pg and Pn (Supplementary Table 1)46. For PmP phases, 50, 75, and 100 ms were respectively assigned based on arrival continuity with high-, mid- and low-level of picking confidence (Supplementary Fig. 6 and Supplementary Table 1). In comparison, SmS phases are present at similar offset ranges but with weaker amplitudes.

Travel time tomographic inversion of P- and converted S-waves
Vp was inverted using TOMO2D71 in a layer-stripping procedure72 by adding data sequentially: (1) first for the sedimentary model using Ps and basement reflections (PbP) picked from DC OBS records56, (2) next for the upper crustal model using Pg2 phases picked from DC OBS records while fixing the sedimentary model56, (3) next by combined inversion of full crustal model and the Moho depth using Pg and PmP phases from original (non-DC) 60 s records, and (4) finally for upper mantle model using Pn phases while leaving the crustal model damped to maintain minimum changes.

The final P-wave sedimentary model (Supplementary Fig. 7) was fixed in the subsequent steps, below which a starting model guided by 1D velocity-depth relations for slow-spreading mature crust from ref. 11 was defined with a top basement velocity of 4.5 km/s56 (Supplementary Fig. 8a). The grid spacing for the inversion was 4x=500m and 4z=50m constantly in the upper crust (~2 km below basement) and then increased linearly to 4z=500m at the base of the model. The smoothing correlation lengths increase from 3 to 5.2 km horizontally and 0.5 to 1.5 km vertically from 5 to 25 km depth. A depth weight kernel of 1 was used, giving an equal weighting between velocity and Moho depth perturbations. In each step, variable damping was applied, defined by ray coverage of the previous model step, to allow velocity perturbations favouring deeper model regions46.

The starting Vs model was derived from the final Vp model using a Vp/Vs ratio of 1.8 below the basement (Supplementary Fig. 8b). The same inversion strategy was applied for both P- and S-wave tomography. First, we applied downward continuation to the radial components (Supplementary Fig. 9). However, this process is less essential than for Vp modelling, as weak water waves on radial and transverse components do not affect S-wave picking at short offsets. SmS phases were picked from less than 30% of OBS records (Supplementary Fig. 6), so the Moho depth was constrained by PmP travel times and fixed in the Vs inversion. The Moho depth was then inverted as a final step using SmS for comparison with the P Moho. Final Vp and Vs models show χ2 close to 1 with reduced RMS misfits between the picked and modelled travel times (Supplementary Fig. 10).

S-wave velocity model inversion using different starting models
To assess the independence of our final S-wave velocity model from the selection of the starting model, we conducted a comparison test using a starting model without prior information about the final P-wave velocity. This starting model is purely from the conversion of the initial P-wave 1D velocity-depth profile into S-wave using a Vp/Vs of 1.8 (Supplementary Fig. 11) and the same initial Moho reflector as Vp modelling. The same inversion strategy and modelling parameters were applied.

The final inverted S-wave model from the simplified starting model (Supplementary Fig. 12b) shows high similarities in ridge-parallel (along-strike) Vs velocity variations with the preferred model (Supplementary Fig. 12a), except for localised differences at topographic highs (at model distances 80 to 120, 240, and 265 km). These differences are possibly due to relatively lower ray coverage of S-waves beneath these basement highs (Supplementary Fig. 14) caused by poor P-to-S conversion at these locations. Without prior Vp-information, the low-velocity anomalies beneath these basement highs are unable to be recovered. The Vp/Vs model (Supplementary Fig. 13) also shows similar patterns despite some localised differences. This indicates that the choice of starting models does not significantly impact the modelling outcome, confirming the reliability of our interpretation based on the preferred Vs model.

Quality analysis and checks for final P- and S-wave models
Following ref. 46, we conducted quality analysis for the final inverted velocity models. In general, rays of P-waves are about double that of S-waves. The derivative weight sum (DWS) shows overall good ray coverage (>10 rays per cell) in the crust and upper mantle except for a lack of ray coverage underneath Barracuda Ridge for the S-wave model (Supplementary Fig. 14). Checkerboard tests performed with a range of velocity anomaly sizes at an amplitude of ±5% and with a gaussian noise level (standard deviation of 10, 50, and 100 ms) scaled by the pick uncertainty for P- and S-waves, reveal a resolution for Vp of 5×1 km to 5×1.5 in the upper crust, 5×2.5 km in the lower crust and 10×5 km in the upper mantle, and for Vs of 5×2.5 to 10×5 km in the crust and 15×5 km in the upper mantle (Supplementary Fig. 15).

In the Monte-Carlo analysis, we randomised the velocities at the top and bottom of the upper, lower crust and top mantle by ±5% of the starting model (Supplementary Fig. 8) with the thickness of the total crust randomised by ±1 km for P-wave inversions. The final Moho reflector inverted from PmP was used and remained unchanged in the S-wave starting models.

Velocity uncertainty of P-wave tomography (Supplementary Fig. 16a) is <0.02 km/s in the sediments, and up to ±0.05 km/s but mostly <0.03 km/s in the upper crust. This uncertainty results in an 80 to 90% reduction in the mean deviation of the upper crust, i.e. from ~±0.2 to ±0.3 km/s (assuming upper crustal velocity of 4.5 to 6.5 km/s with ±5% deviation) in the starting models to ~0.03 km/s deviation in the final models. In the lower crustal depth (between 2 to 5 km below the top basement), Vp uncertainty is ~± 0.07 km/s with peaks at ±0.17 km/s (mean deviation reduced by 30%). This suggests the final model’s resolution is minimally impacted by starting models, resolving velocity within the estimated uncertainty bounds73. The depth uncertainty for the P Moho reflector resolved from PmP phases is ±0.5 km, corresponding to a region of the highest velocity uncertainty up to ±0.25 km/s. In the mantle, Vp uncertainty ranges from ±0.07 to ±0.13 km/s in the south, reducing to ~0.07 or <0.05 km/s at ~17 km depth below the basement under Tiburon Basin, Barracuda Ridge and Barracuda Trough in the north.

The S-wave model shows uncertainties generally below 0.05 km/s (Supplementary Fig. 16b). As a fixed Moho reflector was used for each inversion, in contrast, the uncertainty across the crust-mantle boundary in the S-wave model is comparably lower and reaches a maximum of ±0.1 km/s. The Vp/Vs ratio uncertainties were calculated from the standard deviation by combining 100 different Vp and Vs models randomly (Supplementary Fig. 16c). The uncertainty is mostly <0.05 km/s in the crust with a slightly higher uncertainty (up to 0.07 km/s) below Barracuda Ridge and up to 0.1 km/s within the crust-mantle boundary zone. However, it must be noted that, even in the worst resolved areas, such as the crust-mantle boundary, the control on velocity and velocity gradients is good enough and does not affect the interpretation.

K-means cluster analysis for grouping velocity-depth characters
Davy et al.46 identified two distinct types of P-wave velocity-depth (VZ) profiles south of 15.5°N, which they attributed to magma-robust and magma-poor seafloor spreading modes. Their analysis was done by manual visualisation. This resulted in the categorisation of segments (Segments D to H) into one of the two modes, with the recognition that VZ profiles at segment ends were slightly modified from their centres. Velocity inversions were also noted but not formally categorised. We instead performed a K-Means cluster analysis74,75 of the P- and S-wave VZ profiles, a more robust statistical method, to identify regions with similar velocity structures. To our knowledge, this type of analysis has not been previously applied to classify velocity-depth profiles in the oceanic crust.

To determine the optimal number of clusters (K), we used the silhouette value, which measures how similar a VZ profile is to its own cluster’s centroid compared to others76,77. The optimal K maximises the averaged silhouette value from 20 iterations of K-means algorithms by randomising the initialisation of the cluster centroids each time. To better partition the VZ profiles, we constructed four categories of features for Vp and Vs input to K-means: (1) VZ profiles, (2) velocity gradient, (3) minimum value of the second derivative of depth to velocity (calculated every 50 m in depth), and (4) maximum value of the standard deviation of velocity gradient calculated every 500 m in depth (Supplementary Figs. 17, 18). The last two categories are specially designed for identifying rapid velocity changes with depth. The four categories were then concatenated as one observation at locations every 1 km along the seismic line. Tests showed that the velocity gradient is the key for grouping velocity structures.

We found four clusters (K=4) that best describe Vp and Vs structures with exclusive characters. With two clusters (K=2), velocity inversions are unidentifiable and mixed with two-layer velocity-gradient seismic structures (Supplementary Fig. 19). Both K=3 and K=4 can detect velocity inversions (cyan) but K=4 is preferred as it also distinguishes regions with the lowest velocity gradients (dark blue) (Supplementary Figs. 20, 21), possibly related to pervasive hydration. Both K=5 and K=6 include redundant clusters, which are not central to the interpretation (Supplementary Figs. 22, 23). All the interpretations are based on the preferred four-clusters solution (Supplementary Fig. 21).

Gravity modelling
Our Vp model was checked for consistency against the coincident satellite-derived free-air gravity anomaly (FAA, V29.1)47. The uncertainties of 2D gravity modelling due to the effect of 3D topography were first assessed (Supplementary Fig. 27). The likely error in the observed gravity is ~± 3 mGal. Therefore, the fit to the gravity is deemed acceptable if it matches this range.

To estimate the gravity, a 2D model with homogeneous density blocks was derived from Vp. The block shapes were based on physical boundaries (seabed, basement and Moho) and velocity contours representing lateral and vertical variations. Velocity contours of 2, 2.4 and 2.6 km/s at MFZ and 2.8 km/s at Tiburon Basin were extracted for sediments, and 4.5 to 6.5 km/s for the oceanic crust (Supplementary Fig. 28a). Density was converted from Vp using empirical velocity-density relationships for sediments78 and oceanic crust79. For the mantle, we tested densities from 2.8 to 3.3 g/cm3. The predicted FAA was subsequently computed using a 2D line integral algorithm of ref. 80. The best fitting mantle density is 3 g/cm3, which was used for further analysis.

To evaluate the effect of the high Vp/Vs anomalies on the gravity modelling, we introduced three density bodies defined by Vp/Vs contours of 1.9. We then tested by forward modelling values between −0.1 and −0.2 g/cm3 to account for the drop in density that might be expected if these bodies contained significant quantities of serpentinite as opposed to water-filled fractured rock50,73. Given the issue of real 3D geological structures captured on a 2D profile, the aim was not to determine the best-fit density of these bodies but rather to demonstrate whether the gravity was consistent with the presence of serpentinite or not. We found that a negative density of −0.15 g/cm3 relative to the background density in the crust gave the best overall fit (Fig. 4b–d).

Relationships between lithology, serpentinisation and Vp/Vs
Several studies28,30,81,82 have previously proposed relationships between Vp/Vs and lithology. We re-examined the relationships in the context of our seismic model results and also a recent compilation of laboratory measurements of Vp and Vs of samples from deep ocean drilling83. The laboratory measurements used here to establish the lithology, serpentinisation and Vp/Vs relationship were conducted under pressure at 50 to 200 MPa28,81–83, simulating depths of 2 to 8 km—conditions at the top basement and near the Moho. We broadly followed the classification scheme shown in ref. 30 including adopting a Vp/Vs ratio of 1.9 as the main division between mafic and dry peridotite (below 1.9) and altered, serpentinised peridotite (above 1.9) at crustal pressures but with the following modifications (Fig. 6a and Supplementary Fig. 29). To be more consistent with our seismic results we placed the boundary between basalt/dolerite and gabbro at Vp=6.5 km/s (as opposed to Vp=6.6 km/s). We placed the boundary between gabbro and dry mantle at Vp=7.5 km/s (as opposed to Vp=7.15 km/s) to align with our seismically defined Moho interface. To estimate the excess bound water content due to serpentinisation, we focus on velocity points with Vp/Vs >1.9 only.

Relationship between fractures and Vp/Vs
To investigate the effect of fractures on Vp and Vp/Vs, we include a differential effective medium (DEM) analysis following the methods outlined in previous studies84,85. We used the intrinsic properties of dry dunite (Vp=8.299 km/s, Vs=4.731 km/s, density=3.310 g/cm3) and dry gabbro (Vp=7.138 km/s, Vs=3.862 km/s, density=2.968 g/cm3, measured under 200 MPa, 200 °C from ref. 81) for the host material in the DEM calculations. Note that these host materials were selected based on the lithology of drilled cores of known OCCs53,54. We added increasing proportions of randomly oriented water-filled fractures with aspect ratio of 0.0001 (thin) to 0.05 (thick) to calculate the effective Vp, Vs and Vp/Vs with increasing of porosity (Supplementary Fig. 30). We found that the range of aspect ratios can cover all our observed velocity points (grey small circles in the background in Supplementary Fig. 31b) when a dunite matrix is used, but even with much thinner cracks (aspect ratio = 0.00001), the fracture model with a gabbro matrix cannot fully cover the velocity points with Vp/Vs >1.9 and Vp of ~7 km/s (Supplementary Fig. 31c).

Water content calculations for anomalous crust
We setup and evaluated three kinds of possible hydration models to estimate the water content of the anomalous crust defined by Vp/Vs >1.9. In Model 1, termed as the serpentinisation model, we interpreted velocity points with Vp/Vs >1.9 as serpentinised peridotites. We used the relations linking Vp and Vs to serpentine percentage from ref. 82 and further to bound water content (large white triangles in Fig. 6a and Supplementary Fig. 29) by assuming 13 wt% water content for fully serpentinised peridotites and 0 wt% for non-serpentinised, unaltered peridotites following ref. 50 (Supplementary Table 2). This converts Vp/Vs to the excess bound water content model in Fig. 7b (also Supplementary Fig. 32a). Note that mafic rocks with Vp/Vs <1.9 are considered to be standard composition used in a Penrose hydration model (Supplementary Fig. 33) with 0 wt% excess bound water assigned and not included in our excess bound water model.

In Model 2, termed as the fracture water model, we interpreted velocity points with Vp/Vs >1.9 as either water-filled fractured peridotite (Model 2a; Supplementary Fig. 32b) or water-filled fractured gabbro (Model 2b; Supplementary Fig. 32c). The identical porosity on each Vp/Vs versus Vp trend defined by different aspect ratios is connected and used to divide regions (polygons) of different fracture-water weight content (Fig. 6b, Supplementary Fig. 31b, c and Supplementary Table 3). Velocity points falling into the polygons defined by neighbouring porosity lines are assigned the averaged fracture-water weight content (Supplementary Table 3), converting Vp/Vs to the fracture-water models. The result is shown in Supplementary Fig. 32b, c.

In Model 3, termed the hybrid model, both serpentinisation and fracturing are considered. Velocity points with Vp/Vs >1.9 that lie on the serpentine hydration trend are interpreted as serpentinised peridotite, and those that lie off this trend as fractured peridotite. Velocity points are assigned either excess bound water or fracture water based on rules described in Model 1 and Model 2a (Supplementary Figs. 31d, 32d). Note that we choose only fractured peridotite as an example to set up the hybrid model as the fracture-water template of a peridotite matrix can cover all velocity points with Vp/Vs >1.9 and also the overall fracture water content estimated based on different rock matrices is similar (Supplementary Fig. 32b, c) which therefore has little effect on the hydration estimation in a hybrid mode.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Li and coauthors present new constraints on the hydration state of the Atlantic lithosphere outboard of the Lesser Antilles trench from P-wave, S-wave and Vp/Vs constraints. As the authors write, many constraints on hydration of oceanic crust and upper mantle come from P-wave velocity models, but other factors besides serpentinization can influence Vp, rendering those estimates uncertain. Estimating hydration is especially challenging in slow-spreading crust, which has significant heterogeneity. This paper presents relatively high-resolution models of Vp, Vs and (from those) Vp/Vs that provide much better constraints on both the distribution and extent of hydration in the lithosphere. The profile is relatively long and crosses both relatively magma-rich and magma-poor segments. To my knowledge, this is now the best model we have of Atlantic crustal/upper mantle structure and hydration, and it has important implications for controls on hydration in slow-spreading oceanic lithosphere and the subduction zone water cycle. The analysis, interpretation, writing and illustration are all excellent. I strongly recommend publication and only have minor comments, questions and suggestions described below. My lengthy review reflects my interest in this work.\n-The interpretation of variations in velocity structure in terms of magma-poor and magma rich segments appears convincing, but it would be even stronger if there was other evidence or rationale to support it. Are there any known changes in spreading rate between segments or other constraints elsewhere in these segments that support the classification presented here?\n-The authors interpret variations in upper mantle velocity in terms of composition (depletion/enrichment). What impact would upper mantle anisotropy have on upper mantle Vp, Vs and Vp/Vs? This profile is orthogonal to the spreading direction, and previous studies elsewhere in the Atlantic observe some degree of azimuthal anisotropy in the upper mantle (albeit weaker than in fast spreading Pacific lithosphere) (e.g., Gaherty et al., EPSL, 2004)\n-I think it would be good to expand the discussion of the influence of the Barracuda Ridge on velocities near OCC-3 and OCC-4. Studies elsewhere have shown that there can be significant water stored in the upper crust of volcanic ridges and seamounts (e.g., Chesley et al., Nature, 2021) – could the high Vp/Vs and low Vs and Vp around OCC-4 be explained by a porous, water rich upper volcanic crust of Barracuda Ridge? Also, is the crust in this area thicker than it would be otherwise because of proximity to Barracuda Ridge?\n-Lines 335-340 and abstract. Enhanced hydration around fracture zones is not just assumed or a common view – it has been demonstrated with observations in several regions (e.g., Roland et al., JGR, 2012). I think it would be more interesting to frame the discussion of this topic to explore why hydration does not appear concentrated along fractures zones in this area in contrast to other areas (instead of saying that these results suggest hydration along fracture zones is not as important as previously thought). Perhaps the contrast in hydration between fracture zones and adjacent segments is more pronounced in fast-spreading lithosphere (where many studies of FZ hydration have been done) than slow spreading lithosphere? In the abstract (Line 31), I do not think that previous studies assume hydration is limited to FZ in slow spreading crust, where OCC and thin, hydrated crust/upper mantle have long been observed, so I suggest modifying that.\n-One of the most interesting observations of the model is the “patchiness” of crustal hydration. In addition to contributing to the total water delivered into the Lesser Antilles subduction zone, i think that the irregular distribution could also be important for many processes at depth in the subduction zone. For example, numerical models that include either uniform hydration of variable hydration (but same total water) show different dehydration patterns that could be important for deformation and magmatism (e.g., Wada et al., EPSL, 2012). I think it would be interesting to explore these implications in the Discussion.\nOther minor comments/questions:\n-The title only mentions S-waves, but it is the combination of high-quality Vp and Vs models (and Vp/Vs) that allows the authors to better quantify crustal structure and hydration. Consider changing the title to “….oceanic crust using Vp/Vs”?\n-Lines 70-72. A reference should be provided for this statement.\n-Line 119. This sentence is confusing, contradictory as written. “a single velocity-gradient structure with velocity inversions”. If there is a velocity inversion, then one could not describe it as having a single velocity gradient?\n-Lines 211-213. Is a difference in misfit of only 2 mGal significant? This seems like a very small improvement. Additionally, one of these sentences is confusing as written. “…an overall misfit of 8 mGal, but locally the misfit exceeds ±3 mGal”.\n-Line 252. I was surprised to read that the upper 2 km of the crust have “negligible hydration”. This seems inconsistent with many other studies showing extensive hydration of the upper oceanic crust from hydrothermal circulation and off-axis fluid flow until hydration products and/or sediments prevent it, including from a recent drilling and geophysical transect in the southern Atlantic (e.g., Kardell et al., JGR, 2019).\n-Line 282. Suggest editing to “This results in the highest…”\n-Line 545. I thought the use of the K-means cluster analysis for grouping areas with different velocity characteristics was novel. Has this type of analysis been used for classifying velocities? If so, it would be good to cite those studies. If not, I think the Methods (or main paper) could emphasize the novelty.\n-Suppl Fig 3 – It would be useful is this figure could be made larger and higher resolution so that it is easier to see the data and predicted/observed travel times. I cannot see some of the symbols in the key in the panels.\n-Several of the supplemental figures show a basement interface based on MCS data. Were MCS data used to constrain the basement interface, or did it only come from DC OBS data? If MCS data were used to constrain the models, it would be good to include a plot of the MCS data in the supplement and mention how the MCS data were used in the Methods.\nThank you for the opportunity to review this interesting paper.\nDonna Shillington", "reviewer_2": "Li et al. present a detailed study of oceanic crust created in a slow-spreading environment. The main importance of this work is the measurement of both P and S velocities (Vp, Vs) which are critical to interpreting lithology. Estimation of Vs from active-source experiments is uncommon because it requires identifying P-to-S converted phases (water column source cannot produce a direct S-wave). Using their 2D velocity model, they investigate controls on the hydration state of the crust. They find that serpentinised oceanic core complexes may contribute significantly to subduction zone water budgets. I find the study well-presented and it would be of interest to the Nature Communications audience. However, I have a few concerns regarding the lithologic interpretations and model resolution that require moderate revision before publication.\nGeneral Comments:\n1. Hydration Estimates\nA major result of this work is that oceanic core complexes (OCCs) are highly serpentinized and, thus, contribute significantly to subduction zone water budgets. However, the water content calculations within these zones is likely an over-estimate because the effect of fluid-filled fractures on P and S velocities is not considered.\n1a. While the total volume occupied by fluid-filled cracks is small, they can significantly reduce seismic velocities and alter Vp/Vs ratios (e.g., Kuster & Toksöz; 1974). Appealing to fractures, one could achieve similar velocity reductions with much less water storage in OCCs. Considering that the inferred most hydrated zones are relatively shallow (2-3 km depth; Fig. 5b) and occupy deformed regions of seafloor, this mechanism is likely relevant but not thoroughly considered in the present study. I would like to see a second lower-bound estimate of hydration by assuming all water is stored within fractures.\n1b. Mechanisms for anomalous velocities beyond serpentinisation are also suggested by Fig. 5a. This plot serves as the basis for the lithologic/hydration interpretations. However, the Vp and Vp/Vs values for the inferred hydrated ultramafic model points have a very different trend from that predicted by Carlson and Miller or Horen et al. (bias toward high Vp/Vs at lower Vp relative to the alteration curves). In contrast, the drilled ultramafic samples (magenta points), despite some scatter, do follow the plotted serpentinisation trend. This point warrants some discussion in the text because the velocity trends appear to argue for a different ultramafic hydration mechanism than the one invoked by the authors. It may be informative to compare Vp, Vp/Vs trends resulting from pore/fracture water in Fig. 5 (or elsewhere).\n2. Model Resolution\n2a. I think an important point regarding the resolution is how picking uncertainty effects the recovery of Vp/Vs ratios. The picking error for Sg (~177 ms) is ~5x larger than for Pg (~32 ms). Therefore, I would expect the amplitude recovery of Vs anomalies to be poorer than Vp anomalies; the resulting Vp/Vs ratios would then be biased. Do the synthetic data used in the resolution tests (Supplementary Fig. 15) include this data quality discrepancy (e.g., adding gaussian noise with different standard deviations for P and S travel-times)?\n2b. Considering that the interpretation of Vp/Vs ratios is a key aspect of this work, I would like to see resolution tests specifically targeting Vp/Vs (and include differences in data quality mentioned above). It is difficult to understand from Supplementary Fig. 15 alone how well the target model Vp/Vs ratios are imaged. It also seems that the inversion over- estimates anomaly amplitudes considering many recovered perturbations saturate the color scale.\nSpecific Comments:\nLine 25-27: I suggest revising lines 25-27 to emphasise the lack of shear velocity constraints on oceanic crustal structure as this is the real novelty of the study.\nLine 24: Here, and several other places, the term 'slow-spread crust' is used but I believe 'slow-spreading crust' is the more common phrase. Consider revising.\nLine 52: Can you provide a reference for Penrose stratigraphy?\nLine 245-246: Could you better explain why these pink samples in particular are difficult to assign a composition?\nSpecifically, it is not clear why the boundaries of the pink samples doesn't extend to lower Vp or above the Vp/Vs = 1.9 line (presumably samples near this boundary, above or below, are difficult to identify).\nLine 282-283: Considering these are relatively shallow anomalies in a rather deformed setting, to what extent could these velocity reductions and Vp/Vs anomalies be due to fracturing as opposed to serpentinisation?\nLine 433: Can you clarify here that PSP arrivals are waves that travel through the sediments at source and receiver sides as P but through the crust/mantle as S?\nLine 522; 536: Please clarify that these velocity uncertainty estimates are due to sensitivity to the starting model rather than the true error in the recovered parameter values.\nLine 593-594: Please clarify how you arrived at a density anomaly of -0.15 g/cc for the gravity modelling.\nLine 602-604: As written, it sounds like you've established your own lithologic boundaries based on body wave velocities from field/laboratory data (Fig. S29). However, you do not provide sufficient details on how these boundaries were determined. While you cite Grevemeyer et al. in regards to this, you don't explain how or why you've modified the lithologic boundaries defined in the Grevemeyer study. Please clarify.\nSupplementary Figure S29: It is a bit unclear from the caption or text what exactly the narrow light blue and yellow linear bands are showing. I'm guessing that the light-blue band is the peridotite-serpentinite trend along which the degree of serpentinisation is increasing. However, the fraction of serpentinite (or water content) along this curve is not indicated. Presumably the light-yellow band is the gabbro-dolerite trend along which the fraction of dolerite increases? Can the fraction of dolerite be labeled on this curve? Please clarify this in the cation/figure."}, "author_response": "Response to Reviewers’ comments on Nature Communications\nmanuscript NCOMMS-24-69268\n“Estimating excess bound water content due to serpentinisation in mature\nslow-spread oceanic crust using S-waves” by Li et al.\nWe thank the reviewers for their insightful comments which we believe has significantly\nimproved the clarity and robustness of the manuscript.\nBelow reviewers’ comments are in blue italics and our responses in black. During the revision\nwe also followed the request of the Editor and made some edits to meet the Nature\nCommunications format requirements. As a result, a section about the oceanic core complexes\nthat was in the Discussion has moved to the Results. Below we refer to line/page/paragraph\nnumbers in the revised version. Please note that the separate PDF version of Supplementary\nFigures has higher resolution compared to the PNG file inserted in the merged supplementary\nfile.\nReviewer 1: Donna Shillington\nGeneral Comments:\nLi and coauthors present new constraints on the hydration state of the Atlantic lithosphere\noutboard of the Lesser Antilles trench from P-wave, S-wave and Vp/Vs constraints. As the\nauthors write, many constraints on hydration of oceanic crust and upper mantle come from P-\nwave velocity models, but other factors besides serpentinization can influence Vp, rendering\nthose estimates uncertain. Estimating hydration is especially challenging in slow-spreading\ncrust, which has significant heterogeneity. This paper presents relatively high-resolution models\nof Vp, Vs and (from those) Vp/Vs that provide much better constraints on both the distribution\nand extent of hydration in the lithosphere. The profile is relatively long and crosses both\nrelatively magma-rich and magma-poor segments. To my knowledge, this is now the best model\nwe have of Atlantic crustal/upper mantle structure and hydration, and it has important\nimplications for controls on hydration in slow-spreading oceanic lithosphere and the subduction\nzone water cycle. The analysis, interpretation, writing and illustration are all excellent. I strongly\nrecommend publication and only have minor comments, questions and suggestions described\nbelow. My lengthy review reflects my interest in this work.\nIn making our revision we have borne in mind this very positive review. We believe that the edits\nmade during the revision have further improved the manuscript.\n1.1 Spreading rate estimates\nThe interpretation of variations in velocity structure in terms of magma-poor and magma rich\nsegments appears convincing, but it would be even stronger if there was other evidence or\nrationale to support it. Are there any known changes in spreading rate between segments or\nother constraints elsewhere in these segments that support the classification presented here?\nUnfortunately, at these slow spreading rates, the magnetic anomalies are not separated enough\nto date the seafloor at the resolution that would be needed to recognise local spreading rate\nvariations. There is of course also an inherent asymmetry to magma-poor spreading which\nmeans that half spreading rate may be a poor proxy for magma budget in any case. Numerical\nmodelling by Gerya’s group (Liu et al., 2025; Liu et al., 2022) shows that magma-robust and\n1\nmagma-poor segments emerge self-consistently in a setting of slow-ultraslow spreading, even\nif rates are constant along the ridge. In this case, variations in spreading rate, fault weakening\n(e.g. by variations in fluid infiltration) and mantle temperature in time can explain shifting\npatterns of magma-robust and magma-poor in time (i.e. in ridge-perpendicular direction). We\nhave added this point to the discussion (line 371).\n1.2 Effect of mantle anisotropy\nThe authors interpret variations in upper mantle velocity in terms of composition\n(depletion/enrichment). What impact would upper mantle anisotropy have on upper mantle Vp,\nVs and Vp/Vs? This profile is orthogonal to the spreading direction, and previous studies\nelsewhere in the Atlantic observe some degree of azimuthal anisotropy in the upper mantle\n(albeit weaker than in fast spreading Pacific lithosphere) (e.g., Gaherty et al., EPSL, 2004)\nWe omitted to say that we find very similar mantle velocities on our Line 1 profile (Fig. 1a; Allen\net al., 2022) which is orthogonal to Line 2/3 presented here. We therefore did not consider\nanisotropy a major factor which is why we speculated that depletion/enrichment could be\nunderlying cause of our observations. We have corrected this omission (line 331).\n1.3 Influence of the Barracuda Ridge on velocity structure\nI think it would be good to expand the discussion of the influence of the Barracuda Ridge on\nvelocities near OCC-3 and OCC-4. Studies elsewhere have shown that there can be significant\nwater stored in the upper crust of volcanic ridges and seamounts (e.g., Chesley et al., Nature,\n2021) – could the high Vp/Vs and low Vs and Vp around OCC-4 be explained by a porous, water\nrich upper volcanic crust of Barracuda Ridge? Also, is the crust in this area thicker than it would\nbe otherwise because of proximity to Barracuda Ridge?\nThis comment prompted us to recognise that we should have provided a clearer introduction to\nthe Barracuda Ridge. Whilst is may have originally formed as a transverse ridge, in a similar way\nto those in the nearby Vema, St Paul and Romanche fracture zones (Bonatti, 1978; Bonatti et al.,\n2005), it appears to have been uplifted by later tectonic compression across the North\nAmerican-South American plate boundaries (Bouysse and Westercamp, 1990; Pichot et al.,\n2012; Roest and Collette, 1986; Stein et al., 1982). In this way it is more similar to Gorringe Bank\nin the eastern Atlantic at the African-Eurasian plate boundary. We have added a sentence in the\nIntroduction section and (line 91-95) to explain this. The effect of fracturing on seismic velocity\nis now also included in response to comments from Rev 2 (see below).\n1.4 Fracture zone hydration concepts\nLines 335-340 and abstract. Enhanced hydration around fracture zones is not just assumed or a\ncommon view – it has been demonstrated with observations in several regions (e.g., Roland et\nal., JGR, 2012). I think it would be more interesting to frame the discussion of this topic to\nexplore why hydration does not appear concentrated along fractures zones in this area in\ncontrast to other areas (instead of saying that these results suggest hydration along fracture\nzones is not as important as previously thought). Perhaps the contrast in hydration between\nfracture zones and adjacent segments is more pronounced in fast-spreading lithosphere (where\nmany studies of FZ hydration have been done) than slow spreading lithosphere? In the abstract\n(Line 31), I do not think that previous studies assume hydration is limited to FZ in slow spreading\ncrust, where OCC and thin, hydrated crust/upper mantle have long been observed, so I suggest\nmodifying that.\nmaking the revisions. In particular, we have rewritten the second paragraph of the discussion\nstarting with “Many previous studies have demonstrated that fracture zone…(line 360). We also\nremoved the phrase “is often assumed” and “as traditionally thought” to present the results in a\nmore neutral and open manner.\n1.5 Expand discussion on the effect of “patchiness” of crustal hydration\nOne of the most interesting observations of the model is the “patchiness” of crustal hydration. In\naddition to contributing to the total water delivered into the Lesser Antilles subduction zone, i\nthink that the irregular distribution could also be important for many processes at depth in the\nsubduction zone. For example, numerical models that include either uniform hydration of\nvariable hydration (but same total water) show different dehydration patterns that could be\nimportant for deformation and magmatism (e.g., Wada et al., EPSL, 2012). I think it would be\ninteresting to explore these implications in the Discussion.\nYes, this is a good point. We have reorganised the final paragraph of the Discussion to give more\nprominence to the “patchiness” result and its broader implications.\nMinor Comments:\n• The title only mentions S-waves, but it is the combination of high-quality Vp and Vs models\n(and Vp/Vs) that allows the authors to better quantify crustal structure and hydration.\nConsider changing the title to “….oceanic crust using Vp/Vs”?\nGood point. We have changed the title to “using Vp/Vs” (and so still remain within the required\nnumber of words for the title).\n• Lines 70-72. A reference should be provided for this statement.\nDone (Merdith et al., 2019). As a result of this change, we also replaced the first reference with a\nmore recent overview paper on the question of mantle water budgets over geological time\n(Rüpke and Gaillard, 2024)\n• Line 119. This sentence is confusing, contradictory as written. “a single velocity-gradient\nstructure with velocity inversions”. If there is a velocity inversion, then one could not\ndescribe it as having a single velocity gradient?\nGood point. We have rewritten this sentence as “a single velocity-gradient structure with\noccasional velocity inversions identified as OCCs superimposed” (line 112).\n• Lines 211-213. Is a difference in misfit of only 2 mGal significant? This seems like a very\nsmall improvement. Additionally, one of these sentences is confusing as written. “…an\noverall misfit of 8 mGal, but locally the misfit exceeds ±3 mGal”.\nGood point. While 2 mGal indeed does not seem very significant, this value is averaged across\nthe entire line and so includes the sections between OCCs that remain unchanged in the two\nmodels. These unchanged sections account for approximately 80% of the line length (the parts\nshown with red and blue striped line in Fig 4d). We intended to express that “locally the misfit\nexceeds the assigned uncertainty, i.e., ±3 mGal (the parts indicated with yellow arrows in the\n3\nfigure).” After adding negative density anomalies, the areas with the largest misfit are\nsignificantly reduced. We have revised both the main text and the figure caption accordingly.\n• Line 252. I was surprised to read that the upper 2 km of the crust have “negligible hydration”.\nThis seems inconsistent with many other studies showing extensive hydration of the upper\noceanic crust from hydrothermal circulation and off-axis fluid flow until hydration products\nand/or sediments prevent it, including from a recent drilling and geophysical transect in the\nsouthern Atlantic (e.g., Kardell et al., JGR, 2019).\nWe thank the reviewer for this comment. Line 252 in the original manuscript actually used the\nphrase “negligible excess hydration” which was intended to mean little or no water content\nabove that expected for mature Penrose-style oceanic crust. This reviewer’s comment suggests\nthat our use of “excess” did not come across clearly enough and this is an important point for\nthe overall understanding of the manuscript. We have added the sentence “In other words,\naccording to our classification, the hydration levels of these segments do not surpass what\nwould be expected for Penrose-style oceanic crust of this age” to make our meaning clearer. We\nmade minor revisions elsewhere in the manuscript to reinforce our intended use of the term\n“excess hydration” (line 324).\n• Line 282. Suggest editing to “This results in the highest…”\nDone (line 244).\n• Line 545. I thought the use of the K-means cluster analysis for grouping areas with different\nvelocity characteristics was novel. Has this type of analysis been used for classifying\nvelocities? If so, it would be good to cite those studies. If not, I think the Methods (or main\npaper) could emphasize the novelty.\nTo our knowledge this has not been conducted before. We have added a sentence to the\nmethods section stating this as suggested (line 574).\n• Suppl Fig 3 – It would be useful is this figure could be made larger and higher resolution so\nthat it is easier to see the data and predicted/observed travel times. I cannot see some of the\nsymbols in the key in the panels.\nDone. We also did a general review of all font sizes etc. Please note that the separate PDF\nversion of Supplementary Fig.3 has higher resolution compared to the PNG file inserted in the\nmerged supplementary file.\n• Several of the supplemental figures show a basement interface based on MCS data. Were\nMCS data used to constrain the basement interface, or did it only come from DC OBS data?\nIf MCS data were used to constrain the models, it would be good to include a plot of the MCS\ndata in the supplement and mention how the MCS data were used in the Methods.\nDone. We have added the co-incident, pre-stack time migrated MCS seismic profile as the top\npanel in Supplementary Fig. 7 and revised the figure caption. The initial top basement horizon\nincluded in the P-wave sedimentary starting model (Supplementary Fig. 7b) was picked from\nthis MCS profile and then depth converted using the initial sediment model (Supplementary Fig.\n4\n7b). During the inversion of the sedimentary Vp model, the top basement was further\nconstrained using PbP phases picked from DC OBS data. The comparison between the initial\nand final inverted top basement is shown in Supplementary Fig. 7d (black dashed line and red\nline, respectively). More details about inverting the sedimentary Vp model and top basement\nusing DC OBS data can be found in Li et al. (2024).\nReviewer 2: Anonymous\nGeneral Comments:\nLi et al. present a detailed study of oceanic crust created in a slow-spreading environment. The\nmain importance of this work is the measurement of both P and S velocities (Vp, Vs) which are\ncritical to interpreting lithology. Estimation of Vs from active-source experiments is uncommon\nbecause it requires identifying P-to-S converted phases (water column source cannot produce a\ndirect S-wave). Using their 2D velocity model, they investigate controls on the hydration state of\nthe crust. They find that serpentinised oceanic core complexes may contribute significantly to\nsubduction zone water budgets. I find the study well-presented and it would be of interest to the\nNature Communications audience. However, I have a few concerns regarding the lithologic\ninterpretations and model resolution that require moderate revision before publication.\nWe are grateful for the reviewer’s acknowledgement of this work’s novelty and substantive\ncontributions. The commentary on the analysis fluid-filled fractures and model resolution has\nhelped us to strengthen our interpretation in a rather complete manner.\n2.1. Hydration estimates\nA major result of this work is that oceanic core complexes (OCCs) are highly serpentinized and,\nthus, contribute significantly to subduction zone water budgets. However, the water content\ncalculations within these zones is likely an over-estimate because the effect of fluid-filled\nfractures on P and S velocities is not considered. While the total volume occupied by fluid-filled\ncracks is small, they can significantly reduce seismic velocities and alter Vp/Vs ratios (e.g.,\nKuster & Toksöz; 1974). Appealing to fractures, one could achieve similar velocity reductions\nwith much less water storage in OCCs.\nWe thank the reviewer for this comment and acknowledge that we should have addressed the\neffect of fracturing on Vp/Vs ratios in the original manuscript. We have now incorporated a\ndiscussion of this effect as outlined below. We recognise that by adopting the serpentinite\nendmember as our preferred interpretation, our excess water content estimate is indeed an\nupper bound. We have taken this point on-board and revised the manuscript accordingly.\nWe now include Differential Effective Medium (DEM) calculations for fractured media following\nthe methods outlined in (Mukerji et al., 1995; Norris, 1985; Taylor and Singh, 2002). We show an\nexample of the predictions in Figure 6b, and additional documentation in Supplementary Table\n3 and Supplementary Figs. 30—32. In the main text, we modified accordingly the subsection of\nthe Results section: “Lithology, serpentinisation, water-filled fractures and physical properties”,\n(line 268). We added a new Methods section: “Relationship between fractures and Vp/Vs”\n(starting line 638) and included alternate models in the section titled “Water content\ncalculations for anomalous crust” (starting line 652).\n2.1a Mechanisms for giving anomalous velocity\nThis plot serves as the basis for the lithologic/hydration interpretations. However, the Vp and\nVp/Vs values for the inferred hydrated ultramafic model points have a very different trend from\nthat predicted by Carlson and Miller or Horen et al. (bias toward high Vp/Vs at lower Vp relative\nto the alteration curves). In contrast, the drilled ultramafic samples (magenta points), despite\nsome scatter, do follow the plotted serpentinisation trend. This point warrants some discussion\nin the text because the velocity trends appear to argue for a different ultramafic hydration\nmechanism than the one invoked by the authors. It may be informative to compare Vp, Vp/Vs\ntrends resulting from pore/fracture water in Fig. 5 (or elsewhere).\nWe test dry gabbro and peridotite as the rock matrix materials (to match the dominant rock\ntypes of OCCs from IODP drilling (Blackman et al., 2019; Canales et al., 2008)) and water-filled\ncracks with various aspect ratios. As the reviewer correctly states, fractures can explain the\nspread of Vp/Vs >1.9 values we obtain from our seismic model results about the predicted\nultramafic-serpentinisation trend. Specifically, very thin cracks (with aspect ratios 0.0001) using\na peridotite rock matrix are required to explain our seismic model data samples with Vp above\nthe serpentinite trend, and thicker cracks (with aspect ratios between 0.005 and 0.013) are\nneeded to match data samples with Vp below the serpentinite trend (Fig. 6b).\nHowever, in the original submission we did not make good use of our gravity modelling results.\nIn the revised Fig.6 we do this more explicitly as a means of discriminating between\nserpentinization and fracture mechanisms. For the anomalous Vp/Vs> 1.9 regions, the seismic\ndata samples indicate a range of fracture porosities of between 0.02 and 5.69 % (with the bulk\nof the data samples at the low end of this range). However, the density reduction that would\noccur with such a fracture population is insufficient to account for our gravity model results,\nwhich require a drop in these regions of 0.5 g/cm3. This reduction is easily achieved by\nserpentinisation at the levels indicated by our Vp/Vs observations. For this reason, in the final\npart of the Results section we focus on the serpentinisation model, but alternates are given in\nthe Supplementary Fig. 31-32 (paragraphs starting line 664 and 674).\n2.1b Implications for excess water estimates\nConsidering that the inferred most hydrated zones are relatively shallow (2-3 km depth; Fig. 5b)\nand occupy deformed regions of seafloor, this mechanism is likely relevant but not thoroughly\nconsidered in the present study. I would like to see a second lower-bound estimate of hydration\nby assuming all water is stored within fractures.\nWe now set up and evaluate three hydration models in the final section of the Methods as\nfollows: (i) a model in which Vp/Vs> 1.9 is interpreted as hydrated ultramafic rocks (as\npreviously but we now exclude the pink-coloured data samples from the calculation, Fig. 6a and\nFig. 7b) (ii) a model in which Vp/Vs> 1.9 is interpreted as fractured ultramafic/mafic rocks (Fig.\n6b, Supplementary Figs. 31—32) and (iii) a hybrid model (Supplementary Figs. 31—32) in which\nthe data samples with Vp/Vs > 1.9 that lie on the serpentine hydration trend are interpreted as\nhydrated ultramafic rocks and those that lie off this trend are interpreted as fractured ultramafic\nrocks. Note that we choose only fractured ultramafic rock as an example in the hybrid model as\nthe fracture-water template of a peridotite matrix can cover all velocity points with Vp/Vs>1.9\nand also the overall fracture water content estimated based on different rock matrices is similar\n(Supplementary Fig. 32b, c) which therefore has little effect on the hydration estimation in a\nhybrid mode.\nthe anomalous Vp/Vs bodies, their proximity to elevated or exposed basement and their gravity\nsignature. Model 2 is ruled out from density reduction based on Fig. 6c. This model results in the\nuppermost and central core of the OCCs regions that are dryer than the surrounding area. We\nbelieve it is unlikely to maintain water within fractured peridotite without alteration in a 65 Ma\nlithosphere and given the reactivity of seawater-olivine, and the widespread occurrence of\nserpentinite in the oceans that model 2 is not a realistic model (e.g., Bach and Fruh-Green,\n2010; Klein et al., 2015; Mevel, 2003). Nonetheless, it may serve as a useful end-member as we\nthink reality is much closer to pure alteration (serpentinisation), followed by a combination of\nalteration and fracturing and then the pure fracturing model. We therefore use Model 3 to give\nour minimum bound excess water content estimate. We have added a related description for\nthese models to the main text in the Results section “Relationships between lithology,\nserpentinisation, water-filled fractures and physical properties”, “Preferred lithological model\nand excess hydration due to serpentinisation” and in the Methods section “Relationship\nbetween lithology, serpentinisation and Vp/Vs”, “Relationship between fractures and Vp/Vs” and\n“Water content calculations for anomalous crust”.\n2.2 Model Resolution\n2.2a Vp and Vs pick uncertainties and recovery\nI think an important point regarding the resolution is how picking uncertainty effects the\nrecovery of Vp/Vs ratios. The picking error for Sg (~177 ms) is ~5x larger than for Pg (~32 ms).\nTherefore, I would expect the amplitude recovery of Vs anomalies to be poorer than Vp\nanomalies; the resulting Vp/Vs ratios would then be biased. Do the synthetic data used in the\nresolution tests (Supplementary Fig. 15) include this data quality discrepancy (e.g., adding\ngaussian noise with different standard deviations for P and S travel-times)?\nWe thank the reviewer for this comment, as indeed we had not included the difference in\npicking uncertainty when generating the results for checkerboard tests. We have now done this\nand updated Supplementary Fig.15 and the caption. We have modified lines 529-534 in the\nMethods section (“Quality analysis and checks for final P- and S-wave models”) as follows\n“Checkerboard tests performed with a range of velocity anomaly sizes at an amplitude of ±5%\nand with a gaussian noise level (standard deviation of 10 ms, 50ms, and 100ms) scaled by the\npicking uncertainty for P- and S-waves, reveal a resolution for Vp of 51 to 51.5 km in the upper\ncrust, 52.5 km in the lower crust and 105 km in the upper mantle, and for Vs of 52.5 km to\n105 km in the crust and 155 km in the upper mantle (Supplementary Fig. 15)” and updated\nthe resolution box in Fig.2 and its caption accordingly. Please note that the separate PDF\nversion of Supplementary Fig.15 has higher resolution compared to the PNG file inserted in the\nmerged supplementary file.\nThe results show that the amplitude recovery is indeed poorer for Vs as the reviewer predicted.\nIncreasing the gaussian noise level with different standard deviations added to the synthetic\npicks affects the recovery of small-scale sinusoidal velocity anomalies (e.g., 5 1 km and 5  1.5\nkm) while the differences are only subtle for the recovery of large-scale anomalies (e.g., 10 5\nkm and 15  5 km).\n2.2b Resolution of Vp/Vs\nresolution tests specifically targeting Vp/Vs (and include differences in data quality mentioned\nabove). It is difficult to understand from Supplementary Fig. 15 alone how well the target model\nVp/Vs ratios are imaged. It also seems that the inversion over-estimates anomaly amplitudes\nconsidering many recovered perturbations saturate the color scale.\nWe thank the reviewer for this comment as indeed the interpretation of Vp/Vs ratios is the basis\nof this work. As Vp/Vs model is an indirect product out of the direct tomographic inversion of Vp\nand Vs travel time data, it would be difficult to conduct a similar checkerboard routine, as we do\nnot invert Vp/Vs model from travel time data. However, the resolution of Vp/Vs depends on the\nlower resolution of Vs as the finer details of Vp are “downsampled” to match the Vs when\ncomputing Vp/Vs. In Supplementary Fig. 15, we added black dashed lines to help compare the\nsize of identified Vp/Vs anomalous features in Fig. 2 to the size of the checkers. Vs in the crustal\ncolumn has a resolution in general at 10  5 km (horizontalvertical), finer than the size of\nidentified high/low Vp/Vs features at OCC-2 (x=225-270 km), OCC-3 (x=55-75km), OCC-4 (x=75-\n110km) and OCC-1 (VI-3) (x=210-225km). Especially, although the amplitude is recovered\nrelatively poorly, the Vs resolution achieved at 5  2.5 km at OCC-5 (x=~300km) and VI-2 (x=~135\nkm), which is also finer than the size of these Vp/Vs features. Therefore, we concluded that the\ninterpreted Vp/Vs features are reliable for the analysis.\nSpecific Comments:\n• Line 25-27: I suggest revising lines 25-27 to emphasise the lack of shear velocity constraints\non oceanic crustal structure as this is the real novelty of the study.\nDone\n• Line 24: Here, and several other places, the term 'slow-spread crust' is used but I believe\n'slow-spreading crust' is the more common phrase. Consider revising.\nDone\n• Line 52: Can you provide a reference for Penrose stratigraphy?\nDone (Anonymous, 1972)\n• Line 245-246: Could you better explain why these pink samples in particular are difficult to\nassign a composition? Specifically, it is not clear why the boundaries of the pink samples\ndoesn't extend to lower Vp or above the Vp/Vs = 1.9 line (presumably samples near this\nboundary, above or below, are difficult to identify).\nIn our revised manuscript, we strictly use Vp/Vs=1.9 to differentiate serpentinised peridotite\nfrom others so we now exclude the previous pink-coloured data samples from the calculation to\navoid ambiguity. The modification is applied to Figs. 6-7.\nIn the previous version, the serpentinisation trend lines (blue lines in Supplementary Fig. 29)\nand lithology (dolerite/gabbro, yellow lines in Supplementary Fig. 29) overlap between Vp=6.5-\n7.5 km/s and Vp/Vs=1.85-1.9 (the region of the pink samples). In our original analysis, we\nassigned this region to be partially serpentinised rock based on its spatial position within our\nseismic velocity model (pink areas in original Fig. 5). However, based on the reviewer’s\n8\ncomment, we have now removed these velocity points from our model (i) estimate of excess\nwater content described above. We note, however, that due to their small area and relatively low\nwater contents (3-6 wt% H O), removing these data points only reduces our maximum bound\n2\nwater content estimate from 4.5 wt% to 4 wt%. Please note that in the original manuscript, we\nmade a mistake in calculating the average bound water content over the whole crustal column.\nThe average was not calculated through the whole crustal column but mistakenly over the\nranges where bound water content is above the minimum assigned value. So, the number was\ncalculated as large as 6.6 wt%. We have modified this according to the abstract and the\nDiscussion section.\n• Line 282-283: Considering these are relatively shallow anomalies in a rather deformed\nsetting, to what extent could these velocity reductions and Vp/Vs anomalies be due to\nfracturing as opposed to serpentinisation?\nSee discussion of point 2.1 above.\n• Line 433: Can you clarify here that PSP arrivals are waves that travel through the sediments\nat source and receiver sides as P but through the crust/mantle as S?\nYes, we have done this and added more clarification about the point of conversion throughout\nthis section of the methods. This is now modified as “This delay was subtracted from\nSg/Sn/SmS arrivals to produce “effective PSP arrivals”(Eccles et al., 2009) i.e. as if they had\ntravelled through the sediments at both source and receiver sides as P-waves but through the\ncrust/mantle as S-waves in order to give symmetry for tomographic inversion using Vp in the\nsediments (Supplementary Fig. 4).”\n• Line 593-594: Please clarify how you arrived at a density anomaly of -0.15 g/cc for the gravity\nmodelling.\nWe have rewritten the text and added two references to explain how we obtained this value (see\nMethods section “Gravity modelling”). In the revised manuscript, the gravity results are used\nmore explicitly to assess whether the upper crustal bodies with the anomalous Vp/Vs ratios\nhave densities that would be expected if they contained significant quantities of serpentinite as\nopposed to fractured ultramafic/mafic rocks. See discussion of point 2.1 above.\n• Line 602-604: As written, it sounds like you've established your own lithologic boundaries\nbased on body wave velocities from field/laboratory data (Fig. S29). However, you do not\nprovide sufficient details on how these boundaries were determined. While you cite\nGrevemeyer et al. in regards to this, you don't explain how or why you've modified the\nlithologic boundaries defined in the Grevemeyer study. Please clarify.\nThe reviewer makes a fair point. We have re-written this section of the methods to be more\ndirect. Changes are made to Methods section “Relationships between lithology,\nserpentinisation and Vp/Vs” (starting line 624).\n• Supplementary Figure S29: It is a bit unclear from the caption or text what exactly the narrow\nlight blue and yellow linear bands are showing. I'm guessing that the light-blue band is the\n9\nperidotite-serpentinite trend along which the degree of serpentinisation is increasing.\nHowever, the fraction of serpentinite (or water content) along this curve is not indicated.\nPresumably the light-yellow band is the gabbro-dolerite trend along which the fraction of\ndolerite increases? Can the fraction of dolerite be labeled on this curve? Please clarify this in\nthe cation/figure.\nWe have rewritten the figure caption to explain the rock trends more clearly. We have also\nreplotted the figure to make the numbers indicating the fraction of serpentinite (or water\ncontent) along the light-blue band more visible (previously, some of the values were obscured\nwhich may have led to the reviewer missing them). We prefer not to include the fraction of\ndolerite as the figure is already quite complex, and the main aim of the manuscript is to identify\nand interpret regions that depart from the Penrose stratigraphy.\nReferences\nAllen, R.W., Collier, J.S., Henstock, T.J., 2022. The Role of Crustal Accretion Variations in\nDetermining Slab Hydration at an Atlantic Subduction Zone. Journal of Geophysical Research:\nSolid Earth 127, e2022JB024349.\nAnonymous, 1972. Penrose field conference on ophiolites. Geotimes 17, 24–25.\nBach, W., Fruh-Green, G.L., 2010. Alteration of the Oceanic Lithosphere and Implications for\nSeafloor Processes. Elements 6, 173-178.\nBlackman, D.K., Abe, N., Carlson, R.L., Guerin, G., Ildefonse, B., Kumpf, A., 2019. Seismic\nproperties of gabbroic sections in oceanic core complexes: constraints from seafloor drilling.\nMarine Geophysical Research 40, 557-569.\nBonatti, E., 1978. Vertical tectonism in oceanic fracture zones. Earth and Planetary Science\nLetters 37, 369-379.\nBonatti, E., Brunelli, D., Buck, W., Cipriani, A., Fabretti, P., Ferrante, V., Gasperini, L., Ligi, M.,\n2005. Flexural uplift of a lithospheric slab near the Vema transform (Central Atlantic): Timing\nand mechanisms. Earth and Planetary Science Letters 240, 642-655.\nBouysse, P., Westercamp, D., 1990. Subduction of Atlantic aseismic ridges and Late Cenozoic\nevolution of the Lesser Antilles island arc. Tectonophysics 175, 349-380.\nCanales, J.P., Tucholke, B.E., Xu, M., Collins, J.A., DuBois, D.L., 2008. Seismic evidence for\nlarge-scale compositional heterogeneity of oceanic core complexes. Geochemistry,\nGeophysics, Geosystems 9, Q08002.\nEccles, J.D., White, R.S., Christie, P.A.F., 2009. Identification and inversion of converted shear\nwaves: case studies from the European North Atlantic continental margins. Geophysical Journal\nInternational 179, 381-400.\nKlein, F., Grozeva, N.G., Seewald, J.S., McCollom, T.M., Humphris, S.E., Moskowitz, B., Berquo,\nT.S., Kahl, W.A., 2015. Experimental constraints on fluid-rock reactions during incipient\nserpentinization of harzburgite. American Mineralogist 100, 991-1002.\nLi, L., Collier, J., Henstock, T., Goes, S., 2024. Downward continued ocean bottom seismometer\ndata show continued hydrothermal evolution of mature oceanic upper crust. Geology 52, 717-\n722.\nLiu, M., Gerya, T., Rozel, A., 2025. The Effect of Brittle‐Ductile Weakening on the Formation of\nFaulting Patterns at Mid‐Ocean Ridges. Tectonics 44.\nLiu, M., Gerya, T., Rozel, A.B., 2022. Self-organization of magma supply controls crustal\nthickness variation and tectonic pattern along melt-poor mid-ocean ridges. Earth and Planetary\nScience Letters 584.\nMerdith, A.S., Atkins, S.E., Tetley, M.G., 2019. Tectonic Controls on Carbon and Serpentinite\nStorage in Subducted Upper Oceanic Lithosphere for the Past 320 Ma. Front Earth Sc-Switz 7,\n332.\nGeoscience 335, 825-852.\nMukerji, T., Berryman, J., Mavko, G., Berge, P., 1995. Differential effective medium modeling of\nrock elastic moduli with critical porosity constraints. Geophys Res Lett 22, 555-558.\nNorris, A.N., 1985. A differential scheme for the effective moduli of composites. Mechanics of\nMaterials 4, 1-16.\nPichot, T., Patriat, M., Westbrook, G.K., Nalpas, T., Gutscher, M.A., Roest, W.R., Deville, E.,\nMoulin, M., Aslanian, D., Rabineau, M., 2012. The Cenozoic tectonostratigraphic evolution of\nthe Barracuda Ridge and Tiburon Rise, at the western end of the North America-South America\nplate boundary zone. Marine Geology 303, 154-171.\nRoest, W.R., Collette, B.J., 1986. The Fifteen Twenty Fracture Zone and the North American–\nSouth American plate boundary. Journal of the Geological Society 143, 833-843.\nRüpke, L., Gaillard, F., 2024. The Geological History of Water: From Earth's Accretion to the\nModern Deep Water Cycle. Elements 20, 253-258.\nStein, S., Engeln, J.F., Wiens, D.A., Fujita, K., Speed, R.C., 1982. Subduction seismicity and\ntectonics in the Lesser Antilles Arc. Journal of Geophysical Research 87, 8642-8664.\nTaylor, M.A.J., Singh, S.C., 2002. Composition and microstructure of magma bodies from\neffective medium theory. Geophysical Journal International 149, 15-21.\n11"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have undertaken a thorough revision of the manuscript based on the reviewer comments. All of my questions and concerns and questions have been addressed, and it appears that the authors have included additional calculations to address one of the central comments of the other reviewer, which have also improved the manuscript.\nAs described in my previous review, I think this manuscript is very well written and illustrated, and it presents an important new result with implications for processes at slow-spreading mid-ocean ridges and subduction zones. I recommend publication.", "reviewer_2": "This is my second review of the Li et al. manuscript. The authors have thoughtfully addressed all my comments and I look forward to seeing the work published in Nature Communications."}, "author_response": "Response to Reviewers’ comments on Nature Communications\nmanuscript NCOMMS-24-69268A\n“Estimating excess bound water content due to serpentinisation in\nmature slow-spread oceanic crust using Vp/Vs” by Li et al.\nBelow reviewers’ comments are in blue italics and our response is in black.\nReviewer #1 (Remarks to the Author):\nThe authors have undertaken a thorough revision of the manuscript based on the\nreviewer comments. All of my questions and concerns have been addressed, and it\nappears that the authors have included additional calculations to address one of the\ncentral comments of the other reviewer, which have also improved the manuscript.\nAs described in my previous review, I think this manuscript is very well written and\nillustrated, and it presents an important new result with implications for processes at\nslow-spreading mid-ocean ridges and subduction zones. I recommend publication.\nReviewer #2 (Remarks to the Author):\nThis is my second review of the Li et al. manuscript. The authors have thoughtfully\naddressed all my comments and I look forward to seeing the work published in\nNature Communications.\nWe sincerely thank the reviewers for their insightful comments, which we believe have\nsignificantly improved the clarity and robustness of the manuscript. The positive\nassessments from both reviewers are encouraging, and we are delighted that they recognise\nthe importance of our findings for processes at slow-spreading mid-ocean ridges and\nsubduction zones. Their expertise and time invested in evaluating our manuscript have been\ninvaluable, and we deeply appreciate their support for its publication in Nature\nCommunications.\n1"}]

Decision letter