Deep mantle earthquakes linked to CO_2 degassing at the Mid-Atlantic Ridge

Domain: earth_science
Published: 2025-01-10
Editorial outcome: major_revision

Abstract
Volatiles (CO2, H2O) play a fundamental role in mantle melting beneath oceanic spreading centers, but what role they play during the melt migration remains unknown. Using seismological data recorded by ocean-bottom seismometers, here we report the presence of deep earthquakes at 10–20 km depth in the mantle along the Mid-Atlantic Ridge axis, much below the brittle-ductile boundary. Syntheses of regional basaltic rock samples and their geochemical analyses indicate the presence of an abnormally high quantity of CO2 (~0.4–3.0 wt%) in the primary melts. As the degassing of a high concentration of dissolved CO2 produces volume change, we suggest that deep earthquakes in the mantle result from the degassing of CO2. The large concentration of CO2 in the primitive melt will influence the presence of melt beneath the lithosphere-asthenosphere boundary at sub-solidus temperatures.

Introduction
Oceanic crust is formed by melt derived from the mantle at oceanic spreading centers. A small amount of melting initiates at ~150–300 km depths in the presence of volatiles (CO2, H2O)1–4, but extensive melting commences at 60–70 km depths, driven by mantle upwelling as two diverging plates move apart 5–9. Although partial melting occurs in a wide zone at depth 1, the melt is focused in a narrow zone beneath the ridge axis as it migrates upward and interacts with the host rocks. However, we have no clear understanding of how these melts migrate to the surface, especially in the upper part of the mantle.

At fast- and intermediate-spreading ridges, axial melt lenses are commonly used to separate the cooled, brittle lithosphere above the partially molten crust and mantle below 10, and define the brittle-ductile boundary (BDB). However, at slow- and ultraslow-spreading ridges, in the absence of images of axial melt lenses, the maximum depth of earthquakes, corresponding to the 700 ± 100 °C isotherms 10–12, is used to define the BDB. Based on thermal models, earthquakes are expected to occur at depths <8 km below the seafloor (bsf) beneath slow-spreading ridges and at depths <12 km bsf beneath ultraslow-spreading ridges 13. However, some deeper earthquakes have recently been observed beneath slow- and ultraslow-spreading ridges 13–15 and have been associated with either deep-rooted detachment faults 14,15 or cold thermal regimes 13,16. Deep earthquakes have also been observed along oceanic transform faults (TFs), which have been linked to semi-brittle deformation in high-temperature hydrated mylonite shear zones 17,18. These observations indicate that the maximum depth of earthquakes also depends on other factors, such as tectonic, hydrothermal, and petrological processes.

Here, we present the results of a microseismicity study from the Mid-Atlantic Ridge (MAR) in the equatorial Atlantic Ocean (Fig. 1a). The MAR here spreads at a half-spreading rate of 16 mm/yr 19. The study area is located in the northern part of the ~200-km-long MAR segment between the Romanche and Chain TFs, in the vicinity of the eastern Romanche ridge-transform intersection (RTI) (Fig. 1a). The studied portion of the ridge is ~120 km in length, offset by two non-transform discontinuities (NTD) (Fig. 1b). It can be subdivided into four 20–50 km-long subsections (Fig. 1b): the RTI segment (named RC1), the first NTD (NTD1), a short ridge segment (named RC2), and the second NTD (NTD2) (Figs. 1–3). The ridge segment south of NTD2 is named RC3. The RTI segment is amagmatic and bounded to the east by a westward dipping detachment fault with a prominent oceanic core complex (OCC) on the eastern side of the ridge axis (Figs. 2a and 3a). The extensive observation of peridotites on the seafloor 20,21 (Figs. 1b and 2a) indicate the presence of the exhumed mantle and supports the tectonic origin of this subsection. The surface of this OCC is heavily cut by roughly N20°E- and N20°W-striking normal faults, suggestive of recent active tectonic deformation (Figs. 2a and 3a). The NTD1 is ~35 km long, oriented at N76°E, and is characterized by a large number of N118°E- and E-W-striking faults (Fig. 3a,c). The presence of both pillow basalts and peridotites on the seafloor (Figs. 1b and 2a) 20,21 indicates its magmatic and tectonic origin. The segment RC2 south of NTD1 is ~22 km long, with a typical 10-km-wide median valley and an N154°E-oriented neo-volcanic ridge (Fig. 3c), suggesting a magmatically robust segment. This is further supported by the extensive observation of basalts on the seafloor (Figs. 1b and 2a) 21. The NTD2 has an ~33 km ridge offset, oriented at N110°E, with large areas affected by a complex pattern of normal faults, striking at N115°E and N145°E (Fig. 3e). The segment RC3 is a 50 km-long magmatic segment, oriented ~N165°E.

Results
The microseismicity data were acquired by a network of 19 ocean-bottom seismometers (OBSs) during the SMARTIES cruise in 2019 21,22 (Figs. 1–2), covering the 140 km eastern part of the Romanche TF, the RTI, and 120 km of the MAR axis. The seismic data were recorded continuously for ~21 days, with an instrument spacing of ~30 km (Fig. 1b). We detected initial earthquake arrivals automatically from 17 useful OBSs using a short-term-average/long-term-average trigger algorithm 23 (Supplementary Fig. 1). Both P- and S-wave arrivals were then checked manually. An active-source wide-angle seismic refraction profile 24 was used to find the best one-dimensional (1-D) velocity model to calculate travel times during earthquake locations (Supplementary Figs. 2–5). We used a non-linear earthquake location algorithm 25 to obtain earthquake hypocenters, which were then relocated using a double-difference location method 26. Station corrections (Supplementary Fig. 6) were calculated and updated 25, and S-wave delays due to low-velocity sedimentary layers were removed 18. Extensive depth resolution tests were carried out to assess the reliability of earthquake locations (Supplementary Figs. 3, 5, 7–9). Local magnitudes of earthquakes were also determined (Supplementary Fig. 10). More details on earthquake locations are presented in the Methods section.

We located 514 earthquakes in the vicinity of the Romanche RTI region (Fig. 2). In this study, we focus on the earthquakes along the ridge segments and discontinuities (RC1, NTD1, RC2, and NTD2). The hypocenters locations along each subsection are shown in Fig. 3. On the basis of well-established criteria 27–29, we have classified our earthquake locations into four categories (A, B, C, and D) (See Methods, Figs. 2 and 3, Supplementary Table 1). Categories A and B are considered of good quality and are used for interpretation.

There are three key observations along these four subsections: (1) a majority of shallow earthquakes (0–6 km) occur on the outside corner of the RTI beneath the OCC faulted dome and off-axis west of the segment RC1; (2) deep microseismicity (~10–20 km) lies beneath the ridge axis of the segment RC2; and (3) normal-depth (4–10 km) earthquakes occur beneath the southern NTD2 (Figs. 2 and 3). Note that this microseismicity record is just a brief snapshot in time, and it provides insight into the processes along this MAR supersegment, even if the microseismic activity may vary substantially over years. For a full spreading rate of ~32 mm/yr 19, microseismicity studies 13 indicate that the maximum depth of earthquakes should be less than 10 km (Fig. 4, Supplementary Table 2), corresponding to a brittle lithospheric thickness of <10 km. Such a depth range is only observed beneath the OCC (<6 km, Fig. 3b) and NTD2 (<10 km, Fig. 3f). The absence of deep microseismicity beneath the axial valley floor, near the OCC termination (Fig. 3a), suggests that this detachment fault is inactive. However, the shallow earthquakes recorded directly beneath the OCC dome likely result from ruptures on high-angle normal faults cutting its surface. The earthquake distribution and tectonic observations thus suggest that the ridge axis is being relocated from the OCC termination in the axial valley to the east beneath the OCC faulted dome 20. Beneath the NTD2, the earthquakes are slightly deeper, down to ~10 km (Fig. 3e, f), which is expected for a slow-slipping NTD (Fig. 4, Supplementary Table 2). These results suggest that the brittle lithosphere is at most ~10 km thick at the segment boundaries (NTDs).

The BDB is expected to become shallower southward with increasing distance to the RTI because of the decrease in the cold-edge effect caused by the cold (45 Ma) lithosphere 20,30,31. However, we observed deep earthquakes (16–19 km) beneath the segment RC2 axis (Fig. 3c, d). These are the deepest earthquakes documented to date at a slow-spreading center (Fig. 4). Seismic refraction studies indicate that the 8-Ma-old crust of the western ridge flank is 5.4 ± 0.3 km thick 32, suggesting that these earthquakes mostly occur in the mantle below 10 km depth, with some scattered events in the crust (Fig. 3d), far exceeding the suggested maximum depth range (Fig. 4). An earthquake cluster is also observed on the western side of the axial valley, with shallow focal depths (~2–6 km) (Fig. 3d), suggesting that the unexpected depths of the events beneath the MAR axis are not an artifact of location errors. The depth resolution tests (Methods, Supplementary Figs. 7–9) further support that these events are indeed deep.

Methods
Seismic data
In July and August 2019, we conducted an OBS passive seismic experiment (Figs. 1–3) during the SMARTIES cruise 21,22 (Figs. 1–2). A network of 19 OBSs was deployed, with an instrument spacing of ~30 km (Fig. 2a). We first detected earthquake arrivals automatically using a short-term-average/long-term-average trigger algorithm within the SEISAN package 23 by analyzing the vertical components of 17 useful OBSs (Supplementary Fig. 1). In total, 760 earthquakes were identified and registered into the SEISAN database. Each of these events was then checked manually to make sure they were detected by at least five OBSs. Finally, 514 earthquakes were located (Fig. 2), with depth uncertainties of ≤10 km, horizontal uncertainties of ≤10 km, RMS residuals of ≤0.3 s, and station gaps of <270°. Their mean horizontal and vertical errors are ~2.8 km and ~2.9 km, respectively (Supplementary Table 3).

Wadati diagrams yield a Vp/Vs ratio of ~1.7 (Supplementary Fig. 4), which is used to estimate the S-wave velocity in inversion. To further examine the choice of the Vp/Vs ratio, we used various Vp/Vs ratios ranging from 1.5 to 2.5 to locate earthquakes (Supplementary Fig. 5). The results show that the Vp/Vs ratio of ~1.7 has the lowest RMS residuals and the largest number of located earthquakes among all tests (Supplementary Fig. 5), indicating that the Vp/Vs ratio setting of ~1.7 is reasonable. Theseismic tomographic results 67 also indicate normal Vp/Vs ratios in the present segment RC2.

Earthquake location
We used the non-linear oct-tree search algorithm of the NonLinLoc program 25 to locate initial earthquake hypocenters, of which the maximum likelihood solution was selected as the preferred result. NonLinLoc estimates a 3-D error ellipsoid (68% confidence) from the posterior density function scatter samples 25. Iterative calculations of station corrections (Supplementary Fig. 6) were used to find the best solution as well as to remove the 3-D effects when the average RMS misfit yields a minimum (Supplementary Fig. 2b). At the same time, we removed the S-wave delays, which may be caused by very low velocities associated with unconsolidated sediments 13, from the original S-onsets before inversion 18.

After removing the S-wave delays, double-difference relocations for 364 well-constrained events were determined using the hypoDD program 26. All 364 earthquakes were detected on more than six OBSs, with RMS residuals of <0.25 s, uncertainties of <5 km, and azimuthal gaps of <270°. We performed the relocation using differential travel times from the original catalog and waveform cross-correlation data. Station corrections (Supplementary Fig. 6) obtained by the NonLinLoc program were successfully applied. A minimum of six catalog links per event pair was required to form a continuous cluster. Five iterations were carried out, with a maximum event separation of 6 km. As a result, 276 events were well relocated, and they replaced the NonLinLoc locations in the final catalog. As a result, 317 events were located along the MAR and 197 events along the Romanche TF (Fig. 2b), with an updated average horizontal uncertainty of ~2.1 km (Fig. 2a).

We followed well-established criteria 27–29,68 to assess the reliability of our earthquake locations. For a well-constrained earthquake location, the following criteria can be used 27: (1) the azimuthal station gap is <180° 68; (2) there are more than eight arrivals to locate an earthquake, with at least one S-wave arrival and one arrival within a focal depth distance from the epicenter 29; (3) one S-wave arrival is recorded within 1.4 times of the focal depth distance from the epicenter 28. Based on these criteria, we classified our earthquakes into four categories (Supplementary Table 1). The earthquakes with location category A meet all the above criteria, and those with location quality B-C meet two criteria, whereas the events with location quality D only meet one criterion. We find that ~78% of the located earthquakes meet at least two criteria (see details in Supplementary Table 1), and hence we can confidently interpret these hypocenter locations.

Depth resolution
To gain insight into the effect of uncertainty in velocity on the earthquake location, earthquake depths were determined using five different velocity models (Supplementary Fig. 2a, Supplementary Fig. 7). We find that the depths of most of the axial events remain between 10 and 20 km, and the depth shifts are smaller than the average depth uncertainties (~2.6 km) (Supplementary Table 3). However, the fastest model (Model 1), which is derived from the southern Romanche TF 24, leads to some shallow events in the crust beneath the axial valley (Supplementary Fig. 7d). Notice that this model shows that the P-wave velocity exceeds 7.2 km/s at ~3 km depth, which is unusual for a magmatically accreted young crust (<7.5 Ma 69) (Supplementary Fig. 7b). Although the southernmost flank of the Romanche TF is believed to be composed of mantle peridotites, representing a crust-free lithosphere (Fig. 1b) 18,21,30, this model is not appropriate for locating events beneath the MAR axis. To investigate the effect of velocity uncertainty further, we increased and decreased the velocity by ±0.1 km/s at all depths (Supplementary Fig. 8, Supplementary Table 4). We find that both the lower and higher velocities result in locating a substantial number of deep events (>10 km) beneath the ridge axis (Supplementary Fig. 8c–e). Although the increased velocity model leads to some shallow events, it is also accompanied by a decrease in the number of located earthquakes (Supplementary Table 4). Interestingly, the reduced velocity model leads to a smaller depth uncertainty (Supplementary Fig. 8c–e), and therefore, it is reasonable to use a normal or slightly decreased velocity model in the study area rather than an increased velocity model. To further validate the robustness of the deep earthquake locations beneath the segment RC2, we constructed a subset of 45 events at depths ranging from 10 to 20 km along the cross-section cc′ shown in Fig. 3d. Their depths were then forced to be unchanged at 2.5 km, 5 km, 7.5 km, and 10 km during the relocation process. We find that not only their initial RMS residuals were much higher but they decreased much more slowly, and the final RMS were higher than those obtained when the depths were not fixed. (Supplementary Fig. 9). All these tests indicate that the deep events beneath the ridge axis (RC2) are well constrained, required by the data, and not artifacts (Supplementary Figs. 7–9).

Magnitude estimation
Earthquake magnitudes were determined using the local magnitude scale ML 70:
ML = log A + 1.11 log(D) + 0.00189 D - 2.09
The maximum amplitude A is measured on a seismogram simulating the original Wood-Anderson seismogram using the SEISAN package 23. D is the hypocentral distance in kilometers. The magnitude completeness (Mc, 1.5) and b value (0.87) are also calculated using the ZMAP software 71 (Supplementary Fig. 10a). Earthquakes were divided into three groups, i.e., events along the TF, in the ridge-transform intersection, and along the MAR (Supplementary Fig. 10b–d). Their b values were also determined, with small differences from each other (0.89–0.93) (Supplementary Fig. 10).

Focal mechanism solution
We used the P-phase first-motion polarities to determine focal mechanisms using the HASH package 72, and these polarities were picked from unfiltered earthquake waveform data on the vertical component. As previously noted 18, the resulting mechanism solutions are not very robust, mainly due to the large spacing of OBSs (~30 km) (Fig. 2a). Also, the poor azimuthal ray path distribution limits the quality of the results. Using a selection criterion based on P-wave polarities of >8, an azimuthal gap of <180°, an RMS fault plane uncertainty of <45°, average misfit <20%, station distribution ratio of >0.4, and mechanism probability >60% (Supplementary Table 5), we obtained three new well-constrained focal mechanism solutions (Fig. 2a), which combined with the three previous solutions for earthquake swarms 18, provided six focal mechanisms (Supplementary Table 5).

The maximum depth of earthquakes
We compiled the maximum depth of earthquakes along the ridge axis documented to date at slow- and ultraslow-spreading ridges around the world, as well as the full spreading rate on each site 19 (Fig. 4, Supplementary Table 2). In this study, we selected the maximum depths that were well constrained by several earthquakes instead of only one event to avoid bias in the location process (Supplementary Table 2). In addition, we included information that may influence the maximum depth distribution, such as the presence of oceanic core complexes and/or detachment faults, hydrothermal vents, magmatism (e.g., focused melting and/or hotspots), and adjacent TFs (Fig. 4, Supplementary Table 2). It should be noted that the reported data from the SWIR segment 8, SWIR oblique Supersegment, and the Logachev Seamount of Knipovich Ridge were updated using the newly located data 13,73,74. The Rainbow massif is located at a non-transform discontinuity (Supplementary Table 2), which is also included in the plot for reference (Fig. 4).

CO2 estimation from MORB
We compiled the MORB samples along all the spreading ridge segments from the PetDB database 75 (www.earthchem.org/petdb) within the bounds of our OBS network (Fig. 5, Supplementary Data 1). In particular, we analyzed the samples in the studied segment RC2 and the adjacent segment RC3 in the south (Fig. 5, Supplementary Data 1). The CO2/Rb or CO2/Ba ratios of melt inclusion from the MORBs are a good proxy for the CO2 concentration 50–53. Using a constant CO2/Rb ratio (991 ± 129) 51 (Fig. 5b) and CO2/Ba ratio (81.3 ± 23) 51 (Fig. 5c), we estimated CO2 content in pre-eruptive melts for the segments RC2 and RC3. CO2(Rb) and CO2(Ba) are the CO2 concentrations estimated from Rb and Ba, respectively. For the segment RC2, we find CO2(Rb) = 0.9–4.3 wt% and CO2(Ba) = 0.7–4.6 wt% (Supplementary Data 1). For the segment RC3, our estimated CO2 values are significantly lower, CO2(Rb) = 0.07–1.0 wt% and CO2(Ba) = 0.06–0.8 wt% (Supplementary Data 1). Primary melts are in equilibrium with their mantle source and are, therefore, different from the pre-eruptive melts, which go through various degrees of fractional crystallization.

We calculated Ba90 and Rb90 as the concentrations of those elements in melts in equilibrium with Fo90 olivine 51,57 and used these values to get an estimated CO2 content in primary melts (Fig. 5, Supplementary Data 1). We find that the CO2 content in the primary melts is estimated as CO2(Rb90) = 0.5–2.8 wt% and CO2(Ba90) = 0.4–3.0 wt% for the segment RC2, and CO2(Rb90) = 0.05–0.7 wt% and CO2(Ba90) = 0.04–0.5 wt% for the segment RC3 (Fig. 5, Supplementary Data 1). These results suggest that the CO2 concentration in the primary melts is at least 0.4 wt% along the studied segment RC2. In addition, to investigate whether the seafloor basalts are mostly degassed, we calculated CO2 theoretical solubility for samples with existing CO2 measurement, based on the model of Iacono-Marziano et al. 76, with the following parameters: dredge depth, a temperature of 1200 °C, and major elements concentration. The calculation results show that the CO2 solubility values are very close to the measured CO2 contents (Supplementary Data 1), indicating that those samples are degassed.

Data availability
The raw seismic data and cruise reports are available on the website (https://campagnes.flotteoceanographique.fr/campagnes/18001107/) and can be requested for scientific purposes. The earthquake catalog and picked P- and S-arrivals generated in this study have been deposited in the Zenodo database (https://doi.org/10.5281/zenodo.12696684).

Code availability
The SEISAN software 23 used to pick phases is available at https://www.geo.uib.no/seismo/SOFTWARE/SEISAN/. The NonLinLoc code 25 used for earthquake location is available at http://alomax.free.fr/nlloc/. The VELEST program 66 used for inverting the 1-D velocity model is available at https://seg.ethz.ch/software/velest.html. The HypoDD program (version 1.3) 26 used for double-difference earthquake relocation is available at https://www.ldeo.columbia.edu/~felixw/hypoDD.html. The ZMAP software 71 used for catalog analysis to obtain b-value and magnitude completeness is available at https://github.com/swiss-seismological-service/zmap7. The HASH software 72 (version 1.2) used for determining focal mechanisms solution is available at https://www.usgs.gov/node/279393. The Global Mapper used for structural analysis is available at https://www.bluemarblegeo.com/global-mapper/. The GMT 6 toolbox 77 used for graphing is available at https://www.generic-mapping-tools.org/download/.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The paper investigates the processes involved at MAR by using a high resolution seismic data set recorded over the time period of approx. 3 weeks. Seismicity is used to deduce the brittle to ductile transition zone and this approach has been used widely at MAR to investigate the thermal structure and underlying geodynamic processes. The hypocenter depth is in these kinds of studies the most important parameter and therefore it must be accurately determined. The authors find deep seated seismicity and together with geochemical analysis they infer that CO2 degassing might be a likely process which could facilitate the generation of brittle faulting at these temperature condition.\nThe authors used NonLinLoc and an apriori P-wave velocity model determined from wide-angle data to constrain the hypocenter depth. They investigated the influence of the apriori P-wave model on the earthquake location and due to the relatively small perturbations in depth observed with different models they concluded that their results are accurate. Unfortunately, a thorough investigation of the S-wave model (or alternatively vp/vs model) is lacking in the paper, and it is only state “Wadati diagrams yield a Vp/Vs ratio of ~1.73 (Supplementary Fig. 3), which is used to estimate the S wave velocity in the inversion.” I assume that S-Fig. 3 shows the S, S-P, and P wave arrival after the initial relocations which I also assume has been done using a vp/vs ratio of 1.73. Therefore, there is no surprise that a vp/vs ratio of about 1.65-1.73 is found as this value was used for the initial location. If I look at the material you might find at MAR, water-saturated sediments, basalt, gabbro, peridotite and serpentine I am rather surprised to see a vp/vs ratio of 1.73 as all of them have actually vp/vs ratios greater than 1.73 (e.g. Grevemeyer and Lange, Geology, 2020). Especially the sediments can have vp/vs ratio up to 5 or even larger.\nAs shown by Gomberg (BSSA, 1990) an accurate S-pick close to the epicentre is needed to constrain accurately the depth, but this of cause also implies that you need an accurate S-wave model! The only way forward in areas where you do not have both, accurate P- and S-wave velocities, is to carry out a joint inversion for hypocentre locations, station corrections and minimum 1D model as outlined by Kissling et al. (JGR, 1994). In this paper it is also clearly stated that using apriori models for the earthquake location problem should be done with great care as these starting models might lead to bias in the model and therefore earthquake locations.\nThe authors argue that by iteratively updating the station delay terms in NonLinLoc will lead to more accurate locations. However, this iterative procedure is not addressing the simultaneous inversion problem for velocity model, station correction and hypocentral parameters as it only will lead to the optimum station correction terms for a given velocity model and dependent on the starting model used or in other words dependent on the assumed vp/vs ratio. This problems manifest itself that for the station corrections shown the P-wave residuals tend to more negative corrections and the S-wave corrections to more positive values. As I assume S-wave arrival picks are weight less than P-waves, distance depending damping is also part of NonLinLoc the problem to identify where all these contribution come from is rather complicated.\nAdditionally, the application of the selection criteria for well contained earthquakes is not applied in the right order even for the current case where only stations corrections are iteratively updated. In any joined inversion only the earthquakes which fulfil all criteria (GAP, number of picks, close station, etc…) should be used as otherwise the poorly determined inversion problem becomes completely ill posed (especially for the trade-off between origin time and earthquake depth). For a proper event depth estimate we need a close by station with an S-pick (Gomberg, BSSA, 1990). Looking at table S1 we see that we are then left with 87 usable events for the inversion which are less than 20% of the data set which has been used by the authors to estimate station corrections.\nIn summary, I therefore do not agree with the statement that the event locations are robust and that the observed arrival times can only be explained by deep events. It is interesting to note that there is quite some controversy in the community regarding the depth of earthquakes at slow and ultra-slow spreading ridges highlighted by the recent comment and reply in Geology 2020 (Gevemeyer and Lange, 2020; Schlindwein, 2020). This further highlights that the absolute depth problem of earthquakes at MAR is not conclusively solved yet.\nFinally, I want to highlight that the raw seismic data is restricted on the given WEB site as all other data is as well. I also think it is not acceptable that the underlying travel time data (P- and S-waves), earthquake locations, final velocity model, station locations and corrections are not made accessible to the reviewers for an honest review process. Stating that they will be made available after publication is not helpful for a constructive review process and we should work much more towards Open Data policy.", "reviewer_2": "The authors report an observation of earthquakes below a mid-oceanic ridge at depths where a brittle deformation is not expected in the mantle, based on thermo-mechanical models. Such earthquakes are very interesting because their occurrence reveals some still poorly understood processes in the upper mantle. The authors present a careful study of the earthquake hypocenters and of the accuracy of their determinations clearly demonstrated that the observed “anomalous” depths are not an artifact. Then, they consider several possible hypotheses about the origin of the observed deep earthquake cluster and based on comparison with tectonic and geological settings argue that that the earthquakes in the hot mantle most likely result from the degassing of CO2 in the mantle. This is a very interesting result well supported by observations. The manuscript is well written and I would like to see it published in Nature Communications. I just have a couple of suggestions as detailed below.\nEarthquakes occurring in the deep crust or uppermost mantle where a brittle deformation is not expected are often observed beneath active volcanoes. This class of seismicity is known as deep long period (DLP) volcanic earthquakes (e.g., Shapiro et al., 2017; Wech et al., 2020). In my view, the deep seismicity reported in the present manuscript has many similarities with the volcanic DPL earthquakes and some similar hypotheses have been used for their explanation. In particular, the CO2 degassing has been recently suggested to explain the DLP earthquakes (Melnik, 2020). Therefore, I would suggest to the authors to mention this possible analogy in their manuscript.\nThe model of Melnik et al. (2020) also contain two interesting aspects that might be worth of mentioned. First the CO2 solubility and the intensity of degassing depend of the amount of H2O in the melt. Second, and most interesting, the CO2 degassing can be more than just a “trigger” of earthquakes, as written by the authors in line 194. As shown by Melnik et al., the gas bubble expansion and associated pressure variations can be rapid enough (i.e., occurring over times less than 1 sec) to be itself the main force causing the generation of seismic waves.\nThe DLP earthquakes differ from “volcano-tectonic” earthquakes associated with the faulting by their spectral content (this is why they are called LP) and by their focal mechanisms containing significant non-double-couple components (e.g., Ikegaya and Yamamoto, 2021). Therefore, if the authors might have some information about the distinct spectral characteristics of the earthquakes from the deep cluster of out their focal mechanisms, this might be interesting to mention in the paper.\nReferences:\nShapiro, N., Droznin, D., Droznina, S. et al. Deep and shallow long-period volcanic seismicity linked by fluid-pressure transfer. Nature Geosci 10, 442–445 (2017). https://doi.org/10.1038/ngeo2952\nAaron G. Wech et al., Deep long-period earthquakes generated by second boiling beneath Mauna Kea volcano. Science 368, 775-779 (2020). DOI:10.1126/science.aba4798\nMelnik, O., Lyakhovsky, V., Shapiro, N.M. et al. Deep long period volcanic earthquakes generated by degassing of volatile-rich basaltic magmas. Nat Communications 11, 3918 (2020). https://doi.org/10.1038/s41467-020-17759-4\nTakuma Ikegaya, Mare Yamamoto (2021), Spatio-temporal characteristics and focal mechanisms of deep low-frequency earthquakes beneath the Zao volcano, northeastern Japan, Journal of Volcanology and Geothermal Research, 417, 107321"}, "author_response": "Responses to the review comments\n(The black words show the review comments; the blue words show our responses)\n--------------------------------------------------------------------------------------------------------\nReviewer: 1\nReviewer #1 (Remarks to the Author):\nThe paper investigates the processes involved at MAR by using a high resolution seismic\ndata set recorded over the time period of approx. 3 weeks. Seismicity is used to deduce the\nbrittle to ductile transition zone and this approach has been used widely at MAR to investigate\nthe thermal structure and underlying geodynamic processes. The hypocenter depth is in\nthese kinds of studies the most important parameter and therefore it must be accurately\ndetermined. The authors find deep seated seismicity and together with geochemical analysis\nthey infer that CO2 degassing might be a likely process which could facilitate the generation\nof brittle faulting at these temperature condition.\nThe authors used NonLinLoc and an apriori P-wave velocity model determined from wide-\nangle data to constrain the hypocenter depth. They investigated the influence of the apriori\nP-wave model on the earthquake location and due to the relatively small perturbations in\ndepth observed with different models they concluded that their results are accurate.\nUnfortunately, a thorough investigation of the S-wave model (or alternatively vp/vs model) is\nlacking in the paper, and it is only state “Wadati diagrams yield a Vp/Vs ratio of ~1.73\n(Supplementary Fig. 3), which is used to estimate the S wave velocity in the inversion.” I\nassume that S-Fig. 3 shows the S, S-P, and P wave arrival after the initial relocations which\nI also assume has been done using a vp/vs ratio of 1.73. Therefore, there is no surprise that\na vp/vs ratio of about 1.65-1.73 is found as this value was used for the initial location. If I look\nat the material you might find at MAR, water-saturated sediments, basalt, gabbro, peridotite\nand serpentine I am rather surprised to see a vp/vs ratio of 1.73 as all of them have actually\nvp/vs ratios greater than 1.73 (e.g. Grevemeyer and Lange, Geology, 2020). Especially the\nsediments can have vp/vs ratio up to 5 or even larger.\nWe agree with the reviewer that the Vp/Vs ratio in the vicinity of RTI and ridge axis could be\nhigh, and therefore, we have included several tests with the Vp/Vs ratio ranging from 1.5 to\n1\n2.5 (see Supplementary Fig. 5), but we found that the velocity model with a Vp/Vs ratio of\n~1.7 leads to the lowest RMS residual and the largest number of earthquakes located. Thus,\nthe choice of the Vp/Vs ratio (1.7) from the Wadati diagram (Supplementary Fig. 4) is justified.\nSecondly, the Vp/Vs ratio estimated from the Wadati diagram does not correlate with the\ninitial values. The Vp/Vs ratio from the modified Wadati diagram (Chatelain, 1978) can be\ncomputed using the following equation:\n(cid:1830) (cid:1830)\n(cid:1872) −(cid:1872) (cid:3036) − (cid:3037) (cid:1848)(cid:1868)\n(cid:3020)(cid:3036) (cid:3020)(cid:3037) (cid:1848)(cid:1871) (cid:1848)(cid:1871)\n= =\n(cid:1872) −(cid:1872) (cid:1830) (cid:1830) (cid:1848)(cid:1871)\n(cid:3017)(cid:3036) (cid:3017)(cid:3037) (cid:3036) − (cid:3037)\n(cid:1848)(cid:1868) (cid:1848)(cid:1868)\nwhere for a station pair (i and j), (cid:1872) and (cid:1872) are travel times for P- and S-waves,\n((cid:3017)(cid:3036), (cid:3017)(cid:3037)) ((cid:3020)(cid:3036), (cid:3020)(cid:3037))\nrespectively, (cid:1848)(cid:1868) and (cid:1848)(cid:1871) are P- and S-wave velocities, respectively, and ((cid:1830)(cid:1861),(cid:1830)(cid:1862)) are\nhypocentral distances. From this equation, the onset times have been removed during the\ncomputation. Therefore, the initial setting of the Vp/Vs ratio in the earthquake location does\nnot affect the results from the Wadati diagram.\nThirdly, we agree that the Vp/Vs ratio for unconsolidated sediments can be very high\n(Ferrante et al., 2023; Grevemeyer et al., 2019), and therefore, we have removed S-wave\ndelays caused by the unconsolidated sediments before the inversion, which reduces the\neffects of the local high Vp/Vs ratios on the earthquake determinations. The S-wave delays\nin the sediments are estimated using the method outlined by Yu et al. (2021).\nIt is possible that the Vp/Vs ratios can be highly heterogeneous in a small region. However,\none cannot get a detailed 3-D velocity model (or detailed Vp/Vs ratio) in the whole model\nspace before the earthquake location. Indeed, we just can use an averaged velocity model\nto locate earthquakes. Of course, this kind of model must be carefully selected as we have\ndone in the manuscript.\nIn summary, our choice of the Vp/Vs ratio of ~1.7 is reasonable.\nAs shown by Gomberg (BSSA, 1990) an accurate S-pick close to the epicentre is needed to\nconstrain accurately the depth, but this of cause also implies that you need an accurate S-\nwave model! The only way forward in areas where you do not have both, accurate P- and S-\nwave velocities, is to carry out a joint inversion for hypocentre locations, station corrections\nand minimum 1D model as outlined by Kissling et al. (JGR, 1994). In this paper it is also\n2\nclearly stated that using apriori models for the earthquake location problem should be done\nwith great care as these starting models might lead to bias in the model and therefore\nearthquake locations.\nIndeed, we have been very careful in choosing the 1D velocity. We have referenced a P-\nwave velocity model from an active-source seismic refraction profile (Wang et al., 2021;\nSupplementary Fig. 2). Then we used the Wadati diagram (Supplementary Fig. 4) to obtain\nthe Vp/Vs ratio to construct an S-wave velocity model, which is reasonable from several\nVp/Vs ratio tests (Supplementary Fig. 5). Furthermore, in the revised version, we have\nconstructed a ″minimum″ 1-D velocity model by the VELEST program (Kissling et al., 1994)\nto locate earthquakes and the results show that the RMS residuals of the two models are\nclose, but our selected model resulted in a larger number of located earthquakes than the\ninverted ″minimum″ 1-D velocity model (Supplementary Fig. 3). Therefore, we have\nused ″Model 5″ (see Supplementary Fig. 2a) to locate earthquakes, which is the best fitting\nvelocity model.\nThe authors argue that by iteratively updating the station delay terms in NonLinLoc will lead\nto more accurate locations. However, this iterative procedure is not addressing the\nsimultaneous inversion problem for velocity model, station correction and hypocentral\nparameters as it only will lead to the optimum station correction terms for a given velocity\nmodel and dependent on the starting model used or in other words dependent on the\nassumed vp/vs ratio. This problems manifest itself that for the station corrections shown the\nP-wave residuals tend to more negative corrections and the S-wave corrections to more\npositive values. As I assume S-wave arrival picks are weight less than P-waves,\ndistance depending damping is also part of NonLinLoc the problem to identify where all these\ncontribution come from is rather complicated.\nWe agree with the reviewer that one should perform simultaneous inversion for Vp, Vs and\nhypocentre locations, but given the geometry of the array and earthquakes, the simultaneous\ninversion for the velocity model would be poorly constrained. However, our approach will\nallow a robust relative location, which we can interpret in a more global sense. In fact, this\nprocedure can remove the effects of the sediments or other (Grevemeyer et al., 2019; Parnell-\nTurner et al., 2017, 2021; Yu et al., 2021; Chen et al., 2023).\nAdditionally, the application of the selection criteria for well contained earthquakes is not\napplied in the right order even for the current case where only stations corrections are\niteratively updated. In any joined inversion only the earthquakes which fulfil all criteria (GAP,\nnumber of picks, close station, etc…) should be used as otherwise the poorly determined\ninversion problem becomes completely ill posed (especially for the trade-off between origin\ntime and earthquake depth). For a proper event depth estimate we need a close by station\nwith an S-pick (Gomberg, BSSA, 1990). Looking at table S1 we see that we are then left with\n87 usable events for the inversion which are less than 20% of the data set which has been\nused by the authors to estimate station corrections.\nWe have followed the guideline from Gomberg et al. (1990) and have constructed a sub-\ndataset of 403 earthquakes with location quality A and B (Supplementary Table 1) to perform\nthe earthquake location again, excluding the 112 events that were poorly determined. The\nfollowing figure shows that the earthquake locations using the two different datasets do not\nchange a lot and therefore, our results are robust and reliable.\nFigure Earthquake location test. (a) Bathymetric map and located events. Solid and open\ndots indicate earthquakes with depth uncertainty of ≤5 km and 5-10 km, respectively. The\ncolour of the circle indicates the results using the full dataset and a sub-dataset, respectively.\n(b) The focal depth distribution of earthquakes along the profile (aa′) in (a).\nIn summary, I therefore do not agree with the statement that the event locations are robust\n4\nand that the observed arrival times can only be explained by deep events. It is interesting to\nnote that there is quite some controversy in the community regarding the depth of\nearthquakes at slow and ultra-slow spreading ridges highlighted by the recent comment and\nreply in Geology 2020 (Gevemeyer and Lange, 2020; Schlindwein, 2020). This further\nhighlights that the absolute depth problem of earthquakes at MAR is not conclusively solved\nyet.\nWe respectfully disagree with the reviewer’s judgement; We have performed a thorough\nanalysis and have obtained robust, relative, hypocentral locations, given the geometry of the\nexperiment. We fully appreciate the reviewer's concerns about previous conflicting results\nand have also participated in this debate (Yu and Singh, 2023, Nat. Commun.). Gevemeyer\net al. (2019, Geology) re-analyzed the deep events by Schlindwein & Schmid (2016, Nature)\nand found that these deep earthquakes were caused by large S-wave delays, caused by the\nunconsolidated sediment covering the seafloor. We have proposed a novel method to\nquantify this delay using OBS data (Yu et al., 2021; Ferrante et al., 2023) and have removed\nsuch delays prior to the inversion. A very recent paper by Jie et al. (2022, Nat. Commun.)\nargued that microearthquakes at ultraslow-spreading mid-ocean ridges do not extend to\ndeeper than 15 km.\nFinally, I want to highlight that the raw seismic data is restricted on the given WEB site as all\nother data is as well. I also think it is not acceptable that the underlying travel time data (P-\nand S-waves), earthquake locations, final velocity model, station locations and corrections\nare not made accessible to the reviewers for an honest review process. Stating that they will\nbe made available after publication is not helpful for a constructive review process and we\nshould work much more towards Open Data policy.\nWe appreciate the reviewer’s efforts to Open Data policy, and we agree with it. In the revision,\nwe have provided the raw data for the review process, including P- and S-wave arrivals,\nearthquake locations, 1-D velocity model, station locations, and station corrections.\nReferences\nChatelain, J. L. Etude fine de la sismicité en zone de collision continentale au moyen d’un\nréseau de stations portables: la région Hindu-Kush Pamir. (Université scientifique et\nmédicale de Grenoble, 1978).\namagmatic oceanic detachment fault system. Nat. Commun. 14, 430 (2023).\nFerrante, G. M. et al. Seismically-derived porosity of deep-sea sediments over the last 74 Ma\nin the equatorial Atlantic Ocean: Implications for paleo-climate. Earth Planet. Sci. Lett.\n612, 118163 (2023).\nKissling, E., Ellsworth, W. L., Eberhart-Phillips, D. & Kradolfer, U. Initial reference models in\nlocal earthquake tomography. J. Geophys. Res. Solid Earth 99, 19635–19646 (1994).\nGomberg, J. S., Shedlock, K. M. & Roecker, S. W. The effect of S -wave arrival times on the\naccuracy of hypocenter estimation. Bull. Seismol. Soc. Am. 80, 1605–1628 (1990).\nGrevemeyer, I. et al. Constraining the maximum depth of brittle deformation at slow- and\nultraslow-spreading ridges using microseismicity. Geology 47, 1069–1073 (2019).\nParnell-Turner, R. et al. Seismicity trends and detachment fault structure at 13°N, Mid-\nAtlantic Ridge. Geology 49, 320–324 (2021).\nParnell-Turner, R. et al. Oceanic detachment faults generate compression in extension.\nGeology 45, 923–926 (2017).\nWang, Z., Singh, S. C., Prigent, C., Gregory, E. P. M. & Marjanović, M. Deep hydration and\nlithospheric thinning at oceanic transform plate boundaries. Nat. Geosci. 15, 741–746\n(2022).\nYu, Z. et al. Semibrittle seismic deformation in high-temperature mantle mylonite shear zone\nalong the Romanche transform fault. Sci. Adv. 7, eabf3388 (2021).\nYu, Z. & Singh, S. C. Increase of P-wave velocity due to melt in the mantle at the Gakkel\nRidge. Nat. Commun. 969 (2023).\nThe authors report an observation of earthquakes below a mid-oceanic ridge at depths where\na brittle deformation is not expected in the mantle, based on thermo-mechanical models.\nSuch earthquakes are very interesting because their occurrence reveals some still poorly\nunderstood processes in the upper mantle. The authors present a careful study of the\nearthquake hypocenters and of the accuracy of their determinations clearly demonstrated\nthat the observed “anomalous” depths are not an artifact. Then, they consider several\npossible hypotheses about the origin of the observed deep earthquake cluster and based on\ncomparison with tectonic and geological settings argue that that the earthquakes in the hot\nmantle most likely result from the degassing of CO2 in the mantle. This is a very interesting\nresult well supported by observations. The manuscript is well written and I would like to see\nit published in Nature Communications. I just have a couple of suggestions as detailed below.\nThank you very much for your positive comments.\nEarthquakes occurring in the deep crust or uppermost mantle where a brittle deformation is\nnot expected are often observed beneath active volcanoes. This class of seismicity is known\nas deep long period (DLP) volcanic earthquakes (e.g., Shapiro et al., 2017; Wech et al.,\n2020). In my view, the deep seismicity reported in the present manuscript has many\nsimilarities with the volcanic DPL earthquakes and some similar hypotheses have been used\nfor their explanation. In particular, the CO2 degassing has been recently suggested to explain\nthe DLP earthquakes (Melnik, 2020). Therefore, I would suggest to the authors to mention\nthis possible analogy in their manuscript.\nThe model of Melnik et al. (2020) also contain two interesting aspects that might be worth of\nmentioned. First the CO2 solubility and the intensity of degassing depend of the amount of\nH2O in the melt. Second, and most interesting, the CO2 degassing can be more than just a\n“trigger” of earthquakes, as written by the authors in line 194. As shown by Melnik et al., the\ngas bubble expansion and associated pressure variations can be rapid enough (i.e.,\noccurring over times less than 1 sec) to be itself the main force causing the generation of\nseismic waves.\nWe thank the reviewer for providing useful articles. We have included them in the revised\nmanuscript and have some more discussion of the possibility of deep long-period\nearthquakes. See lines 233-240.\nby their spectral content (this is why they are called LP) and by their focal mechanisms\ncontaining significant non-double-couple components (e.g., Ikegaya and Yamamoto, 2021).\nTherefore, if the authors might have some information about the distinct spectral\ncharacteristics of the earthquakes from the deep cluster of out their focal mechanisms,\nthis might be interesting to mention in the paper.\nGood point. We have added the spectral analysis of the deep earthquakes beneath the ridge\nand found that some deep earthquakes lack high-frequency energy (>5 Hz). We suggested\nthat they may be deep long-period events, see Supplementary Fig. 12. However, not all the\nevents are DLP earthquakes and more work about the source process needs to be done in\nthe future.\nReferences:\nShapiro, N., Droznin, D., Droznina, S. et al. Deep and shallow long-period volcanic seismicity linked by\nfluid-pressure transfer. Nature Geosci 10, 442–445 (2017). https://doi.org/10.1038/ngeo2952\nAaron G. Wech et al., Deep long-period earthquakes generated by second boiling beneath Mauna Kea\nvolcano. Science 368, 775-779 (2020). DOI:10.1126/science.aba4798\nMelnik, O., Lyakhovsky, V., Shapiro, N.M. et al. Deep long period volcanic earthquakes generated by\ndegassing of volatile-rich basaltic magmas. Nat Communications 11, 3918\n(2020). https://doi.org/10.1038/s41467-020-17759-4\nTakuma Ikegaya, Mare Yamamoto (2021), Spatio-temporal characteristics and focal mechanisms of deep\nlow-frequency earthquakes beneath the Zao volcano, northeastern Japan, Journal of Volcanology and\nGeothermal Research, 417, 107321"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Although the authors have put a large amount of work into assessing the robustness of their event depth estimates I am still not convinced by them for the following reasons:\n1) In the rebuttal letter the authors state that by using the “modified” Wadati approach the origin time is cancelled out and that we can directly observe/measure the vp/vs ratio. This is only correct if we have a constant vp/vs ratio in the whole model as otherwise the ray path of P and S are different and then of cause the distance terms in the equation provided are cancelling out. I think we do all agree that vp/vs at MAR will not be constant with depth.\n2) In the panels provided in S4 (Wadati plots) we can clearly see that there are problems with quite a lot of the relative travel time measurements in the standard but also in the modified form as most of the outliers have very large positive deviations. Have they been discarded for the location or left in with the hope that the automatic re-weighting of NonLinLoc gets rid of them? What is concerning me even more that both plots show intercepts of -0.39s and +0.5s which according to the theory/formula you use should not be. This clearly shows that the assumption of a homogeneous vp/vs ratio does not work. 0.5s can easily give a depth change of 4-5 km which is actually the value which is of interest for this paper.\n3) The authors use a lot of layers for their 1D model. Looking at the event distribution provided in the rebuttal letter as well most events are part of one depth cluster and only a handful are shallower. To say the least this is a very unfavorable data set to estimate simultaneously event depth and velocities in the overburden. VELEST needs hypocenters at all depth to robustly estimate the minimum 1D model. By using so many layers as you do the problem becomes completely non-linear and depends strongly on the starting model.\nUnfortunately, I cannot offer a solution of what to do and how to estimate robustly the event depth. I think all epicentre positions are robust and if you can find a way to rely more on the epicentral positions this could be a very interesting paper. However, as event depth is so important, I think the current data set will not be sufficient to locate the events with travel times only to the accuracy needed to support your conclusions.", "reviewer_2": "The authors carefully addressed the issues raised in the reviews. In my opinion the paper can be accepted in its present form.", "reviewer_3": "Review of Yu et al. “Deep mantle earthquakes linked to CO2 degassing at the Mid-Atlantic Ridge”\nsubmitted to Nature Communications\nThis study uses seismic data collected via ocean-bottom seismometers to suggest the presence of deep earthquakes (10-20 km) along a section of the Mid-Atlantic Ridge between the Romanche and Chain transform faults. The authors attribute the source of the deep earthquakes to CO2 degassing from melt beneath the ridge. The manuscript is well written. My expertise is within the field of geochemistry, and I therefore defer to the other reviewers for their judgments on the geophysical methods and interpretations presented in this study. In this review, I will focus on the geochemical interpretations laid forth by the authors.\nTo assess the hypothesis that CO2 degassing is the cause of the deep earthquakes, the authors download data from PetDB (https://search.earthchem.org/) for the region, and use incompatible trace element abundances (Ba, Rb, Nb) to estimate the CO2 concentration of the primary melt. The ratio of CO2/Ba and CO2/Rb has been shown to be approximately constant in undegassed MORB magmas (e.g., Saal et al., 2002; Hauri et al. 2017; Le Voyer et al., 2019), and using these ratios to estimate the primary melt CO2 content is a valid approach.\nHowever, this exercise was conducted incorrectly in this study (discussed below) and, moreover, it was not necessary for them to perform these calculations. Extensive research and discussion about the geochemistry, including CO2 content, of basalts and their mantle sources from this area has been published by Le Voyer et al. (2015, 2019) and Schilling et al. (1994, 1995). The authors do not cite Le Voyer et al. (2015) or Schilling et al. (1994, 1995) in the text and report that there is no CO2 data for the samples pulled from their PetDB search (Legend in Figure 5), but CO2 data (and extensive discussion on volatiles) exists for 11 out of 17 of the samples provided in Supplementary Table 6 (and plotted in Figure 5). I am assuming that the authors were unaware of these studies, although they do cite Le Voyer et al. (2015) and Schilling et al., (1995) in Supplementary Table 6, so I’m not sure how incorporation of these studies or the existing CO2 data got missed. I applaud the authors on providing the sample names from their PetDB search, but I must insist that they incorporate the studies associated with the geochemical data into the main text of the manuscript. This oversight is critical to address, as it has implications to the authors’ interpretation of the source of the deep earthquakes. Therefore, I recommend major revisions, but not rejection. I support this work being published, but the authors must first reconcile with the previous geochemical studies conducted in this area before it can be deemed acceptable for publication.\nSchilling et al., (1994, 1995) presents geochemical data (major elements, trace elements, radiogenic isotopes), as well as estimates of mantle potential temperature, crustal thickness, depth of melting, and melt fraction for the Mid-Atlantic Ridge from ~5oN to ~6oS, including the ridge between the Romanche and Chain transform faults. Le Voyer et al., (2015) add volatile measurements (H2O, CO2, S, F, Cl) to this same suite of samples (5oN to 3oS). In fact, much of Le Voyer et al. (2015) is focused on the ridge segment separating the Romanche and Chain transform faults. They report that the samples from the northern end of this ridge segment have elevated trace element patterns and elevated volatile contents relative to their neighbor samples along the ridge. Le Voyer et al. concludes that these samples are produced through low degrees of partial melting of an enriched source material, which is consistent with the conclusions of Schilling et al. (1995). I strongly recommend that the authors review Schilling et al. (1995) and Le Voyer et al. (2015) because the data and discussion provided in these two studies are directly linked to the interpretations that the authors wish to make about CO2 degassing being the source of deep earthquakes in this region.\nLe Voyer et al. (2019) calculate the primary melt CO2 content of global mid-ocean ridge segments (segments as defined by Gale et al., 2013) via CO2/Ba and CO2/Rb systematics in undegassed MORB. They include an estimation for the primary melt CO2 content of segment MARR168 (Mid-Atlantic ridge between 0.202oN and 0.229oS), which is comparable to Yu et al.’s “North” in Figure 5, and MARR 169 (Mid-Atlantic ridge between 0.223oS to 0.643oS), which is comparable to Yu et al.’s “South” in Figure 5. Le Voyer et al. (2019) calculate the primary melt to have a concentration of 5248 ppm CO2 for segment MARR168 and 2060 ppm CO2 for segment MARR169 (see Supplementary Table 4 in Le Voyer et al., 2019). In Yu et al., the authors calculate a primary melt CO2 concentration of >7000 ppm from the same samples and using the same CO2/Ba and CO2/Rb systematics. The discrepancy can be explained by Yu et al. not correcting for fractional crystallization to a composition in equilibrium with Fo90 olivine before extrapolating the Ba and Rb concentrations to a CO2 content. Correcting the geochemical data to be in equilibrium with Fo90 olivine is necessary because, with progressive crystallization, Ba and Rb concentrations will increase in the melt and extrapolation of these concentrations to a CO2 content will therefore not represent the CO2 content of the primary, unfractionated melt. The authors must either repeat their calculations with the compositions corrected to Fo90 olivine, or simply cite the estimations of primary melt CO2 for these segments from Le Voyer et al. (2019).\nOverall, I believe that incorporation of these previous studies on the geochemistry of basalts and mantle sources along the MAR between Romanche and Chain transform faults will strengthen the arguments made by Yu et al. regarding CO2 degassing being the driver of deep earthquakes in this region. The authors must at least give credit to those who have reported on CO2 and mantle melting in this region previously for this manuscript to be considered acceptable.\nA question does arise, however. Hauri et al., (2019) report that a “normal” primary MORB melt has 621 ppm CO2, based on calculations from the dataset of Le Voyer et al. (2019). The primary melt CO2 content along the ridge from Romanche to Chain transform faults is significantly higher at both the north and south segments. Why doesn’t the south segment also display these deep earthquakes if it also has elevated CO2 relative to “normal” MORB? Some discussion of this may be warranted to affirm CO2 degassing as the source of the deep earthquakes and perhaps be an insightful addition to the manuscript.\nDetailed remarks:\nAbstract\nLine 22: “centers” misspelled as “centres”\nLines 26-28: The calculations used to obtain a primary melt concentration of >0.7 wt% CO2 do not account for crystal fractionation and is thus not representative of the primary melt. Additionally, this sentence reads as though this is a new contribution from this study, whereas it is not. Adjust accordingly following comments above.\nLine 30: Suggest delete “in the mantle” from the end of this sentence.\nResults\nLines 110-112: Should be <10 km because that’s what thermal models predict? Please clarify for reader.\nDiscussion\nLine 193: Recommend deleting the word “(dry)” because CO2 solubility is strongly dependent on pressure in wet melts as well.\nLines 200-203: It is probably prudent to say somewhere in here that estimating CO2 concentrations from trace element abundances relies on the assumption that the trace elements are reflective of the mantle source and have not been affected by secondary processes. This is important to say here because some of the samples plotted as “MAR south” in Figure 5 have been affected by secondary processes (see Le Voyer et al., 2015).\nLines 203-233: Strongly recommend complete rewrite incorporating previous works on the volatile contents of samples, interpretations made about mantle melting and mantle heterogeneity, and estimations of primary melt CO2 concentrations for this section of the ridge.\nLines 203-206: I don’t think K2O/TiO2 needs to be talked about here. It’s not necessary for the discussion.\nLine 214: I disagree that all samples from the southern MAR segment (0.2oS to 0.7oS) show normal values. The southern segment still has elevated trace elements relative to normal MORB (see Le Voyer et al., 2015).\nLines 227-230: Right, you can actually pull out the primary melt CO2 estimation for each specific ridge segment (including the segments relevant to this study) from Supplementary Table 4 in Le Voyer et al. (2019).\nLines 230-233: Recommend deleting sentence starting with “The degassed CO2-rich fluid that would migrate…..”. This is not relevant to this manuscript.\nLines 248-250: This sentence states “Our evidence for a large amount of CO2 in the melt”. Please rephrase because this region having elevated CO2 is not your evidence or a contribution of this manuscript.\nLine 250: Could the text be more specific about what “a large amount of volatiles” refers to? Could you give a concentration limit on what “large” means here?\nLines 252-254: I am not convinced that seismicity at 10-20 km depth means that gabbroic rocks are forming in the mantle. This statement seems completely unfounded. I recommend deletion of this sentence.\nCO2 estimation from mid-ocean ridge basalts (MORB)\nLines 402-411: These calculations are not correct because they do not account for fractional crystallization. Recommend deleting this entire part and referring to the previous works estimating primary melt CO2 concentration. If the authors wish to redo their calculations, they must first correct the compositions to be in equilibrium with Fo90 olivine.\nFigure 5:\n•Having the map be in a different orientation than all the other maps in the manuscript is very confusing for the reader. Please consider having North pointing in the same direction as the other maps in this manuscript.\n•I suggest removing the K2O/TiO2 vs. CO2 and Nb vs. CO2 plots. They are a bit overkill to have in addition to the Ba and Rb plots.\n•Must replot the data titled “MAR north…” and “MAR south…” because CO2 data does exist for most of these samples (see Le Voyer et al., 2015). Additionally, this data needs to be corrected for crystal fractionation before being plotted.\n•Four samples are missing from the Ba vs. CO2 plot. These samples have Ba data, so I’m assuming that was a mistake. If it was not a mistake, please comment in the caption why these data points were excluded from the plot.\n•The original sources of the “MAR” geochemical data must be cited, rather than just saying “PetDB”. These studies can be cited in the figure caption.\n•The melt inclusions in this figure should be cited differently. My understanding of what is being plotted as “melt inclusions” here is that it includes melt inclusions from Figures 4a and b of Le Voyer et al. (2019). The citation for this data needs to either be stated as “summarized in Le Voyer et al. (2019) and Hauri et al. (2018)” or their original data sources should be cited, which can be found in the caption of Figure 4 of Le Voyer et al. (2019) but also includes Saal et al., (2002), Le Voyer et al. (2017) and Hauri et al. (2018). Please confirm and cite appropriately.\n•An alternative suggestion to this figure would be to create a new figure that shows the range of primary melt CO2 estimations for the entire Mid-Atlantic Ridge, highlighting the primary melt CO2 concentration of this segment for comparison.\nAdditional comments:\n•Overall comment about figures including bathymetry: A rainbow colored pallet is not color-blind friendly. Consider changing the pallet for the bathymetry to a pallet that is color-blind friendly.\nReferences to accompany this review:\nGale, A., Dalton, C. A., Langmuir, C. H., Su, Y., & Schilling, J. G. (2013). The mean composition of ocean ridge basalts. Geochemistry, Geophysics, Geosystems, 14(3), 489-518.\nHauri, E. H., Maclennan, J., McKenzie, D., Gronvold, K., Oskarsson, N., & Shimizu, N. (2018). CO2 content beneath northern Iceland and the variability of mantle carbon. Geology, 46(1), 55-58.\nHauri, E. H., Cottrell, E., Kelley, K. A., Tucker, J. M., Shimizu, K., Le Voyer, M., ... & Saal, A. E. (2019). Carbon in the convecting mantle. Deep carbon: past to present, pp. 237 - 275. DOI: https://doi.org/10.1017/9781108677950\nLe Voyer, M., Cottrell, E., Kelley, K. A., Brounce, M., & Hauri, E. H. (2015). The effect of primary versus secondary processes on the volatile content of MORB glasses: An example from the equatorial Mid-Atlantic Ridge (5° N–3° S). Journal of Geophysical Research: Solid Earth, 120(1), 125-144.\nLe Voyer, M., Kelley, K. A., Cottrell, E., & Hauri, E. H. (2017). Heterogeneity in mantle carbon content from CO2-undersaturated basalts. Nature Communications, 8(1), 14062.\nLe Voyer, M., Hauri, E. H., Cottrell, E., Kelley, K. A., Salters, V. J., Langmuir, C. H., ... & Füri, E. (2019). Carbon fluxes and primary magma CO2 contents along the global mid-ocean ridge system. Geochemistry, Geophysics, Geosystems, 20(3), 1387-1424.\nSaal, A. E., Hauri, E. H., Langmuir, C. H., & Perfit, M. R. (2002). Vapour undersaturation in primitive mid-ocean-ridge basalt and the volatile content of Earth's upper mantle. Nature, 419(6906), 451-455.\nSchilling, J. G., Hanan, B. B., McCully, B., Kingsley, R. H., & Fontignie, D. (1994). Influence of the Sierra Leone mantle plume on the equatorial Mid-Atlantic Ridge: A Nd-Sr-Pb isotopic study. Journal of Geophysical Research: Solid Earth, 99(B6), 12005-12028.\nSchilling, J. G., Ruppel, C., Davis, A. N., McCully, B., Tighe, S. A., Kingsley, R. H., & Lin, J. (1995). Thermal structure of the mantle beneath the equatorial Mid-Atlantic Ridge: Inferences from the spatial variation of dredged basalt glass compositions. Journal of Geophysical Research: Solid Earth, 100(B6), 10057-10076.", "reviewer_author_response": "Responses to the review comments\n(The black words show the review comments; the blue words show our responses)\n--------------------------------------------------------------------------------------------------------\nReviewer: 1\nReviewer #1 (Remarks to the Author):\nAlthough the authors have put a large amount of work into assessing the robustness of their\nevent depth estimates I am still not convinced by them for the following reasons:\n1) In the rebuttal letter the authors state that by using the “modified” Wadati approach the\norigin time is cancelled out and that we can directly observe/measure the vp/vs ratio. This is\nonly correct if we have a constant vp/vs ratio in the whole model as otherwise the ray path of\nP and S are different and then of cause the distance terms in the equation provided are\ncancelling out. I think we do all agree that vp/vs at MAR will not be constant with depth.\nWe agree that the Vp/Vs ratio in our study region cannot be constant, but upon testing the\noptimum Vp/Vs ratio selection (see Supplementary Fig. S5), we found that a ratio of 1.7 can\nresult in the lowest RMS values. The Vp/Vs ratio between 1.7 to 1.9 can yield better results\nthan the others, indicating that the study region has a normal Vp/Vs ratio. In addition, our\nprevious seismic tomography results (Yu et al., 2023 EPSL) showed that the Vp/Vs ratio\nalong the present Mid-Atlantic Ridge (MAR) is not very variable, lying around 1.7 (see Figure\nR1 below). To further demonstrate this, we displayed the earthquake depths using the\ndifferent Vp/Vs ratios (1.7-1.9), all of which resulted in deep events beneath the MAR (see\nFigure R2 below). Thus, we believe that our choice of Vp/Vs value is appropriate.\nFigure R1. Vp/Vs results along the MAR from seismic tomography results by Yu et al. (2023) EPSL.\n(a) Bathymetric map shows earthquake locations. (b) The blue diamonds and red squares show Vp/Vs\nratios along the non-transform continuity (NTD) and MAR, respectively.\ndots indicate earthquakes with a depth uncertainty of ≤5 km and 5-10 km, respectively. Coloured circles\nindicate the location results using the different Vp/Vs ratios shown in (b). The earthquake depths along the\ntransect a-a′ are shown in (b). The black star indicates an inactive hydrothermal mound observed during\nthe submersible dive. (b) The focal depth distribution of earthquakes along the transect (aa′). Grey lines\nmark the uncertainties. Coloured circles show the location results using different Vp/Vs ratios from 1.7 to\n1.9.\n2) In the panels provided in S4 (Wadati plots) we can clearly see that there are problems with\nquite a lot of the relative travel time measurements in the standard but also in the modified\nform as most of the outliers have very large positive deviations. Have they been discarded\nfor the location or left in with the hope that the automatic re-weighting of NonLinLoc gets\nrid of them? What is concerning me even more is that both plots show intercepts of -0.39s and\n+0.5s which according to the theory/formula you use should not be. This clearly shows that\nthe assumption of a homogeneous vp/vs ratio does not work. 0.5s can easily give a depth\nchange of 4-5 km which is the value which is of interest for this paper.\nThank you for your concern. For the large positive deviations, yes, they will be reweighted\nand removed automatically during the NonLinLoc calculation. We replotted the Wadati\ndiagram using the travels with a weighting factor greater than 0.6. The results show that many\nlarge deviations were removed, and we can obtain a more linear and narrow slope (see new\nit is unlikely that the P- and S-travels will be perfectly aligned along a line. As we answered\nin the last comment, the Vp/Vs ratios in this study range from 1.7 to 1.9, without much\nvariation. In addition, we wish to clarify that all we have done in this section is try to get a\nbetter 1-D starting velocity model rather than obtaining a true velocity model.\n3) The authors use a lot of layers for their 1D model. Looking at the event distribution provided\nin the rebuttal letter as well most events are part of one depth cluster and only a handful are\nshallower. To say the least, this is a very unfavorable data set to estimate simultaneously\nevent depth and velocities in the overburden. VELEST needs hypocenters at all depth to\nrobustly estimate the minimum 1D model. By using so many layers as you do the problem\nbecomes completely non-linear and depends strongly on the starting model.\nIt should be noted first that we used a continuous velocity model to locate earthquakes with\nthe NonLinloc program (see Supplementary Fig. S2). Second, we fully understand your\nconcerns about the number of layers for the calculation using the VELEST program. We have\nconducted a new test to reduce the layer number of the model; see Figure R3 below. The\nresults show that the earthquake locations did not change much. Thus, we believe that our\nsetting of the starting model is reliable.\nFigure R3. Earthquake depths along the MAR. (a) Bathymetric map and located events.\nThe coloured circles indicate the results using different velocity models shown in (b). The\nother labelling is the same as that shown in Figure R2. (b) The focal depth distribution of\nearthquakes along the transect (aa′) in (a). The velocity models are shown on the right,\n3\nincluding one model (black line) with the decreased layers in the VELEST program.\nUnfortunately, I cannot offer a solution of what to do and how to estimate robustly the event\ndepth. I think all epicentre positions are robust and if you can find a way to rely more on the\nepicentral positions this could be a very interesting paper. However, as event depth is so\nimportant, I think the current data set will not be sufficient to locate the events with travel\ntimes only to the accuracy needed to support your conclusions.\nWe are sorry that our previous revision did not satisfy your requirements. We think that we\nhave now addressed your concerns with the latest revision.\nReference\n1. Yu, Z., Singh, S. C. & Maia, M. Evidence for low Vp/Vs ratios along the eastern\nRomanche ridge-transform intersection in the equatorial Atlantic Ocean. Earth and Planetary\nScience Letters 621, 118380 (2023).\nReviewer #2 (Remarks to the Author):\nThe authors carefully addressed the issues raised in the reviews. In my opinion the paper\ncan be accepted in its present form.\nThank you very much for your positive comments.\nReviewer: 3\nReviewer #3 (Remarks to the Author):\nReview of Yu et al. “Deep mantle earthquakes linked to CO2 degassing at the Mid-Atlantic\nRidge” submitted to Nature Communications\nThis study uses seismic data collected via ocean-bottom seismometers to suggest the\npresence of deep earthquakes (10-20 km) along a section of the Mid-Atlantic Ridge between\nthe Romanche and Chain transform faults. The authors attribute the source of the deep\nearthquakes to CO2 degassing from melt beneath the ridge. The manuscript is well written.\nfor their judgments on the geophysical methods and interpretations presented in this study.\nIn this review, I will focus on the geochemical interpretations laid forth by the authors.\nThank you very much for your positive comments.\nTo assess the hypothesis that CO2 degassing is the cause of the deep earthquakes, the\nauthors download data from PetDB (https://search.earthchem.org/) for the region, and use\nincompatible trace element abundances (Ba, Rb, Nb) to estimate the CO2 concentration of\nthe primary melt. The ratio of CO2/Ba and CO2/Rb has been shown to be approximately\nconstant in undegassed MORB magmas (e.g., Saal et al., 2002; Hauri et al. 2017; Le Voyer\net al., 2019), and using these ratios to estimate the primary melt CO2 content is a valid\napproach.\nHowever, this exercise was conducted incorrectly in this study (discussed below) and,\nmoreover, it was not necessary for them to perform these calculations. Extensive research\nand discussion about the geochemistry, including CO2 content, of basalts and their mantle\nsources from this area has been published by Le Voyer et al. (2015, 2019) and Schilling et\nal. (1994, 1995). The authors do not cite Le Voyer et al. (2015) or Schilling et al. (1994, 1995)\nin the text and report that there is no CO2 data for the samples pulled from their PetDB search\n(Legend in Figure 5), but CO2 data (and extensive discussion on volatiles) exists for 11 out\nof 17 of the samples provided in Supplementary Table 6 (and plotted in Figure 5). I am\nassuming that the authors were unaware of these studies, although they do cite Le Voyer et\nal. (2015) and Schilling et al., (1995) in Supplementary Table 6, so I’m not sure how\nincorporation of these studies or the existing CO2 data got missed. I applaud the authors on\nproviding the sample names from their PetDB search, but I must insist that they incorporate\nthe studies associated with the geochemical data into the main text of the manuscript. This\noversight is critical to address, as it has implications to the authors’ interpretation of the\nsource of the deep earthquakes. Therefore, I recommend major revisions, but not rejection.\nI support this work being published, but the authors must first reconcile with the previous\ngeochemical studies conducted in this area before it can be deemed acceptable for\npublication.\nWe have now corrected this oversight and included the articles by Le Voyer et al. (2015) and\nSchilling et al. (1994, 1995) in the discussion; see lines 220-222. We also added the missing\nCO data in the new Figure 5. The geochemistry part of the discussion went through a\n2\ncomprehensive overhaul. See lines 208-243.\nradiogenic isotopes), as well as estimates of mantle potential temperature, crustal thickness,\ndepth of melting, and melt fraction for the Mid-Atlantic Ridge from ~5oN to ~6oS, including\nthe ridge between the Romanche and Chain transform faults. Le Voyer et al., (2015) add\nvolatile measurements (H2O, CO2, S, F, Cl) to this same suite of samples (5oN to 3oS). In\nfact, much of Le Voyer et al. (2015) is focused on the ridge segment separating the\nRomanche and Chain transform faults. They report that the samples from the northern end\nof this ridge segment have elevated trace element patterns and elevated volatile contents\nrelative to their neighbor samples along the ridge. Le Voyer et al. concludes that these\nsamples are produced through low degrees of partial melting of an enriched source material,\nwhich is consistent with the conclusions of Schilling et al. (1995). I strongly recommend that\nthe authors review Schilling et al. (1995) and Le Voyer et al. (2015) because the data and\ndiscussion provided in these two studies are directly linked to the interpretations that the\nauthors wish to make about CO2 degassing being the source of deep earthquakes in this\nregion.\nWe made some alterations to our discussion to include the conclusions of Le Voyer et al.\n(2015), see lines 228-231. We only selected the data within our OBS network for this study,\nbut we fully agree with the reviewer that the conclusions of Le Voyer et al. (2015) are\nsignificant and very relevant to us.\nLe Voyer et al. (2019) calculate the primary melt CO2 content of global mid-ocean ridge\nsegments (segments as defined by Gale et al., 2013) via CO2/Ba and CO2/Rb systematics\nin undegassed MORB. They include an estimation for the primary melt CO2 content of\nsegment MARR168 (Mid-Atlantic ridge between 0.202oN and 0.229oS), which is comparable\nto Yu et al.’s “North” in Figure 5, and MARR 169 (Mid-Atlantic ridge between 0.223oS to\n0.643oS), which is comparable to Yu et al.’s “South” in Figure 5. Le Voyer et al. (2019)\ncalculate the primary melt to have a concentration of 5248 ppm CO2 for segment MARR168\nand 2060 ppm CO2 for segment MARR169 (see Supplementary Table 4 in Le Voyer et al.,\n2019). In Yu et al., the authors calculate a primary melt CO2 concentration of >7000 ppm\nfrom the same samples and using the same CO2/Ba and CO2/Rb systematics. The\ndiscrepancy can be explained by Yu et al. not correcting for fractional crystallization to a\ncomposition in equilibrium with Fo90 olivine before extrapolating the Ba and Rb\nconcentrations to a CO2 content. Correcting the geochemical data to be in equilibrium with\nFo90 olivine is necessary because, with progressive crystallization, Ba and Rb\n6\nconcentrations will increase in the melt and extrapolation of these concentrations to a CO2\ncontent will therefore not represent the CO2 content of the primary, unfractionated melt. The\nauthors must either repeat their calculations with the compositions corrected to Fo90 olivine,\nor simply cite the estimations of primary melt CO2 for these segments from Le Voyer et al.\n(2019).\nThank you for the nice suggestion. We have recalculated CO concentration in primary melts\n2\nusing Ba and Rb corrected for fractional crystallisation (equilibrated to Fo ). We changed the\nresponse to the reviewer's later suggestion. We also included in the discussion a sentence\nto give credit to Schilling et al. (1995) and Le Voyer et al. (2015) for the interpretation of\nelevated primary CO in RC3 as a result of low melting degrees of an enriched mantle source;\n2\nsee lines 228-231.\nWe feel it is important to note that the ridge segments of this study that we named RC2 and\nRC3 (Romanche-Chain segment numbers 2 and 3) are indeed the same as the ones named\nMARR168 and MARR169 in Gale et al. (2013) and Le Voyer et al. (2019). However, we\ndecided against using the average CO value calculated by Le Voyer et al. (2019) for the\nsamples from CON2806-007-001 and 13-12 49A, both of which are significantly more\nenriched and coming from other stations along the same segment. Thus, we recalculated the\nCO concentration as you suggested.\nconcentration. The reviewer is interested in the causes of the CO enrichment and therefore\n2\nfocuses on the primary CO content in melts in equilibrium with their mantle source (Le Voyer\n2\net al., 2015 and 2019), while we are interested in the effects of the high CO content in the\n2\npre-eruptive melts (which underwent melt storage and fractional crystallisation) as a possible\nsource of the deep earthquakes. Before calculating the CO solubility, we added a sentence\n2\nstating that we are referring to pre-eruptive melts as magmas that underwent various\namounts of fractional crystallisation; see lines 232-241.\nOverall, I believe that incorporation of these previous studies on the geochemistry of basalts\nand mantle sources along the MAR between Romanche and Chain transform faults will\nstrengthen the arguments made by Yu et al. regarding CO2 degassing being the driver of\ndeep earthquakes in this region. The authors must at least give credit to those who have\nreported on CO2 and mantle melting in this region previously for this manuscript to be\n7\nconsidered acceptable.\nWe feel that the new version of the discussion gives better credit to these studies. We also\nmade some changes to how we named the two groups of samples, as we think it was the\ncause of some misunderstanding with the reviewer: the names “MAR south of 0.2” and “MAR\nnorth of 0.2” were probably too vague and were suggesting a boundary within the entire MAR\nat 0.2º N. We are limited by the bounds of our OBS network, so we renamed those groups:\n“RC2 basalts” and “RC3 basalts”; see Figure 5.\nA question does arise, however. Hauri et al., (2019) report that a “normal” primary MORB\nmelt has 621 ppm CO2, based on calculations from the dataset of Le Voyer et al. (2019). The\nprimary melt CO2 content along the ridge from Romanche to Chain transform faults is\nsignificantly higher at both the north and south segments. Why doesn’t the south segment\nalso display these deep earthquakes if it also has elevated CO2 relative to “normal” MORB?\nSome discussion of this may be warranted to affirm CO2 degassing as the source of the deep\nearthquakes and perhaps be an insightful addition to the manuscript.\nYes, there may be some deep events in the southern MAR close to the Chain transform fault,\nbut unfortunately, these earthquakes are outside of our seismic observation network. We are\nunable to verify these suspicions.\nDetailed remarks:\nAbstract\nLine 22: “centers” misspelled as “centres”\nWe used the British English spelling. We have standardized all these similar terms in the text.\nLines 26-28: The calculations used to obtain a primary melt concentration of >0.7 wt% CO2\ndo not account for crystal fractionation and is thus not representative of the primary melt.\nAdditionally, this sentence reads as though this is a new contribution from this study, whereas\nit is not. Adjust accordingly following comments above.\nWe have rewritten this sentence and used the new CO data with crystal fractionation\n2\ncorrections. See lines 22-24.\nLine 30: Suggest delete “in the mantle” from the end of this sentence.\nResults\nLines 110-112: Should be <10 km because that’s what thermal models predict? Please clarify\nfor reader.\nDone. We have clarified this statement. See lines 110-112.\nDiscussion\nLine 193: Recommend deleting the word “(dry)” because CO2 solubility is strongly dependent\non pressure in wet melts as well.\nDone. We have removed it. See line 202.\nLines 200-203: It is probably prudent to say somewhere in here that estimating CO2\nconcentrations from trace element abundances relies on the assumption that the trace\nelements are reflective of the mantle source and have not been affected by secondary\nprocesses. This is important to say here because some of the samples plotted as “MAR\nsouth” in Figure 5 have been affected by secondary processes (see Le Voyer et al., 2015).\nDone. We have added this statement to the discussion. See lines 217-220.\nLines 203-233: Strongly recommend complete rewrite incorporating previous works on the\nvolatile contents of samples, interpretations made about mantle melting and mantle\nheterogeneity, and estimations of primary melt CO2 concentrations for this section of the\nridge.\nWe have rewritten this section, and correcting for fractional crystallisation was also addressed.\nSee lines 208-231.\nLines 203-206: I don’t think K2O/TiO2 needs to be talked about here. It’s not necessary for\nthe discussion.\nWe have removed the statements and the K2O/TiO2 plotting in Figure 5.\nLine 214: I disagree that all samples from the southern MAR segment (0.2oS to 0.7oS) show\nnormal values. The southern segment still has elevated trace elements relative to normal\nMORB (see Le Voyer et al., 2015).\nLines 227-230: Right, you can actually pull out the primary melt CO2 estimation for each\nspecific ridge segment (including the segments relevant to this study) from Supplementary\nTable 4 in Le Voyer et al. (2019).\nWe have added a Supplementary Fig. S13 to show the CO abundance in the equatorial\nLines 230-233: Recommend deleting sentence starting with “The degassed CO2-rich fluid\nthat would migrate…..”. This is not relevant to this manuscript.\nDone.\nLines 248-250: This sentence states “Our evidence for a large amount of CO2 in the melt”.\nPlease rephrase because this region having elevated CO2 is not your evidence or a\ncontribution of this manuscript.\nThis part of the text was rewritten to give better credit to the relevant previous studies. See\nlines 268-270.\nLine 250: Could the text be more specific about what “a large amount of volatiles” refers to?\nCould you give a concentration limit on what “large” means here?\nWe are sorry that we cannot provide such a concentration limit, which is beyond the aim of\nthis manuscript.\nLines 252-254: I am not convinced that seismicity at 10-20 km depth means that gabbroic\nrocks are forming in the mantle. This statement seems completely unfounded. I recommend\ndeletion of this sentence.\nDone. We have removed this statement. See lines 272-274.\nCO2 estimation from mid-ocean ridge basalts (MORB)\nLines 402-411: These calculations are not correct because they do not account for fractional\ncrystallization. Recommend deleting this entire part and referring to the previous works\nestimating primary melt CO2 concentration. If the authors wish to redo their calculations, they\nmust first correct the compositions to be in equilibrium with Fo90 olivine.\nNow, we have recalculated the CO concentration with fractional crystallisation corrections.\n2\nFigure 5:\n•Having the map be in a different orientation than all the other maps in the manuscript is very\nconfusing for the reader. Please consider having North pointing in the same direction as the\nother maps in this manuscript.\nWe have replotted the bathymetry map as you suggested. See Figure 5a.\n•I suggest removing the K2O/TiO2 vs. CO2 and Nb vs. CO2 plots. They are a bit overkill to\nhave in addition to the Ba and Rb plots.\nWe have removed the K2O/TiO2 vs. CO2 and Nb vs. CO2 plots as you suggested.\n•Must replot the data titled “MAR north…” and “MAR south…” because CO2 data does exist\nfor most of these samples (see Le Voyer et al., 2015). Additionally, this data needs to be\ncorrected for crystal fractionation before being plotted.\nThe new version of this figure now includes all CO values from Le Voyer et al. (2015) and\n2\nwe are correcting fractional crystallisation for Ba and Rb to an olivine Fo . See Figure 5b\n90\nand 5c.\n•Four samples are missing from the Ba vs. CO2 plot. These samples have Ba data, so I’m\nassuming that was a mistake. If it was not a mistake, please comment in the caption why\nthese data points were excluded from the plot.\nLooking at the references cited and carefully digging through petDB data, we have not found\nany additional samples in this area for the Ba/CO plot. In Le Voyer et al. (2015), CON2806-\n2\n010 and CON2806-013 are listed as sampled along what they call segment “8” (which could\nbe relevant for us), but those samples are collected further south, outside the bounds of our\nOBS network, and would not plot on our maps.\n•The original sources of the “MAR” geochemical data must be cited, rather than just saying\n“PetDB”. These studies can be cited in the figure caption.\nAgree and changes have been made. See figure legend in Figure 5.\n11\n•The melt inclusions in this figure should be cited differently. My understanding of what is\nbeing plotted as “melt inclusions” here is that it includes melt inclusions from Figures 4a and\nb of Le Voyer et al. (2019). The citation for this data needs to either be stated as “summarized\nin Le Voyer et al. (2019) and Hauri et al. (2018)” or their original data sources should be cited,\nwhich can be found in the caption of Figure 4 of Le Voyer et al. (2019) but also includes Saal\net al., (2002), Le Voyer et al. (2017) and Hauri et al. (2018). Please confirm and cite\nappropriately.\nWe are now citing them together in Figure 5 and the caption.\n•An alternative suggestion to this figure would be to create a new figure that shows the range\nof primary melt CO2 estimations for the entire Mid-Atlantic Ridge, highlighting the primary\nmelt CO2 concentration of this segment for comparison.\nWe plotted the calculated CO values for the entire MAR; see Figure R4 below. However, we\n2\nfeel that the scale is orders of magnitude larger than our studied area, thus we include it as\nSupplementary Material (Fig. S13).\nFigure R4. CO contents in the primary magma along the whole Mid-Atlantic Ridge\n2\nsegments. Segment-averaged CO content is extracted from Le Voyer et al. (2019), and the\n2\nsegment number (1-255) is shown on the top. The inset histogram shows the distribution of\nthe primary melt CO contents. The orange belt shows the CO contents along the MAR\n2 2\nsegments between the Romanche and Chain transform faults.\nAdditional comments:\n•Overall comment about figures including bathymetry: A rainbow colored pallet is not color-\n12\nblind friendly. Consider changing the pallet for the bathymetry to a pallet that is color-\nblind friendly.\nWe are now using a more friendly colour scale (viridis) to show the bathymetry in Figures 1-\n3 and 5.\nReferences to accompany this review:\nGale, A., Dalton, C. A., Langmuir, C. H., Su, Y., & Schilling, J. G. (2013). The mean\ncomposition of ocean ridge basalts. Geochemistry, Geophysics, Geosystems, 14(3), 489-\n518.\nHauri, E. H., Maclennan, J., McKenzie, D., Gronvold, K., Oskarsson, N., & Shimizu, N. (2018).\nCO2 content beneath northern Iceland and the variability of mantle carbon. Geology, 46(1),\n55-58.\nHauri, E. H., Cottrell, E., Kelley, K. A., Tucker, J. M., Shimizu, K., Le Voyer, M., ... & Saal, A.\nE. (2019). Carbon in the convecting mantle. Deep carbon: past to present, pp. 237 - 275.\nDOI: https://doi.org/10.1017/9781108677950\nLe Voyer, M., Cottrell, E., Kelley, K. A., Brounce, M., & Hauri, E. H. (2015). The effect of\nprimary versus secondary processes on the volatile content of MORB glasses: An example\nfrom the equatorial Mid‐Atlantic Ridge (5° N–3° S). Journal of Geophysical Research: Solid\nEarth, 120(1), 125-144.\nLe Voyer, M., Kelley, K. A., Cottrell, E., & Hauri, E. H. (2017). Heterogeneity in mantle carbon\ncontent from CO2-undersaturated basalts. Nature Communications, 8(1), 14062.\nLe Voyer, M., Hauri, E. H., Cottrell, E., Kelley, K. A., Salters, V. J., Langmuir, C. H., ... & Füri,\nE. (2019). Carbon fluxes and primary magma CO2 contents along the global mid‐ocean\nridge system. Geochemistry, Geophysics, Geosystems, 20(3), 1387-1424.\nSaal, A. E., Hauri, E. H., Langmuir, C. H., & Perfit, M. R. (2002). Vapour undersaturation in\nprimitive mid-ocean-ridge basalt and the volatile content of Earth's upper mantle. Nature,\n419(6906), 451-455.\nSchilling, J. G., Hanan, B. B., McCully, B., Kingsley, R. H., & Fontignie, D. (1994). Influence\nof the Sierra Leone mantle plume on the equatorial Mid‐Atlantic Ridge: A Nd‐Sr‐Pb\nisotopic study. Journal of Geophysical Research: Solid Earth, 99(B6), 12005-12028.\n(1995). Thermal structure of the mantle beneath the equatorial Mid‐Atlantic Ridge:\nInferences from the spatial variation of dredged basalt glass compositions. Journal of\nGeophysical Research: Solid Earth, 100(B6), 10057-10076.The authors carefully addressed\nthe issues raised in the reviews. In my opinion the paper can be accepted in its present form.\nThank you very much for providing the useful references.\n14"}, "author_response": ""}, {"round": 3, "reviewer_comments": {"reviewer_1": "This revised manuscript has improved significantly from the original version, especially with regards to the geochemical interpretations and discussion. The authors have addressed by concerns sufficiently and I recommend acceptance of the revised manuscript."}, "author_response": "Responses to the review comments\n(The black words show the review comments; the blue words show our responses)\n--------------------------------------------------------------------------------------------------------\nReviewer: 3\nReviewer #1 (Remarks to the Author):\nThis revised manuscript has improved significantly from the original version, especially with\nregards to the geochemical interpretations and discussion. The authors have addressed by\nconcerns sufficiently and I recommend acceptance of the revised manuscript.\nThank you very much for your positive comments.\n1"}]

Decision letter