Metamorphism of Venus as driver of crustal thickness and recycling

Domain: earth_science
Published: 2025-03-25
Editorial outcome: minor_revision

Abstract
The composition and thickness of the venusian crust and their dependence on thermal gradients and geodynamic setting are not well constrained. Here, we use metamorphic phase transitions and the onset of melting to determine the maximum crustal thickness of basaltic plains in different tectonic settings. Crustal thickness is limited to ~40 km in a stagnant lid regime with a low thermal gradient of 5 °C/km due to density overturn and delamination. In contrast, the maximum crustal thickness in a mobile lid regime with a high thermal gradient of 25 °C/km is restricted to ~20 km due to the onset of crustal melting. The thickest the crust can be is ~65 km for a basaltic crust with a thermal gradient of 10 °C/km. Our models show that a venusian basaltic crust cannot be thicker than 20–65 km without either causing delamination and crustal recycling or melting and producing volcanic eruptions.

Introduction
Venus is unique in our Solar System; its planetary surface is cloaked by omnipresent clouds and a thick ~92 bar atmosphere, which leads to current day surface temperatures exceeding 460 °C1. Observations of Venus’ surface reveal voluminous volcanic plains, thought to be emplaced within the last billion years2,3, with ongoing volcanic activity (inferred from observed surface changes4, infrared, microwave, and thermal emission data5,6, and weathering rates7).

Since surface rocks on Venus are exposed to conditions comparable to contact metamorphism on Earth, the deeper venusian crust is expected to experience phase transitions forming high-grade metamorphic assemblages until partial melting is reached. Metamorphism of the crust significantly changes the geophysical properties of rocks including density and porosity8,9, which in turn has major implications for crustal density, thickness, geodynamics, crustal recycling mechanisms, and the global tectonic state of Venus10–12.

Metamorphic reactions are strongly dependent on the composition of the crust, including its volatile content, and thermal gradients. Direct bulk rock measurements (with substantial error bars) from the Venera and Vega landers indicate that both tholeiitic and alkalic basalt may be present on the surface of Venus e.g., refs. 13–15, and emissivity and radar measurements suggest that the bulk surface of Venus is basaltic in nature16–18. The volatile content in the venusian interior is unknown due to a lack of direct measurements. The high deuterium to hydrogen ratio19 suggests that early Venus had water levels comparable to those on Earth20 but has lost a significant amount of water after its formation; although, there is substantial uncertainty about the timing and mechanism of this process21–27. It is likely that the interior of Venus has retained a small fraction of water and carbon dioxide (and potentially other volatiles), although the initial volatile content will have been depleted due to melting and degassing10 and may be relatively small (ppm range). Furthermore, studies on fault strength and elastic thickness28,29 suggest that the present-day venusian lithosphere is predominantly dry. Estimates of the global average heat flow30–33 are in the range of 10–30 mW/m2, corresponding to thermal gradients of ~5–15 °C/km although regions of higher heat flow (>50 mW/m2) are possible, as has been suggested for coronae, rifts, steep-sided domes, and crustal plateaus11,12,34–36.

The thickness of the venusian crust is also not well constrained and estimates for Venus’ average crustal thickness, based on topography and gravity models, range from 8 to 25 km36–38 with a possible maximum thickness of ~90 km39. Crustal estimates derived from gravity models are particularly sensitive to crustal density40. Both phase transitions and partial melting have significant influence on the thickness of the crust. For example, the transition of basalt to granulite or eclogite mineral assemblages causes a significant increase in density, which limits the topography supported by isostasy41,42. When the crustal root becomes denser than the surrounding mantle, it triggers lithospheric delamination43–46 which is the detachment of the lowermost crust. This process has been suggested as a driving force for gravitational instabilities and magmatism at the crust-mantle boundary in the venusian interior10,47,48, particularly for coronae49,50, and tesserae51. However, phase equilibria modelling for crustal compositions on Earth has demonstrated that the crustal density significantly varies with pressure, temperature (i.e., the thermal gradient, which is interdependent with the tectonic state, the lithospheric thickness, and heat flux) and composition52. Delamination may therefore not always occur at the phase boundary and the assumption of a linear increase in density from gabbro to eclogite e.g., ref. 41 is an inadequate approach. A detailed study of the mineralogical, and associated density changes, expected in the venusian crust is therefore critical to constrain potential crustal thicknesses, as well as crustal recycling via delamination and/or melting.

In this study, we use a phase equilibria modeling approach to calculate the effects of pressure and temperature on the mineralogy for dry basalt and alkali basalt compositions, along with a peridotite mantle composition, to determine the conditions required for melting of the crust and associated density changes of the crust and mantle. Phase equilibria calculations have the advantage in that stable mineral assemblages and their corresponding rock densities reflect gradual changes with depth into the planetary interior, instead of an average, because they are calculated as a function of pressure, temperature, and composition. We then compare the calculated crust and mantle densities to investigate the conditions for potential density inversions (where the crust becomes denser than the underlying mantle). Finally, we compare our results with estimates of crustal thicknesses to predict where either crustal melting or crustal recycling through delamination is likely to occur on Venus. According to our petrological constraints, the basaltic venusian crust cannot be thicker than 20–65 km and is strongly dependent on thermal gradient. For high thermal gradients, the crustal thickness is limited to ~20 km due to the onset of melting which represents a more mobile tectonic regime, while a low thermal gradient of 5 °C/km can cause density overturn and delamination of the crust reflecting a more stagnant-lid tectonic regime.

Results
Maximum crustal thickness
Results presented here focus on mafic crust representing venusian volcanic plains in stable conditions excluding specific regions such as tesserae as well as coronae and rifts with significantly higher thermal gradients.

We find that the maximum crustal thickness is strongly dependent on the thermal gradient and ranges from ~21 to 65 km, where the thinnest crust is a result of high thermal gradients of 25 °C/km and the thickest crust is supported by a 10 °C/km thermal gradient (Fig. 1 and Table 1). The 5 °C/km is the only thermal gradient where density overturn and delamination limit the crustal thickness for both basalt and alkali basalt crustal compositions at ~40 km (Fig. 1A) since the crustal density of both exceed that of the peridotite (Fig. 2A). This is mainly a result of the transition of the dry basalt and alkali basalt compositions from an assemblage dominated by plagioclase and pyroxene (Figs. 3B, C and 4B, C) to garnet-bearing assemblages (basalt: clinopyroxene+plagioclase+garnet±orthopyroxene±quartz; Figs. 3B and 4B; alkali basalt: clinopyroxene+plagioclase+garnet+kalsilite+spinel+ilmenite; Figs. 3C and 4C). Particularly the formation of dense garnet and the gradual replacement of plagioclase with clinopyroxene in the basalt and alkali basalt result in significant densification with depth compared to the peridotite, where the mineralogy is dominated by olivine, orthopyroxene, clinopyroxene at most pressure-temperature conditions (Figs. 3A and 4A). The density overturn occurs before either of the crustal compositions transition to a plagioclase-free mineral assemblage. This metamorphic progression is due to the system being completely dry, which will suppress the formation of amphiboles. The full transition to an assemblage dominated by pyroxene and garnet in our models occurs at depths >45 km for the alkali basalt and >53 km for the basalt, which can result in further densification (>3350 kg/m3) of the already delaminating crust. This process could contribute to the heterogeneity of the venusian mantle, but we find it has no impact on the thickness of the crust. For all other thermal gradients (10–25 °C/km, Figs. 1B–E and 2B–E), the crustal compositions reach the solidus and start melting before reaching the point of density overturn, resulting in a thinner crust with increasing thermal gradient (Table 1). Melt proportions are relatively small close to the solidus of the basalt, but increase rapidly with increasing temperatures (Fig. 4E, H, K, N). The alkali basalt forms significant amounts of melt (26–37 vol%) shortly after melting temperatures are reached (Fig. 4F, I, L, O).

Density of the crust and mantle
Densities were calculated for the solid rock only, thus excluding melt once the compositions reached the solidus. Our models yield crustal densities of >2900 kg/m3 for near surface conditions with the basaltic composition closer to 3000 kg/m3 (Fig. 2). The average density of the upper crust is relatively constant in all our scenarios, even though the absolute density of the crust can vary with crustal thickness, the thermal gradient, and the composition. This can be exemplified by calculating the average density of a basaltic composition for a crustal thickness of 20 km on the 5 °C/km and 25 °C/km thermal gradients: 2993 kg/m3 (5 °C/km) versus 2963 kg/m3 (25 °C/km), and therefore a difference of 30 kg/m3 or ~10% variation in density. For a crustal thickness of 40 km, the average density for the low thermal gradient is 3085 kg/m3. In contrast, there would have been significant melt formation on the 25 °C/km thermal gradient and the average density of the depleted crust would be ~2975 kg/m3. It should be noted that our petrologically constrained densities are significantly higher than the average density values (2800 and 2900 kg/m3) used in some geophysical and geodynamic models37–39,53 while more recent studies using equations-of-state to calculate crustal densities50,54,55 in the range of 2900–3000 kg/m3 are a better match to our results.

The density of the peridotite to a depth of 80 km (upper mantle) is in the range of 3200–3300 kg/m3 for thermal gradients of 10–25 °C/km and only exceeds 3300 kg/m3 by ~40 kg/m3 for the 5 °C/km thermal gradient (Fig. 2). As a result, the density contrast between the crust and mantle is usually in the range of 200–300 kg/m3. However, the density contrast is lower for the 5 °C/km thermal gradient, where the density of the crust increases until it reaches mantle density, and on the 10 °C/km thermal gradient, where the crustal density is comparable to that of the mantle shortly before the onset of melting. Again, these values deviate significantly from the values used in geophysical and geodynamic models (200–300 kg/m3 vs 400–500 kg/m3) used to derive crustal thickness from the relationship between global topography and gravity data37–39.

While the present-day crustal thickness is limited by the onset of melting, earlier stages in Venus’ geological evolution may have recorded melting episodes with implications for crustal compositions, and we have therefore also reported the density of the residual rock assuming that melt is removed from the system (Fig. 2B–E). The restite of the alkali basalt reaches the point of density overturn for thermal gradients of 10 and 15 °C/km at shallower depths than the basalt due to the significant amount of melt formation and subsequent fractionation (Fig. 2B, C). For the 20 and 25 °C/km thermal gradients, the density of the basalt does not exceed that of the peridotite (Fig. 2D, E) and crustal delamination is not feasible even after significant melt extraction.

Methods
We used Perple_X73, a Gibbs free energy minimization software, to calculate equilibrium mineral assemblages for basalt and alkali basalt compositions representative for the venusian crust, and for a peridotite mantle composition. Then we extracted phase proportions and densities for a range of possible lithospheric thermal gradients from 5 °C/km up to 25 °C/km, comparable to estimates for Venus11,12,32,33 and thermal gradients of Earth’s continental crust74 today. To determine the maximum crustal thickness for each thermal gradient and crustal composition, the following two criteria were used: (1) the depth equivalent to density overturn where the calculated density of the crustal composition becomes denser than the modeled mantle density and subsequently delaminates; and (2) the depth equivalent to the onset of melting of the crust.

Starting compositions
Three whole-rock compositions were used as starting compositions for our models (Table 2): a peridotite75, a basalt14,15, and an alkali basalt76. In the absence of a sampled venusian mantle composition, a terrestrial peridotite (a spinel lherzolite from the Kilbourne Hole crater, New Mexico, which is a widely used natural analogue for Earth’s upper mantle75) was chosen since a peridotitic mantle composition has been inferred for Venus based on the bulk density and moment of inertia of the planet77. The basalt composition is derived from analyses of the Venera 14 lander14,78 and terrestrial analogs for elements not analyzed by the lander15. The alkali-rich basalt composition represents a sample from Sverrefjell volcano, Svalbard76, which is chemically similar to the composition measured by the Venera 13 lander14 but provides a complete composition without assumptions and uncertainties. Both crustal samples represent the compositional diversity of the venusian crust.

Phase equilibria modeling
We modeled equilibrium mineral assemblages as a function of pressure, temperature, and composition to infer metamorphism of the venusian crust using the thermodynamic software package Perple_X, which calculates stable phase equilibria by means of Gibbs free energy minimization73. An internally consistent thermodynamic dataset provided the basis for phase endmember calculations79,80. The starting compositions (Table 2) were normalized to 100% by Perple_X at the beginning of each calculation. Models were calculated in the SiO2–TiO2–Al2O3–Fe2O3–FeO–MgO–CaO–Na2O–K2O chemical system. The redox state of the venusian crust and mantle is poorly constrained; however, the FeO/MnO ratio measured at the Venera and Vega landing sites suggests a rather oxidized surface and interior81. Fe2O3 was defined as a new component in Perple_X by the following relationship: 2FeO+0.5O2 and was then set to 0.3 wt% for the peridotite based on the range of 0.1–0.4 wt% Fe2O3 measured in terrestrial peridotites82. For the basalt and alkali-basalt, Fe2O3 was calculated as 10% of the total iron content in the bulk rock compositions to reflect an oxidized crust. Cr2O3, MnO, and P2O5 were not included in our calculations because of their low abundance and/or limited available solid solution models. All calculations were run without the addition of H2O, CO2, S, and halogens since their abundance in the interior are not known e.g., ref. 15 and our study focuses on constraining the conditions of dry phase transitions and melting before adding additional variables. The following activity solid solution models were used for all starting compositions: olivine, orthopyroxene, clinopyroxene, garnet, spinel, and melt83. For the peridotite calculations, we used a plagioclase solid solution from Jennings and Holland84. For calculations with the basalt and alkali basalt compositions, we used a ternary plagioclase solid solution85 and a solid solution model for ilmenite86. Rutile, quartz, coesite, stishovite, kyanite, leucite, nepheline, and kalsilite were treated as pure phases. We excluded redundant endmembers of solid solution models that were not used in our calculations and corundum since it is not expected in the compositions considered. Further, feldspathoids were excluded for the basaltic composition since it represents olivine tholeiite.

Phase diagrams for equilibrium conditions were calculated over a pressure and temperature range of 450–2050 °C and 0.0001–8 GPa. We also calculated one-dimensional equilibrium phase diagrams along five geotherms of 5, 10, 15, 20, and 25 °C/km, based on the range considered in crustal models33,36,38 and using a surface temperature of 450 °C. Rock densities were extracted using the Perple_X application werami73. Where stable, melt was fractionated from the rock before calculating the density. To estimate depth (h) from pressure (P), we used the following equation: h=P/\rho g, with a gravity (g) value for Venus of 8.87 m/s2 and an average crustal density of 3000 kg/m3. Since density changes as a function of depth, all depth estimates given in this study are approximations e.g., ref. 87.

Uncertainties and limitations of the model
Uncertainties in our model are mainly derived from uncertainties on physical properties of mineral endmembers in the thermodynamic dataset and associated with the formulation of activity-composition relationships of solid solutions88. Since these uncertainties affect the phase fields and consequently the density, we assumed an error of 10% on crustal thickness estimates (Table 1). This is reasonable when considering the typically accepted uncertainties reported for pressure (±2 kbar \approx 5–7 km depth on Earth) and temperature (±50 °C) estimates determined via phase equilibria modeling due to geologic uncertainty75. We justify the exclusion of the oxides Cr2O3, MnO, and P2O5 due to their low abundance of <1 wt% in the starting composition and no significant changes in the density profiles are expected. The addition of Cr2O3 could likely cause a slight increase in the rock density since Cr-bearing phases such as chromite are denser than their Cr-free equivalents. P2O5 is mainly incorporated in apatite, which could be present as a trace mineral in the basalt and alkali basalt compositions and may therefore also slightly increase the rock density. The effect of MnO, which is usually included in FeO and MgO-bearing minerals (e.g., clinopyroxene, garnet, olivine) should be extremely small. The peridotite melt model is calibrated up to ~5 GPa and that for basaltic melts up to ~3 GPa83 and the melt curve shown above these pressures is an extrapolation. There is currently no calibration for the alkali basalt although our melt model includes K endmembers. The results shown for the alkali basalt may therefore also show larger uncertainties. We also made the simplification of not including any volatiles since their abundance in the subsurface is currently poorly constrained and the relatively young venusian crust, which is believed to have formed by significant volcanic activity89–92 is likely depleted in volatiles (H2O, CO2, S) due to degassing. However, partial melting of the crust and mantle is not expected to result in the interior to be entirely volatile-free and a fraction of the initial volatile content will be likely retained in the interior10. Therefore, the influence of volatiles on phase transitions and melting should be considered in future studies, particularly since even low abundances of H2O and CO2 can significantly influence melting temperatures of peridotite and basaltic compositions e.g., refs. 93, 94.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "I enjoyed reading this manuscript, which uses phase equilibrium modelling of primitive crust and lithospheric mantle compositions to calculate the density structure and thereby gravitational stability and thickness of the crust on Venus. It is a very powerful approach that I think we can have some trust in. However, while this is very interesting study that I would like ultimately to see published, there are a number of things that concern me. Most importantly:\nPhase equilibria:\nWith reference to the phase equilibrium modelling and Fig. 3, the phase diagram for the peridotite is fine. However, the melt model for the basalts has NOT been calibrated about ~2 GPa. This is clearly indicated by the back bending of the high pressure solidus (dotted pink line) to ~4.8 GPa at 450 C. Does that seem reasonable? You can ONLY show and interpret the phase equilibria for the basalts (Fig. 3b,c) to 2 GPa max. It doesn’t affect much of what you later write, but it is wrong to show it. I also worry that the Green et al. melt model used is not calibrated for alkali basalts, a project I believe Owen Weller and others at Cambridge (and elsewhere) have been working on. Personally, I would model only the venusian basalt and it’s (assumed) lithospheric mantle residue.\nWith respect to the bulk compositions modelled, the sentence “For the basalt and alkali-basalt, Fe2O3 was calculated as 10% of the total iron content in the bulk rock compositions.” needs support. What is based on? If on Earth’s mantle, then is that a reasonable comparison? The statement (l.787) “feldspathoids were excluded for the basaltic composition” also needs support, and the same elsewhere.\nReferences:\nThe theme of the omission of key references gets personal in the introduction, where the authors lay out the utility of phase equilibria in constraining models of lithospheric evolution. It is one I used myself in a 2014 paper…\nJohnson, T.E., Brown, M., Kaus, B.J. and VanTongeren, J.A., 2014. Delamination and recycling of Archaean crust caused by gravitational instabilities. Nature Geoscience, 7(1), pp.47-52.\n…which modelled the gravitational stability of Archean crust on Earth.\nWe did similar for the Moon in 2021.\nJohnson, T.E., Morrissey, L.J., Nemchin, A.A., Gardiner, N.J. and Snape, J.F., 2021. The phases of the Moon: Modelling crystallisation of the lunar magma ocean through equilibrium thermodynamics. Earth and Planetary Science Letters, 556, p.116721.\nThe manuscript reads like this is a new approach, but it isn’t.\nMetamorphic terminology:\nThroughout there is a fundamental misunderstanding of the meaning of a metamorphic facies. As the authors state, an anhydrous rock cannot grow amphibole or any other hydrous mineral. That does not mean it cannot be metamorphosed to amphibolite (or greenschist) facies conditions. Similarly, the use of hornfels and hornfels facies is NOT helpful. These terms have a very specific (contact metamorphic) inference that is at best misleading. Here, you are simply talking about shallow (low P) and deep (higher P) crust, one which contains garnet and another that doesn’t. While Figure 1 is pretty, it would be MUCH more useful to see some of the important phase transitions on it. With these and other aspects, including the discussion of what to call rocks with lots of garnet and clinopyroxene in, I would urge the authors to read Johnson et al. (2014).\nGeodynamics:\nThe authors talk a lot about delamination of crust as a limiting factor, but there is a difference between crust becoming negatively buoyant and crust overturning/delaminating/subducting/whatever. An apple placed on a table is negatively buoyant and wants to sink, but it doesn’t because the table is too strong. Whether or not negatively buoyant crust sinks relates mostly to the viscosity of the underlying mantle, itself a function of T, H2O content, melt fraction, blah blah. Numerical modelling is a good way to look at this problem, as done by Johnson et al. (2014) and others. These 2d models, if sufficiently simply, can run on decent home computers. Persuading somebody to help you with this, in addition to paying attention to the above, would take this to the next level, and permit a few firmer constraints on the possible implications of the phase equilibria to understanding the tectonics of Venus.\nIn case it’s helpful I will try and remember to attach an edited Word doc, where I have edited things (using Track Changes) for clarify and on which I have provided a number of additional comments and suggestions. Sorry, I did not finish the discussion.\nGood luck with it,\nTim Johnson", "reviewer_2": "Review by Ernst of “Metamorphism of Venus: Driver of Crustal Thickness, Recycling, and Volcanism” by Semprich et al.\nGENERAL COMMENTS: I am really pleased to see this manuscript which is very solid piece of work for which I have only minor suggestions.\nThere has been a need for such a study to provide constraints on Venusian crustal thickness and composition, incorporating the factor that is so dramatically different than on Earth—surface temperatures of 450 degree C per km. The background information sections are robust and the manuscript is well written.\nThe modelling seems very through and logical (although the details of such modelling are outside my main expertise). Their approach incorporates metamorphic thermodynamic approach (Gibbs free energy minimization) – using three plausible geothermal gradients (5, 10 and 25 degrees C per km, which are arguably present in different parts of Venus). The paper provides basic constraints on the thickness of crust. Notably, under a high geothermal gradient the thickness of the crust is limited by melting at the base. For the lowest geothermal gradient, there is a potential for delamination related to formation of eclogite. Based on their modelling they infer a range of possible crustal thicknesses between 20 and 65 km. they note that for thicknesses observed greater than 65 km, this would indicate some silicic unit component to the crust.\nThis mention of >65 km requiring silicic units does raise an interesting question which is really outside the scope of their research. The stratigraphically oldest units, tesserae (which also tend to be associated with thick crust) are exposed over about 8% of Venus, but tesserae may have much wider extent under basaltic plains units (e.g. Ivanov and Head, 2007? Or 1997?). Some studies are suggesting that tesserae may have a silicic component (e.g. Gilmore, and I think there is a new paper by Paul Byrne that mentions this)—so perhaps there are regions of Venus with crustal thicknesses less than 65 km for which silicic crust may be present. Adding a modelling component that incorporates some silicic crust is really outside the scope of this paper, but it would be useful for the authors to say a few things about this possibility of silicic units in tessera being more widespread.\nDETAILED COMMENTS;\nLine 28: missing reference on evidence for young volcanism--- Shalygin et al. 2015\nLines 44: could add Way et al. 2022 (which indicates the potential of volcanism in causing a “Great Climate Transition” on Venus\nAlso could add Khawja et al. 2020 (which proposed that the oldest terrains tesserae may preserve evidence of fluvial erosion- consistent with tesserae preserving a protracted geological history during the earlier more habitable climate proposed by Way et al. 2016 for Venus\nLine 59: perhaps add Elkin Tanton (e.g. Elkins-Tanton et al. 2007) on delamination\nLines 190-192: you mention the possibility of more silicic crust involved where the crustal thickness is greater than 65 km. However, there are papers by Ivanov and Head and others suggesting that tesserae terrain (which covers about 8% of Venus could be widespread underlying underlie basaltic units—and there has been a suggest for tesserae having a significant silicic component.\nCould you add some text somewhere in the manuscript acknowledging this possibility\nLines 327-331: you explain your procedure for determining the Fe2O3 amount---- Can you provide a bit more of rationale for this selection\nREFERENCES TO ADD:\nShalygin, E. V., et al. (2015), Active volcanism on Venus in the Ganiki Chasma rift zone, Geophys. Res. Lett., 42, 4762–4769,\ndoi:10.1002/2015GL064088.\nKhawja, et al. (2020). Tesserae on Venus may preserve evidence of fluvial erosion. Nature Communications, 11, article no 5789. https://doi.org/10.1038/s41467-020-19336-1\nWay, M.J., et al. (2022). Large scale volcanism and the heat-death of terrestrial worlds. The Planetary Science Journal, 3:92 (11pp), https://doi.org/10.3847/PSJ/ac6033\nElkins-Tanton, et al. (2007). Continental magmatism, volatile recycling, and a heterogeneous mantle caused by lithospheric gravitational instabilities. Journal of Geophysical Research, 112: B03405", "reviewer_3": "The authors investigate the metamorphism of Venus' crust, with a particular focus on phase transitions and the onset of melting under different lithospheric thermal gradients. They employ a phase equilibria modeling approach to assess the effects of pressure and temperature on the mineralogy of dry basalt, alkali basalt, and peridotite mantle compositions. By calculating phase equilibria as a function of pressure, temperature, and composition, the authors demonstrate how stable mineral assemblages and corresponding rock densities vary gradually with depth, rather than as an average. The study also compares the calculated crust and mantle densities to identify where the crust may become denser than the underlying mantle. Additionally, the authors correlate their findings with estimates of crustal thickness to predict regions on Venus where crustal melting or recycling through delamination is likely to occur. Given the growing scientific interest in Venus and the need for a deeper understanding of its crustal compositions and geodynamics, this paper is particularly timely and relevant to the readership of a high-impact journal such as Nature Communications.\nIn my opinion, this manuscript is interesting, well-written, and structured, and the science is sound. I have only a few minor concerns that I do believe the authors should address before this paper can be published. These minor concerns converge towards the better acknowledgment of other relevant interpretations of geodynamics for Venus and some other needed discussions of some other relevant literature. I am confident the authors can address these comments (or revoke them if they disagree with reasoning) relatively easily.\n• Crustal thickness linked to thermal gradient and global tectonics. The connection between thermal gradients and tectonic settings lacks sufficient elaboration. The authors predominantly consider a binary global tectonic framework of “mobile lid” versus “stagnant lid” regimes, citing mostly their own work on geodynamic regimes they consider appropriate to Venus. This approach overlooks the existence of alternative (or, adapted) global tectonic models for present-day Venus, such as the “sluggish” or “squishy” lid regimes, which suggest partial mobility and laterally variable deformation due to magmatic-induced weakening. These alternative models are particularly relevant to the brief discussion in lines 266-273 and should be acknowledged. In particular, the statement in lines 270-273 (“as a result, crustal thickness may vary significantly as it could be thickened by increased magmatic activity and underplating in regions of higher heat flow and thinned by delamination in regions of lower heat flow”) appears to describe melt generation primarily as a consequence of long-term tectonic regime transitions, with a self-citation. However, there is substantial existing research on these topics that should be considered, such as the work by Byrne et al. (2021 https://www.pnas.org/doi/10.1073/pnas.2025919118 ), Lourenco et al. (2020 https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019GC008756) (as described in the review by Rolf et al. (2022, https://asu.elsevierpure.com/en/publications/dynamics-and-evolution-of-venus-mantle-through-time), which the authors are familiar with).\n• Moreover, surface heat flow estimates for Venus, despite their inherent limitations, are available and show variability across the planet, as noted by Smrekar et al. (2023) (cited as #10 in the manuscript). It would be beneficial for the authors to briefly contextualize their discussion on thermal. gradients within the range of these varying heat flow values across different tectonic structures on the planet, not just stagnant/mobile lid tectonic regimes (the Mead Basin is discussed, but it is not so clearly acknowledged that the wealth of different tectonic structures on Venus, e.g. rifts, coronae, hotspot swells, tessera highlands, may all have different thermal gradient within one geodynamic regime)\n• Phase equilibria have been made for Vega 2, Venera 13, and Venera 14 Venus rock measurements in Chen et al. (2022: https://www.nature.com/articles/s41467-022-35304-3) (see their main manuscript but also supplementary material). This is highly relevant past work that to which results should be (briefly) compared in this manuscript, as well as the conclusions made by those authors (supposedly “light” Venus lithosphere).\n• Average densities of the basaltic crust under different temperature gradients: The high crustal density values are interesting as they indeed contrast to values commonly used in geophysical/geodynamic models. I have a few minor concerns related to this paragraph:\n- Line 146 should be revised, as it references \"geophysical and geodynamic models,\" yet only cites geophysical studies for crustal thickness values (#34-36) without including any geodynamic modeling studies that account for crustal compositions.\n- Indeed, some models like Adams et al. (2022, https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2022JE007460?saml_referrer) use 2900 kg/m3 as Venus crustal density; although a set of other tectonic models (Gülcher et al. (2020; 2023) and Schools and Smrekar (2024 https://www.sciencedirect.com/science/article/pii/S0012821X24000761 ) use equations-of-state that lead to crustal density values around 2900-2970 kg/m³, using the following equation of state: ρ=ρ0[1−α(T−298)][1+β(P−0.1)]ρ=ρ0[1−α(T−298)][1+β(P−0.1)], (T in Kelvin; P in MPa) with thermal expansion α = 3x10 −5 K−1 , compressibility β = 10−5 MPa−1 and ρ0=3000 kg/m³ as the rock density at standard temperature and pressure. Another recent tectonic modeling work by Regorda et al. (2023 https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2022JE007588 ) Therefore, the values mentioned in the text (2800-2900 kg/m³) are not exactly covering the range of geodynamic models.\n• Methods:\no this seems very obvious, but I see nowhere stated what the surface temperature used is for the thermal profiles with the gradients? I assume 460’C based on line 26? It would be good to state in Methods regardless.\no Lines 350: I am under the impression the gravity value for Venus is 8.87 m/s2, and not 8.77 (https://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html). Did you really use 8.77 and if so, how is this motivated? I am of the opinion that the calculations should be done with 8.87 m/s2 for accuracy."}, "author_response": "We thank the reviewers for their comments and have revised our manuscript as described in our responses below. Line references should match those in the revised version.\nReviewer #1:\nI enjoyed reading this manuscript, which uses phase equilibrium modelling of primitive crust and lithospheric mantle compositions to calculate the density structure and thereby gravitational stability and thickness of the crust on Venus. It is a very powerful approach that I think we can have some trust in. However, while this is very interesting study that I would like ultimately to see published, there are a number of things that concern me. Most importantly:\nPhase equilibria:\nWith reference to the phase equilibrium modelling and Fig. 3, the phase diagram for the peridotite is fine. However, the melt model for the basalts has NOT been calibrated about ~2 GPa. This is clearly indicated by the back bending of the high pressure solidus (dotted pink line) to ~4.8 GPa at 450 C. Does that seem reasonable? You can ONLY show and interpret the phase equilibria for the basalts (Fig. 3b,c) to 2 GPa max. It doesn’t affect much of what you later write, but it is wrong to show it. I also worry that the Green et al. melt model used is not calibrated for alkali basalts, a project I believe Owen Weller and others at Cambridge (and elsewhere) have been working on. Personally, I would model only the venusian basalt and it’s (assumed) lithospheric mantle residue.\nResponse: We would like to point out that we used the melt model from Holland et al., 2018 and not the one for metabasic rocks described in Green et al., 2016. This is stated in the phase equilibria section of the Methods. The newer model is calibrated up to ~5 GPa for peridotite (although extrapolation to higher pressure for peridotite seems to be reasonable as R#1 suggests) and up to 3 GPa for eclogite (see Fig. 6 in Holland et al. 2018). It is therefore not clear why we can only show and interpret phase equilibria up to a maximum of 2 GPa.\nWe do acknowledge that the phase diagrams in Fig. 3 showed phase fields in the region to the left of the thermal gradient of 5 °C/km, which we assumed were obviously neither attainable for Venus, nor relevant for the manuscript. We have provided a new Fig 3, where this area is now clearly labelled as representing ‘unattainable conditions’ and have changed the figure caption accordingly (lines 629-631). We have also added the calibration limits in the ‘Uncertainties and limitations of the model’ subsection of the Methods (lines 346-348). The melt model from Holland et al., 2018 also includes a K end member, which is why we have kept the results for alkali basalt in the manuscript and added additional caveats to the same section (lines 348-350).\nWith respect to the bulk compositions modelled, the sentence “For the basalt and alkali-basalt, Fe2O3 was calculated as 10% of the total iron content in the bulk rock compositions.” needs support. What is based on? If on Earth’s mantle, then is that a reasonable comparison? The statement (l.787) “feldspathoids were excluded for the basaltic composition” also needs support, and the same elsewhere.\nResponse: We have added a sentence to clarify the issue with poorly constrained redox state including a reference that summarizes the limited data with the prospect of a relatively oxidized surface and interior (lines 302-304 and 308). Based on these inferred assumptions, we can assume that Fe O is not negligible, but we would also like to point out that variations in a few percent of Fe O will overall not add significant uncertainties to the phase diagrams or extracted densities. We also added more information to support the exclusion of feldspathoids (line 321).\nReferences:\nThe theme of the omission of key references gets personal in the introduction, where the authors lay out the utility of phase equilibria in constraining models of lithospheric evolution. It is one I used myself in a 2014 paper…\nJohnson, T.E., Brown, M., Kaus, B.J. and VanTongeren, J.A., 2014. Delamination and recycling of Archaean crust caused by gravitational instabilities. Nature Geoscience, 7(1), pp.47-52.\n…which modelled the gravitational stability of Archean crust on Earth.\nWe did similar for the Moon in 2021.\nJohnson, T.E., Morrissey, L.J., Nemchin, A.A., Gardiner, N.J. and Snape, J.F., 2021. The phases of the Moon: Modelling crystallisation of the lunar magma ocean through equilibrium thermodynamics. Earth and Planetary Science Letters, 556, p.116721.\nThe manuscript reads like this is a new approach, but it isn’t.\nResponse: According to the author guidelines for Nature Communications, references should not exceed 70 (https://www.nature.com/ncomms/submit/article). The manuscript is already slightly over this limit due to the large range of topics covered. It seems that opinions differ significantly on what ‘key’ references need to be included, especially when looking at the suggestions form R#2 and R#3. We have added the first publication by R#1 (line 59), but we have some difficulties seeing the relevance of the crystallization of the lunar magma ocean to a planet the size of Venus discussing phase equilibria of the solid crust and mantle. It is not clear where the manuscript seems to read like this is a new approach since there are clearly references to delamination and the application of phase equilibria in the introduction. The new aspect of this manuscript is the application of phase transitions to Venus which R#1 seems to acknowledge in their introductory remarks.\nMetamorphic terminology:\nThroughout there is a fundamental misunderstanding of the meaning of a metamorphic facies. As the authors state, an anhydrous rock cannot grow amphibole or any other hydrous mineral. That does not mean it cannot be metamorphosed to amphibolite (or greenschist) facies conditions. Similarly, the use of hornfels and hornfels facies is NOT helpful. These terms have a very specific (contact metamorphic) inference that is at best misleading. Here, you are simply talking about shallow (low P) and deep (higher P) crust, one which contains garnet and another that doesn’t. While Figure 1 is pretty, it would be MUCH more useful to see some of the important phase transitions on it. With these and other aspects, including the discussion of what to call rocks with lots of garnet and clinopyroxene in, I would urge the authors to read Johnson et al. (2014).\nResponse: What R#1 calls a fundamental misunderstanding was, in fact, an intentional decision on our part. We have taken the general guidelines for Nature Communications seriously in writing for a ‘specific research community’. In our case, this is the Venus community, which includes researchers from diverse fields, many of whom are not familiar with the concept of metamorphic facies. Years of experience presenting phase diagrams within this community has shown that a greenschist-facies rock is mostly associated with the presence of water, and we intentionally wanted to prevent these misunderstandings. It seems that our solution of trying to be more inclusive has caused some other issues, so we have decided to remove all references to rock types and metamorphic facies in Fig. 1 (+ caption) and the manuscript (lines 30, 100, 101, 108-109, 110-111). We replaced hornfels with metagabbro and granulite with garnet metagabbro in Fig. 1 and changed the figure caption (line 610) accordingly. Due to the above-mentioned diversity in the Venus research community, Fig. 1 is intentionally a summary and overview, and we believe that phase transitions will add unnecessary complexity, particularly because these details are shown in Fig. 3 and Fig. 4.\nAdditional note: Venus is very likely still volcanically active and most of the surface is less than 1 Ga old due to resurfacing (although there is currently no consensus if this was catastrophic or gradual). This will expose a significant part of the surface and the upper crust to contact metamorphism and therefore hornfels-facies. Without the presence of plate tectonics or water these mineral assemblages would likely be preserved until reaching the garnet stability field.\nGeodynamics:\nThe authors talk a lot about delamination of crust as a limiting factor, but there is a difference between crust becoming negatively buoyant and crust overturning/delaminating/subducting/whatever. An apple placed on a table is negatively buoyant and wants to sink, but it doesn’t because the table is too strong. Whether or not negatively buoyant crust sinks relates mostly to the viscosity of the underlying mantle, itself a function of T, H2O content, melt fraction, blah blah. Numerical modelling is a good way to look at this problem, as done by Johnson et al. (2014) and others. These 2d models, if sufficiently simply, can run on decent home computers. Persuading somebody to help you with this, in addition to paying attention to the above, would take this to the next level, and permit a few firmer constraints on the possible implications of the phase equilibria to understanding the tectonics of Venus.\nResponse: This may be because R#1 did not finish reading the discussion, but the crustal thickness is limited by melting for most thermal gradients and delamination is only likely in cases of extremely low thermal gradients, which may not be very common on Venus. We think that we have made it clear in the manuscript that our conclusions are based on petrological constraints and not on additional parameters. We do agree that numerical modeling is the next step, but we think that the integration of the petrological data into 3D spherical models would be a much better approach, particularly since MBW has done a considerable amount of work in this area.\nIn case it’s helpful I will try and remember to attach an edited Word doc, where I have edited things (using Track Changes) for clarify and on which I have provided a number of additional comments and suggestions. Sorry, I did not finish the discussion.\nGood luck with it,\nTim Johnson\nResponse: Thanks. Unfortunately, we could not locate an additional file.\nReviewer #2 (Remarks to the Author):\nReview by Ernst of “Metamorphism of Venus: Driver of Crustal Thickness, Recycling, and Volcanism” by Semprich et al.\nGENERAL COMMENTS: I am really pleased to see this manuscript which is very solid piece of work for which I have only minor suggestions.\nThere has been a need for such a study to provide constraints on Venusian crustal thickness and composition, incorporating the factor that is so dramatically different than on Earth—surface temperatures of 450 degree C per km. The background information sections are robust and the manuscript is well written.\nThe modelling seems very through and logical (although the details of such modelling are outside my main expertise). Their approach incorporates metamorphic thermodynamic approach (Gibbs free energy minimization) – using three plausible geothermal gradients (5, 10 and 25 degrees C per km, which are arguably present in different parts of Venus). The paper provides basic constraints on the thickness of crust. Notably, under a high geothermal gradient the thickness of the crust is limited by melting at the base. For the lowest geothermal gradient, there is a potential for delamination related to formation of eclogite. Based on their modelling they infer a range of possible crustal thicknesses between 20 and 65 km. they note that for thicknesses observed greater than 65 km, this would indicate some silicic unit component to the crust.\nThis mention of >65 km requiring silicic units does raise an interesting question which is really outside the scope of their research. The stratigraphically oldest units, tesserae (which also tend to be associated with thick crust) are exposed over about 8% of Venus, but tesserae may have much wider extent under basaltic plains units (e.g. Ivanov and Head, 2007? Or 1997?). Some studies are suggesting that tesserae may have a silicic component (e.g. Gilmore, and I think there is a new paper by Paul Byrne that mentions this)—so perhaps there are regions of Venus with crustal thicknesses less than 65 km for which silicic crust may be present. Adding a modelling component that incorporates some silicic crust is really outside the scope of this paper, but it would be useful for the authors to say a few things about this possibility of silicic units in tessera being more widespread.\nResponse: Thank you for the comments. We are very aware of the research related to tesserae since we currently have a manuscript ready for submission, which is focused specifically on the composition to tesserae. We think that this topic is much better covered by itself than adding additional compositions to this manuscript.\nDETAILED COMMENTS;\nLine 28: missing reference on evidence for young volcanism--- Shalygin et al. 2015\nResponse: Thanks for pointing this out. Reference added and text slightly modified (line 28).\nLines 44: could add Way et al. 2022 (which indicates the potential of volcanism in causing a “Great Climate Transition” on Venus\nAlso could add Khawja et al. 2020 (which proposed that the oldest terrains tesserae may preserve evidence of fluvial erosion- consistent with tesserae preserving a protracted geological history during the earlier more habitable climate proposed by Way et al. 2016 for Venus\nResponse: We agree that these suggestions support the reference list from various angles and have added them (line 43).\nLine 59: perhaps add Elkin Tanton (e.g. Elkins-Tanton et al. 2007) on delamination\nResponse: Added (line 59).\nLines 190-192: you mention the possibility of more silicic crust involved where the crustal thickness is greater than 65 km. However, there are papers by Ivanov and Head and others suggesting that tesserae terrain (which covers about 8% of Venus could be widespread underlying underlie basaltic units—and there has been a suggest for tesserae having a significant silicic component.\nCould you add some text somewhere in the manuscript acknowledging this possibility\nResponse: We agree that this is an important topic that needs to be addressed but as mentioned above, we already have a manuscript ready to focus only on the composition of tesserae and have therefore intentionally not included a lot of discussion on this topic. Looking at the comment from R#3 about heat flow and tectonic features, we acknowledge that we should have made it clear that our current approach is relevant for stable mafic crust and not the range of tectonic features for Venus (which is beyond the scope of this manuscript). We have added a sentence at the beginning of the results section to determine the applicability of our results (lines 91-93) and another sentence in the discussion to add the relevance of silica-rich compositions to tesserae (lines 174-175).\nLines 327-331: you explain your procedure for determining the Fe2O3 amount---- Can you provide a bit more of rationale for this selection\nResponse: We have amended the text to reflect the lack of data on redox state and the assumptions behind using this value (see also response to comments from R#1; lines 302-304).\nREFERENCES TO ADD:\nShalygin, E. V., et al. (2015), Active volcanism on Venus in the Ganiki Chasma rift zone, Geophys. Res. Lett., 42, 4762–4769,\ndoi:10.1002/2015GL064088.\nKhawja, et al. (2020). Tesserae on Venus may preserve evidence of fluvial erosion. Nature Communications, 11, article no 5789. https://doi.org/10.1038/s41467-020-19336-1\nWay, M.J., et al. (2022). Large scale volcanism and the heat-death of terrestrial worlds. The Planetary Science Journal, 3:92 (11pp), https://doi.org/10.3847/PSJ/ac6033\nElkins-Tanton, et al. (2007). Continental magmatism, volatile recycling, and a heterogeneous mantle caused by lithospheric gravitational instabilities. Journal of Geophysical Research, 112: B03405\nReviewer #3 (Remarks to the Author):\nThe authors investigate the metamorphism of Venus' crust, with a particular focus on phase transitions and the onset of melting under different lithospheric thermal gradients. They employ a phase equilibria modeling approach to assess the effects of pressure and temperature on the mineralogy of dry basalt, alkali basalt, and peridotite mantle compositions. By calculating phase equilibria as a function of pressure, temperature, and composition, the authors demonstrate how stable mineral assemblages and corresponding rock densities vary gradually with depth, rather than as an average. The study also compares the calculated crust and mantle densities to identify where the crust may become denser than the underlying mantle. Additionally, the authors correlate their findings with estimates of crustal thickness to predict regions on Venus where crustal melting or recycling through delamination is likely to occur. Given the growing scientific interest in Venus and the need for a deeper understanding of its crustal compositions and geodynamics, this paper is particularly timely and relevant to the readership of a high-impact journal such as Nature Communications.\nIn my opinion, this manuscript is interesting, well-written, and structured, and the science is sound. I have only a few minor concerns that I do believe the authors should address before this paper can be published. These minor concerns converge towards the better acknowledgment of other relevant interpretations of geodynamics for Venus and some other needed discussions of some other relevant literature. I am confident the authors can address these comments (or revoke them if they disagree with reasoning) relatively easily.\n• Crustal thickness linked to thermal gradient and global tectonics. The connection between thermal gradients and tectonic settings lacks sufficient elaboration. The authors predominantly consider a binary global tectonic framework of “mobile lid” versus “stagnant lid” regimes, citing mostly their own work on geodynamic regimes they consider appropriate to Venus. This approach overlooks the existence of alternative (or, adapted) global tectonic models for present-day Venus, such as the “sluggish” or “squishy” lid regimes, which suggest partial mobility and laterally variable deformation due to magmatic-induced weakening. These alternative models are particularly relevant to the brief discussion in lines 266-273 and should be acknowledged. In particular, the statement in lines 270-273 (“as a result, crustal thickness may vary significantly as it could be thickened by increased magmatic activity and underplating in regions of higher heat flow and thinned by delamination in regions of lower heat flow”) appears to describe melt generation primarily as a consequence of long-term tectonic regime transitions, with a self-citation. However, there is substantial existing research on these topics that should be considered, such as the work by Byrne et al. (2021 https://www.pnas.org/doi/10.1073/pnas.2025919118 ), Lourenco et al. (2020 https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019GC008756) (as described in the review by Rolf et al. (2022, https://asu.elsevierpure.com/en/publications/dynamics-and-evolution-of-venus-mantle-through-time), which the authors are familiar with).\nResponse: This is a good point and we actually had a more detailed discussion in an earlier version of the manuscript, which we have taken back in together with a discussion of possible tectonic regimes for Venus citing most of the suggested work (paragraph in the discussion, lines: 216-230, and additions in lines 233, 252).\n• Moreover, surface heat flow estimates for Venus, despite their inherent limitations, are available and show variability across the planet, as noted by Smrekar et al. (2023) (cited as #10 in the manuscript). It would be beneficial for the authors to briefly contextualize their discussion on thermal. gradients within the range of these varying heat flow values across different tectonic structures on the planet, not just stagnant/mobile lid tectonic regimes (the Mead Basin is discussed, but it is not so clearly acknowledged that the wealth of different tectonic structures on Venus, e.g. rifts, coronae, hotspot swells, tessera highlands, may all have different thermal gradient within one geodynamic regime)\nResponse: We agree that this is an important issue that should be mentioned in the manuscript, and we have added a sentence about the variability of surface heat flow and tectonic settings in the above-mentioned paragraph (lines 216-230). However, our results will not be applicable to all observed features on Venus (particularly not tesserae – see comment to R#2 above) and this will have to be addressed in future work.\n• Phase equilibria have been made for Vega 2, Venera 13, and Venera 14 Venus rock measurements in Chen et al. (2022: https://www.nature.com/articles/s41467-022-35304-3) (see their main manuscript but also supplementary material). This is highly relevant past work that to which results should be (briefly) compared in this manuscript, as well as the conclusions made by those authors (supposedly “light” Venus lithosphere).\nResponse: Chen et al., 2022 assume the existence of a liquid ocean on early Venus and therefore use completely hydrated crust in contrast to the dry approach of the present-day crust in our manuscript. Due to the significant difference in the two models resulting in completely different mineral assemblages and melting temperatures, a comparison would a) have to include a more detailed discussion about water on early Venus, which we think is not useful for work we are presenting and b) would merely result in us stating that densities and melting temperatures are different due to different conditions.\n• Average densities of the basaltic crust under different temperature gradients: The high crustal density values are interesting as they indeed contrast to values commonly used in geophysical/geodynamic models. I have a few minor concerns related to this paragraph:\n- Line 146 should be revised, as it references \"geophysical and geodynamic models,\" yet only cites geophysical studies for crustal thickness values (#34-36) without including any geodynamic modeling studies that account for crustal compositions.\n- Indeed, some models like Adams et al. (2022, https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2022JE007460?saml_referrer) use 2900 kg/m3 as Venus crustal density; although a set of other tectonic models (Gülcher et al. (2020; 2023) and Schools and Smrekar (2024 https://www.sciencedirect.com/science/article/pii/S0012821X24000761 ) use equations-of-state that lead to crustal density values around 2900-2970 kg/m³, using the following equation of state: ρ=ρ0[1−α(T−298)][1+β(P−0.1)]ρ=ρ0[1−α(T−298)][1+β(P−0.1)], (T in Kelvin; P in MPa) with thermal expansion α = 3x10 −5 K−1 , compressibility β = 10−5 MPa−1 and ρ0=3000 kg/m³ as the rock density at standard temperature and pressure. Another recent tectonic modeling work by Regorda et al. (2023 https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2022JE007588 ) Therefore, the values mentioned in the text (2800-2900 kg/m³) are not exactly covering the range of geodynamic models.\nResponse: Thank you for pointing this out. We have added the references and modified the sentence (lines 135-137) to represent the range of densities in geodynamic models and how they match our results.\n• Methods:\no this seems very obvious, but I see nowhere stated what the surface temperature used is for the thermal profiles with the gradients? I assume 460’C based on line 26? It would be good to state in Methods regardless.\nResponse: Yes, that was an omission on our part. We started with surface temperature of 450 °C, which has now been added (line 325).\no Lines 350: I am under the impression the gravity value for Venus is 8.87 m/s2, and not 8.77 (https://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html). Did you really use 8.77 and if so, how is this motivated? I am of the opinion that the calculations should be done with 8.87 m/s2 for accuracy.\nResponse: Thank you for pointing this out. That was a typo, which is now corrected (line 328)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "It's always nice to devote ones time to doing a review in good faith, signing your name to that review, then receive snippy and and time rude responses from the authors. This is hardly uncommon from a planetary community who seem to begrudge any comments from geologists. Hey ho.\nThe point regarding references was meant to credit those very few studies that have used a phase equilibrium modelling approach used by the authors on other differentiated silicate bodies, even if they were done by geologists. If the authors don't see the relevance and/or that these papers should be referenced, then fine. I thank them for reluctently adding a reference to our 2014 paper on delamination, but they should feel free to remove it if they need to trim the bibliography.\nI am not an expert on Venus, capitalised or not, although I do have plenty of idea to explain some of the features it preserves. However, I do have some expertise in metamorphism and some documented experience of how to express ideas to a broad readership. Regarding metamorphic terminology, if they think a broad readership will understand phrases like 'hornfels and hornfels facies' better than my suggested modifications, then they should revert back to the original. It is their paper upon which I am only providing advice.\nRegarding the geodynamics section, rather than not having read the discussion, perhaps I put together my comments piecemeal and, what with other time pressures, did not find time to cross check everything before submitting my review as close to the deadline (albeit over it) as i could manage. I can only offer my apologies.\nThat the authors think they have \"we have made it clear in the manuscript that our conclusions are based on petrological constraints and not on additional parameters\" does not prevent them from discussing some of those parameters in a section that is specifically designed for such things. If they don't want to then they don't have to.\nAs before, this is a nice study that should be published.\nTim Johnson, Perth, January 2025", "reviewer_4": "Paper summary and key points of interest: The authors investigate the metamorphism of Venus' crust, with a particular focus on phase transitions and the onset of melting of mafic rocks on Venus’ volcanic plains (which cover the majority of the planet’s surface) under different lithospheric thermal gradients that could represent different geodynamic settings. They employ a phase equilibria modeling approach to assess the effects of pressure and temperature on the mineralogy of dry basalt, alkali basalt, and peridotite mantle compositions. Ultimately, they present their calculations of the density structures under various (thermal, compositional) conditions, and asses the possibility of gravitational instabilities, onsets of melting, and ultimately, the maximum possible thickness of the mafic crust. Several findings, such as that delamination may not always occur at the phase boundary, the commonly made assumption of a linear increase in density from gabbro to eclogite is inadequate, and that crustal thickness may be limited by melting rather than delamination under ‘thin’ lithosphere conditions, have significant implications on various Venus science studies.\nAssessment: I enjoyed reading this revised manuscript. I think the authors have addressed the wide range of points made by all three reviewers, pointing out very different aspects, adequately.\nI am happy that the authors have decided to specifically clarify the applicability of their results (i.e., to stable mafic crust and not to other tectonic/dynamic areas), to include a wider discussion of the available concepts of Venus global tectonics, and clarifies some key future work aspects out-of-scope of this paper (e.g. relevance to perhaps silica-rich compositions in tesserae). Overall, I think the paper will be a good contribution to Nature Communication and of high interest to the journals readership and the (Venus) science community."}, "author_response": ""}]

Decision letter