Dislocation creep near the frictional-viscous transition in blueschist: experimental constraints

Hufford, Lonnie Justin; Tokle, Leif; Behr, Whitney Maria; Morales, Luiz Grafula; Madonna, Claudio

doi:https://doi.org/10.5194/egusphere-2024-1507

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https://doi.org/10.5194/egusphere-2024-1507

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29 May 2024

| 29 May 2024

Dislocation creep near the frictional-viscous transition in blueschist: experimental constraints

Lonnie Justin Hufford, Leif Tokle, Whitney Maria Behr, Luiz Grafula Morales, and Claudio Madonna

Abstract. Mafic oceanic crustal rocks at blueschist facies conditions are an important rheological component of subducting slabs and the interface at subduction plate boundaries. However, the mechanical properties and deformation mechanisms of glaucophane, a rheologically-controlling sodic amphibole in blueschists, are poorly constrained. To investigate its mechanical and microstructural properties, we conducted general shear constant rate and strain rate stepping experiments on glaucophane aggregates using a Griggs apparatus at temperatures of 700–750 °C, shear strain rates of ~3x10^-6 to 9x10^-5 s^-1, varying grain sizes, and a confining pressure of ~1.0 GPa. The constant rate experiments show an initial stage of grain-size-dependent strain hardening followed by weakening associated with brittle slip along cleavage planes, kink-band development, cataclasis resulting in a fine-grained matrix, and dislocation glide. These experiments evolved to a steady-state stress that did not depend on starting grain size, showing evidence for subgrain development and dynamic recrystallization by bulge nucleation, interpreted to reflect dislocation creep with limited recovery by climb. The mechanical behavior and microstructures of glaucophane in our experiments are consistent with experiments on other low-symmetry minerals as well as microstructural observations from natural blueschists. The strain rate stepping experiments were used to develop a dislocation creep flow law for glaucophane with values of A = 2.23 x 10⁵ MPa^-n s^-1, n = 3, and Q = 341 ± 37 kJ/mol. A deformation mechanism map comparing our dislocation creep flow law to an existing flow law for blueschist diffusion creep indicates dislocation creep should activate at lower temperatures, higher stresses and larger diffusion lengthscales. Viscosities predicted by our flow law for a typical subduction strain rate of 1 x 10^-12 s^-1 lie between quartz and eclogite dislocation creep for the blueschist stability field, implying that mafic oceanic crustal rocks remain strong relative to quartz-rich metasediments all along the subduction interface.

Received: 22 May 2024 – Discussion started: 29 May 2024

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Lonnie Justin Hufford, Leif Tokle, Whitney Maria Behr, Luiz Grafula Morales, and Claudio Madonna

Status: final response (author comments only)

RC1:
'Comment on egusphere-2024-1507', Anonymous Referee #1, 25 Jun 2024

As main component in blueschist-facies mafic oceanic crust, the quantification of glaucophane’s mechanical behaviour at high-pressure, high-temperature conditions has been the centre of the present study by Hufford et al. To understand its deformation behaviour, the authors conducted several hydrostatic experiments in addition to a series of deformation tests using a Griggs-deformation apparatus. The microstructures of the recovered samples were carefully studied using analytical techniques such as polarised light microscopy and electron-backscatter diffraction. The experimental and microstructural data are all of high quality and support the interpretation proposed by the authors. Furthermore, the well written and structured text makes it easy to follow the authors’ reasoning. I enjoyed reading the manuscript, acknowledge the amount of experimental work presented, and find that the present study is a valuable contribution, which will be of interest to a broad geoscientific community. Therefore, I am highly supportive of this work and suggest publication in EGUsphere after minor revision.
A few minor comments:
Influence of grain size/porosity
The observed relation between grain size of the starting material and peak stress is very interesting (Fig. 2b). Could it, however, be that this apparent relation is only indirectly caused by grain size but rather a product of differences in porosity? As you started with powders, I assume that there is some initial porosity present in the starting material. This initial porosity could depend on grain size or rather the grain size distribution in the sample and influence the peak stress of the samples (e.g., Hirth & Tullis, 1991). Renner et al. (2007) experimentally showed that the measured strength of their quartz-calcite mixtures mainly depended on the amount of quartz, which itself positively correlates with porosity (Figs. 2; 6 in Renner et al., 2007). Even if the authors find no such relation between porosity and strength in their sets of experiments, it would be informative to provide measured/estimated porosities present in the starting material. Related to this comment, I find the statement in line 380-385 a bit too provocative as it does not include any discussion on other aspects that may influence rock strength such as existing textures that could act as planes of weakness.
Hydrostatic tests
It would be good to know whether or not the selected durations for hydrostatic pressing are somewhat related with the duration of deformation or rather with the duration of hot-isostatic pressing (HIP) prior to the onset of deformation. As noted in the manuscript, the hot-pressed powders are important to have an idea about the initial sample thickness, grain size, initial microstructure, etc., with which I fully agree. However, no explanation is given, why different run durations were chosen and it would be interesting to know on which basis you decided to run the hydrostatic tests for 24-88 h. Furthermore, the HIP runs were only conducted at 700 ˚C, although the deformation tests were run at 725 and 750 ˚ C as well. In the supplementary material, it is written that glaucophane becomes unstable at temperatures higher than 725 ˚C and that the run durations were chosen accordingly to avoid an impact of glaucophane breakdown on the mechanical data. I agree that this information belongs to the supplementary material. However, I find that the authors should explain in a bit more detail why they chose these hot-pressing durations/conditions in the main text.
Other comments:
Lines 197-199 and Fig. 2c: I agree that the reproducibility is very good for the 750 ˚C test (shear stress at 1^st and 4^th step), but for the other runs conducted at 700 and 725 ˚C, the reproducibility is far less striking. Please add some more explanation on how you decided that the sample reached steady state.
Lines 250-253: It would be helpful if you could be more specific and provide information on the experimental conditions and the type of material used in the studies by AveLallemant (1978) and Reynard et al. (1989).
Lines 272-275: Just a suggestion: The word ductile can be confusing as I have the impression that this term means different things in different geoscientific communities. Perhaps use: “transition from localized to distributed deformation”?

Citation: https://doi.org/10.5194/egusphere-2024-1507-RC1
- AC1: 'Reply on RC1', Lonnie Hufford, 03 Oct 2024
  
  Reply to Referee 1:
  Influence of grain size/porosity
  The observed relation between grain size of the starting material and peak stress is very interesting (Fig. 2b). Could it, however, be that this apparent relation is only indirectly caused by grain size but rather a product of differences in porosity? As you started with powders, I assume that there is some initial porosity present in the starting material. This initial porosity could depend on grain size or rather the grain size distribution in the sample and influence the peak stress of the samples (e.g., Hirth & Tullis, 1991). Renner et al. (2007) experimentally showed that the measured strength of their quartz-calcite mixtures mainly depended on the amount of quartz, which itself positively correlates with porosity (Figs. 2; 6 in Renner et al., 2007). Even if the authors find no such relation between porosity and strength in their sets of experiments, it would be informative to provide measured/estimated porosities present in the starting material.
  This is a good question and we agree with the reviewer that it’s worthwhile to look into. Based on the reviewer’s question, we returned to the SEM to examine our hydrostatic experiments with different grain sizes using backscatter imaging. The images show grain plucking in some places produced during thin sectioning, but otherwise there is no evidence for significant porosity in the samples and we could not find any differences in porosity between the different grain size fractions. In a revised manuscript, we would include images of the hydrostatic samples in the supplementary material.
  
  Related to this comment, I find the statement in line 380-385 a bit too provocative as it does not include any discussion on other aspects that may influence rock strength such as existing textures that could act as planes of weakness.
  This is a good point, and we agree with the reviewer that we should acknowledge that other textural features (in addition to grain size) may be important for oceanic crust rheology during subduction. In the revised manuscript, we will add a statement to this effect.
  
  Hydrostatic tests
  It would be good to know whether or not the selected durations for hydrostatic pressing are somewhat related with the duration of deformation or rather with the duration of hot-isostatic pressing (HIP) prior to the onset of deformation. As noted in the manuscript, the hot-pressed powders are important to have an idea about the initial sample thickness, grain size, initial microstructure, etc., with which I fully agree. However, no explanation is given, why different run durations were chosen and it would be interesting to know on which basis you decided to run the hydrostatic tests for 24-88 h. Furthermore, the HIP runs were only conducted at 700 ˚C, although the deformation tests were run at 725 and 750 ˚ C as well. In the supplementary material, it is written that glaucophane becomes unstable at temperatures higher than 725 ˚C and that the run durations were chosen accordingly to avoid an impact of glaucophane breakdown on the mechanical data. I agree that this information belongs to the supplementary material. However, I find that the authors should explain in a bit more detail why they chose these hot-pressing durations/conditions in the main text.
  The first experiment we conducted was a coarse-grained hydrostatic at 700˚C for 37 hours as a test for glaucophane stability. The aim of this experiment was for it to remain at hydrostatic conditions consistent with the run-in time for an experiment deforming at a shear strain rate of ~1e-5 s^-1. However, after conducting these strain rate stepping experiments we decided to also conduct constant rate experiments with a slower shear strain rate of ~1e-6 s^-1 at a finer grain size range, which we refer to as the medium grain size range. The longer hydrostatic experiment of 88 hours reflects the longer run-in time for these experiments. The shorter 24 hour hydrostatic experiment on the fine-grained powder was cut short due to technical issues.
  We will do as suggested by the reviewer and include this information about the hydrostatic experiment lengths to the main text, e.g.:
  
  "The different durations of the hydrostatics were intended to mimic the anticipated time to reach the hit point for subsequent constant strain rate experiments. However, one was cut short at 24 hours due to technical issues."
  
  Lines 197-199 and Fig. 2c: I agree that the reproducibility is very good for the 750 ˚C test (shear stress at 1st and 4th step), but for the other runs conducted at 700 and 725 ˚C, the reproducibility is far less striking. Please add some more explanation on how you decided that the sample reached steady state.
  We think the reviewer might be referencing the 700°C strain rate experiment as having good mechanical reproducibility instead of the 750°C experiment because we did not test reproducibility of different deformation steps in the 725 nor 750°C experiments. We determine a mechanical steady in these experiments when the stress is approximately constant with time and strain. We highlighted this region in black in Figure 2 of the original manuscript, where the value chosen in Table 2 is the average of the selected region, with all black regions showing stress variations of < 10 MPa.
  
  Lines 272-275: Just a suggestion: The word ductile can be confusing as I have the impression that this term means different things in different geoscientific communities. Perhaps use: “transition from localized to distributed deformation”?
  Thank you for the comment. We would prefer to keep the terminology as-is. We consistently use brittle-ductile throughout the manuscript (in Sections 4.1 and 4.2) and align with terminology associated with Kohlstedt et al. 1995 Figure 2. We will add a statement to the revised manuscript though about our definition of this term for clarity.
  
  Lines 250-253: It would be helpful if you could be more specific and provide information on the experimental conditions and the type of material used in the studies by AveLallemant (1978) and Reynard et al. (1989).
  We will add text to this sentence highlighting that the Avelallemant (1978) study on clinopyroxene is based on experimental results, while the Reynard (1989) study is based on analyses of naturally deformed amphibole. Given the difference in deformation conditions, we think it might be better to reference the Figure in the Ave Lallemant paper from which we derive our interpretation.
  Here is the modified sentence that we will place in the revised text:
  
  "An explanation for the difference in mechanical behavior between clinopyroxene and amphibole is that experimental results on clinopyroxene show mechanical twinning has a lower resolved shear stress than glide at brittle-ductile transition conditions in clinopyroxene (e.g., Figure 6 in AveLallemant, 1978) than what is suggested by naturally deformed sodic amphibole (Reynard et al., 1989)."
  
  Citation: https://doi.org/10.5194/egusphere-2024-1507-AC1
RC2:
'Comment on egusphere-2024-1507', Anonymous Referee #2, 22 Jul 2024

Comments on the manuscript: “Dislocation creep near the frictional-viscous transition blueschist: Experimental constraints. By Hufford, Tokle, Behr, Morales, and Madonna.
The manuscript describes a set of experiments on polycrystalline glaucophane performed in a solid deformation apparatus. A flow law is derived from the data. The manuscript is concisely written and contains interesting experimental data and observations that seem to be worth publishing. However, there are some major problems with inferring the main conclusion of the present manuscript from the data set presented, i.e. that dislocation creep should be the dominant deformation mechanism in the experimentally deformed samples. For this reason, I do not see the manuscript fit for publication. I suggest to re-write the manuscript with different conclusions after further data collection and to re-submit a completely revised manuscript before it can be published.
The major problems of the manuscript are as follows:
(1) Dislocation creep is inferred for the deformed samples after an initial stage of peak stress and subsequent weakening during which grain size reduction occurs. Dislocation creep is a grain size independent deformation mechanism, so grain size should not matter for the activation of this deformation mechanism. Therefore, it is inconceivable why an initial grain size reduction should be required to activate dislocation creep in these samples. Furthermore, it is questionable whether a flow law can be derived from such experiments that are ambiguous or difficult to interpret.
(2) The authors infer that steady state deformation by dislocation creep is partially achieved by re-orientation of grains for easy slip after early stages of deformation. However, the documented CPO´s of samples with low to high strain and even hydrostatic initial treatment are not substantially different from one another in terms of a distribution of maxima for the different crystal orientations.
(3) The same authors have published a paper recently where they infer that diffusion creep in glaucophane is the rate-limiting deformation mechanism in blueschist samples at temperatures of 650-700C, while the deformation temperatures in the present study is 700-750C at the same pressures and similar strain rates. Flow stresses at 700C are similar in both studies, so how can in one case the rate limiting deformation mechanism be dislocation creep and in the other diffusion creep? Admittedly, the composition of samples in the two studies is different, glaucophane is inferred to be strain rate controlling in the former study, and for glaucophane very different deformation mechanisms are inferred in both studies.
(4) In the present study glaucophane is deformed well outside its thermodynamic stability field. In the light of the fact that glaucophane is a high pressure mineral, it is not clear why such a low pressure was used in the deformation experiments? (The solid medium apparatus typically can be operated routinely at pressures of 1.5 GPa or more) In any case, reconstitution of the glaucophane material is likely to involve some chemical changes (e.g. H2O content), because according to earlier studies, breakdown of glaucophane should occur. Even subtle changes in composition may introduce other driving potentials than strain energy for recrystallisation, which in such a case might not be strictly “dynamic”. Dislocation creep depends on the operation of recovery processes such as dynamic recrystallisation, so that this process (and to demonstrate its operation) is of key importance. In this manuscript, only light micrographs are presented, no SEM backscatter images that may indicate chemical changes. A thorough documentation of microstructures by SEM-BS or EDS maps is required to demonstrate that recrystallisation takes place without chemical changes.
Detailed comments:
Line 24: certainly, one or more works by P. Agard should be cited here.
Line 33: “hierarchy” instead of “heirarchy”
Fig. S2: the labels of the grain size ranges are partly missing (two brown coloured ranges). Furthermore, the 63 to 355 micron grain size fraction (according to the label) includes smaller grains, too. Please correct the figure or the text.
Lines 80-89 and lines 31-42 in supplementary data: the procedure employed here appears problematic for reasons concerning the stability of glaucophane. In a careful early experimental study of glaucophane stability by Maresch (1977) it was found that at 1 GPa the stability of glaucophane extends to about max. 525C. This value is about 200C lower than the temperatures employed in this study. In addition, it is unclear why relatively low pressures of 1 GPa were employed here, when the solid medium apparatus allows higher pressures to be applied routinely. More recent studies by Graham et al. (1989) demonstrated that the H2O content of glaucophane may vary systematically in Na- and other amphiboles with P and T. Corona et al. (2013) and Cheng et al. (2019) and other studies find solid solution effects well below the temperature range of the breakdown reaction of glaucophane. Only the breakdown melting conditions are cited in the manuscript, but other processes at much lower temperature conditions will be relevant here, too. From all of these studies it is obvious that at the experimental conditions employed here glaucophane is deformed well outside its thermodynamic stability field. Even if major decomposition reactions are not observed as indicated in the supplementary data, a more or less subtle shift in composition (e.g. by a change in H2O content) should be detectable in SEM-BS images or EDS maps. Some compositional shift is documented in Fig. S3. Such compositional changes may have important consequences for the recrystallisation process and its driving potentials. As dislocation creep depends on, e.g., recrystallisation as a recovery process, additional driving potentials, such as chemical potentials, affect the reconstitution process, so that the recrystallisation might not be termed “dynamic” (i.e. induced by strain energy only). In any case, the light micrographs presented in this manuscript as the only microstructural documentation are in many ways inadequate to document the full range microstructural features of the samples. Not only is the resolution insufficient, but also any chemical information is missing. This study requires thorough documentation of the microstructures by SEM-BS and possibly EDS maps.
Lines 117-118: Fig. 1 does not show a CPO, Fig. 4 shows CPO´s.
Lines 118-119: these light micrographs and their magnification are insufficient and inadequate to show the features required to document the details of this study (see comment above). SEM-BS and possibly ED maps are required here. For example, compositional changes near the grain boundary regions or along cracks, etc. would indicate additional processes that take place in these samples.
Lines 119-120: the strengths of CPO´s, appears to be approximately the same in all pole figures. The (001) pattern is dominated by a broad single maximum, not a girdle.
Lines 120-122: this sentence is obscure and difficult to understand: If there is crushing during hot pressing, how can the grain size distribution of hot pressed samples correlate with that of the initial grain fraction before hot pressing? Please explain. Furthermore, Fig. 1 does not show grain size distributions.
Line 138: the red arrow points to a discrete low angle boundary - this is not undulatory extinction (which is a change in crystal orientation over a distributed region).
Line 145: there is no undulatory extinction in Fig. 5d, only discrete low angle boundaries.
Lines 146-148: the maxima of (110) and (100) are inclined in an antithetic sense with respect to the shear sense in Fig. 4 for the peak stress sample. Consequently, the weak girdle for (001) is also inclined and not parallel to the shear plane (certainly not parallel to the shear direction!).
Line 152: it is not clear why the pull-apart structures and cracked zones should be kink-bands? Pull-apart and cracked zones adequately describes the observation. Again, it is impossible to observe undulatory extinction in these micrographs.
Line 155: again, there is no obvious indication for kink bands.
Lines 156-158: the point maxima in (110) and (100) are indeed subnormal to the shear plane, while the [001] direction is subparallel to the shear direction. All of these orientations appear slightly rotated from the peak stress orientation in terms of geometry, but they do not differ in, e.g., type or strength of the CPO.
Line 161-171: the low angle boundaries are interpreted here as subgrain boundaries in the sense of ordered dislocation substructures, whereas their morphology and general appearance does not differ form the low angle misorientations described above (which are interpreted as cracking features). There is no evidence presented here why these low angle boundaries should not be interpreted as healed fractures, like the same features above.
Lines 171-175: the (010) maximum is more or less the same as in the other pole figures, only it is more central to the pole figure. In the weakening sample pole figure the (010) point maximum is even better defined than in the steady state sample. The expression of maxima is mainly due to the much larger number of points for the pole figure (3 times as many in the steady state sample).
Lines 186-187: please define: what is “fine grained”? Obviously, there is a wide range of grain sizes in these samples. It is critical to know whether there is a defined range of small grains of, e.g., similar size in all samples or not, i.e., whether the size ranges, especially for the small grains, are identical in different samples or not. If dynamic recrystallization had caused the fine grain sizes, their size should be identical in all samples, because the flow stresses are very similar (piezometric relationship), and they should show a very narrow size range.
Lines 195-205: what is the significance of dashed and solid lines in Fig. 2d? This should be explained in the figure caption.
Lines 208-213: recrystallization is inferred as a grain size reduction mechanism here, while cracking, etc. is inferred to the place in larger grains in the same samples (e.g. crushing at large grains). It is unclear why cracking as a grain size reduction process is not considered here - why should the grain size reduction process change in these samples at the same conditions of P,T, and strain rate?
218-220: the microstructural features of slip on cleavage planes, grain size reduction, and kinking are inferred as the result of dislocation activity without any evidence given. The initial grain size reduction during hot pressing and after peak stress is inferred to take place by fracturing (referred to as “crushing”) above. The observed features in Fig. 6 are consistent with healed cracks producing internal misorientation in grains (“subgrains”), etc. There is no need to infer another processes than cracking (e.g., dislocation processes) if the microstructures are similar in the same samples at the same conditions of deformation. The crushed grains in peak stress samples are termed “core mantle structures” here (Fig. 6) - what are the data or observations to distinguish these core mantle structures from the different interpretation above (“grain crushing”)?
Lines 222-223: how can fewer slip systems be available, if the CPO-type (Fig. 4) looks more or less the same for coarse grained and steady state samples?
Lines 226-227: cleavage slip and granular rotation are consistent with brittle deformation processes, not necessarily with dislocation creep.
Line 228: dislocation creep with limited climb should result in strain hardening, not steady state. Cleavage slip is a type of frictional deformation, not a plastic process. It is difficult to conceive that a steady state stress results from a “transition to dislocation creep”. Why should “a transition stage” from different grain sizes to a fine grain size produce a steady state stress? The reasons given for the transition do not support the interpretation, because:

(a) stress exponents of 2 to 3 (observed here) are lower than 3 to 5 expected for dislocation creep.If frictional processes operate in conjunction with dislocation creep, the expected stress exponent should even be higher than 3-5, certainly not lower.

(b) dislocation creep is a grain size independent deformation mechanism, so grain size should not affect its efficiency and thus the rheology. For this reason, the inference of dislocation creep as the dominant deformation mechanism in the steady state part of deformation following a grain size reduction does not make sense.

(c) “subgrain formation” can easily be explained by non-plastic processes. Core-mantle-structures of Fig. 6 resemble grain crushing structures in “lower strain” samples or hot pressed samples, i.e., brittle deformation processes are inferred for similar microstructures at lower strain.

(d) there are no systematic changes in the CPO patterns that might account for a change in deformation mechanism.
Lines 234-236: no clear evidence is given for inferring the recovery process of bulge nucleation.
Lines 301-304: this statement is quite correct; the cited studies do not document any deformation process that causes the CPO types. For this reason it is interesting to refer to these papers, but their CPO description does not contribute to the understanding or interpretation of the observed CPO patterns.
Lines 315-317: The operation of a (100)[001] slip system has not been demonstrated by observations of this study. The CPO´s shown in Fig. 4 do not change substantially from hydrostatic to steady state samples. Fig. 6 explains some of the “subgrain” misorientations by this and other slip systems, but their resolved shear stresses are extremely low for the large grain in the kinematic framework of the sample, so that it is, for geometric reasons alone, questionable that dislocation activity has caused their origin.
Lines 344-347: Reynard et al. (1989) show an image of small glaucophane grains but do not demonstrate that these have formed by dynamic recrystallisation.
Lines 355-359: As pointed out above, the range of n-values is lower than 3 to 5, so that dislocation creep (in conjunction with other deformation mechanisms such as friction) is unlikely. In the light of ambiguous microstructures presented here, together with an ambiguous database of n-values, the inferred deformation mechanism is one of the least probably deformation mechanisms that can be inferred from this data set. The activation energy given for dislocation creep is indistinguishable from or even lower than that given for diffusion creep by the same authors. This would be a very unusual result compared to known flow laws for silicates where the activation energy of diffusion creep typically is lower than that of dislocation creep, not the same or higher.
Lines 418-420: this statement is questionable, because blueschists contain substantial amounts of weak minerals (white mica, chlorite) that will deform and accommodate strain at considerably lower stresses than glaucophane at low temperatures, so that conditions for dislocation creep may not be reached in glaucophane.

,

Citation: https://doi.org/10.5194/egusphere-2024-1507-RC2
- AC2:
  'Reply on RC2', Lonnie Hufford, 03 Oct 2024
  Reply to Referee 2:
  "The manuscript describes a set of experiments on polycrystalline glaucophane performed in a solid deformation apparatus. A flow law is derived from the data. The manuscript is concisely written and contains interesting experimental data and observations that seem to be worth publishing. However, there are some major problems with inferring the main conclusion of the present manuscript from the data set presented, i.e. that dislocation creep should be the dominant deformation mechanism in the experimentally deformed samples. For this reason, I do not see the manuscript fit for publication. I suggest to re-write the manuscript with different conclusions after further data collection and to re-submit a completely revised manuscript before it can be published."
  We thank the reviewer for their time and extensive comments. To summarize our overall response:
  The reviewer’s comments prompted us to take a closer look at our experimental samples using backscatter imaging and EDS, and we concur with the reviewer that there is evidence for diffusion creep processes of other phases in the fine-grained shear zones in our high strain samples. We appreciate the reviewer’s suggestion to look at this more closely and we consider this a nice example of how constructive peer review can lead to stronger interpretations and science presentation.
  
  In our view, the microstructures and the relatively high stress exponent (2-3) characterizing the mechanical data still requires a significant contribution from dislocation processes, but we acknowledge a simultaneous role of diffusion creep in the localized shear zones.
  
  To summarize our suggested way forward:
  Independent of this manuscript, we have been preparing a separate one presenting load-stepping experiments on the same glaucophane aggregate starting material, and these very nicely constrain dislocation creep and glide flow laws based on both their detailed microstructures and their mechanical data. When we combine this unpublished dislocation creep flow law with the diffusion creep flow law from Tokle et al. 2023 using rheological mixing models, we find a good match to the mechanical data from the strain-rate-stepping experiments presented in this current SE manuscript. We propose, therefore, to submit that manuscript as a companion paper to this one, while simultaneously revising this one to better document the mixed rheology that appears to be represented here.
  
  We think that presenting the whole suite of glaucophane data together in the companion manuscripts (also synthesizing them in the context of Tokle et al. 2023) will provide the community with a more complete picture of glaucophane deformation mechanisms and their mechanical properties.
  Below we respond directly to additional comments from the reviewer point by point.
  
  "The major problems of the manuscript are as follows:
  (1) Dislocation creep is inferred for the deformed samples after an initial stage of peak stress and subsequent weakening during which grain size reduction occurs. Dislocation creep is a grain size independent deformation mechanism, so grain size should not matter for the activation of this deformation mechanism. Therefore, it is inconceivable why an initial grain size reduction should be required to activate dislocation creep in these samples. Furthermore, it is questionable whether a flow law can be derived from such experiments that are ambiguous or difficult to interpret."
  The grain size sensitivity of viscous creep mechanisms pertains to the extent to which their deformation rate is sensitive to grain size, whereas in the text we are arguing that the onset/activation of dislocation creep is sensitive to grain size, NOT its rate. At the coarse grain sizes of our experiments, this is not surprising for several reasons. Firstly, since glaucophane is a tabular/prismatic mineral, the hot-pressed starting materials have a moderate CPO, which (initially) limits the slip systems available for dislocation glide. Secondly, glaucophane is a low symmetry mineral, and there are likely significant differences in the Peierls stress of its various slip systems. As we describe in the text, the combined processes of cleavage-slip, granular rotation, and dislocation motion leads to more variable orientations in the aggregate and therefore unlocks more potential slip systems and produces greater strain compatibility among the deforming grains (closer to satisfying the von Mises criterion). As we discuss in the text, a similar interpretation was made by Tullis and Yund (1985,1987,1992) for feldspar aggregates (also low symmetry).
  
  "(2) The authors infer that steady state deformation by dislocation creep is partially achieved by re-orientation of grains for easy slip after early stages of deformation. However, the documented CPO´s of samples with low to high strain and even hydrostatic initial treatment are not substantially different from one another in terms of a distribution of maxima for the different crystal orientations."
  Shape preferred orientations associated with rotation, and crystallographic preferred orientations produced from dislocation creep, can look identical. Several of the glaucophane slip systems that are likely activated in our experiments are predicted to produce a CPO that is exactly the same as what would be produced by rigid body rotation. This has also been shown recently by Ott et al., 2024. This is in fact one of the main points that we try to make in the text in Section 4.1.3 and with our concluding statement in that section “that CPO in tabular minerals is not a diagnostic indicator of deformation mechanisms such as dislocation or diffusion creep".
  
  "(3) The same authors have published a paper recently where they infer that diffusion creep in glaucophane is the rate-limiting deformation mechanism in blueschist samples at temperatures of 650-700C, while the deformation temperatures in the present study is 700-750C at the same pressures and similar strain rates. Flow stresses at 700C are similar in both studies, so how can in one case the rate limiting deformation mechanism be dislocation creep and in the other diffusion creep? Admittedly, the composition of samples in the two studies is different, glaucophane is inferred to be strain rate controlling in the former study, and for glaucophane very different deformation mechanisms are inferred in both studies."
  
  The reviewer is mistaken regarding our flow stresses and the flow stresses of Tokle et al., 2023 at similar temperatures. At 700°C, the stresses in these two studies vary by more than 100 MPa for a given strain rate. Based on the observation of a diffusion creep component in the high strain shear zones in our samples, we now interpret our mechanical data to be most consistent with mixed diffusion and dislocation creep and would present this in a revised version of the manuscript.
  
  "(4) In the present study glaucophane is deformed well outside its thermodynamic stability field. In the light of the fact that glaucophane is a high pressure mineral, it is not clear why such a low pressure was used in the deformation experiments? (The solid medium apparatus typically can be operated routinely at pressures of 1.5 GPa or more) In any case, reconstitution of the glaucophane material is likely to involve some chemical changes (e.g. H2O content), because according to earlier studies, breakdown of glaucophane should occur. Even subtle changes in composition may introduce other driving potentials than strain energy for recrystallisation, which in such a case might not be strictly “dynamic”. Dislocation creep depends on the operation of recovery processes such as dynamic recrystallisation, so that this process (and to demonstrate its operation) is of key importance. In this manuscript, only light micrographs are presented, no SEM backscatter images that may indicate chemical changes. A thorough documentation of microstructures by SEM-BS or EDS maps is required to demonstrate that recrystallisation takes place without chemical changes."
  It’s true that 1 GPa and our range of temperatures is outside the range of glaucophane stability under water-saturated conditions. The experiments the reviewer notes by Maresch (1977) as well as a number of other more recent experiments (e.g. Corona et al. 2013, Cheng et al., 2019) were conducted under water-saturated conditions, whereas our experiments are conducted with no water added (and certainly not water saturated). Previous studies (Ernst, 1961) on non-water saturated conditions show that glaucophane can be stable up to 800°C (Figure 2 from Maresch, 1977).
  Additionally, the choice of 1 GPa was used for several other reasons, including: a) It allows comparison to the many other experiments conducted at that pressure on glaucophane and/or similar amphibole-bearing aggregates, as shown in Figure 9 of the original manuscript. 2) Also as discussed in the text, we have an initial component of brittle deformation— this means the peak stress in the initial deformation stages is sensitive to confining pressure and going to higher pressures would’ve produced much higher initial yield stresses (producing more technical challenges and risk to the rig components). 3) Our initial hydrostatic experiments showed that glaucophane remained stable for the durations of those experiments, so we were hopeful that it would remain stable during the deformation experiments as well. For the most part, glaucophane did remain stable throughout all of our experiments, except in the fine-grained shear zones.
  
  "Detailed comments:
  Line 24: certainly, one or more works by P. Agard should be cited here."
  We’ve added reference to Agard et al., 2018– a review paper on subduction interfaces.
  
  "Line 33: “hierarchy” instead of “heirarchy”"
  Thank you, this has been fixed.
  
  "Fig. S2: the labels of the grain size ranges are partly missing (two brown coloured ranges). Furthermore, the 63 to 355 micron grain size fraction (according to the label) includes smaller grains, too. Please correct the figure or the text."
  Thank you for the comment. There are only 3 grain size fractions, but the boxes have some transparency and give the appearance of being more than 3. We will make this clearer in a revised manuscript.
  
  "Lines 80-89 and lines 31-42 in supplementary data: the procedure employed here appears problematic for reasons concerning the stability of glaucophane. In a careful early experimental study of glaucophane stability by Maresch (1977) it was found that at 1 GPa the stability of glaucophane extends to about max. 525C. This value is about 200C lower than the temperatures employed in this study. In addition, it is unclear why relatively low pressures of 1 GPa were employed here, when the solid medium apparatus allows higher pressures to be applied routinely. More recent studies by Graham et al. (1989) demonstrated that the H2O content of glaucophane may vary systematically in Na- and other amphiboles with P and T. Corona et al. (2013) and Cheng et al. (2019) and other studies find solid solution effects well below the temperature range of the breakdown reaction of glaucophane. Only the breakdown melting conditions are cited in the manuscript, but other processes at much lower temperature conditions will be relevant here, too. From all of these studies it is obvious that at the experimental conditions employed here glaucophane is deformed well outside its thermodynamic stability field."
  
  As discussed above, amphiboles are difficult materials for experiments and we have been doing our best to characterize their rheological properties with the tools we have, because of how important blueschists are to subduction environments. Independent of glaucophane stability (already addressed above), there are a lot of tradeoffs required in experiments— for example, one always has to use higher temperatures to offset the faster experimental strain rates. This is done for nearly all geological materials (e.g. olivine experiments are routinely conducted at 900-1400°C, but extrapolated to 700-1100°C; quartz, conducted at 700-1000°C, extrapolated to mid-crustal T, etc…). This tradeoff can cause instability problems in many phases; however, at the same time, the short duration of experiments means that reactions commonly do not proceed as far as they would in nature, so one can ideally still characterize the properties of these materials prior to their breakdown (and in some cases it’s quite interesting to examine the mechanical implications of the breakdown process itself, as in many previously published experiments). As noted above, glaucophane did remain stable throughout all of our experiments, except in the fine-grained shear zones.
  
  "Even if major decomposition reactions are not observed as indicated in the supplementary data, a more or less subtle shift in composition (e.g. by a change in H2O content) should be detectable in SEM-BS images or EDS maps. Some compositional shift is documented in Fig. S3. Such compositional changes may have important consequences for the recrystallisation process and its driving potentials. As dislocation creep depends on, e.g., recrystallisation as a recovery process, additional driving potentials, such as chemical potentials, affect the reconstitution process, so that the recrystallisation might not be termed “dynamic” (i.e. induced by strain energy only). In any case, the light micrographs presented in this manuscript as the only microstructural documentation are in many ways inadequate to document the full range microstructural features of the samples. Not only is the resolution insufficient, but also any chemical information is missing. This study requires thorough documentation of the microstructures by SEM-BS and possibly EDS maps."
  
  We thank you for the suggestion to collect BSE and EDS data. When doing so, we noticed the precipitation of a compositionally new amphibole and the precipitation of albite in the finest-grained shear zones (constituting up to 30% total in the shear zones) in the high strain samples. Thus we concur with the reviewer that the steady-state flow is likely associated with some component of diffusion-related creep. We plan to rewrite parts of the manuscript to better document and interpret this.
  
  "Lines 117-118: Fig. 1 does not show a CPO, Fig. 4 shows CPO´s."
  Thank you, the figure call out will be fixed to call out the CPO Figure (Figure 4).
  
  "Lines 118-119: these light micrographs and their magnification are insufficient and inadequate to show the features required to document the details of this study (see comment above). SEM-BS and possibly ED maps are required here. For example, compositional changes near the grain boundary regions or along cracks, etc. would indicate additional processes that take place in these samples."
  
  We concur with the reviewer on this point and will be happy to include BSE and EDS data in a revised manuscript.
  
  "Lines 119-120: the strengths of CPO´s, appears to be approximately the same in all pole figures. The (001) pattern is dominated by a broad single maximum, not a girdle."
  
  We find “weak girdle” describes the pole figure appropriately as there is no break in the spread of data.
  
  "Lines 120-122: this sentence is obscure and difficult to understand: If there is crushing during hot pressing, how can the grain size distribution of hot pressed samples correlate with that of the initial grain fraction before hot pressing? Please explain. Furthermore, Fig. 1 does not show grain size distributions."
  
  As we note in the text: “all three grain size fractions exhibit crushing at grain margins”. The amount of crushing during hot-pressing thus correlates with the starting grain size prior to hot-pressing because the starting grain size affects the distribution of grain boundaries. This is the exact procedure used in hundreds of Griggs experimental studies so we don’t consider more explanation to be needed in the text.
  
  "Line 138: the red arrow points to a discrete low angle boundary - this is not undulatory extinction (which is a change in crystal orientation over a distributed region)."
  Many of the grains show undulose extinction– but in a still photo where the stage cannot be turned, the sweeping extinction can be difficult to see. To prevent confusion, we will remove the red arrow and its reference, but we retain the description of undulose extinction because it is indeed present.
  
  "Line 145: there is no undulatory extinction in Fig. 5d, only discrete low angle boundaries."
  Several of the grains show sweeping low angle misorientations in their interiors, e.g. bottom right, upper left. As we note in the text, they are not common (but they are indeed visible in the mis2mean map) and in the thin sections.
  
  "Lines 146-148: the maxima of (110) and (100) are inclined in an antithetic sense with respect to the shear sense in Fig. 4 for the peak stress sample. Consequently, the weak girdle for (001) is also inclined and not parallel to the shear plane (certainly not parallel to the shear direction!)."
  
  The maxima are in somewhat similar orientations to the hydrostatic, and mechanical steady state orientations and are perpendicular to the shear plane, though we acknowledge that the girdling in the (001) plot should be described better as it is not parallel to the shear direction. This will be addressed in the revised text.
  
  "Line 152: it is not clear why the pull-apart structures and cracked zones should be kink-bands? Pull-apart and cracked zones adequately describes the observation. Again, it is impossible to observe undulatory extinction in these micrographs."
  
  We are not saying pull-apart structures and cracked zones are kink bands, but that in some cases they can localize at kink bands.
  
  "Line 155: again, there is no obvious indication for kink bands."
  The slight misorientation of the grain and the angle of the misorientation band with respect to the shear plane (and other kink bands in the sample) are consistent with this as a kink band.
  
  "Lines 156-158: the point maxima in (110) and (100) are indeed subnormal to the shear plane, while the [001] direction is subparallel to the shear direction. All of these orientations appear slightly rotated from the peak stress orientation in terms of geometry, but they do not differ in, e.g., type or strength of the CPO."
  
  As the EBSD maps vary in size and number of grains, their numerical values must be evaluated carefully. We interpret the girdling in the (010) and (001) to be slightly weaker than the peak stress sample, but agree they do not show a large difference.
  
  "Line 161-171: the low angle boundaries are interpreted here as subgrain boundaries in the sense of ordered dislocation substructures, whereas their morphology and general appearance does not differ form the low angle misorientations described above (which are interpreted as cracking features). There is no evidence presented here why these low angle boundaries should not be interpreted as healed fractures, like the same features above."
  
  We never interpreted any grains as having healed fractures. In the discussion we interpret (some) of these as subgrains that eventually become finally separated through a combination of rotation and fracture (e.g. as suggested for quartz, see Stipp and Kunze, Tectonophysics, 2008, Figure 1).
  
  "Lines 171-175: the (010) maximum is more or less the same as in the other pole figures, only it is more central to the pole figure. In the weakening sample pole figure the (010) point maximum is even better defined than in the steady state sample. The expression of maxima is mainly due to the much larger number of points for the pole figure (3 times as many in the steady state sample)."
  
  As previously mentioned, the EBSD maps vary in size and number of grains, their numerical values must be evaluated carefully. We do not make any quantitative use of these fabrics for this reason.
  
  "Lines 186-187: please define: what is “fine grained”? Obviously, there is a wide range of grain sizes in these samples. It is critical to know whether there is a defined range of small grains of, e.g., similar size in all samples or not, i.e., whether the size ranges, especially for the small grains, are identical in different samples or not. If dynamic recrystallization had caused the fine grain sizes, their size should be identical in all samples, because the flow stresses are very similar (piezometric relationship), and they should show a very narrow size range."
  As we tried to describe in the text, there are multiple grain size reduction mechanisms, including:
  crushing during hot pressing (occurs in all powder experiments, including those that were initially used to produce piezometric relationships);
  
  brittle deformation during peak stress; and
  
  dynamic recrystallization of porphyroclasts during dislocation creep at steady state conditions.
  
  Given this complexity, we do not expect a uniform grain size, on average, because there are mixed mechanisms contributing to grain size reduction (not just dynamic recrystallization). That said, there are well known microstructural criteria when examining host-grain—new-grain relationships that allow us to distinguish brittle grain size reduction (cataclasis) and dynamic recrystallization (involving bulge nucleation and subgrain rotation recrystallization) (e.g. see Passchier & Trouw, Microtectonics). These include, for example, that cataclasis tends to produce angular grain boundaries, commonly fractal grain size distributions, and high misorientations between new grains and host grains. By contrast, DRX tends to produce lobate or sutured grain boundaries, more uniform grain sizes ~similar to subgrain sizes, and progressively increasing (but small) misorientations with respect to their hosts (e.g. see Stipp & Kunze). These distinctions are very clearly on display in our EBSD maps if you compare Fig. 5a,c and 5b,e (cataclastic) to 5c,f,g, and 6 (recrystallization). In the revised manuscript, we will describe these differences in greater detail in the text to help clarify for the reader.
  
  "Lines 195-205: what is the significance of dashed and solid lines in Fig. 2d? This should be explained in the figure caption."
  We explain in the text, but not the figure caption. Thank you, this will be fixed.
  “The solid lines show the stress exponent fit to all data points from an experiment and the dashed lines show only points used from the 725°C, and 750°C data when calculating the flow law.”
  
  "Lines 208-213: recrystallization is inferred as a grain size reduction mechanism here, while cracking, etc. is inferred to the place in larger grains in the same samples (e.g. crushing at large grains). It is unclear why cracking as a grain size reduction process is not considered here - why should the grain size reduction process change in these samples at the same conditions of P,T, and strain rate?"
  
  As discussed in previous responses, we interpret the initial cataclasis to unlock additional orientations for dislocation glide, which then permits a greater contribution from dislocation (and as we now interpret diffusion) creep. A change in mechanism in our experiments is required to explain the mechanical data that shifts from peak stress, to weakening to steady-state. This is quite common in experiments with relatively coarse-grained grain sizes, with our reference to feldspar experiments being a close and robust analog.
  
  "218-220: the microstructural features of slip on cleavage planes, grain size reduction, and kinking are inferred as the result of dislocation activity without any evidence given. The initial grain size reduction during hot pressing and after peak stress is inferred to take place by fracturing (referred to as “crushing”) above. The observed features in Fig. 6 are consistent with healed cracks producing internal misorientation in grains (“subgrains”), etc. There is no need to infer another processes than cracking (e.g., dislocation processes) if the microstructures are similar in the same samples at the same conditions of deformation. The crushed grains in peak stress samples are termed “core mantle structures” here (Fig. 6) - what are the data or observations to distinguish these core mantle structures from the different interpretation above (“grain crushing”)?"
  There are several misinterpretations of our text here that we hope to clarify.
  The term crushing was only used to describe the characteristics of the hydrostatic experiments, and is describing a minor grain size reduction process that occurs in all aggregate experiments that undergo cold and hot-pressing procedures prior to deformation (i.e. every Grigg’s experiment ever conducted on a powdered material).
  
  We do not agree that the observed features in Fig. 6 are consistent with healed cracks. The sweeping, progressive misorientations from core to rim of the grain, the small (less than 10 degree) misorientations defining subgrains within the larger porphyroclasts, and the patches of uniformly-sized, strain free grains with highly sutured grain boundaries are consistent with dynamic recrystallization.
  
  A change in mechanism is absolutely required by the mechanical data, a point that the reviewer seems to have missed here. The brittle features described are only prominent in the peak stress and weakening samples, but the mechanical data clearly show a change to a steady-state flow stress with increasing strain (and simultaneously, brittle deformation features are overprinted in these samples by ductile shear zones). If brittle deformation had continued to dominate throughout the experiments as the reviewer suggests, then the samples should’ve simply continued weakening indefinitely.
  
  "Lines 222-223: how can fewer slip systems be available, if the CPO-type (Fig. 4) looks more or less the same for coarse grained and steady state samples?"
  
  Several slip systems will lead to the same CPO in glaucophane as produced by rotation. This again comes to our point that CPOs in low symmetry minerals have little to do with deformation mechanism.
  
  "Lines 226-227: cleavage slip and granular rotation are consistent with brittle deformation processes, not necessarily with dislocation creep."
  
  We agree and are not suggesting dislocation creep at pre-steady state conditions. We suggest brittle deformation occurs concurrently with dislocation processes (e.g. glide, not yet creep).
  
  "Line 228: dislocation creep with limited climb should result in strain hardening, not steady state. Cleavage slip is a type of frictional deformation, not a plastic process. It is difficult to conceive that a steady state stress results from a “transition to dislocation creep”. Why should “a transition stage” from different grain sizes to a fine grain size produce a steady state stress? The reasons given for the transition do not support the interpretation, because:
  
  (a) stress exponents of 2 to 3 (observed here) are lower than 3 to 5 expected for dislocation creep.If frictional processes operate in conjunction with dislocation creep, the expected stress exponent should even be higher than 3-5, certainly not lower."
  
  We agree that the stress exponents for the higher temperature experiments are a bit low, and now that we see evidence for diffusion processes in the fine-grained shear zones, we agree with the reviewer that this may explain the mixed exponents. We still interpret a contribution from simultaneous dislocation creep.
  
  "(b) dislocation creep is a grain size independent deformation mechanism, so grain size should not affect its efficiency and thus the rheology. For this reason, the inference of dislocation creep as the dominant deformation mechanism in the steady state part of deformation following a grain size reduction does not make sense."
  
  We addressed this point already– we are not implying that dislocation creep rate is grain size dependent, only the onset of dislocation creep is a function of grain size. We believe this is a rational interpretation of our observations.
  
  "(c) “subgrain formation” can easily be explained by non-plastic processes. Core-mantle-structures of Fig. 6 resemble grain crushing structures in “lower strain” samples or hot pressed samples, i.e., brittle deformation processes are inferred for similar microstructures at lower strain."
  
  We do not agree with the reviewer here– the grain boundaries in Figure 6 are highly irregular with evidence for strain-energy driven grain boundary migration (shown in the inset of the original manuscript, figure 5g). The grain sizes are much more uniform than would be expected for cataclasis and they are mantling a host grain that shows progressively increasing misorientations toward the grain margins. The new grains are also nearly strain-free. All of these microstructures are typical of dynamic recrystallization.
  
  "(d) there are no systematic changes in the CPO patterns that might account for a change in deformation mechanism."
  
  As addressed earlier, glaucophane (and many other tabular minerals) have slip systems that will produce the same CPO as rigid-body rotation, so no change in CPO is necessary and CPO cannot be used to say anything about deformation mechanism.
  
  "Lines 234-236: no clear evidence is given for inferring the recovery process of bulge nucleation."
  
  Figure 5g explicitly shows what we interpret as bulge nucleation in some of the fine grains.
  
  "Lines 301-304: this statement is quite correct; the cited studies do not document any deformation process that causes the CPO types. For this reason it is interesting to refer to these papers, but their CPO description does not contribute to the understanding or interpretation of the observed CPO patterns."
  
  Agreed, and that’s pretty much what we wrote in the text.
  
  Lines 315-317: The operation of a (100)[001] slip system has not been demonstrated by observations of this study. The CPO´s shown in Fig. 4 do not change substantially from hydrostatic to steady state samples. Fig. 6 explains some of the “subgrain” misorientations by this and other slip systems, but their resolved shear stresses are extremely low for the large grain in the kinematic framework of the sample, so that it is, for geometric reasons alone, questionable that dislocation activity has caused their origin.
  
  The CPO comment has been addressed in several instances above. We are unclear why the reviewer suggests the resolved shear stresses are low on the boundaries that we interpret as incipient subgrains– most of the ones we highlight with white arrows (e.g. either (010)[001] or (100)[001]) are at moderate to high angles to the shear plane (horizontal in Figure 6), which would mean they have moderate to high resolved shear stresses in the dextral shear kinematic reference frame of our experiments.
  
  "Lines 344-347: Reynard et al. (1989) show an image of small glaucophane grains but do not demonstrate that these have formed by dynamic recrystallisation."
  
  We are not sure which figure the reviewer is referring to. Reynard et al., 1989 suggest that in sample EM-S, there are core and mantle structures and subgrain development near the rims of the larger grains. They indicate that “rigid rotation, together with grain boundary sliding and plastic deformation of the crystal rims, have accommodated the rock deformation”. This is similar to our interpretation of dynamic recrystallization that is enhanced by fracture to help separate bulge-nucleated grains and subgrains from their host grains (also similar to one of the processes proposed by Stipp and Kunze (Tectonophysics, 2008, see their Figure 1) for brittle-plastic transition conditions in quartz).
  
  "Lines 355-359: As pointed out above, the range of n-values is lower than 3 to 5, so that dislocation creep (in conjunction with other deformation mechanisms such as friction) is unlikely. In the light of ambiguous microstructures presented here, together with an ambiguous database of n-values, the inferred deformation mechanism is one of the least probably deformation mechanisms that can be inferred from this data set. The activation energy given for dislocation creep is indistinguishable from or even lower than that given for diffusion creep by the same authors. This would be a very unusual result compared to known flow laws for silicates where the activation energy of diffusion creep typically is lower than that of dislocation creep, not the same or higher."
  
  It is not unusual for a diffusion creep activation energy to be higher than a dislocation creep activation energy for silicates. For example, based on current experimentally-derived flow laws, quartz diffusion creep is 220 kJ/mol (Rutter and Brodie, 2004) whereas dislocation creep is ~110-140 kJ/mol (Hirth et al., 2001, Tokle et al 2019, Lusk and Platt 2021. However, we concur with the reviewer that the stress exponents ranging from 2-3 and the presence of new phases in the fine-grained regions suggest a component of diffusion creep. We now interpret the steady-state increments of these experiments to reflect combined dislocation creep (in coarse-grained regions) and diffusion creep (in fine-grained shear zones) and will present this case in a revised manuscript.
  
  "Lines 418-420: this statement is questionable, because blueschists contain substantial amounts of weak minerals (white mica, chlorite) that will deform and accommodate strain at considerably lower stresses than glaucophane at low temperatures, so that conditions for dislocation creep may not be reached in glaucophane."
  
  What you have written here supports our original text:
  
  “Given that deviatoric stress magnitudes in subduction zones are typically estimated to be under 100 MPa (Lamb, 2006; Behr and Platt, 2013; Penney et al., 2017; Sibson, 2017; Li et al., 2018; Schmidt and Platt, 2022), it seems likely that brittle-plastic deformation or other mechanisms not captured by these two flow laws would dominate in low-temperature mafic blueschists. Overall these deformation mechanism maps provide a quantitative framework for investigating deformation in natural blueschists and predicting which mechanism is expected to dominate under which conditions as a function of rock properties, strain rate, and subduction thermal gradients.”
  
  Citation: https://doi.org/10.5194/egusphere-2024-1507-AC2

Lonnie Justin Hufford, Leif Tokle, Whitney Maria Behr, Luiz Grafula Morales, and Claudio Madonna

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Lonnie Justin Hufford, Leif Tokle, Whitney Maria Behr, Luiz Grafula Morales, and Claudio Madonna

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Short summary

We constrained the rheology of glaucophane aggregates deforming near its brittle-ductile transition with general shear deformation experiments. In the experiments, glaucophane first underwent work hardening and strain weakening associated with brittle grain size reduction and incipient dislocation processes, then evolved to steady-state dislocation creep. We developed a flow law that can be used to approximate the rheological behavior of mafic oceanic crust at blueschist facies conditions.


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