On the self-regulating effect of grain size evolution in mantle convection models: application to thermochemical piles

Schierjott, Jana; Rozel, Antoine; Tackley, Paul

doi:https://doi.org/10.5194/se-11-959-2020

Articles | Volume 11, issue 3

Solid Earth, 11, 959–982, 2020

https://doi.org/10.5194/se-11-959-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Solid Earth, 11, 959–982, 2020

https://doi.org/10.5194/se-11-959-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 11, issue 3

Research article 04 Jun 2020

Research article | 04 Jun 2020

On the self-regulating effect of grain size evolution in mantle convection models: application to thermochemical piles

Jana Schierjott et al.

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Final revised paper (published on 04 Jun 2020)
Preprint (discussion started on 30 Oct 2019)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review', Bradford Foley, 11 Dec 2019
- AC1: 'Response to reviewer 1', Jana Schierjott, 04 Feb 2020
RC2: 'Review of the manuscript On the self-regulating effect of grain size evolution in mantle convection models: Application to thermo-chemical piles by Jana Schierjott, Antoine Rozel, and Paul Tackley', Anonymous Referee #2, 07 Jan 2020
- AC2: 'Reply to reviewer 2', Jana Schierjott, 04 Feb 2020
EC1: 'Editorial assessment of revision 1', Julien Aubert, 06 Feb 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Jana Schierjott on behalf of the Authors (04 Feb 2020) Author's response Manuscript

ED: Publish as is (06 Feb 2020) by Julien Aubert

ED: Referee Nomination & Report Request started (21 Feb 2020) by Julien Aubert

RR by Anonymous Referee #2 (18 Mar 2020)

RR by Bradford Foley (30 Mar 2020)

Suggestions for revision or reasons for rejection

Review of Schierjott et al revised manuscript

The revised manuscript has been improved significantly in terms of its clarity and focus. I do still have one significant comment about the average properties of the piles and the regression used to constrain the effect of input parameters that needs to be addressed. I also have some minor comments about the wording or typos/grammar errors I found. After this the paper will be ready to be published.

Major comment:

The “self-regulating” nature of grain size in the piles is a major focus of the paper, and a regression for how the input parameters influence the resulting pile properties is used in part to demonstrate this self-regulating behavior. But I don’t think the regression really contributes much to the results as presented. The models track the thermal evolution of the Earth, so the mantle and piles are thermally evolving over time. There are also changes in convection regime during the course of the models. Taking a time average over the whole model run time then really doesn’t constrain the governing physics or necessarily even describe the system well. We don’t have oscillations around a mean, but long-term temporal trends, so a simple average is obscuring these dynamic time evolving behaviors.

Moreover, average grain size in the piles in particular is pointed out as being insensitive to the varied input parameters in the regression shown in table 5. But the input parameters varied are not likely to cause grain size in the piles to vary. The parameters that control grain size are the grain growth activation energy and pre-exponential constant (not varied here), and the partitioning factor, f_G. f_top is varied, but not f_bot. The grain size in the piles will be much more sensitive to f_bot than f_top. None of the parameters varied and used in the regression are likely to affect grain size in the piles, so the finding that grain size is insensitive to these parameters does not illuminate the physics controlling how pile grain size evolves.

A better explanation of the long-term trends in pile grain size is still needed. It must be related to the temporal trends in deformational work rate and temperature, as these control the grain reduction and growth terms.

Minor comments:

Lines 9-10: “grain size dominates the viscosity development” and “viscosity can be dominated by temperature.” This is unclear phrasing and should be re-worded. Also, Figure 6 shows both temperature and grain size controlling how viscosity in the piles evolves.

Line 209: “receive” is not the best word here; “determine” or “calculate” would work better

Line 223: Mulyukova & Bercovici 2017 also showed how grain growth activation energy changes the inferred temperature dependence of f_G

Pile detection: It’s not clear why pile material must also be above the given temperature criteria. The example in figure 1 of “lost pile material” still looks attached to the pile. So why is it not counted?

Line 281: piles are strong due to “non-linearity of non-Newtonian fluids” but the pile rheology is typically diffusion creep rather than dislocation creep.

Line 306: Units of deformation work rate should be Pa/s instead of Pa*s

Line 333: “absent” should be “absence”

Line 336: This statement is vague. Which properties and how similar are they? A figure showing actual numbers would be a more clear way to make this point. Though this statement also brings up the issue I raised above, about whether a time average over a model with long-term temporal trends really says much about the underlying physics governing the model results.

Section 3.3: It would be much easier to see the temporal trends with plots showing average mantle temperature (or potential temperature) and other properties evolving over time, rather than trying to tease it out by looking at 1-D profiles plotted at different times

Line 405: “succeed” should be “succeeded”

Line 406: “constant stress” here is overly vague, since constant could refer to constant in space, or constant stress over time. Wording needs to be clarified.

Line 523-525: This discussion of viscosity in high melt fraction regions comes out of nowhere; viscosity of melt fraction regions hasn’t been discussed at all up to now, so there’s context missing for why this discussion is here. Where in the model are the surprisingly high viscosities this sentence refers to?

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ED: Publish subject to minor revisions (review by editor) (30 Mar 2020) by Julien Aubert

AR by Anna Wenzel on behalf of the Authors (27 Apr 2020) Author's response

ED: Publish as is (27 Apr 2020) by Julien Aubert

ED: Publish as is (28 Apr 2020) by Susanne Buiter(Executive Editor)

Short summary

We investigate the size of mineral grains of Earth's rocks in computer models of the whole Earth. This is relevant because grain size affects the stiffness (large grains are stiffer) and deformation of the Earth's mantle. We see that mineral grains grow inside stable non-deforming regions of the Earth. However, these regions are less stiff than expected. On the other hand, we find that grain size diminishes during deformation events such as when surface material comes down into the Earth.