Timescales of chemical equilibrium between the convecting solid mantle and over- and underlying magma oceans

Bolrão, Daniela Paz; Ballmer, Maxim D.; Morison, Adrien; Rozel, Antoine B.; Sanan, Patrick; Labrosse, Stéphane; Tackley, Paul J.

doi:https://doi.org/10.5194/se-12-421-2021

Articles | Volume 12, issue 2

https://doi.org/10.5194/se-12-421-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/se-12-421-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 12, issue 2

Research article

|

22 Feb 2021

Research article |

| 22 Feb 2021

Timescales of chemical equilibrium between the convecting solid mantle and over- and underlying magma oceans

Daniela Paz Bolrão, Maxim D. Ballmer, Adrien Morison, Antoine B. Rozel, Patrick Sanan, Stéphane Labrosse, and Paul J. Tackley

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Final revised paper (published on 22 Feb 2021)
Preprint (discussion started on 17 Apr 2020)

Interactive discussion

Status: closed

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

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RC1: 'Review for: “Timescales of chemical equilibrium between the convecting solid mantle and over-/underlying magma oceans” by Bolrão et al.', Anonymous Referee #1, 26 May 2020
- AC2: 'Response to the Reviewer Number 1', Daniela Bolrão, 11 Jul 2020
RC2: 'Referee comment', Anonymous Referee #2, 03 Jun 2020
- AC3: 'Response to the Reviewer Number 2', Daniela Bolrão, 11 Jul 2020
EC1: 'Topical editor assessment', Julien Aubert, 04 Jun 2020
- AC1: 'Rebuttal letter to the Editor and Reviewers', Daniela Bolrão, 10 Jul 2020

Peer-review completion

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

AR by Daniela Bolrão on behalf of the Authors (11 Jul 2020) Author's response Manuscript

ED: Reconsider after major revisions (20 Jul 2020) by Julien Aubert

AR by Daniela Bolrão on behalf of the Authors (23 Oct 2020) Manuscript

ED: Referee Nomination & Report Request started (31 Oct 2020) by Julien Aubert

RR by Anonymous Referee #2 (20 Nov 2020)

Suggestions for revision or reasons for rejection

The authors have answered most of my questions and comments satisfactorily. I can accept the publication of this study, but I would nevertheless like to emphasize two concerns.
For technical reasons they were not able to test the influence of the buoyancy coefficient B, and it is now argued that the value of B=1 used is a conservative choice. I am not entirely convinced for two reasons: Under the assumption of equilibrium crystallisation as in the current model setup, a large B-value would more strongly impede the downwellings formed at the boundary between TMO and solid mantle, as they are depleted in FeO. On the other hand, if one assumes fractional crystallisation as initial setup, I wonder if the chemical density contrast is not too great to allow efficient convection and mixing when the heavy material is rearranged, i.e. at the bottom of the mantle. However, the authors claim that the preliminary results are similar to the present results, and perhaps that is OK.

I am also not entirely satisfied with their answer to my comment:
“If convection in the solid mantle starts before the solidification of MO, partial melting of the cumulates and ‘feeding’ of the MO with this melt is very likely. Both effects change the chemical equilibrium considerably and do not necessarily require the material to be able to flow through by phase change. However, the latter may further reduce the time scale of chemical equilibrium.”
The authors state on line 345’ “Without this boundary condition, partial melting of solid cumulates could still change the composition of the magma ocean, but we show that with this boundary condition a larger volume can (re-)melt and crystallise efficiently at either or both solid-liquid phase boundaries. “
I wonder how they show or quantify this statement, as they have not considered the melting of the cumulates nor estimated the amount of this melting - or at least it is not shown. I suspect that which effect dominates to obtain chemical equilibrium (i.e. the phase boundary condition or partial melting of the cumulates) depends in turn on the chosen value of the phase change number

Please check Eq. 13, I think it should be minus b - at least to get your values on line 225.

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RR by Anonymous Referee #1 (23 Nov 2020)

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

AR by Daniela Bolrão on behalf of the Authors (07 Dec 2020) Author's response Manuscript

ED: Publish as is (09 Dec 2020) by Julien Aubert

ED: Publish subject to minor revisions (review by editor) (09 Dec 2020) by Susanne Buiter (Executive editor)

AR by Daniela Bolrão on behalf of the Authors (16 Dec 2020) Author's response Author's tracked changes Manuscript

ED: Publish as is (24 Dec 2020) by Susanne Buiter

ED: Publish as is (24 Dec 2020) by Susanne Buiter (Executive editor)

AR by Daniela Bolrão on behalf of the Authors (24 Dec 2020) Author's response Manuscript

Short summary

We use numerical models to investigate the thermo-chemical evolution of a solid mantle during a magma ocean stage. When applied to the Earth, our study shows that the solid mantle and a magma ocean tend toward chemical equilibration before crystallisation of this magma ocean. Our findings suggest that a very strong chemical stratification of the solid mantle is unlikely to occur (as predicted by previous studies), which may explain why the Earth’s mantle is rather homogeneous in composition.