Magnesium isotope fractionation processes during seafloor serpentinization and implications for serpentinite subduction

Nielsen, Sune G.; Klein, Frieder; Marschall, Horst R.; Pogge von Strandmann, Phillip A. E.; Auro, Maureen

doi:https://doi.org/10.5194/egusphere-2023-2933

Preprints

https://doi.org/10.5194/egusphere-2023-2933

Preprints

08 Jan 2024

| 08 Jan 2024

Magnesium isotope fractionation processes during seafloor serpentinization and implications for serpentinite subduction

Sune G. Nielsen, Frieder Klein, Horst R. Marschall, Phillip A. E. Pogge von Strandmann, and Maureen Auro

Abstract. Studies of magnesium (Mg) isotope ratios in subduction zone lavas have revealed small, but significant offsets from the mantle value with enrichments in the heavy isotopes. However, the very high concentration of Mg in the mantle contrasts with much lower concentrations in the subducted igneous crust and oceanic sediments, making these subduction components unlikely vehicles of the Mg isotope anomalies in arc lavas. Only serpentinites, which in various proportions form part of oceanic plates, have high Mg contents comparable to fresh mantle rocks and have thus been considered a potential source of exotic Mg in the source of arc magmas.

In this study we analyzed serpentinite samples from different oceanic settings for their Mg isotopic compositions. The majority of samples are indistinguishable from the depleted mantle (δ²⁶Mg = −0.24 ± 0.04 ‰) irrespective of their origin. Only a small number of seafloor-weathered serpentinites are slightly enriched in the heavy isotopes (up to δ²⁶Mg = −0.14 ± 0.03 ‰), implying that bulk serpentinites are unlikely sources of isotopically anomalous Mg in subduction zones.

We also developed a partial-dissolution method in which 5 % acetic acid for 180 minutes was shown to fully dissolve the minerals brucite and iowaite while leaving the serpentine mineral chrysotile essentially undissolved.

Partial dissolution of 11 bulk serpentinite samples revealed a Mg isotopic composition of brucite (±iowaite) that is systematically ~0.25 ‰ heavier than that of coexisting serpentine. Thus, preferential breakdown of brucite and/or iowaite in a subducted slab prior to serpentine could preferentially release isotopically heavy Mg, which could subsequently be transported into the source region of arc magmas. Such a scenario would require brucite/iowaite breakdown to occur at pressures in excess of 3 GPa and produce fluids with very high concentrations of Mg that could be transported to arc magma source regions. Whether these conditions are met in nature has yet to be experimentally investigated.

Received: 06 Dec 2023 – Discussion started: 08 Jan 2024

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.

Download & links

Preprint (PDF, 734 KB)

Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
Preprint (734 KB)

Supplement (1153 KB)

Download & links

The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Journal article(s) based on this preprint

16 Sep 2024

Magnesium isotope fractionation processes during seafloor serpentinization and implications for serpentinite subduction

Sune G. Nielsen, Frieder Klein, Horst R. Marschall, Philip A. E. Pogge von Strandmann, and Maureen Auro

Solid Earth, 15, 1143–1154, https://doi.org/10.5194/se-15-1143-2024,https://doi.org/10.5194/se-15-1143-2024, 2024

Short summary

Sune G. Nielsen, Frieder Klein, Horst R. Marschall, Phillip A. E. Pogge von Strandmann, and Maureen Auro

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-2933', Anonymous Referee #1, 31 Mar 2024

The paper by Nielsen et al. reported new data for serpentinite from a variety of oceanic settings. The authors found relatively homogenous mantle-like Mg isotopic compositions for bulk serpentinites, but significant heavy Mg isotope enrichment in brucite and iowaite within the serpentinites. Based on these results, the authors proposed that preferential breakdown of brucite/iowaite could release isotopically heavy Mg into the sub-arc mantle, which may explain the heavy-d26Mg observed in some arc volcanic rocks. In general, this is a neat study with some relatively significant results, which I enjoyed reading. I have some minor comments which mostly focus on the lack of information provided in the text which should be easily addressed (detailed below); hence I recommend that this is suitable for publication after minor revisions.

(1) As mentioned in this paper, a few studies have reported Mg isotope data for serpentinites. I would suggest the authors to make a comparison with the literature data visually through figures. It may also be worthwhile to draw a figure for comparison with arc magmas.

(2) The amount of fluid leached from the bulk serpentinite dissolution experiment is not shown. Please specify this amount and the method used to determine it."

Other comments
Line 150: I’ll suggest to cite some method papers here as well (e.g., Teng et al., 2015 GGR).
Line 203: “olivine”
Line 295: The format of (Li, 2023) is not correct.
Table 1: Please clarify how 2se was calculated and how n was defined in this study.

Citation: https://doi.org/10.5194/egusphere-2023-2933-RC1
- AC1: 'Reply on RC1', Sune Nielsen, 31 May 2024
  
  We thank the reviewer for their positive comments on our study.
  (1) We agree with the reviewer that it will be useful to compile literature serpentinite data to compare with our own.
  (2) We do not understand what the reviewer means by 'fluid leached from the bulk serpentinite'. We did not leach fluid from the serpentinites, we dissolved brucite and iowaite, whereas serpentine remained in the residue. We used TGA measurements of the residues to verify that brucite and iowaite had been dissolved. Perhaps the reviewer would like to know the volume of acetic acid that was used to leach the serpentinites? If that is the case, then we will be delighted to supply this information.
  We thank the reviewer for their remaining comments and can make these corrections without any issue.
  
  Citation: https://doi.org/10.5194/egusphere-2023-2933-AC1
RC2:
'Comment on egusphere-2023-2933', Anonymous Referee #2, 22 May 2024

Nielson et al. carried out a systematic Mg isotope study for a set of serpentinites formed both at mid-oceanic ridge settings and at forearc settings in subduction zones. They found that there is no significant Mg isotope variations of serpentinites during serpentinization of peridotite. They also performed partial dissolution experiments that dissolved mainly brucite and found that brucite is preferentially enriched in isotopically heavy Mg than serpentine. Based on these results, they discussed the implications of serpentinite subduction for the generation of arc magmas with heavy Mg isotope compositions. I find the topic is interesting and important for understanding Mg isotope systematics in subduction zones, especially for the formation of arc magmas with heavy Mg isotope compositions. The interpretations are mostly supported by the data, and the writing is concise and clear. However, I still have some concerns on the presentation and interpretations.
My comments are as follows.
Line 61-62: the fluids with heavy Mg isotopes were considered by Teng et al. (2016) to be slab derived, whether it is due to serpentine breakdown is not explicitly expressed.
Line 67-68: it is better to revise “these T and P” to “such P-T conditions”
Line 172-174: the serpentinites from Puerto Rico trench show a large variation of d26Mg (~0.1‰), and the high d26Mg ones were ascribed to chemical weathering. However, there is no further information on the high-d26Mg serpentinites. Do they indeed reflect high degrees of chemical weathering? Please discuss this in the context of petrology and other geochemical data.
Fig. 3 Caption: please note that Mariana forearc serpentinites are not seafloor serpentinites.
Lines 175-180: please explain the data of high-d26Mg serpentinites in Puerto Rico in detail. From the present discussion, we can not evaluate whether the high-d26Mg signature is caused by chemical weathering, or instead, through other process like serpentinization. I suggest to discuss this point by integrating with petrographic observations as well as bulk rock major and trace element compositions. In addition, an evaluation of the Mg mobility during serpentinization/weathering process may be helpful.
Line 200: the Mg isotope variation of Puerto Rico serpentinites is significant, ~0.1‰. Considering the high MgO contents of serpentinites, this Mg isotope variation is essential.
Line 203: Typo: Olivine
Line 204-206: Even we assume that the serpentinization process does not result in serpentinites with heavy Mg isotope compositions, the chemical weathering process does. Please consider the possibility that the subduction can also involve the weathered serpentinites.
Line 265-275: There is large debate on the Mg isotope fractionation between serpentine and brucite. I am curious that if there is abundant brucite in the serpentinite sample, why not separate it and directly measure it? Is it difficult to separate brucite from serpentine in serpentinite? On the other hand, the serpentine is easy to be separated and directly measured. Then, a mass balance can be used to cross-check the fractionation trend and magnitude between serpentine and brucite.
Line 310-315: for the warm to hot subduction zones, the atg + brucite dehydration reaction occurs typically below ca. 2 GPa. Only at the extremely cold subduction zones, this reaction can occur at 3 GPa (See Kendrick et al., 2013-EPSL). Thus, it seems difficult to release fluids through this reaction at subarc depths in most subduction zones.

Citation: https://doi.org/10.5194/egusphere-2023-2933-RC2
- AC2: 'Reply on RC2', Sune Nielsen, 31 May 2024
  
  Line 61-62: Agreed
  Line 67-68: Agreed
  Line 172-180: We will provide more supporting information regarding the weathering experienced by these serpentinites
  Fig 3 caption: Agreed
  Line 200: This statement is in the context that all the variation in the Puerto Rico serpentinites are caused by weathering and, hence, there is no Mg isotope variation in unweathered serpentinites. We will make this clear in a revised version.
  Line 203: Agreed
  Line 204-206: Weathering typically takes place only in the uppermost portion of the crust where cold seawater can interact with serpentinites. The total volume of weathered serpentinites, therefore, remains relatively small compared with overall serpentinite abundances in slabs where this lithology is sufficient to control the Mg budget.
  Line 265-275: Neither serpentine nor brucite can be mechanically separated because they are typically intergrown and very fine grained (which we also noted in line 282 of the original manuscript). Had these minerals been sufficiently large and separate from each other then it would indeed have made sense separate them mechanically.
  Line 310-315: We agree with the reviewer. In the original manuscript we already stated that the P-T conditions required for brucite to break down and become a source of Mg to arc magmas was only found in cold and intermediate subduction zones, which was supported by the most recent subduction zone thermal models (line 314).
  
  Citation: https://doi.org/10.5194/egusphere-2023-2933-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-2933', Anonymous Referee #1, 31 Mar 2024

The paper by Nielsen et al. reported new data for serpentinite from a variety of oceanic settings. The authors found relatively homogenous mantle-like Mg isotopic compositions for bulk serpentinites, but significant heavy Mg isotope enrichment in brucite and iowaite within the serpentinites. Based on these results, the authors proposed that preferential breakdown of brucite/iowaite could release isotopically heavy Mg into the sub-arc mantle, which may explain the heavy-d26Mg observed in some arc volcanic rocks. In general, this is a neat study with some relatively significant results, which I enjoyed reading. I have some minor comments which mostly focus on the lack of information provided in the text which should be easily addressed (detailed below); hence I recommend that this is suitable for publication after minor revisions.

(1) As mentioned in this paper, a few studies have reported Mg isotope data for serpentinites. I would suggest the authors to make a comparison with the literature data visually through figures. It may also be worthwhile to draw a figure for comparison with arc magmas.

(2) The amount of fluid leached from the bulk serpentinite dissolution experiment is not shown. Please specify this amount and the method used to determine it."

Other comments
Line 150: I’ll suggest to cite some method papers here as well (e.g., Teng et al., 2015 GGR).
Line 203: “olivine”
Line 295: The format of (Li, 2023) is not correct.
Table 1: Please clarify how 2se was calculated and how n was defined in this study.

Citation: https://doi.org/10.5194/egusphere-2023-2933-RC1
- AC1: 'Reply on RC1', Sune Nielsen, 31 May 2024
  
  We thank the reviewer for their positive comments on our study.
  (1) We agree with the reviewer that it will be useful to compile literature serpentinite data to compare with our own.
  (2) We do not understand what the reviewer means by 'fluid leached from the bulk serpentinite'. We did not leach fluid from the serpentinites, we dissolved brucite and iowaite, whereas serpentine remained in the residue. We used TGA measurements of the residues to verify that brucite and iowaite had been dissolved. Perhaps the reviewer would like to know the volume of acetic acid that was used to leach the serpentinites? If that is the case, then we will be delighted to supply this information.
  We thank the reviewer for their remaining comments and can make these corrections without any issue.
  
  Citation: https://doi.org/10.5194/egusphere-2023-2933-AC1
RC2:
'Comment on egusphere-2023-2933', Anonymous Referee #2, 22 May 2024

Nielson et al. carried out a systematic Mg isotope study for a set of serpentinites formed both at mid-oceanic ridge settings and at forearc settings in subduction zones. They found that there is no significant Mg isotope variations of serpentinites during serpentinization of peridotite. They also performed partial dissolution experiments that dissolved mainly brucite and found that brucite is preferentially enriched in isotopically heavy Mg than serpentine. Based on these results, they discussed the implications of serpentinite subduction for the generation of arc magmas with heavy Mg isotope compositions. I find the topic is interesting and important for understanding Mg isotope systematics in subduction zones, especially for the formation of arc magmas with heavy Mg isotope compositions. The interpretations are mostly supported by the data, and the writing is concise and clear. However, I still have some concerns on the presentation and interpretations.
My comments are as follows.
Line 61-62: the fluids with heavy Mg isotopes were considered by Teng et al. (2016) to be slab derived, whether it is due to serpentine breakdown is not explicitly expressed.
Line 67-68: it is better to revise “these T and P” to “such P-T conditions”
Line 172-174: the serpentinites from Puerto Rico trench show a large variation of d26Mg (~0.1‰), and the high d26Mg ones were ascribed to chemical weathering. However, there is no further information on the high-d26Mg serpentinites. Do they indeed reflect high degrees of chemical weathering? Please discuss this in the context of petrology and other geochemical data.
Fig. 3 Caption: please note that Mariana forearc serpentinites are not seafloor serpentinites.
Lines 175-180: please explain the data of high-d26Mg serpentinites in Puerto Rico in detail. From the present discussion, we can not evaluate whether the high-d26Mg signature is caused by chemical weathering, or instead, through other process like serpentinization. I suggest to discuss this point by integrating with petrographic observations as well as bulk rock major and trace element compositions. In addition, an evaluation of the Mg mobility during serpentinization/weathering process may be helpful.
Line 200: the Mg isotope variation of Puerto Rico serpentinites is significant, ~0.1‰. Considering the high MgO contents of serpentinites, this Mg isotope variation is essential.
Line 203: Typo: Olivine
Line 204-206: Even we assume that the serpentinization process does not result in serpentinites with heavy Mg isotope compositions, the chemical weathering process does. Please consider the possibility that the subduction can also involve the weathered serpentinites.
Line 265-275: There is large debate on the Mg isotope fractionation between serpentine and brucite. I am curious that if there is abundant brucite in the serpentinite sample, why not separate it and directly measure it? Is it difficult to separate brucite from serpentine in serpentinite? On the other hand, the serpentine is easy to be separated and directly measured. Then, a mass balance can be used to cross-check the fractionation trend and magnitude between serpentine and brucite.
Line 310-315: for the warm to hot subduction zones, the atg + brucite dehydration reaction occurs typically below ca. 2 GPa. Only at the extremely cold subduction zones, this reaction can occur at 3 GPa (See Kendrick et al., 2013-EPSL). Thus, it seems difficult to release fluids through this reaction at subarc depths in most subduction zones.

Citation: https://doi.org/10.5194/egusphere-2023-2933-RC2
- AC2: 'Reply on RC2', Sune Nielsen, 31 May 2024
  
  Line 61-62: Agreed
  Line 67-68: Agreed
  Line 172-180: We will provide more supporting information regarding the weathering experienced by these serpentinites
  Fig 3 caption: Agreed
  Line 200: This statement is in the context that all the variation in the Puerto Rico serpentinites are caused by weathering and, hence, there is no Mg isotope variation in unweathered serpentinites. We will make this clear in a revised version.
  Line 203: Agreed
  Line 204-206: Weathering typically takes place only in the uppermost portion of the crust where cold seawater can interact with serpentinites. The total volume of weathered serpentinites, therefore, remains relatively small compared with overall serpentinite abundances in slabs where this lithology is sufficient to control the Mg budget.
  Line 265-275: Neither serpentine nor brucite can be mechanically separated because they are typically intergrown and very fine grained (which we also noted in line 282 of the original manuscript). Had these minerals been sufficiently large and separate from each other then it would indeed have made sense separate them mechanically.
  Line 310-315: We agree with the reviewer. In the original manuscript we already stated that the P-T conditions required for brucite to break down and become a source of Mg to arc magmas was only found in cold and intermediate subduction zones, which was supported by the most recent subduction zone thermal models (line 314).
  
  Citation: https://doi.org/10.5194/egusphere-2023-2933-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Sune Nielsen on behalf of the Authors (23 Jul 2024) Author's response Author's tracked changes Manuscript

ED: Publish as is (02 Aug 2024) by Andrea Di Muro

ED: Publish as is (07 Aug 2024) by Andrea Di Muro (Executive editor)

AR by Sune Nielsen on behalf of the Authors (08 Aug 2024)

Journal article(s) based on this preprint

16 Sep 2024

Magnesium isotope fractionation processes during seafloor serpentinization and implications for serpentinite subduction

Sune G. Nielsen, Frieder Klein, Horst R. Marschall, Philip A. E. Pogge von Strandmann, and Maureen Auro

Solid Earth, 15, 1143–1154, https://doi.org/10.5194/se-15-1143-2024,https://doi.org/10.5194/se-15-1143-2024, 2024

Short summary

Sune G. Nielsen, Frieder Klein, Horst R. Marschall, Phillip A. E. Pogge von Strandmann, and Maureen Auro

Supplement

https://doi.org/10.5194/egusphere-2023-2933-supplement

Sune G. Nielsen, Frieder Klein, Horst R. Marschall, Phillip A. E. Pogge von Strandmann, and Maureen Auro

Viewed

Total article views: 484 (including HTML, PDF, and XML)

HTML	PDF	XML	Total	Supplement	BibTeX	EndNote
339	116	29	484	53	16	16

HTML: 339
PDF: 116
XML: 29
Total: 484
Supplement: 53
BibTeX: 16
EndNote: 16

Views and downloads (calculated since 08 Jan 2024)

Month	HTML	PDF	XML	Total
Jan 2024	65	32	4	101
Feb 2024	23	6	1	30
Mar 2024	39	8	2	49
Apr 2024	61	18	5	84
May 2024	40	18	5	63
Jun 2024	52	11	6	69
Jul 2024	20	9	1	30
Aug 2024	19	6	4	29
Sep 2024	20	8	1	29

Cumulative views and downloads (calculated since 08 Jan 2024)

Month	HTML	PDF	XML	Total
Jan 2024	65	32	4	101
Feb 2024	23	6	1	30
Mar 2024	39	8	2	49
Apr 2024	61	18	5	84
May 2024	40	18	5	63
Jun 2024	52	11	6	69
Jul 2024	20	9	1	30
Aug 2024	19	6	4	29
Sep 2024	20	8	1	29

Viewed (geographical distribution)

Total article views: 478 (including HTML, PDF, and XML) Thereof 478 with geography defined and 0 with unknown origin.

Country	#	Views	%

Latest update: 17 Sep 2024

Download

The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Preprint (734 KB)
Metadata XML

Short summary

Magnesium isotope ratios of arc lavas have been proposed as a proxy for serpentinite subduction, but uncertainties remain regarding their utility. Here we show that bulk serpentinite Mg isotope ratios are identical to the mantle, whereas the serpentinite mineral brucite is enriched in heavy Mg isotopes. Thus, Mg isotope ratios may only be used as serpentinite subduction proxies if brucite is preferentially mobilized from the slab at pressures and temperatures within the arc magma source region.


Total:	0
HTML:	0
PDF:	0
XML:	0