the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Jan 2024
Magnesium isotope fractionation processes during seafloor serpentinization and implications for serpentinite subduction
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.
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Status: final response (author comments only)
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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
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AC1: 'Reply on RC1', Sune Nielsen, 31 May 2024
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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
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AC2: 'Reply on RC2', Sune Nielsen, 31 May 2024
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