the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
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: open (until 13 May 2024)
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RC1: 'Comment on egusphere-2023-2933', Anonymous Referee #1, 31 Mar 2024
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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
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