海底蛇纹石化过程中的镁同位素分馏过程及其对蛇纹石俯冲的影响

IF 3.2 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Sune G. Nielsen, Frieder Klein, Horst R. Marschall, Philip A. E. Pogge von Strandmann, Maureen Auro
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引用次数: 0

摘要

摘要。对俯冲带熔岩中镁(Mg)同位素比值的研究表明,重同位素富集的镁同位素比值与地幔中的比值偏差虽小,但却很明显。然而,地幔中的镁浓度非常高,而俯冲火成岩壳和海洋沉积物中的镁浓度则低得多,因此这些俯冲成分不太可能是弧熔岩中镁同位素异常的载体。只有以不同比例构成大洋板块一部分的蛇纹岩具有与新鲜地幔岩相当的高镁含量,因此被认为是弧岩浆源中外来镁的潜在来源。在这项研究中,我们分析了来自不同大洋环境的蛇绿岩样本的镁同位素组成。大部分样品与贫化地幔(δ26Mg=-0.24 ‰ ± 0.04 ‰)没有区别,而与它们的来源无关。只有少量海底风化蛇绿岩的重同位素略有富集(最高达δ26Mg=-0.14 ‰ ± 0.03 ‰),这意味着大块蛇绿岩不太可能是俯冲带同位素异常镁的来源。我们还开发了一种部分溶解法,在这种方法中,5%的醋酸作用 180 分钟可完全溶解矿物青金石和碘瓦石,而蛇纹石矿物温石棉基本上不溶解。对 11 块蛇纹石样品进行部分溶解后发现,青金石(± iowaite)的镁同位素组成比共存的蛇纹石重 0.25 ‰。因此,在蛇纹岩之前,俯冲板块中的青金石和/或iowaite会优先分解,从而优先释放出同位素重的镁,这些镁随后会被输送到弧岩浆的源区。这种情况需要在超过3 GPa的压力下发生青金石/刁瓦石分解,并产生镁浓度非常高的流体,从而被输送到弧形岩浆源区。自然界是否满足这些条件还有待实验研究。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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 they have thus been regarded as 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 min 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 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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来源期刊
Solid Earth
Solid Earth GEOCHEMISTRY & GEOPHYSICS-
CiteScore
6.90
自引率
8.80%
发文量
78
审稿时长
4.5 months
期刊介绍: Solid Earth (SE) is a not-for-profit journal that publishes multidisciplinary research on the composition, structure, dynamics of the Earth from the surface to the deep interior at all spatial and temporal scales. The journal invites contributions encompassing observational, experimental, and theoretical investigations in the form of short communications, research articles, method articles, review articles, and discussion and commentaries on all aspects of the solid Earth (for details see manuscript types). Being interdisciplinary in scope, SE covers the following disciplines: geochemistry, mineralogy, petrology, volcanology; geodesy and gravity; geodynamics: numerical and analogue modeling of geoprocesses; geoelectrics and electromagnetics; geomagnetism; geomorphology, morphotectonics, and paleoseismology; rock physics; seismics and seismology; critical zone science (Earth''s permeable near-surface layer); stratigraphy, sedimentology, and palaeontology; rock deformation, structural geology, and tectonics.
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