Geochemistry of serpentinized and multiphase altered Atlantis Massif peridotites (IODP Expedition 357): Petrogenesis and discrimination of melt-rock vs. fluid-rock processes
Geochemistry of serpentinized and multiphase altered Atlantis Massif peridotites (IODP Expedition 357): Petrogenesis and discrimination of melt-rock vs. fluid-rock processes
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蛇纹石化和多相蚀变亚特兰蒂斯地块橄榄岩的地球化学（IODP Expedition 357）：熔岩与流体岩过程的岩石成因和区分
DOI：
10.1016/j.chemgeo.2021.120681
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发表时间：
2022
期刊：
影响因子：
3.9
通讯作者：
Whattam S
中科院分区：
文献类型：
--
作者：
Whattam S
国际海洋发现计划（IODP）第357次考察钻取了17个浅点，分布在亚特兰蒂斯地块洋核复合体（大西洋中脊，30°N）的扩展方向（从西到东）约10公里处。出露在亚特兰蒂斯地块洋核杂岩下盘的地幔主要是几乎完全蛇纹化的方辉橄榄岩，其次是纯橄榄岩。蚀变橄榄岩可分为三类：（Ⅰ）蛇纹岩，（Ⅱ）熔融浸渍蛇纹岩，（Ⅲ）交代蛇纹岩。I型蛇纹岩除了蛇纹石化和局部氧化外，没有熔融浸渍或交代的证据。II型蛇纹岩被辉长岩熔体侵入，在某些情况下可根据宏观和微观观察进行区分，例如，毫米-厘米规模的镁铁质熔体细脉，罕见的斜长石（在一个样品中为0.5模态%）或通过在斜方辉石之后局部存在次生（不稳定）橄榄石;在其他情况下，'隐蔽'熔体浸渍推断的基础上不相容的元素富集。Ⅲ型蛇纹岩的特征是硅交代作用，表现为斜方辉石向滑石和角闪石的蚀变，以及极高的无水SiO2浓度（59- 61wt%）和低的MgO/SiO2值（0.48-0.52）。虽然许多稀土元素和原始地幔标准化的不相容微量元素异常，负Ce异常，可归因于蛇纹岩化，其他成分的不均匀性是由于熔融浸渍。全岩不相容微量元素的基础上，熔融浸渍的主导机制是区分在中部和东部蛇纹岩流体岩石蚀变（主要是蛇纹化）在西部蛇纹岩，随着熔融浸渍表现为西向东增加富集高场强元素和轻稀土。高程度的熔体萃取是明显的，在低的全岩Al 2 O3/SiO2值和低浓度的Al 2 O3，CaO和不相容元素。全岩稀土元素模式的基础上的熔融提取程度的估计建议最大的~20%的非模态部分熔融，网站之间的变化很小。由于一些蛇纹岩样品是非原地碎石，在每个站点观察到的岩浆历史与当地来源（来自断层带）一致，而不是预期表现出更多异质性且无空间模式的筏状碎石。在这种情况下，所研究的网站可能会提供一个增强的熔体岩石相互作用与时间的记录，与建议的地质模型相一致。或者，网站可能意味着在这些过程中的异质性在几公里的空间尺度。
International Ocean Discovery Program (IODP) Expedition 357 drilled 17 shallow sites distributed ~10 km in the spreading direction (from west to east) across the Atlantis Massif oceanic core complex (Mid-Atlantic Ridge, 30°N). Mantle exposed in the footwall of the Atlantis Massif oceanic core complex is predominantly nearly wholly serpentinized harzburgite with subordinate dunite. Altered peridotites are subdivided into three types: (I) serpentinites, (II) melt-impregnated serpentinites, and (III) metasomatic serpentinites. Type I serpentinites show no evidence of melt-impregnation or metasomatism apart from serpentinization and local oxidation. Type II serpentinites have been intruded by gabbroic melts and are distinguishable in some cases on the basis of macroscopic and microscopic observations, e.g., mm-cm scale mafic-melt veinlets, rare plagioclase (˂0.5 modal % in one sample) or by the local presence of secondary (replacive) olivine after orthopyroxene; in other cases, ‘cryptic’ melt-impregnation is inferred on the basis of incompatible element enrichments. Type III serpentinites are characterized by silica metasomatism manifest by alteration of orthopyroxene to talc and amphibole, and by anomalously high anhydrous SiO2concentrations (59–61 wt%) and low MgO/SiO2values (0.48–0.52). Although many chondrite-normalized rare earth element (REE) and primitive mantle-normalized incompatible trace element anomalies, e.g., negative Ce-anomalies, are attributable to serpentinization, other compositional heterogeneities are due to melt-impregnation. On the basis of whole rock incompatible trace elements, a dominant mechanism of melt-impregnation is distinguished in the central and eastern serpentinites from fluid-rock alteration (mostly serpentinization) in the western serpentinites, with increasing melt-impregnation manifest as a west to east increase in enrichment in high-field strength elements and light REE. High degrees of melt extraction are evident in low whole-rock Al2O3/SiO2values and low concentrations of Al2O3, CaO and incompatible elements. Estimates of the degree of melt extraction based on whole rock REE patterns suggest a maximum of ~20% non-modal fractional melting, with little variation between sites. As some serpentinite samples are ex situ rubble, the magmatic histories observed at each site are consistent with a local source (from the fault zone) rather than rafted rubble that would be expected to show more heterogeneity and no spatial pattern. In this case, the studied sites may provide a record of enhanced melt-rock interactions with time, consistent with proposed geological models. Alternatively, sites may signify heterogeneities in these processes at spatial scales of a few km.
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