Alteration Heterogeneities in Peridotites Exhumed on the Southern Wall of the Atlantis Massif (IODP Expedition 357)
Alteration Heterogeneities in Peridotites Exhumed on the Southern Wall of the Atlantis Massif (IODP Expedition 357)
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DOI：
10.1093/petrology/egy065
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发表时间：
2018-06
影响因子：
3.9
通讯作者：
S. Rouméjon;G. Früh-Green;B. Orcutt
中科院分区：
文献类型：
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作者：
S. Rouméjon;G. Früh-Green;B. Orcutt
在国际海洋发现计划（IODP）第357次考察期间，在亚特兰蒂斯地块（大西洋中脊，30°N）南壁钻出了辉长岩和白云岩侵入的蛇纹岩和交代橄榄岩。它们分布在5个地点的7个洞中，构成了一个东西向的扩展平行剖面，横切了这个发掘出来的分离下盘。在这里，我们利用这个采样来研究小于一公里尺度的蚀变的非均质性。我们结合了77个样品的结构和矿物学观察，并对原生和蛇纹石矿物进行了原位主要和微量元素分析，为亚特兰蒂斯地块蚀变非均质性的发展提供了一个概念模型。结构序列和矿物学组合揭示了蛇纹石化的初始普遍阶段和随后的蛇纹石化和交代作用之间的过渡，这些作用集中在热液流体优先使用的局部途径上。我们认为这些局部通路相互连接，并在分离下壁形成100米至1公里大小的细胞。这种流体路径分布的变化伴随着蛇纹岩结构中微量元素的富集：深层、同蛇纹岩化的流体-橄榄岩相互作用被认为是Cu、Zn、As和Sb富集的来源，而U和Sr的富集被解释为较浅的流体-蛇纹岩相互作用的标志。不同岩石圈层位的橄榄岩中的辉长岩和白云岩蚀变导致其周围橄榄岩的蛇纹岩结构发育角闪洞、绿泥石和含滑石结构，并富集LREE、Nb、Y、Th、Ta等元素。结合这些观察结果，我们提出了一个模型，该模型将钻孔置于一个概念框架中，涉及橄榄岩中的基性侵入和海底渐进蚀变和侵位过程中的非均质性。
Serpentinized and metasomatized peridotites intruded by gabbros and dolerites have been drilled on the southern wall of the Atlantis Massif (Mid-Atlantic Ridge, 30°N) during International Ocean Discovery Program (IODP) Expedition 357. They occur in seven holes from five sites making up an east-west trending, spreading-parallel profile that crosscuts this exhumed detachment footwall. Here we have taken advantage of this sampling to study heterogeneities of alteration at scales less than a kilometer. We combine textural and mineralogical observations made on 77 samples with in situ major and trace element analyses in primary and serpentine minerals to provide a conceptual model for the development of alteration heterogeneities at the Atlantis Massif. Textural sequences and mineralogical assemblages reveal a transition between an initial pervasive phase of serpentinization and subsequent serpentinization and metasomatism focused along localized pathways preferentially used by hydrothermal fluids. We propose that these localized pathways are interconnected and form 100 m- to 1 km-sized cells in the detachment footwall. This change in fluid pathway distribution is accompanied by variable trace element enrichments in the serpentine textures: deep, syn-serpentinization fluid-peridotite interactions are considered the source of Cu, Zn, As, and Sb enrichments, whereas U and Sr enrichments are interpreted as markers of later, shallower fluid-serpentinized peridotite interaction. Alteration of gabbros and dolerites emplaced in the peridotite at different lithospheric levels leads to the development of amphibole, chlorite and, or, talc-bearing textures as well as enrichments in LREE, Nb, Y, Th, Ta in the serpentine textures of the surrounding peridotites. Combining these observations, we propose a model that places the drill holes in a conceptual frame involving mafic intrusions in the peridotites and heterogeneities during progressive alteration and emplacement on the seafloor.