Understanding felsic veins in gabbros drilled by IODP at Atlantis Bank (Southwest Indian Ridge): Formation, metamorphism, and their role as multifunctional pathways for fluid and mass transfer

了解 IODP 在亚特兰蒂斯浅滩（西南印度洋中脊）钻探的辉长岩中的长英质脉：形成、变质作用及其作为流体和质量传递多功能途径的作用

• 批准号: 408055224
• 负责人: Professor Dr. Jürgen Koepke
• 金额: --
• 依托单位: Institut für Mineralogie
• 依托单位国家: 德国
• 项目类别: Infrastructure Priority Programmes
• 财政年份: 2018
• 资助国家: 德国
• 起止时间: 2017-12-31 至 2022-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/408055224?language=en
• 关键词: Understanding; felsic; veins; gabbros; drilled

Typical lower crust from slow-spreading mid-ocean ridges, covering about one third of our planet, consists of gabbro which is cut by numerous cm- to dm-sized veins of evolved rocks, so-called "felsic veins". Their mode of formation and their role during hydrothermal alteration of the crust is only poorly investigated. An IODP Expedition drilled recently at Site 1473 on top of the Atlantis Bank (Southwest Indian Ridge, SWIR) into the lower crust, encountering ~ 790 m of massive gabbro, cut by nearly 400 felsic veins, comprising ~1.5% of the drill core, varying in composition from diorites to trondhjemites. The rocks from Hole U1473A offer the unique opportunity for a profound and comprehensive investigation of felsic veins cutting gabbros in slow-spread oceanic crust. The proposed project focusses on three main scientific targets:(1) The objective of theme 1 is to test the model of formation of the felsic veins in Hole U1473A. Do they represent the solidification of melts generated as very last step of MORB differentiation, or are they generated by partial melting of gabbros by percolating fluids, or by liquid immiscibility in an evolved MORB system? Our methodological approach to prove this is bulk rock geochemical analysis, geochemical modeling, and by an experimental simulation of partial melting. (2) Theme 2 focuses on the nature of the transition between latest magmatic processes and the first imprint of hydrothermal activity, which has also the potential to trigger magmatic processes. This theme includes the important question, how deep does water penetrate down oceanic detachment faults. This has broader implications also for the rheology of the just accreted "solid" crust. Key for unravelling these poorly investigated issues in the magmatic-to-metamorphic regime during accretion/uplifting of slow-spread oceanic crust lies in the careful investigation of high-temperature amphiboles, and their inherent information on formation temperature, as well as in the accurate estimation of the wet solidus temperature of the specific, often evolved gabbros from Hole U1473A.(3) Basis of theme 3 is the observation that the felsic veins are much more altered than the host gabbros and that the metamorphic assemblages in the felsic veins suggest varying metamorphic conditions from high temperatures (higher amphibolite facies) down to low temperatures (sub-greenschist facies).These observations raise the questions: what is the role of the felsic veins in the metamorphic development during the hydrothermal cooling of the crust and which are the amounts of fluids and masses transferred along these veins? Key approaches for this topic are the careful analysis of fluid bearing minerals (amphiboles, apatite), accurate temperature estimations by geothermometry in combination with the analysis of the local metamorphic equilibria, estimating the flux of seawater-derived fluids by Sr and oxygen isotope analysis, and thermodynamic computations.

典型的下地壳来自缓慢扩张的洋中脊，覆盖了地球约三分之一的面积，由辉长岩组成，辉长岩被许多厘米至厘米大小的演化岩石脉（即所谓的“长英质脉”）切割而成。它们的形成模式及其在地壳热液蚀变过程中的作用的研究很少。最近，IODP 探险队在亚特兰蒂斯浅滩（西南印度洋中脊，SWIR）顶部的 1473 号地点钻探到下地壳，遇到了约 790 m 的巨大辉长岩，被近 400 个长英岩脉切割，占钻芯的约 1.5%，成分从闪长岩到长岩不等。 U1473A 洞的岩石为深入、全面地研究缓慢扩张的洋壳中切割辉长岩的长英质脉提供了独特的机会。拟议项目重点关注三个主要科学目标：（1）主题1的目标是测试U1473A洞中长英质脉的形成模型。它们是否代表了 MORB 分化的最后一步产生的熔体的凝固，或者它们是由渗透流体部分熔化辉长岩产生的，还是由进化的 MORB 系统中的液体不混溶性产生的？我们证明这一点的方法是块体岩石地球化学分析、地球化学建模以及部分熔融的实验模拟。 （2）主题2重点关注最新岩浆过程与热液活动第一印记之间过渡的性质，这也有可能引发岩浆过程。这个主题包括一个重要的问题，即水渗透到海洋滑脱断层的深度有多深。这对于刚刚增生的“固体”地壳的流变学也具有更广泛的影响。解开缓慢扩张的洋壳增生/隆升过程中岩浆变质体系中这些研究不足的问题的关键在于对高温角闪石及其关于地层温度的固有信息的仔细研究，以及准确的研究估计来自 U1473A 洞的特定的、经常演化的辉长岩的湿固相线温度。(3) 主题 3 的基础是长英质脉多的观察结果比宿主辉长岩的变化更大，并且长英质脉中的变质组合表明从高温（较高角闪岩相）到低温（亚绿片岩相）的不同变质条件。这些观察提出了一个问题：地壳热液冷却过程中变质发育中的长英质脉，沿着这些脉转移的流体和质量是多少？该主题的关键方法是仔细分析含流体矿物（角闪石、磷灰石）、通过地温测量结合局部变质平衡分析进行精确温度估计、通过 Sr 和氧同位素分析估计海水衍生流体的通量，和热力学计算。