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

典型的下地壳来自缓慢扩张的大洋中脊，覆盖了我们地球的三分之一，由辉长岩组成，辉长岩被许多厘米到厘米大小的演化岩石脉切割，所谓的“长英质脉”。它们的形成方式及其在地壳热液蚀变过程中的作用研究得很少。最近，一个IODP探险队在亚特兰蒂斯岸（西南印度洋脊，SWIR）顶部的1473号地点钻探到下地壳，遇到了约790 m的块状辉长岩，被近400条长英质脉切割，占岩芯的约1.5%，成分从闪长岩到奥长花岗岩不等。孔U1473 A的岩石提供了一个独特的机会，深刻和全面的调查长英质脉切割辉长岩在缓慢蔓延的洋壳。本项目主要有三个科学目标：（1）专题1的目标是检验U1473 A孔长英质脉的形成模式。它们是否代表了MORB分异最后一步所产生的熔体的凝固，或者它们是由辉长岩的部分熔融所产生的，或者是由演化的MORB系统中的液体不溶性所产生的？我们的方法学途径来证明这是块体岩石地球化学分析，地球化学建模，并通过部分熔融的实验模拟。(2)主题2侧重于最新岩浆过程和热液活动的第一个印记之间的过渡性质，这也有可能触发岩浆过程。这一主题包括一个重要的问题，即水沿着海洋拆离断层渗透到多深。这对刚刚增生的“固体”地壳的流变学也有更广泛的影响。在缓慢扩展的洋壳增生/隆起过程中，在岩浆-变质机制中，解开这些研究不足的问题的关键在于对高温角闪石的仔细研究，以及它们关于形成温度的固有信息，以及对来自孔U1473 A的特定的、经常演化的辉长岩的湿固相线温度的准确估计。(3)主题3的基础是观察到长英质脉比寄主辉长岩的蚀变更大，长英质脉中的变质组合表明高温变质条件不同（高角闪岩相）下降到低温（亚绿片岩相）。这些观察结果提出了以下问题：长英质脉在地壳热液冷却过程中的变质作用是什么？沿这些脉沿着的流体和物质的量是多少？这一主题的关键方法是流体轴承矿物（角闪石，磷灰石）的仔细分析，结合当地的变质平衡的分析，估计锶和氧同位素分析，和热力学计算的海水流体的流量的地质测温准确的温度估计。

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.