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，覆盖了我们地球的大约三分之一，由辉长岩组成，它被许多厘米到dm大小的演化岩石矿脉切割，即所谓的长英质矿脉。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。IODP探险队最近在亚特兰蒂斯岸(西南印度洋海脊，SWIR)顶部的1473号地点钻探下地壳，遇到了约790米的巨大辉长岩，被近400条长英质矿脉切割，占钻芯的约1.5%，成分从闪长岩到奥长花岗岩不等。来自U1473A孔的岩石为深入而全面地研究缓慢扩张的洋壳中的长英脉切割辉长岩提供了独特的机会。该项目的重点是三个主要的科学目标：(1)主题1的目标是测试U1473A孔中长英质矿脉的形成模式。它们代表了作为MORB分异的最后一步而产生的熔体的凝固，还是由渗流流体使辉长岩部分熔化产生的，还是由进化的MORB系统中的液体不混溶产生的？我们的方法论方法是通过大量岩石的地球化学分析、地化模拟和部分熔融的实验模拟来证明这一点。(2)主题2侧重于最新岩浆作用和第一次热液活动印记之间过渡的性质，这也有可能触发岩浆作用。这一主题包括一个重要的问题，水向下渗透到大洋滑脱断层有多深。这对刚吸积的“固体”地壳的流变学也有更广泛的影响。要解决这些在缓慢扩张洋壳吸积/抬升过程中岩浆-变质过程中研究不足的问题，关键在于仔细研究高温角闪石及其关于地层温度的固有信息，以及准确估计特定岩石的湿固相线温度。通常从U1473A孔演化出来的辉长岩。(3)主题3的基础是观察到长英脉的变化比寄主辉长岩大得多，长英脉中的变质组合表明从高温(高角闪岩相)到低温(亚绿片岩相)的变质条件不同。这些观察提出了这样的问题：在地壳热液冷却期间，长英脉在变质发展中起什么作用？沿着这些脉转移的流体和物质的数量是多少？解决这一问题的关键方法是仔细分析含流体矿物(角闪石、磷灰石)，结合局部变质平衡分析，通过地温测量准确估计温度，通过锶和氧同位素分析估计海水来源流体的通量，以及热力学计算。

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.