The Gabbro/Dike transition in oceanic crust generated at slow- and ultraslow-spreading ocean ridge systems

大洋地壳的辉长岩/脉岩过渡是地球上最重要的界面之一，岩浆作用、变质作用和热液作用都有干扰。在快速扩张的山脊，人们普遍认为地壳是层状的和均匀的，辉长岩/脉岩的转变特征是在变质条件下形成的角辉石形成的十米厚的层位，一直到麻粒岩相。这对应于理论模型预测的导电边界层(CBL)，它是一个薄(&lt；100m)、热(&gt；650°C)的不渗透区，夹在轴向岩浆室和片状岩脉之间，轴向岩浆室的热量通过它进行交换。相比之下，在缓慢和超低扩张海脊形成的地壳变化很大，从地幔直接侵位到广泛区域的海底，到形成与快速扩张海脊大致相似的层状地壳的局部高度岩浆扩张。由于缺乏系统的研究，缓慢和超低扩张脊系的辉长岩/脉岩转变的性质仅受到很少的约束。拟议项目的目的是与伍兹霍尔海洋研究所(美国马萨诸塞州WHOI)的Henry Dick合作，系统地审查WHOI的大量挖泥船和潜水套件，其中包含来自缓慢和超低扩展海脊的板状堤坝根区的岩石。这些将首先根据手部标本的火成岩和变质组合、变形结构、脉络和接触关系来表征。在后续工作中，将选择代表不同构造/岩浆环境(构造侵位模式与高岩浆生产力)的样品。然后，将用岩石学和电子探针对这套岩石进行研究，以揭示原生岩浆、晚期岩浆、次生高温(即花岗岩重结晶)、低温热液和构造作用的岩石学记录。特别关注的将是在相应的脉岩玄武岩中发现具有特征的麻粒岩相花岗岩叠置的存在--这是CBL发展的关键。然后，将获得的数据库与快速扩张系统报告的数据库进行比较，以对辉长岩/岩脉界面及其对这些非常不同的扩张环境中的地壳增生的意义进行一般性比较。工作假说是，对于那些具有较强岩浆活动的缓慢扩张的山脊段，预计辉长岩/脉岩动力学类似于快速扩张的类EPR地壳，具有明确的CBL，而对于那些地壳增长受构造过程控制的段，预计会发展相对简单的切割关系(玄武岩切割辉长岩)。

The gabbro/dike transition of the oceanic crust is one of the most important interfaces of our planet, where magmatic, metamorphic and hydrothermal processes interfere. At fast-spreading ridges, where the crust is generally accepted to be layered and uniform, the gabbro/dike transition is characterized by a decametre-thick horizon made of hornfels formed under metamorphic conditions up to the granulite facies. This corresponds to the conductive boundary layer (CBL), a thin (<100 m), hot (>650 °C), impermeable zone predicted by theoretical models, which is sandwiched between the axial magma chamber and the sheeted dikes and across which the heat of the axial magma chamber is exchanged. In contrast, crust formed at slow- and ultraslow-spreading ridges is highly variable, ranging from emplacement of the mantle directly to the seafloor over broad regions, to local highly magmatic spreading that forms layered crust broadly similar to that from fast-spreading ridges. Due to the lack of systematic investigations, the nature of the gabbro/dike transition of slow- and ultraslow-spreading ridge systems is only poorly constrained. It is the aim of the proposed project, to perform in collaboration with Henry Dick from the Woods Hole Oceanographic Institution (WHOI, Massachusetts, US) a systematic review of the extensive collections of dredges and dive suites of the WHOI containing rocks from the sheeted dike rooting zone from slow- and ultraslow-spreading ridges. These will be characterized in terms of igneous and metamorphic assemblage, deformation textures, veining, and contact relationships first in hand specimen. For the subsequent work samples will be selected representative for different tectonic/magmatic environments (tectonic emplacement mode vs. high magmatic productivity). This suite will then be investigated petrographically and with the electron microprobe, in order to unravel the petrographic record of primary magmatic, late magmatic, secondary high-temperature (i.e. granoblastic recrystallization), low-temperature hydrothermal, and tectonic processes. Special focus will be to discover the presence of the characteristic granulite-facies granoblastic overprint in the corresponding dike basalts - the key for the development of a CBL. The database to be obtained will then be compared with that reported from fast-spreading systems to make a generalized comparison of the gabbro/dike interface, and its implications for crustal accretion in these very different spreading environments. The working hypothesis is that for those segments of slow-spreading ridges with stronger magmatic activity it is expected that the gabbro/dike dynamics are similar to fast-spread, EPR-like crust, with a well-defined CBL, while for those segments where crustal accretion is controlled by tectonic processes, it is expected that relative simple cutting relations are developed (basalt cuts gabbro).