Nature of the lower crust and Moho at slower-spreading ridges: SloMo Leg 1 (IODP Expedition 360)

IODP远征360，'SloMo腿1'，将钻一个深钻孔到辉长岩（从玄武岩结晶缓慢冷却的岩石）形成的海底扩张在下大洋地壳下面的超低速扩张西南印度洋脊。这次考察的总体目标是更好地了解火成岩下地壳是如何增生的：熔体通过何种机制迁移、结晶和演化;板块分离如何同时受到岩浆注入和断层和剪切带上的构造拉伸的影响？首次通过锁定在辉长岩内的矿物中的磁场反向边界进行钻探，将使我们能够限制磁异常（“磁条”，这是海底扩张的第一个明确证据）记录在海洋地壳中。作为远征360的联合首席科学家，我将在综合船上的结果方面发挥核心作用。（以及随后的航行后）科学成果，并解决上述大规模问题。然而，我将充分发挥作用，开展一项具体的研究方案，该方案本身应在若干方面作出重大贡献。我建议的活动有两个不同的方面：（1）将岩芯中的独特特征与从井下探头获得的井壁图像上的特征相匹配，从而将回收的岩芯重新定位到地理参考框架中。岩心的各个棒的水平方向是未知的，因此我们通常不能使用具有空间元素的测量：例如（i）矿脉的方向（重建海水通过裂缝渗透的几何形状），（ii）断层和剪切带（重建岩石变形的历史），（iii）辉长岩中岩石类型和晶体取向之间的边界（重建岩浆运动），及（iv）记录在岩石中的磁化方向的三维定向（以了解地壳中的磁异常是如何记录的）。但是，如果岩心中的一个显著特征（如平面、倾斜裂缝/矿脉）能够与通过电测井或声波测井获得的井壁（定向）图像中的代表性唯一匹配，则我们可以将岩心重新定向到地理坐标。然后，我们可以将该核心部分的所有空间测量恢复到地理参考框架。在实践中，这项技术需要在专门的工作站上进行巡航后的工作，是耗时的，需要高的核心恢复和高质量的日志（我们希望获得这两个）。然而，回报是巨大的，并将为广泛的研究提供信息。我希望将我的研究结果应用于上述四个主题，特别是第（iv）项，并与Exp 360科学家A.莫里斯(2)从一套精选的辉长岩中生成一套全面的矿物分析，以约束熔体如何在下地壳岩浆体中迁移和结晶。特别是，我建议测试我和亲密的同事提出的假设，发生显着的化学交换熔体迁移通过孔隙空间的部分结晶辉长岩在下地壳岩浆房。这种反应性熔体迁移记录不佳，但潜在的巨大意义，作为一种机制，修改熔体从地幔之前，他们爆发在seaflother.I打算利用国家的最先进的新的分析扫描电子显微镜设施在卡迪夫，独特的，使我们能够迅速获得定量图像地图的元素浓度的矿物部分的核心。这些发现揭示了组成中的神秘变化，我们可以使用激光技术进一步探测，以测量单个晶体中的微量元素浓度。由此，我们可以模拟熔体-晶体反应的程度，并严格区分不同的解释。

IODP Expedition 360, 'SloMo Leg 1', will drill a single deep borehole into gabbros (slowly cooled rock crystallised from basalt) formed by seafloor spreading in the lower ocean crust beneath the ultraslow-spreading SW Indian Ridge. The overall goal of the leg is to better understand how the igneous lower crust is accreted: by what mechanisms do melts migrate, crystallise and evolve; how is plate separation simultaneously accommodated by magma injection and tectonic stretching on faults and shear zones? Drilling for the first time through a magnetic field reversal boundary locked into minerals within the gabbros will allow us to constrain how magnetic anomalies ('magnetic stripes', which were the first definitive evidence for seafloor spreading) are recorded in ocean crust.As Co-Chief Scientist of Expedition 360 I will play a central role in synthesising the results of onboard (and later post-cruise) scientific results and addressing the broad scale questions outlined above. I will, however, play a full role in undertaking a specific research programme that should contribute significantly in its own right on a number of fronts. There are two distinct strands to my proposed activities: (1) To reorientate the recovered drill core to the geographical reference frame, by matching distinctive features in core pieces to their representations on images of the borehole wall obtained from probes lowered down the hole. The horizontal orientation of the individual rods of drill core is unknown, so we cannot normally utilise measurements that have a spatial element: e.g. (i) the orientations of mineral veins (to reconstruct the geometry of seawater percolation through cracks), (ii) faults and shear zones (to reconstruct the history of the deformation suffered by the rocks), (iii) boundaries between rock types and orientations of crystals in the gabbros (to reconstruct magma movement), and (iv) the 3D orientation of magnetisation directions recorded in rocks (to understand how magnetic anomalies are recorded in the crust). We can, however, reorientate core to geographical coordinates if a distinctive feature in it, such as a planar, inclined fracture/vein, can be matched uniquely to its representation in (oriented) images of the borehole wall obtained by electrical or acoustic well logging. We can then restore all spatial measurements from that core piece back to the geographical reference frame. In practice the technique, which needs to be done post-cruise on a dedicated workstation, is time-consuming and requires high core recovery and good quality logs (both of which we expect to obtain). The rewards are nevertheless great and will inform a broad range of studies. I wish to apply my results to all of the four topics outlined above, but especially to item (iv), in specific collaboration with Exp360 scientist Prof A. Morris.(2) To generate a comprehensive suite of mineral analyses from a selected suite of gabbros to constrain how melt migrates and crystallises in lower crustal magma bodies. In particular I propose to test the hypothesis proposed by myself and close colleagues that significant chemical exchanges occur as melts migrate through the pore spaces of partially crystalline gabbro mushes in the lower crustal magma chamber. This reactive melt migration is poorly documented but potentially of huge significance as a mechanism of modifying melts from the mantle before they are erupted at the seafloor.I intend to make use of the state-of-the-art new analytical scanning electron microscope facility in Cardiff that, uniquely, allows us to rapidly acquire quantitative image maps of element concentrations of minerals in sections of core. These reveal cryptic variations in composition that we can further probe using laser techniques to measure trace element concentrations in individual crystals. From this we can model the extent of melt-crystal reactions and rigorously distinguish between alternative explanations.