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

IODP Expedition 360，“SloMo Leg 1”，将在辉长岩(由玄武岩结晶而成的缓慢冷却的岩石)中钻一个深孔，形成辉长岩，这些辉长岩是由海底在超低扩张的西南印度洋海脊下方的下洋壳中扩散形成的。该分支的总体目标是更好地了解火成岩下地壳是如何聚集的：熔体通过什么机制迁移、结晶和演化；板块分离是如何同时被岩浆注入和断层和剪切带上的构造拉伸所适应的？首次通过锁定在辉长岩内矿物中的磁场反转边界进行钻探，将使我们能够限制如何在洋壳中记录磁异常(磁条，这是海底扩张的第一个确凿证据)。作为探险360的联合首席科学家，我将在综合船上(以及后来的巡航后)科学结果和解决上文概述的广泛问题方面发挥核心作用。然而，我将充分发挥作用，开展一项具体的研究计划，该计划本身应在多个方面做出重大贡献。我提议的活动有两个不同的方面：(1)将回收的钻芯重新定位到地理参考框架，方法是将岩芯碎片中的明显特征与它们在井壁图像上的表示进行匹配，这些图像是通过向下放下探头获得的。钻探岩心各杆的水平方位未知，因此我们无法正常利用具有空间元素的测量结果：例如：(I)矿脉的方位(重建通过裂缝的海水渗流的几何形状)；(Ii)断层和剪切带(重建岩石遭受变形的历史)；(Iii)岩石类型之间的边界和辉长岩中晶体的取向(重建岩浆运动)；以及(Iv)记录在岩石中的磁化方向的3D取向(以了解地壳中如何记录磁异常)。然而，如果岩心中的一个独特特征，如平面的倾斜裂缝/脉体，能够唯一地与它在电测或声波测井获得的井壁(定向)图像中的表示相匹配，我们就可以将岩心重新定位到地理坐标。然后，我们可以将该核心部分的所有空间测量恢复回地理参考框架。在实践中，这项技术需要在专用工作站上进行巡航后完成，非常耗时，需要高核心恢复和高质量的日志(这两点我们都希望获得)。然而，回报是巨大的，并将为广泛的研究提供信息。我希望将我的成果应用于上面概述的所有四个主题，特别是与Exp360科学家A·莫里斯教授合作的项目(Iv)。(2)从选定的一套辉长岩中产生一套全面的矿物分析，以限制熔体如何在下地壳岩浆体中迁移和结晶。特别是，我建议检验我和我的亲密同事提出的假设，即当熔体通过下部地壳岩浆室中部分结晶的辉长岩的孔隙空间迁移时，会发生重大的化学交换。这种反应性熔体迁移的记录很少，但作为一种在地幔熔体在海底喷发之前进行变质的机制，这种反应性熔体迁移可能具有巨大的意义。我打算利用加的夫最先进的新分析扫描电子显微镜设备，这种设备独特地使我们能够快速获得岩心部分矿物元素浓度的定量图像地图。这些揭示了成分的神秘变化，我们可以使用激光技术进一步探测，以测量单个晶体中的痕量元素浓度。由此，我们可以模拟熔融-结晶反应的程度，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.