Superfast Spreading Rate Crust 4: Magnetism of the lower oceanic crust

洋中脊海底扩张形成的海洋岩石圈是地球上运行的基本地质过程之一。最重要但也最棘手的问题之一，是了解山脊下的岩浆储层如何产生下地壳，尤其是在快速扩张的情况下。来自下地壳的辉长岩通常是难以接近的，但由于以前在太平洋超快速扩张地壳内的一个独特地点进行的科学海洋钻探探险的成功，现在已经可以取样了。综合海洋钻探计划（IODP） 1256站点之前的三次考察已经穿透了1500米的上地壳层，使新的考察首次对下地壳进行了广泛的采样。这将在IODP远征335期间实现，他们将返回1256地点进一步加深洞，希望提供一套独特的海底地壳样本，从而对海底扩张的岩浆和构造过程产生独特的见解。作为这项工作的一部分，古地磁数据将从回收的地核碎片中收集，这对了解该地点下地壳的演化至关重要。这些数据将提供关于锁定在辉长岩中的磁化方向和强度的有价值的信息，提供一个标记，可以用来推断影响该地点的构造旋转的数量，并深入了解下地壳岩石对海洋磁异常的贡献。此外，我们打算结合古地磁数据和井壁内部的地球物理图像来重新定位一些通过钻孔恢复的岩心，从而允许其他定向属性（例如结构数据）恢复到正确的地理参考框架。

Generation of ocean lithosphere by seafloor spreading at mid-ocean ridges is one of the fundamental geological processes operating on Earth. One of the most important yet most intractable problems is to understand how the magma reservoir beneath ridges generates the lower crust, especially at fast spreading rates. Gabbroic rocks from the lower crust are normally inaccessible, but are now within reach of sampling as a result of the previous successes of scientific ocean drilling expeditions to a unique site within superfast spreading rate crust in the Pacific Ocean. A series of three previous expeditions to Integrated Ocean Drilling Program (IODP) Site 1256 have penetrated through 1500m of upper crustal layers, allowing a new expedition to extensively sample the lower crust for the first time. This will be acheived during IODP Expedition 335 which will return to Site 1256 to deepen the hole still further, hopefully providing a unique suite of lower oceanic crustal samples that will yield unique insights into magmatic and tectonic processes involved in seafloor spreading. As part of this endeavour, palaeomagnetic data will be collected from recovered core pieces and will be critical to understanding the evolution of the lower crust at this site. These data will provide valuable information on the direction and strength of magnetization locked into the gabbroic rocks we expect to encounter, providing a marker that can be used to infer the amount of tectonic rotation that has affected the site and insights into the contribution that lower crustal rocks make to marine magnetic anomalies. In addition, we intend to use a combination of palaeomagnetic data and geophysical images of the inside of the borehole wall to reorient some of the core pieces recovered by drilling, thereby allowing other directional properties (e.g. structural data) to be restored to the correct geographical reference frame.