Spatial and temporal scales of crustal accretion in slow-spreading rate oceanic crust (Atlantis Massif, Mid Atlantic Ridge - IODP Site U1309)

大洋中脊海底扩张形成大洋岩石圈是地球上运行的基本地质过程之一。最重要也是最棘手的问题之一是了解山脊下的岩浆库是如何产生下地壳的。来自下地壳的辉长岩通常是无法接触到的，但通过在缓慢扩张的海脊段末端附近的地壳大规模断层作用(在所谓的“海洋核心杂岩”的发展过程中)，这些岩石被带到了科学海洋钻探的触手可及之处。其中一个地点是位于大西洋中脊北纬30度的亚特兰蒂斯地块。这里的钻探最近发现了一段1.4公里长的垂直下部地壳岩石，显示出岩石类型在几米到几十米的范围内迅速变化。船上对这些岩石的自然磁化的初步分析表明，这一部分记录了在地球磁场的不同极性(即在不同时间)获得的复杂的磁化序列。在海洋钻探的历史上，这种多组分的磁化作用以前只遇到过一次。这些数据与一个模型最为一致，在该模型中，下地壳是在一段长时间内以亚水平薄片(或岩床)的形式连续侵入岩浆，跨越几次极性反转而形成的。这种“席状岩床”机制以前是根据蛇绿岩(即造山过程中侵位到大陆上的古代洋壳碎片)的现场证据提出的，但从未在真正的大洋中脊环境中得到明确测试。我们建议通过对亚特兰蒂斯地块样品进行详细的古地磁分析来进行这样的测试，以便准确地描述具有不同磁极的岩心间隔之间的转变的性质，并确定以多分量磁化为标志的间隔的磁性叠印的起源。这些分析将使我们能够明确地确定在不同地磁极性时期注入的离散侵入体/侵入体包之间的边界，并将对地壳建造的时间和亚特兰蒂斯地块的热状态提供宝贵的限制。古地磁数据将得到John博士(怀俄明大学项目合作伙伴)的补充辐射测年和Meurer博士(休斯顿大学项目合作伙伴)从相同岩心样本中获得的矿物化学数据的支持。此外，我们将把岩心中可见的物理结构与井壁定向地球物理图像中观察到的物理结构进行匹配。这将使我们能够将从最初方位不受约束的岩芯获得的古地磁数据恢复到真正的地理参考框架，从而使用完整的磁化矢量(方位方向和倾角)来确定围绕水平轴和垂直轴的构造旋转对地块结构发展的贡献程度。因此，该项目提供了一个独特的机会，可以同时确定下地壳在缓慢扩张轴线上的吸积模式(从而测试关于大洋中脊岩浆室功能的相互矛盾的模型)，并测试目前大洋核心杂岩中下地壳折返的模型。

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. Gabbroic rocks from the lower crust are normally inaccessible, but are brought within reach of scientific ocean drilling by large-scale faulting of the crust near the ends of slow spreading ridge segments (during the development of so called 'oceanic core complexes'). One such locality is at the Atlantis Massif, at 30 degrees north on the Mid-Atlantic Ridge. Drilling here has recently recovered a 1.4 km vertical section of lower crustal rocks that shows rapid changes in rock type on a scale of metres to tens of metres. Preliminary shipboard analyses of the natural magnetizations of these rocks shows that this section records a complex sequence of magnetizations acquired in different polarities of the Earth's magnetic field (i.e. at different times). Such multi-component magnetizations have only once been previously encountered in the history of ocean drilling. The data are most consistent with a model in which the lower crust was built by successive intrusions of magma in the form of sub-horizontal sheets (or sills) over a protracted period of time spanning several polarity reversals. This 'sheeted sill' mechanism has been previously proposed on the basis of field evidence from ophiolites (i.e. fragments of ancient oceanic crust emplaced onto the continents during orogenesis), but has never been definitively tested in a true mid-ocean ridge setting. We propose to carry out such a test by performing detailed palaeomagnetic analyses of samples from the Atlantis Massif, in order to precisely characterise the nature of the transitions between core intervals with different magnetic polarities and to establish the origin of magnetic overprinting in intervals marked by multi-component magnetizations. These analyses will allow us to unequivocally identify the boundaries between discrete intrusions/packages of intrusions injected during different geomagnetic polarity periods, and will provide invaluable constraints on the timing of crustal construction and the thermal state of the Atlantis Massif. The palaeomagnetic data will be supported by complementary radiometric dating by Dr. John (Project Partner, University of Wyoming) and mineral chemistry data obtained from the same core samples by Dr. Meurer (Project Partner, University of Houston). In addition, we will match physical structures visible in the cores to those observed in oriented geophysical imagery of the borehole wall. This will allow us to restore the palaeomagnetic data obtained from the originally azimuthally unconstrained cores to a true geographic reference framework and hence to use the complete magnetization vector (azimuthal direction and dip) to determine the extent to which tectonic rotations about both horizontal and vertical axes have contributed to the structural development of the massif. The project therefore represents a unique opportunity to simultaneously establish the mode of accretion of the lower crust at a slow-spreading axis (thereby testing conflicting models for the functioning of mid-ocean ridge magma chambers) and test current models for the exhumation of the lower crust in oceanic core complexes.