Hydrothermal influences on magnetic mineral assemblages in marine sediments (Guaymas Basin, Gulf of California, IODP Expedition 385)

科学海洋钻探的一个长期目标是了解火山和岩浆过程之间的相互作用和交换，这些过程发生在地球固体地壳形成和海底环境中海水的循环过程中。它们控制着地球系统中海洋地壳、海洋沉积物和海水之间的基本化学交换。国际海洋探索计划第385远征队的目标是对瓜伊马斯盆地（加利福尼亚湾）的一系列海洋沉积物进行取样，这些沉积物在不同深度被称为基岩的亚水平岩浆片侵入。在冷却和结晶过程中，热量传递到宿主岩石中，提供了驱动海水热液循环的能量。这导致热流体与它们所经过的沉积物和岩石之间的化学元素交换，导致岩石中现有矿物的变化，并通过沉淀形成新的矿物。这个鲜为人知的体系的一个重要方面是含铁相（氧化物、氢氧化物和硫化物）的蚀变和生长。由于这些矿物具有天然磁性，因此可以利用对磁性矿物的组成、粒度和浓度高度敏感的岩石磁性技术来研究它们的性质及其在与残岩侵入岩相关的蚀变带中的分布。因此，本项目将检验这样一种假设，即岩浆岩浆岩侵入海洋沉积物导致其主要磁性矿物学发生明显变化（通过热液流体迁移期间产生新的磁性相），这些变化可能通过敏感的岩石磁性实验来表征和区分。为了实现这一目标，在Guaymas盆地钻探过程中回收的岩心样本将以系统的方式在离火成岩断层与其宿主沉积岩接触的距离逐渐增加的地方进行亚采样。最先进的实验岩石磁学技术将被用来全面描述与海洋沉积物中技能驱动的热液循环相关的磁性矿物学变化。这将包括对被称为等温剩余磁化（IRMs）的实验室传授磁化增长的实验数据的分析。IRM采集曲线的形状长期以来被用作不同磁性矿物（如磁铁矿和赤铁矿）存在的指示物。然而，如果这些实验在足够高的分辨率下进行，那么几乎难以察觉的采集曲线形状变化可以使用数据的数值“端元建模”来量化。这种方法可以确定样品之间磁矿物学的非常细微的差异，以及每个样品中不同阶段对总体磁信号的相对贡献。这些新的分析将通过测量样品磁滞特性的实验来增强（磁滞是一种现象，通过这种现象，样品的磁化变化滞后于实验室中施加在它们身上的磁场的变化）。然后可以分析这些数据以产生所谓的“一阶反转曲线”或“forc”，该曲线提供了样品中存在的磁性颗粒尺寸以及这些颗粒与其他磁性颗粒相互作用程度的信息。一旦在远离侵入触点的剖面上收集的样品的磁性实验完成，岩石磁性数据将与385科考队其他成员计划的互补岩石学、地球化学和热模拟分析的结果相结合。这将提供与储层就位相关的热液过程的全面物理和化学表征，这将促进我们对这些动态系统的理解。

A long-term aim of scientific ocean drilling is to understand the interactions and exchanges between volcanic and magmatic processes that take place during the formation of the Earth's solid crust and the circulation of oceanic water through the sub- seafloor environment. These control the fundamental chemical exchanges between oceanic crust, marine sediments and seawater in the Earth system. International Ocean Discovery Program Expedition 385 aims to sample a sequence of marine sediments in the Guaymas Basin (Gulf of California) that have been intruded at various depths by sub-horizontal sheets of magma known as sills. The transfer of heat into the host rocks during cooling and crystallization of such sills provides energy that drives the hydrothermal circulation of seawater. This results in the exchange of chemical elements between the hot fluids and the sediments and rocks they pass through, leading to alteration of existing minerals in the rocks and the formation of new minerals by precipitation. An important aspect of this poorly understood system is the alteration and growth of iron-bearing phases (oxides, hydroxide and sulphides). Since these are naturally magnetic, their nature and distribution across zones of alteration associated with sill intrusions may be investigated using rock magnetic techniques that are highly sensitive to the composition, grain-size and concentration of magnetic minerals.This project, therefore, will test the hypothesis that the intrusion of magmatic sills into marine sediments results in distinctive changes to their primary magnetic mineralogy (via the production of new magnetic phases during hydrothermal fluid migration) that may be characterized and distinguished using sensitive rock magnetic experiments. To achieve this, core samples recovered during drilling in the Guaymas Basin will be sub-sampled in a systematic manner at progressively increasing distances from contacts between igneous sills and their host sedimentary rocks. State-of-the-art experimental rock magnetic techniques will then be used to fully characterise the changes in magnetic mineralogy associated with sill-driven hydrothermal circulation in marine sediments. This will include analysis of experimental data on the growth of laboratory-imparted magnetizations known as an isothermal remanent magnetizations (IRMs). The shape of IRM acquisition curves has long been used as an indicator of the presence of different magnetic minerals such as magnetite and hematite. If these experiments are performed at a sufficiently high resolution, however, then almost imperceptible changes in the shape of the acquisition curves may be quantified using numerical "end member modelling" of the data. This approach allows very subtle differences in magnetic mineralogy between specimens to be determined along with the relative contributions of different phases to the overall magnetic signal in each sample. These novel analyses will be augmented by experiments involving measuring the magnetic hysteresis properties of the specimens (hysteresis is a phenomenon by which changes in the magnetization of samples lag behind those of magnetic fields applied to them in the laboratory). These data may then be analysed to produce so-called "first order reversal curves" or "FORCs" that provide information on the magnetic grain sizes present in a sample and the degree to which these grains interact with other magnetically.Once the magnetic experiments on samples collected in profiles away from intrusive contacts are completed, the rock magnetic data will be combined with results from complementary petrological, geochemical and thermal modelling analyses that are planned by other members of the Expedition 385 Science Party. This will provide a comprehensive physical and chemical characterization of hydrothermal processes associated with sill emplacement that will advance our understanding of these dynamic systems.