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在地球固体地壳形成期间发生的火山和岩浆过程之间的相互作用和交换，以及海洋水在海底以下环境中的循环。它们控制着地球系统中洋壳、海洋沉积物和海水之间的基本化学交换。国际海洋发现计划385旨在对瓜伊马斯盆地(加利福尼亚州海湾)的一系列海洋沉积物进行采样，这些沉积物在不同深度被被称为岩床的亚水平岩浆片侵入。在这种岩床的冷却和结晶过程中，热量转移到寄主岩石中，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。这个鲜为人知的系统的一个重要方面是含铁相(氧化物、氢氧化物和硫化物)的变化和生长。由于它们是天然磁性的，它们的性质和在与岩床侵入有关的蚀变带上的分布可以用对磁性矿物的成分、粒度和浓度高度敏感的岩石磁学技术来研究。因此，该项目将检验这样一种假设，即岩浆岩床侵入海洋沉积物导致其原生磁性矿物学发生显著变化(通过在热液迁移过程中产生新的磁相)，这种变化可以通过灵敏的岩石磁性实验来表征和区分。为实现这一目标，在瓜伊马斯盆地钻探期间采集的岩心样品将在火成岩岩床与其寄主沉积岩之间的接触处逐步增加距离，以系统的方式进行分采样。然后将使用最先进的实验岩石磁学技术来全面描述与海床驱动的热液循环相关的磁性矿物学变化。这将包括对实验室提供的被称为等温剩余磁化(IRMS)的磁化增长的实验数据进行分析。长期以来，IRM采集曲线的形状一直被用作不同磁性矿物(如磁铁矿和赤铁矿)存在的指示器。然而，如果这些实验是在足够高的分辨率下进行的，那么采集曲线形状的几乎不可察觉的变化可以使用数据的数字“末端构件建模”来量化。这种方法可以确定样品之间非常细微的磁性矿物学差异，以及不同相在每个样品中对整体磁信号的相对贡献。这些新的分析将通过测量样品的磁滞特性的实验来加强(磁滞是指样品磁化的变化滞后于实验室施加的磁场的变化的现象)。然后，可以对这些数据进行分析，以产生所谓的“一阶反转曲线”或“FORC”，这些曲线提供了样品中存在的磁性颗粒大小以及这些颗粒与其他磁性物质相互作用的程度的信息。一旦在远离侵入接触的剖面中收集的样品完成了磁性实验，岩石磁性数据将与探险385科学党其他成员计划进行的补充岩石学、地球化学和热模拟分析的结果结合在一起。这将提供与底床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力系统的理解。

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.