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

热液对海洋沉积物中磁性矿物组合的影响（瓜伊马斯盆地，加利福尼亚湾，IODP Expedition 385）

• 批准号: NE/T01234X/1
• 负责人: Antony Morris
• 金额: $ 6.43万
• 依托单位: Plymouth University
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2020
• 资助国家: 英国
• 起止时间: 2020 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FT01234X%2F1
• 关键词: Hydrothermal; influences; magnetic; mineral; assemblages

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.

科学海洋钻探的一个长期目标是了解固体地壳形成期间发生的火山和岩浆过程与海底以下环境中海水循环之间的相互作用和交换。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。国际海洋发现计划第385次考察旨在对瓜伊马斯盆地（加州湾）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岩床的近水平岩浆层侵入。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改变和沉淀形成新矿物。这个知之甚少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。由于这些矿物具有天然磁性，因此可以使用对磁性矿物的成分、粒度和浓度高度敏感的岩石磁性技术来研究它们的性质和在与岩床侵入体有关的蚀变带中的分布。将检验岩浆岩床侵入海洋沉积物导致其主要磁性矿物学发生明显变化的假设（通过在热液流体迁移过程中产生新的磁性相），这可以使用敏感的岩石磁性实验来表征和区分。为实现这一目标，将在距离火成岩岩床及其寄主沉积岩接触面逐渐增加的地方，以系统的方式对在瓜伊马斯盆地钻探期间采集的岩心样品进行二次取样。然后将使用最先进的实验岩石磁性技术来充分验证与海底沉积物中的底床驱动热液循环有关的磁性矿物学变化。这将包括分析实验室赋予的磁化称为一个等温rehistoric磁化（IRMs）的增长的实验数据。长期以来，磁共振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。然而，如果以足够高的分辨率进行这些实验，则可以使用数据的数值“端元建模”来量化采集曲线的形状中几乎不可察觉的变化。这种方法允许样品之间的磁性矿物学的非常细微的差异被确定沿着与每个样品中的不同相的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增强（磁滞是一种现象，样品磁化强度的变化滞后于实验室中施加给它们的磁场的变化）。这些数据然后可以被分析以产生所谓的“一阶反转曲线”或“FORC”，其提供关于样品中存在的磁性颗粒大小以及这些颗粒与其他磁性颗粒相互作用的程度的信息。一旦对远离侵入接触的剖面中收集的样品进行的磁性实验完成，岩石磁性数据将与互补的岩石学、地球化学和热模拟分析，这是由385远征科学队的其他成员计划。这将提供一个全面的物理和化学表征的热液过程与岩床侵位，将促进我们对这些动态系统的理解。