Permeability-Porosity Relationships in Seafloor Vent Deposits: Dependence on Pore Evolution Processes

在海底热液喷口处，富含金属的喷口沉积物是由热（~ 350 ℃）喷口流体、冷（~ 2 ℃）海水和先前沉积的物质之间复杂的相互作用形成的。这些矿床可能类似于陆地上的矿床，并拥有不寻常的生物群落，包括在高温（高达120摄氏度）下茁壮成长的微生物。为了了解这些矿床是如何形成和发展的，以及环境条件（例如，沉积物内的温度、pH值、化学成分、局部流速）随时间变化，我们需要知道喷口流体和海水如何流过沉积物。这需要知道通风口结构的不同部分的渗透性，即流体响应于压力梯度流过沉积物的不同部分的容易程度。由于渗透率与孔隙度密切相关，孔隙度是一个更容易测量的物理参数，因此人们一直在努力建立渗透率-孔隙度关系。虽然不存在单一的“通用”渗透率-孔隙度关系，但对于某些类型的样品，在渗透率和孔隙度之间发现了良好的相关性，特别是当孔隙演化过程（即，改变孔隙空间的过程）。与喷口沉积物有关的孔隙演化过程类型包括沉淀和溶解，以及裂缝的形成（例如，从热裂解）。这些过程破坏和/或产生孔隙。确定哪些过程导致孔隙度和渗透率的变化是通过进行微观结构观察来实现的，即，通过使用反射光显微镜观察纹理细节来检查粒度、孔径、孔分布和连通性。不同演化渗透率-孔隙度关系（EPPR）的识别提供了关于喷口结构的不同部分如何随时间形成的信息，以及什么过程导致孔隙度和渗透率的变化，从而使流体容易流过喷口存款的部分。 在我们的研究中，我们将对各种类型的喷口结构进行渗透率/孔隙度测量和微观结构分析，并从许多不同的活跃海底喷口地点采集样本。这项工作的基础上，我们的第一次研究的成功恢复从一个单一的喷口领域，在那里我们证明了两个不同的EPPR相关性非常好，两个不同的纹理和烟囱生长过程。我们的数据和观察结果将用于确定渗透率的范围和不均匀性，并确定不同的EPPR，我们假设这将与反映不同物理和化学过程的不同纹理相关（例如，热裂化与块状颗粒的沉淀与矿物涂层的沉淀）。结果将用于模型的运输和反应，以检查反馈过程，这是至关重要的模拟喷口结构内的流体流动。我们的研究将解决许多热液结构的一个关键问题，即是否存在导致堵塞的级联反馈，或者，反馈是否使得流体流动在结构的某些部分得以维持。 认识到，与孔隙度不同，渗透率是一个难以完全理解的概念，我们还计划向4年级，8年级和12年级的学生介绍孔隙度和渗透率以及多孔介质中的流动的概念。模块将通过WHSTEP计划在法尔茅斯公立学校（莫尔斯池塘和劳伦斯学校）进行测试，并通过AP物理课程（法尔茅斯高中）和作为一个建议的科学博览会项目。我们还将在我们的实验室内为高中和本科生提供研究机会，收集和分析数据。与过去一样，我们将通过在会议上的演讲和在同行评审期刊上的出版物与科学界分享我们的研究成果，并通过流行的演讲和杂志文章向更广泛的社区分享我们的研究成果。

AbstractOCE-0648337At seafloor hydrothermal vent sites, metal-rich vent deposits form from complex interactions among hot (~350degreesC) vent fluid, cold (~2degreesC) seawater, and previously deposited material. These deposits are possible analogs to ore deposits present on land, and host unusual biological communities, including microorganisms that thrive at high temperatures (up to 120degreesC). To understand how these deposits form and develop, and how environmental conditions (e.g., temperature, pH, chemical composition, local flow rate) within the deposits change over time, we need to know how vent fluid and seawater flow through the deposits. This requires knowing the permeability of different parts of the vent structures, that is, how easily fluid flows through different parts of the deposits in response to pressure gradients. Because permeability is closely related to porosity, a physical parameter that is much easier to measure, efforts have been made to establish permeability-porosity relationships. While there isn't a single 'universal' permeability-porosity relationship, there are good correlations found between permeability and porosity for some types of samples, particularly when pore evolution processes (i.e., the processes that change pore space) are considered. Types of pore evolution processes relevant to vent deposits include precipitation and dissolution, and formation of cracks (e.g., from thermal cracking). These processes destroy and/or create porosity. Identification of which processes are resulting in changes in porosity and permeability is accomplished by making micro-structural observations, i.e., by observing textural details using reflected light microscopy to examine grain size, pore size, pore distribution and connectivity. Identification of different evolution permeability-porosity relationships (EPPRs) provides information about how different portions of vent structures form over time, and what processes are responsible for changes in porosity and permeability and thus the ease with which fluid flows through parts of the vent deposit. In our study we will conduct permeability/porosity measurements and micro-structural analyses on a full range of vent structure types, with samples recovered from many different active seafloor vent sites. This work builds on the success of our first study of vent structures recovered from a single vent field, where we demonstrated that two different EPPRs correlate remarkably well with two different textures and chimney growth processes. Our data and observations will be used to identify ranges and heterogeneities of permeability and to identify different EPPRs, which we hypothesize will correlate with distinct textures that reflect different physical and chemical processes (e.g., thermal cracking vs. precipitation of blocky grains vs. precipitation of mineral coatings). Results will be used in models of transport and reaction to examine feedback processes that are crucial in simulating fluid flow within vent structures. Our study will address a key question for many hydrothermal structures, whether there are cascading feedbacks that lead to clogging, or, alternatively, whether the feedback is such that fluid flow is maintained in certain portions of structures. Recognizing that, unlike porosity, permeability is a difficult concept to fully comprehend, we also plan to introduce the concepts of porosity and permeability, and flow in porous media, to students in grades 4, 8, and 12. Modules will be tested in the Falmouth Public Schools (Morse Pond and Lawrence School) through the WHSTEP program, and through AP physics classes (Falmouth High School) and as a suggested science fair project. We will also provide research opportunities for high school and undergraduate students within our labs, collecting and analyzing data. As in the past, we will share results of our research with the scientific community through presentations at meetings and publications in peer-reviewed journals, and to the broader community through popular presentations and magazine articles.