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

在海底热液喷口，富金属喷口沉积物是由热（~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。这些沉积物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。为了了解这些沉积物是如何形成和发展的，以及沉积物内的环境条件（如温度、pH值、化学成分、局部流速）是如何随时间变化的，我们需要知道喷口流体和海水是如何流经沉积物的。这需要知道喷口结构不同部分的渗透率，也就是说，流体在压力梯度的作用下流过沉积层不同部分的难易程度。由于渗透率与孔隙度密切相关，这是一个更容易测量的物理参数，因此人们努力建立渗透率-孔隙度关系。虽然渗透率和孔隙度之间没有单一的“通用”关系，但对于某些类型的样品，渗透率和孔隙度之间存在良好的相关性，特别是当考虑孔隙演化过程（即改变孔隙空间的过程）时。与喷口沉积相关的孔隙演化过程类型包括沉淀和溶解，以及裂缝的形成（例如热裂解）。这些过程破坏和/或产生孔隙。通过微观结构观察，即利用反射光显微镜观察颗粒大小、孔径大小、孔隙分布和连通性，来确定哪些过程导致了孔隙度和渗透率的变化。通过对不同演化渗透率-孔隙度关系（eppr）的识别，可以了解不同部位的通风口结构是如何随着时间的推移而形成的，以及什么过程导致了孔隙度和渗透率的变化，从而影响了流体流经部分通风口沉积物的难易程度。在我们的研究中，我们将对各种喷口结构类型进行渗透率/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处回收样品。这项工作建立在我们第一次从单个喷口区恢复的喷口结构研究的成功基础上，在那里我们证明了两种不同的eppr与两种不同的纹理和烟囱生长过程有着非常好的相关性。我们的数据和观察结果将用于确定渗透率的范围和非均质性，并确定不同的eppr，我们假设eppr将与反映不同物理和化学过程的不同纹理相关（例如，热裂与块状颗粒的沉淀与矿物涂层的沉淀）。研究结果将用于输运和反应模型，以检验反馈过程，这对于模拟喷口结构内的流体流动至关重要。我们的研究将解决许多热液结构的一个关键问题，是否存在导致堵塞的级联反馈，或者，或者，反馈是否使得流体在结构的某些部分保持流动。我们认识到，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、渗透率和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）进行测试，并作为建议的科学展览项目。我们还将为高中生和本科生提供在实验室进行研究的机会，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议的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与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.