At 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.
