In-situ X-ray Tomography and Chemical Tracer Experiments Examining Hydrothermal Alteration of Peridotite: Pore Scale Studies with Implications for Water-Rock Interaction Models

岩石的孔隙度和渗透率受矿物和通过流体之间的反应影响很大。由此产生的在流体和岩石相互作用的温度、压力和化学条件下不稳定的矿物的溶解，以及更稳定的矿物物种在矿物之间的孔隙、裂缝和晶界中的沉淀，控制着岩石的孔隙度和孔隙度，以及流体流经岩石的难易程度。这项研究使用了新颖的流通式水热反应堆容器，可以在其中实时观察和测量岩石孔隙度和渗透率的变化以及矿物的沉淀和溶解。这些实验的结果将使用一种数学技术(即，格子-玻尔兹曼方法)进行分析，该方法比目前在水-岩相互作用研究中使用的数学技术提供了更好的精细过程模拟，后者仅依赖于连续介质条件。实验中将使用经典海底橄榄岩的完整岩芯，这些岩石通常是海底富含硫化物的热液沉积物的宿主或来源；原始矿物学的变化将使用稳定的钙、镁和硅同位素来追踪，这将记录碳酸盐和硅酸盐矿物的反应和反应速率。这项工作的更广泛影响包括通过开发新的实验和数学方法来改进科学基础设施，以推进水-岩石相互作用的研究，以及地质材料的孔隙度和渗透率如何随着岩石和沉积物与流经它们的水的反应而变化。这项研究在提高我们对碳封存储存库行为、污染物迁移和核废物处置条件的理解方面具有重要的潜在应用。该项目还包括国家科学基金会本科生研究体验(NSF-REU)项目的本科生，从事尖端科学；培养研究生和博士后；帮助开发和验证用于预测天然岩石和流体的化学和物理性质演化的理论和模型模拟。这项研究的重点是了解穿透水流体对地幔橄榄岩的水解和碳化作用，这是影响大洋中脊和俯冲带洋壳的地球化学和地球物理性质的重要过程。将采用实验、分析和理论相结合的方法来研究反应，这些方法强调对橄榄岩-流体系统的化学和物理演化的时间序列监测，使用新型流动水热反应堆和透明反应池，这些反应池连接到X射线计算机断层扫描仪，允许实时和高分辨率地检查矿物溶解和沉淀过程的变化。这使得可以直接研究水-岩石相互作用过程中岩石的3D结构在精细尺度上的变化。使用非传统稳定同位素(硅、镁和钙)的化学示踪剂将用于限制矿物溶解和沉淀速率，并指示矿物表面积的变化。实验及其结果将使用格子-玻尔兹曼多组分、多相、流体流动和溶质运移计算机程序进行模拟，结果将用于开发使用连续介质规模模拟器的更精确的油藏规模模拟方法。这项研究的目的是调查流体-矿物反应和橄榄岩中相关的孔隙空间几何变化之间的反馈，这些变化是由流体流动、化学物种的平流以及反应位置和反应速率等过程引起的。样品将由海底橄榄岩的岩芯组成。

The porosity and permeability of rocks is impacted heavily by reactions between minerals and through-going fluids. The resulting dissolution of minerals that are unstable at the temperature, pressure, and chemical conditions at which the fluids and rocks are interacting and the precipitation of more stable mineral species in the pores, fractures, and grain boundaries between minerals controls where and how much porosity a rock has and how easily fluids can flow through it. This research uses novel, flow-through, hydrothermal, reactor vessels where changes in the porosity and permeability of rocks and the precipitation and dissolution of minerals can be observed and measured in real time. Results of these experiments will be analyzed using a mathematical technique (i.e., Lattice-Boltzmann approach) that provides a better simulation of the fine-scale processes than mathematical techniques presently used in water-rock interaction studies that depend solely on continuum conditions. Intact cores of classic seafloor peridotites, rocks that commonly host or are the source of sulfide-rich, hydrothermal deposits on the seafloor, will be used in the experiments; and alteration of the original mineralogy will be tracked using the stable isotopes of Ca, Mg, and Si, which will document reactions and reaction rates of carbonate and silicate minerals. Broader impacts of the work include improving infrastructure for science by developing new experimental and mathematical methods to advance studies of water-rock interaction and how the porosity and permeability of geological materials change as rocks and sediments react with waters flowing through them. The research has significant potential applications in improving our understanding of the behavior of carbon sequestration reservoirs, the migration of pollutants, and the conditions of nuclear waste disposal. The project also involves undergraduate students from the National Science Foundation Research Experience for Undergraduates (NSF-REU) program in cutting-edge science; trains a graduate student and a postdoc; and helps to develop and verify theoretical and model simulations use to predict the chemical and physical property evolution of natural rocks and fluids.This research focuses on understanding the hydrolysis and carbonation of mantle peridotite by through-going aqueous fluids, an important process affecting the geochemical and geophysical properties of the ocean crust at mid-ocean ridges and in subduction zones. Reactions will be examined using coupled experimental, analytical, and theoretical approaches that emphasize the time series monitoring of the chemical and physical evolution of peridotite-fluid systems using a novel flow-through hydrothermal reactor and transparent reaction cells that are coupled to an X-ray Computed Tomography instrument that allows changes in mineral dissolution and precipitation processes to be examined in real time and in high resolution. This allows direct investigation of changes in the 3D architecture of the rock on a fine scale during water-rock interaction. Chemical tracers using non-traditional stable isotopes (Si, Mg and Ca) will be used to constrain mineral dissolution and precipitation rates and indicate changes in mineral surface area. The experiments and their results will be modeled using a Lattice-Boltzmann multicomponent, multiphase, fluid flow and solute transport computer code with the results being used to develop more accurate reservoir-scale modeling approaches using continuum-scale simulators. Goals of the research are to investigate the feedback between fluid-mineral reactions and associated pore-space geometry changes in peridotites that result from processes like fluid flow, advection of chemical species, and reaction locations and reaction rates. Samples will consist of cores of seafloor peridotite.