ABSTRACT:

Pressure solution, the stress-mediated transport of mass by dissolution and precipitation, is one of the most important deformation mechanisms operating in the earth?s upper crust. It affects both hydraulic and mechanical properties of rocks by redistributing material on relatively short spatial scales. We present here a unique experimental platform designed to study pressure solution by loading small (~3mm) fluid-immersed samples against various flat surfaces, and observing the samples? axial strain and their morphologies in real time. The platform measures axial displacement rates smaller than 5Å/h over weeks, and resolves micron-size structures in 3D. A corner of a cleaved calcite single crystal rhomb was polished into a triangular face (edge length ~ 200µm) and pressed against a muscovite disc to yield a nominal stress of 80MPa. Immersing the sample in a pre-saturated water solution resulted in an axial displacement rate of ~1nm/h, with no significant changes in contact morphology. However, when pre-saturated NH4Cl solution was added, the highly reactive fluid caused an increase in axial displacement rates to ~50nm/h, and the contact roughened, with load-bearing surfaces becoming smaller.

1. INTRODUCTION

Pressure solution is a term encompassing a plethora of mass transport processes that involve dissolution and precipitation of stressed grains [1, 2, 3]. These processes require the presence of a reactive fluid, and the existence of a chemical potential gradient between the site of dissolution and the site of precipitation driven by differences in normal stress, strain energy and surface energy [4, 5]. It is considered to be one of the most important deformation mechanisms in the Earth?s crust because of its impact on the hydraulic and mechanical properties of rocks [6, 7]. Experimental pressure solution studies are traditionally grouped into two broad sets. The first uses granular samples immersed in reactive fluids to investigate constitutive relationships and to probe macroscopic textures [8, 9]. The second loads a fluid-immersed single grain against a flat surface, to evaluate how one contact evolves over time, and how strain rates vary as a function of the contact morphology. Since single contact experiments require considerable thermal and mechanical stability over long periods of time, most investigators have used various rock analogs, primarily salts characterized by high kinetic rates at low temperatures and pressures [10-14]. These rock analogs however have unique mechanical and chemical properties that complicate the applicability of the experimental result to other rock types (e.g., carbonates and siliciclastics). Recently, Zubstsov et al. [15] performed high end pressure solution experiments in calcite that indicated a need for further investigation. We present here a unique experimental platform designed to load fluid-immersed calcite samples against various flat discs, and to observe at high resolution and in real time both vertical strain rates and contact morphology. We show that the deformation of a calcite single-crystal loaded against a mica disc strongly depends on the reactivity of the fluid. An ammonium chloride solution seems to cause contact roughening, and decrease the load bearing surfaces, which in turn causes a dramatic increase in the vertical displacement rates.

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