Abstract
The transport of supercritical fluids is a determining factor for several geological processes and fundamental in predicting natural resource accumulation and distribution. Calcite, ubiquitous in most geological environments, may contain supercritical CO2 trapped under the form of fluid inclusions that may move through grain boundaries affecting the rock physical properties. However, despite macroscopic evidence for this process, until recent it was not possible to characterize this process at the nano-scale due to the difficulty of such observations. In this study, we report nanometer-scale observations on calcite crystal surfaces and demonstrate that stress with absence of visible deformation produces fluid leakage from fluid inclusions. Atomic Force Microscopy scanning experiments on freshly cleaved calcite crystals containing visible fluid inclusions revealed the spontaneous formation of nanometer-scale hillocks on flat crystal terraces in only a few minutes, without evidence of surface dissolution. The fact the hillocks formed on flat surface in a short time was unexpected and suggests deposition of material from the inner crystal to the surface through small-scale fluid migration. We estimated the rate of this fluid mobility is by several orders of magnitude higher than the diffusion rate through vacancies estimated in calcite crystals showing that CO2–rich fluids through micro-pore and nano-pore spaces is in reality much higher than previously assumed using current predictive models.
Highlights
The transport of supercritical fluids is a determining factor for several geological processes and fundamental in predicting natural resource accumulation and distribution
We report nanometer-scale observations made by Atomic Force microscopy (AFM) on natural calcite crystals containing micrometric fluid inclusions and demonstrate stress-induced production of fluid leakage from what most likely are nanometric fluid inclusions, in a very short time under standard temperature and pressure conditions
Optical microscope observations of the calcite crystal denoted the presence of several fracture planes of few millimeters in length, as well as fluid inclusions organized along similar planes or occurring isolated or in small groups (Fig. 1a,b)
Summary
The transport of supercritical fluids is a determining factor for several geological processes and fundamental in predicting natural resource accumulation and distribution. Fluid movement and solid permeability at grain boundaries have often been explained in terms of interfacial energy[7], while when a small amount of fluid is trapped by surface tension, the capillarity pressure is sheared At this scale, fluids must be present in the form of absorbed molecular films, non-equilibrium or wetting film, as well as isolated fluid inclusions maintained by steric force originated from hydrated layer at the mineral surface[13,15]. Fluids must be present in the form of absorbed molecular films, non-equilibrium or wetting film, as well as isolated fluid inclusions maintained by steric force originated from hydrated layer at the mineral surface[13,15] In this contribution, we report nanometer-scale observations made by Atomic Force microscopy (AFM) on natural calcite crystals containing micrometric fluid inclusions and demonstrate stress-induced production of fluid leakage from what most likely are nanometric fluid inclusions, in a very short time under standard temperature and pressure conditions. This study points the way for future similar research on other minerals, helpful for modelling fluid transport of matter, fault lubrication during earthquakes[16] and geological sequestration of CO217
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