Abstract
Replacement reactions commonly alter the multiscale pore structures of rocks during fluid-rock interactions. Analysis of these processes in various model fluid-rock systems during controlled laboratory experiments provides insights into the origins of microstructures found in natural materials. This study focused on understanding the effects of initial starting material permeability and resultant differences in transport pathways on porosity and mineralogical changes during limestone dolomitization. A series of replacement experiments (32–317 days in duration) have been conducted in which 1.59 cm (5/8 in.) diameter cores of two different limestones were reacted with saturated MgCl2 solutions at 200 °C. The Texas Cream (Austin Chalk) is a high-porosity, high-permeability limestone, whereas both the porosity and permeability of the Carthage Marble (Burlington Limestone) are relatively low. Altered limestones were imaged using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), Time-of-Flight Secondary Ion Mass Spectrometry (ToF–SIMS) and electron microprobe analysis (EMPA). A representative grain boundary of the low-porosity limestone was targeted for a focused ion beam (FIB) lift-out and characterized using transmission electron microscopy (TEM). These results were coupled with analyses of radial changes in the porosity distribution of the core derived from X-ray and neutron small- and ultra-small angle scattering ((U)SANS/(U)SAXS). The high-porosity/permeability limestone showed a four times faster bulk replacement rate than the lower-porosity/permeability material, and a different mechanism of porosity development. For the low-porosity limestone, a two-stage replacement occurred, with the reacted region of the core consisting of an inner rim in which the limestone was replaced by two calcite-dolomite solid solutions, and an outer rim in which the dolomite was replaced by magnesite. Elongated pores formed along grain boundaries at the initial limestone/dolomite reaction interface, and additional nanometer-scale porosity was formed at the secondary magnesite replacement rim. Grain boundaries were identified as preferential pathways for transport leading to dolomitization and a grain boundary diffusion rate was calculated based on microstructural characterization. In contrast, replacement in the high-porosity limestone was accompanied by porosity generation through replacement of individual grains by dolomite throughout the sample and, in longer runs, magnesite in outer parts of the core. These observations emphasize that both the mechanisms of the replacement reaction and the microstructure and chemistry of the replaced product are contingent on the initial structure of the starting material.
| Original language | English |
|---|---|
| Pages (from-to) | 137-158 |
| Number of pages | 22 |
| Journal | Geochimica et Cosmochimica Acta |
| Volume | 303 |
| DOIs | |
| State | Published - Jun 15 2021 |
Funding
This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under the contract DE-AC05-00OR22725. We acknowledge the support of the National Institute of Standards and Technology, Center for Neutron Research, U.S. Department of Commerce in providing the research neutron facilities used in this work. Access to both NBG30 SANS and BT5 USANS was provided by the Center for High Resolution Neutron Scattering, a partnership between the National Institute of Standards and Technology and the National Science Foundation under Agreement No. DMR-1508249. Certain commercial equipment, instruments, materials and software are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology or the Department of Energy, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose. A portion of this research used resources at the High Flux Isotope Reactor and Spallation Neutron Source, DOE Office of Science User Facilities operated by the Oak Ridge National Laboratory. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Part of the SEM characterization, FIB preparation and TEM characterization was performed in the Kuiper Materials Imaging and Characterization Facility at the University of Arizona with financial support of the Core Facility Pilot Project Grant Program awarded to Dr. Tom Zega (PI) and Juliane Weber (Co-PI). We acknowledge NASA grants #NNX12AL47G, #NNX15AJ22G and #NNX07AI520, and NSF grants #1531243 and #EAR-0841669 for funding of the instrumentation in the Kuiper Materials Imaging and Characterization Facility at the University of Arizona. We acknowledge Cedric Gagnon for setting up the NGB30 at NIST for us. We would like to acknowledge Allan Patchen from University of Tennessee for the microprobe measurements. ToF-SIMS characterization was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility, and using instrumentation within ORNL's Materials Characterization Core provided by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. We would like to thank Carl Steefel for handling the manuscript and two anonymous reviewers for their detailed comments. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under the contract DE-AC05-00OR22725. We acknowledge the support of the National Institute of Standards and Technology, Center for Neutron Research, U.S. Department of Commerce in providing the research neutron facilities used in this work. Access to both NBG30 SANS and BT5 USANS was provided by the Center for High Resolution Neutron Scattering, a partnership between the National Institute of Standards and Technology and the National Science Foundation under Agreement No. DMR-1508249. Certain commercial equipment, instruments, materials and software are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology or the Department of Energy, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose. A portion of this research used resources at the High Flux Isotope Reactor and Spallation Neutron Source, DOE Office of Science User Facilities operated by the Oak Ridge National Laboratory. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Part of the SEM characterization, FIB preparation and TEM characterization was performed in the Kuiper Materials Imaging and Characterization Facility at the University of Arizona with financial support of the Core Facility Pilot Project Grant Program awarded to Dr. Tom Zega (PI) and Juliane Weber (Co-PI). We acknowledge NASA grants #NNX12AL47G, #NNX15AJ22G and #NNX07AI520, and NSF grants #1531243 and #EAR-0841669 for funding of the instrumentation in the Kuiper Materials Imaging and Characterization Facility at the University of Arizona. We acknowledge Cedric Gagnon for setting up the NGB30 at NIST for us. We would like to acknowledge Allan Patchen from University of Tennessee for the microprobe measurements. ToF-SIMS characterization was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility, and using instrumentation within ORNL's Materials Characterization Core provided by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. We would like to thank Carl Steefel for handling the manuscript and two anonymous reviewers for their detailed comments.
| Funders | Funder number |
|---|---|
| Cedric Gagnon | |
| High Flux Isotope Reactor and Spallation Neutron Source | |
| National Institute of Standards and Technology, Center for Neutron Research | |
| National Science Foundation | DMR-1508249, -0841669, 1531243 |
| U.S. Department of Energy | |
| National Aeronautics and Space Administration | 12AL47G, 15AJ22G, 07AI520 |
| National Institute of Standards and Technology | |
| U.S. Department of Commerce | |
| Office of Science | |
| Basic Energy Sciences | DE-AC05-00OR22725 |
| Argonne National Laboratory | DE-AC02-06CH11357 |
| Oak Ridge National Laboratory | |
| University of Tennessee | |
| University of Arizona | |
| UT-Battelle |
Keywords
- Dolomite
- Dolomitization
- Grain boundary diffusion
- Limestone
- Magnesite
- Neutron
- Porosity analyses
- Replacement reactions
- Small-angle scattering