14-17 May 2018
New Orleans
US/Central timezone

The Effects of Chemomechanical Processes on Limestone Weathering Rates

15 May 2018, 12:29
15m
New Orleans

New Orleans

Oral 20 Minutes MS 1.28: Coupled chemo-mechanical processes in fractured and nano-porous media Parallel 4-C

Speaker

Lawrence Anovitz (Oak Ridge National Laboratory)

Description

Dissolution within porous media is a critical process in many environmental and geological settings. The formation and evolution of soils, rocks, and landscapes (Buhmann and Dreybrodt, 1985; Brantley, 2008; Jin et al., 2010), efficiency of carbon capture in geological reservoirs (Matter and Kelemen, 2009), and the weathering of man-made structures are all highly dependent on the rates at which minerals dissolve. However, the factors governing dissolution in geological media are not yet well understood. In general, rock dissolution rates are governed by fluid composition, fluid flow rates, and the surface area of the mineral–fluid interface (Lasaga, 1998). As a surface becomes rougher, the surface area in contact with a reactive fluid increases, and it is therefore often assumed that rough surfaces dissolve more rapidly than smooth surfaces (Fischer and Luttge, 2007). However, a number of studies have shown a complex relationship between roughness and reactivity (e.g., Anbeek, 1992; Gautier et al., 2001). Emmanuel and Levenson (2014) found that erosion rates in fine-grained micritic limestone blocks are as much as two orders of magnitude higher than rates estimated for coarse-grained limestones. AFM imaging suggested this is the result of rapid dissolution along micron- scale grain boundaries, followed by mechanical detachment of tiny particles from the surface. Such chemo-mechanical processes may be the dominant erosional mode for fine-grained carbonate rocks in many regions on Earth. This erosion extends to the micron scale, and grain detachment can be a crucial mechanism controlling denudation rates in carbonate terrains.
In order to better understand this weathering process we undertook a series of experiments looking at the weathering of carbonates. 5/8” x 5/8” cores of four limestones (Netzer, Shiuvta, Carthage Marble, Texas Cream) were exposed to flowing water at 30°C and several pH values to mimic the weathering process. Annular Cd masks of stepped sizes, and small beam stepped locations analyses, were used to analyze the weathering structure by (U)SANS and (U)SAXS as a function of distance from the edge. SEM analysis was also used to look at the pore structure and surface weathering. The results, both in terms of core/rim variations and pore size dependence, were found to be strongly dependent on initial permeability and rock structure, as well as time and pH.

References

Anbeek C (1992) Geochim. Cosmochim. Acta 56, 1461–1469.
Brantley SL (2008) In: Kinetics of Water–Rock Interaction. Springer, New York, USA, pp. 151– 2010
Buhmann D and Dreybrodt W (1985) Chem. Geol. 53, 109–124.
Emmanuel, S and Levenson, Y (2014) Geology, 42, 751-754.
Fischer C and Lüttge A (2007) Am. J. Sci. 307, 955–973.
Gautier J, Oelkers E and Schott J (2001) Geochim. Cosmochim. Acta 65, 1059–1070.
Jin L, Ravella R, Ketchum B, Bierman PR, Heaney P, White T and Brantley SL (2010) Geochim. Cosmochim. Acta 74, 3669– 3691.
Lasaga AC (1998) Kinetic theory in the Earth Sciences. Princeton University Press, Princeton, New Jersey.
Matter JM and Kelemen PB (2009) Nat. Geosci. 2, 837–841.

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Primary authors

Lawrence Anovitz (Oak Ridge National Laboratory) Dr Timonthy Prisk (Nist Center for Neutron Research) Dr Simon Emmanuel (Hebrew University, Jerusalem) Dr Michael Cheshire (Oak Ridge National Laboratory) Dr Juliane Weber (Oak Ridge National Laboratory) Dr Susan Brantley (Pennsylvania State University) Dr Markus Bleuel (Nist Center for Neutron Research) Dr Jan Ilavksy (Advanced Photon Source, Argonne National Laboratory) Dr David Mildner (NIST Center for Neutron Research) Dr Cedric Gagnon (NIST Center for Neutron Research)

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