Geochimica et Cosmochimica Acta, May 15, 2013, Vol.109, p.400(14)
To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2013.02.012 Byline: Alexis K. Navarre-Sitchler (a)(b), David R. Cole (c), Gernot Rother (d), Lixin Jin (b)(e), Heather L. Buss (f), Susan L. Brantley (b) Abstract: During weathering, rocks release nutrients and store water vital for growth of microbial and plant life. Thus, the growth of porosity as weathering advances into bedrock is a life-sustaining process for terrestrial ecosystems. Here, we use small-angle and ultra small-angle neutron scattering to show how porosity develops during initial weathering under tropical conditions of two igneous rock compositions, basaltic andesite and quartz diorite. The quartz diorite weathers spheroidally while the basaltic andesite does not. The weathering advance rates of the two systems also differ, perhaps due to this difference in mechanism, from 0.24 to 100mmkyr.sup.-1, respectively. The scattering data document how surfaces inside the feldspar-dominated rocks change as weathering advances into the protolith. In the unaltered rocks, neutrons scatter from two types of features whose dimensions vary from 6nm to 40[mu]m: pores and bumps on pore-grain surfaces. These features result in scattering data for both unaltered rocks that document multi-fractal behavior: scattering is best described by a mass fractal dimension (D.sub.m) and a surface fractal dimension (D.sub.s) for features of length scales greater than and less than [approximately equal to]1[mu]m, respectively. In the basaltic andesite, D.sub.m is approximately 2.9 and D.sub.s is approximately 2.7. The mechanism of solute transport during weathering of this rock is diffusion. Porosity and surface area increase from [approximately equal to]1.5% to 8.5% and 3 to 23m.sup.2 g.sup.-1 respectively in a relatively consistent trend across the mm-thick plagioclase reaction front. Across this front, both fractal dimensions decrease, consistent with development of a more monodisperse pore network with smoother pore surfaces. Both changes are consistent largely with increasing connectivity of pores without significant surface roughening, as expected for transport-limited weathering. In contrast, porosity and surface area increase from 1.3% to 9.5% and 1.5 to 13m.sup.2 g.sup.-1 respectively across a many cm-thick reaction front in the spheroidally weathering quartz diorite. In that rock, D.sub.m is approximately 2.8 and D.sub.s is approximately 2.5 prior to weathering. These two fractals transform during weathering to multiple surface fractals as micro-cracking reduces the size of diffusion-limited subzones of the matrix. Across the reaction front of plagioclase in the quartz diorite, the specific surface area and porosity change very little until the point where the rock disaggregates into saprolite. The different patterns in porosity development of the two rocks are attributed to advective infiltration plus diffusion in the rock that spheroidally fractures versus diffusion-only in the rock that does not. Fracturing apparently diminishes the size of the diffusion-limited parts of the spheroidally weathering rock system to promote infiltration of meteoric fluids, therefore explaining the faster weathering advance rate into that rock. Author Affiliation: (a) Geology and Geological Engineering, Colorado School of Mines, CO, United States (b) Center for Environmental Kinetics Analysis, Earth and Environmental Systems Institute Pennsylvania State University, PA, United States (c) Department of Earth Sciences, The Ohio State University, Columbus, OH, United States (d) Geochemistry and Interfacial Sciences Group, Chemical Sciences Division, Oak Ridge National Laboratory, TN, United States (e) Department of Geological Sciences, University of Texas at El Paso, El Paso, TX, United States (f) School of Earth Sciences, University of Bristol, UK Article History: Received 24 June 2012; Accepted 7 February 2013 Article Note: (miscellaneous) Associate editor: Jiwchar Ganor
Terrestrial Ecosystems ; Porphyry ; Porosity ; Metamorphic Rocks
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