How Oxidation and Dissolution in Diabase and Granite Control Porosity During Weathering
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Published January 13, 2015 Soil Chemistry How Oxidation and Dissolution in Diabase and Granite Control Porosity during Weathering Ekaterina Bazilevskaya* Weathering extends to shallower depths on diabase than granite ridgetops Earth and Environmental Systems Inst. despite similar climate and geomorphological regimes of denudation in the Penn State Univ. Virginia (United States) Piedmont. Deeper weathering has been attributed to University Park, PA 16802 advective transport of solutes in granitic rock compared to diffusive transport in diabase. We use neutron scattering (NS) techniques to quantify the total Gernot Rother and connected submillimeter porosity (nominal diameters between 1 nm and Geochemistry and Interfacial Sciences 10 mm) and specific surface area (SSA) during weathering. The internal sur- Group face of each unweathered rock is characterized as both a mass fractal and a Chemical Sciences Division surface fractal. The mass fractal describes the distribution of pores (~300 nm Oak Ridge National Laboratory to ~5 mm) along grain boundaries and triple junctions. The surface frac- Oak Ridge, TN 37831 tal is interpreted as the distribution of smaller features (1–300 nm), that is, David F.R. Mildner the bumps (or irregularities) at the grain–pore interface. The earliest poros- NIST Center for Neutron Research ity development in the granite is driven by microfracturing of biotite, which National Inst. of Standards and leads to the introduction of fluids that initiate dissolution of other silicates. Technology Once plagioclase weathering begins, porosity increases significantly and the Gaithersburg, MD, 20899 mass + surface fractal typical for unweathered granite transforms to a surface fractal as infiltration of fluids continues. In contrast, the mass + surface frac- Milan Pavich tal does not transform to a surface fractal during weathering of the diabase, U.S. Geological Survey perhaps consistent with the interpretation that solute transport is dominated Eastern Geology and Paleoclimate by diffusion in that rock. The difference in regolith thickness between gran- Science Center ite and diabase is likely due to the different mechanisms of solute transport 12201 Sunrise Valley Drive MS 926a across the primary silicate reaction front. Reston, VA 20192 Abbreviations: FIB, focused ion beam; NS, neutron scattering; RZ, reaction zone; David Cole SANS, small-angle neutron scattering; SAP, saprolite zone; SEM-EDS, scanning electron School of Earth Science microscopy with energy dispersive spectrometer; SLD, scattering length density; SSA, Ohio State Univ. specific surface area; USANS, ultra-small-angle neutron scattering; UWR, unweathered Columbus, OH 43219 rock; WR, weathered rock; m-CT, microcomputed X-ray tomography. Maya P. Bhatt Central Dep. of Environmental Science eathering, the transformation of intact rock into soil through physical Tribhuvan Univ. and chemical reactions, is a key process that affects the CO cycle, soil Kathmandu, Nepal 2 formation, and nutrient uptake into ecosystems. Upon reaction with Wmeteoric fluids, pristine parent rocks transform from relatively nonporous mate- Lixin Jin Dep. of Geological Sciences rial to porous weathered material (soil and saprolite), which we term here regolith Univ. of Texas (Brantley and White, 2009; Buol and Weed, 1991; Pavich et al., 1989). The earliest El Paso, TX 79968 weathering-related mineral-fluid interactions are thought to be largely controlled by Carl I. Steefel the distribution of the connected porosity and the topography of the pore interface Earth Sciences Division at the nanometer scale (Hochella and Banfield, 1995). When interconnected, these Lawrence Berkeley National Laboratory pores likely allow solute transport only by diffusion in relatively pristine, low-po- 1 Cyclotron Road Berkeley, CA 94720 rosity crystalline rocks. For instance, in unweathered granite rocks with low poros- ity, microcracks and elongated voids can be the most important pathways of solute Susan L. Brantley transport (Sausse et al., 2001). During weathering and rock disaggregation, solute Earth and Environmental Systems Inst. Penn State Univ. University Park, PA 16802 Soil Sci. Soc. Am. J. 79:55–73 doi:10.2136/sssaj2014.04.0135 Received 6 Apr. 2014. *Corresponding author ([email protected]). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Soil Science Society of America Journal transport by diffusion eventually is outcompeted by transport by porosity without sample destruction. In addition, we use energy advection as fractures interconnect and larger micrometer-sized dispersive X-ray scanning electron microscopy (SEM-EDS) to pores form from smaller pores. However, this transformation from elucidate the mineralogy, chemical composition, and grain dis- a regime dominated by diffusion to one dominated by advective tribution, as well as the shape and grain contact morphology of transport and the role of pore-scale reactions on rock weathering the different mineral phases and pores. are not well understood (Navarre-Sitchler et al., 2013; Rossi and Graham, 2010; Nahon, 1991). DESCRIPTION OF THE WEATHERING PROFILES In our previous work, we investigated aspects of the rela- The weathering profiles on granitic and diabase rocks that tionship between mineral reaction and porosity in two crystal- we study here have developed on hilltops in the Piedmont prov- line rocks weathering in the same climate at similar erosion rates ince in Fairfax County, VA (Fig. 1a) as described in previous (Bazilevskaya et al., 2013; Brantley et al., 2014). These two rocks, publications (Bazilevskaya et al., 2013; Brantley et al., 2014; a diabase and a granitic rock, had been previously cored and stud- Pavich et al., 1989). To summarize, the Piedmont province is ied by workers in the 1990s (Pavich et al., 1989). Both rocks are a rolling upland plain developed on diverse underlying meta- dominated by plagioclase (58% Ca-rich feldspar in diabase; 52% morphic and igneous rocks. The province is characterized by Na-rich feldspar in granitic rock). Although the diabase con- minor relief, moderate elevation (90–200 m), and mature de- tained the more fast-dissolving plagioclase composition (An60), ciduous forest cover. In this region, the summers are generally it was characterized by a much thinner weathering profile: the warm and humid, and the winters are mild with mean average regolith was only about 1 m thick. In contrast, the granitic par- temperature equal to 10°C. Summer temperatures reach 38°C, ent contained the more slow-dissolving plagioclase (An06) but and the minimum winter temperature can be lower than −20°C. had developed a much deeper regolith (20 m). Bazilevskaya et The maximum rainfall intensity occurs in summer with average al. (2013) suggested that oxidation of biotite occurred between rainfall of 1040 mm yr−1. The longest drought period generally the 22- and 20-m depth in the granite, causing fracturing of the occurs during fall. Most groundwater storage in the Piedmont is rock that allowed advection at depth. The advection controlled within the regolith (especially within the saprolite), but minor the solute transport and resulted in formation of deep regolith; storage also occurs in joints and fractures in fresh rock (Pavich in turn, permeability in the deep regolith was maintained by the et al., 1989). The stable erosion rates have been estimated from high concentration of quartz grains in the granitic rock. Similar cosmogenic isotopes, exposure time, and elevation to equal 4 to 8 fracturing was not observed in the diabase profile, and the lack m per 1 ´ 106 yr (Bacon et al., 2012; Price et al., 2008; Pavich et of quartz further precluded maintenance of high permeability. al., 1985). Residual weathering profiles have developed over the Furthermore, by inspection of a compilation of regolith depths parent rocks. As a result, Alfisol soils, Kelly (fine, vermiculitic, as a function of lithology, Bazilevskaya et al. (2013) concluded mesic Aquic Hapludalfs) and Jackland (fine, smectitic, mesic that regolith depth was commonly thicker on granitic mate- Aquic Hapludalfs) series, are present on the granitic and diabase rial compared to diabase when observed at ridgetop locations in rocks, respectively. many locations worldwide. Brantley et al. (2014) explored this We report analyses of cores of diabase that were taken from idea further and pointed out that the high ferrous Fe content of a ridge top (elevation ~91 m asl) adjacent to a diabase quarry rocks such as diabase could deplete downward percolating fluids in the Triassic Culpepper basin in the western Piedmont prov- of O2 before depletion of CO2 when they weathered in ridgetop ince by Pavich and colleagues (Pavich et al., 1989). Samples of locations. In contrast, in felsic granites, CO2 is likely to be de- the granitic rocks were collected in the highest upland (elevation pleted before O2 due to the high content of weatherable base ~135 m asl) immediately north of the Occoquan River as contin- cations but low content of Fe(II). Those authors inferred that the uous cores from surface to unweathered rock. The granitic rock is relative depths of the oxidation