Regolith mass balance inferred from combined mineralogical, geochemical and geophysical studies: Mule Hole gneissic watershed, South India

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Geochimica et Cosmochimica Acta 73 (2009) 935–961 www.elsevier.com/locate/gca

Regolith mass balance inferred from combined mineralogical, geochemical and geophysical studies: Mule Hole gneissic watershed, South India Jean-Jacques Braun a,b,*, Marc Descloitres a,c, Jean Riotte a,b, Simon Fleury a, Laurent Barbie´ro a,b, Jean-Loup Boeglin b, Aure´lie Violette b, Eva Lacarce d, Laurent Ruiz e, M. Sekhar a,f, M.S. Mohan Kumar a,f, S. Subramanian a,g, Bernard Dupre´ b a

Indo-French Cell for Water Sciences (IRD/IISc Joint Laboratory), Indian Institute of Science, 560012 Bangalore, India b LMTG, Universite´ de Toulouse, CNRS, IRD, OMP, 14, Avenue E. Belin, F-31400 Toulouse, France c IRD, Laboratoire d’Etude des Transferts en Hydrologie et Environnement (LTHE), UMR/CNRS-IRD-INPG-UJF, BP53, F-38041 Grenoble, Cedex 09, France d INRA, US1106, Unite´ INFOSOL, 2163 Av. Pomme de Pin, F45075 Orleans Cedex 2, France e Sol-Agronomie-Spatialisation (SAS), UMR INRA, 65, rue de Saint-Brieuc CS 84215, F-35042 Rennes Cedex, France f Indian Institute of Science, Department of Civil Engineering, 560012 Bangalore, India g Indian Institute of Science, Department of Materials Engineering, 560012 Bangalore, India Received 4 February 2008; accepted in revised form 4 November 2008; available online 20 November 2008

Abstract The aim of this study is to propose a method to assess the long-term chemical weathering mass balance for a regolith developed on a heterogeneous silicate substratum at the small experimental watershed scale by adopting a combined approach of geophysics, geochemistry and mineralogy. We initiated in 2003 a study of the steep climatic gradient and associated geomorphologic features of the edge of the rifted continental passive margin of the Karnataka Plateau, Peninsular India. In the transition sub-humid zone of this climatic gradient we have studied the pristine forested small watershed of Mule Hole (4.3 km2) mainly developed on gneissic substratum. Mineralogical, geochemical and geophysical investigations were carried out (i) in characteristic red soil profiles and (ii) in boreholes up to 60 m deep in order to take into account the effect of the weathering mantle roots. In addition, 12 Electrical Resistivity Tomography profiles (ERT), with an investigation depth of 30 m, were generated at the watershed scale to spatially characterize the information gathered in boreholes and soil profiles. The location of the ERT profiles is based on a previous electromagnetic survey, with an investigation depth of about 6 m. The soil cover thickness was inferred from the electromagnetic survey combined with a geological/pedological survey. Taking into account the parent rock heterogeneity, the degree of weathering of each of the regolith samples has been defined using both the mineralogical composition and the geochemical indices (Loss on Ignition, Weathering Index of Parker, Chemical Index of Alteration). Comparing these indices with electrical resistivity logs, it has been found that a value of 400 Ohm m delineates clearly the parent rocks and the weathered materials. Then the 12 inverted ERT profiles were constrained with this value after verifying the uncertainty due to the inversion procedure. Synthetic models based on the field data were used for this purpose. The estimated average regolith thickness at the watershed scale is 17.2 m, including 15.2 m of saprolite and 2 m of soil cover.

* Corresponding author. Address: Indo-French Cell for Water Sciences (IRD/IISc Joint Laboratory), Indian Institute of Science, 560012 Bangalore, India. E-mail address: [email protected] (J.-J. Braun).

0016-7037/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2008.11.013

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Finally, using these estimations of the thicknesses, the long-term mass balance is calculated for the average gneiss-derived saprolite and red soil. In the saprolite, the open-system mass-transport function s indicates that all the major elements except Ca are depleted. The chlorite and biotite crystals, the chief sources for Mg (95%), Fe (84%), Mn (86%) and K (57%, biotite only), are the first to undergo weathering and the oligoclase crystals are relatively intact within the saprolite with a loss of only 18%. The Ca accumulation can be attributed to the precipitation of CaCO3 from the percolating solution due to the current and/or the paleoclimatic conditions. Overall, the most important losses occur for Si, Mg and Na with 286  106 mol/ha (62% of the total mass loss), 67  106 mol/ha (15% of the total mass loss) and 39  106 mol/ha (9% of the total mass loss), respectively. Al, Fe and K account for 7%, 4% and 3% of the total mass loss, respectively. In the red soil profiles, the opensystem mass-transport functions point out that all major elements except Mn are depleted. Most of the oligoclase crystals have broken down with a loss of 90%. The most important losses occur for Si, Na and Mg with 55  106 mol/ha (47% of the total mass loss), 22  106 mol/ha (19% of the total mass loss) and 16  106 mol/ha (14% of the total mass loss), respectively. Ca, Al, K and Fe account for 8%, 6%, 4% and 2% of the total mass loss, respectively. Overall these findings confirm the immaturity of the saprolite at the watershed scale. The soil profiles are more evolved than saprolite but still contain primary minerals that can further undergo weathering and hence consume atmospheric CO2. Ó 2008 Elsevier Ltd. All rights reserved.

1. INTRODUCTION Understanding the relative controls of forcing factors on the long-term silicate chemical weathering rates and the associated atmospheric CO2 consumption remains a major challenge (White and Brantley, 1995; Kump et al., 2000). Several publications based on small to medium granitogneissic watershed studies (1–100 km2) examined the relationships between temperature and runoff for different climatic and tectonic settings (Bluth and Kump, 1994; White and Blum, 1995; White et al., 1999). The authors stressed that the silicate weathering rates were not governed by any single parameter. In addition to climate, the importance of the thickness and nature of the blanket of loose and transportable weathered material, namely regolith, which overlies the intact bedrocks, was also recently invoked, especially in the tropical environment (Millot et al., 2002; Oliva et al., 2003; Braun et al., 2005; West et al., 2005). At the watershed scale the regolith cover is produced either by in situ weathering or by deposition (downslope colluviums and valley-floor alluviums) (Taylor and Eggleton, 2001). However, as the ubiquitous terms saprolite and soil used to describe the regolith compartments from bottom to top have often various meanings in the literature because of their trans-disciplinary usage (Ehlen, 2005; Dethier and Lazarus, 2006; Dewandel et al., 2006) it is germane to give here straightaway consensual definitions. The saprolite corresponds to the lower part of in situ regolith covers. It develops downward (weathering front) at the expense of the underlying fractured parent rock from which it does retain the structure and the fabric, i.e. isovolumetric weathering. The soils develop at the expense of either saprolite or colluviums/alluviums at the uppermost part of the regolith where the perturbation brought by both physical and biological processes lead to (i) the differentiation into horizons and (ii) the loss of the existing isovolumetric weathering features. The regolith thickness depends on the balance between deepening at the weathering front by chemical weathering and skimming off by mechanical erosion at the topsoil (Riebe et al., 2003; Anderson et al., 2007; Burke et al.,

2007, and references therein). Chemical weathering rates are highly sensitive to the availability of fresh mineral surfaces, which would tend to be enhanced by increased physical erosion. A thin, immature regolith still containing a large amount of primary minerals able to weather, would increase the chemical weathering flux while a thick, mature regolith poor in weatherable primary minerals (e.g. strongly depleted lateritic cover) would slow it down (Oliva et al., 2003). Regolith characterization is therefore of fundamental interest to improve the model of the groundwater flow paths and to assess the long-term geochemical mass balance. Regardless, the assessment of the three-dimensional structure of regolith is still challenging (Thomas, 1994; Taylor and Eggleton, 2001; Anderson et al., 2004, 2007). Understanding where chemical weathering takes place within a landscape remains a critical missing piece in the complicated puzzle of this fundamental Earth surface process. Until now, only a few integrated watershed approaches have been carried out in tropical regions. The two most studied sites in terms of contemporary and long-term chemical silicate weathering have focused on humid tropics (i) in the Luquillo mountain tropical forest, Puerto Rico (Rio Icacos site, Water, Energy and Biogeochemical Budget; http://pr.water.usgs.gov/public/webb/) and (ii) in the South Cameroon plateau Nsimi site, developed as part of the project ‘Observatoire de Recherche en Environnement – Bassin Versant Expe´rimentaux Tropicaux, http://bvet.ore.fr/. Both sites are characterized by rather thick regolith, i.e. larger than 5 m, still rich in weatherable minerals in the first case, while strongly depleted in the second case (laterites). A study of the steep climatic gradient and associated geomorphologic features of the Western Ghaˆts rain shadow located on the edge of the rifted continental passive margin of the Karnataka Plateau, Peninsular India was initiated in 2003 (Gunnell and Bourgeon, 1997; Gunnell, 1998a,b, 2000; Gunnell et al., 2003, 2007). This combined gradient from humid to semi-arid provides unique conditions to study the shift from deep mature to shallow immature regolith covers as well as the influence of their thickness and nature on the silicate chemical weathering.

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We started our investigations on the pristine forested Mule Hole Small Experimental Watershed, SEW (4.3 km2). The first results were published on the soil distribution and erosion processes (Barbie´ro et al., 2007), on the performance of Magnetic Resonance Sounding method applied to the hard-rock aquifer (Legchenko et al., 2006) and on the seasonal local recharge processes at the stream outlet (Descloitres et al., 2007). The present paper focuses on the nature and the degree of weathering and the thickness of the regolith developed on the heterogeneous silicate substratum of the Mule Hole SEW. Our key issues are (i) to geochemically distinguish between fresh rocks and weathered materials since the initial parent rock composition is found to be variable, (ii) to spatially characterize the information at the watershed scale and (iii) to evaluate the long-term weathering mass balance. We have adopted a combined approach using geophysics, geochemistry and mineralogy. First, the gneissic protolith is differentiated from its weathering products by comparing geochemical and mineralogical compositions with electrical resistivity at the sample scale. Second, a detailed non-destructive Electrical Resistivity Tomography (ERT) survey is performed on the watershed to produce representative ERT profiles that give resistivity distribution from the sub-surface down to 30 m. ERT uncertainty is then analyzed using a synthetic modeling approach that allows us to spatially characterize the distribution of soil and saprolite at the watershed scale. Finally, the volume of the weathered material is assessed and the main saprolitization and soil chemical processes are deciphered by a mass balance approach based on the ERT mapping. The impact on long-term chemical weathering rates at the watershed scale are then discussed. Forthcoming companion papers will address the contemporary chemical and erosion fluxes based on climatic, hydrological, hydrogeological and geochemical time series and long-term denudation rates determined with cosmogenic nuclides.

Dharwar craton (Naqvi and Rogers, 1987), is dominated by complexly folded, heterogeneous Precambrian peninsular gneiss intermingled with mafic and ultramafic rocks of the volcano-sedimentary Sargur serie (Shadakshara Swamy et al., 1995). The Peninsular gneiss represents at least 85% of the watershed basement. The gneiss foliation is mainly oriented at N75° in the Northern part and at N100°– N120° in the Southern part. The dip angle of the gneissic units ranges from 75° to the vertical. Usually mafic and ultramafic rocks (hornblendite, amphibolite and serpentinite) come into sight as metric enclaves or seams intermingle with the gneiss layers. However an amphibolite body occurs in the southeastern part of the watershed and represents roughly 7% of the whole watershed area. At outcrop level, the basement rocks appear more or less fissured. The soil cover of the watershed has been mapped by Barbie´ro et al. (2007) based on the FAO terminology (IUSS-Working-Group-WRB, 2006). The gneissic saprolite, cohesive to loose sandy, crops out both in the streambed and at the mid-slope in approximately 22% of the watershed area. Shallow red soils (Ferralsols and Chromic Luvisols) from 1 to 2 m in depth cover 66% of the whole watershed area (hillslopes). A stone line, composed of quartz pebbles and ferruginous nodules, often occurs at the boundary between the topsoil and saprolite. The total area covered by black soil is 12%. The lower part of the slope and the flat valley bottoms are covered by, on average, 2 m of black soils (Vertisols and Vertic intergrades). They are developed on both the gneiss and the mafic rocks. The other occurrence of the black soil is lithodependant with development of deeper soils (2.5 m) on gneissic zones rich in amphibolite layers located in the depressions on the crest line.

2. FIELD SETTINGS

Previous soil and geological maps at the Mule Hole watershed scale are based on the observations of the parent rock and the saprolite outcrops and the electromagnetic surveys using a GeonicsÒ EM31 instrument coupled with both structural approach on a selected soil catena and an auger survey (Barbie´ro et al., 2007). The EM31 equipment measures the electrical conductivity in milliSiemens per meter (mS/m) with a penetration depth typically ranging from 4 to 6 m (McNeill, 1980). Conductivities below 2.5 mS/m correspond to fresh rock occurrence between surface and 2 m depth. Values between 2.5 and 10 mS/m are characteristic of red soils, and above 10 mS/m of black soils or weathered amphibolite (Fig. 2). Red soil samples developed on gneiss were collected in both sites S1 and S2 (Fig. 2). S1 is located downslope close to the streambed and including the T1 soil catena described in Barbie´ro et al. (2007) while S2 is located upslope on the North ridge crest. Two soil profiles, namely S1-P and S2-P were sampled down to the top of the saprolite (19 samples). The profiles S1-P and S2-P are 3.2 and 2.4 m thick, respectively. Fourteen soil samples (S1-T1) were also collected at different depths in the T1 soil catena (Barbie´ro et al., 2007).

The Mule Hole SEW (11°430 N–76°260 E) lies in the sub-humid zone of the climatic gradient of the Kabini river basin (Fig. 1). The morphology of the watershed is highly incised by the temporary stream network. The edge slope is relatively low with small depressions. The slope convexity is high upslope and concave by the streambeds. The streambeds are steep-sided up to 2 m down compared to valley floor. The average annual rainfall at the Mule Hole SEW is 1090 ± 230 mm/yr with a dry season lasting an average of 5.5 months. The average annual air temperature is 21.8 °C. The watershed is covered by dry deciduous forest with different facies linked to the soil distribution (Barbie´ro et al., 2007). Currently, the Mule Hole SEW is dedicated to wildlife and biodiversity preservation (Bandipur National Park). The Mule Hole protolith presents high concentration of lithological, structural and compositional heterogeneities, which favor the water circulation and therefore the weathering processes. The lithology, representative of the West

3. MATERIALS AND METHODOLOGY 3.1. Previous studies and sampling

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Fig. 1. Location of the Kabini river basin and the Mule Hole experimental watershed. The shaded area represents the boundaries of the subhumid zone with the 900 and 1500 mm/yr isohyets.

The fresh gneiss samples were collected in the Mule Hole streambed. The deep regolith was studied through a network of thirteen boreholes (BH1–13) distributed on the watershed edges along the main roads (Fig. 2). Composite samples (i.e. cuttings) of saprolite and of protolith were collected for every 2 m along the depth of the eight boreholes (BH1–2–3–4–5–6–12–13). All boreholes were drilled in the gneissic basement but BH6 is mainly in the amphibolite body. Borehole depths range between 20 and 60 m.

3.2. Protolith/regolith geochemistry and mineralogy The mineralogy of 157 powdered composite samples of boreholes BH1–2–3–4–5–6–12–13 was determined by X-ray diffraction (XRD) at LMTG (Laboratoire des Me´canismes de Transfert en Ge´ologie, Toulouse). Thin sections were also prepared from outcrop samples of gneiss and BH6 amphibolite. They were observed with optical microscope and SEM coupled with backscattered electrons and EDX. Major and accessory minerals were analyzed

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Fig. 2. Results of the electromagnetic survey (EM) and location of the 12 ERT profiles. The colored background is the soil electrical conductivity measured with electromagnetic devices (EM31) along N–S oriented profiles implemented every 100 m on the watershed (Barbie´ro et al., 2007). ERT profiles are implemented to sample the main pedological units deduced from electrical conductivity distribution and pedological survey. Shaded areas indicate the zones of occurrence of seams and enclaves of amphibolite mingled with gneiss. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)

with a SX100 Cameca microprobe at the LMTG. Bulk densities (q) were determined by the paraffin method with a SartoriusÒ density kit (10 replicates). Bulk chemical analyses were carried out on BH1, BH5, BH6 and BH12 (109 samples) at the SARM (Centre de Recherche Pe´trographique et Ge´ochimique-CNRS, Vanduvre-le`s-Nancy). After LiBO2 fusion and HNO3 dissolution, Si, Al, Fe, Mn, Mg, Ca, Na, K, Ti, P were analyzed by ICP-AES and Zr, Th and Nb by ICP-MS. The detection limits (in wt%) are 0.8 for SiO2, 0.3 for Al2O3, 0.1 for Fe2O3, 0.03 for MnO, 0.4 for MgO, 0.5 for CaO, 0.08 for Na2O, 0.05 for K2O, 0.09 for TiO2 and 0.2 for P2O5 and (in ppm) 0.5 for Zr, Nb and Th. The detection limit of the Loss on Ignition (LoI), the measure of volatile H2O, CO2, F, Cl and S, is found to be 0.02 wt%. 3.3. Geophysical investigations The bedrock and regolith cover was studied at the watershed scale through Direct Current electrical methods using a SYSCAL R2 resistivitimeter from IRIS Instruments (Descloitres et al., 2007). Our study benefitted from previous attempts to assess the geometry of regolith in the Tropics using these geophysical methods (Robain et al., 1996; Beauvais et al., 1999, 2004, 2007). Electrical resistivity of the regolith varies with porosity (bulk density), amount of clay minerals, temperature and both the water content and the salinity (Telford et al., 1990). These parameters make the electrical resistivity convenient for characterizing the regolith since it presents a lower density than the protolith, as well as significant clay occurrence and water content. Nevertheless, the separation limit between regolith

and protolith cannot be based on uncalibrated resistivity measurements alone as the resistivity of the regolith can be site-specific. Three complementary electrical methods were carried out on the watershed: (i) Resistivity measurements on typical protolith outcrops and soils. For this, a Wenner array with an electrode spacing of 0.20 m was used. The resistivity calculated using such a small array is considered as the true resistivity of the medium. (ii) Resistivity logging in boreholes BH5, 6, 12 and 13 with a pole–pole (also called log ‘‘normal”) array with 0.30 m spacing between electrodes and measurement for each 0.25–0.5 m. The logging was carried out just after drilling, before casing when possible, with an inflatable probe in the vadose zone (Descloitres and Le Troquer, 2004) and with steel electrodes below the water table. The resistivity calculated using such a small electrode spacing is considered as the true resistivity of ground around the probe. (iii) 2D Electrical Resistivity Tomography (ERT) survey (Loke, 2000; Seaton and Burbey, 2002) to investigate the first 30 m of the sub-surface. ERT was carried out with two geometric arrays. The first one is the Wenner array, more sensitive to the vertical variations of the electrical resistivity (Loke, 2000). The second array is the dipole–dipole, more sensitive to the lateral variations of the electrical resistivity. The latter is particularly suitable in fractured hard rock studies (Seaton and Burbey, 2002) because of the 2D distribution of resistivity in such a medium. Twelve ERT profiles totaling 7600 m were setup according to the

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topographical, EM31 and geological surveys (Fig. 2). ERT profiles 1, 2, 3, 4, 7, 8, 12 and 9 are located above gneissic basement, whereas profiles 5, 10, 11 and 6 are mostly above the mingled amphibolitegneiss basement. Profiles 1, 2, 3 and 4 focused on the outlet area and crossed two patches of black soils. For comparison with true resistivity logging in boreholes, ERT profiles 1 and 2 are situated near BH1 and BH12, ERT profile 12 near BH5 and ERT profile 11 near BH6.

4. RESULTS 4.1. Boreholes and soil profiles Table 1 displays the bulk analyses of major and selected trace inert elements (Zr, Th, Nb), borehole electrical resistivity and mineralogy of major minerals for the composite samples of boreholes BH6, BH1, BH5 and BH12. Table 2 shows the same information for the soil samples of the S1 and S2 sites. The compositions of the main gneiss minerals, in wt%, are reported in Table 3. 4.1.1. Fresh gneiss and weathering products Combined XRD patterns, SEM–EDX observations and microprobe analyses indicate that the major minerals of gneiss are quartz, oligoclase (An14), sericite, biotite and chlorite. The accessory minerals are apatite, epidote, allanite, titanite, magnetite, ilmenite, pyrite and zircon. Fig. 3a shows the fine layering of the gneiss with both leucosome and melanosome. Sericitization of oligoclase crystals and chloritization of biotite crystals are frequent (Fig. 3b–d). Titanite and apatite crystals are closely associated with the presence of biotite (Fig. 3e). The zircon crystals observed in thin sections are particularly tiny (10–15 lm in length) (Fig. 3f). The gneiss is veined with epidote-rich quartz seams of hydrothermal origin. The bulk chemical compositions of the fresh to weathered gneiss reflect both the primary and the secondary mineralogical variability. The parent gneiss composition varies from felsic (oligoclase/quartz-rich) to mafic (biotite/chlorite-rich) end-members. Gneiss dominates in boreholes 1, 2, 3, 4, 5, 12 and 13 and is present at a depth of 58 m in BH6. Significant clay mineral contents (kaolinite, smectite) occur up to a depth of 4, 20, 5, 15, 22, 14, 15 m in BH1, 2, 3, 4, 5, 12 and 13, respectively. The range of the chemical compositions for fresh rocks and saprolite in the borehole samples was calculated without taking into account the near surface samples (less than 4 m in depth) because of the blend of soil layers with saprolitic materials. SiO2 ranges between 50 and 76 wt%, Al2O3 between 9 and 16 wt%, Fe2O3 between 1.3 and 16.0 wt%, MnO between the detection limit and 0.2 wt%, MgO between 0.3 and 8.5 wt%, CaO between 0.2 and 9.7 wt%, Na2O between 0.6 and 6.6 wt%, K2O between 0.2 and 9.7 wt%, TiO2 between 0.2 and 2.2 wt%, P2O5 between the detection limit and 0.5 wt% and LoI between 1.0 and 6.8 wt%. Negative correlations, with coefficient r2 P 0.6 (n = 63) exist (i) between SiO2 and Fe2O3, MnO, MgO

and CaO and (ii) between Fe2O3 and Na2O. Positive correlations exist (i) between Al2O3 and Na2O, (ii) between Fe2O3 and MnO and MgO (iii) between MnO and CaO and (iv) between TiO2 and P2O5. The borehole electrical resistivity varies from 50 to 4500 Ohm m. In the soil samples derived from the gneiss (S1-T1, S1P and S2-P), SiO2 ranges between 61.9 and 78.8 wt%, Al2O3 between 8.5 and 16.7 wt%, Fe2O3 between 1.6 and 6.9 wt%, MnO between the detection limit and 0.13 wt%, MgO between 0.3 and 1.5 wt%, CaO between 0.5 and 2.1 wt%, Na2O between 0.4 and 4.5 wt%, K2O between 0.6 and 3.2 wt%, TiO2 between 0.1 and 0.7 wt%, P2O5 between the detection limit and 0.1 wt% and LoI between 2.3 and 11.6 wt%. Negative correlations, with coefficient r2 P 0.6 (n = 32) exist (i) between SiO2 and Al2O3, Fe2O3 and LoI and (ii) between Na2O and TiO2 and LoI. Positive correlations exist between Fe2O3 and MnO, TiO2 and LoI. The electrical resistivity varies from 10 to 100 Ohm m. 4.1.2. Fresh and weathered amphibolite (BH6) The major minerals of the mafic body are labradorite, Mg-hornblende, tremolite and chlorite. Fig. 4a and b portray a fractured seam of amphibolite and the corresponding minor and accessory mineral assemblage as Mg-rich calcite, serpentine and iron oxides containing Cr and Ti. The occurrence of quartz and epidote in the BH6 samples detected by XRD patterns suggests that the amphibolite is also veined with hydrothermal seams. SiO2 ranges between 46.3 and 51.1 wt%, Al2O3 between 10.9 and 16.0 wt%, Fe2O3 between 10.4 and 20.4 wt%, MnO between 0.2 and 0.3 wt%, MgO between 3.2 and 9.6 wt%, CaO between 2.0 and 10.7 wt%, Na2O between 0.8 and 3.6 wt%, K2O between 0.1 and 0.6 wt%, TiO2 between 0.7 and 1.7 wt%, P2O5 between 0.1 and 0.3 wt% and LoI between 1.5 and 14.1 wt%. Two positive correlations, with coefficient r2 P +0.6 (n = 35), exist between Fe2O3 and TiO2 and between TiO2 and P2O5. The borehole electrical resistivity varies from 10 to 10,000 Ohm m. In the borehole samples, large amounts of clay minerals occur between 10 and 12 m in BH6. However, observation carried out in pit shows that the first 3 m are composed of rock with a millimetric to centimetric fissured network filled with loose clayey materials that do not appear in the composite XRD analysis. There is no soil horizon topping this saprolite. 4.2. ERT profiles Twelve ERT profiles were analyzed and a routine inversion method was applied to the apparent resistivity field data. The distribution of the calculated resistivity is displayed along profiles between surface and 30 m depth (Fig. 5). Calculated resistivity ranges from 10 Ohm m near the surface to more than 5000 Ohm m downward with a strong lateral variability, showing high amplitude corrugations. Such complex geometry prevents any simple estimate of regolith thickness based on the resistivity alone: the resistivity limit between weathered and fresh rocks will be determined in the following section.

Table 1 Bulk chemical analyses for major and selected trace inert elements (Zr, Th, Nb), electrical resistivity and mineralogy of major minerals based on XRD patterns for the composite samples of boreholes BH6, BH1, BH5 and BH12 and the soil samples of the S1 and S2 sites. Chemical Index of Alteration (CIA) and Weathering Index of Parker (WIP) are also mentioned. CIA is defined with molecular proportion of major element oxides by CIA = 100[Al2O3/(Al2O3 + CaO* + Na2O + K2O)] with CaO* = CaO  10/3P2O5; CaO is restricted to that derived from silicate minerals. WIP is calculated with the atomic proportion of Na, Mg, K and Ca divided by weighting factors corresponding to the bond strengths of the elements with oxygen: WIP = 100[(Na/0.35) + (Mg/ 0.90) + (K/0.25) + (Ca/0.70)]. The groundwater table level and the conductivity are indicated in each borehole. Key for XRD analysis: empty cell: absence, x: presence (<5%), xx: abundant and xxx: very abundant. Depth, m

SiO2, %

Al2O3, %

Fe2O3, %

MnO, %

MgO, %

CaO, %

Na2O, %

K2O, %

TiO2, %

P2O5, %

LoI, %

Total, %

Zr, ppm

Th, ppm

Nb, ppm

BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6

2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 32.0 34.0 36.0 38.0 40.0 42.0 44.0 46.0 48.0

48.78 51.15 49.37 46.99 49.65 50.74 51.09 50.44 48.97 50.72 49.21 50.84 49.99 50.43 50.68 47.06 47.27 49.53 49.78 50.31 49.71 50.62 50.51 50.86

10.87 11.02 13.53 14.15 12.89 12.78 13.23 13.65 13.50 13.28 13.92 13.41 13.22 13.53 13.31 11.01 13.27 12.93 12.16 13.30 13.50 15.99 13.74 14.65

15.67 14.84 15.81 15.36 14.79 14.95 15.21 16.42 15.52 16.81 16.69 15.36 16.32 15.33 14.63 20.38 17.08 17.02 13.88 11.64 12.71 10.43 12.91 11.77

0.23 0.23 0.23 0.20 0.28 0.19 0.22 0.27 0.23 0.27 0.28 0.24 0.19 0.18 0.20 0.24 0.21 0.20 0.19 0.19 0.17 0.18 0.17 0.16

7.11 7.33 5.20 6.08 3.21 3.63 5.12 5.02 4.92 4.16 4.47 4.17 6.30 5.50 5.60 6.61 7.01 5.29 9.60 7.05 7.05 3.95 7.60 4.87

10.10 8.67 8.20 8.52 2.04 3.68 6.83 8.56 10.41 6.51 10.54 10.60 5.74 7.70 9.11 6.76 8.14 7.13 7.90 10.44 9.92 8.81 10.17 7.17

0.82 2.02 2.26 2.73 1.28 1.65 1.97 2.08 2.09 2.27 2.45 2.33 2.71 3.33 2.95 1.32 3.20 3.60 2.18 2.63 2.73 3.52 2.42 2.35

0.58 0.23 0.17 0.24 0.10 0.11 0.14 0.26 0.14 0.28 0.22 0.15 0.38 0.39 0.27 0.65 0.32 0.45 0.32 0.20 0.31 0.62 0.19 0.10

0.96 1.12 1.30 1.03 1.21 1.22 1.16 1.33 1.22 1.38 1.35 1.27 1.30 1.30 1.26 1.62 1.55 1.67 0.80 0.76 0.74 1.00 0.81 0.89

0.07 0.09 0.13 0.10 0.10 0.15 0.13 0.13 0.12 0.13 0.12 0.12 0.13 0.12 0.13 0.26 0.14 0.18 0.08 0.06 0.09 0.10 0.08 0.10

5.12 3.03 2.74 4.19 14.11 10.76 4.70 1.85 2.53 3.74 1.46 1.73 4.50 2.32 2.85 4.50 2.49 2.69 3.11 2.77 3.66 5.49 1.93 6.58

100.30 99.72 98.94 99.57 99.67 99.85 99.81 100.01 99.65 99.55 100.71 100.21 100.77 100.11 100.99 100.40 100.68 100.69 100.00 99.34 100.58 100.73 100.54 99.50

56 58 86 56 82 85 79 81 87 86 84 74 76 82 72 84 85 96 36 47 41 60 53 68

0.4 0.3 0.8 0.3 0.5 0.5 0.6 0.6 0.5 0.5 0.4 0.5 0.3 0.6 0.4 0.4 0.4 0.5 0.2 0.2 0.2 0.3 0.2 0.3

1.7 1.7 3.1 2.2 2.4 2.3 2.3 2.7 2.7 2.7 2.5 2.3 2.4 2.4 2.4 3.7 3.6 3.8 1.6 1.6 1.6 2.7 2.2 2.4

BH6 BH6 BH6 BH6 BH6

50.0 52.0 54.0 56.0 58.0

46.35 49.52 48.67 57.78 67.22

14.07 14.00 14.13 16.69 15.93

11.41 11.40 13.23 7.01 3.57

0.16 0.19 0.20 0.14 0.06

5.42 5.94 5.91 2.40 1.37

9.13 10.70 10.53 7.79 4.37

2.32 2.25 2.46 4.21 5.18

0.11 0.14 0.24 0.54 0.88

0.89 0.83 1.10 0.83 0.41

0.08 0.08 0.12 0.09 0.11

8.95 4.08 4.04 1.95 1.14

98.90 99.11 100.63 99.42 100.24

59 58 80 56 86

0.3 0.3 0.4 1.2 2.2

2.4 2.1 3.3 2.7 1.6

Borehole

Depth, m

WIP

CIA

ER, Ohm m

BH6 BH6 BH6 BH6 BH6 BH6

2.0 4.0 6.0 8.0 10.0 12.0

58 63 57 65 26 35

35 37 42 41 69 58

10 24 45 11 8 19

Quartz

Oligoclase

XXX XXX

XX XX XX X XXX XXX

X X XX XX XX XX

Anorthite

Biotite

Sericite

Epidote

Mg-hornblende

X X

XXX XXX XXX XXX X

X X

Tremolite

XXX X XX

Chlorite

Calcite

X X X

(continued on next page)

941

Clays 2:1–1:1

Regolith mass balance in a gneissic watershed, South India

Borehole

942

Table 1 (continued) Borehole Depth, m WIP CIA ER, Ohm m Clays 2:1–1:1 Quartz Oligoclase Anorthite Biotite Sericite Epidote Mg-hornblende Tremolite Chlorite Calcite 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 32.0 34.0 36.0 38.0 40.0 42.0 44.0 46.0 48.0

50 57 60 51 63 61 60 68 68 52 72 69 69 72 72 71 71 54

46 42 38 46 38 37 47 41 38 43 40 41 40 36 37 42 38 47

162 1755 206 7547 8532 227 1416 3907 na na na 6329 6071 6461 3829 2654 3047 1450

XXX XXX XXX XXX XX XXX XXX XXX XXX XXX XX XXX XX XX XXX XXX XX XXX

BH6 BH6 BH6 BH6 BH6

50.0 52.0 54.0 56.0 58.0

60 65 67 70 70

41 38 38 44 48

5996 10,222 15,264 17,038 na

XXX XXX XXX XXX XXX

Borehole

Depth, m

SiO2, %

Al2O3, %

BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5

2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 32.0 34.0 36.0

74.28 70.29 72.42 74.81 67.86 73.63 64.89 71.64 68.80 68.99 63.86 63.56 62.07 71.68 74.00 70.93 71.17 68.83

14.00 16.34 13.57 14.57 15.03 13.42 13.48 13.50 12.53 15.14 11.43 15.10 14.16 13.53 13.93 13.96 13.99 13.38

Fe2O3, % 4.05 4.31 4.93 1.91 4.70 3.91 8.47 4.23 5.99 3.32 8.13 7.16 7.49 3.69 2.64 4.05 3.47 4.57

X

XXX XXX XX X XX XXX XX X XX XX

XX XX XX X XXX XX

X X XX X X XX

XX X X

X X

XXX X

XX

MnO, %

MgO, % 0.60 0.50 1.17 0.68 1.84 0.96 2.33 1.53 2.20 1.65 6.57 3.16 3.79 1.60 1.03 1.64 1.44 1.63

XXX XXX XX

XX XX XX X

X X

X X

X X X X X X X X X

XXX XXX X

XXX XXX XX X

XXX XXX

XX X

XX X

X XXX X

XXX XXX

XXX X

X

XX XXX XX XX XXX XXX XXX XX XXX XXX XX XXX

XXX XXX

XXX X

XX


XXX

X

XX

X X XX

XXX XXX

XXX XXX X

Groundwater table XX X

600–700 lS cm1

X

CaO, %

Na2O, %

K2O, %

TiO2, %

P2O5, %

LoI, %

Total, %

Zr, ppm

Th, ppm

Nb, ppm

0.11 0.16 0.15 0.33 0.18 0.66 0.37 0.79 0.65 0.57 1.46 2.31 1.71 0.82 1.51 1.52 2.86

1.79 1.10 2.59 4.15 3.62 3.31 2.64 3.19 4.30 5.95 2.13 4.84 3.72 4.49 4.88 4.27 4.59 5.20

2.11 1.95 2.21 1.55 1.63 1.74 1.59 2.11 1.14 1.08 1.40 1.55 2.54 1.88 1.92 2.35 2.28 0.95

0.34 0.35 0.46 0.18 0.50 0.37 0.80 0.46 0.61 0.31 0.36 0.59 0.84 0.50 0.20 0.47 0.38 0.40

0.04 0.04 0.04

3.57 5.88 3.43 2.86 4.72 3.11 5.90 3.40 3.78 3.04 5.50 3.37 3.42 1.68 1.36 1.62 1.64 2.45

100.77 100.88 100.98 100.85 100.31 100.66 100.86 100.48 100.32 100.29 100.08 101.00 100.55 100.97 100.82 100.99 100.59 100.42

728 610 476 184 334 661 469 521 377 213 85 137 257 257 139 236 243 162

10.6 15.7 10.3 14.9 8.0 9.1 5.8 10.1 5.3 8.2 1.9 1.7 6.1 9.7 9.7 11.7 7.9 2.3

22.3 10.8 12.7 4.3 11.4 15.3 19.2 13.7 14.0 6.3 10.1 13.7 17.1 8.1 7.6 14.3 6.4 8.5

J.-J. Braun et al. / Geochimica et Cosmochimica Acta 73 (2009) 935–961

BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6 BH6

BH5 BH5 BH5 BH5 BH5 BH5 BH5

38.0 40.0 42.0 44.0 46.0 48.0 50.0

72.93 64.99 70.88 57.79 74.43 73.91 76.00

14.14 14.36 13.39 11.01 14.44 13.85 12.90

2.86 5.94 4.26 11.33 1.77 1.86 1.70

0.04 0.05 0.03 0.10 0.04
1.29 3.11 1.84 8.53 0.78 0.80 0.61

0.71 1.20 0.85 1.46 0.48 0.96 0.88

5.31 4.85 5.32 1.77 5.57 5.36 4.71

1.82 2.53 1.33 2.51 1.58 1.38 1.46

0.29 0.59 0.46 0.62 0.18 0.20 0.22

0.08 0.14 0.08 0.11 0.05 0.08 0.06

1.36 2.22 1.86 4.29 1.11 1.04 1.08

100.83 99.96 100.30 99.52 100.41 99.43 99.63

121 175 268 140 137 380 405

14.6 6.5 9.7 17.7 7.2 9.8 32.0

9.2 11.9 10.4 15.0 3.9 5.3 5.1

BH5 BH5 BH5

52.0 54.0 56.0

72.71 68.90 72.77

14.06 10.85 13.89

2.58 6.89 2.36


0.92 5.95 1.00

0.93 0.73 0.72

4.93 2.31 5.39

1.78 2.03 1.62

0.30 0.29 0.19

0.06 0.06 0.03

1.41 2.48 1.04

99.68 100.54 99.00

258 310 256

9.8 16.2 14.8

11.3 9.3 3.7

Borehole Depth, m WIP CIA ER, Ohm m Clays 2:1–1:1 Quartz Oligoclase Anorthite Biotite Sericite Epidote Mg-hornblende Tremolite Chlorite Calcite 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 32.0 34.0 36.0 38.0 40.0 42.0 44.0 46.0 48.0 50.0

36 28 46 54 53 48 46 52 57 70 51 70 72 66 66 67 69 67 69 77 67 65 68 66 60

73 80 66 62 65 64 65 63 57 56 66 56 53 53 55 54 53 48 55 53 54 58 55 54 54

142 106 66 56 108 75 93 81 92 80 127 184 395 490 na na na na na 579 1125 1565 1759 1585 3247

BH5 BH5 BH5

52.0 54.0 56.0

65 57 68

55 60 54

2102 2959 3450

X X X X X X X X X X X

XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX

X X XX XXX XX XX XX XX XXX XXX XX XXX XXX XXX XXX XX XXX XXX XXX XXX XXX X XXX XXX XX

XXX XXX XXX

XXX XX XXX

X X

X X X X XX X

X X X XX XX

X X X X X X X X X X X X XX X X X X XX X X

X X

XX XX X X

X XX X

X XX X

XXX XX X X X X XX X X X XX X X

X

Regolith mass balance in a gneissic watershed, South India

BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5 BH5

X XX X X

XX

Groundwater table 600–700 lS cm1 XX X

X

Depth, m

SiO2, %

Al2O3, %

Fe2O3, %

MnO, %

MgO, %

CaO, %

Na2O, %

K2O, %

TiO2, %

P2O5, %

LoI, %

Total, %

Zr, ppm

2.0 4.0

71.32 69.22

10.47 15.92

4.75 1.97

0.13
1.04 0.64

1.29 1.56

1.37 6.32

0.64 1.31

0.49 0.24

0.04 0.09

7.47 1.64

99.01 98.90

254 114

Th, ppm

Nb, ppm

10.3 5.5 5.1 3.5 (continued on next page)

943

Borehole BH1 BH1

944

Table 1 (continued) Depth, m

SiO2, %

Al2O3, %

Fe2O3, %

MnO, %

MgO, %

CaO, %

Na2O, %

K2O, %

TiO2, %

P2O5, %

LoI, %

Total, %

Zr, ppm

Th, ppm

Nb, ppm

BH1 BH1 BH1 BH1 BH1 BH1 BH1 BH1 BH1 BH1 BH1 BH1 BH1 BH1 BH1 BH1 BH1

6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 32.0 34.0 36.0 38.0

70.36 69.29 70.45 70.85 67.45 72.35 71.88 68.41 50.19 68.08 62.91 63.86 71.69 53.02 53.11 53.14 57.21

16.29 16.16 16.47 15.81 15.52 14.61 14.72 13.27 14.13 13.57 11.82 14.14 14.24 9.74 10.57 10.52 9.03

1.74 2.25 1.59 2.25 2.06 1.58 1.30 3.57 8.31 3.90 6.40 3.98 2.89 11.71 15.83 16.04 10.98


0.47 0.86 0.36 0.34 0.91 0.60 0.46 2.36 4.99 2.61 6.41 1.90 1.57 4.96 5.64 5.73 2.61

0.91 0.87 0.68 0.47 1.86 0.88 1.74 2.20 7.27 1.98 2.72 3.98 1.61 8.88 5.56 5.41 9.68

6.54 6.28 6.63 5.96 5.62 5.50 5.62 4.50 3.37 4.56 2.70 3.80 5.77 0.75 0.64 0.64 1.91

1.50 1.71 1.66 1.93 2.09 1.78 1.60 2.05 3.57 1.98 1.37 2.74 1.44 2.18 3.30 3.29 1.19

0.18 0.22 0.17 0.23 0.26 0.18 0.17 0.32 2.20 0.38 0.29 0.52 0.29 0.84 0.78 0.77 0.43

0.07 0.07 0.07 0.09 0.12 0.06 0.05 0.09 0.46 0.08 0.06 0.15 0.08 0.15 0.16 0.16 0.12

1.34 1.56 1.14 1.21 2.92 1.48 1.75 2.21 4.09 2.03 4.29 4.01 1.30 6.80 4.16 3.89 5.30

99.39 99.27 99.21 99.14 98.80 99.02 99.27 99.02 98.68 99.21 99.05 99.14 100.91 99.21 99.95 99.78 98.68

107 103 99 121 119 103 111 105 213 195 94 114 127 89 102 107 96

10.9 4.5 2.7 6.7 4.5 7.7 8.5 5.1 2.9 5.0 4.8 3.5 7.0 2.1 1.8 2.2 4.3

3.0 3.2 1.8 3.1 3.5 4.0 4.5 4.6 19.5 5.8 4.5 5.5 5.7 5.9 4.6 4.6 4.9

Borehole Depth, m WIP CIA ER, Ohm m Clays 2:1–1:1 Quartz Oligoclase Anorthite Biotite Sericite Epidote Mg-hornblende Tremolite Chlorite Calcite BH1 BH1

2.0 4.0

24 75

67 52

na na

BH1 BH1 BH1 BH1 BH1 BH1 BH1 BH1 BH1 BH1 BH1 BH1 BH1 BH1 BH1 BH1 BH1

6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 32.0 34.0 36.0 38.0

77 77 78 73 76 69 71 71 92 71 61 73 74 61 63 63 59

54 54 54 56 51 54 51 50 40 51 52 47 51 33 42 43 29

na na na na na na na na na na na na na na na na na

Borehole BH12 BH12

Depth, m 2.5 3.0

XXX XXX

XXX XXX

XXX XXX

XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX

XXX XXX XXX XXX XXX XX XXX XXX XXX XX XXX XX XXX XXX X X XX

XX XX X X X X X X X XX XXX XX X X XX XXX XXX XX

XX XX XX XX XX X XX X X X X X XX X

Groundwater table 200–300 lS cm1

X X X XXX X X XX XX X

X X

X XX

SiO2, %

Al2O3, %

Fe2O3, %

MnO, %

MgO, %

CaO, %

Na2O, %

K2O, %

TiO2, %

P2O5, %

LoI, %

Total, %

Zr, ppm

69.64 68.98

12.60 16.12

6.76 3.52

0.22 0.04

0.47 0.67

0.81 1.48

0.96 3.62

0.67 1.06

0.46 0.36

0.07 0.04

7.73 4.90

100.38 100.79

269 156

Th, ppm 6.5 5.3

Nb, ppm 5.6 4.0

J.-J. Braun et al. / Geochimica et Cosmochimica Acta 73 (2009) 935–961

Borehole

4.0 6.0 7.0

72.02 64.51 67.24

14.64 14.85 15.54

2.61 5.59 4.12

0.02 0.06 0.04

1.11 2.42 1.62

1.84 3.24 3.65

4.59 4.54 4.97

1.00 0.39 0.45

0.29 0.49 0.46

0.03 0.06 0.09

2.47 4.22 2.75

100.64 100.37 100.92

139 104 128

3.9 10.7 2.0

2.9 3.7 3.9

BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12

8.5 9.0 10.0 10.5 11.0 13.5 14.0 15.0 17.5 18.5 20.5 22.0 23.5 25.0 26.5 28.0 29.5 31.0 32.5 34.0 35.5 37.0 37.5 40.0 41.0

71.43 66.82 71.04 70.23 70.37 67.93 66.91 71.61 70.81 63.24 62.13 69.97 69.50 70.14 66.52 67.39 70.28 68.63 57.50 64.72 64.59 63.74 67.55 69.72 67.49

14.20 13.28 14.52 14.39 12.78 10.20 10.29 12.87 15.34 12.46 12.24 14.47 13.44 13.14 14.00 13.75 14.52 13.88 13.92 13.00 14.21 13.99 14.17 13.25 14.30

2.76 5.38 3.05 3.22 5.13 8.40 8.07 3.81 2.50 6.57 7.81 3.23 3.83 3.79 5.33 5.02 3.28 4.38 9.59 6.94 5.05 6.43 4.29 4.24 4.56

0.02 0.05 0.03 0.03 0.04 0.05 0.05 0.03 0.02 0.09 0.08 0.03 0.04 0.04 0.05 0.05 0.03 0.04 0.11 0.08 0.06 0.07 0.05 0.04 0.05

1.33 2.65 1.39 1.91 2.03 4.35 5.09 1.14 1.31 3.82 6.77 2.93 3.23 2.73 4.11 3.34 1.89 2.95 4.34 3.38 3.07 3.30 3.05 2.59 2.91

2.23 2.41 2.42 2.01 2.55 1.40 1.26 2.95 1.67 5.41 2.18 0.69 1.10 1.70 1.55 2.28 1.87 2.04 6.43 4.12 3.12 3.93 2.17 1.80 2.39

4.88 4.04 4.86 5.06 3.34 1.54 2.03 4.33 5.78 3.43 2.73 6.16 5.02 4.47 4.16 4.06 5.26 4.88 2.80 3.74 5.30 4.20 4.57 3.94 4.61

0.89 0.42 0.98 0.69 1.62 2.39 1.93 0.96 1.15 1.13 1.99 0.49 0.83 0.92 1.69 1.63 1.29 1.06 1.73 1.18 1.38 1.47 1.39 2.13 1.56

0.31 0.42 0.34 0.26 0.48 0.70 0.46 0.45 0.26 0.60 0.60 0.25 0.39 0.36 0.45 0.39 0.31 0.37 0.85 0.61 0.46 0.58 0.41 0.43 0.46

0.08 0.07 0.09 0.06 0.11 0.08 0.09 0.15 0.08 0.12 0.11 0.06 0.11 0.11 0.12 0.11 0.08 0.10 0.13 0.11 0.13 0.12 0.12 0.13 0.12

2.17 5.29 2.13 2.54 2.32 3.95 4.41 1.45 1.36 3.12 2.82 1.57 1.71 1.59 1.94 1.68 1.47 1.70 2.31 1.98 2.26 2.16 2.18 1.73 1.95

100.30 100.82 100.86 100.39 100.75 100.97 100.60 99.73 100.26 99.98 99.45 99.86 99.18 98.97 99.91 99.70 100.29 100.02 99.71 99.85 99.63 99.98 99.95 100.00 100.39

159 125 221 121 243 500 598 334 175 100 87 61 118 116 107 83 76 136 123 153 178 173 288 244 205

4.5 4.7 6.7 4.0 5.2 11.2 6.0 9.4 8.5 4.1 2.5 1.9 4.5 7.1 6.1 4.7 3.4 7.5 3.8 6.2 11.0 10.6 25.4 14.0 14.7

3.5 4.5 3.6 2.6 7.3 15.6 15.6 6.3 3.8 7.3 8.7 3.2 5.1 3.7 5.7 5.9 4.1 5.0 5.3 4.9 5.5 5.3 5.5 7.1 6.2

Borehole Depth, m WIP CIA ER, Ohm m Clays 2:1–1:1 Quartz Oligoclase Anorthite Biotite Sericite Epidote Mg-hornblende Tremolite Chlorite Calcite BH12 BH12 BH12 BH12 BH12

2.5 3.0 4.0 6.0 7.0

18 48 58 60 63

78 62 59 55 55

60 60 60 90 150

X X X X X

XXX XXX XXX XXX XXX

X XX XX XXX XXX

X

X X

X

X

X X X X

BH12 BH12

8.5 9.0

62 54

57 56

220 300

X X

XXX XXX

XX XXX

X X

X X

X X

BH12 BH12 BH12 BH12 BH12 BH12 BH12

10.0 10.5 11.0 13.5 14.0 15.0 17.5

63 63 56 50 52 58 71

56 58 51 48 50 52 60

400 na na 155 167 165 370

X X X X X

XXX XXX XXX XXX XXX XXX XXX

XX XXX XX X X XX XXX

X X X X XX X X

X X X X X X XX

X X X

X

X X X

X X

Regolith mass balance in a gneissic watershed, South India

BH12 BH12 BH12

Groundwater table 200–300 lS cm1

XX X XX 945

(continued on next page)

946

J.-J. Braun et al. / Geochimica et Cosmochimica Acta 73 (2009) 935–961

XX XXX XX XX XX XX XX XX XX XX XX XX XX XX X XX

5.1. Determination of fresh and weathered materials

X

X X X X X X X

X X X X X X XX X X XX X X XX

Mg-hornblende Biotite Oligoclase

XX XX XXX XXX XX XX XX XXX XX XX XX XXX XXX XX XX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX 43 51 66 61 57 58 54 58 57 42 47 53 49 56 54 55

ER, Ohm m CIA WIP

18.5 20.5 22.0 23.5 25.0 26.5 28.0 29.5 31.0 32.5 34.0 35.5 37.0 37.5 40.0 41.0 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12 BH12

65 66 71 65 61 68 66 69 67 68 64 77 70 68 66 69

Depth, m Borehole

Table 1 (continued)

1221 1703 2425 3481 3128 3445 3952 3823 4538 3003 2445 1303 2221 2475 1621 na

Clays 2:1–1:1

Quartz

Anorthite

X XX X X X X X X X XX X XX XX X X X

Sericite

Epidote

X

Tremolite

Chlorite

Calcite

5. DISCUSSION

Since the nature of the parent lithology is highly variable, we first distinguished the fresh parent material and saprolite samples from the boreholes using both LoI and [Fe2O3 + MgO] contents (Fig. 6). The LoI of fresh parent rocks depends on the relative abundance of the primary hydrated minerals, given the low carbon content of the silicate bedrock. When the rocks weather, the LoI accordingly increases with the formation of clays and clay minerals. Since iron and magnesium are present in all primary hydrated minerals of the watershed bedrocks, the comparison between [Fe2O3 + MgO] and LoI defines theoretical domains for fresh rocks, based on the XRD mineralogy for the borehole samples. LoI, used as a weathering index, will then be compared to the Chemical Index of Alteration, CIA (Nesbitt and Young, 1982) and the Weathering Index of Parker, WIP (Parker, 1970), more classically used. Both indices reflect the mobility of base cations. CIA considers aluminum as a conservative element and reflects the extent of plagioclase weathering, i.e. leaching of K, Na and Ca, and transformation into clay minerals such as kaolinite. As weathering progresses, CIA increases from about 50 for fresh rocks to 100 for optimum weathering. WIP differs from CIA in that it relies on all major mobile alkali and alkaline earth (K, Na, Ca, Mg) and can therefore be applied to both acid and basic igneous rocks. WIP may be upper than 100 for fresh rocks and tends towards 0 for weathered materials. However, its application to highly weathered material such as laterite is not recommended (Price and Velbel, 2003). Finally, the comparison will be carried out with the electrical resistivity measured in boreholes, rock outcrops and soil catena. 5.1.1. Determination of fresh gneiss, gneiss-derived saprolite and red soil The determination of fresh gneiss samples is made with the a priori condition that the samples should be located, in the bivariate plot [Fe2O3 + MgO] versus LoI, in the domain defined by three components: sericite-rich, biotite-rich and chlorite-rich. The sericite-rich component represents the leucocratic pool while both biotite-rich and chlorite-rich components represent the melanocratic pool. In the leucocratic pool, in which the lowest [Fe2O3 + MgO] content is 2 wt%, the LoI content is prominently influenced by sericite (LoI = 4.0 wt%; [Fe2O3 + MgO] = 7.2 wt%). For a [Fe2O3 + MgO] content of 2 wt%, the maximum amount of sericite is 27 wt%, so the corresponding maximum LoI is 1 wt%. Towards the melanocratic pool, in which [Fe2O3 + MgO] reaches 22 wt%, the LoI content is mainly influenced by the abundances and the relative proportions of chlorite and biotite. Knowing that LoI of chlorite is 11 wt% and LoI of biotite is 4 wt%, the maximum LoI values for the fresh melanocratic component range from 2.6 wt% for a biotite-rich sample to 5.9 wt% for a chlorite-rich sample. The samples above the sericite-chlorite mixing line obviously correspond to weathered samples, i.e. gneiss-derived saprolite and red soil. The samples within

Table 2 Bulk chemical analyses for major and selected inert trace elements (Zr, Th, Nb) and electrical resistivity for the soil samples of the S1 and S2 sites. CIA and WIP are indicated. Soils

SiO2, % Al2O3, % Fe2O3, % MnO, % MgO, % CaO, % Na2O, % K2O, % TiO2, % P2O5, % LoI, % Total, % Zr, ppm Th, ppm Nb, ppm WIP CIA ER, Ohm m

downslope T1 catena 40–60 64.52 40–80 63.12 60–80 62.18 80–100 62.34 80–120 63.06 80–120 63.56 100–120 63.72 100–120 63.53 120–140 61.89 120–140 62.53 140–160 64.81 160–180 64.42 180–200 65.66 200–220 73.19

S1-P – profile sampled close S1-P 0–15 78.79 S1-P 25–35 77.10 S1-P 45–55 71.01 S1-P 65–75 67.09 S1-P 95–105 70.07 S1-P 120–130 72.67 S1-P 145–155 73.00 S1-P 170–180 71.22 S1-P 205–215 67.28 S1-P 225–235 67.36 S1-P 230–240 71.09 S2-P – upslope profile S2-P 0–5 71.31 S2-P 8–18 73.29 S2-P 25–35 76.60 S2-P 90–100 64.88 S2-P 190–200 68.85 S2-P 240–250 76.18 S2-P 285–295 76.96 S2-P 310–320 73.41

(Barbie´ro 15.51 15.59 16.57 16.46 15.78 15.50 15.78 16.28 16.67 16.50 15.17 14.78 15.22 14.57

et al., 2007) 6.18 0.10 6.31 0.12 6.65 0.10 6.94 0.08 6.46 0.13 6.49 0.10 6.58 0.10 6.54 0.09 6.92 0.08 6.38 0.06 6.11 0.11 5.82 0.09 6.06 0.11 1.81
0.52 0.51 0.75 0.63 0.58 0.51 0.56 0.58 0.65 0.75 0.50 0.97 0.53 0.70

0.62 0.65 0.68 0.59 0.68 0.70 0.74 0.63 0.56 0.70 0.68 0.83 0.70 0.77

0.45 0.45 0.58 0.50 0.63 0.41 0.46 0.45 0.50 0.60 0.46 0.82 0.57 4.16

0.90 0.87 0.97 0.95 0.94 0.92 0.95 0.94 0.97 0.94 0.88 0.92 0.84 1.35

0.61 0.62 0.67 0.67 0.60 0.63 0.65 0.67 0.67 0.66 0.60 0.64 0.62 0.15

0.05 0.05 0.04 0.04 0.05 0.05 0.05 0.04 0.04 0.04 0.04 0.04 0.04
11.13 11.38 11.56 11.28 10.52 10.61 10.89 11.31 11.30 11.16 9.69 10.62 9.23 2.76

100.59 99.67 100.74 100.48 99.43 99.48 100.47 101.07 100.23 100.31 99.05 99.93 99.57 99.45

283 229 223 232 274 264 236 273 222 265 271 224 295 88

15.1 12.9 8.6 8.4 8.9 10.0 8.5 9.0 9.2 9.5 9.3 9.5 10.5 11.2

3.5 8.9 8.5 8.9 8.4 8.6 8.6 9.1 9.5 9.7 9.7 9.7 9.5 9.3

15 14 17 16 17 15 16 15 16 17 15 20 15 54

85 85 84 85 83 85 84 85 86 84 84 80 83 60

60 60 60 60 60 30 30 30 30 30 30 30 150 150

to the downslope T1 catena 8.50 3.03 0.04 11.26 3.81 0.04 13.31 4.62 0.07 15.21 5.15 0.03 14.57 3.39 0.03 12.69 4.15 0.03 12.30 3.87 0.03 13.06 3.76 0.04 14.81 4.55 0.05 16.69 3.31 0.06 16.51 1.61 0.02

0.61 0.65 0.66 0.66 0.75 0.95 1.34 1.20 1.53 1.13 0.51

0.74 0.68 0.56 0.49 0.55 0.76 1.08 1.32 1.50 1.76 2.08

0.69 0.62 0.48 0.42 0.51 0.90 1.26 1.63 2.02 3.79 4.55

0.79 0.82 0.81 0.79 0.81 0.72 0.81 0.79 0.76 0.58 1.05

0.36 0.42 0.45 0.50 0.51 0.40 0.36 0.37 0.40 0.29 0.21

0.05 0.04 0.05 0.04 0.02 0.03 0.02 0.02 0.02 0.02 0.02

6.76 4.35 8.10 9.01 8.35 6.72 6.15 6.16 6.72 4.67 2.27

100.36 99.80 100.12 99.40 99.55 100.02 100.23 99.56 99.64 99.66 99.92

260 306 230 221 285 234 195 236 144 109 124

na na na na na na na na na na na

na na na na na na na na na na na

16 16 14 14 15 19 25 28 33 47 58

73 79 84 87 85 78 72 69 68 62 57

60 60 60 60 30 30 30 30 150 150 150

0.60 0.64 0.53 1.44 1.17 0.79 0.74 0.35

1.11 0.86 0.69 1.05 1.10 0.81 0.79 0.63

1.15 1.22 1.22 1.11 1.75 1.72 1.73 3.96

0.81 0.77 0.75 0.90 0.85 0.89 0.90 3.16

0.55 0.58 0.51 0.65 0.53 0.35 0.37 0.14

0.08 10.65 0.05 7.56 0.04 5.69 0.02 9.59 0.02 7.28 0.02 4.92 0.02 4.82 0.02 2.44

99.37 99.48 100.27 99.42 99.94 100.09 100.33 100.09

292 304 313 193 170 177 216 79

na na na na na na na na

na na na na na na na na

22 22 21 24 29 28 28 66

66 71 71 75 70 66 67 56

60 60 60 60 150 150 150 150

8.99 10.30 9.77 13.96 13.19 10.22 10.74 14.15

4.03 4.13 4.41 5.77 5.14 4.16 3.23 1.83

0.09 0.07 0.06 0.06 0.06 0.04 0.04 0.01

Regolith mass balance in a gneissic watershed, South India

S1-T – S1-T1 S1-T1 S1-T1 S1-T1 S1-T1 S1-T1 S1-T1 S1-T1 S1-T1 S1-T1 S1-T1 S1-T1 S1-T1 S1-T1

Depth, m

947

99 49 98 71 74 98 97.65 99.70 98.43 11.97

1.50

11.04

0.14

25.67

0.07 12.85 0.03

0.03

9 30 90 77 98.93 99.33 2.07 2.03 0.06 0.21 2.23 10.27

Mafic to ultramafic rocks Tremolite n = 18 Mgn = 10 hornblende Chlorite n=5 Labradorite n = 18 Serpentine n = 47

54.93 46.28

0.22
2.13 4.00

3.87 10.20

0.19 0.21

19.93 12.10

12.78 11.65

0.25 1.28

0.09 0.88

0 100 105 96 43 57 57 99.94 97.57 99.78 96.08 100.09 101.69 #

# n = 30 n = 15 n=7 n=6 n = 16 n=2

100.00 66.61 36.72 49.25 27.25 38.45 31.53

# 20.75 15.22 29.18 20.87 23.19 2.23

#

#

#

#

#

# 1.63 0.12

# 10.75

#

WIP Total, wt% LoI, wt% TiO2, wt% K2O, wt% Na2O, wt% CaO, wt% MgO, wt% MnO, wt% FeO, wt% Fe2O3, wt% Cr2O3, wt% Al2O3, wt% SiO2, wt%

Table 3 Chemical composition, CIA and WIP of the main minerals from gneiss and mafic to ultramafic rocks.
0 50 60 73 100 34 34

J.-J. Braun et al. / Geochimica et Cosmochimica Acta 73 (2009) 935–961 CIA

948

the domain containing chlorite are fresh while those containing only biotite and above the sericite–biotite mixing line can be considered as weathered. Their selection is based on the XRD patterns. Both groups of fresh gneiss and saprolite samples spread on a wide range and show positive correlations (Fig. 6). Most of fresh gneiss and saprolite samples are however in the range of 2–10 wt% [Fe2O3 + MgO]. The average for fresh gneiss and saprolite therefore represents the parent rock at the watershed scale. The estimation of the modal abundances for the average gneiss takes into account the mineral occurrence on the XRD patterns. Assuming that apatite controls 100% of P2O5, its modal abundance is first determined; then the proportion of Ca linked to apatite is deducted from the bulk analysis. To estimate the modal abundances with the apatite-corrected bulk analysis, we apply a linear inverse method using least squares criterion (Tarantola and Valette, 1982). The solution and error is given by equations 47 and 48 in Tarantola and Valette (1982):  1 ^x ¼ AT  C 1  AT  C 1 ð1Þ y0 y0  A y0 y0  y 0  1 C^x^x ¼ AT  C 1 ð2Þ y0 y0  A where y0 is the chemical composition vector of the rock, A is the matrix of the main mineral compositions and ^x the a posteriori solution (modal abundance vector), C 1 y 0 y 0 is the inverse of the covariance matrix and C^x^x is the a posteriori error covariance of the solution. The residuals are calculated by y 0  ^y , where ^y ¼ A  ^x. The minerals selected in the matrix A are quartz, oligoclase, biotite, sericite, chlorite, epidote, titanite for the average gneiss. Both bulk compositions (±r), calculated modal abundances and their errors, estimated bulk compositions (^y ) and associated residuals, and the estimated contributions from each mineral to the whole rock are summed up in Table 4 for the average gneiss. It appears that CIA and WIP are not able to separate the saprolite samples from the fresh gneiss samples, while the red soil samples are distinguished (Fig. 6). The comparison of all three weathering indices to the electrical resistivity shows however a threshold between the fresh gneiss samples and the weathering materials at 400 Ohm m (Fig. 7). Compared to the measurements of electrical resistivity on fresh gneiss outcrops, which are in the range 1000– 2000 Ohm m, this threshold seems to be relatively low. An explanation could be the integration of the fissured unweathered bedrock layers common in hard-rock aquifers (Dewandel et al., 2006). More precise measurements, i.e. drill core sampling instead of cuttings, should be done to characterize with accuracy the boundary between fresh, unfractured rock, fissured rock and saprolite. The 400 Ohm m threshold will therefore be used in the modeling of the ERT profiles in the next section. 5.1.2. Determination of the fresh amphibolite and amphibolite-derived saprolite In the fresh amphibolite, in which [Fe2O3 + MgO] ranges between 17 and 28 wt%, the LoI results from the mixing between chlorite, serpentine and Mg-hornblende. We may

Regolith mass balance in a gneissic watershed, South India

949

Fig. 3. Petrographical features of the gneiss. (a) Handpicked sample of gneiss showing melanocratic and leucocratic parts at the decimetric and centrimetric scale, (b) SEM-BE microphotograph of gneiss section at lower magnification including (c) and (d), (c) detail showing a chlorite crystal, (d) detail showing sericite sticks within an oligoclase crystal, (e) biotite crystal with exsolution of titanite crystals, epidote crystals are also present, (f) apatite and zircon crystals. Note the small size of the latter (10 lm).

consider that the chlorite/serpentine line delineates fresh and weathered samples as both minerals have similar LoI (–12 wt%) while the Mg-hornblende line (LoI – 2 wt%) delineates the lower boundary (Fig. 6). However some fresh samples can contain a large amount of carbonates in which the LoI goes up. Six points related to the fresh samples are clearly observed in the weathered domain. These points correspond to shallow samples up to 10 m in depth. Observations carried out in a 3 m deep pit dug close to BH6 shows clearly

fresh highly fissured rock in which the joints between the angular boulders are filled with clayey materials (similar to the outcrop shown in Fig. 4). We suppose that this conductive material however does not show a difference in terms of chemical signature with the unweathered parent bedrock and is responsible for the low resistivity. The resistivity of the fresh amphibolite ranges between 10,000 and 1000 Ohm m but a clear resistivity limit between weathered and fresh amphibolite cannot be extracted from this data set and cannot be taken into account in the further mass

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Fig. 4. Petrographical features of the BH6 amphibolite. (a) Outcrop of fissured dyke of amphibolite near BH6, (b) detailed SEMbackscattered electron (BE) section of the BH6 amphibolite showing the presence of crystals of Mg-hornblende, chlorite, serpentine, Fe–Cr oxide and Mg-rich calcite.

Fig. 5. Result of ERT survey. Calculated resistivity resulting from ERT inversion of field data is presented versus depth for the 12 ERT crosssections. Bore hole locations (BH1, 5, 6 and 12) are noted.

Regolith mass balance in a gneissic watershed, South India

a

951

Loss on Ignition (wt%)

15

10

Fresh gneiss Red soil Saprolite Fresh amphibolite Weathered amphibolite

chlorite serpentine 1

chlorite

2

5

2 1

3

3 biotite sericite

LoI = 1.7(Fe2O3+MgO) - 1.3 r 2 = 0.78 LoI = 0.3(Fe2O3+MgO) + 1.5 r 2 = 0.62 LoI = 0.1(Fe2O3+MgO) + 0.8 r 2 = 0.87

Mg-hornblende

0 0

10

20

30

Fe2O3+MgO (wt%)

Loss on Ignition (wt%)

b

c

15

10

5

0 0

20

40

60

80

100

Chemical Index of Alteration (CIA)

0

20

40

60

80

100

Weathering Index of Parker (WIP)

Fig. 6. Property–property diagrams for Loss on Ignition versus Fe2O3 + MgO, WIP and CIA.

balance calculation. Besides fresh gneiss and amphibolite have a similar resistivity range. Consequently it is not feasible to differentiate and to quantify the volumes of both lithologies with the resistivity alone at the watershed scale. 5.2. Assessment of regolith thickness with ERT The electrical resistivity measured in four boreholes located on ERT profile 2 (BH13, 7, 8 and 9) was earlier compared to the calculated resistivity of this ERT profile (Descloitres et al., 2007). This comparison shows the good agreement between both measured and calculated resistivities for a restricted data set. But, since the threshold of 400 Ohm m delineates fresh and weathered rocks, the uncertainty linked to the calculation of this resistivity value in the ERT profile has to be assessed. For this purpose, we carried out a modeling, based on typical geometries and resistivity ranges encountered in the watershed. Four geometries have been tested (Fig. 8): (i) one step in the regolith, (ii) three

steps in the regolith, (iii) two thin resistive dykes and (iv) two deep conductive dykes. To fix the model resistivity values we chose the weathered materials with resistivities just below 400 Ohm m, and above 400 Ohm m for the fresh rock. Four materials and corresponding resistivities were defined: (i) thin topsoil of 100 Ohm m, (ii) clayey-sandy materials of 60 Ohm m, (iii) sandy-clayey materials of 350 Ohm m and (iv) fresh bedrock of 5000 Ohm m. The first step of the modeling is to generate a synthetic apparent resistivity data set, similar to field data with RES2DMOD software (Loke, 2000). Then this synthetic data set is inverted with RES2DINV software (Loke, 2000). We calculated simplified ERT profiles with the threshold of 400 Ohm m (Fig. 8). Inversions roughly reproduce the geometry of initial models but they are more reliable to reproduce intrusions of protolith in regolith than to detect intrusions of regolith in protolith. This may be due to the loss of accuracy of ERT with depth. For each model the uncertainty of ERT is noted as a deviation of regolith thickness from the model

952

Density, g/cm3

SiO2, wt%

Al2O3, wt%

Fe2O3, wt%

MnO, wt%

MgO, wt%

CaO, wt%

Na2O, wt%

K2O, wt%

TiO2, wt%

P2O5, wt%

LoI, wt%

Total, wt%

Zr, ppm

Th, ppm

Nb, ppm

WIP

CIA

ER, Ohm m

Average

2.74

68.21

13.68

4.78

0.05

2.60

2.02

4.49

1.67

0.40

0.10

1.93

99.92

172

8.8

6.2

68

53

400– 5000

±r Estimate Residuals

0.05

5.27 68.21 0.00

1.35 13.94 0.25

3.32 5.28 0.50

0.05 0.05 0.01

1.86 2.57 0.04

1.50 1.63 0.39

1.40 3.98 0.51

0.56 1.59 0.08

0.17 0.40 0.00

0.03

0.79 1.65 0.27

83

6.3

2.9

5

5

Mode (%)

±r

Gneissic protolith average gneiss n = 29

Mineral contribution to the whole rock composition SiO2

Al2O3

Fe2O3

MnO

MgO

CaO

Na2O

K2O

TiO2

P2O5

LoI

Major minerals Quartz Oligoclase Biotite Chlorite Sericite

32 ± 7 38 ± 11 10 ± 12 8±6 6 ± 11

47 37 5 3 5

0 57 11 13 14

0 0 46 38 5

0 0 48 38 0

0 0 36 59 5

0 35 1 0 0

0 99 0 0 1

0 2 57 0 41

0 0 43 1 0

0 0 0 0 0

0 0 23 56 16

Accessories Titanite Apatite Epidote

0.60 ± 0.73 0.23 ± 0.08 3.74 ± 6.03

0 0 2

0 0 6

0 0 10

2 0 12

0 0 0

9 7 48

0 0 0

0 0 0

56 0 0

0 100 0

0 0 4

J.-J. Braun et al. / Geochimica et Cosmochimica Acta 73 (2009) 935–961

Table 4 Average composition, modal abundance and contribution to the whole rock for each mineral of the parent rock.

Regolith mass balance in a gneissic watershed, South India

Electrical Resistivity (Ohm.m)

a

953

100000 Fresh gneiss Red soil

10000

Saprolite Fresh amphibolite Weathered amphibolite

1000

400 Ohm.m

100

soil 10

1 0

5

10

15

Loss on Ignition (wt%)

Electrical Resistivity (Ohm.m)

b

100000

10000

1000

400 Ohm.m 100

soil 10

1 20

30

40

50

60

70

80

90

Chemical Index of Alteration (CIA)

Electrical Resistivity (Ohm.m)

c

100000

10000

1000

400 Ohm.m 100

soil 10

1 10

20

30

40

50

60

70

80

Weathering Index of Parker (WIP) Fig. 7. Property–property diagrams for electrical resistivity versus Loss on Ignition, WIP and CIA.

value. In the three-step model, ERT inversion underestimates the regolith thickness by 9.7%. The highest deviation is noted for the dyke model, 21%. Based on these models, the

average underestimation of the regolith thickness by the ERT inversion procedure would be 15.8%. If this value is taken into account for the calculation of the regolith thickness

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ERT forward modelling and inversion

calculated models

synthetic models

average regolith thickness for models

24.10 m

150

0

a

-30

0 -10

0

100

150

150

200

-20

100

150

200

100

150

c 100

150

14.70%

12.55 m

- 3.40 m

9.70%

12.40 m

- 5.55 m

15.80%

16.75 m

- 7.35 m

21.00%

f

-30 100

150

200

-10

-30

- 5.15 m

200

-20

0

-10 -20

16.55 m

e

-30

0

b

-30

ratio to total model thickness

200

-10

-20

0

100

-10

-20

difference with model

Distance (m) 200

-10

depth (m)

17.95 m

0

depth (m)

15.95 m

depth (m)

21.70 m

depth (m)

Distance (m) 100

average regolith thickness for ERT

-20

200

0

-10

g

-30

100

150

200

-10

-20

d

-30

-20

model resistivity (Ohm.m) regolith fresh rock materials 100 60 350

h

-30

5000

total model thickness : 35 m

calculated resistivity with ERT (Ohm.m) regolith

fresh rock

< 400

> 400

Fig. 8. ERT modeling: synthetic models are (a) one step in the regolith, (b) three steps in the regolith, (c) two thin resistive dykes, (d) two deep conductive dykes. The models are computed with ERT forward modeling procedure. The resulting apparent resistivity cross-sections (not shown) are inverted with the same ERT procedure as field data to produce calculated resistivity profile. ERT final results (e), (f), (g) and (h) are presented using the resistivity threshold of 400 Ohm m (deduced from geochemical and mineralogical analysis) that separates regolith domain, in blue, from fresh rock, in red. The differences between the model regolith thickness and calculated ERT regolith thickness and their respective ratio related to the total model thickness (35 m) are indicated on the right for each model. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)

V w qw C j;w V p qp C j;p ¼ þ mj;flux 100 100

in the 12 profiles (Fig. 9), then the regolith thickness would range from 13.5 m at the outlet of the watershed (profile 1) to 23.7 m at the top of the watershed (profile 9). The distribution of the regolith thickness along the 12 ERT sections is not related to the altitude (Fig. 10). The variation in thickness in each profile might be related to the heterogeneity in the structure and the fractures of the gneissic substratum and its availability to weather. On average, the regolith thickness in the Mule Hole watershed is 17.2 m, which corresponds to a volume of 74  106 m3 of weathered materials. As the average thickness of the soil cover estimated from both EM31 investigations and pedological survey is about 2 m, the saprolite thickness can be deduced to be 15.2 m.

where the subscripts p and w refer to the parent and weathered materials, respectively. V is volume in cm3, q is bulk density in g/cm3 and Cj is chemical concentration of any element j in weight percent (wt%). The mj,flux represents the mass of an element j moving into or out of the system. The mj,flux is positive if the element j is accumulating in the system and negative if j is leaching from the system. The volumetric strain (e) or volume change is calculated from the density ratios q and conservative element concentrations Ci in the regolith by

5.3. Mass balance calculation

ei;w ¼

The mass balance equation set is based on the principle of mass conservation (Brimhall et al., 1991; Oh and Richter, 2005). For a chemical element j

Positive values of ei,w indicate expansion, negative ones indicate collapse and values around zero, isovolumic weathering.

qp  C i;p 1 qw  C i;w

ð3Þ

ð4Þ

Regolith mass balance in a gneissic watershed, South India

955

Fig. 9. Interpretation of the 12 ERT profiles and corresponding average thickness of the regolith.

0

830

840

850

860

870

880

890

900

910

UPSLOPE 5 10 15

average thickness

20 25 30 35 40

DOWNSLOPE

average altitude

Regolith thickness inferred from ERT (meters)

Altitude (in meters, above sea level) 820

Fig. 10. Relationship between regolith thickness and altitude along the 12 ERT profiles. The average regolith thickness (17.2 m) and altitude (860 m) lines are also marked.

The addition or subtraction of a chemical element j, either by solute migration or mechanical translocation, is quantified by the open-system mass fraction transport function (sj,w) ! qw  C j;w  ðei;w þ 1Þ  1 ð5Þ sj;w ¼ qp  C j;p Because the calculation of sj,w takes into account both residual enrichment and deformation, a positive value for sj,w reflects a true mass gain in element j of the weathered rock compared to the parent rock and a negative value indicates a mass loss. If sj,w = 0, the element is immobile during weathering with respect to the volume of regolith consid-

ered. Moreover, quantification of the overall mass transfers during both saprolitization and soil processes can be approached by the estimation of the chemical component transfer. The total mass of any mobile element j (DMj) transferred through the weathering system with thickness z (cm), expressed in mol/ha, is given by Z z DM j ðmol=haÞ ¼ 106  qp  C j;p  sj;w  dz ð6Þ 0

Mass balance requires precise verifications regarding the determination of the parent material composition prior to the chemical weathering onset and the choice of an inert element, which should be very insoluble and resistant to weathering. For the mass balance calculation, we propose

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to consider the average thickness of saprolite and soil determined above and the average gneiss composition as parent material. 5.3.1. Selection of the inert element Even if it would be preferable to assess mass balance in weathering profiles based on the usual inert trace elements as Zr, Th and Nb (Braun et al., 1993; White and Brantley, 1995; Kurtz et al., 2000), the heterogeneous distribution of these elements in the Mule Hole parental gneiss prevents them from being used as references (see Table 1). In the gneiss, TiO2 is chiefly controlled by titanite and biotite (Table 4). Both minerals are among the first to breakdown in the incipient weathering stage and this leads to the in-situ precipitation of insoluble Ti-oxides. TiO2 may constitute the most suitable reference for the mass balance calculation even if we cannot dismiss a slight mobility in the weathering profile as shown by Tripathi and Rajamani (2007) in similar weathering profiles from the Mysore Plateau and by Cornu et al. (1999) and Taboada et al. (2006). Chemical weathering fluxes would be therefore slightly underestimated (Table 5). 5.3.2. Strain and elemental gain or loss in the average gneissderived saprolite The mass balance calculation for the saprolite first requires estimating its average bulk density, which obviously spatially varies according to the degree of weathering of the gneissic domains. For instance, the bulk density is 1.9 ± 0.1 g/cm3 for the saprolite samples derived from the gneiss at the bottom of the T1 soil catena. One way to estimate the average saprolite bulk density is to assume isovolumetric weathering. If so, eTiO2 ;w equals to 0 and then qsaprolite ¼ qp  ðC TiO2 ;parent =C TiO2 ;saprolite Þ. The calculated average bulk density of the saprolite is 2.4 g/cm3. Within the Mule Hole saprolite the open-system masstransport functions indicate that all major elements except Ca are depleted with the following sequence: Mg (s = 0.42) > K (0.26) > Mn (0.22) > Fe (0.20) > Na (0.18) > P (0.17) > Si (0.13) > Al (0.11) (Fig. 11). Similar calculations carried out on the Rio Icacos quartz diorite provided different sequences with P > Ca = Na > Fe(II) > K > Mn > Si = Mg > Fe (total) > Al for a spheroid corestone/rindlet system (Buss et al., 2008) and Ca = Na > Mg > Si > K > Al > Fe (total) for the underlying saprolite, respectively (White et al., 1998). Buss et al. (2008) concluded that both sequences indicate (i) the rapid dissolution of plagioclase and apatite and slower weathering of Fe–Mg-silicates in the incipient weathering rindlets and (ii) the further weathering of biotite in the saprolite with loss of Mg. They argue that biotite oxidation is the most likely fracture inducing reaction in the rindlets allowing the solutions to dissolve the other mineral phases. At Mule Hole the s sequence primarily supports that chlorite and biotite, the chief sources for Mg (95%), Fe (84%), Mn (86%) and K (57%, biotite only) are the first to weather during saprolitization. The biotite loss may be estimated with s(K) if we consider (i) the stability of sericite and (ii) the total K leaching. It means that, at least, 49% of the biotite crystals are weathered in the average saprolite and trans-

formed into smectite and kaolinite/smectite interstratified (Bourgeon and Larque´, 1992). The second information borne in the s sequence is that the oligoclase crystals are quite preserved into the saprolite. Oligoclase is the chief source for Na (99%) and Al (57%) and the second source for Si (38%) after quartz (48%) and for Ca (35%) after epidote (48%). As sericite, quartz is stable in the saprolite. Therefore, the chief sources of Al, Na and Si during weathering are the breakdown of oligoclase. The loss of oligoclase in the saprolite can be assessed if we assume that Na is congruently leached from the regolith; if so, s(Na) corresponds to the amount of oligoclase loss in the saprolite, i.e. 18%. The s(P) is also moderate, meaning that apatite, as the only P-bearing mineral, is partly conserved in the saprolite. A significant leaching of Fe and Mn also occurs in the saprolite. Ca is slightly accumulated in the saprolite. In the average gneiss, the chief Ca sources are oligoclase (35%), epidote (48%), titanite (9%) and apatite (7%). All these phases are weathered to some degree and should lead to the leaching of Ca. A differential weathering pathway of the primary Ca-bearing minerals cannot explain the Ca accumulation. Another explanation would be the precipitation of CaCO3 from the percolating solution due to current and/or paleoclimatic conditions; carbonate nodules formed within the saprolite are common in the watershed. Overall when integrated over the average saprolite depth of 15.2 m, the losses by total mass occur for Si, Mg and Na with 286  106 mol/ha (62% of the total mass loss), 67  106 mol/ha (15% of the total mass loss) and 39  106 mol/ha (9% of the total mass loss), respectively. Al, Fe and K account for 7%, 4% and 3% of the total mass loss, respectively. P and Mn account for only 0.04% and 0.10%, respectively. 5.3.3. Strain and elemental gain or loss in the average gneissderived red soil The calculated strain in the average red soil indicates a collapse of 38% of the volume due to bio-pedoturbation processes. The open-system mass-transport functions point out that all major elements except Mn are depleted within the red soil profiles: Na = Mg (s = 0.76) > P (0.70) > Ca (0.65) > K (0.55) > Si = Fe (0.19) > Al (0.17) (Fig. 11). The s sequence indicates that Na-plagioclase weathering is enhanced compared to saprolite; from s(Na), at least 80% of oligoclase crystals have broken down. The low s(K) emphasized that biotite is completely transformed as shown by its absence on XRD soil patterns. The remaining K may be attributed to the persistence of sericite crystals. Fe is slightly leached from the soil and Mn is accumulated. Both elements are precipitated as oxides and oxyhydroxides during soil formation. Their mobility is linked to climate changes (Tripathi and Rajamani, 2007). When integrated over the average red soil depth of 2 m, the most important losses occur for Si, Na and Mg with 55  106 mol/ha (47% of the total mass loss), 22  106 mol/ha (19% of the total mass loss) and 16  106 mol/ha (14% of the total mass loss), respectively. Ca, Al and K account for 7.9%, 5.8% and 3.8% of the total mass loss, respectively. Fe and P account for only 1.9% and

30–100

Loss () or gain (+) % Of the sum (loss only) Loss () or gain (+) % Of the sum (loss only)

Total P Ti K Na Ca Mg Mn Fe Al Si

sj,red soil

Av. thickness = 15.2 m Av. red soil Av. thickness = 2 m sj,saprolite

±r n = 25 Average ±r

mol/ha(106) % mol/ha(106) %

1.60 0.10

286 32 18 0.48 67 7 39 16 NR 62.2 7.0 3.9 0.10 14.7 8.6 3.4 55 7 2 0.04 16 9 22 4 NR 47.1 5.8 1.9 14.0 7.90 19.3 3.8

0.40 0.09 0.49 0.04 0.16 0.01 0.00 0.17 0.00 0.70 1.30 0.73 1.34 0.93 1.24 0.42 0.18 0.26 0.76 0.55 1.19 2.60 0.77 0.88 0.30 0.39 0.42 0.06 0.76 0.65 6.17 1.69 2.68 0.06 68.65 14.04 4.78 0.07 5.15 2.45 1.57 0.03 0.13 0.11 0.20 0.22 0.19 0.17 0.19 0.15

0.56 1.42 1.40 4.23 1.50 2.46 1.86 1.74 0.05 0.04 3.32 4.35 1.35 13.96 5.27 67.99 0.05 2.40 ±r n = 18 Average Av. saprolite

0.21 459 0.05 100 0.12 116 0.10 100

11 25 14 3.2 4.8 10.0 8.7 1.9 1.6 1.39 155 7.95 99.92 223 2.93 65

9 76 10

100– 400 5 62 0.17 0.45

0.03 0.09

0.79 83 3.27 100.01 217

6.3 2.9 6.6 6.8

5 54

400– 5000 53 68 8.8 6.2 1.93 99.92 172 0.10 0.40 1.67 4.49 2.02 2.60 0.05 4.78 13.68 2.74 n = 29 Average Av. gneiss

68.21

Total, Zr, Th, Nb, WIP CIA ER, wt% ppm ppm ppm Ohm m TiO2, P2O5, LoI, wt% wt% wt%

0.1%, respectively. Overall the soil profiles are more evolved than saprolite but still contain primary minerals able to weather. If the mass balance is computed within the soil zone only, 80% of the losses of Si, Al, Fe would be neglected. It thus becomes crucial to assess the weathering across the full depth of the regolith profile.

Chemical weathering rates for landscapes are difficult to quantify because the timescales over which weathering occurs are often unknown. For an eroding landscape where the weathering system is adjusted to hydrobioclimatic conditions it is reasonable to assume that the rate of conversion of rock into saprolite equals the average long-term physical erosion rate, i.e. that the system has reached a steady state. This assumption supposes that the mass of weathered material in storage on the landscape is approximately constant through time (Green et al., 2006). Based on an approach combining landform, vegetation, water balance index, clay mineral and soil studies, the steady state assumption was argued for the landscapes of the rain shadow of the Western Ghaˆts in spite of inevitable fluctuations of erosion rates around median statistical values (Gunnell and Bourgeon, 1997; Gunnell, 2000; Gunnell et al., 2007). Subsequently the erosion rates of the gneissic substratum of the Karnataka Plateau were assessed based on cosmogenic 10Be measurements and steady state assumption (Gunnell et al., 2007). The average erosion rate is 13.6 ± 2.9 mm/kyr (Table 2 from Gunnell et al. (2007)) and consequently the average long-term chemical weathering, i.e. deepening of the weathering front, is of the same order. That supposes an average time span of 1.1 Ma to form 15 m of saprolite at the watershed scale.

Density, SiO2, wt% g/cm3

Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O, wt% wt% wt% wt% wt% wt% wt%

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5.4. Long-term chemical weathering rate and minimum age of the saprolite

Gneissic regolith Av. thickness = 17.2 m

Table 5 Average parent rock, saprolite and soil compositions used in the mass balance calculations. Open-system mass-transport function s and estimated elemental mass flux in mol/ha over the mean sampling depth during saprolite and soil weathering. The percentage of the sum of loss is also indicated. NR: not relevant.

Regolith mass balance in a gneissic watershed, South India

5.5. Consequence of chemical weathering on the alkalinity production potential on the Karnataka Plateau Even if the Mule Hole watershed is representative of only a very narrow bioclimatic transition zone wedged between the comparatively far more extensive humid and semi-arid zones of the rain shadow gradient (Fig. 1) it is worth discussing the potential of alkalinity production of weathering covers according to the regional climatic variability associated with alternating periods of depletion and intensification of the monsoon. Because of low reserves in unweathered base cation-rich primary minerals, we can argue that, whatever the intensity of the monsoon, the deeply depleted lateritic cover of the West end of the gradient will have a limited potential for producing alkalinity. However, in the event of increased mean rainfall over the region, one would assume that both the transition zone and the very extensive semi-arid zone containing a significant stock of unweathered primary minerals would significantly contribute to produce alkalinity and therefore to consume atmospheric CO2. It could be added that the vegetation, at least in the transition zone, would also probably change to evergreen forest instead of moist deciduous, with ecological parameters such as increased biomass and carbon

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reconstructed weathering profile 1

ρbulk 1.5

0

average soil

2

2.5

3

average saprolite

Depth (meters)

2 4 6 8 10 12 14

average parent gneiss

gain

loss -1

-0.8

-0.6

-0.4

16

-0.2

0

0.2 -1

gain

loss -0.8

-0.6

-0.4

-0.2

0

0.2 -1

gain

loss -0.8

-0.6

-0.4

-0.2

0

0.2

0

Depth (meters)

2 4

τ

Si

τ

Mg

τ

Na

τ

τ

τ

Mn

τ

τ

6

Fe

8

Al

10 12 14 16 0 2 4 6 8

τ

K

10 12 14 16 0 2 4 6 8 10

Ca

P

12 14 16

Fig. 11. Estimated s for major elements referenced to Ti in the reconstructed weathering profile developed on gneiss of the Mule Hole watershed. Bulk density profile is also indicated.

storage potential. Quantifying and modeling the contemporary chemical weathering fluxes along this ecocline based on

hydrological and geochemical time series will be the scope of future papers.

Regolith mass balance in a gneissic watershed, South India

6. CONCLUSION By combining investigations of geophysics, mineralogy and geochemistry on the SEW of Mule Hole the following conclusions can be arrived at:  A relationship is found between weathering indices (LoI, WIP, CIA) and electrical resistivity in gneissic weathering profiles, which helps to constraint the ERT profile modeling and to define the most likely limit between protolith and regolith at the watershed scale.  ERT is a suitable method to assess protolith/regolith geometry even in heterogeneous terrains. The average regolith thickness calculated from the 12 ERT profiles is 17.2 m. This result was obtained after correcting routine ERT with an estimate of ERT uncertainty using a synthetic modeling approach. It showed that routine ERT inversion underestimates regolith thickness by 15%. This underestimation is however related to the typical resistivity arrangement encountered in the Mule Hole watershed. For other watersheds, the synthetic modeling approach could lead to a different result.  Saprolitization processes at Mule Hole are limited and lead to an immature material with low porosity and moderate base cation losses. In the incipient stages, biotite and chlorite are broken down leading to the transfer of Mg, Fe and K. Quartz and sericite are stable. Oligoclase moderately weathers as indicated by the Na and Si transfer functions. Nonetheless due to its abundance, Na, Al and Si are the elements that are the most significantly leached away.  Soil processes lead to more mature material with a 90% loss of Na-plagioclase and 100% loss of biotite.  The immature soil and saprolite of the sub-humid zone of the Kabini climatic gradient associated with geomorphologic features have a great potential to produce alkalinity by chemical weathering. Depending on the runoff and therefore climate variability with a more humid gradient (i.e. intensification of the monsoon), the production of alkalinity would increase and consequently increase the atmospheric CO2 consumption.

ACKNOWLEDGMENTS The Kabini river basin is part of the ORE-BVET project (Observatoire de Recherche en Environnement – Bassin Versant Expe´rimentaux Tropicaux, www.orebvet.fr). Apart from the specific support from the French Institute of Research for Development (IRD), the Embassy of France in India and the Indian Institute of Science, our project benefited from funding from IRD and INSU/CNRS (Institut National des Sciences de l’Univers/Centre National de la Recherche Scientifique) through the French programmes ECCO-PNRH (Ecosphe`re Continentale: Processus et Mode´lisation – Programme National Recherche Hydrologique), EC2CO (Ecosphe`re Continentale et Coˆtie`re) and ACIEau. It is also funded by the Indo-French programme IFCPAR (Indo-French Center for the Promotion of Advanced Research W-3000). The multidisciplinary research carried on the Mule Hole watershed began in 2002 under the aegis of the IFCWS (IndoFrench Cell for Water Sciences), joint laboratory IISc/IRD. We

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thank the Karnataka Forest Department and the staff of the Bandipur National Park for all the facilities and support they provided. P. de Parseval (SEM, microprobe), M. Thibaut (XRD), R. Wyns, A. Bost and C. Kumar are thanked for their technical assistance. A special thank to P. Mazzega for his help for the mineralogical inverse problem solutions. We wish to express our sincere gratitude to Y. Gunnell and J.A. West for providing thorough and valuable reviews. M. Novak is thanked for his editorial handling of this paper.

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