Rhenium and osmium isotope and elemental behaviour accompanying laterite formation in the Deccan...

tters 261 (2007) 239–258www.elsevier.com/locate/epsl

Earth and Planetary Science Le

Rhenium and osmium isotope and elemental behaviouraccompanying laterite formation in the Deccan region of India

J. Wimpenny a,⁎, A. Gannoun a,b, K.W. Burton a,c, M. Widdowson a,R.H. James a, S.R. Gíslason d

a Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UKb University of Oxford, Department of Earth Sciences, Parks Road, Oxford, OX1 3PR, UK

c LMTG, Université de Toulouse, CNRS, IRD, OMP, 14 Av. E. Belin, F-31400 Toulouse, Franced Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavik, Iceland

Received 5 November 2006; received in revised form 25 June 2007; accepted 29 June 2007

Available onli

Editor: R.W. Carlson

ne 16 August 2007

Abstract

This study presents major-, trace-element and Re–Os isotope data for two laterite profiles from the Deccan region in India. Thefirst profile from Bidar developed upon Deccan basalt itself, the second from Goa formed on an Archean greywacke. Both profilesshow the typical changes in mineralogy and chemistry accompanying laterite formation; from little altered parent rock at depth, toaltered saprolite (depleted in mobile elements), to a highly altered, Si depleted, Fe-rich laterite capped by an iron-rich crust at thesurface.

Rhenium and Os concentrations show a systematic enrichment with increasing degree of alteration, where the highestconcentrations are found in the Fe-rich laterite crust. The level of Os enrichment is greater than that of Re, and as a consequence187Re/188Os ratios are lower in the laterite than the parent rock. With time this leads to the development of 187Os/188Os ratios that arealso less radiogenic than the parent rock, and this relationship holds irrespective of rock type or age or the time of laterite formation.

The marked enrichment of both Re and Os, and the patterns of change above and below the paleowatertable, indicates that bothelements are highly mobile. High Eh and low pH conditions facilitate Re and Os dissolution, and redistribution occurs viagroundwaters, whereas with increasing pH (higher in the profile) both elements are scavenged out of solution and precipitate eitheras PGE-rich alloys or are strongly partitioned into oxides or oxyhydroxides. Nevertheless, the isotope and elemental data for bothprofiles suggests that at least some of the Re and Os must be externally derived, either from aeolian deposition or from the lateralmovement of solute-laden groundwaters.

Given the high levels of Os in laterites, their extensive geographical coverage of the continents, and their active role in thesurface water cycle, it seems likely that they will exert a significant influence on the continental flux of Os to the oceans, that will,in turn, be sensitive to tectonic or climatic change.© 2007 Elsevier B.V. All rights reserved.

Keywords: rhenium; osmium; laterite formation; basalt weathering

⁎ Corresponding author. Tel.: +44 (0) 1908 655947; fax: +44 (0) 1908 655151.E-mail address: [email protected] (J. Wimpenny).

0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.epsl.2007.06.028

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240 J. Wimpenny et al. / Earth and Planetary Science Letters 261 (2007) 239–258

1. Introduction

The continental crust is severely depleted in manyplatinum group elements (PGE: Ru, Rh, Pd, Os, Ir and Pt)relative to the Earth's core and mantle, and contains lessthan 0.01% of the terrestrial PGE budget (e.g. Esser andTurekian, 1993; Taylor and McLennan, 1995; Wedepohl,1995; Schmidt et al., 1998; Peucker-Ehrenbrink and Jahn,2001). This depletion, in part, reflects the siderophile andchalcophile behaviour of these elements, which meansthat they are highly concentrated in the Earth's metalliccore (Wänke, 1981; Shirey andWalker, 1998), but also, inpart, because some of these elements are compatibleduring silicate melting, and hence are preferentiallyretained in the residual mantle (Allegre and Luck, 1980;Shirey and Walker, 1998).

Osmium (Os) is a platinum group element that isrelated to rhenium (Re) through the beta decay of 187Reto produce 187Os. During partial melting Os behaves as acompatible element and is retained in the residue,whereas Re is moderately incompatible and enters themelt (Hauri and Hart, 1993; Shirey and Walker, 1998).Consequently, the Earth's crust (continental and oceanic)possesses high Re/Os (parent/daughter) ratios anddevelops radiogenic Os isotope compositions overtime, relative to contemporaneous mantle. Under normalcircumstances the Os isotope composition of thecontinental crust is controlled by the Re/Os ratio andage of a given rock type (Esser and Turekian, 1993).However, estimates for the average 187Os/188Os isotopecomposition of the upper continental crust are variable,ranging from 1.4 to 1.9 (from loess (Peucker-Ehrenbrinkand Jahn, 2001) and river sediments (Levasseur et al.,1999), respectively) while rivers themselves yield aglobal mean value of ∼1.4 (Levasseur et al., 1999;Peucker-Ehrenbrink and Ravizza, 2000a).

The behaviour of Re and Os during continentalweathering is unusual because these elements are oftenhighly concentrated in accessory phases or else inparticular rock types. At the mineral scale in silicaterocks, a significant proportion of the Re and Os islocated in phases present at trace levels (accessoryphases) such as sulphide and PGE-alloys (Hart andRavizza, 1996; Burton et al., 1999). It is these phasesthat control the Os isotope composition and elementalabundance of Re and Os in the whole-rock, and oftentheir behaviour during weathering that determines thecomposition of waters and soils. Similarly, certain rocktypes such as organic-rich sediments are highlyconcentrated in Re and Os, possess very high Re/Osratios, and thus with time develop highly radiogenic Osisotope compositions, relative to average continental

crust (Singh et al., 1999; Peucker-Ehrenbrink andHannigan, 2000; Jaffe et al., 2002). Such sedimentsare also highly susceptible to weathering and readilyrelease their radiogenic Os, and this weathering signalmay strongly influence the marine Os isotope compo-sition, despite comprising less than 1% of the continen-tal crust (Peucker-Ehrenbrink and Hannigan, 2000).Notwithstanding, the concentration of Re and Os inaccessory phases and particular rock types, under certaincircumstances both elements may be highly mobileduring weathering, and readily transported in naturalwaters (i.e. ground and river waters).

Laterites result from extreme weathering of thecontinental crust in tropical or sub tropical climates,they are estimated to cover ∼30% of the exposedcontinental crust, and nearly 50% of continental drainageflows through laterite covered terrain (Tardy, 1997).Laterites are commonly over 20 m thick and can havethicknesses of up to 100m (Brimhall et al., 1991). Lateriteprofiles generally show a progression from unalteredbedrock to increasingly altered rock above with an ironrich crust or duricrust at or close to the surface. Lateritesare formed during continuous in situ weathering ofbedrock (Schellmann, 1981; Widdowson and Cox, 1996;Tardy, 1997), which acts to strip away less resistantelements (such as silica, and the mobile elements Na, Mg,Ca) leaving secondary minerals enriched in Fe2O3 andAl2O3. There is also some evidence to suggest that theyare, at least in part, modified by external input andredistribution processes (Schellmann, 1981; Brimhallet al., 1991; Widdowson and Cox, 1996; Kisakureket al., 2004) and that their formation may be initiated bychanging weathering conditions (Dequincey et al., 2002).

It has long been known that laterite soils may beenriched in platinum group elements, located in PGE-richalloys and associated sulphides and iron-oxides (e.g.Bowles, 1986). Early work suggested that these PGE-richminerals were present as resistant phases residual fromweathering of the parent rock (e.g. Cabri and Harris,1975). However, it is now established that under theconditions prevalent in laterite soils (that is, low pH andhigh Eh) some platinum group elements may be taken intosolution, transported and redeposited (Cabri and Harris,1975; Bowles, 1986), consistent with textural evidenceindicating that PGE-minerals in laterites have grown insitu (Mann, 1984;Bowles, 1986). Thus far there have beenfew published Re–Os isotope measurements for laterites.The data that have been obtained indicate a depletion ofReand enrichment of Os in the topsoil part of the profile, andconsequently a relatively unradiogenic Os isotope com-position, relative to the parent rock (Sharma et al., 1998).These results were taken to indicate that the weathering of

241J. Wimpenny et al. / Earth and Planetary Science Letters 261 (2007) 239–258

laterite soils will yield relatively unradiogenic Os whichmay control the composition of dissolved and aeolianmaterial delivered to the oceans (Sharma et al., 1998).

This study presents major-, trace element, and Re–Osisotope and elemental data for two laterite profiles froma similar climatic regime, but having different parent-rock types, from Bidar and Goa in India. These resultsprovide information on the distribution and mobility ofRe and Os during laterite formation and groundwatermovement, the consequences for the Os isotopeevolution of laterites (compared to parent-rock types),and the implications for the behaviour of these elementsduring weathering in tropical and sub-tropical climates.

2. Geological setting and background

2.1. Geological setting

Two laterite profiles from the Deccan region havebeen studied here; the first is from Bidar in central India,

Fig. 1. Map showing the location of the two laterite profiles studied here. Bidaof the Deccan Trap basalts, Goa is on the west coast of India and the profile

and the second is from Goa on the west coast (Fig. 1).Bidar is located near the present-day edge of the DeccanTraps Continental Flood Basalt Province (CFBP). TheDeccan Traps were erupted during a ∼3–4 Maperiod straddling the Cretaceous–Tertiary boundary(e.g. Courtillot et al., 1988; Widdowson et al., 2000),onto a complex Archean–Proterozoic basement com-prising part of the Dharwar craton (Chadwick et al.,2000). Today the Deccan Traps cover an area of over500,000 km2, and have a maximum thickness of ∼2 km(Mitchell and Widdowson, 1991). Each lava package (orformation) has a distinct chemical and isotopic signature,reflecting different stages of the eruptive history(Widdowson and Mitchell, 1992). Of these differentlavas the Ambenali formation is the most volumetric andgeographically widespread (Widdowson et al., 2000),and forms the upper flow units that have beensubsequently laterised at Bidar (Kisakurek et al., 2004).The second profile at Goa developed on a Proterozoicgreywacke that forms part of the Dharwar supergroup.

r is in central India and the profile is located on the Ambenali formationis located on an Archean greywacke of the Dharwar Craton.

Fig. 2. Schematic profiles of the studied laterites at (a) Bidar and (b) Goa. Both profiles show progression from unaltered rock at the baseto increasingly altered rock towards the surface, and both have a hard indurated iron cap at the surface and a distinct paleowatertable horizon. ZonesI–IV illustrate the progressive lithological changes within the laterite. Zone I— unaltered or little altered bedrock, Zone II— high alteration bedrock/laterite (saprolite), Zone III— paleowatertable, Zone IV— Fe rich laterite cap (see text). Includes major element profiles that are characterised by ageneral decrease in Si, and relative enrichment in Fe and Al contents. This pattern of chemical change is interrupted at the paleowatertable where thereis considerable enrichment of Fe, and depletion of both Si and Al.



During the later stages of eruption of the DeccanTraps,northwest drift of India, across the equator (Klootwijk andPeirce, 1979), exposed the Deccan (CFBP) to intensiveweathering leading to the development of deep weather-ing profiles capped by a regional laterite (Widdowson andCox, 1996). At the Bidar site, this major period of lateriteformation was brought to a close between 55 and 45 Ma(Schmidt et al., 1983), when regional uplift raised thelaterite above the watertable (Widdowson, 1997), possi-bly as a result of the collision of India and Asia, therebypreserving this ancient weathering profile. In contrast, thesite at Goa did not experience this uplift, and paleomag-netic data suggests that laterite formation occurred duringthe late Tertiary, ∼20 Ma, and is ongoing today.

Table 1Lithology and mineralogy of the laterite samples from Bidar and Goa and th

Sample Depth Lithology

Bidar (17°54.87′N, 77°32.39′E) (m)BB9 2 Vermiform lateriteBB8 5 Nodular lateriteBB7 6 Base of lateriteBB6 11 Laterite base/top of saprolBB5 13 Saprolized basaltBB4 15 Reddened saproliteBB3 26 Deeply weathered basaltBB2 35 Low alteration basaltBB1 47 Unaltered basalt

Goa (15°28.44′N, 73°52.35′E)SQ14 0 Indurated lateriteSQ13 2.5 Indurated lateriteSQ12 3.5 Indurated lateriteSQ11 7.5 Massive lateriteSQ10 8.5 Semi-indurated laterite,

nodules fusing togetherSQ09 12 Increased nodule density a

sizeSQ08 13.5 Base of nodular laterite,

haematite segregations preSQ07 14 Laterite base/top of altered

greywackeSQ06 15 Top of reddened greywack

soft and well consolidatedSQ05 22.5 Soft weathered greywacke

base of weathered zoneSQ04 25.5 Softer, white weathered

greywackeSQ03 30 Lighter greywacke from st

of weathered zoneSQ02 34 Unaltered greywacke

Ambenali (17°57.10′N, 73°34.58′E) (cm)MPr2a 10 Uniform fine grained ashMPr2b 70 Blocky, lithified, reworkedMPr2c 90 Fine-grained ashMPr2d 140 Weathered flow top mater

The profile at Bidar extends to a depth of∼50mwhereunaltered basalt is located (Fig. 2). In the region of theBidar profile the Ambenali formation represents theuppermost flow units of the Deccan, constructed at thehighest elevations, and not overlain by later eruption.Consequently, any extraneous groundwater influencefrom different lithologies at higher levels can bediscounted. Rather, the huge lateral extent and thicknessof the Ambenali formation suggests that the majority ofelements mobilised and transported by groundwaterswere ultimately derived from the breakdown of thistholeiitic precursor itself. Moreover, the uniform compo-sition of the Ambenali formation offers an ideal substratefor assessing the chemical variations accompanying

e bole samples from Ambenali

Mineralogy

Haematite, goethite, KaoliniteHaematite, goethite, kaoliniteHaematite, goethite, kaolinite

ite Haematite, goethite, kaoliniteHaematite, goethite, kaolinite, magnetiteHaematite, goethite, kaolinite, magnetiteHaematite, goethite, kaolinite, magnetiteAnorthite, augite, illiteAnorthite, augite

Iron, traces of clay and quartzIron, traces of clay and quartzIron, traces of clay and quartzIron 96%, quartz 2%, clay 2%Iron 95%, quartz 2%, clay 3%

nd Iron 50%, quartz 45%, clay 5%

sentQuartz 50%, iron 45%, clay 5%

Quartz 75%, iron 22%, biotite 2%, muscovite 1%

e, Quartz 75%, biotite 12%, feldspar 6%, iron 5%,muscovite 2%

, Quartz 74%, biotite 20%, feldspar 2%, muscovite 2%,chlorite 1%, opaques b1%Quartz 80%, biotite 8%, feldspar 8%, chlorite/muscovite 3%,opaque 1%

art Quartz 75%, biotite 12%, feldspar 8%, opaque 2%, other 3%

Quartz 75%, biotite 15%, feldspar 5%, opaque 2%, other 3%

ash

ial

Table 2Selected major, trace element and Re–Os elemental and isotopic data for Bidar and Goa laterite profiles

Sample 187Os/188Osa 187Re/188Os Re Os SiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O TiO2 LOI Rb Sr Zr Th U V Cr Ni Cu

(ppt) (ppt) (wt.%)

(wt.%)

(wt.%)

(wt.%)

(wt.%)

(wt.%)

(wt.%)

(wt.%)

(wt.%)

(wt.%)

(ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)

Bidar lateriteBB9 0.3313±0.0002 19.7 3405 863 9.59 9.85 77.53 0.23 0.16 0.04 D.L. 0.03 2.03 7.31 1.3 9.7 283 6.9 1.6 2986 692 40 581Dupl.b 0.3375±0.0013 20.0 3540 887

BB8 0.4295±0.0002 1.9 523 1409 31.35 27.22 38.37 0.07 0.10 0.00 D.L. 0.07 2.33 11.1 3.6 7.1 211 6.0 1.8 967 737 106 183BB7 0.4634±0.0003 59.4 10521 901 36.68 31.30 27.70 0.33 0.26 0.05 D.L. 0.03 2.44 11.4 1.3 15 184 3.6 1.2 671 257 63 211BB6 0.4510±0.0007 81.7 1395 86.7 6.12 6.97 84.81 0.03 0.14 0.00 D.L. 0.00 1.40 11.1 0.2 41 77 0.9 4.5 975 766 243 852BB5 0.4382±0.0036 61.8 6193 507 30.61 25.83 36.95 0.06 0.23 0.07 D.L. 0.02 5.76 11.1 0.1 3.5 282 1.4 2.1 1846 250 72 452BB4 0.4695±0.0003 58.1 5613 491 38.78 31.95 21.64 0.06 0.38 1.91 D.L. 0.01 4.78 12.4 0.3 23 246 2.0 1.1 643 213 58 173Dupl.b 0.4725±0.0004 53.0 5731 550BB3 0.4964±0.0006 54.2 883 83.1 38.59 31.54 24.10 0.11 0.40 0.19 D.L. 0.02 5.11 11.7 1.0 13 246 1.9 1.0 687 201 287 394BB2 1.1863±0.0028 2286 2104 4.27 50.06 14.15 12.63 0.22 5.99 11.45 2.78 0.25 2.29 0.72 5.6 222 136 1.1 0.25 375 139 138 185BB1 0.9805±0.0028 896 719 4.35 48.90 13.72 13.40 0.19 6.93 10.99 2.46 0.16 2.16 0.60 1.1 210 128 1.0 0.20 371 148 98 177Dupl.b 0.9876±0.0026 863 736 4.62

‘Bole’ samplesMPr 2a 0.1712±0.0006 37.4 273 35.3 57.78 16.10 16.11 0.10 6.41 2.30 0.04 0.07 2.19 4 69 119 1 1.0 177 131 111 558MPr 2b 0.1713±0.0003 23 97 20.1 57.92 15.63 15.41 0.11 7.17 2.33 0.08 0.37 2.08 24 69 108 D.L. D.L. 217 180 91 394MPr 2c 0.1572±0.0004 119 1078 43.7 57.96 15.36 15.31 0.14 7.18 2.40 0.06 0.34 2.07 22 72 106 D.L. D.L. 191 160 97 179MPr 2d 945 50.59 18.61 21.53 0.31 4.61 2.07 0.06 0.14 3.41 11 62 182 2 1.0 351 110 105 168

Goa lateriteSQ14 0.4771±0.0004 4.44 196 224 13.63 34.15 48.48 0.04 0.19 0.06 0.13 0.723 2.036 16.4 24 38 310 20.4 13.2 765 924 53 41SQ13 0.5479±0.0003 11.2 1071 492 17.50 34.57 44.95 0.04 0.19 0.06 0.21 0.744 1.902 15.4 24 35 296 17.4 5.4 837 1156 52 31Dupl.b 0.5565±0.0006 10.1 983 502

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SQ12 0.5636±0.0006 1.80 142 403 19.62 40.89 36.42 0.06 0.15 0.06 0.12 0.645 2.278 17.2 20 38 339 23.6 9.2 626 908 58 51SQ11 0.8181±0.0012 17.8 393 116 15.20 13.58 68.33 0.57 0.13 0.05 0.07 0.511 0.664 12.1 15 36 99 11.2 7.0 60 153 119 33

Dupl.b 0.7803±0.0011 20.0 428 113SQ10 0.5335±0.0004 2.76 175 323 17.21 30.52 49.64 0.02 0.10 0.05 0.64 0.276 1.842 12.6 7.0 19 257 21.2 4.2 780 839 59 42SQ9 1.7807±0.0017 128 1067 48.5 67.86 18.95 10.78 0.02 0.30 0.05 0.07 1.345 0.730 7.28 26 7.2 219 21.4 5.8 157 149 41 36SQ8 1.6051±0.0002 65.4 610 53.6 67.46 19.62 10.25 0.02 0.32 0.05 0.08 1.428 0.854 7.23 30 9.1 261 22.2 7.2 151 130 42 33

Dupl.b 1.6350±0.0022 69.9 642 53.6SQ7 2.0880±0.0014 23.1 155 40.4 71.19 18.62 7.64 0.06 0.51 0.05 0.09 1.760 0.756 6.59 37 6.5 265 25.1 6.0 124 91 67 30SQ6 4.1502±0.0028 104 559 39.2 75.27 13.43 3.38 0.05 1.62 0.43 3.51 2.445 0.431 1.80 73 109 134 14.2 4.4 65 66 22 37SQ5 4.4108±0.0021 141 801 42.1 64.21 16.38 7.63 0.17 4.13 0.43 2.76 3.954 0.483 2.52 132 81 138 14.2 4.4 88 81 41 65SQ4 2.5774±0.0021 110 519 29.6 79.50 11.41 2.43 0.07 1.29 0.56 3.54 1.632 0.320 1.29 43 100 92 6.7 2.7 44 44 15 38SQ3 5.1968±0.0054 82.0 458 44.4 69.67 13.90 5.58 0.11 2.88 0.66 3.24 3.200 0.577 1.52 100 108 228 18.7 7.5 98 107 43 55SQ2 4.6592±0.0067 123 708 43.4 67.77 14.65 6.08 0.13 3.07 0.95 3.31 3.684 0.562 1.34 118 114 194 15.6 5.7 92 108 38 59

Dupl.b 4.0495±0.0030 139 747 39.4

Dharwar basementIND108 0.7426±0.0011 4.20 71.61 14.40 2.49 0.04 0.64 1.79 3.88 4.43 0.40 0.42 152 233 272 14.8 2.3 30 12 4.7 10IND114 1.7662±0.0070 4.47 74.07 13.92 1.42 0.02 0.29 1.42 3.75 4.54 0.22 0.36 143 220 141 17.4 0.7 11 6.9 2.9 6IND120 1.6692±0.0039 2.40 73.49 14.27 1.15 0.03 0.47 1.65 4.19 3.72 0.13 0.75 94 192 84 9.9 0.2 14 6.8 3.5 3Bhima

Lst0.1752±0.0008 11.5 74 31.0 16.64 2.33 1.34 0.06 0.95 42.29 0.39 0.34 0.34 34.6 17 268 28 4.6 0.1 13 6.9 7.4 8.1

BIF 1.3449±0.0037 786 2825 19.6 55.05 0.18 43.98 0.01 0.09 0.03 0.01 0.01 0.02 0.72 1.4 0.3 5.1 1.6 D.L. D.L. 25 D.L. 21

aErrors shown are 2σmean; 187Os/188Os ratios normalised using 192Os/188Os=3.08271 and corrected using measured 18O/16O and 17O/16O of 0.002047 and 00037, respectively. Given isotope ratiosare blank corrected.bDuplicate results for the same laterite powder split using the high pressure high temperature microwave digestion (HP–HT).D.L.: samples below detection limit.

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laterite formation, because any changes in chemistry areunlikely to be a result of parent-rock heterogeneity. Inaddition, the intracontinental location precludes anymarine influence on the chemistry, through aerosols orprecipitation, but does not rule out the presence ofcontinental derived aeolian material. The profile at Goa is∼35 m thick and developed on a Proterozoic greywackethat forms part of the Dharwar supergroup. In contrast tothe site at Bidar, the relatively low topography of thelaterite and the heterogeneity of the Dharwar craton raisesthe possibility of lateral input of groundwater potentiallysourced by a range of lithologies. Moreover, unlike theAmbenali formation, the parent greywacke shows somechemical variation both laterally and vertically.

2.2. Sample description

The two profiles are shown in detail in Fig. 2 andsample depths and descriptions of their lithology andmineralogy given in Table 1 (previously reported inMason, 1999; Borger and Widdowson, 2001; Kisakureket al., 2004). Both show progression from unaltered rockat the base to increasingly altered rock towards thesurface, and both have a hard indurated iron cap at thesurface (“cuirasse”) together with a distinct paleowater-table horizon a few meters below the cap. The lateriteprofiles can be divided into zones based on the level ofalteration reflected in their physical properties, mineral-ogy and elemental behaviour (Fig. 2). Zone I is unalteredor slightly altered bedrock. Zone II is altered bedrockincreasingly so up the profile. At Bidar the crystallinegroundmass of the basalt is initially replaced by illite andkaolinite clay, similarly at Goa primary quartz, biotite andfeldspar are also replaced by clays in this zone. Movingup, the primary mineralogy is lost, and the groundmassbecomes mottled due to the development of Fe and Aloxides and oxyhydroxides. Zone III is the paleowatertablehorizon, andZone IVis the Fe-rich indurated laterite crust.Within Zone IV no remnant of the protolith texture orprimary mineralogy remains. The high haematite contentof the Zone IV samples indicates that well drainedoxidising conditions prevailed.

There is no petrographic evidence for significantinput of aeolian material to either profile, supporting theview that the laterite profiles across the Deccan regionwere predominantly formed by in situ alteration of theunderlying bedrock. However, for the profile at Bidar,Sr, Nd and Li isotope data have been taken to indicateaeolian input of weathered continental material to the topof the profile (Mason et al., 2000; Kisakurek et al., 2004).Consequently, a number of potential sources of localaeolian material have been examined here including

basement granite, banded iron formation (BIF) and alimestone all from the Dharwar craton.

Finally, a sequence of Deccan ‘bole’ samples havealso been analysed. These are highly weathered basalticmaterial or basaltic ash that developed betweensuccessive lava flows (i.e. during a hiatus in eruption).The bole samples studied are from a locality close to thevillage of Ambenali, and comprise a ∼1.4 m profilesandwiched between two massive lava flows. The baseof the profile is marked by weathered flow top materialthat is overlain by fine-grained ash which is increasinglyweathered towards the top of the bole horizon. Thesebole samples serve to illustrate the behaviour of Re andOs in weathered basalt/ash that has not had sufficienttime to develop into a laterite.

3. Methods

3.1. Sampling and sample preparation

Samples were taken from each of the profiles (samplelocations shown in Fig. 2) chosen as representative ofeach key horizon, or else taken immediately above orbelow (i.e. bracketing) levels where distinct texturalchanges are observed. Because theweathering profiles arehighly heterogeneous large samples (between 1 and 3 kg)were taken, and these large samples were then homo-genised by milling in agate.

3.2. Major and trace element analysis

Bulk materials from the weathering profiles andsurrounding basement rocks were analysed by XRF formajor elements and ICP-MS for trace elements. Fulldetails of the analytical procedures are given elsewhere(Kisakurek et al., 2004; Kisakurek et al., 2005).

3.3. Re–Os isotopic analysis

Rhenium and Os were separated from laterite and rocksamples using techniques previously described (Bircket al., 1997) and analysed using negative thermalionisation mass spectrometry (N-TIMS). Sample powderswere spiked with a mixed 185Re–190Os spike solution anddissolved using amixture of HF and teflon distilledHBr at140 °C for 72 h. Osmium samples were oxidised by CrO3

(in HNO3) and extracted into liquid Br2. The Br2 solutionwas then dried down and the residue purified bymicrodistillation. An aliquot for Re analysis was dissolvedin 2M HNO3 and extracted using iso-amylol. All sampleswere loaded onto high purity Pt ribbon for analysis. Re andOs samples were analysed as OsO3

− and ReO4− oxides by


negative TIMS. The average total procedural blank for theBidar samples was 0.41 pg for Os and 4.75 pg for Re; theOs isotopic composition of the blank (187Os/188Os) was0.179±0.005.Corresponding blanks forGoawere 0.22 pgfor Os and 4.42 pg for Re; the Os isotopic composition ofthe blank was 0.186±0.007. There is a possibility that lowtemperature acid digestion may not recover all the Re andOs present in the samples, particularly when PGE-richphases are present (Meisel et al., 2003). Thus, in order toensure that both Re and Os are fully recovered from thesamples studied here, high pressure and high temperaturemicrowave sample decomposition (Multiwave 3000,Anton Paar®) was also used. The duplicated sampleswere chosen so as to be representative of the entire rangeobserved for both profiles. Approximately 0.2 g of samplematerial was spiked and digested using HF+HCl (3:3 ml)acid mixture in a fluoropolymer vessel at 240 °C and60 bar for 2 h. The solutions were then transferred into aPFA pressure vial, evaporated to dryness and redissolvedwith HF-HBr (2:2 ml) at 150 °C for 48 h. The subsequentstep of sample oxidation and Os extraction was similar to

Fig. 3. Trace element profiles for (a) Bidar and (b) Goa, showing that the immocompared to the parent rock.

that described above. The average total procedural blankwas 0.083 pg forOswith 187Os/188Os of 0.250±0.021 and3.6 pg for Re.

4. Results

4.1. Major and trace element data

Major and trace element data for both profiles aregiven in Table 2. Above the zone of unaltered bedrock(Zone I) both profiles show a decrease in the concen-tration of the more mobile elements (e.g. Ca, Na, K andMg) relative to the parent rock, towards the surface(Zones II, III and IV). The decrease in mobile elements isaccompanied by a decrease in SiO2, and relativeenrichments in Fe and Al contents (Fig. 2). This patternof chemical change is interrupted at the level of thepaleowatertable (~11–14 m depth at Bidar, ∼7–8.5 mdepth at Goa) where there is considerable relativeenrichment of Fe, and a relative depletion of both Siand Al.

bile transition elements such as V, Cr and Sc are enriched in the laterite

Fig. 4. Re and Os concentration profiles for (a) Bidar and (b) Goa. The parent rocks have relatively low Re and Os concentrations. However, bothprofiles show a significant increase in Re and Os concentration with increasing alteration, with the highest values being seen in the Fe-rich laterite cap.For both profiles enrichment of Os is always greater than Re, consistent with previous work. In addition, both profiles show a decrease in theconcentration of Re and Os at the level of the paleowatertable.


The variation of trace elements with depth is alsosimilar for both profiles. The concentration of mobiletrace elements, such as Sr and Rb, is highest in theunaltered bedrock and decreases as the rock becomesmore altered (and permeability increases) (Table 2). AtGoa there is a sharp change in permeability at 15 m depththat is marked by a line of seepage and loss of the mobileelements. Relatively immobile transition elements such asCu, V, Sc and Cr are enriched in the upper half of thelaterite profile, and have lowest concentrations in theunaltered bedrock (Fig. 3).

4.2. Re–Os elemental data

The concentrations of Os and Re at Bidar showsignificant variation with depth (Fig. 4). Osmiumconcentrations increase from 4 ppt at 47 m depth to1410 ppt at 5 m depth, a 300-fold increase. While Reconcentrations are higher than those of Os, increasingfrom 0.7 ppb at 47 m depth to 10 ppb at 6 m depth, thelevel of enrichment is much lower (some 14-fold). SimilarRe and Os variations are observed at Goa. Osmium

concentrations increase from 43 ppt at 34 m depth to492 ppt at 2.5 m depth, an enrichment factor of N11.Rhenium has higher concentrations than Os, increasingfrom 0.7 ppb at 34 m depth to 1 ppb at 2.5 m depth, anenrichment factor of 1.4. The pattern of increasing Re andOs concentration towards the top of the profile isinterrupted by a sharp decrease in the concentration ofboth elements at the level of the paleowatertable.Duplicate measurements made using microwave diges-tion, indicate little difference in Re and Os concentration(10% or less) from those obtained by HF-HBr dissolution(Table 2; Fig. 4). In summary, both profiles show anenrichment in Re and Os, and both show a depletion ofthese elements at the level of the watertable. However, thelevel of enrichment (relative to the parent rock) is an orderof magnitude greater in the profile at Bidar compared tothat at Goa.

4.3. Re–Os isotope data

For both profiles parent-rock 187Re/188Os ratios aregreater than those of the overlying laterite and

Fig. 5. 187Re/188Os and 187Os/188Os ratios shown against depth at (a) Bidar and (b) Goa. The 187Re/188Os and ratios of the laterite are, with oneexception, always lower than the those of the parent rock. The measured 187Os/188Os isotope ratios are also always less radiogenic than the parent rock(see text).


187Os/188Os ratios are also in general more radiogenic;the least radiogenic ratios being found at the surface. AtBidar, 187Re/188Os drops from 2286 at 35 m depth to 2 at

5 m depth. 187Os/188Os ratios show a similar pattern,falling from 0.98 at 45 m depth to 0.33 at 2 m. Bothisotope ratios shift to lower values where the Os

Fig. 6. Initial 187Os/188Os ratios against Os concentration (shown as 1/[188Os]) for the latrite and parent rock at Bidar (open symbols) and previouslypublished data for the Deccan basalts (filled symbols). For the laterite samples there is no significant change in 187Os/188Os composition over the 20Mabetween basalt eruption and the end of laterite formation. However the parent basalt (and other Deccan basalts) rapidly evolve to radiogenic Os isotopevalues and may have possessed an appropriate composition at 45 Ma to source the laterite, albeit that the laterite is significantly enriched in Os. Initialratios calculated using : (187Os/188Os)i=(

187Os/188Os)a− [(187Re/188Os)a×e(λt−1)] where λ is the decay constant of Os (taken as 1.66×10−11 yr−1).


concentration starts to increase (between 35 and 26 mdepth). A similar pattern is observed in the Goa profile;both 187Re/188Os and 187Os/188Os ratios shift to lowervalues towards the surface. The parent rock has a187Re/188Os ratio of ∼100, and a 187Os/188Os ratio of∼4.5; these values are far more radiogenic than those ofDeccan basalt. Both ratios fall as the Os concentrationstarts to rise at 12 m depth. The 187Re/188Os ratio at thesurface is 4.4 and the 187Os/188Os ratio is 0.48, similar to

Fig. 7. Initial 187Os/188Os ratios against Os concentration (shown as 1/[188Ocompositions have been calculated for 5, 10 and 15 Ma of radiogenic gro187Os/188Os ratios of the parent greywacke over these time intervals, and its cindicating that there must have been some external source of Os.

the values recorded at Bidar. Duplicate measurements,given in Table 2 and also shown in Fig. 5, show aconsistent reproducibility to within 5% for 187Os/188Osratio from both dissolution techniques.

4.4. Basement lithologies and bole samples

The three granite samples analysed have Os concen-trations of∼4 ppt; similar to the unaltered basalt at Bidar

s]) for the latrite and parent rock at Goa. The have initial Os isotopewth of 187Os. In contrast, to Bidar there is no significant change inomposition is too radiogenic to exclusively source the laterite material,


but far lower than the laterites at both Bidar and Goa. The187Os/188Os isotope compositions of the granites rangefrom 0.74 to 1.77, and are far more radiogenic than thesurface compositions of either laterite profile. The BIFand limestone samples from the Dharwar craton have Osconcentrations of between 20 and 31 ppt, an order ofmagnitude lower than the highest concentrations recordedat Goa and two orders of magnitude lower than recordedat Bidar. The 187Os/188Os isotope ratio of the Bhimalimestone is relatively unradiogenic (0.175) while the187Os/188Os ratio of the BIF is relatively radiogenic(1.345). The basalt bole samples have Os concentrationsbetween 20 and 44 ppt, and 187Os/188Os ratios of between0.157 and 0.171. The concentration of Re is variableranging from 97 to 1080 ppt, and consequently the187Re/188Os ratio also shows wide variation from 23 to119.

5. Discussion

5.1. Elemental variations

The variation in the chemical composition of bothprofiles is very similar, and consistent with previousstudies of laterite formation on comparable protoliths(e.g. Brimhall et al., 1991; Widdowson and Cox, 1996;Tardy, 1997). The data indicate loss of the more mobileelements (e.g. Ca, Na, Mg, K, Rb and Sr) in the earlierstages of weathering, followed by a decrease in silicacontent, facilitated initially by the breakdown of theprimary mineralogy, and subsequently by the breakdownof neo-formed clay minerals. The depletion in mobileelements is accompanied by a concomitant increase inthe concentration of less mobile elements within thedeveloping laterite profile, in particular Fe, Al and Ti,typically considered as residual. Superimposed on thispattern of enrichment and depletion is element behaviourat the watertable, since many elements are controlled bythe change in redox conditions at this level. For example,iron can exist as reduced Fe2+ that is mobile or oxidisedFe3+ that is immobile. The paleowatertable marks achange in the redox conditions from sub-oxic to oxic sothat Fe2+ is oxidised to immobile Fe3+ that is then de-posited as oxides or oxyhyroxides.

At both Bidar and Goa high Re and Os concentrationsoccur in the upper parts of the profile, coinciding withsignificant Fe2O3 enrichment and SiO2 depletion. En-richment of trace metals, in particular the rare earthelements (REE) in heavily weathered and lateritized soilshas been widely recognised (Walter et al., 1995; Koppiet al., 1996; Nesbitt and Markovics, 1997; Braun et al.,1998). The REE's form mobile complexes with phos-

phates, fluorides and carbonates (Walter et al., 1995), canbe mobilised by organic matter (Braun et al., 1998) andcan be trapped by Fe- and Mn-oxides (Walter et al., 1995;Koppi et al., 1996). Significant PGE enrichment inlateritic soils has also been documented (Mann, 1984;Bowles, 1986; Roquin et al., 1990; Bowles et al., 1994;Eliopoulos and Economou-Eliopoulos, 2000; Brownet al., 2003) and attributed to supergene enrichment andco-precipitation of PGE-rich alloys with iron andmanganese oxides. Lateritic soils are considered todevelop in a high Eh (+200–+800 mV) and low pH(2–5) environment (Mann, 1984; Bowles, 1986), andthese conditions are thought to arise because cations suchas Fe2+ are removed from solution under increasinglyoxidising conditions (Mann, 1984). The release of protonsinto solution causes pH to decrease; and where Fe2+

concentrations are high, such as just below thewater table,pH can be as low as 2 (Bowles, 1986). Under theseconditions the PGEs become mobile by formingcomplexes with species such as chlorides, oxides andhydroxides as well as with organic matter such as oxalatesand humic acid (Bowles, 1986; Mountain and Wood,1988; Bowles et al., 1994; Oberthur et al., 2003).However, not all of the PGEs have the same mobility:for example Pd is considered to be more mobile than Pt(Mountain andWood, 1988; Traore et al., 2006) andmorereadily enters the aqueous phase in solution (Bowles et al.,1994), whilst available data suggests that Os is amongstthe most soluble (Bowles, 1986). The PGEs are thenprogressively removed from solution as conditionsbecome increasingly oxidising and pH increases towardsthe surface. Here there are high concentrations of poorlycrystalline secondary phases such as Mn- and Fe-oxyhydroxides that act as scavengers of metal ions insolution (Kuhnel, 1987). This secondary mineral crystal-lisation accounts for the high PGE, Au and Ag concen-trations in a number of weathering profiles (Bowles,1986; Mann, 1984; Roquin et al., 1990; Bowles et al.,1994; Brown et al., 2003) and the same processes can alsoaccount for the high Re and Os concentrations seen in thelaterites studied here. That is, high Eh and low pHconditions cause Re andOs dissolution, and redistributionoccurs via groundwaters. Subsequently with increasingpH in the upper part of the laterite profile, both elementsare scavenged out of solution and precipitate either asPGE-rich alloys or are strongly partitioned into oxides oroxyhydroxides.

The variation in Re and Os, in particular those seen atthe level of the paleowatertable (Fig. 4, Zone III)indicates that they are mobile. The extent of this mobilitycan be assessed by comparison with an ‘immobile’ indexelement. Elements such as Ti, Th and Zr are considered


to be relatively immobile in surface weathering environ-ments because of their relatively high field strength andlow solubility in water, for this reason they are often usedin the study of weathering processes (Brimhall andDietrich, 1987; Brown et al., 2003). Nevertheless itshould be remembered that they are not totally immobileand can be mobilised in extreme weathering conditions,or in the presence of high levels of organic matter (Olivaet al., 1999; Hodson, 2002). In this study Zr has beenused as the index element because its concentrationshows the least variation over the two profiles, whencompared to the concentration in the parent rock. Re andOs concentrations normalised to Zr and Ti show littledifference in patterns of enrichment or depletion(Supplementary Fig. 1), where enrichment or depletioncan be defined by:

% enrichment = depletion ¼ 100� Rn � Ri

Ri:

In this equation R is the ratio of the elementconcentration to the Zr concentration and the subscriptsi and n indicate the bedrock and weathered horizonrespectively. At Bidar there are substantial enrichments ofboth Os and Re. The highest Os enrichment occurs at 5 mdepth where the laterite is∼19,000% enriched relative tothe parent rock, and Re is enriched by 920%. At Goa mostsamples are enriched in Os relative to the bedrock, thesample at 2–5 m depth being enriched by 630%.Concentrations of Re are very variable; some horizonscontain higher Re and some lower Re than the parent rock.It is important to note that the Zr content at the level of thewatertable is ∼40% lower than the parent basalt,suggesting that this element may also have beenmobilised. The net effect of this relative mobilisation ofZr will be to enhance the relative depletion of Re and Os.However, throughout the remainder of the profile thetrends revealed by Zr normalisation remain unaffected.

The pattern of increasing enrichment in Os towards thelaterite surface has also been observed in a laterite with agranodiorite parent rock from Burkina Faso, west Africa(Sharma et al., 1998) but these authors report low levels ofRe at the surface (lower even than the parent rock) whichis in contrast to the profiles at Bidar and Goa.

5.2. Re–Os isotope variations

Elemental enrichment of Os relative to Re in bothprofiles has an impact on the measured parent/daughter187Re/188Os ratios. Both profiles show a decrease in187Re/188Os with increasing alteration. Because 187Os isproduced from the β-decay of 187Re it can be anticipatedthat with time the altered laterite samples will develop a

less radiogenic 187Os/188Os isotope composition than theparent rock. Accordingly both profiles show 187Os/188Osvalues that are significantly less radiogenic than the parentrock with increasing alteration (i.e. up the profile). As willbe seen for the profile at Goa this is not simply due toin situ decay of 187Re but reflects the sources anddistribution of Re and Os. Therefore, for the profilesstudied here and elsewhere (Sharma et al., 1998) the dataindicate that laterites possess a less radiogenic187Os/188Os isotope composition that the parent rockfrom which they were formed, and this relationship ap-pears to hold irrespective of rock type or age, or the timingof laterite formation.

Figs. 4 and 5 show that any differences between resultsgained from the two dissolution methods used in thisstudy are very small. Microwave dissolution gave slightlyhigher Re concentrations for samples from both lateriteprofiles and higher Os concentrations at Bidar which mayindicate the efficiency of this method. However thesedifferences have little consequence for the pattern ofelement distribution through the profiles, and do not affectthe isotope ratios.

5.3. Basalt bole samples

The basaltic red ‘boles’ are relatively short livedweathering profiles that developed between successivelava flows in the uppermost lava sequences of the Deccan.These boles have been subject to intense tropical weath-ering (hence their red colour), but laterite formation sensu-stricto did not occur (Schellmann, 1981), and thus theyserve to distinguish the affects of element transport in a lowpH, high Eh environment from straightforward basaltweathering at higher pH conditions. TheOs concentrationsin the boles are higher than the parent basalt at Bidar,but within the range observed for other Deccan basalts.However, the Os concentrations remain much lower thanthose observed in the laterite at Bidar. Rhenium concentra-tions are high at the base of the bole profile, similar toconcentrations in unaltered basalt, but Re contents declineat higher levels where they are lower (in most cases) thanthe laterite at Bidar. The low Re and relatively low Osconcentration results in generally low 187Re/188Os ratiosand relatively unradiogenic 187Os/188Os isotope composi-tions, but all are within the range observed for unalteredDeccan basalt (Allegre et al., 1999).

Thus, overall the bole samples appear to haveexperienced Re loss, with little or no Os enrichment,relative to the unweathered parent basalt. However, theydo not show the either the extreme or differentialenrichment of OsNRe seen in the laterite samples. This ispresumably because there has been little external input of


Re or Os to the profile, and pH and Eh conditions havenot resulted in significant mobility of these elements.

5.4. Origin of elemental and isotope variations

5.4.1. In situ parent-rock weatheringThere are a number of different processes that may

have been responsible for the high levels of Re and Osfound in the laterites studied here. It is possible that thisenrichment is simply due to in situ weathering of theparent rock where Re and Os are located in weatheringresistant minerals retained in the residue.

In order to estimate the amount of parent rock neededto supply the Re and Os found in the laterite, the Os (andRe) content of each horizon (Osn) is weighted by thehorizon thickness (hn) and its absolute density (ρn).

Os ¼Xn¼x

n¼1

½Os�nhnqn:

Using this equation the total Os content of the Bidarprofile is 3×10−5 kg/m2 and at Goa it is 7.6×10−6 kg/m2.The total mass of Os that can be supplied from parentbasalt to the Bidar profile is 5.8×10−7 kg/m2, which is1.9% of the estimated Os. Using the same method for Rethe precursor basalt can contribute 35% of the total Re,emphasising the greater level of Os enrichment in theprofile. At Goa, the parent greywacke can supply 45% ofthe Os and all of the Re measured in the profile.

The same approach can be used to calculate themass ofRe or Os in 1 m of bedrock to give an estimate of thenumber of metres of parent rock that would have to beweathered and removed in order to concentrate theestimated masses of Re and Os. The high Os enrichmentin the laterite at Bidar demands the weathering anderosion of some 2600m of parent basalt to account for thelaterite Os content; while 42 m of vertical loss is requiredat Goa. It has been suggested that some two thirds of theinitial volume of the Deccan basalt may have beenremoved by weathering and erosion over the past 65 Ma(Courtillot et al., 1988). However, the Ambenali forma-tion at Bidar represents the uppermost flow units of theDeccan eruptions (Widdowson et al., 2000) and suchvolumes of basalt loss seem unlikely at this locality.Therefore, the elemental data appears to suggest that adirect, vertical, contribution byweathering and removal ofthe parent rock, concentrating Re and Os in the residue, isunlikely to supply the Re and Os at Bidar. However, sucha process could potentially providemuch of the Re andOsin the laterite at Goa.

The Os isotope data can also be used to evaluatewhether the relatively unradiogenic Os at both sites could

be derived from the parent rock. Laterite formation atBidar is thought to have commenced at or soon after thefinal Deccan eruptions at ∼65 Ma, and ceased sometimebetween 55 and 45 Ma, when the profile was elevatedabove the level of the watertable (Widdowson, 1997). Atfirst sight it would appear that laterite and parent rock atthis locality possess very different 187Os/188Os isotopecompositions. However, the laterite has a much lowerparent/daughter 187Re/188Os ratio and thus growth ofradiogenic 187Os from the decay of 187Re may have beenvery different both over the 10–20 Ma of laterite for-mation, and up to the present-day. Assuming that there hasbeen no recent mobility of Re or Os calculations indicatethat the Os isotope compositions of the laterites havenot changed significantly over the past 65 Ma. Initial187Os/188Os isotope ratios have also been calculated forthe parent basalt to take into account the amount of 187Osgrowth since between 65 and 45 Ma, and are shownagainst Os concentration in Fig. 6. The 187Os/188Os of theparent basalt was relatively unradiogenic at the time oferuption at 65 Ma (187Os/188Os≈0.128; this study andAllegre et al. (1999)). However, over the timescale oflaterite formation the basalts will have evolved to muchmore radiogenic values such that the Os isotopecomposition of the parent basalt at 55 to 45 Ma couldhave sourced the laterite at that time (Fig. 6).

At Goa the parent greywacke again possesses muchmore radiogenic 187Os/188Os isotope compositions thanthe overlying laterite and higher 187Re/188Os ratios.Laterite formation at Goa occurred during the late Tertiary(between 20 and 30 Ma) and is ongoing today, providinga 30 Ma maximum timescale for the radiogenic growthof 187Os. However, calculations indicate that even iflaterite formation started at 30 Ma the greywacke willhave been only slightly less radiogenic than at the present-day, and there will have been little resolvable change inthe composition of the laterite (Fig. 7). Therefore thesecalculations indicate that the unradiogenic Os isotopecompositions of the laterite at Goa cannot have beensimply acquired from the parent greywacke.

Overall, these data indicate that there must have beensome form of external input of Re and Os to both profiles.While the Os isotope composition of the laterites at Bidarcan be explained by ageing of the parent rock, the level ofOs enrichment cannot be attributed to concentration(in residual minerals) through massive vertical weatheringand erosion. Whereas at Goa, the level of Os enrichment isless, and could conceivably be supplied by the parentgreywacke. However, the parent greywacke at Goa pos-sesses Os that is too radiogenic to account for theunradiogenicOs isotope compositions seen in the overlyinglaterites.


5.4.2. Aeolian inputPrevious isotope studies of the laterite at Bidar have

concluded that some degree of aeolian input to the top ofthe soil profile must have occurred. With increasinglaterite alteration the 87Sr/86Sr isotope compositionbecomes more radiogenic, shifting from 0.704249 to0.710099 while 143Nd/144Nd becomes less radiogenic,shifting from 0.512818 to 0.512437. This variation farexceeds the natural variation of the Ambenali basalt, andis too extreme to be explained by fractionation induced byweathering over the given timescales. Instead theseisotope shifts can be explained by aeolian input of ancientcontinental material such as the neighbouring DravidianShield (Mason et al., 2000). In addition, Li isotope datasuggests that this continental material must have beenweathered prior to aeolian transport and deposition(Kisakurek et al., 2004). Redistribution of aeolianmaterial input to the top of the profile could occur viadissolution and transport in groundwater and thesubsequent precipitation of secondary minerals.

The Archean to Proterozoic Dharwar craton is onepossible source of aeolian material. For the locality atBidar, during the main period of laterite formation Indiawas located to the south of the equator, and so southeasttrade winds could have carried dust from this terrain(Klootwijk and Peirce, 1979). A number of potentialsource rocks have been studied from the Dharwar cratonincluding granites, banded iron formation and limestone.However, while some rock types (or mixture of rocktypes) possess the appropriate Os isotope compositions,none have a sufficiently high Os concentration to accountfor those measured in the laterite. Further afield anotherpotential source is loess, and samples fromChina, Europeand S. America give an average 187Os/188Os compositionof 1.05±0.23, and Os and Re concentrations of 31 pg/gand 198 pg/g respectively (Peucker-Ehrenbrink andJahn, 2001). If it is assumed that the present day dustflux of 1–10 g/m2/yr (Duce et al., 1991) is similar to thatduring the ∼15–20 Ma period over which both profilesdeveloped then it is possible to estimate the potential massof aeolian derived Os. If it is further assumed that all thisOs dissolved and entered the laterite then such aeoliansources can account for amaximum of 20% of the total Osat Bidar and 81% of the total Os at Goa. This suggests thataeolian material with an average continental crustal Oscomposition and radiogenic 187Os/188Os isotope ratiocannot by itself account for all of the Os in these laterites,although it could account for much of the Os seen in theprofile at Goa. Cosmic (interplanetary) dust is also knownto have a high Os concentration and very unradiogenic187Os/188Os isotope composition. The average flux ofcosmic dust to Earth is 40–80 g/km2/yr (Peucker-

Ehrenbrink andRavizza, 2000b) with anOs concentrationof ∼50 ng/g and 187Os/188Os composition of ∼0.127(Anders and Grevesse, 1989). Over 20 Ma a maximumof 8×10−5 g of Os could be input to the lateriteprofiles, accounting for 0.26% and 1% of the total Os atBidar andGoa, respectively. This suggests that despite thehigh Os concentrations of cosmic dust it is unlikely to besolely responsible for the enrichment seen in the lateriteprofiles.

These results highlight the fundamental difficulty ofsourcing the Os isotope and elemental signature seen inthe laterites directly from crustal rocks. This is becausefirst, average continental crust has a much lower Osconcentration (∼50 ppt) (Wedepohl, 1995; Peucker-Ehrenbrink and Ravizza, 2000a; Peucker-Ehrenbrinkand Jahn, 2001) than that seen in the laterites (80 to1400 ppt). Second, average continental crust also has arelatively high 187Re/188Os ratio (Esser and Turekian,1993; Peucker-Ehrenbrink and Ravizza, 2000a, 2001)and thus ancient continental crust would be expected todevelop a radiogenic 187Os/188Os isotope composition,consistent with the measurements for some of theDharwar lithologies. Therefore, in contrast to Sr and Nd,it is difficult to explain the 187Os/188Os isotope compo-sition of the laterites in terms of straightforward input ofaeolian input of ancient continental material. It is possibleto invoke the input of specific rock types, for example, theweathering and erosion of ultramafic peridotites whichpossess high Os concentrations and radiogenic Oscompositions (e.g. Snow and Reisberg, 1995; Reisbergand Lorand, 1995). However, such peridotites do notnormally possess unradiogenic Sr and radiogenic Ndcompositions and so are unlikely to account for thevariations seen in those systems. Alternatively it ispossible that if the aeolian material was weathered priorto deposition then the Os concentration and isotopeconcentration in this residual material may be verydifferent from that of the bulk rock from which it wassourced.

Therefore, while aeolian input is likely to have hadsome influence on the Re–Os isotope system in thelaterites, the nature and extent of this input is difficult toassess. In any event, if aeolian deposition is responsiblefor the external input to the laterites then this must haveoccurred while the groundwaters were still activelyredistributing elements within the profile or else thepatterns of change, including that at the watertable wouldbe obscured.

5.4.3. Groundwater inputThe chemical variations observed in both profiles

provide clear evidence of the role of groundwater in the


redistribution of elements. In particular, the oxic–suboxicstratification provides a means for dissolution, transportand precipitation of both Re and Os. This raises thepossibility that material may be gained or lost from theprofile, not only by vertical movement from in situweathering or aeolian input, but also by lateral movementof solute-laden groundwaters. The laterite at Bidar isremote frombasement lithologies and developed upon thehighest elevations. Consequently, there is no topographyor adjacent rock types to drive a flowof externally sourcedgroundwater towards the profile. However, this does notpreclude the input of groundwaters from the breakdownof theAmbenali formation itself. Aswas shown in Section5.4.1 the Os isotope composition of the laterite at Bidarcan be accounted for by derivation from the parent basalt,but does demand Os enrichment which could have beenfacilitated by Os-laden groundwaters.

Groundwaters carrying externally derived Os aremuch more likely to be present in the laterite at Goa, notonly because of the low lying topographic position butalso the presence of diverse surrounding rock types.Many of the surrounding rocks are from Archean andProterozoic lithologies of the Dharwar craton, andtherefore be expected to possess radiogenic Os isotopecompositions. However, some lithologies do possess lessradiogenic compositions and it is possible that Os couldbe input via groundwater derived from a mixture of thesesources. Alternatively, the chemical signature of thegroundwaters may have been derived from the nearbyDeccan basalts.

6. Laterites and the surficial Os cycle

The results presented here, together with those ofreported previously (Mann, 1984) suggest that lateritescommonly possess much less radiogenic Os isotopecompositions than their parent rock, and may also havemuch higher Os concentrations. Significantly, the lateritesare also much less radiogenic and have much higher Osconcentrations than average upper continental crust.Given the extensive geographical coverage of laterites(∼30% of the exposed continental crust; Tardy, 1997),and their influence on the surface water cycle (nearly 50%of continental drainage flows through laterite terrain(Tardy, 1997)) it seems likely that they will exert asignificant influence on the movement of Re and Osbetween the continents, oceans and atmosphere.

During their formation laterite soils may initially act tosequester both Re and Osmobilised by weathering (eitherfrom groundwaters or aeolian input). However, over timethe relatively unradiogenic Os isotope composition fromthe weathering and erosion of laterite topsoils is likely to

influence the composition of rivers and dust that theysource. There is little detailed data for rivers draininglaterite terrains, however, in general global river datasuggests if anything that the riverine 187Os/188Os isotopecomposition is actually more radiogenic than the averagecontinental crust (Levasseur et al., 1999), possibly due tothe effects of incongruent weathering. An upper limit ofthe Os contribution to the oceans from mineral aerosolscan be calculated, from estimates of the global aerosolflux (e.g. Duce et al., 1991) and the Os content of averagecontinental crust (Esser and Turekian, 1993; Peucker-Ehrenbrink and Ravizza, 2000a, 2001), which suggestsabout a 10% contribution from aerosols (e.g. Peucker-Ehrenbrink and Ravizza, 2000a). However, if 30% of thataeolian flux is derived from laterite topsoils (reflecting thecontinental coverage) then, at least some, aerosols maycontain a much higher Os content than has previouslybeen considered.

Changes in the balance of Os sequestered or releasedby laterites might be expected to accompany tectonic orclimatic change. For example, uplift or cooling may leadto their enhanced physical weathering, and in particular,periods of global aridity will result in an enhanced dustflux from the continents. Conversely, during periods ofglobal warming laterite formation may be enhanced(Dequincey et al., 2002).

7. Summary

Laterite weathering profiles at Bidar and Goa devel-oped on different rock types,with different ages and a timeinterval of ∼40 Ma between periods of laterisation.Nevertheless, the profiles are very similar, progressingfrom unaltered bedrock upwards to increasingly alteredsaprolite and an uppermost Fe-rich iron crust. Bothprofiles also preserve a watertable that is coincident withsignificant major and trace element variation.

Platinum group element enrichment in laterites is welldocumented, and the data from this study confirms thatboth Re and Os are highly enriched in the laterite relativeto the parent rock. The pattern of Re and Os enrichmentand depletion, particularly at the level of the watertableindicates that both elements are mobile. The level of Osenrichment is far greater than that of Re, which is reflectedin the decreasing 187Re/188Os ratio and correspondingdecrease in 187Os/188Os up profile. Laterite 187Os/188Osratios are always less radiogenic than the parent bedrockthat they developed from, consistent with previous work(Sharma et al., 1998).

The origin of the high Re and Os concentrations inthe laterites remains unclear. Previous isotope studies ofthe laterite at Bidar have concluded that aeolian input to


the profile must have occurred. Nd and Sr isotope datasuggests that the source of this material was ancientcontinental crust (Mason et al., 2000), while Li isotopesindicate that this material has experienced significantweathering prior to deposition (Kisakurek et al., 2004).However, for Os most potential crustal sources have lowOs concentrations demanding significant aeolian depo-sition or the pre-concentration of Os in aerosols (perhapsas residues from weathering). Similarly average crustalsources are too radiogenic to account for the unradio-genic 187Os/188Os compositions observed in both lat-erites. Alternatively, the Re and Os may have beentransported into the profile by the lateral movement ofsolute-laden groundwaters.

The Os concentration in laterites is similar to thatfound in Os-rich rock types, such as organic-richsediments (Singh et al., 1999; Peucker-Ehrenbrink andHannigan, 2000; Jaffe et al., 2002) and ultramaficperidotites (Jacobsen et al., 1984; Rampone et al., 1996),but in contrast to those lithologies laterites comprise asignificant part of the continental land surface, and playan active role in the surface water cycle. Under thosecircumstances laterite formation or weathering is likelyto exert a significant influence on Os behaviour in thesurface environment, and that influence is itself likely tobe highly sensitive to climatic or tectonic change(cf. Dequincey et al., 2002).

Acknowledgements

This work was supported by the Natural EnvironmentResearch Council (under contract NE/B502701/1).

Appendix A. Supplementary material

Supplementary data associated with this articlecan be found, in the online version, at doi:10.1016/j.epsl.2007.06.028. We would like to thank R. Carlson forthe editorial handling and two anonymous reviewers forthe constructive comments.

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