PII S0016-7037(01)00869-9

Germanium–silicon fractionation in the weathering environment

ANDREW C. KURTZ,1,*†, LOUIS A. DERRY,1 and OLIVER A. CHADWICK2

1Department of Geological Sciences, Cornell University, Ithaca, NY 14853, USA2Department of Geography, University of California, Santa Barbara, Santa Barbara, CA 93106, USA

(Received March 8, 2001;accepted in revised form November 14, 2001)

Abstract—We present a detailed study of germanium behavior in the soil weathering environment as animportant step toward using the Ge/Si system as a tracer of silicate weathering processes in both modern andancient environments. Intensely weathered soils developed on Hawaiian basalts have bulk soil Ge/Si ratios 2to 10 times higher than fresh basalt (e.g., 10 to 25�mol/mol vs. 2.5�mol/mol). Soil Ge concentrationsincrease with Si, and decrease with Fe, suggesting that Ge sequestration is related to accumulation ofsecondary soil silicates, rather than retention in soil Fe oxy-hydroxides. Sequential extractions of these soilssuggest that Ge/Si fractionation takes place by Ge sequestration during the initial precipitation of secondarysoil aluminosilicates (principally allophane). Further Si loss and changes in mineralogy as these soils ageresult in little additional Ge/Si fractionation. Ge/Si ratios in granitic soils and saprolites are strongly influencedby relative weathering rates of primary minerals. Kaolinite has a Ge/Si ratio (5.9�mol/mol) higher than theplagioclase from which it forms (3.1�mol/mol), whereas accumulation of primary quartz (Ge/Si 0.5�mol/mol) prevents granitic soils from attaining high Ge/Si ratios. Laboratory synthesis of allophane confirmsthat Ge is preferentially partitioned into the solid phase upon precipitation of secondary aluminosilicates fromsolution. Copyright © 2002 Elsevier Science Ltd

1. INTRODUCTION

Silicate weathering has important roles in soil development,nutrient availability in terrestrial and marine ecosystems, buff-ering of acid rain, and long-term atmospheric CO2 regulation.The development of natural isotopic and elemental tracers (e.g.,strontium, neodymium, and osmium isotopes) has substantiallyaided our understanding of these processes. Such tracers arepowerful because they can be applied to modern weatheringenvironments, and may be used to infer information about pastweathering environments from paleoceanographic or terrestrialrecords. A limitation to many of these approaches is that theymay be strongly affected by processes other than silicate weath-ering. For example, strontium isotope ratios in soil and streamwaters tend to be strongly affected by carbonate dissolution(Palmer and Edmond, 1992; English et al., 2000), whereasstream water osmium isotope ratios may preferentially reflectweathering of organic-rich rocks (Ravizza and Esser, 1993;Peucker-Ehrenbrink and Hannigan, 2000, Pierson-Wickmannet al., 2000). Previous studies of the germanium–silicon ele-mental pair in streams suggest that Ge/Si ratios can offerinsight into silicate weathering processes (Mortlock and Fro-elich, 1987; Murnane and Stallard, 1990; Froelich et al., 1992).The principal potential advantage of the Ge/Si ratio as a tracerof silicate weathering is that it directly involves silicon. To datethere has been little study of Ge-Si fractionation in the soilenvironment where weathering and secondary mineral forma-tion take place.

We present a study directed at understanding Ge behaviorand Ge/Si fractionation in the weathering environment through

measurement of Ge/Si ratios in soils and soil minerals. Thisstudy is based largely on a well-studied basaltic soil chronose-quence in the Hawaiian Islands (Crews et al., 1995; Vitousek etal., 1997; Chadwick et al., 1999). We conducted a series ofsequential chemical extractions of these soils aimed at isolatinga Ge-enriched soil fraction. Additional data on Ge/Si fraction-ation are presented from a soil-saprolite profile developed ongranodiorite (Luquillo, Puerto Rico). These field-based ap-proaches are complemented by a laboratory experiment thatexplores Ge/Si fractionation during neoformation of secondaryaluminosilicates.

2. BACKGROUND

Ge has been termed a “pseudoisotope” of Si, based on theirsimilar chemical properties (Azam and Volcani, 1981). Ge andSi, both group IV elements, have identical outer electron struc-tures and have remarkably similar chemical characteristics. Thesimilarity between tetrahedral Ge-O (1.75 Å) and Si-O (1.64 Å)bond lengths (Martin et al., 1996) allows Ge to substitutereadily for Si in silicate minerals. Isomorphous substitution ofGe for Si in tetrahedral site in the silicate lattice is an exampleof V. M. Goldschmidt’s principle of “camouflage” (Gold-schmidt, 1958). Germanium normally occurs as a trace ele-ment, averaging�1 ppm in crustal rocks. Fresh igneous rocksdo not vary widely in their Ge/Si ratios; the molar Ge/Si ratioin most rocks is 1 to 3�mol/mol (Bernstein, 1985; DeArgolloand Schilling, 1978).

Ge/Si ratios in stream waters appear to show systematicbehavior related to weathering processes. Ge/Si ratios in un-polluted streams are almost always lower than Ge/Si in thesilicate bedrock they drain (Mortlock and Froelich, 1987).Murnane and Stallard (1990) proposed that dissolved Ge/Siratios in streams are related to “weathering intensity,”W, thefraction of Si released to solution from bedrock by chemicalweathering:

* Author to whom correspondence should be addressed(kurtz@bu.edu).† Present address: Department of Earth Sciences, Boston University,Boston, MA 02215.

Pergamon

Geochimica et Cosmochimica Acta, Vol. 66, No. 9, pp. 1525–1537, 2002Copyright © 2002 Elsevier Science LtdPrinted in the USA. All rights reserved

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1525

�Ge/Si�stream � �Ge/Si�bedrock � A�W � 1� (1)

where A is an empirical Ge partitioning factor. When W � 1, allbedrock Si is released to solution, and Ge/Sistream � Ge/Sibedrock. As W approaches 0, only a small fraction of total Siis released to solution (the rest is retained by soil clays), and thedifference between Ge/Sibedrock and Ge/Sistream increases, ap-proaching A. This scenario requires that there is a residualphase or phases, resulting from weathering and soil develop-ment, that is enriched in Ge to satisfy mass balance (Murnaneand Stallard, 1990). Two deeply weathered tropical soils stud-ied by Murnane and Stallard (1990) showed Ge/Si enrichmentsof �2.5 relative to ratios in unweathered bedrock. They sug-gested that clays formed during incongruent weathering ofprimary silicates are the enriched reservoir for Ge/Si, comple-menting the depleted stream waters.

Froelich et al. (1992) expanded on this concept and proposedthat Ge/Si ratios in stream waters could be approximated bymixing of Si from two sources (Fig. 1). The first Si sourcereflects weathering of primary silicates. They proposed thatincongruent weathering is characterized by a clay/bedrock dis-tribution coefficient of 2.5 (Kw � Ge/Siclay/Ge/Sibedrock, result-ing in a high Ge/Si component retained by soils and a lowGe/Si component released to stream waters. The second Sisource (with high Ge/Si) is derived from dissolution of theGe-enriched secondary minerals under more intense chemicalweathering. The Ge/Si ratio appearing in streams is a mixtureof these two sources and varies as a function of weatheringintensity.

Data constraining the nature of the proposed reservoir withhigh Ge/Si ratios in soils are limited. Secondary aluminosilicateminerals, iron oxy-hydroxides, and organic matter are all po-tential reservoirs of a Ge-enriched component. Martin et al.(1996) studied the crystal chemistry of Ge in synthetic talc, in

an effort to understand how Ge might be incorporated into claymineral structures in low temperature environments. They syn-thesized talcs with various amounts of Ge and examined theeffect of Ge substitution on the talc crystal structure throughextended X-ray absorption fine structure (EXAFS) and Fouriertransform infrared radiation (FTIR) spectroscopy. These au-thors found a complete binary solid-solution series between Siand Ge talc end members. Ge occupies the tetrahedral site inplace of Si, without major lattice distortion.

Ge is known to be enriched in some naturally occurring Feoxides (Bernstein, 1985). Bernstein and Waychunas (1987)studied naturally occurring Ge-enriched hematite and goethiteusing EXAFS, and showed that Ge substitutes for Fe in theoctahedral site of these minerals. Octahedral substitution of Gefor Fe(III) in oxy-hydroxides is allowed by similar bondlengths, where Ge-O � 1.88 Å and Fe(III)-O � 2.04 Å (Bern-stein and Waychunas, 1987). However, unlike Ge-Si substitu-tion, substitution of Ge(IV) for Fe(III) results in a chargeimbalance. In hematite, charge balance is maintained by acoupled substitution with Fe(II) such that 2Fe(III) � Ge(IV) �Fe(II). In goethite, charge balance is maintained by loss of oneH� atom for each Ge, such that Fe(III) � H(I) � Ge(IV)(Bernstein and Waychunas, 1987). These observations suggestthat incorporation of Ge into Fe oxides and hydroxides may beredox- or pH-sensitive. Given an octahedral Al-O bond lengthof 1.96 Å, an analogous coupled substitution involving Ge foroctahedral Al in clays is also possible but we are not aware ofany studies of this phenomenon.

Mortlock and Froelich (1987) proposed that preferentialsorption of Ge to Fe oxides during stream transport mightexplain low dissolved Ge/Si ratios in stream waters. However,more recent work did not show evidence for the preferentialsorption of Ge on Fe oxides precipitating in a glacial stream

Fig. 1. Schematic model of Ge/Si fractionation during weathering after Froelich et al., 1992. In this model, Ge ispartitioned into secondary aluminosilicates (clays), resulting in streamwaters with low Ge/Si. More intense weatheringreleases the Si (and) Ge otherwise stored in clays, contributing higher Ge/Si ratios to streamwater. The Ge/Si ratio ofstreamwater reflects the balance of these two processes.

1526 A. C. Kurtz et al.

(Chillrud et al., 1994). Work on marine sedimentary diagenesishas shown that iron redox cycling in opal-bearing organic andiron-rich sediments can lead to sequestration of Ge in anFe-rich phase, although this phase may be a Fe-bearing authi-genic clay rather than an Fe oxide (Murnane et al. 1989;Hammond et al., 2000; King et al., 2000).

Pokrovski and Schott (1998a) studied Ge complexation byaqueous organic species. They suggested that complexation ofGe by dissolved humic acids can increase the dissolved Ge/Siratio of organic-rich stream waters. Previous experimental stud-ies by Manskaya and colleagues (Manskaya et al., 1961; Man-skaya et al., 1972) showed that Ge forms stable organic com-pounds with humic acids in peat. Ge is known to be enriched incoal and other lignitized organic matter (Bernstein, 1985).

3. STUDY SITES

Crews et al. (1995) established a chronosequence of sitesdeveloped on tephra deposits or lava flows with ages of ap-proximately 0.3 ka (Thurston), 2.1 ka (Olaa), 20 ka (Laupa-hoehoe), 150 ka (Kohala), 1400 ka (Molokai), and 4100 ka(Kauai). This chronosequence is referred to as the “Long-Substrate Age Gradient” (LSAG). All soil profiles were sam-pled on primary shield volcano surfaces, where physical ero-sion and groundwater influences were minimal. Parent materialat the two youngest sites (0.3 and 2.1 ka, both on Kilauea) istholeiitic basalt, whereas the older sites are thought to bedeveloped on late-stage caps of post shield-building phasealkalic rocks (hawaiite and mugearite; Kurtz et al., 2000). Atpresent, all six sites are near 1200 m elevation, receive 250 cmannual rainfall, have mean annual temperature of 16°C, andsupport intact rainforests dominated by native Ohia trees(Metrosideros polymorpha). The older sites have been subjectto long-term variations in climate and weathering conditionsdue to Pleistocene climate change and island subsidence (Gav-enda, 1992; Hotchkiss et al., 2000).

Vitousek et al. (1997) described changes in major element

chemistry and soil mineralogy with soil age across the LSAG.Base cations and Si are rapidly depleted by weathering. Only10% of the initial quantities of these elements remain after 20ka. Aluminum is less easily leached, but by 150 ka �50%remains (Fig. 2a). These changes in major element chemistryare accompanied by dramatic changes in mineralogy (Fig. 2b).Primary volcanic minerals are absent by 20 ka, replaced bynoncrystalline minerals (allophane, immogolite, and ferrihy-drite). These metastable minerals slowly recrystallize over �1Ma, forming secondary kaolin and crystalline sesquioxides.Kaolin and sesquioxide minerals begin to dominate the fine(�2 �m) fraction of the soil in the two oldest sites. Eolian dustaccretion is a complicating factor for this study because dustmay play an important role in the silica budgets of these soils.In many of the older soils, soil silica is strongly influenced byaddition of dust (Kurtz et al., 2001).

The Rio Icacos watershed in the Luquillo Mountains ofeastern Puerto Rico is similar in many respects to the Hawaiiansystem in that both are rapidly weathering montane tropicalsystems, remote from anthropogenic inputs. Rio Icacos is a sitein the Luquillo Experimental Forest, part of the U.S. LongTerm Ecological Research Program, and has been extensivelycharacterized in terms of hydrology, soil development, andweathering and erosion rates (Murphy et al., 1998; White et al.,1998). The Rio Icacos watershed has a mean annual tempera-ture of 22°C and receives 420 cm of rainfall per year, whichcombine to produce the fastest chemical weathering rate ofgranitoid rocks yet documented (White et al., 1998). White andcoworkers have studied soils from a knife-edge divide on theeastern edge of the Rio Icacos basin. The samples used in thisstudy are from site LG-1, a 100-cm-thick soil with a thin (�10cm) organic horizon underlain by an argillic horizon. Belowthis bioturbated soil zone is a 200- to 700-cm-thick oxidizedsaprolite zone that retains original bedrock texture. White et al.(1998) estimated that 48% of Si initially present in the saprolitehas been lost to solution. Si, Al, and Fe concentrations are

Fig. 2. Chemical and mineral transformations during development of Hawaii chronosequence soils. (a) The fraction ofparent material Al, Mg, and Si remaining in soils across the sequence. (b) Relative contributions of primary minerals(represented by feldspar, fel.), of noncrystalline minerals (non), and of crystalline kaolin and sesquioxides (cry.) to soilmineralogy. Modified from Vitousek et al. 1997.

1527Ge/Si weathering fractionation

similar in soil, saprolite and parent rocks due to residual accu-mulation of quartz, and formation of kaolinite and Fe oxides(goethite) as the dominant secondary minerals.

4. METHODS

Hawaiian soil profiles were described and sampled by horizon tohard rock or a depth of 1 m. Splits for geochemical analyses were driedovernight at 105°C, weighed, and then ashed for 5 h at 500°C to oxidizeorganic matter. The change in weight during this step was recorded asloss on ignition (LOI wt.%). Samples were then pulverized in a boron-carbide mortar and pestle. 100 mg of powder was weighed into adisposable graphite crucible (SPEX CertiPrep, Inc.) with 700 mg offlux (1:1 mix of Li-metaborate and Li-tetraborate, SPEX CertiPrep,Inc.). Samples were fused for 30 min in a 1000°C muffle furnace. Fusedsamples were dissolved in 100 mL of 10% HNO3. All elementalconcentrations are reported on an “organic-free” (postashing) basis toeliminate differential dilution by soil organic matter.

Hawaiian soils for sequential selective dissolution analysis were not“ashed,” but instead dried overnight at 105°C. Five “operationallydefined” soil fractions were isolated in sequence. Organic matter wasoxidized by gently heating 1 g soil with 0.5 mL H2O2 in a 1M NaOAc(pH 5) buffer solution. This procedure was repeated until no furtherreaction was observed, and the solid and supernatant separated bycentrifugation. Noncrystalline components, dominantly allophane, andalso some poorly ordered Fe and Al sesquioxides were isolated byselective dissolution with acid ammonium oxalate in the dark (AOD)(Jackson et al., 1986; McKeague and Day, 1966). 500 mg of “organic-free” soil was added to 40 mL 0.2M ammonium oxalate (pH 3) in afoil-wrapped centrifuge tube (to exclude light) and was allowed to reactwhile shaking for 4 h. Crystalline Fe and Al-sesquioxides were treatedwith Na-dithionite and Na-citrate (DC) (Ross and Wang, 1993). Analiquot of the AOD-treated solid was mixed with 40 mL 20% Na-citrateand 0.5 g Na-dithionite powder, and allowed to react overnight whileshaking. Kaolinite, halloysite, gibbsite, and opal were selectively dis-solved by heating the DC residue to 500°C for 4 h followed bytreatment with 85 mL of boiling 0.5M NaOH for 2.5 min. We measuredthe change in sample weight at each step and determined Ge, Si, Al,and Fe (see below) in each extracted solution, and in an aliquot of theremaining solid after each extraction. Ge blanks were monitored witha procedural blank that received the same handling and reagents as thesamples.

Luquillo samples were provided by Art White (U.S. GeologicalSurvey [USGS]). Soil and saprolite were sampled from a ridge-top sitewith a hand auger to the saprolite–bedrock interface at a depth of 8.5 m(site LG-1, White et al., 1998). Quartz diorite parent material wassampled from a nearby roadcut. Clean primary mineral separates ofbiotite and hornblende were obtained from the crushed parent rock byhand picking, but we were unable to adequately separate plagioclasefrom quartz. Minerals comprising the saprolite (quartz, kaolinite, al-tered biotite, and minor goethite) were separated by a combination ofsieving and hand-picking. The fine fraction consisted of kaolinite andgoethite. Goethite was removed by dithionite–citrate treatment to ob-tain a reasonably pure kaolinite fraction. X-ray diffraction analysisshowed only kaolinite in the residue, although the measured Al/Si ratio(0.87) indicates that there may be some very fine grained quartz mixedwith the kaolinite.

Allophane synthesis was conducted by J. Chorover (Penn StateUniversity), following a procedure modified from Wada et al. (1979).A 52 ppm Si, 55 ppm Al solution was prepared from hydrolyzingtetraethyl orthosilicate (TEOS) and AlCl3�6H2O dissolved in 5 L ofdeionized water in a polypropylene carboy. This solution was spikedwith Ge to attain a measured initial Ge/Si ratio of 1.0 �mol/mol. Thesolution was titrated with NaOH to a final pH of 3.5 and refluxed at90°C for 5 d. The solution was flocculated with NaCl and the resultinggel was purified by centrifugation and dialysis. FTIR analysis con-firmed the identity of the gel as allophane.

Germanium concentrations were measured by the isotope-dilutionhydride-generation inductively coupled plasma mass spectrometry(ICP-MS) technique developed by Mortlock and Froelich (1996). Dis-solved samples were equilibrated overnight at room temperature with a70Ge enriched spike and buffered to pH 6 with a 2M Tris-HCl solution.

Germanium was reduced by 4% sodium borohydride to germaniumhydride (GeH4), a volatile gas, and stripped from solution with anargon gas stream. The germanium hydride was trapped cryogenicallyby a glass U-tube immersed in liquid argon, and introduced as a “plug”into the torch of a VG Plasmaquad 2� ICP-MS. This produced arelatively large, but transient Ge signal (of �15 s duration), on whichthe 70Ge/74Ge ratio was measured. Our isotopic spike was prepared byfusing 0.7 mg of 70Ge-enriched GeO2 powder in a precleaned platinumcrucible with 7 mg Li-metaborate at 1000°C for 30 min. This powderhas a 70Ge/74Ge ratio of 161.4, compared with a natural 70Ge/74Ge ratioof 0.5616 (Reynolds, 1953). The spike solution was diluted in 1%HNO3 to roughly 500 ppt, and the concentration of the working spikewas determined by reverse isotope dilution with a 100 ppt aqueous Gestandard made from dilution of a 1000 ppm commercial Ge aqueousstandard (High Purity Standards, Charleston, SC).

Our primary check on the accuracy of our technique was USGS rockstandard BIR-1. Eleven analyses of 6 fusions of BIR-1 gave an averageGe concentration of 1.52 ppm, nearly identical to the value reported byMortlock and Froelich (1996) (Table 1). Precision on replicate analysesof a sample is limited by the combination of weighing errors inspike/sample mixing, and instrumental precision in determining 70Ge/74Ge ratios. Precision for 5 replicates of BIR-1 on a single day was 3%relative standard deviation (RSD). We were typically able to reproducethe 70Ge/74Ge ratio of unspiked natural germanium to within 1 to 2%RSD over a single day, although the absolute value of the measuredratio varied with instrument tuning. We are aware of some difficultiesin hydride generation with organic or Fe-rich samples (R. Stallard,personal communication, 1995). We sometimes found low Ge yield onparticularly Fe-rich samples. Provided that samples are well-equili-brated with the spike, 100% yield is not critical for isotope dilutionmeasurements, so difficulties in producing quantitative hydride yieldshould not be serious. We understand that sample-spike Ge-isotopicequilibration can be difficult to achieve in some cases, particularly forseawater samples in which Ge is present largely as organic–Ge com-plexes (Mortlock and Froelich, 1996). In practice, all samples wereallowed to equilibrate with spike overnight before running, but wefound that for dissolved rocks and soils in an acidic matrix (1% HNO3),accuracy of Ge data were not adversely affected even in experimentswith very short sample-spike equilibration times (�1 h).

Si, Al, and Fe concentrations on soils, minerals, and sequentialextraction solids were determined by inductively coupled plasma op-tical emission spectroscopy (ICP-OES) at Cornell University on thesame Li-metaborate fusions used for Ge analysis. We included fivestandard reference materials, prepared identically to samples, in eachrun to define element calibration curves (Table 2). For major elementanalysis of sequential extraction solutions we prepared a series ofstandards matched to each matrix (e.g., ammonium–oxylate, Na-di-thionite/citrate, and NaOH).

5. GE/SI FRACTIONATION IN HAWAIIAN SOILS

5.1. Bulk Soil Results

Given the absence of unweathered volcanic rocks at oursites, we were not able to measure Ge/Si ratios in unweathered

Table 1. Ge data for standard reference materials.

SRM Ge (ppm)* Literature values

BIR-1 1.52 � 0.2 (6) 1.53**; 1.49***SCo-1 1.63 (2) 1.69***BE-N 1.16 (1) 1.17***SDO-1 1.55 (1) 1.47**

* Values by batch hydride generation ID-ICP-MS on lithiummetaborate fusions at Cornell University. Number in parentheses de-notes number of fusions run. Precision on BIR-1 defined as 2 times thestandard deviation of the mean of 11 analyses.

** ID-HG-ICP-MS data from Mortlock and Froelich (1996)*** HG-ICP-OES data from Halicz (1990).

1528 A. C. Kurtz et al.

soil parent material. However, DeArgollo and Schilling (1978)showed that Ge/Si ratios in Hawaiian “basalts” (tholeiitic andalkaline) vary within a narrow range (2.3 to 2.8 �mol/mol),

averaging 2.6 �mol/mol. LSAG soil horizons are enrichedfrom a few percent to as much as a factor of 10 over ratiostypical of Hawaiian basalts (Table 3). The youngest, most

Table 2. Major element data for standard reference materials.

SRM

SiO2 (wt %) Al2O3 (wt%) Fe2O3 (wt%)

measured ref. measured ref. measured ref.

BE-N* 37.7 38.2 9.8 10.0 12.6 12.8STM-1** 65.4 59.6 19.3 18.4 5.6 5.21413*** 86.6 87.8 9.8 9.9 nd 0.2FeR-1**** 16.4 17.0 0.6 0.5 79.9 75.9FeR-2**** 48.9 49.2 5.1 5.2 39.3 39.2

Reported values represent average of three analyses.Reference values in italics from: * Govindaraju (1995), ** Gladney and Roelandts (1988), *** NIST recommended values, **** Canadian CRMP

recommended values.

Table 3. Bulk soil data for Hawaii LSAG soil horizons.*

horizonSiO2

(wt %)Al2O3

(wt %)Fe2O3

(wt %)Ge

(ppm)Ge/Si

(�mol/mol)

Thurston (0.3 ka)3–12 cm A 49.4 12.1 13.7 1.7 2.812–26 Bw1 48.9 12.3 12.5 1.7 2.926–37 Bw2 48.8 13.4 12.7 1.7 2.937–40 B/C 48.8 12.9 12.1 1.7 2.940–49 B/C 48.3 13.4 12.4 1.7 2.9Olaa (2 ka)5–12 cm A 48.1 12.7 13.0 1.7 2.820–28 Bw1 48.2 13.1 13.0 1.6 2.745–62 Bw2 48.6 12.3 12.4 1.8 3.062–85 C 49.5 12.6 12.4 1.6 2.7Laupahoehoe (20 ka)0–5 cm A1 19.2 9.5 32.2 1.4 5.95–12 A2 9.2 6.7 57.1 2.2 19.412–28 Bw1 18.4 29.2 32.1 2.1 9.228–38 Ab 11.7 33.9 32.2 1.7 11.738–54 Bwb1 13.4 39.4 32.1 1.7 10.654–80 Bwb2 21.6 31.4 32.5 2.3 9.080–100 Bwb3 14.5 29.5 40.3 2.1 11.9Kohala (150 ka)0–5 cm A 51.0 15.3 13.2 4.5 7.25–15 Ag 39.1 17.7 15.9 4.5 9.515–24 Bg 50.5 15.0 11.6 5.6 9.224–30 Bw1 47.2 16.3 14.5 5.4 9.530–42 Bw2 34.4 21.9 15.7 4.4 10.742–60 Bw3 13.5 44.2 18.6 4.0 24.3Molokai (1400 ka)5–11 cm A1 42.0 9.2 17.5 4.8 9.411–32 Bw 40.8 13.3 19.5 5.2 10.632–45 Bt1 20.8 27.9 32.4 3.4 13.745–70 Bt2 23.6 34.2 26.3 3.2 11.170–115 Bt3 23.1 43.7 13.1 2.3 8.4115–120 Bt4 23.2 49.7 4.1 1.6 5.5Kauai (4100 ka)11–29 cm Ag 5.5 4.3 80.5 1.3 19.629–37 AB 13.1 15.3 51.4 1.0 6.037–52 Bo1 7.7 25.3 49.0 1.2 13.252–65 Bo2 18.8 30.5 42.2 1.5 6.565–76 Bo3 15.8 32.9 43.3 1.8 9.476–92 Bo4 17.3 27.3 38.7 1.8 8.592–108 Bo5 13.2 23.0 38.0 1.6 10.3

* Concentrations reported on an organic-free basis.

1529Ge/Si weathering fractionation

poorly developed chronosequence soils (0.3 and 2.1 ka parentmaterial age) have Ge/Si ratios of 2.7 to 3.0 �mol/mol. Thesesoils have lost very little Si; Ge/Si (and Al/Si) ratios are notsignificantly affected by incipient weathering. All older soils(20, 150, 1400, 4100 ka) have highly fractionated Ge/Si ratios,ranging from 5.5 to 24.3 �mol/mol. Our discussion focuses onthese four oldest, “ intensely weathered” sites.

Because concentrations of Si, Fe, and Ge are fairly uniformin Hawaiian soil parent material, distributions of these elementsin soils primarily reflect pedogenic processes. Soil Ge concen-trations generally increase with Si concentration, and decreasewith soil Fe (Fig. 3). This relationship suggests that althoughGe is enriched relative to Si in these soils, Ge and Si maintaintheir geochemical similarity, and that Ge is retained by a

Fig. 3. Soil Ge concentration vs. wt.% SiO2 (a) and Fe2O3 (b) for all studied LSAG soils. Primary volcanic minerals havebeen lost by weathering. Concentrations of Fe2O3 greater than the 10 to 15% typical of Hawaiian volcanics reflect residualaccumulation of Fe as secondary sesquioxides. These plots show that soil germanium increases with silica, not with iron.This observation suggests that accumulation of Ge is controlled by a silicate, rather than an Fe-sesquioxide soil phase. Allolder soils fall on a trend in (a) defining a Ge/Si ratio of close to 10 �mol/mol (illustrated by the dashed line). Young,incipiently weathered soils do not fall on this trend.

1530 A. C. Kurtz et al.

silicate soil component. There is no indication from bulk soilcomposition that Ge sequestration in these soils is related toaccumulation of secondary Fe oxy-hydroxides. Soil horizonsvary around an average Ge/Si of roughly 10 at each of the fouroldest sites; there are no systematic changes in Ge/Si withdepth in the soil column. Bulk soil column Ge/Si ratios, cal-culated by weighting each horizon by its thickness, bulk den-sity, and Si concentration, are between 9.5 and 11.5 for each ofthe four oldest sites.

Kurtz et al. (2000) showed that Nb and Ta are the leastmobile elements in these soils. We use Nb as the immobileindex element for geochemical mass balance calculations. Wecompute net losses of Ge and Si by applying the open-chemi-cal-system transport function (Brimhall and Dietrich, 1987;Chadwick et al., 1990). The mass fraction of Si or Ge lost from(or added to) a soil horizon during pedogenesis is calculatedfrom the equation:

�j,w � �Cj,w

Cj,p�

Ci,p

Ci,w� � 1 (2)

where C is concentration, j represents a mobile element (Si orGe) and i an immobile element (in this case Nb), p is parentmaterial, and w is weathered material. The younger soils (0.3and 2.1 ka) have lost very little Si and Ge (�Si, �Ge near 0).Older soils (20 to 4100 ka) are almost completely stripped ofinitial silica (�Si 0.8 to –1) (Fig. 4), whereas Ge is much moreconservative. Some soil horizons show as much as 80% Si loss,without any apparent net loss of Ge. Kurtz et al. (2000) ob-served net gain of Al in these horizons and attributed this toleaching of Al from near-surface horizons and precipitation asclays deeper in the soil column. Accumulation of Ge-enrichedclays in these horizons may explain the observed lack of Geloss. Excluding the young, relatively unweathered sites, most ofthe LSAG soils have Ge/Si ratios of roughly 10 �mol/mol.Ge/Si fractionation in the Hawaii chronosequence must occurrapidly as soils evolve from the relatively unweathered condi-tion represented by the 2 ka Olaa site to the highly desilicated20 ka Laupahoehoe site (Fig. 4a). Further Si loss seems toinvolve very little additional Ge/Si fractionation. Both Si andGe loss from old soils proceed at roughly the same rate, asevidenced by the linear trend in �Ge vs. �Si (Fig. 4b).

5.2. Sequential Extractions

Precipitation of secondary aluminosilicate clays, Fe oxides,and accumulation of soil organic matter are all feasible mech-anisms for concentrating Ge relative to Si (Bernstein, 1985;Froelich et al., 1985, Bernstein and Waychunas, 1987; Mort-lock and Froelich, 1987; Murnane et al., 1989; Murnane andStallard, 1990; Pokrovski and Schott, 1998a). Isolating a Ge-enriched component by physical separation of soil minerals isnot possible in these fine-grained, poorly crystalline volcanicsoils. An alternative is sequential selective dissolution analysis(Jackson et al., 1986), where “operationally defined” soil frac-tions, ideally corresponding to soil mineral components, areextracted by a series of chemical treatments. We selected eightsoil samples for detailed study by sequential dissolution anal-ysis. All eight soils had high Ge/Si relative to Hawaiian basaltbut differed widely in major element composition and miner-

alogy. This sample set included a relatively shallow and adeeper horizon from each of the four oldest chronosequencesites.

Table 4 summarizes the geochemistry of the sequential ex-traction solutions. We report moles of Si, Al, Fe, and Geextracted (per gram of soil) in the AOD (noncrystalline alumi-nosilicates and Fe-Al sesquioxides), DC (crystalline Fe andAl-sesquioxides), and NaOH (kaolin, opal) extraction steps.We also report concentrations of these elements in the bulk soilbefore extraction for reference. Only Ge was determined on thehydrogen peroxide (organic matter) extraction solutions. Per-oxide solutions contained trivial amounts of Ge. Analyses ofperoxide-extracted soil solids show no change in the mass ofsoil Ge, even when this step removed as much as 30% of thetotal soil mass. These results suggest that retention of Ge bysoil organic matter is not an important mechanism for Ge/Sifractionation in these soils.

The AOD extraction is designed to isolate poorly crystallineFe and Al oxy-hydroxides plus poorly crystalline aluminosili-cates, principally allophane. We find that the mass fraction ofAOD-extractable solids generally decreases with soil age, con-sistent with the long-term evolution of soil mineralogy de-scribed by Vitousek et al. (1997) (Fig. 2). In the subsurfacesamples from Laupahoehoe (20 ka) and Kohala (150 ka), theAOD treatment removed nearly half of the bulk soil Ge and Siin proportion such that the Ge/Si ratio of the extracted solutionwas similar to the Ge/Si ratio of the bulk solid. The significantfraction of AOD-extractable Si is consistent with the impor-tance of allophane in these soils. Relatively high Al/Si ratios(i.e., 3.9 to 4.6 vs. �2 in allophane), plus significant amountsof extractable Fe indicate that poorly crystalline Al-Fe oxy-hydroxides were extracted in addition to allophane. We see asimilar pattern from the subsurface Molokai (1.4 Ma) soil,although the mass fraction of AOD-extractable material islower, consistent with the older age of this soil. The shallowersoils from these three sites had much less AOD-extractable Si;the AOD-extractable component was dominated by Al and Feoxy-hydroxides. Very little material was extracted by AODfrom the Kauai (4.1 Ma) soils. In general, Ge and Si appear tobe extracted proportionally by AOD, indicating that most of theGe and Si in the younger soils is in the allophane fraction, witha Ge/Si ratio 10 to 15 �mol/mol. A notable exception is theshallow Laupahoehoe soil, where a significant amount of Gewas extracted without significant Si, indicating that poorlycrystalline Al-Fe oxy-hydroxides may play some role in Ge-sequestration.

The DC extraction is designed to remove more crystallineAl-Fe oxyhydroxides that develop in the older soils throughOstwald ripening of the poorly crystalline Al-Fe componentfound in the younger soils. The DC extraction did not removesignificant quantities of Si from any of the soils. Very highGe/Si ratios measured in the DC extractions (25 to 150 �mol/mol) reflect only a small fraction of total soil Ge, and cannotaccount for high Ge/Si ratios in the bulk soil.

The NaOH extraction removes kaolin minerals (kaolinite,halloysite), gibbsite, and some biogenic opal. The mass fractionof the NaOH-extractable component increases with soil age,reflecting the transformation of poorly crystalline aluminosili-cates in the younger soils to kaolin minerals in the oldest soils(Vitousek et al., 1997). In all cases, Ge/Si ratios in the NaOH

1531Ge/Si weathering fractionation

extractions were similar to, or somewhat lower than bulk soilratios.

In some cases (especially shallow soils) residual solids fol-lowing the sequential extractions represented a significant massfraction of the bulk soil. In most cases, the residual solid had a

Ge/Si ratio not very different from the bulk soil, although theyusually differed greatly in Al/Si or Fe/Si (Table 5). We checkedthe mineralogy of several of the postextraction residual soilsolids with X-ray diffraction to attempt to identify a phaselikely to account for the high Ge/Si ratios (relative to basalt)

Fig. 4. Silica loss vs. bulk soil Ge/Si (a) and germanium loss (b) for all chronosequence soils. Fractional elemental lossis expressed as “�,” and is calculated relative to Nb, which behaves as an “ immobile” element in these soils (Kurtz et al.2000). Calculation assumes parent material with 50% SiO2, 1.6 ppm Ge, 60 ppm Nb (20 ka and older soils; alkalic parentmaterial), 10 ppm Nb (0.3 and 2.1 ka; tholeiitic parent). Young soils (0.3 to 2.1 ka) have lost very little Ge or Si, and haveGe/Si ratios similar to parent material (�2.6 �mol/mol, indicated by arrow). Extensive Si loss in older soils results in highGe/Si ratios, as Ge is retained relative to Si. Fractionation takes place accompanying extensive silica loss and formation ofallophane between 2 and 20 ka. Further Si loss as allophane recrystallizes does not result in additional Ge/Si fractionation.

1532 A. C. Kurtz et al.

measured in the residual solids. X-ray identifiable mineralswere dust-derived quartz and illite (Kurtz et al., 2001), pluspseudobrookite (an Fe-Ti oxide phase), hematite, and magne-tite. The residual fractions of the shallow Kohala and Molokaisoils (�50% SiO2) were dominated by dust-derived quartz andillite (Kurtz et al., 2001). Accumulation of dust should shiftsoils to lower Ge/Si, because slowly weathering quartz has avery low Ge/Si ratio (0.5 to 1 �mol/mol). Ge/Si ratios inresidual fractions from the deeper Kohala and Molokai soils area factor of two higher than the dust-rich residual fractions fromthe shallow soils. A clay mineral tentatively identified as mar-garite (a Ca-smectite) was observed in the residual solid fromthe shallow Kohala soil. Additional (possibly Ge-enriched)aluminosilicates may have been present in all of the residualsolids (cf. Chorover, 1999), but were impossible to identifyagainst the prominent oxide peaks. High Ge/Si ratios in the

residual solids, similar to ratios in bulk soils and extractedsolutions may also result from incomplete removal of extract-able soil components.

Our data are not consistent with an important associationbetween Ge and soil organics or Fe-oxide phases. The majorityof Ge was extracted from each soil component in proportion toSi. Rather than finding one soil silicate component with highGe/Si, we found that all secondary soil silicate fractions havehigh Ge/Si ratios. By 20 ka, silica loss from these soils isprofound, and Ge/Si fractionation has apparently already takenplace. Si is then simply redistributed within soil components assoil mineralogy evolves from dominantly amorphous phases(e.g., allophane) toward more crystalline phases (kaolin). Thereis not much further Ge/Si fractionation with soil development,although there may be some internal fractionation that gives thekaolin fractions in the oldest soils lower (but still highly ele-

Table 4. Geochemistry of sequential extractions.

Si(�mol/g soil)

Al(�mol/g soil)

Fe(�mol/g soil)

Ge(pmol/g soil)

Ge/Si(�mol/mol)

Al/Si(molar ratio)

Fe/Si(molar ratio)

Laupahoehoe 5–12 cmBulk soil 650 648 3500 10700 16.5 1.0 5.4AOD 26.8 336 1380 2250 84.0 13 51DC 12.0 166 344 790 65.8 14 29NaOH 74 0 2.1 960 13.0 - 0.03Laupahoehoe 54–80 cmBulk soil 1490 3660 2750 16900 11.3 2.5 1.8AOD 653 3030 1710 6910 10.6 4.6 2.6DC 51 400 470 3870 75.9 7.8 9.2NaOH 68 45 1.4 1120 16.5 0.7 0.0Kohala 15–24 cmBulk soil 7080 2680 1470 65900 9.3 0.4 0.2AOD 0.68 400 413 966 1420 588 607DC - 251 164 403 - - -NaOH 176 207 3.2 3470 19.7 1.2 0.018Kohala 42–60 cmBulk soil 1410 5040 1870 23300 16.5 3.6 1.3AOD 792 3750 380 11300 14.3 4.7 0.5DC 38 396 231 1410 37.1 10 6.1NaOH 73 421 2.2 1450 19.9 5.8 0.0Molokai 5–11 cmBulk soil 5420 1390 1760 50000 9.2 0.3 0.3AOD 4.3 32 386 980 228 7.4 90DC 18 137 99 377 20.9 7.6 5.5NaOH 402 296 6.4 4600 11.4 0.7 0.0Molokai 32–45 cmBulk soil 2990 4710 3320 37100 12.4 1.6 1.1AOD 374 1460 782 5560 14.9 3.9 2.1DC 47 308 409 1280 27.2 6.6 8.7NaOH 1300 2270 8.4 9390 7.2 1.7 0.0Kauai 29–37 cmBulk soil 1730 2250 5010 10700 6.2 1.3 2.9AOD 0.0 96 383 315 - - -DC 3 249 550 497 166 83 183NaOH 174 678 1.5 839 4.8 3.9 0.0Kauai 76–92 cmBulk soil 2550 4640 4730 21400 8.4 1.8 1.9AOD 4.6 305 134 1280 278 66.3 29.1DC 40 287 428 1070 26.8 7.2 10.7NaOH 1390 2200 24 7940 5.7 1.6 0.0

Sequential extraction data are presented blank-corrected.Major element blanks were insignificant except for Fe blank in NaOH extraction, which was 5–30% of overall signal due to very low Fe

concentrations in these extractions.Ge blanks in AOD and NaOH extractions were �1% of signal.Ge blanks for DC extractions were �10% of signal.

1533Ge/Si weathering fractionation

vated) Ge/Si ratios. Ge may be excluded to some degree duringrecrystallization of allophane to form more ordered alumino-silicates such as kaolinite in the older soils. The data from theseHawaii soils are most consistent with a model involving pref-erential retention of Ge when primary minerals are transformedinto secondary aluminosilicates. This weathering-related highGe/Si signature is then passed along to various soil componentsduring extensive recrystallization of soil minerals.

5.3. Ge/Si in Synthetic Allophane

Allophane is a poorly crystalline aluminosilicate mineralwith variable structure and Al/Si ratio that is common involcanic soils (McBride, 1994). Allophane is an importantproduct of basalt weathering in the LSAG sites (Vitousek et al.,1997), and based on our results, plays an important role inGe/Si fractionation in Hawaiian soils. We synthesized the min-eral allophane from a Ge-doped solution as a test of Ge parti-tioning during the formation of secondary soil aluminosilicates.The experiment was conducted as a batch system, where allo-phane precipitated until dissolved Al became limiting. Precip-itation of allophane consumed roughly 97% of the initial dis-

solved Al, 74% of dissolved Ge, and 54% of dissolved Si. Thesolution evolved from an initial Ge/Si ratio of 1.0 �mol/mol toa ratio of 0.5 �mol/mol in the final solution. Ge/Si in theprecipitate was 1.3 �mol/mol (Table 6).

The experimental data allow for calculation of an empiricalsolution–solid partition coefficient for the incorporation of Geby allophane. Treating this experiment as a Rayleigh distilla-tion:

�Ge/Siclay � Ge/Sisolution0

�1 � f Kd�

�1 � f �(3)

where the left-hand side of the equation indicates the Ge/Siratio of the integrated solid when a fraction f of the initial silicais remaining in solution, given an initial solution Ge/Si ratioequal to Ge/Si°solution. Kd is the bulk partition coefficient,defined as the ratio of Ge/Si of the newly precipitated clayrelative to the instantaneous Ge/Si of the solution:

Kd � � Ge/Siclay

Ge/Sisolution�

inst

(4)

Table 5. Geochemistry of bulk soil and unextractable residual solid.

SiO2

(wt%)Al2O3

(wt%)Fe2O3

(wt%)Ge

(ppm)Ge/Si

(�mol/mol)Al/Si

(molar ratio)Fe/Si

(molar ratio)

Laupahoehoe 5–12 cmbulk soil 3.9 3.3 27.9 0.78 16.4 1.0 5.4residual 13.6 6.0 48.3 2.4 14.7 0.5 2.7Laupahoehoe 54–80 cmbulk soil 8.9 18.7 21.9 1.2 11.3 2.5 1.8residual 25.8 8.8 36.4 2.7 8.8 0.4 1.1Kohala 15–24 cmbulk soil 42.6 13.7 11.8 4.8 9.3 0.4 0.2residual 53.2 12.2 8.9 5.9 9.2 0.3 0.1Kohala 42–60 cmbulk soil 8.5 25.7 14.9 1.7 16.5 3.6 1.3residual 13.3 12.4 53.8 2.8 17.5 1.1 3.1Molokai 5–11 cmbulk soil 32.6 7.2 14.0 3.6 9.2 0.3 0.3residual 47.2 7.4 13.3 5.2 9.0 0.18 0.2Molokai 32–45 cmbulk soil 18.0 24.0 26.5 2.7 12.4 1.6 1.1residual 18.1 13.2 45.8 4.1 18.9 0.9 1.9Kauai 29–37 cmbulk soil 10.4 11.5 40.0 0.78 6.2 1.30 2.9residual 14.6 11.6 53.2 1.1 6.2 0.93 2.7Kauai 76–92 cmbulk soil 15.5 23.6 38.0 1.6 8.3 1.8 1.8residual 7.1 13.4 56.8 1.2 13.4 2.2 6.0

Table 6. Allophane synthesis.

Ge(ppb)

Si(ppm)

Al(ppm)

Al/Si(molar ratio)

Ge/Si(�mol/mol)

initial soln 0.13 51.8 55.1 1.1 1.0final soln 0.03 24.0 1.6 0.07 0.5

Ge(ppm)

SiO2

(wt%)Al2O3

(wt%)Al/Si

(molar ratio)Ge/Si

(�mol/mol)

solid 0.41 26.4 36.6 1.6 1.3

1534 A. C. Kurtz et al.

We calculate a Kd of 1.6 for this experiment, indicating againthat Ge is preferentially partitioned into allophane. We note thatthe exact value of the partition coefficient applies only toallophane under these experimental conditions. Pokrovski andSchott (1998b) argued that Ge/Si fractionation is strongly tem-perature dependent. Our experiment was conducted at 90°C(Wada et al., 1979); Ge should be more strongly partitionedinto the solid (Kd � 1.6) at lower temperature. Our calculationassumes that Ge substitutes for Si and is partitioned into thetetrahedral site, but we acknowledge that substitution for octa-hedral Al is a possibility. Regardless, this experiment demon-strates that Ge/Si fractionation takes place during the precipi-tation of secondary aluminosilicates, with Ge preferentiallypartitioned into the solid relative to Si.

6. GE/SI FRACTIONATION IN GRANITIC WEATHERING:LUQUILLO, PUERTO RICO

We find that Ge/Si is fractionated during weathering ofgranitic rocks such that bulk soils and saprolites have higherGe/Si than the parent rock (Table 7). The Luquillo quartzdiorite has a Ge/Si ratio of 2.4 (�mol/mol), while soil andsaprolite Ge/Si ratios range from 2.6 to 3.6. Although weath-ering is “ intense” in both the older Hawaiian soils and in theseLuquillo soils, the increase in Ge/Si over the parent material issubtle here, where it was extreme in Hawaii.

Minerals comprising granitic rocks have distinct Ge/Si ratiosand weather at different rates. Relative mineral weatheringrates exert a first-order control on Ge/Si in granitic soils andsaprolites. Luquillo parent material is composed dominantly ofplagioclase (56.4%) and quartz (24.9%), with biotite and horn-blende comprising less than 10% of the parent rock (White etal., 1998). Trace K-feldspar and goethite comprise less than 2%each. We measured Ge/Si on bulk parent material, igneousbiotite, quartz, and hornblende (Table 7). A Ge/Si ratio of 3.1�mol/mol for Luquillo plagioclase was calculated by geo-chemical mass balance, given Ge/Si ratios and proportions ofother parent rock minerals, and the Ge/Si ratio of the bulk rock.

Luquillo saprolite is composed of approximately 60% ka-

olinite, 20% each quartz and altered biotite, and 4% goethite.Like the primary igneous minerals, Luquillo saprolite mineralshave distinct Ge/Si ratios (Table 7). White et al. (1998) arguedthat the dominant reaction in the bedrock to saprolite transitionis the incongruent weathering of plagioclase to form kaolinite.Our data suggest that Ge is strongly partitioned into the solidduring precipitation from kaolinite-saturated porewaters. Pla-gioclase, with a Ge/Si ratio of 3.1, weathers incongruently toform kaolinite, with a Ge/Si ratio of 5.9. Bulk soil and saproliteGe/Si and Al/Si ratios reflect a mixture of Ge and Al-poorquartz and Ge and Al-rich kaolinite (Fig. 5).

Although complicated in detail by multiple mineral phasesweathering at different rates, Ge/Si fractionation during weath-ering at Luquillo appears to adhere fairly well to the schematicillustrated in Figure 1. Ge is strongly retained in neoformedkaolinite during weathering of primary minerals. Roughly halfof the Si weathered from primary minerals is retained bykaolinite. The fraction of Si that is released to porewatersshould have a low, highly fractionated Ge/Si ratio. More in-tense weathering resulting in the transformation of kaolinite togibbsite will release Si with the high Ge/Si ratio characteristicof kaolinite.

7. CONCLUSIONS

Several lines of field and laboratory evidence indicate thatretention of Ge by soil clay minerals is the primary mechanismthat controls Ge/Si fractionation in the weathering environ-ment. Young (0.3 to 2.1 ka), incipiently weathered Hawaiiansoils have Ge/Si ratios close to Hawaiian basalt (2.5 �mol/mol). Intensely weathered soils 20 ka and older have Ge/Siratios ranging from 6 to 24 �mol/mol. Our data suggest thatGe/Si fractionation is not related to Ge-Fe or Ge-organic asso-ciations. Sequential extraction experiments indicate that mostof the soil Ge is associated with a secondary silicate fraction.Furthermore, we observed preferential incorporation of Ge byallophane (the most important silicate weathering product inthe Hawaiian soils) during precipitation from aqueous solution

Table 7. Composition of Luquillo parent material, saprolites, and minerals.

SiO2

(wt%)Al2O3

(wt%)Ge

(ppm)Ge/Si

(�mol/mol)Al/Si

(molar ratio)

diorite 60.1 17.4 1.7 2.4 0.3Igneous mineralsbiotite 37.6 14.0 2.8 6.1 0.4quartz 99.2 0.3 0.6 0.5 0.0hornblende 46.9 7.3 3.8 6.6 0.2feldspar* 53.3 27.1 1.8 3.1 0.6Saprolite10 (soil) 2.6 0.422 68.3 17.2 2.2 2.6 0.366 60.7 22.7 2.4 3.3 0.4127 61.5 21.5 2.7 3.6 0.4199 66.9 20.1 2.4 3.0 0.3241 3.3 0.5Saprolite mineralsqtz 99.2 0.3 0.6 0.5 0.0biotite 43.3 29.2 2.7 5.3 0.7kaolinite 42.3 31.1 3.0 5.9 0.9

* Feldspar composition was calculated by mass balance, see text.

1535Ge/Si weathering fractionation

in the laboratory. However, because some Fe-rich extractionscontained very high Ge/Si ratios (although with small amountsof Ge), we cannot rule out some role for Fe in the Ge/Siweathering fractionation. Soils and saprolites developed ongranitic rocks show less pronounced Ge/Si fractionation thansoils developed on basaltic rocks. Granitic rocks do not developextremely elevated Ge/Si ratios due to accumulation of primaryigneous Ge-poor quartz. However, weathering of plagioclasedoes produce a Ge-enriched kaolinite. This study confirms thatsoil is the high Ge/Si reservoir that balances the low Ge/Siratios observed in stream waters globally, and supports thesuggestions of Murnane and Stallard (1990) and Froelich et al.(1992) that high Ge/Si ratios in soils reflect retention of Ge byclays during incongruent silicate weathering.

Acknowledgments—The authors thank Sarah Aciego, Jon Chorover,Lucia Tyler, and Maura Weathers for valuable contributions to thisstudy. We thank David Hendricks, Peter Vitousek, Flip Froelich, RickMortlock, Larry Cathles, and Jen Shosa for interesting discussions ofsoils, germanium, and thermodynamics. Art White provided a completeset of samples for the Luquillo study. Reviews by Jacques Shott, GlebPokrovski, and Flip Froelich greatly improved the manuscript. Thisresearch was supported from Cornell’s NSF-RTG program in Biogeo-chemistry and Environmental Change, and by NSF-EAR 9631740.A.C.K. acknowledges support from the Geological Society of America,Sigma Xi, and the Clay Minerals Society, which were a great helpgetting this research started.

Associate editor: J. Schott

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