The role of disseminated calcite in the chemical ...snobear.colorado.edu/Markw/WatershedBio/Strontium/white_99.pdf · underlain by granitoid rocks. Calcite is commonly associated

PII S0016-7037(99)00082-4

The role of disseminated calcite in the chemical weathering of granitoid rocks

ART F. WHITE,1 ,* THOMAS D. BULLEN,1 DAVISON V. VIVIT ,1 MARJORIE S. SCHULZ,1 and DAVID W. CLOW2

1U.S. Geological Survey, Menlo Park, California 94025, USA2U.S. Geological Survey, Denver, Colorado 80225, USA

(Received June25, 1998;accepted in revised form September11, 1998)

Abstract—Accessory calcite, present at concentrations between 300 and 3000 mg kg21, occurs in freshgranitoid rocks sampled from the Merced watershed in Yosemite National Park, CA, USA; Loch Vale inRocky Mountain National Park CO USA; the Panola watershed, GA USA; and the Rio Icacos, Puerto Rico.Calcite occurs as fillings in microfractures, as disseminated grains within the silicate matrix, and asreplacement of calcic cores in plagioclase. Flow-through column experiments, using de-ionized watersaturated with 0.05 atm. CO2, produced effluents from the fresh granitoid rocks that were dominated by Caand bicarbonate and thermodynamically saturated with calcite. During reactions up to 1.7 yr, calcitedissolution progressively decreased and was superceded by steady state dissolution of silicates, principallybiotite. Mass balance calculations indicate that most calcite had been removed during this time and accountedfor 57–98% of the total Ca released from these rocks. Experimental effluents from surfically weatheredgranitoids from the same watersheds were consistently dominated by silicate dissolution. The lack of excessCa and alkalinity indicated that calcite had been previously removed by natural weathering.

The extent of Ca enrichment in watershed discharge fluxes corresponds to the amounts of calcite exposedin granitoid rocks. High Ca/Na ratios relative to plagioclase stoichiometries indicate excess Ca in theYosemite, Loch Vale, and other alpine watersheds in the Sierra Nevada and Rocky Mountains of the westernUnited States. This Ca enrichment correlates with strong preferential weathering of calcite relative toplagioclase in exfoliated granitoids in glaciated terrains. In contrast, Ca/Na flux ratios are comparable to or lessthan the Ca/Na ratios for plagioclase in the subtropical Panola and tropical Rio Icacos watersheds, in whichdeeply weathered regoliths exhibit concurrent losses of calcite and much larger masses of plagioclase duringtransport-limited weathering. These results indicate that the weathering of accessory calcite may stronglyinfluence Ca and alkalinity fluxes from silicate rocks during and following periods of glaciation and tectonismbut is much less important for older stable geomorphic surfaces.Copyright © 1999 Elsevier Science Ltd

1. INTRODUCTION

Ca release and CO2 consumption during chemical weatheringof crystalline silicate rocks have important geochemical impli-cations. Acidification of watersheds in North America andEurope has led to depletion of base cations in soils to the extentthat Ca has become a potential limiting element in forestproductivity in severely impacted areas (Shortle and Smith,1988; Bailey et al., 1996; Lawrence et al., 1997). Rates of Careplenishment and acid neutralization by weathering of granit-oid rocks is, therefore, an important factor in developing reg-ulatory constraints on atmospheric emissions (Drever, 1988).Chemical hydrolysis of Ca–silicate rocks is also recognized aspotentially controlling long term climate change by providing afeedback with atmospheric CO2 drawdown (Berner, 1991;Brady and Carroll, 1994; Berner and Caldeira, 1997). In con-trast, weathering of carbonate rocks does not have a corre-sponding impact because all the CO2 consumed in weatheringis reintroduced back into the atmosphere by relatively rapidprecipitation of carbonates in the oceans. Thus, the proportionsof silicate and carbonate weathering at the earth’s surface areimportant in long term global CO2 balances.

The mass of Ca found in granitoid rocks resides predomi-

nately in plagioclase feldspar. Garrels (1968) originally pro-posed that proportions of aqueous Ca and Na associated withthe weathering of granitoid rocks approximate the ratio foundin plagioclase. However, recent studies demonstrate that excessCa occurs in many surface and ground waters relative to thisplagioclase stoichiometry (Stauffer, 1990). Short term Ca ex-cesses in water can result from selective loss of exchangeableCa in soils during watershed acidification (Hyman et al., 1998)or loss from biologic storage due to deforestation and fires(Chorover et al., 1994). Longer term excesses in Ca relative toplagioclase stoichiometry have been attributed to acceleratedweathering of other silicate phases such as hornblende (Clow etal., 1993) or to the selective leaching of an anorthite componentof plagioclase (Clayton, 1986; Williams et al., 1993).

Trace amounts of disseminated calcite are also cited assources of Ca weathering of granitoid rocks. Garrels and Mack-enzie (1967) originally introduced calcite dissolution in theirmass balance for ground water emanating from perennialsprings in granitoid rocks in the Sierra Nevada. More recently,Mast et al. (1990), Sueker (1996), and Clow et al. (1997) haveinvoked calcite to explain excess fluxes of Ca in watershedsunderlain by granitoid rocks. Calcite is commonly associatedwith metamorphosed gneisses and schists and hydrothermallyaltered granitoid rocks and is generally recognized as an im-portant contributor to Ca fluxes in associated watersheds(Drever and Hurcomb, 1986; Axtmann and Stallard, 1995,Anderson et al, 1997; Blum et al., 1998). However, the distri-butions and paragenesis of accessory calcite in relatively pris-

*Author to whom correspondence should be addressed. Art F. White,U.S. Geological Survey, Water Resources Division, 345 MiddlefieldRd., MS420, Menlo Park, CA 94025; Tel: (650) 329-4519; Fax: (650)329-4538; E-mail: ([email protected]).

Pergamon

Geochimica et Cosmochimica Acta, Vol. 63, No. 13/14, pp. 1939–1953, 1999Copyright © 1999 Elsevier Science LtdPrinted in the USA. All rights reserved

0016-7037/99 $20.001 .00

1939

tine granitoids, which comprise significant portions of continentalland masses, are not well established. Likewise, the extent towhich this calcite contributes to Ca and dissolved inorganic carbonfluxes in watersheds and large river systems is unknown.

This study investigates sources for the release of excess Ca bydetailing the content and distribution of disseminated calcite ingranitoid rocks and presenting effluent data for long term experi-mental weathering studies on both fresh and naturally weatheredgranitoids. These results are used to interpret solute concentrationsand weathering fluxes observed in associated watersheds.

2. METHODOLOGIES

2.1. Materials

Pairs of fresh and weathered granitoid rock samples were obtainedfrom four research watersheds: the upper Merced River in YosemiteNational Park, California, USA; the Loch Vale watershed in RockyMountain National Park, Colorado, USA; the Panola Mountain Re-search Watershed near Atlanta, Georgia, USA; and the Rio Icacoswatershed in the Luquillo Mountains of Puerto Rio. Fresh samples werecollected from exposures in blasted road cuts and from drill core.Weathered samples were taken from naturally exposed bedrock sur-faces. Nomenclature, mineralogy, sample collection information, andreferences are included in Table 1. The four granitoid rocks will bereferred to based on their geographic association rather than on formalgeologic names, i.e., the Yosemite granodiorite, the Loch Vale granite,the Panola granodiorite, and the Rio Icacos quartz diorite.

2.2. Experimental Setup

Fresh and weathered rock samples were processed through a jawcrusher and a disc mill and size-separated into 0.25–0.85 mm grain sizefractions. Approximately 750 g of sample were introduced into 2.4 cminner diameter (ID) Pyrex columns. During the experiments, distilled/de-ionized water, saturated with a 5% CO2/95% air gas mixture, wasintroduced through fritted glass supports at the column bases. Flowrates were controlled by gravity flow through capillary tubes.

The effluent discharge from the top of each column passed througha 0.45mm filter and was collected in bottles which were sampled at weeklyintervals during the first 3 months of the experiments, every 2 weeks from3 to 12 months and every 3 weeks thereafter. The total duration ofexperimental weathering ranged from 1.03 104 to 1.53 104 h (1.1–1.7yr). The flow rate averaged 10 ml/h and the total effluent volumes thatpassed through the columns ranged between 100 and 150 l.

2.3. Analyses

Two analytical methods, coulometric titration and acid digestion/gaschromatography, were used to determine inorganic carbon in the gran-itoid rocks. Coulometric titration, the standard method for determiningcarbonate contents in silicate rocks (Jackson et al., 1987), consists ofdigestion of 0.5 g of ground sample with 2 M perchloric acid. Theevolved CO2 is converted to a strong titratable acid by ethanolamine.

The acid is titrated with a coulometrically generated base and theendpoint is detected colorimetrically. Analyses performed by XRALLaboratories of Toronto, Canada had a minimum detection of 100 mgkg21 CO2, which corresponds to 227 mg kg21 calcite in the rock.

The total carbon in many of the granitoid rock samples was belowthe detection limits of the coulometric method and a technique withbetter sensitivity was devised based on gas chromatography. Finelyground 10 g rock samples were introduced into 250 ml gas collectingbulbs (Ace Glass #7395-18) and then sequentially evacuated andflushed with high purity N2. Ten ml of 1 N hydrochloric acid wasintroduced with a syringe through a side arm rubber septum. Thesamples were periodically shaken for 24 h after which a 10 ml gassample was extracted with a gas tight syringe and analyzed using aCarle Series 400 AGC gas chromatograph equipped with a thermalconductivity detector. Oxygen was concurrently measured as an indi-cator of potential air contamination. Calculations based on Henry’s lawindicated that the amount of CO2 retained in the liquid relative to thehead space gas was negligible at acid pH and ambient temperature.

A sample of the Yosemite granodiorite was used as a calibrationstandard during the study since no primary carbonate standard isavailable for silicate rocks. Repeated analyses of this sample over aperiod of several months (N 5 5) produced an average value of 530627 mg kg21 CO2. A calibration curve was generated using differentmasses of this standard and by methods of addition using NaHCO3.Acid digestions were also conducted at temperatures of both 25 and50°C to test the effects of temperature on equilibration. Longer evac-uation times were also investigated to access the potential effects ofsorbed CO2. Neither parameter had any discernible effect of the anal-yses. A linear regression fit between the expected and measured calciteconcentrations produced a slope of 1.01 and a correlation ofr2 5 0.99.A detection limit of 10 mg kg21 CO2 was established for the technique.

Cation concentrations in the experimental effluents were determinedby inductively coupled plasma-mass spectroscopy (ICP-MS) using aPerkin Elmer Elan 6000 with a precision of 5%. Major element rockcompositions were determined by x-ray fluorescence spectroscopy (XRF).Alkalinity and pH were measured on collected effluents. Disseminatedcalcite within the granitoids is difficult to detect with plain light opticalmicroscopy. However under a cathodoluminescent source, calcite lumi-nesces a bright orange. Calcite occurrences in thin sections were mappedwith a Cambridge Model CCL 8200 cold cathode luminescence stage.More detailed morphologic features of calcite grains were characterized byelectron backscatter using a Cambridge scanning electron microscope.In situ calcite analyses were performed by a JEOL 8900 microprobe.

3. RESULTS

3.1. Granitoid Mineralogy and Compositions

The major element and CO2 contents of the fresh and naturallyweathered granitoid rocks used in the column studies are presentedin Table 2. The fresh granitoids exhibit a considerable compositionrange from the low-SiO2, high-CaO, MgO Rio Icacos quartzdiorite to the high-SiO2, low-CaO, MgO Loch Vale granite. Theplagioclase compositions of the Yosemite and Panola granodior-

Table 1. Geology and mineralogy associated with granitoids used in the experimental studies.

RockDominantmineralogy Age Source Reference

Upper Merced River,Yosemite, National Park,CA, USA

Granodiorite ofArch Rock

Quartz, oligoclase,microcline, biotite,hornblende

Cretaceous Road cut and weatheredexfoliated surface

(Calkins et al.,1985)

Loch Valle, Rocky Mtn.National Park, CO, USA

Silver PlumeGranite

Quartz, oligoclase,microcline, biotite

Precambrian Fresh and weatheredsurface exposures

(Cole, 1977)

Panola Mtn. ResearchWatershed, GA, USA

Panola Granite(granodiorite)

Quartz, oligoclase,microcline, biotitemuscovite

Mississippian-Pennsylvanian

Drill core, fresh andweathered fractures

(Higgins et al.,1988)

Rio Icacos watershed,Luquillo Mtns., PR

Rio BlancoQuartz Diorite

Quartz, andesine, biotite,hornblende, microcline

Tertiary Road cut and weatheredcore stone

(Seiders, 1971)

1940 A. F. White et al.

ites and the Loch Vale granite correspond to Na-rich oligoclases,while plagioclase in the Rio Icacos quartz diorite is a significantlymore calcic andesine. All the granitoids contain biotite and thePanola granodiorite also contains muscovite. The low K contentsof the Rio Icacos quartz diorite indicate a lack of significantK–feldspar. All the granitoids, except the Loch Vale granite,contain hornblende. The weathered granitoid samples do not ex-hibit any consistent differences in major element compositionsrelative to the fresh samples (Table 2). This indicates that thesesamples represent the initial phases of surficial weathering andhave not undergone significant loss of primary minerals or majorelement mobilization.

The data in Table 2 show a generally good correlationbetween measured CO2 contents using coulometry and gaschromatography. Discrepancies at low concentrations are at-tributable to the higher limits of detection of the coulometrictechnique. For the fresh samples, the Panola granodiorite hasthe highest CO2 content (1460 mg kg21) and the Rio Icacosquartz diorite was the lowest content (140 mg kg21). There isno correlation between CaO and CO2 contents in the freshgranitoids. The weathered samples have much lower CO2 con-

tents (30–40 mg kg21), indicating that carbon loss must havebeen occurred during the initial stages of granitoid weathering.

3.2. Calcite Compositions

The microprobe oxide analyses of selective calcite grains infresh granitoids are reported in Table 3. The compositions aredominated by Ca with minor amounts of Mn, Fe, and Mg(0.03–1 oxide wt%) and trace amounts of Sr (0.01–0.1 oxidewt%). These elements all form carbonate phases that form solidsolutions with calcite and probably reflect partitioning withmagmatic or hydrothermal fluids of differing compositions.Elevated SiO2 and Al2O3 concentrations reflect analytical con-tamination arising from surrounding silicate grains.

3.3. Calcite Morphology

Three calcite morphologies were observed in thin section,disseminated grains in the silicate matrix, calcite replacementof plagioclase, and calcite in fracture fillings. The Rio Icacosquartz diorite contains interstitial calcite as small disseminatedgrains and as larger (up to 200mm) grains (Fig. 1A). In the

Table 2. Selected chemical composition of fresh and weathered granitoid rocks.

CO2a

(mg kg21)CO2

b

(mg kg21)Calcite

(mg kg21)CaO

(wt%)Na2O(wt%)

MgO(wt%)

K2O(wt%)

SiO2

(wt%)Al2O3

(wt%)PO4

(wt%)

Yosemite GranodioriteFresh (An0.33) 400 290 670 3.00 3.32 1.44 2.53 71.0 14.1 0.08Weathered 200 30 68 2.45 3.93 0.75 2.70 69.5 15.2 0.06

Loch Vale GraniteFresh (An0.21) 400 440 1000 0.78 2.56 0.28 6.46 72.7 13.9 0.13Weathered ,100 30 68 0.94 2.85 0.33 5.57 73.2 14.3 0.13

Rio Icacos Qtz DioriteFresh (An0.50) 100 140 320 7.11 3.19 2.43 1.07 59.0 17.8 0.09Weathered ,100 40 91 6.82 3.04 2.59 0.82 58.8 17.5 0.08

Panola GranodioriteFresh (An0.25) 1200 1460 3320 1.93 3.29 0.99 4.43 68.2 14.8 0.19Weathered ,100 30 68 1.61 1.49 1.30 5.77 69.4 14.7 0.19

a Determined by coulometric titration.b Determined by gas chromatography.

Table 3. Electron microprobe analyses of calcite grains in granitoid rocksa.

Granite CaO CO2 MgO Al2O3 SiO2 MnO FeO SrO Total

Panola 55.7 37.6 0.152 0.003 0.002 0.656 0.807 0.053 94.9Granodiorite 54.2 41.3 0.159 0.010 0.006 0.583 0.748 0.028 97.1

53.0 46.9 0.154 0.003 0.005 0.650 0.851 0.087 101.752.7 44.7 0.245 0.003 0.004 0.830 1.178 0.043 99.752.5 45.1 0.175 0.005 0.010 0.722 0.788 0.149 99.553.0 45.5 0.158 0.018 0.012 0.711 0.787 0.038 100.3

Yosemite 54.4 39.1 0.213 0.043 0.044 0.748 0.387 0.032 95.0Granodiorite 54.6 39.2 0.242 0.009 0.035 0.746 0.421 0.028 95.3

54.0 38.8 0.213 0.017 0.028 0.868 0.387 0.091 94.449.5 38.5 0.008 3.290 5.644 0.030 0.009 0.019 96.952.6 40.9 0.004 1.532 2.779 0.033 0.055 0.026 98.0

Puerto Rico 55.2 40.2 0.130 0.018 0.007 0.832 0.301 0.045 96.7Qtz Diorite 54.8 40.5 0.136 0.023 0.049 0.709 0.290 0.013 96.5

55.6 38.1 0.091 0.001 0.022 0.702 0.166 0.043 94.7Loch Vale 53.9 40.9 0.041 0.196 0.518 0.380 0.277 0.027 96.3Granite 47.4 46.4 0.090 1.639 5.344 1.793 0.858 0.062 103.6

51.3 45.4 0.018 0.937 2.744 1.087 0.359 0.019 101.9

a Each value is an average of three analyses on one grain in wt%.

1941Role of disseminated calcite in chemical weathering

Panola granodiorite the calcite is abundant and occurs in rela-tively large interstitial grains (;100–300mm) and as smallergrains between silicate grain boundaries (Fig. 1B). The Yo-semite sample exhibits the most diverse calcite morphologies,with occurrences as small disseminated grains, larger grains,and as replacements of seriticized cores of plagioclase that mayhave been originally enriched in Ca (Fig. 1C). In the Loch Valegranite, calcite occurs as disseminated grains and as fracturefillings that often cut across silicate grains (Fig. 1D) as waspreviously observed by Mast et al. (1990).

There is a lack of documentation in the literature on theorigin of calcite in granitoid rocks. The observations in thepresent study indicate that there may not be a unique paragen-esis. Calcite is associated with both unaltered and sericitizedsilicate phases. Calcite also occurs along obvious pathways forfluid entry such as microfractures, as well as interstitially withinsilicate grains with minimal evidence for fluid interaction. Sharp,crystalligraphically controlled boundaries such as observed be-tween calcite and plagioclase in Fig. 1A suggests a primary origin,while replacement of plagioclase by calcite in Fig. 1C is clearlya secondary reaction. Calcite probably forms from CO2-richfluids associated with the final cooling of the batholiths as wellas during later periods of hydrothermal activity.

3.4. Column Effluent Compositions

Representative effluent compositions from the fresh andweathered granitoids are tabulated in Table 4 at'102, 103, and104 h. The effluent pHs and alkalinities from the fresh grani-toids were significantly higher than those from the weatheredgranitoids at comparable times, indicating more rapid reactionand CO2 uptake. With increasing time, pHs and alkalinitiesdecreased in both the fresh and weathered granitoid effluents,indicating progressively decreasing reaction rates.

For the fresh granitoids, Ca dominated the initial effluentcomposition while the final solutions contained significantlyhigher proportions of Si and K. Principal sources of Ca in thegranitoids are plagioclase and calcite. The much lower concen-trations of Mg in the effluents indicate that hornblende, theother major Ca-containing silicate in all but the Loch Valegranite, was not contributing significant amounts of Ca to theeffluent. Ca concentrations were much lower in both the initial andfinal effluents from the weathered granitoids, which were domi-nated by Si and K. Na concentrations were generally lower than Kin all the effluents, indicating that biotite and/or K–feldspar isreacting faster than plagioclase in the column experiments.

Other accessory minerals can potentially contribute to efflu-

Fig. 1. Electron backscatter images of accessory calcite in granitoid rocks used in this study. (A) Interstitial calcite grainin Rio Icacos quartz diorite surrounded by plagioclase and quartz. (B) Interstitial calcite grains bordering large plagioclasephenocryst in Panola granodiorite. (C) Calcite pseudomorphic replacement within a sericitized plagioclase grain in theYosemite granodiorite. (D) Calcite fracture filling in Loch Vale granite (brightness in upper fracture is due to electricalcharging, it does not contain calcite).


ent Ca concentrations. For example, Irber et al. (1998) reportedthat Sr and Ca were released principally from apatite during theexperimental weathering of highly differentiated granites.However in our experiments, measurable phosphate was belowdetection levels in the effluents (,0.2 mm) indicating negligi-ble apatite dissolution. Anhydrite and fluorite have also beenreported in hydrothermally altered granitoids (Turpault et al.,1992; Savage et al., 1993). Initial sulfate concentrations in theeffluents (max5 5.6mM) decreased with time (Table 4). Muchof this sulfate is probably associated with oxidation of acces-sory pyrite that is observed in the granitoids. Ca associated withanhydrite dissolution at these sulfate levels would not contrib-ute significantly to total Ca in the effluents. Measurable Fconcentrations also occurred in the fresh granitoid effluents(Table 4). These F concentrations remain significantly lowerthan the corresponding Ca concentrations. Much of this F isassociated with the weathering of biotite (Earley et al., 1995).Anion compositions in the effluents from the weathered grani-toids are much lower than for the fresh samples (Table 4).

3.5. Calcite versus Silicate Leaching from FreshGranitoids

Based on the preceding evidence, plagioclase and accessorycalcite are the main sources of effluent Ca in the experiments.The corresponding dissolution reactions are:

CaCO3 1 CO2 1 H2O3 Ca21 1 2HCO32 (1)

and

CaxNa~12x!Al ~11x!Si~32x!O8 1 ~1 1 x!CO2 1 ~2 1 2x!H2O

3 xCa21 1 ~1 2 x!Na1 1 ~3 2 x!SiO2

1 ~1 1 x!HCO32 1 ~1 1 x!Al(OH)3, (2)

wherex is the anorthite component in the feldspar (Anx) and Alis assumed to be conserved in an Al hydroxide phase. Theprincipal aqueous species involved in the above reactions areCa21 Na1, Si, H1 (pH), and HCO3

2 (alkalinity).Rapid changes in Ca, Na, and Si during the initial 5000 h of

reaction of the fresh granitoids are plotted in Fig. 2 and slowerchanges between 5000 and 15000 h are plotted in the corre-sponding inserts. During the first 1500 h, effluents from theYosemite, Rio Icacos, and Loch Vale granitoids are dominatedby high Ca concentrations, which rapidly decrease with time.Ca concentrations from the Panola granitoid are comparable tothe initial effluents from the other granitoids but higher con-centrations persist for longer times before declining after3000 h. In all the experiments Na was much lower than Ca.Silica was also lower than Ca in the effluents for the first 5000 hin the Yosemite, Loch Vale, and Panola granitoids. Si exceedsCa concentrations for the Rio Icacos granitoid after 1000 h.Over longer times (5000–15,000 h), Ca continued to decreasein all the effluents, although at slower rates (Fig. 2, insets). Incontrast, Na and Si levels became essentially constant withtime, indicating relatively steady state silicate reaction rates.Silica equaled or exceeded Ca by the end of all the experimentswhile Na remained significantly lower.

Effluent Ca and alkalinity (HCO32) expressed as micro-

equivalents, in addition to pH, are plotted against time in Fig.3. As expected for calcite and silicate dissolution, decreasing

Table 4. Representative effluent compositions from column experiments using fresh and weathered granitoid rocksa,b.

Sample Time (h) Vol (I) pH Alkc Na Mg K Ca Sr Si F Cl SO4

Yosemite Granodiorite(Fresh)

76 1 8.25 750 14.1 26.9 122.4 258.1 0.3787 54.0 1.5 1.97 2.60983 12 7.35 280 2.2 28.9 520.5 260.2 3.3301 17.1 0.9 0.28 0.58

14,644 151 6.28 37 2.1 2.3 9.2 5.4 0.0000 20.86 0.6 0.01 0.25Rio Icacos Qtz Diorite

(Fresh)141 3 8.21 510 53.8 15.5 76.8 111.5 0.1083 64.9 2.7 51.34 5.63

1144 17 6.73 190 13.5 2.5 91.5 25.1 0.0522 29.9 0.6 1.07 1.1014,008 158 6.37 33 3.3 1.7 10.4 7.8 0.0086 29.3,0.5 0.01 0.06

Loch Vale Granite(Fresh)

144 3 8.11 900 3.9 18.6 38.6 186.6 0.0009 49.4 11.8 11.57 1.291124 19 6.66 163 1.7 1.3 5.4 81.6 0.0028 13.0 32.9 1.21 1.15

13,989 143 6.02 18 2.7 0.9 2.0 7.3 0.0060 15.0 0.6,0.01 0.04Panola Granodiorite

(Fresh)102 2 8.14 460 8.8 13.7 67.0 210.5 0.3248 60.4 30.3 2.20 2.81

1105 18 8.21 1125 3.4 3.7 49.5 502.7 0.3159 82.0 104.3 0.34 1.1813,969 161 6.35 45 1.6 1.1 5.5 29.4 0.0169 23.7 0.7,0.01 0.10

Yosemite Granodiorite(Weathered)

70 1 6.61 80 14.0 7.9 28.5 18.0 0.0788 56.0 ,0.5 nac nac

1004 14 5.76 25 0.6 0.6 5.2 1.5 0.0075 13.6 ,0.5 0.02 0.0713,364 133 5.92 6 0.4 0.2 1.2 0.5 0.0023 9.0 ,0.5 ,0.01 ,0.02

Rio Icacos Qtz Diorite(Weathered)

120 1 7.33 130 161.1 25.0 23.7 25.0 0.0781 53.1 7.3 nac nac

1056 12 6.96 105 31.0 3.7 26.8 14.0 0.0671 52.7 7.9 0.01 0.0610,555 109 6.19 80 3.0 1.7 11.4 6.1 0.0210 27.5,0.05 ,0.01 ,0.02

Loch Vale Granite(Weathered)

60 1 6.62 100 9.3 6.2 40.7 24.7 0.0558 38.8 8.4 nac nac

1004 13 6.31 60 2.3 0.8 12.0 7.9 0.0246 14.3 1.1 0.02 0.0513,364 133 5.73 4 1.3 0.1 1.0 0.5 0.0020 11.6 ,0.5 ,0.01 ,0.02

Panola Granodiorite(Weathered)

73 1 6.41 50 29.5 4.5 18.9 8.5 0.0595 168.9 2.4 nac nac

1004 14 5.67 15 0.6 0.6 8.4 2.1 0.0185 19.1 2.6 0.04 0.1010,293 112 5.62 5 0.5 0.1 1.3 0.3 0.0016 6.5 1.2,0.01 ,0.02

a Units are inmM except as noted.b na 5 not analyzed.c Predominantly HCO32.


Ca concentrations correlate with decreasing pH (Eqns 1 and 2).However, a direct relationship between Ca and alkalinity equiva-lencies is expected only for calcite dissolution (Eqn 1). Thiscorrelation is close to unity throughout the experiments for theLoch Vale and Panola granitoids (Fig. 3), which have the highestcalcite contents and effluent Ca concentrations (Table 4). Effluentalkalinities are elevated with respect to Ca for the Yosemite andRio Icacos granitoids (Fig. 3). These effluents are also higher in Sirelative to Ca (Fig. 2), indicating that alkalinity is derived bothfrom calcite dissolution and silicate hydrolysis. The low Na andhigh K concentrations in the effluents (Fig. 2 and Table 4) indicatethat this “silicate alkalinity” is attributed principally to biotiteweathering. Experimental dissolution rates for K–feldspar areshown to be slower than for plagioclase (Blum, 1994) and are notexpected to contribute significantly to the Si concentrations.

3.6. Calcite Versus Silicate Leaching in WeatheredGranitoids

The importance of silicate relative to calcite dissolution ismuch more evident for experiments on the weathered grani-toids (Table 4). As indicated by the example for the weatheredPanola granitoid (Fig. 4A), Si rather than Ca dominates initialeffluent compositions (0–5000 h). This indicates that eventhough the fresh Panola granodiorite had the highest calcitecontents of any of the granitoids (Table 2), calcite is insignif-icant in the weathered sample. This is also substantiated by high

alkalinity relative to Ca (Fig. 4B). This alkalinity is produced byCO2 consumption during silicate hydrolysis (Eqn 2) and not bycalcite dissolution (Eqn 1). At long times (5000–15,000 h), Naand Ca concentrations remain very low relative to Si. High Kconcentrations (Table 4) indicate that silicate weathering is dom-inated by biotite and not plagioclase weathering.

3.7. Sr/Ca Ratios

Microprobe analyses indicates that the average Sr concen-trations in plagioclase from the Yosemite, Panola, Rio Icacos,and Loch Vale granitoids are, respectively, 1100, 750, 560, and175 mg kg21. The relative Sr/Ca ratios in plagioclase aresignificantly higher than those in accessory calcite based onTable 3. Effluent Sr/Ca ratios should reflect the relative con-tributions of calcite and plagioclase dissolution, assuming thatthese two mineral phases are the major sources of Ca and Sr inexperimental weathering and that releases are congruent. Thisrelationship is demonstrated by the relatively low Sr/Ca ratiosfor calcite relative to plagioclase in the Yosemite and RioIcacos granitoids (Fig. 5, horizontal lines). Effluents from thefresh granitoids have Sr/Ca ratios that fall between these endmember ratios and are dominated by calcite (Fig. 5). A simplemixing model using Sr and Ca concentrations indicates thatcalcite contributes a maximum of 94 wt% Ca to the Yosemiteeffluent at'750 h. The Sr/Ca ratios increase in the effluentwith longer times as expected for decreasing proportions of

Fig. 2. Concentrations of Ca, Na and Si in column effluents from fresh granitoid rocks as functions of reaction time.Inserts show low level concentration data at longer times.


calcium to silica (Fig. 2). After 14,000 h, the contribution ofcalcite is still 80%. The Sr/Ca ratios in the Rio Icacos effluentare initially lower and remain relatively constant with time.This corresponds to a calcite contribution of'65% to effluentCa compared to 35% from plagioclase.

The effluent Sr/Ca ratios from the weathered granitoids werehigher than their fresh counterparts as shown by the examples forYosemite and Rio Icacos granitoids (Fig. 5). This correlates withhigher Si concentrations in the effluents and to the higher propor-tion of Sr to Ca contributed from plagioclase dissolution. Theinitial effluents from the weathered Rio Icacos quartz dioriteexceed the plagioclase Sr/Ca suggesting nonstochiometric ex-change of Sr and Ca from altered biotite or secondary clay min-erals (Bullen et al., 1997).

Accessory calcite may significantly impact the solute87Sr/86Sr ratios derived from granitoid weathering. This effect has beendemonstrated in leaching studies of the Loch Vale granite by Clowet al. (1997). The role of accessory calcite in granitoid rocks hasimportant ramifications in terms of interpreting global weatheringhistory as based on the marine87Sr/86Sr isotopic record (Burke etal., 1982). Details on the distributions of87Sr/86Sr in the presentexperimental studies will be addressed in a companion paper.

4. DISCUSSION OF EXPERIMENTAL RESULTS

4.1. Calcite Reactivity

At near neutral pH, the dissolution rate of calcite ('1025

mole m22 s21; Plummer et al., 1978) is approximately 7 orders

Fig. 3. Ca and alkalinity equivalencies and pH in column effluents from fresh granitoid rocks as functions of reactiontime.

Fig. 4. Effluent compositions from weathered Panola granodiorite.(A) Concentrations of Ca, Na and Si. Inserts show low level concen-tration data at longer times. (B) Ca and alkalinity equivalencies and pH.


of magnitude faster than the dissolution of plagioclase feldspar('10212 mole m22 s21; Blum, 1994). In the column experi-ments, the kinetics of plagioclase dissolution are dominated bysurface reactions far from equilibrium. Speciation indicates thatthe initial effluents (Table 4) are 3–4 orders of magnitudeundersaturated with respect to the albite component of plagio-clase and the final solutions are 5–6 orders of magnitudeundersaturated.

In contrast, initial calcite dissolution is much closer to equi-librium, where the amount of calcite dissolved will be effec-tively limited by solute transport through the column and alonggrain boundaries and interstices (Fig. 1). Calcite saturationcalculated from Ca concentrations, pH, and alkalinity measuredin column effluents is plotted in Fig. 6 as the ratios of log ionicactivity product (IAP)/Ks with time. Initial column effluentsfrom the fresh granitoids are close to calcite saturation (logIAP/Ks ' 0) at less than 500 h (Fig. 6). The Yosemite, RioIcacos, and Loch Vale effluents become progressively moreundersaturated at longer times and reach a log IAP/Ks ' 25after 15,000 h. Decreases in calcite saturation correlate withdecreases in effluent Ca, alkalinity, and pH (Figs. 2 and 3). The

fresh Panola granodiorite contains significantly more carbonatethan the other fresh granitoids (Table 2) and correspondingeffluents remain close to calcite saturation for a much longertime (3000 h). The initial effluents from the weathered grani-toids were significantly undersaturated with calcite (log IAP/Ks , 25) and became even more undersaturated with time(log IAP/Ks , 210). Theextent of this undersaturation cor-relates with lower Ca, alkalinity, and pH relative to the freshgranitoids.

4.2. Mass Transfers

The concentrations of calcite initially present in the freshgranitoid samples and the masses remaining after reaction arereported in Table 5. Experimental leaching removed most ofthe calcite from the columns, with losses ranging from 85% ofthe initial calcite in the Loch Vale granite to 99% in the Panolagranodiorite. These losses contributed significantly to the totalCa released to column effluents, which ranged between 2.63

Fig. 5. Molar Sr/Ca ratios in column effluents from (A) fresh andweathered Yosemite granodiorite and (B) Rio Icacos quartz diorite asfunctions of reaction time. Dashed lines correspond to Sr/Ca ratios incalcite and plagioclase.

Fig. 6. Calcite saturation indices in effluents as functions of reactiontime. IAP is the ionic activity product of CaCO3 and Ks is the calcitesolubility product. A log IAP/K ratio of zero denotes thermodynamicsaturation; a negative value indicates undersaturation. Dashed linecorresponds to solutions saturated with respect of calcite.

Table 5. Mass transfers calculated from granitoid compositions andcolumn effluent concentrations.

YosemiteRio

IcacosLochVale Panola

Total time (h) 14,644 14,008 13,989 13,969Initial calcite (mg kg21) 674 316 1004 3344Final calcite (mg kg21) 47 42 157 17Total effluent Ca (mM) 3.45 2.63 5.26 18.39Total effluent Na (mM) 0.31 1.05 0.20 0.19Final effluent Ca/Na 2.57 2.07 2.70 18.38Plagioclase Ca/Na 0.50 1.23 0.27 0.32Reacted plagioclase (mM) 0.74 1.98 1.43 0.63Reacted calcite (mM) 5.69 2.65 6.58 35.94%Ca from calcite 88 57 96 98%Ca from plagioclase 12 43 4 2% of total calcite loss 96 97 85 114


mmole for the Rio Icacos quartz diorite to 18.39 mmole for thePanola granodiorite (Table 5). Total Na contributed to theeffluents was significantly less and inversely proportional toCa. The final Ca/Na ratios in the effluents remained signifi-cantly higher than the corresponding plagioclase stochiom-etries, ranging from 2.1 for Rio Icacos sample to 18 for thecalcite-rich Panola sample (Table 2). These elevated ratiosindicate that even though most of the calcite has been removed,residual amounts are still impacting the effluent Ca/Na ratios atthe end of the experiments. Correlations also exist between therelative amounts of calcite initially present, the total Ca in theeffluent, and the final Ca/Na ratio (Table 5).

The contributions of calcite and plagioclase to total soluteexports are estimated from the Na effluent concentrations inTable 5 by assuming congruent feldspar dissolution. Amountsof plagioclase reacted in the columns ranged between 0.63mmole for the Panola granodiorite to 1.98 mmole for RioIcacos quartz diorite. The calcite contribution, calculated as thedifference between total Ca and Ca contributed from plagio-clase, ranged from between 2.65 mmol for Rio Icacos to 35.94mmol for Panola. The percentages of total effluent Ca contrib-uted by plagioclase and calcite are tabulated in Table 5. Asindicated, compared to calcite, the plagioclase contributions toleaching of Ca from the Loch Vale and Panola granitoids wereinsignificant. In contrast, plagioclase and calcite contributednearly equally to effluent Ca from the Rio Icacos granitoid.This is due to the higher An component in the Rio Icacosplagioclase and to lower calcite contents.

The above mass balance approach is also used to calculatethe progress of calcite dissolution in the column experimentswith time. These data are plotted in Fig. 7 as the percentages oftotal calcite initially present in the granitoid rocks. At first, therates of calcite dissolution are rapid and then asymptoticallyapproach the amounts of calcite initially present in the freshgranitoids. The predicted masses of calcite reacted after'1.5 y,based on effluent compositions, range between 85% and 114%of the calcite measured in the fresh rocks (Table 5). Thesevalues are only slightly larger than the percentages calculatedfrom residual calcite concentrations measured at the end of theexperiments (Table 5). Overestimation of the mass of calcitereaction based on solute fluxes may have resulted from as-

sumed congruent dissolution of plagioclase and/or from thecontribution of Ca from other silicates such as hornblende.

5. ROLE OF CALCITE IN THE NATURAL WEATHERINGOF GRANITOID ROCKS

The preceding results demonstrate that the dissolution ofaccessory calcite dominates the effluents produced from theexperimental weathering of fresh granitoid rocks. In contrast,experiments on their weathered counterparts indicate the lackof calcite dissolution. These contrasts suggest that the impact ofcalcite in controlling solute Ca and alkalinity concentrations isdependent not only on the initial presence of calcite in thegranitoid rock, but also on the intensity of the weatheringenvironment. The following sections will investigate the impactof calcite dissolution on natural granitoid weathering and theeffects on watershed chemistry.

5.1. Ca/Na Ratios in Watershed Fluxes

Ca excesses associated with calcite weathering in granitoidrocks can be characterized from average annual chemical fluxesin watersheds (mol kg21 y21) based on hydrologic dischargeand normalized to geographical area (White and Blum, 1995).The average of multiyear Ca and Na fluxes for watershedscontaining the granitoid rocks sampled for the experiments aretabulated in Table 6. The specific watersheds are for the upperMerced River in Yosemite, Andrews Creek in Loch Vale,Panola Creek, and Rio Icacos. All flux data are corrected forconcentration inputs from wet precipitation. The Ca/Na dis-charge ratios are plotted against the average molar Ca/Na in thecorresponding granitoid plagioclases in Fig. 8. The diagonalline corresponds to the stochiometric dissolution of plagioclase,assuming that this mineral phase is the principal control onsolute Ca/Na ratios derived from granitoid weathering (Garrels,1968).

The Ca/Na flux ratios of the Yosemite and Loch Vale wa-tersheds plot above the stochiometric line (Fig. 8) indicating anexcess Ca relative to plagioclase weathering. Additional water-sheds in surrounding regions of the Sierra Nevada and RockyMountains exhibit similar Ca excesses (Table 6 and Fig. 8).Stauffer (1990) reported similar Ca excesses for alpine lakes inthese regions. While the Ca/Na ratio of plagioclase in thesewatersheds range between 0.32 and 0.43 (An24 to An30), thecorresponding Ca/Na ratios in watershed fluxes range between0.71 and 2.04 (Table 6). These solute ranges are bracketed bythe final experimental effluent Ca/Na ratios from the fresh(Ca/Na 5 2.07–2.57) and weathered (Ca/Na5 0.38–1.29)Yosemite and Loch Vale granitoids (Table 5). This suggeststhat the discharges are influenced by a combination of the freshand weathered granitoids, which are partially depleted in cal-cite.

In contrast to the Sierra Nevada and Rocky Mountain wa-tersheds, the average annual Ca/Na flux ratio in the Panolawatershed (0.38) approximates the corresponding plagioclasestoichiometry (0.30) (Fig. 8). The Rio Icacos Ca/Na flux ratio(0.62) is significantly lower than the corresponding plagioclasestoichiometry (1.05). This may be due to errors in estimatingwatershed Na inputs from marine aerosols (White et al, 1998).The lack of excess Ca in these watersheds cannot be explainedby lower calcite contents in the fresh granitoids. The Panola

Fig. 7. Percent of original calcite leached from granitoid rocks asfunctions time. Calculations are based on effluent concentrations andstoichiometric ratio of Ca and Na in plagioclase.


granodiorite has by far the highest calcite content of the freshrocks studied (Table 5; 3320 mg kg21) and exhibits the greatesteffects of carbonate dissolution in the column effluents (Figs. 2and 3). The lack of a direct correlation between the calcitecontent of the fresh granitoid and the extent of Ca excess in thestream waters must reflect differences in natural weatheringcondition in the watersheds.

5.2. Excess Ca in Watersheds of the Sierra Nevada andRocky Mountains

Significant seasonal variations in discharge and Ca/Na ratiosare observed in Loch Vale and Yosemite watersheds (Fig. 9).Data are also shown for Emerald Lake (Table 6), a muchsmaller watershed in the Sierra Nevada situated in SequoiaNational Park (Williams et al., 1993). Discharge in these threewatersheds ranges over nearly 4 orders of magnitude (Figs.9A–C) and correlates with their respective basin areas (660,46,000 and 2.7 ha). Discharge in all cases is dominated byspring snowmelt occurring between May and June of eachyear.

Ca/Na ratios during the period of active discharge in thethree watersheds (Figs. 9A–C) always exceed the stochiometricCa/Na ratios of the respective plagioclases (Table 6). Theseseasonal Ca/Na ratios are not corrected for inputs from wetprecipitation as was the case for average annual fluxes (Fig. 8).However, the Ca/Na ratios are lowest in the Yosemite dis-charge and are highest in the Loch Vale discharge, thus fol-lowing the same order of increasing Ca/Na observed in annualfluxes (Table 6). Ca/Na ratios in both the Emerald Lake andLoch Vale watershed decrease continually throughout the ris-ing and declining periods of discharge (Figs. 9A,B). Thissuggests that Ca is preferentially mobilized during the earlystages of snow melt in the spring and becomes progressivelydiminished later in the season. In contrast, the Yosemite Ca/Naratios generally increase with the rising discharge and subse-quently decrease with the declining discharge (Fig. 9C). These

Table 6. Watershed rocks, mineral compositions, discharge fluxes and Ca/Na ratios.

Watershed RockaPlagAn

Plagb

Ca/NaNac

fluxCac

fluxSic

fluxCa/Na

flux ratio Reference

Watersheds associated with experimental studiesYosemite, CA Arch Rock Granodiorite 0.32 0.47 375 247 1160 0.66 This studyLoch Vale, CO Silver Plume Granite 0.27 0.37 71 131 154 1.67 Mast et al. (1990)Rio Icacos, PR Rio Blanco Qtz diorite 0.40 1.05 3958 2120 8066 0.62 Axtmann and Stallard (1995)Panola, GA Panola granodiorite 0.23 0.30 260 103 676 0.38 Hooper et al. (1990)

Other Sierra Nevada watershedsEmerald Lake, CA Granodiorite 0.24 0.32 95 118 392 1.24 Williams et al. (1993)Pear Lake, CA Granite 0.30 0.43 35 46 na 1.30 Lydecker (1998)d

Crystal Lake, CA Granodiorite 0.30 0.43 70 58 na 0.83 Lydecker (1998)d

Log Creek, CA Giant Forest Granodiorite 0.30 0.43 394 281 466 0.71 Williams and Melack (1997)Ruby Lake, CA Quartz Monzonite 0.30 0.43 53 107 na 2.04 Lydecker (1998)d

Spuller Lake, CA Granodiorite 0.30 0.43 70 125 na 1.78 Lydecker (1998)d

Tarp Creek, CA Giant Forest Granodiorite 0.30 0.43 113 105 425 0.93 Williams and Melack (1997)Topaz Lake, CA Granodiorite 0.30 0.43 48 67 na 1.41 Lydecker (1998)

Other Rocky Mountain watershedsFall River, CO Granodiorite 0.27 0.37 98 101 204 1.03 Sueker (1996)Fern Creek, CO Granodiorite 0.27 0.37 150 201 333 1.34 Sueker (1996)Big Thompson River, CO Granodiorite 0.27 0.37 111 115 224 1.04 Sueker (1996)Boulder Brook, CO Granodiorite 0.27 0.37 167 70 384 0.42 Sueker (1996)Gem Lake, CO Chickenfoot Granodiorite 0.35 0.54 29 31 61 1.07 Stoddard (1987)North St. Vrain Creek, CO Granodiorite 0.27 0.37 143 155 341 1.08 Sueker (1996)Rabbit Ears, CO Quartz Monzonite 0.20 0.25 236 186 659 0.79 Peters and Leavesley (1995)Spruce Creek, CO Granodiorite 0.27 0.37 124 162 280 1.31 Sueker (1996)

a Formal geologic units are listed when known.b Molar ratios.c Units in moles ha21 yr21 corrected for wet precipitation.d Personnel communication.

Fig. 8. Ca/Na ratios in average annual watershed fluxes compared tomolar Ca/Na ratios of plagioclase in corresponding granitoid rocks(Table 6). Diagonal line assumes stoichiometric dissolution of plagio-clase. Flux data are corrected for wet precipitation.


differences in watershed behavior may, in part, be attributed tothe much larger scale of the Yosemite drainage and to longersurface and ground water transit times.

Seasonal trends in Ca/Na ratios could directly reflect differ-ences in the relative intensity of silicate versus carbonateweathering in the watersheds. For example, fresh rock surfaceson steep slopes may be exposed to snow melt earlier in theseason than more weathered surfaces overlain by soils andvegetation in valley bottoms. Seasonal trends also probablyreflect temporal differences in additional solute inputs andsecondary storage. In the alpine West Glacier watershed in theRocky Mountains, Finley and Drever (1997) found a strongcorrelation between Ca and SO4 during initial snow melt sug-gesting an additional atmospheric source. Seasonal changes inCa/Na ratios may also reflect exchange dynamics in watershedsoils and biomass uptake during the growing season (Dreverand Clow, 1995).

5.3. Impact of Calcite Dissolution

In cathodoluminescence studies of the Loch Vale granite,Mast et al. (1990) found that calcite occurred both in hydro-thermal alteration assemblages, including sericite and epidote,and along grain boundaries and microfractures of relativelyunaltered minerals. In a mass balance calculation, Mast et al.(1990) attributed 80% of the Ca discharge flux from the LochVale to calcite dissolution with the remainder coming fromplagioclase dissolution. In a later study based on87Sr/86Sr,Clow et al. (1997) attributed 50% of the watershed Ca toweathering of calcite in the granite.

Excess Ca of the magnitude observed in the Yosemite andEmerald Lake watershed (Figs. 9B,C) has long been recognizedin groundwater and surface waters of the Sierra Nevada ofCalifornia. In their classic paper on mass balances for silicateweathering, Garrels and Mackenzie (1967) observed that theCa/Na ratios in ephemeral springs reflected estimated plagio-

clase stoichiometry of the Sierra Nevada batholiths (An38),while the Ca/Na ratios in perennial springs were significantlyhigher. In this latter case, Garrels and Mackenzie apportionedthe largest percentage of solute Ca to calcite (70%) relative toplagioclase (30%). Although not specifically identified, calcitewas suggested as occurring as trace carbonates along the hy-drologic flow path.

Additional chemical analyses have further defined chemicalconcentrations in groundwaters and granitoid rocks of the Si-erra Nevada (Fig. 10A). A linear regression fit through thegroundwater Ca and Na data corresponds to a Ca/Na ratio of1.53. This ratio is significantly higher than that for the averageSierra Nevada granitoids (0.52), which is based on Ca and Naconcentrations plotted along separate axes in Fig. 10A. Thisaverage granitoid Ca/Na ratio corresponds to An34, assumingthat the bulk of Ca and Na resides in plagioclase. This stoichi-ometry is very similar to the Yosemite granodiorite used in thecolumn experiments (An33) and is more sodic than the originalestimate of An38 for the Sierra Nevada made by Garrels andMackenzie (1967).

A significant number of coulometric carbonate analyses havealso been performed on the Sierra Nevada granitoids (Fig.10B). Calcite as the source of excess Ca in the Sierra Nevadawaters is supported by the fact that 85% of the.150 analyseshad measurable CO2 (.100 mg kg21). Corresponding calciteconcentrations range from 0 and 4500 mg kg21 (Fig. 10B). Theaverage calcite content (110 mg kg21) is almost twice as highas measured for the Yosemite granodiorite used in the columnexperiments (670 mg kg21, Table 5). There is no direct corre-lation between the total CaO content of the granitoid and thecalcite concentration (Fig. 10B). These analyses indicate thattrace amounts of calcite, originally proposed to be presentbased on mass balance calculations (Garrels and Mackenzie,1967), are ubiquitous in the Sierra Nevada granitoids.

Fig. 9. Solute Ca/Na ratios in seasonal discharge for three alpine watersheds (A, B, C) and fractions of reacted calciteand silicates required to produce discharge concentrations (D, E, F).


5.4. Calcite Contributions Based on Mass Balances

The amounts of calcite and silicates required to produce theseasonal discharge compositions for Yosemite, Emerald Lake,and Loch Vale (Figs. 9A–C) were determined using a massbalance weathering approach similar to that of Garrels and Mac-kenzie (1967). The calculations are simplifications in that temporalvariations in solutes introduced from other sources such as sea-sonal snow melt, ion exchange, and biomass uptake are not con-sidered. Average mineral stoichiometries for Yosemite granodio-rite determined by microprobe in the present study are plagioclase,Ca0.34Na0.66Al1.34Si2.66O8; K–feldspar, K0.91Na0.09Si3O8; biotite,K1.81(Mg2.39Fe2.00Ti0.26)(Si5.29Al2.82)O20(OH)2; and hornblende(Na0.29K0.144)(Ca1.82Mg2.58Fe1.68)[Si6.59Al1.35O20] (OH)2. TheEmerald Lake and Loch Vale mineral stoichiometries are thosereported, respectively, by Williams et al. (1993) and Mast et al.(1990). The moles of Na, K, and Mg were sequentially subtractedfrom the discharge chemistries using the respective plagioclase,biotite, and hornblende stoichiometries. K–feldspar is assumed toweather significantly slower than biotite and not to contribute to Ksolute concentrations (Blum et al., 1994). Excess Ca remainingafter these calculations is attributed to calcite dissolution.

Mineral contributions to seasonal discharge concentrationsexpressed in mole fractions are plotted for the Loch Vale,Emerald Lake, and Yosemite watersheds in Fig. 9. Calculationsfor Loch Vale indicate that calcite contributes between 75 and85 mole% of the total Ca in the discharge flux with theremainder coming from An27 plagioclase (Fig. 9F). Hornblendeis not a significant mineral in the Loch Vale granite. Theseresults are comparable to mass balance results for annual fluxesreported by Mast et al. (1990).

Mass balances for Emerald Lake (Fig. 9E) indicate thatbetween 36 and 62 mole% Ca is contributed from calcitedissolution with the remaining mass derived from An34 plagio-clase (25–48 mole%) and hornblende (10–26 mole%). Previ-

ously, Williams et al. (1993) constructed mineral weatheringbalances based on An24 plagioclase and hornblende to initiallybalance Ca watershed compositions. Significant excess Ca (60mole%) was then attributed to nonstoichiometric weathering ofanorthite. This approach was based on earlier work showingthat calcic cores of plagioclase were often preferentially weath-ered in granitoid rocks of the Idaho Batholith (Clayton, 1986).However, the Ca excesses observed at the Emerald Lake dis-charge require dissolution of large amounts of Ca-enrichedlabradorite (An53), a calcic component not commonly observedin plagioclase zoning in Sierra Nevada rocks (Bateman andChappell, 1979).

Mass balances for seasonal Ca discharge for Yosemite (Fig.9D) indicate that the calcite contribution (31–60 mole%) iscomparable to that of Emerald Lake. High K relative to Mgconcentrations implies greater biotite weathering as suggestedby Blum et al. (1994) and a lack of a Ca contribution fromhornblende in the Yosemite watershed. The larger size of theMerced drainage contributes to greater uncertainties in theweathering reactions. In a synoptic study of the upper MercedRiver, Clow et al. (1993) found a spatial correlation betweentributary Ca/Na ratios (0.43–1.17) and corresponding bedrockCa/Na ratios, which ranged from between 0.2 and 0.4 for granitesto 0.4 and 1.0 for granodiorites and tonolites. In addition, Clconcentrations in the upper Merced River are significantlyelevated relative to precipitation, suggesting a groundwatercontribution that does not reflect surficial granitoid weathering.

The preceding calculations using realistic weathering reac-tions reproduce observed solute concentrations in dischargefrom the three watersheds. Some of the temporal variationsmay be attributed to nonweathering sources, such as atmo-spheric, deposition in snowmelt, cation exchange in soils, andbiomass uptake. However the interpretation of the data indicatesconsistent Ca excesses that are explained by calcite dissolution.

Fig. 10. (A) Comparison of the Ca to Na concentrations in groundwaters (circles) with Ca to Na concentrations in SierraNevada granitoid rocks (X). Diagonal line is linear regression fit to the groundwater data. (B) Calcite concentrations basedon measured CO2 contents compared to CaO contents of Sierra Nevada granitoids.


5.5. The Role of Weathering Intensity on Ca Excesses

Unlike watersheds in the Sierra Nevada and Rocky Moun-tains, the Panola and Rio Icacos watersheds do not exhibit Caexcesses in discharge that could be attributed to calcite disso-lution (Fig. 8). This lack of Ca is not related to the lack of calcitein the underlying fresh granitoids, which is evident both in anal-yses of the fresh rocks and from experimental effluent composi-tions (Tables 2 and 4). Rather, geomorphic age and climate appearto be the most significant factors that differentiate the Panola andRio Icacos watersheds from the other watersheds. Cosmogenicisotope dating of deep regolith profiles in the Panola and RioIcacos watersheds produced ages.200 kyr (Bierman et al., 1995;Brown et al., 1995). These surfaces are much older than for alpinewatersheds in the Sierra Nevada and Rocky Mountains thathave undergone glaciation during the last 10 kyr. In addition,the Panola and Rio Icacos watersheds are subtropical to tropical(average annual air temperature5 15 and 23°C, respectively)compared to the alpine watersheds. Both the greater age andwarmer climates are expected to promote a greater weatheringintensity in the Panola and Rio Icacos watersheds, which maybe expected to significantly impact the relative weathering ratesof calcite and silicates in the granitoid rocks.

The above hypothesis is tested based on detailed weatheringprofiles for granitoid rocks collected from the Panola andYosemite watersheds. The Panola profile was obtained fromdrill core through a 350 kA ridge top surface which succes-sively penetrated soil, saprolite, and bedrock to a depth of 12 m(Fig. 11A). Both calcite, based on CO2 measurements, andplagioclase, as approximated by total CaO, are completely de-pleted in the soil and underlying saprolite horizons. Plagioclase isalso depleted in the upper granodiorite and only approaches theinitial granitoid composition across a sharp weathering front at adepth of 8–11 m (Fig. 11A). The calcite weathering front, which

mimics plagioclase, is offset downwards by less than a meter. Thiscoincidence implies that plagioclase and calcite are being lostconcurrently during granitoid weathering. Due to its muchlarger mass, plagioclase dissolution dominates solute Ca andNa concentrations derived from this weathering front. Concur-rent plagioclase and calcite weathering explains the close cor-relation between Ca/Na ratios in Panola stream waters andthose based on the plagioclase stoichiometry (Fig. 8).

The weathering profile on the Yosemite granodiorite was ob-tained from a quarry face in the upper Merced River drainage (Fig.11B). The profile consisted of a thin soil/grus horizon (,0.5 m)underlain by a thick sequence of exfoliated granite consisting offractured sheets of approximately 0.5 m thickness. This sequencewas underlain at a depth of about 8 m by unfractured bedrock.Consistent CaO contents with depth in the exfoliated rock denotedno loss of plagioclase relative to the granodiorite composition. Incontrast, approximately two thirds of the calcite was depleted inthe exfoliated granodiorite relative to the fresh bedrock, apparentlyby penetration of water along cracks and fractures within theexfoliation plates. Relative to plagioclase, calcite is being veryselectively weathered from the exfoliated granitoid. Even thoughthe amount of Ca contained in the calcite is very small relative toplagioclase, this weathering selectivity produces the observed Caexcesses observed in stream waters in the Sierra Nevada.

The selectivity in rates of calcite and plagioclase weatheringin granitoid rocks invokes the concept of chemically limitedversus physically limited weathering regimes as originally de-fined by Stallard and Edmond (1983) and discussed in thecontext of selective calcite weathering at Loch Vale by Dreverand Zorbrist (1992). In a chemically limited regime, potentialmechanical erosion is faster than chemical weathering. Undersuch conditions, weathering is highly selective and is depen-dent on mineral-specific reaction rates. Only the most reactive

Fig. 11. Comparison of calcite and CaO contents in profiles of (A) weathered saprolite and granodiorite in the Panolawatersheds and (B) exfoliated granodiorite in the upper Merced watershed in Yosemite National Park.


phases contribute to solute fluxes. Such a situation is occurringin the Yosemite granodiorite weathering profile, which hasundergone relatively recent glaciation. Calcite, which is react-ing orders of magnitude faster than plagioclase, is being selec-tively leached. Comparable conditions exist for the other wa-tersheds of the Sierra Nevada and Rocky Mountains. Excess Cain watershed discharge is expected in such weathering-limitedsystems if accessory calcite is present in the granitoid rocks.

The antithesis of a weathering-limited regime is the physi-cally limited regime. When mechanical erosion is less intense,the most reactive mineral phases have been depleted fromaccessible portions of the exposed rocks. Weathering is no longerdependent on mineral reaction kinetics but on fluid transport. Thissituation is apparent in the deep weathering profile within thePanola granodiorite. Pore waters are probably thermodynamicallysaturated with respect to calcite and the dissolution of both calciteand plagioclase is limited by transport rates of incoming fluidsalong microfractures and grain boundaries. Comparable condi-tions are expected in the Rio Icacos and other watersheds thatare dominated by relatively old weathering surfaces.

These findings suggest accessory calcite dissolution in gran-itoid rock can contribute a significant portion of the total Caand alkalinity exported to watersheds and rivers during orfollowing periods of glaciation and other processes such astectonism, which exposes significant fresh granitoids at theearth’s surface. This process complicates estimates of the ap-parent weathering rate of granitoid rocks based on solute fluxesand may explain many of the apparent discrepancies associatedwith estimates on buffering capacities and climate impacts. Forexample, in a compilation of a large number of well-character-ized watersheds on granitoid rocks, White and Blum (1995)found a good correlation between annual precipitation and tem-perature and chemical weathering fluxes for Si and Na, which arerepresentative of silicate weathering. The lack of a correspondingclimate correlation for Ca and alkalinity fluxes found by theseworkers may be attributed, in part, to variable inputs fromcalcite dissolution, which is watershed and not climate specific.The use of Ca and alkalinity fluxes to delineate weathering andclimate feedback for larger river systems draining dominantlycrystalline rocks is fraught with even greater uncertainty.

6. CONCLUSIONS

Calcite, at levels from several hundred to several thousandmg kg21 occurs in microfractures, as disseminated grains andas replacement of plagioclase in the granitoid rocks studied.Calcite forms either during late stage cooling of granitoidplutons or subsequent periods of hydrothermal activity. Col-umn experiments on crushed fresh granitoids produced efflu-ents that are dominated by equivalent Ca and alkalinity, and areinitially saturated with respect to calcite. At longer times, theseconcentrations decreased and the effluents become dominatedby silicate reactions, principally the weathering of biotite. Mixingcalculations based on Ca/Na and Sr/Ca ratios indicate that calcitecontributes between 50 and 98% of the Ca measured in theeffluents. Calcite is nearly totally removed by the termination ofthe experiments (.1.5 yr). Experiments on corresponding weath-ered granitoids do not exhibit excess Ca, indicating that calcitewas preferentially removed under natural weathering conditions.

Characterization of solute fluxes in watersheds, from which

the granitoid samples were derived, exhibit variable Ca ex-cesses that do not correlate directly with the initial calcite concen-trations nor with the Ca/Na ratios in the experimental effluents.Discharge from alpine watersheds on recently glaciated granitoidbedrock exhibits significant calcite dissolution, while geomorphi-cally older watersheds do not exhibit excess Ca. This pattern isfurther demonstrated in the preferential weathering of calciterelative to plagioclase in an exfoliated granitoid profile, com-pared to concurrent rates of calcite and plagioclase weatheringin an older granitoid profile underlying a saprolitic regolith.

These findings suggest accessory calcite dissolution in gran-itoid rock can contribute a significant portion of the total Caand alkalinity exported to watersheds and rivers basin in whichsignificant amounts of bedrock are recently exposed. Variablecalcite inputs to the weathering signature may explain many ofthe apparent discrepancies that are associated with estimates onbuffering capacities and climate impacts on the weathering ofgranitoid rocks.

Acknowledgments—The authors thank Tom Huntington and MattLarsen of the U. S. Geological Survey for collecting the granitoidsamplesfrom the Panola and Rio Icacos watersheds. The cathode lumi-nescence work was performed at the Monterey Bay Aquarium ResearchInstitute. Robert Oscarson and Lew Calk aided in the scanning electronmicroscope (SEM) andmicroprobe study. The authors also thank JamesDrever, Joel Blum, and an anonymous reviewer for helpful insights.The work was supported by the Water Energy and BiogeochemicalBudget (WEBB) project of the U.S. Geological Survey Program onGlobal Change.

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The role of disseminated calcite in the chemical ...snobear.colorado.edu/Markw/WatershedBio/Strontium/white_99.pdf · underlain by granitoid rocks. Calcite is commonly associated