Prediction of some weathering trends of plutonic and ... - Science Direct

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Prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations H. W. NESBITT and G. M. YOUNG Dept. of Geology, Univ. of Western Ontario, London, Ontario, Canada N6A 5B7 (Received February 6, 1984; accepted in revised form April 16, 1984)

Abstract-The exposed crust consists mainly of plagioclase (35%), quartz (20%). K-feldspar (1 I%), volcanic glass (12%), biotite (8%), and muscovite (5%). Quartz is a resistate. thus feldspan and glass represent approximately 75 percent of the labile minerals. The weathering characteristics of these constituents are summarized in the context of the~~ynamic, mass balance and kinetic considerations. Experimentaily determined release rate constants were used to predict the proportions of Ca. Na and K released by feldspars of plutonic rocks (granites to gabbros) to weathering solutions. The chemical weathering trends of the weathered residues, calculated from the kinetic data, conform closely to the initial trends observed in some recent weathering profiles, demonstrating the accuracy of the predictions. Since the weathering of feldspars is controlled by processes that should not change through geological time. the relative release rates of Ca, Na, and K from the feldspars of granitic rocks can be calculated for future and past episodes of continental weathering. Experimentally determined release rate constants are not available for a wide range of volcanic glass compositions, but the limited data indicate that compositional trends are predictable in weathering profiles developed on volcanic rocks. The kinetic data available for rhyolitic glasses accurately predict the initial weathering trends observed in a recent rhyolite weathering proftle.

WEATHERING,

TRANSPORTATION

and deposition

are

processes influencing the properties of sediments. The physical effects of these processes have received considerable attention as is evident by the advances in sedimentology over the past 50 years, but less emphasis has been placed on the chemical aspects and particularly on the chemical properties of elastic silicate detritus. Some early studies of siliciclastic sediments and sedimentary rocks, however, dealt with their chemistry (CLARKE, 1924) and some of the studies of PETTIJOHN (1963) and MID DLETON(1960) emphasized average com~sitions, but ~nve~i~tions such as those by HIRST( 1962a,b), NANCE and TAYLOR (1976, 1977), and MCLENNAN et al. (1980. 1983) are exceptional in using chemical compositions of individual samples to deduce the history of the sedimentary rocks. This paper is the first of a series presenting results of investigations into effects of weathering, transportation and deposition on the bulk chemical composition of siliciclastic sedimentary rocks. We here emphasize the weathering of phttonic and volcanic rocks, using thermodynamic, kinetic and mass balance constraints. These predicted changes are compared with the chemical com~sitional trends observed in recent weathering profiles in an attempt to gain a better understanding of the weathering process. The agreement suggests that future and ancient trends can be accurately predicted. BLAT”~and JONES (1975, Table 3) determined that approximately a quarter of the exposed crystalline rocks is intrusive (granitic), a quarter is extrusive (volcanic) and a half is metamorphic and “Precambrian”. The average mineralogical composition of the upper crust three of the most important

is estimated (appendix, Table 2, column D). The composition of the exposed crust (Table 2, column E) differs from the upper crustal estimate primarily in the presence of volcanic glass. Considering quartz, and the Feand Ti-bearing oxides as resistates, plagioclase, potash feldspar and glass constitute 75% of the exposed minerals readily susceptible to chemical weathering (labile minerals) so that studies concerning the weathering of the exposed crust necessarily must emphasize the behaviour of these phases. Weathering of phyllosilicates is also important, but the amphiboles, pyroxenes and olivines are volumetrically small; their importance in weathering of exposed crust and their cont~butions to sediments being pro~~onately less. Clay minerals accumulate primarily as muds that ultimately become lutites, the most common sedimentary rock type (PETTIJOHN, 1975, p. 260). Since clays are the major alteration products of feldspars and glass, weathering characteristics of the primary phases must be known to properly interpret the chemical composition of lutites and other sedimentary rocks containing significant amounts of clay minerals. An understanding of these aspects will allow the use of the chemical composition of elastic sedimentary rocks to help deduce paleoenvironments and paleoclimates (NESBITTand YOUNG, 1982). AND MASS CONSIDERATIONS

THERMODYNAMIC BALANCE

The granitic and other feldspathic rocks

The stabilities of some common minerals in aaueous solution are shown on Fig. 1. Thermodynamic data are from HELGESON (1969), NESM~

(1977). and HELGESON et al.

(1978). Labelled fields show the range of water compositions in which the designated minerals are thermodynamically stable. Minerals are not stable in waters that plot outside their

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H W. Nesbitt and G. M. Young

i:?,. /--

i-

3;,

* /

!

+-

!

Ru/~

--’

‘4

‘;I---, t_

=

.

; i

‘ - _-&..__--112....-._.” ..:_: t

FIG. I. The~~ynamic stability relations between some minerals and weathering solutrons. Waters iion; granitic and rhyolitic {circles) and from basaltic and gabbroic terrains (squares) are taken from WFWFIi. al. (1963. Tables 1 and 2 respectively). The dots represent waters from granitic and weathered gramtit terrain (PACES, t972). The dotted curves denote iilite stability field, the composition of which iv K~Al~S4-~I”~,~OH)~ where ‘n’ may vary between 0 and 1. as indicated by the numbers associated with the straight dotted fines. The dashed lines are met&able extentions of stability field boundaries Gib. = gibbsite; Kaoi = kaolinite; Beid = smectite (beidellite component); Pyr = pyrophyllite: MUX = muscovite; Ab = albite; K-spar = microcline stability fields and the potential exists for the minerals and solutions to react. These reactions are realized. The clay minerals are evidence. Although reactions involving silicates may proceed slowly at ambient temperatures, they neces&iy proceed towards an equilibrium state as required by the laws of thermodynamics. The ~~b~urn diagrams (Fig. 1)therefore can be used to predict which minerals will react with natural waters, to provide information about the nature of the reaction products and to predict the directions these reactions take. Fe&pars are the most abundant minerals in the upper continental crnst (Table 2, column D). They are stable only in natural waters of relatively high cation/H+ values and/or high aqueous silica concentrations (Fig. 1). Most ofthe waters of weathering profiles are derived from rainwater with compositions (Fig. 1, rectangles) well removed from the feldspar stability field. The rainwater is made acidic by dissolution of atmospheric CO2 and CO? derived from oxidation of organic

materiais within w~the~ng profiles. The potentmtairherefore exists for reaction between the carbonic acidcharged rainwater and the fetdspars. As reaction proceeds the feldapars are consumed and the soiution approaches equilibrium with the feldspars according to Le Chatelier’s principle. The solution must theEfore approach the stability f&d of feldspar. The precise path fotiowed by the solution during reaction cannot be predicted without kinetic data, but the predicted reaction products (Fig. I) are kaolinite, smectites (montmorillonites and beidellites), iilites and @&site for the solution ~m~sition must migrate through the stability fields of some or all of these clay minerals to reach the stabiity fields of the primiu?; phases HELGESONet al. (1969) predicted the results of reaction of feldspars with dilute CO+harged waters using local equr. iibrium concepts and mass balance constraints. ~Fhese cal. culations will provide reasonable resuits only when near-equi. librium (often metastabie) conditions prevail Krnr~c d&+

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Weathering trends sf! igneous rocks are required to make accurate predictions. The calculations of HELGESONet al. (1969), however, agree with the above qualitative predictions and with available field data. Ground waters(WHInez al., 1963; PA&S, 1972) collected from granitic and rhyolitic (Fig. 1,circlesand dots) and from gabbroic and basaltic terrain (Fig. I, squares) are plotted on Fig. 1. The water data trend across the kaolinite field into the beidellite and illite fields as predicted by HELGE~ON et al. (1969). Kaolinite is the common weathering product of plagioclase and other primary minerals (GRANT, 1963; GARRELS,1967; GARREKS and MACKENZIE, 1967; NESBII-Iet al., 1980). Illite is a common weathering product of granitic rocks (GRANT,1963) and smectites are abundant secondary products after maf~cminerals and basic rocks (PATTERSON,1971). These observations support the calculations and interaction of the weathering processes made by HEVXSON ef at. (f969), but the results are still of a very general nature. Details of many specific mineral-solution reactions are unknown and some important minerals produced during weathering (such as vermiculite) are not included in the calculations. The mechanisms controlling rates of weathering reactions are also poorly understood.

Althou~ albite glass (abfglf) stability field is much smaller than that of crystalline albite (ab(cr)), the clay mineral fields still separate rainwater compositions from the glass field (Fig. 2). As with mystalhne n-i&, the composition of the weathering solutions is predicted to evolve up and across the diagram as cations (Na, K, Ca, Mg) and aqueous silica are leached from the solids and carbonic acid is consumed by the reactions (pH increases). The same weathering products are predicted to form from albite as form from albite( but their proportions may be very different depending on the relative dissolution rates of glass and primary minerals and the rates of formation of the secondary minerals. Thermodynamic data for K-feldspar glass indicate that the stability of potassic phases is essentially similar except that the illite stability field is enlarged (instead of the smectite field), when glass is the w~th~ng precursor. The results suggest that Na-smectite-illite ratios in volcanic rock weathering profiles may reflect Na/K values in the fresh rock. Ground waters from volcanic terrains (Fig. I, see WHITE et al. 1963 for analyses) are similar to waters from granitic terrain in that they are equilibrated with the same clay minerals (NESBI’IT,1977).

The volcanic rocks

Thermodynamic and mass balance considerations suggest that the results of weathering both volcanic and plutonic rocks are similar. As in the case of plutonic rocks, none of the waters derived from volcanic terrain plots close to the stability field of the primary phase (glass) and the same clay minerals are formed. The proportions of secondary minerals produced should be different as a result of differing stability fields of clay minerals, the composition of the fresh rock and the rates at which dissolution and secondary phase production proceeds.

Volcanic rocks contain both glass and minerals. The stabilities of the most abundant primary minerals have been considered. The thermodynamic properties (hence stabilities) of only a few glasses are known, but these allow deductions and predictions about the weathering of glass. Albite glass can be considered a ~e~~ynamic component of glasses (BuR~~M and DAVIS, 1974). Its stabiity in the presence of a soiution and some common clay minerals (and crystalline albite) is shown in Fig. 2 (albite glass data from ROBIEand WALDBAUM,1968). The albite glass is much less stable than its crystalline equivalent at 25°C and I bar pressure (compare size of stability field of glass in Fig. 2 with size of crystalline albite field in Fig. 1). The stability field of Na-smectite is greatly expanded when glass is the primary phase. (compare Fig. I and Fig. 2). Its enlarged stability field suggests that smectites (montmorillonites) may be a more common weathering product after glass (volcanic rocks) than after crystalline materials provided there is no unusual kinetic behaviour. Smectites are common after volcanic material (CRAIG and LOUGHNAN,1964; NADEAUand REYNOLDS,198 1).

ZONATIONS WEATHERING Devefapment

AND NOMENCLATURE OF PROFILES AND SOLUTIONS

of weathering

zones

GARRBLS( 1967) and HEL@SON et al. ( 1969) viewed the weathering process as an acid-base reaction where an acid, commonly H2C03, is neutralized by a solid base such as a feldspar or glass to produce secondary clay minerals and dissolved salts. Plagiocfase, K-feldspar, other alkali- and alkaline earth Al-silicates and volcanic glass weather to clay minerals, the feldspars commonly to kaolinite and illite, the mafic minerals and glass commonly to smectites as well as the above clays. The overall reaction is, using albite as primary phase and kaolinite as the secondary product,

2H&O:, + ZNaAl%Oa + Hz0 carbonic albite acid

A12SirOs(OH), kaolinite

+ 2Na+ + 2HCO; f 4Si02 (aq). dissolved salts

FIG. 2. Thermodynamic stability relations between some common clay minerals and albite glass (At@)). The stability field of crystalline albite (Ah(c)) is delineated by the dashed boundary. Na-M = smectite as in Fig. I (Na-M equivalent to Na-Reid).

(1)

An entirely analogous reaction can be obtained by substituting other primary Al-silicates for albite and other clay minerals such as smectite for kaolinite. For all reactions Al is considered to be essentially insoluble, thus remaining in solid phases. This stipulation is used to construct Fig. 1 and it requires that the ahove reaction be balanced as shown. From reaction ( 1), Na”/

152h

H. W. Nesbrtt and G. M. Young

H”. HC03 and SiOZ (aq) increase sympatheti~i~y in solution as weathering proceeds. As illustrated in Figs. 1 and 2 they behave as predicted in the waters fisted by WHITE et al. (1963) and PACES ( 1972) and as predicted by HELGESON et al. (1969). Reactions of CO,-charged solutions with K-. Ca- and Mg-bearing Al-silicates and natural glasses can be treated and interpreted in an analogous way. Natural weathering solutions therefore evolve from low to high values of Na’/H’, K+/H+. Ca’+/(H)+‘, Mg”‘/(H)+*, HC03 and Si02 (aq) (Figs. 1 and 2). The mineralogy of weathering profiles generally evolves sym~theticaiiy. Waters emanating from the organic zone of profiles contain abundant carbonic acid, thus the most extensively weathered materials, gibbsite and kaolinite, normally occur within and just below this zone. The solutions are progressively neutralized by reaction with solids as they migrate through the profile towards the fresh rock. The mineralogical assemblages and proportions change with illites and smectites increasing in amount towards the fresh rock, ultimately to give way to dominantly primary phases. The weathering profiles and associated waters can be classified in a manner that reflects their genetic evolution

Dilute soil waters charged with CO2 and organic acids are the primary agents of weathering. The acids are neutralized by reaction with primary and some secondary minerals (incongruent reaction). Soil waters that do not react appreciably with solid bases generally remain acidic displaying low Nat/ H+, K’/H’, Ca*+/(H)+*, Mg2+/(H)“‘, HCO, and SiO, (aqj values. These waten are referred to as primirive weather& solufions and they generally are associated with minerals, such as gibbsite. kaolinite and quartz, that are stable in acidic solutions. The residual weathering zone consists primarily of these minerals and primitive weathering solutions. Waters that have reacted to a greater extent with solid bases are iess aggnzsive; some of their dissolved acids are neutralized and they contain greater quantities of dissolved constituent. The above ratios, bicarbonate and dissolved aqueous silica are increased over the values found in primitive soil solutions. These waters approach or achieve equilibrium with illites and smectites, but not with gibbsite or primary minerals (excluding quartz) and they are referred to as evolved weathering solutions and the zone of inlermediarr wearhering contains appreciable quantities of one or more of the mineral groups of illites. smectites and vermiculite (Fig. 11.

Waters that achieve ~u~b~um with respect to ihl: pnrnat~~ phases such as one of the feidspars. volcanic g&s or muscovitc {Figs. 1 and 3) are referred to as mulure weathering .co~t~trms They are commonly associated with weathering zones whew feldspars or glass are little altered and are abundant. the :onr’ of incipient weathering. Mature weathering solutions are not necessarily equilibrated with respect to all priman mineral\ There is a close relationship between the vveatheriny, solutions and the mineralogical zones in space and time. As weathering proceeds the residuai zone encroaches upon the intermediate zone while the intermediate zone encroaches upon the incipiently wearb ered and fresh rock zones. The zones migrate to lowc~ fevels, one displacing the other, as a result of the mgration, through time, of the primitive anJ rvnivrc! solutions to progressively greater depths. KlNETIC CONSIDERATIONS RATES OF CONSTITlJENTS

The precise rates of weathenng of pnmac minerals such as feldspars can be evaluated only from kmetirdata. To date there has been no attempt to ngorou& apply kinetic data to weathe~ng rates or ‘iit cornpar’:predicted changes with those observed uz narurai weathering profiles. BUSENBERC~and CLEMENCY (1976). PE~KOVIC i? al. (1976). HOLDREN and BERNER (1979~ BERNER and HOLDREN ( 1979), and PETROVK ( 198I a,b) argue that comminution produces defects and dtsiocations in crystals and also produces small fragments. ali ut which modify the apparent dissolution rates ,rf mm erals. The studies showed, however, that in the absence of comminution, Na, K and C’a extraction trilrn f&ispars obeyed a linear rate law

f‘i = A.;(*.

.J_

_L._._.._..

-. __ --

.----

:

where Ci is the concentration of‘ species i?i Witi’ tion at time I: Ki is the reiease rate constant And ‘:’ is a constant. Feldspars subject to ~7 ,si/lc usathenng such as occurs during the ii-trmation UT tiprohtet, (GRANT, 1963; NESBI’U tJ[ u!, 1980) arc iir)t i;om minuted and their dissolution rates arc :~pzctcd I(,

ill0 _.

AND RELEASE. FROM ROCKS

+.f[.(]i

.‘.

. .._.~l..

_

bt.. (mgi/!

..-

- -----.--_-

FIG. 3. Relationship between the Na’/H’ ratio and the concentration of bicarbonate ofthe waters plotted in Fig. 1. Waters derived from granitic and rhyolitic terrains (circles) and basaltic and gabbroic terraIn< (squares) are taken from WHITEef al. (1963, see Fig. caption 1). The dots are waters derived from gramt~-. and weathered granitic terrain (PA&X 1972). See text for interpretation of trends.

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Weathering trends of igneous rocks follow a linear rate law (Eqn. 2). The release rate ratio of two species “i” and “j” is, from Eqn. (2): ~d~~~di~~~d~j~d~~ = Ki:Kj,

(3)

hence is independent of time. BUSENBERGand CLEMENCY (1976) determined release rate constants (Ki) and Na, Ca, and K from plagioclase and potassic feldspar. These data are used to calculate the relative rates of release of Na, Ca, and K from plutonic rocks. The average caic-alkali granite (NOCKOLDS, 1954, Table 1) contains in its norm, 32.2 gm K-feldspar, 26.2 gm Ab, 5.6 gm An, and 0.8, 1.3, 1.7 gm of C, En, and Fs. Most of the granites (64 of 72) contain biotite and 30% contain muscovite; consequently normative En, Fs and C have been combined with KAlS$& component to produce annite, phlogopite and muscovite, leaving 29.0 gm normative K-feldspar. There are then, 0.103 moles of K-feldspar (Mks) and 0.122 moles plagioclase (Mpl) per 100 gm of average talc-alkali granite. The plagioclase composition is approximately 83 percent Ab and 17 percent An, so that the release rate constants of Ca and Na from the plagioclase are 1O-‘6-~ and 10-‘5.57(Fig. 4). The release rate constant of K from microcline is 10-‘5~“7. The relative release rates of these elements (considering the number of moles of minerals present in 100 gm of rock) are:

utor of cations to weathering solutions in granitic terrain. Mineralogical and chemical studies of granitic weathering profiles (GRANT, 1963) also show that plagioclase alters rapidly compared with K-feldspar. Equation (5) is rearranged to illustrate better the accordance: (Ca + Naz):Kz = 3.1:1. (6) The average talc-alkali granite composition is plotted on Fig. 5a (dot) as is the proportion indicated by Eqn. (6). Extraction of these elements in the indicated ratio (Eqn. 6, Fig. 5a, open circle) necessitates that the composition of the weathering residues evolves initially in the direction indicated by the arrow emanating from the dot. Analogous calculations have been made using average adamellite, average granodiorite, average tonaiite, and average gabbro (NOCKOLDS, 1954). The predicted chemical weathering trends (Fig. Sa, arrows} of the five rock types are sub-parallel to the (CaO + Na20)-A&O3 boundary, indicating that Ca and Na are removed in preference to K. As weathering proceeds plagioclase may change composition, and thus change the ratio of Ca and Na removed from the feldspar and ultimately the direction of the weathering trend. The only circumstance where this situation would not arise would be if the ratio of the removal rate constants equalled the ratio of Na and Ca in the plagioclase, or; log(&/kc,

= Mpl * luc- :Mpl * KNa:Mks * Kx .

)=

l%fxAbfxAn

)

(7)

(4)

where XA, and X,, are mole fractions of albite and anorthite components in plagioclase. From Eqn. (7) the two release rates should be equal when plagioclase contains equal amounts of Ab and An. The dashed curve (Fig. 4) was calculated using this condition and Eqn. (7), the correspondence with the experimental Sodium constitutes two thirds of the total. GARRELS data indicating that Eqn. (7) is reasonably accurate. ( 1967) and GARRELS and MACKENZIE (1967) con- The first segments of the weathering trends (Fig 5) cluded that plagioclase is the most important contribshould be essentially straight as long as the two feldspars follow their respective rate laws. The proportions of Ca, Na and K extracted from the granite during the initial weathering stages therefore are: Ca:Na:K = 0.6: 1.9: 1.O. (5)

Es~j~~~ion of’& butk ~e~r~erjng rate of biotite in granitie rocks

I

An % -

Abe/o

-

FIG. 4. Relationships between Ca and Na reaction rate constants and plagioclase composition. The dots represent data from BUSENBERC and CLEMENCY (1976). The solid curves are fitted to the data and the dashed curve is calculated using Eqn. (8) and the kt values for Ca.

Most of the Fe0 in the Toorongo Granodiorite resides in biotite (MARKOVICS, 1977) and its decrease in abundance during weathering results primarily from its oxidation to ferric iron (NESBI-IT, 1979). Since Mg and K are retained in the secondary products of biotite (NESBI~ et al., 1980), the oxidation of Fe(B) probably is the best monitor of the weathering rate of the mica. The initial removal rate of Fe0 (oxidation rate) relative to Na20, deduced from the incipiently altered samples of the Toorongo profile is approximately 3: 1, motar basis (MARKOVICS,1977). Almost one half of the octahedral sites of fresh biotite contain Fe and approximately two thirds of a mole of biotite is weathered per mole of Fe0 oxidized. The relative degradation rate of biotite to Ab of plagioclase is near 1:l. The same procedures have been applied to the Flagstaff Granodiorite (WAH~TROM, 1948) and the

g&L&-_

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. .._-.. p_~-_~~ 7 ’

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Y

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(,O-

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_,

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FIG. 5. Calculated and actual weathering trends of some crystalline rocks (molar proportions). Smai$ dots are idealized mineral compositions: PI-plagioclase; Ks-potash feldspar; Mu-muscovite: Ka--. k:+ oh&e. In Fig. Sa, the large dot is average granite, the solid square average adamellite. the solid triangk average granodiorite the solid inverted triangle and diamond average tonalite and gabbro respectively lo Fig. 5b, solid squares, open squares and dots are fresh and weathered materials from the Stone Mountain Granite, the Mazaruni Granite and Toorongo Grancxiiorite profiles respectively. The calculated proportion:. of Ca, Na and K in leachants (derived from rocks with the corresponding symbol) are plotted on Ihe baselines of the two diagrams. The solid arrows (Fig. Sa, b) are the calculated initial trends followed hl the leachates during the initial weathering stages. The dashed arrows mimic the advanced trends of ;hweathering products.

Stone Mountain Granite (GRANT, 1963). The relative weathering rates of biotite to Ab of plagioclase are approximately 1.2:1 for both settings. Biotite apparently weathers rapidly from granitic rocks. Its weathering rate is perhaps slightly more rapid than &bite component of plagioclase. GARRELs (1967) and GARRELSand MACKENZIEf 1967) also suggested that biotite is one of the most rapidly weathered silicate minerals of granitic rocks. Although there is reasonable agreement between the bulk weathering rate of biotite from the three granitoids, the rate may be very dependent upon local environmental factors and particularly to redox conditions. Kinetic aspects of volcanic glass dissolution and weathering of volcanic rocks WHITE et a/. (1980) mC%Euredthe &ease rates of Na, K, Ca, Mg and Si from vitric and crystalline rhyolitic tuffaceous rocks. The order of the release rates changed with time but over an extended period each species followed a parabolic rate law, indicating that the rate-controlling step is different from feldspar dis” solution. It is, however, possible that the experiments

did not proceed long enough for linear ratt~him.frfa 41.22.5-231. NESBIT~’H. W. (1977) Estimatton of the thermodynamri properties of Na- Ca- and Mg-beidellites. C;?n .Mineru! 15.22-30. NESBITT H. W. (1979) Mobtltty and fracttonation oi rare earth elements during weathering of a granodiorite. Nature 279, 206-2 IO. NESBI~ H. W.. MARKOVIC~G. and PRICE K. t (i480) Chemical processes affecting alkalis and alkalme earths during continental weathering. Geochim Cwmorhim. 14nu 44, 1659-1666. NESBII-TH. W. and YOUNG G. M. (1982) Early Proterozoic climates and plate motions inferred from major element chemistrv of lutites. Nature 299. 7 15-7 17 NOCKOLD~S. R. (1954) Average chemical compositions ol some igneous rocks. Geol. Sec. Amer. Bull. 65, 1007- 1032. PA&~ T. (1972) Chemical characteristics and equilibration in natural water-felsic rock-CO? system. Geochim ro.c mochim. Acta 36,2 17-240. PACEST. ( 1983) Rate constants of dissolutron derived from the measurements of mass balance in hydrological catchments. Geochim. Cosmochim. Acta 47, 1R55- 1864. PATTERSON S. H. (1971) Investigations of ferruginous bauxite and other mineral resources on Kauai and a reconnaissance of ferruginous bauxite deposits on Maui, Hawaii I ’ .S. (;eo! Surv. Prof Pap. 656. 65 p

Weathering trends of ignizous rocks PETROVIC R., BERNERR. A. and GOLDHABERM. B. (1976)

Rate control in dissolution of alkali feldspar+--I. Study of residual feldspar grains by X-ray photoelectron spectroscopy. Geochim. Cosmochim. Acta 40, 537-548. &TROVlC R. ( 198 la) Kinetics of dissolution of mechanically cornminuted rock-forming oxides and silicates--I. Deformation and dissolution of quartz under laboratory conditions. Geochim. Cosm~h~m. Acta 45. 1665 1674. PETROV~C R. ( 198 1b) Kinetics of dissoluti& of m~hani~ly comminuted rock-forming oxides and silicates--II. Deformation and dissolution of oxides and silicates in the laboratory and at the Earth’s surface. Geochim. Cosmochim. Acta 45, 1675-1686. PETTUOHN F. J. ( 1963) Chemical composition of sandstonesexcluding carbonate and volcanic sands. U.S. Geol. Sun? Proj. Pap. 440-S 19 pp. PETTLIOHNF. J. (1975) Sedimentary Rocks. 3rd ed. Harper and Row. 628 pp. RANA M. and DOUGL,ASR. (1961) The reaction between glass and water. Part 2, discussion and results. Phys. Chem. of Glasses 2, 196-205. ROBIER. A. and WALDBAUMD. R. (1968) Thermodynamic properties of minerals and related substances at 298.15 K (25 C) and one atmosphere (1.013 bars) pressure and at higher temperatures. US. GeoI. Surv. Bull. 1259. 256 pp. SHAW D. M., REILLYG. A., MUY~SONJ. R., PA~ENDEN G. E. and CAMPBELLF. E. ( 1967) An estimate of the chemical composition of the Canadian Precambrian Shield. Can. J. Earth Sci. 4, 829-853. SHAWD. M., DOSTAL J. and KEAYSR. R. (1976) Additional estimates of continental surface Precambrian shield composition in Canada. Geochim. Cosmochim. Acta 40, 7383. TAYLORS. R. and MCLENNANS. M. (1981) The composition and evolution of the Continental crust: m earth element evidence from sedimentary rocks. Phil. Tram. R. Sot. Land. A301,381-399. WARLSTROM E. E. ( 1948) Pre-Fountain and recent weathering on Fla@aff Moun~in near Boulder, Colorado. Geol. Sot. Amer &dl. 59, 1173-l 190. WEDEPOHLK. H. (1969) The handbook ox ~e~hemist~, v. 1, (ed. K. H. WEDEPOHL),pp. 247-248. Springer-Verlag. WHITEA. F. (1983) Surface chemistry and dilution kinetics of glassy rocks. Geochim. Cosmochim. Acta 47, 805-8 16. WHITE D. E., HEM J. D. and WARING G. A. (1963) The chemical composition of subsurface waters. U.S. Geol. Sure. Prof: Pap. 440-F, 67 pp. WHITEA. F., CLAASSENH. C. and BENSONL. V. (1980) The effect of dissolution of volcanic glass on the water chemistry in a tuffaceous aquifer, Rainier Mesa, Nevada. U.S. Geol. Sure. Water-Sup& Pap. 1535-Q. 34 pp. WILSONR. E. (1978) Mineralogy, petrology and geochemistry of basalt weathering. Wnpubl. Honoun Thesis, La Trobe Univ. Vie. Aust. 55 pp. APPENDIX THE CHEMICAL AND MINERALOGICAL COMPOSITION OF THE UPPER CRUST The response of rocks to weathering is determined largely by their mineralogical and bulk chemical compositions. Data concerning the average compositions of the upper crust are essential to an appreciation of weathering reactions. The average composition of the earth’s upper continental crust (Table 1, column A) is reasonably well established (TAYLOR and MCLENNAN,1981; SHAWef al., 1967, 1976; EADE and FAHRIG, 197 1) but the average mineralogical composition (Table 2, column A) is not well known. WEDEPOHL(1969, p. 247248) estimated the average chemical (Table 1, column 8) and mineralogical (Table 2, column A) upper crustal composition using only data from plutonic rocks.The chemical estimate of WEDEKxiL (1969) is similar to others, but the mineralogical

1533

estimate reflects the average composition of plutonic rocks rather than that of the exposed crust because metamorphic and volcanic rock assemblages were not included. SHAW et al. ( 1967) published an estimate of the average chemical composition of the Canadian Shield (Table 1, column C) and calculated its mesonorm (Table 2, column B). in view of the preponderance of metamorphosed rocks in the shield, a rne~no~ is probably more realistic than a normative composition. The rn~no~, however, is deficient because it excludes chlorite. It can be included by considering the folIo~ng reaction: S.OK(Mg, Fe)jAISi,O,O(OH), (biotite) + 3.0KA1&0,~OH)z (muscovite)

+ 9.0Si02 + 4.0H20 (Qz)

= 3.O(Mg, Fe)JA12Si301dOH)8+ 8.0KAlSi,08. (chlorite) (K-spar)

(,)

The extent to which the reaction proceeds is determined by the quantity of volatiles. SHAWer al. (1967) determined that there are 0.85 gm of Cl, F and HzO+ in average Canadian Shield. The volatiles in excess of the amounts that can be accommodated by the micas are considered to be consumed according to reaction (I ) allowing calculation of the amount of chlorite in the mesonorm (Table 2, column C). There are amphibole- and pyroxene-bearing rocks in the Shield (amphibolites, greenstones, and granulites) but these minerals should be much less abundant than muscovite, biotite and chlorite and no correction is made for their presence. BLATT and JONES (1975) estimated the ratio of metamorphic rocks to intrusive rocks to be 17:9 in exposed crystalline continental crust. Taking this ratio as representative of the upper 10 to 15 km of crust, an average mineralogical composition of the upper crust can be obtained by combining columns A and C of Table 2 in the proportions indicated above and recalculating the result to 100% (Table 2. column 0). The bulk chemical com~sition co~~nding to this mineralogica1 composition is given in Table 1 (column C). It is remarkably close to the estimate of Taylor and McLennan (Table 1, column A), which is the most recent and probably the best estimate now available. The correspondence is still closer if the estimate in Table I (column C) were recalculated on an anhydrous basis as is the estimate of Taylor and

Table

1.

Chemical composition of upper crustal rocks (wt%) A

SiOz TiOn Al203 Fe203 Fe0 W?O CaO Nat0 K20 P205 Others

66.0 0.6 16.0 4.51 2.3 3.5 3.8 3.3

B 66.4 1::; 1.5 ::: 3.8 3.6 3.3 0.18

0.7

C

D

64.93 0.52 14.63 1.36 2.75 2.24 4.12 3.46

65.4 0.6 14.7 1.4 2.8 2.2 4.0 3.5 3.2 0.16 1.8

3.10 0.15 2.37

A Average upper ctust estimated by Taylor and McLennan (1981). B Average upper crust estimated by Wedepohl (1969, Table 7-10). C Average Canadian Precambrian Shield estimate (Shaw et al., 1967) _ D Average upper continental crust estimated using data from Blatt and Jones (1975) and data in columns i3 and C. ’ Total iron given as FeO.

H. W. Nesbitt and G. M. Young

1534 Tabie

._--

2.

Average crustal

Quartz PldCYiOClaSe Gl& Orthoclase Biotite Muscovite Chlorite Amphiboles Pyroxenes Olivines Oxides Others

mlneraioglcal rocks (~01.8)

_---__.-A R

21.0 41.0 0.0 21.0 4.0 0.0 0.0 6.0 4.0

0.6 2.c 0 .5

25.4 39.2.5 0.0 4.5' i5.29 9.77 0.c O.c, 0 . ij

t:ompos~tlo~~ OL

_-. i:

24.42 34.25 '3.0 8 .6 11.23 7.61.

.__,_____.~ _ _. i; E 23.2 19.9

20.3 34.q

0.0

:2.5

12.9 8.7 5.0 1 .2

j.0

‘1.i ii . I:

2.I : .4 !7. 2

11.2 7.6 4.4 i.3

?..?7

i.3-

L.6

I.4

4:

4.7

0. c

?.?i

L.8 : .z 3. ;I

3.0 &,