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Chemical Geology 202 (2003) 313–341

Investigation of in situ weathering of quartz diorite bedrock

in the Rio Icacos basin, Luquillo Experimental

Forest, Puerto Rico

Benjamin F. Turnera,*, Robert F. Stallardb, Susan L. Brantleya

aDepartment of Geosciences, Pennsylvania State University, University Park, PA 16802, USAbU.S. Geological Survey, 3215 Marine Street, Boulder, CO 80303, USA

Received 6 December 2001; received in revised form 29 April 2003; accepted 1 May 2003

Abstract

The Rio Icacos basin, in Puerto Rico, is the site of the highest measured chemical solute fluxes for a catchment on

granodiorite; this is partly attributable to high annual rainfall (4300 mm), high average temperature (23 jC), and moderate

relief. The bulk of these fluxes is contributed by dissolution of plagioclase and amphiboles in zones of partially weathered rock

(0.5–1.5 m thick) underlying saprolite. These zones are characterized by systems of onion-skin ‘‘rindlets’’ (each 3–10 cm

thick) in which porosity development is dominated by weathering of plagioclase to kaolinite. Fe-bearing aluminosilicate

minerals hornblende, augite, and biotite persist in the weathered rock after plagioclase has weathered to completion, but

hornblende and augite disappear between the porous rindlet systems and saprolite. A new watershed-scale solute mass balance

corroborates earlier studies and yields an overall reaction stoichiometry that closely resembles complete weathering of the rock

(except quartz and biotite) to kaolinite and Fe3 + oxides.

Our findings indicate that steady-state assumptions are reasonable and useful in studying weathering at this site. Estimated

steady-state sediment yields (based on net solute fluxes and average bedrock composition) of 3.24� 10� 9 to 3.60� 10� 9 kg

m� 2 s� 1 are smaller than measured values, indicating that sediment generation is currently accelerated with respect to steady-

state; however, sediment generation has recently been closer to steady-state. Since weathering within the rindlet systems

occurs on multiple fronts moving upward and downward, two new steady-state conceptual models are developed to account

for (1) multiple parallel weathering fronts, and (2) differential rates of advance of weathering fronts due to porosity

development. Based on these models, in situ weathering rates for plagioclase at two sample locations of 2.7� 10� 15 and

5.0� 10� 15 mol Na g� 1 s� 1 are calculated, values comparable to BET surface area normalized rates of 2.7� 10� 16 to

5.0� 10� 15 mol Na m� 2 s� 1.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Weathering; Bedrock; Solute flux

0009-2541/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.chemgeo.2003.05.001

* Corresponding author. Now at Department of Civil Engineer-

ing and Geological Sciences, University of Notre Dame, Notre

Dame, IN 46556, USA.

E-mail addresses: bturner@nd.edu (B.F. Turner),

stallard@spot.colorado.edu (R.F. Stallard),

brantley@essc.psu.edu (S.L. Brantley).

1. Introduction

Weathering in the Rio Icacos watershed produces

the highest documented chemical weathering fluxes

in the world for granitic rocks (White and Blum,

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341314

1995). These exceptionally high fluxes have been

attributed to high rainfall and temperature conditions

(White and Blum, 1995; White et al., 1998), as well

as the relatively young age of the rock (47 Ma) and

presumed associated volatiles and lack of metamor-

phic annealing (Stallard, 1995a). Due to the paucity

of data from systems subjected to such extreme

weathering conditions, factors affecting weathering

in the Rio Icacos watershed, as well as other

extreme weathering systems, deserve further study.

Previous research on chemical weathering in the

Rio Icacos basin has focused on watershed-scale

geochemical mass balances, and weathering in the

soils and saprolite of the system (e.g. White et al.,

1998; Murphy et al., 1998; Stonestrom et al., 1998;

Schulz and White, 1999). White et al. (1998)

investigated the geochemical mass balance and

weathering in the watershed as a whole, while

Murphy et al. (1998) and Schulz and White

(1999) investigated weathering of biotite and quartz,

respectively, in the saprolite. Stonestrom et al.

(1998) discussed methods of calculating weathering

rates in the saprolite. However, the saprolite studied

is intensely leached, and is completely depleted with

respect to all but the least reactive primary minerals

(White et al., 1998). Thus, the extreme weathering

fluxes of the Rio Icacos basin result from weather-

ing of bedrock, which has not been studied in

detail. To further understand the underlying factors

that contribute to the extreme weathering fluxes in

this rainforest environment, we have conducted an

examination of chemical weathering occurring in

porous bedrock.

Weathering of joint blocks in granitic bedrock is

believed to start in the joint planes and weathers

inward toward the center of the blocks (Ollier,

1971, 1975). Since the corners and edges of the

blocks have more surface exposed to weathering

fluids than the faces, they tend to become rounded,

resulting in core stones that are spherical or oblong in

shape (Sarracino and Prasad, 1989). Porous rock in

the subsurface of the Rio Icacos basin occurs in

‘‘onion-skin’’ rindlet systems similar to those seen

in spheroidal weathering structures (Ollier, 1971).

Spheroidal weathering (synonyms include onion-skin

weathering, concentric weathering, and spherical

weathering) is most commonly found in well-jointed,

uniform hard rocks such as granite, dolerite, and

basalt (Ollier, 1971), and is characterized by concen-

tric shells that completely surround a core stone. The

cause of spheroidal weathering is unknown; however,

it appears that the boundaries between concentric

shells are formed from the coalescence of solution

channels that tend to form parallel to the fresh rock

contact (Fritz and Ragland, 1980). Some authors (e.g.

Ollier, 1971; Fritz and Ragland, 1980; Fritz, 1988)

have suggested that the micro-cracks that form the

solution channels may result from residual stress

released after erosional unloading. Folk and Patton

(1982) suggested that such cracks may result from

expansion of rocks that are buttressed in the lateral

direction. Twidale (1982) suggested that crack forma-

tion may be aided by ‘‘hydration shattering’’ due to

small volume increases caused by adsorption of

water on mineral surfaces. The shell boundaries

often become Fe3 + oxide stained, and apparently

become the centers of weathering within a core

stone. Due to reasons possibly related to subtle

variations in rock texture or composition (Gardner

and Nelson, 1991), individual core stones may

weather completely to saprolite, or may remain

intact after the surrounding rock has weathered.

Round boulders are commonly seen at the ground

surface within the Rio Icacos basin, often overlying

saprolite, as well as deeply embedded within the

saprolite.

In this paper, we examine in detail the petrology

of weathering in the part of this system where the

bulk of mineral dissolution occurs: the zones of

porous rock, or ‘‘rindlet’’ systems. We then examine

the system as a whole, using a new basin-scale solute

mass balance precise enough to gauge the overall

reaction stoichiometry of the system. Using the

assumption of a weathering system at steady-state,

we apply this approach to estimate a long-term

sediment yield and sediment chemical composition

for the system. At spatial scales smaller than the

basin-scale, the ‘‘conveyor belt’’ model described

below (with a single weathering front) is too simple

to account for the complexities observed in the field.

Thus we present two new conceptual models: the

‘‘parallel’’ and ‘‘differential’’ models for weathering

systems with multiple fronts. The ‘‘differential’’

model is used to calculate dissolution rates for

plagioclase, the most important contributor to dis-

solved weathering products in this system.

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341 315

2. Field site description

The Rio Icacos watershed is located in the Luquillo

Mountains of Eastern Puerto Rico (Fig. 1). The

watershed area contributing runoff to the gaging

station maintained by the U.S. Geological Survey is

3.26 km2. Covered mostly by pristine tropical mon-

tane forest, the basin receives a very large annual

rainfall of 4300 mm year� 1 (McDowell and Asbury,

1994) and has an average annual temperature of 22 jC(White et al., 1998). Average annual runoff from 1987

to 1997 was 3900 mm (USGS, 2000). In a tracer study

investigating natural water movement in the unsatu-

Fig. 1. Location map showing the Rio Icacos watershed (as delineated from t

rated zone over an extended (509 day) period, it was

found that the majority of runoff follows a near-

surface flow path through leaf litter and shallow soil,

while the remaining amount percolates through the

deeper subsurface (Turner, 2001; Turner et al., sub-

mitted for publication).

The watershed is underlain by intrusive rocks of

the Rio Blanco stock. The stock has been dated at

46.2F 1.1 Ma by K–Ar methods (Cox et al., 1977),

and 46–48 Ma by Ar–Ar methods (Smith et al.,

1998). These dates are consistent with Seiders’s

(1971) placement of the stock in the early Tertiary

age based on stratigraphic information. The rock is

he USGS gaging station), the Rio Blanco stock, and sample locations.

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341316

generally considered to consist of quartz diorite with

subordinate diorite (Seiders, 1971). However, Kesler

and Sutter (1979a) sampled rocks classified as diorite,

tonalite, and granodiorite. Volcanoclastic rocks of

Cretaceous age form some ridges bounding the wa-

tershed to the north and east (Seiders, 1971); however,

these rocks underlie less than 10% of the total

watershed area.

Much of the regolith is saprolite, formed from in

situ weathering of quartz diorite bedrock, that retains

much of the structure of the crystalline parent rock

(White et al., 1998; Murphy et al., 1998). In this soft,

thoroughly decomposed residuum, almost all of the

feldspars and amphiboles have been weathered out,

leaving quartz, biotite, and some magnetite, along

with kaolinite and ferric oxides as the secondary

phases (White et al., 1998). The saprolite extends up

to several meters in depth, and is overlain by an ultisol

of similar mineralogical composition with moderate to

high permeability, moderate to low water retention

capacity, and rapid runoff (Boccheciamp, 1977). Be-

tween the saprolite and bedrock are zones of weath-

ered, porous rock up to 1 m or more in thickness.

Since the saprolite is depleted of feldspars and

amphiboles, these zones of porous rock are apparently

where most of the weathering of these minerals

occurs.

Brown et al. (1995) estimated a mean total denu-

dation rate for the basin of 75 m Ma� 1 (F 50%) by a

mass balance based on the water chemistry data

reported by McDowell and Asbury (1994). By a

similar mass balance based on the same data, White

et al. (1998) estimated a total denudation rate of 58 m

Ma� 1. Based on the cosmogenic 10Be content of

fluvial sediments, Brown et al. (1995) estimated a

long-term average total denudation rate of 43 m

Ma� 1, with an uncertainty of 35%. This basin-scale

rate is consistent with localized rates also based on10Be content, which average over about 10,000–

100,000 years, measured on ridges, ridge flanks,

exposed core stones, and landslides (Brown et al.,

1995).

White et al. (1998) calculated watershed-scale

weathering fluxes (N L� 2 T� 1) for the Rio Icacos

basin using mass balance calculations: one based on

changes in bulk chemistry along a deep saprolite

profile, coupled with the long-term denudation rate

estimated by Brown et al. (1995); and one based on

changes in pore water chemistry, coupled with the

flux of water infiltrating through the unsaturated

saprolite. These estimates yielded fluxes of similar

magnitude, comparable with observed net solute

fluxes in stream water. The authors concluded that

weathering rates in the Rio Icacos basin have not

changed substantially in the last several hundred

thousand years.

At outcrops of weathering rock within the Rio

Icacos basin, systems of onion-skin laminae similar

to the ‘‘type I rindlets’’ described by Fritz and Rag-

land (1980) are observed. The term ‘‘rindlet’’ as used

by Fritz and Ragland (1980) and used hereafter refers

to the individual laminar sheets that constitute a

weathering rind. The rindlets discussed in this text

are intermediate in intensity of weathering between

fresh rock and the thoroughly decomposed saprolite.

This state is hereafter referred to as ‘‘weathered rock.’’

The rindlets range in thickness from 3 to 10 cm, and

retain the texture of the fresh rock. Boundaries be-

tween individual rindlets are characterized by cracks

that are usually Fe3 + oxide stained. Generally, indi-

vidual rindlets are more weathered at the inter-rindlet

boundaries than in the rindlet centers. Where rindlet

systems occur close to the ground surface, plant roots

have been observed to grow inside the rindlet shell

boundaries, even though the centers of the rindlets

may be relatively unweathered. Relicts of these inter-

rindlet boundaries are occasionally seen in the sapro-

lite as parallel deposits of black and dark red Mn/Fe

oxides.

2.1. Steady-state ‘‘conveyor belt’’ weathering

Because denudation rates apparently have not

changed substantially over several hundred thousands

of years, the system can be envisioned as being at or

near steady-state. In an ideal steady-state landscape,

the statistics describing landforms and the physical

processes occurring therein remain constant (Stallard,

1995a). In this discussion, we will assume that

physical and chemical erosion remain constant in a

system at steady-state. Under such conditions, the

volume of regolith in the system remains constant,

since processes generating and removing regolith are

balanced.

As suggested by White et al. (1998), the mineral

contributing the largest constituent of dissolved

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341 317

weathering products in this system is plagioclase.

Thus, we will begin by conceptualizing weathering

in the subsurface of the Rio Icacos basin in terms of

plagioclase. The subsurface can be conceptualized as

three zones: (1) competent bedrock, (2) zones of

porous rock (rindlet systems) where weathering of

plagioclase to kaolinite is occurring, and (3) saprolite

where all plagioclase has been converted to kaolinite

(Fig. 2). Since the bioturbated soil is mineralogically

indistinct from the saprolite, we will lump soil and

saprolite together in zone 3. In an ideal steady-state

system, where the generation of new regolith by

chemical weathering must equal the removal of

regolith by physical erosion (on a volume basis,

Fig. 2. Illustration of the ‘‘conveyor belt’’ model of a weathering

system at steady-state. The two plots show the changes in (D0C)/

(DC0) and d(C/D)/dx with depth for an idealized system where

plagioclase is weathering. Variables C and D each represent the

solid-phase concentration of a mobile and immobile element,

respectively, and the subscript 0 denotes concentration in bedrock.

The term (D0C)/(DC0) is the amount of plagioclase per volume of

regolith relative to that of the original rock. The derivative d(C/D)/

dx is proportional to the gross rate of weathering at depth x. Three

idealized zones of weathering, separated by dashed lines, are (1)

competent bedrock, (2) porous regolith containing plagioclase, and

(3) saprolite completely depleted in plagioclase. Within zone 2,

where plagioclase weathering is occurring, weathering rate is

controlled by the relative amounts of porosity and available

plagioclase surface. The spans of sample suites ‘‘GN,’’ ‘‘TC,’’

and ‘‘SS’’ are shown for comparison.

assuming isovolumetric weathering), the average

thicknesses of these weathering zones must stay

constant. Thus, the rate at which the land surface is

lowered due to erosion (i.e. the denudation rate) is

equal to the rate of movement of the boundaries

between zones 1 and 2, and 2 and 3. If weathering

occurs in a steady-state, ‘‘conveyor belt’’ fashion, then

the thickness of the zone 2 is equal to the distance the

weathering front travels in the time needed to dissolve

all the plagioclase from a given volume. Absolute

changes in land surface elevation are caused by uplift

in addition to ‘‘lowering’’ of the land surface by

denudation.

Weathering of plagioclase can be assumed to occur

almost entirely within zone 2, the zone of porous rock

between the unweathered bedrock and the saprolite.

Although small amounts of Na may occur in horn-

blende, most of the primary Na is in plagioclase. For

the purposes of this discussion, we will assume that

changes in Na concentration in the solid phase reflect

plagioclase weathering. It is presumed that physical

erosion removes material primarily from zone 3.

Larsen et al. (1999) found that while overland flow

of water was virtually nonexistent in the Luquillo

Experimental Forest, the vast majority of sediment

generation could be attributed to mass wasting pro-

cesses including landsliding, tree throw, and soil

creep.

White et al. (1998) concluded that the saprolite was

essentially isovolumetric with respect to the parent

rock. Thus, we can assume that weathering in zone 2

and the saprolite portion of zone 3 has occurred under

constant volume conditions (i.e. the structure of the

rock does not expand or contract appreciably during

weathering). Under such conditions, the bulk density

of the regolith decreases with time, as weathering

products are removed in solution. As weathering

progresses, the bulk chemical composition of the

regolith tends to become depleted in elements repre-

sentative of the more soluble minerals (e.g. Na and Ca

in plagioclase), and enriched in elements of relatively

insoluble weathering product (e.g. Al in kaolinite).

Some elements are considered to be immobile, form-

ing minerals so insoluble that appreciable leaching

from the regolith does not occur over long time

periods. White et al. (1998) identified Nb, Ti, and

Zr as possible immobile elements in the saprolite. By

observing the changes in the ratios of mobile elements

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341318

to the immobile ones, the progress of weathering

reactions may be tracked.

Within this conceptual framework, the flux, Ji, of

an element, i, removed by chemical weathering in a

small volume of weathered rock or regolith can be

calculated as

Ji ¼ Rq0D0dðCi=DÞ=dx ð1Þ

where R is the denudation rate (equal to the rate of

advance of the weathering front in this case), q0 is

the density of bedrock, Ci is the concentration of

element i, D is the concentration of immobile ele-

ment, D0 is the concentration of immobile element in

bedrock, and x is distance in the direction of move-

ment of the weathering front (Stallard, 1985). The

derivative d(Ci/D)/dx is proportional to the flux of

weathering products and hence the weathering rate.

The ratio (D0Ci)/(DCi,0), where Ci,0 is the concen-

tration of element i in the parent material, represents

the amount of element i left in the weathered

material relative to the parent material. If element i

is Na, which essentially is contained entirely in

plagioclase, then this ratio reflects the degree of

weathering of plagioclase. The relationships between

d(Ci/D)/dx, (D0Ci)/(DCi,0), and zones 1, 2, and 3 are

illustrated in Fig. 2.

White et al. (2001) suggested that initial weather-

ing in granitoid rocks is extremely slow, being limited

by the low permeability of the primary rock matrix.

The authors further suggested that, under isovolumet-

ric weathering conditions, increased porosity and

permeability from weathering leads to increased rates

of weathering. Thus the fluxes of weathering products

from plagioclase in the Rio Icacos basin in unweath-

ered bedrock (i.e. zone 1) are expected to be minimal,

since the small amount of porosity and permeability

developed in bedrock matrix allows for only a small

amount of plagioclase surface exposed to solution.

Furthermore, the residence time of waters in existing

pore spaces may be large, leading to dissolution

conditions close to thermodynamic equilibrium. How-

ever, as the rock crosses the boundary between zones

1 and 2, significant porosity develops, allowing great-

er contact between plagioclase grains and solution,

and decreased water residence times; these effects are

expected to increase weathering rates, leading to

increasing porosity and decreasing amount of plagio-

clase. At some point, however, further increases in

porosity do not enhance the weathering flux, as gross

dissolution starts to become limited by the availability

of plagioclase rather than the availability of porosity.

Beyond this point, the flux of plagioclase weathering

decreases as the volume fraction of plagioclase de-

creases. Finally, plagioclase weathers to completion

near the boundary of zones 2 and 3. This idealized

situation is conceptualized in Fig. 2.

This conceptual model has the advantage of sim-

plicity and is useful for representing weathering on a

watershed scale. However, in reality, rock weathering

in the Rio Icacos basin is much more complex. Stallard

(1995b) argues that such a ‘‘conveyor belt’’ model is

unrealistic since energetic processes such as debris

flows and gully formation can excavate deeply into

soils. Furthermore, this type of model is limited since

the zone of porous rock is not a single sheet uniformly

underlying the site, but in reality includes multiple

layers with weathering occurring along multiple fronts.

Two additional conceptual models accounting for mul-

tiple weathering fronts are presented in Discussion.

3. Methods

3.1. Field sampling

Three suites of rock samples were collected from

within the Rio Icacos watershed (Fig. 1) in March

1996. Each suite consists of porous, weathered rock,

collected from outcrops of the weathering rindlet

systems (i.e. zone 2). In each case, several cm of

surficial material were removed prior to sampling.

Suite ‘‘GN’’ was collected from a cut stream bank

(Fig. 3), and included a system of more than 20

rindlets. The top of Fig. 3 is approximately 2 m below

the ground surface. This outcrop consists of concen-

tric low porosity rindlets 8 cm thick overlain by milky

white and iron oxide stained rindlets subparallel with

the ground surface. Overlying the white/stained rind-

lets are gray, relatively unstained rindlets in the same

orientation. Compared to the white/stained rindlets,

the gray rindlets are thicker, more uniform in thick-

ness, and have thinner ferric iron stained inter-rindlet

boundary zones. Samples were collected along a line

segment, perpendicular to the onion-skin exfoliation.

This line crossed an apparent joint plane as illustrated

Fig. 3. Photograph of rindlet system ‘‘GN.’’ Dashed white line indicates apparent joint plane. Black line represents the transect line for the

sample suite. White boxes indicate areas from which samples were taken.

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341 319

in Fig. 3. The first sample was taken from a rindlet of

relatively low porosity. Subsequent rindlets located

18, 36, and 76 cm from this point, with the 76-cm

point lying in the gray rindlets, were sampled. Sam-

ples represent the interior of their respective rindlets.

An additional sample was collected from the concen-

tric rindlets below the base of the segment (sample

GN-rock).

At a location 150 m to the northeast of ‘‘GN’’ (Fig.

1), another sample of weathered rock (sample RI) was

taken from an exposed stream cut bank near the Icacos

river.

Suite ‘‘TC’’ was taken from a concentric rindlet

system exposed in a road cut several decades old.

Several cm of material were removed from the

surface of the outcrop prior to sample collection.

The system is roughly 2 m below the ground surface.

The system, roughly 0.5 m in radius, is characterized

by relatively thin, 2–3 cm thick concentric rindlets

(Fig. 4). The rindlets are relatively unweathered, but

the rindlet boundaries are stained by ferric iron

oxides. Samples were collected from along a line

segment running from the center of the rindlet system

to the edge. Integrated samples (i.e. samples repre-

senting the interior and boundaries of more than one

rindlet) were collected from 0 to 5, 10 to 20, 20 to 30,

and 30 to 50 cm from the center of the system

(corresponding to samples TC 2.5, 15, 25 and 40,

respectively). For comparison, a sample of pristine

rock (sample ‘‘TC fresh rock’’) was collected from

the center of an adjacent core stone approximately 2

m to the west.

From the same road cut as TC, another suite of

samples ranging from weathered rock to a near-

surface soil was collected (suite ‘‘SS’’). The weath-

ered rock is 1 m below the ground surface. This

outcrop is characterized by rindlets 2–3 cm thick, and

subparallel with the ground surface, which gradually

degrade into saprolite-like regolith over a range of

about 30 cm (Fig. 5). In the figure, the rindlets

roughly level with the exposed metal part of the

hammer are relatively unweathered at the center, but

the rindlet boundaries are weathered and ferric iron

oxide stained. Rindlets level with the hammer’s

leather wrappings are porous, and become highly

weathered. Above the hammer is saprolite. Integrated

samples were collected along a vertical line segment:

from 15 to 22, 22 to 28, and 28 to 31 cm from the

bottom of the segment (corresponding to samples SS

18.5, 25, and 29.5, respectively). Additional samples

were taken from a relatively unweathered rindlet at

the base of the segment, and from the saprolite at the

top of the segment (samples SS-rock and SS-sap,

respectively).

Fig. 4. Photograph of rindlet system ‘‘TC.’’ Black line represents the transect line for the sample suite. White boxes indicate areas from which

samples were taken.

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341320

In November 1990, an intact weathering rindlet

was sampled from one third of the way up a landslide

(Slide-43 of Guariguata and Larsen, 1990). The rind-

let, roughly 10 cm thick, consists of 6 cm of relatively

unweathered rock overlain by 4 cm of weathered

material. A sample of fresh bedrock was also taken

from the base of Slide-43.

Fig. 5. Photograph of rindlet system ‘‘SS.’’ Hard, relatively unweathered

porous rindlets exist even with the handle. Above the handle is soil with

sample suite. White boxes indicate areas from which samples were taken

3.2. Laboratory methods

All solid samples except for the Slide-43 fresh rock

sample were analyzed for bulk chemistry by lithium

meta/tetraborate fusion. They were ground to less than

100 mesh and fused at 950 jC with a flux composed of

50% lithium metaborate and 50% lithium tetraborate.

rindlets exist below the head of the hammer, while relatively soft,

no rindlet structure. Black line represents the transect line for the

.

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341 321

The resulting glass bead was immediately dissolved in

5% nitric acid. Solutions were analyzed by inductively

coupled plasma (ICP) atomic emission spectropho-

tometry (Leeman Labs PS3000UV).

The Slide-43 fresh rock sample was analyzed using

X-ray fluorescence on fused glass buttons by X-Ray

Assay Laboratories. Oxides (and sulfur) have a 0.01%

detection limit, while Cl, Zr, Rb, Sr, Ba, and Y have a

Table 1

Chemical analyses of bedrock from the Rio Blanco stock

(a) Values in wt.%

‘‘TC’’ ‘‘Slide-43’’ White Seiders Kesler and Sutter (1979b)b

fresh

rock

fresh

rock

et al.

(1998)a(1971)

PR-71-82 PR-71-83 PR-7

SiO2 55.00 62.7 61.20 67.1 60.8 65.1 64.5

Al2O3 18.08 16.9 17.20 15.8 17.6 16.6 16.8

Fe2O3 1.44 2.07 6.81 2.2

FeO 3.86 4.10 2.1 7.7 5.6 4.7

MgO 2.16 2.22 2.18 1.7 2.9 1.9 1.8

CaO 7.27 6.04 7.24 5.2 7.4 6.0 5.9

Na2O 3.29 3.46 3.35 3.3 3.2 3.4 3.4

K2O 0.84 1.23 0.93 0.93 0.8 1.2 1.3

H2O 1.00 1.24

TiO2 0.44 0.47 0.49 0.25 0.46 0.33 0.3

P2O5 0.10 0.12 0.13 0.07

MnO 0.11 0.14 0.18 0.12 0.18 0.15 0.1

CO2 0.02 < 0.05

S 0.07

(b) Values in mg kg� 1

‘‘TC’’

fresh

rock

‘‘Slide-43’’

fresh

rock

White

et al.

(1998)a

Seiders

(1971)

Ba 200 300 300

Co 100

Cr 5

Cu 15

Ga 10

Nb 20 4

Pb 7

Rb 38

Sc 10

Sr 300 200 290 300

V 70

Y 29 20 26 15

Yb 1.5

Zr 79 60 70

Cl 500

a Total Fe expressed as Fe2O3.b Total Fe expressed as FeO.

10 mg/kg detection limit. Igneous and sedimentary

rock standards obtained from the USGS were run as

blind standards.

The Slide-43 rindlet sample was impregnated with

blue-stained low-viscosity epoxy. Petrographic thin

sections were made from some of the solid samples,

including samples ‘‘GN 36,’’ ‘‘TC 25,’’ and the Slide-

43 rindlet. Chemical compositions (i.e. Na, Mg, Al,

1-87 PR-72-109 PR-72-110 PR-72-111 PR-72-112 PR-72-113

59.8 68.7 69.5 64.5 58.8

17.6 15.6 15.5 17.2 18.0

6.6 3.8 3.4 4.5 7.1

3.1 1.7 1.3 1.7 3.4

7.9 4.6 4.1 6.1 8.0

2.9 3.2 3.4 3.7 2.1

0.8 1.7 2.4 0.9 0.6

2 0.51 0.26 0.27 0.35 0.51

4 0.13 0.12 0.09 0.17 0.18

Table 2

Average compositiona of bedrock samples from the Rio Blanco

stock

Oxide Average

weight

percent

Standard

deviation

Element mol/kg

bedrock

SiO2 63.14 4.27 Si 10.509

Al2O3 16.91 0.89 Al 3.316

FeOb 5.39 1.35 Fe 0.751

MgO 2.17 0.64 Mg 0.539

CaO 6.31 1.27 Ca 1.126

Na2O 3.23 0.40 Na 1.041

K2O 1.14 0.50 K 0.241

TiO2 0.39 0.10 Ti 0.049

MnO 0.14 0.03 Mn 0.020

Sum 98.82

a Average of the 12 samples from Table 1.b Total Fe expressed as FeO.

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341322

Si, K, Ca, Ti, and Fe) of individual minerals on the

thin sections were obtained by electron microprobe

(Cameca SX-50).

Samples from each suite were analyzed for phyl-

losilicate minerals by powder X-ray diffraction (XRD;

Rigaku Geigerflex diffractometer). Samples were an-

alyzed under K+-saturated, Mg2 +-saturated, and

Table 3

Chemical compositions of weathered rock and regolith

GNa

0

GNa

18

GNa

36

GNa

76

RI SSa

rock

(0)

SSa

18.5

SSa

25

SSa

29.5

SiO2 55.99 50.86 53.44 57.81 54.70 56.66 54.36 54.19 53.96

Al2O3 17.03 18.65 19.31 17.22 18.55 19.23 18.07 19.30 17.17

Fe2O3 2.16 3.36 3.76 3.46 2.87 2.88 3.22 3.41 11.20

FeO 3.55 3.48 3.55 3.01 3.36 3.45 3.62 3.27 3.60

MgO 2.36 2.78 2.90 2.55 3.09 2.35 2.66 2.67 3.36

CaO 4.08 2.57 3.10 4.68 4.14 6.33 5.39 4.92 3.72

Na2O 2.32 1.30 1.54 2.68 1.95 2.95 2.35 2.08 1.42

K2O 1.05 1.02 1.09 0.97 0.70 0.57 0.46 0.44 0.56

TiO2 0.42 0.50 0.53 0.47 0.57 0.49 0.54 0.49 0.70

P2O5 0.11 0.11 0.12 0.06 0.10 0.05 0.05 0.04 0.01

MnO 0.15 0.17 0.17 0.18 0.17 0.12 0.14 0.13 0.16

Bad 300 200 300 300 300 200 200 100 100

Srd 200 100 100 200 200 300 200 200 100

Yd 27 33 38 28 36 30 31 32 35

Values are expressed in weight percent unless otherwise denoted.a Values in sample identification indicates distance (cm) along transecb Saprolite chemical compositions from White et al. (1998). Depth bec Total Fe expressed as Fe2O3.d mg/kg.

Mg2 +-saturated and ethylene glycol solvated condi-

tions (Whitting and Allardice, 1986).

4. Results

4.1. Bulk chemical analysis

Bulk chemical analysis (Table 1) of the pristine

bedrock sample (TC fresh rock) and previous analyses

of rocks of the Rio Blanco stock (by Seiders, 1971;

Kesler and Sutter, 1979b; White et al., 1998) indicate

compositions typical of diorite, quartz diorite, and

tonalite (Hyndman, 1985). The pristine sample col-

lected during this study has a composition that is

similar to those of previous studies, although the SiO2

concentration is below average. An average of the Rio

Blanco bedrock compositions is presented in Table 2.

Chemical analyses of weathered rock and regolith

samples are included in Table 3, along with saprolite

compositions from White et al. (1998) presented for

comparison. These compositions reflect samples with

degrees of weathering ranging from nearly pristine

(e.g. TC 2.5) to highly weathered (e.g. SS-sap). The

deep saprolites studied by White et al. (1998) are

SSa

sap

(45)

TC

fresh

rock

TCa

2.5

TCa

15

TCa

25

TCa

40

Sap.b

0.76

Sap.b

2.87

Sap.b

7.13

55.59 55.00 61.26 58.78 58.42 58.55 70.67 60.12 54.74

21.93 18.08 18.16 18.12 16.58 17.90 15.35 22.59 26.14

5.05 1.44 2.64 2.47 2.04 2.01 4.42c 6.76c 7.50c

0.64 3.86 4.19 4.10 4.56 4.07

0.90 2.16 2.92 2.96 3.12 2.70 0.00 0.65 0.76

0.49 7.27 7.42 7.24 7.07 7.21 0.00 0.00 0.00

0.15 3.29 3.17 3.08 2.80 3.03 0.00 0.00 0.00

0.16 0.84 0.68 0.61 0.64 0.59 0.88 1.85 2.21

0.66 0.44 0.55 0.50 0.53 0.48 0.39 0.47 0.56

0.00 0.10 0.15 0.11 0.09 0.09 0.00 0.00 0.00

0.06 0.11 0.16 0.16 0.16 0.14 0.02 0.05 0.57

100 200 200 200 200 200

0 300 300 300 300 300 27 22 22

22 29 36 35 33 31 17 15 20

t from which sample was taken.

low surface of sample in meters is indicated.

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341 323

completely depleted with respect to Na and Ca,

reflecting greater intensity of weathering than the

weathered rocks and regolith of this study, including

sample SS-sap.

To illustrate the degree of weathering in the three

sample suites of this study, ratios of the concentrations

of reactive elements (e.g. Si, Mg, Ca, Na, K, and Sr) to

the concentration of an immobile element are plotted

versus position along the sampling transect line (Fig.

6). When a reactive element enters the aqueous phase

and is transported out of the system, its solid-phase

concentration relative to an immobile element concen-

tration will decrease. Elements generally considered to

be immobile include Al, Ti, Y, Nb, and Zr. White et al.

(1998) found no loss of Zr, Ti, or Nb between bedrock

Fig. 6. Chemical compositions of weathered rock samples along the ‘‘GN’

are shown relative to the assumed conservative element, Ti. Vertical dashed

solid line indicates the ratio observed in the ‘‘TC’’ fresh rock sample, whil

weathered sample, ‘‘SS-sap.’’ The horizontal axis indicates distance along

and the saprolite, but observed that the saprolite was

depleted in Y with respect to bedrock. Because Ti was

found to be immobile in the system by White et al.

(1998) and because Ti could be readily analyzed, this

element was chosen for use as the immobile element in

subsequent data analysis.

Concentration ratios of reactive elements to Ti

plotted versus position for the GN suite (Fig. 6) show

that the samples become substantially depleted in Ca,

Na, and Sr between the relatively low-porosity rindlet

(0 cm), and the milky white and Fe-oxide stained

rindlets (18 and 36 cm; see Fig. 3). However, the

unstained gray rindlets at 76 cm are less depleted in

these elements, suggesting that these are less weath-

ered. This suggests that weathering initiated from the

’ suite. Concentrations (in moles per kilogram) of reactive elements

line indicates an apparent joint plane. For comparison, the horizontal

e the horizontal dashed line indicates the ratio observed in the most

the sample transect (see Fig. 3).

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341324

joint plane located at about 10 cm in the transect.

Thus, weathering in the GN suite appears to proceed

in an upward direction (i.e. increasing distance values

in Fig. 6) from the joint plane. The behavior of Si

follows a similar pattern to Ca, Na, and Sr, but the

relative amount of depletion is not as great. Elements

Mg and K do not appear to leach substantially in this

suite. The stoichiometry apparent from this pattern of

depletion suggests that weathering of plagioclase, a

Na- and Ca-aluminosilicate with Sr substituting for

Ca, dominates the observed chemical changes in the

rock.

Similar plots for the SS suite are presented in Fig. 7

for sample locations illustrated in Fig. 5. Like suite

GN, concentration ratios of Mg to Ti in the SS suite

Fig. 7. Chemical compositions of weathered rock samples along the ‘‘SS’’ s

shown relative to the assumed conservative element, Ti. For comparison, th

rock sample, while the horizontal dashed line indicates the ratio observe

distance). The horizontal axis indicates distance along the sample transec

do not appear to change much within the rindlet

system (0–35 cm). Samples collected between 15

and 31 cm appear to be slightly depleted in K with

respect to SS rock (0 cm). However, SS-sap (45 cm) is

substantially depleted in both Mg and K with respect

to the rindlet samples. Also, it appears that Ca, Na,

and Sr become substantially depleted across the rind-

let system, while SS-sap is more than 90% depleted in

these elements with respect to SS-rock. The bulk

chemical changes in the rindlet system appear to be

dominated by plagioclase weathering, while substan-

tial weathering of K- and Mg-bearing minerals occurs

in the saprolite-like regolith above the SS-suite.

Unlike the other two suites, there does not appear

to be any significant depletion of reactive elements in

uite. Concentrations (in moles per kilogram) of reactive elements are

e horizontal solid line indicates the ratio observed in the ‘‘TC’’ fresh

d in the most weathered sample, ‘‘SS-sap’’ (also plotted at 45-cm

t (see Fig. 5).

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341 325

reactive/immobile concentration ratios for the TC

suite (Fig. 8). Differences in composition between

samples from ‘‘TC’’ rindlet system and the nearby

‘‘TC’’ fresh rock sample likely reflect spatial variation

in pristine rock composition.

Relatively steep Na/Ti and Ca/Ti gradients are

observed in the ‘‘GN’’ and ‘‘SS’’ suites (Figs. 6

and 7; corresponding to parts of zone 2 where

D0C/DC0 is significantly less than 1; see Fig. 2),

where substantial porosity had developed in the

weathered rock and substantial amounts of plagio-

clase were available to react. Measurable gradients

were not observed in the ‘‘TC’’ suite (Fig. 8;

corresponding to parts of zone 2 where D0C/DC0 is

Fig. 8. Chemical compositions of weathered rock samples along the ‘‘TC’’ s

shown relative to the assumed conservative element, Ti. For comparison, th

rock sample, while the horizontal dashed line indicates the ratio observed in

distance along the sample transect (see Fig. 4).

not significantly less than 1; see Fig. 2) where

substantial porosity had not developed in the rock

matrix even though a rindlet system had developed.

In the saprolite, corresponding to zone 3, White et al.

(1998) observed Na/Ti and Ca/Ti gradients of zero

since virtually all Na- and Ca-bearing minerals had

been leached away.

4.2. Petrography and mineralogy

Sample TC 25 represents a rindlet in the early

stage of weathering (Fig. 9) where the rindlet interior

is still very similar to the ‘‘unweathered’’ rock.

Modal analysis of the petrographic thin section

uite. Concentrations (in moles per kilogram) of reactive elements are

e horizontal solid line indicates the ratio observed in the ‘‘TC’’ fresh

the most weathered sample, ‘‘SS-sap.’’ The horizontal axis indicates

Fig. 9. Petrographic thin section of sample TC25 under plane-polarized light. A fracture pore (seen as light gray in color) runs vertically from

top to bottom of the photograph. Septa composed of kaolinite run down the middle of the pore. Plagioclase is relatively unweathered. Field of

view in the horizontal direction is 1.2 mm.

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341326

(Table 4) indicates an absence of substantial amounts

of weathering products. Chemical formulae of min-

erals in sample TC 25 (Table 5) are similar to

formulae tabulated by White et al. (1998). Porosity

is not evenly distributed throughout the thin section.

Most porosity is from fractures, as wide as 400 Am,

cutting across mineral grains. Some porosity is devel-

oped along cleavage planes in biotite and hornblende.

Some grains of plagioclase, hornblende, and augite

appear weathered by contact with solution. Alteration

of minerals is most noticeable in areas of confluence

of fractures. Trace amounts of kaolinite appear in

Table 4

Modal analyses (%) for three thin sections

TC 25 GN 36 Slide-43

‘‘unweathered’’

Plagioclase 35 20 40

Quartz 20 30 30

Hornblende 20 < 5 15

Augite 10 10 5

Biotite 5 10 5

Opaques < 5 < 5 < 5

Chlorite < 5 0 < 5

Kaolinite 0 10 0

Porosity 5 15 0

Values represent approximate volume fraction of each component.

some pores. As a whole, the sample appears to have

undergone a relatively small amount of chemical

alteration.

In contrast, sample GN 36 represents a rindlet that

has undergone substantial alteration and is friable

(Fig. 10). Mineralogically, GN 36 is more biotite-rich

and has less hornblende than TC 25 (Table 4). Like

TC 25, there is a system of fractures, as wide as 600

Am, cutting across mineral grains. However, much of

the porosity is developed in plagioclase grains, which

have undergone substantial dissolution. Kaolinite septa

are seen in the center of fractures, and in the center

of former plagioclase grains. In most of the plagio-

clase grains, it appears that more than 50% of the

original grain has been dissolved, with roughly 30%

of the porosity being filled by kaolinite. Porosity

development is slightly greater in biotite, hornblende,

and augite grains relative to TC 25; however, sub-

stantially more weathering has clearly occurred in

plagioclase grains.

The Slide-43 rindlet has a thick highly weathered

shell boundary adjacent to a relatively unweathered

rindlet core. Little porosity is developed in the core;

however, a fracture on the order of 300 Am wide has

developed parallel to the rindlet boundary (Fig. 11).

Associated with this fracture are smaller, subparallel

fractures. Mineralogically, the ‘‘unweathered’’ rindlet

Table 5

Mineral compositions in rock samples determined by electron microprobe

Mineral Formula Sample

Plagioclase K0.01Na0.44Ca0.55Al1.55Si2.45O8 TC 25

Plagioclase K0.01Na0.39Ca0.60Al1.60Si2.40O8 TC 25

Plagioclase K0.01Na0.63Ca0.36Al1.36Si2.64O8 TC 25

Plagioclase Na0.60Ca0.40Al1.36Si2.63O8 White et al. (1998)

K-feldspar Na0.09K0.91AlSiO8 White et al. (1998)

Hornblende (Ca2.21Na0.35)(Mg2.70Fe2.17Al0.13)(Si7.10Al0.90)O22(OH)2 TC 25

Hornblende (Ca1.71Na0.34K0.05)(Mg2.84Fe2.06Al0.89)(Si6.68Al1.00)O22(OH)2 White et al. (1998)

Augite Ca(Fe0.62Al0.38)(Si1.62Al0.38)O6 TC 25

Augite Ca(Fe0.58Al0.42)(Si1.58Al0.42)O6 TC 25

Biotite K0.89(Fe1.41Mg1.28Al0.31)(Al1.20Si2.80)O10(OH)2 TC 25

Biotite K0.89(Fe1.41Mg1.36Al0.23)(Al1.12Si2.88)O10(OH)2 TC 25

Biotite K0.85(Fe2 +1.30Fe

3 +0.05Mg1.25Al0.10Ti0.15)(Al1.20Si2.80)O10(OH)2 White et al. (1998)

Chlorite (Mg3.32Fe2 +0.29Fe

3 +1.00Al1.39)(Si1.61Al2.39)O10(OH)8 TC 25

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341 327

core is similar to the TC 25 sample (Table 4). Although

there are fewer grain-traversing fractures in this rindlet

compared to the previous two, significant porosity is

developed in some plagioclase grains. Minor amounts

of kaolinite appear in some plagioclase grains.

Over a distance of less than a centimeter, the

relatively unweathered rindlet core grades into a zone

similar to sample GN 36. This zone, 3 to 4 cm thick, is

characterized by grain-traversing fractures, the largest

of which run parallel to the rindlet boundary. In

contrast to GN 36, however, weathering of plagioclase

in this zone is near completion, with 90% of plagio-

Fig. 10. Petrographic thin section of sample GN36 under cross-polarized li

almost completely replaced by kaolinite (small white grains) and pore space (

clase removed. A larger fraction of the space voided

by the plagioclase appears to be filled with kaolinite,

approximately 50%. Some relatively large relict frac-

tures are lined with kaolinite, while some smaller

relict fractures are completely filled with kaolinite

(Fig. 12). Porosity of this zone is on the order of

25–30%. While depletion with respect to plagioclase

in this zone is nearly complete, hornblende, biotite,

and augite persist. However, these minerals have

undergone significant alteration and kaolinite has

precipitated in pore space developed in augite. Biotite

has partially altered to an unidentified phyllosilicate

ght. Quartz is relatively unweathered; however, plagioclase has been

seen here as black). Field of view in the horizontal direction is 1.2mm.

Fig. 11. Petrographic thin section, under plane-polarized light, of a relatively unweathered portion of the Slide-43 rindlet. A fracture pore, shown

horizontally traversing the photograph and seen as light gray in color, runs parallel to the inter-rindlet boundary. Field of view in the horizontal

direction is 2.4 mm.

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341328

(refer to Murphy et al., 1998 for a discussion of

possible biotite weathering products). Samples from

each suite, analyzed for phyllosilicate minerals by

XRD, were found to contain biotite, chlorite, and

kaolinite. No evidence of gibbsite was observed.

Fig. 12. Petrographic thin section, under crossed-polarized light, of a weat

horizontally traversing the photograph, runs parallel to the inter-rindlet bou

the relict fracture pore, plagioclase has been almost completely replaced b

Field of view in the horizontal direction is 1.2 mm.

Alteration of these Fe-bearing minerals is most appar-

ent 2–4 cm from the unweathered core.

Although minor alteration of Fe-bearing alumi-

nosilicates appears to occur during the early stages

of porosity development, these minerals tend to

hered portion of the Slide-43 rindlet. A former fracture pore, shown

ndary and is now completely filled with kaolinite. On either side of

y kaolinite (small white grains) and pore space (seen here as black).

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341 329

persist until the rock becomes completely depleted

with respect to plagioclase. However, the saprolite

is completely depleted with respect to hornblende

and augite, and biotite is substantially weathered

(White et al., 1998; Murphy et al., 1998). White et

al. (1998) concluded that weathering of hornblende

was occurring in the zones between saprolite and

fresh bedrock, but did not observe augite in the

bedrock and thus did not consider weathering of

augite in their analysis. Murphy et al. (1998) found

that biotite in the saprolite was weathering to

kaolinite, with no observable intermediate phase.

In the rindlet systems, substantial weathering of

Fe-bearing aluminosilicate minerals is not seen

except at highly weathered rindlet boundaries. In

the case of the SS suite, substantial weathering of

these minerals has apparently occurred in the sap-

rolite-like zone (corresponding mineralogically to

zone 3 near the boundary with zone 2) above the

rindlet system, where discernible rindlet-like struc-

ture has disappeared. There is little evidence for

dissolution of quartz in the rindlets (i.e. zone 2).

However, evidence for quartz dissolution was dis-

covered in the saprolite where silica concentrations

in pore waters are relatively low (Schulz and White,

1999).

5. Discussion

5.1. Plagioclase dissolution and porosity development

It is apparent from both the analysis of bulk chem-

ical data and petrographic thin sections that weathering

in the rindlet systems is dominated by plagioclase

dissolution. White et al. (1998) conducted a solute

mass balance between pore water collected from the

deep saprolite and stream water collected from the

nearby Quebrada Guaba (near ‘‘GN’’ sample location,

Fig. 1), a tributary to the Rio Icacos. The mass balance

indicated that the difference in composition between

the waters could be attributed primarily to dissolution

of plagioclase and precipitation of kaolinite, consistent

with observations reported here. White et al. (1998) did

not observe a significant decrease in Al in saprolite

with increasing age. Murphy (1995) noted that little or

no dissolved organic carbon, which tends to form

complexes with Al, was present in saprolite pore

waters. These observations support the assumption that

Al is conserved in the subsurface.

Dissolution of plagioclase appears to be the pri-

mary process affecting porosity development in the

rindlet systems. Porosity development from weather-

ing of plagioclase to kaolinite is dependent on pla-

gioclase composition. Assuming no aqueous Al, the

stoichiometries of dissolution of the two plagioclase

end members, albite and anorthite, are

NaAlSi3O8 þ Hþ þ 4:5H2O

¼ Naþ þ 2H4SiO4 þ 0:5Al2Si2O5ðOHÞ4 ð2Þ

and

CaAl2Si2O8þ2HþþH2O ¼ Ca2þ þ Al2Si2O5ðOHÞ4;ð3Þ

respectively. Note that dissolution of anorthite results

in twice as much kaolinite per mole of cation released

as compared to albite. Under constant volume con-

ditions, the stoichiometry of plagioclase dissolution

will affect porosity development according to the

relation

/K

/v

¼ 0:5ð1� XAnÞmKGK þ XAnmKGK

ð1� XAnÞmAbGAb þ XAnmAnGAn

ð4Þ

where /K//v is the ratio of voided space filled with

kaolinite to total voided space, XAn is the fraction of

anorthite in plagioclase, m is molecular weight, G is

specific gravity, and the subscripts Ab and K refer to

albite and kaolinite, respectively. For example, assum-

ing typical specific gravities for albite, anorthite, and

kaolinite of 2.62, 2.78, and 2.6, respectively (with

molecular weights being 262.2, 278.2, and 258.2 g

mol� 1, respectively), the amount of space voided by

plagioclase dissolution that is filled with kaolinite is

49% and 87% for albite and anorthite, respectively.

Thus, although Ca-rich plagioclase is generally con-

sidered to weather faster than Na-rich plagioclase,

porosity development will not be as extensive as with

Na-rich plagioclase if kaolinite precipitation occurs.

This may possibly result in a limitation on the

weathering rate of Ca-rich plagioclase relative to

Na-rich plagioclase. A plagioclase composition typi-

cal for diorites, An40, would result in a /K//v of 65%.

The value of /K//v observed in thin section ‘‘GN 14’’

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341330

is on the order of 40%, suggesting that additional pore

space is developed by fracturing possibly related to

rindlet development.

5.2. Weathering at the basin scale

Insight into the overall stoichiometries of weather-

ing in the system can be reached by considering the

fluxes of dissolved products of weathering exported in

stream water. McDowell and Asbury (1994) estimated

inputs of various solutes from precipitation, and out-

puts from vegetative accretion and stream water

(Table 6). White et al. (1998) conducted a similar

mass balance based on the same data, yielding similar

results. However, this approach has limitations, name-

ly an approach to estimating meteoric solute inputs

yielding excessive uncertainty, and an estimation of

net accumulation of solutes in biomass that is con-

ceptually incompatible with a pristine steady-state

forest (which should have zero net solute accumula-

tion). McDowell and Asbury (1994) estimated pre-

cipitation inputs from the averages at the El Verde site

within the Luquillo Experimental Forest (McDowell

et al., 1990), at an elevation of 425 m (compared to

616–800 m for the Rio Icacos watershed), and ad-

justed the magnitude of total precipitation using an

empirical orographic rainfall relationship. The result is

that 38% of exported Cl was unaccounted for and

attributed to chemical weathering, with a Cl to Na

ratio (mol/mol) in weathering inputs of 0.58. If Na-

and Cl-bearing minerals are weathering completely

(which appears to be the case since the saprolite is

completely depleted in these elements), then the Cl/Na

ratio in weathering inputs should equal the ratio in

Table 6

Parameters used in calculation of watershed-scale net solute fluxes

Solute Sea salta

(Amol/l)

Dusta

(Amol/l)

Ci,El Verde*

(Amol/l)

Na 65.2 0 65.2

Cl 76.5 0 76.5

Ca 1.4 2.4 3.8

Si 0 0 0

K 1.4 0 1.4

Mg 7.4 0 7.4

a Sea salt and desert dust components of rain water chemistry at El Vb Solute fluxes exported in stream water per unit basin area (McDowec Net solute fluxes per unit basin area (this study).

unweathered rock (0.013). Although the mass balance

of McDowell and Asbury (1994) has proven to be

useful, the uncertainties are too large for the weather-

ing fluxes to closely resemble the overall stoichiom-

etry of weathering in the Rio Icacos system.

An alternative approach to conducting a solute

mass balance is to use the solute exports measured

by McDowell and Asbury (1994) and do a cyclic salt

correction assuming that all chloride comes from

atmospheric inputs. Stallard (2001) quantified marine

and desert dust sources of ions to atmospheric depo-

sition at the El Verde site (Table 6) using rain water

chemistry data from 1985 to 1999. In his analysis,

Stallard used Cl and Ca as marker ions for sea salt and

desert dust, respectively. The cyclic salt correction is

conducted by assuming atmospheric inputs of Na,

Mg, K, and Ca to the Rio Icacos watershed exist in

ratios relative to chloride equal to those tabulated by

Stallard (2001). Thus, the flux of solute i in atmo-

spheric deposition, Qi,meteoric, is estimated by

Qi;meteoric ¼ QCl;streamCi;El Verde*

CCl;El Verde*ð5Þ

where QCl,stream is the flux of chloride exported in

stream water (19.2� 10� 9 mol m� 2 s� 1; McDowell

and Asbury, 1994), Ci,El Verde* is the concentration of

solute i in rainwater at the El Verde site derived from

marine and desert dust sources, and CCl,El Verde* is the

concentration of chloride in rainwater at the El Verde

site derived from marine and desert dust sources. The

resulting flux is then subtracted from the measured

flux of solute i exported in stream water, Qi,stream, to

Ci,El Verde* /

CCl,El Verde*

Qi,streamb

(mol m� 2 s� 1)

Qi,net,obsc

(mol m� 2 s� 1)

0.852 22.2� 10� 9 5.83� 10� 9

1 19.2� 10� 9 0

0.050 7.59� 10� 9 6.64� 10� 9

0 25.6� 10� 9 25.6� 10� 9

0.018 1.38� 10� 9 1.03� 10� 9

0.097 4.57� 10� 9 2.71�10� 9

erde (Stallard, 2001).

ll and Asbury, 1998).

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341 331

obtain the net solute flux attributable to chemical

weathering, Qi,net,obs:

Qi;net;obs ¼ Qi;stream � Qi;meteoric: ð6Þ

Calculated net flux values are presented in Table 6.

In order to gain additional insights into weathering

in this system from the net fluxes calculated above,

fluxes for two hypothetical weathering scenarios will

be calculated for comparison. These scenarios involve

the assumption that some or all of the rock weathers to

completion. If the minerals containing a certain ele-

ment in the bedrock weather to completion (i.e.

weather so that none of the element in question remains

in a primary mineral phase), then the long-term steady-

state flux of that element from the system,Qi,net,hyp, can

be expressed as

Qi;net;hyp ¼ RCi;0q0 ð7Þ

where R is the denudation rate, Ci,0 is the concentration

of element i in bedrock that will be transported from the

system, and q0 is the original density of bedrock.

In the first scenario, plagioclase (which is absent in

the saprolite) is assumed to weather to completion.

This calculation yields hypothetical fluxes for Na, Ca,

K, and Si (the elements constituting plagioclase) using

Eq. (7) and the following assumptions: (1) all plagio-

clase in the system weathers to completion; (2) all Na

in the bedrock is contained in plagioclase; (3) a

plagioclase composition of An40 (White et al.,

1998), a stoichiometric coefficient for K of 0.01,

and stoichiometric dissolution; (4) bedrock composi-

tion is equal to the average composition from the Rio

Blanco stock (Table 2); (5) bedrock density is 2700 kg

m� 3 (White et al., 1998); (6) the long-term denuda-

tion rate is 43 m Ma� 1 (Brown et al., 1995); and (7)

Table 7

Observed net solute fluxes per unit basin area (Qi,net,obs), and hypothetica

McDowell and

Asbury (1994),

Qi,net,obs (mol m� 2 s� 1)

This study (Table 6),

Qi,net,obs (mol m� 2 s� 1)

Na 12.5� 10� 9 5.83� 10� 9

Ca 6.7� 10� 9 6.64� 10� 9

Si 25.5� 10� 9 25.6� 10� 9

K 1.7� 10� 9 1.03� 10� 9

Mg 3.6� 10� 9 2.71�10� 9

Total 50.0� 10� 9 41.8� 10� 9

Al is conserved in the form of kaolinite. Since Al is

assumed to be conserved, an amount of Si equal to the

Al released would be consumed during precipitation

of kaolinite (i.e. CSi,0 is less than the total concentra-

tion in bedrock). Comparison of these values with

watershed hydrogeochemical fluxes (Table 7) reveals

that plagioclase weathering contributes adequate

fluxes to explain the contemporary watershed hydro-

geochemical mass balances of Na and possibly Ca.

However, K, Mg, and additional Si must come from

other sources.

In the second scenario, the entire rock is assumed

to weather completely to kaolinite and Fe3 + oxides

(i.e. all Al and Fe released from weathering of

primary minerals is conserved as kaolinite and Fe3 +

oxides). Using Eq. (7) and assumptions (4) through

(7) above, fluxes are calculated for Na, Ca, K, Mg,

and Si (Table 7).

A comparison of the hypothetical fluxes with the

observed net fluxes calculated here (Tables 7 and 8)

indicates that the overall stoichiometry of weathering

in the system closely resembles scenario 2 where the

rock is weathering to completion, except that the

relative amount of silica in the observed solute fluxes

is low. This suggests that quartz does not weather to

completion, which is consistent with the observation

of White et al. (1998) that quartz is a major constit-

uent of the saprolite. Also, the slightly lower values of

QK,net,obs/QNa,net,obs and QMg,net,obs/QNa,net,obs relative

to QK,net,hyp/QNa,net,hys and QMg,net,hyp/QNa,net,hyp cal-

culated for total rock weathering may result from

incomplete weathering of biotite in the saprolite as

observed by White et al. (1998) and Murphy et al.

(1998). The overall reaction stoichiometry observed in

the net solute fluxes is consistent with the observa-

tions of complete weathering of plagioclase, horn-

l solute fluxes (Qi,net,hyp)

Scenario 1,

Qi,net,hyp (mol m� 2 s� 1)

Scenario 2,

Qi,net,hyp (mol m� 2 s� 1)

3.83� 10� 9 3.83� 10� 9

2.56� 10� 9 4.15� 10� 9

7.66� 10� 9 26.5� 10� 9

0.06� 10� 9 0.89� 10� 9

0 1.98� 10� 9

14.12� 10� 9 37.33� 10� 9

Table 8

Relative observed net solute fluxes (Qi,net,obs) and hypothetical

solute fluxes (Qi,hyp)

McDowell

and Asbury

(1994),

Qi,net,obs/

QNa,net,obs

This study,

Qi,net,obs/

QNa,net,obs

Scenario 1,

Qi,hyp/

QNa,hyp

Scenario 2,

Qi,hyp/

QNa,hyp

Na 1 1 1 1

Ca 0.54 1.14 0.67 1.08

Si 2.04 4.39 2.00 6.92

K 0.14 0.18 0.02 0.23

Mg 0.29 0.46 0.00 0.52

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341332

blende and augite, and partial weathering of quartz

and biotite. This finding is consistent with a mass

balance on saprolite pore water and stream water

compositions conducted by White et al. (1998) that

indicated that dissolution of plagioclase, hornblende,

biotite, and quartz contributed solutes to solution.

However, the magnitudes of the observed water-

shed-scale fluxes of dissolved Na, Ca, K, and Mg

(Table 7) are greater than those predicted assuming

the long-term denudation rate of 43 m Ma� 1. For

example, QNa,net,obs (as calculated here) is 52% greater

than QNa,net,hyp calculated using the long-term denu-

dation rate. This suggests a contemporary denudation

rate that is 52% greater than the long-term rate, or 65

m Ma� 1. This value is intermediate between the

contemporary denudation rates calculated by Brown

et al. (1995) and White et al. (1998) of 75 and 58 m

Ma� 1, respectively. It is unclear whether the apparent

difference between the long-term denudation rate and

the contemporary denudation rate is due to uncertainty

in the estimates, or reflects actual temporal variation

in denudation rate. However, Brown et al. (1995)

concluded that their long-term and contemporary

denudation rates were not significantly different.

The similarity in magnitude between the total ob-

served solute flux and the flux estimated using the long-

term denudation rate calculated by Brown et al. (1995)

suggests that the contemporary denudation rate is

remarkably similar to the long-term denudation rate.

This is consistent with the conclusion of White et al.

(1998) that weathering fluxes in the watershed have not

changed substantially over the last several hundred

thousand years. Thus, the Rio Icacos basin can be

reasonably envisioned as a steady-state system where

the primary silicate minerals in bedrock (except quartz

and biotite) weather to completion.

5.3. Steady-state sediment export

The approach discussed in the preceding section

can be extended to estimate the flux of sediment

exported from the Rio Icacos system. Assuming a

system at steady-state, the portion of rock exported in

the solid phase is equal to the portion not removed in

solution. For example, assuming our total denudation

rate of 65 m Ma� 1 is correct, a volume of rock 65 m

thick will weather to saprolite within a time period of

1 Ma. Since the volume of regolith (zones 2 and 3 in

the ‘‘Conveyor Belt’’ model) must remain constant

under steady-state conditions, a volume of saprolite

(zone 3) 65 m thick must be removed from the system

in the same time period. Thus, the entire mass of the

volume of rock is removed through chemical and

physical means. This relationship can be expressed

as follows:

Qi;tot* ¼ Qi;net* þ Qi;sed* ð8Þ

where Qi,tot* is the total mass flux of oxide i affected by

weathering, Qi,net* is the net solute export expressed in

terms of mass of oxide, and Qi,sed* is the flux of oxide

removed by sediment transport. A calculation similar

to Eq. (7) can be used to estimate Qi,tot* :

Qi;tot* ¼ RFi;0q0 ð9Þ

where Fi is the mass fraction of each oxide i present in

the parent rock. In order to obtain estimates compa-

rable to the observed net solute fluxes, a total denu-

dation rate of 65 m Ma� 1 is used in this calculation.

Values of Qi,net* are obtained by converting Qi,net,obs

(Table 7) to units of kg m� 2 s� 1, and values of Qi,sed*

are obtained by solving Eq. (8). The value of QCa,sed* is

set to zero since Ca is not exported in the solid phase

in our conceptual model, hence a nonzero value of

QCa,tot* �QCa,net* is attributed to uncertainty in the

estimates of QCa,tot* and QCa,net* . Values of Qi,sed* (Table

9) indicate a sediment composed of 60% kaolinite

(assuming all Al is in kaolinite), with the balance

composed of quartz, Fe-oxides, and weathered biotite.

Table 9

Hypothetical mass flux of total weathering products, Qi,tot* , observed mass flux of dissolved weathering products, Qi,net* , steady-state mass flux

of sediment, Qi,sed* , and an independently calculated steady-state sediment yield, Ys, comparable to the total Qi,sed*

Qi,tot* (kg m� 2 s� 1) Qi,net* (kg m� 2 s� 1) Qi,sed* (kg m� 2 s� 1) Ys (kg m� 2 s� 1)

Na2O 0.18� 10� 9 0.18� 10� 9 0

CaO 0.35� 10� 9 0.37� 10� 9 0

SiO2 3.51�10� 9 1.54� 10� 9 1.98� 10� 9

K2O 0.006� 10� 9 0.005� 10� 9 0.001�10� 9

MgO 0.12� 10� 9 0.11�10� 9 0.001�10� 9

Al2O3 0.94� 10� 9 0 0.094� 10� 9

FeO 0.30� 10� 9 0 0.030� 10� 9

Total 5.47� 10� 9 2.25� 10� 9 3.24� 10� 9 3.60� 10� 9

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341 333

This hypothetical sediment composition is very sim-

ilar to the composition of the saprolite (Table 3).

An upper limit to sediment generation under

steady-state conditions can be obtained by assuming

that the chemical composition of Rio Icacos sand is

representative of bulk sediment composition. The

mass fraction WA of a chemical component A that

dissolves in bedrock can be calculated as

WAðA;BÞ ¼Bb

Ab

� Bs

As

� �H

Bd

Ad

� Bs

As

� �ð10Þ

where A and B are the mass fractions of elements in

(b) bedrock, (d) dissolved load, and (s) solid sediment

(Stallard, 1995b). The fraction of bedrock that dis-

solves, W, is

W ðA;BÞ ¼ Ab

Ad

WcðA;BÞ ð11Þ

(Stallard, 1995b). Stallard (1995a) recommended that

the ideal pair of elements for application of Eqs. (10)

and (11) are Si (as element A) because it is the most

abundant cation in most rocks, and Na (as element B)

because the variance r(Bb/Ab) is small and Bb/AbH

Bs/As. Sediment yield, Ys, can then be predicted from

the total dissolved mass flux, Yd (Table 9):

Ys ¼ Yd1�W

Wð12Þ

(Stallard, 1995a,b). Using values of As (SiO2 in

sediment sand) of 0.7932 and Bs (Na2O in sediment

sand) of 0.0033 (Stallard, unpublished), Ab and Bb

values from Table 2, and Ad, Bd, and Yd values

obtained from Table 9, a value of Ys (see Table 9) is

obtained that is slightly higher than the total Qi,sed* .

These values represent upper and lower limits on the

steady-state sediment yield based on contemporary

solute fluxes: one estimate assumes no Na in sedi-

ment, and the other assumes the Na concentration in

the sand fraction is representative of the bulk sediment

concentration.

The total sediment flux of 3.24� 10� 9 to 3.60�10� 9 kg m� 2 s� 1 represents a hypothetical sediment

yield under idealized steady-state conditions. While a

system may or may not be at steady-state, this value

provides a convenient reference to compare measured

values to. Larsen (1997) estimated a contemporary

sediment yield for the Rio Icacos basin of

30.2� 10� 9 kg m� 2 s� 1; however, 71% of this is

estimated to be due to the presence of the road

through the basin (Fig. 1) and its associated land-

slides. Therefore, sediment transport in the basin is

currently far from steady-state due to the road. How-

ever, Larsen’s background sediment yield of 8.8�10� 9 kg m� 2 s� 1 is much smaller. A comparison

between Larsen’s background sediment yield and our

calculated steady-state yields indicates that the con-

temporary sediment yield in the absence of the road is

significantly accelerated compared to the steady-state

condition. The time scale for this disequilibrium may

be on the order of 100 years; the proximity of the10Be-derived denundation rate (43 m Ma� 1; Brown et

al., 1995) to our contemporary denudation rate (65 m

Ma� 1) indicates that sediment generation was closer

to steady-state in a pre-anthropogenic time frame

(Brown et al., 1998). Reasons for the accelerated

sediment yield over this time scale may include (1)

higher frequency of large storms, (2) vegetation

changes caused by the introduction of plants and

diseases of plants, (3) vegetation changes caused by

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341334

climate shifts or pollution, or (4) subtle human effects

that we have not identified.

5.4. Parallel weathering

It is argued in preceding sections that the ‘‘Con-

veyor Belt’’ model, while useful for conceptualizing

weathering on a basin scale, is limited in its ability to

explain observations at smaller scales due to lack of

consideration of multiple weathering fronts. It is well

documented that the presence of joints in bedrock

enhances the weathering of granitoid rocks (e.g.

Twidale, 1982). Three-dimensional networks of joints

result in the bedrock being divided into joint blocks

Fig. 13. Illustration of the concept of parallel weathering of joint blocks.

(shown as heavy black arrows) is from joint planes towards the center of jo

front is a fraction of the total denudation rate. Figure is not to scale.

bounded by joint surfaces (Fig. 13). Presumably, the

early stages of rock weathering in the system are

dominated by water flow along joint planes. Since

the fracture pores that are suspected to cause the

formation of onion-skin rindlet structures tend to form

subparallel to the ground surface, it is assumed here

that most weathering of joint blocks occur along their

top and bottom surfaces. The GN outcrop (Fig. 3) is

an example of a joint block where the weathering

front appears to be moving in the upward direction

from the joint plane. Since joints typically penetrate

deep into bedrock, more than one vertically stacked

joint block may be weathering in this fashion at the

same time. Under such circumstances, Eq. (1) is not

As water flows along joint planes, movement of weathering fronts

int blocks. The magnitude of the velocity of an individual weathering

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341 335

appropriate to describe weathering on the spatial scale

of a series of joint blocks. It is the purpose of this

section to illustrate that (1) the weathering system

consists of multiple weathering fronts, and (2) the

magnitude of the rate of movement of an individual

weathering front is dependant on its spatial scale.

For a system of joint blocks weathering in parallel

under uniform, steady-state conditions with each

block weathering identically, the system must be

considered as a whole. The rate of advancement,

rblock, of each of several parallel weathering fronts

must each be a fraction of the total denudation rate, R:

rblock ¼R

2nblocksð13Þ

where nblocks is the number of vertically stacked joint

blocks undergoing parallel weathering. The total de-

nudation rate is the sum of all the individual rblockvalues. The parameter nblocks is multiplied by two

since each joint block is assumed to bounded by an

upper and lower weathering front, each moving to-

ward the center of the block.

The concept of parallel weathering can be extended

to a smaller spatial scale. Weathering of a joint block

typically occurs in rindlet systems contained within

the block, with inter-rindlet boundaries parallel to the

joint block boundaries. Rindlet systems typically

consist of parallel onion-skin layers that are weather-

ing from their top and bottom boundaries. Following

the reasoning and assumptions behind Eq. (13), for a

system of rindlets uniformly weathering in parallel

within a joint block, the rate of advance of the

weathering front at an individual rindlet boundary,

rrind, is a fraction of rblock

rrind ¼rblock

2nrindletsð14Þ

where nrindlets is the number of rindlets in the system

weathering in parallel. The parameter nrindlets is mul-

tiplied by two since each rindlet is surrounded by two

weathering fronts: one at the top and one at the bottom

of the rindlet. Since rindlets are observed to occur in

systems of several layers (value of nrindlets varies with

each joint block; nrindlets>20 was observed in the case

of the GN outcrop), the rate of advance of a weath-

ering front on a rindlet-boundary scale, rrind, may be

one or more orders of magnitude slower than the

overall denudation rate, R. Hence, although the solid-

phase Na concentration gradient across the Slide-43

rindlet would be expected to be very large, the in situ

rate of plagioclase weathering is not necessarily larger

than in rindlet systems ‘‘GN’’ and ‘‘SS,’’ since the rate

of advance of the weathering front on an individual

rindlet scale, rrind, is substantially less than the rate at

the scale of a joint block or rindlet system, rblock. This

reasoning could feasibly be extended to an even

smaller spatial scale, where the rate of advance of a

weathering front originating from a fracture within a

rindlet would be a fraction of rrind.

Elements of parallel weathering features are

commonly seen where fractured and porous bedrock

are exposed within the Rio Icacos watershed, such

as road cuts, stream banks, and landslide scars.

Sample suite ‘‘SS’’ appears to be from the interface

between a weathering joint block and the overlying

saprolite, while suite ‘‘TC’’ appears to run from the

center to the upper boundary of a small joint block

undergoing the early stages of weathering, and the

Slide-43 rindlet is presumably from the a rindlet

system at the interface of a joint block and the

overlying saprolite. Suite ‘‘GN’’ appears to traverse

the interface between two adjacent joint blocks. The

suite progresses (top downwards) from relatively

unweathered rindlets to weathered rindlets, then

crosses a sharp boundary, the other side of which

consists of porous, but relatively unweathered rind-

lets. This may be an example of a weathered joint

block underlain by a less progressively weathered

block.

5.5. Differential weathering on multiple fronts

Although the parallel weathering concept may

describe to a greater degree the complexity

exhibited in subsurface bedrock weathering than

the simple ‘‘conveyor belt’’ model, it is unreason-

able to assume that rblock and rrind are constant for

every weathering front. As espoused in the ‘‘con-

veyor belt’’ discussion and in Fig. 2, mineral

weathering rates are greater in porous, moderately

weathered rock than in rock in which little weath-

ering has occurred. When significant porosity devel-

ops in the rock matrix (as with the ‘‘GN’’ and ‘‘SS’’

systems), it is expected that dissolved weathering

products will generally be transported by advecting

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341336

water (thus, plagioclase dissolution in very porous

zones may be surface-controlled rather than limited

by diffusion of reaction products from the rock ma-

trix). When significant porosity has not yet developed

in the rock matrix (as with the ‘‘TC’’ system), advect-

ing water is expected to move primarily through the

rindlet boundaries. Hence, it is reasonable that rblock or

rrind varies according to the degree of weathering of the

subsystem in question, with higher values for moder-

ately weathered systems than system undergoing the

Fig. 14. Illustration of the concept of differential weathering of joint block

of weathering fronts (shown as black arrows of variable weight, with weigh

ground surface, where porosity development is the greatest. Water flow

relative magnitude) is fastest at joints/fractures and where porosity develop

an individual weathering front is a fraction of the total denudation rate. T

conceptual framework are shown for comparison. Figure is not to scale.

early stages of weathering. Therefore, the rate of

advance of the weathering front through a relatively

unweathered rindlet system such as ‘‘TC’’ is slower

than through moderately weathered systems such as

‘‘GN’’ and ‘‘SS.’’ This concept of differential weath-

ering of joint blocks is illustrated in Fig. 14.

Under a differential weathering system at steady-

state, the individual weathering fronts would be con-

strained such that the sum of rates of advance of all

individual weathering fronts must equal the total

s. Symbols are similar to those used in Fig. 13. However, movement

t indicating relative magnitude) is fastest at joint blocks closest to the

(shown as gray arrows of variable weight, with weight indicating

ment is greatest in the rock matrix. The magnitude of the velocity of

he locations of sample suites ‘‘GN,’’ ‘‘TC,’’ and ‘‘SS’’ within this

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341 337

denudation rate. Just as in the case of the parallel

model, this requires that

R ¼Xi

rblock;i ¼Xi;j

rrind;i;j ð15Þ

and

rblock;i ¼Xj

rrind;i;j ð16Þ

where subscripts i and j refer to joint blocks and

rindlets, respectively. It may also be reasonable to

model such a system as having a continuous proba-

bility distribution of rblock or rrind values, such that an

integration of the probability distribution function

(PDF) of rblock, for instance, would yield a value of

R. However, our observations are not sufficient to

fully parameterize such a model.

Presumably, a rock that tends to form onion-skin

rindlet systems would weather more rapidly than one

that does not. This is because the fracturing associated

with rindlet formation would allow water to flow

through rock that would otherwise be essentially

impermeable. Thus, one would expect with all other

factors the same, the total denudation rate at a site

where such systems form to be greater than where

such systems are absent. On a localized scale within

the Rio Icacos watershed, this effect is exhibited by

the presence of unweathered core stones without

rindlet systems but completely surrounded by sapro-

lite. These core stones presumably are the remnants of

joint blocks where rindlet systems did not form, since

rindlet systems were not observed in such stones.

Clearly, for such a core stone, the rate of advance of

the ‘‘weathering front’’ through adjacent joint blocks

was greater than through the joint block from which

the core stone originated.

5.6. Calculation of in situ weathering rates

In a system where the rate of advance of the

weathering front is strongly affected by the weather-

ing-induced porosity of the rock, it can be imagined

that the bulk of weathering occurs at only a small

number of fronts, which have sufficiently high poros-

ities to allow high values of rblock and rrind. The field

observations include porous weathered rock only on

the top (e.g. suite ‘‘SS’’) and bottom (e.g. suite ‘‘GN’’)

sides of joint blocks. Substantial porosity develop-

ment was not observed in rindlet systems associated

with joint blocks below the one closest to the ground

surface. Thus, it can reasonably be assumed that nblockequals 1 (i.e. the bulk of weathering is occurring on

the top and bottom sides of the joint block nearest to

the ground surface). Note that the value of nblock can

be conceptualized as an average value across a large

area, and is thus not necessarily an integer. It can also

reasonably be expected that porosity development and

the flow of water may be greater on the top side of a

joint block than on the bottom side, since there are

presumably fewer barriers to dilute water reaching the

top side (because of the presumed higher hydraulic

conductivity of the saprolite relative to the rindlet

system, etc.). Therefore, since weathering on the top

side of joint blocks nearest the ground surface may be

more prevalent than weathering on the bottom side,

the assumption that nblock equals 1 may result in

conservatively small values of rblock for the front top

side. Using a value of R of 43 m Ma� 1 (Brown et al.,

1995), we obtain from Eq. (13) a value of rblock of

21.5 m Ma� 1.

To calculate weathering fluxes on a joint block

scale, we can use a modified version of Eq. (1):

J ¼ rblockq0D0dðC=DÞdx: ð17Þ

Values of D0 and d(C/D)/dx for the samples suites,

with Na as a reactive element and Ti as a conservative

element, are presented in Table 10. Assuming a value

for q0 of 2700 kg m� 3, values of J for the ‘‘GN’’ and

‘‘SS’’ suites are 1.52� 10� 9 and 3.82� 10� 9 mol Na

m� 3 s� 1, respectively.

The in situ weathering rate, w, can be normalized to

mineral mass via the relationship

w ¼ Jc

qbCmpl

ð18Þ

where c is the number of atoms of Na per formula unit

of plagioclase, mpl is the molecular weight of plagio-

clase, and qb is the bulk density of the weathered rock.Assuming a plagioclase composition of An40 (Na0.6Ca0.4Al1.4Si2.6O8), c is 0.6 and mpl is 268.6 g mol� 1.

Following the reasoning behind Eq. (4) and assuming

that the only process affecting change in bulk density

Table 10

Parameters used to calculate in situ rates of plagioclase dissolution

Series D0 (mol kg� 1) d(C/D)/dx (m� 1) J (mol m� 3 s� 1) C (mol kg� 1) qba (kg m� 3) wb (mol Na g� 1 s� 1)

GN 0.059c 14 1.52� 10� 9 0.50 2490 2.7� 10� 13

SS 0.061d 34 3.82� 10� 9 0.67 2550 5.0� 10� 13

a Calculated (Eq. (19)).b Normalized to mass of plagioclase.c Ti concentration from sample ‘‘GN 76.’’d Ti concentration from sample ‘‘SS rock.’’

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341338

is plagioclase weathering, bulk density of weathered

rock can be estimated by the relationship

qb ¼ q0 þq0mplðC0 � CÞ

c

0:5mKð1þ XAnÞmpl

� 1

� �:

ð19Þ

Using a C0 of 1.041 mol Na kg� 1 (Table 2), and values

of C at the midpoints of the ‘‘GN’’ (GN 36) and ‘‘SS’’

(SS 7–13) suites of 0.50 and 0.67 mol Na kg� 1,

respectively, bulk densities of these points are 2490

and 2550 kg m� 3, respectively. Weathering rates

obtained by solving Eq. (18) thus are 2.7� 10� 15

and 5.0� 10� 15 mol Na g� 1 s� 1 for the ‘‘GN’’ and

‘‘SS’’ suites, respectively (Table 10). These are the first

such estimates of in situ plagioclase chemical weath-

ering rates for the Rio Icacos basin.

Since BET surface areas of weathered plagioclase

are typically in the range of 0.1–1 m� 2 g� 1 (Anbeek

et al., 1994), the rates of plagioclase weathering

calculated above should be comparable to BET surface

area normalized rates of 2.7� 10� 16 to 5.0� 10� 15

mol Na m� 2 s� 1. These rates are comparable to other

rates of plagioclase weathering in the field normalized

to BET surface area. For example, values reviewed by

White (2001) from North America and Europe range

from 0.4� 10� 16 to 7.9� 10� 14 mol Na m� 2 s� 1.

The rates presented here are generally higher than

comparable rates measured by White in Georgia,

Virginia, and California, ranging from 4.0� 10� 17 to

2.0� 10� 16 mol Na m� 2 s� 1 (White et al., 2001).

6. Conclusions

Most rock weathering within the Rio Icacos

basin weathers in 0.5–1.5 m thick zones of porous

rock herein referred to as ‘‘rindlet systems.’’ These

systems have a spheroidal weathering structure,

characterized by planar, onion-skin-like rindlets 3–

10 cm thick. Chemically, porous rindlets are sub-

stantially depleted in Na and Ca relative to bed-

rock, indicating that plagioclase dissolution has

occurred. In addition, in well-developed porous

rock zones, individual rindlets become progressively

more altered away from joint planes. Petrographic

investigation of the rindlets reveals substantial dis-

solution of plagioclase and replacement by kaolin-

ite. Minor alteration of Fe-bearing aluminosilicates

hornblende, augite, and biotite occurs in the early

stages of rock weathering; however, these minerals

tend to persist in the rindlet systems even after

plagioclase has weathered to completion. Since the

overlying saprolite is completely depleted with

respect to amphiboles and pyroxenes, these minerals

weather to completion between the rindlet systems

and the saprolite.

Contemporary fluxes of dissolved weathering prod-

ucts from the watershed were obtained by performing a

cyclic salt correction on previously published water-

shed solute export data. The overall stoichiometry

apparent from the fluxes closely resembles complete

weathering of plagioclase, hornblende, and augite to

kaolinite and Fe3 + oxides, and incomplete weathering

of biotite and quartz. This observation is consistent

with mineralogical investigations of weathered rock

and saprolite, and indicates that localized observations

of mineral weathering are characteristic of the water-

shed as a whole. Calculated steady-state sediment

yields are less than measured values, indicating that

contemporary sediment generation is accelerated with

respect to steady-state. However, the similarity be-

tween the total denudation rate calculated from ob-

served net solute fluxes and a 10Be-derived denudation

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341 339

rate indicates that the system has been close to steady-

state in the recent past.

At a watershed scale, it is useful to envision

weathering as occurring in a ‘‘conveyor belt’’ fashion,

where weathering fronts move downward at a rate

equal to the denudation rate. However, at an outcrop

scale (similar to the spatial scale of an individual joint

block), parallel weathering fronts appear to be moving

both upward and downward from joint planes towards

the center of joint blocks. Thus, a parallel weathering

model is developed where individual weathering

fronts at this scale move at a fraction of the total

denudation rate. Similarly, at the spatial scale of an

individual rindlet, individual weathering fronts are

conceptualized as advancing from the inter-rindlet

boundaries towards the rindlet centers. Thus, the rate

of advance of weathering fronts at a rindlet scale is a

fraction of the rate of advance at a joint block scale.

However, the rate of advance of individual weath-

ering fronts depends on the degree of porosity devel-

opment. In joint blocks deep in the subsurface, where

porosity is low and water flow is minimal, the rate of

advance of weathering fronts may be negligible

compared to joint blocks nearest the ground surface

where substantial porosity has developed. In situ rates

of plagioclase weathering are calculated from within

this conceptual framework in joint blocks near the

surface, assuming that the majority of watershed-scale

fluxes of dissolved weathering products originate

from weathering of the top and bottom sides of the

joint block nearest to the ground surface. Using

chemical data from the outcrop-scale sample suites

‘‘GN’’ and ‘‘SS,’’ weathering rates are 2.7� 10� 13

and 5.0�10� 13 mol Na g� 1 s� 1, respectively (nor-

malized to mass of plagioclase in the weathering

rock). When considering the typical range of BET

surface areas of naturally weathered plagioclase, these

rates are typical for plagioclase weathering in the

field.

Stallard (1988, 1995a) argues that susceptibility to

weathering is a bulk property of silicate bedrock.

Factors include mineral composition, magma-derived

volatiles, jointing, shearing, and metamorphic anneal-

ing. Clearly, the tendency to exfoliate and develop

multiple parallel weathering fronts, as described here-

in, is another factor. Presumably, a rock that tends to

form such exfoliation features would weather more

rapidly than one that does not.

Definitions of symbols:

Dimensions

L length

M mass

N number (e.g. moles)

T time

Variables

c stoichiometric coefficient (dimensionless)

m molecular weight (M N� 1)

n number of joint blocks or rindlets (N)

r rate of advance of weathering front (L T� 1)

w mineral weathering rate (N M� 1 T� 1)

x position perpendicular to weathering front

(L)

A mass fraction of an element (dimensionless)

B mass fraction of an element (dimensionless)

C concentration of reactive substance (N M� 1)

C0 concentration of reactive substance in pris-

tine rock (N M� 1)

C* concentration in aqueous phase (N L� 3)

D concentration of immobile element (N M� 1)

D0 concentration of immobile element in pris-

tine rock (N M� 1)

F mass fraction of an oxide in bedrock

(dimensionless)

G specific gravity (dimensionless)

J element flux from volume (N L� 3 T� 1)

R denudation rate (L T� 1)

Q element flux from area (N L� 2 T� 1)

Q* mass flux from area (M L� 2 T� 1)

W mass fraction of bedrock that dissolves

(dimensionless)

WA mass fraction of element A that dissolves

from bedrock (dimensionless)

X molar fraction (dimensionless)

Y sediment or dissolved yield (M L� 2 T� 1)

/ porosity (dimensionless)

q0 original rock density (M L� 3)

qb regolith bulk density (M L� 3)

Acknowledgements

The authors would like to express their gratitude to

Dr. Matt Larsen and Angel Torres of the U.S.G.S.,

Guaynabo, Puerto Rico, for facilitating work in the

B.F. Turner et al. / Chemical Geology 202 (2003) 313–341340

field. Advice and assistance in the laboratory from Don

Voigt, Henry Gong, Kay Bickle, andMark Angelone of

Penn State University is also appreciated. This work

also benefited from helpful discussion and comments

from Sheila Murphy. Photographs of petrographic thin

sections were taken at Virginia Polytechnic Institute

and State University with assistance from RobWeaver.

The authors would like to acknowledge Art White of

the U.S.G.S.,Menlo Park, CA, for encouragingwork in

the Rio Icacos basin and for providing insightful

discussion and review. B.F. Turner acknowledges

funding by the Penn State Dept. of Geosciences, and

S.L. Brantley acknowledges National Science Founda-

tion Grant #EAR-94-06263. Comments by Suzanne

Anderson, Lee Kump, and Jon Chorover also helped to

improve the manuscript. [EO]

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