The Island Arc (2004) 13, 432–451
Research Article
Character of sediments entering the Costa Rica subduction zone:
Implications for partitioning of water along the plate interface
GLENN A. SPINELLI1,* AND MICHAEL B. UNDERWOOD2
1
Department of Earth and Environmental Science, New Mexico Tech, Socorro, New Mexico 87081, USA
(email: [email protected]) and 2Department of Geological Sciences, University of Missouri, Columbia,
Missouri 65211, USA
Abstract Sediments deposited off the Nicoya Peninsula advect large volumes of water as
they enter the Costa Rica subduction zone. Seismic reflection data, together with results
from Ocean Drilling Program Leg 170, show that hemipelagic mud comprises the upper
~135 m of the sediment column (ranging from 0 to 210 m). The lower ~215 m of the
sediment column (ranging from 0 to 470 m) is pelagic carbonate ooze. We analyzed samples
from 60 shallow (<7 m) cores to characterize the spatial variability of sediment composition
on the incoming Cocos Plate. The bulk hemipelagic sediment is 10 wt% opal and 60 wt%
smectite on average, with no significant variations along strike; the pelagic chalk contains
approximately 2 wt% opal and <1 wt% smectite. Initially, most of the water (96%) in the
subducting sediment is stored in pore spaces, but the pore water is expelled during the
early stages of subduction by compaction and tectonic consolidation. Approximately 3.6%
of the sediment’s total water volume enters the subduction zone as interlayer water in
smectite; only 0.4% of the water is bound in opal. Once subducting strata reach depths
greater than 6 km (more than 30 km inboard of the subduction front), porosity drops to
less than 15%, and temperature rises to greater than 60∞C. Under those conditions,
discrete pulses of opal and smectite dehydration should create local compartments of fluid
overpressure, which probably influence fluid flow patterns and reduce effective stress
along the plate boundary fault.
Key words: biogenic silica, Costa Rica, opal, seismogenic zone, smectite, subduction.
INTRODUCTION
The deposition, compaction and diagenesis of sedimentary rocks controls the distribution of fluids,
fluid pressures and fluid flow patterns within subduction zones. High fluid pressures probably affect
fault strength along various types of plate interface (Sibson 1981; Byerlee 1990; Unsworth et al.
1997). Cyclic dissipation of fluid pressure and commensurate increases of effective stress might
control the spatial distribution and timing of
seismicity (Byerlee 1993; Magee & Zoback 1993).
Sediments constitute a small volumetric fraction of
the oceanic lithosphere, but they host a large frac*Correspondence.
Received 26 November 2003; accepted for publication 24 March 2004.
© 2004 Blackwell Publishing Asia Pty Ltd
tion of the total water volume that enters subduction zones. On a global average, the volume of
pores and sediment grains each account for
approximately 2.5% of the total slab volume, but
approximately 40% and approximately 5%, respectively, of the mass of subducting water (Peacock
1990).
Mechanical compaction and diagenesis control
the release of fluids from sediments (e.g. Athy
1930; Trask 1931; Powers 1967; Bekins & Dreiss
1992; Moore & Vrolijk 1992). The distribution of
sediment-derived fluid sources in any sedimentary
system depends upon 3-D variations in sediment
composition, pressure and temperature (Raymond
1983; Langseth & Moore 1990; Swarbrick et al.
2002). Sedimentary strata gain strength when
porosity is lost, so the amount of consolidation
decreases exponentially as effective stress
Costa Rica sediment: Fluid partitioning 433
increases (Moore & Vrolijk 1992; Bahr et al. 2001;
Holbrook 2002). Therefore, the volume of fluid
expelled from sediment during compaction
decreases with depth in a sedimentary basin (or
distance into a subduction zone). In deep levels of
a basin, water stored within minerals (e.g. opal and
smectite) comprises a greater proportion of the
total fluid budget.
Transformations of opal to quartz and smectite
to illite are controlled by reaction kinetics (Ernst
& Calvert 1969; Pytte & Reynolds 1988; Huang
et al. 1993). The activation energies are low for both
opal to quartz and smectite to illite (Ernst & Calvert 1969; Pytte & Reynolds 1988), so the reactions
usually progress quickly from initiation to completion. However, the smectite-to-illite reaction can be
slowed or halted if the supply of K+ is limited
(Hower et al. 1976; Boles & Franks 1979). Opal-toquartz diagenesis goes to completion at temperatures of 50–100∞C (Murata et al. 1977; Behl & Garrison 1994). Smectite dehydration involves three
discrete stages at temperatures less than approximately 140∞C (Perry & Hower 1972; Bird 1984;
Bruce 1984; Colten-Bradley 1987). Consequently,
one might expect opal and smectite dehydration to
trigger two or three discrete pulses of fluid release
within narrow windows of temperature and depth.
When unconsolidated sediment first enters a
subduction zone, most of the water is stored in
pore spaces, but the pores collapse rapidly near
the deformation front due to vertical compaction
and tectonic consolidation (e.g. Moore & Vrolijk
1992; Kimura et al. 1997; Screaton et al. 2002).
Therefore, hydrous mineral can convey a relatively large volume of water deep into subduction
zones. The loci of diagenetic dewatering reactions
within a subduction zone depend upon a margin’s
thermal structure and its subduction–accretion
geometry (i.e. stratigraphic position and dip of
decollement, offset along out-of-sequence faults,
underplating via duplex structures).
Smectite and opal dewatering are merely two of
several low-grade reactions that influence pore
pressure, effective stress and frictional properties
within subduction zones (Moore & Saffer 2001).
Geochemical evidence of diagenetic dewatering is
substantial within many active margins (e.g. Kastner et al. 1991). The starting composition and temperature history of the subducting sediments,
however, vary dramatically both within and among
individual systems (e.g. Tribble 1990; Underwood
& Pickering 1996; Deng & Underwood 2001;
Underwood 2002; Steurer & Underwood 2003a).
Because of that heterogeneity, linkages between
diagenetic reactions and their chemical or physical
effects must be established independently, and in
3-D, within each study area. The need for sitespecific assessment applies to compositional factors that might, in theory, control the up-dip limit
of seismicity on subduction megathrusts (Vrolijk
1990; Hyndman et al. 1997).
Substantial volumes of sediment-hosted water
enter the Costa Rica subduction zone off the coast
of the Nicoya Peninsula (Fig. 1). Leg 170 of the
Ocean Drilling Program (ODP) demonstrated that
the entire package of incoming sediment is subducted beneath the frontal toe of the margin
(Kimura et al. 1997), but deeper-seated subduction
processes remain poorly documented. In addition,
various interpretations have been offered to
explain profiles in pore water chemistry near the
margin toe (Chan & Kastner 2000; Silver et al.
2000; Saffer & Screaton 2003), but those efforts
have been hampered by a lack of detail regarding
the actual composition of sediment inputs. To help
fill this void in our knowledge of the Costa Rica
subduction system, we completed compositional
analyses of near-surface sediments on the incoming Cocos Plate, with the goal of gaining insight
into how water is partitioned among pore space,
opal and smectite. The nature of fluid partitioning
at the subduction front, and the down-dip evolution of fluid sources, are also important geotechnical considerations, because of their potential
control over fluid pressure and shear strength
along the plate interface.
COSTA RICA MARGIN
Our study focused on the sediment on the incoming Cocos Plate off the Nicoya Peninsula, Costa
Rica (Fig. 1). Along the Pacific margin of Costa
Rica, the Cocos Plate subducts beneath the Caribbean Plate at the Middle America Trench at
approximately 85 mm/year (DeMets 2001). Offshore of the Nicoya Peninsula, a triple junction
trace divides the subducting Cocos Plate into crust
formed at the East Pacific Rise (EPR) to the north,
and crust formed at the Cocos–Nazca Spreading
Center (CNS) to the south (Barckhausen et al.
2001). Seaward of the Middle America Trench, seafloor heat flow on the CNS crust is 105–115 mW/
m2 (Fisher et al. 2003), consistent with conductive
lithospheric cooling models (e.g. Parsons & Sclater
1977; Stein & Stein 1994). Heat flow on the EPR
crust is 20–40 mW/m2. The lateral transition
between the warm and cool crust is abrupt, occurring over a lateral distance £5 km (Fisher et al.
434 G. A. Spinelli and M. B. Underwood
-40
00
10
o
Costa Rica
m
Nicoya
Peninsula
study area
-1
00
0
9.5
-2
o
m
00
0
-3
00
0
m
m
o
ar
/ye
o
mm
8.5
EPR
CNS crust
crus
t
9
o
-87.5
85
50 km
-87
o
o
-86.5
-86
2003). The low heat flow on the EPR crust likely
results from hydrothermal circulation, facilitated
by numerous basaltic outcrops that allow rapid
transfer of water between the ocean and the crust
(Fisher et al. 2003). The location of shallow earthquakes along the plate interface also changes
along-strike, coinciding with the transition from
CNS to EPR crust. Earthquakes occur shallower
(~10 km) and closer to the trench (~60 km from the
trench) on the warm CNS crust than on the cool
EPR crust (~20 km depth and ~75 km from the
trench) (Newman et al. 2002).
Ocean Drilling Program Leg 170 included a reference site on the floor of the Middle America
Trench (Site 1039) and at two sites at the toe of the
margin wedge (Sites 1043 and 1040, ~0.5 and
1.6 km landward of the trench, respectively). The
stratigraphic section at Site 1039 consists of
approximately 225 m of pelagic sediment, primarily composed of siliceous and calcareous nannofossils, overlain by approximately 150 m of
hemipelagic mud containing abundant clays and
diatoms (Kimura et al. 1997). Reflection seismic
profiles, litho-stratigraphy, core-scale geological
structures and borehole logs (gamma-ray, resistivity and density) collectively indicate that very little
sediment (~1%) is scraped off into the wedge
(Kimura et al. 1997; Saito & Goldberg 2001).
o
-85.5
o
Fig. 1 Map showing the location of the 60
gravity and piston cores used in this study from
sediments on the incoming Cocos Plate, offshore
northwestern Costa Rica (). Ocean Drilling Program (ODP) Sites 1039 and 1040 (✩) are approximately 1.5 km seaward and 0.5 km landward of
the Middle America Trench, respectively. Site
1039 spans the entire incoming sediment section
(~350 m thick); Site 1040 penetrates the margin
wedge and the entire underthrust sediment section. Tracklines of reflection seismic data are
shown with the solid gray lines. A triple junction
trace (bold dashed line) separates crust formed at
the Cocos–Nazca Spreading Center (CNS) in the
southeastern portion of the study area from crust
formed at the East Pacific Rise (EPR; northwestern
portion of the study area).
During the TicoFlux program (Fisher et al.
2003), gravity and piston cores, reflection seismic
data and heat-flow data were collected in an area
approximately 120 ¥ 120 nmi offshore of the
Nicoya Peninsula (Fig. 1). The coring program was
designed to capture geochemical evidence of
basement-related fluid flow (e.g. Friedmann et al.
2001), so there is a bias in sample distribution
toward locations where total sediment thickness is
at a minimum.
METHODS
Sediment samples were taken from 60 nearsurface piston and gravity cores distributed
throughout the TicoFlux study area, plus five
intervals at ODP Site 1040 (Fig. 1). Sediment composition was quantified by a combination of chemical and X-ray diffraction (XRD) analyses of bulk
samples and the <2 mm fraction.
OPAL ANALYSIS
We used an alkaline leaching method to determine
opal content, as outlined by DeMaster (1981).
When sediment is placed in an alkaline solution,
siliceous material is dissolved, and amorphous sil-
Costa Rica sediment: Fluid partitioning 435
at a rate of 1∞ 2q/min, and MacDiff software was
used to determine the areas and intensities of a
composite clay mineral peak (smectite + illite +
kaolinite), quartz, plagioclase and calcite (Fig. 3).
Relative percentages of each were calculated using
a set of normalization factors specific to the Costa
Rica mineral assemblage (Table 1), following the
singular value decomposition (SVD) method of
Fisher and Underwood (1995).
Preparation of the clay-sized fraction (<2 mm)
for clay mineral identification started with 3%
H2O2 digestion of organic matter, addition of Nacarbonate and Na-hexametaphosphate dispersant,
and washing by centrifugation at ~6000 g for
20 min. Suspensions were dispersed in de-ionized
water using an ultrasonic cell disruptor, and split
into size fractions by centrifuging at ~320 g for
2.4 min. Preparation of oriented clay slides followed the filter-peel transfer method (Drever
1973), and the clays were saturated with ethylene
glycol vapor before analysis. The clay slides were
scanned at 40 kv and 30 mA over 2q angles ranging
from 3 to 23∞, at a rate of 1∞ 2q/min. We calculated
relative abundances of smectite, illite and kaolinite
(area%) using basal reflections (Fig. 3) and Biscaye (1965) peak-area weighting factors (1¥ smectite; 4¥ illite; 2¥ kaolinite). The percent
expandability of smectite was determined using
the saddle-peak method and the empiric curve for
‘pure’ smectite (Rettke 1981).
ica (opal) is digested more rapidly than clay minerals. Therefore, the concentration of silica in the
alkaline solution increases over time, with a rapid
initial increase overprinted by gradual dissolution
of clay minerals. For each sample analyzed, we
digested approximately 30 mg of freeze-dried sediment in 40 mL of 0.0316 M NaOH (pH 12.5) at
85∞C. A 12.5 pH solution was used to minimize
digestion of clay minerals (Schlüter & Rickert
1998). Aliquots (0.200 mL) of the leachate were
collected 5, 15, 30, 60, 90, 120, 200 and 300 min
after digestion began. The concentration of silica
in the leachate was determined by spectrophotometry (e.g. Grasshoff et al. 1983). The contribution
of opal to the leachate was determined by projecting the flattened tail of the concentration curve
back to time = 0 (Fig. 2). In determining the
weight percent of opal in the bulk samples, we
assumed the water content of opal to be 11% by
weight (Kastner 1981).
X-RAY DIFFRACTION
Bulk sediment samples were analyzed by XRD to
determine relative abundances (wt%) of total clay
minerals, quartz, plagioclase and calcite. Freezedried specimens were powdered for 5 min with a
ball mill and analyzed with a Scintag PAD V diffractometer. Bulk powders were scanned at 40 kv
and 35 mA over 2q angles ranging from 3 to 35∞,
Contribution from opal to Si
concentration of leachate
Fig. 2 Examples of data from the sediment leaching
procedure used to determine the opal content of sediment
samples. After all the opal has been extracted from the
sediment in a hot alkaline solution, silica continues to be
extracted from clays at a slow, constant rate. This rate is
extrapolated back to time = 0 to determine the amount of
silica extracted from the opal alone. The dark olive gray
hemipelagic muds have the highest opal content of the
sediments encountered in the study area; the brown clays
have intermediate opal content; the pelagic sediments
have the lowest opal content.
Leachate Si concentration (mM)
Mass sediment sample (g)
0.8
(h
M05gc 88–90 cm
emipelagic;
8.1% opal)
Si extracted only from
clays (at constant rate)
0.6
All Si from opal
has been extracted
Si extracted from
both opal and clays
0.4
M18gc
0.2
8–10 c
m
clay
(brown
M24gc 68–70 cm (calca
; 2.8%
opal)
reous ooze; 0.6% opal)
0.0
0
60
120
180
Time (min)
240
300
436 G. A. Spinelli and M. B. Underwood
(a)
500
Counts
400
plagioclase
calcite
300
quartz
200
halite
clay
100
0
5
10
15
20
o
(b)
600
30
35
2q
smectite (001)
400
300
illite (001)
200
kaolinite (001)
saddle
100
zeolite
0
4
6
8
10
12
14
o
16
18
20
22
2q
(c)
2000
GRAIN SIZE
Counts
1500
calcite
1000
500
0
TOTAL INORGANIC CARBON
Error analysis of the SVD method shows that calculated abundances of minerals within standard
mixtures are usually within 5% of their true percentages by weight (Fisher & Underwood 1995;
Underwood et al. 2003). To verify the accuracy of
XRD data in an absolute sense, we analyzed for
calcium-carbonate content by acid digestion and a
coulometer using a diverse subset of 18 hemipelagic samples. The absolute wt% of calcite for each
was determined from the total inorganic carbon
content.
peak
500
Counts
25
To discriminate between dioctahedral and trioctahedral varieties of expandable clay, we identified the d(060) value using randomly oriented
powders of the <2 mm size fraction (Brown &
Brindley 1980). Those clay powders were scanned
from 48 to 64∞ 2q at 1∞ 2q/min after adding a spike
of quartz powder to correct for drifts in peak position caused by misalignments of the goniometer
and/or sample holder.
halite
5
10
15
o
20
25
30
35
2q
Fig. 3 Example of X-ray diffraction (XRD) analysis of sediment samples off Costa Rica. Analysis of the bulk sediment powder for a hemipelagic sample indicates that the relative percentage of clay (77%) is much
greater than quartz (4%), plagioclase (16%) or calcite (3%) (a; M17gc
43–45 cm (hemipelagic mud)). Results from an ethylene-glycol solivated,
oriented slide for the same sample indicate that the <2 mm grain size
fraction of the sample is dominated by smectite (b; 84% smectite, 1%
illite, 15% kaolinite). The ratio of the number of counts at the ‘saddle’ and
‘peak’ is used to estimate the proportion of expandable layers in the
smectite. Pelagic samples are dominated by calcite (92%), with very little
clay (<1%), quartz (4%) or plagioclase (4%) (c; M22gc 50–52 cm (calcareous ooze)).
The percentages of sand (>63 mm), silt (4–63 mm)
and clay (<4 mm) were determined by wet sieving
and a Spectrex laser particle counter (LPC).
Before wet sieving, the bulk samples were freezedried, digested with 3% H2O2, and dispersed in
0.01 M Na2CO3. Because of the typical abundance
of biogenic silica, Na-hexametaphosphate (Calgon)
was ineffective as a dispersant. Output from the
LPC (counts/cm3) needs to be converted to equivalent spherical settling diameter (wt%) using a
calibration curve established by pipette analysis
(Steurer & Underwood 2003b). We report here the
average values of percentage silt and percentage
clay (as spherical equivalents) derived from three
replicate LPC analyses.
SEDIMENT THICKNESS
Approximately 1800 km of reflection seismic lines
were collected during 2001 (TicoFlux I expedi-
Table 1 Normalization factors for bulk powder X-ray diffraction (XRD) analysis
Indicator mineral
Target mineral
Total clay
Total clay
Quartz
Plagioclase
Calcite
Quartz
-2
1.3267 ¥ 10
3.8598 ¥ 10-5
2.1620 ¥ 10-4
-5.4600 ¥ 10-5
Plagioclase
-4
-2.3590 ¥ 10
7.9700 ¥ 10-4
-9.6200 ¥ 10-5
6.9900 ¥ 10-5
-4
-4.3760 ¥ 10
5.2500 ¥ 10-5
3.3377 ¥ 10-3
8.5107 ¥ 10-5
Calcite
-1.4851 ¥ 10-3
-2.9600 ¥ 10-6
-5.4700 ¥ 10-5
1.7801 ¥ 10-3
Costa Rica sediment: Fluid partitioning 437
3100
3200
Fig. 4 On reflection seismic lines, the hemipelagic sediment is acoustically more transparent
than the underlying pelagic sediment. Reflection
seismic data is used to map the thickness of both
the hemipelagic sediment and the pelagic sediment. TWT, two way travel time.
TWT (s)
hemipelagic sediment
4.5
pelagic
sediment
3300
3400
3500
3600
5.0
tion). The source was a 10-gun array with a frequency bandwidth of approximately 20–120 Hz.
The receiver was a 6 km long hydrophone
streamer array with 480 channels. An additional
1200 km of reflection seismic data were acquired
during 2002 (TicoFlux II expedition) using a pair
of GI guns with a frequency bandwidth of approximately 20–200 kHz and a four-channel, 200 hydrophone streamer.
The pelagic and hemipelagic sections of the sediment column on Cocos Plate were differentiated
by differences in their acoustic signatures. As
noted during previous geophysical surveys of this
margin (e.g. Shipley et al. 1990), the pelagic section
is characterized by numerous internal reflections
with high degrees of lateral continuity, whereas the
hemipelagic section is relatively transparent
acoustically (Fig. 4). We measured the two way
travel times at the boundaries between hemipelagic and pelagic intervals and acoustic basement.
We converted to sediment thickness using average
velocities of 1541 m/s for the hemipelagic section
and 1581 m/s for the pelagic section (Kimura et al.
1997). Sediment thickness was determined along
the reflection seismic tracklines, then gridded and
contoured over the entire study area.
RESULTS
SEDIMENT THICKNESS
Reflection seismic data and high-resolution swath
mapping indicate that seamounts are more common on crust formed at the EPR than on crust
formed at the CNS to the southeast (e.g. Fisher
et al. 2003). Sediment is thin or absent on the seamounts (Fig. 5). As a result, there is greater spatial variability in total sediment thickness in the
northwestern part of the study area than in the
southeastern part. Aside from the basement highs,
the hemipelagic interval is similar over the EPR
crust and the CNS crust, reaching up to 213 m,
with a median thickness (for the gridded data) of
basaltic basement
3700
Approximate depth (m)
seafloor
135 m. Pelagic sediment is consistently thicker on
the EPR crust than on the CNS crust. The pelagic
interval is 0–472 m thick on the EPR crust
(median = 233 m) and 0–372 m thick on the CNS
crust (median = 193 m).
SEDIMENT COMPOSITION
Shipboard descriptions of split cores and smear
slides led to a classification system with four basic
lithologies: dark olive gray hemipelagic mud (with
abundant biogenic silica); variegated clay (with low
biogenic silica); mixed sediment (with variable carbonate content); and light gray nannofossil chalk.
Many of the cores also contain thin layers of volcanic ash, fragments of Mn-oxide and pieces of
basalt.
Apart from five deeper intervals at ODP Site
1040, our specimens come from within 7 m of the
seafloor. Hemipelagic mud is, by far, the most common lithology throughout the study area. Exposures of variegated clay and chalk are restricted
to basement highs (e.g. seamounts and smaller
basalt outcrops) where total sediment thickness is
at a minimum and the overlying hemipelagic sediment section is thin or absent (Fig. 5). Cores with
pelagic chalk are clustered in the southwestern
portion of the study area and around outcrops,
primarily on the crust northwest of the triple junction trace indicated on Figures 1 and 6 (Fisher
et al. 2003). Winnowing by bottom currents may
have hampered deposition of hemipelagic mud in
those localities. In some cores, the basal chalk
interval grades upward through mixed sediment
into hemipelagic mud. One piston core was collected from the floor of the Middle America
Trench, and it contains turbidites and debris-flow
deposits with remobilized fragments of mudstone.
Grain size analyses show that the hemipelagic
sediment consists primarily of clay-sized particles
(~67% by weight as equivalent spherical diameters). Smaller components are silt-sized (~30% by
weight) and sand-sized (~3% by weight, largely
composed of foraminifers) particles. The pelagic
438 G. A. Spinelli and M. B. Underwood
-87.5o
-87o
-86.5o
-86o
-85.5o
50 km
-87.5o
-87o
-86.5o
-86o
-85.5o
o
10
9.5
9
o
8.5
Total sediment thickness (m)
0
200
400
600
o
o
Hemipelagic sediment thickness (m)
0
50
100
150
200
Fig. 5 Gridded and contoured total sediment thickness and thickness of the hemipelagic sediment section. Sediment thickness (both total and
hemipelagic) is more variable overlying the East Pacific Rise crust in the northwest (where there are more seamounts and rougher basement topography)
than over the Cocos–Nazca Spreading Center crust in the southeast.
E09gc
M38gc E27gc
Opal = 7%
1m
2m
E21gc E34gc
Clay = 70%
Qtz = 4%
Plag = 13%
Calc = 13%
SMEC = 75%
ILLITE = 4%
KAOL = 21%
Opal = 2%
Clay = 81%
Qtz = 2%
Plag = 17%
Calc = trace
SMEC = 90%
ILLITE = 0%
KAOL = 10%
Opal = 8%
Clay = 77%
Qtz = 3%
Plag = 20%
Calc = trace
SMEC = 97% 2m
ILLITE = 0%
KAOL = 3%
Clay = 79%
Qtz = 5%
Plag = 16%
Calc = trace
Clay = 79%
Qtz = 3%
Plag = 11%
Calc = 7%
SMEC = 81%
ILLITE = 2%
KAOL = 17%
Clay = 55%
Qtz = 4%
Plag = 14%
Calc = 27%
Clay = 55%
Qtz = trace
Plag = 45%
Calc = trace 3m
SMEC = 100%
ILLITE = 0%
KAOL = 0%
Clay = 35%
Qtz = 3%
Plag = 16%
Calc = 46%
M02gc
Dark olive gray hemipelagic mud
Brown clay
M38gc
M02gc
Clay = 85%
Qtz = 2%
Plag = 13%
Calc = trace
SMEC = 90%
ILLITE = 0%
KAOL = 10% 1m
Opal = 7%
E34gc
E09gc
Mixed sediment
Carbonate ooze
E21gc
E27gc
Ash
Mn nodule
Fig. 6 Representative core logs and results
from opal and X-ray diffraction (XRD) analyses.
Clay, quartz, plagioclase and calcite percentages
are relative percentages determined by XRD of
bulk powder samples. Smectite, Illite and kaolinite
percentages are relative percentages of the <2 mm
fraction. Opal percentages are weight percentages
determined by sediment leaching. Hemipelagic
sediments (which contain most of the opal and
smectite within the sediment column) are thin in
the southwest and around local basement highs.
Costa Rica sediment: Fluid partitioning 439
plagioclase
(a)
total clay
opal
calcite
(b)
total clay
smectite
chalk appears to be coarser than the hemipelagic
mud, but our LPC results may have been affected
by interference among nannofossils. The average
size distribution for the pelagic sediment is 30%
clay, 47% silt and 23% sand (mostly foraminifers).
Within each sediment type, there is very little
spatial variation in mineralogy (Fig. 6). Of the
samples analyzed by XRD (Table 2; Fig. 7), hemipelagic mud has the highest opal content (mean
(m) = 10% by weight; standard deviation (s) = 3%;
number of samples (n) = 23). Variegated clay (olive
brown) is intermediate in opal content (m = 5%;
s= 2%; n = 11), and the opal content of pelagic
chalk is lowest (m = 2%; s= 1%; n = 4). The hemipelagic mud and variegated clay have similar relative abundances of total clay minerals, quartz,
plagioclase and calcite. The mean values for hemipelagic mud (n = 39) are 77% total clay minerals,
3% quartz, 19% plagioclase and <1% calcite. The
nannofossil chalk contains mostly calcite (n = 6; 1%
total clay minerals, 4% quartz, 5% plagioclase, 90%
calcite), whereas the mixed sediment ranges from
<1 to 46% calcite. Most of the clay-sized fraction is
smectite (Fig. 7). Relative percentages of smectite
average 87% and range from 75 to 100% (Table 2).
The average content of kaolinite is 13%, with a
trace contribution of illite (~1%). The clay mineral
content of pelagic chalk is too small to analyze
accurately by XRD.
calcite
ABUNDANCE OF HYDROUS PHASES
(c)
illite
Dark olive gray hemipelagic mud
Olive brown mud
Calcareous ooze
Mixed sediment
Altered ash
kaolinite
Hemipelagic mud has relatively high total clay and
opal contents, so a considerable volume of fluid
must be contained within the hydrous phases. We
quantified the opal content as an absolute wt%, but
the clay mineral content is relative only to three
other constituents (quartz, plagioclase and calcite).
To determine how closely such relative abundances match absolute weight percentages, we
compared the contents of calcite (from Coulometric analysis) to equivalent XRD values after correcting for the wt% opal. Linear regression shows
that the two sets of values match to within approximately 10 wt% (Fig. 8). The fit would improve if
Fig. 7 Summary of bulk sediment compositional data (a and b) and
<2 mm fraction clay mineralogy (c). The hemipelagic sediment is dominated by clay minerals. The pelagic sediment is mostly calcite. The mixed
sediment samples are mixtures of hemipelagic and pelagic sediment. The
clay minerals in the <2 mm size fraction are mostly smectite. Number of
samples: (a) 39 hemipelagic, 16 olive brown, 6 calcareous, 6 mixed; 1
altered ash; (b) 23 hemipelagic, 11 olive brown, 2 calcareous, 3 mixed;
and (c) 29 hemipelagic, 15 olive brown, 3 mixed, 1 altered ash.
02GC
05GC
05GC
07GC
11GC
12GC
14GC
16GC
17GC
18GC
20GC
21GC
22GC
24GC
25GC
26GC
28GC
33GC
38GC
38GC
40GC
42GC
44PC
45PC
45PC
45PC
48PC
54GC
48–50
45–47
88–90
38–40
42–44
208–210
238–240
35–37
43–45
8–10
40–42
20–22
50–52
68–70
62–64
170–172
198–200
78–80
28–30
112–114
60–62
90–92
520–522
370–372
536–538
686–688
657–659
100–102
Sample ID
Core
Interval
(cm)
9.6855
9.3109
9.3109
9.3408
9.6795
9.6799
9.6066
8.4973
8.5369
8.5303
8.4959
8.7367
8.7391
8.6209
8.7425
8.7420
8.7405
9.0876
9.0828
9.0828
9.0841
9.0809
9.6800
9.8657
9.8657
9.8657
8.4967
8.8191
-86.3937
-86.1877
-86.1877
-86.1740
-86.5711
-86.5718
-86.6524
-85.9677
-85.9042
-85.9149
-85.9675
-87.2125
-87.2107
-87.2845
-87.2150
-87.2137
-87.2146
-87.0963
-87.0999
-87.0999
-87.0989
-87.0965
-86.5718
-86.3547
-86.3547
-86.3547
-85.9668
-87.1688
7
5
8
8
7
14
17
8
7
3
4
5
3
1
7
2
2
10
7
8
6
7
12
8
14
14
10
8
85
80
–
–
84
84
84
–
77
82
79
–
Tr
–
82
–
–
82
–
81
83
–
83
78
79
77
–
83
2
2
–
–
2
3
3
–
4
3
4
–
4
–
2
–
–
2
–
2
2
–
3
3
3
2
–
3
13
18
–
–
14
13
13
–
16
15
17
–
4
–
16
–
–
16
–
17
15
–
14
19
18
21
–
14
Tr
Tr
–
–
Tr
Tr
Tr
–
3
Tr
Tr
–
92
–
Tr
–
–
Tr
–
Tr
Tr
–
Tr
Tr
Tr
Tr
–
Tr
–
–
–
–
3.0
–
–
–
8.8
–
9.1
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
†
90
91
88
87
–
90
87
88
84
–
87
91
–
–
88
93
–
91
91
90
90
90
89
81
82
86
88
88
0
0
1
1
–
0
0
1
1
–
0
1
–
–
1
0
–
0
0
0
1
1
0
0
1
1
1
0
10
9
11
12
–
10
13
11
15
–
13
8
–
–
11
7
–
9
9
10
9
9
11
19
17
13
11
12
62
60
68
61
–
67
59
70
59
–
59
64
–
–
68
51
–
58
63
57
59
61
60
54
51
53
64
59
Y
Y
Y
Y
–
N
Y
Y
Y
–
Y
Y
–
–
Y
N
–
Y
Y
N
Y
Y
Y
Y
Y
Y
Y
Y
–
1.503
–
–
–
1.502
1.506
–
1.509
–
–
–
–
–
–
–
–
1.506
–
–
–
–
–
–
–
1.505
–
1.507
3
–
–
1
–
2
3
2
1
4
2
18
15
36
14
–
17
12
3
8
3
–
2
–
3
4
3
–
25
–
–
22
–
16
24
18
52
39
42
46
34
56
51
–
52
20
20
28
46
–
34
–
26
42
32
–
72
–
–
77
–
82
73
80
47
57
56
36
51
8
35
–
31
68
77
64
51
–
64
–
71
54
65
–
Latitude Longitude Opal
Relative % from XRD
%
Relative % from
Smectite Zeolite Smectite Sand Silt Clay
(wt%) Clay Quartz Plagio- Calcite Calcite
XRD (<2 mm)
%
peak†
060
(%) (%) (%)
clase
from Smectite Illite Kaolinite expanpeak,
TIC
dibility
d-value
–, No data; TIC, total inorganic carbon; Tr, trace (<1%); XRD, X-ray diffraction.
Y, there is a peak ~10∞ 2q in the <2 mm XRD data, likely indicating the presence of zeolite; N, no peak.
‡
Hemipelagic mud = dark olive gray hemipelagic mud.
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
MV0209
Cruise
Table 2 Summary of sediment composition
Hemipelagic mud
Olive brown clay
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Olive brown clay
Hemipelagic mud
Olive brown clay
Calcareous ooze
Calcareous ooze
Olive brown clay
Mixed sediment
Calcareous ooze
Hemipelagic mud
Olive brown clay
Olive brown clay
Olive brown clay
Olive brown clay
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Olive brown clay
Sediment type‡
440 G. A. Spinelli and M. B. Underwood
55GC
59GC
01GC
02GC
05GC
05GC
08GC
09GC
09GC
10GC
11GC
12GC
13GC
14GC
15GC
16GC
17GC
18PC
18PC
19PC
19PC
19PC
19PC
20GC
21GC
21GC
22GC
23GC
24GC
25GC
26GC
26GC
27GC
273–275
88–90
10–12
53–55
10–12
30–32
4–6
72–74
170–172
68–70
108–110
10–12
90–92
176–178
58–60
78–80
30–32
14–16
261–263
110–120
285–295
395–405
540–560
48–50
24–26
140–142
12–14
55–57
96–98
36–38
105–107
262–264
14–16
8.7890
9.1703
7.8601
8.6237
8.6240
8.6240
8.7374
8.7420
8.7420
9.4364
9.4358
9.4808
9.4812
9.4713
9.4710
9.4741
9.4759
9.6762
9.6762
9.6768
9.6768
9.6768
9.6768
8.5156
8.5060
8.5060
8.5014
8.4966
8.2554
8.2553
8.2429
8.2429
8.2433
-87.1970
-87.2582
-85.2732
-87.2770
-87.2800
-87.2800
-87.2081
-87.2137
-87.2137
-86.9320
-86.9332
-86.9966
-86.9964
-86.9946
-86.9975
-86.9959
-86.9937
-86.5687
-86.5687
-86.5613
-86.5613
-86.5613
-86.5613
-85.9867
-85.9765
-85.9765
-85.9718
-85.9670
-86.2432
-86.2426
-86.2322
-86.2322
-86.2323
5
14
–
–
–
–
–
–
–
–
10
–
–
–
–
12
–
–
6
–
–
–
9
–
–
–
2
–
3
–
–
–
2
81
88
72
Tr
69
6
Tr
79
35
74
78
72
77
77
75
76
72
80
59
76
75
73
80
75
79
55
79
–
54
74
75
Tr
70
2
3
5
4
3
3
3
3
3
3
4
4
3
4
4
4
3
3
3
4
3
3
3
4
5
4
4
–
5
2
4
4
4
17
9
18
4
28
8
5
11
16
23
18
24
20
19
21
20
25
17
35
20
22
24
17
21
16
14
17
–
16
24
18
4
13
Tr
Tr
5
92
Tr
83
92
7
46
Tr
Tr
Tr
Tr
Tr
Tr
Tr
Tr
Tr
3
Tr
Tr
Tr
Tr
Tr
Tr
27
Tr
–
25
Tr
3
92
13
1.2
–
6.3
–
0.2
–
–
6.1
31.6
–
–
–
–
–
2.8
–
–
–
2.3
–
–
–
–
–
–
20.4
5.3
–
18.9
–
6.6
–
13.5
†
82
89
–
–
–
–
–
81
–
91
92
–
80
90
–
86
81
85
–
–
–
76
–
90
–
–
–
89
87
–
86
–
75
0
0
–
–
–
–
–
2
–
1
1
–
1
1
–
0
3
0
–
–
–
5
–
0
–
–
–
1
1
–
1
–
4
18
11
–
–
–
–
–
17
–
8
7
–
19
9
–
14
16
15
–
–
–
19
–
10
–
–
–
10
12
–
13
–
21
48
58
–
–
–
–
–
59
–
63
61
–
55
59
–
61
60
54
–
–
–
57
–
62
–
–
–
65
64
–
64
–
56
Y
Y
–
–
–
–
–
Y
–
Y
Y
–
Y
Y
–
Y
Y
Y
–
–
–
N
–
Y
–
–
–
Y
Y
–
Y
–
Y
–
–
–
–
–
–
–
–
–
–
1.505
–
1.503
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
1.506
–
–
–
–
14
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
37
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
49
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
Latitude Longitude Opal
Relative % from XRD
%
Relative % from
Smectite Zeolite Smectite Sand Silt Clay
(wt%) Clay Quartz Plagio- Calcite Calcite
XRD (<2 mm)
%
peak†
060
(%) (%) (%)
clase
from Smectite Illite Kaolinite expanpeak,
TIC
dibility
d-value
–, No data; TIC, total inorganic carbon; Tr, trace (<1%); XRD, X-ray diffraction.
Y, there is a peak ~10∞ 2q in the <2 mm XRD data, likely indicating the presence of zeolite; N, no peak.
‡
Hemipelagic mud = dark olive gray hemipelagic mud.
MV0209
MV0209
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
Cruise
Sample ID
Core
Interval
(cm)
Olive brown clay
Hemipelagic mud
Hemipelagic mud
Calcareous ooze
Mixed sediment
Calcareous ooze
Calcareous ooze
Hemipelagic mud
Mixed sediment
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Mixed sediment
Olive brown clay
Hemipelagic mud
Mixed sediment
Olive brown clay
Olive brown clay
Calcareous ooze
Mixed sediment
Sediment type‡
Costa Rica sediment: Fluid partitioning 441
28GC
30–32
33GC
24–26
34GC
187–188
34GC
210–212
35GC
230–232
36PC
54–55
36PC
163–165
37PC
135–137
38PC
28–30
39PC
20–22
40GC
125–127
42GC
5–6
43GC
30–32
1040C Sec 2R-4
(115–150 cm)
1040C Sec 20R-5
(115–150 cm)
1040C Sec 24R-1
(120–150 cm)
1040C Sec 36R-3
(135–150 cm)
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
EW0104
ODP
Leg 170
ODP
Leg 170
ODP
Leg 170
ODP
Leg 170
54/2
–
4
1
-86.1789
-86.1789
9.6616
–
2
4
-86.1789
9.6616
–
1
-86.1789
9.6616
9.6616
1
–
–
–
11
–
4
–
6
8
–
–
5
2
-85.7779
-86.2304
-86.2503
-86.2503
-86.3117
-86.6826
-86.6826
-86.6852
-86.5689
-86.5692
-86.9098
-86.9048
-86.9141
-86.1789
8.5804
9.2685
9.2501
9.2501
9.2259
8.9375
8.9375
8.9409
9.6765
9.6762
9.1351
9.1254
9.1357
9.6616
80
80
tr
70
81
64
75
77
55
71
67
86
75
71
75
–
73
69
78
4
tr
4
4
4
3
3
3
Tr
5
7
2
4
4
3
–
3
4
5
16
8
4
26
15
25
22
20
45
24
26
12
21
25
22
–
24
27
17
Tr
12
92
Tr
Tr
8
Tr
Tr
Tr
Tr
Tr
Tr
Tr
Tr
Tr
–
Tr
Tr
Tr
–
15.3
–
–
–
6.5
–
–
–
–
–
–
4.1
–
–
–
–
–
–
†
–
–
–
–
–
91
94
97
100
–
86
–
75
89
91
91
81
94
–
–
–
–
–
–
2
0
0
0
–
3
–
0
1
0
0
2
1
–
–
–
–
–
–
7
6
3
0
–
11
–
25
10
9
9
17
5
–
–
–
–
–
–
66
66
52
53
–
61
–
60
58
58
63
61
65
–
–
–
–
–
–
Y
Y
N
N
–
Y
–
Y
Y
Y
Y
N
Y
–
–
–
–
–
–
1.504
1.503
–
–
–
–
–
–
–
–
–
–
1.503
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
Latitude Longitude Opal
Relative % from XRD
%
Relative % from
Smectite Zeolite Smectite Sand Silt Clay
(wt%) Clay Quartz Plagio- Calcite Calcite
XRD (<2 mm)
%
peak†
060
(%) (%) (%)
clase
from Smectite Illite Kaolinite expanpeak,
TIC
dibility
d-value
–, No data; TIC, total inorganic carbon; Tr, trace (<1%); XRD, X-ray diffraction.
Y, there is a peak ~10∞ 2q in the <2 mm XRD data, likely indicating the presence of zeolite; N, no peak.
‡
Hemipelagic mud = dark olive gray hemipelagic mud.
CR
Trench
ODP
1040C Sec 51R-4
Leg 170
(103–107 cm)
Sample ID
Core
Interval
(cm)
Cruise
Table 2 Continued
Diatomite
(underthrust)
Siliceous
nannofossil
chalk
(underthrust)
Calcareous
diatomite
(underthrust)
Hemipelagic mud
Claystone (prism)
Olive brown clay
Olive brown clay
Hemipelagic mud
Altered ash
Hemipelagic mud
Olive brown clay
Olive brown clay
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Hemipelagic mud
Olive brown clay
Olive brown clay
Claystone (prism)
Sediment type‡
442 G. A. Spinelli and M. B. Underwood
Costa Rica sediment: Fluid partitioning 443
Table 3 Weight percentage of hydrous phases in sediment
Opal (wt%)
Smectite
(approximate wt%)
Expandability of
smectite (%)
10
5
2
60
64
<1
59
62
No data
Dark olive gray hemipelagic mud
Olive brown clay
Calcareous ooze
analysis closely approximate the absolute weight
percentages).
To estimate absolute values of total clay (wt%),
we used the wt% opal and the relative abundance
of total clay minerals determined by XRD:
Relative % calcite
(from XRD and opal)
50
40
30
Total clay (wt%) ª [1 - (opal wt%/100)] ·
relative abundance clay (%)
y = 1.40x – 4.53
r2 = 0.91
We then solved for wt% smectite using the value
for total clay and the relative abundance of smectite in the <2 mm size fraction:
20
10
Smectite (wt%) ª (clay wt%/100) ·
relative abundance smectite (%)
0
0
10
20
30
40
(1)
50
% Calcite
(from total inorganic carbon)
Fig. 8 Relative percent calcite from X-ray diffraction (XRD) analysis for
18 hemipelagic sediment samples (dark olive gray hemipelagic mud and
olive brown clay) as a function of the absolute percent calcite calculated
from determination of the total inorganic carbon content of the sediment
samples. The raw relative percentages of calcite () are corrected ()
for the opal content of the sediment samples. For samples with <20%
calclite, the corrected relative percentages closely approximate the absolute percentages of calcite.
we also corrected for the amount of salt in the bulk
powders (precipitated during freeze-drying), but
that would require a parallel program of systematic physical properties measurements (e.g. water
content, grain density, porosity). The XRD analysis slightly underestimates the wt% calcite for
samples with very low calcite content, suggesting
that approximately 2–5 wt% calcite may be the
practical detection limit for the analysis. The
relative wt% calcite closely matches the absolute
wt% for hemipelagic and mixed-sediment samples
with moderate calcite content (~5–15%). For
mixed-sediment samples with >15 wt% calcite, the
XRD analysis overestimates the fraction of calcite
present. The hydrous phases are dominantly contained within the hemipelagic sediment (in which
there is much less than 15 wt% calcite, and the
relative weight percentages determined by XRD
(2)
Limitations of this approach include the possibility of clay mineral partitioning as a function of
grain size and incorporation of non-clay minerals
(e.g. quartz, zeolite) into the clay-sized fraction.
Our XRD results, however, show only traces of
clay-sized quartz and zeolite, and the amount of
smectite should increase in progressively finer size
fractions. On average, hemipelagic mud from the
Costa Rica study area contains approximately 60%
smectite by weight. Smectite in pelagic chalk can
not be determined precisely due to the low clay
content, but it must be less than 1% (i.e. the relative percent of total clay in the pelagic sediment).
CHARACTERISTICS OF SMECTITE
The expandability of smectite averages 59% and
ranges from 48 to 70% (Table 3). These results
indicate that the expandable clay is detrital in origin, rather than a product of authigenic alteration
of volcanic ash, and it probably includes crystallites of illite in a disordered mixed-layer structure.
Accordingly, when making calculations of water
volume within the interlayer site of smectite, we
assumed an intermediate state of hydration with
two layers of water. This hydration state corresponds to approximately 20% water by weight and
approximately 40% water by volume.
Analyses of 13 powders revealed broad smectite
(060) peaks centered near d = 1.503 Å (Fig. 9). This
result indicates the smectite is mostly dioctahedral
(Brown & Brindley 1980; Moore & Reynolds 1997).
444 G. A. Spinelli and M. B. Underwood
Table 4 Volume of water (m3) per m3 of sediment entering subduction zone
Hemipelagic mud
Calcareous ooze
Weighted average for entire sediment column
% of total water
200
quartz
d = 1.82 A
quartz
d = 1.542 A
Counts
160
smectite
060
120
80
40
trioctahedral
0
48
50
52
54
56
dioctahedral
58 60 62
˚2q
Fig. 9 Example of X-ray diffraction (XRD) analysis of randomly oriented <2 mm grain size fraction (E43gc 30–32 cm). The diffractogram is
aligned to the prominent quartz peaks at d-spacings of 1.82 and 1.542 Å.
The smectite 060 peak is close to the typical range (bold lines) for
dioctahedral smectite, suggesting it is detrital in origin.
Dioctahedral members of the smectite group
(montmorillonite, Al-rich beidellite) typically form
through pedogenesis of diverse protoliths under a
wide range of environmental conditions (e.g. Fagel
et al. 2001), although pure bentonite layers also
form through submarine alteration of silicic volcanic ash (Hodder et al. 1990; Naish et al. 1993). In
contrast, trioctahedral varieties (saponite) and Ferich dioctahedral end-members (non-tronite) originate through meteoric weathering and hydrothermal alteration of basalt (e.g. McMurtry et al. 1982;
Parra et al. 1986; Güven 1988; Chamley 1989).
Most of the dioctahedral clay that was deposited
offshore Costa Rica probably formed through
tropical weathering of volcanic and sedimentary
rocks in Central America.
DISCUSSION
FLUID PARTITIONING
Sediment-hosted fluids on the incoming Cocos
Plate are partitioned between a basal pelagic section and an overlying hemipelagic section. Within
Pore space
Opal
Smectite
0.752
0.700
0.727
96.2%
0.0051
0.0015
0.0029
0.4%
0.0653
0.0016
0.0258
3.4%
each of those sections, fluid is partitioned between
sediment pores and hydrous minerals. We estimate sediment porosity using data from ODP Site
1039. The average porosity of hemipelagic samples
from Site 1039 is 75%; the average porosity of
pelagic samples is 70% (Kimura et al. 1997). Porosity averages 73% over the entire sediment column.
The thicknesses of hemipelagic and pelagic intervals cored at Site 1039 (152 and 225 m) are similar
to the medians derived from seismic data (135 and
213 m). The pelagic section, on average, is approximately 2% opal by weight and <1% smectite by
weight. The hemipelagic section is approximately
10% opal by weight and 60% smectite by weight.
We convert the weight percentages of opal and
smectite to volume percentages based on their
densities (ropal ª 2.47 g/cm3; rsmectite ª 2.02 g/cm3),
then calculate the volume of water in opal within a
typical 1 m3 of sediment by:
Volume water in opal ª (1 - [n/100]) ·
vol% opal · vol% water in opal
(3)
where n is porosity, and the vol% of water in opal
is 23% (equivalent to 11% water by weight). A
similar approach is used to determine the volume
of water in smectite within a typical 1 m3 of
sediment:
Volume water in smectite ª (1 - [n/100]) ·
vol% smectite · vol% water in smectite
(4)
where the vol% of water in smectite is 40% (equivalent to 20% water by weight, or two layers of
interlayer water). As initially deposited, the
pelagic and hemipelagic sediments contain more
than 70% water by volume. Distributed over the
entire sediment column, 96% of that water is
within pore spaces, 3.6% is in smectite and 0.4% is
in opal (Table 4).
Once subduction begins, rapid loading consolidates sediment within the first ~5 km from the
trench (Shipley et al. 1990; Kimura et al. 1997;
McIntosh & Sen 2000; Saffer et al. 2000; Saito &
Goldberg 2001). The hemipelagic sediment section
Costa Rica sediment: Fluid partitioning 445
Average porosity (%)
(a)
80
n = –1.34 ln(x) + 64.9
r2 = 0.76
70
60
50
0
1
2
3
4
5
Distance into subduction zone (km)
Average porosity (%)
(b)
80
70
n = 65.1 e–0.055x
r2 = 0.71
60
50
0
1
2
3
4
5
Distance into subduction zone (km)
Fig. 10 Average porosity of underthrust sediment around Ocean Drilling program (ODP) Sites 1039, 1040 and 1043, based on index properties
measurements (Kimura et al. 1997), laboratory consolidation tests (Saffer
et al. 2000), variations in layer thickness (Saito & Goldberg 2001) or
reflection seismic experiments (Shipley et al. 1990; McIntosh & Sen
2000). The hemipelagic section (a) of the sediment column compacts and
looses a large fraction of its porosity in the first kilometer beyond the
Middle America Trench, after which the rate of porosity loss decreases.
The pelagic section (b) compacts less than the hemipelagic section in the
first few kilometers of the subduction zone. , Kimura et al. (1997); +,
McIntosh and Sen (2000); ¥, Saffer et al. (2000); , Saito and Goldberg
(2001); , Shipley et al. (1990).
loses a substantial fraction of its porosity (down to
~65–60%) within the first kilometer of the subduction, but the rate of consolidation decreases over
the next 4 km. Porosity loss in the pelagic section
is more gradual and uniform (Fig. 10). Low permeability within the clay-rich sediment inhibits drainage of the pore water. Consequently, on the Costa
Rica margin, underthrust sediment in the shallow
portion of the subduction zone is underconsolidated due to sustained pore-fluid pressures above
hydrostatic (Saffer et al. 2000).
The amount of additional porosity loss is poorly
constrained beyond the coverage of reflection seismic data (i.e. farther than ~5 km into the subduction zone), but some insights can be gained from
the numerical modeling of fluid by Saffer and
Bekins (1998), who suggested that the underthrust
sediment in Nankai Trough reached a normally
consolidated state by ~30 km into the subduction
zone. If the underthrust sediment on the Costa
Rica margin follows a similar response (normally
consolidated 30 km from the subduction front),
then we can identify a reference point for comparison. The point’s depth below seafloor depends
upon the margin’s subduction geometry. The slope
of the margin wedge off Nicoya is approximately
5.4∞; the decollement dips approximately 6∞ for the
first 30 km, then increases to approximately 13∞
(Christeson et al. 1999). Thus, the wedge is
approximately 6 km thick 30 km landward of the
trench. A global compilation of porosity versus
depth (Bray & Karig 1985) can be used to estimate
corresponding porosity values of approximately
5–15%. If those estimates are correct, then the
switchover in fluid budget from mostly porehosted water to mostly mineral-hosted water
occurs before strata reach 30 km into the Costa
Rica subduction zone (Fig. 11).
SEDIMENT DEHYDRATION
Release of water from the mineral structures of
opal and smectite is controlled by reaction kinetics
(Ernst & Calvert 1969; Pytte & Reynolds 1988;
Huang et al. 1993). Opal dehydration goes to completion between temperatures of approximately 50
and 100∞C (Murata et al. 1977; Behl & Garrison
1994). In typical sedimentary basins, most progress
of the smectite-to-illite reaction (with two pulses of
smectite dewatering) occurs between 60 and 140∞C
(Perry & Hower 1970, 1972; Freed & Peacor 1989).
Geochemical analyses of fluids collected from the
frontal decollement (Kimura et al. 1997) indicate
that key reactions occurred at temperatures of at
least 80–120∞C but less than 150∞C (Chan & Kastner 2000; Silver et al. 2000). This inferred window
overlaps the temperatures for both opal and smectite dehydration. In situ temperature at the frontal
decollement (Site 1040) is <5∞C (Kimura et al.
1997), suggesting a deeper source for the fluids.
Thermal models for the Nicoya subduction zone
yield temperature predictions of 100∞C at horizontal distances 40–75 km into the subduction zone
and 160∞C once strata reach 60–100 km into the
subduction zone (Harris & Wang 2002); the upper
446 G. A. Spinelli and M. B. Underwood
Depth (km)
(a)
0
Plate interface
10
20
Earthquakes
30
40
CNS (warm)
EPR (cool)
20
0
40
60
80
100
Distance into subduction zone (km)
(b)
0
20
40
?
0.6
60
cool
0.8
warm
0.4
?
Proportion of water
in smectite+opal
1
0.2
0
Increased fluid
pressure during
diagenetic dewatering
80
100
Smectite and opal
reactions completed
(cool case)
Dissipating fluid
pressure; increasing
effective stress
and lower limits of the modeling solution depend
on the inferred thermal state of the incoming
crust. On the basis of those numerical simulations,
we suggest that opal and smectite dewater in discrete pulses after porosity drops below approximately 15% and the rate of compaction slows.
Zones of elevated fluid pressures along the plate
boundary fault should coincide with compartments
of diagenetic fluid release (e.g. Saffer & Bekins
1998), thereby decreasing effective stress in a
patchy 3-D pattern. Heat flow also varies alongstrike, with higher values (average ~105 mW/m2)
on CNS crust in the southeast and lower values
(average ~20 mW/m2) on EPR crust in the northwest. There are also local zones of high heat flow
on EPR crust caused by hydrothermal circulation.
This variation in heat flow likely leads to an alongstrike difference in the distance into the subduction zone at which much of the diagenetic
dewatering reaction progress occurs.
IMPLICATIONS FOR OTHER CIRCUM-PACIFIC
SUBDUCTION ZONES
The volumetric contribution and spatial distribution of diagenetic fluid sources along a subduction
thrust depend on the composition and temperature path of subducted and accreted sediment,
Fig. 11 Earthquake locations (a) from Newman
et al. (2002) and proportion of water contained in
hydrous minerals within sediments subducted offshore Costa Rica (b). The volume of water in the
hydrous mineral phases (relative to pore space)
increases with distance into the subduction zone
as porosity decreases. The gray polygon indicates
the range of potential fluid partitioning scenarios
based on a range of sediment porosities from Bray
and Karig (1985). Beyond 20–30 km into the subduction zone, most of the water in the sediment is
contained in hydrous minerals. The location in the
subduction zone at which water is released from
the hydrous minerals is a function of reaction
kinetics; fluid will be released closer to the trench
in a warm subduction zone than in a cool one.
Shallow earthquakes occur closer to the trench in
the warm portion of the subduction zone than in
the cool section (a). The up-dip limit of seismicity
may be related to increased effective stress as fluid
pressures from diagenetic dewatering reactions
dissipate. CNS, Cocos–Nazca Spreading Center;
EPR, East Pacific Rise.
which can vary both between and within subduction zones. To place Costa Rica within a broader
context of circum-Pacific subduction zones, we
compare results from Nankai Trough, the Japan
Trench, the Aleutian Trench and Cascadia.
The Nankai Trough is characterized by sediment with low opal content, highly variable smectite content, and highly variable heat flow. Diatoms
and radiolarians are rare in the upper and lower
Shikoku Basin sections, but are more abundant in
the trench-wedge facies (Shipboard Scientific
Party 1975, 1986, 2001). The content of biogenic
silica is less than 1 wt%. The smectite content of
sediment along the central (Muroto) transect is
modest because of presubduction diagenesis, generally <25% of the bulk sediment by weight
(Underwood et al. 1993; Steurer & Underwood
2003a). Seafloor heat flow near the trench on the
Muroto transect is high, approximately 130–
180 mW/m2 (Hyndman et al. 1995; Shipboard Scientific Party 2001). Along the western (Ashizuri)
transect, smectite is much more abundant in the
lower Shikoku Basin strata, typically 30–50% of
the bulk sediment by weight (Steurer & Underwood 2003a; Underwood et al. 2003), and heat flow
is substantially lower, approximately 63 mW/m2
(Kinoshita & Yamano 1986; Shipboard Scientific
Party 1986).
Costa Rica sediment: Fluid partitioning 447
The sediment inputs to the Japan Trench have
moderate to high opal and smectite contents. Seafloor heat flow is low, approximately 25–65 mW/m2
(Burch & Langseth 1981), because the subducting
Pacific Plate is relatively old. Sediment from Site
436 (outer rise off the Japan Trench) is consistently 5–20% opal (Mann & Muller 1980; Shipboard Scientific Party 1980) and smectite
composes approximately 30–50% of the clay minerals (Mann & Muller 1980).
The Aleutian Trench is characterized by high
opal content and spatially variable smectite content. Heat flow is not well constrained in the
North Pacific, but it is likely moderate to low
because of the age of subducting lithosphere.
Heat flow at Site 183 (off the Alaska Peninsula) is
estimated at 25 mW/m2 (Erikson 1973). On the
basis of lithospheric cooling models, ages of 50–
70 Ma (Muller et al. 1997) correspond to heat flow
of 70–60 mW/m2 (e.g. Parsons & Sclater 1977;
Harris & Chapman in press). The area seaward
of the Aleutian Trench is within a zone of high
diatom productivity (Kastner 1981). Therefore,
those sediments have high opal contents (Shipboard Scientific Party 1973a, 1973b, 1993; Rea &
Ruff 1996). The percentage of smectite in
the <2 mm fraction steadily increases westward
from 10–20% in the Gulf of Alaska to 30–40% in
the central Aleutian Trench (Hathon & Underwood 1991).
Sediment approaching the Cascadia subduction
zone has low opal content and spatially variable
smectite content; heat flow and sediment thickness
are unusually high. At two sites in the Cascadia
Basin, opal accounts for approximately 5% of the
late Quaternary sediment (Heath et al. 1976). Off
southern Washington and northern Oregon, 30–
50% of the <2 mm fraction is smectite, but values
drop to 20–30% off southern Oregon and northern
California (Karlin 1980; Underwood 2002). Heat
flow near the trench off Oregon ranges from
approximately 55 to 150 mW/m2 (Korgen et al.
1971; Moran & Lister 1987). Heat flow at Site 888
(off British Columbia) is 80 mW/m2 (Shipboard
Scientific Party 1994); similar to the southern Cascadia subduction zone, seafloor heat flow on the
incoming plate near the trench off British Columbia ranges from approximately 50 to 150 mW/m2
(Davis et al. 1990).
Subduction zones with relatively large inputs of
opal and/or smectite (Costa Rica, Aleutians, Japan
Trench, southwestern Nankai Trough) have large
diagenetic fluid sources along the plate interface.
Where opal content is higher (Aleutians, Japan
Trench) dehydration will start closer to the trench
(at lower temperatures) than in subduction zones
where smectite content is higher. High heat flow
(southeastern Costa Rica, Cascadia, central Nankai Trough) will push the diagenetic fluid sources
closer to the trench. Where large fractions of the
incoming sediment are subducted (Costa Rica,
Japan Trench) rather than accreted, diagenetic
fluid sources will be concentrated along or beneath
the decollement, not distributed throughout the
prism.
CONCLUSIONS
The hemipelagic sediment subducting beneath the
Nicoya Peninsula, Costa Rica, has moderate opal
content and high smectite content (relative to
other subduction zones and in absolute terms).
These hydrous minerals convey a large volume of
water into the subduction zone. As kinetically controlled diagenetic reactions proceed rapidly at
high temperature (10 s of km into the subduction
zone), that water is released. Hydrous minerals
contain most of the fluid within subducted sediment beyond approximately 20 km into the subduction zone (Fig. 11). Therefore, within loci of
diagenetic dewatering reactions, the volume of
fluid released from the opal-to-quartz and smectite-to-illite transitions is larger than sediment
compaction sources. Once opal and smectite dehydration are exhausted, elevated fluid pressures
should dissipate. The down-dip increase of effective stress might, in turn, affect the up-dip limit of
seismicity.
Along-strike variations in sediment composition,
the thermal state of crust approaching the trench,
and the stratigraphic position of the decollement
lead to spatial variability in diagenetic fluid
sources (and therefore fluid pressure and effective
stress along the plate interface). On the Costa Rica
margin, sediment composition is uniform but there
is a large, abrupt change in seafloor heat flow
along-strike. This thermal variability probably
controls the loci where large amounts of subducted
smectite and opal release their bound water
(Spinelli & Saffer 2004). In the Cascadia and Aleutian subduction zones, along-strike variations in
smectite content modulate diagenetic fluid
sources. In the Nankai Trough, sediment composition and heat flow both vary along-strike. Thus,
each subduction system displays a unique combination of thermal and compositional inputs that
change in 3-D. The influence of those subduction
448 G. A. Spinelli and M. B. Underwood
inputs on fluid pressure and effective stress along
the deeper plate boundaries needs to be evaluated
on a case-by-case basis.
ACKNOWLEDGEMENTS
This work was supported by NSF MARGINS
grants OCE-0304946 and OCE-02106921 and the
University of Missouri Research Council. We
thank the captains and crew of the R/V Maurice
Ewing and R/V Melville for their assistance during the TicoFlux I and II expeditions (funded by
NSF grants OCE-0001892, OCE-0001944, OCE0001941 and OCE-0002031). This research also
used samples provided by the Ocean Drilling Program, sponsored by the US National Science
Foundation and participating countries under
management of Joint Oceanographic Institutions
Inc. E. Silver, P. Costa, and H. DeShon (University
of California at Santa Cruz) acquired and processed the reflection seismic data. Y. Chan
(University of Minnesota–Duluth Large Lakes
Observatory) provided advice on opal analysis.
N. Basu, S. Udas, K. Hoke, J. Dartt and A. Bidesi
assisted in the lab. We thank Yujiro Ogawa and
Tetsuro Hirono for helpful reviews of the
manuscript.
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