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Applied Clay Science 87 (2014) 108–119
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Applied Clay Science
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Research paper
Zeolite occurrence and genesis in the Late-Cretaceous
Cayo arc of Coastal Ecuador: Evidence for zeolite formation in cooling
marine pyroclastic flow deposits
L. Machiels a,⁎, D. Garcés b, R. Snellings a, W. Vilema c, F. Morante b, C. Paredes d, J. Elsen a
a
Applied Geology and Mineralogy, Department of Earth and Environmental Sciences, Katholieke Universiteit Leuven, Celestijnenlaan 200E, B-3001 Heverlee, Belgium
CIPAT — Escuela Superior Politécnica del Litoral, Centro de Investigación y Proyectos Aplicados a las Ciencias de la Tierra (CIPAT-ESPOL), Campus Gustavo Galindo Km. 30,5 Vía Perimetral,
P.O. Box 09-01-5863, Guayaquil, Ecuador
c
Guayaquil, Ecuador
d
Escuela Superior Politécnica del Litoral, Facultad de Ingeniería en Mecánica y Ciencias de la Producción, Campus Gustavo Galindo Km. 30,5 Vía Perimetral, P.O. Box 09-01-5863,
Guayaquil, Ecuador
b
a r t i c l e
i n f o
Article history:
Received 8 June 2011
Received in revised form 26 July 2013
Accepted 15 October 2013
Available online 18 November 2013
Keywords:
Natural zeolite
Ecuador
Cayo Formation
Geoautoclave
Hydrothermal alteration
Pyroclastic flow deposits
a b s t r a c t
This paper describes the quantitative mineralogy, the mineral chemistry and the distribution of natural zeolites
over the outcrop area of the Late Cretaceous Cayo Formation of Coastal Ecuador (N1000 km2) and develops a
model for zeolite alteration in the Cayo volcanic arc. Different zeolite types were identified: Ca-heulanditetype zeolites (clinoptilolite and heulandite), mordenite, laumontite, analcime, stilbite, epistilbite, chabazite,
thomsonite and erionite. Zeolites occur over nearly the entire outcrop area and the entire stratigraphical thickness of the Cayo Formation, in percentages varying between less than 20 and nearly 100 wt.%. A substantial
amount of the analysed samples (1/8) has zeolite contents higher than 50 wt.% and could potentially be used
in agriculture, aquaculture, for waste water treatment or as supplementary cementitious materials. A clear difference in zeolite type and content was observed when comparing the lower and upper units of the Cayo Formation
and the distribution of these units determines the zeolite distribution over the outcrop area. In the upper
unit, Ca-HEU-type zeolites are the main zeolite minerals and rarely laumontite and analcime occur. A
smectite-rich smectite/chlorite (C/S) is the major associated alteration mineral, while quartz contents are
relatively low. In the lower unit, the zeolite mineralogy is more variable and mordenite, Ca-HEU-type zeolites
and laumontite are common. Stilbite, epistilbite and analcime occur rarely. Further quartz, albite, C/S and
celadonite occur as associated alteration minerals. Little burial metamorphism or volcanically induced hydrothermal alteration has affected the deposits of the Cayo formation. Mineral alteration occurred mainly by interaction of hot pyroclastic glass with marine water, present as pressurized steam in cooling pyroclastic flow
deposits on one hand or by low temperature diagenesis of already cooled pyroclastic or epiclastic deposits on
the other hand. A model similar to the “geoautoclave” model is proposed to explain the genesis of zeolites, in
which an autoclave is formed by the hydrostatic pressure exerted by the marine water column overlying the
pyroclastic deposits, preventing gas escape and promoting glass dissolution, zeolite formation and, conversion
to higher-grade phases possible if heat can be maintained for a long enough period.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
In 1994, zeolites were discovered in the coastal region of Ecuador
in Guayaquil, the largest city of Ecuador. Zeolites occur in the Cayo Formation, a Late Cretaceous rock unit composed of marine volcanoclastic rocks.
⁎ Corresponding author at: Research group of High Temperature Processes and
Industrial Ecology, Department of Metallurgy and Materials Engineering, KU Leuven,
Kasteelpark Arenberg 44 room 02.47, BE-3001 Leuven, Belgium. Tel.: +32 494918649.
E-mail addresses: [email protected] (L. Machiels), [email protected]
(D. Garcés), [email protected] (R. Snellings), [email protected]
(W. Vilema), [email protected] (F. Morante), [email protected] (C. Paredes),
[email protected] (J. Elsen).
0169-1317/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.clay.2013.10.018
Initially, clinoptilolite and mordenite were identified (Romero, pers.
com.) and later heulandite was found (Morante, 2004). The main zeolites occurring in Guayaquil are Ca-heulandites (average Si/Al: 3.30)
and Ca-clinoptilolites (average Si/Al: 4.35) and less common are
laumontite, mordenite and analcime (Machiels et al., 2008). Zeolites
compose 10–60 wt.% of the rocks and a large variability in zeolite type
and content exists throughout the beds (Machiels et al., 2008).
Early investigation focussed on the zeolite occurrence in the vicinity of
Guayaquil, but preliminary work has shown that zeolites occur throughout the cordillera Chongón-Colonche, a mountain range stretching out
west from Guayaquil towards the coastal line, 100 km towards the west
(Machiels et al., 2006; Figs. 1 and 2). Nowadays, local zeolitic rocks are
used in agriculture and in aquaculture and during the last decades
L. Machiels et al. / Applied Clay Science 87 (2014) 108–119
109
Fig. 2. Localization of the three main sampling areas for X-ray analysis. The Guayaquil area,
the Río Guaraguao area and the Manabí area are shown. The underlain geological map is
combined from the geological maps of Dirección General de Geología y Minas (1970,
1974a,b,c, 1975a,b, 1980) and ESPOL–ORSTOM (1985). Coordinate system: PSAD 1956.
UTM UPS zone 17S.
Fig. 1. The oceanic basement of western Ecuador. The entire coastal area and the western
Cordillera of the Andes of Ecuador are built up of terranes composed of mafic oceanic crust,
derived from the interaction of the Caribbean oceanic plateau with northern South
America. The outcrop area of the Cayo Formation, west of Guayaquil (G), is indicated.
Modified from Bourdon et al. (2003).
small quarries have appeared near Guayaquil and throughout the cordillera. However, the exploitation and local application of the minerals are
still limited. In the future, zeolitic rocks could be used for the purification
of the waste waters of Guayaquil, a city of more than three million inhabitants (Calvo et al., 2009; Garcés et al., submitted for publication; Morante
et al., 2010), as fertilizer carriers in banana and coffee plantations, or for
the absorption of ammonia from shrimp breeding pools (Morante,
2011). Another possibility is the use of zeolites as supplementary cementitious materials in the local cement industry (Mertens et al., 2009;
Snellings et al., 2009).
The objective of the current contribution is to present an overview of
the mineralogy, mineral chemistry and distribution of the zeolites in the
Cayo Formation over its entire outcrop area throughout the Cordillera
Chongón-Colonche and to develop a genetic model explaining zeolite
occurrence and distribution, thereby contributing to the understanding
of zeolite genesis in the area on one hand, and to the exploitation and
application of the minerals on the other hand.
2. Regional and local geology
The western cordillera of the Andes and the Coastal area of Ecuador
are built up of accreted fragments of Late Cretaceous mafic oceanic
plateau basement (Kerr et al., 2003; Luzieux et al., 2006; Mamberti
et al., 2003; Reynaud et al., 1999; Vallejo et al., 2006 — Fig. 1). An
intra-oceanic island arc was formed at the edge of this oceanic plateau,
which yielded thick sequences of volcanoclastic, volcanic and intrusive
rocks (Benítez, 1995; Lebrat et al., 1987; Luzieux et al., 2006; Machiels,
2010; Machiels et al., 2008; Pichler and Aly, 1983; Thalmann, 1946).
In the Coastal area, the Late Cretaceous basement crops out in the
Cordillera Chongón-Colonche, a NW–SE oriented mountain range occurring west of Guayaquil (Fig. 2). Several rivers cross-cut the cordillera
from NE to the SW, perpendicular to the Late Cretaceous beds.
One of the most complete sections of the Late Cretaceous stratigraphy occurs in the Río Guaraguao, a river located 35 km NW of Guayaquil
(Fig. 2). The stratigraphical section exposed by the river is shown schematically in Fig. 3. From the north to the south, the following rock units
are exposed (Figs. 3–4): the Piñón Formation, of unknown thickness,
which forms the mafic oceanic basement of the area, of late Turonian
to early Coniacian age (Luzieux et al., 2006); the Las Orquídeas Formation (Van Melle et al., 2008), less than 100 metre thick, composed
of submarine volcanic breccia, of early or middle Coniacian age; the
Calentura Formation, less than 50 metre thick, composed of finegrained cherts, limestones and thin-bedded volcanoclastic turbidites,
of middle Coniacian age (Ordóñez et al., 2006); the Cayo Formation,
1700 metre thick, subdivided in two units (Machiels, 2010): the lower
unit, the Río Guaraguao unit, 700 metre thick and composed of marine
pumice-rich pyroclastic flow deposits and minor tuffs and associated
epiclastic rocks, of middle Campanian age at the base (Radiolarian
zone of Amphyphyndax pseudoconolus, Ordóñez et al., 2006) and of
late Campanian age (or younger) at the top (Radiolarian zone of
Amphyphyndax tylotus, Ordóñez et al., 2006); and the upper unit,
1000 metre thick, composed of marine epiclastic rocks, block and ash
flow deposits and water-lain tuffs, of late Campanian (or younger) age
at the base (Radiolarian zone of Amphyphyndax tylotus, Ordóñez
et al., 2006) and of Maastrichtian age (Ordóñez et al., 2006) at the top;
and the Guayaquil Formation, 370 metre thick, is composed of finegrained thin bedded cherts, of Maastrichtian age at the base and of
Danian age at the top (Ordóñez et al., 2006; Vilema, 2008). A more
extensive description of volcanism and deposition in the Cayo arc can
be found in Machiels (2010).
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Fig. 3. Schematic lithostratigraphical column of the Late-Cretaceous stratigraphy with indication of the alteration mineralogy of the main lithologies. Qua: quartz; heu: HEU-type zeolites;
mor: mordenite; lau: laumontite; alb: albite; c/s: chlorite/smectite; cel: celadonite; cal:calcite; c–t: crystobalite/trydymite; ana: analcime; sti: stilbite; pum: pumpellyite.
3. Materials and methods
3.1. Field observations and sampling
Three areas throughout the Cordillera Chongón-Colonche were investigated in this work: the Río Guaraguao cross-section, the Guayaquil
area, and the Manabí area. The sampling locations are shown in Fig. 2.
Individual sample coordinates can be found in Machiels (2010). It was
aimed to collect representative samples of the different lithological
types, to study mineralogical patterns through depositional sequences
and to investigate lithological–mineralogical relations. In order to investigate the zeolite distribution through the stratigraphical thickness of
the Cayo Formation, samples were taken from the top of the Piñón Formation throughout the Calentura and Cayo Formation towards the base
of the Guayaquil Formation in the Río Guaraguao section (220 samples,
Fig. 4). In the Guayaquil area, the results of Machiels et al. (2008), from
the southern part of the area (50 samples) were used. Additional sampling was performed in the northern part of the area (85 samples),
where the Cayo Formation is cross-cut by intrusive and volcanic bodies,
in order to investigate the influence of the proximity to these eruptive
centres. In the Manabí area, mainly the Río Ayampe river section was
sampled (40 samples), and numerous other samples were taken for
qualitative X-ray diffraction analysis.
3.2. Quantitative X-ray diffraction using the Rietveld method
For quantitative X-ray diffraction analysis (QXRD), the sample preparation procedure described in Machiels et al. (2008) was used. Samples
were ground by hand and sieved to b 500μm, spiked with 10wt.% of ZnO
internal standard and ground in ethanol for 7.5 min using a McCrone
Micronizing Mill. Random powder mounds were prepared using the
side-loading or back-loading technique. Samples were placed in an
equilibration chamber with an oversaturated Mg(NO3)2·6H2O solution
for at least 16 h prior to X-ray measurement, in order to obtain comparable hydration states of zeolites in the samples. X-ray diffraction
measurements were performed using a Philips PW1830 diffractometer
with CuKα radiation, graphite monochromator, 45 kV and 35 mA, 0.02°
2θ step size, 5–70° 2θ and a minimum of 2 s counting time per step.
Rietveld quantitative phase analysis (QPA) was performed using the
Topas Academic software (Coelho, 2007). A fundamental parameter approach was used, meaning that instrumental contributions to the peak
shapes were calculated directly (Cheary and Coelho, 1992). The following parameters were refined: the background, sample displacement,
scale factors of all phases, lattice parameters, crystallite size and lattice
strain. The background was refined using a cosine Chebyshev polynomial function of 15 parameters (Snellings et al., 2010). Because of the large
variety in the composition of a single zeolite species, different structural
models were compared. If possible, the chemistry of the model was
compared with the zeolite chemistry, obtained from electron probe
microanalysis (EPMA) measurements. Cell parameters were allowed
to vary between the ranges reported for each mineral in the literature
(for zeolite cell parameters, the ranges given in Passaglia and
Sheppard, 2001 were used). For the ZnO standard, cell parameters
were calibrated against rutile NIST SRM 674 and were fixed to the calibrated values during the analyses (Snellings et al., 2010). Preferred orientation effects were not refined, because it is questionable if refinement of
these factors in complex mixtures leads to a great improvement of the results (Bish and Plötze, 2011; Snellings et al., 2010). Temperature factors,
atom positions and occupancies were not refined because of possible
parameter correlation problems. Identification of clay minerals was
done by combining the results of Rietveld analysis results and optical microscopy observations together with EPMA and scanning electron microanalysis (SEM) measurements. If possible, clay mineral structures were
introduced in the refinement, if not, the total clay mineral content was
L. Machiels et al. / Applied Clay Science 87 (2014) 108–119
111
Fig. 4. Overview of the Río Guaraguao cross-section and variation of the main alteration minerals through the section. The underlain geological map is modified from ESPOL–ORSTOM
(1985). Sample locations are shown. The outcrop area of the Calentura Formation is indicated and a division is made between the lower and upper units of the Cayo Formation. Coordinate
system: PSAD 1956. UTM UPS zone 17S.
calculated from the difference of 100%, as no glassy or other amorphous
phases were present and assuming that all other minerals were introduced into the analysis. Heating tests were combined with quantitative
X-ray diffraction to distinguish between the different HEU-type zeolites.
The procedure of Boles (1972) was adapted for this purpose. The quantitative mineralogy of the samples was analysed with the quantification
method outlined in the previous section. The samples (with ZnO) were
heated at 270 °C and 470 °C for 15 h. Samples were measured in the
range of 9–11.5° 2θ to determine if changes occurred in the (020) X-ray
diffraction peak. If the peak disappeared completely, it was concluded
that the HEU-type zeolites belong to group 1 of Boles (1972), with the
sum of divalent cations (SD) N2.94. If a X-ray diffraction peak was still
present, the heated samples were reanalysed using the Rietveld method.
Because the dehydration of the remaining zeolites leads to significant
shifts in the X-ray diffraction patterns, samples were rehydrated for
16 h prior to measurement. In order to represent the variation in mineralogy throughout Río Guaraguao section, the quantitative mineralogical
data were introduced into a Geographical Information System. The
ArcGIS 9.3 software was used for this purpose. Data were georeferenced
and interpolation was done using Ordinary Kriging Prediction Maps. A
spherical semivariogram and covariance model were used, considering
nugget and anisotropy effects and using a correlation ellipse with a
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L. Machiels et al. / Applied Clay Science 87 (2014) 108–119
mayor axis of 2000, a minor axis of 200 and 120° of inclination, in order to
impose anisotropy along the strike of the layers. The average of the root
mean square values of the prediction method was around 1.
3.3. Mineral chemistry — EPMA and SEM analyses
Scanning electron microscope analyses (SEM) were performed
using back-scattered electron images (BSE) combined with standardless electron dispersive X-ray (EDX) measurements to analyse the
habit, distribution and composition of authigenic phases. All analyses
were performed using an ESEM FEI QUANTA FEG 200 with solid-state
STEM detector and an EDAX Apollo 40 system with Si/Li-detector. An
operating voltage of 14 keV and a beam current of 10 nA were used. As
no standards were used, the SEM analyses should be considered as
semi-quantitative. Electron probe microanalyses (EPMA) were performed with a Camecabox SX50 using accelerating voltages of 15 keV
and beam currents of 20 nA. The following mineral standards were
used: wollastonite for Si and Ca (10 s counting time), synthetic sapphirine for Al (10 s), oligoclase for Na (16 s), leucite for K (10 s), olivine for
Mg (16 s), synthetic hematite for Fe (10 s), rutile for Ti (10 s), rhodonite
for Mn (10 s), strontianite for Sr (10 s) and barite for Ba (10 s). ZAF corrections were used. To avoid dehydration of zeolites or alkali migration,
large beam spot sizes, a rastered electron beam and low counting times
were used. A minimum of 5 μm beam diameter was used, but the beam
spot size was increased if the size of a crystal could permit this. Repeated
analyses were performed. For zeolites the E% value defined by Gottardi
and Galli (1985) was used. E% = [(Al + Fe3+)obs_Alth] / Alth_100, where
Alth = Na + K + 2(Ca + Mg + Sr + Ba). Analyses with an E% value less
than 10% were considered as reliable. For other minerals, the quality
of the analysis was judged by whether the sum of all oxides was within
a narrow range of 100% and by calculating the charge balance. HEU-type
zeolites and laumontite crystals generally permitted the use of large
beam sizes to deliver reliable results. For analcime, only EPMA measurements gave acceptable results. Both EPMA and SEM analyses of
mordenite were difficult, because the mineral occurs in small radial
clusters of crystals smaller than 1 μm in size. However, some acceptable
analyses could be produced.
4. Alteration mineralogy and mineral chemistry of Late Cretaceous
rocks in the Cayo arc
Results of individual QXRD analyses, SEM and EPMPA analyses can
be found in Machiels (2010).
the most commonly associated alteration minerals. Clay minerals
are mainly di- or tri-octahedral mixed layers of chlorite and smectite
(C/S), with varying proportions of smectite, and celadonite. Samples
contain a varying amount of pyrogenetic fraction, represented mainly
by plagioclase (86% of the samples) and minor augite (44%).
4.2. Heulandite-type zeolites
In thin section, HEU-type zeolites can be observed as prismatic
euhedral to subhedral crystals of sub micrometre to 500 μm in size
(Fig. 5.3–4). HEU-type zeolites are often associated with C/S clays and
quartz and in some cases mordenite, stilbite, analcime, celadonite and
calcite. To distinguish between the different HEU-type zeolites, 13 samples from throughout the Cayo Formation in the Río Guaraguao section
were heat-treated as described in Section 3.2. In 6 samples, the (020)
X-ray diffraction peak disappeared after heating at 470 °C. In other
samples, its intensity decreased, but it could be observed that the remaining intensity belongs to other zeolites which possess overlapping reflections and that no HEU-type zeolites remain after heating.
In three samples, a low percentage of HEU-type zeolites (b2 wt.%)
remained after heating. As expected, the amount of not-quantified
material increases after heating, because HEU-type zeolites become
X-ray amorphous. It can thus be concluded that the analysed samples
all belong to group one of Boles (1972), meaning that the HEU-type
zeolites contain a high amount of divalent cations and a low amount
of potassium. These results are comparable to the results of the heating
tests performed by Machiels et al. (2008) in the Guayaquil area.
The high Ca-content and low K-content are confirmed by EPMA and
SEM–EDX analyses.
The results of the SEM–EDX and EPMA analyses of HEU-type zeolites
are shown in Figs. 6 and 7. In Fig. 6, the framework cations Al3+ are plotted against the sum of the extraframework cations Na+, K+, Ca2+,
Mg2+, Sr2+ and Ba2+ (diagram modified from Bish and Boak, 2001).
Samples which plot above the line for Si/(Al+Fe)=4 are clinoptilolites,
and samples which plot below this line are heulandites. Some samples
contain only clinoptilolite, others only heulandite, while in other samples both zeolite types occur. The average Si / (Si + Al) ratio (R-value)
is 0.80 in the EPMA analyses and 0.79 in the SEM analyses with a minimum of 0.75 and a maximum of 0.83. The extraframework cations are
plotted in a ternary diagram in Fig. 7. All samples have a high Cacontent and can be named heulandite–Ca or clinoptilolite–Ca (Coombs
et al., 1997). Ca is the main extraframework cation (2.09–4.17 atoms
per unit cell — a.p.u), while Na is subordinate (0.04–2.33 a.p.u.) and K
is low (0.08–0.94 a.p.u.), as is Mg (0–0.75 a.p.u.).
4.1. An overview of alteration mineralogy
4.3. Other zeolites
Zeolites are common throughout the entire stratigraphical thickness
of the Cayo Formation and in the different areas studied. No zeolites
were found in the Piñón, Calentura, Las Orquídeas and Guayaquil
Formations. Quartz, calcite and a smectite/chlorite mixed layer (C/S)
are the main alteration minerals in these formations. In the Cayo Formation, zeolite contents are strongly variable, ranging from low contents
(b20 wt.%) to very high contents (~100 wt.%). It has to be mentioned
that it was aimed to characterise the mineralogy of all lithologies occurring in the Cayo Formation, rather than to explore for the rocks with the
highest zeolite content. However, it has to be noticed that a substantial
amount of the analysed samples (1/8) has zeolite contents higher
than 50%. Different zeolite types were identified in the samples of the
Cayo Formation: heulandite-type zeolites (clinoptilolite and heulandite,
HEU-type zeolites), mordenite, laumontite, analcime, stilbite, epistilbite,
chabazite, thomsonite and erionite. HEU-type zeolites are the most common zeolites, occurring in 58% of the samples. Mordenite is also common
(22%), as is laumontite (19%). Other zeolites occur only rarely: analcime
(4%), stilbite (3%), erionite (4 samples), stellerite (3 samples),
thomsonite (3 samples), epistilbite (2 samples) and chabazite (1 sample).
Clay minerals (91% of the samples), quartz (84%) and albite (31%) are
Mordenite mostly occurs as fine spherical aggregates of prismatic
crystals. The thickness of the individual crystals is usually smaller
than 1 μm, the length varies from 5 to 200 μm (Fig. 5.1–4, 5.5). Less
commonly, mordenite occurs as separate or clustered fibres, but
no chemical analyses were performed to confirm this. Mordenite
is commonly associated with HEU-type zeolites, quartz, C/S and
celadonite. The chemistry of mordenite is quite constant. The R-value
is 0.79–0.81. Na is the main extraframework cation (3.55–5.18 a.p.u.),
followed by Ca (2.30–3.19 a.p.u.), while K (0–0.22 a.p.u.) and Mg
(0–0.13 a.p.u.) are subordinate.
Laumontite occurs as large euhedral to anhedral crystals (up to
2 mm), which can enclose albite, quartz, recrystallized mordenite aggregate ghosts and other minerals (Fig. 5.6). Laumontite is easily recognized
because of its two cleavages and its higher birefringence compared
to HEU-type zeolites. It is typically associated with albite, quartz and
chlorite-rich C/S. The chemical composition does not differ much from
the end-member composition. Ca is the dominant extraframework cation (3.69–4.11 a.p.u.), while K (0.02–0.21 a.p.u) and Na (0–0.14 a.p.u)
are low.
L. Machiels et al. / Applied Clay Science 87 (2014) 108–119
113
Fig. 5. Optical microscopy pictures of the most common zeolite minerals and associated alteration minerals. 1. Spherical mordenite bundles in C/S altered pumice. 2. Small euhedral stilbite
crystals enclosed in larger stilbite crystals. Crossed polars and gypsum accessory plate inserted. 3. Euhedral intergrowth of HEU-type zeolite crystals filling void in pumice. C/S rims and fills
vesicle remains. Crossed polars and gypsum accessory plate inserted. 4. Mordenite spheres with HEU-type zeolite overgrowth rimming voids. Crossed polars. 5. Large void in C/S altered
pumice filled with mordenite spheres, authigenic silica and HEU-type zeolites. Juvenile quartz grain is rimmed by authigenic silica. Crossed polars. 6. Juvenile K-feldspar (purple) and
albitized plagioclase phenocrysts (dark orange) in a laumontite altered pumice matrix. Small euhedral authigenic albite and quartz grains are enclosed in the laumonite crystals. Crossed
polars and gypsum accessory plate inserted.
Stilbite occurs as small euhedral trapezohedral crystals (100μm) that
are often twinned, and larger anhedral crystals (up to 2mm) that enclose
smaller quartz aggregates and mordenite aggregate ghosts (Fig. 5.2).
Analcime can be easily recognized because it forms isotropic euhedral
to anhedral crystals. It has an R-value of 0.71–0.74 (average 0.72). All
extraframework cations except Na were below the detection limit of
the SEM analyses, in EPMA analyses small amounts of K (b0.05 a.p.u.)
and Ca (b 0.04 a.p.u.) were detected.
plagioclase types were distinguished using the Rietveld refinement
method and by EPMA and SEM analyses. Oligoclase is the most common pyrogenetic feldspar, while all authigenic feldspars are albites.
Albite-rich samples typically contain more quartz compared to albitefree samples, contain laumontite, and contain less clay minerals and
all primary plagioclase is replaced by albite. Albite-free samples contain
pyrogenetic feldspars and in these samples HEU-type zeolites and
mordenite occur.
4.4. Feldspars
4.5. Clay minerals
Pyrogenetic feldspars occur as euhedral fractured crystal clasts in the
matrix of the rocks and as euhedral to anhedral crystals in volcanic rock
fragments, pumice and vesicular lavas. Most feldspars in the samples
are plagioclases and in some cases sanidine occurs. Authigenic feldspars
occur as euhedral to anhedral crystals, as spherical aggregates and
as fracture fillings in pyrogenetic feldspars (Fig. 5.6). The different
In situ chemical analyses were performed with both SEM–EDX and
EPMA analyses. The main clay mineral present is a mixed layer of
smectite with chlorite (C/S), while celadonite is less common. Because
chlorite can accommodate only a minor amount of Ca, analyses with
Ca contents higher than 0.10 cations/28 oxygens per unit cell indicate
the presence of smectite in the mixed layers (Bettison and Schiffman,
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L. Machiels et al. / Applied Clay Science 87 (2014) 108–119
on the contrary is very difficult to recognize by optical microscopy, because of its fine grain size (b1 μm) and its association with iron-oxides
and clay minerals. It forms small (b 1 μm) spherical aggregates and in
many cases a skeletal growth of individual crystals can be observed.
Opal-CT was identified in two samples of the Guayaquil Formation,
and low amounts were identified in several samples of the uppermost
part of the Cayo Formation.
5. Alteration of the Río Guaraguao cross-section
Fig. 6. Framework versus extraframework cations in HEU-type zeolites. Plot of the amount
of framework cations Al against the sum of the extraframework cations Na, K, Ca, Mg, Sr and
Ba (amount of atoms per 72 oxygens, diagram modified from Bish and Boak, 2001). Samples
which plot below the line for Si / (Al + Fe) = 4 are clinoptilolites, samples which plot above
this line are heulandites. A clinoptilolite and a heulandite population can be distinguished.
1988), which is the case for all C/S analyses. The C/S analyses can be
recast into unit cell contents with variable oxygen if both smectite and
chlorite are assumed to be trioctahedral, and if regular interstratification
of the smectite and chlorite components occurs (Bettison and Schiffman,
1988). This resulted in a calculated molar proportion of chlorite in the
mixed layers from 0.34 to 0.74.
Clay minerals can occur as fine grained sub-microscopic pellets
forming iron brown to bluish green aggregates (Fig. 5.1–5). These aggregates are mostly smectites with a small amount of interlayer chlorite.
C/S with higher amounts of chlorite forms larger brown to brownish
green spherical aggregates with first order interference colours. Pure
chlorites are green in colour, have a similar appearance and have anomalous bluish grey interference colours. Corrensite was identified from its
XRD super-reflection at 29 Å. Corrensite exists as greenish to brownish
flakes which are composed of grains of submicroscopic size. Celadonite
is also very fine grained and has a typical bluish green colour.
4.6. Quartz and Opal-CT
Quartz can occur both as pyrogenetic and as alteration mineral.
Pyrogenetic quartz, occurring as crystal clasts, is easily recognized by
its rounded form and uniaxial behaviour (Fig. 5.5). Authigenic quartz
As explained in Section 2, the Río Guaraguao river section composes
the most complete section of Late Cretaceous stratigraphy of coastal
Ecuador. The distribution of alteration minerals through the section is
summarized in Figs. 3 and 4. As indicated above, alteration mineralogy
is highly variable, even on the scale of single beds. However, some general trends can be observed throughout the section. The Guayaquil Formation is composed of cherts composed of quartz and minor cristobalite
and trydymite, which also occur in the uppermost beds of the Cayo
Formation. In the Cayo Formation, an important difference exists
between the upper and the lower unit. The upper unit is composed of
marine volcanic mass flow deposits and water lain tuffs, derived from
an andesitic volcanism. Rocks are composed of glass and crystal fragments, scoraceous pumice and effusive volcanic rock fragments. Glass
is typically palagonitized to smectite and Ca-HEU-type zeolites radiate
from pore walls or fill dissolution voids. Analcime occurs rarely and towards the basal part laumontite occurs in coarse beds. A smectite-rich
C/S is the major associated alteration mineral. Quartz contents are relatively low, except in fine tuff beds near the top.
The lower unit of the Cayo Formation is mainly composed of dacitic
marine pyroclastic flow deposits. These are built up of mainly tube pumice and minor glass and crystal clasts. The zeolite mineralogy is more
variable compared to the upper unit and mordenite, Ca-HEU-type zeolites and laumontite are common, and stilbite, epistilbite and analcime
occur rarely. Further quartz, albite, C/S and celadonite occur as associated alteration minerals. Mordenite–HEU-type zeolite alteration is more
common in thinner and finer depositional sequences or in the upper
part of thicker depositional sequences, while laumonite–albite alteration is more common in thicker, coarser and more welding compacted
depositional sequences or towards the base of depositional sequences.
Beds with HEU-type — smectite alteration, as in the upper unit, occur,
but are less common. Very commonly, it is observed in pyroclastic
flow deposits that mineral replacement reactions occur progressively
towards the base of the deposits, where mordenite is replaced by
heulandite and stilbite. In thicker pyroclastic sequences, a complete sequence of mordenite to heulandite and stilbite replacement and further
replacement to laumontite and albite can be observed from the top to
the base of these sequences, while epiclastic sequences at the same
levels with similar initial permeability lack conversion to higher grade
minerals.
Coarse, welded volcanic breccia at the base of the Cayo Formation is
devoid of zeolites, and is composed of quartz, albite, chlorite and minor
pumpellyite. In the Calentura Formation, mainly quartz and calcite are
present. The Las Orquídeas Formation is composed of coarse volcanic
breccia, altered to quartz, chlorite and albite, while finer beds are mainly
altered to smectite-rich C/S. The Piñón Formation has a similar mineralogy, with alteration to mainly smectitic C/S and quartz.
6. Lateral variation in zeolite mineralogy
6.1. Guayaquil area
Fig. 7. Triangular diagram for Ca, Na and K composition of heulandite-type zeolite, mordenite
and laumontite analyses (in atomic proportions). All analysed samples of HEU-type zeolites
have high Ca contents and are thus heulandite–Ca (Si/Al b4) or clinoptilolite–Ca (Si/Al N4).
In the Guayaquil area, apart from the Cayo Formation, effusive
volcanic and plutonic rocks of late Campanian age are exposed
(ESPOL–ORSTOM, 1985; Pichler and Aly, 1983). The Piñón Formation
crops out in hills and quarries north of the city, where it is intruded by
the late Campanian Pascuales intrusives. The Cayo Formation covers
L. Machiels et al. / Applied Clay Science 87 (2014) 108–119
most of the area, but in the eastern part it is covered by quaternary
sediments of the Río Guayas. In the central area, the Cayo Formation is
intruded by several volcanic conduits, probably related to the same
magmatic phase as the Pascuales intrusives. The southern area is less
disturbed and is exposed well along the Vía Perimetral and at the campus of the ESPOL University (Machiels et al., 2008). A clear distinction
can be made between the mineralogy of the northern and the southern
area. In the southern area, HEU-type zeolites are the most common
zeolites and laumontite, mordenite and analcime occur occasionally
(Machiels et al., 2008). As in the Río Guaraguao, a distinction between
the lower and upper units of the Cayo Formation can be made.
Alteration in the upper unit is dominated by HEU-type zeolites associated with C/S, mainly smectitic, and varying amounts of quartz. In
the lower unit, HEU-type zeolites, laumontite and less commonly
mordenite and analcime occur. The alteration in the central and northern area differs strongly from the southern area and is dominated by albite, clay minerals (C/S) and quartz. Zeolites are absent in most
samples, only few samples contain laumontite, in one sample
epistilbite was found and in some samples traces of HEU-type zeolites and mordenite were detected. In the northern region, around
Pascuales, actinolite, chlorite and albite are common and prehnite
was found. Quartz veins are common and can contain epidote, chlorite, pyrite and galena.
6.2. Manabí area
The Manabí area is tectonically much more disturbed compared to
the eastern area. Along the Cordillera Costera, which is the extension
of the Cordillera Chongon-Colonché running in NNE–SSW direct along
the Coastal line, basaltic lavas occur along fault zones, cross-cutting
the lower unit of the Cayo Formation and cross-cutting, interbedding
and overlying rocks of the upper unit of the Cayo Formation. Alteration
of volcanoclastic rocks of the Cayo Formation in the Río Ayampe
river section is similar to the alteration in the Río Guaraguao, with
HEU-type zeolites being the most common zeolite type and with common occurrence of mordenite and laumontite and occasionally stilbite
in the lower unit of the Cayo Formation. In pillow lavas in the western
part of the section, epistilbite, analcime, chabazite and thomsonite
were found. In the westernmost area, volcanoclastic rocks of the
upper unit of the Cayo Formation have a typical HEU-type zeolite smectite alteration and analcime and erionite occur.
7. Discussion — an alteration model for the Cayo arc
7.1. Marine low-temperature diagenetic alteration
Widespread low-temperature alteration of marine volcanoclastic sequences to zeolites in volcanic basins has commonly been explained by
diagenetic alteration of volcanic glass by seawater, resulting in the formation of clay minerals, SiO2 polymorphs, zeolites, Fe-hydroxides and
carbonates. The presence or absence, composition and relative amounts
of the different alteration phases depend on both glass and pore water
chemistry, permeability, water movement, reactive glass surface, temperature, etc. (e.g. Bish and Ming, 2001). Marine volcanoclastic deposits
are typically very heterogeneous in chemistry, grain size, porosity and
thickness, because they result from a wide variability of eruptive
and depositional processes. Even through depositional sequences, the
parameters determining alteration can vary strongly. As is seen in the
Cayo Formation, this results in a very heterogeneous alteration mineralogy that makes the exploitation of zeolitic rocks of consistent mineralogy and thus properties difficult. The Cayo Formation is subdivided in
two units of different composition and depositional type. In the upper
unit, mainly andesitic marine volcanoclastic depositional sequences
occur. Diagenetic alteration of basaltic and andesitic volcanoclastic
glass by marine water is commonly described as glass palagonitization
with important reaction products being tri-octahedral smectite,
115
phillipsite, analcime, HEU-type zeolites and minor SiO2 polymorphs
(Smith, 1991; Stroncik and Schmincke, 2002). Phillipsite and opalCT are seldom preserved in older or buried sediments (Dibble and
Tiller, 1981). In the upper unit of the Cayo Formation, smectite and
HEU-type zeolites are indeed the major alteration products formed and
quartz, analcime, Fe-hydroxides and calcite occur. Low-temperature
marine diagenetic alteration thus seems indeed to be the dominant
process in the alteration of the sequences. In the lower unit of the Cayo
Formation, glass is more commonly dacitic in composition, and was deposited by submarine pyroclastic flows. When rhyolitic to dacitic glass
is altered in marine systems, clinoptilolite, mordenite and heulandite
are commonly formed (e.g. in Japan, see: Iijima, 2001; Utada, 2001a,b;
Utada and Ito, 1989). Which zeolite type is formed depends mainly on
the glass and pore water compositions, with clinoptilolite being more
common when sufficient K is available from glass and/or pore water,
mordenite being more common when K-poor Na-rich glass is altered
and heulandite being more common in Ca-rich systems with less available silica (Chipera and Apps, 2001). As dacitic glass contains more silica
compared to andesitic glass, when sufficient Si is present in the system,
zeolites occur associated with opal, cristobalite, trydymite or quartz, of
which the presence and contents depend on the Si saturation (Dibble
and Tiller, 1981; Hawkins, 1981). As Mg and Fe contents are lower in
dacitic glass, clay mineral content is generally lower and clays tend to
be di-octahedral smectites or illitic clays. In the lower unit of the Cayo
Formation, mordenite and heulandite are common, while clinoptilolite
is rare. The formation of mordenite rather than clinoptilolite can be
explained by the low K content of the glass and the early formation of
celadonite incorporating K. Additionally, seawater interaction – and
thus K-supply – is probably lower in pyroclastic sequences compared to
water-saturated volcanic debris flows as occurring in the upper unit of
the Cayo Formation. Heulandite–Ca forms after initial mordenite formation at low Si and K saturation, when much of the silica is already
contained in a SiO2 polymorph and mordenite.
7.2. Prograde metamorphism
In depositional basins near volcanic arcs, the replacement of diagenetic alteration minerals by more stable, less hydrous phases has been
explained by progressive exposure to higher temperatures with burial
(Aguirre et al., 2000; Coombs, 1954; Coombs et al., 1959; Levi et al.,
1989; Utada, 2001a,b). Early formed zeolites such as mordenite and
HEU-type zeolites will evolve to laumontite and albite, smectitic C/S
will become more chloritic, and opal-CT will evolve to quartz. At even
greater depths, the zeolitic facies evolves to the zeolite free prehnitepumpellyite facies (Utada, 2001a,b).
The same sequence of replacement seems to occur in the Cayo Formation, as can be observed in the Río Guaraguao section. The first
depth occurrence of laumontite is at the base of the upper unit of the
Cayo Formation, the first depth occurrence of albite in the middle part
of the lower unit, and near the base of the Cayo Formation, only
laumontite, albite, chloritic C/S, quartz and occasionally pumpellyite
occur, while mordenite and HEU-type zeolites are absent.
However, burial metamorphism, where heulandite is replaced by
laumontite and albite and at greater depths by pumpellyite-prehnite
is typically described in thick volcano-sedimentary sequences in forearc or back arc basin, such as the sequences in New Zeeland, where
the zeolite facies was defined (Coombs, 1954; Coombs et al., 1959).
The tectonic setting of the Cayo Formation is somewhat different. The
lower unit of the Cayo Formation is composed mainly of pyroclastic
flow deposits, which makes clear that the Cayo Formation represents
deposition at or very proximal to the volcanic arc rather than deposition
in a fore-arc or back-arc basin. In contrast to the sedimentary basins towards the north and south, the Cordillera Chongón-Colonche remained
as a tectonically elevated area after the Late-Cretaceous, and the
deposits of the Cayo Formation were only slightly affected by burial
(Gems – PetroEcuador, 2007; Machiels, 2010). This can be seen from
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L. Machiels et al. / Applied Clay Science 87 (2014) 108–119
the low reflectivity of organic material in the Calentura Formation,
underlying the Cayo Formation, from which a maximum burial temperature of 75 °C can be deduced (Gems – PetroEcuador, 2007). This
contradicts the temperature that is expected to have occurred in the
overlying lower unit of the Cayo Formation, which is, based on the
mineralogical assemblages, at least the double, 125 °C and probably
even much higher. Additionally, the Piñón Formation, which is underlying the Calentura Formation and was thus buried even deeper,
contains only diagenetic minerals, while zeolites, laumontite, albite,
chlorite and pumpellyite lack completely, which indicates that no
burial related metamorphism affected the sequence in the Río
Guaraguao area. Traditional burial metamorphism can thus not explain the prograde alteration of zeolitic rocks towards higher grade
zeolites, albite, chlorite and pumpellyite in the lower unit of the
Cayo Formation.
7.3. Metamorphism and hydrothermalism proximal to volcanic arcs
Alteration in volcanic arcs is somewhat more complex than in
volcanosedimentary basins, as combined burial metamorphic and hydrothermal processes occur. Because geothermal gradients are typically
high to very high proximal to volcanic arcs (N60°/km to N200°/km near
calderas, e.g. Japanese green tuff area (Utada and Ito, 1989), zonation
from lowest grade phases near the surface to albite–quartz–chlorite
and pumpellyite-prehnite facies can occur over relatively low stratigraphical thicknesses at low depths. In some geothermal areas, depth
zonations are less pronounced, low and high-grade phases occur at
the same burial depth and high grade zones nearly reach the surface
(Kristmannsdóttir and Tómasson, 1978). In these zones, mineral alteration is often considered as hydrothermal, as alteration is strongly dependent on fluid infiltration, circulation and pathways and can vary
strongly laterally (Hall, 2000; Kristmannsdóttir and Tómasson, 1978;
Utada, 2001b). In many cases, difference in alteration can even be observed within a single depositional unit, depending on the degree of
permeability. Typical examples are basaltic flows with a permeable vesicular top, where high-temperature phases occur, while the impermeable core of the same flows can remain glassy because no glass–water
interaction occurs (Neuhoff et al., 1999). In other cases, early alteration
of glassy, fine grained and thus highly reactive beds can result in complete obstruction of pores by clay minerals and zeolites, preventing further fluid flow and reaction to more stable phases later on. This can lead
to alternating stratigraphical occurrence of low and high grade alteration or even reoccurrence of complete low to high grade zonations
with depth. Commonly, this has been explained by reoccurring periods
of volcanic activity (Aguirre et al., 2000; Levi et al., 1989). During volcanically active periods a high geothermal gradient is established,
resulting in a zonation of alteration minerals with depth. Formation of
early alteration minerals such as clay minerals and zeolites strongly reduces porosity and will prevent prograde alteration during a following
volcanically active period, when a new zonation is established in
newly deposited beds, but in which the underlying zonation is unaffected, or alteration only occurs along permeable pathways such as faults
and in permeable beds around faults. Such a hydrothermal system
would thus lead to a very heterogeneous mineral occurrence, with
high-grade minerals being distributed around fluid pathways and permeable beds. This system could be responsible for the northern Guayaquil area, around the intrusive zones of Pascuales and around volcanic
conduits in this area, but is not consistent with the mineral distribution
in the Cayo Formation, where very regular patterns in alteration mineralogy can be observed through depositional sequences and with depth
and in which these patterns can be followed over very large areas laterally. Additionally, it is observed that prograde mineral reactions occur
only in pyroclastic flow deposits, while interbedded epiclastic rocks
show only evidence of diagenetic alteration, even if their permeability
is high.
7.4. Zeolitisation of cooling marine ignimbrites
Several authors have proposed an alternative model for the formation of zeolites and associated alteration minerals in pyroclastic
sequences, in which zeolite formation occurs relatively fast after
deposition of pyroclastic sequences rather than during diagenesis
or burial (Lenzi and Passaglia, 1974 in Aleksiev and Djourova, 1975;
Gottardi, 1989; Matsubara et al., 1978 in Bernhard and Barth-Wirsching,
1999; Christidis and Huff, 2009; Ghiara et al., 1999, 2000; Gottardi,
1989; Leggo et al., 2001; Raynov et al., 1997; Yanev et al., 2006). In
these models, the heat remaining in the pyroclastic material catalyses
glass dissolution causes fluid flow and fluids of temperatures higher
than ambient temperature promote crystallisation of alteration minerals. Both open and closed systems have been proposed to cause
zeolitisation. In closed “geoautoclaves”, an impermeable welded tuff
layer prevents fluid or gas escape (Lenzi and Passaglia, 1974 in
Gottardi, 1989) and alteration occurs by a pressurized vapour phase.
In open hydrothermal alteration systems interaction of water with hot
glass causes glass dissolution and formation of zeolites (Yanev et al.,
2006) or clay minerals. Which minerals are formed, depends not only
on the glass chemistry, but also on ho much dissolved constituents are
transported in or out the system. In these hydrothermal systems, conditions are very similar as in lab scale zeolite synthesis experiments
(Barth-Wirsching and Höller, 1989; Bernhard and Barth-Wirsching,
1999), meaning that zeolitisation can occur relatively fast after deposition in time spans of days, weeks to years, depending on the initial temperature, cooling history, the degree of water interaction and the
internal structure of and position in the pyroclastic sequence. If temperature is high enough and can be maintained for long enough, it can be
expected that – as in lab scale zeolite synthesis experiments – early
formed metastable phases can be converted to thermodynamically
more stable phases.
It is clear that pyroclastic sequences of the lower unit of the Cayo
Formation were hot when deposited, as is obvious from the presence
of welding towards the base of some sequences, the presence of gas
pockets aligned at certain levels of the pyroclastic flow deposits and
the presence of gas oxidation surfaces at the top of some sequences.
As eruptions were submarine, the pressure of the water column resulted in relatively low eruptive columns, pyroclastic flows running over
the sea bottom surface and limited water incorporation and cooling
prior to deposition. The deposits have all characteristics of gassupported pyroclastic flow deposits. If alteration of the deposits was initiated directly after deposition, this would mean that it would be
strongly determined by the degree of cooling and water interaction during eruption, during movement of the pyroclastic flows and after deposition. The internal structure of each depositional unit (e.g. thickness,
particle size, permeability, welding) would thus greatly influence its alteration and would have caused not only different alteration patterns in
different pyroclastic flow deposits but also greatly deferring mineral alteration through single depositional units. Another important factor to
understand alteration of the pyroclastic sequences is the behaviour of
water in the sequences. After deposition, at the high temperatures in
the ignimbrites, incorporated fluids are present as pressurized steam.
As temperature is the highest in the core of the ignimbrite, a gas flow
will be directed outward of the deposit. At deposition, vapour pressure
in pyroclastic flow deposits is estimated to be around 0.1 to 0.5 MPa depending on the temperature at deposition, the permeability, thickness
and geometry of the deposit (Keating, 2005). If deposited on land, this
pressure is much higher than of the atmosphere surrounding the ignimbrite, which will be at atmospheric pressure. An outward directed gas
flow would thus occur until the pressure in the ignimbrite is equal to atmospheric pressure. This permanent flow of gas out of the ignimbrite
would lead to a dry-out of the pyroclastic flow deposit, unless an impermeable cap forms a “geoautoclave” or unless water (e.g. rainwater) infiltrates from the top and forms a barrier preventing gas escape. This
would occur if sufficient water is supplied to form a boiling steam/
L. Machiels et al. / Applied Clay Science 87 (2014) 108–119
water interphase, where heated infiltrated water is redirected towards
the outside. The location of this boiling interphase depends on the initial
temperature at deposition, and will move downwards in the ignimbrite
when it is cooled progressively. This process is shown schematically in
Fig. 8. A similar process occurs at the base of the ignimbrite, where
water from underlying water-saturated beds is pressured downwards
by the pressurized gasses in the ignimbrite. If no permeable cap or permeating rainwater is present and if dry-out would occur, the minerals
formed would be restricted to phases found in devitrification structures
formed at low fluid pressures in ignimbrites, such as feldspars and
quartz and in fact these are also the first minerals to form in the lower
basal parts of thick pyroclastic flow deposits in the lower unit of the
Cayo Formation, or in fine-grained beds directly underling pyroclastic
deposits.
The large difference between ignimbrites deposited on land or submarine is the presence of a thick water column. Apart from the possibility of water incorporation during emplacement, the water column also
has an important role after deposition, having a double function:
water is permanently present to infiltrate the ignimbrite and form a
boiling front and the weight of the water column will easily deliver a
hydrostatic pressure higher than the gas pressure in the ignimbrite at
deposition. In marine environments the hydrostatic pressure increases
~1 MPa per 100 m of water depth, at a depth of 50 m the hydrostatic
pressure would already equal the maximum gas pressure expected in
the ignimbrite, which prevents gasses to escape from the ignimbrite.
This is evidenced in the lower unit of the Cayo Formation by omnipresence of gas pockets concentrated at certain levels in the flow deposits
and by the absence of gas chimneys. The depth at which eruption and
deposition of the Cayo Formation occurred was definitely much deeper.
Pelagic microfauna and ichnofossils indicate a rather deep marine
(N1000 m) depth (Ordóñez et al., 2006; Vilema, 2008). This would
equal a hydrostatic pressure of 10 MPa which would thus be more
than enough to prevent gasses from escaping from the pyroclastic
flow deposits.
It is thus very probable that conditions in marine deposited ignimbrites were ideal to alter the deposits when they were still hot. In
dry parts of the ignimbrites, when temperatures were hot and when
water was pressured outwards, which is typically the case in the basal
part of thick ignimbrites and in the fine-grained layers below ignimbrites, minerals which are typical for devitrification at low H20 contents,
such as quartz, albite and pumpellyite could be formed. At lower
temperatures — below 200 °C and higher H2O pressures, zeolites could
117
be formed. Alteration by overpressured steam rather than water leads
to very high dissolution rates and thus high supersaturation of dissolved
constituents, which results in the formation of metastable zeolite
phases (e.g. mordenite), which were replaced by more stable phases
(e.g. laumontite), given that the temperature could be maintained for
long enough for progressive reactions to be able to occur. Progressive
cooling from the top of the deposits downwards can also explain the
gradual increase of mordenite replacement by other phases, which is
typically seen from top to base of the sequences.
8. Conclusions
Zeolites are common throughout the entire stratigraphical thickness
of the Late Cretaceous Cayo Formation and over almost its entire outcrop area. Although zeolitisation in marine volcanic arc environments
is generally explained by burial metamorphism, which causes prograde
reaction of early formed phases to less hydrous, more stable phases at
greater burial depth, only limited burial seems to have affected the sequences in the Cayo arc and alteration is explained mainly by interaction of hot volcanic glass with seawater and/or diagenesis of already
cooled glass. Although pyroclastic deposition occurred near the centres
of volcanic eruption, the influence of an increased geothermal gradient
or hydrothermalism was also not the main driving force altering the
pyroclastic sequences. The influence of these volcanic centres seems
to be only locally, as in the intrusive zone in the northern Guayaquil
area, or in the close proximity (metres) of volcanic eruptive centres or
faults.
A clear difference in zeolite type and content is observed between
the lower and upper units of the Cayo Formation, because of a clear
difference in volcanism between the two units. In the upper unit, a
Ca-HEU-type zeolite is the main zeolite type. A smectite-rich C/S
mixed layer clay mineral is the major associated alteration mineral,
while quartz contents are relatively low. The upper unit is mainly
build up of volcanic debris flows, directly or indirectly derived from andesitic volcanic eruptions. Deposits were water-saturated and mostly
cooled down or at low temperature at deposition. Alteration was dominated by diagenetic palagonitisation and as the sequences were never
buried, diagenetic mineral was not converted to less hydrous more stable phases. The lower unit of the Cayo formation is mainly build up of
dacitic pyroclastic flow deposits, of which the alteration mineralogy is
greatly influenced by the eruptive, depositional and post-depositional
processes in the pyroclastic flow deposits, which determine the water
Fig. 8. Schematic representation of zeolitisation in marine geoautoclaves. MOR: mordenite, CEL: celanonite, HEU: HEU-type zeolites, STI: stilbite, C/S: smectite/chlorite, LAU: laumontite,
ALB: albite, CHL: chlorite.
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L. Machiels et al. / Applied Clay Science 87 (2014) 108–119
interaction, cooling and alteration of the deposits. The marine water column has a double function, as it delivers sufficient water for alteration
reactions to occur, but its hydrostatic pressure also prevents steam
pressurized in the pyroclastic flow deposits from escaping from the pyroclastic sequences and thereby promotes vapour phase alteration reactions in the sequences. In thick pyroclastic deposits, decreasing
seawater interaction and decreasing cooling rate towards the base of
the sequences result in progressive replacement of initially formed
mordenite by heulandite, laumontite and albite. In dry-low permeability volcanic sequences, albite, quartz and pumpellyite were formed in
the cooling sequences. In thinner, more water saturated pyroclastic
deposits, alteration is limited to mordenite and HEU-type zeolite formation and no progressive conversion reactions occur, or hydrothermal
alteration is halted and diagenetic alteration occurs. When pyroclastic
sequences are nearly cooled down and water saturated at deposition,
alteration resembles the diagenetic HEU-type zeolite–smectite alteration of interbedded epiclastic beds.
Acknowledgements
This research has been conducted at the Applied Geology and Mineralogy research group of the Katholieke Universiteit Leuven (Belgium)
in close cooperation with the Centro de Investigación y Proyectos
Aplicados a las Ciencias de la Tierra and the Facultad de Ingeniería en
Mecánica y Ciencias de la Producción of the Escuela Superior Politécnica
del Litoral of Ecuador. We thank Herman Nijs (Katholieke Universiteit
Leuven) for the careful preparation of the thin sections, Jacques Wautier
(Université Catholique de Louvain) for the EPMA analyses and Veerle
Bams and Pascal de Plus (Wetenschappelijk en Technisch Centrum
voor het Bouwbedrijf) for the help with the SEM analyses. We thank
the Vlaamse Interuniversitaire Raad for the financial support. We
would like to dedicate this article to the memory of Martha Ordoñez,
great palaeontologist and friend.
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