1. Fluid Exchange Between a
Subducting Slab and the Mantle
Wedge in the Guatemala Suture
Zone
By Shafieul Alam,Connor McGrath, and Joanna Schneider
Under the direction of Dr. Celine Martin

2. Abstract
This research focuses on the fluid exchange between the subducting slab in the Guatemala Suture Zone
and the mantle wedge, specifically in the area focusing on the serpentinite mélange. The goal of this
research was to obtain a more detailed understanding of the chemical exchange between a subducting
slab and the mantle wedge by analyzing major and trace element composition of three different samples.
The schist and jadeitite samples represents the chemical makeup of the jadeite veins within the mantle
wedge, and the eclogite sample is representative of the subducting slab’s composition. Experiments
focused on the zoning of mica and the differences in composition of these zones. The zones in the mica
present a chemical snapshot of the fluids that hydrate or crystalize the rock. Over the course of this
project, the methods we used to find and analyze zoning in mica included the use of optical microscope,
electron probe microanalysis, and laser ablation. This data was then organized using Excel® to chart the
data. Using this synthesized information, a history of each rock was created. In the jadeitite and schist
samples, zones were present. However, in the eclogite sample, there was no evidence of zonation,
leading to the conclusion that the eclogite sample was a relatively primitive sample as compared with the
schist and jadeitite which both had three zones each.
Introduction
One of the most important questions in geochemistry today involves the chemical exchange between a
subducting slab and the mantle. Scientists do not yet understand how the chemicals that are entering the
Earth’s mantle at subduction zones are being recycled. These chemicals transported by fluids below
Earth’s crust, and as the circulation of that fluids is recorded in mica crystals within rocks by the time
they are recycled to Earth’s surface. Geologists understand the evolution of Earth’s chemistry before the
slab enters the mantle and they know the chemistry of the rocks emitted in arc volcanoes , but it is

3. p
unclear how it is recycled while the slab is in contact with the mantle. The chemical interaction before the
slab enters the mantle can be understood by analyzing the rocks present in the oceanic crust and the
chemical evolution after the slab leaves the mantle is studied by analyzing volcanic rocks from arc
volcanoes (e.g. Tatsumi, 2005). A possible way for geologists to study the interaction of the mantle and
the slab is by studying the serpentinite mélange, which contains three main components: hydrated mantle
(serpentinite), high pressure rocks (coming from the slab), and jadeitite veins (representing the fluid
circulations) (e.g. Simons et al., 2010; Harlow et al., 2011). An image of a subduction and serpentinite
mélange in figure(g-h).
Understanding these chemical exchanges could hel
to better explain the cause of natural phenomena
such as the formation of volcanoes and volcanic
rings around subduction zones, as well as the
evolution of the mantle chemistry. This study will
add information to the broad body of information
regarding this topic. Thus, a better conclusion can
be drawn from all the information, providing a
better understanding of how the dynamic Earth
operates. The increase of pressure (P) and temperature (T) during the down going path of the slab
triggers the devolatilization of the slab and the release of aqueous fluids into the mantle wedge (e.g.
Bebout et al., 2007). The chemical exchanges that this study elaborates on occur when fluids are
transferred from the subducting slab to the mantle wedge. The hydration of the mantle wedge induces
the formation of serpentinite and the fluid released crystallizes into the jadeite veins. This causes the

4. e
composition of the veins to change, depending upon the chemicals that were expelled by the fluid. Due
to the extremely high temperature and pressure, scientist cannot directly study the chemical exchanges
between the subducting slab and the mantle wedge. Scientist must study rocks that have been exhumed
from ± 40 to 80 km under the Earth’s surface to understand the chemical interactions that take place
below the crust. During exhumation, tectonic processes will mix together the serpentinite (i.e. the mantle
wedge), the jadeitites (i.e. veins), and the eclogites or blueschist (i.e. the slab). This mix is called
serpentinite mélange, and it is a useful proxy in understanding what happens deep below the surface.
Our study is focused on zoning in mica. Mica is a hydrous mineral, commonly formed during fluid-rock
interactions. Successive fluid pulses during mica crystallization could lead to the formation of zones, with
a different chemistry (major or trace elements, or both). Studying these zones provides insight into the
different fluid events that occur within different areas of the subduction zone, or at different time of the
subduction process. Zones of mica crystallize around one another with the same elements that are
released from slab, creating a chemical “snapshot”. In addition, mica is a good carrier of lithium and
boron, which are both very fluid mobile elements due to their small size. These fluid-rock interactions
will help scientists to better understand volcanism related to other subduction zones.
Geological Background
The Guatemala Suture Zone (GSZ) is located in the
Motagua fault system on the North American and
Caribbean plate boundary. In recent years, it has
become clear that the Guatemala Suture Zone
formed differently than the suggested theory that th
zone was formed by a single progressive collision between an island arc related to the Chortis Block

5. g
s
ng
and the Maya Block suggested by Beccaluva et al. (1995). Recent studies, by Kennet Flores et al
(2012), suggest a multistep mechanism in which the first tectonic event was a collision causing the layer
to gain more material as it rose. After a period without tectonic tension, a rift created on the ocean floor
resulted in the exhumation of oceanic crust. As a result of this, the continental high pressure, low
temperature rock was forced up. A shift to the left created the Guatemala Suture Zone, as it is known
today. Figure(e-d) is a map of the GSZ.
Methods
In order to gather data that would better shape ideas about the history of the rock samples, the samples
were first studied by optical microscope. Later experiments required microprobe analysis and laser
ablation ion coupled plasma mass spectrometer (LA-ICP-MS) analysis. Both the microscope and
microprobe analysis were completed at the American Museum of Natural History, while the
LA-ICP-MS analysis was done at Lamont Doherty Earth Observatory (Columbia University). Using all
of these methods, it was possible to better understand the history of the jadeitite, eclogite, and schist.
Optical Microscope:
There are several important steps in identifying mica in
these rocks. The first most basic step involves identifyin
mica, by rotating the sample under polarized light and
watching for parts of the rock to change colors from
extremely vibrant colors to black. It turns black when it i
perpendicular or parallel to the plane, and appears bright
when it is rotated at a 45º angle. The next step used to
identify mica is trying to observe lines of cleavage runni

6. e
parallel to the maximum length through the mineral. These cleavages can be seen more clearly under
non­polarized light; that in combination with the transformation from extremely vibrant colors to black
conclusively proves that mineral is a mica. Once a mica is found rotating the thin section, under polarized
light, it to see if parts within the mica zone become darker or more vibrant while other parts are not
signals that there is a zone within that mica. To confirm this finding, just like in previous steps, the mica is
placed under non-polarized light and checked for cleavages that are not running with the other horizontal
lines of cleavage in the area showing that there is a zone of mica. Once areas that are believed to have
zoning are found, the next step is the Microprobe analysis.
Microprobe:
The microprobe (Cameca SX100), EMPA(electron
microprobe analyzer), helped to detail the chemical
makeup of mica and allowed for further analysis of
major and some minor element composition, as seen
in figure (f-d). The microprobe was used to gather
information on the concentrations of elements that ar
highly abundant in the rocks such as Si, Al, K, etc.
The microprobe cannot be used to measure elements
that are present in low amounts, general anything less
than 0.02 wt%. The X-ray maps acquired on the electron probe can be displayed using XMapTools
software (Lanari et al., 2014). Using this representation, it can be determined if there are any major
element zones present in the sample by checking if the levels of a certain major element or highly
abundant minor element varies. Afterwards we analyze the levels of Si present in each zone. If they

7. he
vary, it reveals an estimation of the P at which the rock crystallized. This is can be concluded because as
the pressure drops so does the level of silicon present in the mineral. However the drop in pressure isn't
always related to the drop in silicon. Due to an occurrence called the Ba-Si exchange, Si levels may
drop because it is being swapped out for Ba.
Laser Ablation:
Trace-element contents (Li, B, Ba, Cr, Ni, etc) have been measured in situ using LA- ICP-MS. This
system couples an ESI New Wave UP-193-FX ArF* (193 nm) excimer laser ablation microscope with
a PQ ExCell (Thermo Scientific) ICP-MS. Traverses are performed in mica (Fig. ) and the elements are
measured simultaneously by the ICP-MS. The detection limits for these elements are a few ppm. When
several zones have been observed on X-ray maps, traverses are performed in each zone. The highly
fluid mobile element (B, Li, Ba, Cs) will then be plotted two by two. If they were present in the same
fluid, they will be covariant (Fig. ), if not, no clear trend will be defined (Fig. ). Additionally, the content
of Cr and Ni of the different samples, and of the different zones of a given are tracked. Indeed, an
increase of Ni and/or Cr content would indicate a mantle circulation of the fluid.
Results
Eclogite: The sample of eclogite taken from the Guatemala
Suture Zone came from the metamorphosed subducting slab, as
opposed to the samples of jadeitite and schist, which were both
taken from the vein rock. It exhibited no signs of mica zones in t
optical microscope, the electron probe microanalysis maps, or in
the data obtained from laser ablation. Under the optical
microscope, it was apparent that there were parts of the sample which contained mica, as sections

8. d
dimmed and brightened when viewed at different angles. The
regions of this sample containing mica brightened and dimme
at the aforementioned angles; however, they did so consistently,
suggesting that these regions of mica were formed at the same
time and do not represent different zones (Fig. 5-1). The
original hypothesis based on this was that the eclogite did not
have contact with any fluids because the microscope analysis
suggested a lack of zoning. Looking at the electron probe
microanalysis maps, the eclogite sample revealed general
uniformity in the concentrations of major elements (Fig. 5-2). The concentrations of these major
elements remains consistent across the sample, suggesting that no fluid interactions occurred. Uniform
element concentration indicates that minerals crystallized at roughly the same time, so no zoning is

9. present. Additionally, after plotting the concentrations of all trace elements against one another, they did
not show any point clustering on the graphs, as represented by the graph of Ba versus Cs (Fig. 5­3);
clusters of points would indicate zones where the concentrations of elements were correlated, suggesting
distinct fluid events, but most of the graphs showed points that were scattered across graphs without

10. n
a
any indication of zoning (Fig. 5-4). It was difficult to gain any conclusive evidence from trace element
composition of this sample because fewer data points were collected. The concentrations were only
recorded for four data points, and while these graphs seemed to agree with the those that plotted the K
and Ba concentrations against one another, it cannot be conclusively determined that trace element
composition proved the lack of zoning. However, another indicator of the primitive nature of this sample
is the B content. The boron content was < 50 ppm across the whole sample. Boron is a light element
and is usually indicative of fluid exchange between the slab and the mantle due to its mobility. The low
concentration of boron across the sample is also an indication that there was no fluid exchange and
therefore no mica zoning. This coupled with the strong evidence from the major element composition
supports the hypothesis that the sample lacks mica zoning that accompanies a fluid exchange or
circulation.
Jadeitite: The optical microscope and
electron probe microanalysis showed
evidence of two zones in the mica see
in the Figure(b-c) and Figure(d-b),
respectively. The laser ablation data
indicate three zones, illustrated in
figure(a-b) when plotting B against B .
On the optical microscope image (Fig.)
the yellow area and the pink area show two different zones. In the EMPA maps, the orange area, of
Mg, shows one zone and the dark red area, of Mg, shows a different zone. In the laser ablation graph,

11. s
u d
the three zones are depicted through difference in
concentration of the compared elements. The first zone ha
low concentrations of B and Ba, 20- 40ppm and
2000-4000ppm respectively. The second has medium
concentrations of 50 ppm- 80 ppm and 6000 to
10000ppm, and the third has high concentrations, with 80
ppm- 90ppm of B and 11,000-13,5000 ppm of Ba.
Schist:
From the Microprobe and Laser Ablation data three
zones are clearly defined. Figure(d-a) shows the three
zones as a representation of the three different amounts
of Barium present in the mica sample. The Microprobe
shows that there is a low Ba concentration, indicated by
the red circle. In addition there is a medium Ba
concentration, indicated by the blue circle. There is also a
high concentration of Ba in the sample, indicated by the
green circle. These varying concentrations are
evidence of three zones epresent in the mica
crystal. Further evidence that proves that there are
three zones is in the Laser ablation data. When
graphing the fluid mobile element B against the fl i

12. mobile Ba, three distinct groups could be seen as represented in figure(d-b). The first zone has a low
concentration of Boron and Barium , 20-45 ppm and 500-1500 ppm respectively. The second zone
has a medium relatively medium concentration of Boron and Barium, 40-60 ppm and 2000- 3500 ppm
respectively. The third zone has a high concentration of Boron and Barium, 80-130 ppm and
6000-9000 ppm respectively. The microprobe and laser ablation data reveal that there are three zones
in the mica crystal found on the schist.
Discussion
General:
From comparing all the samples (jadeitite, schist, and eclogite) it can be concluded that there were no
fluids shared by these three samples. The lack of any zones or grouping within the EMPA maps and
laser ablation in the eclogite clearly shows the sample was not hydrated by any fluids, and most
definitely was not hydrated by any of those affecting the other samples. This is not unusual for the
eclogite because the sample comes from the subducting slab from which the fluid was released (Bebout
et al., 2007). The jadeitite and schist did not share fluid from any of the same releases either. This can
be concluded by comparing the boron and barium, if they is no overlapping concentrations in the
samples that means there was no fluid shared between the two samples, levels within the jadeitite and
the schist samples: jadeitite’s zones in boron (20­ 40ppm, 50 ppm­ 80 ppm, 80 ppm­ 90ppm) and in
barium (2000-4000ppm, 6000 to 10000ppm, 11,000-13,5000) and schists zones in boron(20-45
ppm, 40-60 ppm, 80-130 ppm) and in barium ( 500-1500ppm, 2000- 3500 ppm, 6000-9000 ppm).
Since none of the zones levels of boron and barium lineup exactly or are simple enough to possibly have
been crystallized by the same fluid, it can be concluded that they did not have contact with the same
fluids. None of the sample had any amounts of Ni and/or Cr this implies that none of the leached fluid

13. came from the mantle. This is because Ni and Cr are somewhat abundant in the mantle rock (i.e.
peridotite, e.g. Mercy and O’Hara, 1967) and if any fluid has leached the mantle rocks, there would
have been some Ni and Cr in the samples.
Eclogite:
The general uniformity displayed on all of the EPMA maps as well as the lack of correlation on the
graphs plotting the trace element concentration against one another do not support the existence of
zones in this sample. There is no grouping within the laser ablation graphs plotting K against Ba, and the
graphs of trace elements suggest a similar trend. In building the history of this sample, it can be inferred
that the sample was not hydrated by any fluids, and as a consequence, was not hydrated by any of
those affecting the other samples. From this, it can be concluded that this eclogite was “primitive” in
comparison with schist and jadeitite.
Jadeitite:
The result of having ‘different’ number of zones in the same sample is interesting. There are a few likely
possible reasons for this result: human error, area sample error, and trace element fluid crystallization.
The human error factor hypothesis is that we, as humans, do not see the small variation in the
microscope or EPMA maps leading us to the false conclusions about the two zones. The sample area
error hypothesis states that the microscope, EMPA, and laser ablation were not taken from the same
area each other, leading to a false conclusion on the number of zones. The trace element fluid hypothesis
is the most likely, it states there was three different fluids released from the subducting slab. It continues
to state that two of the three fluid releases were nearly identical in the amount of major elements they
contained, though differ in the amounts trace elements. This very neatly explains why the third zones
were only visible on the trace elements graphs gathered by the laser ablation analysis.

14. Schist:
Looking at the data it can be seen that the mica has three distinct zones meaning three fluid events or
fluid expulsions occurred by the subducting slab into the mantle. These three fluids both varied in major
element composition and trace element composition. This can be seen by the three varieties present in the
electron microprobe data and the three varieties present in the laser ablation data.
Conclusion
From major and trace element composition analysis of samples of eclogite, jadeitite, and schist, the
history of each rock can be built. Zoning acts as a form of indirect evidence that allows for these
conclusions to be drawn. Extrapolating from the data, it can be inferred that the eclogite is in the most
primitive state of the three samples. This is further proven by its dearth of fluid mobile elements.
Additionally, this study concludes that the jadeitite and schist samples were both crystallized from fluids
successively released from the subducting slab during three different events. It is important to conduct
this research in order to understand the chemical exchanges between a subducting slab and the mantle.
It furthers scientists’ knowledge of volcanism in subduction zones and generates a fuller comprehension
of the Earth’s crust and mantle.

15. Bibliography
"Electron Probe Micro-analysis (EPMA)." Natural History Museum. Natural History Museum, n.d.
Web. 18 June 2014.
<http://www.nhm.ac.uk/research-curation/science-facilities/analytical-imaging/micro-analysis/sx100/inde
x.html>
Flores, Kennert. "GEODYNAMIC EVOLUTION AND TECTONIC HISTORY OF THE
OPHIOLITES AND SERPENTINITE MÉLANGES IN THE GUATEMALA SUTURE ZONE." .
N.p., 21 May 2013. Web. . <https://gsa.confex.com/gsa/2013CD/webprogram/Paper219570.html>.
Reyes-Flores K., Brocard G. Y., Harlow G. E., (2013), Continent-Continent Suturing II, Abstract
T13F-03 presented at 2013 Fall Meeting, AGU, San Francisco, Calif., 9-13 Dec.
Robinson, Philip, Savile Bradbury, Mortimer Abramowitz, and Michael Davidson. "Polarized Light
Microscopy." . N.p., 1 Aug. 2003. Web. .
<http://micro.magnet.fsu.edu/primer/techniques/polarized/configuration.html>.
"Microprobe." Microprobe. Http://www.open.ac.uk/, n.d. Web. 19 June 2014.
<http://www.open.ac.uk/earth-research/tindle/AGT/AGT_Home_2010/Microprobe.html>.
"." . N.p., n.d. Web. 19 June 2014.
<http://www.geo.cornell.edu/eas/education/course/descr/EAS302/06Lectures/302_06Lecture29.pdf>.
"Solving a Geological Puzzle in Guatemala." YouTube. YouTube, n.d. Web. 19 June 2014.
<https://www.youtube.com/watch?v=TWofncaF2Ds>.