1. Hydrothermal Alteration of the Mantle Wedge through Subduction
during the Taconic Orogeny
Bailey Flood
Abstract
The serpentinized ultramafic belt in Eastern Vermont, originates from mantle that has
been altered and then uplifted to the surface. This belt is largely composed of antigorite and
opaques. By looking at these minerals in thin section through both reflected and crossed –
polarized light, evidence for hydrothermal processes is found. This is due to the high silica
content of the water that infiltrated the mantle wedge, the complete alteration of the parent rock
and lastly to the alteration of the opaques within the samples themselves. These pieces of
evidence in the thin sections not only confirm the process of subduction in the area, but they also
point to the hydrothermal processes of the region.
Introduction
During the Late Ordovician, an island arc
collided with proto – North America and created
the Taconic mountain range (Bradley and Kidd,
1991). This collision created vast deformation and
introduced rocks that are not native to the area,
which includes modern day New York, Vermont
and New Hampshire. One of the rock types that
became introduced into Vermont are serpentinized
ultramafics. These rocks are developed in the
suture zone of mountain building and if found
unaltered can give a glimpse into the mantle. These
rocks can be used as an indication of subduction in
the area and also of the conditions that were
experienced by the mantle in retrograde
metamorphism. While there is no agreement
among scientists on how these rocks have formed
or been deposited in the area, by looking at new samples from the serpentinized ultramafic belt
Fig. 1 shows the serpentinized ultramafics in
Vermont and the structure that they follow through
the state. (Wigglesworth, 1915)

2. and comparing them to other convergent boundaries throughout the world, a history can be
determined.
Geologic Setting
The serpentinized ultramafics in Vermont are part of a larger belt that runs north – south
from Canada to Alabama. (Wigglesworth, 1915). This band runs parallel to the Appalachian
Mountains and is found on the eastern side of the Green Mountains in Vermont. (Wigglesworth,
1915). Historically there have only been a few localities; Barnes Hill, Waterbury mine and Mad
River, that have been analyzed. These rocks while not wholly serpentinized were described and
analyzed and can give an idea of the types of rocks that have typically been found in the area.
(Chidester, 1962). Fig. 1 shows the serpentinized ultramafics in Vermont, while there are only
seventeen localities shown on the map, connecting them will give an idea as to where the layer
used to lay before erosion.
Outcrop
While the map shown above is from almost a century ago, less outcrops are presumed to
be exposed due to the urbanization. The two
samples that will be analyzed in this paper were
collected along this serpentinized ultramafic belt.
The first sample, VT-13-002, was collected at
William's Quarry along Rt. 103. While there was
an abundance of different lithologies at this site,
there was also serpentinized ultramafics in the
area. The next sample, VT-13-003, was taken off
of Rt. 103, further east and toward New
Hampshire. Fig. 2 shows an approximate location
of the samples that were collected. This outcrop
was on the side of the road where there was
massive formations of the serpentinized
ultramafics. The outcrop consisted of boulders of
the material that were covered with moss and forest deposits. Finding a sample that was clean
and preserved the mineral assemblages inside the rock.
Fig. 2 shows the approximate location of the
serpentinized ultramafics in Chester VT. The area in
the circle is the location of the samples. (Chester
Innkeeper Association, 2008)

3. Experiment
Petrology
Hand Samples
By first looking at the hand samples, it is clear that there are varieties in the samples that
were collected. First, sample VT-13-002 has a light green – gray rind that contains a pale blue
mass with black inclusions. The rock itself is very dense, which suggests heavier cations e.g. Mg,
Fe. Along with the denseness of the rock there seems to be no pore spaces in the rock. There are
small veins in the rock that are due to weathering and the infiltration of fluids. These veins are
not as important as the inclusions and minerals in the rest of the rock. On the other hand sample
VT-13-003, has a brown – green rind that contains a dark blue – green mass, also with black
inclusions. This sample has less inclusions than the other sample, they are also smaller than the
other sample. This sample is also strongly magnetic, suggesting the presence of magnetite.
Thin Section
While looking at the hand sample can be helpful in determining the context of the rock
compared to the outcrop, closer investigation is needed. In this case two thin sections were made,
one for each sample that was collected. Looking at these thin sections can tell what kind of
conditions the rocks experienced. Even
though the thin section can show the minerals
in a small section of the rock, the middle of
sample VT-13-002 could have a different
mineralogy, since it could have resisted the
serpentinization. While the two rocks are
different colors, they both contain the same
minerals which composes the majority of the
sample. Determining what serpentine mineral
is in the sample was difficult due to the size
of the crystals that are in the thin section. Previous research, point – counter analysis, done on the
rocks in the area identifies the serpentine mineral as antigorite. (Chidester, 1962) By comparing
what was seen and described by other scientists, an identification can be made. Fig. 3 shows the
presence of antigorite in the sample and while this is a picture of sample VT-13-002, both
Fig. 3 40x Magnification. Crossed - Polarized Light. VT-13-
002. Antigorite in thin section. (Flood, 2013)

4. samples showed the same minerals with the same habits.
Based on the presence of antigorite in the sample, a graph using the pressure and
temperature conditions can be applied
to the retrograde path that the rock
took. Fig. 4 shows an experimental
graph based on Boron isotope data in
hydrated mantle. This graph shows
the possible pressure and temperature
conditions that rocks in similar
subduction zones have experienced.
While this graph is not based on data
from the Taconic region, pressures
and temperatures similar to the graph
can be expected. As shown on the
graph the temperature ranges from
around 500ºC to 700ºC and the pressure ranges from
0 to 3 GPa. While a range of temperatures and
pressures can be assumed for the reaction, these
assumptions cannot give accurate conditions for the
reaction observed from the samples taken. Only by
knowing the parent rock can the conditions be
further constrained to understand the lineage of the
samples.
Determining what the parent rock of the rock
consisted of, helps understand the path the rock
took. In order to this, certain textures can be located
and examined to determine the original mineralogy
of the samples. Reflected light produced the relict
textures and from these textures, the original
mineral structures are produced. Fig. 5 shows an
original grain boundary of an olivine grain. These
Fig. 5 40x magnification. Reflected light. Olivine
grain relic. VT-13-003. (Flood, 2013)
Fig.4
Olivine to Antigorite P-T diagram, showing the experimental
conditions that of the hydration of the mantle. (Scambelluri and
Tonarini, 2012)

5. grain relics are found all over both thin sections and point to the original mineralogy. Comparing
this grain boundary with other olivine crystals and with other work that has been done in the
area, the data suggests that the parent rock of the samples was largely comprised of olivine and
that the parent rock was almost completely dunite. (Wigglesworth, 1915). This thought is
confirmed both in the thin sections that were made and with other research that was done in the
area. (Chidester, 1962). The dunite in the samples collected was almost completely comprised of
forsterite due to the high magnesium content of the antigorite. (Wigglesworth, 1915). All of the
serpentinized ultramafic rocks share this high magnesium content, which means that the dunite is
from the upper mantle. These finding can further narrow down the pressure – temperature
conditions that the rock experienced.
Fluids
Figuring out how fluids have infiltrated the mantle helps into understanding the
retrograde path of metamorphism that occurred during the Taconic Orogeny. Looking at the
retrograde path will give an idea as to the tectonic implications of the region. Now that the parent
rock is known for the samples that have been collected, evidence of fluids can be found. There
are many forms of evidence for fluid infiltration in the samples that were collected, mainly
showing themselves in the thin sections that were made. There are two main pieces of evidence
that were found to support the hydration of the mantle in the Taconic Orogeny; 1. The process of
serpentinization and 2. The presence of rust in thin section.
Serpentinization
This is probably the largest, easily recognized, piece of evidence in the thin section due to
the fact that the majority of the thin section is comprised of serpentine. Serpentinization is a
process where fluids are introduced to ultramafic rocks and alters them into serpentine. Since the
samples are now fully serpentine and as previously mentioned started off as dunite, it can be
gathered that the sample must have experienced serpentinization. Also the fact that the samples
are completely serpentine means that the rocks must have experienced the presence of fluids for
a long period of time. If more thin sections were to be made of the center of the samples that
were collected, it would be seen that the bulk of the rock, if not all of the rock is serpentine.
(Wigglesworth, 1915)

6. Presence of Rust
Another important evidence for
fluids is the presence of rust in the thin
sections that were made. The type of
rust that is in the samples cannot be
determined based on simple
observations, so chemical analysis
would need to be done in order to
determine which cation present. The
rust that is shown in Fig. 6 is not in the
form of a crystal, but is in fact staining
the minerals around it. If the figure is closely looked at the habit of the serpentine crystals can be
seen through the rust. This rust formed during the introduction of fluids or other volatiles and
there for is evidence of infiltration. While this would not be evidence if the rust was toward the
edge of the sample, because it is seen toward the center of the sample, it gives further weight to
the extent of the fluid infiltration.
Zoning in Opaques
One of the interesting things that was seen in the thin sections was the distinct zoning that
the opaque mineral had in reflected light. This zoning while only shown in one example above,
was present in every opaque in the thin sections. The thicknesses of the zones may have been
Fig. 6 40x magnification. Crossed - Polar Light. VT-13-003
Evidence of Rust in the Samples (Flood, 2013)
Fig. 7 40x magnification. Plain Polarized. VT-13-003.
Opaque mineral (Flood, 2013)
Fig. 8 40x magnification. Reflected light. VT-13-003
Opaque mineral with three distinct and one non - distinct
zone. (Flood, 2013)

7. different, but there were still three very distinct zones and one that was harder to see. Fig. 7 and 8
show the same grain in plain light and then in reflected light in order to show the zones, which
include a gray zone around the entire grain. While this zoning does not appear to have been
documented in the Taconic region of Vermont, it has been documented in other convergent
margins with subduction zones. Similar mineral zoning has been seen in Egypt. While the zoning
is not exactly the same, grade of reflectivity, the work done in this region can be used as a guide
for what occurred in Vermont.
Egypt
In Egypt opaques minerals were found in serpentinites, which had been produced due to a
thrust sheet. When the minerals were placed under reflected light distinct zoning was shown that
could be analyzed under a microprobe to find the ratio of three cations. (Kahlil and El-Makky,
2009) What they found was that the mineral was changing cation content from the core, original
chromite, to the rim, altered chromite to magnetite. The different zones in the minerals that the
scientists looked at was due to recrystallization during deformation and after the alteration. The
scientists found that the grains are increasing in Fe moving from the center of the grain to the rim
and that the zoning is primarily produced by alteration of the grains. (Kahlil and El-Makky,
2009) By comparing the zoning that was seen in the samples collected in Vermont to the zones
established in Egypt, hypotheses can be made. 1. The chromite center has been completely
altered, but the center of the grains in Vermont might still contain Cr. 2. The alteration process in
Vermont was significantly longer than in Egypt due to the large rim of magnetite. 3. The
magnetite must have developed during the alteration process since the boundaries between the
zones are distinct, not gradual. All of these will need to be tested using a microprobe in order to
determine the cation content of the zones.
Discussion
Evidence of Hydrothermal Altering
Since it has been established that there are fluids in the system and that the reaction from
olivine to antigorite occurs at 500ºC to 700ºC, that there are not liquids infiltrating the system.
The water that has been introduced into the system is more likely vapors that still alter the dunite
into antigorite and create the extent of alteration that is seen in the rocks. Since water vapor is
smaller than liquid water, it would be easier for the gas to travel throughout the mantle and

8. completely alter it.
Hydration Reaction
Since the composition of the parent rock and of the samples collected are known, a
hydration reaction can be found. This reaction is given as:
olivine+ Quartz+ Water ⇔ antigorite Or
3(Mg , Fe)2 SiO4+ SiO2+ 4H2O⇔2(Mg , Fe)3 Si2 O5(OH )4
The hydration reaction shows that there needs to be quartz in the system in order for antigorite to
form, but since the parent rock is dunite, there is no quartz present. So the quartz that is needed
must come from the fluids that are being introduced to the system. Meaning that the temperatures
of the fluids needed to be very high in order for the quartz to be dissolved into the fluids. Since
the fluids contain quartz and could not have originated from the mantle, they must have come
from the subducting plate.
Subduction
During the Taconic Orogeny proto – North America was subducting underneath the
Taconic Island arc. During subduction the hydrous phase of minerals lose their water and it
infiltrates the rock above, since water wants to move upwards. This loss of water in the
subducting pate, in the case of the Taconic Orogeny, caused the hydration of the mantle wedge.
While this explains the presence of water in the samples collected, it does not wholly explain the
extent of the serpentinization seen. The amount of water coming from the subducting plate must
have been very high since massive alteration occurred. The angle that the plate was subducting
could also be a factor of the alteration of the dunite. If the angle was shallow, then more of the
subducting plate would have been in contact with the mantle, therefore causing more water to
infiltrate. Speed of the subduction also could affect the amount of water being introduced into the
system. A slow subduction could aid the alteration process by further adding to the water in the
system. In order to figure out these factors more investigation would need to be done in the area.
SiO2 Content
While subduction explains the water that would be needed to alter the dunite of the
mantle, quartz is still needed in the system to alter the dunite to antigorite. Since there is a

9. massive quantity of antigorite in the serpentinized ultramafic belt, which is all mineralogically
identical across the region (Wigglesworth, 1915) there had to have been a consistent supply of
quartz available during alteration. This quartz could have been supplied from the oceanic crust
being subducted or possibly from continental sediments being dragged down to mantle depths.
Water could have then facilitated the transport of the silica – rich sediments into the dunite and
aided in the transformation to antigorite.
Further Research
In order to fully and more completely look at the history that the serpentinized ultramafic
rocks in Vermont more work in the field would need to be done and also more samples from
different localities would need to be taken. While it is not necessary based on the research done
by other scientists, it would help in confirming that all of the rocks had experienced similar
conditions and are comprised of the same material. Also geochemical analysis would aid in the
confirmation of the proposed subduction hypotheses and confirming the history of deformation
based on the zoning in opaques.
Conclusion
While looking at the thin sections that were made from the two samples collected along
the serpentinized ultramafic belt in Vermont, certain evidence became clear that described the
tectonic environment the samples experienced. This included hydrothermal altering and
subduction. The combination of these two environments can encompass the entirety of the
evidence that was seen in the thin sections. The evidence that was found could only be explained
by high temperature and large quantities of fluids. The hydration of the parent rock, dunite,
would have needed to occur in the mantle during the collision event. This leads to the idea that
the water that seems to be present everywhere in the thin sections was vapor. This vapor would
have aided to the complete transformation from dunite to antigorite that is seen. Also the zoning
of the opaques only could have occurred if heat was added to the system, causing them to
recrystallize with a different chemical formula. This rim also can give a relative amount of time
that the alteration took, since recrystallization takes a while to form, requiring sufficient heat and
the amount of magnetite around the rims, points to a long period of heating. From looking at
research done by other scientists, the only explanation is that the dunite was indeed altered
hydrothermally.

10. Acknowledgments
I would like to thank Jolene Fitch for helping with the creation of the thin sections used and to
the Earth Science department for having the necessary equipment needed open to majors.
References
Bradley, D.C. And Kidd, W.S.F, 1991, Flexural extension of the upper continental crust in
collisional foredeeps: Geological Society of America Bulletin, v. 103, p. 1416 – 1438
Wigglesworth, E, 1915, The serpentines of Vermont, Proceedings of the Boston Society of
Natural History, v. 35, p. 95 – 107
Chidester, A.H., 1962 Petrology and Geochemistry of Selected Talc – bearing Ultramafic Rocks
and Adjacent Country Rocks in North – Central Vermont, United States Geological Survey,
Geological Survey Professional Paper 345
Kahlil, K.I, El-Makky, A.M., 2009, Alteration Mechanisms of Chromian – Spinel During
Serpentinization at Wadi Sifein Area, Eastern Desert, Egypt, Resource Geology, v. 59, n. 2, p.
194 - 211