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2. Introduction
The study of heavy minerals, and in particular the study of garnets using Raman
spectroscopy, is an appropriate technique for provenance studies and palaeoenvironmental
investigations in natural climate-tectonic sedimentary archives.
The thesis is born in the world of sedimentary petrology, and it has, first, the aim to valorise
the Raman spectroscopy for the study of sediments and garnets as the best way to represent
the source rocks and as an established method for provenance studies (Morton, 1985). This
modern tool has shown to be very practical thanks to its velocity to get data back, as there is
no need to prepare the sample before its use. It is very important to specify that this tool does
not want to replace the use of the optical microscope, which is always the best way where to
start every study from, but it has to be considered as an implemented device to identify the
chemistry of these common detrital minerals. For this reason, it is very helpful to combine
all the available techniques in a such complex orogenic setting and study. There is not any
well-done study with the use of just one tool: Raman spectroscopy is very efficient for a
quantitative approach and it has the power to identify more clearly some minerals with
similar optical properties that sometimes could result difficult to recognise and could be
easily confused with the only use of an optical microscope. It is also used in the case of
minerals which present solid solutions: this is the studied case, which uses garnets due to
this mineral is a solid solution which consists of six main end members. Garnet is selected
for this study for different reasons. It is very easy to be recognized thanks to its isometric
property. Also, it is a very common mineral and one of the more known: from igneous and
metamorphic rock to sedimentary, from the mantle to the crust. Garnet has high diagenesis
resistance (Morton & Hallsworth, 1999; Morton & Hallsworth, 2007) and its potential to
reflect the chemical composition of the source rock, even after being transported for
thousands of kilometres, is a unique characteristic which makes it more and more interesting
and object of study.
All these instruments and knowledge are then applied to the Laxmi Basin, where the Indus
Fan is located. Its sediments had been sampled during the International Ocean Discovery
Program Expedition 355 because this area is an important geological place to understand
palaeoenvironmental changes. Marine sediments in the Arabic Sea are the best way to
understand climate and tectonic evolutions during geological time. Garnet is here employed
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to examine how its chemical composition had been changed through geological time in the
sediment’s cores from Site U1456.
2.1 Steps performed to realize the project and the structure of the thesis
The first part of the thesis started studying five maps obtained from five different thin
sections which had been already realized in the laboratory of the University of Milano-
Bicocca. The core considered in the report is the U1456 and the five samples examined are
shown in table 1 from the oldest to the youngest. This operation was done thanks to the use
of the optical microscope and consisted of garnets identification (n. garnets = 205).
Later the use of Raman spectroscopy became essential in order to really give a sense to the
study case. The first step is the spectral acquisition for each garnet previously selected. The
second one was focused on numbering the six main peaks of garnets following the protocol
established by Bersani et al., 2009.
At this point, the use of MIRAGEM – MatLab is essential for reconstructing the chemical
composition. The software consists of a script, and it is a very fast way to detect the
chemistry of garnets.
The last part had been done using a program called CoDaPack for the realization of ternary
plots, inspired by the one belonging to Mange & Morton, (2007). To better understand and
have a clear framework of the data, there are also some columns charts and pie charts made
by using Excel.
The structure of the thesis consists of a first explanation of heavy minerals and garnets in
order to really appreciate their important roles in these types of studies. There is also an
explanation related to the methodologies and the tools used to realize the study. Then the
rest of the essay is dedicated to the case of the garnets in the Indus Fan core sediments as the
best window to the past. Last, there is a discussion about all the collected data followed by
the chapter related to future perspectives. This last section is something to not underestimate
due to nature is always evolving and what we study and demonstrate is only a new point of
view regarding a topic. Nature is too complex to attribute to its rules and the end of a job is
always the starting point for a new one.
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Core Site Sample
U1456 19R2W80/82 56E19
U1456 15R1W61/63 56E15
U1456 19R1W83/85 56D19
U1456 57F3W35/37 56A57
U1456 25F2W100/102 56A25
Table 1: schematic representation of the samples examined in the
U1456 core of the Indus Fan.
3. Heavy minerals
Some of the keywords in sedimentary geology are weathering, hydraulic sorting, and
diagenesis. Heavy minerals started to get the attention of scientists thanks to their strength
under diagenesis processes. But what are the heavy minerals? The answer to this question is
not very easy, even though could sound trivial. Accessory minerals with high-density
constituents of siliciclastic sediments can be defined as “Heavy minerals” (Mange &
Maurer, 1992). Because of their very low quantity presence in sandstone thin sections (rarely
makes up more than 1% of the rock), they need to be concentrated to be studied. This can be
done using dense liquids, nowadays generally sodium-polytungstate [Na6(H2W12O40)∙H2O]
which has a density of 2.89 g/cm
3
. Heavy minerals include all detrital components with a
density exceeding 2.90 g/cm
3
(Garzanti & Andò, 2019).
Heavy minerals provide information on both provenance and sedimentological processes; to
reach this goal it is important to realize mineralogical and textural analysis (Garzanti &
Andò, 2019). However, for provenance studies, it doesn’t have to be underrated knowing the
chemical composition and the density of source rocks (Garzanti & Andò, 2007).
Classic provenance studies are traditionally based on compositional analysis of sand-size
sediments because they are much easier to be treated in the laboratory and to be analysed
with standard petrographic techniques. Silt-size fraction is a fundamental component of
sediment transport and for this reason, it is necessary to implement the appropriate
techniques to obtain reliable quantitative mineralogical data from silt (Andò et al., 2011):
compositional information on silt is essential to carry out an unbiased study of fluvial to
turbiditic transport and deposition of sediment.
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The thesis is focused on the study of one heavy mineral: garnet, which is the result of high
cation coordination numbers: its density can be between 3.6 (grossular) and 4.3 (almandine)
g/cm
3
(Morton, 1985).
3.1 Garnet
The name “Garnet” was coined by Albrecht Von Bollstäd from the Latin word “granatus”.
This name is referred to the red seeds in a pomegranate, which resemble the shape and the
colour of the mineral.
“The garnets supergroup includes all the mineral isostructural with garnets regardless of
what elements occupy the four atomic sites”, Grew et al., (2013).
Garnet is a silicate mineral that belongs to the nesosilicate group. This means it is constructed
by isolated silicon (SiO4
4-
) tetrahedra bound together with other cations (figure 1). Which
are these cations? The crystallographer and mineralogist Georg Menzer solved the structure
of garnets using a combination of X-ray diffraction and known atomic radii. The following
formula is now the typical nomenclature to represent the sites of garnets:
X3 Y2 Si3 O12
where X is an eightfold-coordinated site that can host Fe
2+
, Mg, Ca and Mn, while Y is a
sixfold-coordinated site which can host Al
3+
, Fe
3+
and Cr. Combinations of these cations
are shown in Table 2, which represents the most common species of garnets with names and
formulas. Garnet minerals are a solid solution of six end members without considering the
hydrogarnets class where the Si3 is totally replaced by four H
+
atoms in OH
-
groups. The six
end members are isostructural with one another, and the structure has a cubic symmetry.
They are generally found as solid solutions because the pure end members are rare and
referred to specific geological environments.
Different cations affect the colour of garnets, which is usually based on iron, chrome and
manganese content. Almandine garnet has a reddish colour due to the presence of iron;
spessartine has a bright orange-yellow colour. When the quantity of chrome increases, the
mineral tends to be greener and poorly yellow.
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Figure 1: the blue octahedra share corners with the red tetrahedra. The yellow spheres
(X cations) are located in small cavities. (Geiger, 2008)
Pyralspite group Ugrandite group
Almandine Fe
2+
Al2(SiO4)3 Grossular Ca3Al2(SiO4)3
Spessartine Mn3Al2(SiO4)3 Andradite Ca3Fe2
3+
(SiO4)3
Pyrope Mg3Al2(SiO4)3 Uvarovite Ca3Cr2(SiO4)3
Table 2: schematic representation of the garnet series with their chemical compositions
The characteristics of the garnet under the optical microscope are the following:
- Form: subrounded to rounded habit
- Rim: irregular
- Relief: high
- Breakage pattern: conchoidal
- Colour: colourless, but it can have a weak pink or yellow colour
- Zoning: it is quite common
- Pleochroism: absent
- Birefringence: isotropic mineral under Cross Polarized Light (XPL). The solid
solutions of andradite-grossular can be weakly anisotropic with a weak grey colour
- Extinction: always extinct, except the hydrated varieties
- Cleavage: absent
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- Others: it can have inclusions and it can be zoned
Garnet is everywhere. It’s one of the primary constituents of the deep Earth; most of it
derives from crustal metamorphic rocks; it’s an occasional guest in igneous rocks.
Garnet is a common detrital phase in heavy-mineral fractions (Morton, 1985) because of its
strong mechanical properties which make it resistant to weathering processes. Together with
epidote, amphipols and pyroxene is a dominant mineral in sedimentology and the one with
the highest density. The percentage presence of garnet in sediments, its size and its shape
are fruitful ways to understand the power of climate weathering, hydraulic processes and
diagenesis effects. Although heavy minerals are considered high-stable minerals, they can
be removed by strong environmental conditions. Not only the strength of a phenomenon
affects their stability: diagenetic effects cause problems for the interpretation of a sediment
basin when they affect multiple sedimentary cycles. That is the reason why it is difficult to
get information back from an ancient sandstone: heavy minerals can be leached out throw
geological time.
Regarding provenance studies, garnet is one of the best candidates as a provenance tracer: it
provides information for palaeotectonic reconstruction even though sediment-routing system
can destroy provenance tracers. At the same time, these chemical and physical processes are
useful if converted as noise to understand provenance information. Sediments composition
reflects plate-tectonic settings (Win et al., 2007) (e.g., subduction zones on the Earth develop
unique chemical associations of heavy minerals) and represents the mineralogy of source
rock because even after thousands of kilometres they can transport the same chemistry.
Garnet is also used to determine the protolith of metamorphic rocks (Suggate & Hall, 2013)
and thanks to its unique chemical properties characterized by a large compositional range,
garnet is also used to determine metamorphic pressure and temperature because different
cations are stable in different conditions of T-P (Win et al., 2007).
As it has been already said above, there is a correlation between detrital garnet composition
and their source rocks. The authors Maria Mange and Andrew Morton provide a ternary plot
(figure 2) that divides garnets into different Types: Type A, Type Bi, Type Bii, Type Ci,
Type Cii and Type D. Each of these is linked to certain and specific metamorphic conditions
and can be an indicator of a series of different sources areas.
A garnet Type has high Mg and low Ca and is associated with high-grade amphibolite and
granulite facies metasedimentary rocks.
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B garnet Type has low Mg and variable Ca. Sediments can be derived from some amphibolite
facies metasedimentary rocks. The partition into Bi and Bii is based on the Ca content: Type
Bi has low Ca, it is eroded from intermediate-felsic igneous rocks (granitoid and pegmatites);
while Type Bii has high Ca and it is eroded from metasedimentary origins within
amphibolite grade source.
C garnet Type is derived from high-grade metabasic rock sediments. The partition into Ci
and Cii is based on the Mg content: Type Ci has lower Mg content than Cii and this is useful
in assessing the relative contribution from mafic (Ci) and ultramafic (Cii) metamorphic
sources. Type Ci is associated to amphibolite and eclogite facies, while Type Cii with
eclogite, pyroxenite, and peridotite facies.
D garnet Type has high Ca content and low Mg content and it is either from very low-grade
metabasite or high-grade calc-silicate rocks.
Figure 2: subdivision of the garnet Fe+Mn – Mg – Ca ternary plot showing definitions
of garnet Types A, Bi, Bii, Ci, Cii and D
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