Garnet composition in the Indus Fan core sediments from IODP Expedition 355 (Arabian Sea)

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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