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UNIVERSIDADE FEDERAL DE MATO GROSSO
FACULDADE DE GEOCIÊNCIAS
PROGRAMA DE PÓS-GRADUAÇÃO EM GEOCIÊNCIAS
Origin and Evolution of Carbonaceous Phyllites from Peresopolis Deposit, Paraguay Belt,
Mid-West Brazil
Talitta Nunes Manoel
Exame de Defesa Dissertação de Mestrado.
Exame de Defesa realizado junto ao
Programa de Pós-Graduação em
Geociências da Faculdade de
Geociências da Universidade
Federal de Mato Grosso.
Cuiabá, 31 de Março de 2017.
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UNIVERSIDADE FEDERAL DE MATO GROSSO
FACULDADE DE GEOCIÊNCIAS
PROGRAMA DE PÓS-GRADUAÇÃO EM GEOCIÊNCIAS
Origin and Evolution of Carbonaceous Phyllites from Peresopolis Deposit, Paraguay Belt,
Mid-West Brazil
Talitta Nunes Manoel
Orientador: Jayme Alfredo Dexheimer Leite
Banca Examinadora
Professor Doutor Jayme A.D. Leite
________________________________________
Presidente (orientador)
Professor Doutor Francisco Egídio Pinho
________________________________________
1º Examinador Titular
Professor Doutor Nilson Francisquini Botelho
________________________________________
2º Examinador Titular
Cuiabá, 31 de Março de2017
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Dados Internacionais de Catalogação na Fonte.
Ficha catalográfica elaborada automaticamente de acordo com os dados fornecidos pelo(a) autor(a).
Permitida a reprodução parcial ou total, desde que citada a fonte.
N972o Nunes Manoel, Talitta.
Origin and Evolution of Carbonaceous Phyllites from Peresopolis Deposit, Paraguay Belt, Mid-West Brazil / Talitta Nunes Manoel. -- 2017
22 f. : il. color. ; 30 cm.
Orientador: Jayme Alfredo Dexheimer Leitea. Dissertação (mestrado) – Universidade Federal de Mato Grosso,
Instituto de Ciências Exatas e da Terra, Programa de Pós-Graduação em Geociências, Cuiabá, 2017.
Inclui bibliografia.
1. Paraguay Belt. 2. Graphitic Carbon. 3. Biogenic. 4. Geothermometer. I. Título.
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List of Tables
Table 1. Isotopic ratios from Peresopolis carbonaceous phyllites. ....................................................... 16 Table 2. Results of the Raman spectrum analyzes of each sample showing mean values and standard
deviation (1σ). ....................................................................................................................................... 17
Table 3. Temperature results using the FWHM as a parameter and the correlation with depth (m) .... 19
Lis of Figures
Figure 1. 1a- Paraguay Belt in Brazilian territory. 1b- Regional geological map with location of the
studied area. ............................................................................................................................................. 9
Figure 2. Geological map of Brasilândias's Sheet includind the deposit area. ...................................... 10
Figure 3. Litostratigraphic subdivisions of Paraguay Belt. Extracted from Souza et al. (2012) ........... 10
Figure 4. Geologic sketch map and cross section A-A’ of the Peresopolis Deposit. ............................ 11
Figure 5. 5a- The wavy foliation presenting in sericite/quartz-rich bands and the graphitic
carbon/opaque rich bands.5b- quartz grains intergrowth with graphitic carbon/opaque bands. 5c- albite
crystal. 5d- carbonate crystal showing its typical cleavage pattern....................................................... 12
Figure 6. Correlation between isotopic signatures and carbon sources of the most well know graphite
deposits in the world including The Peresopolis’ graphitic carbon deposit for association. Modified
from Luque et al., 2012. ........................................................................................................................ 16
Figure 7. Correlation graphic between the analyzed parameters and temperatures. (a), (b) e (c)
FWHM-D1, G e D2. (d) and (e) Center position of D1 and D2 band. (f), (g) intensity ratios of D2/D1
and D1/G. r² and p values corroborate the results. ................................................................................ 18 Figure 8. Hand sample, petrographic images and the spectrum of A1 and 3 sections. The GL peak
resulting from the combination of the G and D2 band, indicative of low formation temperature, is
observed. Also, is possible observe the narrowing of the band D1 and GL. ......................................... 19 Figure 9. 9a- Diffractogram of graphitic carbon after HF attack, presenting a broad distribution. 9b-
Diagram P/T extracted and modified from Landis (1971) showing the evolution of graphitization on
metamorphic facies (the red rectangle represents the insertion of the temperature obtained by the
Raman Spectrometry). 9c- Diffractogram of carbonaceous phyllite showing the mineral assemblage.20 Figure 10. Frequency distribution of FWHM-D1 e FWHM-D2 for each sample. The dashed line
represents the mean values. The error based on Kouketsu et al., 2014 geothermometer (T°FWHMD1
~30°C E T°FWHM-D2~50°C) are reliable if the distribution is not broad or bimodal. ....................... 24
Appendix 1 ............................................................................................................................................ 27
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Table of Contents
ABSTRACT 6
RESUMO 6
1. Introduction 7
2. Regional Settings 8
2.1 The Peresopolis Deposit 11
3. Methodology 12
3.1 Carbon Isotopes 12
3.2 Raman Spectroscopy 13
3.2.1 Applicability of Raman Spectroscopy as a Geothermometer 13
3.3 X-Ray Diffraction 15
4. Results 15
4.1 Carbon Isotopes 15
4.2. Raman Spectroscopy 16
4.3 X-Ray Diffraction 19
5. Discussion 21
6. Conclusion 25
References 25
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ABSTRACT
The Neoproterozoic Cuiabá Group of the Paraguay Belt corresponds to a thick sedimentary
succession that was metamorphosed under greenschist facies metamorphic conditions during
the Braziliano-Pan African Orogeny. The carbonaceous phyllites, which hosts the graphitic
carbon deposit of Peresopolis, along with metadiamictites and metarenites belong to the Coxipo
Formation, the upper unit of the Cuiabá Group. Light Carbon isotope signature (~-28 per mil)
suggests a biogenic origin in a subaqueous reduced environment. Raman Spectroscopy is an
analytical technique that shows high-resolution and high-sensitivity when applied to
carbonaceous material. One hundred and sixty measurements of graphitic carbon spectrum
returned a well-fit between full width at half maximum parameter (FWHM) and temperature
yielding a range of 285 and 300 °C for the graphitic formation. This temperature range agrees
with the temperature suggested by both the metamorphic mineral assemblage and the graphitic
X-Ray diffraction patterns proving the Raman spectroscopy to be a robust geothermometer.
These temperatures are in agreement with the proposed metamorphic conditions of the Internal
Zone of Paraguay Belt.
KEYWORDS: Graphitic carbon; biogenic; geothermometer; Paraguay Belt
RESUMO
O Grupo Cuiabá pertencente a Faixa Paraguai corresponde a uma espessa sequência sedimentar
de idade Neoproterozóica que em decorrência da Orogenia Brasiliana-Pan Africano apresenta
condições de metamorfismo dentro da fácies xisto verde. Os filitos carbonosos hospedam o
carbono grafítico do depósito de Peresópolis e estão associados a metadiamictitos e metarenitos
da Formação Coxipó, topo do Grupo Cuiabá. As leves assinaturas isotópicas (~-28 permil)
sugerem uma origem biogênica e um ambiente subaquoso redutor. A Espectroscopia Raman é
uma técnica analítica que apresentou uma elevada resolução e sensibilidade quando aplicada
em materiais carbonosos. Foram realizadas 160 medições do carbono grafítico e os espectros
obtidos mostraram excelente adequação ao parâmetro FWHM (full width at half maximum) e
as temperaturas encontradas para a formação do carbono grafítico estão na faixa de 285-300°C
Essas temperaturas são concordantes com a temperatura sugerida tanto pela paragênese mineral
e o padrão da difração de raio-X para o carbono grafítico, provando que a Espectroscopia
Raman atua como um excelente geotermômetro. Essas temperaturas estão de acordo com as
condições de metamorfismo propostas para a Zona Interna da Faixa Paraguai. PALAVRAS-CHAVE: Carbono Grafítico; biogênico; geotermômetro; Faixa Paraguai
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1. Introduction
All solid carbonaceous compounds that are arranged through aromatic carbon lattice are
called graphitic carbon. Graphitic carbon exhibits structural and compositional variety,
including carbon compounds of amorphous nature, carbon-rich metasediments and even
perfectly crystalline graphite (Beyssac & Rumble, 2014). Carbonaceous materials are diffuse
in the most diverse geological contexts in both reduced forms as graphite and diamond and in
an oxidized form as carbonate. Based on geological genesis, carbonaceous deposits are
classified as syngenetic and epigenetic (Weis et al., 1981); in the former, the carbon is of organic
origin and can be completely transformed into graphite through the evolution of metamorphic
processes. In epigenetic deposits, the carbonaceous material is produced by the precipitation of
carbon-rich fluids and through the decarbonatization of carbonate materials.
Carbon isotope geochemistry may reveal the origin of carbonaceous material based on
different ranges of δ13C. Weis et al. (1981), Schidlowski, 1987, 2001 and Luque et al., 2012
attributed mean isotopic ratios of -25 permil to carbon of biogenic origin, ± 2 permil for marine
carbonates (Luque et al., 2012) and -7 permil for carbonaceous materials of mantle origin such
as diamonds (Hahn-Weinheimer e Hirner, 1981).
Raman Spectroscopy and X-ray diffraction techniques have been used to assess the
geological evolution of carbonaceous materials based on variation in crystalline structure.
Raman spectroscopy is a non-destructive analytical technique with high sensitivity that reveals
crystal lattice even in amorphous compounds. Pasteris and Wopenka, 1991; Wopenka and
Pasteris, 1993; and Yui et al., 1996 have shown that the Raman spectra varies in carbonaceous
material according to temperature, which in turn may be related to the degree of metamorphism.
The estimation of metamorphic temperatures and their use as geothermometers is based upon
equations developed by Beyssac et al., (2002a); Rahl et al., (2005); Lahfid et al., (2010) and
Kouketsu et al., (2014). The X-ray technique provides information regarding crystallinity
through interplanar distance. The decreasing of the space between the aromatic carbon layers
indicates a strong correlation with increasing temperature and therefore metamorphic grade
(e.g., French, 1964; Landis, 1971; Wopenka & Pasteris, 1993; Ahn Ho et al., 1999).
In the Peresopolis District of Nova Brasilândia, located in the central-east part of the
state of Mato Grosso, an important deposit of carbonaceous phyllite (> 4.5 Mt Graphitic
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Carbon) was recognized during an exploration program involving trenches and diamond
drilling. The deposit is tabular, trends NE-SW, and is 1.800 m long, 200 m wide and recognized
to a depth of 50 m. The Peresopolis deposit is hosted by the Neoproterozoic-Cambrian Cuiabá
Group of the Paraguay Belt (Almeida, 1965). The purpose of this study is to apply Carbon
Isotopes, Raman spectroscopy and DRX techniques in support of the discussion over the origin
and evolution of the Peresopolis deposit.
2. Regional Settings
The Peresopolis carbonaceous phyllite deposit is located in the Paraguay metamorphic
Belt (Almeida, 1965), a tectonic entity that is in the central portion of Brazil and composed of
a sequence of sedimentary and metasedimentary rocks developed by collision processes during
the Brazilian Orogeny.
The Paraguay Belt (Fig. 1a and 1b) presents a curved arc shape toward the cratonic area,
exposing itself in Mato Grosso and Mato Grosso do Sul states and Paraguay. A portion of this
range extends from Corumbá to the interior of Bolivia with an NW-SE trend. It is known as the
Tucavaca Belt (Litherland et al., 1986) and interpreted by Alvarenga et al. (2000) as an
aulacogen; the intensity of the deformation and metamorphism decreases near the cratonic
section (Alvarenga 1988, 1990).
In the first attempts of stratigraphic subdivision of the Paraguay Belt (Fig 3), Almeida
(1964) denominated the Cuiabá Series pelitic sediments that represent metamorphism in the
greenschist facies and are covered by a sequence of glacial characteristics representative of the
Jangada Group. Above this group, limestones and dolomites were deposited, representing the
Araras Group. Shales and sandstones were located in the upper sequence, identified as the Alto
Paraguay Group.
The subdivision of the Paraguay Belt resulted in four large groups listed in chronological
order: Inferior Unit; Turbiditic-glaciogenic Unit; Carbonated Unit and Superior Unit
(Alvarenga 1988, 1990 and Alvarenga & Saes, 1992). Alvarenga (1988) and Alvarenga &
Trompette (1993) renamed the old tectonic and metamorphic classification of Almeida (1984)
and split the belt into three structural zones: Platform Sedimentary Coverage, External Zone
with weak deformation and little or no metamorphism, and Internal Zone with intrusive granitic
bodies.
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Luz et al. (1980), based on lithostratigraphic information, subdivided the Cuiabá Group
into units 1,2,3,4,5,6,7,8 and Undivided, showing phyllite occurrence throughout the units.
Units 4 and 7 corresponded to sediments of glacial and marine contributions, being composed
of metaconglomerates, classified by Almeida (1965) and Hennies (1966) as tillites. For the
other units, it was suggested that a marine and gradational environment was created by current
turbidity due to tectonism. Pelitic sedimentation is deposited in moments of tectonic quietude.
The study region (Fig. 2) is located at the top of the Cuiabá Group in the Internal Zone
of the Paraguay Belt. The bottom portion corresponds to metadiamictites attributed to Marinoan
Glaciation (age ~ 636 Ma) and by carbonaceous phyllites deposited at the top. As suggested by
Beal (2013) the study area is hosted in the Coxipó Formation, Marzagão Facies and Subunit 7
of Luz et al., 1980. Carbonaceous phyllites are also observed in the basal portions of the Cuiabá
Group, Campina de Pedra Formation by Tokashiki and Saes (2008) and Subunit 1 and 2 of Luz
et al (1980). The shale required for this carbonaceous phyllite may have originated in a moment
of transgression related to deglaciation and the changing elevation level of the Clymene Ocean
(Pimentel, 1992; Tohver et al., 2010; McGee et al., 2015), where the sediments and organic
matter necessary for the subsequent transformation of organic matter into graphitic carbon were
located.
Figure 1. 1a- Paraguay Belt in Brazilian territory. 1b- Regional geological map with location of the
studied area.
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Figure 2. Geological map of Brasilândias's Sheet including the deposit area.
Figure 3. Litostratigraphic subdivisions of Paraguay Belt. Extracted from Souza et al. (2012).
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2.1 The Peresopolis Deposit
The Peresopolis carbon graphitic deposit is a blind deposit revealed by a trenching and
diamond drilling program developed by Lucra Minerals Ltd., between 2014 and 2015.
Integration data from the exploration program allowed the understanding of the geology and
the form of the deposit. Thus, the deposit is interpreted to represent a E-W-trending slightly
folded tabular layer of up to 1.800 m long, 200 m wide and recognized to a depth of 50 m;
preliminary estimations yielded a resource of 40Mt of ore @12% of Graphitic Carbon.
The deposit sits in the normal anticlinal flank and is enveloped by basal diamictites and
top fine-grained metarenites which located in the Cuiabá Group. The deposit itself consists of
a basal sequence of orange to creamy phyllites that grades upward into grey carbon-poor
graphitic phyllite that in turn transition to black carbon-rich graphitic phyllite, the ore. Upwards
the ore grades to grey carbon-poor graphitic phyllite and finally to banded, grey and white,
phyllite (Fig. 4).
Figure 4. Geologic sketch map and cross section A-A’ of the Peresopolis Deposit.
The Peresopolis carbonaceous phyllites are fine-grained rocks composed by quartz,
sericite, albite, carbonate, graphitic carbon and opaque minerals to a lesser extent (Figure 5).
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The graphitic carbon occurs as thin films associated with quartz and sericite. The fine-grained
assemblage turns difficult to find out the mineralogical nature of the rock in most of sections.
Figure 5. 5a- The wavy foliation presenting in sericite/quartz-rich bands and the graphitic carbon/opaque
rich bands.5b- quartz grains intergrowth with graphitic carbon/opaque bands. 5c- albite crystal. 5d-
carbonate crystal showing its typical cleavage pattern.
3. Methodology
3.1 Carbon Isotopes
Four samples considered representative of the graphitic phyllite were collected, without
treatments, and stored in Eppendorf tubes, each containing approximately 1g of sample. The
carbon isotope analysis was carried out at the Geochronological, Geodynamic and
Environmental Studies Laboratory at the Institute of Geosciences, housed at the University of
Brasília. In the laboratory stage, the samples were weighed into tin capsules and then placed in
a sampler introducing the material into a furnace heated between 900 and 1000 °C with a flow
of oxygen and helium. The released carbon binds to oxygen forming CO2, which is then carried
to the Thermo Scientific FLASH 2000 HT spectrometer, where readings were collected. The
standard used is the Vienna Pee Dee Belemnite (VPDB), and the isotope ratio is presented in
permil.
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3.2 Raman Spectroscopy
The analysis was completed in the Center of Electronic Microscopy (CEM) at the
Federal University of Paraná, where the Raman spectrum were achieved using a WITec alpha
300 Raman Confocal Microscope with a @532 nm Nd-YAG laser, 10X objective. The laser
power setting was set at 1 mW, and the acquisition time was 30 s on the thin sections. In this
study, we analyzed eight thin sections representative of the deposit, taking 20 measurements on
different spots by sample for a total of 160 measures. The spectrum were evaluated and
decomposed using PeakFit V4.
3.2.1 Applicability of Raman Spectroscopy as a Geothermometer
Tuinstra and Koening (1970) were pioneers in the use of Raman Spectrometry to study
dark materials. They detected the presence of a band approximately 1575 cm-1 that occurred in
all graphite crystals, called the G band, described as a band of high symmetry.
Several authors have made use of Raman Spectrometry to characterize carbonaceous
materials in metasediments and have observed significant changes in the spectrum according to
their evolution (e.g., Bénny-Bassez & Rouzaud, 1985; Wang et al., 1990; Wopenka & Pasteris,
1993; Pasteris & Wopenka, 1991, Escribano et al., 2001). The authors cited above offered
different explanations about the meaning of the D bands; with regard to the G band, they agreed
that this peak corresponds to the vibrational stretching mode of the carbon bonds in the plane.
As the temperature of metamorphism increases, the G band tends to be robust, showing higher
intensity as a result of the perfect symmetry produced by the increasing compaction between
the material, whereas the bands characteristic of disorder (D4 ~1245 cm-1; D1 ~1350 cm-1; D3
~1510 cm-1 e D2 ~1620 cm-1) tend to disappear, as discussed by Cuesta et al., 1994. In theory,
it would only be possible to identify two of the six vibrational graphite modes (E2 g) that
characterize the G band and D2 band using Raman spectroscopy.
However, with the increase in graphite lattice density by the concentration of
heteroatoms, defects in the layers occur between the hexagonal layers and bonding
discontinuities. Next, there is a breakdown in the Raman selection rules making the bands of
disorder visible (e.g. Wang et al., 1990; Wopenka & Pasterirs, 1993; Pócsik et al., 1998; Ferrari
& Robertson, 2001; Reich & Thomsen, 2004). These bands are more sensitive to the strength
of the laser because the tendency of the defects is to diffuse the light even more through the
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matter due to this arrangement of atomic bonds, resulting in changes in amplitude and intensity.
Thus, it is recommended that researchers use the laser at a low power (e.g., Bokobza & Zhang,
2012; Bokobza et al., 2013; Huang et al., 1998). The D4 band, which occurs in ~1245 cm-1,
represents structures of low crystallinity and is related to organic compounds such hydrocarbon
or aliphatic chains (Bokobza et al., 2015).
Beyssac et al. (2002a) were pioneers in the formulation of a geothermometer model,
with its applicability and reliability being restricted to intermediate temperatures at 330-640 °C
using the peak intensity as parameter. Considering the complexity of the disordered
carbonaceous material, it was necessary to create models that fit current conditions. Researchers
starting with Rahl et al., (2005), followed by Lahfid et al., (2010) and then Kouketsu et al.,
(2014) began formulating equations for the treatment of these less crystallized compounds. Rahl
et al., (2005) changed the Beyssac et al. (2002a) model, presenting only 94% certainty. The
model created by Lahfid et al. (2010) covers temperatures from 200 to 320 °C which is of low
reliability, forming a model of ~96% certainty.
Kouketsu et al., (2014) studied a region of geological background analogous to that of
the Paraguay Belt and analyzed 19 samples collected from the Phanerozoic belts of Shimanto,
Chichibu, Kurosegawa, Kure and Shirataki, which are located southwest of Japan under
greenschist facies conditions. They proposed a more precise geothermometer model (97%
reliability) that was applied to studies of the carbonaceous phyllites of Peresopolis. The model
presents linear correlation at temperatures ranging from 150 to 440 °C, taking into consideration
the FWHM (full width at half maximum) of the D1 and D2 bands: T°C = -2.15 (FWHM-D1) +
478 / T°C = -6.78 (FWHM-D2) + 535. The same authors propose using different methodologies
for each type of carbonaceous material, from the most crystallized to the least crystallized,
totalizing seven adjustments ranging from A to G. The F and G adjustments are at the lowest
temperature, and the ratio D2 / D1 is decisive for choosing the settings; with a D2 / D1 ratio of
less than 1.5 (Table 2), the most adequate adjustment is F. The occurrence of the D4 band makes
the peak approximately 1600 cm-1 wider, thereby interfering in the recognition of the G and D2
bands that characterize a solitary peak (GL). For the treatment of the carbonaceous phyllites
samples, the F-type adjustment was adopted, setting the D4 band at 1245 cm-1 and the G band
at 1593 cm-1 in a Lorentzian function to avoid maximizing interference of these bands and to
minimize errors that occur when using this method and applying a geothermometer.
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3.3 X-Ray Diffraction
The XRD technique was used in one sample (09) that was submitted to disaggregation
with the use of an orbital mill. Some of this pulverized material was digested by HF and another
portion was passed in the 28 + mesh strainer. After this stage, further analysis was carried out
in the X-ray Diffraction Laboratory of the Division of Technology in Civil Engineering -
DTEC.E of Technological Services Management - GST.E, Furnas Centrais Elétricas, in a
Siemens D5000 diffractometer. The samples were analyzed under voltage of 40KV and
amperage of 30 mA, with scan speed of 0,05° / sec in the spectrum of 3° to 70° 2Ɵ. The energy
source was a tungsten filament and the X-ray tube was copper. The analyses were performed
by a computer that was coupled to the diffractometer and using Diffrac Plus version 2.3
software. For data interpretation, we used EVA Software, version 2009, which includes the
2009 International Center for Diffraction Data (ICDD) database.
4. Results
4.1 Carbon Isotopes
The isotopic carbon geochemistry provides important information for the recognition of
changes associated to the transport of carbon in the lithosphere and overall modeling of the
carbon cycle (Walter et al., 2011). For the study of carbonaceous material, the carbon isotopic
geochemistry allows researchers to uncover the origin of the precursor carbon, as well as the
mechanisms that led to the deposition of this material.
According to the precursor material, the occurrences of graphitic carbon can be
classified as coming from organic and inorganic sources (Hahn-Weinheimer,1981). Organic or
syngenetic carbon is resulting from primary organic matter subjected to metamorphic events
(Hoefs & Frey, 1976; Oehler & Smith, 1977). Inorganic or epigenetic carbon is derived from
the precipitation of hydrothermal fluids or through the process of decarbonation (Weis et al.,
1980).
The results of the analysis of carbon isotopes of the Peresopolis carbonaceous phyllites
show values of -28 and -29 permil (Table 1, Fig. 6), which are in the range of those that are
classified as organic (average δ13C close to -25 permil, Schidlowski, 2001).
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Table 1. Isotopic ratios from Peresopolis carbonaceous phyllites.
Samples 13C ‰ Depth (m)
07 -28.70 16.20-18.80
12 -29.01 30.00-31.80
15 -29.23 37.00-40.00
19 -28.36 47.50-50.30
Mean -28.85
Figure 6. Correlation between isotopic signatures and carbon sources of the most well know graphite
deposits in the world including The Peresopolis’ graphitic carbon deposit for association. Modified from
Luque et al., 2012.
4.2. Raman Spectroscopy
The Raman spectroscopy results as well as all parameters used for the application of a
geothermometer are shown in Table 2. In general, there are few differences between sample A1
and sample 3, with values of 83.03 and 88.45, respectively, indicating the narrowing of the D1
band and therefore an increase in temperature. The center position of the D1 band in the higher
temperature samples is slightly smaller due to the low interference of the neighboring D4 band
(Fig. 7 d).
The G band at low temperatures is difficult to decompose, which makes the FWHM-G
an uncertain parameter. The added error (1σ) is practically the same for all samples, showing
G band instability at low temperatures. For the D2 band, few differences are observed regarding
the center position, while FWHM-2 ranges from 32.58 to 34.16 (Fig. 7e). The peak that occurs
around ~ 1600 cm-1 at low temperatures, corresponds to the G and D2 bands, which are read as
a solo peak.
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At temperatures close to 300 °C, the
carbonaceous material is in transition from an
amorphous structure to a more ordered compound
(Kouketsu et al., 2014), making the G band, and
consequently the D2 band, more susceptible to the
frequency emitted by the green laser, which then
causes the intensity of these bands increase (e.g.,
Vidano et al., 1981; Pócsik et al., 1998 Sato et
al., 2006).
The intensity (amplitude) of the D2/ D1
ratio in the studied samples is in the same range >
1; thus, the D2 band with the highest intensity and
the D1 / G > 1<1.5 intensity ratio indicates that the
D1 band has a higher intensity than the G band,
ensuring that these samples are at low
temperatures (Fig. 7 f, g). In the figure 7 are
presented the studied parameters.
The FWHM-D1 (Fig.7a) for all
samples points to a linear correlation between
the width of the D1 bands and temperature. This
finding implies a decrease in the width of the
disordered bands, and as the degree of
metamorphism increases the peaks of D1 and
D2 tend to become narrow as show in figure 8.
The FWHM-D2 (Fig. 7c), as already discussed
in Kouketsu et al. (2014), presents a good linear
correlation at low temperatures, but with the
increase of T °C, the center position and width
of the D as shown band and the G band interact
due to similar wavenumbers, making T °D2 less
accurate. FWHM-G (Fig. 7b) does not provide a
linear correlation with T °C due to the lack of G
Table 2. Results of the Raman spectrum
analyzes of the eight samples showing mean
values and standard deviation (1σ).
D1
/G I
nte
nsi
ty
1
σ
0.3
1
0.1
6
0.1
3
0.1
8
0.1
7
0.2
5
0.2
3
0.2
2
Mea
n
2.3
5
2.1
1
2.0
5
1.8
0
1.6
8
1.9
3
1.9
0
1.6
9
D2
/D1
In
ten
sity
1
σ
0.0
7
0.0
7
0.0
6
0.0
7
0.1
2
0.0
7
0.0
7
0.1
0
Mea
n
1.2
6
1.2
0
1.1
7
1.1
0
1.0
6
1.1
0
1.0
9
1.0
0
FW
HM
-D2
1
σ
0.6
0
0.4
1
0.5
1
0.8
0
0.7
9
0.8
8
0.8
1
0.8
3
Mea
n
32
.58
32
.73
33
.04
33
.82
33
.34
34
.01
34
.10
34
.16
D
2 c
ente
r
1
σ
0.4
8
0.5
4
0.5
4
0.9
1
0.7
4
0.8
8
0.8
7
0.9
2
Mea
n
16
09
.84
16
09
.64
16
09
.95
16
09
.86
16
09
.79
16
09
.51
16
09
.57
16
08
.89
FW
HM
-G
1
σ
10
.61
4.1
6
6.6
9
4.6
9
7.7
0
3.1
1
5.0
7
8.2
0
Mea
n
69
.95
64
.47
69
.58
69
.22
67
.31
62
.31
67
.59
71
.82
G c
ente
r
1
σ
0.0
03
0.0
03
0.0
05
0.0
02
0.0
03
0.0
04
0.0
03
0.0
03
Mea
n
15
93
.00
15
93
.00
15
93
.00
15
93
.00
15
93
.00
15
93
.00
15
93
.00
15
93
.00
F
WH
M-D
1
1
σ
3
.32
2.2
7
2.1
2
1.6
8
2.7
0
1.6
7
1.6
9
2.4
8
Mea
n
83
.05
84
.08
83
.64
87
.39
88
.77
88
.96
89
.36
88
.45
D1 c
ente
r
1
σ
0.6
1
0.7
4
0.7
5
0.9
2
0.7
6
0.8
7
1.1
7
0.8
4
Mea
n
13
38
.81
13
38
.56
13
39
.42
13
40
.00
13
39
.59
13
39
.50
13
39
.79
13
40
.93
Sam
ple
A1
22
19
15
12
9
7
3
18
band stability, with values dispersed at temperatures below 300 °C. The graphitic carbon from
Peresopolis was formed under a temperature range between 285 and 300°C (Table 3) based
on the previous FWHM equations of D1 and D2 bands proposed by Kouketsu et al., 2014. For
more information about temperatures see appendix 1.
Figure 7. Correlation graphic between the analyzed parameters and temperatures. (a), (b) e (c) FWHM-
D1, G e D2. (d) and (e) Center position of D1 and D2 band. (f), (g) intensity ratios of D2/D1 and D1/G.
r² and p values corroborate the results.
19
Figure 8. Hand sample, petrographic images and the spectrum of A1 and 3 sections. The GL peak
resulting from the combination of the G and D2 band, indicative of low formation temperature, is
observed. Also, is possible observe the narrowing of the band D1 and GL.
Table 3. Temperature results using the FWHM as a parameter and the correlation with depth (m).
Samples
FWHM-D1 T°D1 FWHM-D2 T°D2
Depth (m) Mean 1σ Mean Mean 1σ Mean
A1 83,67 2,93 299,45 32,55 0,68 314,13 65.00
22 84,51 1,85 297,22 32,95 0,53 313,06 55.70-58.00
19 85,04 2,44 298,18 33,21 0,47 310,96 47.50-50.30
15 88,16 2,12 290,11 34,26 0,99 305,7 37.00-40.00
12 89,58 2,48 287,15 34,18 0,95 308,93 30.00-31.80
9 89,59 1,7 286,73 34,05 0,79 304,41 21.90-24.00
7 89,27 2,23 285,88 34,09 1,02 303,8 16.20-18.80
3 89,23 2,42 287,84 34,77 1,48 303,41 5.90-8.55
4.3 X-Ray Diffraction
The fully ordered graphite peaks normally occur at 26 ° and 43 ° (diffraction 002 and
100), and they are identified in figure 9, which illustrates the diffractogram of the HF residue.
However, the analyzed material did not present the intense characteristic peak, only a wide
distribution (Fig. 9a). The Landis (1971) scheme was used to determine the crystallinity of the
graphitic carbon arrangement; the carbonaceous material was classified as d3- type, disordered
20
one (Fig. 9b). In addition, it was possible to identify the main rock-forming minerals of the
phyllite, such as quartz, albite, and muscovite as shown in petrographic description and the clay
minerals kaolinite and tosudite (Fig. 9c); subordinate minerals included pyrite and dolomite.
Among these minerals, quartz + albite + muscovite + sericite constitute the equilibrium
graphitic carbon, while clay-minerals are interpreted as alteration products. As related by
Bucher and Grapes (2011), the quartz + albite + muscovite + sericite assemblage constitutes
paragenesis indicative of subgreenschist facies, which are delimited by pressure and
temperature ranges between 1 and 4 Kbar and 100-300 °C, respectively.
Figure 9. 9a- Diffractogram of graphitic carbon after HF attack, presenting a broad distribution. 9b-
Diagram P/T extracted and modified from Landis (1971) showing the evolution of graphitization on
metamorphic facies (the red rectangle represents the insertion of the temperature obtained by the
Raman Spectrometry). 9c- Diffractogram of carbonaceous phyllite showing the mineral assemblage.
21
5. Discussion
The Peresopolis graphitic carbon deposit is hosted in metadiamyctites of Unit 7 of Luz
et al. (1980) and its deposition is related to the melting of Marinoan Glaciation (~ 636 Ma).
Carbonaceous phyllites were found in the most basal portions of the Cuiabá Group, presenting
probable magmatic interference (Luz et al., 1980), but those of Peresopolis are related to the
maturation process of organic matter. Based on the carbon isotopes, values ranging from -28 to
-29 permil were interpreted as originating from a strictly biogenic source (Schidlowski, 2001).
The isotopic signature of the samples is homogeneous, presenting slight or no
fractionation, which leads to the conclusion that the material comes from the same organic
precursor and was not affected by diagenetic and post-diagenetic processes (Santos et al., 1995).
The fine granulometry of the samples and the enrichment of light isotopes can be indicative of
a anaerobic underwater environment (Oliveira et al., 2003) with a strong contribution of organic
matter that fixes in its structure 12C through photosynthesis. Even both the petrography and the
XRD were identified the presence of carbonate, which presents heavy isotopic ratios of 13C
(close to ± 2 permil, Luque et al., 2012), the interference of the carbonate is minimal in view of
the low fractionation and the extremely light values of 13C.
Based upon the isotopic ratios, it is possible that the protholite of the carbonaceous
phyllite was an impermeable rock and that the influence of pressure was negligible because
there was no significant loss of volatiles during the maturation process. This loss of volatiles
(usually methane) would demand removal of the light isotope 12C (Sackett et al., 1976;
Silverman & Epstein, 1958) and enrichment of the heavy isotope, 13C. As the metamorphism
advances, the isotopic composition of the carbon tends to become heavier (e.g., Hoefs and Frey,
1976; Wada et al., 1994; Luque et al., 2012).
Raman spectroscopy determined the crystallinity of the studied material, and the
Peresopolis graphitic carbon was characterized as a compound of transitional structure between
amorphous and crystalline (Kouketsu et al., 2014). The bands D4, D1, D3 and D2 are
characteristic of a disordered material, and the meaning of each band reflects the structure and
crystalline arrangement. The D4 band occurring at ~ 1245 cm-1 characterizes structures of low
crystallinity, and it is correlated to organic compounds such as hydrocarbon or aliphatic fraction
(e.g., Pawlyta et al., 2015; Bokobza et al., 2015). Bény-Bassez and Rouzaud (1985) characterize
22
the D1 band as being either a part of the defect planes of the heteroatoms (O-, H-, N-) or a result
of structural defects in the crystal lattices, which are intense in poorly ordered crystals. These
authors correlate the D3 bands with such defects as the presence of the sp3 carbon (tetrahedral).
The interpretation of the G band is common sense, and referencing the Ferrari (2007)
classification, this band is a result of the bond stretching of the sp²-type bonds in the aromatic
carbons. The D2 peak configures a weak band at ~ 1620 cm-1, appearing as a shoulder on the
G band, arranging a disordered structure between the graphene layers (e.g., Wang et al., 1990).
The temperature difference of ~ 20 °C between T ° D1 and T ° D2 (table 3) occurs
because the material is in a temperature range close to the transition from an amorphous
material to a properly crystalline structure (~ 300 °C). The errors based on the Kouketsu et al.,
2014 geothermometer (T ° D1 ~ 30 °C and T ° D2 ~ 50 °C) are reliable if the distribution of
FWHM-D2 and FWHM-D1 is not broad or bimodal, which is in agreement with the results
found in this study (Fig. 10). We obtained similar results to those of Kouketsu et al., 2014 for
this temperature range. The major variation was detected by the intensity of the GL band in T>
290°C, which is probably related to the instability of this band at these temperatures. The
observed differences in the obtained parameters are minimal, and it can be suggested that the
transformation of the carbonaceous material was homogeneous throughout the entire deposit.
The carbonaceous phyllite comes from an impermeable protholite, probably shale, since
there was no loss of volatiles, which would lead to the progressive depletion of the light isotopes
(Oliveira et al., 2003). The deposition of the graphitic carbon is associated with the Marinoan
deglaciation period (~ 636 Ma), during the transgression of the Clymene ocean (Pimentel et al.,
1992; Tohver et al., 2010; McGee et al., 2015). Closure of the ocean provided anoxic and
sulfidic conditions for the preservation of the organic matter and the mineral composition of
the Peresopolis carbonaceous phyllite.
The petrographic description combined with X-ray diffraction allowed the identification
of minerals associated with low-grade metamorphic pelitic rocks, such as sericite and albite,
and they were observed to be in equilibrium with graphite and kaolinite and tosudite as
weathered alteration products. The metamorphic conditions agree with the conditions proposed
by Bucher and Grapes, 2011 for the transition between subgreenschist facies to greenschist
facies. In addition, the broad XRD spectrum shown by the Peresopolis carbonaceous phyllites
23
seems to reasonably fit within the low-pressure field: ca. 4Kbar, as proposed by Landis (1971).
The temperature range identified in this study agrees with the metamorphic temperature
recorded in the Internal Zone of the Paraguay Belt, not exceeding the greenschist facies for the
Cuiabá Group (e.g., Almeida 1964b, 1965; Alvarenga & Trompette., 1993; Silva et al., 2002).
24
Figure 10. Frequency distribution of FWHM-D1 e FWHM-D2 for each sample. The dashed line
represents the mean values. The error based on Kouketsu et al., 2014 geothermometer (T°FWHMD1
~30°C E T°FWHM-D2~50°C) are reliable if the distribution is not broad or bimodal.
25
6. Conclusion
The homogenous δ 13C isotopic ratios are approximately -28.85 permil for the
Peresopolis graphitic phyllite, which suggests a strictly a biogenic origin for the carbon. The
equilibrium metamorphic temperature for the carbonaceous phyllites is in the range of 285-300
°C, and it was obtained using Raman Spectroscopy. Certain uncertainties in temperature may
arise from the fact that the equilibrium metamorphic temperature is close to the temperature
limit for the F adjustment (280-300 °C) proposed by Kouketsu et al. (2014); this temperature
range is also confirmed by the mineral assemblage in equilibrium with graphitic carbon. The
formation pressure of the carbonaceous phyllite did not exceed 4Kbar based upon the
graphitization diagram of metamorphic rocks of Landis (1971). The P/T range proposed in this
study agrees with metamorphic conditions for the Internal Zone of the Paraguay Belt (e.g.,
Almeida 1964b, 1965; Alvarenga & Trompette., 1993; Silva et al., 2002) and demonstrates the
efficiency of the Raman geothermometer.
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Appendix 1
In the following table are presented the temperatures calculated by the T°C = -2.15 (FWHM-
D1) + 478 and T°C = -6.78 (FWHM-D2) + 535. All the 160 measures are illustrated; showing also the
mean values of each temperature, standard deviation, 2sigma, coefficient of variation and the variance.
These parameters enable the evaluation of homogeneity and consistency of the data. The sample A1
28
shows the highest 1 and the highest temperatures as well. Even the standard deviation presenting high
values, the coefficient of variance still low (< 25%), indicating the data homogeneity.
Sample T°D1 T°D2 Sample T°D1 T°D2 Sample T°D1 T°D2 Sample T°D1 T°D2
A1.1 294.71 309.54 22.1 298.07 315.68 19.1 291.20 318.06 15.1 291.71 305.89
A1.2 310.05 308.37 22.2 296.48 313.97 19.2 293.18 313.24 15.2 288.61 302.86
A1.3 290.79 310.70 22.3 296.61 316.26 19.3 296.49 308.59 15.3 285.38 298.48
A1.4 293.33 312.86 22.4 280.99 311.40 19.4 288.56 313.86 15.4 288.74 302.29
A1.5 299.21 311.90 22.5 300.38 316.48 19.5 301.54 308.98 15.5 288.32 303.52
A1.6 291.98 314.06 22.6 295.99 313.18 19.6 301.64 311.81 15.6 290.17 300.71
A1.7 304.63 316.26 22.7 299.62 309.98 19.7 292.88 312.40 15.7 296.65 302.38
A1.8 308.63 325.84 22.8 298.04 307.90 19.8 298.64 305.83 15.8 284.02 297.66
A1.9 303.33 312.22 22.9 303.72 309.78 19.9 295.06 312.47 15.9 289.13 304.55
A1.10 288.82 315.31 22.10 292.16 313.91 19.10 305.98 314.24 15.10 291.54 307.42
A1.11 306.61 315.35 22.11 296.64 310.14 19.11 301.65 312.46 15.11 282.87 298.35
A1.12 295.45 316.24 22.12 296.21 313.36 19.12 305.47 311.81 15.12 289.59 310.43
A1.13 298.63 313.18 22.13 300.43 309.36 19.13 300.50 317.88 15.13 292.76 311.21
A1.14 309.72 319.19 22.14 298.85 313.92 19.14 298.24 308.41 15.14 289.87 309.78
A1.15 292.91 310.34 22.15 300.63 314.16 19.15 298.08 305.50 15.15 289.94 308.97
A1.16 295.80 310.29 22.16 292.07 312.68 19.16 294.56 308.64 15.16 295.78 307.51
A1.17 289.43 318.08 22.17 299.19 317.37 19.17 300.55 305.57 15.17 288.75 305.53
A1.18 305.83 317.26 22.18 295.39 312.00 19.18 299.95 310.96 15.18 294.48 320.08
A1.19 303.41 311.05 22.19 303.20 317.92 19.19 297.93 308.62 15.19 294.40 311.24
A1.20 305.77 314.47 22.20 299.70 311.73 19.20 301.49 309.92 15.20 289.40 305.20
MEAN 299.45 314.13 297.22 313.06 298.18 310.96 290.11 305.70
STDV 7.14 4.08 4.88 2.80 4.56 3.58 3.61 5.41
2sigma 313.73 322.28 306.98 318.67 307.29 318.13 297.32 316.53
CV 2.38% 1.30% 1.64% 0.90% 1.53% 1.15% 1.24% 1.77%
Variance 50.99 16.63 23.82 7.86 20.77 12.83 13.00 29.30
29
Sample T°D1 T°D2 Sample T°D1 T°D2 Sample T°D1 T°D2 Sample T°D1 T°D2
12.1 286.58 314.07 9.1 292.16 312.82 7.1 284.62 292.46 3.1 296.78 306.83
12.2 287.31 312.53 9.2 288.48 305.11 7.2 288.62 310.54 3.2 289.76 305.15
12.3 286.39 306.90 9.3 289.27 312.08 7.3 286.83 310.60 3.3 290.30 310.16
12.4 285.01 308.19 9.4 292.77 307.02 7.4 285.32 303.66 3.4 293.76 302.30
12.5 284.87 305.91 9.5 288.01 307.86 7.5 286.07 302.82 3.5 291.66 306.43
12.6 284.96 320.84 9.6 283.79 297.94 7.6 280.66 300.74 3.6 294.21 304.19
12.7 290.03 308.20 9.7 292.02 301.93 7.7 281.50 305.94 3.7 288.71 298.17
12.8 289.56 304.20 9.8 284.96 293.98 7.8 280.67 297.18 3.8 294.38 299.52
12.9 292.13 307.47 9.9 286.18 310.21 7.9 288.75 306.66 3.9 275.58 300.44
12.10 268.91 323.04 9.10 286.66 298.48 7.10 281.08 297.90 3.10 286.66 305.81
12.11 283.98 305.29 9.11 287.26 300.14 7.11 286.78 298.54 3.11 283.04 300.30
12.12 282.86 311.29 9.12 289.50 300.42 7.12 284.29 310.96 3.12 282.42 296.68
12.13 291.29 303.22 9.13 280.32 293.75 7.13 286.45 305.89 3.13 288.22 304.59
12.14 286.59 310.03 9.14 287.95 310.06 7.14 286.72 299.46 3.14 286.71 301.84
12.15 285.62 309.97 9.15 282.57 305.00 7.15 292.23 302.08 3.15 284.82 320.07
12.16 283.66 305.36 9.16 280.52 305.31 7.16 286.59 303.95 3.16 282.36 295.85
12.17 295.89 305.51 9.17 283.41 312.67 7.17 293.08 302.66 3.17 282.71 302.64
12.18 293.96 305.28 9.18 284.06 306.71 7.18 288.32 303.52 3.18 285.77 295.16
12.19 295.50 303.38 9.19 288.17 308.07 7.19 288.29 304.82 3.19 285.37 305.28
12.20 287.98 307.95 9.20 286.48 298.71 7.20 280.80 315.54 3.20 293.61 306.77
MEAN 287.15 308.93 286.73 304.41 285.88 303.80 287.84 303.41
STDV 5.81 5.34 3.58 5.96 3.63 5.47 5.32 5.64
2sigma 298.77 319.62 293.89 316.33 293.13 314.73 298.49 314.70
CV 2.02% 1.73% 1.25% 1.96% 1.27% 1.80% 1.85% 1.86%
Variance 33.72 28.54 12.82 35.51 13.14 29.87 28.33 31.86