1

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.

2

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

3

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

6

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

7

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

8

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.

10

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).

11

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).

12

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.

13

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

14

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.

15

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).

16

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.

17

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