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 Originally published as: van Zuilen, M. A., Fliegel, D., Wirth, R., Lepland, A., Qu, Y., Schreiber, A., Romashkin, A. E., Philippot, P. (2012): Mineral‐templated growth of natural graphite films. ‐ Geochimica et Cosmochimica Acta, 83, 252‐262 DOI: 10.1016/j.gca.2011.12.030 Mineral-templated Growth of Natural Graphite Films
Mark A. van Zuilen1,2, Daniel Fliegel3, Richard Wirth4, Aivo Lepland5,6, Yuangao Qu2,
Anja Schreiber4, Alexander E. Romashkin7, and Pascal Philippot1.
1
Equipe Géobiosphère, Institut de Physique du Globe de Paris - Sorbonne Paris Cité, Université
Paris Diderot, UMR CNRS 7154, 1 rue Jussieu, 75238 Paris cedex 5, France.
2
Centre for Geobiology, University of Bergen, Allegaten 41, 5007, Bergen, Norway.
3
National Institute of Nutrition and Seafood Research, Strandgaten 229, 5004 Bergen, Norway.
4
GeoForschungsZentrum Potsdam, Telegrafenberg, Chemistry and Physics of Earth Materials, D-
14473 Potsdam, Germany.
5
Geological Survey of Norway, Leiv Eirikssons vei 39, 7491 Trondheim, Norway.
6
Tallinn Technical University, Institute of Geology, 19086 Tallinn, Estonia.
7
Institute of Geology, Karelian Science Centre, Pushkinskaya 11, 185610, Petrozavodsk, Russia
* To whom correspondence should be addressed:
e-mail: vanzuilen@ipgp.fr , phone: +33(0)1 83 95 73 89
1
ABSTRACT
Organic material in sediments is progressively altered during diagenesis and
metamorphism, leading to the formation of kerogen and ultimately crystalline graphite.
Bulk carbonaceous material in metamorphic terrains typically has attained an overall
degree of structural order that is in line with peak metamorphic temperature. On a micronto nano-scale, however, carbonaceous material can display strong structural variation. The
main factor that drives this variation is the chemical and molecular heterogeneity of the
precursor biologic material. Specific conditions during metamorphism, however, can also
play a role in shaping the microstructure of carbonaceous material. Here we describe the
structural variation of carbonaceous material in rocks of the 2.0 Ga Zaonega Formation,
Karelia, Russia. Raman spectroscopy indicates that bulk carbonaceous matter in these
rocks has experienced peak temperatures between 350-400°C consistent with greenschistfacies metamorphism. On a nano-scale, however, a strong structural heterogeneity is
observed. Transmission electron microscopy (TEM) reveals the occurrence of thin films of
highly ordered graphitic carbon at mineral surfaces. These graphite films – consisting of
20-100 individual layers – completely envelop quartz crystals and occur on specific crystal
surfaces of chlorite. It is proposed that minerals can act as templates for the parallel
ordering of carbon crystallites causing enhanced graphitization within narrow zones at
mineral surfaces. Alternatively, oriented organic precursor molecules could have been
adsorbed onto charged mineral surfaces, leading to thin graphitic films during later
metamorphic heating episodes. Overall the presented observations demonstrate that
mineral surfaces can initiate and accelerate localized graphitization of sedimentary organic
2
material during metamorphism, and therefore cause distinct nano-scale variation in
structural order.
1.
INTRODUCTION
Organic material in Precambrian sediments has invariably been subjected to elevated
temperatures and pressures during progressive diagenesis and metamorphism. This has led to the
loss of functional groups and heteroatoms, to aromatization of the carbonaceous phase, and
ultimately to the formation of crystalline graphite. The steps involved in this maturation process
have been studied in great detail (Bernard et al., 2010; Beyssac et al., 2003a; Beyssac et al.,
2002b; Buseck and Huang, 1985; Bustin et al., 1995; Wopenka and Pasteris, 1993). It is generally
accepted that graphitization is a continuous process in which the overall degree of structural order
of bulk carbonaceous material (CM) – estimated by the average size of sp2-bound carbon
crystallites – can be directly linked to progressive grades of peak metamorphic temperature.
Crystalline graphite phases are typically formed at a metamorphic grade of amphibolite-facies or
higher. On a nano-scale, however, CM of a certain metamorphic grade can display strong
structural variation and can contain a wide variety of sp2-bound carbon nano-structures including
graphitic globules (Kovalevski et al., 2001), carbon onion rings (Beyssac et al., 2002b; Buseck
and Huang, 1985), graphite cones (Jaszczak et al., 2003), rolled graphitic structures (Jaszczak et
al., 2007), and curled graphite structures (Papineau et al., 2010). In general this can be related to
the fact that organic precursor materials are heterogeneous in both molecular structure and
chemical composition. Many organic precursor molecules have a basic planar hexagonal
structure (e.g. highly aromatic compounds) and are easily converted to graphite, while others
contain non-planar parts or 5-membered ring systems which make them particularly resistant to
3
graphitization. In addition, the presence of certain hetero-atoms could either inhibit or catalyze
graphitization (Buseck and Huang, 1985, and references therein). Since natural biologic precursor
materials are complex organic mixtures containing both graphitizing and non-graphitizing
compounds, the resulting metamorphic CM thus commonly shows strong nano-scale structural
variability. The precursor material, however, is not the only factor that governs structural
heterogeneity. A small scale structural variability in CM can also be influenced by external
factors during metamorphism such as the circulation of metamorphic fluids, or the presence of a
specific mineral matrix (Beyssac et al., 2003a; Bonijoly et al., 1982; Bustin et al., 1995; Ross and
Bustin, 1990; Wopenka and Pasteris, 1993). It is currently not well established to what extent
such factors play a role in shaping the microstructure of carbonaceous material in metamorphosed
rocks. A particularly intriguing example of structural heterogeneity in CM is presented by
organic rich rocks of the 2.0 Ga Zaonega Formation, Karelia, Russia (Figs.1-2). The CM in these
rocks has previously gathered wide attention and excitement due to its non-graphitizing nature
(Khavari-Khorasani, 1979), reports of curved globular carbon units (Kovalevski, 1994;
Kovalevski et al., 2001) and the proposed presence of natural fullerenes (Buseck et al., 1992).
The extremely small amounts of reported fullerenes, and lack of a clear understanding of a
formation mechanism (Ebbesen et al., 1995), however, tempered further interest in finding
geologically plausible processes for this type of carbon-based nano-materials. Here we present a
study based on Raman spectroscopy and transmission electron microscopy (TEM) that further
describes the unusual variation in sp2-bound carbon structures in the Zaonega Formation. In
particular the effect of mineral surfaces on the generation of CM nano-structures is explored in
detail. Overall, these results will shed light on the complex processes involved in metamorphic
graphitization of biologic material.
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2.
GEOLOGIC DESCRIPTION
The Paleoproterozoic Zaonega succession of siliciclastic, carbonate and sedimentary
rocks, and mafic tuff, interlayered and intersected by mafic lavas and sills (Galdobina, 1987), is
exceptionally rich in organic carbon (Fillipov, 1994), and represents one of the earliest geological
manifestations of significant petroleum generation in Earth history (Melezhik et al., 2009;
Melezhik et al., 1999). These organic-rich rocks have often been referred to as shungite (after
Shunga village in Karelia), which in the literature appears as a general term for organic carbonrich rocks from the Paleoproterozoic Onega Basin (Buseck et al., 1997; Fillipov, 1994;
Kovalevski et al., 2001), although many authors (Kovalevski, 1994; Melezhik et al., 1999)
describe shungite as a type of CM found in these rocks following the original terminology of
Inostranzev (1879). The minimum age of ca. 1.98 Ga for the Zaonega Formation is based on the
dating results from the mafic-ultramafic body in the overlying volcanic sequence (Puchtel et al.,
1998; Puchtel et al., 1999). CM occurs in rocks of the Zaonega Formation as autochtonous
kerogen residues and allochtonous (migrated) pyrobitumen. Several intervals are pervasively
impregnated with pyrobitumen that results in obliteration of sedimentary layering, and in places
massive appearance of the host sedimentary rock (Fig.2). Numerous veins consisting largely of
pyrobitumen or containing also quartz and carbonate can be observed throughout the Zaonega
Formation as well as in overlying sedimentary and volcanic rocks (Melezhik et al., 2009;
Melezhik et al., 1999). Pyrobitumen-rich intervals that are in places rich in silicates (Melezhik et
al., 2004), are found in the contact zones of magmatic bodies, but also occur as layers within
sedimentary successions. CM in the Zaonega Formation display a large overall range in δ13Corg (45 to -17 ‰) and a low average H/C-ratio (0.005-0.2), indicating various precursor materials and
5
a complex history of oil generation, migration, and subsequent thermal alteration (Buseck et al.,
1997; Fillipov, 1994; Khavari-Khorasani, 1979; Melezhik et al., 1999). The association of
pyrobitumen with mafic lava flows and co-magmatic sills is consistent with a contact-heatinduced hydrocarbon generation and migration (Rankama, 1948), coeval and/or shortly postdating the sedimentation. Isolated pyrobitumen-rich intervals that do not associate with magmatic
bodies are typically massive exhibiting cryptic fluidal textures and signs of multiphase,
syndepositional micro-brecciation. These intervals appear to represent organic-rich sediment
masses generated during the course of hydrocarbon migration that have been injected into the
host sediment or expelled onto the sea-floor as mud-volcanos (Melezhik et al., 2004). The highsilica content (quartz, feldspars) of some pyrobitumen-rich rocks is likely related to wide-scale
hydrothermal circulation and silica-leaching during hydrocarbon formation and migration. This is
consistent with the water-rock-organic matter interaction in shallow depths of a geothermally
active sedimentary basin where heat was provided by lavas and sills. Hydrocarbons, particularly
the ones occurring in veins may have also been generated during burial and regional
metamorphism that reached greenschist-facies (chlorite-actinolite-epidote paragenesis indicates
300-350ºC) during the 1.8 Ga Svecofennian orogeny (Melezhik et al., 1999).
3.
MATERIALS AND METHODS
Samples 12A-52-55, 12B-140-06, 12B-410-00, and 12B-413-11 were derived from
various depths from overlapping drill cores 12A and 12B (62° 29.711N, 31° 17.460E, Figs.1-2)
that was obtained in the frame of the International Continental Scientific Drilling Program's
(ICDP) Fennoscandian Arctic Russia – Drilling Early Earth Project (FAR-DEEP). The combined
cores of 12A and 12B provides 504 m long continuous rock record, and intersects several
6
organic-rich intervals and igneous bodies within the Zaonega succession. The samples were
collected from pyrobitumen-rich intervals from contact zones with magmatic bodies (12A-52-55,
12B-410-00, 12B-413-11) and from a sedimentary rock-hosted, pyrobitumen-type mush unit
(12B-140-06). An additional field sample (SHU-06-19) was collected at Krasnaya Gorka (62°
31.206N, 31° 15.926E, Fig.1) and represents a mudstone-hosted pyrobitumen-rich vein. All
samples were studied by Raman spectroscopy, and samples SHU-06-19, 12B-140-06, and 12B413-11 were studied in detail by transmission electron microscopy (TEM).
A Horiba-Jobin Labram 800 HR Raman spectrometer and an Olympus BX41
petrographic microscope at the Centre for Geobiology, University of Bergen, Norway, were used
on polished thin sections (200 µm thick) of the rock samples. Polishing of a rock surface during
thin section preparation causes breakup of the carbon crystal structure which leads to a higher
degree of disorder (Beyssac et al., 2003b), and therefore to shifts in peak intensities in the Raman
spectrum. In order to avoid these disorder-related artifacts measurements were performed by
focusing the laser beam through the transparent crystals on the thin section surface.
Measurements were made with a 100 mW Argon ion Laser, by focusing a 1 µm spot of 514 nm
(green) light on the sample through a 100x objective. The laser light intensity was diminished to
ca. 15-20 mW by using a density filter. Comparative tests at lower laser power (e.g. 0.1 mW)
confirmed that laser heating of the sample did not cause any change in the structure of the CM
phases. The spectrometer was calibrated daily before each analytical session by ‘zero-point’
centering, and analysis of a Si-standard with a characteristic Si Raman band at 520.4 cm-1.
Spectra were obtained for 2x10 seconds in multi-window mode over a range of 150-4,000 cm-1,
using an edge filter for 514 nm excitation wavelength with a 100 cm-1 cut-off, a 100 µm entrance
slit, a 1,800 lines/mm grating, and a Peltier-refrigerated (-70°C) 1024x256 pixel CCD array
detector. Although background due to fluorescence was minimal, Raman spectra were baseline7
subtracted by automatic polynomial fitting. Subsequently Raman peak characteristics were
obtained by peak de-convolution using a Gauss-Lorentian function with 100 iterations per fit,
with the program Labspec version 5.58.25. Peak widths were derived at full width half maximum
(FWHM).
Focused ion-beam (FIB) milling was used as a site-specific and contamination-free
sample preparation method for measurements by transmission electron microscopy (TEM). FIB
was conducted on Pt-coated sample thin sections using a FEI FIB2000-TEM equipped with a Gaion source at GeoForschungsZentrum Potsdam (GFZ; Germany). FIB-slide dimension were on
average 15x10x0.10 µm. Milled FIB slides were lifted out of the FIB sputtering site and
subsequently placed on standard Cu TEM grids covered with perforated amorphous carbon film
(lacey carbon). Details of this FIB preparation technique are given in Wirth (2009), and are
shown in electronic annex EA-1. Transmission electron microscopy was carried out at the GfZ,
Germany. The TEM used for this study was a FEI TecnaiG2 F20 X-TWIN equipped with a Gatan
Imaging Filter GIF (Gatan Tridiem, Gaatan, CA, USA), EDAX X-ray analyzer and a Fishione
high-angle annular dark-field detector (HAADF). The electron source of the TEM was a field
emission gun emitter, operated at an acceleration voltage of 200 kV. Electron energy-loss
spectroscopy (EELS) was performed to study the distribution of silicon and carbon within
selected areas of the FIB sample foils. For this purpose electron images (using a window of a
specified width, 20-40 keV) were obtained both above and below the C-edge or Si-edge
respectively. Subtraction of both images resulted in a jump ratio image displaying the relative
abundance of the element of interest (Williams and Carter, 2009).
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4.
RESULTS
4.1
Raman spectroscopy
We used Raman spectroscopy to study the degree of structural order of the bulk CM in
our samples. In order to avoid artifacts due to polishing, the laser beam was focused through
quartz crystals on the surface (Fig.3). The Raman shift at 1580 cm-1 (G-peak) represents E2g bond
stretching of hexagonal ring structures that make up crystalline graphite. The Raman shifts at
1350 cm-1 (D1-peak) and 1620 cm-1 (D2-peak) represent vibrations associated with the A1g
breathing mode of sp2 rings. This vibration becomes possible in ‘disordered’ or nano-crystalline
structures where the in-plane crystal domain size is limited by defects. Tuinstra and Koenig
(1970) initially showed that the intensity ratio of the D1- to G-peak varies inversely with the
domain size (La) of various graphitic materials. This ratio should thus describe the
transformations that organic material undergoes during progressive metamorphic-induced
graphitization. However, in natural disordered CM many out-of-plane defects occur, which are
related to tetrahedral carbons, dangling bonds and hetero-atoms. This gives rise to a broad shift at
1500 cm-1 (D3-peak) causes a broadening of the D1-peak and leads to partial overlap between the
G-peak and D2-peak (Beyssac et al., 2002a; Wopenka and Pasteris, 1993). The average degree of
structural order of metamorphosed CM is therefore typically described by empirically derived
indicators, such as the peak-area based ratio R2= AD1/(AD1+AD2+AG) (Beyssac et al., 2002a;
Wopenka and Pasteris, 1993). Studies of variously metamorphosed CMs have shown that the
relationship TMax (°C)= -445* R2+641 (+/- 50°C) most faithfully describes the degrees of
structural order at peak-metamorphic temperatures in the greenschist to granulite metamorphic
range (Beyssac et al., 2002a). Shungite samples SHU-06-19 and 12B-140-06 have an R2 of ca.
9
0.61-0.63 while 12B-413-11 has a slightly lower R2 of 0.53 (Table 1). Such R2 values indicate
that the CM in these rocks experienced a peak temperature between ca. 360-400°C. Sample 12B413-11 is derived from a 4 m thick layer of highly bituminous rock directly above a gabbroic
intrusive sill (Fig.2). The slightly higher degree of structural ordering in this carbonaceous phase
is thus likely caused by contact metamorphism. The degree of structural order of CM is also
reflected by overtone scattering of the D1-peak at ca. 2700 cm-1 (Fig.3). In poorly ordered CM
this band is absent while in pure crystalline graphite it occurs as a doublet of two partially
overlapping peaks (Wopenka and Pasteris, 1993). The splitting of these peaks is only observed in
CMs that have reached three-dimensional order during the final stage of graphitization. In most
partially graphitized CMs a prominent symmetric 2700 cm-1 peak (relative to the first-order Gband) is observed (Fries and Steele, 2008; Lepland et al., 2011; Papineau et al., 2011; Wopenka
and Pasteris, 1993). This has generally been attributed to the presence of turbostratic graphite
(graphitic material in which tri-periodic order is lacking) or to the presence of carbon crystallites
with highly variable dimensions (Wopenka and Pasteris, 1993). In the studied shungite samples –
12B-413-11, 12B-410-00 and 12B-140-06 – this peak is symmetric and quite intense relative to
the G-band (Fig.3, Table 1), which indicates that highly ordered carbon crystallites are present
but the tri-periodic order is lacking. Finally, a small additional band at ca. 2940 cm-1 occurs in all
studied samples (Fig.3, Table 1). It can be attributed either to C-H stretching or to the presence of
poorly ordered carbon (Beyssac et al., 2002a; Wopenka and Pasteris, 1993).
4.2
Transmission electron microscopy
4.2.1 Graphite films on quartz surfaces
10
Bright field (BF) images of the samples revealed the occurrence of quartz grains within a
poorly ordered carbonaceous (POC) matrix that are fully enveloped by thin graphite films (GFs)
(Fig.4a, and EA-2 to EA-4). EELS jump ratio images of silicon (Fig.4b) and carbon (Fig.4c)
confirm the presence of a pure carbon-based structure in contact with the quartz crystal surface,
and show that the surrounding POC matrix contains minor silicon. High-resolution transmission
electron microscope (HRTEM) images show lattice fringes that represent the extent and
interlayer spacing of individual carbon layers. The GFs around quartz crystals are between 7 and
35 nm thick, corresponding to stacks of ca. 20 to 100 layers (Figs.5a, 5e and 6b, details of
thickness measurements are shown in electronic annex EA-5). In sample 12B-140-06 the GFs are
relatively thin (Fig.5a, EA-5) and the POC matrix consists of small and variously oriented
crystallites displaying a diffuse electron diffractogram (Fig.5d). In contrast, sample 12B-413-11
contains GFs that are relatively thick (Fig.5e, EA-5) and the POC matrix consists of larger subparallel crystallites displaying individual spots in the electron diffractogram (Fig.5h). As was
discussed in the previous paragraph sample 12B-413-11 was likely influenced by contact
metamorphism from an underlying gabbroic sill (Fig.2). The presence of relatively welldeveloped GFs and a slightly more ordered POC matrix in this sample thus appears to be the
result of extended heating. These TEM observations can also explain the relatively low Ramanbased R2 value and the relatively enhanced peak-intensity of the 2700 cm-1 peak in this sample
(Table 1). During Raman analysis the laser beam was focused through the crystal, directly
irradiating the GF that is attached to its surface. The depth of Raman analysis in CM is typically
less than 100 nm (Wopenka and Pasteris, 1993), suggesting that thick GFs of ca. 30 nm formed a
significant scattering contribution to the obtained Raman spectrum.
Within the GFs the highest structural order occurs at the direct contact with the quartz
surface where individual layers can be traced for several nanometers (Fig.5a,e). Defects and
11
distortions are progressively more common away from the quartz surface, as recorded in the
electron diffractograms by a shift from sharp spots representing aligned 0002 lattice planes
(Fig.5b,f) to more diffuse spots and rings (Figs.5c,g). This shows that growth of highly ordered
GF was initiated at the mineral surface, and additional carbon layers accumulated progressively
more defects and dislocations.
4.2.2 Graphite films on chlorite surfaces
Many mineral phases other than quartz are commonly found in shungites, including
calcite, dolomite, muscovite, albite and chlorite. In this study we describe the occurrence of GFs
on the surface of chlorite crystals. In contrast to the ubiquitous presence of GFs enveloping
quartz crystals, only specific faces of chlorite crystals are associated with this carbonaceous
phase. Films of graphite develop along the 110 surface and not along the 001 surface of the
crystal (Fig.6a, additional pictures are shown in the electronic annex EA-6). A HRTEM image
(Fig.6b) reveals that GFs on chlorite are equal in thickness as those on quartz in the same sample
(compare with Fig.5e), and that the degree of stacking order varies from highly aligned defectfree graphitic sheets at the contact with the 110 plane to short and distorted graphitic units at the
contact with the 001 plane. This decrease in structural order is also observed in the electron
diffractograms (Fig.6c,d,e) where individual spots representing well-aligned 0002 planes of
graphite change to diffuse rings representing strongly misaligned graphitic units. These
observations, taken together, imply that the occurrence of GF is specifically associated with a
growth process on a chlorite 110 surface and not a 001 surface (schematic representations of
these surfaces are shown in electronic annex EA-7). This strongly suggests that emerging carbon
12
crystallites were not merely aligning along a random flat surface, but were guided by a templatedependent process.
5.
DISCUSSION
The specific occurrence of graphite films on the surface of quartz and chlorite crystals in
the studied carbonaceous samples of the Zaonega Formation is unusual and deviates from the
expected structural changes that occur during metamorphic graphitization. During progressive
diagenesis and metamorphism organic macromolecular aggregates gradually lose functional
groups causing aromatic parts to grow in size until small sp2-bound carbon crystallites of
approximately 4-10 stacked layers are formed (Bustin et al., 1995). Sustained conditions of
elevated temperature and pressure cause in-plane growth, assimilation, alignment, and stacking of
such crystallites leading ultimately to the formation of crystalline graphite with 3D structure
(Beyssac et al., 2002b; Buseck and Huang, 1985; Bustin et al., 1995). The efficiency of this
process depends on the initial micro-texture and the presence of strong cross-linking bonds
(Franklin, 1951) within the organic precursor, the liberty of the formed crystallites to rearrange
and align themselves, and the degree to which defects are eliminated and aromatic layers are
annealed (Bustin et al., 1995). Metamorphosed CMs are therefore complex heterogeneous
mixtures on a nanoscale, and can include micro-porous domains and structures such as carbon
onion-rings (Beyssac et al., 2002b; Buseck and Huang, 1985). Earlier studies on shungites have
shown that this type of carbonaceous material typically contains highly variable domains of
structural order, and it has been generally assumed that this mainly reflects the chemical and
molecular heterogeneity of the organic precursor material (Buseck and Huang, 1985; Kovalevski
et al., 2001). The mineral-associated graphite films which are described here clearly show that
13
specific additional factors in the metamorphic process can cause nano-scale structural variability
of a carbonaceous phase.
The organic-rich rocks of the Zaonega Formation have been influenced by petroleum
generation and migration, hydrothermal circulation, hydrocarbon-gas release, mineral
authigenesis, and have experienced contact and regional metamorphism. Under these
circumstances the rock matrix was exposed to a mixture of organic macromolecular material and
silicate-rich aqueous fluids. Authigenic mineral growth (Fig.7a) within this bitumen-rich fluid
could have caused localized domains of strain and stress that facilitated carbon-based crystallite
alignment and preferred graphitization. This general process of re-orientation near mineral
surfaces was likely enhanced by circulating hydrothermal fluids, which cause aggregation and
reorganization of graphitic clusters (Calderon Moreno et al., 2000; Sevilla and Fuertes, 2009).
Moreover, hydrocarbon gas release during cracking of organics could have introduced additional
surfaces (gas bubbles) for passive graphite crystallite alignment.
There are also indications that minerals were actively influencing graphite nucleation. In
the specific case of chlorite, the GFs are absent on the 001 face, but are present on the 110 face of
the mineral (Fig.6, electronic annex EA-6), implying that nucleation and growth of graphite was
template-dependent. This could be a simple result of closely matching lattices and inter-atomic
distances, facilitating effective growth of graphite where nucleation is most easily initiated.
However, it is also possible that liquid organic precursors were adsorbed onto charged mineral
surfaces. Chlorite is an electrokinetically anisotropic phyllosilicate (Fuerstenau and Pradip, 2005)
for which overall surface charge is dependent on the crystal face. The extended sheets of silica
rings in the 001 plane are electrically neutral and are held together by weak van der Waals forces.
Charge on this surface predominantly arises from lattice defects or incongruent dissolution
(Alvarez-Silva et al., 2010). In contrast, the edge of a chlorite crystal – e.g. the 110 plane – has an
14
enhanced surface charge due to broken covalent bonds and exposed hydroxyl groups (Fig.7b).
Specific chlorite surfaces thus exhibit a strong affinity for polar organic macromolecules.
Adsorption processes are generally complex phenomena, depending on charge transfer
interactions, electrostatic interactions, van der Waals interactions, steric interactions, hydrogen
bonding, molecular aggregate formation, and the presence of solvent molecules. Despite this
multitude of factors, adsorption of polar organic molecules such as asphaltenes has been observed
on many different mineral surfaces in petroleum-rich rocks (Balabin and Syunyaev, 2008;
Buckley; Lebedeva and Fogden, 2011; Mendoza de la Cruz et al., 2009; Pernyeszi et al., 1998;
Zheng et al., 2001). It is thus possible that adsorption also occurred on the surface of quartz
crystals in the Zaonega organic-rich rocks, for instance, by attraction of polar macromolecules to
negatively charged silanol (Si-O-) groups (Fig.7c) (Zheng et al., 2001). Once thin zones of
parallel alignment were established on these surfaces, heat-induced graphitization would be
highly efficient leading to template-induced graphitization (Fig.7d), and the formation of GFs at
mineral surfaces (Fig.7e). The general metamorphic history of these rocks is complex and active
mineral adsorption of precursor organic macromolecules could only have occurred during the
early episodes of petroleum generation and aqueous fluid circulation. Minerals that formed
during late episodes of fluid circulation in a highly viscous bitumen matrix may have simply
constituted a matching lattice for alignment of emerging carbon crystallites. It remains to be
established whether this type of graphite film also exists on mineral surfaces other than chlorite
and quartz.
6.
CONCLUSIONS
15
It has been often suggested that the nano-scale structural variability of metamorphic CMs
is in large part reflecting chemical and molecular heterogeneity of the organic precursor. The
detailed study presented here of carbonaceous material in lower greenschist-facies rocks of the
Zaonega Formation, Russia, clearly shows that nano-scale variation in structural ordering can
also be caused by mineral template induced graphitization. Quartz and chlorite mineral surfaces
can initiate and/or accelerate graphitization of sedimentary organic matter in relatively low-grade
metamorphic terrains. It remains to be determined whether this type of surface-induced
graphitization also occurs on other mineral phases. The difference in templating effect among
minerals can have implications for the geologic interpretation of carbonaceous structures, e.g.
Raman-based determination of peak-metamorphic temperature. For instance CM in a quartz- or
chlorite-dominated greenschist-facies rock could have obtained a different degree of structural
variability than CM that is embedded in a greenschist-facies carbonate rock. Future studies on
CM in different mineral matrices can possibly solve these issues, and likely shed light on the
generation of intriguing sp2-bonded carbon nanostructures that found in nature. Finally, this
mineral-template-induced process may be of interest for the synthesis of sp2-bonded carbon
structures. It can possibly lead to innovations in carbon-based nano-materials such as 3D-fiberlattice networks (Lee et al., 2011), meso-porous thick films (Kao and Hsu, 2008), thin graphite
films (Obraztsov et al., 2007), and carbon-microtube rings (Wang et al., 2009).
7.
ACKNOWLEDGMENTS
This study was based on samples from the ICDP FAR-DEEP drilling project in Karelia, Russia.
We thank members of the central science team T. Fallick, T. Prave, H. Strauss, E. Hanski, L.
Kump, M. Philipov, and especially principal investigator V. Melezhik for coordinating this
16
project and for providing geological background. M. Filippov is thanked for the overview of the
Zaonega Formation field geology, and stimulating discussions in the field. Funding for TEM and
Raman analysis (MvZ, DF) was provided by the Centre for Geobiology at the University of
Bergen. The NFR grant 191530/V30 to Victor Melezhik fully funded the work of AL and YQ,
and in part the work of MvZ. Field work in Karelia (MvZ) was funded through an ANR-BLANC
grant to PP. We are grateful to Marc Fries and two anonymous reviewers, as well as the associate
editor Tom McCollom, for their detailed reviews. This is IPGP contribution No. 3254.
8.
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24
9.
TABLES
Table 1
Raman spectral characteristics of carbonaceous matter in studied samples of the Zaonega
Formation, Karelia, Russia.
Sample
R2
Tmax(°C)
I2700-G
I2700-2940
SHU-06-19-1
0.61
370
0.42
2.49
SHU-06-19-2
0.60
372
0.40
2.35
SHU-06-19-3
0.64
358
0.33
2.56
SHU-06-19-4
0.61
369
0.43
2.49
SHU-06-19-5
0.61
367
0.40
2.35
SHU-06-19-6
0.63
360
0.32
2.57
12A-52-55-1
0.66
346
0.27
1.64
12A-52-55-2
0.63
361
0.27
2.10
12A-52-55-3
0.62
366
0.41
2.26
12A-52-55-4
0.63
361
0.36
1.72
12B-140-06-1
0.63
363
0.33
2.21
12B-140-06-2
0.61
369
0.30
2.60
25
12B-140-06-3
0.60
373
0.37
2.88
12B-140-06-4
0.60
375
0.45
2.38
12B-140-06-5
0.59
380
0.41
2.80
12B-410-00-1
0.56
391
0.50
5.69
12B-413-11-1
0.53
406
0.38
5.12
12B-413-11-2
0.52
411
0.46
4.56
12B-413-11-3
0.55
396
0.46
3.91
12B-413-11-4
0.50
418
0.40
4.39
R2= AD1/(AD1+AD2+AG), where A is the peak-area. Tmax (°C) is the temperature at peak
metamorphism, calculated using the relationship TMax (°C) = -445* R2+641 (+/- 50°C) (Beyssac
et al., 2002). I2700-G is the peak intensity ratio of the 2700 cm-1 peak over the first-order G-peak,
I2700-2940 is the peak intensity ratio of the 2700 cm-1 over the 2900 cm-1 peak.
10.
FIGURE CAPTIONS
Figure 1. Simplified geological map of the Paleoproterozoic Onega Basin (modified after
(Koistinen et al., 2001)) showing the distribution of rocks of the Zaonega Formation, and
positions of FAR-DEEP drill holes 12A and 12B and the locality of Krasnaya Gorka.
26
Figure 2. Simplified lithologic column of FAR-DEEP Cores 12A (0-94.57 m) and 12B (94.57504.26) with sample positions and TOC abundances (total organic carbon, in wt%). Compilation
by Alenka Črne and Aivo Lepland, Norwegian Geological Survey.
Figure 3. a) Raman spectrum of carbonaceous matter of Zaonega sample 12B-140-06, measured
through a quartz crystal on the surface of a polished thin section. Peak G (1580 cm-1) represents
the E2g bond stretching and peaks D1 (1350 cm-1) and D2 (1620 cm-1) the A1g breathing mode of
sp2 hybridized hexagonal ring structures. Peak G’ (2705 cm-1) represents the second harmonic of
the D1-peak, while the small peak at ca. 2900 cm-1 can be attributed either to strong disorder or
C-H bond stretching, but its origin is still not fully understood (Beyssac et al., 2002a; Wopenka
and Pasteris, 1993). Quartz is represented by a peak at 461 cm-1.
Figure 4. Transmission electron microscopy (TEM) images of quartz and carbonaceous matrix in
a 15x10x0.1 µm foil of sample SHU-06-19, prepared by Focused Ion Beam (FIB) milling. Scale
bar is 100 nm. a) Bright field (BF) image illustrating the structural contrast between GFs
occurring as thin bands around quartz grains and the POC matrix. The light grey band running
from upper right to lower left (see arrow) is an artifact, representing the underlying carbon-based
support film. b, c) Electron energy loss spectroscopy (EELS) jump ratio images of silicon (b) and
carbon (c) from the same area as in (a). Bright tones represent higher abundance of the analyte.
Additional TEM images of quartz crystals in other sample foils are shown in electronic annexes
EA-2 to EA-4.
Figure 5. High-resolution transmission electron microscope (HRTEM) images and selected area
electron diffractograms (SAEDs, as seen by fast Fourier transform FFT from HRTEM images) of
27
quartz- carbonaceous phase contacts. Scale bars are 5 nm. a) In sample 12B-140-06 a 14 nm thick
GF band occurs on the quartz surface, and is surrounded by POC matrix. b,c,d) SAEDs
representing a GF at the contact with quartz (b), at the contact with POC (c), and bulk POC itself
(d). e) In sample 12B-413-11, a 22 nm thick GF band occurs on the quartz surface, and is
surrounded by POC matrix. f,g,h) SAEDs represent a GF close to the contact with quartz (f), GF
close to the contact with POC (g), and bulk POC matrix (h). The blurring in the SAEDs indicates
an increase in disorder.
Figure 6. BF-TEM, HR-TEM images and SAEDs from sample 12B-413-11 representing the
chlorite-GF contact. a) BF-image showing structural contrast between a GF that is in contact with
the chlorite 110 face and POC forming the wedge between GF and the chlorite 001 face. Scale
bar is 100 nm. b) HRTEM image of the 22 nm wide GF band shown in (a). Carbon layers are
perfectly aligned with the 110 surface but not with the 001 surface. Scale bar is 5 nm. c, d, and e)
The SAEDs illustrating the decrease in structural alignment within the GF band with increasing
distance from the chlorite 110 face. More detailed TEM images of these chlorite crystals are
shown in electronic annex EA-6.
Figure 7. Models for the mineral-induced formation of graphite film (GF) in carbonaceous rocks
of the Zaonega Formation. a) Passive alignment model. Compression and heating of bitumen
causes emerging sp2-bonded carbon crystallites to align along mineral surfaces. Hydrothermal
circulation and bi-directional expansion of crystal surfaces during mineral authigenesis would
have further enhanced this parallel alignment. b-c) Active, adsorption-controlled model. b)
Chlorite [(Mg,Fe2+)5(Al,Fe3+)2Si3O10(OH)8] consists of alternating talc-like tetrahedral sheets and
brucite-like octahedral sheets (blue = Si, red = O, black = OH, purple = Al or Fe, green = Mg).
28
Broken bonds at the edge of these stacked sheets are a source for surface charge. This
electrokinetically anisotropic behavior (Alvarez-Silva et al., 2010) leads to preferential
adsorption of polar organic macromolecules on specific oriented faces (e.g. the 110 plane as
observed in this study). c) Structure of quartz [SiO2] showing Si-OH (silanol) groups (blue = Si,
red = O). Large organic macromolecules containing positively charged functional groups (Zheng
et al., 2001) can be adsorbed on such a surface. d) A surface film of aligned precursor aromatic
molecules or emerging carbon crystallites is highly susceptible to heat-induced graphitization. At
a mineral surface a graphite film (GF) will thus be generated, while in the surrounding matrix a
lack of templates leads to a poorly ordered carbon (POC). e) Lower greenschist-facies
metamorphism resulted in formation of GFs that envelope quartz crystals and occur on specific
chlorite surfaces.
29
Figure 1
D
Figure 2
Figure 3
G
Laser
Quartz crystals
D1
Quartz
2700
D2
D3
500
1,000
2900
1,500
2,000
Raman shift (cm-1)
2,500
3,000
3,500
Figure 4
b
a
POC
Quartz
Artifact
c
Quartz
POC
GF
Figure 5
14 nm
a
Quartz
GF
b
22 nm
e
POC
Quartz
GF
POC
c
d
f
b
c
d
f
g
h
g
h
Figure 6
a
b
22 nm
Chl-001
e
d
c
b
Chl-110
Chl-001
Chl-110
c
GF
POC
d
e
Figure 7
a
b
c
R
δ+
-
Mineral
R
δ+
-
R
δ+
-
-
R
δ+
H
-
H
e
d
R
P,T
R
GF
R
POC
R
R
R
R
R
R
-
H
R
R
R