New textural evidence on the origin of carbonado diamond:
An example of 3-D petrography using X-ray computed tomography
Richard A. Ketcham1 and Christian Koeberl2
1
High-Resolution X-Ray Computed Tomography Facility, Jackson School of Geosciences, The University of Texas at Austin, Austin,
Texas 78712, USA
2
Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria, and Natural History
Museum, Burgring 7, A-1010 Vienna, Austria
ABSTRACT
Three-dimensional textural observations
of inclusion and porosity patterns in a 23carat carbonado diamond using high-resolution X-ray computed tomography reveal
new information bearing on the nature and
origin of this enigmatic material. A prominent patinaed surface is texturally linked to
a banding and grading of inclusions and pore
space beneath, extending several millimeters
into the specimen. In situ observation demonstrates that almost all inclusions are polymineralic and show replacement textures,
corroborating previous work indicating that
the pore network is fully three-dimensionally
(3-D) connected, and that virtually all macroinclusions are secondary. Large metal inclusions are only found immediately adjacent
to the margin of the specimen, and are thus
also likely to be secondary or even tertiary.
However, we also report pseudomorphs after
a phase forming pristinely euhedral rhombic
dodecahedra, individually and in clusters
from 0.3 to 1 mm in diameter; although we
could find no evidence of this phase persisting, it nevertheless represents the first “true”
macro-inclusion reported in carbonado,
which almost certainly formed syngenetically
with the diamond material. The pore system
is essentially trimodal, consisting of single
and clustered pseudomorphs, oblate pores
0.1–0.3 mm in length with a clear preferred
orientation, and 20 µm to <1 µm pores that
form the connected network. Our observations support recent work suggesting that
carbonado crystallized from a carbon-supersaturated fluid and suggest that the second
stage may correspond with the creation of
the pore alignment fabric. We further postulate that, although the present-day macroinclusions are certainly secondary, the bulk
material that comprises them may not be,
and may instead be broken-down remains of
the original included phase(s). While further
verification is needed, a model built around
this hypothesis may provide the simplest
explanation to many of the unusual features
of carbonado.
INTRODUCTION
Carbonado is an enigmatic polycrystalline
diamond variety found in placer deposits in
Brazil and the Central African Republic (Trueb
and Butterman, 1969; Trueb and De Wys, 1969,
1971). Carbonados are commonly millimeter to
centimeter sized, and the “Carbonado of Sergio”
from Brazil is the largest diamond known (Haggerty, 1999) at 3167 carats. Age determinations
range from 2.6 to 3.8 Ga, with large uncertainties
(Ozima and Tatsumoto, 1997; Sano et al., 2002).
The confined geographic distribution of carbonados, on land masses that were likely adjacent
at times in the Archean, suggests that they may
all have been created in a single event (Heaney
et al., 2005), although they are in some respects
similar to other polymineralic diamond varieties
such as framesites and yakutites (McCall, 2009).
Carbonado is set apart from other diamond
varieties by several unusual characteristics. The
diamond material is texturally diverse, being
dominantly porphyritic and consisting of small
(10–250 µm) diamonds cemented together by
even smaller (<1 µm) microdiamonds (De et al.,
1998; Petrovsky et al., 2010). The smaller crystals can have subplanar dislocations that may
be interpreted as defect lamellae, whereas the
larger ones are mostly defect free. Other, rarer
textures found in subregions are homogeneous,
flow texture (Yokochi et al., 2008) and columnar crystals reminiscent of vein infill (Rondeau
et al., 2008). Carbonado carbon isotopes are very
light, with δ13C values ranging from –21‰ to
–32‰, with major modes at –24‰ and –26‰,
rare for diamonds and suggestive of organic carbon (De et al., 2001; Kagi et al., 2007), although
the source for light carbon in the mantle remains
an open question (Deines, 2002). The diamond
material contains H (hydrogen) defects indicative of a hydrogen-rich environment (Garai et al.,
2006; Nadolinny et al., 2003), and N (nitrogen)
defects in various states of aggregation (Fukura
et al., 2005; Garai et al., 2006). Carbonado hosts
extensive porosity ranging from the nanometer up
to the millimeter scale. Some carbonado material
has been found to be enriched in isotopes characteristic of the products of spontaneous fission
of uranium (Ozima and Tatsumoto, 1997), but
cathodoluminescence imaging suggests that this
may be a secondary feature caused by infiltration
of the pore network by fluids enriched in radioactive elements (Kagi and Fukura, 2008; Kagi
et al., 2007; Rondeau et al., 2008).
The inclusion suite in carbonado has also
proven enigmatic. It hosts enclosed nanoinclusions of various metal alloys (Fe, Fe-Ni,
Ni-Pt, Si, Ti, Sn, Ag, Cu, and SiC), as well as
other minerals (calcite, sylvite, smithsonite)
that may be original (De et al., 1998) and some
(augite, ilmenite, phlogopite) that may predate the diamond-forming event (Sautter et al.,
2011). It also hosts larger metal particles tens
of micrometers in diameter within open pore
space (De et al., 1998; Fitzgerald et al., 2006).
Its macro-inclusion suite principally features
minerals of ostensibly crustal origin, such as
orthoclase, goethite, quartz, kaolinite, hematite,
serpentine, and florencite, a hydrous rare-earth
phosphate that commonly forms as an alteration product of monazite. The fact that extensive
leaching can extract virtually all compositional
impurities (Dismukes et al., 1988) and magnetic
components (Fitzgerald et al., 2006; Kletetschka
et al., 2000) suggests that all macro-inclusions
are probably secondary products precipitated
Geosphere; October 2013; v. 9; no. 5; p. 1336–1347; doi:10.1130/GES00908.1; 9 figures; 14 animations.
Received 8 February 2013 ♦ Revision received 22 May 2013 ♦ Accepted 25 July 2013 ♦ Published online 14 August 2013
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For permission to copy, contact [email protected]
© 2013 Geological Society of America
3-D petrography of carbonado
by fluids moving through a connected pore network. None of the inclusion phases found thus
far is typical of kimberlitic diamond, such as
pyrope, pyrrhotite, and chromian clinopyroxene
(Heaney et al., 2005), although olivine has been
reported (Trueb and De Wys, 1971). Ishibashi
et al. (2012) report a 0.3 µm void with euhedral walls that they interpreted as a former fluid
inclusion, though it was not preserved as such
due to sample preparation.
The origin of these diverse features in carbonado has eluded consensus for over 40 yr.
Hypotheses have included meteorite impact
(Smith and Dawson, 1985); formation during hot
Archean subduction of early organic material;
volcanic hydrothermal systems with metal-catalyzed diamond growth (McCall, 2009); extraterrestrial origin (Garai et al., 2006); and irradiation
of carbonaceous materials (Ozima and Tatsumoto, 1997). Recent work has favored formation
at mantle conditions, likely in the presence of a
C-O-H fluid supersaturated in carbon (Ishibashi
et al., 2012; Petrovsky et al., 2010), a C-H–rich
fluid in lower crust transiently metamorphosed at
mantle conditions (Sautter et al., 2011), or associated with komatiite magma intrusion into the
continental lithosphere (Cartigny, 2010).
Because carbonado is among the toughest substances in nature, it is difficult to study
using traditional petrographic techniques. With
the exception of early X-ray imaging (Trueb
and Butterman, 1969; Trueb and De Wys,
1969, 1971), most studies have concentrated
on microchemical analyses of small pieces
of material that were powdered, fractured,
Focused Ion Beam– (FIB) extracted, laser-cut,
or polished with laborious effort, providing a
spatially limited and largely two-dimensional
perspective. Most macro-inclusions have been
studied by disaggregating carbonado specimens. It is particularly challenging to examine
the macro-inclusions and diamond in concert on
anything but a fractured or laser-cut surface, due
to the extreme range of material hardness. As a
result, relating these features to each other has
been problematic. Here, we report on a textural
analysis using high-resolution X-ray computed
tomographic (HRXCT) imaging of a relatively
large specimen. Three-dimensional nondestructive imaging of pores and inclusions in situ
enables us to look at various textural details in
their spatial context, providing a holistic view,
and essentially allowing “three-dimensional
(3-D) petrography” to be performed.
SPECIMEN AND METHODS
Our sample is from the Central African
Republic, and it was purchased from a dealer in
Belgium. It is 23.45 carats (4.690 g), ~18 mm ×
13 mm × 10 mm, with a deltoid shape (Fig. 1).
One broad side and a smaller adjacent edge have
a silvery, vitreous patina, a common though not
ubiquitous feature in carbonados that has been
interpreted by some as a fusion crust (e.g.,
Shelkov et al., 1997), although overall the side
is rough. The other side is a dull dark gray and
rougher. Pores are visible to the naked eye on
all surfaces.
We imaged the specimen using the Xradia
MicroXCT scanner at the University of Texas
High-Resolution X-Ray CT Facility (http://
www.ctlab.geo.utexas.edu). The MicroXCT
has a unique design for a micro–computed
tomography scanner. Whereas most instruments utilize large detectors and geometric
magnification from a small X-ray focal spot to
achieve high resolution, the MicroXCT derives
its resolution from a set of specialized detectors consisting of a 35 mm camera lens or various microscope objectives coated with scintillating material. This arrangement produces
very sharp imagery and facilitates “zooming
in” to image small subvolumes within larger
specimens by simply switching detectors, in
much the same manner one switches objectives with a petrographic microscope (Figs.
2A, 2B, and 2C). With each such increase in
magnification, however, acquisition time rises
considerably, due to lower signal and the need
to acquire more densely sampled data (i.e.,
more views) to reduce the effect of parts of the
sample not in the region of interest but still in
the X-ray path.
The specimen was scanned at a range of resolutions with different detectors, using 29.2 µm
voxels to image the whole sample and 5.4 µm
and 1.0 µm voxels to image selected subvolumes within it. X-ray energies were varied from
40 to 140 kV, and some scans were repeated at
different energies to help with discrimination of
phases. Crystals of almandine, apatite, and calcite were also included in the field of view when
imaging the entire specimen to aid in interpretation of computed tomography (CT) numbers
(image gray levels).
Data were reconstructed as up to 900 ~1000 ×
1000 16 bit grayscale images per scan, and software corrections were applied to compensate
for ring and beam hardening artifacts. The volumetric data were visualized with Avizo (VSG,
Inc., versions 6.1 and 7.0) and VG Studio Max
(Volume Graphics, Inc., version 2.0). Measurements of metal inclusions were carried out with
Blob3D software, accounting for partial-volume
effects to compensate for their small size (Ketcham, 2005a, 2006), and 3-D determination of
inclusion preferred orientation was conducted
using star volume analysis with Quant3D software (Ketcham, 2005b).
Geosphere, October 2013
5 mm
Figure 1. Carbonado specimen as mounted
for micro–computed tomography (CT)
imaging, along with samples of calcite, apatite, and almandine garnet for comparison
of X-ray attenuation. The patinaed side of
the specimen is shown.
Following scanning, a single cut was made in
the sample using a laser at the Carnegie Institute. After polishing the surface on a diamond
wheel, scanning electron microscopy (SEM)
images and chemical analyses were obtained
using the JEOL JSM-6490LV at the University
of Texas.
Visualization was a particularly important
aspect of our methodology. It is not an exaggeration to say that many of the insights in this study
can only be obtained by viewing the CT imagery
in motion, either as slice animations stepping
through the data or as rotating volume renderings allowing structures and fabrics to be viewed
from many orientations. We thus provide a number of these animations as supplemental material.
These animations are also available at http://www
.ctlab.geo.utexas.edu/pubs/ketcham_koeberl
/ketcham_koeberl.htm.
Interpretation of CT Numbers for
Dual-Energy CT Scans
CT numbers reflect the linear X-ray attenuation coefficient of the materials being scanned,
which are a function of density, atomic number
(Z), and X-ray energy (Ketcham and Carlson,
2001). Due to the complexities of scanning, such
as the polychromatic nature of the X-ray source
and beam hardening and other artifacts, as well
as the variability of natural materials, which can
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Ketcham and Koeberl
A
B
2 mm
1 mm
C
D
0.2 mm
E
0.2 mm
0.2 mm
Figure 2. (A) 29.2-µm-resolution computed tomography (CT) image encompassing entire specimen cross section, with patinaed surface on
right. Circle indicates position of part B. (B) 5.3-µm-resolution image showing polymineralic nature of most inclusions. Circle indicates
position of C and D. (C) 1.0-µm-resolution, 140 kV scan showing pseudomorphs; bright material in center is probably florencite. (D) Same
field of view as C, imaged with X-rays set at 40 kV, increasing contrast of inclusion to lower right (further imagery in Animation 14). (E) Volume rendering of the high-attenuation material in pseudomorph in C, indicating replacement of a rhombic dodecahedron. Slice data are
given in Animations 1 and 2.
feature zoning, impurities, and microporosity,
absolute mapping of CT number to phases is
often not straightforward. As a result, CT data
in general do not have information sufficient
for independent mineral identification, necessitating prior knowledge of which phases are
likely to be present. The effective attenuation
coefficient for a mineral is essentially the integral of all attenuation coefficients over the range
of X-ray energies used, weighted by the relative
intensity of the X-ray flux at each energy, and
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other machine-specific factors such as detector efficiency as a function of energy. It is also
affected by beam hardening within the sample
and measures taken to correct for it.
The relationship between X-ray energy and
linear attenuation coefficient for the comparison
minerals used and some of the phases discussed
in this study is shown in Figure 3. The step
function for florencite is caused by the X-ray
absorption K edge of Ce at 40.4 keV; heavier
rare earth elements (REEs) have their K edges
Geosphere, October 2013
at progressively higher energies, up to 63.3 keV
for Lu. By scanning below and substantially
above 40 kV (Animations 1 and 2), the effective
attenuation coefficient of REE-rich phases will
change dramatically with respect to other minerals. In this case, Figure 3 suggests that when
the entire X-ray spectrum used is below 40 keV,
then florencite will have an effective attenuation coefficient similar to almandine, whereas
when a large portion of the spectrum is above
40 keV, then florencite will be substantially
3-D petrography of carbonado
10000
diamond
florencite
1000
spinel
kaolinite
Fe
Animation 1. Individual slice animation for
a 29.2-µm-resolution data set of carbonado
acquired with X-rays set at 40 kV, at two
contrast levels. Patinaed side of specimen is
on right side of images. Images are 17.9 mm
wide (1224 × 612 pixels; 10.4 MB). If you are
viewing the PDF of this paper or reading it
offline, please visit http://dx.doi.org/10.1130
/GES00908.S1 or the full-text article on
www.gsapubs.org to view Animation 1.
almandine
100
calcite
μ (cm–1)
apatite
10
1
0.1
10
60
110
160
Energy (keV)
Figure 3. X-ray linear attenuation coefficient µ vs. X-ray energy for
minerals discussed in this study, over the X-ray spectrum used.
more attenuating. Because previous carbonado
work indicates that only REE-rich phases will
have heavy elements in sufficient quantity to
affect their X-ray attenuation, and other REErich minerals aside from florencite have also
been reported (e.g., rhabdophane, xenotime; De
et al., 1998), in this study we generically refer
to minerals identified by reduced relative X-ray
attenuation below 40 keV as REE rich.
At energies below 40 keV, the most attenuating macrophase we expect to find based on prior
investigations of carbonado is native Fe. The
diamond material itself has very low attenuation at low kV because of its extremely low
mean atomic number relative to other phases.
As X-ray energy increases, and the dominant
attenuation mechanism transitions from photoelectric absorption to Compton scattering, the
attenuation coefficient becomes a linear function of energy, and other phases with higher
mean atomic numbers but lower density (i.e.,
spinel, kaolinite) come to have similar or lower
attenuation coefficients compared to diamond.
For example, a euhedral inclusion faintly visible
in the lower right corner in Figure 2D, which
was imaged at 40 kV, is almost indistinguishable from the diamond matrix when imaged at
140 kV in Figure 2C.
OBSERVATIONS
Mineralogy
Figure 4 and Animations 3–6 show a series of
volume renderings of the inclusion phases in the
29.2-µm-resolution data, in which the diamond
is rendered transparent except for the sample
boundary, and other phases are rendered transparent or partially or fully opaque based on CT
number. The highest-attenuation phases in the
data gathered at 140 kV (Fig. 4A; Animation 3)
are expected to be REE-rich minerals such as
florencite, and native Fe, the latter of which
is far less abundant. REE-rich phases occur
throughout the sample. When data are acquired
at 40 kV (Fig. 4B; Animation 4), the REE-rich
phases are indistinguishable from the almandine
crystal included in the scan field, as predicted
from the relations shown in Figure 3. The similarity of the inclusions rendered opaque in Figures 4A and 4B indicates that there is little material similar to almandine (i.e., a relatively dense
Fe-bearing phase) in the carbonado, and this
corroborates the interpretation that the phases
shown in Figure 4A are REE-bearing due to
their substantial change in attenuation relative to almandine. As the range of CT numbers
Geosphere, October 2013
Animation 2. Individual slice animation for
29.2-µm-resolution data sets of carbonado
acquired with X-rays set at 40 kV (left) and
140 kV (right). Patinaed side of specimen is
on right side of images. Images are 17.9 mm
wide (1224 × 612 pixels; 6.0 MB). If you are
viewing the PDF of this paper or reading it
offline, please visit http://dx.doi.org/10.1130
/GES00908.S2 or the full-text article on
www.gsapubs.org to view Animation 2.
rendered opaque is expanded to encompass the
values for the apatite and calcite samples (Figs.
4C and 4D; Animations 5 and 6), more inclusion
phases become visible.
Native metal inclusions were identified
by their high CT numbers in the 40 kV,
29.2-µm-resolution data (Fig. 5A, upper left). In
total, 16 were identified and measured in the complete data set, with sphere-equivalent diameters
ranging from 80 µm to 148 µm, and a mean value
of 107 µm. Every metal inclusion was directly
adjacent to the boundary of the specimen, with
the largest distance from sample exterior to inclusion being 300 µm (see Animation 7). All of these
inclusions appear to be linked to the outside of the
specimen via open porosity, though the local artifacts cast by the inclusions in the HRXCT data,
as seen for example in Figure 5A, prevent this
statement from being made unequivocally. No
smaller metallic inclusions were observed within
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Ketcham and Koeberl
Figure 4. Volume renderings
from 29.2 µm resolution data
sets showing various inclusion
populations based on their effective attenuation relative to the
index minerals scanned with the
carbonado. Diamond is rendered
transparent with purple boundary; inclusion colors are based
on index minerals (green—REErich minerals more attenuating
than almandine at X-ray energies above ~40 keV; red—almandine; yellow—apatite; white—
calcite). Rotation Animations
3–6 allow viewing from a range
of angles. (A) 140 kV, computed
tomography (CT) number >
almandine. See also Animation 3
( h t t p : / / d x . d o i . o r g / 1 0 . 11 3 0
/GES00908.S3); 903 × 1013
pixels; 7.8 MB. (B) 40 kV, CT
number ≥ almandine. See also
Animation 4 (http://dx.doi.org
/10.1130/GES00908.S4); 903 ×
1013 pixels; 7.2 MB. (C) 40 kV,
CT number ≥ apatite. See also
Animation 5 (http://dx.doi.org
/10.1130/GES00908.S5); 903 ×
1013 pixels; 6.7 MB. (D) 40 kV,
CT number ≥ calcite. See also
Animation 6 (http://dx.doi.org
/10.1130/GES00908.S6); 903 ×
1013 pixels; 6.0 MB. If you are
viewing the PDF of this paper or
reading it offline, please visit the
individual doi links or the fulltext article on www.gsapubs.org
to view Animations 3–6.
A 140 kV, μeff > almandine
B 40 kV, μeff ≥ almandine
C 40 kV, μeff ≥ apatite
D 40 kV, μeff ≥ calcite
the specimen in any of the higher-resolution subvolume scans; the nano-inclusions observed in
transmission electron microscopy (TEM) studies
(De et al., 1998; Sautter et al., 2011) are below
the resolution of the HRXCT data.
SEM analysis on the cut section showed
kaolinite and possibly other clay minerals, with
minor iron, to be by far the most common inclusion mineral in this specimen. Disseminated
florencite was often distinguishable by significant Ce peaks. Other probable phases included
ilmenite and quartz.
Macrotextures
The volume directly beneath the patinaed
side of the specimen has an evident rim texture (Fig. 5A, right side; also see Animations
1340
1 and 2) with up to three components distinguished by the abundance of small inclusions:
a 300–500-µm-wide inclusion-depleted outer
rim; an ~500-µm-wide inclusion-rich middle
band that features overall slightly higher X-ray
attenuation than surrounding material; and an
inner inclusion-depleted zone with variable
thickness. The outermost zone may correspond
to the 200-µm-thick outer rind investigated with
SEM by Shelkov et al. (1997), who found it to
be free of microporosity, which they interpreted
to be a result of annealing just below what they
interpreted to be the fusion crust.
Large, millimeter-scale inclusions occur
throughout the specimen, with no readily discernible pattern, even crossing the three-layer
rim texture. These inclusions are responsible
for most of the megaporosity observed on the
Geosphere, October 2013
patinaed surface, and in all other faces. However, smaller inclusions with maximum dimensions in the tens to hundreds of micrometers
have an apparent gradation from sparse near the
patinaed surface to increasingly abundant with
increasing distance from it, as seen both in the
example slice image (Fig. 5A) and most clearly
in a 3-D volume rendering (Fig. 5B; Animation
6). There is also an apparent dearth of small
inclusions along the non-patinaed surface, but
closer inspection reveals both open porosity on
the same size scale as the small inclusions, and
many inclusions of lower attenuation that do not
show up as clearly in the volume rendering, so
this is most likely a surface leaching effect.
When the region near the patinaed surface is
observed more closely with 5.4-µm-resolution
data (Fig. 6), it is also clear that there are low-
3-D petrography of carbonado
B
A
5 mm
Figure 5. Example micro–computed
tomography imagery of carbonado
specimen based on 40 kV, 29.2 µm data.
Both images have patinaed side on
right, normal to image plane. (A) Slice
image showing the three-layered texture
(divided arrow marking zone boundaries), with small inclusions becoming
more abundant from right to left. Top
arrow indicates native metal; all such
grains were close to the edge of the specimen. (B) Volume rendering showing
graded small inclusion pattern. Diamond
is rendered transparent, and inclusion
colors are based on scheme in Figure 4.
X-ray attenuation of inclusions increases
from purple to white to yellow to orange.
2 mm
Animation 7. Three-dimensional (3-D) volume rendering animation for 40 kV, 29.2 µm
data set, with only computed tomography
(CT) numbers ≥ metal shown (small orange
particles around specimen edges) (903 ×
1013 pixels; 6.7 MB). If you are viewing the
PDF of this paper or reading it offline, please
visit http://dx.doi.org/10.1130/GES00908.S7
or the full-text article on www.gsapubs.org
to view Animation 7.
attenuation inclusions and open pores, but the
overall grading pattern of lower-inclusion-number density toward the patinaed surface remains
clear. The inclusion-rich band near the patinaed
surface seen in Figure 5A can also be detected
in some regions of the 5.4-µm-resolution data
(Animation 8). The band has a higher density
of small inclusions near the resolution limit of
those data, but it retains an overall brightening
that cannot be ascribed to individual inclusions.
Based on the progression observed across data
resolutions, however, it can be inferred that the
brightening is probably caused by yet smaller
inclusions.
Cursory viewing suggests that the inclusions
have a preferred orientation that is oblique to
the overall apparent grading direction. This is
supported by 3-D fabric analysis that combines
all phases significantly more attenuating than
diamond in the 40 kV, 29.2-µm-resolution data
(Fig. 6B). As suggested by the fabric analysis
and corroborated by data visualization, the predominant shape of elongated pores corresponds
to blades (ellipsoid axes a > b > c). Similar
fabrics were also observed in early X-ray imaging (Trueb and Butterman, 1969; Trueb and
De Wys, 1969, 1971).
Microtextures
The higher-resolution 3-D images (Fig. 6;
Animations 8–10) of the inclusions show that
nearly all of them (>99%) are composites with
multiple minerals. They are in many cases a mix
between low-attenuation and high-attenuation
phases, the latter in most cases likely to be
florencite. Grain boundaries of phases within
inclusions are diffuse and intergrown or mottled,
indicating alteration or replacement and suggesting disequilibrium and material exchange
after the diamond had formed.
Geosphere, October 2013
There are two distinct populations of inclusions evident in the 5.4-µm-resolution HRXCT
data (Fig. 6C; Animations 9 and 10). One is
smaller and irregular, but often roughly ellipsoidal and with a preferred alignment. The other
is larger and features many perfectly euhedral
boundaries in contact with the diamond matrix.
In other words, there are within the diamond
material perfect negative crystal forms much
larger than the diamond crystallites, which host
polymineralic assemblages with disequilibrium
textures. These latter minerals are thus clearly
pseudomorphs replacing an earlier phase. The
most distinct pseudomorphed habit is rhombic
dodecahedral {011}, which tends to occur in
clumps of 0.2–1 mm crystal forms (Figs. 2E
and 7; Animations 11–13).
Aside from the near-edge occurrences of
inclusions interpreted to be native metal, all
inclusions containing high-Z phases that we
observed in detail were polymineralic; the only
inclusions we observed that appeared to be
monomineralic were relatively large and contained low-Z material. The 1-µm-resolution
images, which include one low-Z inclusion
(Fig. 2D, lower right; Animation 14), reveal
extremely sharp grain boundaries and a rhombic
dodecahedral habit. We positioned our laser cut
to expose this latter inclusion and found it to consist of kaolinite, and thus another pseudomorph.
The polymineralic nature of many inclusions
complicated 3-D visualization by volume rendering, and thus required segmentation by hand
to fully discern pseudomorph faces. Selected
examples are shown in Figure 7 and Anima-
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Ketcham and Koeberl
B
1
SVD/SVDmax
A
0
Figure 6. Computed tomography data of patinaed border
region and inclusion preferred
orientation. (A) Index image
from 29.2 µm data, showing position of C. (B) Threedimensional (3-D) rose diagram
of inclusions from center of
specimen, showing preferred
orientation; north is up in A,
east is left, etc. Colors and
distance from the origin indicate relative strength of fabric
according to the star volume
distribution metric (Ketcham,
2005b). (C) 5.4-µm-resolution
data showing close-up of region
near patinaed surface, showing
preferred orientation of pores
and inclusions, and increasing
inclusion density from right to
left. Full data set is provided in
Animation 10.
5 mm
C
patinaed
surface
internal
open pores
1 mm
1342
Geosphere, October 2013
3-D petrography of carbonado
Animation 8. Individual slice animation for
longer-acquisition 40 kV, 5.4 µm data set
of specimen interior. Denser angular sampling reduced interference artifacts, making porosity more apparent. Patinaed side
appears along right edge. Images are 5.3 mm
wide (992 × 1013 pixels; 57 MB). If you are
viewing the PDF of this paper or reading it
offline, please visit http://dx.doi.org/10.1130
/GES00908.S8 or the full-text article on www
.gsapubs.org to view Animation 8.
tions 11 and 12. Among the 13 inclusions thus
processed, some were undeterminable, but the
only habit identified was rhombic dodecahedral.
SEM imagery of the laser-cut surface (Fig. 8)
corroborates the inferences from the CT data
concerning the two distinct morphologies of
inclusions. Figure 8A shows a euhedral pseudomorph, in which the infilling minerals include
A
Animation 9. Individual slice animation
for 40 kV, 5.4 µm data set encompassing
different regions of the specimen interior.
Images are 5.3 mm wide (1006 × 1024 pixels;
15.1 MB). If you are viewing the PDF of this
paper or reading it offline, please visit http://
dx.doi.org/10.1130/GES00908.S9 or the fulltext article on www.gsapubs.org to view
Animation 9.
Animation 10. Individual slice animation
for 40 kV, 5.4 µm data set encompassing
different regions of the specimen interior.
Images are 5.3 mm wide (1006 × 1024 pixels;
14.9 MB). If you are viewing the PDF of this
paper or reading it offline, please visit http://
dx.doi.org/10.1130/GES00908.S10 or the
full-text article on www.gsapubs.org to view
Animation 10.
florencite and kaolinite. Figure 8B shows a pair
of elliptical, irregular inclusions that are also
polymineralic.
specimen, though some are present in the region
~1 mm inward from the patinaed surface. Some
of these megapores have a similar size and shape
to nearby inclusions (Fig. 6C), suggesting dissolution of previous grains or infilling of previous
pores, or both. Similarly, open pores in a smaller
size range (tens of micrometers) often have
the same preferred orientation as nearby inclusions (Fig. 6C). With increasing HRXCT scan
Porosity
Open megapores hundreds of micrometers
and larger were relatively sparse, and these
were principally observed near the rim of the
B
Figure 7. Volume renderings of pseudomorphs after dodecahedral phase based on 5.4 µm resolution data. Opaque
white is REE-rich phase (probable florencite); semitransparent green indicates minerals with lower attenuation
coefficients. Scale bars are 50 µm. (A) See also Animation 11 (http://dx.doi.org/10.1130/GES00908.S11); 512 × 512
pixels; 2.7 MB. (B) See also Animation 12 (http://dx.doi.org/10.1130/GES00908.S12); 1024 × 620 pixels; 4.2 MB.
If you are viewing the PDF of this paper or reading it offline, please visit the individual doi links or the full-text
article on www.gsapubs.org to view Animations 11–12.
Geosphere, October 2013
1343
Ketcham and Koeberl
Animation 13. Individual slice animation
for 40 kV, 1.0 µm data set encompassing
smaller subvolume of specimen interior.
The animation shows various pseudomorphs
containing abundant florencite (bright
material). Images are 1 mm wide (341 × 318
pixels; 4.3 MB). If you are viewing the PDF
of this paper or reading it offline, please visit
http://dx.doi.org/10.1130/GES00908.S13 or
the full-text article on www.gsapubs.org to
view Animation 13.
resolution, smaller pores become visible, and
are observed in every 1-µm-resolution scan we
obtained. However, their appearance is isolated,
and the HRXCT data were not able to resolve
directly continuous channels forming a connected network. The SEM imagery documented
micropores of various sizes throughout the diamond matrix along the cut surface. Pores from
tens of micrometers down to <1 µm width can be
seen, and there were no broad areas that lacked
pores at some level. Only the megapores were
observed to have other minerals within them.
DISCUSSION
Inclusions
The polymineralic nature and disequilibrium
texture common to all REE-containing inclusions indicate that they are exclusively secondary. Florencite is often a hydrothermal alteration product of monazite, which also tends to
host high amounts of thorium and uranium. The
occurrence of fission products (Fukura et al.,
2005) and radiogenic lead in carbonado diamond
matrix (i.e., Ozima and Tatsumoto, 1997) can be
attributed to recoil implantation from these inclusions and pore fluids carrying components that
have been mobilized. Overall, the observations in
this study support the conclusion that all megainclusions in carbonado are secondary, and that
1344
Animation 14. Individual slice animation for
40 kV, 1.0 µm data set encompassing smaller
subvolume of specimen interior. The animation shows a pseudomorph containing exclusively kaolinite. Images are 1 mm wide (454 ×
495 pixels; 17.8 MB). If you are viewing the
PDF of this paper or reading it offline, please
visit http://dx.doi.org/10.1130/GES00908.S14
or the full-text article on www.gsapubs.org to
view Animation 14.
the pore network is almost fully 3-D connected,
as suggested previously based on leaching experiments (Dismukes et al., 1988).
However, the rhombic dodecahedral pseudomorphs almost certainly represent a phase that
did form as an original inclusion in carbonado
but has since been lost. Insofar as these pseudomorphs are many times larger than the diamond
crystallites comprising their boundaries, we
consider it unlikely that they could be some form
of negative diamond crystal; we also consider it
unlikely that they are themselves resorbed diamonds. The pristine faces and corners of the
dodecahedra, combined with their presence
throughout the sample volume, strongly suggest
that they formed coevally with the diamond,
and moreover did not suffer any resorption during diamond growth. Of the 30 or so minerals
previously identified in carbonado, none has a
rhombic dodecahedral crystal form. A variety
of minerals may form rhombic dodecahedra
(including, rarely, diamond or spinel), and this
habit is commonly associated with garnet, a
common inclusion phase in mantle diamonds
(e.g., Spetsius and Taylor, 2008). However, it
also seems evident that this phase must have
subsequently become unstable, insofar as it has
been entirely removed from this large specimen, and all other carbonados studied to date.
Moreover, insofar as wide varieties of pseudo-
Geosphere, October 2013
morphing minerals are present, from almost
pure kaolinite in some to florencite mixed with
other phases in others, at least some chemical
components from the original phase must have
migrated through the pore network.
The only large metallic inclusions were
observed extremely close to the outside of the
specimen, and probably connected to the exterior via relatively wide open porosity. The resolution of our data is not sufficient to image the
nanoscale metallic inclusions documented by
De et al. (1998), which were not observed to be
in contact with any pore network. We thus infer
that there are two populations of metallic particles in carbonado: Megaparticles are probably
secondary pore fillings, whereas fully enclosed
nano-inclusions are primary. The observation
that the metal inclusions are confined to the very
edge of this specimen suggests that they may
even be a tertiary feature acquired after the other
interior inclusions had already formed.
Implications from Textural Observations
A striking feature of this sample is the spatial coherence of the patinaed surface paralleled
with banding just underneath and apparent grading of pores extending throughout the specimen.
This coherence implies that a single process or
event was responsible for all of these textural
features. The patinaed surface has been interpreted in other carbonados as possibly representing a fusion crust, which may be formed by
passage through the atmosphere at high velocity, either as an incoming meteorite (Shelkov
et al., 1997; Smith and Dawson, 1985), or as
ejecta from an impact (Kletetschka et al., 2000).
However, we consider it unlikely that either of
these mechanisms would result in the observed
bulk rearrangement of the internal volume of the
diamond material, extending several millimeters
inwards, i.e., much thicker than typical fusion
crusts. For this to occur, there would have to be a
transfer of carbon from the rim several millimeters into the interior through the confined pore
network, and recrystallization there as diamond,
without forming graphite, a similar physical
argument to the one that rules out an impact
origin (DeCarli, 1997).
A more likely alternative is that the smallinclusion patterns reflect an original pore network, which grades from almost nonporous near
the patinaed surface to progressively more abundant and larger pores with increasing distance
from this surface. The connectivity of the network implies that the pores were formed with a
common, intercommunicating mechanism, such
as a trapped grain-boundary fluid phase consistent with recent conclusions of carbonado forming in a fluid medium (Ishibashi et al., 2012).
3-D petrography of carbonado
A
B
Figure 8. Scanning electron microscope images of (A) euhedral and
(B) irregular-oriented inclusions in carbonado. Scale bars are both
50 µm.
Overall, the porosity in carbonado has a distinctive trimodal character. The first two modes
consist of large euhedral pores up to millimeter scale caused by dissolution of originally
included minerals, and preferentially elongated
pores from 0.1 up to 0.3 mm in length with
irregular boundaries, which appear to be isolated
islands of original porosity. The third is microto nanoscale pores that constitute the connected
network. Precipitation of secondary minerals
appears to be almost exclusively within the large
pore classes, probably owing to larger spaces
being more favorable due to surface energy considerations. The principal exception is within
the bright band adjacent to the patinaed surface,
in which smaller pore spaces may be filled. This
may be due to the possible presence of a locally
pore-free, impermeable boundary immediately
below the patinaed surface as identified by Shelkov et al. (1997), which could have formed a
local trap for mineral-laden fluids, forcing them
to precipitate their material in-place.
If the pore network indeed bears some
genetic relation to the patinaed surface, the
question arises of whether growth was toward
or away from this surface. We favor the idea that
growth was away from it. One possible explanation is that the initial layers of carbonado could
more easily precipitate porosity free, and that as
carbon was consumed from the fluid, residual
fluid remained trapped inside the pores, inhibiting infill. An alternative possibility is that the
large, irregular pores represent bubbles of fluid
trapped in a diamond crystal mush during the
early formation stages of carbonado, and that
these bubbles buoyantly rose from the patinaed
Geosphere, October 2013
surface, which was down-facing. Also of note is
that euhedral pores and elongate irregular pores
both intersect the patinaed surface, and that the
patinaed surface, while relatively smooth, is still
rough in the manner of an impression on rock
rather than rounded.
The elongation of the irregular inclusions and
pores also requires explanation, as it implies
that either these pores formed with this shape
and orientation or were deformed by strain
in the diamond matrix. We can speculatively
link this texture with the two-stage carbonado
growth model suggested by Petrovsky et al.
(2010), in which the diamond material begins
as a loose mush of 10–100 µm crystals, which
is then sintered with smaller diamonds and
cryptocrystalline material by a large increase
in nucleation rate caused by a sudden change in
some environmental factor, such as temperature
decrease, pressure increase, or crystallization of
a new phase. If this mush contained bubbles of
liquid, which were then sheared during a relatively sudden event with an associated pressure
increase that drove pressure significantly away
from the diamond-graphite stability boundary, the deformation of the bubbles could be
frozen in place by the rapid crystallization of
microdiamonds. The contemporaneity of the
shear strain and sintering provides an explanation for deformation lamellae (De et al., 1998)
and high residual stress (Kagi et al., 2007) being
confined to the smaller, later diamond population, which would be deforming even as it was
crystallizing, transiently being a weaker and
more mobile material, while the larger crystals
remained undamaged. Another important constraint is provided by the pristine nature of the
pseudomorphs, which show no signs of strain,
suggesting that the diamond medium surrounding the dodecahedral phase was weaker than
the dodecahedral phase itself. The two-stage
hypothesis also explains the different carbon isotopes of the small and large diamond populations
(De et al., 2001; Petrovsky et al., 2010). Opening
of fractures in the crystalline mush during deformation and contemporaneous crystallization
may also provide a mechanism for the formation
of columnar diamond (Rondeau et al., 2008).
Implications of the Present-Day
Inclusion Suite
Even if the present-day mega-inclusions in
carbonado are secondary, the unusual abundance
of U, Th, REEs, and other incompatible elements
in carbonado still calls for explanation. We suggest that for the sake of parsimony, this explanation should optimally be linked to the formation
event. For this evidently rare diamond creation
mechanism to be followed by a similarly rare
1345
Ketcham and Koeberl
incompatible-element enrichment event in a
different environment and location, millions or
billions of years later, in samples now distributed between two continents, is akin to lightning
striking twice. On the other hand, carbonatitic
high-density fluids trapped in fibrous diamonds
are frequently rich in incompatible elements
(Rege et al., 2010; Weiss et al., 2011).
An obvious place to look is to the now-absent
dodecahedral phase. If that phase was sufficiently enriched in U and Th, over time it could
become unstable due to metamictization from
radiation damage, causing the crystal structure to
collapse and new minerals to form in its place.
As one possibility, garnet is capable of hosting
high concentrations of U, Th, and REEs in hydrothermal conditions; Smith et al. (2004) document
local U concentrations of up to 358 ppm and
REE concentrations up to 4724 ppm in zoning
bands within skarn garnets from the Beinn an
Dubhaich granitic aureole in Skye, Scotland,
which they attributed to periods of closed-system
crystallization. Garnet retains all fission damage
at temperatures below 200 °C (Haack and Potts,
1972), and this ability to accumulate radiation
damage without annealing facilitates metamictization. After 1.7 b.y., the age of some conglomerates bearing carbonados (Sano et al., 2002), a
garnet with 350 ppm U and Th/U ratio of 4 will
accumulate a dose of 4.2 × 1018 alpha decays per
gram, which is enough to cause significant swelling in zircon (Weber, 1990). However, whatever
phase housed U and Th would have to grow in the
highly reducing conditions implied by the metal
nano-inclusions; Ishibashi et al. (2012) placed the
likely oxygen fugacity of the diamond-forming
fluid at ~3 log units below the FMQ buffer, while
Sautter et al. (2011) placed it even lower, at ~15
log units below the IW buffer based on the presence of metallic Ti.
We thus postulate that the carbon-supersaturated fluid from which the diamond grew was
also enriched in incompatible elements, perhaps
in the manner of a pegmatite from the fluid
residuum of a crystallizing carbonatitic magma
body, albeit in a possibly very unusual setting,
and that these elements partitioned into the
dodecahedral phase, and perhaps others that we
are not able to identify from the pseudomorphs.
The enrichment may have also included other
elements that are also represented in the
present-day carbonado inclusion suite, such as
K, P, and Al, with the first now helping comprise
the kaolinite, the second the florencite, and so
on. In other words, even though the present-day
inclusion minerals in carbonado are almost cer-
tainly secondary, it does not necessarily follow
that the materials that comprise them are. The
radiogenic elements in the fluid may have also
helped in diamond crystallization (e.g., Ozima
and Tatsumoto, 1997), but it is not clear if such
catalysis is necessary.
Figure 9 shows a schematic summary of carbonado formation as postulated here. In addition
to incorporating all of the textural observations
in this study, we believe that it is also consistent with the multitude of observations by other
authors discussed in the foregoing text.
Left unspecified in this narrative is whether
the carbonado formation process took place in
the crust or the mantle. Most evidence points
to a mantle origin, as the known processes that
catalyze diamond formation at crustal pressures either include features not observed in
carbonado or do not explain features that are
observed (Petrovsky et al., 2010); an alternative
possibility is a transient event producing mantle conditions in the deep crust (Sautter et al.,
2011). However, the positing of a deep liquid
so highly enriched in incompatible elements as
to allow precipitation of millimeter-scale grains
of a U-, Th-, and light (L) REE–rich phase is
unconventional. A possible compensating circumstance is that carbonado formation occurred
100 μm
A Accumulation
B Deformation and sintering
C Redistribution of included material
Figure 9. Schematic illustration of model for formation and evolution of carbonado diamond based on textural observations in this study.
(A) In a fluid-filled cavity associated with a crystallizing magma, crystallizing solids including 10–100 µm diamond crystallites (orange
circles), the dodecahedral phase (pink hexagons), and assorted nano-inclusions settle to the floor, trapping fluid between mineral grains
and occasionally as bubbles in the process (white ovals). While the assemblage remained a mush, the bubbles would tend to slowly rise
due to positive buoyancy. (B) A shearing event then deforms the bubbles, but not the dodecahedra, while catalyzing crystallization of
micrometer-scale diamond from the trapped fluid (light orange shading). (C) Over the subsequent billions of years, once the carbonado
has been emplaced in a near-surface environment, accumulation of radiation damage from U and Th breaks down the dodecahedral phase,
and material migrates through the pore network to form new minerals that are stable at crustal conditions (mottled ovals and hexagons).
1346
Geosphere, October 2013
3-D petrography of carbonado
relatively early in the evolution of Earth, possibly at 3.8 Ga or even earlier, at a time when the
segregation of incompatible elements into the
crust was far from complete (Rollinson, 2006).
CONCLUSIONS
High-resolution X-ray CT enables “3-D
petrography” to reveal a number of previously
unrecognized features in a large specimen of
carbonado diamond. These include a grading
of oriented inclusions that is spatially associated
with the patinaed surface, and the occurrence
of dodecahedral pseudomorphs reflecting the
first syngenetic inclusion phase above the nanometer scale recognized in carbonado, albeit in
its absence. From these observations, we build
upon recent work proposing that carbonado
grew from a C-supersaturated fluid, and we propose that the irregular pores may correspond to
trapped bubbles in a 10–100 µm loose or lightly
bound diamond slurry, and that the observed
fabric in these pores may reflect a strain event
that sparked the growth and deformation of the
smaller, <1 µm diamond population that holds
carbonado together. We further postulate that
this fluid may have also been highly enriched
in a number of incompatible elements that were
partitioned into primary inclusion phases, which
subsequently broke down due to their high U
and Th contents. While some rough edges and
loose ends remain, this combined model encompasses a great number of the enigmatic observations that have made the origin of carbonado a
mystery for several decades, and it does so with
a single formation mechanism and setting. In
addition, the proposed incompatible-element–
rich fluid may provide a compact explanation
for the observed differences between carbonado and other polycrystalline diamonds such
as yakutites and framesites, suggesting that, in
other respects, the mechanism for their origin
may have been similar.
ACKNOWLEDGMENTS
This research was supported in part by National
Science Foundation grant EAR-0948842, and by the
John A. and Katherine G. Jackson School of Geosciences and the Owen-Coates Fund of the Geology
Foundation at the University of Texas at Austin. We
thank Joseph Lai for performing the laser cut, Travis
Clow for helping with inclusion segmentation, and
J. Barnes, J. Lin, and D. Smith for helpful discussions.
We appreciate the efforts of the editors and two anonymous reviewers in helping to improve the manuscript.
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