Meteoritics & Planetary Science 49, Nr 11, 2095–2103 (2014)
doi: 10.1111/maps.12312
Diamond xenolith and matrix organic matter in the Sutter’s Mill meteorite
measured by C-XANES
Yoko KEBUKAWA1,2,8*, Michael E. ZOLENSKY3, A. L. David KILCOYNE4, Zia RAHMAN5,
Peter JENNISKENS6,7, and George D. CODY1
1
Geophysical Laboratory, Carnegie Institution of Washington, Washington, District of Columbia 20015, USA
2
Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
3
NASA Johnson Space Center, Houston, Texas 77058, USA
4
Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
5
Jacobs-Sverdrup, Houston, Texas 77058, USA
6
SETI Institute, Mountain View, California 94043, USA
7
NASA Ames Research Center, Moffett Field, California 94035, USA
8
Present address: Faculty of Engineering, Yokohama National University, Yokohama 240-8501, Japan
*
Corresponding author. E-mail: kebukawa@ynu.ac.jp
(Received 12 September 2013; revision accepted 14 April 2014)
Abstract–The Sutter’s Mill (SM) meteorite fell in El Dorado County, California, on April
22, 2012. This meteorite is a regolith breccia composed of CM chondrite material and at
least one xenolithic phase: oldhamite. The meteorite studied here, SM2 (subsample 5), was
one of three meteorites collected before it rained extensively on the debris site, thus
preserving the original asteroid regolith mineralogy. Two relatively large (10 lm sized)
possible diamond grains were observed in SM2-5 surrounded by fine-grained matrix. In the
present work, we analyzed a focused ion beam (FIB) milled thin section that transected a
region containing these two potential diamond grains as well as the surrounding finegrained matrix employing carbon and nitrogen X-ray absorption near-edge structure
(C-XANES and N-XANES) spectroscopy using a scanning transmission X-ray microscope
(STXM) (Beamline 5.3.2 at the Advanced Light Source, Lawrence Berkeley National
Laboratory). The STXM analysis revealed that the matrix of SM2-5 contains C-rich grains,
possibly organic nanoglobules. A single carbonate grain was also detected. The C-XANES
spectrum of the matrix is similar to that of insoluble organic matter (IOM) found in other
CM chondrites. However, no significant nitrogen-bearing functional groups were observed
with N-XANES. One of the possible diamond grains contains a Ca-bearing inclusion that is
not carbonate. C-XANES features of the diamond-edges suggest that the diamond might
have formed by the CVD process, or in a high-temperature and -pressure environment in
the interior of a much larger parent body.
INTRODUCTION
The Sutter’s Mill (SM) meteorite fell in El Dorado
County, California, on April 22, 2012. The SM find
numbers were assigned to the 77 fragments found, with
a total mass of 943 g. The first three pieces (SM1, 2,
and 3) were recovered on April 24, before heavy rain
fell in the area of the fall. The observed preatmospheric
orbit of Sutter’s Mill points to an origin in either a low-
inclined family of Main Belt asteroids near the 3:1
resonance, or perhaps in a Jupiter-family comet
(Jenniskens et al. 2012).
The Sutter’s Mill meteorite is classified as a
carbonaceous chondrite, and is a regolith breccia of
CM-type materials, containing at least one highly
reduced xenolithic clast that contains oldhamite existing
as separate mineral grains. There are also several
xenolithic minerals, including abundant enstatite laths,
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© The Meteoritical Society, 2014.
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Y. Kebukawa et al.
and Cr-bearing troilite. Unlike most other CM
chondrites, the brecciated nature of this meteorite is
evident, e.g., SM2 contains angular to rounded clasts
embedded in a fine-grained comminuted matrix
(Jenniskens et al. 2012).
Organic chemistry also distinguishes Sutter’s Mill
from typical CM chondrites. For example, Sutter’s Mill
has lower N/C and d15N than other CM2 chondrites,
suggesting that it contains different N-bearing organic
components (Jenniskens et al. 2012). In addition, C and
N isotopic ratios vary widely between fragments, and
there is a distinct bimodality in the C release that
suggests there are two separate organic components:
one that is volatile-rich and one that is volatile-poor
(Jenniskens et al. 2012). The Raman G-band center and
full width at half maximum (FWHM) of sample SM12
indicated that this stone has been heated to an
intermediate degree between CO3 and CV3 chondrites.
SM2 has not experienced the same degree of annealing,
but has been heated more than other CM2 chondrites
(Jenniskens et al. 2012).
Sample SM2-5 contains clasts of incompletely
altered CM material, whose matrix consists largely of
submicrometer-sized olivine grains that formed from
phyllosilicates and troilite, an observation that,
combined with a complete lack of carbonate and
tochilinite, indicates that this particular clast has
experienced thermal metamorphism to approximately
500 °C, unlike other SM samples, which contain
abundant
carbonate
grains
and
phyllosilicates
(Jenniskens et al. 2012).
Carbon and nitrogen X-ray absorption near-edge
structure (C-XANES and N-XANES) spectroscopy
using scanning transmission X-ray microscopy (STXM)
is a well-established technique that allows us to obtain
molecular structural information on micrometer-sized
samples with high spatial resolution (pixel sizes of
50 9 50 nm are routine). This work presents a STXMXANES study of a focused ion beam (FIB) milled slice
from Sutter’s Mill SM2-5, to survey the nature of
carbon-bearing phases in this meteorite.
ANALYTICAL PROCEDURE
Sample Preparation
STXM-XANES analysis requires a sample thickness
on the order of 100–150 nm. An approximately 100 nm
thick section was extracted from a fragment of SM2-5
using a focused ion beam (FIB) mill at NASA/JSC,
which isolated two large potential diamond grains
found in backscattered electron (BSE) studies (Fig. 1).
The extracted section is shown in Fig. 2 as seen in BSE,
optical, and scanning transmission X-ray microscopy.
Diamond
Enstatite
10μm
Fig. 1. Backscattered electron (BSE) image of fragment SM25, containing coarse grains of possible diamond and enstatite.
A focused ion beam (FIB) milled section was extracted along
the line across two possible diamond grains.
The identification of the large grains as potential
diamonds was based on their pure carbon compositions
and optical transparency. However, unfortunately now
we cannot verify this as the FIB section was damaged
during preparation for TEM analyses.
X-Ray Absorption Near-Edge Structure Spectroscopy
C-XANES and N-XANES microspectroscopy was
performed using the scanning transmission X-ray
microscope (STXM) at beamline 5.3.2.2 of the
Advanced Light Source, Lawrence Berkeley National
Laboratory (Kilcoyne et al. 2003). The beamline is
particularly designed for light elements, such as C, N,
and O K-edge energy regions, but we are also able to
obtain L-edge absorptions of some heavier elements,
such as Ca and Fe. Soft X-rays generated by a bending
magnet provide a useful photon range spanning 250–
700 eV with a photon flux of 107 photons s 1. Energy
selection is performed with a low-dispersion spherical
grating monochromator, affording an energy resolution
(E/DE) of 5000; most of our data were at an energy
resolution of approximately 3000, i.e., at approximately
100 meV. STXMs utilize Fresnel zone plate optics for
X-ray focusing providing a theoretical spot size of
31 nm; in optimum cases, smaller structures
(approximately 15 nm) can be resolved. C-XANES and
N-XANES spectra were typically acquired using a
multispectral imaging method (“Stacks”; Jacobsen et al.
2000). In the fine-structure regions of the near edge, the
energy step size (DE) employed was typically 0.1 eV; in
the less featured pre-edge and postedge regions, energy
steps of 1–2 eV are sufficient for spectral resolution.
C-XANES analyses of Sutter’s Mill meteorite
A
Diamond
(A)
2097
#2
Diamond
#1
10μm
B
C
Fig. 2. The approximately 100 nm thick FIB section,
including the two possible diamonds. A) Backscattered
electron (BSE) image, B) optical microscope image, and C)
image from scanning transmission X-ray microscope (STXM)
obtained at E = 390 eV. Note the circular inclusion in the
possible diamond on the left that is observed clearly in the
SEM, optical, and STXM images. In the optical image it is
clear that there exists a large difference in the index of
refraction of the inclusion relative to the diamond.
5μm
(B) C map
1μm
RESULTS
The two large 10 lm diameter potentially diamond
grains, which appear dark in the backscattered electron
(BSE) and STXM images (Figs. 2A and 2C) and
transparent in the optical microscopy image (Fig. 2B),
are surrounded by a fine-grained matrix. These grains
are most likely diamond based on their carbon-rich
compositions as determined by energy-dispersive X-ray
spectroscopy (EDS) analysis and their optical
transparency. A C-XANES and N-XANES spectrum of
the fine-grained matrix was measured from an area
close to the diamond (circled area in Fig. 3A). Note
that it was not possible to obtain C-XANES spectra of
the potential diamond grains directly, due to the
following two reasons: (1) during the process of
applying FIB, the hard possible diamond grain
remained thick relative to the softer matrix material,
and (2) the absorption was too great, consistent with
the expected high density of carbon atoms in diamond.
We were able to acquire C-XANES spectra at the very
edges of the possible diamond grains.
Fig. 3. Scanning transmission X-ray microscope (STXM)
image (A), and carbon K-edge map at 290.4 eV (B) of the
matrix of SM2-5. The carbon X-ray absorption near-edge
structure (C-XANES) spectrum of the circled area is shown in
Fig. 4 (SM2-5 matrix). The carbon-rich spots are indicated
with arrows. The C-rich spot on the top (B) may be an
organic nanoglobule (see Fig. 4 for C-XANES), and the
C-rich spot in the middle is probably a Ca-bearing carbonate
grain (see Fig. 6).
The C-XANES spectrum of the SM2-5 matrix
(Fig. 4) reveals absorption features at 285 eV that are
assigned to the 1s–p* of aromatic and olefinic carbon
(C=C); a distinct absorption feature at 286.5 eV is
assigned to the 1s–p* bond transitions of aryl,vinylketone (C=C–C=O), while absorption at 288.4 eV is
assigned to the 1s–p* bond of carboxyl groups (O–C=O).
Absorption at 290.2 eV is assigned to the 1s–p* bond of
carbonate (CO3). The identities and the photon energies
of these characteristic C-XANES absorptions are
summarized in Table 1 (Cody et al. 2008a). There is no
graphene 1s–r* exciton feature at 291.6 eV, indicating
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Y. Kebukawa et al.
1 2 3 56
Table 1. Characteristics of carbon X-ray absorption
near-edge structure transitions.
1
2
3
4
5
6
Murchison
IOM
SM2-5
Matrix
4
Globule-like
Energy (eV)
Transition
Functional group
284.8–285.2
286.5
288.4
288.6–288.9
289.5
290.2
1s–p∗
1s–p∗
1s–p∗
1s–r∗
1s–3p/s
1s–p∗
Aromatic/olefinic
Aryl,vinyl-ketone
Carboxyl
Diamond exciton
Alcohol/ether
Carbonate
C=C
C=C–C=O
O–C=O
C–O
CO3
These assignments are derived from Cody et al. (2008a) for organics,
and from Jaouen et al. (1995) for diamond.
Diamond edge
#1
Diamond edge
#2
SM2-5
Matrix
Diamond reference
280
285
290
eV
295
300
Fig. 4. Carbon X-ray absorption near-edge structure (CXANES) spectra of the SM2-5 matrix as measured in the
circled area in Fig. 3A, in the globule-like C grain (Fig. 3B,
top arrow), and in the diamond-edge regions (#1: Fig. 3A,
left, #2: Fig. 3A, right). Results are compared with the
C-XANES spectrum of insoluble organic matter (IOM) from
Murchison and a diamond reference spectrum taken from
Jaouen et al. (1995). Table 1 identifies the absorption features
(labeled 1–6).
that the SM2-5 matrix has never experienced extensive
thermal metamorphism (<~200 °C) (Cody et al. 2008b).
Although the N-XANES spectrum clearly reveals the
presence of nitrogen, there are no distinct XANES
absorption features (e.g., imine, nitrile, or amine peaks)
as are typically observed in carbonaceous chondritic
IOM (Fig. 5). Note that whereas nitrile (CN) or
heterocyclic imine (C=N) exhibit significant absorption
features in the C-XANES region at approximately
286.5 eV (Apen et al. 1993), the lack of corresponding
absorption features consistent with such N-bearing
functional groups in the N-XANES spectrum indicates
that the peak at 286.5 eV cannot be due to nitrile or
imine.
STXM carbon “maps” of this matrix area were
obtained by acquiring pairs of images below and on the
carbon K-edge, e.g., 280 and 290 eV, respectively,
taking the log(I290/I280) for each pixel (Fig. 3B). Three
carbon-rich grains, each much smaller than the
diamonds, are observed in this area. The C-XANES
spectrum obtained from the grain to the upper left in
Fig. 3B, potentially an organic nanoglobule, exhibits
weaker C=O peaks (including ketone and carboxyl at
286.5 and 288.4 eV, respectively) compared with the
C-XANES spectrum of the matrix (Fig. 4). A very weak
395
400
405
410
415
420
eV
Fig. 5. The nitrogen X-ray absorption near-edge structure
(N-XANES) spectrum of SM2-5 matrix as measured in the
circled area in Fig. 3A. No distinct absorption bands related to
specific N-bearing functional groups (e.g., imine) are observed.
peak corresponding to carbonate at 290.2 eV is also
observed in the spectrum of this grain, possibly
contamination from the surrounding matrix region. The
carbon-rich grain in the middle of Fig. 3B shows X-ray
absorption features at 290.2 eV that are assigned to the
1s–p* bond transition of carbonate carbon (CO3)
(Fig. 6A), as well as corresponding calcium L-edge
absorption features at 349.0 and 352.2 eV (Fig. 6B),
consistent with this grain being Ca-bearing carbonate.
We did obtain C-XANES spectra from two
boundary regions between diamond and matrix
(Regions #1 and #2 in Fig. 3A). Figure 4 presents the
diamond-edge #1 C-XANES spectrum, which contains
some aromatic/olefinic C=C at 285 eV, and C=O at
286.5 eV, in addition to the 1s–r* diamond core
exciton at 288.7 eV. Compared with spectrum of region
#1, the diamond-edge #2 C-XANES spectrum contains
no aromatic/olefinic C=C and little C=O, but does
exhibit an absorption feature at 285.6 eV, which is not
clearly identified. Imine (C=N) is expected to have an
absorption feature near 285.6 eV (Dhez et al. 2003;
Shard et al. 2004), but the presence of considerable
imine would be unambiguous in the N-XANES, where
no significant imine signal is observed. Alternatively, a
C-XANES analyses of Sutter’s Mill meteorite
2099
Ca map
(A)
Carbonate CO3
C K-edge
5μm
290.2
280
285
eV
290
295
(B)
Ca L-edge
Fig. 7. Scanning transmission X-ray microscope (STXM)
calcium L-edge map at 349.0 eV of SM2-5 where bright
regions correspond to calcium-rich entities. The diamond grain
(upper left) contains a circular Ca-rich inclusion that is not
carbonate.
diamond phase. Note that this inclusion was also clearly
seen in the optical microscope image (Fig. 2B), where a
large difference in refractive index between the
“diamond” and the inclusion is indicated. C-XANES
shows no indication that this calcium-rich inclusion
contains any carbon (data not shown).
DISCUSSION
Matrix
349.0 352.2
340
345
350
eV
355
360
Fig. 6. (A) Carbon and (B) calcium X-ray absorption spectra
of the carbonate grain (Fig. 3B, middle arrow). A) The peak at
290.2 eV is assigned to 1s–p* of carbonate (CO3), and B) the
peaks at 349.0 and 352.2 eV are assigned to calcium L-edge.
285.6 eV peak might be attributed to the 1s–p*
transition of alkynes (CC) (Hitchcock and Brion
1977). The broad features at around 292–294 eV in
diamond-edges #1 and #2 are also probably due to the
1s–r* transition of diamond C–C (see the diamond
reference spectrum in Fig. 4 taken from Jaouen et al.
1995). The lack of graphene 1s–r* exciton feature at
291.6 eV indicates that no graphite is associated with
the edge of the diamond at either location.
Figure 7 presents a calcium L-edge “map” ( log
[I290/I280]) of the FIB section obtained by STXM. The
calcium map reveals a round Ca-bearing inclusion of
approximately 2 lm diameter inside the potential
The combination of abundant C=O groups and lack
of exciton features indicates that the organic matter in
the matrix has a primitive molecular structure similar to
insoluble organic matter (IOM) found in other CM and
in CI and CR chondrites. The fact that the N-XANES
peak is so weak is consistent with the low N/C ratio
measured by other techniques (Jenniskens et al. 2012).
This feature makes organic matter in Sutter’s Mill
different from that in other carbonaceous chondrites
(fig. S24 of SOM document in Jenniskens et al. 2012).
The C-XANES spectrum of SM2-5 matrix is
essentially identical to that of the CM2 Murchison
insoluble organic matter (IOM), except for a distinct
peak corresponding to alcohol and/or ether groups
(C–OR) at 289.5 eV, which only appears on the
Murchison IOM spectrum (Fig. 4). A lack of alcohol/
ether groups is also observed in C-XANES of bulk
Murchison matrix before IOM purification (Yabuta et al.
2010a); thus, it may be a typical feature of carbonaceous
chondrites before IOM purification. The N-XANES
spectrum of Murchison IOM has weak features near 400
and 402 eV (Feser et al. 2003), probably assigned to
nitrile/imine in imidazoles and amine, respectively, while
2100
Y. Kebukawa et al.
these features are not observed in SM2-5 (Fig. 5).
Therefore, organic matter in SM2-5 matrix probably has
been subjected to similar chemical processes as organic
matter in other CM chondrites, but may have been
synthesized in an N-poor environment. The spectrum of
SM2-5 does not show the 1s–r* exciton feature at
291.6 eV, indicating that this matrix has never
experienced long-term thermal metamorphism above
approximately 200 °C (Cody et al. 2008b). This might
appear inconsistent with the mineralogical observation
that the SM2-5 has been heated up to approximately
500 °C (Jenniskens et al. 2012). However, if the heating
of SM2-5 was short term, the development of graphene
1s–r* exciton would not have occurred as the kinetics
take over millions of years (Cody et al. 2008b). For
example, Yamato-86720 appears to have been heated in
the range of >750 °C for short duration (Nakamura
2005), but the 1s–r* exciton indicates 265 42 °C
(Cody et al. 2008b). Yabuta et al. (2010b) also concluded
that WIS 91600 has experienced short duration heating at
<500 °C due to the highly aromatic nature of the IOM
with a very weak 1s–r* exciton. Jenniskens et al. (2012)
reported that, based on Raman spectra of
macromolecular carbon, SM2-9 (probably a fragment
from the same stone with SM2-5) has experienced
temperatures no higher than 153 27 °C, which is
consistent with our C-XANES results. This is probably
due to the similarity of spectral characteristics of
graphene structure obtained using Raman and C-XANES
(Cody et al. 2008b). Note that it is possible that carbonate
grains (as shown in Fig. 6) survive the temperature over
500 °C, which sample SM2-5 probably experienced, as the
thermal decomposition of dolomite occurs in the
temperature range of 600–800 °C, and that of calcite
occurs at approximately 900 °C (Tonui et al. 2003).
Globule-Like Organic Carbon Grains
Organic nanoglobules are reported in the postrain
Sutter’s Mill sample, SM8, a carbonate- and
phyllosilicate-free lithology (Nakamura-Messenger et al.
2013). The FIB section of SM2-5 analyzed here contains
minimal carbonate and phyllosilicates. The globule-like
organic matter found in SM2-5 is in the form of a grain
approximately 300 nm in diameter, and contains less
ketone and carboxyl groups than the matrix organics,
indicating a more aromatic nature of this material. De
Gregorio et al. (2013) reported that there are two types
of nanoglobules in terms of chemical signatures, where
one type has IOM-like chemical structure, and the other
has more aromatic material. The relative abundance of
highly aromatic nanoglobules is lower in more
aqueously altered meteorites, suggesting the formation/
modification of IOM-like nanoglobules during parent
body processing (De Gregorio et al. 2013). The
aromatic nature of the globule-like organics found in
SM2-5 is consistent with the observation that SM2-5
has not experienced significant aqueous alteration
(Jenniskens et al. 2012; Zolensky et al. 2014). However,
TEM observation would be required to determine
definitively if this grain is a nanoglobule.
Possible Diamond
The large (10 lm) possible diamond grains found in
SM2-5 are unique among the diamonds observed
previously in any meteorite, although we cannot verify
until additional diamonds can be located for analyses.
Meteoritic nanodiamonds are common in primitive
chondrites, and typically have diameters that range
between 0.1 and 10 nm (Lewis et al. 1987). Possible
mechanisms for the formation of these nanodiamonds
include chemical vapor deposition (CVD) (Clayton
et al. 1995), grain–grain collisions of amorphous or
graphitic C in interstellar shocks (Tielens et al. 1987),
and UV irradiation of graphite (Nuth and Allen 1992);
the distribution of twin microstructures and an absence
of dislocations suggest that most are formed by CVD
(Daulton et al. 1996).
Grady et al. (1995) reported a large polycrystalline
diamond plate (approximately 8 9 5 lm) in the unusual
chondrite Acfer 182, in addition to twelve individual
angular crystals of diamond (1–4 lm). The hexagonal
outline of the plate indicates the shock-induced
transformation of graphite, although the angular crystals
appear to have different origins (Grady et al. 1995).
Russell et al. (1992) reported a unique type of diamond
isolated from acid residues of the Abee (EH4) chondrite.
These diamonds are much coarser grained (100 nm to
1 lm) than the typical presolar nanodiamonds, and may
have either formed within the solar nebula (Russell et al.
1992), or been produced by shock on the parent body
(Rubin and Scott 1997).
In the Canyon Diablo iron meteorite, some graphite
has been transformed into diamond and lonsdaleite, a
hexagonal high-pressure polymorph, due to shock
(Mittlefehldt et al. 1998). These diamond fragments,
with sizes up to a few hundred micrometers, have a
border of graphite (Carter and Kennedy 1964), and
probably are the result of the transformation of
graphite during the crater-forming impact on Earth
(Chabot and Haack 2006). In some ureilites, diamond
and lonsdaleite occur as small (<1–3 lm) anhedral to
subhedral grains within a fine-grained matrix of
graphite, possibly the result of parent body collisions
(Marvin and Wood 1972; Berkley et al. 1976).
There are some C-XANES measurements on
meteoritic nanodiamonds extracted from Allende (Flynn
C-XANES analyses of Sutter’s Mill meteorite
et al. 2000), Orgueil (Garvie 2006), and Murchison
(Berg et al. 2008) meteorites. These spectra either do
not show a prominent diamond core exciton absorption
(Flynn et al. 2000; Garvie 2006), or the exciton peak is
largely broadened and blueshifted (Berg et al. 2008).
The absence of the diamond exciton in these meteoritic
nanodiamonds may result from the size effect (Chang
et al. 1999). The diamond-edge spectra of SM2-5 show
broader peaks at 288.6–288.7 eV, compared to the
sharp diamond exciton peak at 288.9 eV in the diamond
reference spectrum (Fig. 4). This may be attributed to
irradiation, which results in a broader exciton peak at
lower energy, as shown in hydrogen-plasma-exposed
CVD diamond films (Guenette et al. 2013). Additional
peaks are also found in the diamond-edge spectra of
SM2-5, including 285, 285.5, 290.2 eV. The peaks
around 285 eV are often found in CVD diamond
surfaces, and are assigned to 1s–p* of sp2 (C=C),
including the contribution of sp (CC); they may
originate from a graphitic phase (Lenardi et al. 1999;
Duda et al. 2008). However, we cannot exclude the
possibility of a contribution from aromatic carbon in
the surrounding matrix. The peak at 290.5 eV is
probably a part of the r* resonances of diamond (St€
ohr
1992), but might also be due to the contribution of
carbonates in the surrounding matrix.
Considering the Ca-bearing spherical inclusion
found in one of the diamond grains, the diamonds in
SM2-5 might have formed around spherical inclusions
by the CVD process and been subjected to cosmic ray
irradiation. Another possibility is that the SM2-5
diamonds may have originated in a high-temperature
and -pressure environment, where the grains had
sufficient time to grow. Diamonds from Earth’s mantle
are formed under extreme conditions within the deep
keels of cratons, at very high pressures and
temperatures, 45–55 kbar and 1050–1200 °C, which is
equivalent to depths of 150–200 km (Erlich and Hausel
2002). Diamonds can also grow at shallower depth
under tectonic pressure, such as along subduction zones
or deep shear zones (Erlich and Hausel 2002). The
minimum parent body radius R required for in situ
diamond formation is roughly estimated as over
approximately 1700 km, which is close to the size of the
Moon, and larger than the size of Pluto, which is
1153 km in radius. This value is calculated using the
equation P = 2pGq2R2/3, where P is the central
pressure, which is approximately 50 kbar required for
the diamond formation, G is the gravitational constant
(=6.67 9 10 8 cm3 g 1 s 2), and q is the density (here
we use the values of 3.5 g cm 3 for Vesta; Hilton 2002).
Considering that the diamonds in SM2-5 are
surrounded by matrix material that has not experienced
similar thermal processes, and the fact that other CM2
2101
chondrites do not suggest such conditions occurred on
the CM parent body, this suggests that these diamonds
must be xenolithic. They presumably were produced in
a high-temperature and -pressure environment in a large
parent body that was then destroyed, after which the
liberated diamonds were incorporated into the Sutter’s
Mill parent body, as a breccia.
CONCLUSIONS
We have investigated the carbon-bearing phases in
a FIB section of Sutter’s Mill fragment SM2-5 using
C-XANES and N-XANES. The section was isolated at
the position of two large approximately 10 lm possible
diamond grains. CM chondrite-like primitive organic
matter is present in the fine-grained matrix material
surrounding these diamond grains. This organic matter
contains no distinct nitrogen-bearing functional groups.
The matrix contains a globule-like organic carbon grain
that is more aromatic in nature than the matrix region.
A single carbonate grain was also found. One of the
diamond grains contains a Ca-bearing inclusion that is
not carbonate. The diamond grains may have formed
by a CVD process, or in a high-temperature and
-pressure environment, after which they were
transported to the Sutter’s Mill parent body from the
disintegration of a larger parent body.
Acknowledgments—We thank George Flynn, Monica
Grady, Hikaru Yabuta, and the Associate Editor
Christine Floss for their careful reviews and
constructive comments. STXM-XANES data were
acquired at beamline 5.3.2.2 at the ALS, which is
supported by the Director of the Office of Science,
Department of Energy, under Contract No. DE-AC0205CH11231. We gratefully acknowledge support from
NASA Astrobiology and Origins of the Solar System
Programs. Y.K. gratefully acknowledges support
through the JSPS Postdoctoral Fellowships. The
Sutter’s Mill recovery was supported by NASA Ames
Research Center.
Editorial Handling—Dr. Christine Floss
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