Meteoritics & Planetary Science 45, Nr 1, 73–90 (2010)
doi:10.1111/j.1945-5100.2009.01006.x
Matrix and whole-rock fractionations in the Acfer 094 type 3.0 ungrouped
carbonaceous chondrite
John T. WASSON* and Alan E. RUBIN
Institute of Geophysics and Planetary Physics, Departments of Earth and Space Sciences and Chemistry and Biochemistry,
University of California, Los Angeles, California 90095–1567, USA
*Corresponding author. E-mail: [email protected]
(Received 30 January 2009; revision accepted 30 September 2009)
Abstract–We used the electron microprobe to study matrix in the ungrouped type
3.0 carbonaceous chondrite Acfer 094 using 7 · 7-point, focused-beam arrays; data points
attributable to mineral clasts were discarded. The grid areas show resolvable differences in
composition, but differences are less pronounced than we observed in studies of CR2 LaPaz
Icefield (LAP) 02342 (Wasson and Rubin [2009]) and CO3.0 Allan Hills A77307 (Brearley
[1993]). A key question is why Acfer shows an anomalously uniform composition of matrix
compared with these other carbonaceous chondrites. Both whole-rock and matrix samples
of Acfer 094 show enhancements of Ca and K; it appears that these reflect contamination
during hot desert weathering. By contrast, the whole-rock abundance of Na is low.
Although weathering effects are responsible for some fractionations, it appears that nebular
effects are also resolvable in matrix compositions in Acfer 094. As with LAP 02342, we
infer that the observed differences among different areas were inherited from the solar
nebula and may have been carried by porous chondrules that experienced low (about 20%)
degrees of melting. Acfer 094 has been comminuted by one or more impact events that may
also have caused volatile loss. Thus, despite preserving evidence (e.g., an exceptionally high
content of presolar SiC) implying a high degree of pristinity, Acfer 094 is far from pristine
in other respects. This evidence of comminution and an O-isotopic composition similar to
values measured in metamorphosed CM chondrites suggest that Acfer was hydrated before
being outgassed by the inferred impact event. Convection within the plume associated with
the impact event probably also contributed to the homogenization of the Acfer 094 matrix.
5–12% in enstatite chondrites (Rubin et al. 2009) to
40% in CR chondrites (Weisberg et al. 1993). In the CI
chondrites, the initial fraction may have approached
100%, but aqueous alteration has destroyed the nebular
record. Matrix studies have been reported in numerous
papers; these are comprehensively reviewed by Brearley
and Jones (1998).
This is the second in a series of matrix studies of
carbonaceous chondrites in which we apply a new
electron-probe technique; our first study was of the
relatively pristine CR2 chondrite LaPaz Icefield (LAP)
02342 (Wasson and Rubin 2009). As the interchondrule
matrix consists of a mixture of nebular (including
presolar) fines and fragments of anhydrous pristine
chondrules (including small metal grains), it is a
challenge to devise techniques to maximize information
INTRODUCTION
The solid matter in the solar nebula consisted of
chondrules (silicate-rich objects as well as those
composed of metal and ⁄ or iron sulfide), chondrule
fragments, refractory and mafic inclusions, and
submicrometer fines. The chondrules formed by melting
(in most cases, incomplete melting) of pre-existing solids.
Chondrule fragments formed by collisional disruption of
solidified chondrules. Fine-grained materials (‘‘fines’’)
are a major constituent of the interchondrule matrix in
chondrites. An important goal is to characterize the
nebular component of these nebular fines, some fraction
of which consists of relatively unprocessed presolar
materials. In ‘‘anhydrous’’ chondrite groups, the
proportion of matrix in the whole-rock ranges from
73
The Meteoritical Society, 2010. Printed in USA.
74
J. T. Wasson and A. E. Rubin
about nebular fines. Wasson and Rubin (2009) took the
approach of analyzing 49 points on a rectangular grid
of individual clastic matrix patches and discarding
points having compositions dominated by a single phase
or having low totals indicating the presence of holes or
other artifacts. Although the grid areas were chosen to
be predominantly on fine-grained regions, most grid
areas included several holes or small (3–10 lm) rock
fragments and mineral grains that produced deviant
compositions. Our screening process resulted in a data
set that was relatively homogeneous in composition
within each area but with resolvable compositional
differences among areas.
Acfer 094 (shortened to Acfer in some of the
following text) is widely held to be one of the most
primitive carbonaceous chondrites (e.g., Scott and Krot
2005). This conclusion was based initially on the high
degree of disequilibrium within its silicates, its very high
contents of presolar SiC and diamond grains (Newton
et al. 1995) and its high content of amorphous
materials (Greshake 1997). Greshake (1997) studied
Acfer matrix with scanning electron microscopy (SEM)
and analytical transmission electron microscopy and
noted similarities to matrix in Kakangari and Allan
Hills (ALH) 77307. Bischoff and Geiger (1994) assigned
Acfer to shock stage S1 and to weathering grade W2.
Dreibus et al. (1995) measured a low S content in Acfer
and in several other oxidized carbonaceous chondrites
from hot deserts, and inferred that weathering processes
removed a large fraction of its S. Rubin et al. (2007)
reported a bulk INAA (instrumental neutron-activation
analysis) study of Acfer that showed anomalously high
Ca and K contents and a low Na content. Sakamoto
et al. (2007) found Acfer to contain a rare component
with a highly anomalous O-isotopic composition
(D17O 90&). Newton et al. (1995) noted that Acfer
has a high content of spherical radial-pyroxene ⁄
cryptocrystalline chondrules, about two-to-three times
higher than typical in carbonaceous chondrites.
Greshake (1997) observed high contents of amorphous
materials and ferrihydrite (Fe5O3(OH)9) in the Acfer
matrix.
SAMPLES AND PETROGRAPHIC TECHNIQUES
We studied thin section M9324 of Acfer 094 from
the Naturhistorisches Museum in Vienna. The UCLA
LEO 1430 SEM was used to create back-scattered
electron (BSE) images of the entire 15 · 13 mm section;
these were combined into a poster-size mosaic (Fig. 1).
This composite image formed the basis for choosing
target areas for detailed study. A grid with 1-mm spacing
was superposed on the mosaic and used to label the
chondrules and other features. Each square is subdivided
into a 5 · 5 subgrid designated by letters. The top row
has the letters a through e, with a at the upper left
position; the last letter on the bottom right is y.
Two kinds of studies using the UCLA JEOL
electron microprobe (EMP) were carried out: (a)
elemental maps for five elements and (b) analyses of 10
elements using an electron beam 3 lm diameter electron
beam over 7 · 7 point grids with interpoint spacings of
8 to 9 lm.
Maps
We created BSE and five wavelength dispersive X-ray
maps (Mg, Al, S, Fe, Ni) for a 6 · 4 mm region with the
upper left corner near the bottom center of cell F08 and
the lower right corner near the bottom center of cell L12.
These maps were produced by moving the sample stage
below a finely focused (d 1.5 lm) electron beam. Point
spacings were 2 lm and dwell times were 50–55 ms.
Matrix Grids
Quantitative information about matrix was
obtained by using an EMP beam 3 lm in diameter to
analyze rectangular grids about 50 · 50 lm in size; the
spot size was chosen to be larger than matrix grains but
small enough to allow recognition of single-phase grains
larger than 3 lm. Each grid area and its immediate
surroundings were imaged in BSE, then the 3 lm
diameter beam was used to determine 10 elements: Na,
Mg, Al, Si, S, K, Ca, Cr, Mn, and Fe; this set includes
all elements having concentrations >1 mg ⁄ g except O
and Ni (and O contents can be roughly inferred from
the concentrations of metal cations and stoichiometric
parameters). Natural standards were used: chromite for
Cr, forsterite for Si and Mg, albite for Na, Mn-rich
garnet for Mn, orthoclase for K, millerite for S,
grossular for Al and Ca, and magnetite for Fe. The
beam current was limited to 15 nA to minimize loss of
alkalis.
We used ZAF corrections. As the matrix material is
fine grained, it approaches the homogeneity of a single
mineral phase; we conclude that errors introduced by
applying this correction procedure should be minor.
RESULTS
Element Maps and the Choice of Regions for Detailed
Study
The element that is the best tracer for interchondrule
matrix is S; our S map (Fig. 2a) shows the contrast
between the high S content of the matrix and the
minimal S contents of the common, low-FeO chondrules
Acfer 094 matrix and bulk fractionations
75
Fig. 1. Backscattered electron (BSE) image of the investigated section of Acfer 094. The large rectangle shows the region
mapped using wavelength dispersion for five elements and BSE. Sampled regions are labeled using the superposed grid with lines
spaced 1.0 mm apart.
and the rare metal grains. The color scale concentrations
were inferred by comparing the counting rate intensity of
FeS with that in the appropriate peak in the intensity
histogram. A BSE image of the 6 · 4 mm elementmapped region of the Acfer 094 section is shown in
Fig. 2b; an approximate X-Y grid from Fig. 1 is
superposed.
Compositional Data Sets for Matrix (Grid) Areas
We investigated 10 grid areas; the locations are
shown on Fig. 2a; because the section was rotated
slightly between the production of Figs. 1 and 2b, the
grid-area location names do not exactly fit the Fig. 2b
lines. In Fig. 3, we illustrate the textures of these matrix
areas on detailed BSE images; dimensions of the
rectangles representing the grid areas are about
50 · 50 lm. Particles in these areas are relatively fine;
with the exception of areas H09h, J08w, and L11u, £1
particle per area has long dimensions as large as 6 lm.
Mean compositions and standard deviations for the
investigated grid areas are listed in Table 1. All elements
except S (elemental) were calculated as oxides. Totals
were corrected by removal of moles of O equal to the
moles of S (the assumption being that S is almost
entirely in the form of FeS, perhaps with a minor
fraction forming other sulfide phases). Analytical totals
for our selected points are low, in the range 77 to 88 wt%
with a mean of 84 wt%.
Totals, Porosity, and Surface Roughness
Three properties of matrix are potentially responsible
for low analytical totals: porosity and surface roughness,
water of hydration, and the presence of carbonates.
In part because CR chondrites have moderately high
H2O contents and most of the water is in the fine matrix,
76
J. T. Wasson and A. E. Rubin
Wasson and Rubin (2009) inferred that hydrated phases
were mainly responsible for the low totals of matrix
regions in LAP 02342. By contrast, Acfer is a dry
meteorite in which hydrated phases are rare; thus, the low
totals must have another explanation.
The totals for Acfer 094 grid areas showed a
different pattern than observed in LAP 02342. In LAP,
our totals are slightly lower, with a mean of 82% and a
larger fraction of the totals are below the 75% cutoff.
Even if we neglect one CaCO3-rich region in LAP, the
fraction of low totals was about 8%.
Only 21 (4.3%) of our 490 Acfer matrix points have
totals lower than our minimum limit of 75%, but 10 of
these are below 65%, implying cracks or holes in the
section at these locations. Given the lower fraction of low
totals, the higher fraction of those that are <65% and
the large number of cracks visible in the Acfer section, we
suggest that cracks and surface roughness are the
dominant source of low totals in Acfer 094 matrix
analyses.
Exclusion of Analyses with Extreme Values from Mean
Matrix Compositions
To avoid the contributions of coarse mineral grains
(and other anomalies such as holes) to our mean
compositions, we excluded analyses having extreme
compositions
before
calculating
matrix
mean
compositions and standard deviations. The rejected data
are also not included in scatter diagrams. This screening
is useful both because it avoids the effect on mean
compositions of the chief constituents of mineral grains
(e.g., Mg or Si in olivine, Fe or S in FeS) and because it
avoids the depression of mean concentrations of other
elements that are not present in the coarse grains. Both
the accepted and the rejected data are tabulated in the
electronic annex.
In Table 2, we list the limits for acceptable
analyses that were chosen for ten elements and for
totals. The guiding philosophy for the limits was to
eliminate concentrations that deviated ‡3r from the
mean and to use the same limits for all matrix regions.
As elevated concentrations of the least-abundant
elements (such as Cr and Mn) had no appreciable
effect on the totals of other elements, we discarded
only these extreme values from plots and elemental
means but continued to include the remaining elements
on plots and in means.
The standard deviations varied among the different
areas; the screening limits were therefore adjusted on
the basis of trial and error. In the EXCEL files
provided in the electronic annex, we flag one property
of each rejected point that was outside our chosen
limits, but it was not uncommon that two or more
properties were outside the limits.
Fig. 2. Maps of the investigated 6 · 4 mm region of Acfer 094
in terms of S X-rays and back scattered electrons. a) The
colorized S X-ray map shows the location of the (S-rich)
matrix regions. b) On the BSE map, we show as small squares
the locations of the 10 regions in which we determined 10
elements in 7 · 7 grid arrays; on the right side of the image,
the grid is slightly rotated relative to that on the master in
Fig. 1. c) The Ca X-ray map shows the location of Ca-Al-rich
inclusions and chondrules; with the exception of a crack in
G11, there is no evidence of Ca deposits within cracks that
would be associated with Ca introduction during weathering;
excess Ca introduced during weathering seems to be located
within the matrix or as fine structures within or surrounding
chondrules and inclusions.
Acfer 094 matrix and bulk fractionations
77
Fig. 3. Detailed BSE images of the 10 matrix grid regions in Acfer 094. Almost all the regions are crossed by cracks, but
accumulations of iron oxides generated by weathering are confined to a small fraction of these cracks. It appears that most
cracks opened only during sample preparation.
For most elements, the means of the entire
matrix-grid sets were within 5% of the means
for the sets consisting of selected points. Thus,
rejecting the extreme points mainly affected the
variance, but had a minor-to-negligible effect on the
means.
1.92
0.40
0.236 0.07
2.58
16.6
29.3
0.28
0.248 0.041
3.20
84.7
33
Al2O3
Cr2O3
MnO
CaO
MgO
FeO
Na2O
K2O
S
Total
n, disc
0.19
0.23
0.05
3.1
3.1
0.24
40
84.3
2.84
0.18
2.1
0.81
0.241 0.039
0.27
29.4
16.4
2.48
0.216 0.07
0.43
1.87
2.4
0.13
0.20
2.1
I11w
SD
0.05
2.0
2.3
0.20
36
84.4
2.57
0.27
2.68
0.85
0.221 0.033
0.26
29.6
16.6
2.44
0.248 0.08
0.44
1.79
31.5
H11k I11w
SD
Mean
0.07
0.22
2.0
0.05
2.2
1.6
0.19
35
82.9
2.98
0.29
1.8
0.78
0.223 0.026
0.29
30.5
15.3
2.41
0.09
0.22
2.5
J09t
SD
0.04
2.8
2.3
0.20
34
84.4
2.93
0.31
2.4
0.87
0.200 0.042
0.26
30.3
15.9
2.44
0.253 0.13
0.40
1.79
31.4
J08w J09t
SD
Mean
0.215 0.05
0.38
1.86
30.2
J08w
Mean
0.09
0.20
2.0
J10n
SD
0.05
2.4
2.6
0.20
38
83.9
2.85
0.22
2.6
0.68
0.253 0.033
0.29
30.2
15.7
2.70
0.197 0.06
0.41
1.97
30.8
J10n
Mean
0.13
0.25
2.0
J12l
SD
0.06
2.6
2.6
0.19
32
83.4
3.03
0.35
3.2
0.69
0.224 0.031
0.28
29.8
16.0
2.55
0.170 0.06
0.42
1.61
30.7
J12l
Mean
0.07
0.23
1.8
0.05
2.4
1.9
0.18
41
83.5
2.80
0.16
1.2
0.91
0.217 0.031
0.26
30.0
14.8
2.65
0.07
0.19
2.1
0.05
2.7
2.8
0.16
36
84.3
3.02
0.27
2.3
0.94
0.188 0.037
0.27
30.0
15.8
2.59
0.14
0.20
2.5
0.06
2.8
2.7
0.26
39
84.2
3.40
0.20
2.6
0.65
0.213 0.030
0.28
28.5
17.2
2.52
2.22
0.105
0.162
29.6
20.7
2.18
0.232
0.488
2.63
31.0
L11u
SD
Bulk
0.236 0.11
0.44
1.86
31.3
L09p L11u
SD
Mean
0.202 0.06
0.37
1.63
31.7
K11a L09p
SD
Mean
0.207 0.07
0.36
1.84
31.7
K11a
Mean
Totals include corrections for Fe bound to S as FeS. Final row shows the number of points (n) averaged and the discarded fraction (disc) of the 49 grid points.
0.33
2.7
0.78
0.06
2.9
2.6
0.25
0.09
0.24
31.7
31.5
SiO2
2.5
H09h H09h H11k
Mean SD
Mean
Element
Table 1. Mean compositions (in wt%) of matrix areas in the ungrouped carbonaceous chondrite Acfer 094.
78
J. T. Wasson and A. E. Rubin
Acfer 094 matrix and bulk fractionations
Variable Scatter and Resolvable Means for Different
Matrix Areas
Compositional Scatter within Analyzed Areas
In Fig. 4, we show S versus Fe and Mg versus Fe
scatter plots for the 10 matrix grid areas (two areas on
each on five diagrams). As discussed above (and
documented in the last row of Table 1), 18 to 35% of
the 49 grid points were discarded. Mean concentrations
are listed in the legends. In most diagrams the bulk of
the data forms an ellipsoidal scatter field, with a
handful of points scattering more widely. Some fields
are relatively compact (e.g., areas I11w and J12l in
Fig. 4f,j); others show much scatter (e.g., area H11k in
Fig. 4a,f).
The five diagrams on the left (Fig. 4a–e) show S
versus Fe scatter fields. Most of the data sets show
weak positive trends. To aid recognition of unusual
S ⁄ Fe ratios, we show a reference line corresponding to
0.4 · CI (CI = the ratio in CI chondrites). Note, for
example, the low S ⁄ Fe ratios in grid area J09t (Fig. 4b)
and the high S ⁄ Fe ratios in area L11u.
The five diagrams on the right (Fig. 4f–j) show MgFe distributions. Negative trends are present in each of
the 10 grid areas. Most of the scatter fields are similar.
Although the relative ranges of Fe are particularly large
in areas H11k and J08w (Fig. 4f), the absolute ranges of
Mg (calculated as MgSiO3 or Mg2SiO4) are larger, and
the high Fe range seems mainly to reflect variations in
abundance of mafic silicates.
In Fig. 5, we show K versus Al and Ca versus Al
scatter plots for the 10 matrix grid areas (two each on
five diagrams). There is a weak positive trend for some
grid areas (for L09p and J10n on K-Al diagrams, for
J08w and J10n on Ca-Al diagrams), but most diagrams
show no significant correlations.
Table 2. Criteria for rejection of grid points.
Element
S
CaO
FeO
MgO
Cr2O3
SiO2
Al2O3
Na2O
K2O
MnO
Total
Concentration (%)
Lower
Upper
Concentration
(mg ⁄ g)
Lower
Upper
1.2
1.8
21
11.3
12
13
163
68
20.5
1.19
0.15
0.108
0.100
75
5.5
3.2
38
22.7
1.5
39
2.4
0.45
0.335
0.55
102.5
96
6.3
1.1
0.90
0.77
Limits are listed both in wt% oxide and in mg ⁄ g element.
55
23
295
137
10.3
182
12.7
3.3
2.78
4.3
79
The five diagrams on the left (Fig. 5a–e) show K-Al
distributions. For reference purposes, a 3 · CI line is
shown; points for all matrix grid areas scatter around this
line. Although no lines are plotted in the K-Al diagrams
shown by Wasson and Rubin (2009), the mean ratio in
those was 1.5 · CI, 2· smaller. Only J12l (Fig. 5e)
shows K ⁄ Al ratios appreciably higher than the 3 · CI
line. Area L09p (Fig. 5b) shows a remarkably compact
distribution and low mean values of both elements.
The five diagrams on the right (Fig. 5f–j) show
Ca-Al distributions for the 10 grid areas. The reference
line shows the 2 · CI mass ratio. As the Ca ⁄ Al ratio
in Acfer matrix is higher than that in whole rock (or
in CI chondrites), it is interesting to see which grid
areas show especially high ratios. There are two, L09p
(Fig. 5g) and J12l (Fig. 5j). The same two areas show
higher Na ⁄ Al and K ⁄ Al ratios than the other areas.
These two areas have the lowest mean Al contents,
8.5 mg ⁄ g.
Compositional Differences among the Analyzed Areas
Although the inter-area differences are appreciably
smaller than those observed in CR2 LAP 02342
(Wasson and Rubin 2009), with the exception of one
pair the matrix grid areas of Acfer have their own
distinctive compositions. This shows up both in the
means and in the ranges. To emphasize the differences
in composition, on Fig. 6, we plot means and 95%
confidence limits for the ten areas on S-Fe (Fig. 6a),
Mg-Fe (Fig. 6b), K-Al (Fig. 6c), and Ca-Al (Fig. 6d)
diagrams. Note that smaller error limits reflect more
compact distributions on diagrams such as those cited
in the discussion of Figs. 4 and 5. On these diagrams, a
few grid areas are resolvable from all other areas; some
are resolvable from all except one or two; on each
diagram, several areas cluster together, but the clusters
have different members on different diagrams.
We summarize the compositional discrimination of
six elements on Fig. 7; six elements are represented by
dots in a 2 · 3 array placed within a cell at the
intersection of two matrix grid areas. A filled dot
indicates that the two grid areas are different at >95%
probability if compared only in terms of that element.
The diagram shows that, with the exception of the pair
J08w and J09t, all pairs are resolved from each other by
at least one element. Figure 7 shows that six area pairs
are resolved in terms of only one element; the remaining
38 area pairs are resolved by two or more of the six
elements.
Abundance Patterns in the Matrix of Acfer 094
In Fig. 8, we show Mg- and bulk-Acfer-normalized
abundance ratios of the 10 studied elements in the
80
J. T. Wasson and A. E. Rubin
Fig. 4. Plots of EMP analyses of S versus Fe and Mg versus Fe are shown for 10 grid areas, two each on 5 diagrams; legends
give means (mn). a–e) A line showing the position of 0.4 · CI is shown on each S versus Fe plot as a reference; the data scatter
around this line. Although most areas show considerable scatter and there is little difference from area to area, one can see
that points in region K11a (c) have high S and low Fe and those in area L11u (c) have low S and average Fe. Weak positive
correlations between S and Fe are observed. f–j) All areas show strong negative correlations between Mg and Fe implying that
these are mainly in different nebular components. Most scatter fields show a tail toward high Mg, low Fe values that is
especially pronounced in areas H11k and L11u.
Acfer 094 matrix and bulk fractionations
81
Fig. 5. Plots of EMP analyses of K versus Al and Ca versus Al are shown for 10 grid areas, two each on 5 diagrams; legends
give means (mn). a–e) On each diagram, a reference line showing the position of 3 · CI is shown as a visual guide. Visual
examination shows that centroids are above this line in areas H11k (a), H09h (d) and J12l (e), and below this line of J09t
(b), K11a (c), and L11u (c). Area L09p (b) shows a compact distribution with low values of both K and Al. f–j) Reference lines
at 1 · CI and 2 · CI are drawn on each diagram. Although Ca ⁄ Al ratios are commonly low in the matrix of primitive
chondrites (Huss et al. 2005), in Acfer 094, centroids of scatter fields are in the range 1.7 to 2.0 · CI; those for L09p (g) and J12l
(j) are especially high.
82
J. T. Wasson and A. E. Rubin
Fig. 6. Means and 95% confidence limits for the ten areas on S-Fe, Mg-Fe, K-Al, and Ca-Al diagrams. Note that small error
limits here (e.g., Al in L09p) reflect compact distributions on diagrams in Figs. 4 and 5. Two areas were considered to be
compositionally distinct if the elemental means were not overlapped by the 95% uncertainty of either area.
Fig. 7. In this truth table, we show whether six elements show
resolvable differences between 10 Acfer 094 matrix grid areas.
Dots at the intersection boxes show which elements resolve the
two areas at 95% confidence based on Student’s t distribution.
The six employed elements can be distinguished either by
position within the box or by color ⁄ shading. Only one area
pair (J09t and Jo8w) cannot be resolved on the basis of these
six elements whereas 23 of the 45 pairs are resolved by three
or more elements.
matrix grid areas. The same general pattern is observed
for each grid area: Cr and Mg ratios are similar (but
note the arbitrary bulk-Acfer Mg value), Si (an
estimated bulk value), Fe, and Mn ratios are slightly
Fig. 8. Whole-rock- and Mg-normalized abundance ratios of
ten elements in the ten grid areas in Acfer 094. The pattern is
similar to the one we observed in CR LAP 02342 with the major
exception of the low Al and the high Ca. The high Ca appears
to be a weathering feature associated with a long residence in
the Sahara desert. The low Al ⁄ Mg ratio may reflect asteroidal
or, less likely, nebular processing. As discussed in the section
Nebular Processes to Account for the Fractionated
Composition of Acfer 094 Fines, the bulk-Acfer Mg and Si
values chosen for normalization are arbitrary, thus limiting the
conclusions that one can make regarding these elements.
elevated (ratios near 1.2), and Na, K, and S are high
(ratios of about 2.1, 2.6, and 1.7, respectively). Calcium
and Al are described below.
Acfer 094 matrix and bulk fractionations
83
Table 3. Mean matrix composition in Acfer 094; comparison of relative standard deviations for Acfer with those
of ALHA77307 (Brearley 1993) and LAP 02342 (Wasson and Rubin 2009).
Element
Acfer
Mean
wt%
SiO2
Al2O3
Cr2O3
MnO
CaO
MgO
FeO
Na2O
K2O
S
Total
Ca ⁄ Al
Na ⁄ K
31.30
1.81
0.409
0.214
2.526
16.23
29.62
0.270
0.222
2.96
84.12
1.88
1.09
SD
wt%
Rel. SD
%
0.51
0.12
0.026
0.019
0.093
0.67
0.54
0.013
0.020
0.22
0.64
1.62
6.57
6.39
8.68
3.68
4.14
1.82
4.86
8.94
7.54
0.76
ALHA77307
Mean
SD
wt%
wt%
30.07
4.15
0.346
0.170
0.78
14.46
33.3
0.245
0.115
2.91
90.2
0.25
1.89
0.69
0.30
0.036
0.059
0.19
0.92
2.0
0.055
0.040
0.65
1.1
Rel. SD
%
2.29
7.28
10.4
34.6
24.4
6.33
6.04
22.6
34.9
22.2
1.23
LAP
Mean
wt%
28.3
1.87
0.341
0.204
0.79
16.19
31.6
0.80
0.119
3.36
81.8
0.57
3.76
SD
wt%
Rel. SD
%
3.0
0.23
0.038
0.029
0.35
0.96
1.5
0.18
0.030
0.61
2.3
10.5
12.2
11.1
14.3
44.1
5.95
4.79
23.1
25.5
18.0
2.80
The ALHA77307 S data are calculated as SO3.
In Table 3, we compare mean Acfer matrix
compositions with those for CR2 LAP 02342 matrixgrid areas (Wasson and Rubin 2009) and for matrix and
rim areas in CO3.0 ALHA77307 (Brearley 1993). The
Acfer data differ from those in the other two sets in
terms of some ratios and in terms of the relative
standard deviations (see the Discussion section).
A most striking feature in Fig. 8 is that the Ca and
Al patterns are reversed compared to patterns
commonly observed in other chondrites (e.g., Huss et al.
2005): Ca is high (mean ratio about 1.4 and Al is low
(mean ratio about 0.9). The Ca ⁄ Al ratios in the grand
means of Acfer, LAP, and ALHA77307 are listed in
Table 3; the Acfer ratio is 3.3· higher than that in LAP
and 7.5· higher than that in ALHA77307.
We examined whether this difference could be the
result of the poorly constrained Acfer bulk composition
that we are using for the normalization. Our chosen
bulk composition (Table 1) has a Ca ⁄ Al mass ratio of
1.12 g ⁄ g, a typical chondritic ratio, and our mean
Al ⁄ Mg ratio of 0.098 g ⁄ g is only 4% lower than the
value we measured in CR LAP 02342 matrix (Wasson
and Rubin 2009). The big differences are in the
exceptionally high Ca concentration in Acfer matrix and
in the high Al contents that Brearley (1993) reported for
the ALHA77307 matrix.
Although in Fig. 8 the abundance ratios for K and
Na are similar to those we observed in LAP 02342 matrix
(Wasson and Rubin 2009), note that we are normalizing
to an Acfer composition that has a very low Na content.
The Na ⁄ K ratio in Acfer is 6.2· lower than the CI ratio.
The Na ⁄ K ratios are compared in Table 3; the Acfer
ratio is 0.58 that in ALHA77307 and 0.29 that in LAP.
DISCUSSION: WEATHERING AND ASTEROIDAL
EFFECTS
Acfer 094: Matrix and Whole-Rock Observations That
Need Explanations
As discussed above, Acfer 094 matrix shows various
compositional anomalies. The two most striking are (1)
the exceptionally low Na ⁄ K ratio and (2) the
exceptionally high Ca ⁄ Al ratio. In addition, Acfer
matrix grid areas have relatively uniform mean
compositions compared with the large inter-regional
variations observed in CR2 LAP 02342, which was
studied by the same method.
Acfer 094 also shows major fractionations in wholerock compositions. Dreibus et al. (1995) noted that it
has a low S content compared with most carbonaceouschondrite falls, and that its S ⁄ Se ratio is anomalously
low (0.59 · CI). By contrast, their measured bulk Se
content (14.9 lg ⁄ g) was relatively high, as high as
observed in most CM chondrites and higher than that
in CO and CV.
The Se content reported by Rubin et al. (2007) was
only 11.3 lg ⁄ g, which is 24% lower than that reported
by Dreibus et al. (1995) and 12% lower than the mean
content in CM chondrites. As Se analyses by the UCLA
and MPI Mainz teams commonly agree to within about
3%, we suspect that the discrepancy between the two
analyses reflects sampling heterogeneities. Nonetheless,
it diminishes the strength of the Dreibus et al. (1995)
conclusions regarding selective depletion of S and casts
some doubt on the relatedness of Acfer to CM
chondrites.
84
J. T. Wasson and A. E. Rubin
As noted above, the bulk INAA data in Rubin
et al. (2007) revealed a low bulk Na and a low Na ⁄ K
ratio (0.16 · CI). The effect is mainly a depletion in Na;
the K ⁄ Al ratio reported by Rubin et al. was slightly
(1.06·) elevated compared with CI and higher by a
factor of 2 than the ratio in CM chondrites.
Weathering Effects in Acfer 094
Dreibus et al. (1995) attributed the low S content of
Acfer 094 to weathering that occurred at the find
location in the Sahara. They pointed out that many
carbonaceous chondrites from hot deserts have S
contents tens of percent lower than those observed in
falls of the same group. Their examples are mainly from
the CO and CV groups. It is a common cosmochemical
assumption that Se substitutes more or less ideally for S
in FeS and other sulfides, but the Dreibus et al. bulk Se
content in Acfer was close to the mean concentration in
CM chondrites. They therefore inferred that the
fractionation reflected weathering and suggested that,
when S is oxidized to mobile SO2 (g), SO3 (g) or to
SO4=, it is leached or lost as a volatile whereas Se
forms insoluble SeO2 and remains within the rock.
However, as discussed above, the lower Se
concentration reported by Rubin et al. (2007) decreases
the fractionation between these two elements; the
CI-normalized S ⁄ Se ratio is revised from 0.59 to a
less-dramatic value of 0.78. Nonetheless, this ratio is
still significantly lower than unity.
Terrestrial weathering may also be responsible for
the anomalous concentrations of these two alkalis. If
so, then Na has been extracted and K added during
this process. Low Na ⁄ K ratios are known in some
weathered chondrites and even in one CM fall,
Murray (which was, however, possibly subjected to
some rain prior to recovery; Kallemeyn and Wasson
1982), but the K abundance is low in these samples.
This leads to the expectation that leaching is likely to
remove both elements (Kallemeyn and Wasson 1982).
Dreibus et al. (1995) showed evidence of Na depletion
in several carbonaceous chondrites but did not report
K data.
In a study of CK chondrites, Huber et al. (2006)
found that most Saharan CK samples had K values
enhanced by factors of 1.2 to 1.8 compared with
Antarctic samples and to the Karoonda observed fall; in
the same samples, they found that Na was variable, in
some cases enhanced, and in some cases depleted, but
that the Na abundance ratios were, on average, 20–30%
lower than those of K. Huber et al. (2006) observed
some scatter in CK Na values, but no evidence of
systematic loss or gain attributable to weathering. We
conclude that it is likely that K was introduced in Acfer
094 during its Saharan residency and that weathering is
responsible for the low Na ⁄ K ratio in the Acfer matrix.
Might weathering also be responsible for the
enhanced Ca in the Acfer matrix? It seems possible that
CaCO3 or CaPO4 was deposited in cracks and pores of
the matrix. Huber et al. (2006) observed enhanced Ca in
Saharan CK chondrites and noted that Keller (1992)
and Al-Kathiri et al. (2005) inferred that Ca
enhancements in CK and in ordinary chondrites
resulted from Ca deposition during desert weathering. If
Ca was moving into the meteorite, one might expect
‘‘caliche’’ enhancements along cracks. Although, with
the exception of one crack, our Ca X-ray map (Fig. 2c)
does not show Ca-rich veins (although our BSE image
in Fig. 2a shows that iron oxide veins are common), we
nonetheless consider weathering to be the most
probable source of the high Ca in the matrix.
Dreibus et al. (1995) argued that weathering is
responsible for the low (40% based on their Se data,
22% based on that of Rubin et al. 2007) S content of
whole-rock Acfer and suggested that the absence of a
depletion in Se reflected a difference in mobility during
weathering-induced leaching. One would expect that it
is the fine-grained sulfides in the matrix that are most
susceptible to attack during weathering and it is
therefore interesting to compare Acfer and LAP S ⁄ Mg
and S ⁄ Fe mass ratios (multiplied by 10). Our matrix
S ⁄ Mg ratios are 3.0 and 3.5, respectively, and our S ⁄ Fe
ratios are 1.29 and 1.39, respectively. The similarity in
these ratios indicates that Acfer started out with CRlike matrix composition and that weathering loss of 10
to 15% of the S is responsible for the difference. This is
a much smaller effect than inferred by Dreibus et al.
(1995). Although one can challenge the assumption that
CR chondrites offer a reasonable comparison standard,
at this time, there is only minor evidence for the
leaching of S from the matrix.
Brecciation and Matrix Homogenization in Acfer
Although Acfer 094 is, in several respects, one of
the most primitive known meteorites, its texture shows
evidence of parent-body processing. As can be seen in
the images in Figs. 2 and 3, Acfer contains many
chondrule and mineral fragments having shard-like
shapes, implying that Acfer was crushed in an impact
event. Newton et al. (1995) noted that 63% of the area
consists of grains <50 lm in size and attributed this
low mean size to impact-induced crushing.
Wasson and Rubin (2009) observed striking textural
differences among regions in CR2 LAP 02342. By
contrast, the textural differences among different grid
areas in Acfer are small compared to the compositional
variances of these areas.
Acfer 094 matrix and bulk fractionations
The texture of LAP is that of a gently compacted
chondrite. Although some shards are present, most of
the large chondrules are intact (and large chondrules are
more vulnerable to fragmentation than small ones; e.g.,
Rubin and Pernicka 1989). Although LAP has suffered
minor asteroidal aqueous alteration, Wasson and Rubin
(2009) argued that Ca is the only major or minor
element that shows resolvable amounts of elemental
transport during aqueous alteration. Our working
hypothesis is thus that inter-area compositional
heterogeneities observed in the LAP matrix are nebular
in origin and are therefore to be expected in the most
pristine chondrites.
The introduction of K and Ca during desert
weathering processes is probably responsible for some
fraction of the compositional homogeneity observed in
Acfer matrix. We nonetheless suggest that the more
uniform matrix textures in Acfer also reflects
homogenization on the asteroid, probably as a result of
convective mixing within the plume associated with the
impact event that produced the small fragmental grains.
An event that produced such thorough crushing would
be expected to produce turbulent mixing of matrix and
comminuted particles and thus obscure pre-existing
compositional heterogeneities.
Possible Impact Heating Effects in Acfer
Acfer 094 has a low bulk water content. Newton
et al. (1995) stated that the phyllosilicate content of the
matrix is <2%. Greshake (1997) reported <1 vol%
serpentine and rare fibrous ferrihydrite (<2 vol%) in
the matrix and stated that these are the two most
abundant secondary minerals in Acfer. Ferrihydrite is
very rare in unweathered chondrites and the new COS
(cosmic symplectite) hydrous phase that Seto et al.
(2008) found in Acfer is even stranger.
A key question is why Acfer is so dry. Did it form
as an anhydrous chondrite in the solar nebula? Or,
based on the fact that it shares several key primitive
features with CM chondrites, might it have formed as a
hydrous chondrite in the nebula and been dehydrated
on the parent asteroid, presumably by an impact event?
We find the connections to CM chondrites strong
enough to warrant a detailed discussion of the latter
possibility.
The ferrihydrite of Acfer 094 is present as clusters
of 300–500 nm elongated acicular crystals (Greshake
1997). This phase also occurs in CI Orgueil (Tomeoka
and Buseck 1988) and the ungrouped chondrite
Kakangari (where it constitutes 10 vol% of the matrix
and exhibits a similar morphology; Brearley 1989). By
contrast, ferrihydrite was not reported in the finegrained matrix regions of LL3.0 Semarkona or LL3.1
85
Bishunpur (Alexander et al. 1989). Greshake (1997)
interpreted ferrihydrite to be an asteroidal aqueous
alteration product, but Tonui and Zolensky (personal
communication) informed us that no studies have firmly
established the extraterrestrial origin of ferrihydrite;
they suspect that it is a terrestrial weathering artifact.
However, ferrihydrite occurs in both falls (e.g.,
Kakangari and Orgueil) and finds (Acfer 094 and H3.7
ALH 77299; Brearley 1989). On balance, we think it
probable that the ferrihydrite in Acfer 094 is an
asteroidal aqueous alteration product. One possibility
that A. Brearley suggested in a review is that it might
be an oxidation product of ferrisaponite, another
hydrous phase, and thus equally useful as an argument
for Acfer having experienced aqueous alteration.
Although the peculiar COS (cosmic symplectite)
phase in Acfer reported by Seto et al. (2008) seems to
be a product of aqueous alteration, its highly
anomalous O-isotopic composition shows that its
formation did not occur in the Acfer parent asteroid.
The main arguments that Acfer had a higher water
content earlier in its history and may have experienced
asteroidal aqueous alteration are circumstantial. Choe
et al. (2010) showed that its bulk composition is
closely similar to that of CM chondrites with the
exception of Na, K, Se, and S. Acfer matrix has an
exceptionally high content of amorphous materials
(rich in SiO2, FeO, and MgO) and Brearley and Rubie
(1990) have shown that flash heating of fine hydrated
silicates offers a good mechanism for forming the
amorphous materials. One peculiarity of Acfer is that
olivine in some chondrules (mainly fragments) have
fayalitic contents as high as 85% Fa and Mn contents
high enough (10 mg ⁄ g) to qualify for the name
ferrohortonolite (Newton et al. 1995). Fayalitic olivine
is commonly an indicator of aqueous alteration (e.g.,
Krot et al. 1997, 1998).
The bulk O-isotopic composition of Acfer 094
(d18O = 1.17&; d17O = )3.91&; D17O = )4.52&) lies
about 1.2& to the right of the CCAM line, roughly
between the fields of CO and the metamorphosed CM
chondrite Yamato-82054 (e.g., Kitajima et al. 2002) on
the standard three-isotope diagram (Clayton and
Mayeda 1999). The close resemblance to Y 82054
supports the conclusion that Acfer 094 experienced
extensive aqueous alteration followed by dehydration.
Two other CM chondrites, one (PCA 91008) known to
be metamorphosed (e.g., Wang and Lipschutz 1998) and
the other (Lewis Cliff [LEW] 87016) uncharacterized,
plot a similar distance to the right of the CCAM line at
slightly higher D17O ()3.5 to )3.0&, respectively). We
infer that an O-isotopic composition in this area to the
right of the CCAM line implies origins by degassing of
primitive hydrated carbonaceous chondrites.
86
J. T. Wasson and A. E. Rubin
The model we envision is that, prior to the impact
heating event, Acfer was more like the high (moreprimitive) type-2 (or type 3.0) CR chondrites such as
Meteorite Hills (MET) 00426 (Abreu and Brearley
2006) that has D17O )3.0& (Choe et al. 2010), and
that the rapidly outgassed H2O had a D17O value of 0&
or above, leaving a residue with D17O <)3&.
Acfer may have had a complex asteroidal history.
Although one commonly thinks of chondrites as having
experienced either aqueous alteration or thermal
metamorphism, a few chondrites show evidence that
both processes occurred. These include several
metamorphosed CM chondrites (e.g., Zolensky et al.
1993; Lipschutz et al. 1999; Tonui et al. 2002, 2003).
We have therefore considered the following
scenario: (1) Acfer 094 experienced aqueous alteration
that led to minor elemental transport and redeposition;
(2) a short impact heating event removed most of the
water and selectively allowed the escape of volatiles
such as S and the alkalis as well as the constituents of
water-bearing matrix phases that exploded during rapid
heating (e.g., Scott et al. 1992). Tomeoka et al. (1999)
reported that, in about half the experiments they
conducted on CM Murchison at calculated shock
pressures of 30 GPa or larger, explosion of the samples
prevented recovery.
To help gain perspective, we consider the
anomalous but relatively primitive chondrite Kakangari
that also seems to have had a complex regolith history.
The matrix of Kakangari differs from that in Acfer 094
in its lack of amorphous material. The Kakangari
matrix consists mainly of enstatite and olivine; also
present are ferrihydrite (discussed above), albite,
anorthite, Cr-spinel, troilite, and metallic Fe-Ni
(Brearley 1989). Phyllosilicates are absent. Berlin et al.
(2007) pointed out that Kakangari has an anomalous
enrichment of S compared with normal chondrites (i.e.,
members of the large groups). This anomalous
enhancement of an element that can experience
transport as a volatile or a liquid suggests fractionation
associated with an impact, perhaps onto a somewhat
hydrated protolith.
As presolar diamonds and SiC are relatively
refractory, they can probably withstand brief impactheating events. Evaporation can occur only at the
surface of the grain. During a brief heating event, a few
surficial layers might evaporate with grain cores
surviving. A very brief excursion to a gas temperature
of 2000 K seems unlikely to diminish appreciably the
contents of presolar SiC and diamonds.
Bischoff and Geiger (1994) assigned Acfer a low
(S1) shock stage indicating that <25% of the olivine
showed undulose extinction. We suspect that some
annealing of olivine could occur within the impact
cloud, and suggest that a more detailed study is needed
of residual shock effects.
Fine serpentine would have been destroyed at high
temperatures during an impact event. Thermodynamic
calculations show that, in a vacuum, chrysotile
(serpentine) breaks down to forsterite, enstatite, and
H2O vapor at temperatures above 350 K. Although
moderately higher temperatures would be required to
cause serpentine grains to evaporate at appreciable
rates, rapid heating in a vacuum to a temperature
several hundred degrees higher would probably result in
explosive loss of H2O.
The unequilibrated nature of Acfer chondrules
demonstrates that the time-temperature product in
Acfer 094 was far smaller than that experienced by
subtype 3.4 ordinary chondrites in which Huss and
Lewis (1995) observed incipient loss of SiC. We
suggest that the only constraint on the minimum
duration of the Acfer heating event is that the timetemperature product needs to have been sufficient to
dehydrate serpentine and other fine-grained waterbearing phases.
The abundant amorphous material in Acfer seems
consistent with this scenario. We suggest that a brief
heating event causing rapid loss of H2O from hydrated
silicates could be the mechanism for producing some or
much of the amorphous materials. Brearley and Rubie
(1990) noted that rapid heating can cause hydrous
phases to melt in disequilibrium fashion with only
partial loss of H2O.
An important requirement of the impact model is
that an impact plume was produced and that the plume
materials cooled down while still suspended. Residual
temperatures in the resulting deposit must have been
too low to produce appreciable diffusional transport at
the Acfer location.
DISCUSSION: NEBULAR CARRIERS POSSIBLY
ACCOUNTING FOR THE HETEROGENEITIES
OBSERVED IN ACFER 094 MATRIX
We showed above that, with the exception of one
pair, the 10 Acfer 094 matrix grid areas are resolvable
from one another at ‡95% confidence. This is similar to
the results from our study of matrix in CR2 LAP 02342
(Wasson and Rubin 2009), but differences among the
Acfer 094 grid areas are less pronounced than those in
LAP 02342. As discussed above, we suggest that
compositional differences among matrix areas in Acfer
were initially larger, more similar to those observed in
LAP, and that they have been homogenized by a
combination of impact redistribution and terrestrial
weathering (particularly recognizable in the introduction
of Ca and K).
Acfer 094 matrix and bulk fractionations
As noted by Wasson and Rubin (2009), appreciable
differences are inconsistent with the common view that
chondrite matrix material consists of nebular fines that
were relatively well mixed before they were
agglomerated (together with chondrules and other
coarse materials) into chondritic assemblages. If the
fines had existed throughout the nebular period as freefloating, submicrometer grains (a period that is
commonly estimated to have had a duration of >1 Ma;
e.g., Wadhwa et al. 2006), the fines would have achieved
near-complete mixing.
To emphasize that Acfer matrix is anomalously
uniform, in Table 3, we compare it to means and
standard deviations for LAP 02342 (Wasson and Rubin
2009) and to ALHA77307 (Brearley 1993). Some details
regarding this compilation are given in the Appendix.
The important message is that the LAP and
ALHA77307 data show much larger relative standard
deviations (shown bold in Table 3) than are observed in
Acfer. The LAP and ALHA77307 relative standard
deviations are similar for most elements but there are
significant differences. For example, in LAP, they are
appreciably higher for two elements (Si and Ca) that
show evidence of local transport during asteroidal
aqueous alteration on the CR parent body. The
elements Mn, Mg, Fe, and K show appreciably larger
deviations in the ALHA77307 data set compared with
LAP.
Nebular Processes to Account for the Fractionated
Composition of Acfer 094 Fines
In Fig. 8, we show that the Mg- and whole rock
normalized abundances of all elements except Al and
(of course) Mg are greater than unity. We used the
Rubin et al. (2007) data for Acfer 094 for the
normalization; because these authors did not determine
Si, we made a rough estimate of 140 mg ⁄ g, about
15 mg ⁄ g below a mean CO value. And, because the
INAA Mg value reported by Rubin et al. (2007) of
100 mg ⁄ g seems implausibly low, we increased this to
120 mg ⁄ g (Mg concentrations in both CO and CV are
145 mg ⁄ g; Wasson and Kallemeyn 1988). This would
require a fairly large analytical or sampling error; the
former we can check by rerunning the sample through
the INAA procedure, but the latter is more difficult to
assess, particularly because the amount of research
material in the world’s museums is only about 35 g.
Elements showing high abundance ratios in Acfer
matrix include moderate volatiles (Na, K, and S) with
enhancements in the range of 1.6 (one at 1.4) to 2.8,
Ca in the range of 1.3 to 1.6, elements that normally
partition into olivine and pyroxene (Si, Fe, and Mn)
with enhancements ranging between 1.1 (one at 0.9) and
87
1.4. Two elements show abundance ratios below unity;
Cr ratios are between 0.95 and 1.09 and Al ratios range
from 0.75 to 0.95.
The moderate volatiles can be further subdivided
into alkali elements and sulfur; both sets form nebular
phases that evaporate (i.e., have 50% condensation
temperatures) in the range 600 to 1000 K (Wai and
Wasson 1977). It seems probable that, in the nebula, the
enhancement of these elements in fines was mainly
associated with volatilization during chondrule
formation and recondensation as fine particles or on the
surfaces of pre-existing fine particles. As discussed
below, the enhancements may also be related to the loss
of mesostasis microdroplets from partially molten
chondrules. We infer that the Cr and Al abundances in
Acfer are low compared with LAP because of some
impact-induced loss.
The enhancements of Si, Fe, or Mn cannot be
explained by volatility. For CR LAP 02342, we
suggested that a plausible model to account for the
enhancements of Al and Si in matrix areas is that fine
droplets of plagioclase-rich mesostasis melt are lost
from molten chondrules. However, in Acfer, Al is
depleted. Although the apparent depletion is affected by
the uncertainties in our choice of bulk composition of
Acfer, we do not expect this effect to be >10%.
The loss of melt droplets from chondrules could
also have played a role in the enhancement of S and Fe
in matrix areas. As Fe-S liquids form at temperatures
above the eutectic temperature of 1260 K, it seems
probable that some tiny droplets of an Fe-S melt would
form as spray during chondrule formation.
As discussed above, the negative correlation
between the major elements Mg and Fe in Acfer
matrix appears to result from stochastic sampling of
mafic silicates, presumably tiny chondrule fragments.
The Mg is mainly in these fragments, the Fe mainly
in fine dust as well as FeS, etc. Although tiny
chondrule fragments existed in the nebula, the main
source in Acfer appears to be impact comminution on
the asteroid.
Comparison of the Compositional Variations of Acfer 094
and CR LAP 02342
In our study of matrix in LAP 02342, we
documented compositional differences on a scale of 0.1
to 0.5 mm. If nanoparticles were free floating in the
solar nebula, even minor amounts of turbulence would
have averaged out the compositional variations within
the course of a few orbital periods. In LAP 02342, we
ruled out the production of these differences by
asteroidal aqueous alteration and concluded that
plausible models require the existence of nebular
88
J. T. Wasson and A. E. Rubin
structures that maintain these compositional signatures.
The scale of these nebular carrier structures might be
around 10 to 100 lm. Wasson and Rubin (2009)
suggested three ways in which the compositional
variations in nebular fines could have been preserved
prior to the agglomeration of the first generation of
planetesimals: (a) as porous ‘‘chondrules’’ formed by
low (20%) degrees of melting; (b) as fused
nanoparticle clusters related to compound chondrules;
and (c) as sets of nanoparticles trapped in mesh-like (or
net-like) structures present in the dusty nebular
midplane. The first possibility seems the most plausible.
In this study of Acfer 094 matrix, there are
resolvable differences among the various grid areas but
they are less well defined than in LAP 02342 (i.e., the
differences are smaller relative to the calculated 95%
confidence limits). Some of the variations may be
related to terrestrial weathering and asteroidal
processing but our working model is that these
processes mainly introduced ‘‘noise’’ and that nebular
carriers are mainly responsible for the resolvable
differences among the Acfer grid areas. We suggest that
the nebular processes inferred to explain our LAP
matrix results remain viable, but we caution that, in
Acfer, these cannot be fully resolved from weathering
effects and asteroidal transport. Although we infer that
the uniformity of Acfer matrix is anomalous, we cannot
rule out the possibility that it originated in the nebula.
SUMMARY
This is our second compositional study of matrix in
carbonaceous chondrites using a new approach. In our
first study, Wasson and Rubin (2009) showed that the
use of a focused electron-microprobe beam to analyze
rectangular grids allowed improvements in the precision
with which matrix compositions can be determined;
these authors reported resolvable heterogeneities among
matrix regions of the primitive CR2 chondrite LAP
02342.
In this study of ten elements in 10 50 · 50 lm
areas of the primitive ungrouped type 3.0 carbonaceous
chondrite Acfer 094, we again observed compositional
variations among the analyzed areas, similar to but
somewhat smaller than those observed by Wasson and
Rubin (2009) in CR2 LAP 02342. The composition
changes on scales smaller than 1 mm; we thus infer that
the dimensions of compositionally anomalous regions
are in the range 10 to 100 lm.
Although Acfer 094 is widely regarded as highly
primitive, it has suffered appreciable terrestrial
weathering (e.g., it has high Ca ⁄ Al and K ⁄ Na ratios, as
previously observed in other chondrites from hot
deserts) and there is also evidence of elemental
transport during asteroidal processes. Although we
suggest that the resolved matrix compositional
anomalies mainly reflect friable nebular carriers that
were disaggregated during asteroidal compaction, the
strength of this conclusion is compromised by elemental
redistribution during terrestrial weathering and by
asteroidal aqueous and impact processes.
Acknowledgments—We
thank
Frank
Kyte
for
assistance with electron-microprobe studies, Tram Le
for help with image preparation, and Jeff Grossman
for assistance with the colorization of our X-ray
maps. Detailed reviews by Adrian Brearley and
Michael Velbel are appreciated; the manuscript is
improved because of their comments. We are
especially grateful to Alex Ruzicka for his scientific
and organizational suggestions. This research was
largely supported by NASA grant NNG06GG35G
(JTW) with additional support by NASA grant
NNG06GF95G (AER).
Editorial Handling—Dr. Alex Ruzicka
REFERENCES
Abreu N. M. and Brearley A. J. 2006. Early solar system
processes recorded in the matrices of CR2 chondrites
MET 00426 and QUE 99177. (abstract #2395). 37th Lunar
and Planetary Science Conference. CD-ROM.
Alexander C. M. O., Barber D. J., and Hutchison R. H.
1989. The microstructure of Semarkona and Bishunpur.
Geochimica et Cosmochimica Acta 53:3045–3057.
Al-Kathiri A., Hofmann B. A., Jull A. J. T., and Gnos E.
2005. Weathering of meteorites from Oman: Correlation
of chemical and mineralogical weathering proxies with
14
C terrestrial ages and the influence of soil chemistry.
Meteoritics & Planetary Science 40:1215–1239.
Berlin J., Jones R. H., and Brearley A. J. 2007. A closer look
at chondrules and matrix in Kakangari: evidence for
widespread reduction and sulfurization (abstract 2395).
38th Lunar and Planetary Science Conference. CD-ROM.
Bischoff A. and Geiger T. 1994. The unique carbonaceous
chondrite Acfer 094: the first CM3 chondrite. 25th Lunar
and Planetary Science Conference. pp. 115–116.
Brearley A. J. 1989. Nature and origin of matrix in the unique
chondrite, Kakangari. Geochimica et Cosmochimica Acta
53:2395–2411.
Brearley A. J. 1993. Matrix and fine-grained rims in the
unequilibrated CO3 chondrite, ALHA77307: Origins and
evidence for diverse, primitive nebular dust components.
Geochimica et Cosmochimica Acta 57:1521–1550.
Brearley A. J. and Jones R. H. 1998. Chondritic meteorites.
In Planetary materials, edited by Papike J. Reviews in
Minerology and Geochemistry, vol. 36. Chantilly, VA:
Mineralogical Society of America. pp. 313–398.
Brearley A. J. and Rubie D. C. 1990. Effects of H2O on the
disequilibrium breakdown of muscovite + quartz. Journal
of Petrology 31:925–956.
Choe W. H., Huber H., Rubin A. E., Kallemeyn G. W., and
Wasson J. T. (forthcoming) Compositions and taxonomy
Acfer 094 matrix and bulk fractionations
of 15 unusual carbonaceous chondrites. Meteoritics &
Planetary Science 45: in press.
Clayton R. N. and Mayeda T. K. 1999. Oxygen isotope
studies of carbonaceous chondrites. Geochimica et
Cosmochimica Acta 63:2089–2104.
Dreibus G., Palme H., Spettel B., and Wänke H. 1995.
Sulfur and selenium in chondritic meteorites. Meteoritics &
Planetary Science 30:439–445.
Greshake A. 1997. The primitive matrix components of the
unique carbonaceous chondrite Acfer 094: A TEM study.
Geochimica et Cosmochimica Acta 61:437–452.
Huber H., Rubin A. E., Kallemeyn G. W., and Wasson J. T.
2006. Siderophile-element anomalies in CK carbonaceous
chondrites: implications for parent-body aqueous
alteration and terrestrial weathering of sulfides. Geochimica
et Cosmochimica Acta 70:4019–4037.
Huss G. R. and Lewis R. S. 1995. Presolar diamond, SiC, and
graphite in primitive chondrites: Abundances as a function
of meteorite class and petrologic type. Geochimica et
Cosmochimica Acta 59:115–160.
Huss G. R., Alexander C. M. O. D., Palme H., Bland P. A.,
and Wasson J. T. 2005. Genetic relationships between
chondrules, fine-grained rims, and interchondrule matrix.
Astronomical Society of the Pacific Conference Series
341:701–731.
Kallemeyn G. W. and Wasson J. T. 1982. The compositional
classification of chondrites-III. Ungrouped carbonaceous
chondrites. Geochimica et Cosmochimica Acta 46:2217–2228.
Keller L. P. 1992. Petrography and mineral chemistry of
calcium- and aluminum-rich inclusions in the Maralinga
CK4 chondrite (abstract). 23rd Lunar and Planetary
Science Conference. pp. 671–672.
Kitajima F., Nakamura T., Takaoka N., and Murae T. 2002.
Evaluating the thermal metamorphism of CM chondrites
by using the pyrolytic behavior of carbonaceous
macromolecular matter. Geochimica et Cosmochimica Acta
66:163–172.
Krot A., Scott E. R. D., and Zolensky M. 1997. Origin of
fayalitic olivine rims and lath-shaped matrix olivine in the
CV3 chondrite Allende and its dark inclusions. Meteoritics
& Planetary Science 32:31–49.
Krot A. N., Petaev M. I., Scott E. R. D., Choi B.-G.,
Zolensky M. E., and Keil K. 1998. Progressive alteration
in CV3 chondrites: more evidence for asteroidal alteration.
Meteoritics & Planetary Science 33:1065–1085.
Lipschutz M. E., Zolensky M. E., and Bell M. S. 1999. New
petrographic and trace element data on thermally
metamorphosed carbonaceous chondrites. Antarctic
Meteorite Research 12:57–80.
Newton J., Bischoff A., Arden J. W., Franchi I. A., Geiger T.,
Greshake A., and Pillinger C. T. 1995. Acfer 094, a
uniquely primitive carbonaceous chondrite from the
Sahara. Meteoritics 30:47–56.
Rubin A. E. and Pernicka E. 1989. Chondrules in the
Sharps H3 chondrite:
Evidence
for intergroup
compositional differences among ordinary chondrite
chondrules. Geochimica et Cosmochimica Acta 53:187–195.
Rubin A. E., Trigo-Rodriguez J. M., Huber H., and Wasson
J. T. 2007. Progressive aqueous alteration of CM
carbonaceous chondrites. Geochimica et Cosmochimica
Acta 71:2361–2382.
89
Rubin A. E., Griset C. D., Choi B. G., and Wasson J. T.
2009. Clastic matrix in EH3 chondrites. Meteoritics &
Planetary Science 44:589–601.
Sakamoto N., Seto Y., Itoh S., Kuramoto K., Fujino
K., Nagashima K., Krot A. N., and Yurimoto H. 2007.
Remnants of the early solar system water enriched in
heavy oxygen isotopes. Science 317:231–233.
Scott E. R. D. and Krot A. N. 2005. Chondrites and their
components. In Meteorites, comets and planets, edited by
Davis A. M. Treatise on Geochemistry, vol. 1. Amsterdam:
Elsevier. pp. 143–200.
Seto Y., Sakamoto N., Fujino K., Kaito T., Oikawa T.,
and Yurimoto H. 2008. Mineralogical characterization
of a unique material having heavy oxygen isotope
anomaly in matrix of the primitive carbonaceous
chondrite Acfer 094. Geochimica et Cosmochimica Acta
72:2723–2734.
Tomeoka K. and Buseck P. R. 1988. Matrix mineralogy of
the Orgueil CI carbonaceous chondrite. Geochimica et
Cosmochimica Acta 52:1627–1640.
Tomeoka K., Yamahana Y., and Sekine T. 1999.
Experimental shock metamorphism of the Murchison CM
carbonaceous chondrite. Geochimica et Cosmochimica Acta
63:3683–3703.
Tonui E., Zolensky M., and Lipschutz M. 2002.
Petrographic, mineralogy and trace element chemistry of
Yamato-86029, Yamato-793321 and Lewis Cliff 85332:
aqueous alteration and heating events. Antarctic
Meteorite Research 15:38–58.
Tonui E. K., Zolensky M. E., Lipschutz M. E., Wang M.-S.,
and Nakamura T. 2003. Yamato 86029: Aqueously altered
and thermally metamorphosed CI-like chondrite with
unusual textures. Meteoritics & Planetary Science 38:269–
292.
Wadhwa M., Amelin Y., Davis A. M., Lugmair G. W., Meyer
B., Gounelle M., and Desch S. J. 2006. From dust to
planetesimals: Implications for the solar protoplanetary
disk from short-lived radionuclides. In Protostars and
planets V, edited by Reipurth B., Jewitt D., and Keil K.
Tucson, AZ: University of Arizona Press. pp. 835–848.
Wai C. M. and Wasson J. T. 1977. Nebular condensation of
Ga, Ge and Sb and the chemical classifcation of iron
meteorites. Nature 282:790–793.
Wang M.-S. and Lipschutz M. E. 1998. Thermally
metamorphosed carbonaceous chondrites from RNAA
data. (abstract #1028). 29th Lunar and Planetary Science
Conference.
Wasson J. T. and Kallemeyn G. W. 1988. Compositions of
chondrites. Philosophical Transactions of the Royal Society
of London 325:535–544.
Wasson J. T. and Rubin A. E. 2009. Composition of matrix in
the CR chondrite LAP 02342. Geochimica et Cosmochimica
Acta 73:1436–1460.
Weisberg M. K., Prinz M., Clayton R. N., and Mayeda T. K.
1993. The CR (Renazzo-type) carbonaceous chondrite
group and its implications. Geochimica et Cosmochimica
Acta 57:1567–1586.
Zolensky M., Barrett R., and Browning R. 1993. Mineralogy
and composition of matrix and chondrule rims in
carbonaceous chondrites. Geochimica et Cosmochimica
Acta 57:3123–3148.
90
J. T. Wasson and A. E. Rubin
APPENDIX
Comparison of Matrix Data for Three Highly
Unequilibrated Carbonaceous Chondrites
As shown in the Discussion section, in Table 3, we
compare the means and standard deviations of these
Acfer 094 data with those for CR2 LAP 02342 (Wasson
and Rubin 2009) and those for ALHA77307 (Brearley
1993).
The Acfer 094 means, standard deviations, and
relative
standard
deviations
(=(100 · standard
deviation) ⁄ mean) were calculated from the 10 individual
means listed in Table 1.
For LAP 02342 matrix, we calculated means and
standard deviations for 16 of the 18 area means listed in
Table 1 of Wasson and Rubin (2009). We did not
include the CaCO3-rich points from area G4d nor the
FeS-rich points from area K5y. We include both points
of the five duplicate analyses; a test in which the
duplicate (second) value was deleted gave results that
were indistinguishable.
For ALHA77307, we included the eight rims and
the three matrix analyses from Table 1 of Brearley
(1993). Brearley stated that there is no significant
difference between rims and matrix, a conclusion that
we have also reached in our studies of CR2 02342.
Mean compositions of 10 of the 11 areas are based on
the analysis of 9 to 19 points; matrix area 9 is based on
only 5 points. A test showed that deletion of the area 9
mean caused no significant difference in the grand
means or in the standard deviations.