45th Lunar and Planetary Science Conference (2014)
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ELEMENT REDISTRIBUTION IN METAMORPHISM OF CO CHONDRITES: IMPLICATIONS FOR
EMERGING WORLDS. D. S. Ebel1, M. K. Weisberg2,1, E. J. Crapster-Pregont3,1, 1American Museum of Natural
History, Central Park West at 79th St., New York, NY 10024 ([email protected]), 2Kingsborough College, CUNY,
Brooklyn, NY 11235, 3Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, 10964, USA.
Introduction: The CO carbonaceous chondrites offer the clearest metamorphic sequence of the carbonaceous chondrite groups [1]. This sequence may be distinguished using a variety of criteria [2, 3]. Here, we
provide data on inclusion and matrix chemical compositions for large areas of CO samples across petrologic
grades from CO3.0 to CO3.7, and for Acfer094 (2ungr). Our analyses shed light on the chemical exchanges resulting from metamorphism, demonstrating
apperent element mobility or lack thereof.
Methods: Samples are polished thin and thick sections of CO chondrites, and the ungrouped carbonaceous chondrite Acfer 094, with affinities to the CO
group [3-5]. Surfaces were mapped at 1 to 3 µm/pixel
resolution for the x-ray intensities of Mg, Ni, Ti, Al,
CA (WDS) and S, Si and Fe (EDS) on the Cameca
SX100 electron microprobe at AMNH. Conditions
were 15KeV, 30 or 20 nA beam current, and 20 or 15
ms dwell time. Hand-drawn inclusion outlines formed
the basis for detailed, pixel-by-pixel image analysis.
We infer volume % of chondrite components directly
from area % measurements [6, 7]. Fine-grained rims ,
small isolated olivine and metal/sulfide grains are considered part of the matrix. We have previously reported
abundances of matrix and inclusions in Colony
(CO3.0) and Kainsaz (CO3.2) [8], and Acfer094 (C2ung) [9].
Table 1: Samples. See text for notes.
CO
Table 1 reports the CO samples, origin, type as determined by [13, 2, 3], spatial resolution (microns/pixel), total area mapped and analyzed (mm2),
and AMNH sample number. Acfer094 is from MfN,
Berlin. Porosity has been reported by [14, 15].
Fig. 1: Change in Mg and Si with petrologic grade
(arrows). Solid black line joins bulk composition with
the origin.
2
fall/find type [13] type [2] type [3] res A(mm ) sample
Colony
find
3.0
3.2
3.0
2
32.516 4595-1
Kainsaz
fall-1937
3.1
3.5
3.2
2
46.042 4717-1
Ornans
fall-1868
3.3
3.5
3.4
1
14.019 520-1-r4
Lancé
fall-1872
3.4
3.5
3.5
3
45.891 618-1
Warrenton fall-1877
3.6
3.6
3.7
2
14.468 4151-1
Acfer 094
n.d.
n.d.
C2-ung
1
10.453 from MfN
find
Our analyses now include bulk chemical data for
each individual inclusion, and of matrix [10-12]. These
data are reported as counts per pixel. We assume that
the mean counts/pixel for each element across every
pixel in a sample represents the bulk composition of
that sample (e.g., for 1.0453329x107 pixels in
Acfer094, mean Si/pxl ∝ wt% Si). For comparison of
element distributions, we normalize all bulk compositions to bulk Colony composition. The same factor is
used to normalize inclusion and matrix abundances.
The assumption is that all these CO have identical major element bulk compositions.
Fig. 2: Change in Fe and Mg with petrologic grade
(arrows). Symbols as in Fig. 1.
Results: The Mg-Si and Fe-Mg relations between
CO chondrite matrix and inclusions (clasts) are shown
in Figs. 1 and 2. Inclusions (clasts) include all chondrules, CAIs and AOAs. Matrix includes isolated olivine and metal grains, and dark inclusions. The Mg/Si
ratios of clasts and matrix approach the bulk CO value
(~solar, black line) with increasing petrologic grade
from Colony (3.0) to Warrenton (3.7), although Ornans
(3.4) and Lancé (3.5) deviate slightly from this pattern.
No such correlation of Mg/Si ratio with oxidation/reduction is observed in the CV chondrites [15]. In
45th Lunar and Planetary Science Conference (2014)
the CO chondrites, the mobile element appears to be
Mg, rather than Si.
In all chondrites, it is known that with increasing
petrologic grade, Fe and Mg are increasingly redistributed between inclusions and matrix [16]. In Warrenton
(3.7), and in CO 3.4 Ornans (but not in CO 3.5 Lancé),
Fe is nearly equilibrated between inclusions and matrix. A reasonable interpretation is that Fe-Mg exchange between inclusions and matrix becomes more
complete with increasing petrologic grade. This is
qualitatively understood through petrological examination of CO chondrites. Here, we are able to quantitatively demonstrate these changes in chemical balances
among components.
Another way to understand these data is to calculate
the total contribution to element counts across entire xray maps, from refractory inclusions (CAIs and
AOAs), chondrules (including Al-rich chondrules), and
matrix (Table 2). Normalized to area fraction, chondrule/matrix Fe is 0.45 (Colony) and 0.53 (Kainsaz)
for the least equilibrated CO, and 0.99 (Warrenton)
and 1.22 (Ornans) for the most equilibrated, with
Lancé anomalous (0.62).
Table 2: Fraction of total element counts in sample accounted for by each chondrite component, and
area fraction occupied by each component.
Si
Colony 3.0
ref. i nclsns. 0.068
chondrules 0.604
matrix 0.328
Kainsaz 3.2
ref. i nclsns. 0.070
chondrules 0.611
matrix 0.319
Ornans 3.4
ref. i nclsns. 0.086
chondrules 0.643
matrix 0.271
Lancé 3.5
ref. i nclsns. 0.046
chondrules 0.561
matrix 0.393
Warrenton 3.7
ref. i nclsns. 0.068
chondrules 0.664
matrix 0.269
Acfer094
ref. i nclsns. 0.064
chondrules 0.453
matrix 0.483
Mg
Al
Ca
Ti
Fe
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compositions are chondritic in major elements. Others
have remarked on the fact that CO [10], CV [15, 18],
CR[12, 17, 18] and other chondrites [11, 18] consistently have solar ratios of major elements independent
from inclusion and matrix compositions or abundances
of all components. This suggests that emerging planets
formed from planetesimals that accreted locally from
dust mixtures of solar composition that were variably
processed, producing the heterogeneous inclusion populations observed in different chondrite classes. The
chondritic meteorites represent such planetesimals.
area
0.078 0.150 0.164 0.120 0.050 0.072
0.754 0.494 0.515 0.525 0.367 0.541
0.168 0.356 0.322 0.355 0.583 0.387
0.072 0.197 0.165 0.134 0.038 0.073
0.684 0.428 0.533 0.501 0.387 0.520
0.244 0.375 0.302 0.365 0.575 0.407
0.072 0.303 0.226 0.158 0.055 0.064
0.693 0.503 0.508 0.554 0.469 0.418
0.235 0.193 0.266 0.288 0.477 0.517
0.041 0.177 0.144 0.106 0.028 0.048
0.609 0.465 0.546 0.471 0.364 0.468
0.350 0.358 0.310 0.422 0.609 0.484
0.059 0.214 0.142 0.135 0.057 0.066
0.668 0.563 0.674 0.586 0.584 0.580
0.274 0.223 0.185 0.279 0.359 0.354
0.084 0.297 0.225 0.152 0.030 0.063
0.554 0.357 0.297 0.349 0.280 0.372
0.363 0.346 0.478 0.499 0.690 0.565
Finally, we may examine the same dataset to assess
the complementary nature of chondritic components.
Fig. 3 illustrates the spread in inclusion compositions
in Colony, matrix bulk composition (orange diamond),
and bulk (all pixels, yellow square). A similar spread is
observed in Kainsaz (CO3.2).
Discussion: Chondrule and matrix compositions
differ strongly (Figs. 1, 2, 3), yet CO chondrite bulk
Fig 3: Inclusion, matrix, and bulk major element
compositions in Colony. Points represent counts/pixel
in each component, for Σ(Ca+Mg+Al+Si)=100.
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