GE v|tome|x s Operating Manual

THREE DIMENSIONAL PETROGRAPHY OF KERNOUVÉ: A STORY OF VEIN FORMATION,
COMPACTION, AND METAMORPHISM. J. M. Friedrich

Rudolph
NY 10458,
10024 (email: [e-mail]),
University, 1721 SW Broadway, Portland, OR, 97207,
Natural History, New York, NY 10024,
gonne, IL 60439,

4*

, M. L. Rivers5, R. J. Macke6, and D. T. Britt6, 1Department of Chemistry, Fordham University, Bronx,

2

Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY

6

Department of Physics, University of Central Florida, Orlando, FL 32816. *Current Address: GE

3

Cascadia Meteorite Laboratory, Department of Geology, Portland State

5

Consortium for Advanced Radiation Sources, University of Chicago, Ar-

1,2

, A. Ruzicka3, D. S. Ebel2, J. Thostenson4, R. A.

4

Microscopy and Imaging Facility, American Museum of

Inspection Technologies, 50 Industrial Park Road, Lewistown, PA 17044.

Introduction: Metallic veins in ordinary

chondrites, like those found in Kernouvé (H5, S1),
have been attributed to impact or shock processes on

rior porosity in Kernouvé indicates that the meteorite
was incompletely compacted.

the parent body [1-8]. However, Kernouvé is widely
known to exhibit few traditional signs of shock load­ing [5,9-11] and has a very old

39

Ar-40Ar age (4.46 Ga)
[1], making the significance of vein formation in
Kernouvé unclear. One possibility is early vein for­mation followed by partial obliteration due to thermal
metamorphism [2, 6]. We examine this scenario here.

Samples and Methods: We examined two sam-

ples of the Kernouvé chondrite with x-ray
microtomography (µCT): USNM 359 (0.7 g) and
AMNH 470 (4.7 g). The former was imaged at the
GSECARS 13-BM beamline located at the Advanced
Photon Source of the Argonne National Laboratory
and the latter was imaged with the GE phoenix
v|tome|x s µCT system at the American Museum of
Natural History. µCT resolutions were 11.9 and 11.2
µm/voxel respectively. To extract visualization and
quantitative compaction data from our data, we used
techniques described previously [12-16]. We deter­mined the porosity of two specimens (#499 and #1379)
from the Vatican collection (29.7 and 18.0 g) with
ideal gas (He) pycnometry. We also used traditional
petrographic methods to infer the shock stage of
Kernouvé.

Results: Our petrographic observations verify the

S1 classification of Kernouvé [9]. Each sample con­tained a large (up to cm-sized) metallic vein (Figure 1)
like those observed by others [2,4,6,7], but which we
imaged in 3D. Besides veins, μCT data also indicate
that Kernouvé has considerable interior porosity (Fig­ure 1).

Kernouvé porosity. Mean porosity by ideal gas

pycnometry was 6.8%. µCT-visible porosity at the
resolutions described above was 4.4 ± 1.3%. Our re­sults are consistent with previous determinations of
Kernouvé porosity [17]. The slightly lower value of
porosity derived from µCT compared to that from
pycnometry is also consistent with previous studies
that used both techniques [13-15], and can be ex­plained by the presence of voids smaller than the imag­ing resolution. More importantly, the presence of inte-

Figure 1. A µCT slice of the AMNH 470 Kernouvé

(H6, S1) chondrite sample. A metallic vein and
intragraunlar porosity can readily be imaged with reso­lutions of ~11 µm/voxel.

Degree of compaction. We used techniques de-

scribed in [12] to quantitatively evaluate the degree of
compaction experienced by Kernouvé and an addition­al 16 ordinary chondrites of varying chemical classes
and petrographic types. Results are shown in Figure 2.
Here we use the ratios of ellipsoid tensors, summed
over all metal grains, as an indicator for overall pre­ferred orientation (as indicated by the strength factor).
Our expanded database increases the number of S1
shock stage samples and includes chemical classes
other than L chondrites, which made up our original
dataset [12]. Our results show that Kernouvé does not
possess significant preferred orientation of metal
grains, consistent with other low shock stage meteor­ites (Figure 2). These data suggest that the meteorite
was not significantly compacted by a late shock event.

Vein visualization and 3D shape. The 3D structure

of metal veins in one of our Kernouvé samples is
shown in Figure 3. The vein in this sample is discon­tinuous with the two pieces appearing to be offset. The
other sample also contains a vein of similar propor­tions and structure, but is continuous. Each vein has

an overall prolate to triaxial (or scalene) structure and
a striking complexity of shape.

Figure 2. Degree of compaction, as given by the

strength factor determined by collective orientation of
metal grains in chondritic samples [12], versus shock
stage. Kernouvé is not only mildly shocked, but also
poorly compacted. Data in black [12] incorporated
only L chondrites; new data (red symbols and
Kernouvé) incorporating H and LL chondrites, demon­strate that the scheme holds for chondrites other than
L.

Discussion and Conclusions: The metal vein in

Kernouvé can be explained as a pre-metamorphic
shock-induced structure, given the evidence for current
weak compaction and low shock stage. However, it is
not clear how one explains high porosity and low
shock stage if the meteorite experienced early shock
sufficiently strong to mobilize metal into a vein-like
structure. Some combination of unusual conditions
during shock (e.g. high temperatures) or mobility of
metal during metamorphism may be needed.

One possible mechanism occurring early on the H
chondrite parent body follows: blocks of slightly indu­rated but porous material were disturbed by an impact
which caused a physical concentration of metallic
phases akin to slickensides. Shearing of these materials
caused the warm, ductile metal grains to be concen­trated on rock surfaces. This process would have im­parted the rocks with indicators cited by [4,5] as shock
induced. This event happened early in the history of
the chondrite otherwise shearing forces would have
imparted a preferred orientation upon the metal grains.
Subsequent annealing due to radiogenic heating erased
any preferential orientation and further coalesced the
vein metal. The final compaction of the material oc­curred yielding the porosity seen in the samples today.
The material escaped other major shock episodes.

Figure 3. 3D visualization of two discontinuous metal

veins in Kernouvé (AMNH 470). Views are rotated
90° on a vertical rotation axis; an “edge-on” view is
shown at left and a “face on” view at right. The com­plexity of shape and degree of interconnectivity indi­cates a pre-metamorphic origin for the metallic vein
structures.

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