IMCA Insights – January 2009
3D Chondrules Revealed
by Jürgen Otto & Norbert Classen

The fragments of a so far unpublished chondrite from Oman show a rather intriguing feature that we would like to report in this article – free-standing chondrules that have been partly separated from the matrix and layed open to view by natural weathering processes. These “3D chondrules” reveal new aspects which don't get obvious in two-dimensional thin section studies, and which might help to improve our understanding of chondrule formation, and the accretion history of chondrite parent bodies.

Figure 1: Free-standing Chondrule

Figure 1: A free-standing chondrule that has been separated
from the matrix by weathering and wind erosion. Small amounts of
matrix material are still attached to its surface, and fine vertical
lines give an impression of its interior (chondrule Ø 1.1mm)

Find Description

The meteorite was found in August 2006 in the desert of Oman by an anonymous prospector, and its fragments were scattered about an area of about 12 x 12 meters. A total of 62 fragments were recovered from the find location with a total known weight of about 21 kilos.

The bottom sides of the fragments, and especially those parts that were buried below the sandy surface, show a higher degree of corrosion due to chemical weathering, and they are partly covered with rust and a whitish patina (caliche). The top sides of the fragments are dark-brown, and mostly polished flat by sand blasting and wind erosion. Only in areas near to the ground where there has been a moderate balance of chemical weathering and wind erosion we found an abundance of well preserved chondrules that have been separated from the matrix and layed open to view. Their average diameter varies from about 1.1mm to 2.3mm.

Figure 2: Small chondrules firmly attached to a larger one

Figure 2: Two smaller chondrules still firmly attached to a larger
chondrule (Ø 2.2mm). They obviously escaped the breakup process
that separated most of the other chondrules in this meteorite

A thin section reveals the characteristic texture of a relatively unequilibrated H4 chondrite with well defined chondrules in a fine-grained to dense matrix (Fig. 7). The stone has only been weakly shocked, and we determined a shock stage of S1-2.

3D Study of Chondrules

Basically, a thin section only reveals a two-dimensional view of a very thin, sectional plane of any given material, and hence it will also only reveal a rather restricted section through any given chondrule. On the other hand, free-standing chondrules such as in this particular sample often reveal the greater part of the chondrules surface (Fig. 1), and allow us a new type of three-dimensional study, revealing some interesting new aspects of these primordial objects.

Figure 3: Chondrule with a prominent dent

Figure 3: A chondrule with a prominent dent of another, formerly
attached chondrule. The surface of the dent shows a distinct,
parallel lining pattern (chondrule Ø 1.4mm)

We were surprised to find that most chondrules in our sample revealed rather distinct dents, obviously produced by neighboring chondrules (Fig. 3), and sometimes we also found compound chondrules, still closely connected to each other (Fig. 2, Fig. 6), supporting this interpretation. But generally – and this is the puzzling aspect of these observations – the single chondrules featuring such dents are spatially separated from each other by the chondrite’s matrix, and mineral fragments within the matrix.

This all points to a former state in which the chondrules must have been much more densely packed – a compound texture that must have been broken up in a later phase of formation, a phase in which the chondrules were separated by what is now the chondrite’s matrix. Another interesting observation is that those chondrules must have been in a semi-plastic state in the “compound phase”, and that they obviously solidified prior to the later textural breakup, and re-formation.

Figure 4: Light-colored chondrule with tiny facets

Figure 4: A light-colored chondrule with several tiny facets on its
surface. This is probably a crypto-crystalline chondrule (Ø 1.7mm)

The Need for More 3D Studies

There are a lot of possible implications that can be drawn from our observations: They could imply that, e.g., the so-called “chondrule conglomerates” (a loose term coined for certain highly unequilibrated NWA chondrite finds that are lacking any matrix and which consist more or less solely of densely packed chondrules) might be no conglomerates at all but are more likely to represent most pristine chondritic matter that escaped a later breakup and re-formation process.

Of course, this is pure speculation at this point, and it would need further studies and more serious research to strengthen such a case. But we firmly believe that the three-dimensional study of chondrules will provide us with a great wealth of additional information that can’t be easily drawn from traditional, two-dimensional studies such as thin section microscopy. A first step of a future 3D study of chondrules would have to show if other ordinary chondrite samples do also contain such an abundance of dented chondrules as our Omani sample, and if they also show signs of a later textural breakup and re-formation.

Figure 5: Light-colored chondrule with dendritic inclusions

Figure 5: Another light-colored, crypto-crystalline chondrule with
dark dendritic inclusions on and below its surface (Ø 1.6mm)

Besides that, the study of free-standing chondrules is great fun as their surfaces often reveal structural traits that tell us a lot about their internal makeup. We often found fine-lined areas, e.g., that lead us to the conclusion that we were looking at radial pyroxene chondrules (RP chondrules; Fig. 3). We also observed an abundance of light-colored chondrules with fine facets on their surface, and dark dendritic inclusions that could be identified as crypto-crystalline chondrules (C chondrules; Fig. 4, Fig. 5).

Figure 6: A more eroded chondrule with dents

Figure 6: A more eroded chondrule (Ø 1.3mm) exhibiting more or
less prominent dents of other chondrules, and a small chondrule,
still firmly attached to its surface

These facets are another feature that won't get too obvious in traditional thin section studies, and we are convinced that future 3D studies of chondrules will reveal a lot more of intriguing details that might help us to improve our understanding of chondrule formation, and the accretion history of chondrite parent bodies.

Figure 7: Thin section photo

Figure 7: Thin section photo of a well defined, multi-phase olivine
chondrule surrounded by several smaller chondrules and a near
to opaque matrix. Clear olivine fragments and small voids appear
in white (picture taken in plane polarized light)

Jürgen Otto is a Professor emeritus of Mineralogy and Petrology at the University of Freiburg, Germany, and he studied and classified hundreds of meteorites during his active career as a meteoriticist.

Norbert Classen is working as a freelance writer and editor for various publishing houses in Germany. Besides that, he is a passionate meteorite collector, and the current President of the International Meteorite Collectors Association, IMCA.

This article was first published in Meteorite, May 2008, and is featured in IMCA Insights with friendly permission. If you want to subscribe to Meteorite, the International Quarterly of Meteorites and Meteorite Science, please visit the Meteorite Magazine Homepage - highly recommended!

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