Seeing Processing in a CV3

Not all CV3 carbonaceous chondrites are alike. More than most, NWA 6207 makes it easy for us to see that its components went through a succession of processes before they were delivered to us. Existing large mineral grains were incorporated into chondrules. Chondrules formed, solidified and then were included in subsequent chondrules. Mineral grains evolved, chondrules gained layers and their internal glasses crystallized. Here are some examples.

The yellow barred olivine chondrule obviously formed early. Then it became part of the large porphyritic chondrule either through a low speed collision or through accretion and partial melting. The apparent diameter of the compound chondrule is 2.5 mm. Thin section in cross-polarized light, XPL. NWA 6207 CV3.

 

Porphyritic chondrules are generally thought to be the product of the partial melting and subsequent cooling. Since some of the precursor material is not melted there are plentiful ‘seed crystals’ from which grains can grow. The partial melting argues well for the survival of an incorporated chondrule. Thin section in XPL. NWA 6207 CV3.

 

Here a barred olivine chondrule is embedded in an aggregate. The chondrule has several sets of bars so it could be called polysomatic, many bodied. It is not clear that the rounded aggregate was formed through melting so it might not be considered a true chondrule. Thin section in XPL. NWA 6207 CV3.

 

Yet another BO chondrule, looking like a large mineral grain, is incorporated into a large and interesting chondrule. See the closer view below. Thin section in XPL. NWA 6207 CV3.

 

There are numerous poikilitic grains in this chondrule – small rounded grains of olivine included in, probably, twinned clinopyroxene. Here, the horizontal striping expresses the twinning. Thin section in XPL. NWA 6207 CV3.

 

CAIs, calcium-aluminum-rich inclusions appear cloud-like here. Thin section in XPL. NWA 6207 CV3.

 

A variety of textures. Thin section in XPL. NWA 6207 CV3.

 

A chondrule that has been built up with multiple concentric layers including a thin dark one near the outside, best seen at the lower right. The entire assemblage is jacketed by opaque matrix. Transmitted light. NWA 6207 CV3.

 

A barred olivine chondrule with an apparent diameter of 0.85 mm. Thin section in transmitted light. NWA 6207 CV3.

 

Looking closer we see that thermal metamorphism has altered the glass that was between the bars. Thin section in transmitted light. NWA 6207 CV3.

 

The feldspathic glass has devitrified, that is, it has crystallized (often termed “recrystallized”). Thin section in transmitted light. NWA 6207 CV3.

About the Author

John Kashuba
John is a natural history enthusiast living in Oregon.
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