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NWA 869 Inclusions

Hey Chuck,

Please look for fine grained inclusions, too, when you’re slicing those NWA 869s. They look like little gray patches of unmixed Portland cement. Folks have been calling them achondritic inclusions but that might not be what they are. There’s a recurring rumor that something’s going to be published about them but I haven’t seen it yet.

NWA 869 is getting more study. I understand its classification has been amped up to L3-6 chondritic breccia. It is a coarse breccia with chunks of up to 5.5 cm. Nobel gas measurements show that some portions of the mix had been on the surface of the parent body so it is a regolith breccia – think “asteroid soil”. Plowed soil. The light colored, rather well metamorphosed parts might be from deep down. Clasts as primitive as type 3 have seen little heat. There are shock darkened and shock melted bits and the rare foreign carbonaceous fragment. Mostly it is its grey green self. Impacts plowed it up and impacts compacted – lithified – it.

Scientists figure the meteoroid that was blasted off the parent body was maybe 4 to 5 meters in diameter before atmospheric entry. Even with ablation loss of 90 to 95% about 7 tons dropped on the Sahara Desert. This was 4.4±0.7 kyr ago – after the mammoths but well before iPhones. [Metzler et al. (2008) LPS and Welten et al. (2010) LPS]

Slices with fine grained inclusions shouldn’t go to the kids. We’ll keep them. They’ll be good trading material if that paper ever comes out.

I’ve got some pictures.

– John

A breccia for sure. The long diagonal of the squarish white clast is 5 cm. The slice is 19 cm long.

This inch long slice has a nice fine grained inclusion. Looks like cement, doesn’t it?

This is one of the thin sections made from that slice. The fine grained inclusion stands out pretty well here.

Up close to it now. The fine grained portion is on the left and the regular lithology is on the right. PPL FOV 5.2 mm.

Same view. XPL

Same slice, different thin section. Is that a carbonaceous clast there on the right edge?

Maybe it is. Same clast, different thin section. XPL FOV 2 mm.

Different NWA 869 thin section. There’s a patch sitting a little above center with dark matrix and well defined chondrules. It’s probably one of the low metamorphic grade areas.

Here is a close-up of that part. Neat and orderly. PPL FOV 3 mm.

Same with another filter thrown in. Not as distinct, but prettier. XPL FOV 3 mm.

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