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Happy Canyon and Mount Egerton

Hello Mike, David, Alex, Martin, and List,

> I remember recently discussing this on the list. I gave a reference
> published in GCA vol. 61, 1997, written by Dr. Alan Rubin et al.
> that reestablishes all enstatite chondrite classifications. Happy
> Canyon is classified as an EL melt breccia in agreement with this
> classification earlier by McCoy et al.


Here are the sources that David kindly mentioned:

McCOY T.J. et al. (1995) Origin and history of impact-melt rocks of
enstatite chondrite parentage (GCA 59, 161- 175).

RUBIN A.E. et al. (1997) Shock metamorphism of enstatite chondrites (GCA
61, 1997, pp. 847-858).

-------- snip --------

And another:

ZHANG Y. et al. (1996) Pyroxene structures, cathodoluminescence and the
thermal history of the enstatite chondrites (Meteoritics 31-1, 1996, pp.
87-96, excerpt):

Happy Canyon, which was considered an impact melt of EL composition by
McCoy et al. (1995), also contains disordered orthopyroxene. Despite
this high-temperature episode, the low equilibration temperatures of the
cubic sulfides for these two meteorites indicate that some systems
closed at lower temperatures (Zhang et al., 1992).
It seems that the transition between petrologic type 3 to 5,6 in EL
chondrites is not continuous, either because we have not yet recovered
an EL4 chondrite or because there was some mechanical process to
separate these EL5,6 chondrites from original EL3 chondrites before they
underwent high peak temperatures that destroyed chondrules. Experience
shows that it is probably due to the lack of complete sampling of the
enstatite meteorite classes.

With regard to Mount Egerton, the following paper might be the one
mentioning its reclassification - but I am not sure and perhaps David
can back-check:

CASANOVA I. et al. (1993) Composition of metal in aubrites: Constraints
on core formation (GCA 57, pp. 675-682).


Best wishes,

Bernd

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