[meteorite-list] Unshocked or Weakly Shocked Ureilites
From: Bernd Pauli HD <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 09:49:02 2004
> Ureilites containing diamonds were always thought to have been
> formed by impact shock ... The features of DaG 868 seem to
> suggest that reconsiderations about the origins and building
> history of Ureilites are now necessary ... The ureilite contains
> fine grained diamonds, but the olivines appear to be unshocked,
> showing no evidence of undulose extinction - graphite found
> in pigeonite crystals contain diamonds ... It suggests that the
> diamonds were not formed by shock impact or chemical vapor
The DaG 868 ureilite is not the first that is virtually unshocked.
The three Antarctic ureilites FRO 95028*, ALHA 78019 and ALH 83014,
are also classified as very low-shock because of their exceptionally
large, euhedral, graphite crystals - features that indicate shock
pressures <0.2 Mb. RC 027 is described as a low-shock ureilite.
But Cyrena Anne Goodrich cautions:
These criteria for degree of shock are q u a l i t a t i v e, and the
extent to which individual ureilites display them is s u b j e c t i v e
Hence, there is some d i s a g r e e m e n t in the literature, and
some designations have changed over the years. For example, Vdovykin
(1970) classified ureilites only as high or low shock. Since the
discovery of ureilites such as ALHA 78019, all the ureilites classified
as, "low shock" by Vdovykin have been regarded as medium shock. Also,
some of the criteria are c o n t r o v e r s i a l. For example, if
diamonds are formed by vapor deposition as suggested by Matsuda et al.
(1991), then they cannot be an indicator of shock level. Also, it has
not been shown that cloudy pigeonite is a result of shock melting
...Development of a more informative shock classification for ureilites
could be a fruitful area of study, because of the various roles that
impacts and shock melting have been proposed to play in the petrogenesis
of ureilites ...For example, it would be useful to distinguish shock
features due to late-stage events, such as ejection from or breakup of
the parent body, from shock features that could be associated with
significant impact mixing or injection of foreign material.
* The graphite in FRO 95028 shows prominent pressure lamellae,
which suggests that it was deformed after crystallization.
> I had proposed a theory that ureilites
> may have originated from Venus ...
Cyrena Anne Goodrich in her MAGPOX article:
Ureilites have characteristics that appear to have been established in
the solar nebula and that seem unlikely to have survived extensive
planetary igneous processing, particularly the multi-stage history
required if ureilites are cumulates.
GOODRICH C.A. et al. (1987b) Roosevelt County 027:
A low- shock ureilite with interstitial silicates and high
noble gas concentrations (Meteoritics 22, 1987, 191-218).
GOODRICH C.A. (1992) Ureilites: A critical review
(Meteoritics 27, 1992, 327-352).
GOODRICH C.A.(1999) Are ureilites from partial melting
of chondritic materials? The answer from MAGPOX
(MAPS 34-1, 1999, 109-119).
Received on Sun 23 Sep 2001 07:58:49 AM PDT