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Here is a just released abstract from Meteoritics and
Planetary Sciences, June issue.
An anomalous eucrite, Dhofar 007,
and a possible genetic relationship with mesosiderites
Akira YAMAGUCHI1, 2*, Takehiko SETOYANAGI3,
and Mitsuru EBIHARA1, 3
1Antarctic
Meteorite Research Center, National Institute of Polar
Research, Tokyo 173-8515, Japan
2Department of Polar Science, School of
Multidisciplinary Science, Graduate University for Advanced
Studies, Tokyo 173-8515, Japan
3Department of Chemistry, Tokyo Metropolitan
University, Hachioji, Tokyo 192-0397, Japan
*Corresponding author. E-mail:
yamaguch@nipr.ac.jp
We studied
the texture, mineralogy, and bulk chemical composition of
Dhofar 007, a basaltic achondrite. Dhofar 007 is a polymict
breccia that is mostly composed of coarse-grained granular
(CG) clasts with a minor amount of xenolithic components,
such as a fragment of Mg-rich pyroxene. The coarse-grained,
relict gabbroic texture, mineral chemistry, and bulk
chemical data of the coarse-grained clast indicate that the
CG clasts were originally a cumulate rock crystallized in a
crust of the parent body. However, in contrast to monomict
eucrites, the siderophile elements are highly enriched and
could have been introduced by impact events. Dhofar 007
appears to have experienced a two-stage postcrystallization
thermal history: rapid cooling at high temperatures and slow
cooling at lower temperatures. The presence of pigeonite
with closely spaced, fine augite lamellae suggests that this
rock was cooled rapidly from higher temperatures (>0.5 °C/yr
at ~1000 °C) than typical cumulate eucrites. However, the
presence of the cloudy zone in taenite and the Ni profile
across the kamacite-taenite boundaries indicates that the
cooling rate was very slow at lower temperatures (~1–10 °C/Myr
at <600–700 °C). The slow cooling rate is comparable to
those in mesosiderites and pallasites. The two-stage thermal
history and the relative abundance of siderophile elements
similar to those for metallic portions in mesosiderites
suggest that Dhofar 007 is a large inclusion of mesosiderite.
However, we cannot rule out a possibility that Dhofar 007 is
an anomalous eucrite. |
