ANGRITE
Angrites
are a rare group of achondrite meteorites (= meteorites lacking rounded
structures called chondrules) with a distinctive chemistry and mineralogy. Dates on
some of the 25 known angrites are between 4.557 to 4.562 billion years.
Published dates on the NWA 2999 angrite shown below range from 4.558 to 4.562
billion years.
Angrites have been interpreted as originating from the
mantle of Planet Mercury. This idea was based on, for one thing, the
lack of volatile elements in the rocks. The full reasoning behind
angrites having a Mercurian origin is summarized in Irving et al. (2005) and
Kuehner et al. (2006).
Other meteorite researchers (e.g., Ruzicka &
Hutson, 2006; R. Malcuit, pers. comm.) have not accepted the Mercurian
provenance interpretation. Data from
Planet Mercury that has been acquired by the MESSENGER spacecraft in 2011 &
2012 now show that angrites are not from Mercury (Irving et al., 2013). [Note: an olivine gabbro meteorite,
designated Northwest Africa 7325, has recently been identified as being more
consistent with the known geochemistry of Mercury rocks - see Irving et al.,
2013]
The lack of volatile elements in angrites does argue
for an origin in proximity to the Sun, possibly in the vulcanoid belt. The term “vulcanoids” refers to the
hypothetical existence of several bodies (how many?) that used to orbit the Sun
closer than Mercury. They have left
their original orbits by one or more planetary migration events.
Angrite
(slice, 2.4 cm across), NWA 2999 Meteorite.
This angrite was discovered in 2004 in Northwest Africa. Based on chemical and physical similarities,
this meteorite has been paired with the NWA 3164, NWA 4662, and NWA 4931
Meteorites. The rock is a very dark brown peridotite (ultramafite).
“Peridotite” is an igneous-sounding name, and it is ultimately plutonic in
origin, but is has been altered and metamorphosed since it originally
crystallized from melt. Available
mineral analysis data has indicated that the NWA 2999 angrite contains ~65%
olivine, ~23% clinopyroxene, ~8% metallic iron, ~4% spinel, ~1% anorthite
(calcium plagioclase feldspar), plus trace amounts of other minerals.
Additional info. on this particular meteorite can be
found in Gellissen et al. (2007) and Humayun et al. (2007).
Info. mostly synthesized from:
Irving et al. (2005) - American Geophysical Union
Fall Meeting Abstracts.
Kuehner et al. (2006) - 37th Lunar and Planetary
Science Conference Abstracts.
Ruzicka & Hutson (2006) - 69th Annual
Meteoritical Society Meeting Abstracts.
Gellissen et al. (2007) - 38th Lunar and Planetary
Science Conference Abstracts.
Humayun et al. (2007) - 38th Lunar and Planetary
Science Conference Abstracts.
Markowski et al. (2007) - Earth and Planetary
Science Letters 262(1-2): 214-229.
Amelin & Irving (2007) - Workship on Chronology
of Meteorites abstract 4061.
Shukolyukov & Lugmair (2008) - 39th Lunar and
Planetary Science Conference Abstracts.
Irving et al. (2013) - Ungrouped mafic achondrite
Northwest Africa 7325: a reduced, iron-poor cumulate olivine gabbro from a
differentiated planetary parent body. 44th Lunar and Planetary Science
Conference Abstracts.
Adam Hupé (pers. comm.)
Robert Malcuit (pers. comm.)