Geology Reference
In-Depth Information
Brachinite
4.58 g
Found December 26, 1984
2.0 × 1.5 × 0.8 cm
Weathering = A/Be
having minor amounts of metal, sulfide, chromite, clinopyroxene, and pla-
gioclase. They are thought be from the deeper part of an asteroid-sized
body, perhaps representing a residue from partial melting or an olivine-
rich cumulate from early differentiation. These are rare meteorites with
some affinity to the HED meteorites (similar oxygen isotopes) but not
much coherency in the membership.
10000
ALH84025
10
GRA06129
Brachina
Plag
melt
Ureilites
1000
Chossigny
Btc
+
Acapulcoites
+
Shergottites and
nokhlites
Dunties
1
100
Winonaites
Y75097c
Brachina
MB
Moon
rocks
Pyx
residuum
EN
10
Eucrites
0.1
Lodranites
Ureilites
1
A84
2
5
10
Magnesium (wt%)
20
50
0.01
0.1
(Sm/Sc)
CI
1
10
Plate 48
MiNERALogy
SigNiFiCANCE
Allan Hills 84025 has a coarse-grained (0.5 to 1.5 mm
olivines), equigranular recrystallized texture with 80%-
90% Fo
68
olivine, 4%-15% clinopyroxene, chromite, sul-
fide, and metal. The olivine contains no melt inclusions
as do many other olivine-bearing achondrites.
As the second member of the small brachinite group
of achondrites, ALH 84025 has helped define the char-
acteristics of this group. Brachinites are distinct from
other achondrite groups such as acapulcoite/lodranites,
eucrites, and shergottites, and they have distinct chemical
features such as high Ni/Mg (right, [357]). ALH 84025
may be linked to the ungrouped achondrite gRA 06128
by melting relations (left, [364]), where ALH 84025 repre-
sents a residue from melting. The parent asteroid is a dif-
ferentiated body and not a primitive achondrite that
might still contain some relict chondritic material or
residues. The differentiation event was very ancient, just a
few Ma after the start of the solar system.
References [354-357]
