Geology Reference
In-Depth Information
Lherzolitic shergottite
78.8, 204.6 g
Found December 25 and 28, 2004
4.0 × 3.5 × 2.5; 6.5 × 5.5 × 3.5 cm
Weathering = B
olivine, orthopyroxene, chromite, and plagioclase (maskelynite). A distinc-
tive characteristic of lherzolitic shergottites is the abundant evidence for
shock, including maskelynite, mosaicised olivine, and pockets of melted
rock in the interior. Lherzolitic shergottites may represent the residue from
melting of the martian mantle during production of basaltic rocks. They
also are among the most heavily shocked of all the martian meteorites.
Thus, they hold information about the interior and differentiation history
of Mars as well as constraints on its impact history.
RBT 04…
261
262
data-point error elipses are 2
σ
0.28224
Age = 225±21 Ma
Initial
176
Hf/
177
Hf = 0.2821349±0.0000038
MSMD = 0.74
'CPX-rich'
0.28222
Leached WR
0.28220
0.28218
'Ol-rich'
Chromite
'Mask-rich'
-19.50
0.28216
CPX
WR
-20.50
-21.50
0.28214
Oxide
OI
Mask
-22.50
-23.50
0.28212
0.0
0.5
1. 0 .5
2.0 .5
1/[Hf]
0.28210
0.000
0.004
0.008
0.012
176
Lu/
177
Hf
0.016
0.020
Low
High
2mm
Plate 73
MiNERALogy
SigNiFiCANCE
RBT 04262 is composed of poikilitic and nonpoikilitic
areas. The poikilitic areas (up to 5 mm in size) are com-
prised of pyroxene oikocrysts enclosing small olivine
grains (~0.5 mm). Pyroxene oikocrysts in poikilitic areas
are low-Ca pyroxene (En
73
Wo
5
to En
60
Wo
10
) in the cores
and augite in the rims (En
52
Wo
30
-En
45
Wo
40
). Kaersutite-
bearing magmatic inclusions are found within pyroxene
oikocrysts. The nonpoikilitic areas comprise olivine,
maskelynite, and pyroxene, the latter mostly pigeonite
with small amounts of augite and slightly higher in Fe
contents than those in poikilitic areas (En
62
Wo
7
-En
58
Wo
12
and En
50
Wo
30
-En
44
Wo
39
). The olivine grains in poikilitic
areas are more magnesian and show wider compositional
variation (Fa
28-39
) than those in nonpoikilitic areas (Fa
32-41
).
Maskelynite is An
58
or
3
to An
30
or
7
. K-rich feldspar is
An
10
or
25
to An
2
or
70
. Minor amounts of Ca phosphates,
chromite, ilmenite, and FeNi sulfide are also present.
The presence of large pyroxene oikocrysts in RBT
04261/262 and its overall mineralogy show that it is a
member of the lherzolitic shergottite group (left, [770]).
However, the relatively high abundance of plagioclase
(10%-15%) indicates that it has affinity to the basaltic
shergottites as well. The nonpoikilitic areas have the most
ferroan olivine composition, highest modal abundance
among lherzolitic shergottites, and more albitic maskel-
ynite than the others. Thus, RBT 04262 may be located
near the upper region of the igneous complex in which
the lherzolitic shergottites formed (right, [769]), where
cumulus phases were less abundant.
References [764-781]
