Geology Reference
In-Depth Information
(a)
GRO 95551
2 cm
(b)
(c)
Oxygen isotopes
2.0
GRO 95551, 51
Δ
17
O =
δ
17
O - 0.52 (
δ
18
O)
1.0
0.0
TF
-1.0
-2.0
-3.0
NWA
5492
GRO
95
CR
CH
CB EH
EL
HLLL
20 kV
X15
1 mm
0000
29 61 BES
Figure 4.4.
(a) A cut surface of GRO 95551 showing large cm-sized silicate chondrules (grey) and metal nodules (white), and
numerous smaller metal and silicate chondrules. (b) A backscattered electron image of GRO 95551, 51 showing a large barred
olivine (BO) chondrule in the interior. Surrounding the large central BO chondrules are smaller (1 mm) chondrules (grey) and metal
grains (white). (c) Diagram showing the Δ
17
O values of silicates in GRO 95551 compared with average values for chondrites from
other groups. The gray bar shows the range of values for GRO 95551. Data for ordinary chondrites are averages for the H, L, LL.
Data are from
Weisberg et al.
[2012] and references therein.
against impact as major process for the history of EL chon-
drites include the low shock stage of many EL3 chondrites,
chondrules and metal with sharp boundaries, the lack of
high-pressure minerals, and a wide range of
53
Mn/
55
Mn ra-
tios and excess of
53
Cr in EL3 sulfides, which should have
been equilibrated if they were impact modified [e.g.,
Weisberg and Kimura
, 2012;
El Goresy et al
., 1992].
Notable in many of the EL3 chondrites are metal-rich
nodules (Figure 4.5b). Nodules in the MAC 88136 EL3
commonly occur as 400-µm nodules intergrown with lath-
shaped or euhedral enstatite and schreibersite, Cr-bearing
troilite, daubreelite, and graphite [
Van Niekirk and Keil
,
2011;
Weisberg et al.
, 1994]. These intergrowths of metal
and enstatite have been interpreted to be the result of
