PROCESSES AFFECTING THE CR CHONDRITES PARENT BODY

46th Lunar and Planetary Science Conference (2015)
2561.pdf
PROCESSES AFFECTING THE CR CHONDRITES PARENT BODY: PETROLOGY, MINERALOGY
AND CHEMICAL COMPOSITION OF THE MATRICES OF ANTARCTIC CR CARBONACEOUS
CHONDRITES. N. M. Abreu1, 1Earth Science Program, Penn State DuBois [email protected]
Introduction: Scales devised to quantify the degree of aqueous alteration of CR chondrites have generated contradictory classifications [1,2]. CRs record a
broad range of asteroidal alteration features [e.g., 1-6].
Most CRs are classified as type 2. The least altered
CRs have been argued to be to petrologic type 3 [5],
whereas the CRs that record most extensive signs of
aqueous alteration are type 1 [6]. Fine-grained mineralogical and compositional studies of the matrices of
10 Antarctic CRs were collected via FEG-SEM, FEGEPMA, and TEM. Samples names are abbreviated as
follows: EET 96259, EET96; GRA 95229, GRA95;
GRA 06100, GRA06; GRO 95577, GRO95; GRO
03116, GRO 03; LAP 02432, LAP02; LAP 04516,
LAP045; LAP 04720, LAP047; MIL 07525, MIL07;
and MIL 090001, MIL09. Observations are compared
the aqueous alteration scales.
Results: Less than half of chondrules in the CRs
show signs of mesostasis replacement by chlorite or
serpentine. Matrix is texturally heterogeneous, containing abundant chondrule fragments and small clasts that
are enriched in elongated, feathery sulfides and framboidal and platelet magnetite grains. Opaques in some
clasts are oriented. Chemical composition of 8 CRs
was determined using via EPMA, using 10µm beam
(Table 1). Large intra- and inter-chondrite variations
are observed in all elements. Through an ongoing
study of CRs, 24 FIB sections were extracted from
representative fine-grained regions from 10 CRs. Observed mineral assemblages are given in Table 2. Owing to marked differences in mineralogy the details of
the mineralogy of GRA06 matrix are discussed elsewhere [7]. CR matrices are dominated by amorphous
Fe-Mg-silicates. Neither increased phyllosilicates
abundance near chondrules nor phyllosilicates layers
around chondrules have been observed. Phyllosilicate
abundance increases with aqueous alteration. However,
no correlation between the textural characteristics of
matrix and phyllosilicates abundance has been observed; more extensively altered chondrites contain
more phyllosilicates in all fine-grained regions.
Discussion: The following sub-µm secondary
phases are used to determine the degree of aqueous
alteration: (1) ratio of Fe-Mg amorphous silicates to
phyllosilicates, (2) size of phyllosilicates, (3) abundance of magnetite, (4) replacement of Fe-Ni sulfides
(partial oxidation to replacement by tochilinite). The
sequence from the least to the most altered CRs is:
GRA95, LAP02, EET96, MIL09, MIL07, LAP047,
LAP045, GRO95. GRA06 and GRO03 are excluded
due to significant differences in the matrix mineralogy,
which have been attributed to the heating [7].
Sub-µm observations are not in agreement with
proposed compositional indicators of aqueous alteration (Figs. 1-2) or with alteration scales. [8] suggested
that increasing degrees of aqueous alteration resulted
in higher Mg concentration relative to Fe in matrix,
owing to replacement of Mg-phenocrysts by phyllosilicates. There is no correlation between aqueous alteration and Mg matrix concentration (Fig. 1). As CR
aqueous alteration proceeds, Fe is also mobilized into
to the matrix via oxidation and hydration of Fe-Ni
metal, which are abundant in CR chondrites. Thus the
correlation between Mg and aqueous alteration is
weak. Decreasing and heterogeneous distribution of S
have been suggested to accompany aqueous alteration,
owing to coarsening of nanophase Fe-sulfides [2,8].
However, Fig. 1 shows that there is no simple correlation between matrix sulfide content and aqueous alteration. This could be explained by incipient formation of
nanophase tochilinite observed in some extensively
altered CRs (e.g., LAP047, LAP045, MIL07) that do
not contain large amounts of micron-sized sulfides.
Table 1. Average composition of matrices of CRs.
b.d.: below detection; n.m. – not measured.
Matrix
Al2O3
CaO
TiO2
MgO
SiO2
Cr2O3
MnO
P2 O5
Na2O
K2 O
NiO
FeO
S
Total
EET96 GRA95 GRA06 GRO03 LAP045 LAP047
2.0
2.0
2.1
1.4
1.7
1.7
1.2
1.0
1.5
1.1
1.1
2.5
b.d.
0.1
0.1
0.1
0.1
b.d.
17.1
14.9
14.8
12.9
14.6
17.2
30.6
30.1
27.3
23.0
27.1
27.7
0.4
0.2
0.3
0.4
0.4
0.4
0.2
0.3
0.3
0.2
n.m.
0.3
0.3
0.2
0.2
0.2
n.m.
0.2
0.3
0.9
0.4
0.3
0.4
1.4
0.1
0.1
0.1
0.1
b.d.
0.1
1.5
1.3
2.0
1.8
1.8
2.2
30.3
29.2
36.8
46.5
29.7
32.6
2.8
2.2
0.6
1.9
3.6
0.5
86.8
82.6
86.5
89.8
80.4
86.7
MIL07 MIL09
2.1
3.7
2.0
0.9
0.1
0.1
15.8
10.4
29.0
28.2
0.4
0.3
0.3
0.2
0.4
0.2
0.3
0.3
b.d.
0.1
2.1
3.4
30.1
36.6
3.0
1.4
85.6
85.6
Clusters of meteorites (around petrologic type 2.52.6 in [1] and around petrologic type 2.8 in [2]) have
widely divergent secondary mineralogies. Based on
bulk water/OH and phyllosilicate abundances measured by [1], the sequence is: LAP047(2.6);
GRA95(2.5); LAP02(2.5); GRO95(1.3). Differences
between this classification scheme and the petrologic
record were explained by heterogeneities in (more altered) dark inclusion abundance and by differences in
46th Lunar and Planetary Science Conference (2015)
the temperature of aqueous alteration [1]. Another possibility is that since unaltered amorphous silicates may
be hydrated [9], wt.% H may not be a direct proxy
asteroidal aqueous alteration.
2561.pdf
[2] overestimated the presence of phyllosilicates in
weakly altered CRs based on BSE and low total
EMPA. This may explain the cluster of weakly and
moderately altered CRs around petrologic sub-type
2.8. Smooth rims described in LAP02 are assumed to
be rich in phyllosilicates [10]. A FIB section extracted
from such rim (Fig. 2) does not contain phyllosilicates.
Fig. 1. Average Mg/Si v. Fe/Si and Fe v. S. Petrologic
sub-types from [5]-red; [2]-blue; [1]-purple.
Table 2. Sub-µm CR matrix mineralogy (TEM/EDS).
Meteorite
EET 96259
Common Matrix Minerals
Fe-Mg amorph silicate, Ferrolizardite, Fe-oxide (prob. Wustite)
GRA 95229 Fe-Mg amorph silicate, Ferrolizardite, FeNi-sulfide
GRA 06100 Fe-Mg amorph silicate, Fe-rich Serpentine, FeNi-sulfide, FeNi metal,
Fe-silicide, Fe-oxide (prob. Wustite
and Magnetite), Hisingerite
GRO 95577 Fe-Mg amorph silicate, Fe-rich Serpentine, FeNi-sulfide
GRO 03116 Fe-Mg amorph silicate, Fe-rich Serpentine, FeNi-sulfide, FeNi metal
LAP 02432 Fe-Mg amorph silicate, Fe-rich Serpentine, FeNi-sulfide
LAP 04516 Fe-Mg amorph silicate, Fe-rich Serpentine, Tochilinite, FeNi-sulfide
LAP 04720 Fe-Mg amorph silicate, Fe-rich Serpentine, FeNi-sulfide
MIL 07525 Fe-Mg amorph silicate, Fe-rich Serpentine, FeNi-sulfide
MIL 090001 Fe-Mg amorph silicate, Fe-rich Serpentine, FeNi-sulfide
Scarce Minerals
Fe-sulfide
Forsteritic Olivine,
Enstatite, Fe-oxide,
C-nanoglobules
Forsterite, Fayalite,
Enstatite, Garnet
Forsterite, Ferrosilite
Fe-oxide (prob.
Wustite,Magnetite),
Diopside, Pigeonite
Forsterite, Fayalite,
Enstatite, Fe-silicide
Tochilinite
Based on petrologic and O-isotopic indicators, [2] obtained the sequence: LAP02 (2.8); MIL07 (2.8);
LAP045 (2.8); GRA95 (2.7); EET96 (2.4); LAP047
(2.4); GRO95 (2.0). TEM observations suggest that
Fig. 2. TEM images of (a) smooth rim in LAP 02432;
(b) representative mineralogical assemblages of CR.
Conclusions: Aqueous alteration scales for the CRs
need to be consistent with mineralogical changes recorded by matrices. Bulk compositional and >µm petrologic indicators of aqueous alteration are not good predictors for the abundance of secondary matrix phases
and thus cannot substitute sub-µm observations.
References: [1] Alexander et al. (2013) GCA 123,
244–260. [2] Harju et al. (2014) GCA 139, 267–292.
[3] Zolensky et al. (1993) GCA 57, 3123-3148.
[4] Weisberg et al. (1993) GCA 57, 1567. [5] Abreu &
Brearley (2010) GCA 74, 1146-1171. [6] Weisberg &
Huber (2007) MAPS, 42, 1495-1503. [7] Abreu et al.
(2014) LPS XLV, Abstract # 2753. [8] Abreu (2007)
Ph.D. Dissertation. [9] Le Guillou & Brearley (2014).
GCA, 131, 344-367. [10] Rubin & Wasson (2009)
GCA, 73, 1436-1460.
Funded by NNX11AH10G to NMA.