Abyaneh, M.K.

Araki, T.

Bonato, E.

Kaulich, B.

King, A.J.

Lee, M.R.

Russell, S.S.

Schofield, P.F.

English

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THE OXIDATION STATE OF IRON IN SILICATE MINERALS FROM THE MATRICES OF CO 
CARBONACEOUS CHONDRITES.  
E. Bonato
1,2
, A. J. King
1
, P. F. Schofield
1
, B. Kaulich
3
, T. Araki
3
, M. K. Abyaneh
3
, M. R. Lee
2
 and S. S. Russell
1
. 
1
Department of Earth Science, Natural History Museum, London, SW7 5BD, UK (e-mail: e.bonato@nhm.ac.uk), 
2
School of Geographical and Earth Sciences, University of Glasgow, Glasgow, G12 8QQ, UK, 
3
Diamond Light 
Source, Didcot, OX11 0DE, UK. 
  
 
Introduction: Carbonaceous chondrite meteorites 
are amongst the most primitive extra-terrestrial materi-
als available for study. They consist of chondrules and 
calcium-aluminium-rich inclusions (CAIs) set within a 
matrix of fine-grained (<1 μm) materials including 
amorphous and crystalline silicates, metal, Fe-
sulphides and Fe-oxides. In many “type 3” carbona-
ceous chondrites the matrix has remained largely unal-
tered since it was accreted into their parent bodies. The 
matrix therefore preserves the original building blocks 
of the solar system and is an important tracer of the 
formation and evolution of asteroids.  
Amorphous silicates can comprise up to 50 vol% of 
the matrix in the most pristine carbonaceous chondrites 
[e.g. 1]. However, the origin of the amorphous silicates 
remains enigmatic; analyses are challenging due to 
small domain sizes and the hetrogeneous nature of the 
matrix. The amorphous silicates could be a presolar 
phase [2], a product of nebula condensation [3, 4], or 
result from later parent body alteration [5, 6].  
Hopp and Vollmer [7] recently used transmission 
electron microscopy (TEM) combined with energy-
dispersive X-ray (EDX) analysis and electron energy 
loss spectroscopy (EELS) to investigate amorphous 
silicates in the pristine ungrouped carbonaceous chon-
drite Acfer 094. They interpreted mobilization of Fe 
and high (>0.6) Fe
3+
/Fe ratios to show that the amor-
phous silicates had reacted with fluids in the parent 
body. Similarly, using TEM and scanning transmission 
X-ray microscopy (STXM), [8] analysed Fe
3+
-rich 
hydrated amorphous silicates in pristine CR chondrites, 
and showed that their mineralogy and Fe
3+
/Fe ratios 
varied as a function of hydration. 
We are investigating the effects of thermal meta-
morphism on crystalline and amorphous silicates in the 
matrices of CO carbonaceous chondrites, which range 
in petrologic type from 3.0 to 3.8. Our aim is to better 
understand the initial formation conditions of the sili-
cates, and to quantify how the degree of asteroidal 
heating has influenced their abundance, structure, 
chemistry and transformation behaviour. Here we pre-
sent results from the CO chondrites DOM 08006 (CO 
3.0), Kainsaz (CO 3.2) and Moss (CO 3.6). 
Methodology: Suitable areas of matrix were iden-
tified in each meteorite by scanning electron micro-
scope (SEM)-EDX mapping and analysed by electron 
microprobe. Electron transparent foils (~10 × 5 × 0.09 
µm) were cut and extracted from each area using the 
focused ion beam (FIB) technique and attached to a Cu 
TEM grid. 
The Fe
3+
/Fe ratios of matrix materials in the FIB 
sections were measured using STXM on beamline I08 
at Diamond Light Source, UK. Energy-dependent im-
age stacks (~2 × 2 µm) were collected over the Fe L3-
edge (705 ‒ 715 eV) with an energy resolution of 0.1 
eV, a spatial resolution of ~50 nm and a dwell time per 
pixel of 5 ms. Fe L3-edge spectra where then extracted 
from specific grains/regions within the energy stacks 
ranging in size from ~50 to 500 nm in size. In addition, 
we also analysed several standards including San Car-
los olivine, magnetite and an Fe-(oxy)hydroxide.  
The FIB sections were subsequently imaged by 
TEM using an FEI T20 instrument at the University of 
Glasgow operated at 200 kV. 
Results: Electron microprobe results clearly show 
chemical variations in relation to the degree of meta-
morphism. Plotting data on a Fe-Si-Mg ternary dia-
gram reveals a decrease in Fe within the matrix, of 
about 10 wt%, in the more highly metamorphosed 
samples and an enrichment in Mg in the matrix (Fig. 
1). 
Bright-field TEM images of DOM 08006 and Moss 
show how the matrix texture changes with increasing 
thermal metamorphism (Fig. 2). The matrix in DOM 
08006 has an amorphous groundmass embedded with 
~0.1 µm silicate, sulphide, metal and phyllosilicate 
grains. The matrix of Moss by comparison consists 
mainly of micron to sub-micron euhedral crystals that 
are interpreted to have formed by recrystallisation of 
the amorphous groundmass, metal grains and other 
matrix silicates.  
At the Fe L3-edge there are two absorption peaks, 
the relative intensities of which are related to the abun-
dance of Fe
2+
 and Fe
3+
 [e.g. 8, 9]. Fig. 3 shows typical 
Fe L3-edge spectra extracted from silicates in DOM 
08006, Kainsaz and Moss. Also included in Fig. 3 are 
spectra for San Carlos olivine (Fe
2+
) and an Fe-
(oxy)hydroxide (Fe
3+
). Based on our mineral standards 
we can qualitatively infer that silicates in the matrix of 
DOM 08006 are rich in Fe
3+
, Kainsaz contains about 
10% Fe
3+
, and Moss is dominated by Fe
2+
. The spatial 
variation in the spectra was much greater for DOM 
1917.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)
08006, with some regions found to be less Fe
3+
-rich 
than other areas.  
Discussion: Matrix is one of the first components 
of primitive carbonaceous chondrites to be affected by 
parent body alteration. Highly reactive amorphous 
silicates can therefore be used as an indicator of the 
progression of metamorphism. We find that amorphous 
silicates are abundant in the matrix of DOM 08006 and 
are typically Fe
3+
-rich, although there is some variation 
in the Fe oxidation state within the sample. If the 
amorphous silicates condensed in the solar nebula, we 
would predict that they should be rich in Fe
2+
 due to 
the reducing environment. However, while DOM 
08006 is believed to be one of the most primitive car-
bonaceous chondrites, there is recent evidence that it is 
a breccia and has experienced minor aqueous alteration 
[10, 11]. Even low levels of parent body processing 
would have oxidized the amorphous silicates. This 
finding is in agreement with the model proposed by [7] 
for Fe
3+
-rich amorphous silicates in Acfer 094, and in 
aqueously altered CR chondrites [8]. In addition, we 
note that DOM 08006 is an Antarctic find of weather-
ing grade A/B [10, 12]. Terrestrial alteration may ex-
plain why the silicates in DOM 08006 show greater 
variation in the  Fe
3+
/Fe ratios compared to the rela-
tively minor differences observed between grains in 
Acfer 094 [7]. 
In Kainsaz and Moss very few amorphous silicates 
are present because they were readily destroyed by 
parent body thermal metamorphism. The crystalline 
silicates are all Fe
2+
-rich, which might be because the 
environmental conditions became increasingly reduced 
with thermal metamorphism due to the removal of oxi-
dizing H2O and the initial prescence of reducing agents 
such as H and C [14]. 
Acknoledgements: Thank you to STFC for the 
funding support, ANSMET and NHM Meteorite Col-
lection for providing the meteorite samples. 
References: [1] Greshake A. (1997) GCA 61, 437‒
452. [2] Vollmer C. et al. (2009) GCA 73, 7127‒7149. 
[3] Brearley A. J. (1993) GCA 57, 1521‒1550. [4] Kel-
ler L. P. & Messenger S. (2012) LPSC XLIII, #1880. 
[5] Chizmadia L. J. & Brearley A. J. (2008) GCA 72, 
602‒625. [6] Abreu N. M. & Brearley A. J. (2010) 
GCA 74, 1146‒1171. [7] Hopp T. & Vollmer C. (2017) 
M&PS. [8] Le Guillou C. et al. (2015) EPSL 420, 162‒
173. [9] Bourdelle F. et al. (2013) Contrib. Mineral. 
Petrol. 166, 423‒434. [10] Davidson J. et al. (2014) 
LPSC XLV, #1384. [11] Krot A. N. et al. (2017) LPSC 
XLVIII, #1084. [12] Alexander C. M. O’D. et al. 
(2018) GCA 221, 406‒420. [13] Menzies O. N. et al. 
(2005), M&PS 40, 1023‒1042.   
 
 
Figure 1. Fe-Si-Mg ternary diagram from the matrix 
of DOM 08006 (CO3.0), Kainsaz (CO3.2) and Moss 
(CO 3.6).  
 
 
Figure 2. Bright-field TEM images of DOM 08006 
(CO3.0) matrix on the left, and of Moss (CO3.6) ma-
trix on the right. (Scale bar 1µm). 
 
 
Figure 3. Fe L3 edges of CO chondrites that under-
went different degrees of thermal metamorphism. The 
dashed lines respectively indicates the peak corre-
sponding to the Fe
2+
 at ~708 eV and the peak of the 
Fe
3+
 at  ~710 eV. 
1917.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)


The Oxidation State of Iron in Silicate Minerals from the Matrices of CO Carbonaceous Chondrites

http://eprints.gla.ac.uk/157634/1/157634.pdf

The Oxidation State of Iron in Silicate Minerals from the Matrices of CO Carbonaceous Chondrites

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