Boyce, A.

Cohen, B.E.

Lee, M.R.

Mark, D.F.

English

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EPHEMERAL NEBULAR COMPONENTS IN THE MILDLY AQUEOUS ALTERED CM CARBONACEOUS CHONDRITE LEWIS CLIFF (LEW) 85311. M. R. Lee1, B. E. Cohen1,2, D. F. Mark2, A. Boyce2 1School of Geographical and Earth Sciences, University of Glasgow, G12 8QQ, U.K. (Mar-tin.Lee@Glasgow.ac.uk). 2Scottish Universities Environmental Research Centre (SUERC), Rankine Avenue, East Kilbride G75 0QF, U.K.  Introduction: The CM carbonaceous chondrites have been aqueously altered to various degrees in a parent body environment. The least altered members of the group are highly sought after as they can provide valu-able insights into the nature of the nebular materials that were accreted to form their parent asteroid(s). Those materials that were most susceptible to aqueous alteration were glass, melilite, and the precursors to the phyllosilicate-rich matrices and fine-grained rims, which included amorphous silicates [1]. Various criteria have been used to classify the de-gree of aqueous alteration of the CMs, which include the modal abundance of phyllosilicates [2,3], degree of hydration (as expressed by the weight % H in wa-ter/OH) [4]), and a suite of mineralogical and chemical properties including the abundance of Fe,Ni metal [5]. Using these criteria, the least aqueously altered CMs that have been identified to date include Yamato (Y) 791198 [1], Queen Alexandra Range (QUE) 97990 [5], Paris [6], and Elephant Moraine (EET) 96029 [7]. Here we ask whether Lewis Cliff (LEW) 85311 is another minimally aqueously processed CM that may poten-tially contain the ephemeral nebular components. LEW 85311 is a 199.5 g Antarctic find. Its degree of aqueous alteration has been classified as CM2.6–2.7 [8], which is similar to Paris and QUE 97990. LEW 85311 has a petrologic subtype of 1.9 according to the criteria of [4], and only one of the 48 other (unheated) meteorites that was classified in the same study was found to be less altered (i.e., subtype of >1.9). Here we ask whether LEW 85311 has been unusually mildly aqueously altered, as its petrologic subtype suggests, or whether its low concentration of H in water/OH is due to post-hydration heating and concomitant dehydrox-ylation of phyllosilicates. Alternatively, LEW 85311 could be an anomalous carbonaceous chondrite, as suggested by its bulk oxygen isotopic composition [8], so that the properties and metrics that are conventional used to quantify the degree of aqueous alteration of CMs are not applicable. Materials and methods:  One thin section of LEW 85311 was studied by SEM, and the fine-grained rims were quantitatively chemically analysed by EDX. Electron-transparent foils for transmission electron microscopy (TEM) were extracted from selected sites using the Focused Ion Beam (FIB) technique. The hy-drogen isotopic composition of a bulk sample was de-termined by stepwise pyrolysis at SUERC. Results:  LEW 85311 comprises ~30 vol. % chon-drules, chondrule fragments and CAIs, 36 vol. % fine-grained rims that enclose these objects, and 34 vol. % matrix. The thin section lacks a strong petrofabric, although mild compaction is demonstrated by a linea-tion that is developed within selvages of the matrix between chondrules. The abundance of Fe,Ni metal is 0.30 vol. %. Calcite is very rare in the matrix and ab-sent from fine-grained rims, but occurs in one melilite-bearing CAI (Fig. 1a) and is the main constituent of several elongate objects, each of which is enclosed within a fine-grained rim (Fig. 1b). The scarcity of calcite in LEW 85311 is consistent with the 0.03 vol. % that was recorded by [9], and they also showed that its isotopic composition differs to that of many of the other CMs (δ13C 28.6 ‰, δ18O 19.4 ‰).   Fig. 1. Backscattered electron SEM images. (a) A CAI containing melilite (M) and calcite (C). (b) An object with an unusually high aspect ratio that is almost en-tirely composed of finely crystalline calcite. This ob-ject has a fine-grained rim (FGR). 1287.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) Fig. 2. (a) Backscattered electron SEM image of a chondrule with a micropore-rich fine-grained rim. (b) Bright-field TEM image of serpentine in a fine-grained rim. The chemical composition of the fine-grained rims is listed in Table 1. These rims characteristically con-tain abundant micropores that are an average of ~6 µm in size (Fig. 2a). TEM shows that the rims contain ser-pentine crystals of a variety of sizes and habits, but amorphous silicates were not observed (Fig. 2b). Stepwise pyrolysis of LEW 85311 evolved 6.88 wt. % H2O with a δD of 83 ‰. If water evolved in the 100 °C step is assumed to be a terrestrial contaminant and re-moved from the totals, the values change to 6.19 wt. % H2O and δD of 102 %.   Table 1. Chemical compositions of rims and matrix (wt. %) Wt. % LEW 85311   rims EET 96029 matrix [7] Paris     matrix [10] SiO2 20.7 28.0 27.1 Al2O3 3.1 3.1 2.5 FeO 32.5 34.3 34.0 Cr2O3 na 0.38 0.28 MnO 0.2 0.15 0.21 MgO 11.2 12.1 13.2 CaO 1.1 0.31 0.34 Na2O 0.6 0.59 1.0 K2O 0.1 0.06 0.08 S 3.3 2.2 4.2 Total 72.8 83.55 80.9 MgO/FeO 0.34 0.35 0.39 Discussion: Several of the properties of LEW 85311 are consistent with mild aqueous alteration. Fe,Ni metal is highly susceptible, and so assuming that all of the CMs originally contained a similar volume, inter-meteorite differences in its abundance reflect loss by aqueous alteration. The 0.3 vol. % of Fe,Ni metal in LEW 85311 is relatively high, falling between QUE 97990 (CM2.6–2.7; 0.98 vol. %) and Murchison (CM2.5; 0.11 vol. %) [5]. Melilite is also highly vul-nerable to aqueous alteration, and has been previously described only from Murchison [11], Paris [6] and EET 96029 [7]. Its occurrence in LEW 85311 is again consistent with mild aqueous alteration. The presence of serpentine in the fine-grained rims confirms that they have been aqueously altered, and their chemical compostion is similar to EET 96029 and Paris, alt-hough lower in silica (Table 1). The MgO/FeO ratio of matrices and fine-grained rims should increase with progressive alteration as magnesium is liberated from chondrule silicates. Values for LEW 85311 are compa-rable to or lower than EET 96029 and Paris (Table 1). The origin of the micropores that characterize these rims is unknown, but they could have formed from dissolution of highly soluble grains, or even ice. The amount of water that was evolved during stepwise py-rolysis (~6.2 wt. %) is low relative to most other un-heated CMs [12], but is unlikely to be due to heating as there is no evidence for dehydroxylation or recrystalli-zation of serpentine in the fine-grained rims (Fig. 2b).  Conclusions: Relative to CM carbonaceous chon-drites, LEW 85311 has been mildly aqueously altered. The preservation of melilite demomstrates that it con-tains nebular components that have been lost from most other meteorites by aqueous alteration, and fur-ther work will assess whether this primitive material includes an amorphous precursor to the phyllosilicate-rich matrix and rims.  References: [1] Chizmadia L. J. and Brearley A. J. (2008) GCA, 72, 602–625. [2] Howard K. T. et al. (2009) GCA, 73, 4576–4589. [3] Howard K. T. et al. (2011) GCA, 75, 2735–2751. [4] Alexander C. M. O’D. et al. (2013) GCA, 123, 244–260. [5] Rubin A. E. et al. (2007) GCA, 71, 2361–2382. [6] Hewins R. H. et al. (2014) GCA, 124, 190–222. [7] Lee M. R. et al. (2016) GCA, 187, 237–259. [8] Choe W. H. et al. (2010) Meteoritics & Planet. Sci., 45, 531–554. [9] Alexander C. M. O’D. et al. (2015) Meteoritics & Planet. Sci., 50, 810–833. [10] Rubin A. E. (2015) Meteoritics & Planet. Sci., 50, 1595–1612. [11] Arm-strong J. T. et al. (1982) GCA, 46, 575–595. [12] Lee M. R. et al. (2018) LPS, this volume. Acknowledgements: We thank the NHM London and NASA ANSMET for meteorite loans, and the U.K. STFC for funding. 1287.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)

Ephemeral Nebular Components in the Mildly Aqueously Altered CM Carbonaceous Chondrite LEW 85311

Enlighten: Publications

EPHEMERAL NEBULAR COMPONENTS IN THE MILDLY AQUEOUS ALTERED CM 
CARBONACEOUS CHONDRITE LEWIS CLIFF (LEW) 85311. M. R. Lee1, B. E. Cohen1,2, D. F. Mark2, A. 
Boyce2 1School of Geographical and Earth Sciences, University of Glasgow, G12 8QQ, U.K. (Mar-
tin.Lee@Glasgow.ac.uk). 2Scottish Universities Environmental Research Centre (SUERC), Rankine Avenue, East 
Kilbride G75 0QF, U.K. 
 
Introduction: The CM carbonaceous chondrites have 
been aqueously altered to various degrees in a parent 
body environment. The least altered members of the 
group are highly sought after as they can provide valu-
able insights into the nature of the nebular materials 
that were accreted to form their parent asteroid(s). 
Those materials that were most susceptible to aqueous 
alteration were glass, melilite, and the precursors to the 
phyllosilicate-rich matrices and fine-grained rims, 
which included amorphous silicates [1]. 
Various criteria have been used to classify the de-
gree of aqueous alteration of the CMs, which include 
the modal abundance of phyllosilicates [2,3], degree of 
hydration (as expressed by the weight % H in wa-
ter/OH) [4]), and a suite of mineralogical and chemical 
properties including the abundance of Fe,Ni metal [5]. 
Using these criteria, the least aqueously altered CMs 
that have been identified to date include Yamato (Y) 
791198 [1], Queen Alexandra Range (QUE) 97990 [5], 
Paris [6], and Elephant Moraine (EET) 96029 [7]. Here 
we ask whether Lewis Cliff (LEW) 85311 is another 
minimally aqueously processed CM that may poten-
tially contain the ephemeral nebular components. 
LEW 85311 is a 199.5 g Antarctic find. Its degree 
of aqueous alteration has been classified as CM2.6–2.7 
[8], which is similar to Paris and QUE 97990. LEW 
85311 has a petrologic subtype of 1.9 according to the 
criteria of [4], and only one of the 48 other (unheated) 
meteorites that was classified in the same study was 
found to be less altered (i.e., subtype of >1.9). Here we 
ask whether LEW 85311 has been unusually mildly 
aqueously altered, as its petrologic subtype suggests, or 
whether its low concentration of H in water/OH is due 
to post-hydration heating and concomitant dehydrox-
ylation of phyllosilicates. Alternatively, LEW 85311 
could be an anomalous carbonaceous chondrite, as 
suggested by its bulk oxygen isotopic composition [8], 
so that the properties and metrics that are conventional 
used to quantify the degree of aqueous alteration of 
CMs are not applicable. 
Materials and methods:  One thin section of LEW 
85311 was studied by SEM, and the fine-grained rims 
were quantitatively chemically analysed by EDX. 
Electron-transparent foils for transmission electron 
microscopy (TEM) were extracted from selected sites 
using the Focused Ion Beam (FIB) technique. The hy-
drogen isotopic composition of a bulk sample was de-
termined by stepwise pyrolysis at SUERC. 
Results:  LEW 85311 comprises ~30 vol. % chon-
drules, chondrule fragments and CAIs, 36 vol. % fine-
grained rims that enclose these objects, and 34 vol. % 
matrix. The thin section lacks a strong petrofabric, 
although mild compaction is demonstrated by a linea-
tion that is developed within selvages of the matrix 
between chondrules. The abundance of Fe,Ni metal is 
0.30 vol. %. Calcite is very rare in the matrix and ab-
sent from fine-grained rims, but occurs in one melilite-
bearing CAI (Fig. 1a) and is the main constituent of 
several elongate objects, each of which is enclosed 
within a fine-grained rim (Fig. 1b). The scarcity of 
calcite in LEW 85311 is consistent with the 0.03 vol. 
% that was recorded by [9], and they also showed that 
its isotopic composition differs to that of many of the 
other CMs (δ13C 28.6 ‰, δ18O 19.4 ‰). 
 
 
Fig. 1. Backscattered electron SEM images. (a) A CAI 
containing melilite (M) and calcite (C). (b) An object 
with an unusually high aspect ratio that is almost en-
tirely composed of finely crystalline calcite. This ob-
ject has a fine-grained rim (FGR). 
1287.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)
 
Fig. 2. (a) Backscattered electron SEM image of a 
chondrule with a micropore-rich fine-grained rim. (b) 
Bright-field TEM image of serpentine in a fine-grained 
rim. 
The chemical composition of the fine-grained rims 
is listed in Table 1. These rims characteristically con-
tain abundant micropores that are an average of ~6 µm 
in size (Fig. 2a). TEM shows that the rims contain ser-
pentine crystals of a variety of sizes and habits, but 
amorphous silicates were not observed (Fig. 2b). 
Stepwise pyrolysis of LEW 85311 evolved 6.88 wt. % 
H2O with a δD of 83 ‰. If water evolved in the 100 °C 
step is assumed to be a terrestrial contaminant and re-
moved from the totals, the values change to 6.19 wt. % 
H2O and δD of 102 %.  
 
Table 1. Chemical compositions of rims and matrix (wt. %) 
Wt. % LEW 85311   
rims 
EET 96029 
matrix [7] 
Paris     
matrix [10] 
SiO2 20.7 28.0 27.1 
Al2O3 3.1 3.1 2.5 
FeO 32.5 34.3 34.0 
Cr2O3 na 0.38 0.28 
MnO 0.2 0.15 0.21 
MgO 11.2 12.1 13.2 
CaO 1.1 0.31 0.34 
Na2O 0.6 0.59 1.0 
K2O 0.1 0.06 0.08 
S 3.3 2.2 4.2 
Total 72.8 83.55 80.9 
MgO/FeO 0.34 0.35 0.39 
Discussion: Several of the properties of LEW 
85311 are consistent with mild aqueous alteration. 
Fe,Ni metal is highly susceptible, and so assuming that 
all of the CMs originally contained a similar volume, 
inter-meteorite differences in its abundance reflect loss 
by aqueous alteration. The 0.3 vol. % of Fe,Ni metal in 
LEW 85311 is relatively high, falling between QUE 
97990 (CM2.6–2.7; 0.98 vol. %) and Murchison 
(CM2.5; 0.11 vol. %) [5]. Melilite is also highly vul-
nerable to aqueous alteration, and has been previously 
described only from Murchison [11], Paris [6] and 
EET 96029 [7]. Its occurrence in LEW 85311 is again 
consistent with mild aqueous alteration. The presence 
of serpentine in the fine-grained rims confirms that 
they have been aqueously altered, and their chemical 
compostion is similar to EET 96029 and Paris, alt-
hough lower in silica (Table 1). The MgO/FeO ratio of 
matrices and fine-grained rims should increase with 
progressive alteration as magnesium is liberated from 
chondrule silicates. Values for LEW 85311 are compa-
rable to or lower than EET 96029 and Paris (Table 1). 
The origin of the micropores that characterize these 
rims is unknown, but they could have formed from 
dissolution of highly soluble grains, or even ice. The 
amount of water that was evolved during stepwise py-
rolysis (~6.2 wt. %) is low relative to most other un-
heated CMs [12], but is unlikely to be due to heating as 
there is no evidence for dehydroxylation or recrystalli-
zation of serpentine in the fine-grained rims (Fig. 2b).  
Conclusions: Relative to CM carbonaceous chon-
drites, LEW 85311 has been mildly aqueously altered. 
The preservation of melilite demomstrates that it con-
tains nebular components that have been lost from 
most other meteorites by aqueous alteration, and fur-
ther work will assess whether this primitive material 
includes an amorphous precursor to the phyllosilicate-
rich matrix and rims.  
References: [1] Chizmadia L. J. and Brearley A. J. 
(2008) GCA, 72, 602–625. [2] Howard K. T. et al. 
(2009) GCA, 73, 4576–4589. [3] Howard K. T. et al. 
(2011) GCA, 75, 2735–2751. [4] Alexander C. M. 
O’D. et al. (2013) GCA, 123, 244–260. [5] Rubin A. E. 
et al. (2007) GCA, 71, 2361–2382. [6] Hewins R. H. et 
al. (2014) GCA, 124, 190–222. [7] Lee M. R. et al. 
(2016) GCA, 187, 237–259. [8] Choe W. H. et al. 
(2010) Meteoritics & Planet. Sci., 45, 531–554. [9] 
Alexander C. M. O’D. et al. (2015) Meteoritics & 
Planet. Sci., 50, 810–833. [10] Rubin A. E. (2015) 
Meteoritics & Planet. Sci., 50, 1595–1612. [11] Arm-
strong J. T. et al. (1982) GCA, 46, 575–595. [12] Lee 
M. R. et al. (2018) LPS, this volume. 
Acknowledgements: We thank the NHM London 
and NASA ANSMET for meteorite loans, and the U.K. 
STFC for funding. 
1287.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)


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Ephemeral Nebular Components in the Mildly Aqueously Altered CM Carbonaceous Chondrite LEW 85311

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