THE GLUE THAT HOLDS WORLDS TOGETHER: ABUNDANCE

46th Lunar and Planetary Science Conference (2015)
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THE GLUE THAT HOLDS WORLDS TOGETHER: ABUNDANCE, ORIGIN & SIGNIFICANCE OF
AMORPHOUS FE,MG SILICATE IN CARBONACEOUS CHONDRITES. K. T. Howard12, J. Davidson3,
C.M.O’D. Alexander3 and N. M. Abreu4. 1Kingsborough Community College. 2American Museum of Natural History. 3Department of Terrestrial Magnetism, Carnegie Institute of Washington. 4Earth Science Program, Penn State
DuBois. Email: [email protected]
Introduction: X-ray and crystallographically
amorphous Fe,Mg silicates, located in matrix, distinguish primitive chondrites from those that were significantly altered in asteroids and planets [1]. By understanding the origin of amorphous Fe,Mg silicates, we
can explore the origin of matrix, the material that
bound chondrules together to form planetesimals and
we can contribute a mineralogical perspective on the
chemical relationship between chondrules and matrix.
Position sensitive detector X-ray diffraction (PSDXRD) allows us to quantify the abundance of amorphous Fe,Mg silicates, transmission electron microscopy (TEM) provides us with images, structural constraints and compositional data. With PSD-XRD large
sample volumes can be studied, enabling us to constrain how representative the TEM observations are.
Amorphous Fe,Mg silicate appears to have been the
main component of the pristine matrix. Abundance and
compositional data indicate that after extraction of Mgrich components to form chondrules, amorphous silicate quenched from the residual of a single, chondritic,
reservoir.
Sample selection: Targeted for quantification of
modal abundances were samples of carbonaceous
chondrites (CCs) that TEM studies [e.g., 2,3,4] show
contain amorphous silicates in their matrices, ALHA
77307 (CO), Acfer 094 (C2-ung) and Adelaide (C2ung). We also report on an additional CR (MIL
090657) and 2 CO (MIL 090073, MIL 090010) samples from the Antarctic collection that we have determined are low petrographic types. These data are in
addition to abundances determined for another set of
COs [8] and CRs, including MET 00426 and QUE
99177 [5], which have been studied extensively by
TEM [6,7].
Methods: Abundances of phases present in
amounts >1 wt.% are determined by PSD-XRD, using
a pattern fitting technique [5]. In addition to crystalline
phases, we are able to quantify the total abundance of
Fe-bearing X-ray amorphous material [5]. Modal determinations are for 100 mg aliquots of larger of powdered samples.
Results: The mineralogy of Acfer 094 is approximated as olivine (37 vol.%), pyroxene (31 vol.%), sulfides (3.1 vol.%), Fe,Ni-metal (1 vol.%) and Febearing X-ray amorphous material (26 vol.%). ALHA
77307 is estimated to contain olivine (35 vol.%), py-
roxene (36 vol.%), sulfides (2.2 vol.%), Fe,Ni-metal
(1.3 vol.%) and Fe-bearing X-ray amorphous material
(16 vol.%). In Adelaide we resolve olivine (32 vol.%),
pyroxene (45 vol.%), sulfides (4 vol.%), Fe,Ni-metal
(<0.5 vol.%) and Fe-bearing X-ray amorphous material
(16 vol.%). Olivine compositions are heterogeneous
and range from Fo100-Fo40, the main population of
olivine is forsterite (>Fo90). In Acfer 094 and ALHA
77307, there are roughly equal parts enstatite and clinoenstatite. In the new CR and CO samples studied
here, the abundance of Fe-bearing amorphous material
(11-23 vol.%) is similar to the CRs, QUE 99177 (15
vol.%) and MET 00426 (25 vol.%) [5] and also to the
type 3.0 CO samples reported previously [8].
Identifying amorphous Fe-Mg-silicate. Amorphous
materials in CCs are not restricted to Fe,Mg silicates,
Fe-oxides, FeO(OH),±H2O and sulfides can also be Xray amorphous. In addition, the amorphous silicate is
not a homogenous material. Extensive TEM studies of
CR and CM matrix show that it is variably hydrated
and contains nano-crystallites of many different phases, along with organics [6,7]. Even though we are careful to study interior chips from samples with low
weathering grades, rusts may represent a component of
the Fe-bearing amorphous material that we resolve, but
petrography excludes its presence in amounts as large
as 25 vol.%. Petrography and CC bulk and matrix
compositions also exclude 25% of the bulk sample
mineralogy from being amorphous sulfide. Therefore,
we can be confident that the majority of amorphous
material that we detect is Fe,Mg silicate.
Petrographic context. Chondrules also contain
glassy, mesostasis material, most often its composition
is similar to plagioclase [9]. With a maximum FeO
content of a few percent [9], chondrule mesostasis will
not contribute significantly to residual X-ray counts
from fluorescence. This gives us confidence that the
majority of amorphous Fe,Mg silicate that we detect by
XRD is in matrix (including fine grained rims on
chondrules), consistent with TEM [6]. If it is assumed
that all of the detected amorphous Fe,Mg silicate is in
matrix, the proportion of matrix that is amorphous
Fe,Mg silicate is determined to range from 30-80
vol.%.
Discussion: Since first studied by TEM, matrix in
the most primitive meteorite samples (e.g., Acfer 094)
has been described as dominated by amorphous mate-
46th Lunar and Planetary Science Conference (2015)
rials [2,3,6,7]. Quantifying the abundance of amorphous silicates magnifies the significance of the TEM
observations and the need to explain this material.
Mg/Si and the origin of amorphous Fe,Mg silicate.
In Fig. 1, amorphous Fe,Mg silicate defines a trend
connecting the Mg/Si composition of average bulk
chondrules with that of average matrix, the average
matrix Mg/Si composition being approximately equal
to that of average amorphous silicate. If Mg-rich components in chondrules are first to crystallize, extracting
Mg from the bulk reservoir, the composition of the
residual would evolve along a path towards Mgdepletion (i.e., fayalite), as the trend defined by the
amorphous Fe,Mg silicate compositions does.
Chemical relationship to chondrules. Amorphous
silicates have sub-chondritic Mg/Si ratios. In Fig. 1, a
linear mixing line has been constructed between average chondrules (super-chondritic Mg/Si) and average
amorphous silicate. Addition of 25-30 wt.% amorphous Fe,Mg silicate (≤ the upper limit of the amounts
detected), to the bulk chondrule compositions [10],
gives a chondritic bulk Mg/Si ratio, but at higher absolute Mg and Si abundances than measured for bulk CR
and CO samples [10]. However, these bulk chondrule
data are only for silicates and the bulk rock data includes Fe from metal and sulfide in matrix [10]. Therefore, only the Mg/Si ratio can be used to explore evidence for a genetic relationship [10].
The Mg/Si ratios suggest that amorphous silicates
formed in the nebula by the same type of heating event
that formed chondrules. This interpretation appears
contrary to available O-isotopic data from pristine CR
matrix [11], but we are not suggesting that all matrix
materials are by-products of chondrule formation.
Matrix is a mixture of materials from different origins,
including phases (i.e., organics, presolar grains) that
did not undergo the kind of high-temperature processing that formed chondrules. The conclusion that
pristine CR matrix is a distinct component, formed
separately from chondrules [11], is based on one of
three bulk matrix analyses for QUE 99177 plotting
outside the field of type I chondrules. In light of the
diverse origins of matrix, the fact that not all matrix Oisotope analyses are collinear with type I chondrules is
not surprising and does not necessarily place constrains
on the origins of individual matrix components.
Secondary processing of amorphous silicates.
Amorphous Fe,Mg silicate was initially assumed to be
highly reactive to water [6]. However, TEM shows that
amorphous silicate still exists in the matrices of highly
altered samples, including the CM2 samples Y 791198
[12] and Paris [13] and the CR1 GRO 95577 (this
study). This may be explained by experiments showing
that high Fe content ≠ high reactivity [14]. Structurally
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bound OH may also stabilize the material in the presence of fluid [7]. In the event of its hydration, amorphous Fe,Mg silicate is predicted to form Fe-serpentine
[6]. CM2 chondrites contain ~35 vol.% Fe-serpentine,
this could be produced by hydration of ~20 vol.%
amorphous silicate (assuming a 40% increase in volume results from hydration). Thermal metamorphism
of amorphous Fe,Mg silicate can form ferrous olivine
[15]. Thermally metamorphosed samples, e.g., CVs,
contain between 14-28 vol.% ferrous olivine (<Fo60)
[16]. These values agree closely with the bulk abundance of amorphous Fe,Mg silicate in pristine samples.
Overlapping Mg/Si ratios in all of these phases mean
that speculated alteration pathways would preserve the
primary sub-chondritic Mg/Si ratio of amorphous silicate, provided that mobilization of Mg and Si was limited [c.f., 17].
Conclusion: Abundant amorphous Fe,Mg silicate
is not a just feature of anomalous primitive meteorites
[1], it is a signature of type 3.0 matrix. At the onset of
CC accretion, amorphous Fe,Mg silicates must have
dominated the fine-grained material. Therefore, we
must not only ask what nebular processes formed
mafic chondrule phenocrysts, but also under what nebular conditions crystal growth was inhibited.
References: [1] Nuth J. A. et al. (2005) Chondrites
and the Protoplanetary Disk, 675–700. [2] Greshake
A. (1997) GCA 61, 437–452. [3] Brearley A. J. (1993)
GCA 57, 1521–1550. [4] Brearley A. J. (1991) LPS
22nd, p. 133. [5] Howard K. T. et al. (2015) GCA 149,
206–222. [6] Abreu N. M & Brearley A. J. (2010)
GCA 74, 1146–1171. [7] Le Guillou C. & Brearley A.
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(2014) LPS 45th Abstract #1830. [9] Kurahashi E. et al.
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R. H. et al. (2014) GCA 124, 190–222. [14] Chizmadia
L. J. et al. (2006) LPS 37th, Abstract #2187. [15] Huss
G. R. et al. (2006) MESS 2, 567–586. [16] Howard K.
T. et al. (2010) GCA 74, 5084–5097. [17] Bland P. A
et al. (2005) PNAS 102, 13755–13760.
Bulk CO chondrules [10]
forsterite
Bulk CR chondrules [10]
Mg (%)
Amorphous Fe,Mg silicate
in MET 00426 [6]
enstatite
Amorphous Fe,Mg silicate
in MET 00426 [7]
CR Matrix [10]
CO Matrix [10]
+
Bulk CR [10]
CI Mg/Si ratio
fayalite
Si (%)
Fig. 1.
Mixing line b/w avg. chondrules
and avg. amorphous silicate
Mixing line b/w Mg-serpentine
and Fe-serpentine