Samples and petrographic thin sections of the Orgueil, Ivuna, and Alais chondrites were examined by optical, X-ray, and SEM techniques. Mineral species identified as primary vein constituents were epsomite, gypsum, and a calcium-magnesium carbonate. Relative abundances and textural relationships have suggested that fracture mineralization was a multi stage process, with individual mineralizations closely associated with impact brecciation events. Mass balance considerations of carbonaceous chondrite matrix support the prevailing view that the source of the fracture filling minerals was local. By inference they also suggest that the phyllosilicate matrix has been chemically altered and that there are probably very few primitive mineral phases in the primitive Cl chondrites

Wood, J. A.

English

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Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=19770013017 2020-03-22T11:12:24+00:00Z
MINERALOGIC AND PETROLOGIC STUDY OF THE LOW—TEMPERATURE
MINERALS IN CARBONACEOUS CHRONDRITES
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GRANT NSG 7132
Final Report
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Principal Investigator
	
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Dr. John A. Wood
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The NASA Technical Officer for-this grant is Dr. Bevan M. French, Code SL,---
Office of Space Science, Lunar Planetary Programs, NASA Headquarters,
Washington, D.C. 20546.
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Final Report: Grant NSG 7132
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	 "Mineralogic and petrologic study of the low-temperature minerals
in carbonaceous chondrites"
I
With the support of grant NSG 7132, "Mineralogic and petrologic study of the low-
temperature minerals in carbonaceous chondrites," McSween, Richardson, and Wood have
i
investigated a number of chemical and mineralogical problems in the carbonaceous
i
fchondrites. We have attempted to place all of our studies, from those of low-temperature
phases in matrix and in fracture-fillings to those of high-temperature chondrules and
ii
,
inclusions, into a framework which allows us to model chemical processes in the early
i
^i	 solar nebula.
`I	 The type I carbonaceous chondrites are characterized by a network of narrow
mineralized fractures, which have long been known to contain hydrated magnesium sulfate.
One p'iase of the work supported by this grant involved a study of the individual mineral
i.
species in these veins, in an attempt to understand the mineral deposition sequence and
the effect on the late-st^t e chemical environment on the meteorite parent body. Samples
and petrographic thin sectio p 7 of the Orgueil, Ivuna, and Alais chondrites were examined
}i
by optical, x-ray, and SEM techniques. Mineral species identified as primary vein
t;
constituents were epsomite (MgSO4-' li20), gypsum (CaSOq.2H 2O), and a calcium -magnesium
carbonate. Relative abundances and textural relationships have suggested that fracture
mineralization was a multi-stage process, with individual mineralizations closely
;r
associated with impact brecciation events. Mass balance considerations raised by c,bser-
'" o
	
vations in this study and in our subsequent investigation of carbonaceous chondrite
matrix (see below) support the prevailing view that the source of the fracture-filling
^ i
	
	minerals was local. By inference they also suggest that the phyllosilicate matrix has
been chemically altered and that there are probably very few "primitive" mineral phases
in the "primitive" Cl chondrites. This work is currently being prepared for publication
(Richardson, 1977). Some preliminary observations and modeling were reported at the
1976 Spring meeting of A.G.U. (Richardson, 1976).
r?'i
2.
During the tenure of this grant, we also undertook a broad study of the ma:or
element ;hemistry of carbonaceous chondrite matrices. We borrowed petrographic thin
sections of 32 carbonaceous chondrites, representing all petrographic types, and
performed broad beam analyses of selected areas of matrix in each by electron micro-
probe. Statistical interpretations of the analytical data indicate that carbonaceous
chondrite matrices can be separated into chemical subgroups which .correspond closely
to petrographic-chemical subgroups for the bulk meteorites [i.e., Cl, C2, C3(V), C3(0)1.
We also infer the presence of a dispersed Ni-S-bearing phase in C2 matrix material,
and have been able to draw comparisons between our inferred phase and observed but
uncharacterized phases reported in other studies. The chemical effects of metamorphism
on C3(0) and C3(V) matrices were also studied in conjunction with a broader set of
investigations of metamorphism in C3 meteorites (see below). Metamorphosed matrices
were all shown to be depleted in magnesium relative to unmetamorphosed matrices. Con-
sideration of our analytical data in the light of currently known isotopic, chemical,
and mineralogical constraints on early solar system processes have led us to conclude
that matrix is chiefly a solar system condensation product which contains a small amount
of admixed interstellar dust. The results of this work were summarized at the 1976
annual meeting of the Meteoritical Society (Richardson and McSween, 1976), and have
recently been accepted for publicatirn in extended manuscript form (McSween and Richardson,
1977).
A systematic survey of the type III carbonaceous chondrites was also undertaken.
The two subtfetsses of C3 chondrites (the Ornans and Vigarano subtypes) were surveyed and
reasons for the petrographic and chemical variations observed were explored. Meteorites
of the Ornans subtype constitute a metamorphic sequence (McSween, 1976, 1977a). varia-
tions among chondrites of the Vigarano subtype are more complex; although some meteorites
are slightly metamorphosed, there appear to be several previously unrecognized subgroups
within this classification (McSween, 1977b).
___^—,__ _.._ _ --w,-----,—.-„-tea,---..-•°_-- - --- r .:—
After establishing which C3 chondrites have been altered by metamorphism, the
various components which comprise the unaltered meteorites were isolated and studied
individually. Matrix compositions were reported by McSwn_en and Richardson (1977), as
discussed previously. The petrographic and chemical properties of isolated olivine
grains in C3 chondrites were studied by McSween (1977c), who concluded that these grains
•	 were derived from broken chondrules, and thus represented crystallization from a melt
rather than condensation from a vapor. Similarities between olivine grains in C3
P
chondrites and those of Cl and C2 chondrites suggest a common origin.
The constraints imposed by petrographic and chemical features on the origin of
chondrules and inclusions were investigated by McSween (1977d, 1977e). The bulk compo-
sitions of inclusions follow the trajectory of equilibrium condensation, and chondrule
compositions in the same meteorites apparently define an extension of this trend.
Chondrule compositions cannot be easily rationalized in terms of dust fusion or impact
sampling models. A condensation model in which the compositional trajectory dips into
the liquid stability field at a certain point is advocated, Chondrule liquids can be
stabilized by altering the composition of the gas with which chondrules are in equili-
brium through dust fractionation (Wood and McSween, 1976). These studies thus lead to
the philosophically satisfying conclusion that all the components of C3 carbonaceous
chondrites are related by a common process.
o	
In the closing months of the grant a new project was begun, which will continue
	
1	 under the new meteoritic grant. The most widely used system for classifying chondritic
	
II '	 meteorites was developed about 10 years ago by Van Schmus and Wood (1967). This system
puts chondrites into a matrix determined by the composition (primarily iron content)
and the metamorphic history of the meteorite. Approximately 80% of the known meteorites
	 f
are chondrites, and only about half of these
i
have been classified by the Van Schmus and
Wood system.	 We have searched the meteorite literature, and have catalogued the known
chondrites (by name; place and date of fall or find; classification and any controversy 	 aL
)
3.
4.
concerning the same; and possible sources of sample material for our study). In the
I
near future this catalogue will be circulated to the international meteoritic communi-
	 j
ty in an attempt to solicit additional classifications and/or material for study in
this laboratory. There are many anomalous meteorites among the chondrites, and we
hope that an examination of the unclassified or controversially classified chondrites
f
will lead to additional knowledge about their formation and possible evolution.
References
McSween H.Y. (1976) Carbonaceous chondrites of the Ornans subtype, paper and abstract
	 j
presented at the 39th Meeting of the Meteoritical Society, Bethlehem, Pa.
	 lj
McSween H.Y. (1977a) Carbonaceous chondrites of the Ornans subtype. a metamorphic
	 i
sequence. Geochim. Cosmochim. Acta, in p ress.
McSween H.Y. (1977b) Petrographic variations among carbonaceous chondrites of the
Vigarano subtype, Geochim. Cosmochim. Acta, submitted.
McSween H.Y. (1`.^,70 On the nature and origin of isolated olivine grains in carbonaceous
chondrites. Geochim. Cosmochim. Acts, vol. 41, pp. 411-418.
McSween H.Y. (1977d) Chemical and petrographic constraints on the origins of chondrules
and inclusions in carbonaceous chondrites, in preparation.
McSween H.Y. (1977e) The bulk composition of refractory inclusions in carhonaceous
a
chondrites. Lunar Science VIII, Lunar Sci. Inst., 652-654.
McSween H.Y. and Richardson S.M. (1977) The composition of carbonaceous chondrite matrix.
Geochim. Cosmochim. Acta, in press.
Richardson S.M. (1976) Paragenesis of vein sulfates in the Orgueil (Cl) carbonaceous
chondrite [abstr.7. Trans. Am. Geophys. Union, vol. 57, p. 277.
Richardson S.M. (1977) Paragenesis of vein minerals in carbonaceous chondrites, in
preparation.(
i.A
Richardson S.M. and McSween H.Y. (1976) The matrix composition of carbonaceous chondrites.
Paper and abstract presented at the 39th annual meeting of the Meteoritical Society,
Bethlehem, Pa.
Van Schmus, W.R. and Wood J.A. (1967) A chemical-petrologic classification for the
chondritic meteorites. Geochim. Cosmochim. Acta 31, pp. 747-765.
Wood J.A. and McSween H.Y. (1976) Chondrules as condensation products. Proc. I.A.U.
Colloquim No. 39, "Relationships between comets, minor planets, and meteorites,"
Lyon, France, in press.
n
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Mineralogic and petrologic study of the low-temperature minerals in carbonaceous chondrites

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