Mineralogy and Petrology of Amoeboid Olivine Inclusions in CO3 Chondrites: Relationship to Parent-Body Aqueous Alteration

16 Pages·2003·13.7 MB·English

by  Chizmadia, Lysa J.

Mineralogy and petrology of amoeboid olivine inclusions in C03 chondrites:
Relationship to parent-body aqueous alteration
LYSAJ . CHIZMADIAlt*, ALANE . RUBIN2 AND JOHNT . WASSONLV
'Department of Earth and Space Sciences, University of California, Los Angeles, California 90095-15 67, USA
2Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095-15 67, USA
3Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, USA
tPresent address: Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 8713 1, USA
*Correspondence author's e-mail address: [email protected]
(Received 2001 June 6; accepted in revised form 2002 August 20)
Abstract-Petrographic and mineralogic studies of amoeboid olivine inclusions (AOIs) in C03
carbonaceous chondrites reveal that they are sensitive indicators of parent-body aqueous and thermal
alteration. As the petrologic subtype increases from 3.0 to 3.8, forsteritic olivine (Fao-1) is
systematically converted into ferroan olivine (Fa60-75). We infer that the Fe, Si and 0 entered the
assemblage along grain boundaries, forming ferroan olivine that filled fractures and voids. As
temperatures increased, Fe+2 from the new olivine exchanged with Mg+* from the original A01 to
form diffusive haloes around low-FeO cores. Cations of Mn+2, Ca+2 and Cr+3 were also mobilized.
The systematic changes in A01 textures and olivine compositional distributions can be used to
refine the classification of C03 chondrites into subtypes. In subtype 3.0, olivine occurs as small
forsterite grains (FaGI), free of ferroan olivine. In petrologic subtype 3.2, narrow veins of FeO-rich
olivine have formed at forsterite grain boundaries. With increasing alteration, these veins thicken to
form zones of ferroan olivine at the outside A01 margin and within the A01 interior. By subtype 3.7,
there is a fairly broad olivine compositional distribution in the range Fa63-70, and by subtype 3.8, no
forsterite remains and the high-Fa peak has narrowed, Fa64-67. Even at this stage, there is incomplete
equilibration in the chondrite as a whole (e.g.,d ata for coarse olivine grains in Isna (C03.8) chondrules
and lithic clasts show a peak at Fa39). We infer that the mineral changes in A01i dentified in the low
petrologic types required aqueous or hydrothermal fluids whereas those in subtypes 23.3 largely
reflect diffusive exchange within and between mineral grains without the aid of fluids.
INTRODUCTION Kerridge (1 972) suggested that iron mobilization in chondrule
olivine phenocrysts along cracks was due to fluid alteration.
McSween (1977a) was the first to organize the C03 Kojima et al. (1995) ascribed similar iron enrichment in
UI chondrites into a metamorphic sequence. He defined stages 1-111 amoeboid olivine inclusions (AOIs) of Antarctic C03
on the basis of the degree oftextural recrystallization, decreasing chondrites to aqueous processes.
I
, heterogeneity in mafic mineral compositions and increasing FeO Rubin (1 998) noted that several systematic changes of C03
enrichment in the mafic minerals. With increasing chondrite properties with increasing petrologic subtype could
metamorphism, the amount of chondrule glass and the be interpreted as evidence of fluid alteration. These include
abundances of metallic grains, presolar diamonds, bulk volatiles the erosion of chondrule surfaces, the formation of
and noble gases decrease (McSween, 1977a,b; HUSS, 1990). phyllosilicates and hydrous ferric oxides (Ikeda, 1983; Keller
Scott and Jones (1990) refined the metamorphic series and and Buseck, 1990; Brearley, 1993), an increase in the apparent
defined subtypes 3.0-3.7 on the basis of igneous zoning in percentage of "rimmed" AOIs, a decrease in the abundance of
chondrule phenocrysts and increasing FeO concentration at the small chondrules, and an increase in the whole-rock A170
edges and cracks of these phenocrysts. Keck and Sears (1987) (=dl70 - 0.52 x ~3180)v alues, an apparent indication of
used peak thermoluminescence (TL) temperatures to classify increasing degree of alteration by water having higher A170
the C03 chondrites into subtypes that differed slightly from (Clayton, 1993; Clayton and Mayeda, 1999; Clayton et al.,
those of Scott and Jones. 1976; Hiyagon and Hashimoto, 1999).
Several authors have attributed some of the alteration of Amoeboid olivine inclusions in C03 chondrites are
C03 chondrites to hydrothermal processes. For example, irregularly shaped, fine-grained objects that constitute a few
1781
PvelLcde preprint MS#4566 Q Meteoritical Society, 2002. Printed in USA.
1782 Chizmadia et al.
volume-percent of individual meteorites in most carbonaceous and Grossman, 1987). Kornacki and Wood (1984) defined
chondrite groups. They consist of major olivine and minor two types ofAOIs in Allende, "rimmed" and "unrimmed", based
amounts of moderately refractory minerals such as anorthitic on the presence or absence of coarse ferroan olivine near the
plagioclase, aluminum- and titanium-bearing diopside, and in A01 margin. Broad-beam electron probe analyses of AOIs in
some cases, spinel (MgA1204) (Fig. 1). Low-Ca pyroxene is C03 chondrites by McSween (19 77b) showed that the FeO,
absent in all subtypes. MgO and MnO contents vary with whole-rock metamorphic
It is important to note that there are different names for what subtype. In a study of AOIs in reduced CV3 chondrites,
we call AOIs. Some researchers call them amoeboid olivine Komatsu et al. (2001) found a ferroan enrichment pattern
aggregates (AOA), but AOIs are relatively compact objects similar to the one we report below, which they attributed to
whereas the term "aggregate" implies high-intergranular secondary thermal metamorphism. They also ascribed the
porosity. Interlocking grains are common in AOIs in C03.0 anorthite-diopside-spinel assemblages in the A01 to I
chondrites. We, therefore, prefer the more neutral, less model- incorporated refractory inclusions.
dependent term, AOIs. The AOIs in C03 chondrites are similar to those in CV3
Most past A01 studies were of inclusions in the Allende chondrites in terms of mineralogy, abundance, shape and
CV3 chondrite (e.g., Grossman and Steele, 1976; Hashimoto texture. However, the AOIs in C03 chondrites are much
"
FIG. 1. Backscattered electron images of the relationships between anorthite and diopside in C03 amoeboid olivine inclusions. In every case,
these phases are surrounded by olivine. (a) Isolated grains of diopside and anorthite in C03.7 Warrenton. (b) Diopside (light gray) rimming
anorthite (medium gray) in C03.0 ALHA77307. (c) Intergrown diopside (medium gray) and anorthite (dark gray) in C03.2 Y-82050.
(d) Intergrown diopside (medium gray) and anorthite (dark gray) in C03.8 Isna.
Mineralogy and petrology of amoeboid olivine inclusions in C03 chondrites 1783
smaller and have smaller grain sizes than their counterparts in origin that have rounded outlines were counted as voids. The
CV3 chondrites. sizes of the inclusions, constituent phases and voids were
To document the role of alteration, we investigated AOIs in measured on the BSE images.
12 C03 chondrites that span the range of petrologic subtypes.
C03 AOIs are fine-grained and thus have high surface-area/ RESULTS
volume ratios and small lattice-diffusion path lengths. These
characteristics lead to the expectation that AOIs should be Mineralogy and Petrology of Amoeboid Olivine Inclusions
affected in the early stages of metamorphic or aqueous-alteration
processes (McSween, 1977a) and should be sensitive indicators AOIs constitute 7-16 vol% of the C03 chondrites (Table 4
of such alteration. AOIs are present throughout the C03 of McSween, 1977a) and range in size from -20pm to -1 mm.
I
metamorphic sequence, and it should be possible to use their The petrographic and mineralogic characteristics of AOIs
textural and mineralogical characteristics to quantify C03 change with petrologic subtype. In all C03 subtypes AOIs are
alteration. fine-grained and have irregular shapes, but these properties
change systematically with subtype. AOIs in C03.0 chondrites
ANALYTICAL METHODS tend to have somewhat angular outlines; some are moderately
equant (Fig. 2). AOIs in subtypes 23.4 tend to have scalloped
Polished thin sections of 12 C03 chondrites (Table 1) were (amoeboid) morphologies (Fig. 2). Some AOIs have
studied microscopically in transmitted and reflected light. Five embayments at their surfaces that enhance their irregular
AOIs in each meteorite were analyzed for 10 elements with the morphologies. In C03.4-3.8 chondrites, chondrules and
Cameca Tamebax-microbeam" electron microprobe at UCLA refractory inclusions that are in the same size range as AOIs
using crystal spectrometers, natural mineral standards, 20 s exhibit fewer irregular surface features and do not have
counting times, PAP corrections (the Cameca version of ZAF amoeboid outlines. It is difficult to measure the sizes of A01
corrections), a sample current of -1 1 nA and an acceleration silicate grains because most grains are appreciably smaller than
voltage of 15 kV. Backscattered electron (BSE) images were the -30pm thickness of the thin sections. Nevertheless, based
made of each A01 at a range of magnifications with the LEO on BSE images of ferroan olivine outlines in AOIs from C03.3-
1430vP scanning electron microscope (SEM) at UCLA using 3.5 chondrites and olivine crystal surfaces seen invoids, we estimate
a 15 kV acceleration voltage and a working distance ranging indwidual C03 A01 olivines to range in size from -2 to 10 pm.
from 17 to 20 mm. In C03.0 chondrites, the best-documented AOIs (Table 2)
The BSE images were used to determine the modal are about 100-500 pm in mean diameter. AOIs in Yamato
abundances of low-FeO olivine, ferroan olivine, anorthitic (Y)-8 1020 (the least-equilibrated C03 chondrite) contain
plagioclase, diopsidic pyroxene, spinel, voids and opaque about 8-20 vol% voids that range from about 0.5 to 60 pm in
minerals. A 1 x 1 cm acetate grid was laid over the images and size. As discussed above, it is probable that some of the voids
the phase (or void) at each intersection was tabulated. Holes resulted from plucking during thn-section preparation. Voids
clearly due to pluclung were excluded, but holes of ambiguous tend to be rounded with olivine crystal edges protruding into
them. No glass was observed in AOIs in C03.0 chondrites or
TABLE1 . C03 chondrites and section numbers. in those of higher petrologic subtype.
On a void-free basis, AOIs in C03.0 chondrites contain
Meteorite Subtype Thin section 63.6 f 8.5 vol% olivine, 16.7 f 6.4 vol% anorthitic
*
plagioclase, 12.0 2 4.5 vol% Al- and Ti-bearing diopsidic
Yamato-8 1020 3.0 NIPR 56-4
pyroxene, and 7.7 & 5.0 vol% opaque phases having a range
Allan Hills A77307 3.0 ALHA773 07,8 8
from 1.7 to 13.9 vol% and a peak at 2-4 vol% (Table 2). A
Colony 3.0 USNM 6264-3
minor fraction of the AOIs also contain spinel. Opaque grains
Kainsaz 3.2 USNM 2486-8
Rainbow 3.2 LC 998 in C03.0 AOIs are composed of metallic Fe-Ni (mainly
Yamato-82050 3.2 NIPR 101-1 kamacite); troilite is rare to absent. The metal grains range in
Felix 3.3 USNM 235-1 diameter from 4 to -8 pm.
Ornans 3.4 USNM 1105-7 The outermost regions of the C03.0 AOIs are composed of
LancC 3.5 USNM 380-1 olivine; olivine also commonly surrounds anorthite-diopside
Allan Hills A77003 3.6 ALHA77003,115 patches in the A01 interior. Olivine in C03.0 AOIs is forstentic,
Warrenton 3.7 USNM 43 Fa0-3. Because of some enhancement of measured FeO values
Isna 3.8 USNM 5890-1
due to oxides produced during terrestrial weathering, we suggest
that the initial (i.e.,n ebular) range in mole percent Fa was
Abbreviations: NIPR = National Institute of Polar Research, Japan;
narrower (ie.,F awl).
USNM = National Museum of Natural History, Smithsonian
Anorhte and diopside are ubiquitous in AOIs from all C03
Institution; LC = UCLA Leonard Collection; ALHA sections from
subtypes. These phases tend to form irregular and interlocking
NASA Johnson Space Center.
1784 Chizmadia et al.
t
FIG.2 . Backscattered electron images ofthe irregular surfaces of amoeboid olivine inclusions. (a) A01 kom C03.0 Colony. The A01 is much darker
gray than the adjacent portion ofthe matrix. (b) A01 from C03.0 ALHA77307. (c) A01 from C03.4 Omans. (d) A01 from C03.3 Felix. (e) A01
from C03.6 ALHA77003. The inclusion-matrix boundary is outlined in black. (f) A01 from C03.7 Warrenton. In C03.0 chondrites, the edges of
AOIs appear to reflect fragmentation as opposed to the rounded "amoeboid" shapes present in CO chondrites that experienced more alteration.
Mineralogy and petrology of amoeboid olivine inclusions in C03 chondrites 1785
TABLE2 . Sizes of selected AOIs and modal abundances of their constituent phases.
Meteorite A01 # Size Olivine Anorthite Diopside Voids Opaques
@m) (vol%) (vol%) (vol%) (vol%) (VOW)
Yamato-8 1020 1 IO6 x 153 50.4 22.7 15.1 8.4 3.4
2 413 x 538 53.1 6.2 6.2 21.9 12.5
4 155 x 329 47.7 17.4 11.9 12.8 10.1
5 364 x 536 56.8 9.5 8.4 20.0 5.3
7 171 x400 48.6 17.1 10.5 20.0 3.8
Allan Hills A77307 1 211 x 389 32.9 14.1 11.8 36.5 4.7
2 188 x 270 45.3 12.8 9.4 30.8 1.7
3 156 x 236 45.9 18.8 13.5 19.5 2.3
4 413 x 638 52.3 19.8 15.1 5.8 7.0
5 103 x 215 54.0 16.0 10.0 18.0 2.0
Colony 1 132 x 203 50.6 6.2 4.9 34.6 3.7
2 68 x 144 53.3 8.2 4.1 20.5 13.9
4 74x 122 64.5 6.0 4.1 16.2 9.2
Omans* 6 306 x 375 72.8 2.6 6.1 13.2 3.5
Lance* 13 39 x 173 56.3 16.9 4.2 15.5 4.2
Warrenton 1 529 x 757 47.1 17.3 11.5 23.1 1. o
2 250 x 488 41.1 24.0 20.0 14.0 0.9
4 289 x 529 44.7 16.7 17.5 20.2 0.9
5 282 x 306 58.1 18.8 12.8 10.3 0.0
6 524 x 1029 64.0 12.0 8.8 15.2 0.0
Isna 1 137 x 457 47.1 14.3 10.9 27.7 0.0
3 240 x 287 55.3 11.7 7.8 25.2 0.0
4 122 x 141 68.5 14.4 7.2 9.9 0.0
5 206 x 382 65.6 16.4 11.5 4.9 1.6
6 179 x 385 49.5 22.0 17.6 8.7 2.2
*Omans-6 and Lance-13 also contain 1.8 and 2.8 vol% spinel, respectively
grain assemblages ranging from -5 to 60 pm (Fig. 1). In some of anorthte. In the most altered AOIs (Fig. la,d), there are discrete
assemblages, diopside rims anorthite; in others, the phases are grains of diopside and anorthite.
intergrown. Backscattered electron images show that individual In the most altered C03 chondrite (subtype 3.8, ha), all
grains of anorthite and diopside in AOIs are rounded and range of the A01 olivine is FeO-rich (Fa6447); in chondrites that
in diameter from<l to -3 pm in the least (3.0) and mostaltered have experienced intermediate degrees of alteration (e.g., Felix,
(3.7-3.8) subtypes (Le., there does not appear to be a systematic Ornans, Land), the olivine compositional distributions are
increase in diopside and anorthite grain size with increasing bimodal (e.g.,F a3-16; Fa62-70).
c
subtype). From the BSE images it appears that each of the Diagrams of minor elements in olivine (Le., wt% MnO,
assemblages of anorthite and diopside are composed of wt% CaO, wt% Cr2O3 vs. mol% Fa; Fig. 3) show that the
numerous smaller crystallites. MnO, CaO and Cr2O3 contents in A01 olivine in C03
On a void-free basis, the best-documented AOIs in C03.3- chondrites of low petrologic subtype are quite variable
3.8 chondrites contain 66.5 ? 10.6 vol% olivine, 18.6 & 6.2 vol% whereas those in A01 olivine of high petrologic subtype are
anorthtic plagioclase, 13.5 & 5.8 vol% Al- and Ti-bearing much more uniform.
diopsidic pyroxene, and 1.4 -+ 1.7 vol% opaque phases (Table 2). AOIs typically contain several volume-percent opaque
Spinel (MgA1204), pleonaste ((Fe,Mg)A1204), and a Ca-P minerals. The observed phases include blebs of kamacite,
phase (probably merrillite) also occur in some of the AOIs. taenite and troilite. Troilite is rare to absent in AOIs from C03
The principal difference in modal mineralogy between AOIs chondrites of subtype 3.0 and is appreciably more common in
in C03 chondrites of different subtypes is in the abundance of AOIs from more-altered C03 chondrites (subtype 3.4-3.8).
opaque phases which are particularly low in subtype 3.7-3.8. Metallic Fe-Ni grains occur in AOIs from all petrologic
The texture of the anorthite-diopside intergrowths changes with subtypes. Opaque grains typically range in diameter from 4
increasing subtype (Fig. 1). In the least altered AOIs the anorthte to -10 pm, although a few rare grains in AOIs from the higher
is completely enclosed within the diopside (Fig. 1b ). In moderately subtypes range up to -25 pm. These large opaque grains are
altered AOIs (Fig. IC), diopside contains roughly linear patches almost exclusively troilite.
1786 Chizmadia et al.
$
0.3
1
0.2
0.1
0
0.8
Y-81020
0.7 1 ALHA77307
A Colcoy
Kaimaz
0.6
v Y-82050
x Rainbow
0.5 i- Felix
n0 Omans
$ Lance'
0.4 A ALHA77003
B 0 Warrenton
v 0.3
to 0
0.2
0.1
0
0.7
0.6
0.5
hs
0.4
$
v
an 0.3
uL-
0.2
0.1
0
4 0 5 0 60 70 80
ml%F a mol% Fa
FIG. 3. Minor element concentrations (wt%) vs. mol% Fa in A01 olivine grains. All plotted points are fiom different olivine grains. Individual
meteorites are plotted with distinct symbols. (a,b) MnO vs. Fa in low-FeO and high-FeO olivines, respectively. (c,d) CaO vs. Fa in low-FeO and
high-FeO olivines, respectively. (e,f) Cr2O3 vs. Fa in low-FeO and high-FeO olivines, respectively. With increasingp etrologic subtype, corresponding
to increasing degrees of alteratiodmetamorphism, the variability in minor element concentrations decreases as equilibrium is approached.
Spinel-Bearing Amoeboid Olivine Inclusions The Ornans A01 is an irregularly shaped 306 x 375 pm
object that contains many cracks and voids (Fig. 4a). The left
Several AOIs containing spinel have been identified in Y- side of the inclusion is completely normal; it consists of olivine,
8 1020, Y-82050, Felix, Ornans, Lance, Allan Hills diopside and anorthite with typical textural relationships. The
(ALH)A77003 and Warrenton. Two spinel-bearing AOIs (one amount of alteration in this A01 is the same as that found in
each from Ornans and Lance) are illustrative of the phase other AOIs in Ornans. Low-FeO olivine grains are adjacent to
relationships (Fig. 4). -0.7 to 1.3 pm thick veins of ferroan olivine (see below). At
Mineralogy and petrology of amoeboid olivine inclusions in C03 chondrites 1787
FIG.4 . Backscattered electron images of spinel-rich A01 Omans-6 and LancC-13. (a) The entire A01 from C03.4 Omans. Elongated spinel
laths (light gray) occur within the right side of this inclusion. (b) Enlargement of one of the spinel laths. The spinel grain (medium gray) is
surrounded by anorthite (black) that is itself surrounded by diopside (dark gray), indicative of the crystallization sequence. (c) The entire A01
, from C03.5 LancC. The spinel-rich patches occur in the center of this inclusion. (d) Enlargement of one of the spinel-rich patches. The
spinel (light gray) is surrounded by anorthite (dark gray) that is itself surrounded by diopside (medium gray). The degree of olivine alteration
in these AOIs is similar to those of non-spinel-bearing AOIs in these same meteorites.
least 12 spinel grains (constituting 2 vol% of the whole AOI) The Lanc6 A01 is an elongated, 39 x 173 pm sized, spinel-
are located on the right side of the inclusion. Most spinel grains bearing object (Fig. 4c) containing many voids (15.5 ~01%)
are elongated (Fig. 4b), ranging in maximum dimension from and cracks. Most of the olivine in the inclusion is ferroan with
-5 to 47 pm, appreciably larger than typical individual olivine the low-FeO olivine concentrated mainly in the left part of the
grains in the AOI. Spinel is rimmed by anorthite that is itself inclusion. In this portion of the A01 there is a rounded 47 x
mantled by diopside. The anorthite layers range from -0.9 to 52 pm object that superficially resembles a type-I porphyritic
2.7 pm in thlckness; the thickness of the outer diopside rims olivine chondrule. It consists of a dark low-FeO olivine core
range from -0.4 to 6 pm. The anorthite and diopside surrounded by anorthite.
assemblages are about 5-60 pm in size, similar in texture to Spinel-anorthite-diopside assemblages occur in the center
those in non-spinel-bearing AOIs. of this inclusion (Fig. 4d). The spinel is mantled by anorthte
1788 Chizmadia et al
that is itself rimmed by diopside. There is a larger concentration 12
of anorthite and diopside toward the right part of the A01 where 0 Y-81020
spinel is absent. There are small (-4 pm size) grains of a Ca-P IO A0 COomloannys
phase (probably merrillite). U Lance'
The spinel in AOIs tends to increase in FeO content with
68
increasing petrologic subtype (Fig. 5). Russell et al. (1998)
found a similar trend in the spinel grains within C03 refractory
inclusions and attributed it to thermal metamorphism. If parent-
body alteration processes are responsible for the FeO increases
N
in the olivine of AOIs and type-I chondrules, it is plausible that
64
these processes are also responsible for the FeO increases in
spinel. 0 0 1
Using Amoeboid Olivine Inclusion Observations to
Refine the C03 Chondrite Subtype Classification 6 0 t " " " " ' " " ' " " ' " " ' " "
0 10 20 30 40 50 60
Several investigators have devised classification schemes Hercynite (mol %)
for the C03 chondrites. McSween (1977a) assigned six C03
chondrites to three metamorphic grades on the basis of bulk FIG.5 . A01 spinel compositions plotted as wt% AI203 vs. mol%
hercynite (FeA1204) in four C03 chondrites. The hercynite
olivine and pyroxene compositions in chondrules. The Scott
component of the spinel is represented by the ratio FeO/(FeO + MgO).
and Jones (1990) subtype classification is based on the FeO With increasing alteration/metamorphism,F eO/(FeO + MgO)
content and igneous zoning in chondrule olivines and pyroxenes increases as A1203 decreases. This compositional trend is similar to
in 11 C03 chondrites. The Keck and Sears (1987) subtype that of FeO/(FeO + MgO) vs. Si02 in olivine. The cluster near Hc50
classification is based on peak TL temperatures of whole-rock may be an alteration phase rather than an altered nebular product.
samples of nine C03 chondrites. Kojima et al. (1995) based
their classification on FeO in chondrule olivines in three maps of Grossman and Rubin (1999) and J. N. Grossman (2000,
Antarctic C03 chondrites. Table 3 summarizes these pers. comm.). These maps show abundant S in the matrix of
classification schemes and presents the petrologic subtypes we the least-altered C03 chondrites (Y-81020 and ALHA77307)
recommend. and appreciably less S in the matrix of more-altered C03
The C03 subtypes recommended here range from3.0 (least chondrites (subtype >3.2).
equilibrated) to 3.8 (most equilibrated). The changes we We increased Kainsaz's subtype from 3.1 to 3.2 because
propose (Table 4) are based mainly on observations that A01 numerous ferroan olivine veins are present, in contrast to the
textures and mineral compositions reflect the substitution of complete absence of such veins in the AOIs from C03.0
Fe2+ for Mg*+ during whole-rock alteration more precisely than chondrites. We assigned Rainbow to the same 3.2 subtype on
do other C03 chondrite properties. The scheme is also based the basis of the abundance and thickness of the ferroan olivine
on the chondrule data of Scott and Jones (1990) and the S x-ray veins in its AOI. Bulk S distributions are similar in Rainbow
TABLE3 . Classification of CO3 chondrites by present and previous investigators.
Meteorite Recommended McSween Scott and Sears Kojima
(1 977a) Jones (1 990) et al. (1 99 1 b) et al. (1 995)
Yamato-8 1020 3.0 - - - 3.0
Allan Hills A77307 3.0 - 3.0 3.3 -
Colony 3.0 - 3.0 3.2 -
Kainsaz 3.2 I 3.1 3.5 -
Yamato-82050 3.2 - - - 3.1
Rainbow 3.2 - - - -
Felix 3.3 I1 3.2 3.4 -
Ornans 3.4 I1 3.3 3.5 -
LancC 3.5 I1 3.4 3.5 -
Allan Hills A77003 3.6 - 3.5 3.6 -
Warrenton 3.7 111 3.6 3.6 -
Isna 3.8 111 3.7 3.7 -
Mineralogy and petrology of amoeboid olivine inclusions in C03 chondrites 1789
TABLE4 . The A01 characteristics used to refine the C03 Variation of Amoeboid Olivine Inclusions with Increasing
petrographic subtypes. Petrologic Subtype
~
Petrographic Thicknesses of ferroan Thicknesses of Figure 7 shows BSE images of AOIs of different subtypes.
subtype olivine veins (urn) diffusive haloes (urn)
Olivine in AOIs from C03.0 chondrites (Y-81020,
ALHA77307, Colony) is forsteritic (Fao-3); virtually no ferroan
3.0 da nla
olivine is present (Fig. 7a,b). At petrologic subtype 3.2
3.1 <2 nla
(Kainsaz, Rainbow, Y-82050), forsteritic olivine is rimmed by
3.2 -2.5 rda
abundant -0.25 pm thick veins of ferroan olivine (Fig. 7c).
3.3 -4 -0.5
3.4 -6 -0.6 The ferroan olivine tends to conform to the shape of the outlines
< 3.5 10-15 -0.8 ofpre-existing cracks and grain boundaries. The ferroan olivine
3.6 20-30 2-3 veins thlcken systematically as the subtype increases (Fig. 7d-f)
3.7 rda 5-8 until subtype 3.7 (Warrenton, Fig. 7g) is reached. Here, the
3.8 da nla low-FeO olivine has been entirely replaced by ferroan olivine
~~ in all but the single largest A01 observed in this study (524 x
Abbreviations: da = not applicable. 1029 pm). In C03.8 Isna, the most altered C03 chondrite
(Fig. 7h), the olivine is entirely ferroan (Fa64-67).
and Kainsaz (J. N. Grossman, unpubl. data). We infer that As the ferroan olivine veins thicken and replace the low-
C03.1 chondrites would show incipient vein formation (ie., FeO olivine in subtypes 3.2-3.7, the boundaries between the
ferroan olivine veins would be present but appreciably lower low-FeO olivine and the ferroan olivine become more poorly
in abundance and thickness than those in Kainsaz, Y-82050 defined. At about subtype 3.3, diffusion halos develop around
and Rainbow). There are currently no known members of the relic low-FeO cores (Fig. 7d). With increasing subtype,
subtype 3.1. We reclassified Y-82050 as subtype 3.2 rather the FeO gradients become shallower (Fig. 7d-f). These halos
than 3.1 as originally proposed by Kojima et al. (1995). This separate the low-FeO olivine cores from the ferroan olivine
change is indicated by the olivine compositional distribution veins, particularly in C03.5 LancC and C03.6 ALHA77003
of AOIs in Y-82050 (Fig. 6): some grains with Fao-1 (Fig. 7e3. The halos are -0.5 pm thick in Felix (subtype 3.3),
compositions that are common in subtype 3.0 are still found in -0.8 pm thck in LancC (3.5) and -5 to 8 pm thick in Warrenton
subtype 3.2, similar to those found in Kainsaz and Rainbow. (3.7). It appears that the low-FeO olivine cores themselves
Furthermore, subtypes 3.3-3.8 have a peak at Fa60-75 whch is become richer in FeO with increasing petrologic subtype,
absent in Y-82050. typically from Fa<l in type 3.0 to Fa-10 in type 3.6 (Fig. 6).
We recommend that the subtypes of all the other C03 Although we analyzed the apparent centers of olivine grains
chondrites in this study be increased by 0.1 to reflect the full with the electron microprobe, because of uncertainties in the
spectrum of their alteration. For example, what was previously location of grain boundaries and the location of grain centers
classified 3.2 (Felix) should now be 3.3 because it shows relative to the plane of the section, many of the analyses are
systematically more advanced alteration than that shown by not of actual grain centers. Nevertheless, these data reveal
Kainsaz, Y-82050 and Rainbow. This trend should continue systematic changes in olivine composition with petrologic
into the higher subtypes so that Warrenton and Isna would now subtype (Fig. 6). In Y-81020 and ALHA77307 essentially all
be 3.7 and 3.8, respectively. We argue that these changes make the olivine compositions plot in low-FeO peaks (0-3 mol%
r our classification scheme less arbitrary than previous schemes Fa). About 20% of the olivine analyses in Colony are in the
because the new classifications of Warrenton and Isna are Fa12-15 range; because many olivine grains in Colony AOIs
consistent with the uniform olivine compositions in these contain several volume-percent limonite veins, we suggest that
,
meteorites, which are similar in their uniformity to those of these ferroan olivine compositions reflect contaminationb y iron
type 3.7-3.8 ordinary chondrites (e.g.,S ears et al., 1991a,b). oxides during the severe terrestrial weathering of this meteorite
We try to quantify the differences among C03 subtypes in the (Rubin et al., 1985). In Kainsaz and Rainbow, the low-FeO
following section. olivine peaks are between 5 and 10 mol% Fa, indicating that
Although we have no peak-temperature or duration-of- even these subtype 3.2 rocks have experienced appreciable
heating data to corroborate the C03 reclassification, our alteration. Felix (subtype 3.3), Ornans (3.4) and Lanct (3.5)
proposed refinement produces a classification scheme that show bimodal olivine compositional distributions representing
differs only slightly from those of Scott and Jones (19 90) and low-FeO olivine cores (3 to 10 mol% Fa) and ferroan olivine
Kojima et al. (1995). Their classifications are based on Mg/Fe veins and rims (58 to 75 mol% Fa).
ratios in type-I and type-I1 chondrules which have typical grain The olivine compositional distributions for Kainsaz,
sizes of 5-100 pm, and are thus much coarser grained than Rainbow and Y-82050 (Fig. 6) show intermediate Fa values
AOIs. The chondrules are therefore less sensitive to alteration between 10 and 45 mol% Fa. Because of the small size of the
than the smaller grains in AOIs. veins (-0.25 pm) relative to the electron beam (1-3 pm),t hese
1790 Chizmadia et al.
I
' 1 ' ' 1 '
I I I
Felix (3.3)
3
2
I
4
ALHA 77003 (3.6
4
n=21
3
2
I
0
12
9
6
3
0
-
-
8 20 lsna (3.8)
--
6 15 n=23
-
-
4 I O
2 5 --
0 0 I I 1 1 1 1 1 1 1 1
l ~ l ~ l ~ l ~ l ~ 1 ~
0 10 20 30 40 50 60 70
mole % Fayalite
FIG. 6. Compositional distributions of olivine (in mol% Fa) in AOIs in C03 chondrites. The petrologic sequence begins with all of the
olivine occurring as forsterite (as in the C03.0 chondrites Y-8 1020, ALHA77307 and Colony). The olivine grains in Colony with compositions
ofFa,&ls were probably oxidized due to terrestrial weathering. The subtype 3.2 chondrites have a broad olivine compositional distribution
that peaks at Fa3-10. As the petrologic type increases, a secondary peak develops at Fa6670 and no forsteritic olivine remains. At subtype 3.6,
the only peak is at Fa6670, but a long low-Fa tail remains. At subtype 3.8 (ha)o nly a narrow peak of ferroan olivine at Fa45 occurs.