Hibonite-bearing microspherules: A new type of refractory inclusions

0016.7037/91/13.00
Geochimica et CosmDchrmrca Ada Vol. 55, pp. 361-319
Copyright 0 1991 Pergamon Press pk.Printed
inIJ.S.A
Hibonite-bearing
+ .M)
microspherules: A new type of refractory inclusions
with large isotopic anomalies
T. R. IRELAND,*
McDonnell Center for the Space Sciences and
A. J. FAHEY,+and E. K. ZINNER
Physics Department, Washington University, St. Louis, MO 63 130, USA
(Received February 8, 1990; accepted in revisedform November I, 1990)
Abstract-Reported
are petrographic descriptions, major and trace element chemistry, and Mg, Ca, and
Ti isotopic compositions of a new class of refractory inclusions that consist of spherules composed of
hibonite and a silicate glass. The distinctive features of these inclusions are excesses in 48Ca and “Ti in
both glass and hibonite, and 26Mgdepletions relative to terrestrial isotopic compositions. Three spherules
have been examined and analyzed, one from the Lance CO3 meteorite and two from the Murchison
CM2 meteorite. Lance 34 13- l/3 1 (LA34 13- 1/ 3 1) and Murchison 7-228 (MUR7-228) have euhedral
to subhedral hibonite crystals enclosed within glass. Murchison 7-753 (MUR7-753) has a rounded hibonite
core with several small inclusions of perovskite. A small fragment of glass is attached to the hibonite and
an Fe-silicate rim is imperfectly preserved around the grain. LA34 13- 1 / 3 1 has a Group II REE pattern;
MUR7-228 a refractory pattern with depletions in the relatively volatile elements Sr, Ba, Nb, V, and Eu;
and MUR7-753 a pattern characterized by the prior removal of an ultrarefractory component and overall
fractionation of all REEs. The partitioning of the LREEs between hibonite and glass in MUR7-228 is
consistent with equilibrium hibonite-liquid partition coefficients previously determined; LA34 13- 1 / 3 1
shows much less partitioning, while MUR7-753 shows no evidence for partitioning and preserves an
unequilibmted refractory component highly enriched in Gd. All spherules have initial magnesium depleted
in 26Mg by around 3% relative to terrestrial Mg, but only MUR7-228 shows evidence for in situ decay
of 26A1,with an initial 26A1/27A1of (1.7 ? 0.7) X 10-j. Both hibonite and glass in all three spherules
show excesses of 48Ca and 50Ti, ranging up to f40 and +20%0, respectively, relative to terrestrial Ca and
Ti. Spherules such as these are rare and the only other occurrence is in the unique chondrite ALH85085.
The hibonite in the spherules shows similarities to isotopically anomalous hibonite crystal fragments
(PLACs), but it is unlikely that the spherules formed by remelting of PLACs. The precursors include
isotopically anomalous Ca-Ti carriers but also isotopically normal refractory components that probably
formed as condensates. The spherules formed by melting of these precursors under disequilibrium conditions and rapid cooling after hibonite crystallization. These inclusions must have formed early, prior
to the dilution of isotouic anomalies bv mixine nrocesses and in an area characterized by excesses of48Ca
and 50Ti, depletions of 26Mg, and lack of ‘“Al: L
1.
INTRODUCIION
1984, 1986; BRIGHAM et al., 1986), but no large anomalies
in Ca and Ti have been found to date ( FAHEY et al., 1987d;
CALCIUM-, ALUMINUM-RICHINCLUSIONS (CAIs) from CM
and CV meteorites are believed to be among the first solids
to have formed in the early solar system and thus to carry
information about the physical and chemical conditions that
prevailed in the solar nebula (for reviews see, e.g., L. GROSSMAN, 1980; MACPHERSON et al., 1988). Of special interest
among them are hibonite-bearing inclusions. Hibonite is one
of the most refractory minerals found in primitive meteorites.
Because it is only second in sequence, after corundum, of
the minerals predicted to condense from a cooling gas of
solar composition (L. GROSSMAN, 1972, 1980; KORNACKI
and FEGLEY, 1984; GEIGER et al., 1988), its origin is thought
to be associated with high temperature processes.
In CV meteorites hibonite appears mostly as a minor primary phase in CAIs consisting largely of melilite. Hibonite
in these inclusions normally shows 26Mg excesses (LORIN
and CHRISTOPHEMICHEL-LEVY, 1978; HUTCHEON et al.,
FAHEY, 1988).
In contrast to CAIs in CV chondrites, the mineralogy of
CM refractory inclusions is dominated by the oxide minerals
spinel, hibonite, and perovskite, while melilite and pyroxene
are generally only minor constituents when present. Hibonite
in many of these inclusions shows large anomalies in Ca and
Ti ( ZINNER et al., 1986; FAHEY et al., 1987b; HINTON et al.,
1987; IRELAND, 1988, 1990), exceeding those seen in FUN
inclusions (LEE et al., 1978, 1979; NIEDERER et al., 198 1;
PAPANASTASSIOUand BRIGHAM, 1989) by an order of magnitude. There exist general correlations between the morphology, Ca-Ti anomalies, 26Mgexcesses, and refractory trace
element abundances of the hibonite-bearing, silicate-free CAIs
(mostly from CM chondrites). Based on these systematics,
we have previously divided these inclusions into four different
classes: PLACs, SHIBs, BAGS, and HAL-type hibonites
(IRELAND, 1988, 1990: IRELAND et al., 1988a, 1989; see Table 1).
In addition to its occurrence in inclusions of the first five
classes listed in Table 1, hibonite has been found in association
with silicate in igneous spherules containing a pyroxene-like
glass rather than melilite. The first example of such an object
* Present address.. ResearchSchool of Earth Sciences,The Australian National University, Canberra ACT 260 I, Australia.
’ Present nddre.w IBM Corporation, Hopewell Junction, NY
12533, USA.
367
T. R. Ireland, A. J. Fahey, and E. K. Zinner
368
Table 1.
Classification and general systematics of hibonite-bearing inclusions.
Mineralogy/
Morpholopv?
Silicate-bearing
Tie
concentration
Mg isotopes
REEabundances
Ti-Caisotopes
*6~g*
ti ocC”ISas
phase
variable
PLACs
platyhib crystals
0.5-2.0
SHlBs
bladedhib crystals
0.5-9.0
depletedultrarefractories 26~~. = 5x10-W'AJ
blue hib pIares
5.5-6.5
lowconcentrations,
depletedultrarefractories
bib crysrals
<0.2
variable
variable
small anomalies
accessory
26Mg*lowOIabsent
depleledEu.Yb
large%a and%i
excessesanddepletions
small anomalies
in spl with pvs
BAGS
HAL-w
hib-glass
GlassSpherules
f hib = bibonile,spl =
1.5-2.3
depletionsof 48Caand5%
massfractionated
massfractionated
depletedCe, Eu. Yb
low 26MgV’Al
variable
*6Mg*lowor absent
*6Mgdepletions
excessesin ‘Wa andSOri
spinel,pvs = perovskile
was discovered in the Lank CO3 chondrite by KURAT ( 1975 )
and consists of two hibonite laths within a 50 pm glass spherule. Subsequently,
J. N. GROSSMAN et al. ( 1988) described
similar inclusions from the unique chondrite ALH85085.
We
noted the similarity in the morphology of Lanck 34 13- 1/ 3 1
to Murchison 7-228 and subsequently identified glass in another Murchison inclusion, 7-753.
In this paper we report ion microprobe measurements
of
Mg, Ca, and Ti isotopic compositions, as well as trace element
concentrations,
of these three hibonite-glass spherules: inclusion 34 13- l/3 1 from the Lank CO3 chondrite, and Murchison 7-228 and 7-753. A petrographic and mineral-chemical
description of Lank 34 13- 1/ 3 1 was given by KURAT ( 1975 ) .
Chemical and trace element analyses as well as Mg and Ti
isotopic compositions ofthe hibonite in inclusions 7-228 and
7-753 were previously reported by IRELAND ( 1988) and IRELAND et al. ( 1988a).
2. EXPERIMENTAL
TECHNIQUES
The three inclusions were studied in polished sections, La& 34 l31/ 3 1in a thin section on loan from the Museum of Natural History,
Vienna. Back-scattered electron images of the inclusions were obtained on a JEOL JSA 840 electron microscope, which was also used
to obtain major element compositions and X-ray maps by energy
dispersive X-ray analysis.
The procedures for the measurement of trace element concentrations with the Washington University Cameca IMS-3f ion microprobe
have previously been described by FAHEYet al. ( 1987b,c) and IRELANDet al. ( 1988a). The secondary ion extraction voltage is offset
by 100 V from the 10% low energy edge in order to collect high
energy ions, thereby discriminating against molecular ions. The rare
earth element (REE) spectrum is deconvolved into atomic and monoxide species. All 14 stable REEs were measured as well as eight
additional refractory trace elements ( RTEs): Sr, Ba, SC, Y, Zr, Nb,
Hf, and V. These elements have been found to be useful indicators
of physicochemical conditions in the solar nebula (DAVIS and
L. GROSSMAN,1979; KORNACKIand FEGLEY,1986). Concentrations
were obtained from ion intensities relative to the Ca signal and the
relative ion yields listed in Table 2 (cf. also ZINNERand CROZAZ,
1986; FAHEYet al., 1987b). The relative ion yields were determined
by measurements of perovskite and of silicate standards for which
the concentrations of Ca and the trace elements of interest could be
determined by independent means (instrumental neutron activation,
electron microprobe analysis).
The techniques for measuring Mg isotopic ratios with the Cameca
IMS-3f have previously been described by MCKEEGANet al. ( 1985)
and FAHEYet al. ( 1987b,c). The isotopic mass fractionation, A25Mg,
expresses the deviation of the measured 25Mgf/24Mg+ from the terrestrial 25Mg/24Mgvalue of 0.12663 (CATANZAROet al., 1966), i.e.,
A25Mg = [(25Mg+/2“Mg+)/0.12663 - I] X IO00 (%O/AMU)
and includes both instrumentally induced and intrinsic fractionation.
An estimate ofthe intrinsic Mg-isotopic mass fractionation, FM,, was
obtained by subtracting the mean total mass fractionation of - 11.9%0/
AMU, measured on a suite of terrestrial samples (IRELAND,1988),
from A*‘Mg. The 26Mg+/24Mg+ was corrected for linear isotopic
mass fractionation as determined from the 25Mg+/24Mgt ratio and
is expressed in delta notation relative to the terrestrial 26Mg/24Mgof
0.13932 (CATANZAROet al., 1966):
626Mg = [26Mg+/24Mg+/0.13932 - I] X 1000 - 2 x AZ5Mg(%o).
27Al/24Mgratios were determined by multiplying the measured *‘Al+/
*“Mg+ by 1.37 ( FAHEYet al., 1987b).
The techniques for measuring isotopic ratios of Ca and Ti with
the ion microprobe have previously been described by ZINNERet al.
(1986) and FAHEYet al. (1987b). Calcium and titanium isotopic
ratios were measured in a single collection cycle that ranged from
mass 40 to mass 52. All isotopes of Ca except “‘Ca (masses 40, 42,
43, 44, 48) and all isotopes of Ti (masses 46 through 50) were mea-
Table 2. Ion Yields
Relative
to Calcium
Hibonite
Sr
Silicate
Ba
SC
Y
zr
0.86
0.63
1.51
0.85
0.46
0.97
0.70
2.00
0.98
0.54
Nb
Hf
V
0.32
0.40
0.72
0.37
0.53
0.95
La
ce
Pr
Nd
Sm
:
0.67
0.62
0.74
0.77
0.92
0.88
1.08
0.74
0.65
0.74
0.82
0.93
0.87
1.03
2
Er
Tm
Yb
Lu
0.70
0.62
0.68
0.60
0.60
0.60
0.47
0.77
0.79
0.71
0.75
0.74
0.57
Isotope anomalies in refractory inclusions
369
sured. Unresolved isobaric interferences of s”V and “Cr with ?f’i
were monitored bv measurina “V and 52Cr. respectively. Mass 43.5
was checked for the presence of 87Sr2+which would indicate Sr*+
interferences at masses 42, 43, and 44. The signal from this species
was below detection in all the meteoritic samples, implying that contributions to the Ca isotopes are negligible (co.1 %O). A correction
was apphed to 4aCa3 for ~ont~butions from the tail of ‘“Ti+.
Isotopic mass fractionation was monitored via the *‘Ca/“Ca and
46Ti/48Ti ratios. with A@Ca and Ad6Ti expressing the deviation (in
L/AMU) from the terrestrial 4oCa/44Caand 46Ti/48Tiratios, respectively, as measured by NIEDERER and PAPANASTASSIOLI
( 1984). and
NIEDERERet al. ( 198 I ): i.e..
A‘Ya = -[(40Ca*/44Ca+)/47.153
A46Ti = -[(~Ti+/4*Ti+)/0.108S48
- 11 X 1000/4(!G~/~MU)
- l] X 1000/2 (~~/A~~).
With this convention, a positive mass fractionation is defined as an
enrichment in the heavy isotopes. The intrinsic isotopic mass fractionations of Ca and Ti, F,, and Fr,, were estimated from the measured A4”Ca and A46Ti bv subtractinn the mean A%Ca and A46Ti
measured on Madagascar hibonite ( Aqa = -0.6 %O/AMUand A46Ti
= -14.5 ~/AM~I).
The Ca and Ti data were normalized according to ex~nential
isotopic mass fractionation Iaws of the form
(‘Ca/Ya),,,
(‘Ti/48Ti),,
= (‘Ca/YZa),,,
X (m,/44)-”
= (‘Ti/48Ti),,,,
X (m,/48)-A
where “OCaand 46Ti are the secondary reference isotopes used to
determine n and 0, (IQ,,_ and (I&,, are the measured and corrected
ratios, respectively. and mj and m, are the isotopic masses. The normalized ratios. Ka and 6’Ti (for i = 42, 43, and 48, and j = 47,
49, and 50), are reported as deviations (in WO)from the terrestriaf
values of NIEDERERand PAPANASTASSIOU
( 1984) for Ca, and NIEDERERet al. ( 198 I ) for Ti.
The counting-system dead time was determined within 1 ns from
Ti isotopic measurements on a Ti metal sample. The ‘%a+ count
rate was limited to 100.~0 c/s during the isotopic measurements of
the hibonites and glasses, thus making the error from the un~~ainty
in the determination of the dead time negligible. Counting times on
the Ca and Ti isotopes were adjusted to yield similar precisions for
all normalized ratios. Since the samples were relatively small. and
the Ca and Ti isotopic anomalies relatively large, isotopic ratios were
measured only to a precision of approximately 5% ( 2~).
3. RESULTS
3.X. Sample Description, Major Element Chemistry
The common feature of the three refractory inclusions reported here is the association of hibonite and glass. Lance
3413-l/31 (LA3413-l/31;
Fig. la) was first described by
KURAT ( 1975) and consists of two euhedral hi&mite laths
(20 X 10 rrn) within a glass spherule (50 pm in diameter).
Murchison 7-228 (MUR7-228; Fig. lb) is texturally very
similar to LA34 13- 1/ 3 1, being a glass spherule w 120 pm in
diameter with several euhedral to subhedral laths of hibonite
near the core. Murchison 7-753 (MUR7-753; Fig. Ic) is quite
different. This inclusion consists of two distinct regions: a
core composed of a rounded mass of hibonite with several
small perovskite inclusions, and a rim of Fe-silicate material
in which a small glass fragment is preserved.
The hibonite major-element compositions are remarkably
uniform amongst the three inclusions, as are the glass compositions (Table 3). The hibonite compositions are typical
for meteoritic hibonite in that they deviate from the ideal
composition of CaA1,z0r9 by way of a coupled substitution
of Mg*+ and ‘Ti4f for 2AI 3+. The TiOz concentration in meteoritic hibonite can reach 9%. but the hibonites in the in-
FIG. 1. Backscattered electron photomicrographs of hibonite-glass
spherules: (a) LA3413-l/31, (b) MUR7-228, (c) MUR7-753. (Legend: HI = hibonite, GL = glass. PV = perovskite. FE = Fe-silicate:
Scale bar = 10 pm).
cfusions studied here have relatively low substitution of Ti
with I .5 to 2.3% TiOz and corresponding MgO concentrations
of 0.9 to 1.4%. Hibonite and glass from the two Murchison
spherules have very similar compositions, respectively. The
T. R. Ireland, A. J. Fahey. and E. K. Zinner
370
Table 3.
Si@+
Ti@
Al203
Sr
::
zr
Chemical
compositions
Hibonite
Bulk
Hibonite
Glass
Bulk
Hibonite1t Hibonite2
Glass
10’
90
100
2
98
100
32
68
100
40.1
3.0
27.0
22.6
7.3
27.3
2.6
47.0
17.9
5.3
:IJ
6.6
2.1
2:
0.4
33.1
1.7
35.1
23.8
6.3
0.0
2.3
87.9
8.6
1.2
40.2
1.8
27.7
22.9
7.3
39.4
1.8
28.9
22.6
7.2
72
:“9
32
1.5
0:;’
OZl
17
59’9
z!
103
z;
;;
1.242.34
525
1.2
557
::::
263
89’9
0.68+0.56
864
:.;
0.72
852
6.4
6.9
10.8
:.‘5
13
4.3
0.49
4.4
0.83
5.8
0.99
2.8
0.45
3.0
0.16kO.03
:“7
3”.“9
19
5.3
0.28
8.8
1.8
5.6
0.86
4.7
1.8
0.66
2.9
0.60
4.5
1.02kO.13
4.3
0.76
5.9
0.89kO.10
2.0
6.0
0.92
5.0
1.9
0.65
3.0
0.61
4.5
1.0
4.3
0.75
5.8
0.88
36.7
:9;:2
25.5
6.8
8?3
46
307
:t
21
81
169
18
A’3
8.6
Nb
Hf
V
o.9z29
la
Ce
k
::
4.9
Sm
Murchison
l-753
Murchison
7-228
Lard
3413-l/31
Glass
t?t
&
0.42
6.3
B
Er
Tm
Yb
LU
::‘:
0.78
0.90
0.11
~0.24
CO.05
i4
4.5
0.48
4.6
0.89
Cz
2.6
0.42
2.7
0.14
:::
1.1
2.5
0.26
0l.i
0.0
8b::
7.8
0.9
40
10
33
4.6
81
4.8
29
3.2
0.48+0.06
189
0.442i;.16
223
0.24
0.20
:;oo
0.66
0.84
0.37
1.4
0.20
1.3
0.32
0.68
0.43
1.1
0.07*0.01
3q16
21
d.;‘2
0.48
0.66
0.33
0.45
0.11
1.o
0.24
0.50
0.25
0.91
0.05f0.01
91.:
<o. 17
1154
0.25
1.2
0.16
0.7 1
0.7 1
0.25
0.18+0.05
0.07f0.01
0.41
0.1 lf0.02
0.41
0.26f0.04
3.4
0.08f0.02
Bdk
G
18.9
7.5
0.26
850
0.24
1.3
0.16
0.67
0.72
0.28
0.43
0.10
0.65
0.16
0.46
0.29
2.6
0.07
10 errors are less than 10% unless otherwise stated; Upper limits 20
I electron probe analysis of major elements from Kurat (1975)
$ analysis from Irelander al. (1988b)
* calculated proponions in % for assumed spherical geometry
+ major elements analyzed by elecaon probe, oxide concentrations in wt %; trace element concentrations in pg/g
composition of hibonite from LA34 13- 1 / 3 1 is similar to that
of the Murchison hibonites, but the glass has lower MgO and
SiOz and higher CaO and A1203. The glasses have compositions close to that of pyroxene but tend to have more AlzO,
(2 1.3 to 29.2% ) than is typical for meteoritic clinopyroxene
(up to 22%; L. GROSSMAN,1980). They also have less TiOz
than is typical for CA1 pyroxene (~6%; BECKETT, 1986).
3.2. Trace Element Analyses
Trace-element
analyses of hibonite and glass are presented
in Table 3 and Fig. 2. Also given in Table 3 are the bulk and
trace element compositions
calculated from the observed
modal proportions of hibonite and glass assuming spherical
geometry. The trace-element
measurement
of hibonite in
LA34 13- 1 / 3 1 was difficult because of the small size of the
crystals and because it was carried out after the isotopic analyses. The previous analyses had consumed a significant portion of the sample and had also resulted in a heavily cratered
surface, and thus some difficulty was experienced
in positioning the primary beam entirely onto a hibonite lath. From
Ca+/Si+ and Ca+/Mg+,
it is estimated that the hibonite
analysis consisted of 70% hibonite and 30% glass. The hibonite
compositions presented in Table 3 and Fig. 2a were corrected
for this contribution
of glass.
* Eu/Eu* is the ratio of the measured
of Eu.
to interpolated
abundance
The LA34 13- 1/ 3 1 hibonite is light REE ( LREE) enriched
up to 60-80 X CI, with a gradual decrease to Dy and a sharp
fall after Tb with Er-Lu below the limits of detection. Europium is depleted with Eu/Eu* = 0.23 ? 0.05.* Ofthe other
refractory trace elements, Zr and Nb enrichments
are close
to those of the LREEs; the other elements are enriched to
20-30 X CI, except for Ba which has the lowest enrichment
at 6 X CL The Sr is much higher in this analysis (29 X CI)
than is typical of meteoritic hibonites, which generally have
3-6 X CI Sr (IRELAND et al., 1988a). Hf also appears significantly depleted relative to Zr. The glass REE pattern shows
relatively flat LREEs at just under 30 X CI, with the heavy
REE (HREE) abundances also relatively constant at around
20 X CI, except for Lu which is only 7 X CI. Europium is
depleted with Eu/Eu * = 0.34 f 0.05. There is evidence for
a modified Group II pattern in the slight depletion of the
HREEs relative to the LREEs, the Tm, Yb, and Nb enhancements similar to those seen in Murchison perovskites and
SHIB hibonites, and the fractionation
between Y and Ho
(IRELAND et al., 1988a). The Group II pattern is even more
evident for the bulk composition of this inclusion (Table 3).
The RTE pattern in the LA34 13- I/ 3 1 glass is similar to the
hibonite pattern, except that Zr, Nb, and V are a factor of
two lower, and Sr is only 8 X CI.
Hibonite in MUR7-228 has previously been analyzed by
IRELAND et al. ( 1988a), and a replicate analysis is presented
in Table 3 and Fig. 2b along with the analysis of the glass.
The hibonite analysis presented here is in excellent agreement
Isotope anomalies in refractory inclusions
I
Land 3413-1131
Hibonite
Glass
Errorbars are 1cz
-o-
rIIIIIlll
L
z
“’
IIII
Sr~SclZr~HfI
Ba Y Nb
0
V
Iflrllrlrll
LalPrl
SdGd(Dyl
Eu Tb
Ce Nd
ErlYtj
Ho Tm Lu
Murchison 7-228
Hibonite
-a-
Glass
Error bars are lo
Ba
Y
Nb
V
Ce Nd
SmlGdiDylErlYb
Eu Tb Ho Tm Lu
Murchison 7-753
-e-Hibonite 1
--A-
Hibonite 2
+I--
Glass
371
ation; i.e., it is enriched in the HREEs that are depleted in
the hibonite. It is smooth and increases from 8 X CI for La
to 36 x CI for Lu, and does not have a pronounced Eu
anomaly (Eu/Eu* = 0.87 f 0.18). The RTEs in the glass
have similar abundances to those in the hibonite, except that
Nb and V are a factor of 3 enriched and Ba is depleted by a
factor of 5 relative to the hibonite.
Hibonite in MUR7-753 has also been analyzed by IRELAND
et al. ( 1988a); the data of that previous analysis are represented as “Hibonite # 1” in Table 3 and Fig. 2c. Since then,
the mount had been repolished and another analysis of
MUR7-753 hibonite was made (Hibonite #2), along with
the measurement of the glass. IRELANDet al. ( 1988a) found
MUR7-753 hibonite to have the most unusual pattern ofany
of the hibonites they analyzed. The LREE abundances are
very low, with Ce and Sm being the most abundant at 3X
and 6 X CI, respectively, and La and Pr being the lowest at
around chondritic abundance. There is no Eu anomaly. The
HREEs are enriched overall relative to the LREEs, but there
is a steady fall from Gd at 7 X CI to Lu at 3 X CI with positive
anomalies at Tm ( 18 X CI) and Yb (7 X CI). The abundances
of the RTEs are also low and are within the range of 2-4
X CI, except for SC, Zr, and Nb which are approximately a
factor of 4 higher. The second hibonite analysis (Hibonite
#2) shows slightly lower overall abundances in the REEs
relative to Hibonite # I; however, there is a pronounced depletion for Gd and Tb. Thulium is again the most abundant
REE in the hibonite at 10 X CI, with Yb at 6 X CI, and the
ultrarefractory REEs are rather flat at 2-4 X CI. The glass of
MUR7-753 shows the same REE pattern as hibonite analysis
#2, except that the HREEs from Gd to Er are further depleted
by a factor of 2, and Yb is a factor of 3 higher at 21 X CI.
The glass RTE pattern also differs from the hibonite in having
higher Nb and V abundances (by a factor of 4) and lower SC
(factor of 10).
3.3. Isotopic Analyses
1
I~lllllllllrll
‘.’
SrlScIZrIHfl
Ba Y Nb
IlWlHlll
V
LalPrl
SrtlGdlDyl
Ce Nd
Eu Tb
ErlYbl
Ho Tm Lu
FIG. 2. Trace element abundance patterns for hibonite and glass:
(a) LA3413-l/31, (b) MUR7-228, and (c) MUR7-753 with two
hibonite analyses plotted-Hibonite 1 from IRELAND et al. ( 1988a);
Hibonite 2 from Table 3.
with the previous measurement. In the hibonite, the REE
pattern is fractionated with enriched LREEs relative to
HREEs. The LREEs decrease from 46 X CI for La to 37
X CI for Sm, with a steeper decrease in the HREEs from 45
X CI for Cd to 8 X CI for Yb and Lu. The less refractory
REE Eu is depleted (Eu/Eu * = 0.12 + 0.02), as are the
relatively volatile RTEs, Sr, Ba, Nb, and V. The highly refractory RTEs, SC, Y, Zr, and Hf, are generally below the
enrichments of the LREEs in the hibonite with 15-30 X CI
abundances. The REE pattern of the glass of MUR7-228 is
complementary to that of the hibonite in its overall fraction-
Magnesium isotopic compositions of hibonite and glass
are presented in Table 4 along with analyses of neighboring
olivine grains which were used to monitor the measurement
conditions. Isotopic mass fractionation is a function of instrumental conditions which must be held constant in order
to get a reliable measurement of intrinsic mass fractionation
in the samples. While the precision of a measurement is usually less than ~%O/AMLJ, accuracy at this level can only be
achieved under optimum conditions ( FAHEYet al., 1987b,c).
The CA1 samples had already been sputtered during Ca and
Ti isotopic measurements, and so smooth surfaces were not
always available because of the small size of the samples.
The Fhlg values of the olivine grains show good agreement
with each other, and the near-zero values indicate good
agreement with the mean instrumental Mg-isotopic mass
fractionation previously determined (IRELAND, 1988). Mass
fractionation effects are clearly apparent in the glasses of
LA3413-l/31 and MUR7-228 with heavy isotope enrichments of $8.2 and +~.~%o/AMu, respectively. The hibonite
ofLA34 13-I /3 1 and glass of MUR7-753 have only marginal
effects with FMgof +3.7 and -~.~%o/AMu, respectively, while
hibonite from MUR7-228 and MUR7-753 have mass frac-
T. R. Ireland, A. J. Fahey, and E. K. Zinner
312
tionation effects less than 2%/AMU and are not resolved from
normal.
Normalization
removes the effects of isotopic mass fractionation, and the precision and accuracy of the 626Mg values
are therefore frequently higher than those of the measured
Fhlg. A notable feature of the Mg isotopic analyses is the
presence of 26Mg depletions, resolved by more than 4u from
normal, in all three spherules. LA3413-l/3
1 has clearly resolved 26Mg deficits of more than 3%0 in both hibonite and
glass. Glass from MUR7-228 also has a well-resolved 26Mg
deficit, but the hibonite has a small excess of 26Mg at + 1.5
k 1.1 %o. Since the hibonite has a higher *‘Al /24Mg than the
glass, the difference in 626Mg could be due to in situ 26Al
decay. In this case the inclusion would have had a (26A1/
27A1)0 ratio of 1.7 (kO.7) X 10m5. MUR7-753 hibonite has
a clearly resolved depletion in 26Mg, but the composition
of
the glass is normal within error, although the hibonite and
glass compositions do overlap within 20 error bounds as well.
Calcium- and titanium-isotopic
compositions
of hibonite
and glass are presented in Table 5 and Fig. 3. The Ti isotopic
composition of MUR7-753 hibonite reported here is in good
agreement with the measurement
by IRELAND ( 1988) which
was made with the Australian National University ion microprobe (SHRIMP).
Intrinsic isotopic mass fractionation
effects in Ca and Ti are not clearly resolved in any of the
analyses. The main feature of the analyses is the presence of
large enrichments
in the heaviest isotopes of Ca and Ti, 48Ca
and 50Ti, in both hibonite and glass. Enrichments
in 48Ca
range from + 11 to +42%0, and 50Ti excesses range from within
error of normal up to +2 I %Orelative to the terrestrial Ca and
Ti isotopic compositions.
The other normalized isotopes of
Ca and Ti also tend to show slight enrichments,
but these
are generally not clearly resolved from normal. The hibonite
and glass from each of the two Murchison inclusions have
the same isotopic compositions,
and the two spherules differ
only marginally in their mean composition ( Fig. 3 ). The Caisotopic composition
of LA 3413-l/31
is the exception to
this since the Ca compositions
of hibonite and glass do not
overlap within the stated 20 error bounds, but differ by 3.2~.
Table
Sample
4.
Mg isotopic
Phase
FM~~
626Mg$
(“/oo)
Hibonite
Glass
Olivine’
3.7k1.4
8.2f1.8
-0.5kl.l
-3.2f0.8
-3.7M.8
0.3kO.4
31.8f2.7
3.6fO. 1
<o.o 1
7-228
Hibonite
Glass
Olivine’
1.8kO.9
9.2fl.O
0.2kO.6
1.5kl.l
-2.9f1.4
-0.7f1.2
38.9f1.2
0.72kO.03
<o.o 1
Hibonite
Glass
Olivine’
0.3+1.0
-3.2f1.2
-0.4f1.8
-4.7f2.3
-1.2f2.4
O.Of1.3
30.9fl.O
3.6&O. 1
co.01
7-7 16
Fca+
(o/w/AMU)
Hib
Glass
Hib
Glass
Hib
Glass
3413l/31
7-228
7-753
Sample
34131131
7-228
7-753
Hib
Glass
Hib
Glass
Hib
Glass
h4*Cat
(S)
+1.8*1.8
+0.4*1.4
-3.OkO.7
+0.1*1.4
-2.5kl.l
-3.0*1.8
3.2f6.0
2.5k3.1
1.6k3.5
0.5k3.8
0.2k4.1
5.9k4.1
0.9k6.5
-1.3f2.8
-1.7k3.6
1.6k3.9
1.6f3.6
6.5k4.0
24.5f7.8
10.8f3.6
41.9f3.5
35.5f4.3
32.5k4.5
33.8k5.0
FTi+
(%d.WU)
h4’Ti*
(“/w)
h49Tit
(W
@Wi*
(“/w)
1.6k5.9
2.Ok4.3
0.3k3.6
-1.2f5.2
-0.8k4.2
-0.lk4.9
6.Ok6.8
0.6f4.3
5.1f3.8
4.4f4.8
2.1k4.4
7.9f4.5
6.9f8.8
8.2f5.3
21.1f4.9
21.4f7.0
11.8f5.9
15.1f6.1
+5.3+6.4
-1.lf1.5
-2.4f1.5
-1.lf2.2
-0.8k2.0
-0.4k1.8
All errors are 20.
7 F& and FTi are inbinsic isotopic mass-fractionation estimates
based on deviation of measured isotopic mass-fractionation
from that measured on Madagascar hibonite.
$ Data normalized with exponential mass-fractionation law to
normal Ca and Ti isotopic ratios of Niederer and
Papanastassiou (1984) and Niederer er nl. (1981) respectively.
4. DISCUSSION
Hibonite-glass
spherules
are very uncommon,
and, apart
from the three inclusions studied here, they have only been
described from the unique chondrite
ALH85085 (J. N.
GROSSMAN et al., 1988; MACPHERSON
et al., 1989). As a
group, the three spherules present unique isotopic systematics
not previously found. The glass contains the largest isotopic
effects yet measured in a silicate phase, with up to 40%0 excess
in 48Ca and 20%0 excess in 50Ti, as well as deficits in 26Mg.
The major-element
composition
of the glass in the three
spherules is remarkably constant. However, the spherules are
not identical to each other and differ in morphology
and
2’A1/24Mg
3413
-l/31
7-753
Sample
compositions
compositions
(%damu)
7-198
Table 5. Ca and Ti isotopic
All errors are 20.
t Fhlg is an estimate of intrinsic Mg-isotopic mass-fractionation
after correction for insmnnental fractionation of -11.9 o/odAMu
(Ireland, 1988).
$ 6%Mg is normalized with a linear-mass-fractionation law to
normal isotopic ratios of Catanzaro er al. (1966).
* Olivine analyses from neighboring grains as a check on
instrumental conditions; Olivine is not present in the the
refractory inclusions.
-20
-10
0
10
20
30
40
50
S’“ri (%o)
FIG. 3. 48Ca and ‘@T’ianomalies of coexisting hibonite (solid symbols) and glass (open symbols) in the three microspherules.
Also
shown are the compositions of hihonite crystals MUR-H7 and MURH8 (ZINNER et al., 1986), FUN inclusions HAL and EKl-4-I (LEE
et al., 1978; NIEDERER et al., 1981; FAHEY et al., 1987b), the field
of SHIB compositions,
and trajectories to the extreme compositions
shown by PLACs and one unusual SHIB (IRELAND, 1990). Error
bars are 20.
Isotope anomalies in refractory inclusions
trace element chemistry, as well as in their isotopic compositions. But, in each spherule, the difference between the glass
composition and the hibonite composition is much less than
the variations amongst the three spherules. Thus, the major
element chemistry alone indicates that the glass and hibonite
are related in each spheeruie.
We will first consider the question of the formation of the
spherules and follow with a discussion of the isotopic Systematics and the relationship of the spherules with other hibonitebearing CAIs.
4.1 Formation of Hibonite-Glass Mierospherufes
The most prevalent framework for the interpretation of
the chemistry of refractory inclusions has been the equilibrium condensation model whereby phases condense from a
cooling gas according to the volatility of their components.
Since the refractory oxides of Al, Ti. and Ca condense at
higher temperatures from a cooling gas of solar com~sition
than those of Mg and Si. it has been suggested that the refractory inclusions from CM meteorites are an earlier, highertemperature assemblage than those from Allende (EKAMBARAM et al., 1984). This is consistent with the finding of
large isotopic anomalies in hibonite; hibonite, as one of the
earhest minerals to condense, would preserve larger anomalies
than the less refractory Allende inclusions.
A premise for the condensation model is that gas condenses
directly to solid without any liquid phase (L. GROSSMAN,
1972). The presence of glass in an inclusion indicates that
the inclusion, or at least part of it, was once molten and is
therefore incompatible with direct ~onden~tion, and may
indicate the formation of metastable liquids. However, melting is often regarded as a secondary process in the solar system,
and glasses could also result from remelting of earlier condensates. In this case the chemical composition of the inclusion reflects the composition of the precursors, i.e., is an inherited feature, and does not result from the melting event,
which is only responsible for the r~ist~bution of the elements
between different phases.
An alternative to the condensation model is a distillation
model whereby a less refractory aggregate is heated, resulting
in the evaporation of volatiles. Besides volatile elements, the
light isotopes are preferentially evaporated, and thus a signature of distjllation would be isotopic mass fractionation
enriching the heavy isotopes. The experiments of DAVIS et
al. ( 1991 ) show that forsterite has to be molten for isotopic
fractionation to occur during distillation. Glass would thus
be a natural feature of distillation, provided the melt is
quenched, since the inclusion is likely to be melted during
the heating event.
These formation models have been discussed in relation
with the formation of meteoritic hibonite ( HIN’TON et al.,
1988; IRELANDet al., 1988a: MACPHERSON et al., 1988; IRELAND, 1990).
MACDOUGALL (1981)
has advocated an ig-
neous origin for spine]-hibonite spherules (SHIBs, according
to the terminology of IRELAND, 1988; see Table 1) based on
their texture and morphology. However, most SHIBs have
REE patterns (IRELAND et al., 1988a; IRELAND, 1990) which
show close affinities to Allende Group II patterns (MARTIN
and MASON, Z974), indicating that they experienced a com-
373
plex condensation history ( B~YNTON, 1975; DAVIS and L.
GROSSMAN,1979), followed by remelting in order to produce
the observed petrographic characertistics. The clearest signature of distillation is found in four HAL-type hibonites
(IRELAND et al., 1989a: IRELANDand ZINNER, 1989): isotopically heavy 0, Ca and Ti; extremely low Mg and low Ti
concentrations; and a pronounced Ce depletion.
Most CAIs appear to have a complex multistage history
with different features being established at different stages.
For example, the morphology and petrography of SHIB inclusions reflect melting, while their Group II patterns were
established by prior hip-tem~rature
nebular processes. The
situation seems to be similar for the spherules of this study.
Their morphology and mineralogy, and that of the
ALH85085 microspherules, indicate an igneous origin from
molten droplets (J. N. GROSSMANet al., 1988). The first
question, therefore, is whether or not the hibonite and the
glass in these unique inclusions are genetically related. There
are several indications that this is the case for the microspherules of this study. The strongest argument for a genetic
link between glass and hibonite comes from the isotopic
compositions. Both 648Ca and 650Ti are identical within the
errors in the glass and hibonite for both Murchison spherules
(Fig. 3). In addition, these compositions are extremely
anomalous, more than any that have previously been measured in silicate-bearing inclusions (LEE, 1988).
For LA34 13-l / 3 1, the b5’Ti is the same in hibonite and
glass. albeit with large errors which are the result of the small
size of the hibonite in this inclusion and the multitude of ion
probe analyses performed on it. The 64*Ca values, on the
other hand. are signi~cantly different in hibonite and glass
(3.2 c). This could be the result of Ca exchange in the glass.
We shall argue below that the spherules LA341 3-l/31 and
MUR7-228 were distilled during hibonite crystallization. This
makes it possible that Ca was exchanged in the liquid portion
and, if the surrounding reservoir had normal Ca, that the
4xCa excess was lowered in this process. An alternative possibility is that the Ca exchange took place later, during metamorphism on the parent body. The higher Ca concentration
in La3413-I /3 I compared to the glass of the other two
spherules might be related to an exchange process. Such an
event has not necessarily affected the Ti in the glass. FAHEY
et al. ( 1986) observed that in a heavily altered hibonite-bearing inclusion. Lance HH- I, Ca had been in part completely
replaced by Fe (perovskite was changed to ilmenite, hibonite
to hercynite) without disturbance of the Ti isotopic system
(in all these four phases Ti has the same highly anomalous
isotopic composition), attesting to the relatively high mobility
of Ca. Furthermore, these workers found a gradient in the
4’Ca and 48Ca anomalies from the hibonite in the core of the
inclusion to an inner diopside and an outer pyroxene layer
of the rim. The difference in 648Ca between hibonite and
glass of LA34 I3- I /3 1 is reminiscent of this gradient in La&
HI-I-I. In addition, HASHIMC~I-oet al. ( 1987) found that Ca
is apparently much more volatile than Ti. and even Mg and
Si. in the presence of water.
The non-linear 62hMg in LA34 13-I /3 1 could be measured
with much higher precision than in the other microspherules
(Table 4). It is the same in both phases but is significantly
different from zero, the average solar value. This uniformity.
374
T. R. Ireland, A. J. Fahey, and E. K. Zinner
however, cannot be the result of isotopic equilibration
after
formation, since the Mg in the glass is much more fractionated
than in the hibonite, and is additional evidence for the common origin of hibonite and glass. This interpretation
of the
behavior of Ca and Mg implies that Ca was more mobile in
the glass and possibly points to parent body metamorphism.
It has already been noted that the glass has an unusual
composition,
with more Al203 than is typical for pyroxene
found in other CAIs, and that this composition is rather constant among the three spherules. In their bulk major-element
compositions
the spherules are much more refractory than
other types of silicate-bearing
inclusions (see Table 3). It
therefore appears that the association of the hibonite and the
glass is not accidental but that the hibonite grew from an
unusually refractory melt. The bladed crystals in glass seen
in LA34 13- 1 / 3 1 and MUR7-228 are the textures one would
predict for hibonite growing in a melt. MUR7-753 is distinctly
different. On first inspection there is little to link the glass
fragment in the rim with the rounded hibonite core in this
inclusion. However, the major element composition
of the
glass in MUR7-753 is remarkably similar to that in the other
two spherules.
Crystallization
of hibonite from a liquid should produce
a predictable fractionation
of elements between the liquid
and hibonite. DRAKE and BOYNTON ( 1988) determined partition coefficients for La, Sm. Eu, Cd, Yb, as well as Sr, between hibonite and silicate liquid. They found that the LREEs
were favored over HREEs in the hibonite, with the partition
coefficients showing a smooth decrease from 7.2 for DLa to
0.10 for DYbr according to the sizes of the trivalent ions. The
only deviation from this trend is Eu, which can also exist in
the Eu2+ oxidation state; DQ+ = 0.52, as estimated from
the Sr2+ partition coefficient, whereas Dru3+ is estimated to
be 1.57.
The distribution
coefficients between hibonite and glass
for the trace elements in the three spherules are presented in
Table 6 along with the experimental partition coefficients for
the elements determined
by DRAKE and BOYNTON ( 1988).
Two analyses are presented for hibonite in MUR7-753 based
on the hibonite data # 1 and #2 in Table 3. We shall consider
first the distribution coefficients for the trivalent REEs. The
distribution
coefficients for LA34 13- 1 / 3 1 are roughly constant from La to Tb, with values within the range 1.4-2.2,
and then fall off to values less than unity after Dy. MUR7228 shows a stronger degree of fractionation, with distribution
coefficients steadily falling from 6.0 for Db to 0.22 for Dv,,
and DLu. MUR7-753 shows no evidence for partitioning in
the LREEs, with distribution coefficients close to unity. The
distribution coefficients for Gd to Ho are significantly greater
than unity and Dad is also clearly different in the two analyses,
with values of 2.6 f 1.3 ( la) and 7.7 + 2.6. Dyb is again
down near the values shown by the other spherules at approximately 0.3.
For the trivalent REEs it appears that only MUR7-228
has distribution coefficients that are close to those determined
experimentally
for equilibrium crystallization.
Even for this
inclusion there is not perfect agreement since Dad and Dy,,
are a factor of 2 higher than the predicted values. LA3413l/3 1 shows a much lower degree of partitioning of LREEs
into hibonite, while MUR7-753 shows no such fractionation.
The partitioning
of ELI between glass and hibonite of
Table 6. Hibonite-glass distribution coefficients
Lane6 Murehison
3413-l/31 7-228
Murehison
7-753
PI
WI
HibLiqt
1:1
0.86
2.5
1.4
0.93
1.22
0.78
12.2
1.8
1.48
1.69
9.2
1.3
:b
2.0
1.5
0%
0!3”3
O!;:
Hf
V
0.79+j;g
1.6
4.0+;ro
0.30
<1.8
0.16
<1.6
0.19
2:
4:5
0.96
7.2 It 1.0
0.93
;.:
0.8
1.0
0.68
0.75
1.2
0.93
2.7 f 0.2
$1:
2.9
3.2
2.9
1.7
2.5
1.3
1.6
2.4
2.2
1.2
1.57
1.49 -f 0.53*
0.06
0.27
0.96
0.10 + 0.03
Sr
!:
6.1
24
1.9
Y
La
1.7
FF
2.0
2.2
0.53 f 0.01
4.0
::
Fz
1.5
0.86
9
;::
Er
Tm
Yb
d;b
0:32
0.24
<O.lO
LU
CO.3
rz2
3.0
2.2
1.4
1.1
0.58
0.34
0.22
O!!z2
0.22 f 0.05 0.882:;;
0.63$$
[l] uses hibonite analysis of Ireland et al. (1988b);see Table 3
[2] uses bibonite analysis from this work ; seeTable 3
t partition coefficients from Drake and Boynton (1988)
* dependent on oxygen fugacity
10 errors less than 20 I unless otherwise stated, upper limits 20
MUR7-228. the only inclusion whose distribution coefficients
are in general agreement with equilibration
crystallization,
places constraints on the ambient oxygen fugacity in the formation region of this spherule. DRAKE and BOYNTON ( 1988)
measured DEu at varying oxygen fugacities in their hiboniteliquid partitioning experiment and determined
Eu2+/Eu3+
from the partition coefficients. Dsr was used to obtain a limit
for infinitely reducing conditions (i.e., Eu’+/[ Eu2+ + Eu’+]
= 1 ), given the similarity in the size of Sr2+ and Eu2+. The
DEu value for MUR7-228 is near the Dsr value of 0.52 determined by DRAKE and BOYNTON ( 1988). This indicates
that the Eu present in this spherule is predominantly
Eu2+
and is in agreement with the predictions for the oxygen fugacity of the solar nebula and with direct measurements
of
Ti3’/Ti4’ from meteoritic hibonite (BECKETT et al., 1988).
However, the Dsr for MUR7-228 is clearly in excess of the
experimental value. The reason for this behavior is not clear,
but it may not be a primary feature. Strontium is a relatively
mobile element, and it is possible that Sr was introduced into
the spherules during meteorite metamorphism.
It is apparently favored in the hibonite relative to the predicted partitioning behavior but may be present in secondary minerals
often present in refractory inclusions.
The distribution
coefficients for the remaining trace elements are fairly consistent amongst the three spherules. Since
MUR7-228 is the only spherule that has distribution coefficients for the trivalent REEs consistent with the experimental
values of DRAKE and BOYNTON ( 1988), the trace element
data for this spherule should therefore give the best indication
of the RTE distribution coefficients. Barium is favored in the
hibonite ( Daa = 2.5); the refractory elements SC, Y, and Zr
have values close to unity; while Nb and V are favored by
the liquid, with Dv and 4\lb being 0.30 and 0.19, respectively.
Isotope anomalies in refractory inclusions
The Dnrvalue has large errors, but, since Zr4+ and Hf 4+ have
nearly the same size, it is likely that Dur is close to unity as
well. This is also consistent with the Dur values measured
from the other two spherules.
In conclusion, the three spherules probably formed from
molten droplets. The hibonite morphologies
of LA34 13- I/
31 and MUR7-228 are clearly indicative of crystallization;
however, MUR7-753
has an unusual texture that is not
readily interpretable in terms of igneous processes. A common
origin of hibonite and glass is further indicated by the Mg,
Ca, and Ti isotopic compositions
as well as by the trace element patterns in these two phases which are either almost
identical (in MUR7-753) or differ by varying degrees of solidliquid fractionation.
Hibonite crystallization
apparently did
not proceed under equilibrium
conditions. The bulk composition of MUR7-228 is similar to the glass studied experimentally by REID et al. ( 1974). These authors found that
such glasses must be quenched very rapidly to prevent crystallization. Thus, the hibonite crystallization must have been
followed by fast cooling.
4.2. Isotopic Mass Fractionation and REEs
In this section we discuss the evidence for distillation during
formation of the microspherules
and its possible effect on
REE distributions.
Isotopic mass fractionations
are common
in many silicate-bearing
CAIs and are most clearly seen for
Mg (R. N. CLAYTON et al., 1985 ). Coarse-grained CAIs usually have heavy Mg and fine-grained CAIs light Mg, implying
that the former were effected by distillation, the latter by
condensation
( NIEDERER and PAPANASTASSIOU, 1984; ESAT
and TAYLOR, 1984a,b). However, there is no obvious relationship between the sign and magnitude of the Mg fractionation and the REE patterns in Allende CAIs, either in “normal” inclusions or in FUN inclusions ( NIEDERER and PAPANASTASSIOU, 1984). A clear relationship between isotopic
fractionation
(of 0, Ca, and Ti in this case) and REE abundances appears to exist only in the HAL-type inclusions
(IRELAND et al., 1989; IRELAND and ZINNER, 1989).
The glass of the microspherules LA34 13- l/3 1 and MUR7228 is enriched in the heavy isotopes of Mg (Table 4). The
magnitude of these enrichments
is within the range of Mg
frationations
displayed by “normal” Allende CAIs ( NIEDERER and PAPANASTASSIOU, 1984; BRIGHAM et al., 1985;
PROMBO and LUGMAIR, 1986) and can be accounted for by
a distillation model such as advocated by KURAT ( 1975).
The hibonite, on the other hand, is only marginally enriched
in LA34 1%1 / 3 1 and not clearly enriched in MUR7-228.
The difference in the fractionation
between hibonite and
glass could be the result of progressive volatilization
of the
liquid during hibonite crystallization. The hibonite originally
crystallizes because the bulk composition
lies within the hibonite stability field. The bulk composition
of MUR7-228
can be estimated in two ways, either from the relative proportions of hibonite and glass exposed on the surface of the
polished section, or by assuming that the REE pattern was
originally flat. Hibonite comprises approximately
7% of the
exposed area; for an assumed spherical geometry of the
spherule, the volume fraction would be 2%. On the other
hand, a flat REE pattern is obtained at 24 x CI for approximately 45% hibonite and 55% glass. The composition of the
375
spherule inferred from modal mineralogy (Table 3) would
be SiOz 39.4, TiOz 1.8, Al103 28.9, MgO 7.2, and CaO 22.6
wt%, while trace element inventory implies a composition
of SiOz 22.1, TiOz 2.0, A&O3 54.8, MgO 4.6, and CaO 16.5
wt%. The first composition is slightly less refractory than the
initial composition
of DRAKE and BOYNTON ( 1988), while
the latter is substantially more refractory.
In either case, hibonite crystallized as a response to the
cooling of a liquid somewhat less refractory than the bulk
composition of the spherule. The reason is that while the first
hibonite to crystallize must have the original Mg-isotopic
fractionation
of the precursor, the heavier Mg in the glass
indicates Mg loss by distillation from the remaining liquid
during hibonite crystallization. The hibonite therefore should
be zoned in its Mg isotopic fractionation,
but the crystals are
too small to verify this behavior. If we assume that the measured F,, in the hibonite represents the starting composition,
roughly 40% of the initial Mg must have been lost to account
for the isotopic mass fractionation
in the glass. DAVIS et al.
( I99 1) found that forsterite evaporates stoichiometrically,
i.e., that the evaporated fractions of MgO and SiOz are the
same. This, however, is not the case for more complex mixtures containing Ca, Al, and Ti, from which Mg is removed
preferentially over Si ( ESAT, pers. comm., 1990). This is also
supported by the observation that in many CAIs the Mg isotopes are more fractionated than the Si isotopes compared
to what would be expected based on Rayleigh fractionation
during stoichiometric
distillation (see DAVIS et al., 1990).
Thus, it is unlikely that the original liquid from which MUR7228 formed had a much higher Si02 content than the final
glass. As already discussed, quenching must have been very
rapid to prevent crystallization of the glass, not allowing further distillation after the hibonite crystallization stage.
While the Mg isotopic fractionation
can be explained by
distillation that took place during the igneous formation of
LA34 13- l/3 1 and MUR7-228, the major element composition cannot be explained by distillation, and only some features of the REE and RTE abundances could possibly result
from it. The original material from which these two inclusions
formed must have been already very refractory and could
not have been anywhere near chondritic in initial composition. For example, the Mg/Al ratio of 0.28 in MUR7-228 is
only 2.5% of the chondritic ratio of 11.4. If Al is assumed to
be involatile during distillation, then Rayleigh fractionation
involving the loss of 97.5% of Mg should result in a Mgisotopic mass fractionation of around +~~‘$xJ/AMIJ, which is
clearly not observed.
The depletion of the relatively volatile RTEs, Sr and Ba,
in MUR7-228 and of Eu in LA34 I3- I / 3 1 could be taken as
distillation signatures. However, they could also be inherited
from the precursor material. The chemical composition
of
these two inclusions must therefore, to a large extent, be the
result of processes acting before the last event (melting and
solidification)
responsible for their formation; in particular,
the Group II characteristics
of LA34 13- I / 3 1 require multistage condensation
processes (DAVIS and L. GROSSMAN,
1979).
This is even clearer for MUR7-753, whose trace element
systematics cannot be derived from distillation from an initial
chondritic reservoir or even from a reservoir which originally
had relative chondritic abundances
of the REEs. The bulk
T. R. Ireland, A. J. Fahey. and E. K. Zinner
376
composition
is depleted in the most refractory REEs, and, if
anything, the glass in MUR7-753 appears to be isotopically
light rather than heavy. The REE pattern is similar to the
Allende Group II pattern in its low ultrarefractory
element
abundances relative to the trend defined by the LREEs, but
differs in the positive Eu and Yb anomalies and the volatilitycontrolled fractionation between the LREEs. It most resembles the perovskite patterns described by IRELAND et al.
( 1988a) but displays an overall fractionation
superimposed
on the perovskite pattern. It can only be produced by condensation from a gas from which an ultrarefractory
component had been separated. This could also account for isotopically light Mg, but at the temperatures
required to fractionate the REEs, Mg would probably be entirely in the gas
phase. However, formation as a direct condensate from such
a depleted gas is an oversimplification
since the RTE pattern
does not show the elemental fractionation
expected. In particular, there is no tendency in the RTE pattern for the ultrarefractory elements Zr, Hf. SC, and Y to be systematically
depleted relative to the more volatile elements Sr, Ba, Nb.
and V, as is the case for the Allende Group II pattern.
It appears that besides showing a REE inventory depleted
in elements according to their volatility, a refractory component rich in Gd is preserved in MUR7-753.
While the
abundance of Lu is relatively constant in all three measurements (2-3 X CI), the Gd/Lu ratio varies from 18.5 in the
first hibonite analysis to 9.7 in the second, and 2.2 in the
glass (relative to the CI value of 8.0). Gadohnium-enriched
compositions
have previously been reported from hibonite
in Murchison inclusion GR- 1 and hibonite inclusions within
corundum
in Murchison inclusion BB-5 by HINTON et al.
( 1988 ). In both of these cases, Gd is the most abundant REE
followed by a smooth drop in the abundances to Lu. However,
in GR-I and BB-5, the LREEs also exhibit enrichments
in
the relatively refractory LREEs La and Nd, whereas they are
depleted in MUR7-753. The REE systematics of MUR7-753
appear to be produced predominantly
by the removal of a
refractory component
(cf. Allende Group II ), but a Gd-enriched component
has been incorporated
into the hibonite.
The unique morphology of MUR7-753 does not seem to
be related to the REE signature. MACPHERSON et al. ( 1989)
analyzed four hibonite-glass spherules from ALH85085 that
have igneous textures and no excess 26Mg, similar to LA34 131/3 1 and MUR7-228, and three of them have REE patterns
depleted in the ultrarefractory
REEs. positive Eu, Tm, and
Yb anomalies, and volatility-fractionated
LREEs. However,
the lack of RTE partitioning
may be related to the process
that produced the unusual morphology of MUR7-753.
In conclusion, the morphology and Mg isotopic fractionation of microspherules
do not seem to be directly associated
with a particular trace element pattern. While the first two
properties are most likely the result of igneous events (melting
and distillation).
the last has been inherited and is the result
of processes preceding the final formation.
4.3. Isotopic Systematics
and Precursors
We have discussed previously ( FAHEY et al., 1987b; IRELAND, 1988, 1990) the preservation of isotopic anomalies in
terms of the chemical memory model (e.g., D. D. CLAYTON,
1978). The signatures of various nucleosynthetic
sites are
preserved in refractory interstellar dust, and the isotopic
composition
of an inclusion is the average composition
of
the component
dust. It is generally the case that the largest
anomalies are found in the smallest inclusions (hibonites,
and the microspherules of the present study). This correlation
could just be the result of the statistics of mixing, i.e., the
larger the refractory inclusion formed, the more likely the
average is going to be isotopically normal. But this is most
likely not the main reason, since the distributions of Ca and
Ti anomalies are by no means statistical; e.g., there are differences in the systematics between 48Ca and “Ti excesses
on one hand and deficits in these isotopes on the other
( NIEDERER et al., 1981; NIEDERER and PAPANASTASSIOU,
1984; IRELAND, 1990). One reason must have to do with the
temporal evolution and mixing of material and with varying
formation mechanisms
of large and small inclusions. Apparently, small inclusions containing large isotopic anomalies
formed preferentially early in the solar nebula when thorough
mixing of the original material had not yet been achieved.
Large anomalies are expected in Ca and Ti because of
their refractory nature and their resistance to homogenization
by distillation and recondensation.
Magnesium is abundant
as Mg,SiO, grains in the interstellar medium and in dust
clouds. But since it is a much more volatile element there is
a much greater likelihood for the homogenization
of Mg by
distillation/recondensation
and also for dilution by normal
material. Primary Mg isotopic anomalies can only be unambiguously identified if they are in the form of 2hMg deficits
since 26Mg excesses can be produced by “Al decay. Negative
626Mg anomalies are rare. The FUN inclusions C 1 and EK l4-l are characterized
by 26Mg depletions of about -3.5 and
-1.7%0, but also by large fractionation
($20 and +30%0/
AMU, respectively), as are FUN purple, spinel-rich inclusions
(PAPANASTASSIOU and BRIGHAM, 1989). Two hibonites
measured by IRELAND et al. ( 1986) were also characterized
by 26Mg depletions of 2%0 and isotopic mass fractionation of
+ 1~%~/AMu. The application ofan inappropriate
mass-fractionation law could produce apparent anomalies in 26Mg
during normalization: however. the application of any known
law still results in 26Mg deficits in these inclusions. In addition,
Egg-3 has a 26Mg depletion in the intercept of its AI-Mg isochron ( ESAT et al., 1980).
As a group, the spherules of this study, and apparently also
similar microspherules
from ALH85085 (A. DAVIS, priv.
comm.), are highly unusual in that they contain depletions
in 26Mg. Like the FUN inclusions, they are characterized by
deficits in 26Mg, coupled with the presence of large Ti and
Ca isotopic anomalies, but unlike the FUN inclusions, they
do not have large Mg isotopic mass fractionation.
Also like
FUN CAIs and most PLAC hibonites, the spherules did not
have 26A1 or had ( 26A1/27Al)o < 1.7 X 10-5, below the “canonical” ratio of 5 X lO_‘. This observation has been discussed previously for these two types of refractory objects in
terms of (a) late formation or isotopic resetting (i.e., within
a chronological
framework for *‘Al) and (b) heterogeneity
of 2hA1 in the early solar system ( WASSERBURG and PAPANASTASSIOU, 1982; IRELAND et al., 1988a). There is no definitive proof for either alternative offered by the data on the
spherules. However. we shall argue below, on the basis of the
large Ca and Ti isotopic anomalies, that these inclusions
Isotope anomalies in refractory inclusions
formed early relative to “normal” CAIs. Such an interpretation would require that their formation region was devoid
of 2hAl.
The Ca-Ti anomalies in the spherules are similar in magnitude to those found in PLAC hibonites, but there are significant differences between the spherules and the PLACs
(Fig. 3). Ofthe PLACs. around two out ofthree have negative
anomalies in 4RCa and 50Ti (IRELAND, 1990). whereas all
three spherules show excesses in these isotopes. The relative
sizes of the 48Ca and 5”Ti anomalies are also unusual. PLACs
with negative anomalies generally have 6”%Za/d“Ti of around
unity, while the hibonites with large positive excesses, MYH3/H4 and MUR13-13, have S4*Ca/6”Ti of around 0.5.
Only two PLACs, MUR-H7 and MUR-H8, out of 3 I analyzed have similar compositions to the three spherules ( ZINNER et al., 1986; FAHEY et al., 1987a; IRELAND. 1990).
Therefore, even with the small number of spherules analyzed,
they appear to show much less diversity than three randomly
picked PLACs. The spherules are also distinct from any other
isotopically anomalous silicate-bearing inclusions. Figure 3
also shows the 4RCa-50Ti composition of EK I -4- 1 (LEE et al.,
1978; NIEDERERet al., 198 I), the FUN inclusion with the
largest 48Ca and “Ti excesses found to date. Excesses in the
microspherules of this study are up to 3X larger.
It has to be kept in mind that differences in the 6”‘Cai
FtSoTiratios could either reflect the primary nucleosynthetic
signature of the n-rich component (CAMERON, 1979; HARTMANN et al., 1985) or result from chemical fractionation of
the carriers or from mixing of the carriers with material of
different Ca/Ti ratios. The comparison of &values implies
mixing of the original carriers of anomalous Ca and Ti with
a reservoir of solar Ca/Ti. Under such an assumption different
64XCa/650Ti ratios can be explained by different values of
the maximum neutron enrichment in the multizone mixing
model of HARTMANNet al. ( 1985 ). Correlated variations of
648Ca/650Ti and 6s0Ti/S49Ti seem to confirm this interpretation: while in PLACs MYH3/H4 and MUR13-13 the
former ratio is 0.5 and the latter -25 ( ZINNER et al.. 1986;
IRELAND,1990), in MUR-H7 and EKI-4-l the &4RCa/650Ti
ratios are 3.2 and 3.6 and the fi50Ti/649Ti ratios are 3.4 and
2.0, respectively (LEE et al., 1978; NIEDERER et al., 1981;
ZINNER et al., 1986). We note that 650Ti/649Ti in MUR7753 determined by IRELAND( 1988) is 6.4 _t 3.2.
The 648Ca/650T’i in the mi~rospherules is approximately
2.0 (Fig. 3), but unfortunately the large errors do not allow
precise determinations of 6 50Ti/ 649Ti, so we cannot exclude
a mixing origin for this high ratio. If, indeed, anomalous CaTi carriers with 648Ca/650Ti of -0.5 were mixed with very
refractory material, a Ti/Ca ratio 4X the solar ratio in this
precursor coutd have resulted in the 648Ca/6s0Ti ofapproximately 2.0 we now observe in the microspherules.
Since PLACs are the only type of refractory inclusions
with Ca-Ti isotopic anomalies ofat least the same magnitude
as those measured in the microspherules, we can ask whether
there are any relationships between these types of inclusions.
We have argued above that the spherules formed from molten
droplets of precursor material that must have included condensates. Could the spherules be derived from molten PLAC
hibonites?
From a representative composition of PLACs (Al*@ 87.8,
CaO 8.5. TiOl 2.5, MgO 1.25) and the composition of a
377
spherule (e.g.. MUR7-228, based on trace element inventory.
with Si02 22.1, A1203 54.8, CaO 16.5, TiOz 2.0, MgO 4.6),
we can calculate the composition of the additional mixing
component required. If the sole source of Al203 is hibonite,
the mixing component would have the composition Si02
59.1, CaO 29.9, Ti02 1.2, MgO 10.2 wtlo: and the spherule
composition is a mixture of roughly 37% of this Ca-silicate
component with 63% of the PLAC hibonite component. If
the Ca-silicate component is isotopically normal, Ca, Ti, and
Mg isotopic anomalies will be diluted by factors of 3.2, 1.3,
and 5.9, respectively. Therefore, given anomalies such as
648Ca = 40%0, 650Ti = 20%0,and h2”Mg = -3%0 in the spherules, the hibonite precursor would need to have anomalies
of 64RCa = 128%0,6~*Ti = 26%0, and 62hMg = - IS%,. If the
spheruie composition based on modal abundances (Table 3)
is used, the mixing component would have almost the same
composition, but the mixing fractions would be 67 and 33%,
and the dilution factors would be 8.1,2.2, and 17.4, resulting
in even more extreme required anomalies. Such isotopic
compositions have never been measured in a PLAC hibonite.
This does not strictly preclude a hibonite precursor, but it is
very unlikely that PLACs were the source of the isotopic
anomalies in the spherules.
We can of course turn the question around and ask: Could
PLACs be derived from hibonite-glass spherules? To first order, PLACs do appear to be similar to hibonite that might
be released from the breakup of glass spherules. However,
there are differences. PLACs are generally much larger than
the hibonite crystals in the spherules. The overwhelming majority of PLACs have trace-element patterns depleted in the
less refractory elements Eu, Yb, V, Ba, Sr, and Nb. They are
not dissimilar to the hibonite in MUR7-228, and, in fact,
IRELANDet al. ( I988a) originally classified MUR7-228 with
the PLACs because of the similarities. But it was noted that
MUR7-228 is indeed different: it does not have a Yb anomaly,
and it is monotonically fractionated from La to Lu. PLACs
normally have Yb anomalies larger than Eu anomalies, and
they do not show the same degree of fractionation as MUR7228 and generally have flat LREE patterns and roll-off only
in the HREEs. This roll-off may be due to solid-liquid partitioning, but it does not appear to be directly attributable to
the same igneous fractionation process as for the spherules.
The spherules appear to be distinct from the PLACs in terms
of their formation.
4.4. Conclusion
The microspherules seem rather to constitute a separate
class of refractory inclusions, characterized by (a) distinct
mo~holog~ and mineralogy, (b) large *%a and “Ti excesses
with d4*Ca/b5% = -2.0, the excesses being larger than those
found in any other silicate-bearing inclusions, and (c) 26Mg
depletions. It is curious that nucleosynthetic signatures are
associated with the igneous formation event which is likely
to be only the last step of other physicochemical processes
experienced by most of the material from which these inclusions formed. This is a problem similar to that encountered
in the interpretation of the association of certain (but not
all, e.g., not trace element patterns) mineralogical-physicalchemical properties of the FUN inclusions (large mass fractionation% distinct mineralogy of the purple spinel-rich in-
T. R. Ireland, A. J. Fahey, and E. K. Zinner
378
elusion; PAPANASTASSIOU
and BRIGHAM,1989) with isotopic
anomalies in the Fe-peak elements.
A model to explain the characteristic features of the microspherules would invoke mixing of interstellar dust grains,
the carriers of anomalous Ca and Ti, with preprocessed isotopically normal material. The preprocessing is responsible
for the major element composition (highly refractory) and
the trace element signature of this material. It most likely
includes condensation, but could also include the preferential
enrichment of refractory precursor grains by the evaporation
of more volatile phases, It is likely that all three spherules
formed from molten droplets, but while the morphologies of
LA341 3-l /3t and MUR7-228 are consistent with such an
origin, MUR7-753 has an unusual morphology that is not
readily interpretable as an igneous texture. Only MUR7-228
has distribution coefficients consistent with the equilibrium
hibonite-liquid partition coefficients of Dn.Am and BOWTON
( 1988) but the other two do not. If these spherules did solidify
from melts, they did not all do so under equilib~um conditions. This is consistent with the presence of glass, requiring
rapid cooling which is apparently a consequence of the small
size of these CAIs.
It is unclear whether the melting of the droplets occurred
by the same mechanism that is invoked for other igneous
CAIs and for chondrules (the enigmatic “chondrule forming
process”). One possible distinction is that it must have occurred early, before isotopic anomalies were obliterated by
continued mixing, and, apparently, it occurred in this form
only in a part of the solar nebula preferentially containing
carriers with 4*Ca and “Ti excesses, and lacking 26A1.
However, it may be premature to build a complicated formation model on the basis of the analysis of three inclusions.
Microspherules are exceedingly rare, and only ALH85085
seems to have sampled them preferentially. It remains to be
seen whether those spherules also have the unique Ca and
Ti isotopic characteristics of the inclusions studied here.
.4c~nf~wiedg~7ent,~-We are greatly indebted to Gero Kurat for drawing our attention to the Lance inclusion 3413-I /3 I, for providing
us with the thin section Lance 3413, and for extensive discussions.
Helpful comments by Glenn MacPherson and one anonymous reviewer improved this manuscript considerably. We thank Robert
Walker for his interest and support. This work was funded by NASA
grant NAG9-55.
Editorial handling: K. Marti
REFERENCES
BECKETTJ. R. ( 1986) The origin of Ca-, Al-rich inclusions from
carbonaceous chondrites; an experimental study. Ph.D. thesis,
Univ. Chicago.
BECKETTJ. R., LIVED., FUN-DOWT., GROSSMANL., and STOLPER
E. ( 1988) Ti3+ in meteoritic and svnthetic hibonite. Geochim.
Cosmochim. Acta 52, 1479-1495. .
BOYNTONW. V. ( 1975) Fractionation in the solar nebula: Condensation of yttrium and the rare earth elements. Geochim. Cosmoc&m. Acta 39, 569-584.
BRIGHAMC. A., PAPANASTASSIOZI
D. A., and WASSERBURGG. J.
( 1985) Mg isotopic heterogeneities in fine-grained Ca-Al-rich inclusions. Lunar Planet. Sci. XVI, 93-94.
BRIGHAMC. A., HUTCHEON1. D., PAPANASTASSIOU
D. A., and
WASSERBURGG. J. ( 1986) Evidence for 26Aland Mg isotopic
heterogeneity in a fine grained CAL Lunar Planet. Sci. XVII, 8%.
86.
CAMERONA. G. W. (1979) The neutron-rich silicon-burning and
equilibrium processes of nucleosynthesis. Ap. J. (Let&) 186, L.53L57.
CATANZAROE. J., MURPHY T. J., GARNER E. L.. and SHIELDS
W. R. f 1966) Absolute isotopic abundance ratios and atomic
weights of magnesium. J. Res. Natf. Bur. Stand. 70a, 453-458.
CLAYTOND. D. ( 1978) Precondensed matter: Key to the early solar
system. Moon and Planets 19, 109- 137.
CLAYTONR. N., MAYEDAT. K., and MOLINI-VELSKOC. A. ( 198.5)
Isotopic variations in solar system material: evaporation and condensation of silicates. In Profostars and Planets II (eds. D. C. BLACK
and M. S. MATTHEWS),pp. 775-77 i Univ. Arizona Press.
DAVISA. M. and GROSSMANL. ( 1979) Condensation and fmctionation of rare earths in the solar nebula. Geochim. Casmochim.
Acta43, 161 l-1632.
DAVISA. M., HASHIMOTOA., CLAYTONR. N., and MAYEDAT. K.
( 1990) Correlated isotopic mass fractionation of oxygen, magnesium and silicon in forsterite evaporation residues. Nature 347,
655-658.
DRAKEM. J. and JWYNTONW. V. ( 1988) Partitioning of rare earth
elements between hibonite and melt and implications for nebular
condensation of rare earth elements. Meteorifics 23, 75-80.
EKAMBARAM
V., KAWABEI., TANAKAT., DAVISA. M., and GROSSMANL. ( 1984) Chemical compositions of refractory inclusions in
the Murchison C2 chondrite. Geochim. Cosmochim. Acia 48,20892105.
ESAT T. M. and TAYLORS. R. ( 1984af Correlated REE and Mg
anomalies in Allende inclusions. tutPar Planet. Sci. XV, 252-253.
ESAT 7. M. and TAYLORS. R. (1984b) FREE FUN with Mg in
Allende Group II inclusions. Lunar Planet. Sci. XV, 254-255.
ESATT. M., PAPANASTASSIOU
D. A.. and WASSERBURG
G. J. ( 1980)
The initial stage of 26AIand 26Mg/24Mg in the early solar system:
Lunar Planet. Sci. XI, 262-263.
FANEYA. J. ( 1988) Ion microprobe measurements of Mg, Ca, Ti.
and Fe isotopic ratios and trace element abundances in hibonitebearing inclusions from primitive meteorites. Ph.D. thesis. Washington Univ., St. Louis.
FAWEYA., ZINNER E., and KURAT G. ( 1986) Anomalous Ca and
Ti in a hercynite-hibonite inclusion from Lance. Meteoritics 21,
359-361.
FAHEYA. J., GOSWAMIJ. N., M~KEEGAN K. D., and ZINNER E.
( 1987a) I60 excesses in Murchison and Murray hibonites: A case
against a late supernova injection origin of isotopic anomalies in
0, Mg, Ca, and Ti. Ap. J. [Letfs.) 323, L91-L95.
FAHEYA. J., GOSWAMIJ. N., MCKEEGAN K. D., and ZINNER E.
(1987b) 26Al, 244Pu,‘“ri, REE and trace element abundances in
hibonite grains from CM and CV meteorites. Geochim. Cosmochim. Acta 51, 329-350.
FAHEYA. J., ZINNERE. K., CROZAZG., and KORNACKIA. S. ( 1987~)
Micr~ist~butions of Mg isotopes and REE abundances in a Type
A calcium-aluminium-~ch inclusion from Efremovka. Geachim.
Cosmochim. Acta 51,32 15-3229.
FAHEY A., ZINNER E., and MACPHERSONG. (1987d) Where do
hibonites with anomalous Ti and no excess 26Mgcome from?
Meteoritics 22, 377-378.
GEIGERC. A., KLEPPA0. J., MYSEN B. O., LATTIMERJ. M., and
GROSSMANL. ( 1988) Enthalpies of formation of CaA&O, and
CaA1i20i9 (hibonite) by high tem~mture, alkali borate solution
calorimetry. Geochim. ~osmochim. Acta 52, 1729- 1736.
GROSSMANJ. N., RUBIN A. E., and MACPHERSONG. J. (1988)
ALH85085: a unique volatile-poor carbonaceous chondrite with
possible implications for nebula fractionation processes. Earth
Planet. Sci. Lett. 91, 33-54.
GROSSMANL. ( 1972) Condensation in the primitive solar nebula.
Geochim. Cosmochim. Acfa 36,597-619.
GROSSMANL. ( 1980) Refractory inclusions in the Allende meteorite.
Ann. Rev. Earth Planet. Sci. 8, 559-608.
HARTMANND., W~~SLEY S. E., and EL EID M. F. (1985) Nucleosynthesis in neutron-rich supernova ejecta. Ap. J. 297, 837-845.
HASHIMOTO
A., WOODJ. A., and WEINBERGA. ( 1987) Experimental
Isotope anomalies in refractory inclusions
determination of Ca( OH), vapor pressure as a key to understanding
the alteration of Ca, Al-rich inclusions. Meteoritics 22, 405-406.
HINTON R. W.,DAVIS A. M.,and SCATENA-WACHEL D.E.(1987)
Large negative roTi anomalies in refractory inclusions from the
Murchison carbonaceous chondrite-evidence
for incomplete
mixing of neutron-rich supernova ejecta into the solar system. Ap.
J. 313,420-428.
HINTONR. W., DAVISA. M., SCATENA-WACHEL
D. E., GROSSMAN
L., and DRAUS R. J. (1988) A chemical and isotopic study of
hibonite-rich refractory inclusions in primitive meteorites. Geochim. Cosmochim. Acta 52.2573-2598.
HUTCHEONI. E., ARMSTRONGJ. T., and WASSERBURG
G. J. ( 1984)
Mg isotopic studies of Leoville “compact” Type A CAI. Meteoritics
19,244-245.
HUTHCEONI. D., ARMSTRONGJ. T., and WASSERBURG
Cl. J. ( 1986)
Mg isotopic studies of CA1 in C3V chondrites. Lunar Planet. Sci.
XVII, 372-373.
IRELANDT. R. ( 1988) Correlated morphological, chemical, and isotopic characteristics of hibonites from the Murchison carbonaceous
chondrite. Geochim. Cosmochim. Acta 52,2827-2839.
IRELANDT. R. ( 1990) Presolar isotopic and chemical signatures in
hibonite-bearing refractory inclusions from the Murchison carbonaceous chondrite. Geochim. Cosmochim. Acta 54,32 19-3237.
IRELANDT. R. and ZINNER E. K. ( 1989) Oxygen isotopic compositions of Murchison refractory inclusions. Meteoritics 24, 279280.
IRELANDT. R., COMBTON W., and ESATT. M. ( 1986) Mg isotopic
compositions of olivine, spine1 and hibonite from the Murchison
carbonaceous chondrite. Geochim. Cosmochim. Acta 50, 14 131421.
IRELANDT. R., FAHEY A. J., and ZINNER E. K. (1988a) Traceelement abundances in hibonites from the Murchison carbonaceous
chondrite: constraints on high-temperature processes in the solar
nebula. Geochim. Cosmochim. Acta 52,2841-2854.
IRELANDT. R., FAHEYA. J., and ZINNERE. K. ( 1988b) Petrogenesis
of a hibonite-pyroxene spherule from Murchison. Meteoritics 23,
275-276.
IRELANDT. R., FAHEYA. J., and ZINNERE. K. ( 1989) Isotopic and
chemical constraints on the formation of HAL-type refractory inclusions. Lunar Planet. Sri. XX. 442-443.
KORNACKIA. S. and FEGLEY B., JR. ( 1984) Origin of spine]-rich
chondrules and inclusions in carbonaceous and ordinary chondrites. J. Geophys. Rex Suppl. 89, B588-B596.
KORNACKIA. S. and FEGLEYB., JR. ( 1986) The abundance and
relative volatility of refractory trace elements in Allende Ca, Alrich inclusions: implications for chemical and physical processes
in the solar nebula. Earth Planet. Sci. Lett. 79, 217-234.
KURAT G. ( 1975) Der kohlige Chondrit Lance: Eine petrologische
Analyse der komplexen Genese eines Chondriten. Tschermaks
Mineral. Petrogr. Mitt. 22, 38-78.
LEE T. ( 1988) Implications of isotopic anomalies for nucleosynthesis.
379
In Meteorites and the Early Solar System (eds. J. F. KERRIDCE
and M. S. MATTHEWS),pp. 1063-1089. Univ. Arizona Press.
LEE T., PAPANASTASSIOU
D. A., and WASSERBURGG. J. (1978)
Calcium isotopic anomalies in the Allende meteorite. Ap. J. (Letts.)
220, L21-L25.
LEE T., RUSSELLW. A., and WASSERBURGG. J. ( 1979) Calcium
isotopic anomalies and the lack of aluminium-26 in an unusual
Allende inclusion. Ap. J. (Lefts.) 228, L93-L98.
LORINJ. C. and CHRISTOPHEMICHEL-LEVYM. ( 1978) Radiogenic
26Mgfine-scale distribution in Ca-Al inclusions in the Allende and
Leoville meteorites. In Short Papers of the 4th Intl. Conf Geochronology, Cosmochronology, and Isotope Geology (ed. R. E.
ZARTMAN); USGS Open-file Rept. 78-701, pp. 257-259.
MACDOUGALLJ. D. ( I98 I ) Refractory spherules in the Murchison
meteorite: Are they chondrules? Geophys. Res. Lett. 8, 966-969.
MACPHERSONG. J., WARK D. A., and ARMSTRONGJ. T. (1988)
Primitive material surviving in chondrites: refractory inclusions.
In Meteorites and the Early Solar System (eds. J. F. KERRIDGE
and M. S. MATTHEWS),pp. 746-807. Univ. Arizona Press.
MACPHERSONG. J., DAVIS A. M., and GROSSMANJ. N. (1989)
Refractory inclusions in the unique chondrite ALH85085. Meteoritics 24, 297.
MARTINP. M. and MASONB. ( 1974) Major and trace elements in
the Allende meteorite. Nature 249, 333-334.
MCKEEGANK. D., WALKERR. M., and ZINNERE. ( 1985) Ion microprobe isotopic measurements of individual interplanetary dust
particles. Geochim. Cosmochim. Acta 49, I97 I - 1987.
NIEDERERF. R. and PAPANASTASSIOU
D. A. ( 1984) Ca isotopes in
refractory inclusions. Geochim. Cosmochim. Acta 48, 1279-1293.
NIEDERERF. R., PAPANASTASSIOU
D. A., and WASSERBURGG. J.
( I98 I ) The isotopic composition of titanium in the Allende and
Leoville meteorites. Geochim. Cosmochim. Acta 45, 1017- 103 I.
PAPANASTASSIOU
D. A. and BRIGHAMC. A. ( 1989) The identification
of meteorite inclusions with isotope anomalies. Ap. J. (Letts.) 338,
L37-L40.
PROMBOC. and LUGMAIRG. W. ( 1986) Search for correlated isotopic
effects in Allende CAIs. Lunar Planet. Sri. XVII, 685-686.
REID A. M., WILLIAMSR. J., GIBSONE. K., and FREDRIKSSON
K.
( 1974) A refractory glass chondrule in the Vigarano chondrite.
Meteoritics 9, 35-45.
WASSERBURG
G. J. and PAPANASTASSIOU
D. A. ( 1982) Some shortlived nuclides in the early solar system-a connection with the
placental ISM. In Essays in Nuclear Astrophysics (eds. C. A.
BARNESet al.), pp. 77-140. Cambridge Univ. Press.
ZINNER E. and CROZAZG. ( 1986) Ion probe determination of the
abundances ofall the rare earth elements in single mineral grains.
In Secondary Ion Mass Spectrometry (SIMS V) (eds. A. BENNINGHOVEN et al.), pp. 444-446. Springer-Verlag, Berlin.
ZINNER E. K., FAHEYA. J., GOSWAMIJ. N., IRELAND T. R., and
MCKEEGAN K. D. ( 1986) Large 48Ca anomalies are associated
with 5% anomalies in Murchison and Murray hibonites. Ap. J.
(Letts.) 311, L103-L107.