Geachzmrca ef Cosmwhrmrca Acm Vol. 49, pp. 1989-1993
0 Pngsmon Press Ltd. 198S. Rinted in U.S.A.
LETTER
Titanium isotopic anomalies in hibonites from the
Murchison carbonaceous chondrite
T.R. IRELAND, W. COMPSTON
Research School of Earth Sciences, The Austraiian National University, Canberra ACT 260 I. Australia
and
H. R. HEYDEGGER*
Department of Chemistry and Physics, Purdue University Calumet, Hammond, IN 46323 U.S.A. and
Enrico Fermi Institute, University of Chicago, Chicago. IL 60637 U.S.A.
(Received d4u.v27, 1985: uccepred in rev~sed.f~r~ Augwr 8,
1985)
Abstract-The isotopic compositions of titanium in eight grains of hibonite fCaAl,20,9) from the carbonaceous
chondrite Murchison have been determined by high precision secondary ion mass spectrometry using an
ion microprobe. The titanium in the hibonites varies greatly in Ti. from about -42 to +8 permil (relative
to terrestrial) with smaller (up to 4 pennil). but clearly resolvable, effects in ‘6Ti and 48Ti. These results
complement ion probe measurements by FAHEYet al. (1985) of a 100 permil excess of “ri in a hibonite
grain from the carbonaceous chondrite Murray, and confirm the presence of widespread negative anomalies
suggestedby the results of HUTCHEONet af. (1983) on hibonites from Murchi~n. The magnitude of these
variations seems explicable only in terms of nucleogenic pmcesses which produced extremely variable titanium
isotopic abundances in the hibonite source materials. The hibonites evidently did not participate to the same
extent as most material in the mixing and homogenisation processes that accompanied the formation and
later evolution of the solar system. Thus, significant source materials of the hibonites may be the supernova
condensates of CLAYTON(1978) and may support the concept of “chemical memory” (CLAYTON,
1978;
NIEMEYERand LUGMAIR,1984).
THE ISOTOPIC
composition of titanium in refractory
covered using ion microprobes on a small number of
inclusions from meteorites is an important indicator
hibonite grains from C2M chondrites.
of nucleosynthetic processes. Titanium is on the lower
HUTCHEON et al. ( 1983) first analysed four hibonite
mass side of the iron abundance peak and therefore
inclusions from Murchison using an ion microprobe
its composition can reflect variations in the quasiset at low mass resolution. Two of the hibonites apequiIib~um conditions in stellar interiors giving rise peared to have a normal titanium isotopic com~sition
to that peak, provided nuclear processing during in- but the other two were character&d by large ‘@lYdefjection into the interstellar medium is minimal
icits of -8 f 2 permil (DJ-6). and -16 + 3 permil
(WOOSLEYand WEAVER, 1982). Titanium is important
(DJ-5). The precision of the data was not sufficient to
also in having one of the most refractory oxides and
resolve anomalies in the other isotopic ratios. FAWEY
is concentrated in refractory inclusions by either conet af. (1985) have recently reported the titanium isoden~tion or evaporation processes (GROSSMAN,1972). topic com~sition of three hi~nit~
at high mass resA large number of titanium isotopic analyses has olution to separate all significant isobaric interferences,
including 4sCa, from 48Ti. Two Murchison hibonites
been carried out on individual calcium aluminiumrich inclusions (CAI) from Allende and other carbowere character&d by excesses of - +8 permil in connaceous chondrites by thermal ionisation mass spec- trast to the deficits recorded by HUTCHEON et al.
(1983). A hibonite inclusion from Murray has a retrometry. The great majority of these show a 1 permil
enhancement
of qi
relative to terrestrial ratios
markable signature in having a 100 permit excess in
(NIEDERER ef al., 1980, 1981, 1985; NIEMEYERand
% with effects at the other two normal&d ratios
LUGMAIR, 198 I, 1984; HEYDEGGER et al.. 1982). clearly resolved.
In this letter we document the presence of wideAnalyses of the so called FUN (Fractionation and Unknown Nuclear) inclusions show more diversity, but spread titanium isotopic heterogeneities in Murchison
even here the anomalies are limited to a range of PIi
hibonites, and independently confirm the large deficits
from -5to f4 pertnil (NIEDERER~~al.,
I981.19851. in ‘@fi indicated by the first ion micropro~ measureLargeranomalies, up to 100 permil, have been dis- ments of titanium in Murchison hibonites by HUTCHEON et al. (1983).
The great virtue of ion microprobe analysis is that
* N.R.C. Resident Associate, NASA Johnson Space Center,
SN3, Houston, TX 77058, U.S.A.
isotopic compositions
can be determined
in situ on a
few nanograms of sample. In this study titanium iso-
1989
I990
i K Ireland. W iompston
topic analyses have been carried out on a JO-75 pm
size fraction of hibonite grains separated from the
Murchison (GM) chondrite. (~‘onventional titanium
isotopic analysis of such small individual grains would
be extremely difficult
Hibonite is an ultrarefractory mineral composed ofcalcium
hexaluminate (CaAl,,O,o) with minor but significant substitution of titanium and magnesium. The Murchison hibonites
were extracted by crushing 67 g of small meteorite fragments
to pass a 400 pm mesh, with further sieving to collect the 4075 pm fraction. The denser fraction (p > 3.3 g/cm’) was separated in methylene iodide and the non-magnetic fraction
was collected after magnetic separation in an alcohol suspension. The resulting concentrate was handpicked for blue inclusions. which were mounted in epoxy, and polished in prep
aration for analysis. Of seventy blue grains separated and
mounted. eight were identified by electron microprobe as
dommantly hibonite and these were selected for isotopic analysis. The eight hibonite grains are morphologically divided
mto three inclusions and five crystal fragments and range from
1.7 to 6.4% TiOz. Most of the remainder were spine1 with
<;15 pm inclusions of hibonite and perovskite.
The titanium isotopic analyses were carried out on the ion
microprobe. SHRIMP. designed and constructed at the Australian National University (CLEMENTer al.. 1977). A negative
oxygen primary beam of l-3 nA focused to a spot 25 pm in
diameter sputters a sufficiently intense beam of Ti’ ions for
precise titanium analysis at a mass resolution of 7OClO.
A fraction of the secondary ion beam was monitored using a Car)
401 electrometer at a suppressed aperture before the source
slit. and the counts on each isotope were divided by the charge
collected simultaneously on this monitor. This procedure ap-
and H. K. Heydegger
proximates to a double collecting mode ol operation ano IIsulted in increased precision. by removing the cllects ol‘ an:
low frequency Instability in the primary beam The iso~opc~.
of interest were measured sequentially by cyclic magnetic liclti
stepping. each peak being electrostatically centered hy dellec
tion plates located in front of the collector slit Flat-topped
peaks. in which the top IS equal to or wider tharr one-third
the base width. were maintained for each isotope h! automanr
mechanical displacement of the collector assemhi\ along the
beam path. This compensated for small mass-dependent zhrti%
in the focal pomt of the magnetic analyser
High precision titanium isotopic analyses b> ion micropior*.
require an intense secondary ion beam for good counting ststistics and. stmultaneously. sufficient mass resolution to scl,arate isobaric interferences. At pi mass resolution of 7000 21
A.%{(I% valley), all molecular interferences. rnriudinp iii
drides. are resolved from the titanium isotopes. (ini! 3tomii
isobaric interferences from &C‘a.%V. and “Cr are not resolved
these may be estimated by monuoring %t. “a’ :md “f!
and then stripped from the respective titamum iroban 1 h
resolution is sufficient to fully separate ‘*Ca from “‘Ti %hen
the latter is centered in the collector slit: during 4 mass scan
48Ca does not enter the collector slit until “‘Ti just begins tt’
leave it. In addition. because the “Ca intensity IS ver) iov.
compared with ‘8Ti in the hibomtes, there is no observable
contribution (<IO-? to the @Ti count due to scattered 4cC~~
ions.
Where titanium is a major component m the mmerrl under
investigation. e.g. rutile or ilmenite. the secondary beam 15
sufficiently intense to make superior analyses using the Farada)
cup (Table I ). The limitation of the Faraday cup is zem point
noise and for the hibonites, containing only a few percenr
Ti02. better precision is obtained by ion counting. System
deadtime is a major limitation of ion counting rspeciailv nr
TABLE I. Iitanium isotopic analyses of Murchison hibonite grams and terrestnal standards.
-_--.
..~
(I)’
Sample
20 1
2
31 I
43 I
52 I
305
(I?
3.
-----~.-~~-
1312
rTdhTi
14)’
,?‘TI
(5)’
f?‘OTi
(h?
i’a
,.---.-_.-
329
326
324
320
-17.1
- 13.2
- 15.4
- 15.3
-14.0
0.26 -c 0.82
0.80 t I.22
/ 26 c 0.94
-3.36 +_0.86
-I 75 rt 0.90
‘.4Y rt 0.50
; 44 ‘r 1.14
I .08 + 0.62
0.03 t 0.48
0.24 2 0.36
-42.46 f 0.50
4 I .78 2 0.48
I .‘1! + 0.62
17.21 t 0.76
---0.0I f 0.44
RR0
: I
iz
6_
,
54 I 303
55 I 302
2 330
-17.5
-- 14.6
-14.0
-3.67 + 0.54
2.34 + 0.80
I.24 1 0.98
2.45 ? 0.28
3.61 _’ 0.50
7.34 i- 0.84
I .92 f 0.50
7.89 -+ 0.46
7.77 + 1.08
61 I 325
70 I 304
2 318
3 319
-15.3
-13.5
- 13.9
-14.1
3.33 fr 0.96
-~0.46 f 0.64
- 1.41 2 0.82
-0.55 $: 0.84
1 __
‘9 *2.63 i
3.66 ii 23 i-
ks 1
2
3
hi I
I
3
ru I
il I
-18.3
--17.3
-17.6
-14.9
-15.1
- 15.9
-21.6
-19.5
0.23
0.20
-0.17
-0.35
- 0.67
-0.52
0.13
--0 I3
300
.39’
_328
301
317
321
277
295
0.x4
0.36
0.46
0.80
18.28 & 0.72
2 I .48 t 0.44
71.34 i-O.&l
22.40 & 0.56
5.t>
- ‘i
_I
._
4.0
16
8.i
‘X
2 0.8:
+ 1.10
+ 0.8X
2 0.G
-+ 0.94
+ 0.94
I 0.26
~0.04 _t 0.56
0.0’ t 0.96
-0.36 + 0.48
0.3 I :’ 0.50
0.54 I 0.40
0.05 t 0.50
0.03 ?z 0.08
+ 0.13
-0.3’ c 0.22
0.09 f 0.50
~-0.45 F 0.50
0.20 + 0.52
0.20 + 0.52
0.31 ? 0.46
0.71 i 0.54
0.16 t 0.16
0.24 & 0.33
2.x
2-h’
2.’
.’ 6
I.
Z.’
2x
ii..?
ri.(i
\
iti
i
’ Sample identification: grain no., spot number. consecutive data tile number. Terrestrial standards: ks -: kaersurue (Glen
Innes. N.S.W.), hi = hibonite (Antani Mora, Madagascar), ru = rutile (beach sand. Rutile and Zircon Mines (Newcastle)
Ltd.). il = ilmenite (St Urbain. Quebec). Rutile. ilmenite collected on Faraday cup. Files 300-306 deadtimc = 22 ’ ns. Files
3 17-330 deadtime = 2 I .4 ns.
’ A = - 1[(*‘Ti/?‘i).,J I .336] - I ; x lOOO/:!
which ISthe fractionation per atomic mass umt relative to ai lii4” I r i .33X)
(HEYDEGGERel a/.. 1979) heavy enrichment = positive. Typical analytical error 0.3 permil (2cr,).
’ Delta notation deviations from terrestrial standards of HFYDECXERL*Iuf (1982). 6’Ti = [(‘Ti!4’ri),,,/(‘ri/4”l ii,,,,,,
Ij
Y 1000. Errors for Murchison hibonites are 20, of 7 sets of IO ratios.
’ Ca and V + Cr are corrections in permil applied to 46Ti and “Ti respectively due to unresolved isobaric interferences
from calcium. vanadium and chromium
1991
Ti isotopic anomalies
high count rates and for ratios that differ significantly from
unity (HAYESand SCHOELLER,1977). The titanium isotopes
have a rather fo~u~tous abundance pattern in this regard, with
only 48Ti being signi~cantly more abundant than the other
masses. During the Murchison hibonite, terrestrial hibonite.
and kaersutite analyses, the count rate was kept close to 0. I
MHz for 49Ti to minim&e biassing due to error in the estimation of deadtime. However even at this count rate, a 0.1
ns error in dete~ining the deadtime would have a 0.12 permil
effect on the ?i,@% ratio. Deadtime errors have no detectable
effect on the other ratios owing to the similar count rates at
these peaks. Deadtime was determined initially by measuring
48Ti/4vi over a wide range of beam intensities. and later. by
setting the (normalised) 48Ti/49Tito the standard value.
The data were corrected for deadtime. ratioed to the secondary beam monitor, ratioed to mass 49. stripped to remove
the isobaric interferences, and normal&d to I .336 for 4’Ti/
‘?i to correct for mass fractionation. Mass fractionation always accompanies the sputtering process with a systematic
enhancement of the light isotopes (SHIMIZLIand HART. 1982).
For titanium, the effects are large and are also matrix dependent. ranging from 0.8%/amu for Ti metal to 2.8%/amu for
pcrovskite. In addition, instrumental parameters, such as the
secondary ion extraction. may also contribute. but the fractionation for Ti isotopes using SHRIMP has been found to
be constant to within %2 permil for a given phase in a given
mount. The fmctionation is adequately described by a linear
mass fractionation law allowing no~ali~tion
to an arbitrary
datum point, in this case to 1.336 for 4’Ti/+‘9Ti(HEYDEGCXR
et al.. 1979). The actual normalisation law used will have little
effect owing to the limited range of fractionation recorded in
this study. The fractionation corrected ratios of the unknowns
(unk) are then expressed as deviations from the standard (stdf
terrestrial ratios in delta notation as
6’Ti = { [(‘Ti/4%),,,/(‘Ti/49Ti),]
- I ) X 1000
where I’may be 46,48, or 50 and the standard terrestrial values
from HEYDEGGERet al. (1982).
To verify the ability of SHRIMP to make titanium analyses
of high precision and accuracy we cite results for a pyroxene
646Ti[4
separate from the Allende inclusion 3529. 32 which had previously been determined by thermal ionisation mass spectrometry as having a qi excess of 1.O ? 0.2 permil (HEYDECGERet al., 1982). The ion microprobe analyses of individual pyroxene fragments were carried out with 49Ti at 0.2
MHz: 4sTi was not collected during this experiment to avoid
possible overloading of the ion counter. The mean ion microprobe result for three grains was found to be 1.3 -t O-3(20,)
permil excess for %. and a*Ti within error of terrestrial. A
more complete description of these data will be given elsewhere.
The analyses reported here are weighted means of 7 sets of
IO scans of the peak sequence. Each set takes approximately
25 minutes. resulting in a total collection time of -3 hours
per spot analysis. The uncertainties quoted in Table 1as two
standard errors of the mean are propagated from the dispersion
observed amongst the seven set means for each spot. and are,
m general. very close to those expected from counting statistics.
The hibonite titanium analyses were carried out during two
collection periods separated by a week during which SHRIMP
was used for other applications. The first four hibonites were
analysed in a single 24 hour period along with terrestrial kaersutite and hibonite standards. The second period was for 3
days when the remaining hibonites plus three of the previous
four grains were analysed, again with interspersed kaersutite
and hibonite standards. The deadtime determined for the first
set of analyses was 22.2 ns and for the second set 21.4 ns.
However because all the isotopes have similar abundances
except 48Ti. only the 48Ti/4% ratio is sensitive to deadtime
variations of this magnitude. However, during both periods,
interspersed standard runs agree within error with the terrestrial
norms and replicate analyses of samples 20, 55. and 70 show
excellent agreement (Table I), despite the differing deadtimes
used.
The
are the
grains
viduals
most striking features of the hibonite analyses
diversity of the compositions measured between
and the contrasting
homogeneity
within indias analysed in different spots (Fig. 1). As with
all other reported titanium
I
I
isotopic anomalies.
n
FIG. I. isotopic data of hibonites presented in Table i plotted as S*Ti (upper) and S48Ti{lower) vs. S’@Ti;
note difference in vertical and horizontal scales. Italicised numbers refer to Murchison hibonite grains listed
in Table I. Boxes are two sigma uncertainties of the analyses. Replicate analyses of grains 20, 55, and 70
show excellent agreement suggesting intragrain homogeneity as opposed to the diversity of compositions
between individual crystals.
the
I. K. Ireland. W. Compston and H. R. Heydegger
1992
dominant effects are in “Ti. in this case ranging from
-42 to +8 permil. The range of ds% anomalies reported by HUTCHEON ef al. ( 1983). FAHEY PI~1. ( 1985 ).
and HINTON EI ~1. (1985) from Murchison and Murra!,
hibonites is represented in the analyses reported here.
and the increased precision of the SHRIMP results and
the expanded data base have allowed clear resolution
of widespread anomalies in 46Ti (HEYDEGGER e/ al
1982) and 4*Ti. Despite the variety of isotopic compositions, the mass fractionation
is ciose to that observed for terrestrial hibonite. Our experience with the
consistency of sputtering fractionation suggests that an\;
intrinsic isotopic fractionation
should be less than 3
permil/amu. This observation corresponds with the low
values of fractionation (< 1 permil/amu)
reported from
Allende inclusions (NIEDERER et al. 1985).
intriguing.
The deficits in 50Ti are especially
HUTCHEON et al. ( 1983) described 50Ti deficits in two
hibonite grains measured by ion microprobe.
Recent
measurements
reported by HINTON et al. (1985) also
using the Chicago machine included ‘% deficits up
to 60 permil. Thermal ionisation analysis of an “anhydrous” separate from Murchison led NIEMEYERand
LUGMAIR (1984) to report a “hint” of negative ‘vi
anomalies. However. of all of the thermal ionisation
results. only FUN inclusion Cl has yielded an unambiguous deficit (NIEDERER et al., 1985).
Enhancements
of 5% have been much more common in the analyses reported (HEYDEGGER et al.. 1979.
1982; NIEDERER et al., 1980, 198 I. 1985: NIEMEYER
and LUGMAIR, 198 I, 1984). The addition
of a “Tirich component
to a terrestrial-type
composition
was
suggested from the first indication of the + I permit
anomaly in the Allende inclusions (HEYDEGGER CIu/..
1979). The observation
of FAHEY ef u/. ( 1985 1recently
of a 100 permil enhancement
of “Ti in the hibonite
grain from the Murray carbonaceous
chondrite is consistent with such a model. However. the large negative
anomalies are more consistent with models involving
more complex mixtures of isotopically different titanium (NIEDERER el ~1.. 1981. 1985: NIEMEYER and
topes shown to exist amongst hlbomtes tram a slngir
meteorite. we may infer the presence of still greater
isotopic heterogeneity
in their source materials. The
latter may well be the micron-sized grams ofrefractor!
oxides. etc. that condensed
from gases qlected from
one or more stellar objects predating the formatIon ui
planetary bodies in the solar system (CL.&\-TOY, I978 :.
The great variability in titamum isotopic zompositlon
amongst these grains would reflect spatial and/or tern
poral heterogeneities
in the composition
ofmtersteltar
dust from a number of diverse nucleosynthctlc
en\!.
ronments in one or more stellar objects. The accrerior;
of the primary objects into the hundred micron-sired
objects we see today took place with a lesser degree oi
homogenisation
(“chemical
memory”)
than. apparently. characterises
most solar system materials, 17~
with sufficient heating at some stage to cause the degre:.
of intragrain titanium isotopic uniformit! observed.
The diversity of compositions
in Murchison hibon
ites is in marked contrast to the titanium in 4llendc
refractory inclusions. Nine out of ten Allerxie C’AIs
have the + 1 permil 5@lYsignature (HE~IXXXER 1’1U:
1982; NIEMEYER and LUGMAIR. 1984) which requires
a relatively homogeneous
source for these objects.
Thus. the reported (NIEMEYER and L%M*IR.
1383.
NIEDERER VI a/., 1985) variability in titanium Isotopic
composition
in bulk meteorites could be due to varration in the small proportion of highly anomalous hibonite rather than large variations in the proportiori
of slightly anomalous CA1 pyroxene.
,4ch-noM,/~d~~,menls-I. S. Williams and J. J. f-ester partly.
pated in the exploratory use of SHRIMP for precise titanium
isotopic analyses (COMPSTON n al.. 198 I ) and together with
N. Schramm and L. All&on provided essential instrumental
support. This manuscript has benefitted greatly from criticai
examination by 1. S. Williams. M. 7. Esat. and A. E. Ringwood
and from thorough reviews by R. Walker and two anonymou+
referees. One of us (HRH) wishes to acknowledge supper!
from the US-Australia Co-operative Science Program through
NSF grant 83 I 1890. from the Australian National I !nivenit).
and from Purdue University. Calumet.
LUGMAIR. 1981. 1984: HEYDEGGER CI ui.. 1982).
The minimum number of components
necessary to
account for the observed isotopic variations has increased as the precision of the measurements
and the
size of the data base has increased. NIEDERER et N/
( 1980)
found that three components were required. as
did HEYDECGER ef al. (1982).
while NIEMEYER and
LUGMAIR (1984). using essentially the same data base.
argued for the requirement
of four components.
The
hibonite
measurements
using the ion probe have
greatly expanded the volume in “three-isotope
ratio”
space in which data reside, but have not yet resolved
the issue because the number of components
required
is sensitive both to the precision of the experimental
data (which is significantly degraded under renormalisation) and to the normalisation
basis adopted.
Even though the hibonite titanium is dominated by
the terrestrial isotopic signature, from the large variations in ‘@I3abundance relative to other titanium iso-
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