GEOPHYSICALRESEARCHLETTERS, VOL. 7, NO. 4, PAGES 275-278,
A SEARCH FOR ISOTOPIC
ANOMALIES
APRIL 1980
IN URANIUM
J. H. Chen and G. J. Wasserburg
The LunaticAsylum,Divisionof Geological
andPlanetary
Sciences,
CaliforniaInstituteof Technology,
Pasadena,
California91125
Abstract.The 23aU/23$U ratiosfor ninebulk chondriticmeteorites
examiningour measurements.
An exampleof anthropogenic
contamina-
anda terrestrial
basalt
weremeasured.
Thetotalrange
in 235U/235U
tionwasreported
by Piepgras,
Wasserburg,
andDasch[1979]for •48Nd
determined
at the level of 106 atoms/gmof seawater
due to the unrecognized
for both total meteorites and for acid leaches was from 137.2
presence
of thisisotopein the laboratoryfrom previouswork.
to 138.3 and to within the error limits indistinguishable
from normal
A 233U.236Udoublespikeprepared
fromhighpurity233U and
terrestrial
U. Thetypical
errors
in a single
determination
are+6•oo
(2aM)
$ 235U for each measurementthe maximum amount of excess235U that
236U wasusedto monitormassdependent
isotopefractionation.
The
absolute
ratio of 233U/236U andthe concentration
in themixedspike
canbe allowed
to bepresent
ranges
from2 x 108to 2 x 109 atomsper
were determinedusinga solutionof mixed spikeand the NBS U-500
for a 2 ng U samplefrom a chondrite.Taking the extremevaluesof
gram of bulk meteorite. These resultsdo not supportthe claims of
standard
[99.94%U3Os,235U/238U
= 0.9997+0.001,
Garner,
Machlan,
variations
in 238U/235
U at the percentage
levelsor numberof excess
235U atomsin someof thesamemeteorites
asreported
by several
other
and Shields,1971] to correctfor fractionation.
The ratio in the double
previousworkers.
spike,233U/236U = 1.0066is knownto within+1•ooexcluding
the
errorsof 1•ooreportedfor NBSU-500. Following
Arden [1977], we
usedgraphiteas an emitter. The procedureusedby us wasto load a
suspension
of graphitein wateronto a previously
outgassed
Re filament.
This was evaporatedyieldinga thin uniform layer of graphite.The U
samplesin 0.1 N HNO3 were then loadedon the filamentand dried.
Anotherloadof graphitesuspension
wasthendeposited
anddriedmaking
a sandwichwith U betweenthe graphitelayers.Thistechniqueroutinely
yields"•1% ionizationefficiencyand low loadingblank (<0.5 pg). The
Introduction. The purposeof this investigationis to measurethe
relative abundancesof 238U and 23 sU in various classesof chondrites.
Until recentlyisotopicvariationswerenot expectedandonly limitedhigh
precision
workhasbeendonein search
of them.In addition,
as23sU is
relatively rare in meteoritic material, there has been very little effort
directed
towarddetermining
the238U/23sU ratioin thesematerials
with
high precisionand reliability. Numerousmeasurements
on terrestrial
materials show no large variations except in special circumstances
[Naudet and Renson, 1975]. The "absolute"ratio of terrestrialU is
(23aU/235U)(•=137.88[Shields,
1960;Cowanand Adler, 1976],
possibly
beinguncertainto +1•oo.Measurements
onlunarmaterials
show
a 238U/23sU ratio indistinguishable
from normalU [Rosholtand
Tatsumoto,1971' Fieldset al., 1972;Barneset al., 1972] .Arden [1977]
reportedanalysesof five chondriteswhich showedthat somesamples
ion beamintensity(U+) for 2 ng U was 5 x 10-•3 A between
1700ø-1750øC for 2-3 hrs. At each massthe zeroes and peaks were
measured
with an integration
timeof 1 secanda settlingtimeof 3 sec
priorto signalintegration.
To determine
instrumental
mass
fractionation,
we measured
the ion beams(I) at masses
233 and 236 in conjunction
with 235 and238. Themassjumpingsequences
werecycledin theorder
238U-236U-233U-238U
and 236U-235U-235U-233U-236U
and data
takenin setsof 10 ratios.Theratios(relative
to 236U)in a setwere
averaged
andcorrected
fordiscrimination
(<2%0/mu)
andcontributions
contained
an excess
of 23sU. In particular,
excesses
of 23sU (290•oo)
were reported for some bulk chondritesand certain fractions of
of 238U and 235U from the double spike which were less than
dissolution
residues
gaveexcesses
up to 2400%0.Theresults
reported
by
4 x 10-2•ooand 7•oo respectively.
In general,threesetsof 10 ratios
(238U:236U:233U) werefirstmeasured,
theillament
current
thenraised
Arden were astoundingboth in terms of the magnitudeof the isotopic
shifts and the number of excessnuclei required.Theseeffectswould
suchthat I236 wassimilarto thatof I238 in thepreceding
setsandthree
setsof 236U'-235U:233Uwere measured.
The 238U/235U ratio was
calculated
fromthenormalized
238U/236Uand235U/236Uratiosin the
differentsetsand the errorgivenasthe vectorsumof the errorsof the
require4x10 9 to 7x10 •ø excess
2ssU atomsper gramof bulk
chondriteand would.implya highyield(x40 to 300) at Z = 92 for the
injected
material
compared
to the valueof 10-3 - 10-4 estimated
for
intermediateZ assuming
solar abundances.
[See Wasserburg,
Papanastassiou
andLee, 1979]. Arden'sreportstimulatedwork on U isotopes
in
meteorites. Chen and Tilton [1979] and Unruh, Hutchison and
Tatsumoto [!979a,b] did not observelarge effects,but did report
2$8U/23SUratiosa fewpercentlowerthanthe terrestrial
valuein some
chondrites and Allende inclusions.However, Tatsumoto and Shimamura
lattertworatiosin 2oM (mean).
Totestlinearity
ofthedetection
system
the 238U/235U ratiowasdetermined
asa functionof ionbeamintensity.
On the Lunatic III mass-spectrometer
systemwe observeda decrease
in the measured238U/236U ratios with increasingsignal. All
experiments
showedthe samebehaviorwhichwasprobablycaused
by
saturationof the detector.The error from this apparentdeficiencyin
[1979] reported238U/235U ratiosfrom 128 to 145 for several
meteorites.
I
To clarify this matter, we carriedout analyses
on nine meteoritesand
a terrestrialrock. Prior to carryingout the measurements,
we considered
that the major technicalproblemsshould first be addressed.Our
approach
was(1)to improvethe ionizationefficiencyof U' (2)to firmly
I
i
I
I
I
NATURAL U STANDARD
NBS 950o
139
-
establish the limits of detection and reliability by measurementof
enrichedstandards;and (3)to eliminatesourcesof contaminationfrom
"exotic" U isotopesof anthropogenic
origin.
ExperimentalProcedures.Becauseof our concernovercontamination,
our first stepwasto cleanthe laboratoryarea,establish
cleanerchemical
procedures,
useonlynewchemicalprocessing
equipment,andto stopany
handling
of 23sU during
theperiodof thework.Theusual
procedure
for
measuring
U isbyisotope
dilution
using
pure23sU asa tracerandresults
in thepresence
of pure23sU in thelaboratory.
Thecontamination
levels
necessary
to produce
a onepercent
shiftin 23sU abundance
for a gram
of chondrite
are 2 x 109 atomsof 23sU (equivalent
to a particleof
2000A). The possibilityof contamination
from suchsources
mustbe a
causeof greatconcernsincethe vogueof isotopicvariationsprovokesa
ki•dof isotopic
cred,ulity
which
inhibits
some
of usfromcritically
Copyright
1980 by the American Geophysical Union.
Paper number 80L0118.
0094-8276/80/0080L-0118501.00
275
I.O
2.O
3.0
238U(I(•3AMPERE
)
Fig. ]. NaturalU standard,linearitytest of the electronmultiplier
(currentintegrationmode) on the LunaticI massspectrometer.
The
238U/23sU ratiosarecorrected
for fractionation
usingthedoublespike.
The normalized238U/235U ratiosmeasured
with intensities
of • 38U
between
0.2 and2.5 x ]0-• 3 A agreeMth thenormalv•ue.
276
Chenand Wasserburg:Isotopic Anomaliesin Uranium
ALLENDE
256
U 25õ
U
HCI LEACH
/•$U
235U
=5.7x I0•øATOMS
I -•5 xI0
(:5.1x I(•9gmU)
258U
1.3x I0
BEAM /• x400
|
A
234
/
ß
BEAM
2:59
2:38
237
2:56
235
2:54
233
23?_AI ,'IU
Fig.2. U mass
spectrum
ofHC1leach
onAllende.
Arrows
show
where
thezeroes
weremeasured.
Notetheshiftinscale.
23Su
would
correspond
to'"35%o
enrichment
of23SU
when
I23swas
8x 10-•3A.Forcomparison
weperformed
thesame
experiments
onthe
Lunatic
I equipped
withthesame
typeof multiplier
(gain
= 4 x 103at
2300volts.)
Theresults
(Fig.1)indicate
thattheproblem
ofsaturation
is
HC1,7M HNO3,andH:O. Thechemical
yieldof U ranged
from50 to
95%.Thetotalprocedural
blankwas'"2x 10•ø atoms
("-8pg).Acid
leachexperiments
weredoneon two meteorites
in the orderlisted.An
aliquot("-10%)wasspiked
to determine
theamountof U andthebalance
absent
ontheLunatic
I within
limits
oferror.
Allexperiments
reported then spikedto give• 3sU/• 36U • 12. Eachsetof meteoritemeasureherewereperformed
ontheLunatic
I which
waspreviously
never
used mentswaspreceded
by a linearitytestasshownin Fig.1. Results
are
forU measurements.
During
allexperiments,
wescanned
themass
range shown
in Table1 listedchronologically.
230-240(seeFig.2) anddidnot observe
anysignals
besides
U. To
establish
thereliability
withwhichshiftsin isotopic
abundance
canbe
RESULTS AND DISCUSSION
determined,
weprepared
fourstandards
enriched
in :3SUbymixing
differentamounts
of: 3sU spikeintoa standard
solution
of NBS950a
and:36U_:33U tracer.
Theisotopic
abundance
of U in theenriched
U
standards
wasmeasured
by loading
differentamounts
of U foreachsetof
experiments.
A complete
setof comparisons
wasmadeforsamples
of
enriched
standards
ranging
from2 to 20 ng.Themeasured
ratiosafter
correction
are compared
with gravimetrically
determined
23sU/•3sU
ratiosin Fig.3. Theseresults
wereindistinguishable
for all the dataand
indicatethat (1)at least5•ooU isotopic
effectscanbe resolved
for
samples
assmallas2 ng,and(2) allmass
spectrometric
measurements
for
Results
aregivenin Table1 as2ssU/2ssU andasfractional
deviations
(/5
•ssU)oftheratios
from
thenominal
(2ssU/2ssU)• value
inparts
in
l0s.Theamount
ofmeteoritic
Umeasured
peranalysis
ranges
from
0.2to
10 ng. The resultson terrestrial
samples
NBS950aandbasaltBCR-1
show
good
agreement
withtheassumed
terrestrial
value
forsamples
ofU
ranging
from1.7x 10
-9to2x 10-sgm.Twoaliquots
ofUfrom
Dhajala,
containing
1.8and4.4ngU wererunandareingood
agreement
with
eachother and the valueis indistinguishable
from terrestrial.
These
analyses
followed
ananalysis
of NBS950aunderthesamerange
of
theenriched
standards
agree
withthevalues
calculated
fromgravimetry operating
conditions.
A similarsequence
of experiments
weredoneonall
wellwithinerrors
(20M).
the meteoritesamples,
which includea fragmentof Richardton
Samples
of meteorite
fragments
wereprepared
by removing
all
(originally
fromASU)transferred
to thislaboratory
fromUCSBfor
exterior
surfaces
of thesamples
astheywerereceived
inthelaboratory. analysis.The only apparentdifferencebetweenthe meteortiesandthe
Samples
(1-1.5g) were digested
in HF in a 50ml FEP teflontube
followed
by HNO3andHC104attackin a 30 ml FEPteflonbeaker
until
allresidues
dissolved
in2MHC1.
Uwas
coprecipitated
withother
group
II
standards
liesin the factthat theprecision
of themeteorite
datawasnot
usually
of thequalityobtained
ontheverypurestandard.
majorelements
withNH4OHandseparated
bypassing
through
Dowex
AG1x 8, 100-200
mesh
anion
exchange
resin
using
7MHNO3andH:O.
A second
anion
exchange
resin
wasused
topurify
U fromThusing
6M
•4o
,,,
•
•
•o
METEORITE
U
235U
ENRICHED
½
2xI•9gm
U
-8
i '1
-4
0
?
TERRESTRIAL
• 20-
SLO• = I
• ,o
+8
U
BCR-I
NBS 950a
•
•
I
+4
lBxI08
0
,b
4'o
8 •S•U,%o(GRAVIMETRIC)
Fig.3. Comparisonbetweengra•metric•ly determined5•3•U and
massspectrometric•ly
measured
•SSU for experiments
on 2 ng U
samples.
All dataplotona45ølineanddemonstrate
theability
toclearly
resolve
U isotopic'effects
above5• for 2 ngU.
,
-8
, IIIIII,
-4
0
,
+4
+8
;825u(%0)
Fig.4. Histograms
showingthe measured•523SUon meteoriteand
terrestrialU. The distributions
of both classes
of data shownormal
distribution
withmeans
close
to/52
ssU= 0, andasmall
dispersion.
Chenand Wasserburg' Isotopic Anomalies in Uranium
Onthebasisof theenriched
U standard
experiments,
we areconfident
Table 1. U isotopicCompositions
a
Samples
b
U (ng) 23sU/23sU
N1 NBS 950a
N2 NBS 950a
N3 NBS 950a
N4 NBS 950a
N5 NBS 950a
1
2
3
BCR-1
2. ?ultusk(HS)
+1.16_+5.28
138.12_+0.55
- 1.74_+3.98
1
1.6
137.58_+0.99
+2.18_+7.16
2
3.3
137.69_+1.29
+1.38_+9.33
10
2.7
6.3
2.0
9.2
10
5.1
8.8
10
1.0
2.6
10
4.5
5.7
2.8
5.4
137.68_+0.55
138.14_+0.65
137.20_+1.12
137.50_+0.50
138.18_+1.10
137.83_+0.51
138.28_+0.69
137.87_+0.69
137.67_+0.48
138.09_+1.39
137.50_+1.10
137.63_+0.29
138.02_+0.69
137.98_+0.74
138.03_+0.72
137.81 _+0.51
+1.45_+3.99
-1.88_+4.67
+4.96_+8.17
+2.76_+3.60
-2.17_+7.92
+0.36_+3.69
-2.89_+4.96
+0.07_+4.98
+1.53_+3.49
- 1.57_+9.90
+2.76_+7.96
+1.82_+2.10
- 1.00_+4.97
-0.72_+5.37
-1.08_+5.17
+0.51_+3.69
10
137.85_+0.40
137.62_+0.50
+0.22_+2.89
+1.86_+3.60
137.76_+0.47
+0.88_+3.39
138.09_+1.38
- 1.52_+9.88
137.88_+0.41
0.00_+2.99
+0.58_+3.59
2
N 10 NBS 950a
1
2
1
2
N 11 NBS 950a
9. Richardton(H5) HF+HC1
c
HNO3
HC104
HC1a
3.1
137.80_+0.49
HNO3
2.6
137.65_+1.13 +1.67_+8.15
HF
5.6
137.88_+0.41
HC104
4.5
137.75_+0.83 +0.94_+5.97
N13 NBS 950a
11. Richardton
(H5)
10
1
2
N14 NBS 950a
12. Barwell(L5)
1.9
5.4
1.2
10
N12 NBS 950a
10. Allende(CV3)
+0.14_+3.09
4.4
1
8. Leedey(L6)
+0.29_+3.00
+0.36_+5.97
2
N9 NBS 950a
7. St. Severin(LL6)
137.84_+0.41
137.83_+0.83
137.72_+0.73
1
2
6. Murchison(CM2)
11
1.7
137.86_+0.43
N8 NBS 950a
5. Allende(½V3)
+0.22+2.99
+0.15_+2.39
+2.83_+3.90
+0.80_+3.19
-0.14_+2.89
+0.36_+2.69
1.8
1
2
1
2
4. Bruderheim
(L6)
137.85+0.41
137.86+0.33
137.49_+0.54
137.77_+0.44
137.90_+0.40
137.83_+0.37
5
N7 NBS 950a
3. Richardton(H5)
•i23sU (•oo)
20
20
5
2
5
20
1
N6 NBS 950a
1. Dhajala(H3)
2.2
138.03_+0.28
thatwe canresolve
anyU isotopic
effectabove5%o.Theresults
show
that the 23aU/23su ratiosfor the individualmeteoritesstudiedare
similarwithin error limits to that for normalterrestrialU. The averageof
all meteoritic
U isotopic
measurements
is 23sU/•3sU - 137.84_+0.52
(2OGM)
or b23sU
- +0.3_+3.8%0,
whichis indistinguishable
fromthe
average
for the U standard,
137.79_+0.28
(2OGM)or •i23SU+0.7_+2.1%o.
Heretheerrors
are2o of thegrand
means
of eachclass
of
samples.
Toillustrate
better
thedistribution
ofthese
results,
the•i23sU
valuesare shownin histograms
in Fig.4. Most of the meteoriticU
measurements
haveuncertainties
about6%0 (seea bargraphin Fig.5).
Takingthe extreme•i23sU valueof +13.13for bulk Richardton
(Aliquot
2), theamount
ofexcess
23sU pergram
which
canbepresent
is
2 x 109 atoms. Similar calculationsfor the rest of the data indicatethat
the maximum
amountof excess
23sU pergramwhichcanbe allowed
withinthe errorsranges
from2 x l0 s to 109 atoms.In contrast,
the
results
of Arden[1977]indicate
a 23SUexcess
of from4 x 109to
7 x 10• o atomsper gramfor somemeteorites.
We didnot observe
the
shiftsin 23SU/23sU in somemeteoritesreportedby Chenand Tilton
[1979], Unruh et al., [1979a,b], and Tatsumotoand Shimamura
[1979]. A directcomparison
with the results
of ChenandTiltonwas
possible.
The solutionaliquotsof all thesemeteorites
on whichU
isotopes
hadbeenmeasured
weretransferred
fromUC SantaBarbarato
thislaboratory
andmeasured.
These
solutions
yielded
238U/235
U ratios
ranging
from133.8to 137.3,confirming
eachof thevalues
of Chenand
Tilton.These
moreprecise
results
clearly
showed
b23suranging
from
30.5 to 4.2. Throughthe courtesyof G. R. Tilton,fragments
of the
originalmeteorites
(Dhajala,Pultusk,andBruderheim),
whichshowed
excess
23sU in the solutions
preparidat UCSB,weretransferred
to CIT
andprocessed
asdescribed
here.Theresults
(Table1, nos.1, 2 and4),
clearlydemonstrate
a normalvalue.We conclude
that the abundance
measurements
reportedby ChenandTiltonwerecorrectbut that the
effectsarespurious
andprobably
dueto laboratory
contamination
with
23sU. Fromour observations
we conclude
that 23sU/23sU in the bulk
meteorites studied is similar to the terrestiral value to well within an
extremeerror of _+5•oo
and considering
the dispersion
aboutthe mean
probably
towithin
anerror
of_+2•oo
ßTheuncertainty
inthe23sU/235
U
S 235U(%o)
0.00_+3.00
- 1.08_+2.09
137.69_+0.44 +1.38_+3.19
10.4
138.07_+0.33
- 1.36_+2.39
18
137.85_+0.26
+0.22_+1.89
1
0.18 137.58_+1.24 +2.18_+8.94
2
5.1
137.51_+0.39
277
+2.70_+2.81
-12
'
-8-4
I
'
0
*4
*8
I
'
I
-12
•
I
DHAJALA
II %
I•
PULTUSK
I
, I
RICHARDTON
BRUDERHEIM
ALLENDE
aErrors
are2oM. All datacorrected
for mass
fractionation
anda small
contribution
of 23sU fromthe233U.236U doublespike.
•j23su=
((235U/238U)meas
X137.881)x103
bSamples
no.1-4 fromG.R. Tilton;Richardton
(nos.3, 9 and 11),
#100h, ArizonaStateUniversity;
Murchison
(no.6), #Me 2642, Field
Museumof NaturalHistory,Chicago;St. Severin(no.7), #H2402-b,
Museumof NaturalHistory,Paris;Leedey(no. 8), #489.1, ArizonaState
University.
Numbers
aftersample
namedenote
repeatrunsonaliquots
of
samesolution.cSample(1.48gm) decomposed
in HF. Dried residue
heatedin 6M HC1(10 ml), andclearsolutioncentrifuged
givingentry
labelledHF+HC1.The residuewasdriedagain,and 7M HNO3 (10 ml)
added and heated. Solution removedand remainderbroughtinto total
MURC+ISON
,I
I
I
LEEDEY
RICHARDTON
LEACHES
ALLENDE
LEACHES
I
I
I
14o
139
,
i
I
I
I
I
137.88 137
,
I
136
238U/235
U
solution
with70%HC104(2 ml). dSample
(1.75gm)leached
in 6M HC1
(20ml) for fivedays.Thesolution
centrifuged
andtheresidue
washed
O,
ST. SEVERIN
Fig.5. Bargraphshowing
therange
of •i• 3sU onbulkmeteorites
and
twicewith H20. The washes
werecombinedwith the HC1extractand
labelledHC1.The residuewasleachedwith a new acid(15 ml) in each
leaches.The center dashedline corresponds
to the nominal terrestrial
successive
cycle.AfterHC104(2 ml)leach,noresidue
wasobserved.
value
of (23sU/23s)•= 137.88.
278
Chen and Wasserburg: Isotopic
ratioin thisstudyconstrains
theamountof anyexcess
23sU to lessthan
2 x 109- 2 x 108atomspergramof bulkmeteorite.
Leachingexperimentswere carried out on whole rock samplesof
Richardtonand Allende. The Richardtonsamplewas from the same
parent sample analyzed by Arden [1977]. The leachingprocedure
generallyfollowedthe samesequence
of acid treatmentsusedby Arden
[1977], but the conditionswere differentand the fractionof total U
leachedin eachstepdifferedfrom the yieldsobtainedby him. The results
obtained
on
Richardton
show
that
two
successive leaches and the
dissolutionof the final residue contained 22, 64, and 14% of the U and
that the isotopiccompositions
wereall the sameasthe terrestrialvalueto
within error. A modified leachingprocedurewasusedfor Allendewith
yields of 20, 16, 35, and 29 percent,respectively,
and againgivingthe
terrestrialvalue. We find no evidencefor the existenceof isotopically
peculiarU in eitherbulk samplesor leachesusingthe procedures
followed
here. From data reportedby Arden on Richardton(no. 2), it would
appear that 57% of the total U is isotopically anomalouswith
$23sU - 500•oo
ßIf only10%of thisexoticU werepresent
in oneof our
Richardtonleaches,we would have observeda shift of •i23sU =
45-200%owhichwasnot observed.
Similarly,43% of the total U in
Arden'sAllende(no.2) appearto have$23Su=42•oo.If 10%of this
Anomalies in Uranium
[1977], it is still possible
that effectsmayexistin phases
whichhavenot
been studiedin this laboratory.However,unlessthe mostextremecareis
madeto eliminatelaboratorycontaminationandinstrumental
bias,it will
be difficultto avoidthe appearance
of spurious
shiftsin isotopic'
abundances
in nature.The level of isotopicshiftswhichare anticipatedin
bulk samplesmay be estimated.If we assumethat the deficiencyin
'•SBa of"- 2 partsin 104or the '29I/' 27I ratioof'"10-4 isa measure
of the number of exotic nuclei added to the solar systemand that the
chemicalabundanceof the injectedmaterialis roughlysimilarto solar,
thenwewouldexpect
shiftsof $2ssU "- 0.2•ooin bulkmeteorites.
This study supportsone of the basic assumptionsused in U-Pb
chronology
of meteorites
regarding
the nearconstancy
of 238U/2S
sU.
The improvedanalyticaltechniquesdescribed
herehavedirectapplication
to the precise measurementof U concentrationsand the reliable
determinationof U-Pb agesfor lunar andmeteoritesamples.
Acknowledgements.We havebenefited from extensivediscussion
both
philosophical
and scientificwith D. A. Papanastassiou.
Wearegratefulto
G. R. Tilton for generously
givingus meteoritesamples
andaliquotsof
meteoritic U solutions. This work was supported by NSF grant
PHY76-83685 and NASA grant NGL 05-002-188. Contribution3351
(334) of the Divisionof Geological
andPlanetarySciences.
anomalousU were present in our Allende leaches,we would have
observed
a shift of $23SU.,, 12•oowhichwasalsonot observed.
In
addition we analyzed another larger sampleof Richardton(#11) to
obtain high precision data. Aliquots of U from this sample are
indistinguishable
fromnormalU to within1.4•oo
ßOurrepeated
analyses
of different fragmentsof Richardtonthus showno hint of a U anomaly.
After submissionof this paper we also analyzeda sampleof Barwell
chondrite
for whichUnruhet al., [1979] reported•s/•Su ratioof
135.24-+0.17.
The results
(#12) showthatthe •S•U/•ssu ratiofor the
bulk
Barwell
are similar within
errors to terrestrial.
We can find no
evidencein supportof isotopicallyanomalousU in eitherbulk meteorite
samplesor in any of the leacheson two meteoritesin which anomalies
havebeenreported.
In previousstudiesof Pb-U agesone of the limiting experimental
errorslies in determining
the numberof 2S•U atomsin a sample
(typicallyerrorsof '•1-1.5%). Thiserroris an orderof magnitudelarger
thanerrorsin theratioof 2ø?Pb/2ø6Pb.
Asa result,uncertainties
in age
determinations
maybegoverned
byerrors
in the2o6pb/2ssU ratio.This
is most seriousin the datingof lunar sampleswherethe dataoften form a
linear array nearly tangent to the concordiacurve [cf. Tera and
Wasserburg,
1974].In theexperiments
reported
here,theratioof 2s•U
to 2s6U (or• 3sU) hasalways
beenbetterthan1-2•oo
(2o•). It follows
that the useof a mixed• ssU-•s6Uspikecalibrated
against
a good
standard
canreadilypermit2ssU determinations
at a precision
of at least
1%0onsamples
downto 10-• o gmof U.
Arden, J. W., Isotopiccompositionof uraniumin chondriticmeteorites,
Nature, 269, 788-789, 1977.
Barnes, I.L., B.S. Carpenter,E.L. Gamer, J.W. Gramlich, E.C.
Kuehner,L.A. Machlan,E. J. Maienthal,J. R. Moody,L.J. Moore,
T.J. Murphy, P.J. Paulsen,K.M. Sappenfield,and W.R. Shields,
Isotopic abundanceratios and concentrationsof selectedelementsin
Apollo 14 samples,
Proc.ThirdLunarSci. Conf., 1465-1472,1972.
Chen, J.H., and G.R. Tilton, Uranium Isotopic Compositionin
ChondriticMeteorites,Lunar and Planetarysci. X, 192-194, 1979.
Cowan,G. A., and H. H. Adler, The variabilityof the naturalabundance
of2•SU,Geochim.
Cosmochim.
Acta,40, 1487-1490,
1976.
Fields,P. R., H. Diamond,D. N. Metta,D. J. Rokop,andC. M. Stevens,
23?Np, 2s6U, andotheractinides
onthemoon,Proc.Ttu'rd
LunarSci.
Conf, 1637-1644, 1972.
Garner, E.L., L.A. Machlan,and W.R. Shields,StandardReference
Materials:Uranium IsotopicStandardReferenceMaterials,National
Bureauof StandardsSpecialPublication,260-27, 1971.
Naudet,R., and C. Renson,R•sultatsdesanalyses
syst•matiques
de
teneursisotopiques
de l'uranium,Proc.Int. Syrup.OkloPhenomenon,
Libreville, 265-291, 1975.
Piepgras,D.J., G.J. Wasserburg,
and E.J. Dasch,The isotopic
compositionof Nd in differentoceanmasses,
Earth Planet.Sci.Lett.,
45, 223-236, 1979.
CONCLUSIONS
We find that the mean238U/2ssU ratiosfor ninebulkmeteorites
to
is indistinguishable
fromnormalterrestrial
uranium
to within4•oo.The
maximum amount of excess 23sU which can be allowed for these
samples
within the extremelimits of errorsrangesfrom 2 x 108 to
2 x 109atomspergramof meteorite.
Analysis
of differentleachfractions
of two meteoritesshowsthe isotopiccompositions
to be indistinguishable
fromnormalterrestrial
U. Theupperlimitto excesses
of 235U in all of
the leach fractions is 3-9 x 108 atoms out of a total of '--10"
REFERENCES
atoms.
From these observations we conclude that bulk chondrites have a U
isotopiccompositionwhich is the sameas terrestrialto within limits of
error(2-4%0).We,further
conclude
thatthereisnoevidence
in support
of
variations
in the 238U/2SsU abundances
at the percentage
levelsor in
termsof the excess
2SSUatomsas reportedby Arden [1977] or in
supportof the smallshiftsin 238U/2sSUreported
by ChenandTilton
[1979], Unruhet al. [1979a,b],andTatsumoto
andShimamura[1979].
It is our belief that someof the effectsmay be due to contaminationin
the laboratorywith exotic uraniumof anthropogenic
origin.Somesmall
isotopicshiftsmay alsobe the resultof instrumentalerrors.The scientific
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and T. Lee, Isotopicheterogeneitiesin the solarsystem,VarennaVol.,Italian PhysicalSoc., 1979.
significance
of variations
in 238U/2s5U is sufficiently
greatthatfurther
and moreintensiveinvestigations
mustbe pursued.Insofaraswe havenot
repeated the specificacid decompositionproceduresused by Arden
(Received January 4, 1980;
accepted January 28, 1980.)