Nagoya University
NagoyaUniversity

Protocoland pitfallsof CHIME dating
KazuhrroSUZUKI
AlagqyaUhivemsi4x
Centerfor
ChronologicalReseareh,
mpan

CHIME datingoriginated by theresearch groirpof Nagoya Universityet al.,1991: Suzulci(Suzukiof monazite,
zircon and other Th- andand Adachi,1991)isa novel method fctmieron-scale
datingU-bearingminerals
on thebasisof theEPMA
analyses. Itconsists inmeasuring many spots withinconstantage domains showing
sufficientcompositionalvariatioq and in constructing anl}omwhich an age can be obtained by regression.
Analyseswere carTied out on a JEOL JXA-733EPMA equipped with4 wayelength-dispersiveof Rowland circle
140 rrrrn). The instrument
was Qperated
at 15 kV(WD)spectrometers (radiusaecelerating
voltage, 2oo nA probecurrent and defbcused
3-6 um prebe diarnetez
ThMa, UM6,
PbMa (PbMP)and YLq 1ines
were measured
sirnultaneously
withPET crrystals and sealed Xe X-ray
detectors,CaKa, SKa, PKct. SiKa,KKa, (ZtLa)and NbLSI lineswere alsomeasured. For theanalysesof Th, U, Pb and Y X-ray intensity
was integrated
over 400s periodforthelme peak[[bensure thatpossible
posinonand over 200s periodfortwo qptirnumbackgroundpositions.
changes of thesample sur facehad only a mmirnal elfecton theresults,we rqpeated measurernent of
fivetimesforeach analysis.Fortheanalyses of other elements, X-ray
positionspeak and background
intensities
were integrated
over 40s on each line
and 20son two backgroundpositions,peakposition
The baclcgroundvalue fbreach 1ine
was estimatedof two readings, because
bylinearinterpolation
WD step spectra examlriod priQrto spotanalyses suggests few discrepancies
betweenlmearandforTh, U and Nb,
exponential fits,
The standards were euxenite providedby Smellieet al. (1978)
fbrPb, Si and Cq synthesized YLglass
synthesized glass
providedby Suzukiand Adachi(1998)
fbr
1O wtPfo and K20 2 wt9fo) forY and K barite S, zircon forZg and xenotime for P(Y203==Standards
forrare eanh elemept when analyzed are synthesized glassprovidedby Drake and Weill
A highTh monazite withlinie
U and a highU xenotime withlittle
Th are used to determine(1972),
weretheX-rayinterference
conection factors.
The corTection factorsforNb and Y inierferences31-36wtPfo) and thesynthesized YLglass,estimated by using inhomogeneouscolumbite(INb20s=respectively. The X-rayintensity
datawere converted intoconcentrations by theBenoe and Albee's
methodusing analyses of natural minerals
as thematrix composition,
The small differencein thetheThO,,
UO,andPbO
matrix
between analyzed and reference minerals has little
effect ondeterminations.
The detection
limitsat a 2o confidence levelare O,oo9,O.Ol1and O.O06wt. % fdr
ThO,,U02 and PbO, respectively. Relativeeri;Qrsare about 10% for O.03 wt.% PbO, 2% fbr O.6
coupledwithwt,%UO, and O.5% fbr7.0 wt.% ThO,. [lheconstruction of "pseudo-isochion",wt.%) anddiscrimination
ofpossibly
concordant age databy chernjcal criteriaIikeS content (<0.005
ratio (O.95-1.05)
fbrmonazite and Ca and S contents (<O.O1and O,O05wt. %,(Ca+Si)f(ThHJ[+Pb)
to wotk
respectively)
fbrzircon, hasthepotemialadvantage of sigriificant procision,and theal)ility
with minerals
havingsubstaritial mitialPb, and can identify
two or more homogeneousdomainsthat
are separated by aii age gap smaller thanthe error on individual
spot analyses of age.
elementanalysis
can have rnajor manifestation
inMany features
thatare insignificant
inmajor"pseudo-isochnn"=-2LNII-Electronic Library Service

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NagoyaUniversity

includethe reles of collimator slit,detector
gas,
accelerating veltage, prdbe current, X-rayintembrences
and count-rate inatlectingtheaccuracy, and away
to access theTh and U interiierence
correction without pureTh- and U-oxides
or synthesized
The PbMa lme as measured on theJCXA-733 isapproxirnately 25 % intensethanthepure [[hSi04,PbMP line.Furtherthe PbMB linehas a demeritin evaluation of backgroundbecausethe
high-energy
side ofthe PbMB peak isoccupied by a series ofThM-,
UM- and PbM-lmes.The PbMa
'Ihe
1ineispreferai)leforthePb deterrninatien
ofmonazite
and zircon.
inter[ferencesare corrected by:- I}gbi.
- I'thMa(ObS)
XjlgbLp - IyLa(ObS)×
XfThMig
IpbM.(neOIpbMa(ObS)(ObS).f{Lrwhere]gJbig,fyi.,
andfThMig are thecorrection factor
and are defined
as:atPbMa detected!INbi.
]laB I(obs)position(obs) (incolumbite)I(obs)
synthesmi
YLbearingglass
fyL,= at PbMa detected
positionfIyi.(dbs) (in)MZ-Std)MZ' Std)
IpbM.(ObS,Pb-Std)[foMe(ObS,Pbstd))1.fihMt {IlhMa(6bS,(IlhMp(ObS,IThlut.(obs,mz-std)(ina highTh and low U monazite standard and ina U- and Th-freePtrstandard)
BecffuseYLy interference
withPbMa issevere forxenotime and polycrase,
thePbM6 1inewas used
forPb deterrniriationin theseminerals. [[heNbliyand UM2;2 intederenceson thePbMP lme areaccluirod

traoe elements, Criticalcomments=-=×correctedbM

Xjgfolv - Imvi,(obs)
xfuMt
IpbMe(ncoIpbM3(obs)INbi.(obs)
wherejgfoh andfuMt are thecorrection factor
and are definedas:!INbla(obs)(incolumbite).ITibLcI(dbs)atPbM6 detectedposition
Txe-std) (IpbM6(obs,xe-std)
Pb-std)ApbMB(obs,Pb-std)}!
IpbM.
fuMc{Ipt}},Q,(obs,(obs,=====×

Iur,Q,(obs,xe-stdja highU and low Th xenotime(instandard and ina U- andIlh-lhee
Pb-standardj
The age-mapping procedureforyoung monazite and zircon includes
acquireimg
PbMa (orPbMP) intensityof individual
correcting
baclrgroundwithpixelswith multiple spectrometers,
backgroundmap computed froma measui ed backgroundintensity
thiough theintensity
relationShips
deterrnined
inadvance of therneasurement, calibrating of intensitywith staridard and calculating ofages ffomtheTh, U and Pb concentrations. Thistechniqueprovidesage maps
showingdiIIerence
inage domainson theorder of20
Ma withinmonazite as young as 1OO Ma. Considerationof sarnple
damage by irradiation
ofintense
shown.
probewas briefly
Rerefenoes
Drake,M.J.,
Well1,D.F.,1972.New raJe earth element standards foreleenon mieroprobe analysis. Chemical
GcologylO,179-181.
Smellie,
J.A.T,,
Cogger,N.,Herrington,J.,1978,Standards
forquantitative
mieroprobe
deteiminadon
ofururtium
andthoriumwithadditional
informatienon thechemical
foJmulaeofdayidite and cuxenite-Ix)Iyerase. Chemical Gcology 22, 1-10.
SuulciL
K.,Adachi,M,, 199i.Precambrianand Silurian
metamorphism
provenanceterrane,NonheastJapan,
revealedof theTlsubonosawaparagneissintheSouinKItakamiby thechemica1 Th-U-toalPb isochiDnagesof monazite. zircon and xenotime. Geochamical

Jouma125,357-376.
Suniki,K.,Adchi,M., 1998.Denudation
historyof thehighTtP Ryoke metamorphic

bek Southwest Japan: constraints fiomCHIME

monaziteagesofgneissesandgranicids.JoumalofMetamorphicGeology16,23-37.

MiddleProcambrianof Juiasslc
sandstone(1991),
intheMino Terrane,
central Japan:
pravenRnee
Th-U-toelPbevidencefromaneloctronmieropiobemenazitestudy.SedirneritaryGeology75,141-147.

Suzuki, K., Adachi, M.. Thriaka,r-22NII-Electronic Library Service