JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 102, NO. B12, PAGES 27,285-27,295, DECEMBER
10, 1997
Paleomagnetismand paleothermometryof the SydneyBasin
2. Origin of anomalouslyhigh unblockingtemperatures
D. J.DunlopandO. Ozdemir
Geophysics
Laboratory,Departmentof Physics,Universityof Toronto,Toronto,Canada
P. W. Schmidt
Divisionof ExplorationandMining, CSIRO, NorthRyde,New SouthWales,Australia
Abstract. The Milton Monzoniteof southeastern
Australiawasthermoviscously
remagnetizedas a resultof Cretaceous
burialanduplift. Thermaldemagnetization
separates
the low unblockingtemperature(LT) overprintfrom the high unblockingtemperature(HT)
primaryremanence,with a relativelysharpjunctionbetweenLT andHT components
in
vectorprojections.For single-domaingrains,thejunctiontemperatureT• betweentwo such
vectorscorresponds
to the maximumblockingtemperatureTrreactivatedin nature,apartfrom
a correctionfor the differencebetweennaturaland laboratorytimescales.However,
measuredT• valuesare distributedoveran implausiblywide range(>250øC) for burial
remagnetization
of an untiltedintrusionlike the Milton Monzonite.Furthermore,manyT•
valuesare anomalouslyhigh comparedto the predictionsof single-domaintheory.
Multidomaingrainsare the causeof theseanomalies.Samplespretreatedbeforethermal
demagnetization
by zero-fieldcyclingto liquidnitrogentemperature,
soasto erase
multidomainremanenceandisolatesingle-domain
remanence,do havethe theoretically
expectedTi•values. In thesesamples,realisticremagnetization
time andtemperature
(tr, Tr)
conditionsin natureare predictedusingthe t-T contoursof Pullaiah et al. [1975]. The
anomalouslyhigh T• valuesbeforelow-temperature
treatmentaredueto multidomaingrains,
which carry >_50%of the LT overprint. The LT thermaldemagnetization
curvein samples
dominatedby multidomaingrainsis quasi-exponential
in shapewith a high-temperature
tail
extendingalmostto the Curiepoint,aspredictedby multidomaintheory. ThesehighLT
unblockingtemperatures,
whicharemuchgreaterthanplausibleremagnetization
temperatures
reachedin nature,overlapandmaskthe lowerpartof theHT unblocking
temperature
spectrum,drivingup Ti•valuesandleadingto inflatedestimates
of Tr. Although
multidomainremanenceis a sufficientexplanationof anomalouslyhigh unblocking
temperatures
of thermoviscous
overprintsin theMilton Monzonite,chemicaloverprinting
maybe a factorin otherlithologiesandtectonicsettings.
1. Paleomagnetism of the Milton Monzonite
Dunlopet al. [thisissue](hereinafterreferredto aspaper1)
showedthatthe Milton Monzonite,an Early Triassicintrusion
into flat-lying sedimentsof the SydneyBasin of southeastern
Australia, has a naturalremanentmagnetization(NRM) with
four distinctcomponents.(1) The high unblockingtemperature (HT) component,carriedby magnetiteor occasionally
pyrrhotite,is interpretedto be the primaryNRM, (2) the low
unblockingtemperature(LT) component,carriedby magnetite
or pyrrhotite,is believedto be a thermoviscous
overprintof
HT acquired during Cretaceousuplift and cooling, (3) a
chemicalremanentmagnetization(CRM) carriedby magnetite
(CRM1) hashigherunblockingtemperatures
thaneitherLT or
HT; CRM1 and LT have similar directionsand ages but
widely separatedunblockingtemperatureranges,and (4) a
secondCRM (CRM2), carriedby hematite,has the highest
unblocking temperaturesof all. CRM2 and HT have similar
directionsand ages.
Papernumber97JB02478.
The widelyseparated
LT andCRM1 unblocking
temperatures (100-150øC of nonoverlap,flanking HT below and
above)makeit unlikelythatLT is a chemicaloverprint.It is
difficult to imaginechemicalprocesses
that would generate
two types of magnetitewith no overlap in grain size and
unblocking
temperatures.
However,therearetwoproblemsin
the interpretation
of LT asa purelythermoviscous
overprint.
First,junctionsbetweenLT andHT on vectordiagramsare
somewhatrounded,not perfectlysharpas they shouldbe for
a partial thermoremanent
magnetization(TRM) carriedby
single-domain grains overprinting another remanence.
Second,maximumlaboratoryunblockingtemperatures
TLof
the LT overprint,measuredasjunctiontemperatures
between
the LT and HT vectorsin orthogonalvectordiagrams(e.g.,
paper1, Figures4 and5), areoftenunexpectedly
high in view
of theknownlimitson remagnetization
temperatures
Trin the
0148-0227/97/97JB-02478509.00
SydneyBasin [Middleton and Schmidt,1982].
Copyright1997by theAmericanGeophysical
Union.
27,285
27,286
DUNLOP ET AL.: ANOMALOUSLY HIGH UNBLOCKING TEMPERATURES
step),while trcanbe millionsof yearsfor deepburialandslow
uplift, T[ canbe asmuchas 100øChigherthanthe remagnetiAnomalouslyhigh unblockingtemperaturesof magnetic zationtemperatureTr the rock experiencedin nature.
overprintsarewell documented
elsewhere.Chamalaun[1964]
The resultsof our work and severalpreviousstudiesare
was the first to study thermoviscousremagnetizationand plottedin Figure 1. The two setsof pointslabeledSD come
constructtime-temperature
(t-T) contoursto correctlaboratory from thermaldemagnetization
of laboratoryviscousremanent
TL data for the effect of slow coolingin nature. With the magnetizationsin single-domainmagnetite [Dunlop and
differing timescalesof coolingtakeninto account,TL values Ozdemir,
1993].Thedataparallelthecontours,
verifying
the
for remagnetized
Britishred bedsmatchedplausiblegeologi- Pullaiah-N6elequationsoverthe longesttimescalesavailable
cal remagnetization
temperatures
T•. However,in calculating in thelaboratory.Othersingle-domain
data(not shown)which
his t-T contours,Chamalaunomittedthe key factor •(T) = follow the Pullaiah et al. contours over more extended
M•(T)/M•o, where M• is spontaneousmagnetization and timescales include remagnetized Appalachian red beds
subscriptzero indicatesroom temperaturevalue. If we use containingsingle-domainhematite[Kent and Miller, 1987]
contourscalculatedwith •(T) included[Pullaiahet al., 1975], andblocksof fine-grainedColumbiaRiver basalttumbledin
Chamalaun'sTL data predict T• valuesthat are higherthan a landslide[TysonSmithand Verosub,1994].
expectedon geologicalgrounds.
However,theresultsof otherstudiesarein seriousdisagree-
2. Time-Temperature Relations
Pullaiah et al. [ 1975] basedtheir calculationson the classic
Ndel [ 1949]theoryof thermoviscous
magnetization
in singledomaingrains.N6el hadshownthattheblockingtemperature
T• of a partialTRM duringcoolingor its unblockingtemperature Ttmduringheatingaregovernedby the thermalvariation
of the magneticrelaxationtime
1/•:(T)= C exp[-goVMsoHKol3n(T)/2kT],
(1)
whereC = 109s-j, go= 4•r x 10-7H/m, V andHKaregrain
volume and microcoercive force, and k is Boltzmann's
constant.For shapeanisotropy,
n= 2. OmittingIt(T) from (1)
gives•: valuesthat are too high at all temperaturesandorders
of magnitudetoo high nearthe Curie point Tc.
Because•: dependsexponentiallyon [3n(T)/T,temperature
changesof a few degreesCelsiushavea large effect on •:. At
TB, a slight coolingwill renderthe magnetizationrelaxation
sluggishor blocked,while at TuB(which is equalto TBfor a
particularsingle-domain
assemblage
in weakfieldslike thatof
the Earth),a slightheatingwill causeinstantaneous
relaxation,
i.e., within a time shorter than the observation time t.
TB or Ttm mark a sharpboundarybetweenblocked and
unblockedmagnetizationstates.PartialTRM acquiredby a
single-domain
ensembleat TBduringcoolingshoulddemagnetize completelyduringzero-fieldreheatingat TuB= TB. For
this reason,the unblockingtemperaturespectrumof a partial
TRM shouldbe sharplyseparatedfrom that of the NRM it
overprints,giving sharpjunctionson vectordiagramsrather
ment with the Pullaiah et al. [1975] contours. The points
marked A and B in Figure 1 are from Kent's[1985] studyof
Appalachianlimestones.A denotesbouldersfrom a till which
have acquiredpostglacialviscousoverprints. B denotes
bedrockmaterialthathasbeenregionallyremagnetized
during
mild heating. In both cases,the linesjoining laboratoryand
natural (t, T) pointshave muchlower slopesthan predicted.
That is, laboratoryheatingto much higher temperaturesT[
thanpredictedis requiredto completelyerasethe thermoviscousoverprint.
Middletonand Schmidt's[1982] reportof similarlyelevated
laboratoryunblockingtemperatures
T[ in the Milton Monzonite wasthe incentivefor the presentstudy. Our TLdata,from
much more detailed thermal demagnetizationof Milton
Monzonite samples(seepaper 1), are plottedin Figure 1 as
groups1, 2, and3. Group 1 includessamplesfrom site2 and
samplesfrom other sitesthat were pretreatedby zero-field
cyclingto liquid nitrogentemperature
to isolatesingle-domain
remanence.Group2 includesresults(withoutlow-temperature pretreatment)from sites 1, 3, 4, 5, 6, and 9 and agrees
quite well with Middleton and Schmidt'sresult. Group 3
includes results from sites 10 and 11, where multidomain
magnetitecarriesthe remanence.
During burial, samplesfrom all sitesexperienceda maximum temperatureTr • 200øC for tr • 3 Myr (Middletonand
Schmidt[ 1982]:basedon coalgrade,regionalmetamorphism,
andfissiontrackdata). When thispointis joined to eachof
the
threegroups,we canseethatgroup1 resultsareconsistent
than rounded ones.
with thePullaiah et al. [ 1975] contours,group2 resultshave
Usingasa blocking/unblocking
condition•: = t (relaxation higher T[ valueswhich are compatible(coincidentally,we
time equal to observationtime), (1) gives the following believe) with a differentsetof dashedcontoursproposedby
relationbetweenthe maximumlaboratoryunblockingtemperMiddleton and Schmidt on the basis of a single-domain
ature TL of overprintingpartial TRM, measuredin thermal remagnetizationtheory by Walton [1980], while group 3
demagnetizationover a relatively short time tL, and the
resultshavestill higherT[ valuesthatareincompatiblewith
originalblockingtemperature
or remagnetization
temperature either set of contours.
Tr of the partial TRM during long exposurefor time t rin
Samplesfrom site 10 contain large, truly multidomain
nature:
3. Remagnetization Diagrams and Results
grainsof magnetite,whereasthe magnetiteat other sitesis
smallerin size(moderateto largepseudo-single-domain,
_>10
gm approximately)
(seepaper1, Figure3 andTable 1). Partial
TRMs and viscous remanencescarried by multidomain
magnetitehavelongthermaldemagnetization
tails, extending
essentiallyto the Curie temperatureTc [Bol'shakovand
Shcherbakova, 1979; Worm et al., 1988; Xu and Dunlop,
Contourscalculatedfrom (2) are plottedas solidcurvesin
Figure 1. The valueof goVM•oH•o/2k
determinesthe contour
value of the quantityT •-n(T) ln(Ct) for a particularsingledomaingrainensemble.Largeror morecoercivegrainshave
higherblocking/unblocking
temperatures.SincetLis typically
a few minutes(residencetime at peaktemperaturein a heating
16.11]. Thusthe elevatedmaximumunblockingtemperatures
of the LT componentin group 3 samplescould be due to
multidomainmagnetite.The compatibilityof group1 results
with thePullaiahet al. [ 1975]contours
is not surprisingsince
low-temperaturepretreatmentwas intendedto isolatesingle-
YL [3L
-nln(CtL)= Tr [3r
-nln(Ctr)= goVMsoH•o/2k. (2)
This is the Pullaiah et al. [ 1975] time-temperature
relation.
1994;Dunlopand Ozdemir,1997,Figures10.11,10.13,
DUNLOP ET AL.: ANOMALOUSLY HIGH UNBLOCKING TEMPERATURES
27,287
1Ga
10Ma
105a
lO3a
• 1oa
lm
ld
lh
100s
ls
50
150
250
350
450
550
Temperature,
T (øC)
Figure1. Thermoviscous
remagnetization
contours
predicted
byPullaiah
etal. [1975]onthebasis
ofNdel[1949]
single-domain
blocking
theory
(solid
curves)
andbyMiddleton
andSchmidt
[1982]
onthebasis
ofthesingle-domain
relaxation
theory
of Walton
[1980](dashed
curves).
Datashown
areshort-term
andlonger-term
(t, T) pairsfor
single-domain
magnetites
(SD,Dunlop
and•zdemir[1993]),
Appalachian
limestones
(A,B,Kent[1985]),
andthe
MiltonMonzonite
(1,2, 3 andopencircleattop,thispaper).OnlySDandourgroup1 dataagree
withPullaiah
et
al.'spredictions.
A, B,andourgroup
2 dataagree
withtheMSWcontours.
Group
3 haslaboratory
unblocking
temperatures
toohightobeexplained
byeither
setof contours.
Webelieve
group
1 isduetosingle-domain
remanence,
group3 is dueto multidomain
remanence,
andgroup
2 is dueto a mixture
of single-domain
and
multidomain
remanence
or to pseudo-single-domain
grains
whose
remanence
hasbothsingle-domain-like
and
multidomain-likeaspects.
domain
magnetite.
Group2 results
mayrepresent
a mixture
of
single-domain-like
andmultidomain
carriers,
or it maybe
characteristic
of pseudo-single-domain
carriers
of a certain
grainsize.We shalltestthese
ideasin theremainder
of this
paper.
remanence
in magnetite[Ozimaet al., 1964;Kobayashi
and
Fuller, 1968; Merrill, 1970]; the memoryratio R of remanenceafter LTD to remanencebeforeLTD decreases
rapidly
asgrainsizeincreases
[Heideret al., 1992;Halgedahland
Jarrard,1995]. The otherTLdeterminations
in thisgroupare
forsamples
fromsite2, whichdidnothavesingle-domain-like
4. Thermal DemagnetizationResults
hysteresis
properties
(paper1, Table1 andFigure3). HowFigure2 shows
theresults
of 194determinations
of T•,the ever,thesesamplesdo haveunusuallysmallLT components
(Figures
4, 5, and6) andwewill
maximumunblocking
temperature
of theLT component,
from relativeto theHT component
argue
later
that
this
circumstance
brings
the single-domain
sharpor slightlyrounded
junctions
betweenLT andHT
vectors
onorthogonal
vectordiagrams
(e.g.,Figures4, 6, and fraction in both HT and LT into prominence, Thus initial
to
7 orFigures4 and5 of paper1). TheT• valuesfall intofour indicationsare that the secondTL peak, corresponding
remanence.
groups.Manysamples
fromsites7 and8 haveT• between group1 in Figure1, is dueto single-domain
The thirdandlargestpeakin T[ valuesis between350 and
200 and230øC.Thispeakis probably
dueto pyrrhotite
(Tc=
320øC),whichis knownto carrytheLT andHT remanences 460øC, especiallybetween400 and440øC. Theseresults,
in someof thesesamples(seepaper1, Figure5), andnotto from sites1, 3, 4, 5, 6, and9, constitutegroup2 in Figure 1.
The fourth and highestgroupingof T[ valuesis between
magnetite.Thesedatahavetherefore
notbeenincluded
in
Figure 1.
500 and 550øC. These results are from sites 10 and 11 and
group3 in Figure1. Theseunblocking
temperatures
The secondT• peakis between280 and330øC. Mostof comprise
theyaretoohighto be explainedby either
thesevaluesarefor samples
thatwerepretreated
by zero-field arecrucialbecause
cyclingthroughthe magnetite
isotropictemperature
(T• • the ?ullaiah et al. [1975] or the Middleton-Schmidt[1982J- 150øC)tounpindomain
wallsanddemagnetize
multidomainWalton [ 1980] (MSW) t-T contoursunlesstotallyunrealistic
temperatures
areassumed.In anycase,the very
remanence.
Thisprocess
of low-temperature
demagnetizationgeological
temperatures
in
(LTD) is a standardmethodof isolatingsingle-domainwidespreadbetweengroupsof unblocking
27,288
DUNLOP ET AL.: ANOMALOUSLY
15--
HIGH UNBLOCKING TEMPERATURES
Sites 1, 3,
Total
ß194Tt•determinations
-
4,5,6&9
Site 2
Sites
10 & 11
& LTD
-
Sites
•
•
-
,
0
150
•
,/,
200
250
300
350
400
450
500
550
MaximumLT unblocking
temperature,
TL(øC)
Figure 2. Histogramof 194 valuesof the maximumlaboratoryunblockingtemperature
TL of the LT overprint,
determinedfromjunctionsbetweenvectortrajectories
of LT andHT thermaldemagnetization
data. The four TL
groupingscorrespond
to samplescontainingpyrrhotite(sites7 and 8); samplesdominatedby single-domain
remanence(LTD treatedsamples)or with unusuallysmallLT components
(site2); sampleswith mixedsingledomain-likeandmultidomaincarriersof LT (sites1, 3, 4, 5, 6, and9); andsampleswith predominantly
multidomain
carriersof LT (sites 10 and 11).
Figure1 or 2 showsthatsomethingis amissin ourunderstanding of thermoviscous
remagnetization.
Two other possible explanationsof anomalouslyhigh
unblockingtemperatureswere consideredin paper 1. First,
long-term averagegeologicalreheatingtemperaturesdeter-
This
fortunate
circumstance
allows
us to examine
the
thermal demagnetizationof an almost single-component
multidomainremanence.The resultsshownin Figure3 for
10d1 and 10e2are typical. The LT componentdemagnetizes
slowly in almost exponentialfashion up to N500øC, above
mined from vitrinite reflectance (a measureof coalification) which the very small HT componentdemagnetizes.The tail
might be lower than thermal pulses which could cause of the LT demagnetization
curveis in goodagreementwith
remagnetization.However,differentsitesin an untiltedbody multidomainpartial TRM theory [Xu and Dunlop, 1994] and
the sizeof theMilton Monzonite(•50 km2in cross-section) with measuredthermal demagnetizationtails for 135
couldnot havereachedpeaktemperatures
differingby several magnetite
grains[DunlopandOzdemir,1997,Figures
10.11,
hundredøC. Second,possiblythe LT overprintis not wholly 16.11].
thermoviscous
butis partlychemical.The 100-150øCwindow
The resultsin Figure3 showthat whenLT is carriedby
betweenunblockingtemperatures
of LT andCRM1, which is multidomaingrainsand its thermaldemagnetization
is not
definitelya chemicaloverprint,is theninexplicable.
obscured
by underlying
HT magnetization,
theLT unblocking
We therefore believe that LT is indeed a thermoviscous
temperaturesextendup nearlyto the Curie point. It may be
overprint.Its anomalously
highaveragevalueof TL (anoma- thatthetwo conditionsarerelated: HT maywell be localized
lous accordingto conventionaltheories)is not the only, or in single-domain
or smallpseudo-single-domain
grainswhich
eventhe main,problem. The wide spreadof TLvaluesshows are rare at sites10 and 11 becauseof the largeaveragegrain
that thermoviscous
remagnetizationis not explicablein terms size. However, whetheror not HT is single-domain-like,
a
of single-domaingrains only, whether the Pullaiah et al. largeHT component
demagnetizing
overthesamerangeasLT
[1975] or the MSW model is used.
wouldmasktheLT tail (sinceLT andHT arealmostantiparallelvectors)andwouldresultin a lowerjunctiontemperature
T• betweenLT and HT. That is, T• shoulddecreaseas the
5. Thermal Demagnetization of Multidomain
Grains
We now return to the fourth cluster of T• values, for
samplesfrom sites10 and 11. Thesesamplesare specialin
two ways. First,Table 1 andFigure3 of paper1 showthatsite
10 samplescontainlargemagnetitegrains(> 100 gm approximately) with truly multidomain hysteresisproperties(no
unheatedsite 11 material remained for hysteresismeasurements). Second,all 12 samplesfrom sites10 and 11 havevery
smallHT componentscomparedto samplesfrom othersites;
an estimate of the HT direction was only possiblefor 2
samples(paper 1, Table 3).
HT/LT
ratio increases. We will test this idea in section 6.
6. Correlation Between T L and the HT/LT Ratio
The idea that a large HT componentmasksthe full tail of
theLT demagnetization
curvewhile a smallor vanishingHT
componentrevealsthe entire tail is borneout by the datafor
groups1, 2, and3. Figure4 andTable 1 showthatsite 10 and
11 samples,with HT/LT • 0, have LT unblockingtemperaturesextendingto 500øC and above,as alreadyillustratedin
Figure 3. Samplesfrom site 6 and othergroup2 sites,with
HT/LT in the range0.2-0.5, haveLT unblockingspectrathat
DUNLOP ET AL.: ANOMALOUSLY HIGH UNBLOCKING TEMPERATURES
27,289
1.o[
M/Mo
0.9
0.8-
0.7•'\
0.6-
0.5-
0.4-
oo
ß
Miltonmonzonite
•
samples10dl (ß)
\x ø\
•and
Oe2
(ø)
Multidomain
xX o•
pTRM
theory
xXx•x•
0.3-
•'•_
0.2-
T•.:
maximum
Tus
'-••• •o.•_
ofLTcomponent
0.1o
0
100
I
I
200
I
I
300
I
I
400
I
I
500
ß
•1(•0
ß0600
I
Temperature,
T (øC)
Figure3. Thermal
demagnetization
oftwosamples
fromsite10,where
LT iscarried
bytrulymultidomain
grains
andHT isverysmall
compared
toLT. Thedemagnetization
tailsoffexponentially
andagrees
withpredictions
of
multidomain
partial
TRMtheory
[Dunlop
andXu,1994].Thefinaldecrease
inmagnetization
justbelowTcisdue
to demagnetization
of thesmallHT vector.
aretruncated
or maskedaboveTi•= 400-450øCby demagneti- LTD-treatedoverprintis a partialTRM in single-domain-like
was due to
zationof HT. Samples
fromsite2 (group1) haveHT/LT • 2 grainsand that the originalrounding/overlap
and HT unblockingoutweighsLT unblockingaboveTi• = multidomaingrains.
330øC.
This is thecasein oursamples.Specimen2a2 hasrounded
Thecorrelation
is clearin a plotof Ti•versusHT/LT (Figure junctionsbetweenLT andHT vectorsin its demagnetization
5). As HT/LT changes
overthefull range0 -• 2, averageTi• trajectories(Figure6). Its companion,2a3, was pretreated
beforethermaldemagnetization
by LTD to erasemultidomain
valuesdecreaseby about200øC.
At thesametime,low-temperature
memoryratiosR for the remanence. As a result, the LT vector lost about one-half its
LT component
(i.e., LT intensityafterLTD dividedby LT intensity(the segmentbetweenthe NRM and20øC steps).
memory,now
intensity
initially)arein therange0.3-0.55andareuncorrelat- The remainingLT vector,the low-temperature
junctionwith HT.
ed with Ti•or HT/LT (Table 1). Multidomainremanence hasa sharpdemagnetization
forms a similarfractionof LT (one-halfto two-thirds)at all
Sharpjunctionsarea featureof single-domain
partialTRM
earlier,theyresultfrom
siteswheremagnetite
carriesLT. Sites2 and10 havethe or viscousoverprints.As discussed
grainwith blockingtemperature
lowestR values,in accordwith theiroverallmoremultidomain thefactthata single-domain
temperature,
Ttm= TB. Multidohysteresis
(paper1,Figure3). In section
7, wewill seethat TBhasa uniqueunblocking
the HT vectoris lessaffectedby LTD andis thereforemore main partialTRM behavesquitedifferently. A particular
blockingtemperature
TBdoesnothavea singlecorresponding
single-domain-like.
unblocking
temperature
TuBbutrathera broaddistribution
of
essentiallyto Tc
7. Effectsof Low TemperatureDemagnetization TuBextendingfrom room temperature
7.1 Rounded and Sharp Junctions
[Dunlop and Xu, 1994]. LT-HT junctionswhich become
sharpafterLTD showthattheoriginalrounding
wasa result
Anyoverlap
between
theunblocking
temperature
spectra
of of distributedTtmin multidomaingrains.
tworemanence
vectors,
e.g.,between
LT andI-IT asdiscussed A multidomainfractioncarryingeitherLT or HT would
themajor
in section6, will resultin roundedjunctionsbetweentheir resultin Ttmoverlapandrounding.If LT contains
the main
vectortrajectories
duringthermaldemagnetization.
Such multidomainfractionandHT is single-domain-like,
effect
will
be
to
push
junction
temperatures
up,
because
LT
overlap
androunding
arenotnecessarily
proofofmultidomain
grains:
a CRM overprint
wouldhavethesameeffect.How- will continueto demagnetizeaboveits originalmaximum
in singleever, if roundedvectorjunctionsbecomesharpwhenthe blockingtemperature.If LT wereconcentrated
grains,thejunction
composite
NRM ispretreated
byLTD,wecanbesurethatthe domaingrainsandHT in multidomain
27,290
DUNLOP ET AL.' ANOMALOUSLY HIGH UNBLOCKING TEMPERATURES
Sample:2a2
Sample:6d4
W,Up
Sample:10f2
W,Up
NRM
N,Up
NRM
lOOO
HT/LT
=0.17
HT/LT
=0
163
f36
HT/ LT
NRM
446
-lOOO
LT
=2.2
161
228
192
423
222
281
518
LT
NRM
136
280
348
16•
4O
400
•
176234 2•2 •
192
øC
/
302
163
LT
1000
NRM
- •32aOc
294
Figure 4. A demonstration
thattherelativemagnitude
of theHT andLT vectorsinfluences
thejunctiontemperature
TL betweentheir demagnetization
trajectories.The larger LT becomesrelativeto the single-domain-like
HT
component,
thehigheris TL: 328øCfor 2a2 with HT/LT = 2.2; 420øC for 6d4 with HT/LT = 0.17; and>500øC
for 10f2 with HT/LT = 0. The effectis dueto maskingof partof theHT unblockingspectrum
by LT unblocking
temperatures
spillingover abovetheiroriginalblockingtemperature.Sample10f2 andsamples10d1 and 10e2in
Figure3 areextremeexamples
of themaskingeffect,in whichdemagnetization
of a verysmallHT vectoris hidden
until closeto Tc.
temperaturewould tend to be pusheddown as a resultof HT
is destroyedby LTD. The HT vectors,on the otherhand,do
demagnetizing
belowits originalminimumblockingtempera- not decreasesignificantlyasa resultof LTD (e.g.,Figures6
ture. These trends are in addition to those due to the relative
intensities of LT and HT.
The experimentalevidenceis that a largeproportionof LT
(one-halfto two-thirds)is carriedby multidomaingrainsand
and 7). HT is single-domain-like.
Thereforewe anticipate
thatjunctiontemperatures
in mostsampleshavebeenpushed
up,i.e.,areanomalously
high,andwill tendto be loweredby
LTD. This hypothesisis testedin section7.2.
Table 1. Component
RatiosandLT UnblockingTemperatures 7.2 Reduction of T L
LTD reducesthe LT-HT junctiontemperatureT•.in speciSite
HT/LT
R = LT(LTD)/LT
Ti•, øC
mensof sample2a from about314øC to 302øC (Figure6).
The reductionis muchlargerin othersamples,particularly
1
0.184 _+0.057
0.486 _+0.027
401 _+35
from sitesotherthan2. Figure7 givestwo examples.Sample
2 (a-d)
1.90 _+0.36
0.268 _+0.020
333 + 15
2e has a much lower HT/LT ratio than samples2a-2d and
3 (b-g)
0.169 _+0.035
0.427 _+0.028
395 +_18
therefore
more elevatedjunction temperaturesbeforeLTD.
4
0.280 _+0.090
0.408 _+0.019
428 _+22
LTD lowersthe T•.from 352øCto 288øC. Sample6b, from
5
0.504 _+0.054
0.577 _+0.105
440 _+ 15
group2, has a still smallerHT/LT ratio. Its T•.value,origi6
0.289 _+0.070
0.564 _+0.055
394 _+27
7
220 _+ 12
nally 420øC, is reducedto 294øC by LTD. Notice thatLTD
8
267 _+61
does not affect the HT-CRM1 junction temperature,which
9 (c)
9 (d-f)
0.080 _+0.002
0.259 _+0.045
0.459 _+0.075
484 _+6
427 _+6
0.388 _+0.076
0.477 _+0.072
516 _+ 14
538 _+ 8
was 518øC without LTD and 510øC with LTD.
The preponderanteffect of LTD is to reduceT•. values,
which were originallyelevatedrelativeto the predictionsof
Pullaiah et al. [1975] and widely spread(over a rangeof
250øC:groups1, 2, and3 in Figure1), to a narrowconcentraHT and LT are the magnitudesof the high-temperature
and
tion
around300øC(secondT•.peakin Figure2). This value
low-temperatureNRM vectors,respectively. LT(LTD) is the
magnitudeof the LT vectorafter low-temperaturedemagnetiza- of T•.is as predictedby Pullaiahet al. We havethusverified
or single-domain-like
cardersof thermovistion. T[ is apparentremagnetization
temperature
asdeterminedin thatsingle-domain
cousmagnetizationdo obey the predictionsof conventional
thelaboratoryasthejunctiontemperature
betweentheLT andHT
vectors.
single-domainremagnetizationtheory on geologicaltime10
11
=0
=0
DUNLOP ET AL.' ANOMALOUSLY HIGH UNBLOCKING TEMPERATURES
27,291
11
lO
l
• 9c
400[ ,
3000
0.1
0.2
0.3
2
0.4
0.5
0.6
1.6
1.8
2.0
2.2
HT/ LT componentratio
Figure5. Correlation
between
T[ andtheHT/LTratio.Valuesshown
aresiteorsubsite
averages
withstandard
deviations
(Table1). Themasking
effectof multidomain
grains
produces
a >250øCspread
in T[ values
frompurely
single-domain
values
(z300øC,including
LTD-treated
samples)
topurely
multidomain
values
(500-550øC).
Sample:2a2 (withoutLTD)
Sample:2a3(withLTD)
N,Up
N,Up
5•o•
570
E
E
0
302
373
294
20
NRM
LT
373
HT
346
328
rounded
302 sharp
Figure6. A demonstration
thatrounded
junctions
between
LT andHT vectors,
indicating
overlap
in theirunblocking
temperature
ranges,
become
sharp(single-domain-like:
TuB= TB)asa resultof LTD pretreatment.
27,292
DUNLOP ET AL.' ANOMALOUSLY
Sample:2e
2
(without
LTD)
HIGH UN-BLOCKING TEMPERATURES
Sample:2e
I
(with
LTD)
N,Up
ßß NRM
Sample:6b3
(withLTD)
Sample:6b
2
(without
LTD)
N,Up
W,Up
NRM
W,Up
NRM
O0
• NRM
900
- - 1000
2O
W
11•570•352
ß.•
w
510 øC
372
518øC
/
'•
f
600
37•
S:Down
196
269
35•oc
S:DOwn
•
2•c
•
1000
E:Down
N
Illk551 x294 øC
-•88
NRM
E:Down
20
• NRM
Figure 7. Two examplesof decreases
in TLasa resultof LTD pre-treatment.The TLvalueafterLTD is ~300øC in
bothcases.The reductionin TLis greaterfor 6b thanfor 2e becausethe largerLT vector(relativeto HT) in 6b had
a greatermaskingeffectandpushedtheoriginalTLhigherthanin 2e. NoticethattheHT-CRM 1junctiontemperature
in 6b is unaffectedby LTD (518øC beforeLTD, 510øC after LTD). The intensityof the HT vectoris similarly
unchangedby LTD in either2e or 6b. Bothobservations
demonstrate
the single-domain-like
natureof HT.
scales.
The Middleton-Schmidt-Walton
contours are not
overprints
in theMilton Monzonitewereanomalously
high,on
necessary
to explainsingle-domain
remagnetization.
Elevated the basisof existingsingle-domain
remagnetization
theory
unblocking temperaturesare a consequenceof varying [Ndel,1949;Pullaiahet al., 1975]. A newsetof time-temperadmixtures of multidomain remanence carriers, which are aturecontours,proposedby Middletonand Schmidt[ 1982] on
particularlyprominentin theMilton Monzonitebecauseof the the basisof a theoryby Walton[1980], explainedthe data
coarseaveragegrain size.
moresatisfactorily.Walton'sanalysisalsousedsingle-domain
This picture is of coursean oversimplification.Figure 8 relaxationtheorybuthiscriterionfor remagnetization
wasthat
documentssample-by-sample
changesin TL valuesas a result the overprintingNRM shouldhavethe sameintensityasthe
of LTD. Althoughthereis a strongtendencyfor TLto decrease overprintedNRM. In reality,thermoviscous
overprintingof
asa resultof LTD, thereareexamplesof no significantchange single-domain
grainsoccursby replacement
of theoverprinted
or evenincreases(la, lf, 3g). Theseincreases
couldbe due to NRM, whateverits intensityor direction,by theoverprinting
variabilityin HT/LT betweenspecimensof the samesample, NRM up to TB= TuB= Tr(TLon a laboratorytimescale).This
to unusual mixtures of multidomain and single-domain is the criterionusedby Pullaiah et al., and it is the correct
carders(e.g.,multidomaincardersof HT), or possiblyto some picture of remagnetizationin a geologicalcontext (see
degree of chemical overprinting in these samples. For discussion
by Enkin and Dunlop [1988]).
example,specimen1a3 hasHT/LT- 0.240 while its companOur restudyof the Milton Monzonite,with broadersamion 1a5, pretreatedby LTD, has HT/LT- 0.133. The lower pling andmoreexhaustivethermaldemagnetization
than in
HT/LT ratio will tend to increaseTL (seeFigure 5). Further- Schmidtand Embleton's[1981] study,has shownthat the
more,morethanone-thirdof HT in both specimensis carried apparentsuccessof the Middleton-Schmidt-Walton(MSW)
by multidomainmagnetite.The ratio of HT vectorintensities remagnetization
contourswascoincidental.Although6 of our
in 1a5 (after LTD) and 1a3 (no LTD) givesa memoryratio R 11 sites give TL values similar to those of Schmidt and
= 0.637, an unusuallylow R value for the HT componentin Embleton (the third TL groupingin Figure 2) which are
the Milton samples.
compatible
withMSW contours
(group2 in Figure1), samples
from othersitesgive very differentresults.
Samplesfrom sites10 and 11 yieldTL valuesbetween500
8. Discussion
and550øC(highestgroupingin Figure2, group3 in Figure1),
It was previouslythought[Schmidtand Embleton,1981] muchtoohighto explainby the MSW contours.On the basis
that maximum unblocking temperaturesof thermoviscous of hysteresisdata and low-temperaturememories,these
DUNLOP ET AL.' ANOMALOUSLY HIGH UNBLOCKING TEMPERATURES
500
without
with
LTD
LTD
Site
27,293
lO
Site 1
480
lOc
46O
_la
Site 3
It'
440
Site 6
420
3g
of
400
380
Site 2
360
340 lb
2d
2c
• 2f
320
300
2a
2f
6e
2e b
of
280
6b 6c
Figure8. A sample-by-sample
comparison
of T•.values
without
LTD (left-hand
points
in eachpair,usually
two
specimens)
andwithLTD (right-hand
point,onespecimen),
asindicated
bythemodelat top. In all butthree
instances,
T•.decreases
asa resultof LTD,andinmanycases
(everysample
forsites2 and6), theT•.valuefollowing
LTD is in therangeexpected
for single-domain
grains(~300øC).
At the otherextreme,samplesfrom site 2 andmanyfrom
sampleshave multidomainmagnetiteas their principal
magnetic
constituent.
Furthermore,
thermaldemagnetizationother sites which have been low-temperaturecycled to
curvesof theirLT overprints,
unobscured
bytheconsiderable demagnetizemultidomainmagnetite,have much lower T•.
underlying
HT remanence
thatsurvives
atothersites,extend values, around300øC (secondpeak in Figure 2, group 1 in
almost
to Tc, withtailsof theformpredicted
bymultidomain Figure 1). Most site 2 sampleshave HT >> LT, unlike
HT does
partialTRM theory[DunlopandXu, 1994;Xu andDunlop, samplesfrom anyothersite. With few exceptions,
1994]. If LT werea CRM, it wouldlikely have"thermally not lose significantintensityas a resultof LTD: it is singlediscrete"
unblocking
temperatures,
concentrated
justbelowTo domain-like. BecauseLT is of low intensity at site 2, its
tail doesnot significantlycloudthe
ratherthanthe observed"thermallydistributed"unblocking thermaldemagnetization
demagnetization
of HT. For thisreason,
temperatures
witha quasi-exponential
spectrum
characteristicsingle-domain-like
of multidomain
partialTRM. A thermally
discrete
chemical site2 samples(aswell asmanyLTD-treatedsamples)giveT•.
overprint,
CRM1,withthesamedirection
asLT, doesexistat values that are close to true single-domainvalues. The
of group1 resultswith thePullaiahet al. contours
most sites but the LT and CRM1 unblocking ranges are agreement
cleanlyseparated,
with 100-150oc of nonoverlap
(paper1, (Figure1) confirmsthatthePullaiah-N6eltheoryis a correct
description
of single-domain
thermoviscous
remagnetization.
Figure6).
27,294
DUNLOP ET AL.: ANOMALOUSLY HIGH UNBLOCKINGTEMPERATURES
At other sites,LT >> HT. The low-temperaturememory 9. Conclusions
ratio betweenLT after and beforeLTD rangesfrom ~0.3 to
1. In the case of single-domainNRM, thermoviscous
~0.55. Thus aboutone-halfto two-thirdsof LT is carriedby remagnetization
is accuratelydescribedby the theoryof Ndel
multidomainmagnetite. BecauseLT is of high intensity [1949] and Pullaiah et al. [1975]. Geological conditions
compared
to HT, themultidomain
demagnetization
tail of LT during remagnetization,(tr, Tr), are correctlypredictedfrom
outweighs
theintensitydecrease
of thealmostantiparallelHT short-termlaboratoryobservations,(t[, T0, by using the
vectorin thelowerpartof theHT demagnetization
range,with Pullaiah et al. contours, and not the Middleton-Schmidtthe resultthat HT appearsto begindemagnetizing
at higher Walton contours(Figure 1).
temperatures
thanit actuallydoes. This maskingeffect,in
2. In the caseof multidomainNRM, unblockingtemperawhich LT unblocking overwhelmsHT unblocking and ture spectraof overprintingand overprintedNRMs overlap,
artificiallyinflatesthe measuredLT-HT junctiontemperature leadingto roundedratherthansharpjunctionsbetweenvector
TL, growsasLT/HT increases
(Figure5). We believethisis demagnetizationtrajectoriesof the two NRM components.
theexplanation
of anomalously
highunblocking
temperatures Thejunctiontemperature
T[ betweenthe vectorprojectionsof
in the Milton Monzonite(Figure 1, groups2 and 3).
The tremendous
spreadin laboratory
TLvaluesin Figures1
the two componentsmay be pushedeitherup or down comparedto the single-domain
caseby the overlapof unblocking
and2, for a singlevalueOf Tr (andof tr)in nature,demon- temperatures.In theMilton Monzonite,wheretheoverprinted
stratesthateithertheoverprinting
is notpurelythermoviscous HT primaryNRM is single-domain-like
andthethermoviscous
or it is notpurelysingle-domain.We havearguedabovethat LT overprint is at least 50% multidomain,TL valuesare
LT is verydifferentin character
from theknownCretaceous pushedup in general(Figure2).
chemicaloverprintCRM1 andhasall theearmarks
of a partial
3. When HT is verysmall,LT exhibitsa quasi-exponential
TRM overprint.Thereforemultidomain
careersof NRM must thermal demagnetization
curve with a high-temperature
tail
be the key factor.
extendingalmostto Tc (Figure3). The thermallydistributed
If this idea is correct, demagnetizingas much of the unblockingtemperatures
andthe shapeof thedemagnetization
multidomainremanence
aspossiblebeforebeginningthermal tail arein accordwith predictions
of multidomainpartialTRM
demagnetization
shouldyield moresingle-domain-like,
i.e,. theory [Dunlop and Xu, 1994].
lower,T• values.Thisis whathappens
in thegreatmajorityof
4. When LT is largerthanHT, theLT demagnetization
tail
caseswhen the NRM is treatedinitially by low-temperature masks the lower part of the HT unblockingtemperature
demagnetization
(Figures7 and 8). This simpleprocedure spectrum,pushingthe junction temperatureT[ up. The
requires
nomorethan30 minandcanbecarriedoutin blanket amountby whichT[ exceedstheexpectedsingle-domain
value
fashionon largenumbersof cores. It is recommended
as a correlateswith the LT/HT ratio (Figure 5).
standardpretreatment.If LTD is observedto sharpen
junc5. When LT is lessthan HT, the maskingeffect is small
tions between NRM componentsin vector projections, andlaboratoryT[ valuesare closeto expectedsingle-domain
comparedto roundedjunctions in untreatedspecimens values(Figure 1, group1).
(compare
Figure6), onehasindependent
evidence
thatsingle6. When the multidomainfraction of LT is removedby
domain-like remanence has been isolated.
priorlow-temperature
demagnetization,
theobserved
T[ values
Is multidomainremanence
a generalexplanation
of anoma- are generallyreduced(Figures7 and 8), often closeto exlouslyhighunblocking
temperatures?
The Milton Monzonite pectedsingle-domainvalues(Figure2, secondTLgrouping).
is an ideal test case because (1) the grain size is coarse,
7. Multidomain remanenceexplainsthe "anomalously"
favoringlargepseudo-single-domain
to multidomain
grains, highunblockingtemperatures
of LT in theMilton Monzonite,
(2) variableamountsof theolderHT remanence
survive,and but chemicaloverprintingmay be a factorin otherlithologies
(3) at mostsitesthereis a chemicaloverprintof the sameage andtectonicsettings.
as LT which is clearly distinctfrom LT in its properties,
8. Most of the Milton Monzonitesampleshavehysteresis
specificallyits range of unblockingtemperatures.Other propertiescharacteristic
of moderateto largepseudo-singlepublishedcasehistoriesarenot soclearcut.
domaingrainsof magnetite(10-100gm approximately).
Only
For example, in Kent's [1985] study of Appalachian a few approachtruly multidomainsizes (>100 gm). The
limestonesamples,the A overprint(Figure1) is certainlya multidomain and single-domain-likefractionsof NRM are
viscousremanencebecausethe glacialbouldersthat carryA thereforelikely a propertyof pseudo-single-domain
grains,
must have remainednear room temperaturein nature and ratherthanof separatemultidomainandsingle-domain
grains.
could not have been chemically altered except surficially.
Multidomainmagnetiteis thelikely culpritin thiscase.The
B overprintresultedfrom mild burial reheating(<250øC
accordingto conodontalterationdata)in a tectonicsetting
where chemicalremagnetizationis known to have been
widespread
[McCabeandElmore,1989]. However,thereis
nohigh-TuB
magnetiteCRM, andprimaryNRM, if presentat
all, is carriedby hematitewhichunblocksabovethemagnetite
Curiepoint. The unblocking
temperature
spectrum
of theB
overprintis peculiarin this and someothereasternNoah
Acknowledgments.D.J.D. andO.O. aregratefulto CSIRO for
coveringthe expensesof their researchvisit. Most of the measurements
weremadeat CSIRO; mostof the analysiswasdoneat
Toronto with supportfrom NSERC researchgrant A7709 to
D.J.D. We thankGillian Turner, Bob Butler,and an anonymous
refereefor their helpful reviewsand editorialsuggestions,
and
David Clark, RandyEnkin, and Dennis Kent for usefuldiscussionsaboutremagnetization.
Americancarbonatestudies.Unblockingdoesnotcontinueto References
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oursite
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HIGH UNBLOCKING
TEMPERATURES
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(ReceivedJanuary16, 1997; revisedJuly 7, 1997;
acceptedAugust27, 1997.)