JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 98, NO. B5, PAGES 7977-7986, MAY 10, 1993
Early Dolomitizationof Platform Carbonatesand the Preservation
of Magnetic Polarity
DONALD
F. MCNEILL
Division of Marine Geologyand Geophysics,
RosenstielSchoolof Marine and AtmosphericScience,
Universityof Miami, Florida
JOSEPH L. KmSCHVn,•K
Division of Geologicaland PlanetarySciences,CaliforniaInstituteof Technology,Pasadena
Resultsfrom a combinationof techniques
are presentedto evaluatethe natureof magnetization
in shallowwater platformcarbonates
which haveundergonerecrystallizationduringearly calcificationand dolomitization.
Magnetic grain separates,coercivityspectra,modifiedLowtie-Fuller tests, magnetizationefficiency, and
magnetostratigraphic
constraintsindicatethat the ultrafine-grainedmagnetiteis preservedduring early burial
geochemicalregimes,inversionfrom aragonite/high-magnesium
calciteto low-magnesiumcalcite, and even
pervasivedolomitization.These single-domain
crystalsare thoughtto occuras interactingmultigrainclusters,
someof which may exceed1 /amin diameter. Theselarge clustersmay help prohibitmagneticreorientation
during diagenesis. Furthermore,during both fabric preservingand fabric destructivedolomitization,the
ultrafine-scalereplacement
processrestrictsreorientationof the clusters,thuspreservingdepositionalor early
postdeposition
magneticorientation.This early dolomitization(matrix stabilization)may evenhelp protectand
extendthe subsurface
lifespanof the originalpolarity.
INTRODUCTION
Recognitionof depositionalor early postdepositional
remanent
magnetizationin Tertiary and older shallow-watercarbonatesis
becoming increasinglymore critical. These results are now
commonlyused for magnetostratigraphic
studies,paleomagnetic
poles, and for timing of remagnetizationrelated to major
diageneticevents such as: dolomitizationby relatively lowtemperaturefluids;very early recrystallization;dolomitizationby
deep basinal fluids related to tectonic/orogenic events;
dedolomitization;
base metal emplacement;and early hematite
formationfrom iron hydroxides[Horton et al., 1984; McCabe et
al., 1984; McCabe et al., 1985; Elmore et al., 1985; Jacksonand
shallow-water
carbonates,
this reportwill focuson the natureof
magneticremanence
in rockshavingundergone
early, pervasive,
near-surfacedolomitization. We will addressthe questionof
magneticgrainpreservation
throughrecrystallization,
andwhether
the magneticgrainsare susceptible
to reorientationduring the
recrystallization
process. Samplesfor this study, and data on
which comparisons are based, come from a larger
magnetostratigraphic
studywhere magneticreversalshave been
correlatedto the geomagnetic
polaritytime scale,with the aid of
biostratigraphic
markers [McNeill et al., 1988; McNeill, 1989].
The core borings for this study are from several different
Bahamianplatforms(Figure 1), and penetrateinto Plioceneand
lateMioceneagerocksat their base. Core locationsincludeLittle
Vander Voo, 1985;Bachtadseet al., 1987; Hurleyand Van der BahamaBank (GB-2, WC-1, SC-1); Great BahamaBank (Unda,
Voo, 1987; Jacksonet al., 1988;]. In general, magneticgrains U-l, U-3); and Ocean Drilling Program site 632. Holocene
were collectedfromGreatBahamaBankat JoultersCay,
incorporatedin shallow-watercarbonatesare often subjectedto samples
Andros
tidal
flat (GBB-3),Tongue-of-the-Ocean
(GBB-4),andLee
severalcarbonatediageneticfluid and recrystallizationregimes
during deposition, initial dewatering, early compaction, StockingIsland. Magnetizationin shallow-waterrockswhich have
cementation, mineralogic inversion, and recrystallization. undergone very early dolomitization (within 1-2 m.y.
are of especialinterestsince(1) theserockshave
Shallow-waterlimestonesare especially prone to numerous postdeposition)
undergone
several
(minimum of two) of the common carbonate
diageneticenvironments
due to their proximityto sealevel, with
extremesrangingfrom an entirelymarinefluid burial, to meteoric diageneticenvironments(initial burial, normal marine fluids,
vadoseandphreatic,mixedmarineandfreshwater);(2)
fluid exposure,to a complicated,often repetitivemixed fluid freshwater
stablematrixfor the magnetic
burial history. Secondly,the original mineralogiesare often theymayprovidea mineralogically
remanence
carriers
which
could
help
extend the lifespanof the
completely altered shortly after deposition, with complete
depositional
magnetization,
shieldingit fromcompletedestruction
recrystallization
fromanoriginalaragonite/high-magnesiumcalcite
or remagnetization;(3) they comprisea large portion of late
mineralogyto low-magnesium
calcite,and dolomite.
As a result of a better understanding
of these carbonate Tertiary carbonateplatforms and atolls which contain a rich
events;and(4) there
diageneticsettings[Longman, 1980; Mcllreath and Morrow, archiveof sealevelandregionaldepositional
1990], and an expanding application of paleomagnetismto are similar depositsin the ancient record that may contain
importantpaleomagnetic
polesif an early magnetization
can be
confirmed.
Copyright1993by the AmericanGeophysical
Union.
LaboratoryMethods
All rock-magnetic
andmagnetostratigraphic
measurements
were
conducted
at the CaliforniaInstituteof Technologyusinga 2G
Papernumber93JB00353.
0148-0227/93/93 JB-00353 $05.00
7977
7978
MCNEILL AND KIRSCHVINK: MAGNETIC POLARITY IN EARLY DOLOMITE
I
I
79ø
78ø
I
I
77ø
76ø
I
75ø
SC-1
LITTLE
BAHAMA
BANK
27 ø-
GB-
26 ø-
JOULTER•"
•BB-3
25 ø-
UNDA
ODP SITE
632
ANDROS
24 ø-
GREAT
BAHAMA
BANK
LEE
STOCKING
•L
23 ø-
L
Fig. 1. Locationof core boringscontaininglimestoneand dolomite,and sedimentsamplesused in this study. Core borings
includeGB-2, WC-1, SC-1 from Little BahamaBank; coresUnda, U-l, U-3 from Great BahamaBank; and OceanDrilling
Program site 632 in Exuma Sound. Holocene sample localities includeJoultersCay ooid shoal, Andros tidal flat (GBB-3),
Tongue-of-the-Ocean
periplatform (GBB-4), and Lee StockingIslandplatform sediments.
Enterprises 760 and SCT superconductingmagnetometers,
respectively. Additional measurementfor the magnetization
efficiency tests [Fuller et al., 1988] were done using a 2G
Enterprises 755 magnetometerat the University of Miami.
Coercivity and anhystereticremanentmagnetization(ARM) tests
were both conductedon bulk samples,about 3 cc in size. The
magneticseparateswere isolatedfollowingthe techniqueof Chang
and Kirschvink [1985], and examined on a Phillips 300
transmissionelectronmicroscopeat 100 kV. Magnetic samples
were collected from zones that have not undergoneobvious
massivedissolutionand reprecipitationassociatedwith meteoric
diagenesis. These thin, vertically restrictedzones of obvious
diagenesis,usually associatedwith subaerialexposure,included
void filling calcite and secondaryiron-oxides associatedwith
ancientsoil horizons(paleosols).
polaritycontinuity,and reversaltests)and rock-magnetictestscan
be used to characterizeand assessprimary versus secondary
remanence. In core boringsfrom recentcarbonateplatformsand
atolls, the rock-magnetictestsbecomesignificantlymore critical
for assessing
a primary remanence.
Several recent studies [McNeill et al., 1988; McNeill, 1990;
Aissaoui et al., 1990] have suggestedthat biogenicallyformed
magnetiteis the primary remanencecarrier in isolated,shallowwater carbonatesettings. Definitive recognitionof biogenic
magnetitein sedimentsolder than a few thousandyearsis otten
not possibledue to destructionof the characteristicorganic
componentsof the bacteria, mainly the magnetosomeand the
break-up of crystal chainswhich containprogressivelysmaller
crystals in the formative stages at the end of the chain.
Recognitionis howeverbasedon a combinationof characteristics
suchas limited single-domain
size rangeassociated
with known
bacteria[Kirsc:hvink,
1982;McNeill et al., 1988], a titanium-poor
crystalshape,and orientation
in chains
Sourceof Magnetic Minerals and CarbonateMagnetostratigraphy crystalcompø•ition,
(althoughrarely preservedin platform carbonatesexceptfor 3 to
Confirming the preservation of an original remanent 4 grain chains,and bearingin mind that the separationtechnique
magnetizationand polarity data is critical to applying magnetic may realignthe grainsin chains). Carbonaterocksshownto have
reversalstratigraphy
to platformcarbonates.In outcrop,both beenremagnetized,haveauthigenicmagnetitegrainssignificantly
conventionalfield tests(suchas fold tests, brecciatests, lateral differentto those from modernsedimentsand nonremagnetized
MCNEILL AND KIRSCHVINK:MAGNETIC POLARITYIN EARLY DOLOMITE
7979
the hostphaseminerals(aragonite,high-magnesium
calcite,lowmagnesiumcalcite)drivesthe replacementprocess. The pressure
the growingcrystalexertsagainstthe hostphaseis responsiblefor
Horton et al., 1984; Bachtadseet al., 1987; E#nore et al., 1987;
McCabe et al., 1987; Hart and Fuller, 1988]; pyrite/magnetite the hostphasedissolutionat the stressedface througha solution
film (several nanometersthick) at the boundary. At the stressed
spheres[Suket al., 1990]; noninteracting
single-domain
grainsof
contact
hostphasedissolutionoccursthroughincreasedsolubility
perhapsspheroidalmorphologies
[Jackson,1990]; or asextremely
resultingfrom increasedGibbs free energy. This mechanismis
fine-grained
(< 300 •) magnetite
nearthe superparamagneticintriguing for fine fabric preservationin that the pore waters
singledomainboundary[Jacksonet al., 1992].
during dolomitizationdo not have to be undersaturatedwith
Dolomitization Process
respectto the hostmineral. Severalfundamentaltexturalcriteria
have been presentedto supportthis mechanism[Maliva and
of grain/crystalcontacts
The mechanismof reerystallizationto dolomite, and the $iever, 1988]. Theseincludepreservation
controllingfactorsof the resultingcrystaltype, remainsomewhat (fabricpreservation),thepresenceof precursorcrystalghosts,and
of an enigma. Three generalpathwaysto dolomitization(see sometimesthe presenceof euhedralauthigeniecrystal facesin
hosts. Clearly,the mostimportant
below) are commonlyinvokedto explain the dolomitefabrics planarcontactwith unreplaeed
of theseis the highdegreeof fabricpreservation,especiallyacross
[Morrow, 1990]:
crystal boundaries,which suggeststhat no space gap existed
2CaCO3+ Mg2+ = CaMg(CO3)2
+ Ca:+
(1) betweenhostdissolutionandauthigeniecrystalprecipitation.This
fabric preservationsuggeststhat the rate of host dissolution
CaCO3+ Mg•+ + CO3
:'= CaMg(CO•):
(2) equaledthe rate of authigenieprecipitation.
The effect of pervasivereerystallizationon magneticgrains
(2-x)CaCO3
+ Mg:' + xCO32-=CaMg(CO3)
:- + (1-x)Ca
:+ (3)
maintainingtheir original remanentorientationis of particular
Each equationinvolvesdifferent formationconditionsand interest for assessing the potential for remagnetization,
reactant by-products,but most importantly contain volume reorientationand may even help constrainthe mechanicsof the
changes.For example,a volumelossis experienced
in equation replacementprocess. Clearly, two factors are important in
(1), an increased
volumein equation(2), andvolumeconservation retaining the initial magnetic orientation, the preservationof
can occurin equation(3) undercertainconditions.Any volume original magneticgrains, and restrictedgrain rotationduringthe
process.
changewould be importantwith respectto potentialmagnetic reerystallization
grainreorientation
and the inclination/declination
record. Thus,
the individualdolomitizationmechanismshave someimpacton the Remagnetization
AssociatedWith Dolomitization
resulting dolomite fabric, with either fabric preservingor
It is importantto realize that dolomitization,resultingfrom
destructivetypes [Sibley, 1982; Dawans and $wart, 1988].
can occurat almostany
Photomicrographs
of thedifferentdolomitefabrictypesare shown severalcompletelydifferentmechanisms,
byDawansand$wart[1988]. Theprecursor
mineralogy,
rateof point alongthe diageneticpathway. SeveralPaleozoicexamples
dolomitecrystallization,and the precipitatingfluid composition of remagnetizationassociatedwith regional dolomitizationand
are thoughtto be the main controlsof fabricpreservationin fluid tectonismhavebeenreported[McCabeet al., 1983;Horton et al.,
1984; Bachtadseet al., 1987; Jackson, 1990]. Remagnetization
dominatedsystems[Lippman, 1973; $ibley, 1982]
Lippman [1973] and Folk and Land [1975] suggested
that the in thesedolomiteswas thoughtto occur throughhydrothermal
magnesiumcontent(ordering) was a reflectionof crystallization and/orbasinalfluidscoevalwith the dolomitization,as opposedto
rate, with poorly orderedand ealeiandolomitesformingat faster a very early dolomitizationby near normal marine waters
rates relative to the more ordered stoiehiometrie
dolomites.
consideredhere. Hart and Fuller [1988] reported both
Subsequently,
Dawans [1988] andDawansand $wart [1988] have preservationand destructionof primary remanence in a
suggested
that facies/permeability
controlsinfluencethe rate of dolomitizedbed of the Monterey formation. In this ease, a
crystallization. Thus, sediments possessinginitially high chemicalremanentmagnetizationassociatedwith dolomitization
permeabilitieshadaccessto greaterfluid flow. The dolomitizing (cements)occurredirregularlyin the unit. In the Monterey,the
fluids in these sedimentshad a higher saturationrelative to lithologies,organic content, and fluid migration history are
dolomite,but due to relatively rapid precipitation,produceda considerablymore complicatedthan the almost pure carbonate
here, and may haveaided in supplyingand mobilizing
morecalciandolomite. Their datashowa very distinctseparation discussed
of the mol percentMgCO3in fabricpreservingmimetiedolomite iron.
As the mechanismsfor dolomitizationare highly variable
(42 95-4695)and fabric destructive,mierosuerosiedolomite(47 954995). At the same time, sedimentswith more uniform textures (normalseawater,mixed fluids, hypersalinebrines,deepbasinal
and lower permeabilitieswould allow reducedfluid flows and a fluids, tectonic related thermal events), some recrystallization
slower rate of dolomitization, thus more stoiehiometriedolomite. events (i.e. basinal fluids, hydrothermal)are likely to have
Traditionally, most dolomitization models invoke a fluid considerablygreater remagnetizationcapabilities than others.
dominated mechanism to account for dolomitization and dolomite
Very early dolomitization and calcification of predominantly
aragoniticsedimentsmay help retain the depositionalpolarity
texturaltypes.
shallow-and deep-burialcarbonatediagenetic
More recently, Maliva and $iever [1988] proposeda very duringsubsequent
different mechanism for mineral diagenesis, that of forced events. In the Bahamiandolomites, the main dolomitization event
crystallization-controlled
replacement. This mechanismis of was thoughtto occurwithin 1-2 m.y. after deposition[$wart et
86 isotopicdates
considerable
interestwith respectto the preservationof magnetic al., 1987; McNeill, 1989] basedon SrS7/Sr
grainssinceit invokesa very thin solutionfilm at the replacement [Vahrenkampand Swart, 1988]. Horton et al., [1984] described
carbonatesshow
boundary.Nonhydrostatic
stressresultingfromthe formationand an interestingsituationwhere Mississippian-aged
growthof authigeniecrystals(calciteor dolomite)in contactwith magneticpreservationrelatedto carbonatemineralogyand were
limestone/dolomites.Magnetic grains in thesecarbonatesoccur
as either: large (1-100 #m) spheres[Wisniowieckiet al., 1983;
7980
MCNEILL AND KIRSCHVINK: MAGNETIC POLARITY IN EARLY DOLOMITE
usedto constrainthe timing of dolomitization. Severaltypes of
dolomitewere shownto retaina late Paleozoicmagnetization
with
single-domain or pseudo-single-domainmagnetite carriers.
Limestones were shown however, to have been completely
remagnetizedbearinga Tertiary pole position. Second,work on
the Lower Ordovician Oneotadolomite in the upper Mississippi
River valley indicatesthat a very early magnetizationis preserved
in the dolomite [Jackson and Van der Voo, 1985]. The Oneota
dolomite is thought to have been depositedwithin a 5 m.y.
interval,and probablyexperienceddolomitizationrelativelyearly
in its diagenetichistory. The relatively quiet tectonichistoryof
the regionhasprobablypartiallycontributedto this preservation
of early magnetization.Thesereportssuggestthatbothearly and
late dolomitizationcan help preserve the original and early
remagnetizedremanencefrom subsequentcarbonatediagenetic
changes.A similarmineralogicallystablematrix (low magnesium
calciteto dolomite)in the youngBahamiandolomitesmay serve
a similar protectivepurpose. The averagelifespanof primary
single-domainmagnetiteis unknownat present, but would be
dependent
on burial, thermaleffects,anddiageneticfluid regimes:
it is apparenthoweverthat secondarysingle-domain
magnetitecan
have an extremely long preservationhistory [Jackson,1990]
RESULTS
AND
.........
........
•......
•
..........
:•::
DISCUSSION
•:::.•::
....• ..........
..
. ....:/..•:•
.... :•-::::::::•::?•:•:•,•:•:•::•........•;
:.'•:•:.•:.:::•:::•:::.:•:::•.•:.•..•::::::"•:•:'•$•'•:
....
:::::::::::::::::::::
.4:.•..
In order to address the effects of early carbonate
recrystallization
on magneticremanence,severaltechniqueshave
been employed:magnetostratigraphic
constraints,intensityand
inclinationcharacteristics,examinationof magneticseparates,
modified Lowrie-Fuller tests, Cisowski tests, magnetization
efficiencytests,andARM tests. Resultsfrom thesetestssupport
the preservationof original magnetic grains and remanence
throughearly calcificationand dolomitization.
MagnetostratigraphicConstraints
The vertical sequenceof Bahamiandolomitescontainseveral
•::
'"=•*::•:"•'"'":::::::::::.•g•'
•:•.?•::•:..:•:':•37:•'7
•;•...:.:•'•"•'•.
•.:.:::•
.....::::::::::::::::::::::::::::::::::::
-:":'-4:.:•:z
.... ::-•'
.:
Fig. 2. TEM photomicrograph
of single-domainmagnetitecrysgls. •e
grains often occur as large clusterswhen separatedfrom •e carbonate
matrix of loose sediment, limestones,and dolomites. •e effect of •e
separationtechniqueon clusteringis unceflain. Sample from Ocean
Drilling Program•g 101, site 632-5H, 125-129 cm. Scalebar 0.5 •m.
cluster,however, on a rare occasiona shortchain configuration
is observedin some separates. Grain clustersseparatedfrom
Bahamiandolomitesare similar in size (up to about 1 #m) and
shape(irregular ovoid) to thoseshownby McNeill et al. [1988],
polarity reversalsthat with biostratigraphic
markers, are
also from the Bahamas subsurface. In addition, the cluster can
correlative to the geomagneticpolarity time scale. Since
dolomitizationis believedto occurwithin a fairly restrictedtime
range(2.5-3.5 Ma) [Swart et al., 1987; Vahrenka•npand $wart,
1988], remagnetization
wouldlikely haveresultedin a pervasively
normalpolaritysequence.This hasshownnot to be the casefor
Little Bahama Bank where correlation is possible in the
dolomitized
sectionacrosstheplatformin theEarly Pliocenerocks
(Gilbert reversedchron) [McNeill, 1989].
sometimesbe slightly more elongate and irregular (branching
form) when fewer crystals are containedwithin the aggregate.
Shapes similar to these have been reported from ancient
limestohes[Chang et al., 1987]. To date, no report of similar
sized and shapedmagnetitecrystals have been reported from
carbonatesshownto have undergoneremagnetization.
Grain Extracts
Magnetic intensityat natural remanentmagnetization(NRM),
and variousalternatingfield (AF) and thermal demagnetization
Magneticgrainseparates
extractedfrom bothfabricpreserving
and fabric destructivedolomitescontainultrafine-grainedsingledomain magnetite as confirmed by transmission electron
microscopy(TEM) (Figure 2) and electron diffraction. The
single-domain
grainshave crystaldimensions
and morphologies
similar to known biogenic magnetites[McNeill et al., 1988;
McNeill, 1990]. The extractedgrainsusuallyoccurin large (>
1 #m) multigrainclusters(Figure2 and seeMcNeill et al. [1988]),
however, often show no signs of the original bacterialchain
configuration.Alternatively,the clustersare somewhatsimilarto
levels are similar for both the limestone and dolomitized
Intensity Characteristics
sections
(usuallybetween1.0x10-• and5.0x10'• A m2/kg)of a Bahamian
core boring (Figure 3).
The intensitiesare also comparableto
thosemeasured
in Holocenecarbonate
sediment(10-• to low 10-6
A m2/kg)for similaralepositional
settings
[McNeill,1990;also
unpublished
data, 1991]. Remagnetized
limestones
anddolomites
commonly
exhibitmuchstronger
intensities,
about10-2to 104
A/m (> 10-3 A m2/kgNRM for comparison)
[Johnson
et al.,
1984; Horton et al., 1984; Dunn and Elmore, 1985; Bachtadseet
al., 1987; Tucker and Kent, 1988].
single-domainmagnetite aggregatesfound in colonial A second indication of original remanence, and nonmagnetotactic
organisms,
althoughtheseaggregates
areonlyabout remagnetizationlies in the inclinationrecord of the limestonesand
0.5 #m in diameter. Magneticgrainsextractedfrom cemented dolomites. The distributionof inclinationanglesis similar for
limestonesand dolomitesusuallyoccur in sometype of grain bothrock types,with a broadscatteringof valuesusuallyranging
MCNEILLAND KIRSCHVINK:MAGNETICPOLAmTYIN EARLYDOLOMITE
INTENSITY (Am:/kg)
løø
t
AF 0
ALL SAMPLES
N=471
6O
40
20
from about10ø up to 60ø. This rangeof inclinationsis consistent
with some relatively young carbonates(125 kyr oolitic Miami
Formation)composedof aragoniteand low magnesiumcalcitein
several0.5 m thick beds, thoughtto have beendepositedwithin
several thousandyears, with varying amountsof subsequent
bioturbation(Figure 4). These results are inconsistentwith
pelagiclimestonescontainingfine-grainedmagnetite,and likely
reflect differences in the nature of carbonate deposition.
Depositionof platformcarbonates
is ot•enpunctuated
ascompared
to the more consistent,low accumulationrate, time-averaged
record in pelagic limestones. This diversity of depositional
environments,
andassociated
physicalprocesses,
is interpretedto
give rise to the wide-rangeof inclinations,and is not specificto
predominantlybiogenicremanences.
Likewise, precisionparameters(k) betweenthe youngMiami
formation(k = 18) and the Bahamiandolomiticrocks(meank = 12)
are withinthe samerange(usingmethodof Kono [1980]). The k
valuesfor remagnetizedcarbonates[McCabe et al., 1984;Jackson
et al., 1988] are usually,althoughnot always,significantlyhigher
than the Bahamian carbonates.
These low k values in Cenozoic
carbonates
canbe interpretedas eitherpreservation
of the original
magnetizationrepresentinga secular variation signal from
punctuated
deposition,or one which hasbeenaffectedby various
physicalprocesses
beforeand at•er signal"lock-in".
50
AF 0
40
7981
LIMESTONE
N=242
MagnetizationE•ciency
30
Support for preservation of depositional or early
postdepositionalremanence also comes from comparing
magnetizationefficiency through a test recently describedby
Fuller et al. [1988] and Hart and Fuller [1988]. By comparing
the ratio of AF demagnetizationof NRM versus the AF
demagnetizationof IRM (isothermal remanent magnetization),
different efficienciesor magnetizationregimes, can be isolated.
Data from the Bahamasfor several different types of dolomite
2O
10
indicates
efficiencies
similar
to
those
of
modern
carbonate
sediments,cementedcalcite/aragonite
rock, and completelylowmagnesiumcalcite limestones(Figure 5). The positioningalong
the same slope confirms a uniform remanence type and
preservational
history.
40
Coercivilyand Modified Lowrie-Fuller Tests
AF 0
Coercivity spectra and the ARM Lowrie-Fuller test for
dolomites, low-magnesiumcalcites, aragonite-low magnesium
calcite, and aragonitebearing rocks and sedimentall exhibit
similar characteristics(Figure 6), supportinga single-domain
20
mineralogythatis preservedthroughseveralstagesof aliagenesis.
Using the criteria determinedby Cisowski[1981] the dominant
remanencecarrier is single-domainmagnetite. In addition, the
10
coercivityspectraare almostidenticalto the ultrafine-grained
magnetitestandardand Holocenecarbonatesediments(Florida
Keys)measured
by Changet al. [1987]. The coercivitypatterns
reflect a combination of interacting single-domainparticles
•.
•.
o,.
o
(confirmedby TEM examination)and partial oxidationof the
magnetitegrains. Relativelybroadcoercivityranges(10-150 mT)
for both the limestoneand dolomiteare indicativeof partial grain
corrosionor grain oxidation(Figure 6), perhapsto maghemite
[Heider and Dunlop, 1987; Vali and Kirschvink,1989]. In the
Fig. 3. Histogramsof NRM magnetic intensity in limestonesand
separated
crystals,thispartialoxidationis represented
by a fuzzy,
dolomitesfrom severalBahamiancoreborings. Note similarityin intensity
distributionbetweenthe two mineralogies.These intensityrangesoverlap roundedcrystalboundarylikely composedof maghemite. The
with valuesmeasuredin sedimentsfrom moderncarbonatedepositional significanceof maghemitein Bahamiandolomitesfor carrying
magneticremanencehas not yet been quantitativelyassessed.
settings.
3O
N=229
ß
o
ß
o
7982
MCNEILL AND KIRSCHVINK: MAGNETIC POLARITYIN EARLY DOLOMITE
lO
level, after AF and thermaldemagnetization,
showsno consistent
decrease with depth which would suggest progressive
maghemitization
[McNeill et al., 1988]. The intensityvalues,do
however, contain considerable downcore sample-to-sample
variationswhichmay representbed-scaledifferencesin magnetite
oxidation and maghemite formation. Thermal sourcesor a
thermalgradientthatwouldenhancemaghemiteformation,are not
thought to be significant in these shallow settings. Detailed
facies/dolomite
fabric and maghemiterelationships
remainto be
U-1
N=64
determined.
0
10
20
30
40
50
60
70
The coercivity values (intersection of IRM curve and AF
demagnetizationof IRM, Figure 7) for Bahamiandolomites
average27.8 mT (n = 13, standarddeviationV=4.8) as compared
to limestoneat 30.1 mT (n=19, V=5.2) and loose sedimentat
38.6 mT (n=10, V=7.54). The higher coercivityaveragefor
unconsolidated,
uncompacted
sedimentsreflectsa combinationof
rapid submarinecementation,perhapshelpingto preservea less
interactinggrain configuration,and lack of grain oxidation. The
similaritybetweenthe limestoneand dolomitecoercivityvalues
suggest
a commonpreservational
history. Uponinitial dewatering
and burial many of the loose sedimentsalready approachthe
80
12
10
U-3
N=80
cemented rock values.
The percentslRM at the intersection
of IRM acquisition
andAF
decay curves (R value) can be used as an indicator of
magnetostatic
interactingbehavior[Cisowski,1981; Moskowitzet
0
10
20
30
40
50
60
70
80
al., 1988]. For limestoneand dolomite,percentsIRM at crossover valuesrange from 26%-39%, considerablylower than the
ideal 50% often measuredin less interactingsingle-domain
magnetiteextractsand unconsolidated
sediment[Chang et al.,
1987; Moskowitz et al., 1988; Vali and Kirschvink, 1989].
-4
lO
/I
/
/
/
/
/
/
MIAMI
LIMESTONE
Late Pleistocene
165
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/
/
/
/
/
/
/
/
/
/
//
/
/
/
//
7
.e/•//
/
/
/
/
/
/
10-7
/
//
D
/
-
/
o
/
/
/
0
10
20
30
40
50
60
70
/
/
80
/
INCLINATION
(Degrees)
/
measurements
after
chemical
treatment
to dissolve
maghemitesuggests
that it playsa partial, althoughminor role in
total remanencefor Bahamiandolomites. For example,in a core
from San Salvador, Bahamas, magneticintensity at the NRM
/
/
/
/
168_
,
/
/
/
/
/
/
/
of Andros Island, Bahamas, and those from several beds within the late
Pleistocene
Miami Formation. The wide rangeof inclinations
seemsto be
characteristicof late Tertiary and QuaternaryBahamiancarbonatesand
likely represents
the intermittentnatureof platformdeposition,bioturbation, lithification,and the associatedchangesin morphologyof the finegrainedmagnetite.
/
/
/
/
Fig. 4. Histogramof inclinationanglesafter principalcomponent
analysis
from the upper60 m from two cores(U-1 and U-3) on the northernend
/
/
/
/
/
Rcmanence
/
/
/
/
/
//
/
/
/
/
/
106
!
/
/
AF & Thermal
!
/
!/I
//
/
N=53, 10 discard
/
!
/
1(•9 /
10-6
/
/
/
/
Ir/
10-5
1(•4
1(•3
1• 2
slRM
Fig. 5. Resultsof a testof magnetization
efficiencydevisedby Fuller et
al. [1988] whichexaminesthe AF demagnetization
ratiosof the NRM and
IRM. Values include unconsolidatedHolocene sediments(C,D), calcitic
limestone(A), and both fabric preserving(E) and fabric destructive
dolomite(B). Magnetization
alongthe 10'3 ratioline suggests
a common
sourceand preservationin the recrystallizedrocks.
MCNEILL AND KIRSCHVINK: MAGNETIC POLARITY IN EARLY DOLOMITE
7983
lO0
•oo
ARM
80
-
70
-
C
BO
.•-
rl'-
•_
D
90
-- ß
-
-
40
-
30
--
PO
--
•o
'10
--
•o
o
U
aJ
60
60
50
o
loo
ry
90
--
BO
--
70
-
60
-
50
--
40
--
m
lO0
90
80
•
•
70
60
o
aJ
a,.1
50
• .o
30
--
PO
--
20
lO
--
lO
•
3
5
•.o
30 50
•oo
aoo
,•ooo
30
•.
Magnetic Field [mT)
3
-.6
•.o
ao 50
•.oo
-300
•.ooo
MagnetzcF•eld (mT)
Fig. 6. Coercivityspectrafor limestones
anddololnite:(a) modernsubmarine
cementedsediments,
JoultersCay Bahamas;(b)
calcitic limestone,core UNDA, 28.8 m; (c) fabric preservingdolomitefroIn core GB-1, 74.3 m; and (d) fabric destructive
dolomite from San Salvador, 96 m.
Modern carbonatesedimentsexhibita slightlyhigherR cross-over
value than the limestoneand dolomite, likely the result of less
grain interaction[Moskowitzet al., 1988; Vali and Kirschvink,
1989] prior to dewatering, compaction,and cementation. It
shouldbe rememberedthat these values may be influencedby
depositional facies controls that often dictate the original
concentrationof single-domain magnetite and its subsequent
preservation/loss
throughreactiveearly burial conditions.
any constraintson the size of the specific clusters. This test,
calibratedwith endmembersof stronglyinteractingsingle-domain
magnetitecrystalsin chiton teeth (respondingmagneticallyas
multidomain)versusfixed, noninteracting
chainsof single-domain
crystalsfrom culturedmagnetotactic
bacteria,indicatesthat much
of the magnetitewithin the carbonaterock is tied up as interacting
multigrain clusters(Figures 2 and 8). Many of the clusters
separatedfrom the carbonateare in excessof one micrometer,
althoughthe effectsof separation
on creatingsuchclustersare still
unknown.
MAGNETIZATION
"LOCK-IN"
AND
RESISTANCE
TO
Single-domaingrain clusters may result naturally in the
magnetosomesof magnetotacticbacteria [Towe and Moench,
1981] and in colonial magnetotacticorganisms[Lins de Barros
The location and size of the magnetic grains within the and Esquivel,1985], or are perhapsformedduringthe decayof
carbonatefabric is especially intriguing in light of potential the organic membrane containing the chain(s) of magnetite
physicalprocesses
acting
preservationthrough multiple recrystallizationevents. Since crystals. Alternatively,postdepositional
physicalseparation
of the ultrafine-grained
magnetitecrystalsfrom on the sediment [Verosub, 1977] may help promote grain
the rock likely destroys the natural size and morphology of clustering. In platform carbonates,the combinationof initial
compositegrain clusters, until now it has been impossibleto dewatering,compaction,and burrowingboth before and after
dewatering,may act to not only homogenizethe sedimentsbut
assesshow the magnetizationresistsdiageneticreorientation.
A test originally describedby Cisowski [1981] and recently may also provide an avenuefor magneticgrain clustering. The
calibratedwith biogenicmagnetitecrystals,and reportedby Diaz dataindicate(Figure 8) that clusteringoccurssometimeafter the
Ricci et al., [1991] and McNeill eta!., [1991], comparesthe periodof high water content(> 50%), and beforeand/or during
ARM/maxIRM momentratio at ARM fieldsup to 2.0 mT in order initial cementation.Thus, the postlock-inARM test indicatesa
to elucidatethe interacting/non-interacting
statusof single-domain stronginteractionof magnetitegrains for the cementedlimestone
magnetite crystals. This ratio provides some indication of and dolomite, but little to moderate interaction in the carbonate
magneticgrain habitafter "lock-in", however,it doesnot provide sediments(Figure 8).
REORIENTATION
7984
MCNEILLANDKIRSCHVINK:
MAGNETICPOLARtTY
IN EARLYDOLOMITE
COERCIVITY
0
10
20
I
(mT)IRM-AFCross
30
i
I
40
I
I
0-
ß
ß
ß
ß
50-
&l
ß
ß
&
ß
150-
ß ARAGONITE
ß CALCITE
The capabilityof a limestoneunit to resistgrain reorientation
is interpreted
to stemfrom a combination
of the largermultigrain
clustersand the fine scale of the replacementprocess. The
clusteringof single-domain
crystalsis probablycriticalin retaining
the original"lock-in"orientationthroughrecrystallization
because
it increasesthe effective size of the magneticgrains within the
carbonatematrix. This increasedsize, and perhapsan irregular
shape,alongwith interactingmagneticforceswithin the cluster,
assist in physical stability during recrystallization. The
dissolution/reprecipitation
process through either a fabric
preserving
migratingsolutionfilm at stressed
boundaries,
or fabric
destructiverecrystallization,must operateat sucha fine scale
throughprogressivereplacementso as to disallowreorientation
from rotation. Interacting grain clusters within the matrix
structureare perhapslocked-inupon dewatering,and basedon
morphologyandsizeare restrictedfrom reorientationby carbonate
grain contact inhibition. Thus, during dolomitization, the
magneticclustersare physicallyheld in place by the carbonate
grainsalthoughtheyareconcurrently
undergoing
recrystallization.
ß DOLOMITE
250-
Fig. 7.
Coercivity values versusdepth for varying mineralogiesfrom
Bahamian
sediments and rocks.
The
limestone
and dolomite
fluctuate
between22 and 40 mT, the resultof grain interactionsand perhapspartial
grain oxidation. Many of the Holocenesediments(e = eolianite,m= mud)
fall within this same range, exceptthose which have undergonerapid
submarinecementation(s).
ß
!
.9
NON
.8
I
J
K
L
.2
o
IS
0.5
1.o
1.5
2.0
ARM BIAS (mT)
Fig. 8. Compilation
of resultsfrom the ARM teststo assess
the naturalgrainconfiguration
in cemented
rocksandsediments.
All samples
passthe modifiedLowrie-Fullertestfor single-domain
crystalsandindicatean interacting
magneticconfiguration.
TEM examination
of samples
A, G, H, L andS haveconfirmedmagneticcrystalsin thesingle-domain
sizerange. SamplesA-H
representuncemented,unconsolidated,
non-compacted
carbonates,except for sampleG which is a submarinecemented
hardground. SamplesI-S cementedlimestoneand dolomite.A, GBB-2, Holocenepeloid/mud,Great BahamaBank (GBB),
Bahamas;B, LSI-3, Holocenemud/oolite,Lee StockingIsland(LSI), Bahamas;C, GBB-6, Holoceneperiplatformmud, Tongue
of theOcean,Bahamas;
D, GBB-3,Holocenetidalflat peloidmud,GBB, Bahamas;
E, LSI-2, Holoceneooidsand,LSI, Bahamas;
F, LSI-4, Holocenereef sands,LSI, Bahamas;G, Holocenesubmarinecementedooids,Joulters,Bahamas;H, GBB-4, sameas
C; I, GB-2-198, Miocene dolomite, Little BahamaBank (LBB), Bahamas;J, U-1055, Pliocenedolomite,core Unda, Bahamas;
K, U-1055, Pliocene, core Unda, Bahamas;L, U-1050, Pliocene, core Unda, Bahamas;M, U-1062, Pliocene, core Unda,
Bahamas,N, WC-167, Pliocene,LBB, Bahamas;O, U-1051, Pliocene,core Unda, Bahamas;P, GB-1-244, Miocene dolomite,
LBB, Bahamas;Q, U-94.5, Pleistocene
limestone,coreUnda, Bahamas;R, U-678, Pleistocenelimestone,coreUnda, Bahamas;
S, SC-162, Pliocene dolomite, LBB, Bahamas.
MCNEILL AND KIRSCHVINK:MAGNETICPOLARITYIN EARLYDOLOMITE
So far, besides the ARM tests, confirmation of clusters would
have to include locating and identifyingsingle-domaincrystal
clusterswithin a sectionedcarbonaterock: several attemptsin
Bahamiandolomitesusingthe scanningelectronmicroscopehave
been unsuccessful.
In the Bahamiandolomites,many sectionsof the core showan
extremelyhigh degreeof minute fabric preservation,suggesting
a forced crystallization-controlledreplacement. Other core
sections,mainlythe fabricdestructive(?) sucrosicdolomiteseither
replacedsimilar calcite grainsor directly replaceda fine-grained
micritictexture. This type of dolomitization,which Sibley[1982]
termedimpingementreplacementis alsoa contactinitiatedforced
recrystallization [Maliva and Siever, 1988]. The critically
important part of either of these dolomitization mechanisms
remains that recrystallizationin itself would restrict the free
movement of these grain clusters, thus precluding magnetic
reorientation.It is possible,however,that subsequent
changesin
the rock matrix, usuallycementation,may contributeto magnetic
grain clustering.
CONCLUSIONS
7985
Chang, S.-B. R., J.F. Stolz, and J.L. Kirschvink, Biogenicmagnetiteas
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Cisowski, S., Interactingvs. non-interactingsingle-domainbehavior in
naturalandsyntheticsamples,Phys.Earth Planet. Inter., 26, 56-62, 1981.
Dawans,J.M.L., Distributionand petrographyof Late Cenozoicdolomites
beneath San Salvador and New Providence Islands, the Bahamas, M.S
thesis,91 pp., Univ. of Miami, Coral Gables, 1988.
Dawan, J.M.L., and P.K. Swart, Textural and geochemicalalternationsin
Late CenozoicBahamiandolomites,Sedimentology,
35, 385-403, 1988.
Diaz Ricci, J.C., Woodford, B.J., Kirschvink, J.L., and Hoffman, M.R.,
Alterationof the magneticpropertiesofAquaspirillummagnetotacticum
by
a pulsemagnetization
technique,Appl. andEnviron.Microbiol., 57, 32483254, 1991.
Dunn,W.J., andR.D. Elmore, Paleomagnetic
andpetrographic
investigation
of the Taum Sauk Limestone,southeastMissouri,J. Geophys.Res., 90,
11,469-11,483, 1985.
Elmore, R.D., W. Dunn, and C. Peck, Absolutedatingof dedolomitization
by meansof paleomagnetic
techniques,Geology,13, 558-561, 1985.
Elmore,R.D., M.H. Engel, L. Crawford, K. Nick, S. Irabus,and Z.Sofer,
Evidence for a relationshipbetween hydrocarbonsand authigenic
magnetite,Nature, 325, 428-430, 1987.
Folk, R.L., and L.S. Land, Mg/Ca ratio and salinity:Two controlsover
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Fuller, M., S. Cisowski, M. Hart, R. Haston, E. Schmidtke, and R. Jarrard,
NRM:IRM(S) demagnetizationplots:
An aid to the interpretationof natural
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Geophys.Res. Lett., 15, 518-521, 1988.
Hart, M., and M. Fuller, Magnetizationof a dolomitebed in the Monterey
Formation:Implicationsfor diagenesis,Geophys.Res. Lett., 15, 491-494,
A combinationof rock-magnetictest data, orientationdata,
magnetostratigraphic
and Sr-isotope age data, and magnetic
separatessuggeststhat ultrafine-grainedsingle-domainmagnetite
can be preserved through early carbonatealiageneticregimes.
Inversion to low-magnesiumcalcite and dolomitization shortly
1988.
after depositiondoesnot necessarilydestroyor remagnetizethe Heider,F., andD.J. Dunlop,Two typesof chemicalremanentmagnetization
platform carbonates. More importantly, the single-domain
duringthe oxidationof magnetite,Phys.Earth Planet. Inter., 46, 24-45,
1987.
magnetitebearingrockscanresistreorientation
duringthe various
and
recrystallizationevents due to the presenceof relatively large Horton,R.A., Jr., J.W. Geissman,andR.J. Tschauder,Paleomagnetism
rock magnetismof the MississippianLeadville (carbonate)formationand
multigrain clusters versus the ultrafine-grainedreplacement
implication
for theageof sub-regional
dolomitization,
Geophys.
Res.Lett.,
processoperatingat a thin solutionfilm.
11, 138-146, 1984.
This recrystallization
from metastablearagoniteto either lowHurley, N.F., and R. Van der Voo, Paleomagnetism
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magnesium
calciteor dolomiteshortlyafter deposition
may indeed
reefallimestones,
Canningbasin,WesternAustralia,Geol. Soc.Ant. Bull.,
help prolongthis weak, but stableremaneneeduring platform
98, 138-146, 1987.
subsidence and burial in marine fluids.
The window of remanence
Jackson,M., Diagenetic sourcesof stable remanencein remagnetized
preservationwill vary considerablydependingon subsequent Paleozoiccratoniccarbonates:A rock magneticstudy,J. Geophys.Res.,
95, 2753-2761, 1990.
exposureto basinalfluids, excessivegeothermalgradientsduring
Jackson,M., and R. Van der Voo, A lower Ordovicianpaleomagnetic
pole
burial, or dedolomitization
associated
with reemergence.
from the OneotaDolomite,Upper MississippiValley, J. Geophys.Res.,
Acknowledgments. This study was made possiblethroughthe partial
supportof NSF grantsEAR-8817060andEAR-9005354to DFM. Several
newdolomitesampleswere availablefrom a recentlydrilledcoreon Great
BahamaBanksupported
by the NSF grantOCE-8917295andthe industrial
partners. The Ocean Drilling Program made samplesavailablethrough
request 11921A. The RSMAS PaleomagneticLaboratory was made
possible through NSF Grant EAR-8804957 and the W.H. Keck
Foundation.Constructivereviewsby D. Elmore, an associateeditor, and
an anonymous
reviewerare acknowledged
for improvingthe manuscript.
A contributionfrom the RosenstielSchoolof Marine and Atmospheric
Science. Contribution5185 from the CaliforniaInstituteof Technology.
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MCNEILL AND KIRSCHVINK: MAGNETIC POLARITY IN EARLY DOLOMITE
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J. L. Kirschvink,
Divisionof Geological
andPlanetary
Sciences
170-25,
CaliforniaInstituteof Technology,Pasadena,
CA 91125
D. F. McNeill, Divisionof MarineGeologyandGeophysics,
Rosenstiel
Schoolof Marine and Atmospheric
ScienceUniversityof Miami, 4600
Rickenbacker
Causeway,Miami, FL 33149.
(ReceivedJune26, 1992;
revisedJanuary20, 1993;
acceptedFebruary5, 1993.)