OceanCrustalMagnetizationandMagneticAnomalies

• Anomalyandmagnetizationbasics• Lavasaslargestmagneticsource– whatcontrolstheirmagnetization?

• Dolowercrustallayerscontribute?• Magneticanomaliesandplatereconstructions• Skewness polesandanomalousskewness• Howdoescrustalaccretionaffectmagnetizationsourceandanomalies?

• Vectormagneticanomalies• Doestheoceancrustrecordintensityvariations?

AnomalyBasics

• Earthfilter:shorterwavelengthsmoreattenuated

• Geomagneticsignalincludesbothdirectionandintensity

• Crustalaccretionprocessesasfilter

Sourcemagnetizationandseasurfaceandnearbottomanomaliesgeneratedatpole(i.e.withremanence andambientfieldbothvertical).55km/m.y.halfspreadingrate.

• Shape(skewness)ofanomaliesdependsongeometryofspreadinglineation,Mdir,Bdir

• Skewness =phaseshifttomakesimilartopatternatpole

• Anomalousskewness =notpredictedbygeometry

MagneticMinerals

• Twopropertiesofmagneticmaterials(ferromagnets)– LongrangeorderingbelowCurietemperature

– Magnetichysteresis=magnetizationisafunctionoffieldhistory

• Curietemperatureisafunctionofmineralogy

• Magneticpropertiesastrongfunctionofgrainsize

Thermoremanent Magnetization

• Magnetizationofoceancrustisprimarilythermalinorigin

• Duringcooling,thermalenergydecreasesandanisotropybarriersincrease

• Smallparticles(<micron)arethemoststable

Tauxe (2010)

LavasasDominantSource

• Thinextrusivelayerisdominantsource

• AlongaxisprofilefitwithNmagnetizedbathymetry

• Crossaxisprofiletodeterminelayerthickness(~400m)

Talwani etal.(1971)

RidgeparallelprofileoverRcrustshowsnegativecorrelationwithtopography.

MagneticGrainSize&Remanence

• Coolingatpillow/flowmarginsresultsinlargegradientingrainsize

• Smallestmagneticgrains(nearmargin)havelargestmagnetization

• Compositionoflargetitanomagnetites (TM60)butmorevariablenearmargin

afterKent&Gee(1996);Zhouetal.(2000)

Remanence andGeochemistry

• Fractionationofmeltinshallowmagmachambers(FeO*increases)

• HigherFeO*shouldresultinmoretitanomagnetiteandhighermagnetization(magnetictelechemistry)

• SignatureofhigherFeatwakesofpropagatingrifts(andnearfracturezones)

Carbotte &Macdonald(1992)

Magnetizationpattern9-10°NonEPR.Maximum~40A/m.

MagneticTelechemistry

• Magnetizationofaxiallavasdeterminedfromslabsperpendiculartochilledmargin

• Reasonablecorrelation(R=0.81)forFeO*&NRM

• Alongaxismagneticanomalyvariationsgenerallymatchsamplegeochemistry

afterGee&Kent(1997,1998)

Variationsinmagnetization(calculatedfromanomalydata)arebestmodeledbylavathicknessvariations

Williamsetal.(2008)

Along-AxisMagnetizationVariations

TemporalChangesinMagnetization

• Anomalyamplitudesandsampleremanence higheratridge

• AttributedtolowToxidationoftitanomagnetite

sampleNRMdatafromIrving(1970) oxidizedTM(fromFuruta,1993)

CrackinginlargetitanomagnetitetypicaloflowToxidation

SpreadingRateDependence

Gee&Kent(2007)

Largedecreaseinamplitudeanomalyoff-axisconfinedtofullspreadingrateslessthanabout30km/m.y.

CentralAnomalynotsuchapronouncedfeatureatfasterspreadingrates.

EvidenceforLow-TAlteration

datafromWangetal.(2005,2006),Zhouetal.(2001)

• DirectdeterminationofoxidationstateofTMgrainsbyTEM

• Veryheterogeneousoxidation,withlowvaluesatoldestseafloor

Oxidationparametermeasuredonseafloorlavas.Avalueof1correspondstocompleteoxidationandwouldbeassociatedwitha4xreductioninmagnetization.

TemporalMagnetizationChanges

Gee&Kent(2007)Age(Ma)

DrillsiteswithlargenhavelowerNRMthanaxialdredgesbutnosystematicchangewithage.

AlterationandAnomalyAmplitudes

Gee&Kent(2007)

A2xto4xalteration-relatedreductioninmagnetizationiscompatiblewithslowandfastspreadprofiles.Thetimescaleofalterationispoorlydefined.

fullrate80km/m.y. fullrate20km/m.y.

MagnetizationofSheetedDikes

• Remanence indikes~5xlowerthaninlavas

• UniformlyhighCurietemp.indicatepuremagnetite

Datacompliled fromvarioussources;seeGee&Kent(2007)

NRM,inclinationandCurietemperaturevariationsinHole504B.

GabbroMagnetization

• Lowercrusthasbeensampledatseveralshallowholesandtwoholespenetrating~1.5km

• Remanence averagesabout1A/m

datacompiledfromvarioussources;seeGee&Kent(2007)

MagnetizationofDikesandGabbros

Pangetal.(2007)

Lowercrustalrockstypicallyhavenearlypuremagnetite(Curietemperature580C)eitherasaresultofsubdivisionoflargergrainsorastinymagnetitegrainsincludedinsilicateminerals.

Ilmenite lamellaeintitanomagnetite

TwogenerationsofmagnetiteexsolvedfromCPX(fromFeinbergetal.,2004).

ThreeLayerModel

• Lavashavehighestmagnetization(~5A/m)

• Thickerdikes(2.3A/m)andgabbro(1.2A/m)generatecomparableanomalies

Gee&Kent(2007)

LavaAccumulationPattern

Macdonaldetal.(1983)

Brunhes/Matuyama boundaryat21°NonEPRillustratinglavaspillover.Brunhes lavasextend250-500mbeyondtheverticallyaveragedreversalboundarydeterminedfromnearbottomdata.

• Transitionzonewidth(4sigma)of1.5to3kmlimitstheultimatetemporalresolution

MagneticanomaliesfrommodernGPTS

FromGeeandKent,2007,TreatiseonGeophysics,v.5.12

MarinemagneticanomalyskewnessShape(skewness)afunctionoftheremanentandambientfieldprojectionsandspreadinglineation

θmeas = ʹIr + ʹI f −180ʹIr = tan Ir( ) sin Az−Dec( )

EffectiveRemanent Inclination

Notethateffectiveinclinationchangesmostrapidlyforspreadingcentersnearthepaleoequator

Variationofeffectiveremanentinclinationasafunctionoflineationandpaleolatitude (fromActonetal.,1996)

Skewness Lunes ofConfidence

AsingleIr’estimatecouldbeproducedbyafamilyofremanent declination,inclinationpairs

Exampleoflunes ofconfdence fromLateCretaceousanomaliesinthenorthandsouthPacific(thesehavebeenadjustedforanomalousskewnessof14°;Cande,1976)

AnomalousSkewness

• Anomalousskewness notpredictedbyblockmodelgeometry

• WhartonBasin:differentphaseshiftneededfortwoflanks

Dyment etal.(1994)

Anomalousskewness estimatesfromIndianOcean(halfdifferenceofconjugateflanks)andanexampleofthesequenceeffect(right).FromDyment etal.(1994)

Possiblecausesofanomalousskewness

1) outwardtiltofsourceblocks2) slopingboundaryinthelowercrust3) systematicvariationsinfieldintensity

TiltingofLavas

Tivey etal.(2005)

LavatiltingmodelforODPHole801C.Dipsinferredfromloggingdataareusedtosuggesttiltfromlavaloading.

LavatiltfromloadingplusblockrotationsinferredforSites417/418.

Schouten(2002)

LavaTiltandSkewness

• Anomalyskewness issensitiveindicatorofsuchtilting(1° tiltcorrespondsto1° differenceinskewness)

• Lavatiltfromloadingresultsinanomalousskewnessoppositetothatobserved

Gee&Kent(2007)

Nonvertical BoundariesinGabbro

• Slopingisochrons inlowercrustcansignificantlyaffectanomalyshape

• Phaseshiftof~45°,insamesenseasglobalobservationsofanomalousskewness

Gee&Kent(2007)

RichardGordon’svideopresentationprovidessomeexamplesofapplicationofskewness polestoPacificplatemotion

Best-fittingpaleopoles fromanomaly12r(~32Ma)forthePacificplatefromanomalyskewness (blue)comparedtoestimatesfromsedimentaryfacies,paleomagnetic dataandhotspots

Exampleofaeromagneticvectordataatlowlatitude

Horner-JohnsonandGordon(2003)

Testsfortwodimensionality

Notethatmagnitudeofvectoranomaliesisalwaysgreaterthanorequaltototalfieldanomaly.

GeeandCande (2002)

Vectoranomaliescanallowidentificationofanomaliesingeometrieswithverylowamplitudetotalfieldanomalies

Engelsetal.(2008)

Vectordataalloworientationofmagneticcontraststobeestimatedfromasingleprofile

Engelsetal.(2008)

DoesOceanCrustRecordIntensityFluctuations?

• Potentialhigherresolutiontimemarkers

• Consistentpatterninseasurfaceanomalies(anom 5)

• Wavelengthsafunctionofearthfilter

Cande andKent(1992)Bowersetal.(2001)

82mm/yr

31mm/yr

54mm/yr

82mm/yr

54mm/yr

NearBottomAnomalyData

• Anomaly5profilesacquiredafew100mabovesource

• Nearbottomanomaliesalsolineatedbutmorecomplexpattern

• Unlikelytorepresentreversals

• Globallypresent?

afterBowersetal.(2001)

AnIndependentRecordofIntensityFluctuationsintheBrunhes

afterGeeetal.(1996)Tauxe andYamazaki(2007)

Sedimentarycoresproviderelativeintensityrecord,thatmatchseasurfaceanomalyprofilesreasonablywell.

Brunhes NearBottomAnomalies

• Stackof8profilesseparatedbyupto50km

• Topographicvariationscanbeaccountedforwithinversions

• Severalsimilaritieswithsedimentaryrelativeintensityrecords

Geeetal.(2000)

AnAdditionalCheck

• Candetermineabsoluteintensityofthefieldfromsurface(glass)samples

• Similarpatternwithaxiallowandboundinghighsinmagneticanomaliesandglasspaleointensities

Geeetal.(2000)