Crustal magnetization and magnetic petrology from the IDDP-drilling
Ocean Crustal Magnetization and Magnetic...
Transcript of Ocean Crustal Magnetization and Magnetic...
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)