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ROCK MAG N ETISM AN D THE INTERPRETATION OF AN ISOTROPY OF MAG N ETIC SUSCEPTI BI LITY

P.Rochette, • M. Jackson,2 and C. Aubourg Laboratoirede G•ophysiqueInterne et Tectonophysique Observatoirede Grenoble, Grenoble, France

Abstract.The conventional rules, derived from empirical and isotropyto makequantitative inferences on the intensityof theoreticalconsiderations, for theinterpretation of anisotropy the preferredorientation and consequently on strain.In fer- of magneticsusceptibility (AMS) in termsof microstructure romagneticgrains, AMS mayalso be influencedby themag- and deformationare subjectto numerousexceptions as a neticmemory of the grains(including natural remanence). resultof particularrock magneticeffects. Unusual relation- The effectof alternatingfield or thermaldemagnetization on shipsbetween structural and magnetic axes (so-called inverse AMS is brieflydiscussed. Various rock magnetic techniques, or intermediatemagnetic fabrics) can occurbecause of the specificto AMS interpretation,have to be developedfor a presenceof certainmagnetic minerals, either single domain betterassessment of thegeological significance of AMS data. magnetiteor variousparamagnetic minerals. When more than Thesetechniques mainly rely on measurementsof suscep- onemineral is responsiblefor magneticsusceptibility, various tibility versusmagnetic field and temperature, together with problemsappear, in particularthe impossibilityof usingan- anisotropyof remanence.

1. INTRODUCTION grain(maximum susceptibility parallel to the long axis and minimumsusceptibility parallel to the shortaxis). Magneticsusceptibility K is definedby M = [K] x H, The anisotropyof magneticsusceptibility (AMS) is a where M is the inducedmagnetization of the materialand physicalproperty of rockswhich is usedfor petrofabricand H is the inducingmagnetic field. As both M and H are ex- structuralstudies (see Owens and Barnford[ 1976], Hrouda pressedin amperesper meter,volumetric susceptibility K is [ 1982], Borradaile [ 1988], Lowrie [ 1989], and Jacksonand dimensionless(written as "SI"), while masssusceptibility X Tauxe[1991], for recent reviews). Its principle is simple: = K/p is in cubicmeters per kilogram.Susceptibility varies AMS arisesfrom the preferredorientation of anisotropic in the generalcase according to field andtemperature values magneticminerals, in otherwords, the magnetic fabric. The and may vary accordingto the measurementdirection re- preferredorientation of crystallographicaxes, themselves sultingin a nonparallelismbetween H and M vectors.K is often controllinggrain shape,determines the AMS for the most often measuredat room temperatureand low-field vastmajority of minerals.The shapepreferred orientation of strength (•<1 mT) because in such conditions, individualgrains or of grain clusters,the secondcase cor- (1) experimentalprocedures are the easiest,allowing high respondingto the "distributionanisotropy" introduced by sensitivityand rapid measurements,even in the field; (2) K Hargraveset al. [1991], playsa directrole in AMS essen- is a goodestimate of the in situ inducedmagnetization of a tially in the caseof magnetite. rock in the Earth's field, thereforeserving as a basis for The AMS techniqueis gainingnumerous users for a wide magneticanomaly interpretations; and (3) K canbe correctly range of applications in Earth sciences because of approximatedby a second-ordersymmetric tensor, a situation (1) applicabilityto practicallyevery rock and soft sediment which greatly facilitatesthe measurementof susceptibility type; (2)high sensitivity,which allows one to determine anisotropy.At the grain scale the anisotropyof magnetic fabricsin rockspreviously seen as isotropic(for example, susceptibility,as manyother physical properties such as op- rocks with a fabric due to flow of magma, bottom water tical refractive index, mechanicalconstant, conductivity, currents,weak deformation), therefore opening new research etc., is controlledby the type and directionsof the crystal- fields;(3) timely operation(about 15 min per specimenin- lographicsystem. However, other phenomena overcome the cludingfield samplingand orientation,preparation, meas- crystallographiccontrol in mineralswith very high suscep- urement, and processing) allowing statistical and tibility suchas magnetite.In sucha casethe anisotropyof cartographicfabric investigations, particularly useful to map a multidomaingrain is simplythe imageof the shapeof the complexstructures such as thosefound in magmaticbodies 'Permanentlyat Facult•St-J•r0me, Marseille, France. or in tectonicunits; (4) possibilitiesfor semiquantitativeand 2Permanentlyat Institutefor RockMagnetism, University of quantitativeapplication in termsof fabricand deformation Minnesota,Minneapolis. intensityand symmetry; and (5) useas a newtool to constrain

Copyright1992 by the AmericanGeophysical Union. Reviewsof Geophysics,30, 3 / August1992 pages 209-226 8755-1209/92/92 RG-00733 $15.00 ß 209. Papernumber 92RG00733 210 ß Rochetteet al.' ROCKMAGNETISM 30, 3 / REVIEWSOF GEOPHYSICS

paleomagneticinterpretations in termsof age of the natural the relative lengthof the susceptibilityaxes) and the inten- remanentmagnetization (NRM), better definitionof struc- sitiesof linear or planarpreferred orientations, respectively. tural corrections,possible deviation of NRM from the geo- In the case of solid-statedeformation this implies a direct magneticfield, etc. relationshipbetween AMS and strain. Thereforea quanti- The outputof AMS measurementsis an ellipsoidof mag- tativeapplication of AMS is possibleonce a calibrationwith netic susceptibilitydefined by the lengthand orientationof strainhas been performed [e.g., Borradaile, 1991].In many its threeprincipal axes, K• •> K2 •> Ks, i.e., the threeeigen- cases,only a semiquantitativeinterpretation is possible;i.e., vectorsof the susceptibilitytensor. The parametersusually a more anisotropicrock is more strained. presentedare the meansusceptibility Km = (K• + K2 + Ks)/ 3. The AMS measurementis not affectedby naturalor 3 and the anisotropyratios L = K•/K2, F = KdKs, and P artificialremanent magnetizations. = K•/Ks (lineation,foliation, and degreeof anisotropy,re- Theseassumptions can be comparedto the basicrequire- spectively).The choiceof theseparameters and a standard- mentsto obtaina paleomagneticpole: The naturalremanent izedway of presentingAMS data,used throughout this paper, magnetization(NRM) of a rock is parallel to the field in are discussedby Ellwood et al. [1988]. The sensitivityof which it is acquired;the age of NRM is the formationage the individualmeasurements depends on the instrumentused of the rock; the ancientfield is representedby a geocentric (eithera low-fieldtorquemeter, a spinnermagnetometer (both axial dipole. In fact, as in paleomagnetism,the basic as- measuringdirectly a susceptibilitydifference) a suscepti- sumptionsof AMS interpretationare not alwaysfulfilled, bility bridge, or an inductivemagnetometer), the value of and one needsvarious techniques to assesstheir validity in Km, and the anisotropyratio L for the sensitivityon K• di- eachcase study. rection and F for Ks (see discussionby Ellwood, [1984a] Comparisonswith casestudies or modelsare not sufficient andSchmidt et al., [ 1988]). In the authors'opinion the most by themselvesto establishtruly scientificAMS interpreta- reliable, sensitive,and practical instruments presently avail- tions.Models are usuallytoo simpleto accountfor the com- able appear to be the torquemeterand the susceptibility plexity of naturalrocks, and empiricallybased deductions bridgeKLY-2 (manufacturedby GeofyzikaBrno). They al- stronglylimit the field of AMS applications.One can only low one to determinereliable AMS directionsin practically concludein termsof likelihoodwhen dealing with AMS data everyrock type down to an anisotropyratio of 1.002. that are not supportedby structuralmeasurements. When Statisticaltreatment of AMS datafrom a setof specimens suchstructural measurements exist, the needfor AMS study to define a site mean AMS tensorrequires specific tech- is minor becauseit bringsfew new information.Finally, a niques,either analytical(the tensorialmean method of Je- strongbias in the generalityof an empiricallaw may arise linek [1978]) or numerical (the bootstrap method of from the natural tendencyof workersin the field to only Constableand Tauxe[1990]). Thesetechniques must be pre- publishresults that fit in the usualinterpretation grid. ferred becausethey weigh each sampleby its anisotropy Amongthe independentpieces of evidenceused to estab- ratios,provide mean anisotropyratios more significantthan lish the validity of paleomagneticresults, rock magnetic the arithmeticmean of specimenvalues, and provide ellipses criteriaplay a major role, in particularthrough the identifi- of confidenceon the mean directions.Recently, Lienert cationof magneticminerals responsible for the NRM (i.e., [ 1991]has demonstrated by Monte-Carlosimulations that the magneticmineralogy). As in paleomagnetism,where it has ellipsesgiven by Jelinek's[ 1978] simplermethod are very become standardpractice to produce supplementaryrock goodestimates of the dispersion.However, when differently magneticevidence, AMS interpretationshould also be based orientedfabrics are superposedin a large set of samples, on rock magneticcriteria. their orientationcan be betterseparated by usingprocessing One of the main breakthroughsin that directionin the methodsfor structuraldirections (e.g., densitycontours on last decadehas been the wide recognitionof a specificmag- a stereonet). netic mineralogyrelated to AMS. In particular,there are This paperreviews the applicabilityof the followinggen- importantmineral sources of susceptibilitythat are not car- eral assumptions,often put forwardwhen interpreting AMS tiers of NRM (ferromagneticminerals sensu lato): the dia- data: magnetic, paramagnetic,and antiferromagneticminerals, 1. The AMS ellipsoidis coaxialto the petrofabric;the referredto as matrix mineralsbecause they constitutethe Ks axis is perpendicularto the foliation,which may be the main volume fraction of common rocks [Owens and Barn- beddingplane in sedimentaryrocks, the magmaticfoliation ford, 1976; Borradaile et al., 1985; Borradaile, 1987; Ro- plane in magmaticrock, or the flatteningplane for solid- chette, 1987]. The toolsof magneticmineralogy developed statedeformed rocks; K• is parallelto the petrofabricline- for paleomagnetism,mainly basedon propertiesof artificial ation, which may be a tectoniclineation, a magmaticflow remanence[e.g., Lowrie and Heller, 1982], thereforehave directionor a paleocurrentdirection for sediments.Therefore to be complementedby other techniquesbased on induced the Ks axis is namedthe pole of magneticfoliation, and K• magnetizationmeasurements as a functionof field and tem- givesthe magneticlineation direction. perature[Rochette et al., 1983;Rochette and Fillion, 1988]. 2. The shapeof the AMS ellipsoidis directlyrelated to Linear high-field susceptibility(Kay) measuresthe con- rockfabric. In a givenrock type there is a simplequantitative tribution of matrix minerals due to the saturation of ferro- relationshipbetween L or F (or otherparameters linked to magneticmoment in highfield. Thereforecomparison of 30, 3 / REVIEWSOF GEOPHYSICS Rochetteet al.: ROCKMAGNETISM ß 211 with low-fieldsusceptibility is amongthe bestways to assess the mineralogicalorigin of AMS. Propertiesof the matrix susceptibility,which is essentiallyof paramagneticorigin, have been reviewedalready [Rochette,1987; Joveret al., 1989]. Variation of susceptibilitywith temperatureallows the identificationof mostparamagnetic minerals, whose sus- ceptibilityvaries according to the Curie-Weisslaw K = C/

(T - 0). C is a constantproportional to the amountof o paramagneticions, and0 is the paramagneticCurie constant, I-I• K30 linked to the concentration of ions in the mineral and to the NORMAL typeof magneticinteraction. Therefore the studyof low-field

susceptibilityversus temperature also allows one to estimate •::::::::::::::::::::: REVERSE OTHERS :::::::::::::::::-- the paramagneticcontribution [e.g., Schultz-Krutischand ...... iiiiii?•"?•½ INTERMEDIA TE Heller, 1985]. Indirect estimates can also be derived from mineralogicaland geochemicaldata. For paramagneticsus- ceptibilitydue to silicatesor carbonateswhere Iol < 20 K a very good agreementis found betweenhigh-field meas- urement and the formula

KHF = -- 14.6 +

Kill d(25.2tFe2++ 33.4tFe3++ 33.8tMn2+)10-a SI 0 • K30 where the diamagneticsusceptibility of quartz is taken as the one of all minerals,d is the densityof the rock, and the Figure 1. Examplesof thefour AMS fabrictypes encountered in the dikes from Oman, with their respectivefrequency in a pie iron and manganeseamounts are in atomicweight percent. diagram.Magnetic anisotropy data (maximum and minimum sus- Only Fe and Mn ions are taken into accountbecause the ceptibilityaxes, respectively, squares and circles) are plottedin otherparamagnetic ions (from Cr, Ni, Co, Cu, U, etc.) are "dike"coordinates, i.e., with the dike plane(dashed line) tiltedto practicallyalways negligible compared to Fe. the vertical and rotated to a NS strike; L indicatesthe flow line Anotherimportant recent development has been the use derivedin this site from bubbleelongation. AMS data presented of thefollowing various types of magneticanisotropy, besides in all the figures(except Figure 7) wereobtained with the high- low-fieldsusceptibility at roomtemperature, to helpin AMS sensitivitybridge KLY-2 manufacturedby GeofyzikaBrno. interpretation:(1) anisotropyof inducedmagnetization under high fields [Rochetteand Fillion, 1988], which implement the formertechnique of high-fieldtorquemeter [e.g., Coward casestudies would suggest that K2 shouldbe parallelto the and Whalley,1979; Parma, 1988;HroudaandJelinek,1990]; flowline insteadof K, becauseof the "rolling"effect [Khan, (2) anisotropyof low-fieldsusceptibility at low temperature 1962](also reported for sedimentarycurrents by Reesand [e.g., lhmld et al., 1989]; and (3)anisotropy of various Woodall[ 1975]). This situationwith K3 perpendicularto the artificial remanences[Stephenson et al., 1986; Jackson, dikeplane, referred to as"normal magnetic fabric" (because 1991]. The use of anhystereticremanence (ARM) is quite themagnetic axes correspond one by oneto the petrofabric practical[McCabe et al., 1985] but more or lesslimited to axes), is encounteredin only 50% of the Omandikes, as magnetite-beatingrocks, while anisotropymeasurements exemplifiedin Figure1 (wherethe dike plane is verticaland with isothermalremanence (useful for goethiteor hematite) NS in all cases).In 25% of the casesthe inverserelationship [e.g., Tauxeet al., 1990]or thermoremanence[Cognd, 1987] is foundbetween the expectedpetrofabric and the observed are more difficult experimentally.- magneticfabric, with K• perpendicularto thedike pole. Ten The main purposeof this paperis to discussthe validity percentof thesites exhibit "intermediate magnetic fabrics," of the three assumptionsstated above in the light of rock definedby the occurrenceof K2 axesperpendicular to the magneticevidence, to presentexamples where these as- dikeplane. The remaining"other" cases correspond to sites sumptionsfail, and to proposesome rock magnetictests for whereanisotropy axes are scatteredor whereseveral types suchexceptions. The first and mostbasic assumption, con- of magneticfabric (normal,inverse, or intermediate)are cerningthe directionalinterpretation of AMS, will be dis- foundin the samedike. The purposeof the followingtwo cussedstarting from a case studyof 67 basalticdikes from sectionswill be in particularto discussthe possiblerock the ophioliteof Oman[Rochette et al., 1991].The AMS due magneticexplanations of suchabnormal magnetic fabrics, to the preferredorientation of early crystallizedelongated keepingin mindthat they can also be dueto "anomalous" titanomagnetitegrains in the magmaticflow of dikes is ex- petrofabric.In the caseof our dikesthe inversemagnetic pectedto showK3 axesperpendicular to the dike plane and fabricsare in factalso likely to be dueto secondaryprocesses K• axesparallel to the flowdirection of themagma [Ellwood, that haveerased the flow fabric, suchas coolingstresses or 1978;Knight and Walker,1988], althoughsome models and hydrothermalism. 212 ß Rochetteet al.' ROCKMAGNETISM 30, 3 / REVIEWSOF GEOPHYSICS

a) L : 1.02 F : 1.11 N b) L = 1.02 N c) L = 1.01 F = 1.02 F : 1.06

/ / / I

L = 1.02 - ß L = 1.01 F = 1.02

/ /

/ / / / /

Figure 2. Stereoplotsof AMS data,with averageL andF values, (d) tourmaline-bearingfacies from the samemassif; (e) mixedfa- from individualsites in variousrocks whose susceptibility is due cies; (f)cordierite- and biotite-bearinggranite from Spain [after,. to paramagneticminerals according to high-fieldmeasurements: Amice, 1990]. Structuralfoliation (horizontal in Figures2a and2b) (a) phyllosilicate-rich beds from a western alpine schist; is indicatedby dashedlines, structurallineation when visible is (b) carbonate-richbeds from the samesite [afterRochette, 1988a]; indicatedby a star. (c) biotite-bearing facies from the Everest leucogranite;

2. MINERALOGICAL ORIGIN OF INVERSE MAGNETIC rhombohedralstructure) of the crystalis the axis of maxi- FABRICS AND THEIR IDENTIFICATION mum susceptibility.Experimental deformation of calciteag- gregates[Owens and Rutter, 1978] as well as X ray texture 2.1. Matrix Minerals goniometryin naturallydeformed marbles [lhmld et al., Examplesof matrixminerals whose preferred orientation 1989] demonstratesthat c axes of carbonatestend to cluster leadto a normalmagnetic fabric have been listed by Hrouda aroundthe flatteningaxis, thusexplaining why K• appears [1982]. Theseinclude phyllosilicates such as chlorite or bio- perpendicularto schistosityin our case.Iron-bearing car- tite, as well as pyroxenesand amphiboles(see Table 1). bonatescan be easilydetected in a limestoneby staininga Crystallographiccontrol of both susceptibilityand grain thin sectionwith potassiumferricyanide. Inverse magnetic shaperesults in maximum(minimum) susceptibility along fabriccan be attributedto suchminerals by establishingthat the long (short)dimension of suchparticles. For example, the susceptibilityis of parmagneticorigin, i.e., by high- in an alpine schistwhose susceptibility is entirelydue to fieldor low-temperaturemeasurements. Usually, at leastone parmagneticphyllosilicates the K3 axes appearclustered structuralelement (foliation or lineation) is measurableand aroundthe poleof schistosity(vertical in Figure2a) andthe canbe comparedwith K• andK3 orientations. Once an inverse K• axesare parallelto the measuredstretching lineation in relationshiphas been demonstrated, the AMS resultscan be the rock. However, interbedded limestone levels show the reliablyinterpreted in termsof petrofabric.The meaningof oppositerelationship (Figure 2b from Rochette[1988a]). F andL are exchanged;a planarfabric is now characterized Studyof the magneticmineralogy revealed that the suscep- by goodgrouping of K• axes,and a valueof L •> F (e.g., tibility of theselimestones is carriedby parmagneticiron- Figure2b). Suchinverse magnetic fabrics due to carbonates beatingcalcite or dolomite.Measurements on singlecrystals are probablymainly encounteredin deformedmetamorphic of iron-bearingcarbonates including siderite [Rochette, limestones[Rochette, 1988a; lhmld et al., 1989], although 1988a] haveshown that the c axis (the revolutionaxis of the Rochette[1988a] also described an examplefrom a siderite- 30, 3 / REVIEWSOF GEOPHYSICS Rochetteet al.' ROCK MAGNETISM ß 213

TABLE 1. SelectedAMS Data for Rock-FormingMinerals cementedsandstone where only compactionhad occurred (see alsoHirt and Gehring [1991]). Mineral SymmetryType Km P Reference More recently,other matrix mineralshave been found to Diamagnetic produceinverse magnetic fabric. In a studyof the high Hi- Quartz 0 .... 14.5 < 1.01 1,2,13 malayanTertiary leucogranites[Rochette, 1988b; Scaillet, Calcite lc N(C) - 13.0 1.13 1,2 1990] a large differenceappeared between biotite-bearing Paramagnetic Biotite lc N(CS) 1-3 1.35 2,3,4,5 outcrops(Figure 2c), which exhibit a well-definednormal Other fabric, and tourmaline-bearingoutcrops where an inverse phyllosilicates lc N(CS) O.05-1 1.2-1.35 2,3,5 fabricis detected(Figure 2d). High-fieldmeasurements dem- Pyroxenes 4 N(S)7 0.5-5 1.2-1.4 2 onstratethat susceptibility,varying from 10-5 to 10-4 SI, is Amphiboles 4 N(S)7 0.5-5 1.08-1.3 2 entirelydue to matrix minerals.AMS measurementson var- Riebeckite la 9 2.6 1.16 0 Orthoferrosilite 1a 5 1.21 14 ioussingle crystals of tourmaline,which occuras rodselon- Staurolite 2b 0.8 1.06 0 gatedalong the c axis showminimum susceptibility along Garnet 0 3 1.001 0 this axis (as alreadymentioned by Foex [1957, p. 278]). Tourmaline 1c •(s) 0.9 1.12 0 Thereforerocks whose AMS is dominantlydue to tourmaline Cordierite 1c •(s) 0.6 1.15-1.31 0 are expectedto showK3 parallelto the linear preferredori- Siderite 2c •(c) 3.8--4.2 1.7 6 Other Fe entationof tourmalinecrystals and K• perpendicularto the carbonates 2c I(C) 0.05-0.7 1.08-1.45 6 foliationplane. This situationis well demonstratedin some Ordered of the Himalayantourmaline facies samples where lineation Goethite 1c I(S) 1.3-5 2? 7,8,9 and foliationare readily visible. Again, the significanceof Hematite 3 N(CS) 2-50 2.5-100 2,9,10 the anisotropyratio is inverted.The fabricof thesegranites Pyrrhotite 3 N(CS) 50-300 > 100 9,11 MagnetiteMD 4 N(S) •<3000 <5 2,12 is dominantlyplanar, as shownby L < F for thebiotite facies MagnetiteSD 3 I(S) •<1500 o•? 12 (Figure3 afterScaillet [1990]), whileL is similaror slightly MagnetiteSP 4 N(S)? •<5000 ... 12 larger than F on the averagefor the tourmalinefacies. The Kmis in 10-3 SI, exceptfor the diamagneticminerals (10-6). valuesof L and F are also consistentlysmaller for the tour- Symmetrycode: 0, isotropic;1, uniaxialoblate; 2, uniaxialprolate; malinefacies. This agreeswell with single-crystalmeasure- 3, triaxialoblate; 4, triaxialprolate. For uniaxial,the symmetry ments:black tourmaline monocrystals of the kind presentin is indicatedby thecrystallographic axis of revolution.Type code: thosegranites show Km around 9 x 10-4 SI and P • 1.12 normalN or inverseI with mechanismof preferredorientation in contrastto P = 1.35 for biotite [Zapletal, 1990]. eithercontrolled by shapeS or by intracrystallinegliding during ductiledeformation C. References:0, unpublishedor Rochette Thus when studyingthose graniteswhere lineationori- [1988b];1, Rochette[1987]; 2, Hrouda[1982]; 3, Ballet [1979]; entation is the chief interest, the dominance of tourmaline 4, Zapletal [1990]; 5, Borradaile et al. [1987]; 6, Rochette or biotite in the susceptibilityhas to be determinedin order [1988a];7, ttedley [1971]; 8, Rochetteand Fillion [1989]; 9, to equatethe structurallineation to K• or K3. In severalcases, Dekkers[1988]; 10, Dunlop [1971]; 11, Rochette[1988a]; 12, visual and thin sectioninspection unfortunately reveal both Maher[1988]; 13, Hrouda[1986]; 14, Wiedenmannet al. [1986]. of theseminerals. Actually, sucha site has been foundto bearsamples with normaland inverse magnetic fabric, within 1.03 one 10-cm block (Figure 2e). It thus appearsdesirable to find a rock magneticmethod to differentiatebetween specific paramagneticminerals. This has been attemptedby using the variationof KH• at low temperature.Fitting by a Curie-

1.02 Weisslaw, correctedfor a diamagneticcontribution, allows calculationof the paramagneticCurie temperature0, which L [3 is the interceptof the inversesusceptibility with the tem- peratureaxis (Figure4). Our resultsfor a tourmalinesingle crystalshow that 0 is - 10 K alongthe c axis and - 1 K in 1.01 the perpendicularplane. On the otherhand, data on biotite monocrystals[Ballet, 1979] indicate0 valuesof 5 to 20 K and near-zerovalues in muscovitemonocrystals. In fact, 0 is a measureof the strengthof the magneticinteractions i .00 , , I ' ' I ' ' I ' ' (roughlyincreasing with increasingiron amountin the crys- 1.00 .04 1.06 1.08 tal)and of theirtype (0 < 0 fordominantly antiferromagnetic F interactions)[e.g., Coey,1988]. Inverse susceptibility curves (Figure4a) on samplesfrom the tourmalineand biotite facies Figure 3. Anisotropyratio L versusF for the GangotriHimalayan leucogranite(redrawn from $caillet, [1990]), with open and full of the Himalayanleucogranite, as well as on samplesfrom symbolsfor the tourmalineand biotite facies,respectively. Solid purelymuscovite-bearing Hercynian granites (see Figure 1 anddashed lines indicate the maximumvalues for completeor 50% in the work by Rochette[ 1987]and Figure 4 in the workby preferredorientation of tourmaline,respectively, according to the Joveret al. [1989]) show0 valuesin very goodagreement relationship(5) in section3.3. with the monocrystalvalues. Therefore this technique,al- 214ß Rochetteet al.' ROCKMAGNETISM 30, 3 / REVIEWSOF GEOPHYSICS a) TOURMALINE FACIES of AMS in thoserocks with typicallyparamagnetic suscep- tibility [Amice and Bouchez, 1989]. Cordieriteoccurs in thesegranites as rods elongatedparallel to the c axis in the orthorhombicsystem, and a preferredorientation of those D,, ---15 10-6• rodswas occasionally observed. AMS measurementson sin- gle crystalscut from the granitesamples again show a min- imum susceptibilityparallel to the c axis, with P from 1.15 to 1.31and Km • 6 X 10-4 SI. Thereforeinverse fabric may be expectedagain. In onecordierite-rich site (Figure 2f) the AMS resultson four samplesindeed show that the K• and K3axes are exchanged in two samples,indicating that normal fabric (due to biotite) and reversefabric (due to cordierite)

I i i T IKI are coexistingin the samples. 100 200 300 To completethe list (see Table 1) of matrix mineralsas possiblecarriers of inversefabric, the antiferromagneticoxi- BIOTITE FACIES hydroxideetFeOOH, i.e., goethite,must be mentioned.This mineraloccurs usually as needles parallel to thec axis,which is alsothe direction of spinalignment. Therefore, as observed D.-17 10-6 • on a singlecrystal by Hedley [ 1971], one can expecta min- imum antiferromagneticsusceptibility along the needleaxis. At roomtemperature the P ratio observedby Hedleyis 2.04. Low-field susceptibilitydue to the weak ferromagneticbe- haviorof thismineral may changethe situationfor AMS but this contributionis probablynegligible [e.g., Rochetteand Fillion, 1989]. However,because of the very low meansus- ceptibilityof goethite(about 2 x 10-3 SI) comparedto F1 associatedminerals such as maghemite,hematite, or many o I I I T IK• paramagneticminerals, an AMS carried by goethitewill o 100 200 300 probablybe very rarely encountered.

b) TOURMALINE MONOCRYSTAL 2.2. FerromagneticMinerals Amongthe remanence-carrying minerals, hematite, pyr- 0.3 -- //C:D---910-6 J rhotite,and multidomain (MD) magnetite(or relatedcubic ironoxides) are well known to produce normal fabric [e.g., Hrouda,1982]. On theother hand, Daly [1970],O'Reilly [1984,p. 70], andStephenson etal. [1986]have pointed out thatsingle domain (SD) uniaxial prolate grains of magnetite havea zerosusceptibility along their easy axis of magneti- zation,i.e., theirlong axis. The resulting inverse AMS fab- ricshave been exemplified in limestones[Rochette, 1988a], in sampleswith syntheticSD maghemite,in basalticrocks [Potterand Stephenson, 1988], and more recently in exper- 0 I I I T (K) imentallydeformed synthetic samples [Borradaile and Pu- 0 lOO 200 300 umala,1989]. As recentlyreviewed by Jackson[1991], the bestway to detectsuch inverse fabric is to studythe ani- Figure 4. Inverseof high-fieldsusceptibility K,,,• corrected for the sotropyof anhystereticremanence (AAR). AsSD grains have diamagneticterm D versustemperature for (a) two samplesof tour- theirARM parallelto the easyaxis, the AAR fabricshould maline- and biotite-bearingleucogranite from the Everestmassif; be normal[e.g., Potterand Stephenson, 1988]. (b) a tourmalinemonocrystal parallel and perpendicularto the c However,as found in severalcases including the dikes of axis. Fitted D and 0 values indicated. Oman(see also Rochette [1988a]; and Aubourg [1990]), the AAR fabric appearssimilar to the anomalousAMS fabric (e.g.,Figure 5a). Thismay be related to anabnormal pre- thoughit requireshigh precisionto define an accurate0 ferredorientation of themagnetite grains or to a specific value,seems to be ableto distinguishdifferent paramagnetic behavior of pseudosingle domain (PSD) grains, which are minerals. characterizedby a few domainwalls pinned on crystalline In studyingHercynian granites from Spain, Amice [1990] defects.Indeed, it isnot impossible from the theoretical point wasled to questionthe possiblerole of cordieriteas a carrie of viewthat some kind of PSDgrains may have a minimum 30, 3 / REVIEWSOF GEOPHYSICS Rochetteet al.: ROCKMAGNETISM ß 215 axisparallel to theirlong dimension both for AMS andAAR: AMS AAR if theremanent magnetization of thepinned domain walls is a) Site M36 N N largerthan that due to the differentsizes of the domains,it will accountfor inverseAAR, while for AMS, suchPSD grainscould exhibit the same behavior as SD grains.How- ever,whether this situationmay be encounteredin nature remains to be demonstrated. i• •'. E • E In thisoverview of the grainsize dependence of the in- trinsicAMS of uniaxialmagnetite grains, one may also considerthe caseof superparamagnetic(SP) grains.ASP grainhas a veryhigh susceptibility along its easyaxis, be- causethermal activation allows the spontaneousmoment to N N jumpfreely from one to theother equilibrium position. On theother hand, the perpendicular susceptibility is similar to that of an SD grain as the spontaneousmoment has to be rotatedaway from the easy axis. Thereforea uniaxialSP H E • E grainshould show a normalAMS fabric.Whether SP grains are ableto showany preferredorientation in natureremains speculative.

2.3. The Problem of Intermediate Fabrics Figure 5. Comparisonof AMS and AAR (measuredin Minne- When dealingwith inversefabrics, one oftenencounters apoliswith a cryogenicmagnetometer and ARM acquiredin 100- intermediatecases, i.e., withK2 perpendicular to thefoliation mT AF and0.1-mT biasfield; see McCabe et al., [ 1985])maximum planeas exemplified in Figure1, or samplesthat interchange andminimum directions: (a) for a dike with inverseAMS fabric; theirprincipal axes with respectto othersamples from the (b) for a shale-limestonesite from the UpperJurassic of Western samesite [Rochette,1988a; Aubourg, 1990; Rochette et al., Alps with the beddingplane corrected to the horizontal(site N6 1991].Another example is foundin Figure5b, whichshows afterAubourg [ 1990]). Shale samples appear in opensymbols. the AMS datafrom a shale-limestonesite from the Upper Jurassicof the WesternAlps in a foldedarea with no schis- tosity[Aubourg, 1990]. All sampleshave apparently normal ing shalylayers or to a specificdeformation mechanism such fabricwith K3 perpendicular tobedding, except some samples as selectivedissolution of calcitegrains under deviatoric from a singlelimestone bed which tendto be intermediate stresses)and anomalousmagnetic fabric (due to particular with K3 andK• within the beddingplane. The moreshaly PSD grains)may possibly explain the present data set. samplesyield NS K• axes(the observedstructural lineation Twoparamagnetic silicates, both in themonoclinic crys- parallelto fold axis), while the limestonesamples have EW tallographicsystem, have been found to exhibit an AMS K• axes. The EW magneticlineation could be anotherun- ellipsoidneither related to the normal nor the inversesitu- detectedstructural lineation [Aubourg et al., 1991] or the ation:staurolite and riebeckite (a Na andFe 3+ rich amphi- artifactof an exchangebetween K• and K2 axes, the NS bole),which crystallize as rodselongated along the c axis. directionbeing the truly normallineation for the wholesite. The AMS measuredon selectedmonocrystals is uniaxial Another indication of abnormal fabric is that the AMS el- prolatewith K• parallelto the b axisin stauroliteand uniaxial lipsoidshape is prolateor nearneutral for the sampleswith oblatewith K3 parallelto the a axis in riebeckite.The latter EW lineationor intermediatefabric, while the other samples situationis in factalso encountered in theFe-rich orthopy- havestrongly oblate shapes. AAR measurements,with a peak roxene (orthoferrosilite)studied by Wiedenmannet al. alternatingfield of 100mT, on theselimestones duplicate the [1986], in contradictionwith pyroxenedata reportedby AMS resultsin almostperfect detail for eachsample (only Hrouda [1982] (seeTable 1). Thereforethe preferredori- K3 axes appeara bit more dispersed,Figure 5b). ARM entationof the longaxis (i.e., c axis)of suchgrains does ellipsoidsare almostall prolate,as the susceptibilityellip- notproduce a simplenormal or inversemagnetic fabric, but soidsare, with P valuesnot largerthan 1.03. Thuswe can the resultingAMS dependson the b or a axespreferred rule out anomalous AMS in these rocks as due to the "SD" orientation(see Table 1). It mayproduce a complexrela- effect. tionshipbetween AMS andpetrofabric with a possibilityfor Hysteresismeasurements indicate a predominanceof fine- an intermediatemagnetic fabric. grainedferrimagnetic material in thesesamples. The ratio However,it doesnot seem possible to finda singleminer- of saturationremanence to saturationmagnetization is ap- alogicalorigin for intermediatemagnetic fabrics. On the proximately0.15; accordingto the empiricalframework of other hand, their close association with inverse or normal Day et al. [ 1977],this indicates a meanmagnetic grain size magneticfabrics, often in thesame site, suggests an expla- withinthe PSD range. Thus both anomalous petrofabric (due nationby a mixingof differentmineralogical components. to mechanical contrast between the carbonate and surround- This will be discussedin the followingsection 3.2. 216 ß Rochetteet. al.- ROCK MAGNETISM 30, 3 / REVIEWSOF GEOPHYSICS

3. OTHER EFFECTS OF MINERALOGY ON THE urableparameters, was foundto fit quite well the datafrom SUSCEPTIBILITY ANISOTROPY a weaklymagnetic granite with a diamagneticisotropic (PA - 1) contributionand a variableparamagnetic phyllosilicate 3.1. QuantitativeSignificance of the AnisotropyRatio in amount,responsible for the anisotropy[Rochette, 1987; see a Two-MineralSystem Hrouda, 1986]. It waspossible to correctthe anisotropyratio The effects of variable mineralogicalcomposition on to get a value independentof the phyllosilicateamount. In AMS ratios have been consideredin a numberof previous examplessuch as in Figure 6 it is possibleto qualitatively studies[e.g., Borradaile, 1987; Hrouda, 1987; Rochette, fit the P versusKm relationship using both laws. 1987]. These studieswere generally concernedwith the Althoughthe sectionon inversefabrics emphasized the widely encounteredcase where susceptibility is dueto more extremecase of completeinversion of principalaxes, a more than one mineral, with coaxial normal fabrics but different subtleand probablyfar more commoneffect associated with anisotropyratios. It is obviousthat if the proportionbetween the presenceof "inverse"minerals is simplya reductionin thesecontributions changes in a formation,the global ani- the degreeof anisotropy[see Potter and Stephenson,1988]. sotropyratio will showvariations independent of strainin- This may be an importantsource of error in "straingauge" tensitybut relatedto composition,i.e., lithology.One should applicationsof AMS. A possibleexample of this effectcan expecta correlationbetween Km and P if the differentcon- be seenin the studyof mylonitesfrom the Brevardzone by tributionshave different Km as well asP valuesas exemplified Goldsteinand Brown [ 1988]. They observedthat meansus- in Figure 6. The differentexamples gathered in that figure, ceptibilitydecreases with increasingintensity of mylonitic from pyrrhotite-bearingschists [Hrouda, 1987; Rochette, foliationand interpretedit in termsof decreasinggrain size 1987] (see also Fuller [1963], and Borradaile and Sarvas of the magnetitegrains. This would imply an increasing [1990], for very similar P versusKm relationshipin such proportionof SD grains.They also observedan associated rocks), magnetite-bearinggranitoids [Damm, 1988], and decreasein anisotropydegree; this could plausiblybe the basalticdikes [Rochetteet al., 1991], clearly demonstrate resultof coexistingnormal and inverseanisotropies, due to the wide occurrenceof the controlof anisotropyratio by the MD and SD particles,respectively. proportionof ferromagneticversus paramagnetic contribution It is thereforequite obviousthat directquantitative use of to the susceptibility.In suchplots the increasingtrend of P AMS is not possiblewhen the susceptibilityis not due to a valuesmainly appearsfor Km > 3-5 10-4 SI and P > 1.2 singlemineral or evena singlegrain sizefraction in the case (for stronglydeformed rocks). These limits correspondto of magnetite,a possiblyquite rare situationin rocks. To the usual upper limits of the paramagneticcontribution overcomethis problemwithout using a more specificani- [Rochette, 1987]. sotropytechnique such as high-field (for thematrix minerals) For a two-component(A andB) systema formularelating or remanenceanisotropy (for the ferromagneticfraction), P andKm for the rock as a functionof the anisotropydegree Henry [1983] has proposedan indirectmethod for the case of eachcomponent (PA and PB) has been proposed [Rochette, of an AMS with two contributions, one called matrix and 1987] in the casewhere the contributionKA of A component theother ferromagnetic. An applicationof thismethod, based to the mean susceptibilityis constam(i.e., independentof on a correlationbetween diagonal terms K•i of the suscep- the fractionf of the B component): tibility tensorand mean susceptibilityKm on the site level, is shownin Figure 7, which illustratesAMS resultsfrom P = [Km(PA -Jr-1)PB- KA(P•- PA)]/[Km(PA-Jr- 1) Proterozoicslates of northernMinnesota [Johns,1990]. Hen- -Jr-KA(P a -- PA)] (1) ry's methodallows one to derive from the observedlinear correlationsbetween Ki andKm the matrix andferromagnetic The assumptionmade is valid if A is the diamagnetic susceptibilitytensors, with an algorithmbased on the fol- contributionand B is a variableparamagnetic or ferromag- lowingassumptions: the matrix andferromagnetic fabric are netic contribution or if A is a constant matrix term and B is constant(in orientationand intensity)within the site andthe a ferromagneticmineral with large susceptibilitybut small variationof Km and K• is only due to a variableamount of amount(f < 0.05). In suchcases we can equateKm to KA ferromagneticgrains while the matrix contributionremains + fK•. To derive (1), f and K• were eliminatedfrom the constant.The dataof Figure7 yield a goodlinear correlation basicequations (Ki = KAi + fKBi)usingthe relationshipsfor that allows derivationof apparentlysignificant matrix and neutralellipsoids; For example,Km= (K1 + K3)/2 - K2. ferromagneticsusceptibility mean tensorsfor the site, with Similarrelationships were derived for purelyoblate or prolate quite differentanisotropies. While L valuesare similar for ellipsoidsbut computations showed that the resulting curves thematrix, ferromagnetic,and AAR (ARM measuredin 100- are in fact shapeindependent for smallf values.When such mT peak alternatingfield) tensors,the F valuesare 1.07, assumptionsare not valid; i.e., when we haveKi - (1 - 1.92, and 1.18,respectively. In particular,the computedfer- J•Kni -F-fKai, Borradaile and Sarvas [1990] proposedthe romagneticsusceptibility F ratio appearsto be unrealistically followingrelationship: high, comparedto the AAR ratio. The interpretationgiven by the modelis both nonunique P = [fKB1+ (_1 -- J•KA1]/•B 3 + (-1 -- J•KA3] (2) andtestable. In particular,it impliesthat the ratio of ferrom- agneticto totallow-field susceptibilities increases with mean The firstproposed law (1), whichuses more readily meas- susceptibility,from a value near zero up to about30% for 30, 3 / REVIEWSOF GEOPHYSICS Rochetteet al.' ROCKMAGNETISM ß 217

ß ß 2.0- 2.1 -

e•e ß 2.0- #e e ß ß eee e ß ß 1.8- ß ß e• e 1.6 t

1.4 -

1.2 -

1.0 1.0 I i i O 500 1000 1500

Km(10-6) Log (Km)

P 1.4- 1.08 -

1.3- 1.06 -

ele l e•ee • ß 1.2- 1.04 - o mmmo m o ,,.•OA 1.02 - ' ø mm mmm øm•• z•z•mmO• A 1.0 1.00,

-5 -4 -3 -2 -1 Log (Km)

Figure6. P versusKm plots for variousregional studies: (a) and 1991].In Figure6b, sampleswith pyrrhotitenot detected, present, (b) pyrrhotite-bearingschists from the Carpathiansafter Hrouda andabundant are shown with squares,open circles, and triangles, [1987] and from the Swiss Alps after Rochette [ 1987], respectively; respectively. In Figure6d the sitemean values for normal,reverse, (c) granitoids from the hercynian chain [Datum, 1988]; andother fabric types appear as squares, open circles, and triangles, (d) magnetite-bearingbasaltic dikes from Oman[Rochette et al., respectively. the highestsusceptibility samples (Figure 7a). However,di- verifiedby an independentmethod such as high-field meas- rectmeasurement of thisratio using high-field susceptibility urements.Unfortunately, all rockformations with mixedcon- measurementsshows that it remainspractically zero (•<2%) tributionsto AMS andvariable Km that have been studied so on the wholerange of Km;in thoseslates, susceptibility far with this technique[e.g., Rochette,1987] revealeda appearsin fact entirelyof paramagneticorigin on the whole variabilityof the matrixcomponent as largeas the oneof rangeof Kmvalues, thus ruling out the conclusionsderived the ferromagneticcomponent. from Henry'smodel. The apparent"ferromagnetic" fabric derivedfrom the seeminglyconclusive correlation in Figure 3.2. A Modelfor IntermediateMagnetic Fabrics 7 hasno physicalsignificance, while the "matrix" fabric is To find a possibleexplanation for the occurrenceof these just somesort of averageof the AMS data. abnormalbut not truly inverseAMS fabricsoften associated In fact,it canbe concludedthat results of Henry'smethod with inversefabric, whereK2 exchangesits directionwith can only be trustedif its basicassumption, i.e., constant K• or K3, let us considerwhat happens for a mixtureof two matrix contributionand variableferromagnetic amount, is mineralswith coaxialpetrofabrics, but normaland inverse 218 ß Rochetteet al.: ROCK MAGNETISM 30, 3 / REVIEWSOF GEOPHYSICS

rection), and one that can be named as a "false" normal or

5- • KxxR2= .743 • inversemagnetic fabric, becauseit looksnormal (or inverse) [] KyyR2= .901 ..••"'• from the point of view of Kz but the horizontalaxes are not 0 KzzR2= .904 0 0 • -- in the expectedpositions. In particular,the modelof Figure 8c, with a stagewhere K3 is verticalbut K• is perpendicular 4- to lineation(false normal case), exhibits the samesequence of fabricsas the naturalexample of Figure 5b, wherewhat could be interpretedas normal, false normal, and interme- diate fabricscoexist in the sameoutcrop. In that casethe 3 1 two componentsare likely to be due to MD and SD mag- 3 4 Km (10-4 SJ) 5 netite.In particular,the ratio ARM/Kmincreases from normal Figure 7. Plot of principalsusceptibilities K,, versus mean suscep- to false normaland intermediatesamples, while the shape, tibility Km[after Johns, 1990] in Proterozoicmetasediments from stronglyoblate in normalsamples, becomes prolate or neutral Minnesota,according to the methodof Henry [ 1983]. in false normaland intermediatesamples. However, a com- plex petrofabricmay as well explain the resultsof Figure 5b. magneticfabrics, for example,either MD and SD magnetite Using partial ARM anisotropies,Jackson et al. [1989a] or somematrix minerals.The followingmodels start from wereable to demonstratein a compactedshale that the coarser the exampleof Figures2a and 2b with a petrofabricdefined soft grainshave a much more oblateAAR fabric than the by a horizontalfoliation (perpendicular to the z axis) andan finer highercoercivity grains, which haveneutral shape in EW (y axis) lineation.The mixingparameter p is definedas that case. Assumingthat the finer fractionhas an inverse the proportionof the inversecomponent in the mean sus- AMS fabric, we obtainthe caseof Figure 8c; in fact, this ceptibility.It is possibleto calculatethe susceptibilities along casewith falsenormal stage is probablymore likely to occur. theNS, EW, andvertical directions (Kx, Ky, Kz) as a function If such false normal magneticfabrics are not rare, it ofp andgiven values for theprincipal susceptibilities of both stronglycomplicates the interpretationof weakmagnetic lin- components,chosen to give Km = 1 (Figure 8). As both eationssuch as thosefound in the WesternAlps [Aubourg end-membermagnetic fabrics reflect the same petrofabric et al., 1991].The testof K3 perpendicularto foliationis often foliationand lineation, we havefor p = 0 a maximumKy usedas a criterionfor evaluatingthe reliability of K• as a and a minimumKz (normalmagnetic fabric), while the op- structurallineation indicator;the modelingresults above posite(reverse magnetic fabric) holds for p = 1. Stereonets showthat this criterionmay be misleading.Such problems on the fight of the figure illustratethe resultingorientations againstrongly indicate the needfor additionalrock magnetic of principalAMS axes. informationin interpretingAMS results. In Figure 8a thereis a uniquep valueproducing a fully The intermediatemagnetic fabrics encountered in thebas- isotropicpoint (Kx = Ky = Kz) andnormal or inversemag- altic dikesof Oman(Figure 1) maybe well explainedby the netic fabric on each side. This situation occurs when the presentmixing model, the normal end-memberbeing the shapesof the end-memberellipsoids are exactly"inverse," primaryflow fabric and the inverseend-member being the for example,one oblate and the otherprolate or bothneutral secondaryfabric with K• perpendicularto the dikeplane. In as in Figure 8a. Formally,it correspondsto En = (Ln - 1)/ fact, severalsites are found with normal and intermediateor (Fn - 1) = (Fi - 1)/(Li - 1) = Ei, wherethe subscripts reverseand intermediate samples in the samedike. The pro- n and i refer to the normal and inversecomponents. This portionof normal:reverse:intermediatedikes, about 5:2:1, is conditionmay often not be met in rocks. As discussedby also similar to what can be derived from the model: end- manyprevious authors [Owens, 1974; Hrouda, 1982;Roch- membersare more likely to occurthan the intermediate stages ette, 1987; Borradaile, 1987], the AMS due to an individual of the mixing model.This modelalso predicts a loweran- mineral dependsupon both its orientationdistribution and isotropyratio in the intermediatecase, as foundin the dike its characteristicanisotropy. Thus the symmetricsituation data set (Figure 6d and Rochetteet al [ 1991]). describedabove could arise only when the susceptibilityis due to two minerals with similar shape-orientationdistri- 3.3. Symmetryof MagneticFabric Versus Mineralogy butions and similar but inverted shapesof their intrinsic An additionalmineralogical effect that has not been ex- susceptibilityellipsoids. However, this is clearly a special plicitlydiscussed is the sensitivityof AMS symmetry(pro- case.Thus it is appropriateto introducesome asymmetry in lateversus oblate) to mineralogicalcomposition. An extreme the end-membermagnetic fabric shapesfor our mixing exampleof thiseffect can be seenin the caseof a mixing model. betweennormal and inversemagnetic fabrics. In the model Figures8b and8c showthe AMS behaviorresulting from of Figure 8a, the end-memberellipsoids and mixturesfor E n > E i (i.e., the normalcomponent is lessoblate than the all valuesofp areneutral and only the degreeof AMS varies inversecomponent is prolate) and En < Ei, respectively.It with p. However,in the morerealistic models b and c the is easyto observefrom the evolutionof K•, Ky,Kz that four AMS symmetrychanges drastically with p. For example,in different stagesappear: normal and inverseend-members, Figure8c the shapeof the ellipsoidevolves from strongly one truly intermediate(i.e., Kz being the intermediatedi- oblateto purelyoblate, then through neutral to purelyprolate, 30, 3 / REVIEWSOF GEOPHYSICS Rochetteet al.' ROCKMAGNETISM ß 219

Petrofabric axes

(a) ß Max. ß Int. i Min. 1.1

•/ ßß e eee (•Yx I ß eee Resulting AMS axes e eee I

0.9 0.0 0.2 0.4 0.6 0.8 1.0 (e.g. ß 100% p (e.g!100% MD magnetite) SDmagnetite) (b) 1.1 I

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I © ' (•), , © ,© I I I 0.9- i I i i i I 0.2 0.4 p 0.6 0.8 1.0

(c)

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O.g 0.0 . 0.2 0.4 0.• 0.8 1.0 P

Figure 8. Model of normalizedprincipal susceptibilities along x, susceptibility.Three differentrelative shapes of the SD and MD y, z for a mixtureof MD and SD grainswith coaxialpetrofabric ellipsoidsare represented(see text). Stereonetson the fight show (grain alignmentalong y and pole of foliationalong z as indicated theresulting K,, K2, andK3 directions (same convention as in Figure in the first stereonet)versus the proportionp of SD in the total 1) for thedifferent domains defined by theintersections of thelines. andfinally back through neutral to purelyoblate, Note that losilicateswill resultin an oblateAMS and converselyfor this sequencestrongly resembles the patterncommonly as- prolatemineral grains. For example,magnetic fabric due to sociatedwith progressive deformation of sedimentaryrocks: oblateminerals will be more sensitiveto planarthan linear beddingplane foliation, lineation along the bedding-cleavage deformation regimes. This mustbe takeninto accountwhen intersection,and finally cleavageplane foliation. consideringthe variation of anisotropydegree P of therock. A morecommon effect may be producedby thedifferent For purelyoblate grains with a mineralanisotropy degree ellipsoidalsymmetries associated with differentminerals. Pmthe maximumpossible anisotropy of a rock submittedto Consideran initially isotropicrock undergoing plane strain, pureflattening will correspondto L = 1 andP - F = Pm; so that the K• axesof individualparticles rotate toward the on the other hand, triaxial deformation cannot achieve a stretchingdirection and K3 axes rotate toward the shortening maximumP valueequal to Pm.To obtainthe upperlimit of direction. If these axesrotate at the samerate, then the AMS L and F in the triaxial case, let us assumethat the rock symmetryof the rockis emirelydetermined by thoseof the containsN platyparticles, whose mean susceptibility is (2Pm sourceparticles: magnetically oblate particles such as phyl- + 1)/3 and with their plane perfectlyaligned along the 220 ß Rochetteet al.: ROCKMAGNETISM 30, 3 / REVIEWSOF GEOPHYSICS

and thereforethe maximum anisotropyin a purely linear fabric (F = 1) will be P = L = 2; goingfrom a planarto a linear fabric, a decrease of P, unrelated to a decrease in strain,will be observedin a stronglydeformed rock. A study in strongly triaxially deformedschists with AMS due to phyllosilicates(Pro • 1.35) or pyrrhotiteillustrates this re- lationshipbetween the maximumpossible L and F (Figure 9 after Rochette [ 1988c]). Available L and F fields have been delimitedusing (5) for Pmequal to 1.35 or largerthan 100. Site mean values of L and F with their standard deviations are below the curve for Pm= 1.2 in purely paramagnetic sites,while they are in betweenthe two curvesfor the pyr- rhotite-bearingsites. Such considerationsalso accountfor the observationquite often made[e.g., Hrouda, 1982]that P valuestend to be smallerfor prolatethan for oblatefabrics. Relatingthe evolutionof the symmetryof magneticfabric to the symmetryof petrofabric,or deformation,is therefore precludedwhen susceptibilityhas multipleorigins or when comparingmagnetic fabrics due to minerals of different symmetry. Ii

4. INTERACTION BETWEEN REMANENCE AND AMS

The third assumptionwhose exceptions we wantto discuss is the independenceof AMS on remanence.It has been recognizedsince the beginningof anisotropystudies that the 1 1.2 1.4 anisotropyof a rock influencesits remanenceby deviating the remanencevector from the applied field towardthe di- Figure9. Sitemean values of L andF withtheir standard deviation rection or plane of maximum susceptibility[e.g., Fuller, in schistsfrom the SwissAlps [afterRochette, 1988c]. Pyrrotite- 1963]. Usingthe anisotropyof remanence,it is possibleto bearingsites appear with squaresand sites with paramagnetic sus- quantitativelycorrect this deviationin the particularlyim- ceptibilitywith circles.Limit curvesare drawnaccording to (5) portantcase of NRM; it was exemplifiedin a graniteusing for mineral anisotropydegree of biotite (1.35) and pyrrhotite (> 100). thermoremanentanisotropy by Cognd [1988] and in sedi- mentsusing AAR by Collombatet al. [1990]. On the other hand,the influenceof remanenceon AMS, noticedin early stretchingdirection, and possible misalignment around this studies[e.g., Bathal and Stacey,1969], hasbeen somewhat axis. The maximum susceptibilityK• along the stretching neglected. directionis simply The first way in whichthe remanencecan influence AMS measurementsis throughinstrumental bias. For example,in K, = NPm (3) the caseof a spinneror torquemeasurement the AMS meas- ured correspondsto the 20 componentof the signal, while On the otherhand, the meansusceptibility of therock should remanenceis measuredas the 0 component.However, strong be and inhomogeneousremanence also resultsin 20, 40, etc. harmoniccomponents that can bias the AMS signal. Km = (K, + K2 + K3)/3 = N(2P m -I- 1)/3 (4) Even usingmethods unaffected by this problem,such as an alternatingbridge, it wasnoticed that the AMS of a sample Dividing (4) by (3) we obtain maybe changedwhen submitting it to a directfield [Schmidt and Fuller, 1970;Zapletal, 1985] or a staticalternating field 1 + 1/L + 1/(LF)--(2Pm + 1)/Pm, [Violat and Daly, 1971]. In the first case a hematitemono- crystalwas found to havea minimumsusceptibility parallel which can be transformedinto to the directionof saturatedisothermal remanent magneti- zationor IRM (as in the magnetiteSD casewhere suscep- L = Pm(F q- 1)/F(P m q- 1) (5) tibility is zero along the easy axis). In the secondstudy concerningMD magnetite-bearingbasalts the directionof This equationgives us the saturationvalues of L and F appliedfield becamea maximumaxis. This can be inter- for a given mineral anisotropyPm. For example,in a pyr- pretedby consideringthat the alternatingfield has introduced rhotite-bearingrock we havePm • 100[e.g., Hrouda, 1982], a metastabledomain structurewith walls parallelto the ap- 30, 3 / REVIEWSOF GEOPHYSICS Rochetteet al.: ROCK MAGNETISM ß 221

SITE : M34 SITE : M2 SITE : M36

N N

EN EN Ill I I I ß I I I

N N N

E N E N

Figure 10. Examplesof the changeof AMS directionsin dikes from Oman (a) in natural stateand (b) after ramblingAF demagnetizationin 100 mT. pliedfield. More recently, such field-impressed magnetic fab- detectedby increasesin meansusceptibility. Therefore the ric hasbeen thoroughly investigated [Potter and Stephenson, observedchanges were not due to the disappearanceof an 1990a, b]; these authorshave discussedquantitatively the NRM-dependentAMS but to the creationof a new AMS, effect of IRM, thermoremanence,static, and tumbling al- wherethe fabric of the newly formedferromagnetic grains ternatingfield on AMS, dependingon grainsize. They also "mimics"the fabricof the primaryweaker magnetic grains. showhow IRM acquisitionis affectedby previousalternating For example,Hirt and Gehring[ 1991]found that an inverse fieldhistory, pointing out the difficultyof measuringIRM fabric in a siderite-bearingore was transformedto normal anisotropy.One can conclude that anhysteretic or isothermal fabric after thermaltreatment due to creationof magnetite remanencesstrongly interact with AMS in somecases where with a mimetic normal fabric. Therefore "thermal enhance- AMS is cardedby ferromagneticgrains; therefore one should mentof magneticfabric" [Perarnauand Tarling, 1985], al- not use artificiallymagnetized samples for magneticfabric thoughit may help in gettingwell-defined AMS directions studies(see, for example,Kodama and Sun [ 1990]).Further, from an initially hardly measurableformation, should be thesestudies suggest that the naturalmagnetic state of the handledwith carewhen interpreting the newlycreated fabric rock, visiblethrough NRM andwhich correspond to a par- in termsof geologicallysignificant phenomena. ticular metastable domain structure, could also have some Recently,Park et al. [ 1988]reported that AMS of basaltic noticeable effects on AMS. dikes of the Canadian shield were scattered in the natural The solutioncould be to demagnetizethe NRM in an stateand becamewell groupedwith stmcturallysignificant isotropicway, i.e., eitherwith tumblingAF or thermalde- directionsafter heating above the Curie pointof thosemag- magnetization.The effectof thermaldemagnetization on netite-bearingrocks. Other changes of AMS directionswere AMS hasprovoked numerous papers because it wasfound observedduring AF demagnetization.They interpreted these that the AMS was better defined after thermal treatment resultsby assumingthat the naturalAMS is constitutedby [Urrutia-Fucugauchi,1981; Perarnau and Tarling, 1985; a primarycomponent linked to petrofabricand a piezomag- Schultz-Krutischand Heller, 1985; Jelenskaand Ka&ialko- neticcomponent (particular metastable domain structure due Hofrnokl,1990]. However, these studies concerned sedimen- to tectonicstresses) which was erasedby demagnetization. taryrocks where heating produces new magnetic phases, as Anotherway to considerthe resultsis to saythat therecould 222 ß Rochetteet al.: ROCKMAGNETISM 30, 3 / REVIEWSOF GEOPHYSICS

be an interaction between the natural state of the domains ically align the easyaxes of the grains,thus resulting in an in the ferromagneticgrains and AMS. To checkthat such anisotropycontrolled by the directionof theacquisition field. spuriousAMS was the reasonfor someof the abnormalor In the caseof deposition(DRM) or postdeposition(pDRM) dispersedfabrics in our studyof Oman dikes [Rochetteet remanence of sediments the remanence is indeed the result al., 1991] tumbling alternatingfield and then thermalde- of sucha physicalalignment. Thus if magnetictorques are magnetizationswere performedon eight selectedsites with not negligiblecompared to hydrodynamicand mechanical low anisotropyratio (P < 1.01). In threenormal sites with forcesacting on AMS carryinggrains, one shouldexpect a well-groupedAMS directionsin the naturalcase, demag- K• directiontending to the acquisitionfield direction.It has netizationschange neither the meandirection nor the scatter. been observedin experimentalDRM [Reesand Woodall, On the other hand, siteswith initially dispersedAMS di- 1975; LOvlie and Torsvik, 1984a] and possiblyin experi- rectionsexhibit good grouping after alternating field demag- mentalpDRM [Ellwood, 1984b;LOvlie and Torsvik,1984b] netization(examples of Figure 10) andthen no changeafter thatwhile K3 axeskeep confined to the vertical,K• axestend thermaltreatment. In siteM34, K3directions become clearly to the fielddirection. This effect,which may hamper the use normalto the dike planeand K• directionsclose to the meas- of magneticlineations in sedimentaryrocks for paleocurrent ured subverticalmineral lineation, as expectedfor the flow estimation,has not been clearly evidencedin natural ex- fabric.However, the demagnetizationsensitive part of AMS amples.This is probablybecause the AMS carryinggrains is notresponsible for reverseor intermediatemagnetic fabric are usuallycoarser than the NRM carryinggrains, thus hav- but only for a scatterof directionsin the initial state(e.g., ing experiencednegligible magnetic torques compared to the siteM2 andM36). Thermoremanencewas imparted to sam- otherprocesses leading to preferredorientation. plesof threesites but no effecton AMS wasobserved, while Recentexperiments on chemicalremanence (CRM) ac- impartingan ARM resultedin large changesof AMS di- quisitioncoupled with subsequentmeasurements of the an- rections and ratios. isotropyof isothermalremanence (AIR) havebeen performed Tumblingin 100-mTpeak AF appearedquite efficient for on syntheticfine-grained hematite [Stokking and Tauxe, 1990] removingthe scatterdue to presumablyNRM-dependent andmagnetite [Pick and Tauxe,1991]. They found that max- AMS. Therefore thermal treatment, which has the defectsof imum axesappear parallel to the field in which the grains being time consumingand often creatingnew magnetic havebeen grown, suggesting that anisotropy(such as AMS, phases,can be avoidedin rocks with soft ferromagnetic AAR, or AIR) directionsmay be controlledby the NRM grains.There is probablyno needfor a generalizationof such directionin rocks with a CRM. Two naturalpossible ex- demagnetizationas a routine before AMS measurements. amplesof this effect may be presented. However,some tests are desirablein eachcase where very 1. In theOnodonga Appalachian limestone, 'whose AAR weak anisotropyis found. is carriedby fine-grainedmagnetite grown during a remag- The field-impressedanisotropy discussed above corre- netizationevent [Jackson eta!., 1989b],K• directionsappear spondsto the alignmentof domainwalls or spontaneous parallelto theremagnetization field direction. Unfortunately, magnetizationwithin the grains.However, another possible it is alsoclose to the shorteningdirection, and the interpre- mechanism for an effect of NRM on AMS has been left tationremains ambiguous. apart:the possibilitythat NRM acquisitionprocesses phys- 2. Chemicalsediments from the Omanophiolite revealed

N

W w

S s

Figure11. Stereoplotsof AMS maximumand minimum directions in stratigraphiccoordinates in two sitesof hematite-bearingumbers from Omanafter Thomas [ 1991]compared with the declinationof characteristicremanence, shown as a star. 30, 3 / REVIEWSOF GEOPHYSICS Rochetteet al.: ROCKMAGNETISM ß 223 primaryNRM carriedby very fine grainedhematite with AMS in rocksusing appropriate rock magnetictechniques shallowinclination [Thomas et al., 1988];subsequent AMS in orderto makereliable geological interpretations. measurements[Thomas, 1991] on unheatedsamples show a cleargrouping of K1perpendicular to thecharacteristic NRM APPENDIX: TOWARD A MINERALOGICAL DATA BASE direction(Figure 11). This agreeswith the observationof FOR AMS STUDIES the maximumsusceptibility axis perpendicular to therema- nencedirection within the easyplane of hematite[Schmidt and Fuller, 1970]. Numerouspapers and reviews on AMS have included tableswith a list of characteristicparameters of minerals, 5. CONCLUSIONS including mean susceptibility(Kin), anisotropyratio (P), symmetry,and orientation of the susceptibilityellipsoid with 1. Inverse magneticfabrics can be found in samples respectto thecrystallographic axes. Another attempt to sum- whosesusceptibility is carriedby iron-bearingcarbonates, marizethese features is presentedin Table1, with the addition tourmaline,cordierite, goethite, and SD magnetite.Various of fabric type encounteredin rocksbearing these minerals. stagesof intermediatemagnetic fabric or interchangedprin- Althoughthese compilations are very useful for the inter- cipalaxes can be foundwhen a mixtureof normaland inverse pretationof AMS in rocks, many problemsarise in their mineralsis present.Therefore it is stronglyrecommended to completionand the valuesgiven may be usedwith variable investigatethe mineralogicalsource of AMS andto compare confidence.A recentpaper by Zapletal[ 1990]on biotitewell AMS with other types of anisotropy(mainly AAR) when exemplifiesthe various problemsencountered in defining findingAMS directionsthat cannotbe correlatedwith ob- suchcharacteristic parameters. vious structuralmarkers. Rocks with fine-grainedmagnetite The first problemis that somefeatures are intrinsic;i.e., suchas limestonesare particularly prone to anomalousAMS they are constantfor a given mineral, while otherscan vary fabric, whichis difficultto interpretprobably due to a wide with chemicalcomposition, crystallographical defects, size, rangeof grain size. shape,and interactionof the grains. For diamagneticmin- 2. Manyrocks owe their magnetic susceptibility to more erals, all featuresare intrinsic. For paramagneticminerals, thanone mineral. In suchcases it is not possibleto use the only the symmetryis intrinsicand can be deducedfrom the AMS anisotropyratio for structuralinterpretation as their crystallographicsystem. For example,.a cubic mineral is variationis likely to be mainly due to compositionalvaria- isotropic;hexagonal, rhombohedric, and orthorhombicsys- tions. A P versusK,n plot is the main techniqueto detect temscorrespond to uniaxial (i.e., of revolutionsymmetry), sucha problem.The Henry's method,designed to delineate ellipsoidswith the revolutionsusceptibility axis strictlypar- a two-componentAMS, appearsin someexamples to pro- allel to the revolutioncrystallographic axis; other systems duce resultsthat have no physicalsignificance. The only leadto a triaxial ellipsoid,where susceptibility axes are not secureway to get quantitativeresults from a rock with multi- necessarilyparallel to crystallographicaxes. componentlow-field susceptibility appears to be a morespe- Kmvalues of paramagneticminerals can be estimatedfrom cific anisotropytechnique such as high-fieldor remanence chemicalanalysis using a simpleformula as a functionof anisotropy. Fe2+, Fe3+ , Mn 2+ amountsand mineral density[Rochette, 3. Stmcturallysignificant AMS can be in somecases 1987]. This parameterwill thereforeshow a rangeof values biasedby remanence.One shouldstrictly avoid measuring for natural mineralswhich showa rangeof composition.P samplesused for ARM or IRM acquisitionor sampleswhich valuescan also vary with composition,although in the ex- have been submittedto static alternatingfield demagneti- ampleof biotiteit appearsquite constant.In ferromagnefic zation. When AMS directionsare dispersedin the natural mineralssuch as pyrrhofiteand hematite,only a part of the state, this scattermay be due to interactionof AMS with symmetryis intrinsic;K3 is parallelto the c axis, while the NRM. In that case, tumbling alternatingfield demagneti- anisotropywithin thebasal plane is alsocontrolled by various zation or thermaltreatment may help in recoveringthe in- factorsincluding shape, defects, and domainstate. trinsic magneticfabric. This effect is probablysignificant The secondproblem is experimental.In principle,the only only for large NRM and very low anisotropysuch as those properreference data for mineralsshould come from well- foundin basalticrocks. However,in sedimentaryrocks with characterized,untwinned, and undeformedsingle crystals depositionalor chemicalremanence the paleomagneticfield where good evidenceis foundfor a lack of impurities,for may physicallyalign the easy axes of the grains,mainly example,ferromagnefic inpufities often found in paramag- resultingin a magneticlineation parallel to the geomagnetic neficminerals. Such evidence, mainly from high-fieldmeas- declinationand independent of currentor tectoniclineations. urements,is here only availablefor quartz, phyllosilicates, All thesecomplications with respectto the usualway of orthoferrosilite,carbonates, and tourmaline.It may be pos- interpretingAMS datawill probablybecome more and more sible that the contradictorydata basefor chain silicates(see importantto take care of as a resultof the progressof the section2.3 and Table 1) may be linked to ferromagnetic AMS techniquetoward higher sensitivity,quantitative use, impurities. new rock types, and domainsof application.This again Unfortunately,due to lack of large crystals,poor instru- underlinesthe importanceof understandingthe sourcesof mentalprecision, or fine grainsize, oneis forcedto usedata 224 ß Rochetteet al.: ROCK MAGNETISM 30, 3 / REVIEWSOF GEOPHYSICS

from orientedaggregates or to havepoorly defined single- of magneticsusceptibility in a slate, Earth Planet. Sci. Lett., crystalvalues. The resultsmust then be constrainedusing 76, 336-340, 1985. Borradaile,G.J., W. Keeler,C. Alford, and P. Sarvas,Anisotropy theoreticalconsiderations. For example,pseudohexagonal of magneticsuscept. ibility of somemetamorphic minerals, Phys. phyllosilicatessuch as biotite, chlorite, and muscovite should Earth Planet. Inter., 172, 215-222, 1987. have a uniaxial symmetry,as found in carefullyselected Coey,J. M.D., Magneticproperties of iron in soil iron oxydes crystalsby Zapletal [1990]. Thereforethe triaxial results and clay minerals, in Iron in Soils and Clay Minerals, edited (i.e., L =/: 1) obtainedby Borradaileet al. [1987]are prob- by J. W. Stucki, B. A. Goodmanand U. Schwertmann,893 pp., D. Reidel, Hingham, Mass., 1988. ably due to impuritiesor imperfectorientation in their ag- Cogn6, J. P., TRM deviationin anisotropicassemblage of MD gregates.Another problem arises when one wants to extrap- magnetite,Geophys. J. R. Astron. Soc., 91, 1013-1023, 1987. olateto fine grain size the referencevalues for hematiteor Cogn6, J.P., Strain-inducedAMS in the granite of Flamanville pyrrhotitewhich are derivedfrom largemultidomain single and its effect upon TRM acquisition,Geophys. J. R. Astron. crystals.The verylarge P valueusually quoted for hematite Soc., 92, 445-453, 1988. Collombat, H., P. Rochette, and M. Jackson, Possiblecorrection canbe indirectlyquestioned considering the following facts. of theinclination error in deepsea sediments using the anisotropy Hematiteshows an intrinsicisotropic susceptibility of anti- of anhystereticremanence (abstract), Eos Trans.AGU, 71, 1288, ferromagneticorigin equalto 1.1 x 10-3 SI and a variable 1990. ferromagneticsusceptibility confined to thebasal plane [Ndel Constable,C., and L. Tauxe, The bootstrapfor magneticsuscep- and Pauthenet,1952]. In fine disorientedgrains the second tibility tensors,J. Geophys.Res., 95, 8383-8395, 1990. Coward,M.P., andJ. S. Whalley,Texture and fabric studies across one is the order of 10-3 SI [Dunlop, 1971], which corre- the KishornNappe, Western Scotland, J. Struct.Geol., 6, 33- spondsto a P valueof the orderof 2.5. This loweringof P 38, 1979. valuein fine grainsis probablyless critical in pyrrhotite. Daly, L., Etude des propri6t6smagn6tiques des rochesm6tamor- phiquesou simplementtectonis6es, thesis, 340 pp., Univ. of Paris, Paris, 1970. ACKNOWLEDGMENTS. For this review we are deeplyin- Damm, V., GesteinsmagnetischeAnisotropien in Magmatitenund debtedto numerouscolleagues who have provided comments, ideas, derenstrukturgeologische Bedeutung, Z. Geol. Wiss., 16, 739- 752, 1988. samples(in particularthe monocrystals) and data, in sucha number Day, R., M. Fuller, and V. A. 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