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Rock Magnetism

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In earlier chapters we discussed the theory of the magnetic method ofgeophysical exploration. By applying interpretation procedures such as thosedescribed in Chap. 11 to data obtained from magnetic surveys we draw detailedconclusions about the magneti!ation of rocks within the ground and assume ihatthis property is ostensibly related to the lithologv of the subsurface."nfortunately the connection between a rock#s geological properties and itsmagneti!ation is rarely simple and it is at this stage of our knowledgeimpossible to lay down any strict rules upon which to pattern a relationship. $venso it is worth investigating the very considerable fund of information available onthis sub%ect so that any geological inferences to be drawn from magneticinterpretations may at least be well&informed. In the past fifteen years a

considerable effort has been directed toward research in rock magnetism '1( ')(mostly with a view to helping decide upon such large&scale geophysical andgeological *uestions as whether or not continental drift and polar wandering havetaken place. It is the purpose of i his chapter to select from this research thepoints of greatest significance +o magnetic interpretations.12-1Ferromagnetism

If a rock has an appreciable magneti!ation it is because it is ferromagnetic. ,llmaterials are diamagnetic i.e. when placed in a magnetic field they will ac*uirea small moment which is in a direction opposing the field. -he cause of thiseffect is the armor precession of electron orbits '1( but the phenomenon is anexceedingly weak one and is totally negligible in geological surroundings. /erymany materials are also paramagnetic i.e. when placed in a magnetic field theyac*uire a magneti!ation which is proportional to and in the same direction asthe external field. -his is usually much stronger than the diamagnetic effect but

even so it is of such small magnitude that it may virtually always be disregarded.0aramagnetism has its origin in the intrinsic magnetic moments of

individual ions. ormally these have a random orientation within the materialbecause of thermal agitation of the lattice structure. If however the bondingbetween the atoms of a crystal is of such a nature that it affects the orientationof the orbital structures an alignment of the magnetic moments may take place.It will be opposed by the thermal agitation of course but below a certaintemperature called the Curie point a natural alignment and therefore aspontaneous macroscopic magneti!ation will take place. -he phenomenon is notuncommon in crystalline materials but it is most easily and was earliestrecogni!ed in iron. It is thus known as ferromagnetism.

,lthough the underlying principle is simple enough to understandferromagnetic phenomena can be extraordinarily complicated in detail. -o start

with we must distinguish three ways in which alignment may take place. If themagnetic moments of all the ions in the lattice tend to point in the samedirection the material is said to be truly ferromagnetic. If however some of themoments tend to lie in a direction *uite different from that of the others 'usuallyin exactly the opposite direction( the material is said to be ferrimagnetic.In thespecial case in which the ions of the lattice are divided into two exactlye*uivalent groups or sublattices which are magneti!ed oppositely the material

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is said to be antiferromagnetic.,ccording

2ig. 1)&1 -he spontaneousmagneti!ation of a ferromagnetic crystal lattice below its Curie point.

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to simple theory then the macroscopic moment of an antiferromagnetic materialmust be !ero.

$xamples of each of these three types of ferromagnetism can be foundamong materials familiar in geology. Metallic iron nickel and cobalt are trueferromagnetic substances. Magnetite '2e345( is the classic example of aferrimagnetic material. 4ther ferrimagnetic substances that have a commonoccurrence in geological environments are pyrrhotite '2e678( ulvospinel'2e)-i95( and a form of hematite known as maghemite '2e)93( 4f theantiferromagnetic materials hematite '2e)43( is probably the best known butanother example from among the common minerals is ilmenite '2e-i4%(.

7ince in a ferromagnetic material the magnetic moments of all atoms arecoupled together we might expect crystals of these substances to exhibit largespontaneous magnetic moments at all times. ,s is so often the ease in physicshowever this is not what is observed. 4nly when placed in a moderately strongmagnetic field does a ferromagnetic crystal exhibit its maximum moment. -hereason for this is that the crystal subdivides itself into numerous regions knownas domains,whose magnetic moments are oppositely or at least differentlydirected. -his allows alignment of the electron&spin vectors of neighboring atoms'known as exchange coupling( to take place except in the boundaries betweendomains. ,t the same time it prevents the formation of strong magnetic fields inand around the crystal which would lead to large amounts of storedmagnetostatic energy. -he crystal thus compromises in fixing its internalstructure between a minimal exchange coupling energy 'uniform alignment

everywhere( and a minimal magnetostatic energy 'random alignment(. In thisway the total free energy of the crystal is held to a minimum value.

-he domains which form in a ferromagnetic crystal are rathervariable in shape and si!e. 7ome schematic examples are shownin 2ig. 1)&) In

' C)

is#

2ig. 1)&) 7chematic diagrams showing how the division of aferromagnetic crystal into domains can reduce its magnetostatic energy.

-he series+ ':( to id )shows a successive reduction iu energy; (e)show s adomain pattern similar in efiert to 'd( hut which would he more likely to

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occur if they direction ol i+i3e& neti!ation were preferred to the i direction.

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6 Gravity and magnetic methods

large nearly perfect crystals rains larger thanabout 9.1 a are likely to contain at least two domains. -he crystallographicdirections in which the domains can be magneti!ed depend very much on thecrystal structure and on the nature of the exchange coupling. In general there areseveral directions in which the domain magneti!ation may lie without causing alarge increase in the internal energy. ,ll crystals arc& aeolotropic however and ina crystal of low symmetry a single most favored direction '&%& and ?( willgenerally exist. ,ccording to the picture %ust described a material made up of alarge number of isolated ferromagnetic grains distributed throughout an inert

matrix behaves in the following way@ If the substance is isolated from externalmagnetic fields the domains in each crystalline grain will arrange themselves insuch a way that their internal field will vanish to a high degree. -hus the netmoment of the material will be almost !ero. If a magnetic field is then applied anet magneti!ation will appear. -hose domains whose magneti!ations lie in thedirection of the field will enlarge themselves and the others will diminish by amovement of the domain boundary surfaces. -he net magneti!ation whichappears in each grain will then be %ust sufficient to produce an internal field e*ualand opposite to the external field since this maintains the free energy at aconstant minimal amount.

-o gain a rough idea of what the netor bulA moment induced in the materialmight be we can think of the grains as spheroids and use the results discussed in7ec. 11&. If each spheroid is uniformly magneti!ed in its axial direction with a

magneti!ation M, a uniform magnetic field of intensity ? NMawill be createdinside it. -he demagneti!ing factor Xdepends on the ellipticity of the spheroidand varies from 5tt for a thin circular disk through 5tt3 for a sphere to !ero for avery long needle. -hus if the external field II,is to be canceled in the interior of agrain the magneti!ation will be Ho N.-he net magneti!ation of the material canthen be found if the volume fraction of magnetic material is known and if a meanvalue can be assumed for ,+ 'averaged on a volume basis(. If these parametersarep and respectively the net magneti!ation Mof the bulk material will be

3 D vHo '1)&1(N

and thus the material will exhibit a bulk susceptibility k,which is given by

kD 5. '1)&)( ,

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:Ui'i Gravity and magnetic methods

Imenite'2e-i93( iias a rhonibohedral structure. It is antiferromagnetic with aYel temperature1 of about 199 to 19CC although it usually shows a smallunbalance in the theoretically perfect cancellation which gives it a saturationmagneti!ation of about 9.) emu.

-here are two forms of 2e)9.%@ *ematite 'E2e)4s( which has a rhornbo&hedral structure and is fundamentally ant Zferromagnetic and mag*emite't2c%4s( which has an inverse spinel structure like magnetite but has one&ninth ofthe iron sites vacant. =ematite like ilmenite has a slight ferrimagneti!ation ofabout 9. emu and a eel temperature of X6LC. Maghemite is ferrimagnetic andhas a saturation magneti!ation 'per gram( of 83 emu and an unmeasured Curietemperature. -his is because pure maghemite is unstable at temperaturesexceeding about 399LC when it converts spontaneously to hematite. It appearshowever that impurities can stabili!e it at much higher temperatures and its

Curie point is then well above 99LC.-he other minerals of this group are comparatively rare and less is known of

their properties.1 -he temperature above which thcrmai agitation prevents ordering in an sntiferro&

magnetic substance is usually known as the /iel temperature rather than the Curiotemperature.

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:Ui'i Gravity and magnetic methods

Much less is known geochemically about the iron sulfur group. -hecompositional se*uence is usually written +ei-. [hen 0, we have themineral p1rite, which has a cubic structure ami is paramagnetic 9corresponds to rhe mineral trodite,which is antiferromagnetic and has a Curietemperature of about i399LC -roilite has a hexagonal structure. -he range 9 \ 1.9 includes the p1rr*otites, natural pyrrhotites occurring mostly from themiddle to the lower end of this range. 0ure pyrrhotite is often stated to have thecomposition 2er7E but also is sometimes identified with 2e7. Between x D 9.1and 9.N5 the mineral is ferrimagnetic having a saturation magneti!ation of aboutX9 emu 'per gram( and a Curie temperature of 399 to 3)LC. -he range of solidsolubility seems to be complete between ?9.1X \ 1.9 but the mineralogyhas been very little investigated. -he properties of pyrrhotites in sulfide ores arecompletely unstudied.

12-4 The Gencheinistry of Magnetic Minerals

In order to discuss the origin of the magnetic minerals in rocks we may considerthe three fundamental geological processes@ magmatic or volcanic crystalli!ationsedimentation and metamorphism. [e begin first with the crystalli!ationprocess since this involves no reference to either original minerals or structure.

-he ultimate form of any igneous crystalline rock depends on three factors@the original composition of the melt the rate of cooling during crystalli!ation and

the occurrence of macroscopic changes in melt composition due to the additionor subtraction of material such as the loss of volatile li*uids that sometimesoccurs while the melt is relatively permeable. 2or a melt having a given chemicalconstitution there wall be a certain mineral compositional structure that is ine*uilibrium with the residual li*uid at each temperature. -his structure isdetermined by the thermodynamic properties of the minerals themselves andchanges continuously with the decreasing temperature. =owever compositionalchanges in the already solidified mineral components take place extremelyslowly and e*uilibrium is approximated only in the li*uid component and onmineral surfaces exposed to it. >rowing ciErstals therefore become !oned andpreviously solidified crystals may dissolve until complete solidification iseventually attained. Meanwhile some of the crystals which had formed at highertemperatures become unstable at lower temperatures and may endeavor topurify themselves by slowly exsolving certain substances. Complex inter& growthsof allied minerals may therefore develop and small grains of secondary mineralsmay be precipitated within the larger host crystals. .Magnetic minerals may befound in either form or as original unexsolved primary crystals. >enerallyspeaking the faster the rate of cooling the fewer ] he number of secondarycrystals relative to primary . since the pri&

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:Ui'i Gravity and magnetic methods

Rock typeumber ofsamples

0ercent showing only aspinel or an orthorhombic

2e -i oxide lattice

0ercent sho#singexsoluiion intergrown.hbetween 2e -i oxides

Bisalt 16 X )3

^olerite 3X )) 68>abbro )9 )X 65

mary minerals have had no time in which to exsolve their unstable constituents-able 1)&1 illustrates this.

,s we have previously stated the bulk of the magnetic material in igneousrocks is in the form of iron&titanium&oxygen compounds or of iron& sulfurcompounds. It is noteworthy that oxides and sulfides of other elementsalthough often of economic importance are rather rare. -he chemicalexplanation for the tendency of these two mineral groups to form from complexsilicate melts does not seem to be known at present and information relatingthe composition of the oxide or of the sulfide mineral fractionates to thepetrology of the whole rock is very sparse indeed. -he few published analysessuggest that the composition of the oxide minerals tends to lie in a fairlyrestricted area on the 2e&-i&4 diagram the oxides in mafic rocks containingrather more titanium than those in silicic rocks. 2igure 1)& shows the regions.

0erhaps the only other generali!ation that can be made is that mafic rocks tendto carr_& larger *uantities of oxides and sulfides than silicic rocks.

Metamorphism can cause considerable change in the oxide constituents ofan igneous rock. Magnetite may be oxidi!ed toward hematite or to maghemitethe latter process occurring at lower temperatures than the former. 4ften theoxidation is incomplete and only parts of the magnetite crystals 'along surfacessuch as cracks( are affected. Balsley and Budding& ton '8( have studied thecomposition of the oxides of some ,ppalachian metamorphic rocks which fallinto *uite a different region of 2ig. 1)& than most igneous rocks. It is howeverpossible that many of these are highly metamorphosed sediments in whichhematite has been reduced toward magnetite by regional metamorphism ratherthan altered igneous rocks in which magnetite has been oxidi!ed towardhematite by hydrothermal solutions. 7tress often accompanies the higher

temperatures of metamorphism promoting the regrowth of many crystals. Curietemperatures may even be exceeded in very high&grade metamorphism.-he magnetic minerals of sedimentary rocks are largely the same as those ofigneous rocks. -he iron oxides occur either as detrital particles or

-abie 1)&1 Influence of Cooling Rate on $xsolution 7tructures+

+ ,fter >. ^. icholls '5;. ^ata from [. =. ew ho use. 4pa*ue 4xides and 7ulphides in CommonIgneous 2locks 2eo. oc. America 34.,vol. 56 pp. 1?6 1N3X.

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5ock magnetic mMJS

else they are created from iron which has been precipitated into the sedimentmainly as ferric hydroxide. Magnetite ilmenite and hematite are resistant to low&temperature weathering and are likely to occur in rather lar!e particles in detritalsediments such as sandstones. -hey may also occur ia glacial clays as minutegrains with dimensions of the order of a. micron or less. 2erric hydroxide tends toprecipitate in both marine sandstones and clay sediments which have formed nearshorelines since it is insoluble in sea water. It is rarely found in limestones or indeep&water sediments. 2erric hydroxide ages to goethite or 'much more rarely( tolepidocrocite which may in the process of lithification dehydrate to hematite ormag& hernite respectively. -he grain si!es within these materials are extremelyfine being of the order of 9.1 Mor less. Metamorphism tends to promote thegrowth of these crystals and if the environment is a reducing one it will change

the bulk composition of the oxides toward the 2e4 side of the composition scale.Iron sulfide is also created during the lithification of sediments particularly

the argillaceous muds. "sually pyrite is formed in the process but occasionallyother sulfides are found. Metamorphism may mobili!e this material and redepositit elsewhere in the ad%acent rocks.

12-5 The Magnetisation of Rocks

-he ferromagnetic minerals in a rock can become magneti!ed in a wide variety ofways. -here is of course the induced magneti!ation that is determined by theearth#s field and by the present state of the magnetic grains but this is very oftenaccompanied by a permanent magneti!ation which has been ac*uiredcontinuously or intermittently during the rock

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5ock magnetic mMJS

>ranites anda ie 65 X9 )3 19 1

>neissesschists.3lates 5 61 )) 6 9

7edimentaryrocks

57 63 1N 5 5

direction parallel to it. /ariations of several percent are not uncommon andextreme cases arise in finely banded magnetite or pyrrhotite. -here thedemagneti!ation factor is for the most part 5a& in the direction transverse to thestructure and !ero in a direction parallel to it. -hus the transverse susceptibilitywill be %ustp 78,while the parallel susceptibility will be limited only by the defectstructures in the ferromagnetic grains and by the appropriate demagneti!ingfactor of the outer envelope of the minerali!ed region.

7everal attempts have been made to formulate statistical lawsabbro 36 9. 19>ranite 31 9 194ther acid intrusivas 16 9. 19

$ly g-eensr.&n\3 1 9. 197lates )X 9. 19

+ 2rom . B. 7liehter 'X(.

-able 1)&3Rock type o. of samples Mean k, emu

2ig. G.)&X ^ata from which the empirical formula for susceptibility k %).8N 19S+ f& w a s derived. Mooney and Bleifuss ($(.I

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5ock magnetic mMJS

-hey also have derived a formula relating the bulk susceptibility to the volume ofmagnetite found by crushing magnetic separation and chemical analysis for iron.It is

kD ).8N l4&E21 91 '1)&3(

where 9is tlie volume percentage of magnetite and the data an which it is basedare shown in 2ig. 1)&X.

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Balsley and Buddingtnn '8( have related the susceptibility of a suite of ,dirondackrocks to the fractional volume of all the minerals visually ident ified aH`magnetiteV which would generally include any 2e&-l oxide minerals of spinel sructure. -heir empirical formula for the hulk suscepti& ,1 4 1 5 Gravity and maaneticmeth!c"sbility is

kD ).X 19T32111 '1)&5(

where # is the volume percentage of magnetite.` -he data upon which thisformula is based are shown in 2ig. 1)&6. It is interesting to note that throughoutthe range of # for which these two formulas are valid '9.1 \ # 19 they givevalues which do not differ greatly from those predicted by $*. '1)&)(. [hen # 1 percent for instance $*. '1)&5.( gives the same value for kas $*. '1)&).( when

the average demagneti!ing factor is 3.8 a figure very close to the theoreticalvalue for spherical grains. -he nonlinearity of '1)&5( can probably be explainedas being due to a diminishing grain si!e as #becomes much less than 1 percentand a general

Mo*netife conlepf vol percent2ig. 1)&6 l(a ta from which tiie empirical susceptibility formula kT ).X 19&3r111was derived. Balsle.v aul '8(.

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decrease in the effective demagneti!ing factor when the concentration Isrelatively large.

, similar formula has been determined by Bath 'N( fromanalyses ofmagnetite&bearing iron ores. It is

kD 1.1X 19T+-T1&3U '1)&(where 9is the volume percentage of magnetiteV determined by magneticseparation. -he similarity in form between this expression and '1)&5( is ratherremarkable and the difference in the proportionality constants can probably beexplained by the tendency for the separation process to overestimate theferrimagnetic mineral content and for a visual or microscopic examination tounderestimate it. Moreover '1)&( ignores any contribution to the bulksusceptibility by hematite and other less susceptible oxide minerals which arevery likely to be present.

Isothermal remanent magneti!ation 'IRM(. -his is %ust the residualmagneti!ation which is left after an external field is applied and removed from amagnetic material. ,s was explained in 7ec. 1)&1 it is of negligible intensity whenthe field is weak. -hus the earth

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?+? 119LC for #days&&&? N99LC for 199 days 'sco#e x /%U?+& & 899LC for 199 days&&&& 699LC for 199 doys&&&L? natural crystal 'scale x /s(' component aioig2ig. 1)&8 -hermoremanencc versus temperature for ilmenito&hematitecrystals from ,llard ake uebec. Carmichael '1)(.G Compare thesewith the ideal simple curve of 2ig. 1)&1.

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5ock iriasneti7tm18

a weak magnetic field it will ac*uire a relatively strong and very stableremanence. -he properties of this phenomenon are rather remarkable and werefirst described by -hellier '19(. In fields up to a few oersteds the magneti!ation isproportional to the magneti!ing field strength. Moreover partial -RMs areadditive. By partial -RM we mean a magneti!ation produced by cooling a materialfrom its Curie temperature to room temperature while the magneti!ing field hasbeen applied only over a imited temperature interval. -hus it is found that theaddition of two partial -RMs the first having been ac*uired between the Curiepoint and temperature : and the second between :and room temperature yieldsa sum that is e*ual to the total -RM produced in the specimen by cooling itdirectly from the Curie point down to room temperature in the same magneticfield. -his is one of the very few cases in ferromagnetism where different

processes add in a linear fashion and it has considerable importance in paleo&and archeo& magnetism. =owever -RMs formed within a given temperatureinterval will not be the same for all temperatures. Materials made up of multi&domain grains ac*uire most of their -RM within a few lens of degrees of the Curiepoint and change little thereafter. 4n the other hand substances containingmonodomain grains tend to ac*uire their -RM over a comparatively broadtemperature range.4ne extraordinary property of -RM is its ability co become incertain special cases an inverted magneti!ation i.e. one which is directedoppositely to the ambient magneti!ing field. -he circumstances and explanationsof this phenomenon are rather complicated and have been outlined by eel ill.'.-he essential feature i3 an interaction between tvro ferromagnetic componentswhich must be intimately associated either in a ferri& magnetie crystal lattice or incrystal intergrowths. -he substance may then ac*uire a normal -RM near theCurie point but as the temperature falls it can change into a reversedmagneti!ation. 2igure 1)&8 shows some measurements made by Carmichael '1)on a suite of ilmenite minerals which bear this out.

Chemical remaaent magneti!ation 'CRM . , magneti!ing process somewhatakin to -RM is chemical remanent magneti!ation. It has not been as carefullystudied as -RM but the intensity of the effect seems to be proportional to themagneti!ing field strength and it appears to be a rather stable form ofmagneti!ation. It takes place whenever ferromagnetic grains grow or aretransformed from one form to another a@ a temperature below their Curie point. Itis probably the most important mechanism leading to permanent magneti!ationin many sedimentary and metamorphic rocks. $ven in igneous rocks the ironoxide minerals may undergo a transition from one form to another during a slowexsolution or unmixing process and may then ac*uire a CRM.

etrital remanent magnetization '^RM(. -his process can take place

during the sedimentation of fine&grained almost colloidal particle.:. Magnetite iswell preserved during the weathering of rocks and may be among the detrituswhich slowly settles out of suspension. If the particle is a monodomain grain itwill have a rather large magnetic moment which will tend in some degree tobecome aligned with the earth#s field. Moreover since the direction of themoment in the particle is likely to be fixed by its shape the settling process rnavtend to deflect the direction of magneti!ation either toward or away from the

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5ock iriasneti7tm1!

hori!ontal. -hus even if the grains are not closely aligned with the earth

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5ock iriasneti7tm"#

during successive stages of its history. ew magneti!ations need not necessarilywipe out old ones. -hus a rock

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Ro)*k magnetismGG6&3

In studying the RM of rocks. a numerical constant known as Aoenigsberger

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Ro)*k magnetismGG6&3

-heir mean direction is also rather more nearly parallel with the present earth#sfield. In both serE however some anomalous magneti!ations have been found7everal of the underground samples exhibit reversed magneti!ations almostcertainly due to physical or chemical processes and not to a reversal of theearth

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Ro)*k magnetismGG6&3

-he most likely explanation of these results is magneti!ation of the outcropsby lightning. 7ince the four outcropping regions apparently stand out as smallhills above the surrounding terrain it does not seem

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improbable that they may have been struck several times during the past fewhundreds or thousands of years.

Books '16( has analy!ed a number of aeromagnetic anomalies in Montanaand has measured the magneti!ation of specimens of the rocks whichpresumably caused them. -wo of his examples are shown in 2igs. 1)&1) and 1)&13. Both anomalies occur over buttes which are capped by igneous rocks the firstby a mafic lava and the second by a layered laccolith of so&called syenite&

shonkinitc.-he volcanic rock samples were found to have a very strong RM thedirections of which were somewhat scattered but distinctly grouped about ana!imuth of 15NL and an inclination of 3L upward. -he mean intensity of theirmagneti!ation was N.N1 19&3emu and their mean ;ratio was of the order of .-o see if this would account for the observed aeromagnetic anomaly a theoreticalprofile was calculated by the method of =enderson and eit! '18(. -he induced

EC,MBRI,

Gh ?$flrrt-s nr.acnetvc#f[ld N$,R^: ?Meandirectiu.i

rg . 1)&19 RM anrf susceptibility of the 7udbury irrupt ivc on a traverseat =lfard. ,fter =ood '13(.G

U769 2ravit1 and tnagneiic met*oda

2ig. 1)&11 RM and susceptibility of the 7paviuaw granite. f,fter =awes

'1X(.

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magneti!ation.was neglected in the calculation because it was relatively small.-he fit with the observed profile is sufficiently close that there seems to be nodoubt that the remanence observed in the specimens is chiefly responsible forthe aeromagnetic anomaly. It would seem however that the mean intensity ofthe RM throughout the formation as a whole is somewhat larger than thatestimated from the sample measurements.

-he second example reveals a different situation. -he RM has a directionsimilar to that of the present earth#s field and a mean intensity of 5.X 19T5emu.-he induced magneti!ation has almost the same intensity .1 19&5 emu. ,theoretical profile calculated on the basis of the sum of

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$ brusiva naeM Magneticnorf

o outn&iaekrn* pofcnEtanon $art/i 3res:nf tieiat

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)6 GravityMirtei magnetic methoc"s

19N+5#

R1

2ig. 1)&1) ,eromagnetic anomaly directions of remanence andihcoretiEI anomaly $ro%ile over Bo& elder Butte. ,fter 'oo(s 'I-(.

-j.)6> %ommi%i Magneticncr&.-

Na r* h - se

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Rock magr0itiim )*%t

these uo magneti!ations fits the observed anomaly rather well although again itwould seem that the formation magneti!ation is somewhat larger than theestimate derived from the sample measurements.

,s a final example it is worth noting some of the evidence whichdemonstrates that soils are often appreciably magnetic. e Borgne '1N( hasshown that in many cases the uppermost soil layer has a much higher volumesusceptibility than all the rest often of the order of 19T5emu. -his he feels isdue to the syngenetic formation of very fine&grained mag& hemite. Cook andCarts ')9( have sampled several hundred soils in the "nited 7tates and 0anamaand they also find that moderately high soil susceptibilities are common. Inagreement with e Borgne they conclude that there is little or no relationbetween the susceptibility of a soil and that of its parent rock unless the soilcontains large *uantities of parent&rock fragments. , summary of theirsusceptibility measurements is shown in 2ig. 1)&15.

In -able 1)&5 we show Cook and Carts# tabulation of the results of ten

7enire&i:+#onkmite laccolith

1999 ).99> 3.99C t=f

2ig. 1)+13 ,eromagnetic anomaly direction+ of Q R 1. und theoreticalanomaly profile over 7*uaw Butte& %,&fter Boofcs '16(.

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Rock magr0itiim )*%t

magnetic analyses by which they have attempted to discover the origin of

,rea of origin

>reat soil group k.,em

u

& io+s

Mag

neti

Composition of the mostmagnetic fraction

4rigi

nal

Residual

C. -exas Reddishchestnut

) )9 33 .3magnetite

.X *uart!

7. -exas Rend!ina 19 16 .5 . *uart!

-exas coast 7and I7 . magnetite . *uart!-exas coast 7emi&bog ' 1 .) . 1 limonite

. *uart! .1 gypsum$. -exas ellow pod!olie 19 3 .'magnetite .5 *uart!

.9N ilmenite .91 gypsum$. -exas ellow pod!olie 1 ) )1 .6 .) *uart!C. -exas Rend!ina 39 339 19 .1 magnetite .N *uart![. -exas Reddish brown 189 3 6 .7 magnetite .1 i menite

.9 *uart! and gypsum. ew ,lex. 7and 89 699 .N .9 *uart!C. Colo. Brown 198

9)99 )59

9.6 magnetite .3 *uart!

soil susceptibilities. -he ma%ority of the large magnetic particles 'i.e. those visibleunder the microscope( were extracted from each soil specimen and identified.early all particles turned out to be magnetite. -he susceptibility of the residuumwaa then measured to see what fraction of the original susceptibility had beencontributed by the extract. In four of the ten cases it was found to have producedless than half and thus the susceptibility of these soils is probably due to ultra&fine&grained maghemite as suggested by e Borgne. In the other six cases themagnetite content was more important.

Cook and Carts made no direct measurements of the R,I of the soils theysampled but their work did indicate that it may be appreciable. e Borgne hasfound the soils containing ultra&fine&grained magnetic material to be verysusceptible to "M indicating that the particles are rnono& domain grains on theverge of superpararnagnetism. 7uch material unless disturbed regularly byoperations such as cultivation will almost certainly exhibit an appreciablemagnetic remanence.

Midwest /o. and [. /a. 0anarne

'Zon&free( 0arent matericiU 4.igh Znan content(fric+ )&15& Correlation of toil 8u:cepihilitv Ei t l parent material. r4+kW+nd Carts ')9(.

-able 1)&5

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Rock magr0itiim )*%t

5eferences

1. -. agata #Rock MagnetismV rev. ed. Maru!en -okyo 1NX1.

). $. Irving `0aleomagnetism and Its ,pplications to >eological and >eophysical0roblemsV [iley ew ork 1NX5.

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