Magnetic Properties

Magnetic Properties

Chapter 10 Magnetic Properties 10.1 FUNDAMENTALS AND UNITS Magnetic properties describe the behavior of any substance under the influence of a magne...

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Chapter 10

Magnetic Properties

10.1 FUNDAMENTALS AND UNITS Magnetic properties describe the behavior of any substance under the influence of a magnetic field. There are two main effects and phenomena: 1. Induced magnetization results when a magnetic field is applied to a material with a magnetic susceptibility. 2. Remanent magnetization exists regardless of the presence of an applied field and occurs within ferri- and ferromagnetic substances, which are characterized by a natural alignment of magnetic moments. For induced magnetization, the magnetic susceptibility characterizes the magnetic response of a material to an external magnetic field. The volume susceptibility κ is defined as the ratio of the material magnetization M per unit volume to the external magnetic field strength H: κ¼

M H

ð10:1Þ

The volume susceptibility κ is a dimensionless unit. The mass susceptibility κg, measured in units of m3 kg21, is defined as the ratio of the material magnetization per unit mass to the magnetic field strength, and therefore: κ ð10:2Þ κg ¼ ρ where ρ is the bulk density. In general, the susceptibility is a tensor of rank two. Unless otherwise mentioned, the symbol κ means a “mean, quasi-isotropic” susceptibility. For magnetic anisotropy studies, see, for example, Tarling and Hrouda (1993).

Physical Properties of Rocks. © 2011 Elsevier B.V. All rights reserved.

373

374

Physical Properties of Rocks

TABLE 10.1 Magnetic Units and Conversions Symbol

SI Unit 21

H

Am

CGS Unit

Conversions

Oe (Oersted)

1 A m21 5 4π 1023 Oe 5 1.257 1022 Oe 1 Oe 5 103/4π A m21 5 79.6 A m21

A m21

M

1 Gauss 5 103 A m21

Gauss

1 A m21 5 1023 Gauss B

Tesla

1 Gauss 5 1024 T

Gauss

1 T 5 1 V s m22 κ

dimensionless

1 T 5 104 Gauss dimensionless

κ [SI] 5 4 π  κ [cgs] κ [cgs] 5 (1/4 π )  κ [SI]

In addition to susceptibility, magnetic permeability μ is used to describe magnetic properties. Permeability relates magnetization to magnetic induction B: B ¼ μ0 ðH þ MÞ ¼ μ0 ð1 þ κÞH ¼ μ0 UμUH

ð10:3Þ

where μ0 5 4 π 1027 V s A21 m21 is the magnetic permeability for vacuum μ is the relative magnetic permeability of the material. μ¼1þκ

ð10:4Þ

In SI units, H and M are in A m21, and B in Tesla (1 T 5 1 V s m22). Table 10.1 shows the SI units with their equivalents in the older CGS system and their respective conversions. There are three main groups of materials with regard to magnetic properties. Diamagnetic materials: Diamagnetism is the general property of materials that create a magnetic field in opposition to an externally applied magnetic field in conformity with Lenz’s law (Figure 10.1). Diamagnetic materials therefore have a negative (but very low) magnetic susceptibility; for common rock-forming minerals, it is often in the region of 21025. Diamagnetic susceptibility is independent of temperature. In materials that show ferromagnetism or paramagnetism, the diamagnetism is completely overpowered. In paramagnetic substances, a magnetic field results in a magnetic moment that has the same direction as the applied field (Figure 10.1). Paramagnetic substances therefore have positive susceptibilities that extend

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375

Magnetic Properties

(A)

(B) Diamagnetic No field

Applied field

No field

Paramagnetic Applied field

H

H

FIGURE 10.1 Diamagnetic and paramagnetic material without magnetic field (A) and with magnetic field (B).

Ferromagnetic

Ferrimagnetic

Antiferromagnetic

FIGURE 10.2 Ferromagnetic, ferromagnetic, and antiferromagnetic material.

over a range between 1024 and 1022 (SI) for the common rock-forming minerals (Tarling and Hrouda, 1993). The susceptibility of paramagnetic materials is inversely proportional to absolute temperature (Curie’s law or CurieWeiss’s law). Diamagnetism and paramagnetism exist only in an applied magnetic field; the magnetization is linear in relation to the field strength. If the field is removed as a result of thermal motion, the spins become randomly oriented. Ferro-, antiferro-, and ferrimagnetic substances show a much higher positive susceptibility than paramagnetic materials and might also have a remanent magnetization. The magnetic behavior is characterized by magnetic volume elements termed “magnetic domains” (single domain, multidomain). The three groups are (Figure 10.2): 1. Ferromagnetic material with parallel orientation of neighboring intrinsic moments and a resulting macroscopic external moment. 2. Antiferromagnetic material with an equal but antiparallel orientation of the intrinsic moments and, therefore, a zero macroscopic external moment. 3. Ferrimagnetic material with antiparallel intrinsic moments of different magnitudes and, therefore, a resulting external moment. This type of magnetization and susceptibility is temperature dependent. When the temperature is higher than the Curie temperature TC for

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Physical Properties of Rocks

M in 102 A m–1 M

1.0

Ms

0.5

Mr Hc

H –4

–2

2 H in

4 105

A m–1

–1.0 FIGURE 10.3 The magnetic hysteresis curve: (A) schematic, (B) hysteresis curve of a volcanic rock, after Nagata (1961); values are converted to SI units. Ms is saturation magnetization, Mr is remanent magnetization, and Hc is coercitive field strength.

ferro-/ferrimagnetics or the Ne´el temperature TN for antiferromagnetics, the material has paramagnetic properties. Table 10.4 gives some values for TC. The magnetization depends on the field strength and the “magnetic history” and shows the phenomenon of remanent magnetization (“hysteresis loop,” Figure 10.3). In general, for ferro- and ferrimagnetic substances, the magnetization M is the sum of the induced magnetization Mi and the remanent magnetization Mr: M ¼ Mi þ Mr

ð10:5Þ

The ratio of the remanent magnetization and induced magnetization is called the “Koenigsberger Q-ratio,” a dimensionless quantity defined as: Q¼

Mr Mr ¼ Mi κUH

ð10:6Þ

where Mr is the magnitude of the (natural) remanent magnetization (per unit volume), κ is the volume susceptibility, and H is the magnitude of the Earth’s magnetic field at the site.

10.2 MAGNETIC PROPERTIES OF ROCK CONSTITUENTS 10.2.1 Magnetic Properties of Minerals Minerals can also be classified as: G G G

diamagnetic minerals; paramagnetic minerals; ferromagnetic minerals, ferrimagnetic minerals, and antiferromagnetic minerals.

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Magnetic Properties

TABLE 10.2 Mass Susceptibility κg and Volume Susceptibility κ of Some Diamagnetic Minerals Mineral

κg [1028 kg21 m3]

κ [1026]

Reference

Anhydrite

22.11

259.3

BP

20.5 to 22.0

214 to 260

H

20.48

213.0

BP

20.3 to 21.4

27.5 to 239

H

238.0

TH

20.58

212.4

BP

20.5 to 20.6

213 to 217

H

213.4 to 215.4

TH

Calcite

Dolomite Quartz

Fluorite

20.79

224.0

BP

Halite

20.48

210.4

BP

20.48 to 20.75

210 to 216

H

20.58

212.5

BP

20.49 to 20.67

213 to 217

H

21

29

H

Orthoclase

Ice

Reference key: BP, Bleil and Petersen (1985); TH, Tarling and Hrouda (1993); H, data compilation from Hunt et al. (1995).

Table 10.2 shows susceptibility values for selected diamagnetic minerals. For more detailed data, see Clark (1966), Lindsley et al. (1966), Melnikov et al. (1975), and Bleil and Petersen (1982). Table 10.3 shows susceptibility values for paramagnetic minerals. Ferro-, antiferro-, and ferrimagnetic minerals: The most important and abundant groups are iron and iron-titanium (Fe-Ti) oxides. Iron oxyhydroxides and iron sulfides are significant but not abundant (Bleil and Petersen, 1982). Fe-Ti-oxides are the dominant magnetic substance, particularly in magmatic rocks; they are components of the ternary system (Figure 10.4), implementing: G

Simple oxide minerals: FeO (wu¨stite), Fe3O4 (magnetite), γ-Fe2O3 (maghemite), α-Fe2O3 (hematite), FeTiO3 (ilmenite), Fe2TiO4 (ulvo¨spinel), Fe2TiO5 (pseudobrookite), and FeTi2O5 (ilmenorutile, ferropseudobrookite).

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Physical Properties of Rocks

TABLE 10.3 Mass Susceptibility κg and Volume Susceptibility κ of Some Paramagnetic Minerals Mineral

κg [1028 kg21 m3]

Olivine

5130, mean 29

Amphibole

κ [1026]

Reference BP

36

990

H

1130

1600

D

16100, mean 49 1669

BP 1570

D

Pyroxene

494

D

Hornblende

6100

BP

Smectite

2.75

D

Biotite

52

BP

5298

1500

H

595

15002900

D

226

BP

122, 165

TH

Muscovite 026, mean 8 Illite

Montmorillonite

15

H

15

410

D

1314

330350

H

70, 358, 370, 1550

TH

Chlorite Bentonite

5.8

D

Siderite

100

D

Dolomite

1.1

D

The range is mostly due to ferrimagnetic impurities. Reference key: BP, Bleil and Petersen (1982); TH, Tarling and Hrouda (1993); H, data compilation from Hunt et al. (1995); D, Dearing (1994).

G

Four series (solid solution series) of the system: titanomagnetite, ilmenitehematite, pseudobrookite, titanomaghemite.

This system gives “the most basic knowledge of understanding the ferrimagnetic characteristic of general rocks” (Nagata, 1966). The strongest contribution to rock magnetism comes from magnetite, titanomagnetite, and maghemite. A detailed description is given, for example, by Nagata (1961), Stacey and Banerjee (1974), and Bleil and Petersen (1982).

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Magnetic Properties

TiO2

Ti2FeO5

a

TiFe2O5

TiFeO3 b

TiFe2O4 c

d

FeO Fe3O4

Fe2O3

FIGURE 10.4 The ternary system FeOFe2O3TiO2 with typical series: a—pseudobrookite, b—ilmenitehematite, c—titanomagnetite, d—titanomaghemite. Arrows represent the directions of oxidation.

The series occur in different, preferred rock types: G

G

G

G G

Titanomagnetites “are the most common magnetic minerals in igneous rocks. Magnetite occurs in a great variety of igneous, metamorphic, and sedimentary rock types. Typically, it is formed in various types of subsolidus reactions. As a carrier of rock magnetism, magnetite is the most abundant and important oxide mineral. Magnetite occurs on the continents and in the oceanic crust in igneous, sedimentary, and metamorphic rocks. Ulvo¨spinel is a rare natural crystal present in terrestrial rocks and is almost always intergrown with magnetite. It is frequently observed in lunar samples” (Bleil and Petersen, 1982). Ilmenitehematite yields the following naturally occurring characteristic orientations: Hematite is a carrier of remanent magnetization in sediments (mainly in specular grains and the pigment). In igneous rocks, the primary composition of the series relates to the bulk chemistry of the rock. With decreasing total basicity, the content of ilmenite decreases; subsolidus reactions lead to ilmenite enrichment. This series also occurs in a wide variety of metamorphic rocks. Pseudobrookite occurs naturally in igneous and metamorphic rocks. Titanomaghemites are the main magnetic constituents in the basaltic oceanic basement, but they also occur in continental igneous rocks (Bleil and Petersen, 1982).

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Physical Properties of Rocks

TABLE 10.4 Mass Susceptibility κg and Volume Susceptibility κ of Some Ferri- and Ferromagnetic Minerals κg [1028 kg21 m3]

Mineral Magnetite

κ [1026] SI

TC in  C Reference

1,200,00019,200,000 mean 6,000,000

575585 T

20,000110,000 1,000,0005,700,000 Maghemite

40,00050,000

Haematite 10760 Ilmenite

H

2,000,0002,500,000

B600

H

50035,000

675

T

50040,000 300,0003,500,000 mean 1,800,000

H 2233

T

4680,000

2,2003,800,000

H

2,50012,000

130,000620,000

H

Titanomaghemite 57,000

2,800,000

H

Goethite

26280

1,10012,000

B120

H

Ulvo¨spinel

100

4,800

2153

H

Pyrrhotite

1030,000

4601,400,000

320

H

Titanomagnetite

Reference key: T, Telford et al. (1990); H, data compilation from Hunt et al. (1995).

Pyrrhotite (FeS11x) is ferrimagnetic and a common accessory mineral in rocks and a representative of iron sulfides. Representatives of iron oxyhydroxides are goethite α-FeOOH and lepidocrocite γ-FeOOH. Nagata (1966) analyzed samples of eruptive rocks and showed that more than 90% of the magnetically effective substance are parts of the titanomagnetite and the ilmenitehematite series. The dependence of susceptibility on magnetic field strength results in the difficulty to give “representative mean values” for ferri- and ferromagnetic minerals. Thus, the values in Table 10.4 are only for general orientation.

10.2.2 Magnetic Properties of Fluids Most fluids are diamagnetic and have only a very small influence on the magnetic rock properties. For liquids, Kobranova (1989) gives the following susceptibility values: water κwater ¼ 20:9 3 1025

oil κoil ¼ 21:04 3 1025

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Magnetic Properties

381

Potter (2007) reports a mass susceptibility for formation water (Forties Field/North sea) of κg,water 5 20.87 3 1028 kg21 m3 and for crude oil κg,oil 5 21.02 3 1028 kg21 m3. The water mineralization has a small effect because most salts are also diamagnetic. Ice has a mean susceptibility of κ 5 29 3 1026 (Hunt et al., 1995). Gas components are also diamagnetic, except oxygen, which is paramagnetic. The low value for air therefore is approximately κair 5 20.04 3 1025. For hydrocarbon gases, Kobranova (1989) gives susceptibilities of about κgas  21028.

10.3 MAGNETIC PROPERTIES OF ROCKS Fundamental publications and comprehensive reviews about rock magnetism are, for example: Hunt et al. (1995), Carmichael (1989), Mooney and Bleifuss (1953), Nagata (1961, 1966), Angenheister and Soffel (1972), Stacey and Banerjee (1974), Bleil and Petersen (1982), Petersen and Bleil (1982), and Tarling and Hrouda (1993). Literature is available on palaeomagnetism and its applications for geology, geophysics, and archaeology, for example, Tarling (1983).

10.3.1 Overview—Rock Magnetization The magnetic properties of rocks are controlled by those mineral constituents that have a magnetic effect. The fraction of these minerals with respect to the total rock volume may be small. Therefore, two consequences result (Carmichael, 1989): 1. “Magnetic properties can be quite variable within a rock type, depending on chemical inhomogenity, depositional and/or crystallization, and postformational conditions. 2. Magnetic properties are not necessarily closely predictable by the lithologic rock type (geologic name). This is because the geologic rock name and the geologic classification are generally given on the basis of the genesis and the gross mineralogy, but a minor fraction of the mineral constituents controls the magnetic properties”. The most abundant minerals in common rocks are paramagnetic or diamagnetic. The magnetic rock properties are controlled by the ferrimagnetic minerals, although their concentration “in major rock types rarely exceeds 10% vol.” (Bleil and Petersen, 1982). Minerals of the Fe-Ti-system are dominant. In sedimentary rocks, the Fe-hydroxides are also important. Figure 10.5 shows schematically the mineral contribution to the susceptibility of a rock after Tarling and Hrouda (1993). The authors state: “All mineral grains within a rock contribute to its total susceptibility, but their individual

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Physical Properties of Rocks

101

100

tite

Susceptibility, K (SI)

10–1

ne

g Ma

10–2 ite

en

-ilm

o

m ae

10–3

H

ite

en

IIm

Biotite, hornblende and pyroxene

te

ati

10–4

m ae

Muscovite

H

10–5

10–6 0.01

0.1

1 10 Concentration (wt%)

100

FIGURE 10.5 Mineral contributions to the rock susceptibility (Tarling and Hrouda, 1993).

influence depends on their intrinsic susceptibility, as well as on their concentration.”

10.3.2 Susceptibility Range for Rock Types—Induced Magnetization Susceptibility has a wide range of values for the individual rock types and more or less distinct tendencies and rules as demonstrated in Table 10.5 and Figure 10.6. Obviously, G G G

susceptibility for each rock type varies over orders of magnitude, susceptibility of magmatic rocks increases from acid to basic rocks, susceptibility of sedimentary rocks increases with increasing clay content. Gueguen and Palciauskas (1994) give the following general orientation: Sedimentary rocks κ , 1024 Granites and gneisses κ 5 10241023 Intrusive basic rocks κ . 1023.

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Magnetic Properties

TABLE 10.5 Magnetic Susceptibility of Some Selected Rock Types Volume Susceptibility κ [1026]

Reference Mass Density Susceptibility [103 kg m23] κg [1028 kg21 m3]

3882,000

1.43,100

2.61

H

550120,000

204,400

2.79

H

Average basic igneous rocks

170,000

6,500

2.61

H

Andesite

250180,000

8.46,100

2.99

H

Basalt

8,50079,100

Diabase

1,000160,000

Rock Type

Igneous rocks Average acidic igneous rocks

J 355,600

2.91

98052,780

H J

Diorite

630130,000

224,400

2.85

H

Gabbro

1,00090,000

263,000

3.03

H

5,530051,500 Granite

050,000

J 01,900

2.64

38033,900

H J

Peridotite

96,000200,000 3,0006,200

3.15

H

Porphyry

25038,000

2.74

H

9.27,700

2906,300

J

Pyroxenite

130,000

4,200

3.17

H

Rhyolite

25038,000

101,500

2.52

H

Average sedimentary rocks

050,000

02,000

2.19

H

Clay

170250

1015

1.70

H

Coal

25

1.9

1.35

H

Dolomite

(210)940

(21)41

2.30

H

Sedimentary rocks

0900

T (Continued )

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Physical Properties of Rocks

TABLE 10.5 (Continued) Rock Type

Volume Susceptibility κ [1026]

Reference Mass Density Susceptibility [103 kg m23] κg [1028 kg21 m3]

Limestone

225,000

0.11,200

03,000

0.55

Red sediments

10100

0931

2.24

H

Sandstone

020,900

3886

2.24

H

Shale

6318,600

2.10

H

2.11

H T

1018,000

T

Anhydrite

4125

K

Rock salt

up to 100

K

Gypsum

1.51,250

K

Metamorphic rocks Average 073,000 metamorphic rocks

02,600

2.76

H

Amphibole

750

25

2.96

H

Gneiss

025,000

0900

2.80

H

1,30025,100

J

Granulite

3,00030,000

1001,000

2.63

H

Phyllite

1,600

60

2.74

H

Quartzite

4,400

170

2.60

H

Schist

263,000

1110

2.64

H

3273,000

110630

Serpentine

3,10018,000

01,400

Slate

038,000

J 2.78

H

2.79

H

Reference key: H, taken from a data compilation of Hunt et al. (1995); J, Jakosky (1950) (converted from cgs); T, Telford et al. (1976); K, Kobranova (1989).

Alteration processes can greatly influence magnetic behavior. As an example, Henkel and Guzman (1977) reported martization (oxidation of magnetite to hematite) at an outcropping fracture zone with a negative magnetic anomaly. Other studies, such as of the Svaneke granites (Sweden),

Chapter | 10

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Magnetic Properties

Volume susceptibility [SI] 1.0E–06

1.0E–05

1.0E–04

1.0E–03

1.0E–02

1.0E–01

1.0E+00

Andesite Basalt Diabase Diorite Gabbro Granite Porphyry Rhyolite Sandstone Dolomite Limestone Shale Anhydrite Gneiss Granulite Serpentine

FIGURE 10.6 Volume susceptibility for some rock types, compiled after data from Hunt et al. (1993).

show that the alteration of mafic minerals (hornblende and biotite) into chlorite and magnetite results in increasing susceptibility with an increasing degree of alteration (Platou, 1968).

10.3.3 Correlations Between Susceptibility and Content of Magnetic Substances The susceptibility of rocks is strongly controlled by the magnetic mineral type and its concentration in the rock. Because “magnetite is the most common and the most magnetic mineral of the iron-titanium oxide series” (Hearst and Nelson, 1985), there is a distinct correlation between rock susceptibility and magnetite content. This can be expressed by a relationship of the general form: b κ ¼ aUVmagnetite

ð10:7Þ

where Vmagnetite is the magnetite volume fraction (mostly in %). Parameters a and b are empirical values. Normally, b ranges between 1.0 and 1.4 (Grant and West, 1965; see Hearst and Nelson, 1985). Table 10.6 shows some values for the empirical parameters.

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Physical Properties of Rocks

TABLE 10.6 Empirical Parameters a,b in Equation (10.7) Rock Type

a

b

Reference

Rocks, general with 180% magnetite

0.0140

1.39

J

Basalt, Minnesota

0.0475

1.08

M

Diabase, Minnesota

0.0336

1.14

M

Granite, Minnesota

0.0244

0.47

M

Gabbro, Minnesota

0.0155

0.36

M

All rocks, Minnesota

0.0363

1.01

M

Volume content in %, properties in SI converted. Reference key: M, Mooney and Bleifuss (1953); J, Jahren (1963).

10 Magnetic susceptibility

Iron formation 1 Diabase 0.1

0.01 0.1

1

10

100

Magnetite content % FIGURE 10.7 Correlation between magnetic susceptibility and magnetite content (in %) of rocks and ores from Minnesota (data: Mooney and Bleifuss, 1953).

For example, data from Mooney and Bleifuss (1953) in Figure 10.7 give the correlations: diabase

1:14 κ ¼ 0:0336UVmagnetite

iron formation

1:43 κ ¼ 0:0116UVmagnetite

ð10:8Þ ð10:9Þ

Parasnis (1973) commented about such relations: “Many other relations have also been suggested which make it clear that no universally valid relation between the susceptibility and the Fe3O4 content of rocks exist. Furthermore, where a relation does exist, the same susceptibility value may correspond to different Fe3O4 contents and vice versa so that a great caution

Chapter | 10

387

Magnetic Properties

1.50

κ

1.00

0.50

0.00 1.0

10.0 100.0 Grain diameter in µm

1000.0

FIGURE 10.8 Susceptibility versus grain diameter of magnetite particles; filled symbols are data after Nagata (1961); open symbols are data after Spravocnik Geofizika (1966).

must be exercised in predicting one from the other. It is therefore advisable to directly determine the susceptibilities of rocks and ores within the area of interest and not rely on formulas of the above type.” Not only the volume fraction and intrinsic susceptibility of ferro- and ferrimagnetic substances control the rock susceptibility, but also mineral grain size and shape are of influence as a result of domain interactions. Susceptibility decreases with decreasing grain size of magnetic minerals in the rock matrix (Hunt et al., 1995; Nagata, 1961). Figure 10.8 shows two examples. For disseminated ores with larger grain sizes (multidomain grain size range), susceptibility is influenced by the effect of demagnetization: κ o ¼ Vm

κ 1 þ NUκ

ð10:10Þ

where κo is the bulk susceptibility κ is the intrinsic susceptibility Vm is the volume fraction of the magnetic substance N is the demagnetization factor. The demagnetization factor is 1/3 for spheres. Carmichael (1989) has published demagnetizing factors for ellipsoids, cylinders, and rectangular prisms of various dimension ratios. The influence of the internal rock structure on the magnetic properties creates magnetic anisotropy. The susceptibility tensor can be represented by a susceptibility ellipsoid. A detailed description of the magnetic anisotropy of rocks was published by Tarling and Hrouda (1993). Siegesmund et al.

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Physical Properties of Rocks

(1993) have studied the fabric-controlled anisotropy of KTB (German Continental Deep Drilling Project) core samples.

10.3.4 Natural Remanent Magnetization The total magnetization M of any rock is the sum of two vectors: 1. induced magnetization Mi, dependent on the external field 2. remanent magnetization Mr, independent of the external field. The natural remanent magnetization (NRM) is the field independent and irreversible part of the total magnetization. There are different types and origins of this phenomenon in rocks as follows: 1. Thermoremanent magnetization (TRM): TRM is the remanence acquired by a rock (containing ferrimagnetic substance) when it is cooled from a temperature above its Curie temperature to a lower temperature in the presence of a magnetic field. Generally, most of the magmatic and hightemperature metamorphic rocks are characterized by a distinct TRM higher than the induced magnetization; thus, the Koenigsberger ratio (Equation (10.6)) is Q . 1. 2. Chemical remanent magnetization (CRM): CRM occurs during the formation of a magnetic mineral (origin and growing process), for example, as the result of a chemical reaction or phase transition below its Curie temperature under a magnetic field. CRM is therefore related to processes such as oxidation of magnetite to hematite or maghemite, oxidation of titanomagnetite to titanomaghemite, dehydration of iron hydroxide to hematite, precipitation of ferromagnesian minerals (biotite, hornblende, augite), and recrystallization of ferrimagnetic minerals below Curie temperature (Bleil and Petersen, 1982; Hunt et al.,1979). Hunt et al. (1979) remark that CRM is “due to the unusually large volumes of hematite in the form of either pigmentation or specularite . . . the most probable source of magnetization in red beds.” 3. Detrital or depositional remanent magnetization (DRM): DRM originates from the oriented deposition of previously magnetized mineral grains under the influence of the earth’s magnetic field. The magnetic moments of the particles are aligned to the field direction, so that this direction is “conserved” in the sediment. This is a process that depends on the depositional environment (low turbulence) and also on the sediment type (obvious relations are shown by clays). After deposition, minor changes are possible upon compaction (postdepositional remanent magnetization, or PDRM). The DRM can be important in marine sediments, lake sediments, and varved clays (Carmichael, 1989). Remanent magnetization is described by the magnitude of Mr (in A m21) or by the Koenigsberger ratio Q (Equation (10.6)). Q shows a wide scattering

Chapter | 10

389

Magnetic Properties

TABLE 10.7 Koenigsberger Ratio Q of Some Selected Rock Types Rock Type

Q

Reference Rock Type

Igneous rocks

Q

Reference

Metamorphic rocks

Average

140

H

Granulite

0.00350 H

Intrusions

0.120

H

Volcanics

3050

H

Sedimentary rocks

Granite

0.128

H

Average

0.0210

H

Granite

0.31

C

Marine sediments

5

H

Granodiorite

0.10.2

H

Red sediments

1.66

H

Dolerite

23.5

H

Red sediments

24

C

Diabase

0.24

H

Siltstone

0.022

H

Diabase

23.5

C

Silty shale

5

H

Gabbro

19.5

H

Limestone

0.0210

C

Oceanic gabbro 0.158.4 H Basalt

510

C

Seaflor basalt

140

C

Subaerial basalt

1116

H

Oceanic basalt

1160

H

Seamounts

857

H

Reference key: C, Carmichael (1982), H, taken from a data compilation of Hunt et al. (1995).

of values for a rock type in Table 10.7. Carmichael (1989) gives as average values: typical igneous rocks Q 5 140, typical sedimentary rocks Q 5 0.0210. Gueguen and Palciauskas (1994) give the following general orientation for igneous rock types: acidic intrusive veins Q 5 01, basic intrusive veins Q 5 110, basaltic lava Q 5 100. Figure 10.9 shows the magnitude of the remanent magnetization Mr versus susceptibility κ for some rock types. The two straight lines indicate a

390

Physical Properties of Rocks

1×104

Remanent magnetization in A m–1

1×103 Magnetite (ore)

1×102 1×101 Basalt 1×100 1×10–1

t

al iss as ne a-b G et m Schist

1×10–2 1×10–3

e ite as tin ab en i D rp se

Sediments

1×10–4 1×10–5 1×10–4 1×10–3 1×10–2 1×10–1 1×100 1×101 1×102 Susceptibility

Remanent magnetization (A m–1)

FIGURE 10.9 Remanent magnetization Mr versus susceptibility κ for some rock types. The two straight lines indicate a Koenigsberger ratio Q 5 1 for the field strength at the pole and the equator, respectively (modified after a figure of Angenheister and Soffel, 1972).

100

10

Basalt

1

0.1 0.0001

Rock, sampling area (lobe) 3 Rubble, sampling area (lobe) 3 Rock, sampling area (lobe) 4 Rubble, sampling area (lobe) 4

0.001

0.01

0.1

Susceptibility (SI) FIGURE 10.10 Remanent magnetization versus susceptibility crossplot; samples from two different lobes at The Barrier (Garibaldi Provincial Park, BC/Canada); Scho¨n (2011).

Chapter | 10

10

2

5

391

Magnetic Properties

NRM, mA m–1 102 103 2

5

2

5

104

Susceptibility, 10–3 SI 1 10

10–1 2

0

5

2

5

2

102 5

M/G 500

BG 1000

M 1500

2000

BG

FIGURE 10.11 NRM and susceptibility measured at cores of the KTB (Continental Deep Drilling Program, Germany). Vertical axis: depth in meters; M, metabasite; BG, biotite-gneiss; G, gneiss.

Koenigsberger ratio Q 5 1 for the field strength at the pole and the equator, respectively. Figure 10.9 demonstrates the high contribution of remanent magnetization particularly for basalt. Remanent magnetization versus susceptibility crossplots can be used for discrimination of different basalt and lava types as demonstrated in Figure 10.10. Figure 10.11 shows as a logging example the NRM and susceptibility measured at cores of the KTB (Continental Deep Drilling Program, Germany) with a separation between the main rock types metabasite, biotitegneiss, and gneiss.