ANISOTROPY OF MAGNETIC SUSCEPTIBILITY -...

CHAPTER 5

ANISOTROPY OF MAGNETIC SUSCEPTIBILITY

(AMS)

Page No.

5.1 Introduction 111

5.2 Magnetic Mineralogy of Lava Flows 113

5.3 The Effects of Time, Temperature and Weathering 114

5.4 Magnetic Foliation 115

5.5 Magnetic Lineation 116

5.6 Instrument Used 116

5.7 Measuring of Anisotropy of Magnetic Susceptibility 117

5.8 Methodology 118

5.9 Analysis of the AMS results 121

5.10 References 141

111

CHAPTER 5

ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS)

5.1 Introduction

Important works on the magnetic anisotropy of rocks were carried out during

the 1940s and 1950s (Ising, 1942; Graham, 1954). These authors first realized that

magnetic methods may be used to characterize the preferred orientation of minerals

within the rock samples. Ising studied varved clays in Sweden and noticed that the

magnetic susceptibility was higher on the bedding plane than orthogonally to it.

Graham recognized that the anisotropy of magnetic susceptibility (AMS) may be

regarded as a petrofabric element, he later extended the analysis to various

sedimentary rocks of the Appalachian Mountains and pointed out the existence of

distinct and systematic relationships of the magnetic properties with structural setting

(Graham, 1966). The studies progressively developed the following decades and a

first comprehensive review on magnetic anisotropy and its application in geology and

geophysics was published by (Hrouda, 1982). Over the past 20-30 years, researches

on magnetic anisotropy gained widespread use and were extended to examine the

fabric in a variety of sedimentary, igneous, and metamorphic rocks (Lanza and

Meloni, 2006; Hrouda, 2007).

Two principal mechanisms control the magnetic anisotropy of rocks. 1. Lattice

alignment of crystals with magnetocrystalline anisotropy, 2. Shape alignment of

ferromagnetic grains.

The magnetic anisotropy of rocks is often determined by means of the analysis,

of the anisotropy of the magnetic susceptibility (AMS), which is the significant

property that has found the most geophysical applications. The AMS of rocks is

controlled by preferentially oriented magnetic mineral grains and, therefore, it

contains information about both the grains susceptibilities and their orientation-

distribution. Knowing the composition of the rock forming minerals and their

magnetic anisotropy characteristics, it is possible to determine the spatial distribution

of the grains and the geological processes inference. A preferential orientation

distribution of mineral grains is in fact typical of almost all rock types and it develops

during various geological processes, such as water flow in sediments, magma flow in

112

igneous rock, ductile deformation in metamorphic rocks, and even incipient strain in

the paramagnetic clay matrix of apparently undeformed fine-grained sediments.

All the materials are “susceptible” to become magnetized in the presence of an

applied magnetic field, and the magnetic susceptibility describes this transient

magnetism within a material sample. If the magnetic field is relatively weak, the

magnetization of a rock is a linear function of the intensity of this field. The low field

magnetic susceptibility is defined as the ratio of the induced magnetization (M, dipole

moment per unit volume or J, dipole moment per unit mass) to the applied low-

intensity magnetic field (H). Only for isotropic substances the induced magnetization

is strictly parallel to the applied field, and the magnetic susceptibility is a scalar. In

the general case of anisotropic media, like minerals and rocks, the induced

magnetization is not parallel to the applied field.

The magnetization of a rock induced in a weak magnetic field is a linear

function of the intensity of field

M1 = k11 H1 + k12 H2 + k13H3 M2 = k2l H1 + k22 H2 + k23 H3 M3 = k31 Hl + k32 H2 + k33 H3

where Mi (i = 1, 2, 3) are the components of the magnetization vector (in the

Cartesian coordinate system), Hj (j = 1, 2, 3) the components of the vector of the

intensity of magnetic field and the set of the constants kij (kij = kji) represents the

components of the symmetric tensor of second rank, called the susceptibility tensor.

The components of the susceptibility tensor are in general non zero, but there exists

such a Cartesian coordinate system in which the non-diagonal components of the

susceptibility tensor are zero and the above equations change into

M1 = k11 H1 M2 = k22 H2 M3 = k33 H3

The components k11, k22, k33 are called the principal susceptibilities and their

directions, as the principal directions. The principal susceptibilities are usually

referred to as the maximum, intermediate and minimum susceptibilities respectively.

Low-field anisotropy of magnetic susceptibility (AMS) is an increasingly used

petrofabric tool that allows us to solve a large number of problems of geological

interest. Although the sources of the AMS of any rock type ultimately reside in their

113

mineral components, whether associated with their crystalline structure, the shape of

individual mineral grains or their distribution within a more or less isotropic matrix

(Hrouda, 1982, Tarling & Hrouda, 1993) The orientation of principal susceptibilities

represents distribution of magnetic minerals fabric in rock; however, the rock

composition and metamorphic grade may affect the anisotropy of magnetic

susceptibility and bulk susceptibility of rock type (Borradaile and Henry, 1997,

Nakamura and Borradaile, 2004). The bulk susceptibility (Km), anisotropy

susceptibility and other forms of magnetic anisotropy commonly play a great roll to

determine the state of strain and petrofabric, and are also used as strain indicators. The

AMS may be represented by magnitude ellipsoids, geometrically shaped by three

magnetic principal axes (K1≥ K2≥K3) those are closely related to the strain axes (λ1>λ2

>λ3). There are two elements, magnetic foliation ‘F’ (K1- K2 Plane) and magnetic

lineation ‘L’ (K1) (Tarling and Hrouda, 1993) and their anisotropy parameters are

described by eccentricity ‘Pj’ and its shape ‘T’.

5.2 Magnetic Mineralogy of Lava Flows

The magnitudes of bulk magnetic susceptibility (km) have large differences

among different lava flows or within the single lava flow. It depends upon the

different factors as source of lava flows, dominance and contribution of different

magnetic minerals. Different stages of lava flows also play vital role in contribution of

magnetic anisotropy. Exact discrimination of magnetic mineralogy and their

contribution for rock anisotropy property is difficult and it required detail analysis

although (Borradaile, 1988) and (Hrouda & Kahan, 1991) have shown that a small

amount of ferromagnetic minerals (~l volume %) is enough to dominate the magnetic

properties of a rock including its characteristic (km). Further (Tarling & Hrouda,

1993) suggest that, if km > 5x10-3SI is very likely that the contributions from

ferromagnetic minerals rather than paramagnetic and diamagnetic minerals. In other

case if km < 10-4SI the ferromagnetic contribution can be negligible and the

interpretation of the anisotropy is only relevant to the paramagnetic and diamagnetic

minerals fraction. For rocks with a bulk susceptibility intermediate between these two

extremes, a more detailed examination of the mineral content might be required

before drawing a valid petrofabric interpretation of the measured AMS (Canon-Tapia,

E 2004.) All mineral grains with in a rock contribute to its total susceptibility but

their individual influence depends upon the intrinsic susceptibility as well as on their

concentration (Hrouda and kahan, 1991)

Fig. 5.1 Mineral contribution to the susceptibility of a rock (After Hrouda

and kahan ,1991)

5.3 The Effects of Time, Temperature and Weathering

It is difficult to understand the importance of the changes that are induced

both temperature and time in the magnetic mineralogy and, hence, in the anisotropic

properties of the rock types. Temperature increases are usually the most effective, but

changes that occur as a consequence of the passage of time can be very significa

yet, being much more subtle, difficult to detect. In general, it is sediments that are

most sensitive to both temperature and the passage of time, reflecting their

heterogeneous composition and multifarious environmental history.

Most mineralogical changes induced by increasing temperature are likely to be

associated with an increased degree of metamorphism, particularly as the greenschist

grade is approached at some 250

also result in major fluctuations in local Eh

modifications of the mineralogy. Such changes will almost always result in a

noticeable modification of the mean susceptibility of the rocks affected, reflecting

either the creation or destruction of f

fabric is likely to be drastically altered. These thermochemical effects are most

114


(Hrouda and kahan, 1991) as shown in Fig. 5.1.


and kahan ,1991)

5.3 The Effects of Time, Temperature and Weathering

It is difficult to understand the importance of the changes that are induced



changes that occur as a consequence of the passage of time can be very significa



heterogeneous composition and multifarious environmental history.

ogical changes induced by increasing temperature are likely to be


grade is approached at some 250-300˚C, but the migration of hot or cold fluids can

also result in major fluctuations in local Eh-pH conditions, with consequent



either the creation or destruction of ferromagnetic minerals, and the mean shape of the




It is difficult to understand the importance of the changes that are induced by



changes that occur as a consequence of the passage of time can be very significant,



ogical changes induced by increasing temperature are likely to be


˚C, but the migration of hot or cold fluids can

pH conditions, with consequent



erromagnetic minerals, and the mean shape of the


115

pronounced in sediments in which the mineralogy of the magnetic constituents is

prone to major change. In an extreme case when temperature increases the magnetic

fabrics may even be inverted, as when predominantly single-domain magnetite or

maghaemite grains are produced.

Additionally, the drying out of sediment results not only in chemical changes

due to oxidation but also to the physical realignment of minerals as water menisci

pass through the sample (Noel, 1980).

Weathering marks the peak of readjustment to the environment at the Earth’s

surface. It is primarily a chemical process of oxidation and hydration, but pressure

release can also cause the physical disintegration of a rock. Generally, it is the

chemical changes that are of the most profound importance as these alter both the

composition and grain size of individual minerals. Few studies have been undertaken

specifically to identify the effects of weathering-induced changes on magnetic

anisotropy; usually, however, such effects appear to be disruptive, although there are

situations in which newly formed magnetic minerals pseudomorph the original

paramagnetic grains and thereby enhance the fabric.

5.4 Magnetic Foliation

The Magnetic anisotropy of foliated rocks of several types has been measured

by the torque-meter method, and shows that the alignment of long axes magnetic

grains normally follow the pattern of foliation is evident in field observations. In a

sharp fold in lit-par- lit formation the magnetic anisotropy indicated an otherwise

undetected lineation independent of the bedding and superimposed upon the foliation

determined by the layering.

Magnetic foliations close to the northern and southern margins of the pluton

are similar to the mesoscopic foliations of the surrounding gneisses (Pratheesh et al.,

2013). The orientation of magnetic foliation can also be used to distinguish among

different structures, sills and dykes, a task sometimes impossible simply by field

observations (Halvorsen 1974).

116

5.5 Magnetic Lineation

Rock fabrics can reflect the strain state of rocks and are a rich source of

information of tectonic evolution. Lineation is a common fabric element in rocks and

is particularly useful in deciphering the history of deformation. Lineation is

commonly defined as any linear feature that occurs penetratively in a rock and

includes form lineations (crenulation, rods, elongate pebbles) and mineral elongation

(stretched grains, linear aggregates of equidimensional grains, subhedral grains with

an elongate crystal shap, etc.). Lineation introduces anisotropy in the physical

properties of rocks and therefore it can be determined by methods capable of sensing

rock anisotropy.

The most obvious means of determining the origin of magnetic lineation in

deformed rocks is to obtain AMS records from rocks deformed in different tectonic

environments in order to compare the magnetic anisotropy tensor and its axes

distribution with the structural elements (Josep M Pares, et al., 2002).

5.6 Instrument Used

The MFK1 (Multi Function Kappabridge1) Kappabridges are the most

sensitive commercially available laboratory instruments for measuring magnetic

susceptibility and anisotropy of magnetic susceptibility (AMS). The same

Kappabridge is used in the present work of AMS. The Kappabridges have the

following features:

� Automatic zeroing over the entire measuring range.

� High sensitivity

� Automatic compensation of both real and imaginary susceptibility

components.

� Auto-ranging.

� Measuring at three different frequencies (version FA and FB).

� Measuring of in-phase and relative change of out-of-phase component.

� Slowly spinning specimen (version FA and A).

� Quick AMS measurement (FA and A).

� Easy manipulation.

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� Automated field variation measurement (FA and A).

� Only three manual manipulations for measuring AMS (FA and A).

� Built-in circuitry for controlling the furnace CS4 and cryostat CSL

� Full control by computer.

� Sophisticated hardware and software diagnostics.

The Kappabridge apparatus consists of the Pick-Up Unit, Control Unit and

Computer. In principle the instrument represents a precision fully automatic

inductivity bridge. It is equipped with automatic zeroing system and automatic

compensation of the thermal drift of the bridge unbalance as well as automatic

switching appropriate measuring range. The measuring coils at frequency F1 are

designed as 6th-order compensated solenoids with remarkably high field

homogeneity. Special diagnostics was embedded in MFK1 Kappabridges, which

monitors important processes during measurement with MFK1 and also with CS4 or

CSL unit. The MFK1-FB and MFK1-B versions measure the AMS of a static

specimen fixed in the manual holder. In the static method, the same as in KLY-2,

KLY-3 and KLY-4 bridges, the specimen susceptibility is measured in 15 different

orientations following rotatable design. From these values six independent

components of the susceptibility tensor and statistical errors of its determination are

calculated. The specimen positions are changed manually during measurement.

Different component of KLY-2 is showing by arrow in the figure.

5.7 MEASURING OF ANISOTROPY OF MAGNETIC SUSCEPTIBILITY

The measurement of AMS using static specimen method with manual holder

is available with the MFK1-FB or MFK1-B Kappabridges and also with MFK1-FA or

MFK1-A. During measurement process, the susceptibility of the specimen is

measured subsequently in 15 directions following the rotatable Jelinek's design in

exactly the same way as in the KLY-2, KLY-3 and KLY-4 Kappabridges. Using the

least squares method, the susceptibility tensor is fit to these measurements of the 15

directional susceptibilities (Fig. 5.2) and the errors of the fit are calculated.

118

Fig.5.2 MFK1-FA – KLY-2 showing different component.

5.8 Methodology

Sampling procedure in collecting rock samples for AMS is same as used for

paleomagnetic studies (Cox and Doell, 1960; Collinson, 1983; Tarling, 1983; Tarling

and Hrouda, 1993). In both cases oriented block sample collected. Rock sample can

be collected by two ways either drilling in the field itself or oriented block sample

collected for laboratory treatments. Specific size of rock specimen is required for

various measurements because most of the instruments have standard size specimen

holder which are identical for both AMS and paleomagnetic study. However,

magnetic anisotropy is more sensitive to the shape of the specimen than

paleomagnetic. These two methods viz: AMS & palaeomagnetic are most common

standard shape with diameter 2.5 cm and height of 2.1 cm and for cube 2.0 cm side.

Optimum height/ diameter ratio for cylinder can vary between 0 .8 to 0 .9 (Porath et

al., 1966; Noltimier, 1971a; Scriba and Heller, 1978; Collinson, 1983; Tarling and

Hrouda, 1993). In the present study, 20 dyke sites were sampled for AMS study

during the field work, which was held from April - 2009, May - 2010 and November -

2011. In most cases sampling was restrained to middle and lower parts of the flows in

119

order to detect reheating effects and partial alteration of magnetic oxides. Weathered

and jointed rocks are very susceptible for oxidations and hence could have been

affected by remagnetization. Total 200 standard size specimens are measured with the

help of MFK1-FA – KLY-2 for AMS study and there details are given in the Table

5.1. All the measurements have done with pattern of the Jelinek's design of the 15

directions measurements as shown in the Fig. 5.3 given below. The position design is

the same for the cubic and cylindrical specimens.

Fig 5.3 Jelinek's design of the 15 directions measurements of cubic and

cylindrical specimens.

120

Table 5.1

Anisotropy of Magnetic Susceptibility Parameters

Sample

No.

of

sam

ples

K1

K2

K3

Kmean

SI Units

K1

K2

K3

Pj

T

L

F

D I D I D I

S1 26 1.003

1 0.996 2.04E-02 340.5 1.1 250 28.2 72.6 61.8 1.025 -0.164 1.014 1.01

RP2 26 1.006

0.998 0.996 1.88E-02 279.8 40.4 164.5 26.7 51.5 37.9 1.039 -0.256 1.025 1.013

JP3 26 1.001

1 0.999 3.75E-03 223.3 3.8 315.5 30 126.7 59.7 1.018 -0.075 1.010 1.007

KG4 25 1.001

1 0.999 1.19E-03 35.3 4 130.4 51.9 302.2 37.9 1.01 -0.053 1.005 1.005

GK5 25 1.003

0.999 0.998 3.16E-03 168.1 77.5 347 12.5 77.1 0.2 1.011 0.161 1.005 1.006

KS6 26 1.001

1.001 0.998 9.18E-04 8.8 12.5 242.4 69.5 102.4 16 1.009 0.287 1.003 1.006

KS7 25 1.001

3 0.999 0.988 1.26E-02 253.5 9.6 348.8 28.7 146.8 59.4 1.057 0.163 1.024 1.031

KS8 25 1.014

1 0.986 1.51E-02 73.3 10.1 168.3 25.9 323.8 61.9 1.073 0.387 1.018 1.05

KRP9 26 1.002

1.001 0.998 1.20E-03 255.5 40.3 9.8 26 122.8 38.7 1.014 0.377 1.004 1.01

IBM10 25 1.002

1 0.998 1.93E-02 102.7 35.1 246.2 48.9 359 18.7 1.025 -0.235 1.015 1.009

NML11 26 1.002

1 0.999 9.65E-04 202.8 44.4 338.8 36.3 87.5 23.6 1.016 0.021 1.008 1.008

MCL12 25 1.001

1 0.999 1.47E-02 355.1 10.2 263.3 9.7 130.7 75.8 1.018 0.004 1.009 1.009

SNG13 26 1.003

1.001 0.996 1.70E-02 110.8 24 266.8 64 16.6 9.4 1.022 -0.076 1.011 1.01

MBN14 26 1.002

1 0.998 2.68E-02 70.4 10.7 329.7 44.7 170.7 43.3 1.21 -0.148 1.011 1.009

KKR15 25 1.003

0.999 0.998 3.80E-03 175.5 51.6 49.8 24.8 306 27.3 1.02 -0.019 1.01 1.009

CPG16 25 1.008

0.999 0.992 8.72E-03 118.1 82.3 353.7 4.4 263.2 6.3 1.031 -0.024 1.016 1.014

NVP17 21 1.001

1 0.999 9.68E-04 166.1 52.5 348.5 37.5 257.2 0.9 1.008 0.004 1.004 1.004

CTL18 25 1.101

0.994 0.905 5.79E-02 166.1 52.5 348 37.5 257 0.9 1.008 0.004 1.004 1.004

EKT19 25 1.002

1 0.998 6.00E-03 62.8 60 332.4 0.2 242.3 30 1.019 0.195 1.008 1.011

K1- Maximum Susceptibility, K2- Intermediate Susceptibility, K3- Minimum Susceptibility, D –

Degrees, I- Degrees, T- Shape Parameter, L – Magnetic Lineation,

F – Magnetic Foliation, Pj – Corrected Degree of Anisotropy,

121

5.9 Analyses of the Anisotropy of Magnetic Susceptibility (AMS) Results

The AMS study of the 20 dykes of the study area specimens revealed the mean

susceptibility values ~1.23 x 10-2 SI Units, indicating the dominance of ferromagnetic

minerals in the studied samples. The degree of anisotropy values are found as 1.033

matching well with the dolerite dykes of other regions of Precambrian terrains.

Whereas another property of anisotropy of magnetic susceptibility lineation and

foliation values are found as 1.003 and 1.25 (Minimum- Maximum) and 1.004 and

1.05 (Minimum- Maximum) respectively showing mostly prolate fabrics.

In most sites of the study magnetic susceptibility is dominantly, shows the

presence of ferromagnetic minerals. Mainly coarse to fine-grained Ti-poor

titanomagnetite up to pure magnetite carry the magnetic fabrics. Three primary AMS

fabrics are recognized in the present work.

Normal AMS fabric (where the greater circle joining K1 and K2 is parallel to

the dyke trend) is dominant in 12 dykes. This fabric is interpreted as a result of

magma flow in which the analysis of Kmax inclination permitted to infer that the

majority of dykes were fed by inclined flows. Intermediate AMS fabric (where the

greater circle joining K1 and K3 is parallel to the dyke trend) was found in 5 dykes.

This type of fabrics is interpreted as due to vertical compaction of a static magma

column with the minimum stress along the dyke strike. Inverse AMS fabric (where

the greater circle joining K2 and K3 is parallel to the dyke trend) is also observed in

the other 2 dykes. This type of fabrics is due to the presence of fine grained

ferromagnetic minerals in the dykes samples. Inclined and horizontal flows allow us

to infer the relative position of at least three magma sources (or magma chambers).

122

S-1 NW-SE

Fig. 5.4: a. Lower hemispheres Stereoplot of AMS data

b. Mean Susceptibility (km) Vs Degree of c. Degree of Anisotropy (Pj) Vs Shape

Anisotropy (Pj) diagram Parameter (T) diagram

Oblate

Prolate

123

RP-2 N-S




Oblate

Prolate

124

JP-3 NW-SE




Oblate

Prolate

125

KG-4 NNW-SSE




Oblate

Prolate

126

GK-5 NE-SW




Oblate

Prolate

127

KS-6 NE-SW

Fig. 5.4: a. Lower hemispheres stereoplot of AMS data



Oblate

Prolate

Oe

128

KS-7 NE-SW




Oblate

Prolate

129

KS-8 NE-SW

Fig. 5.4: a.Lower hemispheres stereoplot of AMS data



Oblate

Prolate

ate

130

KRP-9 NNE-SSW

Fig. 5.4: a.Lower hemispheres stereoplot of AMS data



Oblate

Prolate

131

IBM 10 E-W




Oblate

Prolate

132

NML-11 N-S




Oblate

Prolate

133

MCL-12 NW-SE




Oblate

Prolate

134

SNG-13 E-W




Oblate

Prolate

135

MBN-14 NE-SW (ENE-WSW)




Oblate

Prolate

e

136

KKR-15 E-W




Oblate

Prolate

137

CPG-16 NE-SW




Oblate

Prolate

138

NVP-17 NW-SE




Oblate

Prolate

139

CTL-18 NE-SW




Oblate

Prolate

e

140

EKT-19 NE-SW




Oblate

Prolate

141

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