The magnetization of clay-rich rocks in sedimentary basins: low-temperature experimental formation...

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Earth and Planetary Science Le

The magnetization of clay-rich rocks in sedimentary basins:

low-temperature experimental formation of magnetic

carriers in natural samples

M.G. Moreaua,*, M. Adera,1, R.J. Enkinb,2

aInstitut de Physique du Globe de Paris, FrancebPaleomagnetism, Geological Survey of Canada-Pacific, POB 6000, Sidney, B.C., Canada, V8L 4B2

Received 30 June 2004; received in revised form 15 November 2004; accepted 17 November 2004

Available online 4 January 2005

Editor: E. Bard

Abstract

Rocks are often magnetically altered during burial, thus providing a useful method to study their thermo-chemical history. In

order to improve the understanding of burial diagenetic magnetization, we experimentally synthesized magnetite in natural

samples. Lower Jurassic argillites from the Paris Basin were heated to 150 8C, under a confined atmosphere of argon. When a

magnetic field was applied during the heating, a chemical remanent magnetization was acquired. Rock magnetism studies reveal

that most of the newly formed magnetic carriers were magnetite. Using anhysteretic remanent magnetization as a measure of the

quantity of magnetite, an increase of up to 150% after heating was observed. Ferrimagnetic iron sulphide and hematite are

observed to have formed in samples composed of less than 3% calcite, and new magnetite is sometimes completely absent in

samples containing less than 0.5% calcite. Hysteresis measurements suggest that the new magnetite is in the pseudo-single

domain state. Scanning electron microscope studies indicate that the new magnetic carriers are associated with pre-existing

pyrite. The magnetic transformations during the experiments involved a fluid which reacted with iron-bearing and sulfur-

bearing minerals in ways depending on local redox conditions. This study shows that magnetite can be produced in argillaceous

sediments at the normal temperatures and pressures found in sedimentary basins, without any external supply from long-range

fluid flow.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Argillites; Burial remagnetization; Magnetite; Fluids

0012-821X/$ - s

doi:10.1016/j.ep

* Correspon

05; fax: +33 1 4

E-mail addr1 IPGP, T142 Tel.: +1 2

tters 230 (2005) 193–210

ee front matter D 2004 Elsevier B.V. All rights reserved.

sl.2004.11.013

ding author. IPGP, T14 Laboratoire de Paleomagnetisme, 4, Place Jussieu, 75252 Paris Cedex 05, France. Tel.: +33 1 44 27 26

4 27 74 63.

esses: [email protected] (M.G. Moreau)8 [email protected] (M. Ader)8 [email protected] (R.J. Enkin).

Laboratoire des Isotopes stables, 4, Place Jussieu, 75252 Paris Cedex 05, France. Tel.: +33 1 44 27 60 90; fax: +33 1 44 74 63.

50 363 6431; fax: +1 250 363 6565.

M.G. Moreau et al. / Earth and Planetary Science Letters 230 (2005) 193–210194

1. Introduction

Paleomagnetists usually search for primary mag-

netizations, which are synchronous with the origin of

the rocks measured. Often, late magnetizations are

recognized but these can be difficult to interpret.

These remagnetizations may have been caused by

heating, which produces a thermo-viscous remanent

magnetization (TVRM), or by circulation of fluids,

which produces a chemical remanent magnetization

(CRM).

In the case of CRM, the magnetic carriers are

usually hematite (hydrated or not), magnetite, or

ferrimagnetic iron sulphide (FIS). Hematite can be

produced by remobilization of iron in an oxidizing

environment by the circulation of meteoric or oro-

genic fluids (e.g., [1]). The mechanisms leading to a

remagnetization carried by magnetite or FIS are more

difficult to explain. Magnetite formation may be

linked to hydrocarbon migration, with examples

found in North America [2,3] and Europe [4]. In

contrast, there are several cases in which secondary

magnetite exists where there is no evidence of fluid

circulation [5–7].

Fruit et al. [8] and Banerjee et al. [9] showed, for

the Belden Formation in Colorado, that magnetite is

formed by replacing pyrite. They observed that this

magnetite is spread throughout the rock matrix rather

than being concentrated near calcite-filled veins.

Furthermore, they observed that the magnetite was

absent in zones lacking organic matter. The age of the

remagnetization in four different tectonic settings

agrees well with the age of organic maturation at

between 2 and 3 km burial, for which Banerjee et al.

[9] speculate a causal connection. Brothers et al. [10]

have experimentally demonstrated that organic acids

can be responsible for magnetite crystallization, and

suggest that in sedimentary basins, organic acids

produced by maturation of organic matter are respon-

sible for remagnetization.

The transformation of clay minerals, in particular

from smectite to illite between 2 and 4 km depth may

also produce magnetite. Woods et al. [11] proposed

this model for Jurassic sediments from the Isle of

Skye (Scotland), as did Katz et al. [12,13] for Jurassic

and Cretaceous sediments from the Vocontian trough

(south-east of France). In the Mississippian Deseret

Basin, Blumstein et al. [14] found that remagnetiza-

tion occurred as the rocks were within the oil window,

whereas smectite-to-illite transformation apparently

predated both events. Whether the creation of magnet-

ite is linked to the transformation of organic matter or

clay minerals, it does not seem necessary to have

exterior contributions.

Late diagenetic magnetizations are often observed

in rocks rich in organic matter and pyrite, and authors

usually note the total absence of a primary component

of magnetization. Possibly these rocks contained no

primary magnetite of single or pseudo-single domain

grain size, capable of preserving a magnetic signal.

Alternately, the original single or pseudo-single

domain magnetite grains may have dissolved during

early diagenesis [15,16], with the iron precipitating as

pyrite—a paramagnetic mineral incapable of holding

a remanence. If there are no primary fine grains of

magnetite, then in the absence of subsequent dia-

genesis these sediments cannot hold a stable magnet-

ization. The key question concerns what happens to

such rocks when they are buried within a sedimentary

basin. Can burial, without any exterior contributions,

lead to the production of fine-grained magnetite and a

stable magnetization where there had not been one

before?

While researchers often note that a fluid is required

for the dissolution and reprecipitation of iron, in most

cases the origin of this fluid remains unknown [6,17–

21]. Long range transport of fluids has been invoked

to explain the broad regions which are observed to

have been remagnetized ahead of orogenic deforma-

tion fronts [22], however the hydrodynamic condi-

tions for this flow have been disputed.

In order to improve our understanding of the

mechanism controlling this magnetization we have

heated natural samples to investigate the magnetic

transformations that can occur during burial.

2. Choice of samples

For several years, we have been studying the

Lower Jurassic levels of an industrial bore-core

situated in the north-east of the Paris Basin in northern

France. Magnetostratigraphic studies of the Hettan-

gian-Sinemurian, between 1075 and 920 m depth,

where the proportion of calcite is almost always larger

than 40%, show that the primary paleomagnetic signal

M.G. Moreau et al. / Earth and Planetary Science Letters 230 (2005) 193–210 195

is well-preserved [23]. Nevertheless, Moreau and

Ader [24] showed that, in clay-rich and organic-rich

layers (with less than 20% calcite), fine magnetite

grains are largely absent.

Higher up, in the Pliensbachian section of the

same core between 920 and 770 m depth, the

proportion of calcite decreases whereas that of clay

increases. In these rocks, the paleomagnetic signal is

less well preserved, and for 10% of the samples no

primary remanence was discernible [25]. For the

Toarcian (the target for this study), between 770 and

680 m depth, the proportion of calcite can decrease

below 1%, while that of clay occasionally reaches

clays

quartz

calcite

700

720

740

760

Dep

th(m

)

Major components Clay comp

chlor

kaoli

illite

smec

illite/

chlor

cumulative %0 100 0 cumulati

Fig. 1. Framework of the study, data obtained by X-ray analysis on po

composition and the depths of sampled levels in cumulative percentages;

increase in anhysteretic remanent magnetization (ARM) after heating of s

80%, and very little stable remanent magnetization

could be characterized in a reconnaissance study of

300 samples (unpublished). Mineralogical study of

this level (unpublished Agence nationale pour la

Gestion des dechets radioactifs (ANDRA) report by

Thiry and Schmidt, 1992, and [26]) has shown that

these rocks have not undergone any significant

transformation. Thus they provide interesting samples

for the study of authigenesis of magnetite during

burial diagenesis.

Fig. 1 shows the global composition as well as the

clay spectrum of the b2 Am fraction. The sediments

contain between 0.5% and 2% pyrite as isolated

onents ARM Gain

0 200

ite

nite

tite

smectite

ite/smectite

100ve % % gain

wdered samples every 5 m on average: Column 1—global rock

column 2—composition of the clay fraction (b2 Am); column 3—

ample at 150 8C for 1 week confined in a neutral atmosphere.


crystals or framboidal clusters (unpublished report by

Thiry and Schmidt, 1992). The total organic carbon

varies between 1% and 6% [26].

Standard size paleomagnetic cores were drilled

perpendicular to core axis from the 11 cm diameter

bore-core, and samples are labeled after their depth

level in meters. Most samples were selected between

690 and 722 m depth where the reconnaissance study

showed that no stable remanent magnetization could

be characterized; the rest of the samples were selected

between 722 and 770 m deep where a weak primary

remanent magnetization was occasionally identified.

Obviously heterogeneous levels were avoided.

3. Experimental conditions for crystallization of

magnetic carriers

3.1. Methods

A suite of experiments was performed to determine

the conditions under which magnetic minerals could

be formed in the laboratory in these sedimentary

rocks. All measurements of remanent magnetization

were made using a 2G cryogenic magnetometer.

Monitoring several samples over 3 h showed that

viscous magnetic changes never observed after 15

min. Thus all samples were held in zero field for 15

min before remanence measurements were made.

We used the magnitude of anhysteretic remanent

magnetization (ARM) to quantify the amount of fine-

grain magnetite in the samples [27]. ARM acquisition

curves were determined using a continuous field of

0.1 mT and stepwise application of a parallel

alternating field (AF) with peak fields of 10 to 90

mT. While other measures, such as isothermal

remanent magnetization, have desirable properties

for quantifying the amount of magnetite, ARMs have

the advantage of being removable using AF demag-

netization. Before every ARM curve acquisition, the

samples were AF demagnetized at 90 mT, leaving a

residual magnetization always b2% of total ARM. In

this paper, total ARM refers to the magnetization

using a peak alternating field of 90 mT. By doing

these measurements before, ARM(I), and after,

ARM(II), each heating experiment, we could measure

the change in the quantity of newly formed magnetite.

Given the sources of noise and the experimental

reproducibility of these experiments, we find that

ARM gains can be considered significant when they

are greater than 10%.

3.2. Preliminary experiment

As a preliminary experiment to determine if it were

feasible to produce magnetite in natural sediments

subjected to burial conditions, we performed experi-

ments in an autoclave which was large enough for

samples which next required ARM measurements, but

which was unfortunately not well-adapted to holding a

constant temperature below 200 8C. Two samples

(699.76 and 700.45) were subjected to a 100 MPa

heating experiment to simulate the effects of ~3 km

burial. Samples (1.5 cm diameter, 1.5 cm long) were

wrapped in aluminum foil, and held at pressure under

argon gas at a temperature of 180F20 8C for 15 days,

using the apparatus at the Centre de Recherche sur la

Synthese et la Chimie des Mineraux in Orleans,

France. ARM acquisition curves before and after

treatment show an impressive increase of more than

3000% (Fig. 2a; Table 1 column 3).

3.3. Choice of experimental parameters for acquis-

ition of CRM

Having shown that formation of magnetic minerals

is possible under experimental conditions, we set out

to determine which experimental parameters control

the amount of this formation: the confining fluid, the

hydrostatic pressure, the duration, and temperature of

the experiment.

Since performing high pressure experiments with a

controlled magnetic field proved to be very difficult,

most trials were done at atmospheric pressure.

Samples were heated in a confined atmosphere by

placing them in Teflon air-tight containers. The

container manufacturer (Savillex) guarantees a good

seal at 150 8C up to 0.5 MPa. This maximum

temperature corresponds to a burial of about 5 km,

which is quite reasonable for a sedimentary basin.

Since the preliminary experiment showed that no

additional fluids are needed to produce mineralogical

transformations, argon was chosen as the only

experimental fluid in order to avoid chemical reac-

tions with the sediments. The containers were placed

in a standard paleomagnetic furnace (Pyrox) for 1

150 II

I

a10

I

IIb5

I&II-75-100-125oC

II-150oCc

AR

M (

10-6

Am

2 /kg

)

0 100 0 0alternating field(mT) alternating field(mT)alternating field(mT) 100100

AR

M (

10-6

Am

2 /kg

)

AR

M (

10-6

Am

2 /kg

)

Fig. 2. Anhysteretic remanent magnetization (ARM) acquisition curves (using a bias field of 0.1 mT) for replicate specimens from level 699.76,

before (I) and after (II) heating confined in argon atmosphere. (a) 180 8C heating under 100 MPa pressure 2 weeks duration; (b) 150 8C heating

at atmospheric pressure, 1 week duration; (c) stepwise heatings at 75, 100, 125, and 150 8C.


week. The ARM measurements (Table 1 column 4,

and Fig. 2b) show that magnetite growth occurred,

however the increases were far less than seen with the

preliminary high pressure experiment.

In order to determine the temperature at which the

transformation took place, samples from four levels

were heated in 25 8C steps of 7 days duration each,

from 75 8C to 150 8C. After each heating step, the air-

tight container was opened, the ARM acquisition

curve was measured, the sample was AF demagne-

tized at 90 mT, and then resealed with argon before

Table 1

Experimental conditions for acquisition of CRM

ARM(I) High pressure Atmospheric pressure,

confined

Step

180 8C 150 8C 75 8

Sample (10�6 Am2/kg) gain (%) gain (%) gain

693.74 4.3 0.0

697.67 4.3 155

699.12 3.5 67

699.76 4.5 3100 150 0.4

700.45 4.6 3500 95 0.2

739.48 3.5 19

754.53 2.8 58

759.83 4.0 15 0.2

762.91 3.9 28 0.0

766.02 3.1 17

769.15 3.4 14

Gain in ARM, measuring growth in magnetic carriers, after various experim

meters; column 2, ARM(I) is the ARM acquired before all experimentatio

were negligible. Columns 3–8, gain in ARM after heating confined in a con

8C during 15 days; column 4, under atmospheric pressure for 7 days at 1

successive temperature steps: 75 8C, 100 8C, 125 8C, and 150 8C; colum

being put in the furnace for the next week-long

heating step. A twin sample of each level was heated

directly to 150 8C for 7 days. The magnetic trans-

formation during this stepwise heating experiment

(Table 1 columns 5–8; Fig. 2c) was much smaller than

when a single heating at 150 8C was done (Table 1

column 4; Fig. 2b). For three samples, no significant

change was observed, while the fourth (699.76),

which saw the largest change during the single heating

(150%), saw only a 9% change and that only on the

final (150 8C) step.

wise heating at atmospheric pressure, confined Open air

C 100 8C 125 8C 150 8C 150 8C

(%) gain (%) gain (%) gain (%) gain (%)

�4.0

0.0

6.0

0.2 0.0 8.8

1.1 2.8 �2.7

1.0

�3.0

0.0 0.5 2.7

2.5 0.5 3.6

6.0

2.0

ental heatings. Column 1, sample identification refers to the depth in

n. Only one value is given as the variations among sister specimens

tainer filled with argon; column 3, under pressure of 100 Mpa at 180

50 8C; columns 5–8, heating confined in a container for 7 days at

n 9, gain in ARM after heating in open air for 7 days at 150 8C.


Apparently breaking the confinement during the

heating limits the creation of magnetic carriers.

Following this observation, seven pairs of samples

were heated in argon under confinement and in open

air, respectively. The increase in ARM in samples

heated in open air was always negligible (below 6%,

see Table 1 column 9). This result compares well to

the usual paleomagnetic observation that standard

thermal demagnetization in open atmosphere to 150

8C or even 200 8C only removes a viscous present

field overprint and seldom leads to observable

magnetic mineralogical changes.

The implication from these experiments is that the

samples themselves produced fluids during the experi-

ments. These fluids are necessary for the mineralog-

ical transformations because when the atmosphere is

not strictly confined throughout the heating, there is

little or no transformation. Moreover, a deposit of

anhydrite (CaSO4) was usually observed on the

sample surfaces, uncorrelated to the increase in

ARM after heating experiments, whereas there was

little or no anhydrite deposit on the samples heated in

open air or the samples heated in steps. (Such deposits

had also formed on cracks after the core was brought

to the surface). These deposits indicate that fluid

circulations, which took place during the confined

heating experiments, mobilized calcium and sulfate

ions. The fluids have not been characterized in this

study.

Heatings done over 50 h, 7 days (168 h), and 1

month (720 h) on six sets of three samples from the

same level show irregular gains in ARM. The

variability is probably due to lateral heterogeneities

at the centimeter scale. A large part of the increase in

ARM was usually acquired in the first 50 h, however

the duration of 7 days was used for the following

experiments to allow the maximum amount of the

transformation in a reasonable experimental time.

The increases in ARM obtained by heating at 150

8C in a confined atmosphere for 7 days varied

between 0% and 155% (Table 1 column 4). In order

to understand this variability, experiments were done

on 10 more levels, with results shown in Fig. 1

column 3. These results show that in this type of

sediment, the creation of magnetic carriers is not

exceptional, and that almost all levels are susceptible

to remagnetization under these conditions. It appears

however that the lithology has an influence on the

degree of remagnetization: in the zone between 690

and 710 m (with b5% calcite) the gains in ARM vary

widely from zero up to 170%, while in the zone

around 760 m (with 10 to 30% calcite) the ARM gains

are generally weaker but never zero (5% to 60%).

4. Properties of the CRM

The preliminary experiments showed that most of

the Toarcian part of the core was susceptible to

acquisition of a CRM when heated in a confined

atmosphere. The next stage consisted therefore in

studying the properties of this CRM.

4.1. Experimental protocol

At 7 levels (3 around 700 m and 4 around 760 m)

sister specimens (1 cm long) were cut from 2.5 cm

diameter cores: specimens (a) and (b) were given

CRMs at atmospheric pressure, while specimen (c)

was left untreated as a control. When the sediments

were poorly consolidated (9 levels between 690 and

710 m) a single specimen (a) was collected. Before all

experiments, specimens were demagnetized by alter-

nating field (AF) with a peak field of 100 mT. Unused

rock went towards chemical analysis of carbonate

content.

After the initial ARM curves were measured,

specimens (a) and (b) were confined in a Teflon

container purged with argon and heated at 150 8Cduring 7 days with a continuous 0.1 mT field applied

along the sample axis. The field was zeroed during the

cooling. The remanent magnetization was measured,

and then the specimens were heated a second time at

150 8C for 7 days, but this time in zero field and in

open atmosphere in order to demagnetize any

contribution due to thermo-viscous remanent magnet-

ization acquired during the first heating. The (a)

specimens were AF demagnetized and the (b) speci-

mens were thermally demagnetized to characterize the

nature of the magnetic carriers.

A fourth sister specimen (d) was heated at high

pressure (100 MPa) for 7 days, using an apparatus

specially designed and built at the Geology Labo-

ratory of the Ecole Normale Superieure de Paris (with

better temperature control than the one used in the

preliminary experiment). This new autoclave, sealed

Table 3

Chemical remanent magnetization (CRM) obtained in the laboratory

Sample CRM+TVRM CRM0 CRMRES

(10�6 Am2/kg) (10�6 Am2/kg) (%)

694.77a 4.41 3.4 22

700.28a 102 62 1.0

704.39a 47 31 5.5

759.90a 19 12 7.8

759.45a 21 12 2.0

762.58a 44 25 0.4

765.35a 1.7 1.0 2.5

694.77b 1.5 1.1 8.8

700.28b 1.3 0.8 7.5

704.39b 23 16 2.0

759.90b 2.1 1.5 10

759.45b 1.3 0.83 8.3

762.58b 2.8 1.6 5.3

765.35b 1.0 0.6 7.8

693.80a* 0.90 0.67 82

693.88a* 0.37 0.24 60

695.12a* 10.7 8.4 13

696.96a* 5.8 3.7 1.2

699.00a* 20.2 11.6 2.4

700.57a* 2.1 1.3 2.0

703.49a* 0.52 0.32 32

706.22a* 7.7 6.2 12

706.69a* 7.7 5.6 11

CRM+TVRM is magnetization measured after heating samples for

7 days at 150 8C with a continuous 0.1 mT field followed by

cooling in confined atmosphere and zero field; CRM0 is magnet-

ization measured after reheating samples in open air for the same

duration at the same temperature and zero field; CRMRES is

percentage of magnetization after demagnetization at 70–100 mT

alternating field for sample baQ or 500–570 8C for sample bbQ.Samples with asterisk are poorly consolidated.


with a high-temperature Bridgman-type closure and

with argon as the pressure medium, was positioned

vertically in a topless furnace box. The temperature

was held at 150F5 8C, monitored with a Ni–NiCr

thermocouple placed 3 mm within the external

autoclave wall at the sample level. At the end of the

heatings, the pressure was reduced to atmospheric

pressure and the autoclave was removed from the

furnace box to cool at a rate of 100 8C/h. The

magnetic field unfortunately could not be controlled.

4.2. Results

4.2.1. Pressure effect

The increases in ARM in specimens heated under

high pressure are comparable to those observed in

sister specimens heated at atmospheric pressure

(Table 2). Apparently, the extremely large increases

observed in the preliminary experiments were caused

by the higher temperature and longer duration of the

heating rather than by the higher pressure of the first

autoclave. Since pressure does not greatly affect

formation of magnetite, we consider the atmospheric

pressure experiments to be representative of the

remagnetizations caused naturally by burial.

4.2.2. Demagnetization of CRM

The CRM intensities are listed in Table 3. The

difference between the remanence after the first

heating in a confined atmosphere and the second

heating in open air varies between 20% and 45%. This

difference is interpreted to be a TVRM since the

mineralogical study discussed below shows no evi-

dence of magnetic carriers with Curie temperature

b150 8C (e.g., goethite).

Table 2

Comparison of the acquisition of ARM in samples heated at atmospheric

Sample (a) ARM (I) ARM (II) ARM gain S

(10�6 Am2/kg) (10�6 Am2/kg) (%)

694.77a 3.18 5.30 67 6

700.28a 3.91 11.7 405 7

704.39a 3.50 46.1 1220 7

759.90a 4.04 20.1 397 7

759.45a 3.66 24.3 563 7

762.58a 4.08 41.2 910 7

765.35a 3.77 5.87 56 7

Samples from the same level heated at atmospheric pressure (sample baQ,ARM(I) is ARM before heating and ARM(II) is ARM after heating.

The thermal and AF demagnetization character-

istics of the CRMs separate the samples into two

categories. In the first, which includes almost all of

and high pressure

ample (d) ARM (I) ARM (II) ARM gain

(10�6 Am2/kg) (10�6 Am2/kg) (%)

94.77d 3.39 8.13 140

00.28d 3.88 11.7 201

04.39d 3.78 32.7 766

59.63d 3.94 4.37 11

60.54d 3.63 4.18 15

62.58d 4.00 40.5 910

65.35d 3.75 4.13 10

columns 1–4) and under high pressure (sample bdQ, Columns 5–8),

694.77(a)

90

694.77(b)

180

0

704.39(b)

90

0 200 400 600

704.39(a)

90

M/M

max

0

1.0

alternating field(mT)

temperature(ºC)


540ºC

0 200 400 600

0 20 40 60 80 100

0 20 40 60 80 100

M/M

max

0

1.0

M/M

max

0

1.0

M/M

max

0

1.0

759.45(b)

90

540ºC

759.45(a)

90

0

703.49(a)

90

180

0

0 200 400 600



0 20 40 60 80 100

0 20 40 60 80 100

M/M

max

0

1.0

M/M

max

0

1.0M

/Mm

ax

0

1.0

90

temperature(ºC)

temperature(ºC)

Fig. 3. Representative intensity and stereograph plots of AF (a) and thermal (b) demagnetization of CRM acquired in 0.1 mT field along sample

axis with 1 week heating at 150 8C confined in an argon atmosphere. The samples were reheated in open air for the same duration at the same

temperature and zero field to eliminate thermo-viscous remanent magnetization that was concurrently acquired during the first heating.


0

1

0 1

M/M

max

0 0

4010-4

0

1002*10-4

IRM

(A

m2 /

Kg)

0

702*10-3

0 0 0

20

3*10-5

0

a

b c

d e

Applied field (Tesla)

IRM

IRM

(A

m2 /

Kg)

IRM

(A

m2 /

Kg)

IRM

(A

m2 /

Kg)

susc

eptib

ility

(10

-5 S

I)

susc

eptib

ility

(10

-5 S

I)su

scep

tibili

ty (

10-5

SI)

susc

eptib

ility

(10

-5 S

I)

0 200 400 600temperature oC

694.77a696.96a

704.39a 703.49a

693.80694.77696.96703.49704.39759.45




Fig. 4. IRM acquisition (a) and three-component IRM thermal demagnetization [27] and corresponding weak field susceptibility evolution (b–e)

for representative samples. IRM absolute intensities are listed Table 4, column 4. Solid circles indicate the soft component, 0.12 T field; open

squares indicate the medium component, 0.4 T field; small diamonds indicate the hard component, 1.2 T field, and black triangles indicate

susceptibility. In most specimens, the soft component shows unblocking temperatures around 580 8C, the Curie temperature of magnetite but

inflexions around 230–270 8C (c and d) suggest the presence of greigite and others FIS. Coexisting with magnetite and iron sulphide is a high

coercive component with unblocking temperatures of 680 8C, most likely hematite (c and e).



the well-consolidated samples, at least 95% of the

remanence is demagnetized by heating to 570 8C or

by 50 mT AF (e.g., Fig. 3, samples 704.39 and

759.45). The dominant magnetic carrier is therefore

identified to be magnetite. The second category has

samples which do not demagnetize completely by 100

mT AF and thus include a significant quantity of high

coercivity magnetic carriers (e.g., Fig. 3, samples

694.77 and 703.49). Some of these samples, such as

694.77b were not entirely demagnetized by 570 8C,suggesting that the magnetic carrier is hematite. The

percentage of residual remanence after AF demagnet-

ization is tabulated in Table 3 (CRMRES). Type 1

sample 704.39b shows a magnetite unblocking

temperature spectrum between 500 and 580 8C.Chemical instabilities occurring during heating above

500 8C prevent a better characterization of the

magnetic carriers in many samples. For several

samples, a slight break in slope in the thermal

demagnetization curve around 300 8C suggests the

presence of FIS.

Table 4

Occurrence of the magnetic carriers and calcite content

Sample ARM (I) ARM (II) ARMgain

(10�6 Am2/kg) (10�6 Am2/kg) (%)

693.80a* 2.77 2.82 1.8

693.88a* 2.41 2.41 0.0

694.77a (694.77c) 3.18 5.30 67

695.12a* 3.12 10.4 230

696.56 4.22 8.90 110

696.96a* 4.19 10.6 150

699.00a* 3.80 21.6 470

700.28a (700.28c) 3.91 19.7 400

700.57a* 4.69 6.71 43

700.67 4.77 11.9 150

703.43 4.82 5.01 3.9

703.49a* 4.24 4.28 0.9

703.59 4.71 4.99 5.9

704.39a (704.39c) 3.50 46.1 1200

706.22a* 4.28 12.6 190

706.69a* 3.90 10.8 180

759.90a (759.90c) 4.04 20.1 400

759.45a (759.45c) 3.66 24.3 560

762.58a (762.58c) 4.08 41.2 910

765.35a (765.35c) 3.77 5.87 56

Column 1, samples bracketed (index c) are as control, samples without in

with asterisk are poorly consolidated. Column 2, ARM(I) is ARM before

ARMgain=(ARM(II)�ARM(I))/ARM(I)�100; Column 5, IRMMAX is IRM

T and 0.3 T; column 7, FISTEST =(IRM(230 8C)�IRM(320 8C))/IRM(320

content is obtained by quantification of CO2 released by phosphoric acid

Sample 700.28a broke after being demagnetized at

5 mT AF. The sample was reglued, however several

fragments could not be placed in their original

position. The magnetic moment of the sample

dropped from 6.5�10�7Am2 to 1.04�10�7Am2, a

drop of 80% while the sample only lost 6% of its

weight. We conclude that the poorly consolidated part

of the sample holds a high concentration of the

magnetic carriers. The unglued fragments were recu-

perated for further experiments, and this new sample

was called 700.28CHIPS.

4.2.3. Identification of the magnetic carriers

Isothermal remanent magnetization (IRM) acquis-

ition curves were measured for all the (a) specimens in

order to better characterize the magnetic carriers. For

most well-consolidated samples, 95% of IRM satu-

ration was acquired by 0.3 T, while for most of the

rest, saturation did not occur in applied fields up to

1.25 T (Fig. 4a). The ratio IRMHEM (defined as the

IRM acquired in 1.2 T over the IRM acquired at 0.3 T)

IRMMAX IRMHEM FISTEST CaCO3 F5% rel

(10�6 Am2/kg)

120 2.60 0.18 0.026

73.9 1.88 0.37 0.058

209 (56) 1.48 (1.06) 0.24 2.5

336 1.17 0.37 0.48

152 1.07 0.39 1.1

213 1.05 0.39 2.3

450 1.02 0.40 1.9

732 (52) 1.03 (1.06) 0.30 1.9

124 1.08 0.45 1.6

166 1.07 0.32 1.6

138 1.80 0.39 0.44

88.9 1.30 0.36 0.33

94.5 1.39 0.32 0.44

1682 (42) 1.01 (1.10) 0.29 0.77

328 1.28 0.33 0.77

383 1.09 0.41 0.34

386 (52) 1.03 (1.08) 0.29 10.0

319 (43) 1.02 (1.08) 0.29 14.6

653 (49) 1.01 (1.09) 0.32 12.2

96.3 (47) 1.05 (1.10) 0.33 12.9

dex where used for the choice of experimental parameters, samples

treatment; columns 3, ARM(II) is ARM after treatment; column 4,

acquired in 1.2 T; column 6, IRMHEM is ratio of IRM acquired in 1.2

8C) is used an indicator of the presence of FIS; column 8, calcite

. Precision for the calcite content is F5% of the calcite content.

-0.8

-1

110-3Am2/Kg

0.8T

-60

6010-3Am2/Kg

-0.8

-60

6010-3Am2/Kg

-0.8

a

b

e

fd

10-3Am2/Kg

-80

80

-0.8 0.8T

10-3Am2/Kg

-0.8

6

-6

g

h

40010-3Am2/Kg

-0.8

-400

10-3Am2/Kg 300

-300

15

-15

10-3Am2/Kg

-0.8 -0.8

c

Applied field(T) 0.8

Applied field(T) 0.8

Mag

netiz

atio

n

Mag

netiz

atio

n

Mag

netiz

atio

nM

agne

tizat

ion

Mag

netiz

atio

n

Mag

netiz

atio

n

Mag

netiz

atio

n

Mag

netiz

atio

n

Fig. 5. Hysteresis curves in fields up to 0.8 Tesla: total curve in (a), (c), (e), and (g), and with the paramagnetic component removed in (b), (d), (f), and (h). (a) and (b) for the unheated

sample 699.76; (c) and (d) for the sample 704.39 heated at 150 8C for 7 days confined in argon at atmospheric pressure; (e) and (f) for sample 699.76 heated at 180 8C for 2 weeks

under 100 MPa pressure; (g) and (h) for 700.28CHIPS.

M.G.Morea

uet

al./Earth

andPlaneta

ryScien

ceLetters

230(2005)193–210

203


has a large range from 1 to 2.6 in the (a) specimens,

while the unheated (c) specimens never have

IRMHEMN1.15 (Table 4, column 6). Thus carriers

with a large range of coercivity have been created by

the laboratory heatings.

Following the method of Lowrie [28], three

orthogonal IRMs (hard: 1.2 T, medium: 0.4 T, and

soft: 0.12 T) were applied to samples which were then

thermally demagnetized. Three different behaviors

were observed. For the first group (e.g., Fig. 4b), the

soft component is dominant, with unblocking temper-

atures up to 580 8C and a slight break in slope

observed between 230 and 330 8C typical of FIS

(ferrimagnetic iron sulphide). There is little medium

and hardly any hard component, and they demagnetize

by 600 8C. These samples, dominated by magnetite,

are the most consolidated of the samples with the

exception of sample 694.77. The second group (e.g.,

Fig. 4c and d) is similar to the first group except that

the remanence drops suddenly at 270 8C, principallyvisible in the soft component, and probably owing to

alteration of greigite [29,30]. The medium and hard

components are sometimes bigger than in the first

group, and they demagnetize between 630 and 690 8C,indicating the presence of hematite. This group

includes all the friable samples except for 693.80a.

The third group, consisting of samples 693.80a et

694.77a (Fig. 4e), hold hematite but show little

evidence for any FIS. The hard and medium compo-

nents are larger and they have unblocking temper-

atures up to 690 8C.Hysteresis curves were measured using a vibrating

sample magnetometer built by Maxime LeGoff

(Institut de Physique du Globe de Paris). Only

samples with relatively high magnetic concentrations

gave usable results, so we studied the two samples

Table 5

Hysteresis parameters

Sample JS(Am2/kg)

JRS(Am2/kg)

699.76 0.012 0.0041

700.45 0.038 0.012

704.39 0.0051 0.0018

700.28 CHIPS 0.256 0.116

Two samples heated to 180 8C with 100 MPa pressure argon, sample sho

heating experiments and 700.28CHIPS. JS: saturation magnetization; JRS:

coercivity of remanence.

heated to 180 8C under high pressure (from the

preliminary experiments, Fig. 5e and f) and their

unheated pairs (Fig. 5a and b), the sample showing the

greatest increase in ARM during the atmospheric

pressure heating experiments (Fig. 5c and d), plus the

700.28CHIPS sample (Fig. 5g and h). In the untreated

samples, magnetization at 0.8 T is N95% due to

paramagnetic minerals (Fig. 5a). On subtraction of the

paramagnetic component, the ferromagnetic magnet-

ization is seen to be weak and the curves are too noisy

to determine the hysteresis parameters (Fig. 5b). In

contrast, the heated samples have paramagnetic

magnetization below 75% of the total under an

applied field of 0.8 T (Fig. 5c, e, and g). The

ferromagnetic components offer noisy (Fig. 5d) or

well-defined hysteresis cycles (Fig. 5f and g) which

reach saturation by 0.3 T, indicating magnetite as the

dominant ferromagnetic mineral. The form of the

curve, without apparent inflections, suggests the grain

sizes are homogeneous [31]. The saturation magnet-

ization ( JS), saturation remanence ( JRS), coercive

field (HC), and coercive field of remanence (HCR) are

reported in Table 5. The ratios JRS/JS near 0.3 and

HCR/HC near 1.5 indicate the grains are in the single

to pseudo-single domain size range (around 0.1 Am,

by comparison with the data compiled by Dunlop

[32]). Since the hysteresis cycles shown in Figs. 5d, f,

and h are very similar, we propose that the magnetic

carriers are the same in these samples.

4.2.4. Occurrence of the magnetic carriers

The magnetic carriers of CRM apparently vary

according to the samples’ consolidation. As this

property seems to be related to the calcite content,

calcite was quantified in the levels of the studied

samples. The calcite content (Table 4, column 8) is

JS/JRS HCR

(mT)

HC

(mT)

HCR/HC

0.34 28 17 1.6

0.31 23 15 1.5

0.35 25 18 1.4

0.45 32 23 1.4

wing the greatest increase in ARM during the atmospheric pressure

remanence magnetization after saturation; HC: coercive force; HCR:


obtained by manometric quantification under vacuum

of the CO2 released from the powdered sample by

phosphoric acid at 25 8C for 4 h. Reproducibility is

better than F5% of the calcite content. Blanks are

negligible. The possible occurrence of dolomite was

checked by quantifying the subsequent CO2 released

at 100 8C for 2 h. Dolomite content was lower than

10% of the calcite content.

In order to examine the relation between calcite

content and the occurrence of different magnetic

calcite content (%)

150 105

1.15

2.5

IRM

HE

M

calcite con

1

10

100

1000

AR

M g

ain

(%)

0 1

a

b c

0

0

0

FIS t

est

0

Fig. 6. Occurrence of magnetic carriers as a function of the concentration o

the symbols used depend on the nature of the magnetic carriers identified in

the presence of FIS (ferrimagnetic iron sulphide), and a diamond marks t

shaded zone indicates the samples for which the ARM gain is below 1

significant growth of hematite or FIS. Above the 0.5% calcite threshold, th

and (c) FISTEST indicate the appearance of FIS and hematite below 3% c

carriers of CRM, we defined criteria for the recognition

of these carriers.

The gain in ARM acquired after heating [i.e.,

ARMgain=(ARM(II)�ARM(I))/ARM(I)] depends

mostly on the quantity of magnetite created (Table 4,

columns 2–4).

The IRM acquired above 0.3 mT is mostly held by

hematite. Thus we use IRMHEM=IRM(1.25 T)/

IRM(0.3 T) as a measure of the relative contribution

of hematite (Table 4, column 6).

tent (%)152 10

FIS

magnetitehematite

.15

.35

.45

calcite content (%)

150 105

.25

f calcite (Table 5). (a) ARMgain is plotted on a logarithmic scale and

the samples. A cross marks the presence of hematite, a circle marks

he presence of magnetite. In samples containing b0.5% calcite, the

0%, where formation of new magnetite is uncertain, but there is

ere is no apparent dependence on calcite concentration. (b) IRMHEM

alcite concentration.


In the IRM thermal demagnetization curves, the

drop in intensity at 270 8C (Fig. 4) is distinctive and

typical of demagnetization or alteration of ferrimag-

netic iron sulphides, FIS. FIS Curie temperatures range

up to 320–330 8C, so we define FISTEST=(IRM(230

8C)�IRM(320 8C))/ IRM(320 8C). When there is an

observable inflection point in the thermal demagnet-

ization plots (e.g., Fig. 4, samples 696.96 and 706.49),

FISTEST is higher than 0.35, conversely when the drop

is not visible FISTEST is lower than 0.35 (Fig. 4, sample

704.39). Therefore, in this study the FISTEST is used as

an indicator of the presence of FIS (Table 4, column 7).

The relative quantity of magnetite created

(ARMgain, Fig. 6a), the relative contribution of he-

matite (IRMHEM, Fig. 6b), and the test of presence

of FIS (FISTEST, Fig. 6c) are plotted as a function of

the quantity of calcite. Magnetite is clearly present

when ARMgainN10%. FIS seems to be present when

FISTESTN0.35. There is new hematite when IRMHEMN

1.15, because the highest value of IRMHEM in unheated

samples is 1.14 (Table 4, column 6). Thus, Fig. 6a

shows that when there is less than 0.5% calcite, the

creation of magnetite is rare (2 of 7 samples). Above

0.5% calcite, magnetite is formed, however the

quantity bears little dependence on the percentage of

calcite. Figs. 6b and c show that FIS and hematite are

never present in samples with N10% calcite, but this

threshold might be as low as 3% since we have no data

between 3% and 10% calcite.

5. Microscope study of the magnetic minerals

Magnetic characterization of the newly formed

magnetic minerals was complemented by a scanning

electron microscope study to investigate their shapes,

sizes, growth habits, and composition. We performed

back scattered electron imaging using a Jeol

JSM840A, with semi-quantitative elemental compo-

sition measured by electron microprobe X-ray fluo-

rescence using energy dispersive spectrometry with a

Si–Li detector.

It is almost impossible to image single-domain or

pseudo-single-domain magnetite grains in sediments

because (1) the grain sizes are almost always smaller

than the best resolution of the SEM (~1 Am), and (2) the

quantity of magnetite is usually extremely low. Since

the saturation magnetization of magnetite at room

temperature is 92.4Am2/kg, the untreated samples with

saturation magnetizations below 10�3 Am2/kg (Fig. 5)

have magnetite concentrations below 1/100 000. The

hysteresis curves for the strong samples (the two

samples heated under high pressure at 180 8C during

the preliminary experiments, and residual sample

700.28CHIPS) display saturation magnetizations (Table

4; Fig. 5), which correspond to magnetite concen-

trations of 0.01% to 0.03% for the samples heated

under pressure and 0.3% for 700.28CHIPS.

These three samples were used for the microscope

study because of their relatively larger magnetite

concentrations. The largest piece of 700.28 CHIPS was

imaged directly. For both levels 699.76 and 700.45 we

looked at magnetic concentrates from 3 sister samples:

one untreated, one heated at 150 8C confined at

atmospheric pressure with argon, and one heated at

180 8C confined at 100 MPa by argon. These were

desegregated in distilled water with acetic acid to

dissolve the carbonate minerals. The pH of the

suspension was maintained above 4 to preserve the

iron minerals from dissolution. The powder was

sieved, and the coarse fraction was broken with

mortar and pestle until all could pass through a 50

Am mesh. The samples in suspension were circulated

through an apparatus similar to that described by

Hounslow and Maher [33], with which the magnetic

minerals are gathered on a glass sleeve containing a

permanent magnet.

No magnetic material was extracted from the

untreated sample using this method, so a strong rare

earth magnet held in a thin rubber sleeve was moved

through the suspension while it was agitated in an

ultrasonic bath. This method collected some material,

of which a small fraction was ferromagnetic. For the

three samples of each of the two levels 699.76 and

700.45, the extracted quantities (almost none for the

natural sample, a small amount for the sample heated

at atmospheric pressure, and about 10 times more for

the sample heated under high pressure) correlate well

with the ARM(II) (Table 1).

In the untreated sample, a few oxide and sulphide

grains were located among the mass of clays. The

magnetic grains, with sizes ranging from a few to

30 Am, have been identified as titanomagnetites and

ilmenites as well as one cuprite. The fine grains up

to 10 Am are isolated crystals or framboids of pyrite.

In the heated samples, there are also grains of


titanomagnetite or ilmenite and a few pyrite cubes.

In contrast, the most numerous grains were pyrite

framboids. These framboids form almost the entire

collection of grains extracted from the sample heated

at high pressure. Pyrite in its pure state is para-

magnetic, with a susceptibility 104 times weaker

than that of magnetite. In the low fields produced by

the permanent magnet used for magnetic extraction,

no pyrite should have been isolated. Both the fact

that pyrite was isolated and that the amounts of

pyrite extracted magnetically correlate with the

amounts of magnetite determined independently,

suggest that these pyrite framboids have been partly

oxidized to magnetite. Magnetite couldn’t be iden-

tified using the electron microprobe (~1 Am)

Fig. 7. Scanning electron microscope images of sample 700.28CHIPS. (a) P

pyrites are bright; (b) detail of a zone with concentrated pyrites; (c) rhom

probably because they are too small to be resolved by

this technique. However the amount of pyrite extracted

apparently reflects the amount of newly formed

magnetite.

A piece of 700.28CHIPS was examined as a whole

in order to study the environment in which magnetite

has the greatest probability of crystallizing. The

images show that, as well as a groundmass of clay

(Fig. 7a), there is a large concentration of pyrite

(approximately 20% as opposed to the 0.5 to 2%

estimated from the other samples; Fig. 7b). These

pyrites are found both in the form of rhombohedra

(Fig. 7c) and framboids (Fig. 7d). This observation

confirms that the formation of magnetite is intimately

associated with the pyrite in these samples.

hoto of the largest piece. The clay minerals are imaged dark and the

bohedric pyrite, (d) framboidal pyrite.


6. Discussion

The results outlined above show that magnetic

minerals, mostly magnetite, are formed at temperatures

as low as 150 8C in these argillaceous sedimentary

rocks when they are confined in an inert gas at

atmospheric pressure. This study shows that (i)

magnetic carrier formation involves a fluid produced

by the sample, (ii) magnetic carriers are closely

associated to pyrite, and (iii) the mineralogy of the

new magnetic carriers is linked to the abundance of

calcite in the sample.

Occurrences of secondary magnetite grown at the

expense of pyrite have already been observed

[8,9,17]. In the case of the Belden formation

(Colorado) the magnetite is present as a rim in

replacement of the pyrite in some coprolites con-

stituting a reducing micro-environment. In an exper-

imental study undertaken to explain the replacement

of pyrite by magnetite in the Belden formation,

Brothers et al. [10] have experimentally demonstra-

ted that organic acids when complexed to ferric

(Fe3+) ions, allow the oxidation of pyrite. They

further propose that the neutralization of the acidity

generated by this oxidation by carbonate dissolution

would allow dissolved ferrous iron to precipitate as

magnetite. A process of this type may account for

the formation of new magnetic minerals in our

samples.

The samples studied here are mainly composed of

clay minerals, contain organic matter (between 1% and

6% [26]) and have never been buried below 1 km or

heated above 50 8C throughout their geological history

[26]. Heated at 150 8C, they are expected to produce

water by dehydration of clays [34] and organic

molecules (organic acids and/or hydrocarbons) by the

maturation of organic matter [35–40]. Such a mixture

of water and organic molecules with iron or sulfur

bearingminerals is expected to produce redox reactions

[41,42]. The source of ferric ions, necessary for the

proposed process [10], could be the clay transformation

of smectite to illite [43,44]. The formation of magnetite

only in samples containing more than 0.5% calcite

suggests that the acidity was buffered by calcite

dissolution as suggested by Brothers et al. [10]. When

there was insufficient calcite to neutralize the acidity,

only FIS could precipitate [45]. Such a precipitation of

FIS was not observed by Brothers et al. [10] because

the experimental conditions were too basic and too

oxidizing for FIS formation. During their experimental

magnetite formation stage, the presence of nitrate in the

solution they used increased the pH.

The coexistence of hematite and FIS is more

surprising. From a thermodynamic point of view they

should not be present together. Two hypotheses could

account for this observation: an oxidation of iron

sulphides during the heating step in open air at 150 8Cfollowing the experiment (designed to remove any

thermo-viscous remanent magnetization), or, local

heterogeneous control on the formation of the iron-

and/or sulphide-bearing minerals during the experi-

ment. Since the samples containing hematite are

unconsolidated, and are therefore highly permeable

to air, hematite formation could be due to some air

remaining in the porosity of the sample at the

beginning of the experiment or to the recrystallization

of oxidized phases formed during the 12 years

between core extraction and these experiments.

7. Conclusion

In summary, the major results obtained in this

experimental study are

1. magnetic carriers are produced at temperatures as

low as 150 8C in these argillaceous sedimentary

rocks when they are heated while being confined

in an inert gas,

2. fluids generated by sample heating are necessary

to the synthesis of the magnetic carriers,

3. the location and amount of newly formed

magnetic carriers are highly heterogeneous and

physically associated with pyrite crystals,

4. the calcite content controls the nature of the

newly formed magnetic carriers.

Even if the exact hydrothermal conditions of burial

have not been reproduced in these laboratory experi-

ments, and if the samples had been altered since their

coring, these observations can be used to improve the

understanding of some cases of remagnetization. Our

experiments suggest that sedimentary rocks can be

remagnetized during burial diagenesis with fluids

produced in situ during burial. In this hypothesis,

both organic matter maturation and clay transforma-


tion appear to be key elements for such a remagne-

tization. The new magnetic carriers form locally in

micro-environments inherited from early diagenesis,

probably where organic matter and pyrite are con-

centrated; the nature of the newly formed magnetic

carriers being controlled by lithologic parameters.

Characterization of (i) the chemistry of the fluid

produced, (ii) the claymineral transformations, and (iii)

the organic matter maturation during the experiments

would help in understanding the processes associated

with the formation of new magnetic carriers in our

laboratory experiments and in sedimentary basins.

Acknowledgments

Core samples and unpublished reports were gen-

erously made available by ANDRA. High pressure

preliminary experiments were performed by J. Roux at

the Centre de Recherche sur la Synthese et la Chimie

desMineraux in Orleans (France). We are grateful to G.

Marolleau for his help with the high pressure experi-

ments in the Laboratoire de Geologie at Ecole Normale

Superieure de Paris. Fruitful discussions with F.

Brunet, V. Courtillot, M. Thiry, J. Trichet and B. Velde

were much appreciated. The manuscript was much

improved due to the careful reviews done by Mark

Dekkers andMaria Cioppa. This is a IPGP contribution

No. 2020 and G.S.C. publication 2002260.

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The magnetization of clay-rich rocks in sedimentary basins: low-temperature experimental formation...

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