continuous drift correction and separate identification of ferrimagnetic and paramagnetic...

Geophys. J . h i . (1993) 114, 663-672

Continuous drift correction and separate identification of ferrimagnetic and paramagnetic contributions in thermomagnetic runs

T. A. T. Mullender, A. J. van Velzen and M. J. Dekkers Paleornagnetic Laboratory ‘Fort Hoofddijk’, Budapestlaan 17, 3584 CD Utrechi, The Nerherlands

Accepted 1993 February 22. Received 1993 February 22; in original form 1992 April 6

S U M M A R Y The principle of a Curie balance was changed by using a sinusoidally cycling applied magnetic field instead of a fixed applied field. This was done with a horizontal translation type Curie balance. By cycling between field values B,,, and B,,,, the output signal is amenable to Fourier analysis. Partial Fourier analysis yields the fundamental harmonic and the second harmonic, termed SIG, and SIGz respectively. These are related to the saturation magnetization (Ms) by M, = (2 SIG, - 8 SIG2 [ ( B m a x + B m i n ) / ( B m a x - Bm, , , ) ] } / [A”(Bmdx - Bm,,,)] and to the paramagnetic susceptibility (xpdr) by xpar = 8 SIGz/[A”(B,,, - B,,,J2], whereby A” is a calibration constant. Through the Fourier analysis continuous drift correction is achieved simultaneously. A personal computer takes care of field control, temperature control and data acquisition in real time mode, as well as processing the data, to yield SIG, and SIGz. After the experiment, SIG, and SIG, are processed further with a separate transversal filtering program that improves the signal-to-noise ratio. The working temperature range of the adapted horizontal translation type Curie balance is between room temperature and 900°C. Its noise level corresponds to a magnetic moment of 2 x lop9 Am2, making it a very powerful tool for thermomagnetic analysis of weakly magnetic material. Examples demonstrating this potential of the device are shown.

Key words: Curie balance, drift correction, ferrimagnetism, ferromagnetism, paramagnetism, thermomagnetic analysis.

1 INTRODUCTION

A detailed magnetomineralogical analysis is increasingly required for a sound interpretation of palaeomagnetic data. To this end, thermomagnetic analysis is a commonly applied technique (e.g. Schwarz 1975; Gehring & Heller 1989; Snowball & Thompson 1990). Determination of the Curie temperature yields valuable information about how to interpret magnetic hysteresis data. In addition, chemical reactions involving magnetic minerals can be assessed in order to discriminate true blocking temperature spectra from actual removal of magnetic minerals, which is crucial for the interpretation of the behaviour of the natural remanent magnetization (NRM). An example is the inversion of maghemite and/or magnetite to haematite (e.g. van Velzen & Zijderveld 1992). A large portion of palaeomagnetic data is obtained from sedimentary rocks, which often are only weakly magnetized. With conventional magnetic balances it is usually impossible to extract meaningful information unless tedious pre-concentration procedures are followed. In addition, the large paramag-

netic contribution in sedimentary samples obscures the extraction of the ferrimagnetic component.

Thermomagnetic analysis, the determination of the magnetic moment of a sample as function of temperature, can be carried out in several ways (e.g. Zijlstra 1967; Foner 1981). Two techniques are used most frequently, involving either: (1) the vibrating sample magnetometer, where the sample is placed in a homogeneous magnetic field (recently, superconducting magnets in combination with a SQUID sensor have been used (Foner 1981)); or (2) the force balance, where the sample is placed in a magnetic field gradient in addition to the main magnetic field (Lewis 1974; Flanders 1990). The mechanical force experienced can be measured using either a vertical or horizontal balance. Horizontal balances suffer from offset due to imperfect levelling of the instrument; levelling should remain horizontal within f slope.

We present a horizontal translation type Curie balance that makes use of a sinusoidally cycling applied magnetic field instead of a static magnetic field. The output signal of the Curie balance is then amenable to Fourier analysis. This

663

664 T. A . T. Mullender, A . J . uan Velzen and M . J . Dekkers

makes continuous drift correction possible as well as the separation of the ferro-/ferrimagnetic and paramagnetic contributions. The balance is two to three orders of magnitude more sensitive than conventional Curie balances, making it extremely powerful for thermomagnetic analysis of weakly magnetic material such as sedimentary deposits.

2 BASIC PRINCIPLES A N D THEORY

The typical magnetization curve of a rock sample at a certain temperature is composed of a ferrimagnetic part, reaching saturation above a certain field, and a paramagnetic part, which shows a linear field dependence. Both the ferrimagnetic and paramagnetic contributions to the magnetization curve change with temperature. Hence, the contribution of the ferrimagnetic minerals to the magnetization curve remains unsolved. By cycling the applied field between B,,, and B,,,, both contributions can be separated for each temperature, provided that the ferrimagnetic contribution is saturated. The change in magnetization that is caused by the field change will then depend on the paramagnetic behaviour only. With this information the paramagnetic and ferrimagnetic contributions to the total magnetization can be calculated. The sample experiences an inhomogeneous magnetic field cycling between Bmin and B,,,. Because the sample is virtually fixed relative to the magnet, the field gradient Bgrad(t) = dB(, , /dz will cycle according to the magnetic field B(,). The time(,) dependence of B(,) can be described by:

B(,) = B, t B , cos at, (1) where w is the cycling frequency, B, = (B,,, + Bmi,)/2 and B , = (B,,, - Bmin)/2. Provided that B,,, is large enough to saturate the ferrimagnetic contribution to the total signal, the magnetic moment A4(r) can be described by:

M(r) = J s g , + XhfgZB(t)/PoJ (2) where J, and xhf stand for the specific saturation magnetization and the specific paramagnetic susceptibility and g , and g, represent the mass of ferrimagnetic minerals and paramagnetic minerals present in the sample respectively. Only the total mass of the sample is needed. If we substitute J s g , by M , and XhfgZ by xpar, eq. (2) is written as

M(r) = Ms + XparB(r)/Po. (3)

The field gradient along the z-axis dB( , ) /dz = AB,,,, where A is a constant depending on the geometry of the pole shoes and z the horizontal axis perpendicular to B. It causes a mechanical force, &r) = Mcl) dB,)/dz, along the z-axis which is available as the (electronic) output signal SIG,,, = A'F,,, where A' is a calibration constant. Substitu- tion of eq. (3) into 4,) = A M ( , ) B ( , ) yields

Fr)/A = MsB(r) + XparB:r)/Po, (4)

and subsequent substitution of (1) in (4) yields

&,) /A = MsBo + MsB, cos wt + ~parB: /~<)

+ 2XparB,,B, cos W t l P O

t Xpar(BI cos wt)*/Po. ( 5 )

This can be represented by

4,) = 6) + F, cos wt + F2 cos 2wt,

where

4 = ABO(M, + Xp,,(Bo + o.jB;/Bo)/Po)> (6)

6 =AB,(Ms + 2XparBdPO)r (7)

F' = O.jAXp,,B:/Po. (8)

and

Partial Fourier analysis of the output data produces SIGo = A'4) , SIG, = A'F, and SIG, = A'F,. Thus, SIG, and SIG, depend on the minimum and maximum values of the cycling field. Substitution in (7) and (8) relates M , and xpZlr to the instrumentally extractable parameters SIG, and SIG,:

M s = [SIG, - SIG, (4Bo/B,)]/(A"B,)

Xpar = [2 SIG,/(A"B3I PO

(9)

(10)

where A" is a constant equivalent to AA' . Alternatively, it is possible to give the relation to the

'classical' thermomagnetic signal. Here a constant field Bsteady is applied. According to the assumed conditions the magnetization M,,,,, would be expected to be:

Mtotal= Ms + XparBstcadyIPo.

S I G , / ( A " B , ) = M , + Xpar2Bo/Po

Since F, = S I G , / A ' , it follows from (7) that:

= Ms + Xpiir(Bmin + B n m . x ) / ~ o . (11)

This shows that SIG, is equivalent to the classical thermomagnetic signal in a constant applied field of

Surprisingly, the equivalent applied constant field is even greater than any momentary value of B ( r ) that really occurs during cycling. Eq. (7) shows that the paramagnetic induced moment x ~ ~ ~ B , / ~ , contributes twice to F, and thus equally to SIG,. It should be noted that SIG, is also an important component in the calculation of M,.

Bmnn + B m a x .

3 THE INSTRUMENT A N D CALIBRATION

3.1 The instrument

Our Curie balance (Fig. 1) is based on a horizontal translation type balance described in Lebel (1985) and was manu- factured by the ETH in Zurich (Switzerland). It makes use of an automatic force compensator system. The operating transducer is able to measure displacements in the order of 0.1-1 nm. The balance is equipped with a water-cooled Oxford Instruments magnet (N 38) that has a maximum field of 0.4T. Samples are put in a small (diamagnetic) quartz-glass cup mounted on a quartz-glass rod and are heated in air. Temperature is measured by a Pt=PtRh thermocouple very close to the end of the sample holder. Approximately 300mg of sample material can be put into the holder. The translation arm of the original balance was replaced by a much lighter one and the suspension was changed into a six-point triangle suspension, which considerably reduces undesired sideward movements of the arm. A unit (Fig. 2) was added to control the field cycling. The complete system, i.e. field control, temperature regulation, data acquisition and data processing, is

Ferrimagnetism and paramagnetism 665

0 SIG -- 0

LVDT electron ics

Figure 1. Instrument configuration. The translation beam, approximately 45 cm long, is suspended by polyester threads 30 cm long. The quartz-glass sample holder has a length of approximately 16cm. The balance is protected against air draughts by an acrylate box. The permanent magnet assembly, providing the negative feedback, and the operation transducer (LVDT), providing the signal, are situated at the left side of the beam. The mobile assembly of the electromagnet, furnace and thermocouple is shown in the position when exchanging the sample holder.

controlled by a personal computer (PC; IBM compatible). Two 16-bit analogue to digital converters (ADC) are used for acquisition of the balance signal and sample temperature. With the computer program the minimum and maximum field values for the cycle (any set of values between 15 and 400mT) can be selected, the maximum heating temperature (up to 9OO0C), and the heating and cooling rates (between 0.5 "C min-' and SO "C min-'). Routine instrumental settings are a field cycling between 200

thermo couple 0

Thyristor

v out onioff furnace

electro magnet I I

Power Supply

flM4out v rJ

and 400mT to meet the requirement of saturated ferrimagnetism as much as possible without loosing too much signal, a heating rate of 6 "C min-' and a cooling rate of lO"Cmin-'. With these fairly low heating and cooling rates the thermal hysteresis of the sample holder is negligible. One complete field cycle lasts 1.2s. Each fifth-cycle partial Fourier transformation is carried out by the PC on a set of 1024 data sampled during four preceding cycles, so that every 6 s a data point consisting of

16 bit

I

<*I 12 bit

PC

I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

I I I I I I I I I I I

Figure 2. Functional diagram of the controlling unit and data processing units used

666 T. A. T. Mullender. A. J . van Velzen and M . J . Dekkers

temperature, SIG, and SIG, is calculated and stored. Field and temperature are continuously monitored (respectively 1024 and four times per cycle). The maximum magnetic moment that can be controlled amounts to approximately 150 @Am2, which is equivalent to 1.7 mg magnetite.

3.2 Accuracy and calibration

Because the instrument is very sensitive, the potential for quantitative thermomagnetic analysis is large. Therefore, considerable effort was put into establishing a high accuracy of the instrument. A n important feature is the ability to extract the saturated moment from the total magnetic moment. The accuracy depends largely upon the accurate calibration of both SIG, and SIG, signals in relation to each other. Owing to the low-pass frequency response of the system, both a phase error and amplitude error will occur between signal SIG, a t frequency 2w and signal SIG, at frequency w. Therefore, a compensation routine was implemented in the computer program to adjust for these errors. To examine the possible mechanisms that may cause errors, tests were performed on consequences of: magnitude and phase error between the extracted SIG, and SIG, signal, non-sinusoidal cycling of the magnetic field, offset, possible distortion of the signals of strong samples, drift and noise in the SIG, and SIG, signal.

3.2.1 Response of SIC, versus SIC,

In order to measure the (constant) phase and amplitude errors, a force signal was simulated by a small calibration coil, attached to the sample holder. This coil was placed inside the slot of a calibration magnet. The computer program that normally synthesizes and controls the cycled current for the electromagnet a t frequency w was adapted to synthesize this cycled current by the operator's choice at frequency w or 2 0 together with specified magnitude and phase. Instead of feeding this current into the electromagnet it was fed into the small coil to create a precisely known Lorentz force. Under these conditions a program run provided data of SIC, or SIG, only, where the magnitude is dependent on the (chosen) phase and magnitude of the synthetized current. Repeated runs with various values for phase and magnitude of the synthetized current provided the information to determine the errors in phase and magnitude. It appeared that these errors were in good agreement with the calculated values based upon the overall response of the system. Afterwards, several test runs were performed with a variety of magnitudes to simulate both weak and strong samples. The departure from the correct response SIC, versus SIG, remained within 0.1 per cent.

SIC, signal of frequency 4w that is of no consequence. This means that correction of SIC, for a relative amount of SIG, is necessary. To achieve an accuracy of 1 per cent in the overall calculations, this correction must be known within an accuracy of 0.1 per cent. To measure this correction, a Hall probe was placed in the sample position to measure the main field and its distortion directly. The output of the Curie balance was replaced with the output of the Hall probe. The normal measurement procedure was followed and repeated for a number of field cycle settings. The distortion appears in the signal SIG, = (relative distortion x SIC,). The distortion figures obtained are used in the data filtering program in order to carry out a mathematical correction on SIG,. Thereafter, tests were performed on samples of mumetal ( f0 .5 mg). This material will be (virtually) saturated for the complete field range in use, implying that SIG, be zero. This test showed a maximum residual cross-talk of 0.1 per cent over the measuring range of the instrument.

3.2.3 Noise and remaining drift

It appears that the noise in the measurements is dominated completely by the movements of the instrument, with frequencies around the working frequencies. Apparently these are due to movement of the building. This movement has an amplitude of 10-20nm and is probably caused by wind, traffic and/or tidal movements. The compliant soil in this part of the Netherlands causes these to b e passed on, leading to noise levels of 45-90 nAmZ, which can be reduced to < 10 nAm2 by filtering. The SIC, component especially is prone to a fairly large noise contribution because it often constitutes a weak signal. Because SIC, has to be multiplied by 4-12 depending on the field-cycle setting, the extraction of the ferromagnetic contribution from SIC, unfortunately is also sensitive to this noise. Tests with no sample holder show that there is no remaining residual drift (Fig. 3). The time-dependent offset in SIG, and SIG; appears to be caused by the residual noise only.

DRIFT

Cowent ional versus Cycling Mode

T Iv

i ~~ ,_) Fleld 400 rnT Stattc

f

Conventional Made,/

_I-'-

3.2.2 Cross-tafk between SIC, and SIC,

In the present state the field cycling in the balance is current-controlled, where the current is cycled sinusoidally. When using fairly large field cycles, a slight deviation of a true sinusoid occurs (approximately 1 per cent). This causes harmonic distortion of the main magnetic field, mainly introducing second harmonics. A magnetic field of frequency 2 0 introduces an additional SIG, signal of frequency 2 0 that adds to the normal SIG, signal and a

2c 1

Figure 3. Comparison of the drift in a static applied field (conventional mode) and in a cycling field (cycling mode) The drift in the cycling mode remains withm f 2 x 10 "Am', I e the saturation magnetization of 22 ng magnetite only


magnetization (NRM) intensity is 2 mA m-'; after thermal demagnetization at 200°C usually 50 per cent of the initial NRM intensity remains. Fig. 4(a) shows the unfiltered data points. The SIG, and SIG, output signals are presented in absolute magnetic moments, by means of the instrumental calibration factor. In addition, SIG, is multiplied by 12 (4B,,/B, with field cycling between 0.2 and 0 .4T) to enable a visual check of the separation of the ferrimagnetic contribution. The SIG, curve already approaches a more or less smooth thermomagnetic curve. The SIG, signal cannot be interpreted without the filtering program. Apparently, it suffers to a much larger extent than SIG, from noise. which is conceivable because SIG2 IS at least one order of magnitude smaller. The filtered results in Fig. 4(b) show that the paramagnetic contribution, although still slightly noisy, now has more or less a T-' temperature dependence, as expected (Curie's law). It is not reversible, however, indicating chemical alterations in the paramagnetic mineral fraction. The ferrimagnetic contribution, i.e. SIG, corrected for the paramagnetic contribution, is shown by the dashed line. At temperatures above 580-600°C (the Curie point of magnetite), the ferrimagnetic contribution is virtually zero, a good illustration of correct instrumental calibration. The cooling curve lies below the heating curve, therefore a considerable amount of the magnetite in the sample does not survive heating up to 700 "C.

Table 1. Instrument parameters measured and calculated

constant method measured calculated

A Hall probe 33

A' feedback piimmeters I .5 x I O - ~

A A ' e q . A x A 3.70 x 10"

A ' force simulation 1.51 x 10-8

A A ' Nickel rample 1.73 tn: 2 1 8 x 109

Some results obtained by the prototype of the instrument illustrate the potential of the balance. The effect of the field cycling principle on the drift is shown in Fig. 3. The drift was measured in a cycling field (referred to as cycling mode) and in a static field (referred to as conventional mode), both without sample holder. The voltage output has been converted to a virtual magnetic moment in lo-" Am'. In the cycling mode the drift remains within f 2 X lo-" Am2 or f 2 0 0 p V (peak-to-peak value for most of the time being within *lo-' Am'), whereas in the conventional mode it is at least two orders of magnitude larger. The drift curve shown for the conventional mode was recorded during a quiet period, other curves showed, for a similar duration, up to five times more drift. The drift curve for the cycling mode was recorded during normal operation conditions, i.e. in a laboratory where no special precautions were made to avoid vibrations.

3.2.4 Calibration

The calibration involves calibration of the magnetic moment, the magnetic field at the sample position and the temperature of the sample. The magnetic field was measured by means of a calibrated Hall probe. Subseq- uently the gradient of the magnetic field a t and nearby the position of the sample holder was measured for a number of field values. The accuracy of these measurements is limited and is estimated to be approximately 5 per cent. The Lorentz force in a coil with a well-defined cycled current, situated in the slot of a calibration magnet (see also Section 3.2.1) was used to determine the sensitivity of force versus output (see also Section 3.2.1). The combination of this sensitivity together with the field gradient allowed us to calculate indirectly the calibration constants for SIG, and SIG,. Moreover, the instrument was calibrated directly using nickel powder with well known magnetic properties. The results of these independent calibrations were in good agreement with each other. Finally, the error of a change in sample mass was checked. Again, the nickel powder was used as the sample. The measured signals were compared with and without an extra weight of 500 mg attached to the beam. The change in calibration was less than 0.3 per cent, The temperature in the furnace is accurate to within 1.0"C throughout the complete temperature range.

4 SOME CASE STUDIES

4.1 Marine marl sample

The results of a Pliocene marine marl sample from Eraclea Minoa (Sicily) will be shown (van Velzen & Zijderveld 1990; Langereis & Hilgen 1991). Typical natural remanent

4.1. I Ferrimagnetism

Two magnetic concentrates were prepared from this sediment using a Frantz isodynamic separator. The first concentrate was collected with a current of only 0.25A, hence only relatively strongly magnetic grains should have been extracted (Fig. 5a). The second concentrate was collected with an increased current of 1 . 5 A (Fig. 5b). Indeed, the magnetization displayed in Fig. 5(a) is larger than that in Fig. 4(b), certainly when taking into account the ratio in sample mass of 20mg versus 8.7mg. As expected, the paramagnetic contribution to SIG, is much smaller. The Curie temperature is higher than 600 "C, implying slightly oxidized magnetite (cation-deficient spinel), probably due to heating up to 700 "C. In the 1.5 A concentrate (Fig. 5b) a reaction occurs after heating at approximately 420 "C, very likely the oxidation of pyrite to magnetite (van Velzen & Zijderveld 1992). This reaction could not be traced in the run of the original sediment sample. Again, this magnetite is oxidized further upon heating at higher temperatures. Pyrite is present in trace amounts in Eraclea Minoa samples. It is remarkable that pyrite is extracted in the 1.5 A magnetic concentrate, possibly reflecting an oxidized (maghemitized) surface layer.

4.1.2 Paramagnetism

The susceptibility signal of the original sediment can be split into a temperature independent diamagnetic part and a paramagnetic part. By fitting the measured curve with a hyperbola the diamagnetic and paramagnetic parts (Fig. 6) can be separated. It is reasonable to assume that no chemical matrix reaction will occur during cooling from 700°C. Indeed, the fitted hyperbola follows the measured curve quite well throughout the total temperature range.

668 T. A . T. Mullender. A . J . van Velzen and M . J . Dekkers

(4 1000

0

3000 E 6 0)

0 7

c_ C - 0 2000

t - m 2 2 1000

- m

0

-1ooc

EMA36 SEDIMENT

. . * . ::. . ..

I t - :... *-, . . . 12 * SIG,

. .

SIG,

Cycling 200 400 m i Mass 120 mg Maximum temperature 700 OC Healing rate 3 OCimin Cooling rate 6 OCimin

1 " " I ~ ~ " I " ' ~ l ' ~ ' ' I ' ' ~ ' ~ ' " I

100 200 300 400 500 600 700

Temperature in OC

EMA36 SEDIMENT

d Ferrimagnetism (dashed)

? m

2 zooo/

i

Total Magnetism (solid)

Cycling 200 400 m i Mass 1 2 0 m g Maximum temperature 700 'C Heating rate 3 OC/min

to00 0 100 200 300 400 500 600 700

Temperature in OC

Figure 4. (a) SIC, and 12 SIG, (the contribution of the paramagnetic minerals to SIC,) before filtering. The field cycling interval is between 0.2 and 0.4T. The sample'is a Pliocene marine marl from Eraclea Minoa (Sicily, labelled EMA 36) without any magnetic concentration (van Velzen & Zijderveld 1990; Langereis & Hilgen 1991). Heating and cooling runs are indicated by arrows. Note the negative values for the magnetization-caused by the diamagnetic sample holder-at high temperatures. (b) The same sample after filtering. Heating and cooling runs are indicated with arrows. The calculated ferrimagnetic contribution is represented by a dotted line.

For the heating curve, this does not apply; when fitted from room temperature upwards (dash-dotted line) the measured curve starts to deviate from the paramagnetic hyperbola from approximately 400 "C. From about 550 "C upwards it follows the cooling curve, implying that a chemical reaction occurs in the matrix between 400 and 550 "C, possibly due to a dehydration reaction in clay minerals and/or chlorite. At high temperatures the measured signals become negative, i.e. diamagnetism of the sample holder dominates (see also Fig. 7). The calculated diamagnetism is compared to the measured diamagnetism of the empty sample holder in Fig. 6. It is apparent that the sample itself has a small diamagnetic contribution, compared to the sample holder alone. This clearly shows that the diamagnetism of the sample holder should be taken into account when dealing with weak signals.

Dominant diamagnetism does not hinder the acquisition of meaningful results, as illustrated (Fig. 7) by a thermomagnetic run using a sandstone from the Campo section in the Pyrenees, Spain (Pascual et al. 1992). The maximum field cycling interval (15-400 mT) was selected.

The complete run is dominated by diamagnetism of both the sample and sample holder. Nevertheless, a (slightly oxidized) magnetite clearly shows up. No major chemical reaction is apparent. Unlike the mark, the magnetite from the sandstone seems to survive heating up to 700 "C rather well because the cooling curve is largely reversible.

4.2 Chemical alteration

An example of how to discriminate chemical alteration from thermal demagnetization is given using a sample of late Miocene clay from the Crotone basin (southern Italy) (Fig. 8). The NRM is typically 15 rnAm-' . The magnetic mineralogy is likely dominated by an iron-sulfide. The thermomagnetic runs A , B , C and D were recorded one after another, each time to a higher maximum temperature of 300, 350, 450 and 600"C, respectively. It appears that in the temperature range between 250 and 350 "C chemical alteradon occurs because the heating and cooling curves A and B are not reversible. During the second heating (B) the alteration reaction sets in at a lower temperature: the heating

EMA36 MAGNETIC CONCENTRATE 0.25 A

2000 -'

12000--

10000 4

N

E 2 8000-- 0 .-. c C 6000-- 0

N - m m -

4000-- z: - m - I-" 2000:-

0 --

2000

. . . . . . . . . (4 2000 T

.-.

Ferrimagnetism (dashed)

Total Magnetism (solid) -~

Cycling 200 400 mT Mass 8 7 rng Maximum temperature 700 'C Healing rate 5 'Cimin Cooling rate 10 'Cirnin

. . l l ( l l . . , . I . . , . I . . . . . . I . . . , , , , . . i I .

0 100 200 300 400 500 600 700

(b) 1000

0

3000

N

E a m

0 2000

... B

2 too0

E

0 C - m 0)

- m 0 c -

0

1000


EMA36 MAGNETIC CONCENTRATE 1.5 A

.. .

Paramagnetism - Ferrimagnetism (dashed) Total Magnetism (solid)

Cycling 200 400 mT Mass 2 0 0 mg Maximum temperature 700 OC Heating rate 5 OCimin

I ' " ' l r ' ' ~ I ' ' ~ ~ , ~ , , ' , ' I , . , , . . . / S I

100 200 300 400 500 600 700 Temperature in OC

Figure 5. Thermomagnetic runs for two magnetic concentrates of Pliocene marine marl from site EMA 36 at Eraclea Minoa, heating and cooling curves are indicated with arrows. (a) Run of a strongly (ferri-) magnetic concentrate (0.25 A current in a Frantz isodynamic separator). (b) Run of a (less magnetic) concentrate extracted with a much higher current of 1.5 A . Chemical reactions involving oxidation processes occur when heating over 420 "C. For further explanation, see text.

EMA36 SEDIMENT

Paramagnetism and Diamagnetism

Figure 6. The paramagnetic contribution of the Pliocene marine marl sample from Eraclea Minoa fitted with hyperbolas to distinguish diamagnetism (temperature-independent) and true paramagnetism (obeying Curie's Law). The contribution of the diamagnetic sample holder to the signal is substantial. For further explanation, see text.

IC1403C SANDSTONE IN AIR

Temperalure I" OC o j , , , 1 y C , , , , 2?0 , , , , 3?0 , , , 4g0 , , , , 5e0 , , , , 6e0 , , , , 7 g C , ,

Cycling 15 400 rnT

Mass 100 mg Maximum temperature 700 "C Heating rate 5 oCimin

Cooling rate 10 OClmin

Figure 7. Thermomagnetic run of a sandstone from the Campo basin (Pyrenees, Spain, Pascual et al. 1992) showing that the instrument can also measure diamagnetism-dominated samples. From the Curie temperature, the ferromagnetic mineral in the sandstone is an oxidized magnetite.

670 T. A . T. Mullender, A. J . uan Velzen and M . J . Dekkers

Figure 8. An example of repeated thermomagnetic runs of late Miocene clay from the Crotone (South Italy) basin to illustrate the discrimination between true thermal demagnetization and chemical alteration of the magnetic mineralogy. Heating and cooling curves are indicated with arrows. Maximum temperatures are: for run A, 300°C: for run B, 350°C; for run C , 450°C; for run D, 600°C. Heating has been performed in air.

curve of run B starts to deviate from the cooling curve of run A at 260°C and not close to 300°C, the maximum temperature reached before. Between room temperature and 250 "C the cooling curve of run A and the heating curve of run B are exactly identical, an indication of the accuracy of the instrument (the same applies to the low temperature curves of runs B and C, and of C and D). From 400°C upwards magnetite is being produced, which is not oxidized as in the previous samples (run C). This may cause aberrant NRM behaviour during thermal demagnetization of palaeomagnetic samples. In run D, magnetite continues to be produced from 400 "C until its maximum at 480 "C, not only because of the approaching magnetite Curie temperature, but also as a result of the physical removal (oxidation) of the magnetite, because run D is clearly not reversible.

4.3 Metallic iron

Two thermomagnetic runs with material containing metallic iron are shown in Fig. 9. A high-temperature blast furnace slack shows a complex thermomagnetic behaviour; the Curie temperature of 770°C indicates metallic Fe (Fig. 9a). During the heating run, a minute discontinuity a t 580°C indicates the presence of magnetite; upon cooling this bump has disappeared, implying that the magnetite has been oxidized. The decay at approximately 200°C may be indicative of the presence of an Fe-Mn alloy with approximately 5 per cent Mn (Bozorth 1951). The Fe-Mn alloys are known to exsolve to some extent depending on their preparation method (notably the cooling rate), leading to different amounts of metastable phases. The increase at slightly higher temperatures (from 250 "C to 350 "C) may be due to the exsolution of Fe, bur needs further investigation.

Upon cooling the curve is largely reversible, implying that the metallic Fe is t o a large extent present as inclusions protected from contact with the surrounding air. A small

CS+ Pt + Pt contaminated

. ~~.

0 , I , . , , . , , , ? , , 7 . . . . / - , . . ; . , . . j , . . . i . . . . 7 0 100 200 300 400 500 600 700 800

Temperature in OC

Figure 9. Thermomagnetic runs of samples containing metallic Fe. (a) Industrial high-temperature blast furnace slack containing as little as 0.012 ml per cent metallic iron. (b) Pure platinum and (c) iron-contaminated platinum from a capsule for high-temperature- high-pressure experiments in a reduced atmosphere.

change in slope at 420°C suggests the presence of another (undetermined) magnetic phase in trace amounts. The amount of Fe in the original sample was 13.5 f 1 pg, based upon the curve between 600°C and 770°C (Bozorth 1951). In this way interference with the other (low temperature) magnetic phases is avoided. After the complete run 9.1 f 1 pg Fe remained in the sample. These quantities correspond to, respectively, 0.012 and 0.008ml per cent of the sample mass.

In Fig. 9 (b and c) the magnetic behaviour of Pt from a capsule used for high-pressure-high-temperature hydrother- mal experiments is shown before and after an experiment with Fe-bearing minerals. Platinum is known to take up iron rapidly, especially under reducing conditions. Before the experiment, no traces of any metallic Fe were detected, implying a high purity Pt. The susceptibility of the Pt is 1.2 X lo-' m3 kg-'. After the experiment (heating at 1200°C at 1 bar in an atmosphere of 70 per cent H, and 30 per cent CO, for 24 hr) the magnetic behaviour changed dramatically. The presence of a Pt-Fe c6mpound with a


Curie temperature of 400 "C can be clearly demonstrated; Fe forms a Pt-Fe compound with a distinct Curie temperature. Trial runs, not shown here, indicated completely reversible thermomagnetic behaviour up to 500°C. At higher temperatures diffusion takes place and changes the structure to a more ordered one. The Curie temperature indicates a composition of approximately 58atom per cent Pt (Gaf & Kussmann 1935; Fallot 1938), which would be in the two-phase area of the Pt,Fe and PtFe tetragonal superstructures (Crangle & Shaw 1962). This may explain the large difference in the saturation magnetization given by Graf & Kussmann (1935) and Fallot (1938): 51-61 and 33 Am2 kg-', respectively. To our knowledge, more recent data on compositional dependence of the saturation magnetization and Curie temperature d o not exist for the Fe-Pt compositional range of interest. Platinum solid solutions that contain only a few per cent Fe have Curie temperatures far below room temperature. More data on the compositional dependence of the saturation magnetization and Curie temperature in the Pt-Fe system is needed in order to quantify the Fe uptake by Pt by means of thermomagnetic analysis.

5 DISCUSSION The instrumental accuracy is tested elaborately and promises meaningful results. Yet the application of the separation method will only be valid if true ferrimagnetic saturation is reached. In order to estimate the possible errors in relation to the magnetic field values used we will investigate on the simplified assumption of easy-axis single domain grains. Briefly we will consider the effects on the signals of highly coercive minerals and finally will discuss possible improvements to the balance.

5.1 Condition of saturation We will analyse how an incompletely saturated ferrimagnetic mineral fraction interferes with the calculation of the saturation magnetization, M,, from SIG, and SIG,. For this purpose the field-dependent magnetization of the ferrimagnetic part M ( H , with H = B l c ) / p o is described by the law of approach to saturation. Furthermore, we will assume that our sample consists of an assembly of randomly oriented single domain (SD) grains with uniaxial anisotropy. In this case the law of approach to saturation can be described (K, is the uniaxial anisotropy constant) as (e.g. O'Reilly 1984):

M ( H ) = Ms(l - b / H 2 ) ,

with

b = KZ sin' 2 @ / p ; M i ;

where sin22@ has a mean value of 4/15 for directions uniformly distributed over a sphere. The relation between coercive force H, and K, can be expressed by H, = 0.958K,/poMs (O'Reilly 1984), hence b = 1.0896 x 4/15 x Hf, which provides a straightforward estimation for the value of b.

To estimate the spectral content of the time dependency of the signal M ( H ) , we will use a Taylor expansion at B,, [M;B) equals the nth derivative of M ( B ) ] :

M ( B o + AB) = M ( B o )

+ 2 M"(Bo) (AB)" /n! ( n = 1, 2, 3, . . .)

Because AB = B , cos of:

M@) = M(B( , ) + 2 M"(B, ) ) (B , cos wt)"/n!

According to Section 2, we qbtain the force < B , by multiplying M(B, with the field gradient dB,,)/dz = A(B,, + B , cos wt). Subsequently we rearrange in terms of (cos wt)". Thereafter, the terms (cos wt)" are substituted with their equivalent representation (cos wt)" = co, a,,, cos iwf. At the end this will result in:

x B , cos iwt,

where i, rn = 1, 2, 3, and 0 < h,ni < 2. If the ratio B , / B o < 1, the summation over rn will

converge. In that case, we may define c i = C(,m,[l + h,i(B,/BO)Z'". The force F B , can now be expressed as (where FB, = 4,)):

qf) = 4) + 2 (Ai + Bi(b/B;)ciB, cos iwt), (1 )

where i = 1, 2, 3, . . .

This can also be written as:

<,,=&)+ F, cos ~ j t + F,c0~2wt + . . . It follows that:

SIG, = MsBl cos wt{l + [ (b /B i )c , ] }

and

SIG,= M , B , C O S ~ ~ ~ [ - ~ / ~ ( ~ / B ~ ) ( B , / B ( , ) C , ] .

According to Section 2, the measured saturation magnetization M, and the measured susceptibility xpar are then equivalent to:

M, = Ms(l + 2b/B;c ' )

and

xpZlr = -O.jb/B;~+o.

The constants c,, c2 and c' will approach 1 for B I / B O < 1. Apparently, the relative error for M, will be 2b/B&'. For the more or less routine B , and B, values of 100mT and 300 mT respectively, the values of the constants c,, c, and c' will be approximately 1.1. Hence, it is straightforward to estimate the error in M, dependent on the value of H,. When H, amounts to 3 0 m T an error in M, will occur of approximately +1 per cent. Also an incorrect value of SIG, will occur, amounting to SIG, = 0.06 per cent of SIG,. This error is smaller than the instrumental resolution. Assuming a H, value of 50mT, this yields an error in M, of +2.5 per cent and a false contribution to SIG, of SIG, = -0.14 per cent of SIG, , which again is barely noticeable. A rather high H, value of 100 mT would cause an error in M, of + 10 per cent and a false contribution to SIG, of SIG,= -0.6 per cent of SIG,.

5.2 Highly coercive minerals

In the case of highly coercive minerals, approximation of saturation will not occur. The magnetization will be

612 T. A . T. Mullender, A . J . van Velzen and M . J . Dekkers

determined by the acquisition curve and reach a maximum value at B,,,. During cycling the magnetization will follow a minor hysteresis loop with moderate width and an effective positive slope (much) less than the slope of the acquisition curve at B,,,. The width of the minor loop will account for a phase shift in the signal contribution of the slope of the minor loop, this has little consequence for the signal strength. Following the same reasoning that counted for the comparison with the ‘classical’ signal, we may conclude that SIG, compares to the classical value at a constant applied field of between B,,, and (B,,, + Bmin). Therefore, SIG, represents the slope of the minor loop, not the slope of the acquisition curve between Bmi, and B,,,. The calculated M, has n o simple relevance to the magnetic characteristics and in the case of a very highly coercive mineral (goethite) can have a negative sign. W e conclude that both the ‘classical’ method and the proposed method fail equally in finding a quantitatively correct analysis.

5.3 Possible improvements

It is apparent that a n accurate determination of SIG, is crucial for the reliable calculation of the ferro-/ferrimagnetic contribution. Because the present design is based upon a modified conventional balance, additional reduction of noise is not possible. However, pilot experiments based upon a new compact design with extra isolation against low- frequency vibrations yielded a SIG, signal with at least an order of magnitude less noise than currently is the case. This would allow a very precise calculation of the ferrimagnetic contribution, even for the weakest samples. Also, sample holders that can be evacuated or filled with an inert atmosphere a r e currently being designed. Measurement of the third harmonic, to estimate the unknown parameters in the law of approach t o saturation, could relax the stringent criterion of saturation of the ferrimagnetic component. Model calculations showed that in this case a prolonged measurement duration (in the order of minutes) would be required.

CONCLUSIONS

The present contribution shows a novel method to measure thermomagnetic curves. Separation of the ferrimagnetic and paramagnetic components in the thermomagnetic runs can be realized, based on partial Fourier analysis of a cycling field. Because the field is cycled, the influence of noise and instrumental drift is strongly reduced. The noise level of the present instrument is only f 2 X lo-” Am2peak-to-peak value, corresponding to the saturation magnetization of some 20 ng of magnetite; it is two to three orders of magnitude lower than that of conventional horizontal Curie balances. This can be achieved in a normal laboratory environment. In the case of weakly magnetic rock samples (sediment, limestone) it is possible t o measure whole rock samples without the need of (tedious) concentration procedures. The high sensitivity allows separation of the susceptibility signal into a

true paramagnetic part and a diamagnetic part. When measuring weakly magnetic samples, the (well known) correction for the diamagnetic quartz-glass sample holder is crucial.

ACKNOWLEDGMENTS

MJD acknowledges the Royal Netherlands Academy of Sciences (KNAW) for a research fellowship. The comments of J . D. A. Zijderveld and C. G. Langereis on an earlier draft are appreciated. We thank two anonymous referees for their helpful comments.

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