EPSL, 2004, 229/1-2 pp. 31-43.

Maxime LE GOFF and Yves GALLET

A new three-axis vibrating sample magnetometer for continuous high-temperaturemagnetization measurements :

Applications to paleo- and archeointensity determinations

Paleo- and archeointensity studiesare fastidious and often unproductive

due to poor data quality

It is possible to make them fasterby measuring the magnetization

at high temperatures

TRIAXE SENSOR

must take into account the thermal dependence of the spontaneous

magnetization

High-temperature version of the Thellier and Thellier method revised by Coe

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Temperature (°C)

Mto

tal (

10

-8 A

m²)

NRM

T1 = 148°C

H = 0

High-Temperature Magnetization Measurements

Heating Troom to T1 (H=0)

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Temperature (°C)

Mto

tal (

10

-8 A

m²)

H = 0NRM

T1 = 148°C

T2 = 448°CCurve #1

High-Temperature Magnetization Measurements

Heating T1 to T2 (H=0)

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900

0 50 100 150 200 250 300 350 400 450 500

Temperature (°C)

Mto

tal (

10

-8 A

m²)

H = 0NRM

T1 = 148°C

T2 = 448°C

Curve #1

Curve #2

Curve #3

High-Temperature Magnetization Measurements

Cooling to T1 andHeating T1 to T2 (H=0)

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0 50 100 150 200 250 300 350 400 450 500

Temperature (°C)

Mto

tal (

10

-8 A

m²)

H = 70µT

H = 70 µTNRM

T1 = 148°C

T2 = 448°CCurve #1

Curve #2

Curve #3

High-Temperature Magnetization Measurements

Applying Hlab = 70µT

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900

0 50 100 150 200 250 300 350 400 450 500

Temperature (°C)

Mto

tal (

10

-8 A

m²)

NRM

T1 = 148°C

T2 = 448°C

Curve #1

Curve #2

Curve #3

Curve #4

High-Temperature Magnetization Measurements

Cooling T2 to T1 (H = 70µT)

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600

700

800

900

0 50 100 150 200 250 300 350 400 450 500

Temperature (°C)

Mto

tal (

10

-8 A

m²)

H = 0NRM

T1 = 148°C

T2 = 448°C

Curve #1

Curve #2

Curve #3

Curve #4

High-Temperature Magnetization Measurements

Hlab = 0µT

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400

500

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700

800

900

0 50 100 150 200 250 300 350 400 450 500

Temperature (°C)

Mto

tal (

10

-8 A

m²)

NRM

T1 = 148°C

T2 = 448°C

Curve #1

Curve #2

Curve #3

Curve #4Curve #5

High-Temperature Magnetization Measurements

Heating T1 to T2 (H=0)

R(Ti) = Hlab * 1(Ti) / 5(Ti)

R’(Ti) = Hlab * 1’(Ti) / 5’(Ti)

Remaining NRM (1) and TRM (5) fractions between Ti and T2

R ratio

Lost NRM (1’) and TRM (5’) fractions between T1 and Ti

R’ ratio

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TiT1 = 150°C

T2 = 450°C

Two possibilities to derive intensity data

NRM

TRM

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Temperature (°C)

R, R

' (µ

T)

R

R'

Test on a pseudo NRM acquired in a field of 50µT in the same conditions as the laboratory TRM (Hlab=50 µT)

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R

R'

Ancient NRM : R increasing, R’ much more constant

Ti, (°C)

R, R’ (µT)

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65

150 200 250 300 350 400 450

Temperature (°C)

R,

R' (

µT

)

NRM 0.5°/mn; TRM 25°/mn

NRM 0.5°/mn, TRM 6°/mn

NRM 0.5°/mn, TRM 2°/mn

Cooling rate effect

R’ must be considered

Correction for the TRM anisotropy

Comparison between archeointensity

data obtained from the Thellier and

Thellier method revised by Coe and

from the Triaxe

(Samples from Mesopotamia previously studied by Genevey et al., JGR, 2003)

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R

R'

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150 250 350 450

Selection criteria

• A large NRM fraction must be involved

• Expected behaviour of R and R’ curves

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Temperature (°C)

R

R'

Example of rejected sample

MR 15

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150 200 250 300 350 400 450

Temperature (°C)

R' (

µT)

Archeointensity results from a site of baked bricks from Mesopotamia (Mari, Syria)

6 samples

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-4000 -3000 -2000 -1000 0

Thellier Thellier - Coe

Triaxe Same Site

Triaxe New Site

Comparison between archeointensity results obtained from the Thellier and Thellier and Triaxe methods

H (µT)

Age (BC)

CONCLUSIONS

• The experimental procedure was successfully tested with archeological baked materials. We need now to work on volcanic samples.

• Other applications of the Triaxe :- Viscosity at high temperature.- Alteration.- Coupling between different magnetic phases.- etc.