We did several rock magnetism measurements on meteorites in order to get the information about the magnetic minerals in meteorites and its character.
Instruments and Methods
Paleomagnetic Laboratory at Institute of Geology, Academy of Sciences of the Czech Republic
The paleomagnetic laboratory of the Institute of Geology,
Academy of Sciences of the Czech Republic is located close to Průhonice,
southeast from Prague, Czech Republic. The laboratory is equipped with
modern
instruments for paleomagnetic and rock magnetic studies including
state-of-art MAgnetic Vacuum Control System (MAVACS). MAVACS system is used for
sample precise thermal demagnetization. The construction of the system enables
instant response to external field variations to keep high magnetic vacuum (2 nT)
in the sample holder area during the heating and cooling phase. In general the
MAVACS construction is ideal for experiments in zero or low magnetic field,
however MAVACS can be also operated in manual mode enabling to manually trim the
magnetic field in the sample holder area. Manual mode is suitable for thermal
magnetic acquisition simulations and for magnetization stability research.
Magnetic susceptibility study of meteorites was measured using the KLY-2 and KLY-4S kappabridge (Geofyzika / Agico) instruments. The Agico KLY kappabridges enable the measurement of magnetic susceptibility in range from 0 SI up to 0.2 SI with precision of 3 x 10-8 SI. The frequency is 920 Hz and the field intensity is can be changed between 4 A/m and 300 A/m. This feature allows us to study field dependence of the magnetic susceptibility in some minerals. In combination with CS-L and CS-3 control units the instrument is capable of the measurement of magnetic susceptibility at low temperatures (from 80 K up to room temperature) and at high temperatures (from room temperature up to 973 K). This method is used for magneto-mineralogy studies and is very popular in geology and environmental problems.
As a cooling medium at low temperatures the liquid nitrogen is used. However the liquid nitrogen and gaseous nitrogen at low temperatures is paramagnetic. Due to this fact all nitrogen should be completely blown out using gaseous argon before starting the measurement and the influence of paramagnetic phase should be considered in interpretation especially at magnetically weak sample measurements. In general, the measurement of magnetic susceptibility at low temperatures is a non invasive method for sample magnetic composition study. The advantage is that the measurement is repeatable and the method prevents the phase changes in magneto mineralogy and lost of the sample magnetic record due to the extreme heating. These reasons make this method highly suitable for rare precious sample measurements as meteorites are. However, the sample should be strong enough to obtain sufficient signal to noise ratio. Meteorites are magnetically much intense than terrestrial rocks. However this condition is met using minimum 3 – 4 mm sized samples.
The interpretation of the measurements of the magnetic susceptibility at high temperatures is more complicated. In comparison to low temperature measurements, sample oxidation, mineralogical phase changes and magnetic record loss occurs at high temperatures. To reduce the material oxidation the gaseous argon can be applied into the sample holder during the measurement. However the flow of argon in the sample chamber is not ideal and some oxygen may still remains. The oxygen inducing the oxidation can be also in sample fractures and in crystal structure present. According to those facts, the measurement process is not repeatable and is less suitable for rare sample measurements.
The spinner magnetometers
JR-5A and JR-6 (Agico) are constructed for remanent magnetization measurements
of terrestrial rock samples shaped to cube (a = 1 cm) or cylinder (d = 2.54 cm,
h = 2.50 cm). However the meteorite samples subjected to measurements were from
1 mm to 5 mm in size. This problem has been solved using adjusted plastic box
for loose sample measurements. The box was filled up to half of its height with
non-magnetic plastic foam matter. The sample was placed in the middle of the box
onto foam fill and covered with the cover made of the same material. Once the
box is closed, the sample is fixed properly in the center of the box and the
change of the sample orientation due to the spinning measurement process is
avoided. The measuring routine for the sample in the adapter is the same as for
cube samples. The sample in the adapter can be easily replaced with another one
and the adapter can be used to measure multiple sample set. The magnetization of
the empty adapter is the same order of magnitude as the sample holder and is
compensated from all measurements.
The spinning speed of the sample holder during the measurement is 89.3 rev / s (JR-5A) and 89.3 rev / s or 16.7 rev / s (JR-6). The sensitivity of the instrument is 2.4 µA / m. The samples are measured in three positions in automatic mode and in six positions in manual mode. The integration time of the measurement is 10 s. The three measured components are used to calculate the vector of magnetization (absolute value, declination angle and inclination angle).
During measurements done on meteorite all of the samples were strong enough (same order of magnitude as terrestrial cube or cylinder samples) allowing us to measure with a precision from approx. 1 % (NRM) up to approx. 3 % (weak samples at highest demagnetization fields).
To test magnetization stability and to determine its components the demagnetization process is used. According to the fact, that the meteorite samples are rare, in most studies we used AF (Alternating magnetic Field) demagnetization process. During AF demagnetization process there is no change in magneto-mineralogy and no oxidation occurs. The Průhonice paleomagnetic laboratory is equipped with two LDA-3 AF demag. units (Agico). This instrument allows us to expose sample to the alternating magnetic fields from 1 mT up to 100 mT in steps of 1 mT. The sample is demagnetized in three perpendicular axes. The change of the position is done automatically. We used fast taper feature of the AF field and the time of residence in the AF field 10 s for each position. The AF demag field intensity steps were selected according to the magnetic character of each meteorite material. This instrument is also equipped with the selection of the shape of field taper (linear, parabolic, hyperbolic). On all samples the linear shape has been applied.
More information about the installed instruments can be found on laboratory pages.
Laboratory for Extraterrestrial Physics, NASA’s Goddard Space Flight Center
The Laboratory for Extraterrestrial Physics is located in NASA’s Goddard Space Flight Center, Greenbelt, Maryland, United States of America and is equipped with state-of-art instruments for rock magnetism and interplanetary matter research including vertical Superconducting Rock Magnetometer (2G Enterprises) and Vibrating Sample Magnetometer (Lake Shore) in combination with DC magnet (Varian) allowing real time automatic measurement of hysteresis loops.
Superconducting Rock Magnetometer is used for sample magnetization measurements. This state-of-art instrument has been build as a prototype for measurements of small particles as meteorite fragments are. The sensor is made of three perpendicular coils cooled with liquid helium to superconducting temperature. The electrical current in the coils is balanced to the rest state. The body of the instrument is shielded from terrestrial magnetic field variations and the only factor that can change the rest state of electrical current is the magnetized sample lowered among the coils. The change in the electrical current is proportional to the sample sample’s magnetization.
The vertical construction of the
instrument allows us to measure the magnetization under the temperature of
liquid nitrogen and to study the time stability of sample’s natural remanent
magnetization in different external fields by rising the sample step by step
from zero field inside the shielded instrument (sample holder in lower most
position) up to terrestrial field intensity (sample holder in upper most
position above the instrument). The intensity of magnetic field penetrating
inside the magnetometer’s tube in different levels has been measured prior to
the experiment for calibration purposes. The advantage of this instrument is
that the measuring routine is fast, static (sample is not rotating as in
spinning magnetometer), and that all three components of the sample’s
magnetization vector are measured at once (no need to change sample position
within the holder during measuring routine. Those facts allow us to measure
irregular (no need to balance the sample) or fragile samples.
Vibrating Sample Magnetometer is designed to measure hysteresis parameters and to construct hysteresis loops of rock samples. The external magnetic DC field can be set from 0 T up to 2 T. The precision of sample’s magnetization measurement on external field background is 10-4 Am2/kg. In combination with Windows® based control software the instrument is able to work completely automatically and to construct the hysteresis loop plots in real time. The fast measuring routine and quantity of user settings are great advantages of this system. In addition, the instrument can work in full manual mode suitable to saturate the rock samples for further experiments. For measurements of hysteresis parameters at the temperature of liquid nitrogen special adapter has been constructed.
The laboratory is also equipped with Schonsted furnace for sample thermal demagnetization. The temperature in sample’s chamber can be controlled from room temperature up to 800°C with precision of 1°C. The residue field within the heating and cooling chamber is 10 nT. In cooling chamber the magnetic field can be controlled from 10 nT up to 200000 nT. This feature allows us to manage lots of different magnetic acquisition experiments.
For AF demagnetization routine the laboratory is equipped with 2G AF demag. unit. Operation range of the instrument is from 1.3 mT up to 240 mT in steps of 0.1 mT. The time of residence in the AF field have been set to 3 s. In comparison to LDA-3 instrument the field control is more precise, the maximum AF field intensity is more than double (what is important to demagnetize very stable samples as Allende meteorite) and the field taper is more rapid, however the field control options are weaker (absence of taper shape selection and the AF taper speed selection).
Solid Earth Geophysics Laboratory at Division of Geophysics, University of Helsinki
Information about the installed instruments can be found on laboratory pages.
All three facilities are equipped with unique combination of modern instruments. The diversity of instrument types and principles is essential for running complex and diverse experiments.