interpretation of magnetic noise component in lunar sample magnetic
(1), Kletetschka, G. (2)
(1) Faculty of Science, Charles University, Czech
Republic; Institute of Geology, Academy of Sciences of the Czech
Republic, Czech Republic; (2) Institute of Geology, Academy of
Sciences of the Czech Republic, Czech Republic; Faculty of Science,
Charles University, Czech Republic; Department of Geology and
Geophysics, University of Alaska Fairbanks, USA,
measurements of the lunar samples were based on Natural Remanent
Magnetization (NRM). Because it was shown that any heating of Lunar
samples causes irreversible changes to their paleomagnetic record, new
methods were designed, analyzing NRM without heating . Magnetic
characteristics stem from magnetic minerals involved in sample, their
grain size, temperature, strain and aspect ratio [2, 3]. There are two
existing ways for the crustal rock sample how to record the
paleomagnetic information. The chemical process when magnetic grain is
growing through the blocking volume of homogeneously distributed
magnetic dipoles. The magnetic minerals will interact with the
magnetic field, in case the field will be present at the stationary
temperature. The acquired magnetization by this process is called
chemical remanent magnetization (CRM). The second process is for
cooling magnetic grain of constant volume through the blocking
temperature, when fluctuation of the magnetic moments interacts with
the external magnetic field, in case the field is present. This
acquired magnetization is called thermal remanent magnetization (TRM).
Both magnetizations achieve similar efficiency for specific magnetic
This work develops a new method which don’t involve the sample
heating and estimates an amount of magnetic noise in the Lunar
samples. This method is based on the following logic.
Lets assume that the sample A has not seen magnetic field, when it was
formed. The sample should be completely demagnetized and contain just
magnetic background M(A). The first step of our approach is to take
sample A and demagnetize it by 1 mT, 10 mT, 100 mT, 1000 mT and the
overall magnetization M(A(AF)) should be constant magnetic background
, magnetic noise.
Then we check if this sample contains magnetizing magnetic carriers.
When the sample is saturated by pulse or constant external homogeneous
magnetic field, all of the demagnetized magnetic grains of the sample
are contributing towards one overall magnetic dipolar field that can
be detected from out side of the sample. Such sample contains maximum
saturation remanence MS(A). When sample is step-wise demagnetized,
observation of its monotonous magnetic decay is evidence that sample
contains magnetizable magnetic carriers. Demagnetizing curve itself
from its saturated value is monotonous down to its demagnetized state
MS(A(AF)). Ratio between these two sequences M(A(AF))/MS(A(AF)) has a
special case when M(A(AF)) function is constant (=magnetic noise) and
is divided by monotonously decreasing function MS(A(AF)). Then the
overall result is function that monotonously increases. And this
monotonous trend is central for our test for magnetic noise presence
in lunar samples . The proposed method is modification of the
method for establishing paleomagnetic field intensity .
The lunar rocks magnetic carrier is mostly iron minerals . In case
these minerals contain superparamagnetic grains, they are vulnerable
to viscous magnetization when is exposed to geomagnetic field.
Carriers of this magnetization have very low magnetic coercivity. Such
magnetization is removed when demagnetizing the sample by using the
lowest amplitude of the demagnetizing alternating field (usually up to
5 mT) .
We tested sample of lunar breccia chipped by Apollo 15 mission. The
sample 15445.277 was fragmented. We had 7 subsamples, one thin
section, one of these subsamples contained only dust as a residue from
separation for control of magnetic noise.
The noise/viscosity detection procedure was applied for all fragments.
Surprisingly, all subsamples displayed monotonously increasing
function. The 4 fragments and thin section showed a magnetic noise
only (monotonously increasing function), 3 fragments with the highest
sample masses were partly induced by viscous magnetization and
contained superparamagnetic component overprinted on magnetic noise.
It was possible to show with magnetic data that all sub-fragments of
15445 without SP that they contain magnetic noise and did not record
any level of magnetic field during their formation . We discuss
that the level of magnetic noise depends on magnetic carrier .
Kamacite provides noise level at 30000 nT, taenite 10000 microtesla,
and Troilite 3 nT.
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