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The interpretation of magnetic noise component in lunar sample magnetic measurement |
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Kamenikova, T. (1), Kletetschka, G. (2) |
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(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,
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The paleomagnetic 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 [1]. 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 mineral [4]. 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 [5], 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 [5]. The proposed method is modification of the method for establishing paleomagnetic field intensity [6]. The lunar rocks magnetic carrier is mostly iron minerals [7]. 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) [5]. 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 [5]. We discuss that the level of magnetic noise depends on magnetic carrier [8]. Kamacite provides noise level at 30000 nT, taenite 10000 microtesla, and Troilite 3 nT.
[1] Weiss B. P. and Tikoo S. M. (2014) Science 346, 1198. [2] Kletetschka G. et al. (2004) EPSL 226, 521-528. [3] Kletetschka G. and Wieczorek M. A. (2017) PEPI 272, 44-49. [4] Kletetschka G. et al. (2002) Tectonophysics 347, 167-177. [5] Kamenikova T. et al. (2018) LPSC 49, No. 1389. [6] Kohout, T., et al, (2008) Studia Geophysica Et Geodaetica, 52(2), 225-235, doi:10.1007/s11200-008-0015-1. [7] Oliveira J. S. et al. (2017) JGRPlanets 122, doi:10.1002/2017JE005397. [8] Kletetschka G. et al. (2017) PEPI, 272, 44-49, doi:https://doi.org/10.1016/j.pepi.2017.09.008 |
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