Lunar and Planetary Science XLVIII (2017)
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MODEL OF THE GALACTIC COSMIC RAY INTENSITY VARIATIONS DURING THE LAST BILLION
YEARS. V.A. Alexeev, Vernadsky Institute of Geochemistry and Analytical Chemistry, RAS, Moscow 119991
Russia; e-mail: [email protected]
Abstract: The analysis of the distributions of the
К/К exposure ages (TK; N = 80) and radionuclidestable nuclide exposure ages (TRS; N = 106) of the iron
meteorites is executed. The model of variations in the
intensity of galactic cosmic rays (GCR) during the past
~1-1.5 billion years is proposed.
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Distributions: The distribution of TRS ages has exponential form, similar to the same for the ordinary
chondrites [1], but it is totally different from the distribution of the TK ages (Fig. 1).
The method of dating with the use of the short-lived
nuclides has substantial higher sensitivity. This allows
to determine the ages up to TRS ~100 Ma with the accuracy ~5% [8]. And this fact, apparently, is determining
in completely different picture of the distribution of the
ages, calculated by this method (Fig. 1b). Such “artificial” selection of meteorites with respect to the ages
makes the set of the meteorites, for which were determined the 40K ages, unrepresentative. This fact calls
doubt both on the possibility of the attraction of this set
of ages for studying the variations in the intensity GKL
and on correctness of the conclusions published in [911].
Difference of the ages: TK ages are systematically
higher than the TRS ages approximately in 1.5 times
(Fig. 2).
Fig. 1. Distributions of the exposure ages of iron meteorites. (a) Ages are determined by 40K/K method; 1 –
all meteorites (N = 80, according to the data [2-4]; 2 –
the same after the exception of paired meteorites (N =
29). (b) Ages are determined according to the shortlived radionuclides; 3 – all meteorites (N = 106, according to the data [5-8 and other]; 4 – the same after
the exception of paired meteorites (N = 45).
The difference between TK and TRS distributions,
most probably, is caused by high errors in the T K ages
(up to 100% and more) at the low abundances of the
cosmogenic 40K. For this reason, for example, Voshage
et al. [3] could determine the exposure age only of 10
meteorites out of 31 investigated meteorites. According
to [3], "spectra with 40K-abundances ≤4% can barely
provide useful information", and such spectra were not
used for age determination.
Fig. 2. Comparison of the exposure ages, determined
by 40K/K method (TK) with ages based on short-lived
radionuclides (TRS). Solid line is regression line (T K =
kTRS). Dotted line corresponds to the value k = 1.
Voshage and Hintenberger [12, 13] proposed to explain this effect by change of the cosmic-ray intensity
according to relationship IT = Io∙exp(-γT) in entire interval of ages. (Here Io is a modern cosmic ray intensity.) However, we found that this model does not give
the exponential distribution of TRS ages shown in Fig.
1b. For explaining of difference between TK and TRS
ages, we proposed the model according to which the
intensity of GCR (IT) in the time interval of 0-1500
millions years exponentially increased according to the
relationship: IT = IT=1500(1 + α·exp(-βT)) (Fig. 3).
Lunar and Planetary Science XLVIII (2017)
Fig. 3. Relative change in the intensity GKL according
to the dependence IT = IT=1500(1 + α·exp(-βT)). Values
of β (in Ma-1) and α for the curves 1 – 5 are equal to: 1.
0.008 and 2; 2. 0.01 and 1.5; 3. 0.012 and 1.2; 4. 0.016
and 1; 5. 0.03 and 0.8 respectively.
This model satisfies to experimental data – to difference of the TK and TRS ages in ~1.5 times and to the
exponential form of the distribution of T RS ages. For
one of the versions of this model, the intensity of GCR
exponentially increased two times during last ~300
million years with the approximately constant intensity
in the remaining time (Fig. 4). The data obtained according to this model testify about the proximity of the
measured TK ages to the real time of the presence of
meteorites in space.
Conclusions: The distribution of all values of
K/K ages (N ~80) is not representative. Therefore the
use only of this set of ages for studying of the galactic
cosmic rays intensity variations is unacceptably and so
the correctness of conclusions is doubtful. The features
of the distributions of the 40K/K exposure ages (TK)
and radionuclide-stable nuclide exposure ages (TRS) of
the iron meteorites can be explained according to the
proposed model of variations of the galactic cosmic ray
intensity as IT = IT=1500 (1 + α·exp(-βT)) during the last
~1500 million years.
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Acknowledgement:This work was supported in
part by Fundamental Research Program no. 7 of the
Russian Academy of Sciences.
References: [1] Alexeev V.A. (2005) Solar System
Res., 39, 124-149. [2] Voshage H., Feldmann H.
(1979) EPSL, 45, 293-308. [3] Voshage H. et al.
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Fig. 4. Distributions of the radiation ages of iron meteorites. (a) The model set of “true” values of ages (N =
200), randomly distributed in interval 0-1500 Ma; the
number of meteorites decreases with the age according
to equation (1) for the mean life of iron meteorites in
space τ = 700 Ma; (b) “measured” values of TK- ages
for the model set on the assumption that intensity GCR
changed according to the version, shown in Fig. 3
(curve 4). (c) Solid line is the same for “measured”
values of TRS-ages. Dotted line is the normalized exponential curve, which characterizes the real distribution
of the TRS-ages of iron meteorites (see Fig. 1b, curve
4). Distributions are approximated by the “best” exponential curves according to the equation (1).
(1983) Z. Naturforschg., 38a, 273-280. [4] Voshage H.
(1984) EPSL, 71, 181-194. [5] Lipschutz M.E. et al.
(1965). J. Geophys. Res., 70, 1473-1489. [6] Chang
C.T., Waenke H. (1969) In: Meteorite Research (ed.
by P. Millman). D.Reidel Publ. Company, Dordrecht,
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(1979) EPSL, 42, 348-358. [8] Lavielle B. et al. (1999)
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