Noble gases and cosmogenic radionuclides in the Eltanin Pacific meteorite Page: 3 of 4
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NOBLE GASES AND COSMOGENIC RADIONUCLIDES IN THE ELTANIN PACIFIC METEORITE
SUGGESTED AUTHOR ORDER: NISHIIZUNI, CAFFEE, BOGARD, GARRISON, KYTE
D. D. Bogard, and D. H. Garrison NASA-Johnson Space Center, Planetary Sciences, Houston, TX 77058. don-
ald.d.bogardl@jsc.nasa.gov
M. W. Caffee, Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, CA
94550. Caffeel@llnl.gov
F. Kyte, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567.
kyte@igpp.ucla.edu
K. Nishiizumi, Space Sciences Laboratory, University of California, Berkeley CA94720-7450.
Kuni@ssl.berkeley.eduAbstract: A 1.5 cm long, 1.2 g specimen of the
Eltanin meteorite was found at 10.97 m depth in Polar-
stern piston core PS2704-1 [Kyte, 2000 #4486]. The
early studies indicated that the small fragments of the
Eltanin meteorite was debris from a km-sized asteroid
which impacted into the deep-ocean basin [Kyte, 1985
#2195; Gersonde, 1997 #4030]. In this study, we
measured 3yAr-4 Ar age, noble gases, and cosmogenic
radionuclides in splits of specimen as a part of consor-
tium studies of Eltanin meteorite. We concluded that
the specimen was about 3 m deep from the asteroid sur-
face. The exposure age of the Eltanin asteroid was
about 20 Myr.
39Ar- Ar Age: A 45 mg sample of Eltanin was neu-
tron irradiated to determine its 3yAr-4 Ar age (Fig. 1).
The first ~35% of the 3yAr release suggests K contami-
nation (high K/Ca ratio) and major diffusion loss of
4 Ar. Most of the Ar was released in a broad peak over
850-13000C, where the measured Ar-Ar ages scatter
considerably and are all less than ~2 Gyr. Thus, all
Eltanin phases have been extensively degassed of 40Ar,
either by terrestrial weathering or by impact prior to
falling on earth or during impact with the earth. The
measured [Cl] concentration of our sample (determined
using 38Ar) was <10 ppm, so significant contamination
by seawater is not indicated.
Figure 1. 39Ar-40Ar ages and the K/Ca ratio as a
function of fraction of 39Ar releaseda
U'20 5
0.5.
0.00.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Cosmogenic and Trapped Noble Gases: Noble gases
were extracted in three temperature steps (500, 750, and1550 C) from a separate 22.5 mg sample of Eltanin and
analyzed on a different mass spectrometer (Table 1). 3He
signals were at blank levels, and the values given in
Table 1 are upper limits. The 4He concentration, how-
ever, is comparable to that observed in some other
mesosiderites [Schultz, 1989 #1516]. Neon and Ar are
primarily released at the highest extraction temperature,
which suggests that terrestrial noble gases do not con-
stitute a significant portion of the totals. The 20Ne/22Ne
ratio decreases from 12.45 at 500 C to 11.23 at 1550 C.
These ratios are higher than the atmospheric ratio of
9.81 and indicate the presence of solar wind or SEP Ne.
It is not clear how this solar Ne was introduced into
Eltanin.
Table 1. Noble gases in Eltanin (10-y cm3 STP/g)
Isotope 500 C 750 C 1550 C Total
3He 0.07 0.07 0.07 0.22
4He 1100 2800 650 4600
20Ne 2.4 2.5 19.6 24.5
21Ne 0.004 0.003 0.175 0.182
22Ne 0.19 0.22 1.74 2.16
Ne, -0 -0 0.13 0.13
36Ar 1.48 0.76 12.0 14.3
38Ar 0.37 0.17 2.38 2.92
40Ar 654 475 2717 3846
38Arc 0.10 0.03 0.14 0.27
132Xe n.m. n.m. 0.02 0.02
19Xe/132Xe 1.08 0.10
Eltanin contains cosmogenic 2Ne and 38Ar (Table
1).. These concentrations were calculated assuming two
component mixtures with trapped 21Ne/22Ne= 0.029 and
36Ar/38Ar =5.32, and cosmogenic (21Ne/22Ne) =0.9 and
(36Ar/38Ar)c =0.67. Only the 1550 C extraction released
significant amounts of cosmogenic 21Ne Calculation of
cosmogenic 38Ar at each temperature is sensitive to the
choice of trapped 36Ar/38Ar, and the values in Table 1
are rather uncertain. The low temperature release of
cosmogenic 38Arc may derive from oxidized metal, as
little metal phase remains in Eltanin. The 19Xe/132Xe
ratio at 1550 C is slightly higher than, but within un-
certainty the same as, this ratio in terrestrial Xe. The
low (3He/21Ne), ratio suggests that more than 50 % of
3He has been lost from Eltanin.. --
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Bogard, D D; Garrison, D H; Caffee, M W; Kyte, F & Nishiizumi, K. Noble gases and cosmogenic radionuclides in the Eltanin Pacific meteorite, article, January 14, 2000; California. (https://digital.library.unt.edu/ark:/67531/metadc702017/m1/3/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.