Records of cosmogenic effects were studied in a large suite of Antarctic meteorites. The cosmogenic nuclide measurements together with cosmic ray track measurements on Antartic meteorites provide information such as exposure age, terrestrial age, size and depth in meteoroid or parent body, influx rate in the past, and pairing. The terrestrail age is the time period between the fall of the meteorite on the Earth and the present. To define terrestrial age, two or more nuclides with different half-lives and possibly noble gases are required. The cosmogenic radionuclides used are C-14, Kr-81, Cl-36, Al-26, Be-10, Mn-53, and K-40

Nishiizumi, K.

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N86-28824
TERRESTRIAL AND EXPOSURE HISTORIES OF ANTARCTIC METEORITES: K. Nishiizumi,
Dept. of Chem., B-017, Univ. of Calif., San Diego, La Jolla, CA 92093 (U.S.A.)
Records of cosmogenic effects (noble gases, radionuclides, and cosmic ray
tracks) have been studied in a large suite of Antarctic meteorites. The cosmo-
genic nuclide measurements together with cosmic ray track measurements (if pos-
sible) on Antarctic meteorites provide us. with information such as exposure age,
terrestrial age, size and depth in meteoroid or parent body, influx rate in
the past, pairing, and so on. The first two ages are of special interest.
The exposure age is the time period between meteorite ejection from its parent
body and capture by the earth. The cosmogenic nuclides are produced during
this period by interraction of cosmic rays in space. The terrestrial age is
the time period between the fall of the meteorite on the earth and the present.
To define both ages, two or more nuclides with different half-lives and pos-
sibly noble gas data are required.
In the present work, we emphasize the terrestrial age of Antarctic mete-
orites based on their cosmogenic radionuclides. The cosmogenic radionuclides
for this study are 14C (t^/2 = 5740 years) [e.g. 1,2,3], 81Kr (2.1xl05 years)
[e.g. 4,5], 36C1 (3.0xl05 years) [e.g. 6 7,8], 26A1 (7.05x10^  years) [e.g. 9,
10], 10Be (1.6xl06 years) [e.g. 11,12], 53Mn (3.7xl06 years) [e.g.13,14,15] and
40K (1.28xl09 years) [e.g. 16].
Terrestrial age; Figure 1 is a histogram of terrestrial ages of Yamato,
Allan Hills, and other Antarctic meteorites. The figure shows the average age
for same fall (pair) meteorites and does not include the so-called 3.5x10^
year peak for l^C age. [2], This figure indicates several features. The ter-
restrial ages of Allan Hills meteorites are widely distributed between 1x10^
and 7x10^ years and clearly show longer ages than both Yamato and non-Allan
Hills meteorites. This is expected because the meteorite surface density on
the Allan Hills blue ice is much higher than that at the Yamato mountain area.
All six Meteorite Hills meteorites which we have so far measured show terres-
trial age less than 1x10^  years. The shape of the two distributions is dif-
ferent. The terrestrial ages of Allan Hills meteorites show poisson distri-
bution and others show exponential distribution. It is not clear that the
difference is due to different accumulations mechanisms or sampling bias for
measurements. For Allan Hills meteorites, there is no clear correlation be-
tween the terrestrial age and their weathering features. It may be that wea-
thering is mainly dependent on exposure time at the ice surface. Meteorites
found on the west and north part of the Allan Hills blue ice region have shor-
ter terrestrial ages than meteorites found on the south and east part. This
trend is similar to the flow direction of the surface ice. Figure 1 also in-
dicates that the terrestrial ages of Allan Hills L-chondrites are widely dis-
tributed up to 7x10^ years compared to other types of meteorites. We are not
confident that this indicates a different influx rate of L-chondrites in the
past, because of inadequate statistics and sampling artifacts which were the
result of non-random selection of samples for measurement of terrestrial age
determination. Additional studies are required to clarify this.
Cosmic ray exposure history: There is no distinguishable difference be-
tween Antarctic meteorites and non-Antarctic meteorites with regard to expo-
sure age. The distribution of l^^ e exposure ages of Antarctic meteorites
[e.g. 17,18,19] is very similar to that of non-Antarctic meteorites [20].
Figures 2-a and 2-b show the histogram of -^ Mn results in L-chondrites (2-a)
and H-chondrites (2-b) of both Antarctic and non-Antarctic meteorites. The
https://ntrs.nasa.gov/search.jsp?R=19860019352 2020-03-20T14:43:15+00:00Z
, TERRESTRIAL AND •'EXPOSURE HISTORIES
72 * ' K. Nishiizumi X
•- *
~~* " % •
• - % •
values have not been corrected for undersaturation. These two histograms for
both classes of meteorites are surprisingly similar to each other. It is com-
pletely different from the histogram of nuclides with shorter half-lives, °^A1
[9,10] and 36C1. Figures 2-a and 2-b indicate that the terrestrial ages of
the Antarctic meteorites are usually short compared to the half-life of ^Mn,
and also that the 5^Mn contents of Antarctic meteorites are useful for the
study of meteorite exposure history. We found several interesting exposure
histories among Antarctic meteorites, for example, two stage irradiations for
ALHA 76008, Yamato 7301, 74028 and 74116 [6,14] and Solar Cosmic Ray effects
for ALHA 77005 [unpublished]. These discussions will be described elsewhere.
In general, we found no distinguishable differences between the average expo-
sure histories for the two sources of meteorites.
New methods; We have started two .new dating methods for measuring ter-
restrial ages of Antarctic meteorites and the history of the glacier. The
cosmogenic nuclide ^Ca with half-life of 1.0x10^  years can fill the gap be-
tween -^C and °1Kr, also ^Cl for measurements of the terrestrial age of An-
tarctic meteorites. The ^Ca in the metal phase is produced by high energy
spallation such as ^ ^Cl in the metal phase. The production rate of ^Ca is
insensitive to size and depth for a meteorite of normal size. The first ac-
celerator mass spectrometry measurement of this nuclide in the Bogou iron me-
teorite [21] demonstrates that ^lea-can be measured in 100-200 mg of meteor-
itic metal. The sample size will be reduced after improvements of the ion
source of the accelerator. We also demonstrated in situ production of -"-^Be
and- "Al in quartz samples [22]. The concentrations of both nuclides and the
ratio show the erosion rate and cosmic ray exposure age of the quartz on the
earth's surface [23] . The measurement of this pair in Antarctic quartz in-
dicates that how long the quartz was exposed on ice or snow. The first mea-
surements of 10ge and ^ Al in a quartzite which was collected at the peak of
Allan Hills shows that the top of Allan Hills was not covered by ice for the
last 1 million years [22]. The measurements of l^Be and 2"A1 in other quartz
collected at different locations are in progress.
The author wishes to thank colleagues D. Elmore at the Univ. of Rochester
and J. R. Arnold. This work was supported by NASA Grant NAG 9-33.
Refs; (1) Fireman E.L. (1980) Proc. Lunar Planet. Sci. Conf. 11th, 1215-1221.
(2) Brown R.M. et al. (1984) EPSL 67, 1-8. (3) Jull A.J.T. et al. (1984) Proc.
Lunar Planet. Sci. Gonf. 15th, JflR 89. C329-C335. (4) Freundel M. et al. (1983)
Meteoritics 18, 299-300. (5) Schultz L. and Freundel M. (1984) Meteoritics 19,
310. (6) Nishiizumi K. et al. (1979) EPSL 45. (7) Nishiizumi K. et al. (1981)
EPSL.52, 31-38. (8) Nishiizumi K. et al. (1983) EPSL 62, 407-417. (9) Evans
J.C. et al. (1979) Proc. Lunar Planet. Sci. Conf. 10th, 1061-1072. (10) Evans
J.C. and Reeves J.H. (1984) (abstract) LPS XV, 260-261. (11) Nishiizumi K. et
al. (1982) Meteoritics 17, 260-261. (12) Moniot R.K. et al. (1982) NIM 203.
495-502. (13) Nishiizumi K. et al. (1978) Mem. Natl. Inst. Polar Res., Spec.
Issue 8. 209-219. (14) Nishiizumi K. et al. (1980) ibid., r7_, 202-209. (15)
Goswami J.N. and Nishiizumi K. (1983) EPSL 64. 1-8. (16) Nitoh 0. et al. (1980)
Mem. Natl. Inst. Polar Res., Spec. Issue 17. 189-201. (17) Takaoka N. et al.
(1981) ibid. ^ 0, 264-275. (18) Nagao K. et al. (1983) ibid. 30, 349-361. (19)
Weber H.W. et al. (1983) Z. Naturforsch. 38a, 267-272. (20) Schultz L. and
Kruse H. (1983) Compilation, Mainz, pp.88. (21) Kubik P.W. et al. (1985)
Nature (submitted). (22) Nishiizumi K. et al. (1985) Nature (to be submitted).
(23) Lai D. and Arnold J.R. (1985) Proc. Indian Acad. Sci. 94, 1-5.
TERRESTRIAL AND EXPOSURE HISTORIES
K. Nishiizumi 73
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. 1 Terrestrial Age (10 y)
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Terrestrial and exposure histories of Antarctic meteorites

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