Full text of DOI:10.1007/BF02900577

NOTES
ries and shock metamorphism of their parent bodies, and
hint at early evolution of the solar system. On the basis of
petrological and mineral chemical study of the Guang-
mingshan chondrite[5,6], here we report its cosmic-ray ex-
posure ages and gas retention ages.
Cosmic-ray exposure and gas
retention ages of the Guang-
mingshan (H5) chondrite
This meteorite fell in Guangmingshan Village,
WANG Daode1, LIN Yangting1 & LIU Jinyuan2
Zhunghe City, Liaoning Province (39°48Ą15ąN, 122°45Ą
50ąE) at about 08ꢀ00 am (Beijing time) on December 30,
1996. The stone broke into three pieces with a total mass
of 2.91 kg. According to its petrography and mineral
chemistry[5,6], the Guangmingshan chondrite consists
mainly of olivine (average Fa of 19.5 mol%), low-Ca py-
roxene (average Fs of 17.3 mol%), plagioclase (average
An of 12.1 mol%), kamacite (0.39%ü0.55% Co). Minor
minerals are troilite, diopside, apatite, whitlockite, taenite
and chromite. Observation of the polished thin sections
indicated that the chondrules are less abundant, but readily
delineated. No glass was found in the chondrules. Silicate
matrix is well recrystallized. The sample shows weak
shock metamorphism, and only some grains of olivine and
low-Ca pyroxene exhibit undulose extinction. Accordingly,
the shock grade was referred to as S1. Based on the taxo-
nomic parameters of olivine-kamacite compositions (oli-
1. Guangzhou Institute of Geochemistry, Chinese Academy of Sciences,
Guangzhou 510640, China;
2. Dalian Museum of Natural History, Dalian 116023, China
Abstract
Isotopic compositions of noble gases from the
Guangmingshan chondrite were analyzed. Based on the
analyses of cosmogenic nuclei, cosmic-ray exposure age of
the meteorite is (65f10.0) Ma (3He), (80f12) Ma (21Ne) and
(65f10.0) Ma (38Ar), with an average of 70 Ma. This is the
highest exposure age of H-group ordinary chondrites. Gas
retention ages of K-Ar and U, Th-4He are (4230f100) Ma
3
and (3300f60) Ma, respectively. The smaller ages of He
4
3
4
than 21Ne and He than 40Ar suggest that both He and He
lost together. This is probably related to a solar heating effect
of a meteorite with a small perihelion during the last expo-
sure period.
Keywords: noble gas, cosmic-ray exposure age, meteorite, chondrite,
Zhuanghe, Liaoning.
vine: 16.9 20.4 mol% Fa; kamacite: 0.40% 0.52%
ü
ü
Co)[7], the Guangmingshan meteorite was classified as H5
ordinary chondrite.
Cosmic-ray exposure ages of meteorites are deter-
mined from abundances of stable cosmogenic nuclei and
activities of radioactive ones[1]. The stable nuclei inte-
grated cosmic-ray flux during the exposure age of a mete-
orite in the space, and the radioactive nuclei recorded in-
tensity of the flux in a meteorite. In order to determine the
exposure ages of a meteorite, it is required to have con-
tents of certain noble gas isotopes and activities of radio-
1
Sample and experiments
Analyses of noble gases of the whole rock were
conducted in Institute of Physics, University of Bern,
Switzerland, following a method described by refs. [8, 9].
Sample of the meteorite (2ü3 g) without fusion crust was
crushed in a stainless steel martor to a grain size of <1
mm (usually 750 µm). The whole analysis system was
pre-heated and vacuumized. The sample, standard of
Mocs chondrite (L6) and an aluminium-slice were in-
stalled in the sample frame, and the system was heated to
3
active cosmogenic nuclei (e.g. He, 10Be, 22Na, 26Al, 36Cl,
39Ar, 40K, 41Ca, 53Mn, 60Co and 81Kr)[1, 2]. Chondrites are
the most primitive and complicated meteorites of the solar
system. They experienced varies of events in the solar
nebula (e.g. formations of chondrules, fine-grained matrix,
interstellar grains in the matrix, Ca-Al-rich inclusions and
mafic inclusions), and secondary processes in their aster-
oidal parent bodies (aqueous alteration, thermal and shock
metamorphisms)[3]. Up to date, cosmic-ray exposure ages
of a large number of meteorites have been determined[4].
The exposure ages of stony meteorites are less than 100
Ma. Among them, H-group has two peaks around 7 Ma
300ć and vacuumized again. After that, the sample was
heated in vacuum to 100ć to desorb any atmospheric
4
gases, and backgrounds of He and 40Ar were checked.
Before loading the sample, the molybdenum crucible was
heated by a heatronic furnace at 1700ć. Mocs standard
was first loaded in the molybdenum crucible and heated at
1700ć. Extracted He, Ne and Ar were analyzed using
mass spectrometers, and the results were compared with
the recommended values to check the measurement. The
sample of the meteorite was then treated and analyzed
under an identical condition as the standard. Extraction
and measurement of the noble gases were carried out us-
ing two different systems of A and B. System A has two
60° sector-type mass spectrometers and a Faraday cup. It
was used to analyze He, Ne and Ar. System B consists of
a noble gas extracting part and two metal-tube mass spec-
trometers equipped with secondary electron multipliers.
and 33 Ma, L-group varies between 20ü30 Ma, and
LL-group shows a peak of 15 Ma. Exposure ages of
stone-iron meteorites are within 10—100 Ma, and those of
iron meteorites are the highest (100—1000 Ma). The ex-
posure age peaks of chondrites probably indicate primary
breakup events of their parent bodies. In addition, the ex-
posure ages also give hints for burial depths of the mete-
orites in their parent bodies. Studies of noble gas isotopes
and exposure ages of meteorites will clarify thermal histo-
1544
Chinese Science Bulletin Vol. 46 No. 1 January 2001

One of the mass spectrometers is used to analyze He and
Ne, and the other for Ar. Analytical results of noble gas
isotopes of the Guangmingshan chondrite are given in
table 1. Analysis errors correspond to a confidence level
of 95%.
Table 1 Isotopic compositions of He, Ne and Ar of the Guangmingshan chondrite (cm3/g(CSTP))
4He
20Ne
40Ar
4He/3He
20Ne/22Ne
22Ne/21Ne
36Ar/38Ar
40Ar/36Ar
1650.6
1750f80×10ꢀ8
21.4f0.8×10ꢀ8
5150f100×10ꢀ8
17.10
0.863
1.143
0.923
3He 102.3f5.0, 21Ne 21.7f0.8, 22Ne 24.8f0.8, 36Ar 3.12f0.2, 38Ar 3.38f0.2.
lowing equations[1]:
2
Exposure ages
P3 = FH[2.09ꢀ0.43(22Ne/21Ne)c], FH = 0.98;
(4)
P
Cosmic-ray exposure ages of meteorites are deter-
P21 = 1.61FH[21.77(22Ne/21Ne)cꢀ19.32]ꢀ1, FH = 0.93; (5)
P
mined based on abundances of the cosmogenic unclei
produced by reactions between meteorites and galactic
cosmic ray. They can be calculated using the following
equation:
P38 = FH[0.125ꢀ0.071(22Ne/21Ne)c], FH = 1.08.
(6)
P
The production rates are related to the target compo-
sitions, and here FH is the correction factor for H-group
chondrites. The F values vary between 0.67ü1.00 for
Ts = Cs/Ps,
(1)
P
21Ne and 0.75ü1.10 for 38Ar among various chondrite
groups (CI, CM, CO, CV, H, L, LL, EH and EL). But, the
production rate of 3He is insensitive to the target composi-
tions, with a relatively constant F value (0.97—1.01). In
addition, the widely used shielding parameter is
(22Ne/21Ne)c ratio, which is correlated with (3He/21Ne)c.
This is based on reactions of 24Mg (n, D)21Ne and 25Mg (n,
D)22Ne. Production of 21Ne increases with the secondary
neutron flux, whereas that of 22Ne the is insignificant be-
cause relative abundance of 25Mg (10%) is much less in
comparison with 24Mg (79%). In case of ordinary chon-
drites, a high (22Ne/21Ne)c ratio (>1.20) corresponds to
irradiation of a small object with a radius of <10 cm,
whereas a low ratio (<1.10) suggests a burial depth of >10
cm in a body with a radius of >30 cm.
where Ts is the exposure age, C s is the concentration of
stable cosmogenic nuclide s (s can be He, 21Ne, or 38Ar,
3
cm3 S/g (STP)), Ps is the production rate (cm3/ggMa
P
(STP)).
First, the measured concentrations of noble gas iso-
topes were deconvolved into cosmogenic (c), trapped (tr)
and radiogenic (r) components, using the following expe-
riential ratios[9]: Her = 0, (4He/3He)c = 5, (20Ne/21Ne)c = 1,
(20Ne/22Ne)c = 0.8, (36Ar/38Ar)c = 0.65 and (40Ar/38Ar) =
0.2. Concentrations of cosmogenic 22Nec and 38Arc can be
calculated based on the following equations, respectively.
22Nec = 22Ne[1.1053ꢀ20Ne/22Ne/7.6],
(2)
(3)
38Arc = 38Ar[1.1343 ꢀꢁ0.2132(36Ar/38Ar)],
In order to determine the exposure ages, the produc-
tion rates of given cosmogenic nuclei are also required,
which can be determined by isotopic ratios between stable
and radioactive nuclide couples. However, the production
rates can also be changed by different sizes of the aster-
oids and shielding depths of meteorites buried in them.
Eugster[2] reported that the ratio of (20Ne/21Ne)c is the most
sensitive to the shielding effect, and suggested that the
ratio could be used as a shielding parameter or indicator to
Table 2 shows the calculated production rates and
3
exposure ages of He, 21Ne and 38Ar, based on eqs. (4)ü
(6). The exposure age of Guangmingshan is 65, 80, or 65
Ma, according to 3He, 21Ne and 38Ar, respectively, with an
average of 70 Ma. This is the highest exposure age of or-
dinary chondrites. Other ordinary chondrites fallen or col-
lected in China have exposure ages of 2.1ü40.9 Ma for
correct the production rates of cosmogenic He, 21Ne and
3
18 H chondrites, 2.58ü53.9 Ma for 11 L chondrites and
9.4, 16.8 and 41.3 Ma for three LL chondrites[8,11]. The
unusually high exposure age of Guangmingshan is proba-
bly related to its larger asteroidal body and resistant of
weathering in space. Iron meteorites have the highest ex-
posure ages (100—1000 Ma) of the known meteorites, an
example for such correlation between the exposure ages
and resistance against weathering.
38Ar. Nishiizumi et al.[10] reported a least squares fit equa-
tion with (3He/21Ne)c=21.77(22Ne/21Ne)cꢀ19.32, based on
the data of 138 chondrites. This correlation is true within
the range of 1.06—1.30 of (22Ne/21Ne)c ratio. The contents
of He and 21Ne can be used to calculate the production
3
rates or exposure ages if the analyses are plotted on or
close to the above correlative line. Others plotted below
the line suggest defusion loss of He, complicated expo-
U, Th-4He and 40K-40Ar gas retention ages are calcu-
3
4
sure histories, or effects of 2S exposure geometry of me-
teorites.
lated from concentrations of radiogenic Her (1238.5h
10ꢀ8cm3/g (STP)) (by subtracting cosmogenic He) and
4
Using the depth-sensitive (22Ne/21Ne)c ratio as the
shielding parameter and abundances of the target elements,
production rates of 3He, 21Ne and 38Ar (i.e. P3, P21 and P38
40Arr (5150h10ꢀ8cm3/g (STP)), and average bulk abun-
dances of K (780 Pg/g)[12], U (0.013 Pg/g) and Th (0.04
Pg/g)[13] of H chondrites. The gas retention ages of Her
4
P
P
P
cm3/ggMa (STP)) are calculated according to the fol-
(T4) and 40Arr (T40) are calculated using the following
Chinese Science Bulletin Vol. 46 No. 1 January 2001
1545

NOTES
equations[14]
:
3300 Maf60 Ma (T4). The smaller age of T4 than T40 is
T4 = 4Her(1238.5) = 75200[238U](e0.155[ t]ꢀ1)
usually due to a faster loss of 4He than 40Ar during heating.
+ 474[235U](e0.985[ t]ꢀ1)+56400[232Th](e0.0492[ t]ꢁꢀ1) (7)
The concentrations of cosmogenic He, 21Ne and 38Ar,
3
T40 = 1.805ln[40Arr/0.701hK+1],
(8)
their production rates, corresponding exposure ages, and
the gas retention ages (4Her, 40Arr) are given in table 2.
where t = T4. Using the data of the Guangmingshan chon-
drite, gas retention ages are 4230 Maf100 Ma (T40) and
Table 2 Concentrations of cosmogenic nuclides (cm3/g (STP)), production rates (cm3/ggMa (STP)) and cosmic-ray exposure ages (Ma)
3Hec
21Nec
38Arc
22Ne/21Ne
P3
P21
P38
T3
T21
T38
Tav
P
P
P
102.3h10ꢀ8
21.7h10ꢀ8
3.17h10ꢀ8
1.143
1.566
0.269
0.0486
65.3
80.7
65.2
70.4
4
4Hec = 511.5h10ꢀ8, Her = 1750h10ꢀ8ꢀ511.5h10ꢀ8 = 1238.5h10ꢀ8, T4 = (3300f60) Ma, T40 = (4230f100) Ma; errors of T3, T21 and T38 are
15%; Tav average cosmic-ray exposure age.
and 126Xe in chondrites based on 81Kr-Kr exposure ages, Geochim.
Cosmochim. Acta, 1988, 52(6): 1649.
3
Discussion and conclusion
2. Terribilini, D., Eugster, O., Mittlefehldt, L. W. et al., Mieralogical
and chemical composition and cosmic-ray exposure history of two
mesosiderites and two iron meteorites, Meteorit. Planet. Sci., 2000,
35(3): 617.
3. Brearley, A. J., Jones, R. H., Chondritic Meteorites (ed. Papike, J.
J.), Planetary Materials, Washington D. C.: Mineralogical Soc.
America, 1998, 1—5.
4. Marti, K., Graf, T., Comic-ray exposure history of ordinary chon-
drites, Annual Review of Earth and Planetary Sciences, 1992, 20:
221.
5. Lin, Y., Wang, D., Liu, J. et al., Two meteorite fall in Zhuanghe
City, Liaoning Province, China, Antarctic Meteorites, Tokyo:
NIPR, 2000, 67—68.
In the previous study of cosmic-ray exposure ages
and gas retention ages of 33 chondrites[14], we found that
some meteorites have T3 ages on the low side of the
analysis errors and the lower than exposure ages deter-
mined using other cosmogenic nuclei. The lower T3 ages
indicate the loss of 3Hec, and their T4 ages are smaller than
T40. Some other meteorites show lower T4 than T40. But,
their T3 ages are consistent with exposure ages of other
cosmogenic nuclei within analysis errors (r15%). On the
T3/T21ꢀT4/T40 diagram[8], analyses cluster into two groups:
(ν) meteorites with loss of both 3Hec and 4Her plotted on
6. Lin, Y., Wang, D., Liu, J. et al., Petrological and mineralogical
study of Zhunghe and Guangmingshan chondrites, Chin. J. Space
Sci. (in Chinese), 2001, 21(1): 1.
or close to a dotted line with a slope of 1. These meteor-
3
4
ites lost comparative fractions of Hec and Her together,
probably due to degassing by the solar heating in the case
of their small perihelions or intensive impact events
within their exposure ages. (ξ) Those with identical T3
and T21 within the analysis error (r15%), but the radio-
7. Wang, D., Rubin, A. E., Petrology of nine ordinary chondrites
falls from China, Meteoritics, 1987, 22(1): 97.
8. Wang, D., Yi, W., Eugster, O., A study on noble gases in the chon-
drites from China, Geochimica (in Chinese), 1992(4): 313.
9. Eugster, O., Michel, T., Niederman, S., Guangnan (L6) and
Ningqian (CV3): Exposure ages and radiogenic ages of two un-
usual chondrites, Meteoritics, 1988, 23(1): 25.
10. Nishiizumi, K., Regnier, S., Marti, K., Cosmic ray exposure ages
of chondrites, pre-irradiation and constancy of cosmic ray flux in
the past, Earth Planet. Sci. Lett., 1980, 50(1): 156.
4
genic He lost significantly. They are plotted within two
horizontal dotted lines on the T3/T21ꢀT4/T40 chart. Losing
4
the radiogenic He from the meteorites probably took
place before breakup of the asteroidal parent bodies.
Summarily, the Guangmingshan chondrite has a
T3/T21 ratio of 0.80 and T4/T40 ratio of 0.78, and it is close
to the dotted line with a slope of 1 on the T3/T21ꢀT4/T40
diagram. In addition, this meteorite has a relationship of
T3<T21 and T4<T40. Accordingly, we refer to Guangming-
shan as a meteorite of type (1). Furthermore, the Guang-
mingshan chondrite shows no significant shock effects.
We suggest that the loss of both 3Hec and 4Her may be due
to its small perihelion that makes it heated by the sun
within the exposure age of 70 Ma.
11. Wang, D., Wang, R., Noble gas and cosmic-ray exposure age of
Juancheng chondrite, Chin. Sci. Bull., 1999, 44(12): 1142.
12. Wang, D., Chen, Y., Petrology and chemistry of ordinary chon-
drites from China and their classification, Geochimica (in Chi-
nese), 1991(1): 13.
13. Mason, B., Data of Geochemistry, 6th ed., I Meteorites in Geo-
logical Survey (ed. Fleisscher, M.), 1979, 118.
14. Wang, D., Chen, Y., Li, Z. et al., Introduction to Chinese Meteor-
ites (in Chinese), Beijing: Science Press, 1993, 505.
(Received February 5, 2001)
Acknowledgements The authors are greatly indebted to Prof. Otto
Eugster and his colleagues in Institute of Physics, University of Bern,
Switzerland, for their support and help. This work was supported by the
National Natural Science Foundation for Distinguished Young Scholars
(Grant No. 40025311).
References
1. Eugster, O., Cosmic-ray production rates for 3He, 21Ne, 38Ar, 83Kr,
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Chinese Science Bulletin Vol. 46 No. 1 January 2001