Geochimica et Cosmochimica Acta, Vol. 63, No. 2, pp. 175–192, 1999
Copyright © 1999 Elsevier Science Ltd
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0016-7037/21801 $20.00 1 .00
Pergamon
PII S0016-7037(99)00012-5
Relationships among lodranites and acapulcoites: Noble gas isotopic abundances, chemical
composition, cosmic-ray exposure ages, and solar cosmic ray effects
ANDREAS WEIGEL1,* Otto EUGSTER,1 Christian KOEBERL,2 Rolf MICHEL,3 Urs KRÄHENBÜHL,4 and SONJA NEUMANN3
1
Physikalisches Institut, Universität Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland
2
Institut für Geochemie, Universität Wien, Althanstrasse 14, A-1090 Wien, Austria
Zentrum für Strahlenschutz und Radioökologie, Universität Hannover, Am Kleinen Felde 30, D-30167 Hannover, Germany
4
Departement für Chemie und Biochemie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland
3
(Received April 2, 1998; accepted in revised form January 8, 1999)
Abstract—Noble gas isotopic abundances of ten lodranites (EET84302, FRO90011, Gibson, LEW86220,
LEW88280, Lodran, MAC88177, QUE93148, Y74357, Y791491) and four acapulcoites (Acapulco,
ALH81187, ALH81261, ALH84190), as well as major, minor, and trace element compositions of six
lodranites (EET84302, Gibson, LEW88280, Lodran, MAC88177, Y791491), are reported. Because existing
empirical production rate models for cosmic-ray-produced nuclides in achondrites could not account for the
effects of bulk chemical composition and for the unique shielding conditions in lodranites and acapulcoites,
we modeled the production rates of cosmogenic nuclides in lodranites and acapulcoites by galactic and solar
cosmic rays using a purely physical model. All lodranites and acapulcoites are relatively small meteorites
having preatmospheric radii # 200 mm, one-half of them even #75 mm. Evidence was found for solar-cosmic
ray produced nuclides in the acapulcoites ALH77081, ALH81187, ALH81261, and ALH84190. The derived
cosmic-ray exposure ages of all lodranites (with the exception of QUE93148 with 15 Ma) and all acapulcoites
cluster around 6 Ma, suggesting, supported by the similar abundances of cosmogenic nuclides, similar
shielding conditions, and similar chemical compositions, that they all originate from one ejection event from
the same parent body.
Within error limits identical abundances of cosmogenic nuclides, identical shielding conditions, and
identical cosmic-ray exposure ages support pairing between ALH77081 and ALH81261, and ALH81187 and
ALH84190. Copyright © 1999 Elsevier Science Ltd
concluded from their petrological study that Lodran is a partially differentiated meteorite, which—to a great extent— has
lost minerals that have a low melting point, and which, due to
a reverse zoning of olivine, must have gone through a process
of reduction. Thus far the only chemical analysis of Lodran was
done by Fukuoka et al. (1978). The lack of alkaline elements
and rare Earth elements indicated to them a loss of partial,
feldspar rich melt. The achondritic texture and the primitive,
non-differentiated composition similar to chondrites prompted
Prinz et al. (1978) to classify Lodran as a primitive achondrite.
The Acapulco meteorite fell in 1976 on the outskirts of
Acapulco, State of Guerrero, Mexico (Sean, 1978; Graham,
1978). Preliminary petrographical and mineralogical studies
(Christophe Michel-Lévy and Lorin, 1978, 1979) revealed a
highly recrystallized texture, a unique mineral composition,
and similarities to ALH77081 in many respects (Mason, 1978).
The Gibson meteorite was found in October 1991 north of
Esperance, Western Australia (Wlotzka, 1992). The Monument
Draw meteorite was found in 1985 near Monument Draw,
Andrews County, Texas, USA (Wlotzka, 1992).
All other acapulcoites and lodranites were recovered in Antarctica: FRO90011 and FRO95029 were found by the European Search for Meteorites consortium (EUROMET ) and is
curated at the Open University, Milton Keynes, UK. The Yamato meteorites Y74063, Y74357, Y75274, Y791491,
Y791493, and Y8002 were collected by the National Institute
of Polar Research, Tokyo, Japan. ALH77081, ALH78230,
ALH81187, ALH81261, ALH81315, ALH84190, EET84302,
GRA95209, LEW86220, LEW88280, MAC88177, and
QUE93148 were collected through the antarctic search for
1. INTRODUCTION
A decade ago, the two unique samples Lodran and Acapulco
were the only meteorites besides the angrites (Papanastassiou,
1970) and the winonaites (Prinz et al., 1980) that were classified as primitive achondrites. Thanks mainly to the Antarctic
meteorite collection programs the collection of lodranites now
includes EET84302, FRO90011, FRO95029, Gibson,
GRA95209, LEW86220, LEW88280, Lodran, MAC88177,
QUE93148, Y74357, Y75274, Y791491, Y791493, Y8002,
and the collection of acapulcoites Acapulco, ALH77081,
ALH78230, ALH81187, ALH81261, ALH81315, ALH84190,
Monument Draw, and Y74063.
Based on petrographic descriptions, the lodranites Y75274,
Y8002 (Yanai et al., 1984), and Y791491, Y791493 (Mason,
1983), and the acapulcoites ALH77081, ALH78230,
ALH81261, ALH81315, and ALH81187, ALH84190 (Mason
et al., 1989) are considered to be paired, meaning that they
originate from the same meteoroid.
The Lodran meteorite fell in 1868 (Oldham, 1869). Tschermak (1870) described it as a granular, friable meteorite composed of roughly equal amounts of metal, olivine, and pyroxene
with minor amounts of troilite, chrome-diopside, and chromite.
Prior (1916) stated that Lodran is a unique stony-iron meteorite
that is closely related to the ureilites. Zähringer (1968) was the
first to analyze noble gases in Lodran. Bild and Wasson (1976)
*Author to whom correspondence should be addressed
([email protected]).
†
Present address: Department of Chemistry, University of California
San Diego, La Jolla, CA 92093-0317, USA.
175
176
A. Weigel et al.
meteorites program (ANSMET) and are made available
through the Meteorite Working Group of NASA/NSF, Houston, Texas, USA.
On the basis of petrological observations, Nagahara and
Ozawa (1985) pointed out that there may be a genetic connection between acapulcoites and lodranites. Acapulcoites and
lodranites very likely originate from the same parent body and
represent zones of various degrees of melting. Acapulcoites
were heated slightly above the eutectic temperature of Fe,NiFeS (;980°C) (McCoy et al., 1992); the silicates in lodranites
were partially melted at somewhat higher temperatures (1000
to 1050°C). Numerical models were used to determine whether
or not acapulcoites and lodranites could have originated from
the same precursor material and to provide models that explain
the measured amounts of trace elements and their evolution
(Petaev et al., 1994a, 1994b, Morikawa and Nakamura, 1994).
Lodranites and acapulcoites can be differentiated from other
classes of meteorites by their mineralogical and petrological
characteristics (McCoy et al., 1993) and their oxygen isotope
ratios (Clayton et al., 1992). In an oxygen isotope plot, all
samples of Martian meteorites, all howardites, eucrites, and
diogenites (HED) and all aubrites lie on their respective mass
fractionation lines with a slope of 0.52 whereas no mass fractionation trend is seen for the acapulcoites and lodranites.
Lodranites and acapulcoites can be distinguished from each
other in three ways: (a) acapulcoites are more finely grained
than lodranites; (b) acapulcoites have a higher plagioclase
content than lodranites; (c) lodranites have less troilite than
acapulcoites (McCoy et al., 1993).
Asteroids located between the orbits of Mars and Jupiter are
the most likely parent bodies of many meteorite classes. They
are classified by the reflection spectra of sunlight on their
surfaces. The only presently known direct method of determining the parent body of meteorites is to compare the reflection
spectra of meteorites measured in the laboratory with those of
asteroids. This method was used when 4-Vesta was suggested
to be the parent body of howardites, eucrites, and diogenites
(McCord et al., 1970). Until recently, S-type asteroids, which
are the second most commonly observed ones, were assumed to
be possible parent bodies for the ordinary chondrites (Wetherill
and Chapman, 1988). However, the reflection spectra of the
S-type asteroids do not compare well with those of the ordinary
chondrites (McFadden et al., 1985). Recent research on the
reflection spectra of primitive achondrites, especially lodranites
and acapulcoites, indicate that these correspond more closely to
the S-type asteroids (Hiroi and Takeda, 1991). However, asteroid surface layers due to space weathering as thin as 1 mm
already alter the reflectance spectrum of an asteroid (Wetherill
and Chapman, 1988). Therefore, similar reflectance spectra of
asteroids and meteorites are not conclusive evidence for a
common origin.
We analyzed 10 lodranites (EET84302, FRO90011, Gibson,
LEW86220, LEW88280, Lodran, MAC88177, QUE93148,
Y74357, and Y791491) and four acapulcoites (Acapulco,
ALH81187, ALH81261, and ALH84190). Our aims were to:
1. Determine the cosmic ray exposure age from the concentrations and the production rates of stable cosmogenic isotopes
3
He and 21Ne.
2. Determine the chemical composition necessary to refine the
production rate calculations.
3. Calculate the production rates of cosmogenic nuclides with
a purely physical model describing the depth- and sizedependent production of cosmogenic nuclides in meteoroids
by galactic cosmic ray protons (Michel et al., 1991).
4. Explore whether lodranites and acapulcoites contain solar
cosmic ray (SCR) produced nuclides.
5. Identify a possible genetic relationship between lodranites
and acapulcoites and among them (pairings).
Preliminary results of the analyses presented here have been
discussed in abstract form by Eugster and Weigel (1992, 1993),
Weigel and Eugster (1993, 1994), Weigel et al. (1994, 1995,
1996, 1997), and Nagao et al. (1995).
2. EXPERIMENTAL PROCEDURES AND RESULTS
All meteorite samples were taken more than 10mm away
from the fusion crust and were crushed in a stainless steel
mortar to a grain size of ,750 mm.
One sample of Lodran and one sample of ALH81261 were
received in powder form, and the QUE93148 sample, due to its
heterogeneity, was delivered as silicate and metal fractions.
After having analyzed a whole rock chip of Lodran for He,
Ne, and Ar isotopic abundances, the remaining samples of
Lodran were separated into major (olivine, pyroxene, metal)
and minor (chromium diopside, troilite) mineral fractions and
residues (silicates . 50 mm, silicates , 50 mm).
2.1. Noble Gases
He, Ne, and Ar concentrations and isotopic ratios were
measured on whole rock samples with the exceptions mentioned above. Before noble gas extraction, the samples were
heated at about 100°C in a vacuum for 10 days in the storage
arm of the extraction system to degas adsorbed atmospheric
gases. The noble gases were extracted from the sample by
inductive heating to 1700°C or as indicated in the tables.
Details on our mass spectrometer systems, blank corrections,
etc., were described by Eugster et al. (1993).
2.2. Bulk Chemical Composition
For most lodranites no literature data on the chemical composition that is relevant for the calculation of the production
rates were available. Therefore, aliquots of the samples for the
determination of the noble gases were used for the determination of the chemical abundances.
2.2.1. Bern analyses
Sample aliquots of Lodran metal, Lodran silicate,
MAC88177, and Y791491 were sealed in suprasil vials and
irradiated for 48 h by thermal neutrons at 2 3 1013 n cm22s21.
After the measurement of the g-activity of Na, K, Ca, Sc, Cr,
Mn, Fe, Co, Ni, Zn, As, and La the two samples were dissolved
in a mixture of concentrated HF and HClO4 in presence of the
respective carriers followed by a radiochemical separation for
the measurement of the other elements listed in Table 5. Due to
the delayed measurement of the radiochemically separated ac-
Lodranites and acapulcoites
177
Table 1. He, Ne, and Ar concentrations and isotopic ratios of lodranites.
4
20
He
Weight
(mg)
EET84302
FRO90011
Gibson
Gibson
Gibson
LEW86220
LEW88280
Lodran
MAC88177
MAC88177
MAC88177
QUE93148
Fe/Ni
QUE93148
Silicate
QUE93148
Y74357
Y791491
Y791491
Y791491
Y791491
40
Ne
28
10
Ar
4
20
22
40
3
22
21
36
He
He
3
cm STP/g
Ne
Ne
Ne
Ne
Ar
Ar
36
38
Ar
Ar
31.18
20.18
999.14
31.64
Av.
21.86
20.43
36.13
33.63
20.39
Av.
19.95
537 6 27
826 6 46
733 6 22
777 6 40
743 6 19
216 6 8
116 6 4
48 6 1
960
560
760
56 6 2
1.60 6 0.09
1.47 6 0.08
1.36 6 0.04
1.72 6 0.10
1.41 6 0.04
1.15 6 0.06
1.13 6 0.04
1.38 6 0.04
1.34 6 0.03
1.46 6 0.08
1.35 6 0.03
0.48 6 0.02
2968 6 110
512 6 35
1311 6 50
2008 6 75
1525 6 42
3514 6 89
72 6 3
60 6 2
17 6 1
78 6 14
17 6 1
46 6 10
57.58 6 1.52
87.53 6 2.35
83.8 6 2.5
85.70 6 0.90
85.5 6 0.8
30.37 6 0.48
12.53 6 0.33
4.39 6 0.10
11.10 6 0.31
6.84 6 0.38
9.4 6 0.2
3.11 6 0.03
1.041 6 0.052
0.856 6 0.019
0.867 6 0.017
1.021 6 0.044
0.887 6 0.016
0.814 6 0.048
0.912 6 0.029
0.992 6 0.026
0.819 6 0.022
0.856 6 0.014
0.845 6 0.012
0.894 6 0.035
1.223 6 0.026
1.203 6 0.016
1.254 6 0.025
1.275 6 0.028
1.263 6 0.019
1.285 6 0.029
1.231 6 0.028
1.163 6 0.027
1.240 6 0.028
1.300 6 0.027
1.271 6 0.019
1.065 6 0.041
175.4 6 1.7
488.1 6 16.8
1075.0 6 76.0
1184.0 6 16.0
1179.0 6 16.0
3400.0 6 371.0
9.38 6 0.37
2.71 6 0.04
117.3 6 2.2
299.7 6 14.0
121.7 6 2.2
45.53 6 8.60
4.81 6 0.05
3.11 6 0.07
4.01 6 0.37
4.08 6 0.08
4.08 6 0.08
3.54 6 0.30
4.38 6 0.08
4.85 6 0.05
1.18 6 0.02
2.14 6 0.28
1.18 6 0.02
0.76 6 0.03
28.00
138 6 4
6.00 6 0.19
288 6 10
4.64 6 0.06
0.834 6 0.014
1.102 6 0.014
304.0 6 14.6
2.78 6 0.21
102 6 2
214 6 11
537 6 16
106 6 6
103 6 5
128 6 4
3.57 6 0.11
2.38 6 0.14
1.03 6 0.03
1.66 6 0.09
1.17 6 0.06
1.11 6 0.03
181 6 7
668 6 36
672 6 20
647 6 36
158 6 7
228 6 6
4.15 6 0.04
21.78 6 0.41
57.20 6 1.60
9.74 6 0.19
10.26 6 0.10
10.29 6 0.09
0.837 6 0.017
1.573 6 0.129
0.821 6 0.015
0.835 6 0.023
0.844 6 0.016
0.832 6 0.010
1.100 6 0.020
1.551 6 0.203
1.192 6 0.027
1.198 6 0.033
1.198 6 0.032
1.195 6 0.017
186.0 6 15.9
798.4 6 55.2
1096.0 6 21.0
1108.0 6 75.0
390.3 6 8.5
498.8 6 7.8
1.26 6 0.12
2.51 6 0.15
1.80 6 0.03
2.70 6 0.12
1.36 6 0.03
1.61 6 0.02
1
bulk
4.17
24.30
21.31
20.50
Av.
Weighted errors represent 2s level.
Av.: weighted average according to analytical errors.
1
Calculated values for bulk material adopting 44% Fe/Ni and 56% silicates.
2 3 1012 n cm22s21 for about 8 h. Accuracy and precision of
the analyses varied between 1 and 30 rel.% depending on the
elements, the abundance, and the sample weight. More details
on the analytical procedures are given in Koeberl et al. (1987)
and Koeberl (1993). After completion of the g-counting and a
suitable decay period, the mineral samples were fused on a
tungsten strip in an Ar atmosphere to produce fused beads.
These were analyzed by electron microprobe analysis (Cameca
Camebax, at the NASA Johnson Space Center) using standard
procedures.
tivities and low yields in the chemical separation, the resulting
detection limits for Zr, Te and Ba are quite high.
2.2.2. Vienna analyses
Sample aliquots of Lodran silicate, Lodran olivine, Lodran
pyroxene, Gibson, Y74357, Y791491, LEW88280,
MAC88177, and EET84302 were weighted into clean polyethylene vials that were then heat sealed. Minor and trace elements
were analyzed by INAA. The samples were packed together
with geological reference materials (Govindaraju, 1987), Allende meteorite standard powder (Smithsonian Institution) and
synthetic multielement standards (absorbed on high-purity
quartz powder) into larger irradiation containers. Irradiations
were performed at the Triga Mark II reactor of the Atominstitut
der Österreichischen Universitäten at a neutron flux of about
2.3. Results of the Noble Gas Analyses
The results are given in Tables 1– 4. All errors correspond to
a 95% confidence level (2s errors). The mean values and their
errors were determined by the weighted mean method. Multiple
Table 2. He, Ne, and Ar concentrations and isotopic ratios of acapulcoites.
4
Acapulco
ALH81187
ALH81261
n. magn.
ALH81261
magn.
ALH81261
ALH81261
ALH84190
20
He
Weight
(mg)
28
10
Ne
40
Ar
3
cm STP/g
4
20
22
40
36
3
22
21
36
38
He
He
Ne
Ne
Ar
Ar
Ar
Ar
21.27
20.55
34.72
16198 6 522
1089 6 43
3401 6 179
1.54 6 0.07
1.38 6 0.06
1.13 6 0.06
3400 6 86
1682 6 39
7266 6 315
1506.0 6 19.0
116.4 6 2.3
503.8 6 6.0
0.832 6 0.023
0.902 6 0.020
0.879 6 0.038
1.184 6 0.014
1.367 6 0.057
1.327 6 0.024
635.4 6 29.0
370.8 6 4.6
1261.3 6 29.8
4.22 6 0.06
4.51 6 0.04
4.87 6 0.18
23.80
1739 6 87
1.23 6 0.04
3889 6 152
250.1 6 5.1
1.000 6 0.023
1.338 6 0.032
544.8 6 7.5
4.86 6 0.07
2703 6 110
2173 6 87
1000 6 39
1.17 6 0.0
0.98 6 0.04
1.52 6 0.05
5848 6 194
4967 6 132
1421 6 33
395.4 6 6.3
296.0 6 5.0
105.0 6 1.3
0.929 6 0.025
0.931 6 0.028
0.943 6 0.015
1.332 6 0.028
1.306 6 0.031
1.346 6 0.025
922.4 6 16.6
772.1 6 12.6
358.9 6 4.9
4.87 6 0.14
4.78 6 0.12
4.31 6 0.06
1
bulk
22.32
21.02
Weighted errors represent 2s level.
Calculated values for bulk material adopting 58% nonmagnetic and 42% magnetic material.
1
Ne
Ne
178
Table 3. He, Ne, and Ar concentrations and isotopic ratios of Lodran mineral separates.
3
He
Weight
(mg)
Cr2O3 1200°C
Cr2O3 1700°C
Cr2O3 1800°C
Cr2O3 1820°C
Cr2O3 Total
Olivine 1200°C
Olivine 1700°C
Olivine 1800°C
Olivine 1820°C
Olivine Total
Pyroxene 1200°C
Pyroxene 1700°C
Pyroxene 1800°C
Pyroxene Total
Troilite 1200°C
Troilite 1700°C
Troilite 1800°C
Troilite Total
Errors represent 2s level.
0.95
0.95
0.95
0.95
0.95
10.33
10.33
10.33
10.33
10.33
15.18
15.18
15.18
15.18
8.31
8.31
8.31
8.31
10
11.5 6 0.4
,0.03
,0.04
,0.03
11.5 6 0.4
12.5 6 0.5
0.05 6 0.01
0.01 6 0.00
,0.005
12.57 6 0.50
12.4 6 0.4
0.05 6 0.01
,0.002
12.5 6 0.4
8.60 6 0.35
0.07 6 0.00
,0.10
8.77 6 0.35
20
He
Ne
28
36
Ar
3
cm STP/g
4.88.0 6 15.0
1.81 6 0.35
2.01 6 0.38
1.83 6 0.35
490.0 6 15.0
65.0 6 2.0
0.74 6 0.12
0.38 6 0.07
0.40 6 0.07
66.52 6 2.01
75.0 6 2.0
0.68 6 0.10
0.15 6 0.03
75.8 6 2.0
34.0 6 1.0
0.50 6 0.09
0.19 6 0.04
34.69 6 1.01
1.80 6 0.12
0.28 6 0.06
2.82 6 0.16
0.12 6 0.06
2.08 6 0.13
1.91 6 0.08
0.28 6 0.02
0.33 6 0.01
0.08 6 0.01
2.60 6 0.08
1.42 6 0.02
0.81 6 0.02
2.23 6 0.03
1.07 6 0.11
0.05 6 0.01
0.02 6 0.01
1.14 6 0.11
7.59 6 0.87
0.02 6 0.02
8.16 6 0.82
5.62 6 1.74
7.61 6 0.87
2.87 6 0.11
1.15 6 0.24
0.11 6 0.02
0.02 6 0.02
4.15 6 0.27
6.07 6 0.61
2.22 6 0.36
0.02 6 0.02
8.31 6 0.71
27.0 6 1.9
1.75 6 0.44
0.02 6 0.02
28.77 6 1.95
20
22
40
22
21
36
Ne
Ne
Ne
Ne
1.020 6 0.070
2.715 6 0.733
10.150 6 1.650
1.162 6 0.156
1.336 6 0.250
20.760 6 13.266
1.114 6 0.077
0.864 6 0.036
0.903 6 0.038
0.865 6 0.072
0.918 6 0.122
0.870 6 0.029
0.892 6 0.015
0.875 6 0.026
1.170 6 0.163
1.128 6 0.041
1.257 6 0.081
1.135 6 0.031
1.148 6 0.143
1.142 6 0.051
1.172 6 0.038
1.143 6 0.041
0.886 6 0.014
1.574 6 0.113
1.540 6 0.205
1.436 6 0.353
1.570 6 0.106
1.161 6 0.038
1.251 6 0.035
1.179 6 0.140
1.148 6 0.267
1.245 6 0.037
Ar
Ar
36
38
Ar
Ar
24.60 6 0.32
3.21 6 0.06
309.9 6 3.8
24.45 6 0.33
34.84 6 0.54
5.03 6 0.08
5.13 6 0.12
3.22 6 0.06
4.80 6 0.05
4.81 6 0.06
2.69 6 0.50
24.09 6 1.48
2.04 6 0.04
0.00 6 0.00
4.73 6 0.06
5.27 6 0.13
4.27 6 0.07
1.49 6 0.08
1.68 6 0.02
0.00 6 0.00
4.97 6 0.10
5.08 6 0.10
4.97 6 0.10
1.58 6 0.03
5.08 6 0.09
A. Weigel et al.
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
4
Lodranites and acapulcoites
179
Table 4. He, Ne, and Ar concentrations and isotopic ratios of Lodran mineral separates.
4
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Fe/Ni
Fe/Ni 1000°C
Fe/Ni 1200°C
Fe/Ni 1400°C
Fe/Ni 1600°C
Fe/Ni 1800°C
Fe/Ni Total
Pyroxene 1000°C
Pyroxene 1200°C
Pyroxene 1400°C
Pyroxene 1600°C
Pyroxene 1800°C
Pyroxene Total
Silicate .50 mm
50.85
80.05
80.05
80.05
80.05
80.05
80.05
98.36
98.36
98.36
98.36
98.36
98.36
30.34
20
He
Weight
(mg)
28
10
32 6 1
40
Ne
Ar
3
cm STP/g
0.71 6 0.02
4
20
22
40
36
3
22
21
36
38
He
He
Ne
Ne
Ne
Ne
36 6 1
3.76 6 0.06 1.638 6 0.052 1.199 6 0.027
15 6 1
260
961
460
260
32 6 1
761
861
560
560
260
27 6 1
244 6 13 3.48 6 0.18 191 6 20 18.67 6 0.32 1.218 6 0.018 1.159 6 0.015
Ar
Ar
1.11 6 0.01
125.6 6 5.1
0.10 6 0.02
1.85 6 0.10
58.07 6 7.12
243.2 6 21.4
81.85 6 8.26
221.3 6 14.1
4.90 6 0.41
1.02 6 0.08
3.01 6 0.25
127.7 6 15.6
69.07 6 7.25
12.36 6 0.79
Ar
Ar
4.82 6 0.05
2.35 6 0.04
5.15 6 0.05
4.11 6 0.04
3.14 6 0.05
3.10 6 0.40
3.59 6 0.02
4.59 6 0.22
5.17 6 0.14
5.22 6 0.07
4.15 6 0.04
3.98 6 0.13
4.43 6 0.03
5.03 6 0.05
Errors represent 2s level.
analyses of aliquot samples show similar He, Ne, and Ar
concentrations. The reason that the results lie sometimes outside the error limits maybe due to sample heterogeneity.
2.4. Results of the Chemical Analyses
Our results for the chemical composition are given in Tables
5 and 6 for the two different data sets. Different splits of two
meteorites (Y791491 and MAC88177) were analyzed at both
laboratories, while the other samples cannot be compared due
to selection of different phases. Differences between the two
data sets are caused by the coarse-grained nature of the meteorites and the resulting heterogeneity, as well as by the small
sample size and low abundance of lithophile elements. While
Table 5. Major and trace element composition of 4 lodranite samples
analyzed in Bern.
Weight (mg)
Fe
Mn
Ca
Na
K
Sc
Cr
Co
Ni
Zn
As
Y†
Zr†
Te†
Ba†
La
Hf (ppb)†
Th
Lodran Iron Lodran Silicate MAC88177 Y791491
19.28
40.83
34.73
41.13
82.7
0.027
,0.5
18.0
3.0
0.39
400.0
4380.0
53000.0
56.0
13.0
2.1
,3.0
3.0
,1.0
0.09
9.0
1.2
17.4
0.60
1.0
305.0
27.0
14.2
3100.0
380.1
7100.0
350.0
0.75
0.85
,3.0
,0.2
,1.0
0.04
155.0
0.3
11.0
0.36
1.2
255.0
9.7
8.8
4100.0
125.4
2400.0
52.0
0.15
1.7
,3.0
0.23
,1.0
0.12
8.0
0.2
26.6
0.28
0.85
425.0
22.0
6.2
7200.0
1030.0
11000.0
155.0
2.8
0.53
,3.0
0.27
,1.0
,0.7
47.0
1.2
Major elements in weight%, trace elements in ppm, except as noted.
All measurements by INAA, except † by RNAA.
Analytical 2s uncertainties for major, minor, and trace elements are
,10%, ,20%, and ,10%, respectively.
some elemental abundances agree within the precision of the
analyses (up to 30% relative uncertainty), others are significantly different. However, this can be expected because of the
irregular distribution of metals and associated siderophile trace
elements. Given the sample weights that vary between about 6
and 23 mg, the agreement between the two data sets is actually
surprisingly good. The lower values for the fused bead Fe
analyses relative to the INAA may be the result of incomplete
transfer of Fe to the melt. While the data are representative of
the samples analyzed, caution should be applied to extrapolate
from a 20 mg sample to the whole meteorite.
Figure 1 shows chondrite-normalized rare earth element
(REE) plots of some of the samples. In all cases a depletion of
the light REEs is obvious, and in one case (Gibson) a striking
negative Eu anomaly is present. The slightly non-chondritic
REE patterns indicate that some fractionation during (partial)
melting has occurred during the formation of these meteorites.
The Eu anomaly is indicative of separation of a mineral (e.g.,
plagioclase) that was separated from the meteorite after the
melting event. On average, most other refractory elements have
near-chondritic abundance patterns, although some of them are
depleted as well. The siderophile elements show only minimal
depletions compared to chondritic values (Fig. 2). The volatile
element abundances show considerably more scatter than the
refractory or siderophile element abundances, with in most
cases significant depletions compared to average chondritic
values. This observation provides further evidence for a highertemperature event and fractionation.
Table 7 attempts to summarize the best available data that
are relevant for the discussion of the exposure ages, as derived
from literature data and our own data.
3. NOBLE GAS COMPONENTS
For the partitioning of the noble gas components the following assumptions were made (for references to the adopted
values see Eugster et al., 1993): trapped (tr) He 5 0, cosmogenic (c) 4He/3He 5 5.2, (20Ne/22Ne)tr 5 8.4, (21Ne/22Ne)tr 5
0.035, (20Ne/22Ne)c 5 0.8, (36Ar/38Ar)tr 5 5.32, (40Ar/36Ar)tr
5 0, (40Ar/36Ar)c 5 0.2. Tables 8 –10 give the trapped, cos-
180
A. Weigel et al.
Table 6. Major, minor, and trace element composition of 9 lodranite samples analyzed in Vienna.
Weight (mg)
SiO2†
TiO2†
Al2O3†
FeO†
MnO†
MgO†
CaO†
Na (wt.%)
K
Sc
Cr
Co
Ni
Zn
Ga
As
Se
Br
Rb
Sr
Zr
Ag
Sb
Cs
Ba
La
Ce
Nd
Sm
Eu
Gd
Tb
Tm
Yb
Lu
Hf
Ta
W
Ir (ppb)
Au (ppb)
Hg
Th
U
Lodran Silicate Lodran Olivine Lodran Pyroxene
18.51
19.43
20.14
47.1
0.09
0.34
14.8
0.48
36.9
1.67
0.019
127.0
8.13
2620.0
118.0
1970.0
288.0
11.0
0.71
5.35
2.96
1.6
,20.0
,15.0
0.03
0.022
0.046
3.3
0.032
0.11
0.08
0.03
0.03
0.11
0.025
,0.02
0.14
0.029
0.03
0.01
0.25
118.0
18.0
,0.05
0.008
0.009
40.1
0.02
0.01
10.9
0.46
47.6
0.05
0.012
73.0
3.27
805.0
22.1
153.0
252.0
0.28
0.15
0.62
2.85
0.3
60.0
,50.0
0.04
0.033
0.006
4.0
0.013
0.05
,0.1
0.02
0.008
,0.1
0.01
,0.03
0.049
0.0086
0.015
0.01
0.022
6.9
10.0
,0.05
0.005
0.003
52.3
0.14
5.50
9.2
0.33
28.7
3.11
0.033
121.0
14.6
2740.0
13.4
127.0
300.0
2.8
0.1
0.19
2.94
1.6
80.0
20.0
0.02
0.058
0.012
,3.0
0.025
0.08
,0.1
0.021
0.0063
,0.2
0.03
0.035
0.28
0.059
0.09
0.01
0.036
11.2
24.0
,0.02
0.02
0.01
Gibson
23.44
Y74357
6.42
Y791491
19.32
47.3
48.8
38.2
0.12
0.20
0.06
0.64
0.39
0.59
20.5
27.7
31.5
0.54
0.48
0.29
31.2
30.4
35.1
1.28
1.18
1.33
0.28
0.11
0.071
190.0
150.0
150.0
10.5
10.6
6.07
2800.0
14500.0
2350.0
720.0
714.0
1250.0
8100.0
9600.0
12900.0
62.0
550.0
105.0
8.1
7.5
4.4
2.12
3.97
4.28
0.51
0.84
0.53
3.2
8.1
4.4
2.6
1.5
1.4
,70.0
,80.0
,80.0
,100.0
,200.0
,90.0
0.01
0.01
0.02
0.15
0.22
0.13
0.058
0.04
0.09
,50.0
,60.0
,50.0
0.17
0.068
0.014
0.7
0.3
0.1
0.55
0.22
,0.2
0.21
0.084
0.03
0.02
0.06
0.013
0.26
0.3
,0.1
0.05
0.08
0.013
0.03
,0.1
,0.03
0.27
0.38
0.12
0.05
0.051
0.018
0.05
0.12
,0.1
0.014
0.015
0.011
0.19
4.7
0.54
1220.0
2020.0
2020.0
90.0
2210.0
291.0
,0.1
,0.2
,0.04
0.034
,0.08
0.041
0.05
,0.03
,0.04
LEW88280 MAC88177 EET84302
19.79
17.76
20.46
35.4
0.03
0.56
38.5
0.46
32.5
1.94
0.049
110.0
5.81
2740.0
3200.0
45000.0
83.0
2.0
16.4
0.36
3.2
1.9
,80.0
,80.0
0.04
0.39
0.51
11.0
0.051
0.3
0.3
0.11
0.051
0.15
0.03
,0.03
0.13
0.015
0.09
0.022
0.23
842.0
423.0
,0.03
,0.08
,0.05
47.4
0.08
0.43
13.1
0.53
36.6
0.98
0.029
17.0
8.97
3850.0
100.0
2160.0
79.0
0.6
0.17
0.88
3.3
1.1
,70.0
,100.0
0.03
0.025
0.045
5.0
0.011
0.07
,0.4
0.021
0.017
0.13
0.035
,0.05
0.16
0.024
0.07
0.014
0.14
31.0
8.4
,0.1
0.008
0.01
38.5
0.10
0.20
34.5
0.53
34.9
0.71
1.05
980.0
7.35
1500.0
1620.0
20500.0
62.0
3.0
5.31
0.68
2.9
2.8
,80.0
,100.0
0.01
0.077
0.01
9.1
0.053
0.4
0.5
0.22
0.089
0.31
0.065
0.034
0.25
0.049
,0.1
0.01
0.51
2125.0
204.0
,0.1
0.07
0.07
Major elements in weight%, trace elements in ppm, except as noted.
All measurements by INAA, except † by fused bead/electron microprobe analysis.
mogenic, and radiogenic noble gas components.
In this work, we shall focus on the cosmogenic component
and discuss the trapped and radiogenic noble gases in a separate
publication. Cosmogenic 3He in MAC88177 is an order of
magnitude lower than in all other lodranites, whereas 21Nec and
38
Arc are not depleted. This indicates He diffusion loss for
MAC88177. With one exception, all other lodranites and acapulcoites show similar 3He, 21Ne and 38Ar concentrations, respectively (3He: 9.31 6 1.20, 21Ne: 1.28 6 0.38, 38Ar: 0.21
6 0.13, units: 1028cm3STP/g). Only the lodranite QUE93148
has considerably higher concentrations compared to the mean
values of all other lodranites. The acapulcoite ALH81261
yields identical concentrations within error limits as
ALH77081 (Schultz et al., 1982). Mason et al. (1989) describe
these two acapulcoites and ALH81315 as paired. Our noble gas
analyses support this conclusion. Pairing between ALH81187
and ALH84190 (Mason et al., 1989) is also supported by our
analyses.
Comparison of the data for Lodran mineral separates and
their sum according to weight proportions with the analyzed
whole rock sample indicate that the latter is metal-rich: 21Nec is
32% lower and 38Arc is 39% higher in the whole rock sample
than in the bulk calculated from the mineral separates. Because
the total mass used for the measurements of the mineral separates is about three times higher than the bulk sample and the
mineral assemblage of our whole sample (about 30 times larger
than the bulk sample) is well known, we consider the bulk data
calculated from the mineral separates to be representative for
the Lodran meteorite. The same conclusion is reached for the
acapulcoite ALH81261: the whole rock sample (22.32 mg) is
15% lower in 21Nec and 23% higher in 38Arc relative to the
Lodranites and acapulcoites
181
Fig. 1. Rare earth element concentrations of EET84302, Gibson, LEW88280, MAC88177, Y74357, and Y791491,
normalized to those of the C1-chondrite Orgueil (Palme et al., 1981b).
calculated bulk that is based on a magnetic and non-magnetic
separate.
4. PRODUCTION RATES
The production rates of cosmogenic nuclides depend on the
flux, energy, and composition of the cosmic-ray particles, the
chemical composition of the meteoroid, its size and the shield-
ing depth of the sample within the meteoroid. Because most
meteorites found on Earth are chondrites, an empirical model
has been set up especially for them that takes these parameters
into account (Eugster, 1988). This model is based on the
abundance of a stable and the activity of a radioactive cosmogenic nuclide. The stable nuclide integrates the cosmic-ray flux
over time, whereas the radioactive nuclide measures the inten-
Fig. 2. Elemental abundances of EET84302, Gibson, LEW88280, MAC88177, Y74357, and Y791491 normalized to
those of the C1– chondrite Orgueil (Palme et al., 1981b) and Mg.
182
A. Weigel et al.
Table 7. Chemical abundances of lodranites and acapulcoites relevant for the interpretation of the noble gas data (in weight %).
EET84302
EET84302
FRO90011
Gibson
Gibson
LEW88280
LEW88280
Lodran1
Lodran
MAC88177
MAC88177
MAC88177
Y74357
Y74357
Y791491
Y791491
Y7914911
Acapulco
ALH77081
ALH78230
ALH81187
ALH81261
Monument Draw
Y-74063
Na
Mg
Al
Si
K
Ca
Ti
Cr
Mn
Fe
Co
Ni
1.05
0.474
0.077
0.28
0.195
0.049
0.058
0.017
0.032
0.029
0.026
0.044
0.11
0.11
0.071
0.043
0.029
0.657
0.752
0.753
0.542
0.692
0.671
0.705
20.9
0.106
18.0
15.5
15.0
22.0
0.51
0.234
0.228
14.8
2.05
2.64
1.26
0.81
1.03
4.5
1.22
1.57
2.25
0.22
0.24
0.186
0.207
22.1
22.8
0.071
0.96
0.313
17.8
0.125
1.29
1.1
17.3
15.1
15.7
13.4
16.3
15.5
14.2
15.5
0.101
1.21
1.2
1.16
0.99
1.12
1.16
1.28
15.2
17.5
19.1
26.8
38.6
24.3
16.0
18.2
30.0
18.6
30.0
28.7
10.2
11.0
7.86
21.6
29.9
24.5
26.6
35.3
24.1
24.8
24.0
18.1
22.8
25.5
19.8
0.162
0.176
0.071
0.072
0.063
0.32
0.048
0.005
0.123
0.01
23.2
18.2
23.2
21.1
0.15
1.24
0.521
0.28
0.163
0.274
0.604
0.173
0.39
0.385
0.41
0.33
1.45
0.411
0.18
0.339
0.48
0.297
0.51
1.15
0.49
0.91
1.04
1.39
0.83
1.03
0.57
0.70
1.20
1.60
0.84
1.25
0.95
0.85
0.30
1.13
0.59
1.48
1.33
1.38
1.04
1.405
0.06
17.5
18.7
17.5
19.5
0.098
0.015
0.007
0.019
0.018
0.011
0.064
0.044
0.077
0.040
0.043
0.009
0.046
1.81
1.56
1.48
1.11
1.41
2.34
1.19
22.1
16.5
22.1
0.0079
0.0014
0.0017
0.001
0.008
0.015
0.0037
0.015
0.0022
0.0005
0.050
0.069
0.054
0.020
0.057
0.035
0.058
0.09
0.072
0.06
0.018
0.036
0.095
0.048
0.042
0.12
0.036
0.031
0.077
0.09
0.235
0.72
0.131
0.569
0.719
0.5
0.382
0.565
0.215
0.27
0.290
0.419
0.231
0.357
0.231
0.260
0.411
0.35
0.372
0.225
0.255
0.301
0.303
0.276
0.27
0.246
0.263
Ref.
9,
11,
13,
12,
12,
13,
1
3
4
1
4
1
3
1
5
1
2
7
1
8
1
2
6
10
12
14
15
15
12
14
1
Calculated based on mineral assemblage and abundances therein.
References: 1: this work, Vienna analyses, 2: this work, Bern analyses, 3: Mittlefehldt et al. (1996), average of 2, 4: Zipfel and Palme (1993), 5:
Fukuoka et al. (1978), 6: Hiroi and Takeda (1991), 7: Prinz et al. (1991), 8: Fukuoka and Kimura (1990), Yamamoto et al. (1991), Torigoye et al.
(1993), 9: Palme et al. (1981a), 10: Zipfel et al. (1995), 11: Schultz et al. (1982), 12: Zipfel and Palme (1994), 13: Torigoye et al. (1993), 14: Fukuoka
and Kimura (1990), 15: Mittlefehldt et al. (1996).
sity of this flux in the meteorite. The radioactive species used
was 81Kr. Thereupon, the production rates for 3He, 21Ne, 38Ar,
83
Kr, and 126Xe as a function of the shielding parameter (22Ne/
21
Ne)c was derived. The production rates for 3He and 21Ne are
linked through a correlation of (3He/21Ne)c with the shielding
parameter (22Ne/21Ne)c.
Because achondrite classes have other and more variable
elemental abundances than chondrites, and because their production rates have other shielding dependencies, a similar production rate model was developed for howardites, eucrites, and
diogenites (Eugster and Michel, 1995), which is used here. The
obtained production rates are given in Table 11. Their uncertainty (;14%) is obtained by quadratically adding the uncertainties of the chemical analyses (#10%), the uncertainty in the
correlation of the aliquot for chemical analyses with that of the
noble gas abundance analyses (;10%), and the uncertainty of
the production rate model itself (;5%; Eugster and Michel,
1995). P3’ and P21’ are production rates based on target element
concentrations without taking shielding dependence into account [see Eugster and Michel (1995) for details]. Because of
the larger spread of the target elements and the shielding
conditions of the lodranites and acapulcoites relative to the
HEDs, an average of P21 for HEDs was used here. Sample
heterogenity and, therefore, highly variable target element concentrations (Fe, Ni, Ca) for the production of 38Ar, as well as
large corrections for trapped Ar result in large uncertainties for
P38 and T38, which are, therefore, not given.
To use an independent method of testing whether the production rate model of Eugster and Michel (1995) is usable in
our case or not, the production rates of cosmogenic nuclides in
lodranites and acapulcoites were also calculated by a purely
physical model describing the depth- and size-dependent production of cosmogenic nuclides in meteoroids by galactic cosmic ray protons (Michel et al., 1991). These calculations combine depth- and size-dependent spectra of primary and
secondary GCR particles in the irradiated objects with thintarget cross sections of the underlying nuclear reactions. The
spectra are calculated by Monte Carlo simulations using the
HERMES code system (Cloth et al., 1988). The present calculations make use of a recent version of the model (Michel et al.,
1995a) applying the same set-ups and cross section data as used
for calculating production rates for the interpretation of cosmogenic nuclides in differentiated Antarctic meteorites (Herpers et al., 1995). In particular, new cross section measurements for the production of rare gases (Michel et al., 1995b)
were used in these calculations. Flux normalization and details
of the actual status of both the GCR and SCR model calculations may be found elsewhere (Michel et al., 1996). The production rate calculations were done using units of g/cm2 for
radii and depths; those were converted to millimeters using an
average density of 5.25g/cm3 for all meteorites. The theoretical
production rates have an accuracy of better than 10% for
production rates and better than 3% for production rate ratios.
Those accuracies result from the overall quality by which the
production rates from thick-target experiments were reproduced (Michel et al., 1994; Leya and Michel, 1997). It is a
general observation that the calculated GCR production rates
tend to be smaller for small meteoroids than those derived from
Lodranites and acapulcoites
183
Table 8. Trapped, radiogenic and cosmogenic He, Ne, and Ar of lodranites and acapulcoites.
20
Net
36
Art
4
EET84302
FRO90011
Gibson
Gibson1
LEW86220
LEW88280
Lodran
Lodran
MAC88177
QUE93148
Y74357
Y743572
Y743573
Y791491
Acapulco
Acapulco4
ALH770815
ALH81187
ALH81261
ALH81261
ALH84190
Monument Draw6
Y-740637
31.18
20.18
Av.
21.86
20.43
36.13
Bulk
Av.
Bulk
4.17
2.39
3.87
Av.
21.27
20.55
Bulk
22.32
21.02
40
Her
Weight
(mg)
Arr
28
3
Hec
21
Nec
38
Arc
10 cm STP/g
0.41 6 0.07
0.11 6 0.03
0.12 6 0.00
0.55
,0.09
0.15 6 0.03
0.30 6 0.03
0.80 6 0.10
0.08 6 0.02
0.19 6 0.03
1.29 6 0.13
16.67 6 0.64
0.95 6 0.07
1.224 6 0.03
2.76
0.96 6 0.11
7.48 6 0.42
21.88 6 0.73
18.10 6 1.10
0.08 6 0.00
0.54 6 0.04
0.71 6 0.06
0.36
0.21
0.05 6 0.01
0.31 6 0.01
0.07 6 0.05
5.16 6 0.27
0.66
4.12
0.17
4.85
0.17 6 0.03
4.42 6 0.12
0.18 6 0.04
6.26 6 0.28
0.15 6 0.03
6.33 6 0.20
0.25 6 0.02
3.83 6 0.10
0.07
3.22
1.22
150.41
488 6 27
777 6 46
697 6 19
868
179 6 8
68 6 5
0
77 6 6
360
0
163 6 12
2968 6 110 9.32 6 0.53 1.26 6 0.09
512 6 35
9.43 6 0.58 1.43 6 0.08
1525 6 42
8.82 6 0.23 1.26 6 0.04
2400
8.52
1.35
3514 6 89
7.12 6 0.26 1.10 6 0.09
72 6 3
9.23 6 0.32 1.01 6 0.05
60 6 2
10.98 6 0.32 1.19 6 0.05
114 6 9
11.40 6 0.50 1.75 6 0.10
17 6 1
0.79 6 0.02 1.26 6 0.04
181 6 7
24.58 6 0.66 3.87 6 0.13
668 6 36
9.83 6 0.55 0.97 6 0.16
10.90 6 1.10 1.12 6 0.11
196
1027
8.27
4.59
79 6 4
228 6 6
9.90 6 0.27 1.12 6 0.03
16142 6 522 3400 6 86 10.75 6 0.37 1.56 6 0.08
12572
4213
11.58
1.71
2409
5135
6.99
0.90
1040 6 043 1682 6 39
9.36 6 0.41 1.12 6 0.07
2667 6 148 5848 6 194 6.83 6 0.37 0.94 6 0.05
2135 6 87 4967 6 132 7.34 6 0.32 0.80 6 0.05
951 6 39 1421 6 33
9.53 6 0.39 1.19 6 0.05
3590
3420
9.52
1.36
2097
5242
11.21
1.08
S D
22
Ne
Ne
21
3
0.38 6 0.04
0.16 6 0.02
0.08 6 0.00
0.23
0.11 6 0.03
0.36 6 0.04
0.46 6 0.06
0.33 6 0.10
0.12 6 0.01
0.67 6 0.16
0.20 6 0.03
0.10 6 0.01
0.13
0.23 6 0.01
0.30 6 0.02
0.28
0.11
0.17 6 0.01
0.13 6 0.03
0.16 6 0.04
0.20 6 0.02
0.18
0.53
c
1.185 6 0.026
1.195 6 0.017
1.241 6 0.021
1.235
1.283 6 0.030
1.214 6 0.028
1.135 6 0.027
1.110 6 0.050
1.264 6 0.019
1.091 6 0.020
1.401 6 0.187
1.326 6 0.026
1.073
1.190 6 0.017
1.179 6 0.014
1.188
1.295
1.349 6 0.056
1.310 6 0.027
1.284 6 0.031
1.322 6 0.025
1.244
1.320
Weighted errors represent 2s level.
Av.: Weighted average according to analytical errors.
Bulk: Calculated from mineral separates data.
1
McCoy et al. (1997), average of 3 samples.
2
Nagao et al. (1995).
3
Takaoka et al. (1993).
4
Average of Evans et al. (1982) and Palme et al. (1981a), 2 samples each.
5
Schultz et al. (1982).
6
McCoy et al. (1996).
7
Average of Takaoka and Yoshida (1991) and Takaoka et al. (1993).
empirical models (Leya, 1997). This is attributed to the influence of SCR production in small meteoroids (Merchel et al.,
1998). Although the empirical production rates by Eugster and
Michel (1995) are based on 81Kr-Kr-exposure ages, that are
unlikely affected by any SCR-contribution (Reedy and Marti,
1991), small meteorites are likely to contain SCR-produced He
and Ne. Therefore, the empirical production rates by Eugster
and Michel (1995) for He and Ne already implicitly contain a
slight correction for SCR-produced He and Ne whereas our
GCR-only calculations do not.
For each cosmogenic nuclide all relevant elemental production rates are calculated separately, which allows to account for
the non-chondritic bulk chemical composition of lodranites and
acapulcoites. The GCR production rates in Table 11 and the
GCR preatmospheric radii and depths in Table 12 were obtained by matching the calculated and measured (3He/21Ne)c
and (22Ne/21Ne)c ratios. As MAC88177 experienced diffusive
loss of 3He, only (22Ne/21Ne)c was used for the matching. The
calculated 3He and 21Ne production rates by galactic cosmic
rays are lower than the production rates by Eugster and Michel
(1995) by 6% and 34% for lodranites and 11% and 20% for
acapulcoites.
The influence of solar cosmic rays on the production of
cosmogenic nuclides was also investigated. Neglecting secondary SCR particles, the depth-dependent SCR-spectra in mete-
oroids were calculated taking only into account electronic
stopping and nuclear attenuation. The production rates were
then evaluated using thin-target excitation functions of the
underlying reactions and are normalized to a solar cosmic ray
spectrum with a rigidity of 125 MV and a flux density of 55
cm22s21 (Michel et al., 1996). No heliocentric distance dependence was assumed. Again, the GCR&SCR production rates in
Table 11 and the GCR&SCR preatmospheric radii and depths
in Table 12 were obtained by matching the calculated with the
measured (3He/21Ne)c and (22Ne/21Ne)c ratios, except for
MAC88177, for which only (22Ne/21Ne)c was used because of
3
He diffusion loss. The calculated 3He production rates by
galactic and solar cosmic rays are on average 11% higher than
the production rates by Eugster and Michel (1995) for lodranites and acapulcoites whereas the average 21Ne production rate
for lodranites is 17% lower and the one for acapulcoites is 5%
higher than the production rates by Eugster and Michel (1995).
4.1. Shielding
Fig. 3 shows the correlation between the routinely used
indicator for the shielding depth (22Ne/21Ne)c, and (3He/21Ne)c
for lodranites and acapulcoites. Because the flow of secondary
neutrons increases with shielding, the 21Ne production is increased, mainly through the reaction 24Mg(n,a)21Ne. As a
184
A. Weigel et al.
Table 9. Trapped, radiogenic, and cosmogenic He, Ne, and Ar of Lodran mineral separates.
20
Net
36
Art
4
Her
Weight
(mg)
Lodran Cr2O3 1200°C
Lodran Cr2O3 1700°C
Lodran Cr2O3 1800°C
Lodran Cr2O3 1820°C
Lodran Cr2O3 Total
Lodran Fe/Ni
Lodran Fe/Ni 1000°C
Lodran Fe/Ni 1200°C
Lodran Fe/Ni 1400°C
Lodran Fe/Ni 1600°C
Lodran Fe/Ni 1800°C
Lodrdan Fe/Ni Total
Lodran Fe1
Lodran Olivine 1200°C
Lodran Olivine 1700°C
Lodran Olivine 1800°C
Lodran Olivine 1820°C
Lodran Olivine Total
Lodran Pyroxene 1200°C
Lodran Pyroxene 1700°C
Lodran Pyroxene 1800°C
Lodran Pyroxene Total
0.95
0.95
0.95
0.95
0.95
50.85
80.05
80.05
80.05
80.05
80.05
80.05
10.33
10.33
10.33
10.33
10.33
15.18
15.18
15.18
15.18
40
3
Arr
28
Hec
21
38
Nec
Arc
10 cm STP/g
0.43 6 0.11 6.96 6 0.18
0.21 6 0.04 0.00
2.87 6 0.14 8.16 6 0.23
0.13 6 0.05 5.59 6 0.10
0.64 6 0.12 6.96 6 0.18
0.40 6 0.02 31.73 6 1.03
0.10 6 0.01
19.64 6 6.40
4.44 6 0.40
0.07 6 0.01
0.01 6 0.00
24.26 6 6.41
0.99
18.57
0.16 6 0.08 2.74 6 0.09
0.03 6 0.02 1.14 6 0.02
0.03 6 0.03 0.10 6 0.01
0.01 6 0.01 0.00
0.23 6 0.09 3.98 6 0.09
0.16 6 0.02 6.07 6 0.19
0.08 6 0.02 2.14 6 0.07
0.00
0.00
0.24 6 0.03 8.21 6 0.20
428 6 15 188 6 4
260
0
2 6 0 2548 6 66
260
0
430 6 15 188 6 4
0
36 6 1
15 6 1
260
961
461
260
32 6 2
202
54
,3
97 6 3
060
0
060
0
060
0
460
97 6 3
10 6 3
12 6 0
060
0
060
0
11 6 2
12 6 0
S D
22
Ne
Ne
21
3
11.50 6 0.44 1.52 6 0.24
,0.03
0.07 6 0.01
,0.04
,0.02
,0.03
0.00
11.53 6 0.44 1.59 6 0.24
8.51 6 0.21 0.36 6 0.02
9.50
12.52 6 0.45
0.05 6 0.01
0.01 6 0.00
0.00
12.59 6 0.45
12.43 6 0.41
0.05 6 0.00
0.00
12.48 6 0.41
0.49
1.96 6 0.13
0.24 6 0.02
0.33 6 0.03
0.08 6 0.01
2.61 6 0.41
1.35 6 0.05
0.81 6 0.03
0.00
2.16 6 0.06
1.07 6 0.06
0.034 6 0.001
0.103 6 0.029
0.043 6 0.032
1.104 6 0.060
0.72 6 0.08
0.03 6 0.00
0.14 6 0.06
0.29 6 0.03
0.01 6 0.00
0.00
0.47 6 0.07
0.51
0.064 6 0.008
0.026 6 0.006
0.024 6 0.001
0.000 6 0.000
0.114 6 0.010
0.013 6 0.031
0.116 6 0.022
0.00
0.129 6 0.038
c
1.130 6 0.152
1.011 6 0.232
1.125 6 0.146
1.072 6 0.026
1.300
1.119 6 0.041
1.241 6 0.095
1.126 6 0.098
1.131 6 0.208
1.131 6 0.035
1.158 6 0.038
1.132 6 0.041
1.101 6 0.110
1.148 6 0.028
Errors represent 2s level.
Zähringer (1968).
1
reference the correlation line for chondrites from Eugster
(1988) is shown; this line is extrapolated for the low shielding
of the lodranites and acapulcoites to values (22Ne/21Ne)c . 1.3.
A correlation line for all lodranites except for MAC88177
would have a negative slope and a poor correlation coefficient
(r 5 0.24). The reason for this is the high iron content of
lodranites: according to Table 7 lodranites have (22 6 13%)
metallic Fe; H chondrites, which otherwise are chemically
similar to lodranites, have only 11% metallic Fe (Dodd, 1981).
Little 21Ne is produced by spallation in metallic Fe and, therefore, the 3He/21Ne versus 22Ne/21Ne correlation is generally
applicable only for silicate rich meteorites. This is demonstrated by the silicate separates of two lodranites (‘L’: Lodran
mineral separates, ‘Q’: QUE93148 mineral separates): their
data points lie closer to the correlation line than those of the
whole rock analyses.
From the model calculations of the cosmogenic nuclide
production rates for lodranites and acapulcoites the correlation
of (3He/21Ne)c versus (22Ne/21Ne)c for single lodranites or
acapulcoites and various shielding conditions can be derived.
Fig. 4 shows this correlation for meteorites with the same
chemical composition as the paired acapulcoites ALH81187
and ALH84190, different radii (mm) and shielding depths
(s 5 surface, c 5 center) and the measured concentrations.
Neglecting any SCR contribution and comparing the measured
with the calculated (3He/21Ne)c and (22Ne/21Ne)c a pre-atmo-
Table 10. Trapped, radiogenic and cosmogenic He, Ne, and Ar of Lodran mineral separates.
20
Net
Weight
(mg)
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Lodran
Pyroxene 1000°C
Pyroxene 1200°C
Pyroxene 1400°C
Pyroxene 1600°C
Pyroxene 1800°C
Pyroxene Total
Silicate .50 mm
Silicate1
Troilite 1200°C
Troilite 1700°C
Troilite 1800°C
Troilite Total
Errors represent 2s level.
Zähringer (1968).
1
98.36
98.36
98.36
98.36
98.36
98.36
30.34
8.31
8.31
8.31
8.31
36
Art
4
Her
40
3
Arr
28
Hec
21
Nec
38
Arc
10 cm STP/g
1.32 6 0.08
2.69
0.58 6 0.04
0.03 6 0.01
0.01 6 0.00
0.62 6 0.04
0.03 6 0.00
761
1.57 6 0.12
861
4.91 6 0.60
560
1.44 6 0.18
460
0.02 6 0.00
260
7.97 6 0.64
26 6 1
15.29 6 1.86 176 6 13 190 6 20 13.04 6 0.72 2.46 6 0.13
9.83
322
126
22.50
5.32
27.13 6 0.83
0
46 6 1
8.60 6 0.35 0.54 6 0.04
1.73 6 0.04
060
0
0.07 6 0.00 0.03 6 0.00
0.00
060
0
0.00
0.01 6 0.00
28.86 6 0.83
060
46 6 1
8.68 6 0.35 0.58 6 0.04
S D
22
Ne
Ne
21
3
0.00
0.01 6 0.01
0.02 6 0.02
0.09 6 0.01
0.00
0.12 6 0.02
0.19 6 0.04
0.07
0.28 6 0.12
0.027 6 0.011
0.00
0.307 6 0.121
c
1.098 6 0.015
0.942
1.129 6 0.037
1.069 6 0.131
1.056 6 0.252
1.125 6 0.035
Lodranites and acapulcoites
185
Table 11. Comparison of empirical production rates of cosmogenic He and Ne in lodranites and acapulcoites using the model by Eugster and Michel
(1995) with simulated production rates (GCR: galactic cosmic ray only, GCR&SCR: galactic and solar cosmic ray).
S D
Eugster and Michel ’95
22
Ne
21
Ne
P93
P921
GCR
P3
P21
c
P3
GCR&SCR
P21
P3
P21
155
148
148
21.9
21.8
20.9
172
167
172
26.1
24.5
24.9
10210cm3STP/g/Ma
EET84302
FRO90011
Gibson
LEW86220
LEW88280
Lodran
MAC88177
QUE93148
Y74357
Y791491
1.185
1.195
1.241
1.283
1.214
1.110
1.265
1.091
1.401
1.191
157
159
165
159
157
156
168
159
160
159
40.6
29.2
38.2
38.3
37.9
25.3
43.3
38.3
37.7
40.8
154
156
159
160
153
148
161
160
148
156
34.1
23.9
28.0
28.9
29.6
32.8
30.1
42.7
20.4
33.8
146
149
131
16.5
22.7
16.2
172
154
170
22.1
23.5
25.7
134
148
12.9
20.0
192
169
23.8
23.6
Acapulco
ALH770811
ALH81187
ALH81261
ALH84190
Monument Draw2
Y740633
1.179
1.295
1.349
1.311
1.322
1.244
1.32
157
159
164
159
159
158
162
31.3
33.0
34.1
32.4
32.4
29.8
32.3
153
151
153
151
151
152
153
26.3
21.6
20.1
20.5
20.1
21.7
20.2
144
145
128
141
132
142
115
20.4
18.7
14.9
18.9
16.4
20.0
11.4
158
175
171
174
171
168
166
22.7
22.5
22.2
23.0
22.2
24.7
21.3
Adopted production rates are emphasized. P93 and P921 are production rates based on target element concentrations without taking shielding
dependency into account (see Eugster and Michel (1995) for details).
Production rate uncertainties are ;14%
1
Calculated from data given by Schultz et al. (1982).
2
Calculated from data given by McCoy et al. (1996).
3
Calculated from data given by Takaoka and Yoshida (1991).
spheric radius of 50 –75 mm and a shielding depth of ;4 mm
are obtained for ALH81187 and ALH84190.
rates for the individual meteorites, some of the calculated
production rate ratios of (3He/21Ne)c and (22Ne/21Ne)c match
the measured ratios rather poorly, especially for meteorites with
(22Ne/21Ne)c . 1.28.
Taking SCR contribution into account, better agreement
between calculated and measured (3He/21Ne)c and (22Ne/
21
Ne)c ratios for ALH81187 and ALH84190 is found for a
4.2. Solar Cosmic Ray Effects
Even having taken into account the effects of bulk chemical
composition in the calculation of the production rates for lodranites and acapulcoites by simulating the GCR production
Table 12. Solar cosmic ray efects in lodranites and acapulcoites and derived preatmospheric radii and depths (in mm). Adopted radii and depths
are emphasized.
S
3
22
He
Ne
vs. 21
21
Ne
Ne
EET84302
FRO90011
Gibson
LEW88280
Lodran
MAC88177
Y74357
Y791491
n
y
y
n
n
p
n
Acapulco
ALH77081
ALH81187
ALH81261
ALH84190
Mon. Draw
Y74063
y
y
y
y
y
y
u
D
10
26
Be vs. 21Al
~T21Ne
T Ne!
n
u
n
p
n
u
n
n
(y)
y
y
y
S
22
Mg
Ne
vs. 21
Si 1 Al
Ne
D
GCR
GCR&SCR
R
d
R
d
u
u
p
p
n
y
y
u
75
200
125
50
200
25
25
50
24
5
6
9
45
14
1
29
75
125
75
50
150
50
25
50
73
19
19
47
71
22
25
46
u
y
y
y
200
125
50
150
75
200
25
10
3
4
4
4
15
1
125
50
25
50
25
200
25
34
14
25
10
25
10
23
p
y
y: yes; p: possibly; u: unlikely; n: no. (Y): using radionuclide data from ALH81261.
186
A. Weigel et al.
Fig. 3. (a) Cosmogenic 3He/21Ne versus 22Ne/21Ne plot for lodranites. Filled squares: this work; open squares:
Gibson—McCoy et al. (1997a), average of 3 samples, Y74357—Nagao et al. (1995). MAC88177 with (3He/21Ne)c 5 0.59
is not shown. ‘L’ and ‘Q’ designate mineral separates from the Lodran and the QUE93148 meteorite. For reference purpose,
the correlation line for chondrites from Eugster (1988) is shown. (b) Cosmogenic 3He/21Ne versus 22Ne/21Ne plot for
acapulcoites. Filled squares: this work; open squares: Acapulco—Average of Evans et al. (1982) and Palme et al. (1981a),
two samples each, ALH77081—Schultz et al. (1982), Monument Draw—McCoy et al. (1996), Y74063—Average of
Takaoka and Yoshida (1991) and Takaoka et al. (1993). For reference purpose, the correlation line for chondrites from
Eugster (1988) is shown.
preatmospheric radius of 25 mm and a shielding depth of 25
mm, corresponding to the center position of the meteoroid. The
same calculations were done for all other lodranites and acapulcoites with known chemical composition and noble gas con-
tent. The resulting production rates are summarized in Table 11
and the preatmospheric radii and depths in Table 12. Column 2
in Table 12 indicates which simulation (pure GCR or
GCR&SCR) fits the measured ratios best.
Lodranites and acapulcoites
187
Fig. 4. Cosmogenic 3He/21Ne versus 22Ne/21Ne plot for the paired acapulcoites ALH81187 and ALH84190. For reference
purpose, the correlation line for chondrites from Eugster (1988) is shown. ‘GCR only’ designates the calculated correlation
line for meteorites with the same chemical composition as ALH81187 and ALH81261 for different sizes (25–200 mm radii)
and shielding conditions (S–surface, C– center) for galactic cosmic ray produced 3He, 21Ne, and 22Ne. ‘GCR&SCR’ denotes
the calculated correlation lines for meteorites with the same chemical composition as ALH81187 and ALH81261 for
different sizes (25–200 mm radii) and shielding conditions (S–surface, C– center) for galactic and solar cosmic ray produced
3
He, 21Ne, and 22Ne.
10
Be and 26Al are also a suitable radionuclide pair to detect
SCR contributions to the production of cosmogenic nuclides.
10
Be is a higher energy product than 26Al. 10Be is produced by
protons or alpha particles with energies above 50 MeV, while
the cross sections for the production of 26Al have their maxima
between 15 and 45 MeV (Michel et al., 1995a, Schiekel et al.,
1996, Michel et al., 1997). Therefore, SCR are much less
responsible for the production of 10Be than of 26Al. Using 10Be
and 26Al from Xue et al. (1994) and our 21Nec data we have
calculated 10Be/21Ne and 26Al/21Ne exposure ages (Table 13)
following the model of Graf et al. (1990). A SCR contribution
would increase the production of 26Al and, therefore, decrease
the 26Al/21Ne-age relative to the 10Be/21Ne-age. Therefore, the
ratio of the 10Be/21Ne- to the 26Al/21Ne-age is indicative of a
SCR contribution (Table 12, column 3). As there is still a
discrepancy between SCR fluxes derived from 26Al and 21Ne
(Michel et al., 1996), SCR contributions can not be determined
quantitatively yet.
Another diagnostic plot for the GCR versus GCR&SCR
issue is shown in Fig. 5: Mg/(Si1Al) is plotted versus (22Ne/
21
Ne)c following Begemann and Schultz (1988) and Garrison et
al. (1995). The area between the dashed lines represents the
boundary between essentially pure GCR-Ne on the left and
increasing SCR Ne on the right. However, the slope and the
width of the boundary area was obtained from chondrites and
the effect of non-chondritic chemical composition of the lodranites and acapulcoites is not taken into account. Therefore,
we consider this plot only as indicative and not as conclusive
for solar proton produced nuclides (see also summary in Table
12, column 4).
Using all diagnostic tools summarized in Table 12, we conclude that only the paired acapulcoites ALH77081 and
ALH81261, and ALH81187 and ALH84190, show clear evidence of SCR produced nuclides.
(3He/21Ne)c vs. (22Ne/21Ne)c plots (e.g., Fig. 4) suggest SCR
contributions for FRO90011, Gibson, Acapulco, and Monument Draw. However, the T(10Be/21Ne)/T(26Al/21Ne) age ratios, which we believe to be the best indicator of possible SCR
contributions, as well as the Mg/(Si1Al) versus (22Ne/21Ne)c
plot, do not support this conclusion.
5. COSMIC-RAY EXPOSURE AGES
The single-stage cosmic-ray exposure ages are given in
Table 13. The analytical errors (;15%) are estimated from
those of the production rates (;14%) and those of the noble gas
concentrations (,5%). The average exposure ages, TAv., are
weighted averages of T3 and T21 according to their analytical
errors. For MAC88177 no 3He age is given due to He diffusion
loss. No T38 are given because of large corrections for trapped
Ar and highly variable target element concentrations (Fe, Ni,
Ca), due to sample heterogenity. Because no chemical concentrations are available for LEW86220 and QUE93148, we
adopted the average lodranite composition. For QUE93148 we
obtain an exposure age that is about twice as high as that of the
other lodranites.
188
A. Weigel et al.
Table 13. Cosmic-ray exposure ages of lodranites and acapulcoites (Ma). Adopted ages are emphasized.
E. & M. ’951
GCR
GCR&SCR
10
26
T3
T21
TAv.
T3
T21
TAv.
T3
T21
TAv.
Be2
T21Ne
Al 2
T21Ne
6.0
6.1
5.6
4.5
6.0
7.1
4.6
5.4
5.0
4.3
4.4
5.9
4.4
11.2
6.3
4.0
6.0
6.4
6.0
5.8
6.6
6.0
5.4
5.7
5.1
4.8
5.8
5.1
5.2
5.7
5.1
4.3
5.3
5.9
4.2
4.7
6.0
6.3
7.7
6.1
7.7
8.2
5.9
6.5
6.0
(6.0)
6.3
7.7
8.2
5.4
7.4
4.6
7.5
5.1
5.0
7.4
5.1
5.9
7.0
7.2
6.0
6.5
6.0
15.4
6.6
6.4
3.7
5.0
4.5
3.8
3.4
5.0
4.4
9.0
4.8
3.3
7.4
6.7
7.5
5.8
7.4
6.1
5.1
5.9
4.1
4.8
5.0
5.2
4.2
4.0
6.0
0.8
4.2
0.7
5.0
0.8
6.6
0.6
6.5
0.8
6.5
0.7
5.7
0.8
5.2
1.1
5.5
0.9
Acapulco
ALH770817,8
ALH812618
ALH8118710
ALH8419010
Monument Draw11
Y7406313
7.0
4.6
4.5
6.1
6.3
6.2
7.3
5.8
4.2
4.6
5.6
5.9
6.3
5.4
6.5
4.5
4.5
5.9
6.1
6.3
5.9
7.5
4.8
5.9
7.3
7.2
6.7
9.7
7.6
4.8
6.2
7.5
7.3
6.8
9.5
7.5
4.8
6.0
7.4
7.2
6.8
9.6
6.8
4.0
4.3
5.5
5.6
5.7
6.8
6.9
4.0
3.8
5.1
5.4
5.5
5.1
6.8
4.0
4.1
5.3
5.5
5.6
5.6
5.1
4.2
4.5
5.9
6.3
5.4
2.2
2.2
3.3
3.7
Acapulcoites Av.
6
6.0
1.1
4.8
0.9
5.3
0.9
6.7
1.2
6.7
1.3
6.7
1.2
5.6
1.0
5.2
1.0
5.4
0.9
EET84302
FRO90011
Gibson
LEW86220
LEW88280
Lodran
MAC88177
QUE93148
Y74357
Y791491
Lodranites Av.
6
5
T213
6.24
5.06
5.69
6.812
6.214
Value in ( ) is adopted (see text for explanation). Exposure age uncertainties are ;15%. 1Using production rates for HED’s by Eugster and Michel
(1995). TAv. are weighted averages according to analytical errors.
2
Calculated from 10Be and 26Al data given by Xue et al. (1994) following the model by Graf et al. (1990).
3
T21 exposure age given by the cited authors (4,7,10,12).
4
McCoy et al. (1997).
5
Average excluding LEW86220, MAC88177, and QUE93148.
6
Palme et al. (1981a).
7
Calculated from data given by Schultz et al. (1982).
8
Paired.
9
Schultz et al. (1982).
10
Paired.
11
Calculated from data given by McCoy et al. (1996).
12
McCoy et al. (1996).
13
Calculated from data given by Takaoka and Yoshida (1991).
14
Takaoka and Yoshida (1991).
Using the production rates by Eugster and Michel (1995), the
He ages are about 30% higher than the 21Ne ages. The original
publications (Table 13) confirm that this is an artifact of the
model: there too, the 3He ages listed are greater than the 21Ne
ages but have not been discussed. It seems that the production
rate ratio P3/P21 was underestimated. Because the production
rates P3 and P21 of all empirical models are closely linked to
each other by the correlation (3He/21Ne)c to (22Ne/21Ne)c,
limitations come into play that were discussed in the section on
production rates and on shielding. We consider the 3He age to
be the most reliable age obtainable with the production rates of
Eugster and Michel (1995), because it is the least dependent on
chemistry and shielding characteristics.
Using the calculated GCR production rates the 3He and the
21
Ne ages agree well on average (T3/T21 5 1.008 6 0.051).
This clearly shows that the calculated GCR production rates
account much better for the chemical composition of the lodranites and acapulcoites than the empirical production rates
for HEDs by Eugster and Michel (1995).
Using the calculated GCR and SCR production rates, the
difference between the 3He and the 21Ne ages increases
3
(T3/T21 5 1.086 6 0.119), but still is within the assumed
uncertainty of the exposure ages of ;15%.
10
Be/21Ne exposure ages (Table 13) were calculated from
10
Be data on bulk or silicate from Xue et al. (1994) and on 21Ne
data from this work following the model by Graf et al. (1990).
They are on average 17% lower than the 21Ne exposure age.
The terrestrial ages (,20 ka, except for Y791491 with ;110
ka) based on 36Cl data (G. Herzog, 1998, personal communication) are short compared to their exposure ages and can,
therefore, not explain the systematically lower 10Be/21Ne exposure ages. In two cases, Gibson and LEW88280, the 10Be/
21
Ne and the 21Ne exposure ages are identical within error
limits. This suggests that the model parameters for the calculation of the 10Be/21Ne exposure ages that were derived from
chondrites do not account for the non-chondritic chemistry, the
small size, and the low shielding conditions found for lodranites and acapulcoites.
The 26Al/21Ne exposure ages (Table 13) were calculated
from 26Al data on bulk or silicate from Xue et al. (1994) and on
21
Ne data from this work following the model by Graf et al.
(1990). They show a larger variation relative to the 21Ne
Lodranites and acapulcoites
189
Fig. 5. Mg/(Si 1 Al) elemental concentration ratios versus the ratio of cosmic-ray produced 22Ne/21Ne for lodranites
(squares) and acapulcoites (circles), following Begemann and Schultz (1988) and Garrison et al. (1995). The area between
the dashed lines represents the boundary between essentially pure galactic cosmic-ray Ne (GCR only) on the left and
increasing solar-cosmic ray Ne (SCR) on the right.
exposure ages, possibly due to the contribution of SCR produced 26Al. For Gibson and LEW88280, the 26Al/21Ne exposure ages agree within error limits with the 21Ne exposure ages,
proving its usefulness to determine possible contributions of
SCR-produced 26Al to other meteorites. The only other independently determined exposure age for Acapulco (5.7 Ma) by
the 36Cl/36Ar method (Graf et al., 1995) is 24% lower than our
adopted age. For LEW86220 an exposure age of 6 Ma was
adopted based on the fact that GCR calculated production rates
are, on average, 30% lower than those of Eugster and Michel
(1995).
Another explanation for non-matching radionuclide and noble gas exposure ages are multi-stage (complex) exposure
histories, that are shown to occur also for meteorites with
exposure ages ,10 Ma (Herzog et al., 1997). However, we do
not find the typical indications (T3/T21 , 1, T10 > T26 , T21)
for complex exposure histories in lodranites and acapulcoites.
Figure 6 shows a histogram of the cosmic-ray exposure ages
of lodranites and acapulcoites. H chondrites are chemically
similar to lodranites and acapulcoites. In spite of this similarity,
H chondrites have cosmic-ray exposure ages covering the range
from 0 to 80 Ma (Marti and Graf, 1992) while lodranites and
acapulcoites have very low exposure ages (generally ,10 Ma).
Their distribution does not show as wide a range as that of H
chondrites. The average cosmic ray exposure age for all lodranites, except for QUE93148, is 6.8 6 0.9 Ma whereas the
one for the acapulcoites is 6.1 6 2.1 Ma. Using GCR ages for
the ALH acapulcoites the average exposure age or acapulcoites
is 7.0 6 1.5 Ma. This would emphasize the exposure age peak
of the lodranites and acapulcoites around 6 –7 Ma even more.
This age cluster suggests that all lodranites (except for
Fig. 6. Exposure age histogram for lodranites and acapulcoites using
the adopted ages from Table 13.
190
A. Weigel et al.
QUE93148) and all acapulcoites originate from the same ejection event and, therefore, from the same parent body. Whether
the lodranite and acapulcoite exposure age cluster only accidentally coincides with that of the most frequently occurring
exposure age of the H chondrites (Marti and Graf, 1992),
remains to be explored.
6. SUMMARY AND CONCLUSIONS
Noble gas isotopic abundances of ten lodranites (EET84302,
FRO90011, Gibson, LEW86220, LEW88280, Lodran,
MAC88177, QUE93148, Y74357, Y791491) and four acapulcoites (Acapulco, ALH81187, ALH81261, ALH84190) as well
as major, minor, and trace element compositions of six lodranites (EET84302, Gibson, LEW88280, Lodran, MAC88177,
Y791491) are reported. All lodranites (except for QUE93148)
and acapulcoites yield similar abundances of cosmogenic noble
gases 3He, 21Ne, and 38Ar. Cosmogenic 3He in MAC88177 is
an order of magnitude lower than in all other lodranites indicating diffusion loss.
Existing empirical production rate models for cosmic ray
produced nuclides in achondrites rely on a silicate-rich bulk
chemical composition and commensurate shielding conditions.
The high metallic iron content in lodranites and acapulcoites
(22 6 13)% [H-chondrites: 11% (Dodd, 1981)] affects not only
the bulk chemical composition, but also the shielding: Little
21
Ne is produced by spallation in metallic Fe which makes the
3
He/21Ne versus 22Ne/21Ne correlation that is used in empirical
production rate models unusable for lodranites and acapulcoites
and metal rich meteorites in general. Because the conditions for
the application of existing empirical production rate models for
cosmic ray produced nuclides on lodranites and acapulcoites
were not fulfilled, we modeled the production rates of cosmogenic nuclides in lodranites and acapulcoites by galactic and
solar cosmic rays using a purely physical model. It was shown
that this model accounts for the effects of the high iron content,
for the special shielding conditions, and for the contribution of
solar cosmic ray produced nuclides in lodranites and acapulcoites. All lodranites and acapulcoites are relatively small meteorites having preatmospheric radii # 200 mm, about one-half
of them even #75 mm.
Multiple evidence was found for solar-cosmic ray produced
nuclides in the acapulcoites ALH77081, ALH81187,
ALH81261, and ALH84190. Solar-cosmic rays increase the
total production of cosmogenic 3He and 21Ne in these meteorites by ;27% and ;32%.
The cosmic-ray exposure ages derived for all lodranites (but
QUE93148 with 15 Ma) and all acapulcoites cluster around 6
Ma, suggesting, supported by the similar abundances of cosmogenic nuclides, similar shielding conditions, and similar
chemical compositions, that they all originate from one ejection
event from the same parent body.
Acknowledgments—We thank the NSF/NASA Meteorite Working
Group and the National Institute of Polar Research for providing the
Antarctic meteorites. We thank R. Hutchison (Natural History Museum, London) and G. Kurat (Naturhistorisches Museum, Wien) for the
Lodran samples, and Kurt Marti for the Acapulco sample. We are
grateful for the thorough reviews by U. Ott, R. Reedy, and T. Swindle.
We thank M. Zuber for preparing the samples and P. Guggisberg and
A. Schaller for their help in mass spectrometry. This research was
supported by the Swiss National Science Foundation and the Deutsche
Forschungsgemeinschaft, Bonn. A. Weigel acknowledges support by a
ESA Research fellowship during preparation of this manuscript.
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