Meteoritics & Planetary Science 46, Nr 7, 989–1006 (2011)
doi: 10.1111/j.1945-5100.2011.01205.x
Cosmogenic helium and neon in individual chondrules from Allende and Murchison:
Implications for the precompaction exposure history of chondrules
A. S. G. ROTH*, H. BAUR, V. S. HEBER, E. REUSSER, and R. WIELER
Department of Earth Sciences, ETH Zürich, NW D84, CH-8092 Zürich, Switzerland
*
Corresponding author. E-mail: [email protected]
(Received 20 May 2010; revision accepted 16 April 2011)
Abstract–We analyzed cosmogenic He and Ne in more than 60 individual chondrules
separated from small chips from the carbonaceous chondrites Allende and Murchison. The
goal of this work is to search for evidence of an exposure of chondrules to energetic
particles—either solar or galactic—prior to final compaction of their host chondrites and
prior to the exposure of the meteoroids to galactic cosmic rays (GCR) on their way to Earth.
Production rates of GCR-produced He and Ne are calculated for each chondrule based on
major element composition and a physical model of cosmogenic nuclide production in
carbonaceous chondrites (Leya and Masarik 2009). All studied chondrules in Allende show
nominal exposure ages identical to each other within uncertainties of a few hundred thousand
years. Allende chondrules therefore show no signs of a precompaction exposure. The majority
of the Murchison chondrules (the ‘‘normal’’ chondrules) also have nominal exposure ages
identical within a few hundred thousand years. However, roughly 20% of the studied
Murchison chondrules (the ‘‘pre-exposed’’ chondrules) contain considerably or even much
higher concentrations of cosmogenic noble gases than the normal chondrules, equivalent to
exposure ages to GCR at present-day fluxes in a 4p irradiation of up to about 30 Myr. The
data do not allow to firmly conclude whether these excesses were acquired by an exposure of
the pre-exposed chondrules to an early intense flux of solar energetic particles (solar cosmic
rays) or rather by an exposure to GCR in the regolith of the Murchison parent asteroid.
However, we prefer the latter explanation. Two major reasons are the GCR-like isotopic
composition of the excess Ne and the distribution of solar flare tracks in Murchison samples.
INTRODUCTION
Young stars emit copious amounts of energetic
particles (Feigelson and Montmerle 1999). Meteorites
may have preserved traces of such a presumed high
activity of the young Sun. It is widely believed that an
early solar cosmic-ray (SCR) irradiation caused some of
the isotopic anomalies found in meteorites, in particular
10
B excesses from the decay of 10Be (McKeegan et al.
2000; Chaussidon and Gounelle 2006). It has also been
suggested that calcium-aluminum-rich inclusions (CAIs)
in carbonaceous chondrites with atypically high initial
abundances of 53Mn and perhaps 26Al may have been
irradiated by energetic protons from the early Sun
(Nyquist et al. 2009). However, the group of elements on
which early irradiation effects in meteorites may have left
their fingerprints most conspicuously are the noble gases.
It is conceivable that noble gases produced during an
early irradiation of nebular solids by energetic
particles—either SCR or galactic cosmic rays
(GCR)—may still be recognizable in specific phases in
meteorites. If so, this would not only provide
information about the energetic particle environment in
the early solar system but conceivably might also help us
to constrain the lifespan between chondrule formation in
the solar nebula and their incorporation into larger
objects.
Much of the previous work searching for evidence
of precompaction exposures concentrated on chondrules
(Allen et al. 1980; Polnau et al. 1999, 2001; Eugster et al.
989
The Meteoritical Society, 2011.
990
A. S. G. Roth et al.
2007; Das and Murty 2009; Das et al. 2010). This is
because chondrules are abundant and possibly formed
close to the Sun (Shu et al. 1996). Because they formed
at high temperature, they are also poor in potentially
interfering primordial noble gases (Vogel et al. 2004)
and can be expected to have lost any putative
cosmogenic noble gas component present already in
their precursor material. Allen et al. (1980) searched for
excesses of nuclear tracks in chondrules induced by
energetic particles, but found no evidence of an exposure
of chondrules as independent objects in space. The
other studies searched for excesses of cosmogenic noble
gases in chondrules relative to matrix samples from the
same meteorites. Some chondrules indeed showed such
excesses. Although they were often relatively minor, the
authors concluded that the respective chondrules had
seen a higher fluence of energetic particles than the rest
of the meteorite and excluded alternative explanations
like larger than expected differences in production rates
between chondrules and matrix or loss of noble gases
from matrix. The interpretation of the excesses remained
ambiguous. Polnau et al. (1999, 2001) mentioned the
possibility that chondrules had suffered an exposure in
their parent body regolith prior to the final compaction
of the meteorite but seemed to prefer the idea that
chondrules or some of their constituents had been
subjected to a high fluence of energetic particles from the
early Sun, as may be expected in the scenario by Shu
et al. (1996). Eugster et al. (2007) found modest excesses
of cosmogenic noble gases in chondrules from only one
of 15 meteorites they studied, the H chondrite Dhajala.
These authors were not able to decide whether these
excesses had been acquired while the chondrules were
floating in free space or during an exposure on their
parent body regolith. Das and Murty (2009) also
reported occasional excesses in chondrules from some of
the 10 meteorites they studied, most notably again
in Dhajala and in Murray, a regolith breccia
containing implanted solar noble gases. These authors
suggested that chondrules might have been exposed
sporadically to a high fluence of solar energetic particles
as freely floating particles close to the early Sun. In a
recent preliminary publication, Das et al. (2010)
report occasional differences in the nominal
exposure ages of different fragments from the same
chondrule. They interpret these age differences as
evidence for an SCR irradiation from the early Sun,
because the mean attenuation length of SCR particles in
solid matter of a few millimeters (e.g., Hohenberg et al.
1978) is comparable to chondrule sizes, whereas the
mean attenuation length of GCR particles of some
50 cm is much larger.
In contrast to the mostly relatively minor noble gas
excesses reported for bulk chondrules so far, partly very
large excesses were found in individual olivine grains
separated from CM chondrites (Hohenberg et al. 1990).
Grains rich in nuclear tracks induced by heavy SCR
particles (solar flare tracks) also showed much higher
concentrations of cosmogenic 21Ne (21Necos) than grains
without tracks. If these excesses were caused by a GCR
irradiation on the surface of a parent body (2p
irradiation) with a flux similar to that today, the most
gas–rich grains would have suffered a precompaction
irradiation exceeding a hundred million years. Such long
parent body exposures seemed unlikely to these authors.
This and the correlation between cosmogenic noble gas
excesses and solar flare tracks led them to prefer the
hypothesis that the gas- and track-rich olivine grains in
CM chondrites testify to a much enhanced flux of SCR
in the early solar system. Already earlier, track-rich
grains in solar gas–rich meteorites had been interpreted
as probable evidence for a solar irradiation while the
grains were floating in the solar nebula (Lal and Rajan
1969; Pellas et al. 1969), although these authors also
considered the possibility of an irradiation of the grains
on a parent body.
The hypothesis that the gas- and track-rich
individual grains in CM chondrites testify to an early
active Sun, as well as similar earlier hypotheses based on
analyses of individual grains from solar gas–rich
ordinary chondrites and achondrites (e.g., Caffee et al.
1987), were challenged by Wieler et al. (2000). They
noted that a correlation between solar flare tracks and
excess cosmogenic noble gases does not necessarily imply
that the latter had also been produced by solar particles.
The correlation could also result from a mixture of
grains having been exposed to both SCR and GCR in a
well-mixed parent body regolith with grains having been
exposed neither to SCR nor GCR. The tracks would be
the result of SCR, the noble gases overwhelmingly the
result of GCR, just as it is the case in lunar soil samples.
Meteorites rich in solar noble gases indeed often
show effects of a differential precompaction exposure of
their constituents in an asteroidal regolith (e.g., Wieler
et al. 1989, 2000; Lorenzetti et al. 2003; Matsuda et al.
2009). In these cases, the observed excesses do not
allow to draw any conclusions about an early active
Sun. In hindsight, it also seems more straightforward to
interpret the early observations of track-rich grains in
solar gas–rich meteorites (Lal and Rajan 1969; Pellas
et al. 1969) to be the results of parent body
irradiations.
In summary, while precompaction effects are clearly
present in some meteorites, it remains unclear whether
any of them actually testify of an early active Sun and ⁄ or
an early irradiation by galactic or solar particles while
the respective meteorite phases were freely floating in the
solar nebula. For chondrules in particular, observed
Cosmogenic He and Ne in chondrules
excesses on top of concentrations of cosmogenic noble
gases acquired during the recent meteoroid exposure are
mostly relatively minor and often ambiguous. In this
work we study chondrules of the CV chondrite Allende
and the CM chondrite Murchison. Rather than to
compare analyses of a few chondrules from several
meteorites with data from respective matrix samples, our
approach is to compare data of a large number of
chondrules from the same meteorite, to look for possible
differences in the precompaction irradiation history of
chondrules from the same small meteorite piece. This
avoids potential problems due to uncertain differences in
cosmogenic noble gas production rates between
chondrules and matrix samples and possible noble gas
losses either from fine-grained matrix or perhaps also
from chondrules (e.g., Das and Murty 2009).
METEORITE SELECTION AND SAMPLE
PREPARATION
We analyzed 64 chondrules from the carbonaceous
chondrites Allende (CV3.2) and Murchison (CM2). Both
meteorites have low cosmic-ray exposure ages of a few
million
years,
i.e.,
acquired
relatively
small
concentrations of cosmogenic noble gases during their
meteoroid stage. This should facilitate the detection of
precompaction exposure effects. Both meteorites also
largely escaped thermal metamorphism on the parent
body, which might have led to a loss of previously
acquired noble gases. As discussed below, we will only
compare data for chondrules extracted from the same
centimeter-sized split with each other, to assure negligible
shielding-induced variations of cosmogenic noble gas
production rates. Production rate differences due to
variable chemical composition of chondrules are
controlled by major element analyses. Both
unequilibrated
meteorites
contain
well-defined
chondrules that can be extracted without difficulties.
Murchison was also selected because of the large
precompaction effects in single olivine grains reported by
Hohenberg et al. (1990).
Chondrules and a few matrix samples from three
chips of Allende (A1–A3) and two chips of Murchison
(M1 and M2) were studied, the original position of the
chips relative to each other being unknown. We will
show below that our chips of Allende are free of solar
noble gases. On the other hand, our Murchison chips
contain solar noble gases, as is common for this
meteorite (Schultz and Franke 2004), indicating that it
was once exposed to the solar wind in its parent body
regolith. The sample nomenclature is as follows: the first
letter is the meteorite identifier (A = Allende,
M = Murchison), followed by the chip number and
sample type (Ch = chondrule, Ma = Matrix). The last
991
two digits represent the specific sample number. For
example, M2-Ch-03 means chondrule Nr. 3 from chip
Nr. 2 of Murchison.
Noble gases in chondrules from chip Nr. 1 of
Allende were analyzed by in situ UV-laser ablation on a
20 · 16 · 2 mm sized hand-polished section. All other
analyzed chondrules were separated from their
centimeter-sized host chip, and noble gases were
extracted by melting samples with an infrared laser. This
allowed a much larger sample mass than in situ laser
ablation. Meteorite chips were disaggregated by freezethaw cycling. Each chip was individually enclosed in a
small container filled with deionized water and
alternately immersed in boiling nitrogen and warm
water, respectively. After a dozen cycles, chips had
decayed into unconsolidated fine-grained material.
Unbroken chondrules were handpicked under a
stereomicroscope based on their spheroidal and
polycrystalline morphologies. Special care was taken to
avoid detrital olivine grains in Murchison samples. These
grains were identifiable by their distinctive angular and
monocrystalline morphologies. Chondrules were then
individually abraded in a corundum abrasion cell, to
remove possible noble gas–rich rims, which could have
compromised the determination of the cosmogenic
component in the chondrules. Abraded chondrules were
ultrasonically cleaned with acetone and then split using a
razor blade. The largest fragment of each chondrule was
weighed and reserved for noble gas analyses, while
smaller fragments were analyzed by electron microprobe
(EPMA) for major elements.
NOBLE GAS AND MAJOR ELEMENT ANALYSES
Noble Gas Extraction by UV-Laser Ablation
Sample chip Allende A1 was preheated in vacuum
(approximately at 100 C, 24 h) before analysis, to
remove loosely bound atmospheric noble gases. Gases
from chondrules and matrix samples were released by
ablation with a quintupled Nd:YAG laser with a
wavelength of 213 nm (Heber et al. 2009). The laser
beam with a spot size of 65 lm was rastered over
rectangular areas of typically 500 · 500 lm at
100 lm s)1 with a pulse repetition rate of 20 Hz. Some
20 passes led to a pit depth of 25–30 lm. Cracks and the
ablation of matrix material were avoided. The ablated
masses (Table 1) were estimated by measuring laser pit
depths with a NewView 5000 Zygo white light
interferometer, while pit perimeters were mapped on
photomicrographs. The assumed density of Allende
chondrules was 3.2 g cm)3, slightly higher than the bulk
density of Allende of 2.9 g cm)3 (Consolmagno and
Britt 1998).
A1-Ch-01
A1-Ch-02
A1-Ch-03
A1-Ch-04
A1-Ch-05
A1-Ch-06
A1-Ch-07
A1-Ch-08
A1-Ch-09
A1-Ch-10
A1-Ch-11
A1-Ch-12
A1-Ch-13
A1-Ma-01
A1-Ma-02
A1-Ma-03
A1-Ma-04
A1 average
A1 SD
A2-Ch-01
A2-Ch-02
A2-Ch-03
A2-Ch-04
A2-Ch-05
A2-Ma-01
A2-Ma-02
A2 average
A2 SD
A3-Ch-01
A3-Ch-02
A3-Ch-03
A3-Ch-04
A3-Ch-05
A3-Ch-06
A3-Ch-07
A3-Ch-08
A3 average
A3 SD
A average
A SD
M1-Ch-01
M1-Ch-02
M1-Ch-03
M1-Ch-04
Sample
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
174.8
294.9
303.4
958.9
1997.2
677.8
526.6
156.6
175.7
258.8
398.7
634.2
680.6
1028.9
1300
5.5
4.1
7.1
2.3
3.7
4.1
4.5
25.2
21.6
12.1
18.8
24.4
25.2
18.9
29.2
24.6
38.0
Weight
(lg)
0.1
0.1
0.1
0.1
0.1
1.1
0.6
0.1
0.9
0.3
0.6
0.5
1.1
0.2
0.4
0.5
0.1
0.2
0.4
6.82
0.74
6.70
8.035
8.034
7.993
7.735
7.301
7.311
7.70
0.57
7.75
8.43
8.187
7.778
8.566
7.706
7.144
7.693
7.91
0.46
7.39
0.78
3.025
0.782
2.906
2.37
21.74
9.47
9.43
5.77
7.46
6.98
6.63
6.060
8.09
6.48
7.32
6.11
7.326
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
0.067
0.022
0.056
0.11
0.10
0.11
0.070
0.050
0.042
0.024
0.030
0.022
0.10
0.064
0.066
0.026
0.027
0.040
0.030
0.86
0.69
0.49
0.15
0.17
0.25
0.18
0.089
0.20
0.24
0.11
0.15
0.094
He
(10)8 cm3STP g)1)
3
Ne
1.550
1.074
1.273
1.79
2.952
3.050
2.883
2.716
3.027
2.779
2.244
2.430
2.039
2.519
2.630
2.480
2.384
2.145
2.262
19.80
5.73
6.21
2.54
3.53
4.32
3.49
2.59
3.86
3.30
3.51
2.99
3.51
22
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
0.068
0.056
0.051
0.15
0.050
0.037
0.029
0.024
0.027
0.025
0.020
0.021
0.030
0.030
0.023
0.021
0.020
0.021
0.028
0.81
0.37
0.40
0.12
0.16
0.15
0.13
0.11
0.10
0.18
0.12
0.10
0.12
He
He
399.4
2415
603.5
882
433.4
327.2
426.0
661.6
334.1
222.6
726.8
426.8
683.2
426.8
420.2
517.0
500.1
329.8
376.9
298
138
144.7
783
188.8
842
547
681
899
595
379.3
617
580.1
504
442
598
494.9
3
4
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
4.9
34
6.8
75
5.4
3.8
3.6
5.6
2.5
1.5
4.7
2.5
8.9
3.5
3.5
3.2
3.1
2.5
2.5
12
10
7.5
21
4.3
30
15
10
22
22
5.7
16
7.4
22
13
14
8.9
Ne
Ne
3.84
7.03
4.19
6.00
0.969
0.958
0.894
0.935
0.8811
0.9437
1.0111
0.9345
1.024
0.9310
0.892
0.8915
1.0158
3.300
3.848
7.73
5.61
5.43
2.55
2.33
5.32
3.19
2.17
3.51
4.43
3.37
3.18
2.350
8.33
5.74
5.99
6.10
22
20
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
0.30
0.43
0.30
0.72
0.038
0.012
0.010
0.010
0.0071
0.0068
0.0093
0.0050
0.029
0.0091
0.010
0.0049
0.0039
0.025
0.033
0.35
0.41
0.40
0.14
0.12
0.23
0.14
0.13
0.12
0.26
0.14
0.13
0.093
0.41
0.15
0.16
0.19
Ne
Ne
0.60
0.27
0.66
0.36
0.932
0.912
0.9227
0.8720
0.9351
0.9313
0.8706
0.9085
0.867
0.8870
0.8870
0.8909
0.8819
0.6217
0.5856
0.242
0.632
0.614
0.738
0.820
0.495
0.747
0.789
0.681
0.605
0.724
0.699
0.743
0.275
0.463
0.347
0.375
22
21
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
0.018
0.011
0.016
0.022
0.019
0.015
0.0066
0.0085
0.0090
0.0064
0.0064
0.0066
0.012
0.0071
0.0095
0.0085
0.0047
0.0042
0.0081
0.013
0.054
0.043
0.039
0.043
0.030
0.033
0.039
0.026
0.043
0.031
0.033
0.027
0.021
0.021
0.012
0.014
Table 1. He and Ne concentrations and isotopic compositions of Allende and Murchison chondrules and matrix samples.
2.32
0.35
1.765
2.233
2.332
2.209
2.101
1.318
1.305
2.13
0.22
2.751
2.780
2.660
2.368
2.830
2.587
1.952
2.206
2.52
0.31
2.34
0.33
0.920
0.264
0.833
0.609
4.35
3.53
3.73
1.86
2.87
2.08
2.58
2.03
2.60
1.96
2.51
2.07
2.59
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
0.050
0.019
0.040
0.067
0.072
0.056
0.033
0.031
0.037
0.029
0.023
0.025
0.037
0.032
0.032
0.028
0.021
0.016
0.025
0.33
0.39
0.37
0.13
0.20
0.15
0.15
0.14
0.12
0.18
0.14
0.12
0.13
Necos
(10)8 cm3STP g)1)
21
992
A. S. G. Roth et al.
He
(10)8 cm3STP g)1)
3
22
Ne
3
4
He
He
22
20
Ne
Ne
Ne
Ne
22
21
Necos
(10)8 cm3STP g)1)
21
M1-Ch-05
2.7 ± 0.1
3.60 ± 0.14
2.07 ± 0.12
546 ± 42
4.58 ± 0.39
0.53 ± 0.016
1.063 ± 0.070
M1-Ch-06
3.0 ± 0.1
0.606 ± 0.028
1.170 ± 0.086
1167 ± 257
7.91 ± 0.71
0.25 ± 0.015
0.261 ± 0.028
M1-Ch-07
10.4 ± 0.1
14.20 ± 0.21
5.66 ± 0.15
82.7 ± 3.1
1.249 ± 0.038
0.86 ± 0.008
4.89 ± 0.13
M1-Ch-08
11.5 ± 0.1
2.908 ± 0.044
1.048 ± 0.028
535 ± 14
2.93 ± 0.17
0.85 ± 0.014
0.887 ± 0.028
M1-Ch-09
1.6 ± 0.1
2.95 ± 0.19
2.57 ± 0.21
531 ± 59
5.98 ± 0.57
0.39 ± 0.020
0.97 ± 0.10
M1-Ch-10
16.3 ± 0.1
2.339 ± 0.031
0.590 ± 0.023
821.8 ± 8.1
3.68 ± 0.30
0.98 ± 0.025
0.571 ± 0.027
M1-Ch-11
29.6 ± 0.1
2.095 ± 0.026
0.744 ± 0.019
1681.3 ± 9.1
2.519 ± 0.090
0.75 ± 0.011
0.554 ± 0.016
M1-Ch-12
49.6 ± 0.1
3.212 ± 0.036
0.755 ± 0.019
260.0 ± 1.8
1.979 ± 0.061
1.24 ± 0.013
0.936 ± 0.025
M1-Ch-13
1.1 ± 0.1
2.91 ± 0.27
4.10 ± 0.43
5.42 ± 0.51
0.41 ± 0.017
1.61 ± 0.19
M1-Ch-14
2.6 ± 0.1
2.42 ± 0.10
1.90 ± 0.11
357 ± 17
5.96 ± 0.43
0.41 ± 0.019
0.747 ± 0.058
M1-Ma-01
56.5 ± 0.1
6.509 ± 0.074
8.59 ± 0.20
3150.6 ± 6.4
10.53 ± 0.14
0.06001 ± 0.00050
0.274 ± 0.013
M1 average
2.37
0.71
M1 SD
0.98
0.28
M2-Ch-01
11.0 ± 0.1
2.246 ± 0.035
0.830 ± 0.028
305.5 ± 3.3
3.32 ± 0.20
0.700 ± 0.017
0.574 ± 0.024
M2-Ch-02
12.4 ± 0.1
2.193 ± 0.032
0.906 ± 0.029
861.4 ± 5.6
3.01 ± 0.16
0.719 ± 0.014
0.645 ± 0.024
M2-Ch-03
16.7 ± 0.1
2.702 ± 0.035
1.043 ± 0.026
411.0 ± 2.0
2.132 ± 0.088
0.7638 ± 0.0080
0.792 ± 0.022
M2-Ch-04
41.0 ± 0.1
2.631 ± 0.031
0.150 ± 0.007
911.1 ± 2.7
8.54 ± 0.82
3.12 ± 0.12
0.465 ± 0.028
M2-Ch-05
53.6 ± 0.1
2.404 ± 0.027
0.788 ± 0.018
662.1 ± 1.8
1.605 ± 0.040
0.8319 ± 0.0056
0.654 ± 0.016
M2-Ch-06
8.8 ± 0.1
2.343 ± 0.041
0.792 ± 0.028
1312 ± 10
3.65 ± 0.25
0.748 ± 0.019
0.585 ± 0.026
M2-Ch-07
8.5 ± 0.1
2.835 ± 0.049
1.225 ± 0.042
376.1 ± 4.0
2.68 ± 0.17
0.735 ± 0.015
0.893 ± 0.036
M2-Ch-08
8.0 ± 0.1
3.078 ± 0.053
1.149 ± 0.048
595.1 ± 4.0
2.96 ± 0.24
0.706 ± 0.019
0.803 ± 0.040
M2-Ch-09
46.9 ± 0.1
2.841 ± 0.033
0.850 ± 0.021
590.7 ± 1.6
1.671 ± 0.043
0.8146 ± 0.0074
0.690 ± 0.018
M2-Ch-10
58.3 ± 0.1
4.723 ± 0.053
1.548 ± 0.036
600.9 ± 1.2
1.057 ± 0.022
0.8688 ± 0.0056
1.343 ± 0.032
M2-Ch-11
12.2 ± 0.1
3.400 ± 0.049
1.253 ± 0.035
591.8 ± 3.2
2.39 ± 0.10
0.717 ± 0.011
0.892 ± 0.028
M2-Ch-12
0.2 ± 0.1
15.6 ± 7.8
20 ± 10
233 ± 27
6.71 ± 0.34
0.281 ± 0.015
5.2 ± 2.8
M2-Ch-13
0.5 ± 0.1
8.1 ± 1.6
7.3 ± 1.6
699 ± 30
6.35 ± 0.71
0.338 ± 0.020
2.33 ± 0.54
M2-Ch-14
4.8 ± 0.1
2.497 ± 0.064
1.192 ± 0.054
1146 ± 13
4.48 ± 0.36
0.506 ± 0.017
0.590 ± 0.034
M2-Ch-15
42.6 ± 0.1
2.454 ± 0.028
0.751 ± 0.020
803.5 ± 2.4
1.403 ± 0.059
0.831 ± 0.010
0.622 ± 0.018
M2-Ch-16
173.0 ± 0.1
2.606 ± 0.029
0.953 ± 0.022
1719.9 ± 1.8
1.637 ± 0.018
0.8191 ± 0.0052
0.778 ± 0.019
M2-Ch-17
3.8 ± 0.1
1.743 ± 0.054
1.216 ± 0.056
614 ± 18
5.16 ± 0.33
0.390 ± 0.014
0.457 ± 0.028
M2-Ch-18
14.6 ± 0.1
41.90 ± 0.54
17.16 ± 0.41
74.79 ± 0.18
0.890 ± 0.013
0.8943 ± 0.0062
15.34 ± 0.39
M2-Ch-19
14.8 ± 0.1
2.779 ± 0.039
1.513 ± 0.046
373.9 ± 4.6
4.64 ± 0.14
0.5398 ± 0.0095
0.798 ± 0.029
M2-Ch-20
16.2 ± 0.1
2.527 ± 0.034
1.157 ± 0.033
760.2 ± 5.5
3.34 ± 0.11
0.660 ± 0.011
0.753 ± 0.025
M2-Ch-21
18.9 ± 0.1
37.25 ± 0.46
9.32 ± 0.22
38.13 ± 0.24
1.148 ± 0.016
0.8668 ± 0.0063
8.07 ± 0.20
M2-Ch-22
16.7 ± 0.1
2.405 ± 0.033
1.092 ± 0.030
1434.5 ± 9.6
4.72 ± 0.11
0.5535 ± 0.0083
0.591 ± 0.019
M2-Ch-23
60.7 ± 0.1
21.39 ± 0.24
7.36 ± 0.17
76.51 ± 0.20
1.090 ± 0.014
0.8752 ± 0.0059
6.43 ± 0.16
M2-Ch-24
14.5 ± 0.1
12.14 ± 0.16
5.28 ± 0.13
196.7 ± 1.5
2.936 ± 0.045
0.7084 ± 0.0059
3.70 ± 0.10
M2-Ma-01
67.8 ± 0.1
9.18 ± 0.10
8.24 ± 0.19
2910.0 ± 2.9
11.22 ± 0.14
0.0547 ± 0.0006
0.203 ± 0.011
M2 average
2.62
0.71
M2 SD
0.33
0.11
M average
2.52
0.71
M SD
0.66
0.19
Notes: Discarded samples are shown in italics, preirradiated Murchison chondrules (see text and Fig. 2) in bold face. Average values do not include matrix samples, and in
the case of Murchison the averages refer to the ‘‘normal’’ chondrules only (not in bold face).
Sample
Weight
(lg)
Table 1. Continued. He and Ne concentrations and isotopic compositions of Allende and Murchison chondrules and matrix samples.
Cosmogenic He and Ne in chondrules
993
994
A. S. G. Roth et al.
Noble Gas Extraction by IR-Laser Heating
Gases from all other samples were released by
melting with an Nd:YAG infrared laser (k = 1064 nm)
in continuous wave mode (Heck et al. 2007). All samples
were preheated in vacuum (at about 100 C, 24 h) prior
to analysis. The laser power was raised until the sample
started to glow on the video monitor and subsequently
melted to form a sphere. Then, lasering was continued at
a slightly higher power level for another 3–5 min. In
most re-extractions of previously melted samples no
additional 21Necos was observed, indicating efficient
degassing in the main extraction step. In a few cases, a
small amount of 21Necos—never exceeding 8% of the
amounts found in the main extraction step—was released
in the re-extraction and added to obtain the total
concentration of 21Necos.
Noble Gas Mass Spectrometry
The evolved gases were first cleaned on hot Zr-Ti
pellets and commercial Zr-Al getters. He and Ne
isotopes were analyzed in two noncommercial sector field
noble gas mass spectrometers. The spectrometer used for
the Allende samples is equipped with an electron
multiplier operated in counting mode and a Faraday cup
for higher ion currents. The electron energy in the ion
source was 45 eV only, to suppress interferences by
doubly charged 40Ar and CO2 on the 20Ne and 22Ne
signals, respectively. To further reduce interferences, Ar,
Kr, and Xe and other condensable gases were frozen
onto charcoal traps cooled by boiling nitrogen, one of
them being connected to the mass spectrometer volume
during He and Ne analysis. Argon, Kr, and Xe were not
analyzed. Tests had shown that to guarantee efficient Ar
extraction would have required higher laser power which
would have compromised Ne blanks.
Due to their much smaller sizes, the Murchison
chondrules were analyzed in a special purpose mass
spectrometer with about two orders of magnitude higher
sensitivity for He and Ne than the spectrometer
described above. A molecular drag pump concentrates
the noble gases into the ion source, which is a
miniaturized version of a conventional Baur–Signer ion
source (Baur 1999). The ionization energy has been set
to 40 eV, again to minimize doubly charged 40Ar and
CO2. Apart from an electron multiplier in counting
mode, this spectrometer is equipped with a Faraday cup
collector in a separate arm of the flight tube. This leads
to a high abundance sensitivity on 3He, allowing the
detection of small amounts of this isotope in the presence
of large amounts of 4He. Gas cleaning is achieved by
similar procedures as described for the first analysis
system. The very low sample gas amounts were measured
by a modified version of the method described by Heck
et al. (2007). Helium was separated from Ne by freezing
the latter gas on metal frits at 14 K. First, both He
isotopes were detected simultaneously (4He on the
Faraday cup, 3He on the electron multiplier). In a second
step, Ne was analyzed in peak-jump mode by monitoring
mass spectrometer signals before and after sample gas
inlet. The Ne amount in a sample was determined by
extrapolation of the respective signals to gas inlet time.
In both systems, spectrometer sensitivities were
determined by peak height comparison of sample signals
with signals from accurately known quantities of
mixtures of pure He and Ne (Heber et al. 2009). Ar, Kr,
and Xe cannot be analyzed in this system because, due to
high ion pumping rates, signal intensity drops too fast.
Blanks, Interference Corrections, and Uncertainties
Hot blanks (firing the laser on an empty spot of the
target holder) were frequently measured. Apart from
interference corrections discussed in the next paragraph,
sample Ne data were not blank-corrected, however, since
we are mainly interested in concentrations of cosmogenic
21
Ne. Noncosmogenic contributions on 21Ne were
corrected for by assuming trapped Ne to be of
atmospheric isotopic composition (or solar composition
for the Murchison matrix samples, see the Results
section). This is justified because Ne blanks and trapped
Ne in those samples where the correction was sizeable at
all were essentially of atmospheric isotopic composition
(except for Murchison matrix samples). Data for 3,4He
were corrected for hot blank contributions. These
corrections amounted to <2% for 3He. Blanks for 4He
were <1% for all Allende samples and <3% for most
Murchison samples. Some low-mass Murchison
chondrules required 4He blank corrections of up to 50%,
but this does not compromise the calculations of 3He
exposure ages.
Stated uncertainties of gas concentrations in Table 1
(1r) include ion counting statistics, uncertainties in the
sensitivity variations of the mass spectrometers (<1%),
He blank corrections, and uncertainties in sample masses
of all samples analyzed by IR-laser extraction. For the
A1 chondrules uncertainties of the estimated ablated
sample masses are difficult to estimate and were
therefore not propagated.
Despite the low electron energies in the ion sources,
corrections for doubly charged 40Ar and CO2 on 20Ne
and 22Ne signals, respectively, were applied. We
determined a 40Ar++ ⁄ 40Ar+ ratio of 0.0012 and a
CO2++ ⁄ CO2+ ratio of (6 ± 1) · 10)4 at 45 eV electron
energy. Corrections for 40Ar++ never exceeded 0.3% for
Allende samples and 1.5% for Murchison samples.
Corrections for 44CO2++ typically were <1% but
Cosmogenic He and Ne in chondrules
995
amounted to up to about 5% for some low-mass
Murchison chondrules, and were even on the order of
10% for the samples of Allende chip A1 measured by
UV-laser extraction. Uncertainties of the 44CO2++
corrections may be a reason for the few anomalously
high 21Ne ⁄ 22Ne ratios reported below.
Major Element Concentrations
To determine chondrule-specific production rates of
cosmogenic 3He and 21Ne, concentrations of major
elements were measured for all chondrules analyzed by
IR-laser extraction in the chondrule fragments reserved
for this purpose. Samples were mounted in epoxy,
polished, and carbon-coated. An average of 24 spots per
fragment was measured by EPMA. A defocused
approximately 100 lm-sized beam and an energydispersive spectrometer (EDS) acquisition mode were
used for Allende chondrules because of their relatively
large size, while the smaller Murchison chondrule
fragments were analyzed in wavelength-dispersive
spectrometer (WDS) mode with a 10 lm-sized beam. Mg
and Si concentrations of single spots (the two elements
contributing most to 21Ne production) typically showed
a standard deviation of 9% and 6%, respectively. We
conservatively adopt this spot-to-spot scatter as the
uncertainty of the element concentrations in each
chondrule. To test the method, three Allende chondrules
were split into three to four fragments each and analyzed
(Table S1). Mean major element concentrations for each
fragment generally agree within one standard deviation
with the averages of the paired fragments. We will
demonstrate in the next section that the major element
concentration data obtained here allow reliable
chondrule-specific noble gas production rates to be
deduced.
RESULTS
Noble Gas Concentrations
He and Ne concentrations and isotopic compositions
for 26 chondrules and two matrix samples of Allende
and 38 chondrules and two matrix samples of Murchison
are given in Table 1. For another four Allende matrix
samples only the isotopic composition is reported. The
first three chondrule samples of Allende chip 1 (analyzed
by UV-laser ablation) given in italics had very low
masses and correspondingly large uncertainties of their
noble gas concentrations. Therefore, they are not further
considered. For the same reason, the data of Murchison
chondrules M2-Ch-12 and M2-Ch-13 are not considered
below. Another small sample (M2-Ch-17) is discarded
due to a lack of major element data.
Fig. 1. Ne three-isotope diagram of Allende (upper panel) and
Murchison (lower panel) chondrule and matrix samples
analyzed in this study. Sample labeling is described in the text.
Three potential trapped Ne components are indicated by
diamond symbols: ‘‘solar wind’’ (Heber et al. 2009), ‘‘air,’’ and
‘‘primordial’’ (the Ne-HL composition from Ott 2002). The
inset in the two panels is a blow-up of the area on the lower
right of the main panel, emphasizing data points representing
essentially pure cosmogenic Ne. Black squares and circles in the
lower panel represent the seven Murchison chondrules with
noble gas excesses. Not shown are the data points of the
discarded samples A1-Ch-01, A1-Ch-02, A1-Ch-03, M1-Ch-10,
M1-Ch-12, M2-Ch-04, M2-Ch-12, M2-Ch-13, and M2-Ch-17.
Figure 1 shows the Ne data. All Allende chondrules
measured by IR-laser melting (chips A2 and A3) contain
almost pure cosmogenic Ne with measured 20Ne ⁄ 22Ne
ratios <1. The measured 21Ne ⁄ 22Ne ratios of these
chondrules are therefore essentially identical to the
values of their cosmogenic component. Chondrules from
chip A2 on average have slightly lower 21Ne ⁄ 22Ne ratios
than those of chip A3 (Table 1), indicating slightly lower
shielding of chip A2 during Allende’s meteoroid
exposure. The lack of noncosmogenic Ne in the IRmelted chondrules (devoid of gas–rich rims) strongly
996
A. S. G. Roth et al.
indicates that the minor amounts of noncosmogenic Ne
in the UV-laser-ablated chondrules in Allende chip A1
are not indigenous to the chondrules but are blank Ne
with atmospheric composition (see previous section).
This also becomes evident by the fact that amounts of
noncosmogenic Ne in these samples are similar to blank
Ne values. The correction for noncosmogenic 21Ne in the
A1 samples is very small and does hardly depend on the
exact composition of the noncosmogenic component.
In contrast to Allende, all analyses of Murchison
chondrules (obtained all by IR-laser heating) revealed
some noncosmogenic Ne. Both Murchison matrix samples
contain trapped Ne of approximately fractionated solar
wind composition (20Ne ⁄ 22Ne = 11.2; Grimberg et al.
2008). However, as shown in Fig. 1b, the noncosmogenic
Ne in the (abraded and rim-free) chondrules is
predominantly atmospheric, and, like for the Allende A1
samples, derived mostly from the blank. Also for the
Murchison chondrules the correction for noncosmogenic
21
Ne does not critically depend on the exact isotopic
composition of the noncosmogenic Ne. Exceptions are the
three chondrules M1-Ch-10, M1-Ch-12, and M2-Ch-04
with unexpectedly high (21Ne ⁄ 22Ne)cos ratios, for which we
have no clear explanation except for a possibly inaccurate
correction for doubly charged 40Ar or CO2. The data of
these three chondrules are not considered any further.
The blank-corrected 3He in all chondrules is
assumed to be entirely cosmogenic. This is justified also
for Murchison since the Ne data strongly indicate no
contamination of the Murchison chondrules with solar
Ne, and hence solar 3He. As we will see below, the
Murchison He data by themselves make it unlikely that
solar 3He constitutes a sizeable fraction to the measured
3
He in any chondrule.
Major Element Concentrations and Production Rates of
Cosmogenic He and Ne
high concentrations of Na, Al, and Ca contain large
proportions of feldspathic mesostasis. Low totals may
reflect the presence of hydrated mineral phases that
replace chondrule mesostasis. For all chondrules from
Allende chip A1, analyzed by UV-laser ablation, we
adopted the average chemical composition of the
chondrules from chips A2 and A3 (Table S1). For
the matrix samples of both meteorites, we adopted the
elemental composition given by Jarosewich (1990).
The production rates of cosmogenic 3He and 21Ne
for each chondrule are shown in Table 2. They are based
on the major element concentrations given in Table S1
and depth- and size-dependent elemental production
rates provided by Leya and Masarik (2009) for a
carbonaceous chondritic composition of the bulk
meteorite. Because the dependence of the (22Ne ⁄ 21Ne)cos
ratio on shielding is ill-constrained for carbonaceous
chondrites and also not very precisely defined for our
samples containing noncosmogenic Ne, we had to
assume meteoroid radii and especially shielding depths of
our samples. For Allende we adopted a radius of 65 cm
as proposed by Nishiizumi et al. (1991) and—
arbitrarily—for all chips the same shielding depth
of 20 cm. For Murchison we assumed a radius of
35 cm—reasonable for a recovered mass of about
100 kg—and a shielding depth of 10 cm. The choice of
shielding depths is explained in the next section. Note
that the uncertain shielding will introduce an additional
uncertainty in the calculated absolute exposure ages.
However, this is of no concern herein, since we are
primarily interested in differences between nominal
exposure ages of chondrules from the same sample chip.
These chips were small enough (about 1–2 cm) that
variations of noble gas production rates due to different
shielding during the meteoroid exposure to GCR are
negligible.
Exposure Ages
The concentrations of 13 elements in all chondrules
in which noble gases were analyzed by IR-laser
extraction are reported in Table S1 in the electronic
annex. Oxygen concentrations are given as complement
of the respective oxides. The table also shows the results
of the reproducibility test on multiple fragments from
three Allende chondrules. Concentrations of the most
important elements for production of cosmogenic He
and Ne (Mg, Si, Al, and Fe) in each fragment from the
same chondrule mostly agree to within better than
±15%, although concentrations of these elements in
different chondrules vary by up to a factor of about 3 for
Mg and about 6 for Al. The major element
concentrations determined on one chondrule fragment
and the resulting noble gas production rates are therefore
representative for the entire chondrule. Chondrules with
Table 2 shows the exposure ages of all samples
analyzed calculated as explained in the previous section.
Figure 2 shows all ages considered reliable (i.e.,
excluding the samples in italics in Tables 1 and 2). Note
again that these nominal ages do not all necessarily have
a physical meaning, as the underlying assumption that
the samples acquired all their cosmogenic noble gases
during a single 4p exposure is exactly what is tested here.
The most salient features of Fig. 2 are:
1. All Allende chondrules have similar concentrations
of cosmogenic 3He and 21Ne, respectively. All 3He as
well as all 21Ne exposure ages, respectively, are
therefore essentially identical within uncertainties.
The overall average (3He and 21Ne) age of all
Allende chondrules is approximately 4.3 Ma. For
Cosmogenic He and Ne in chondrules
997
Table 2. 3He and 21Ne production rates and exposure ages.
Sample
A1-Ch-01
A1-Ch-02
A1-Ch-03
A1-Ch-04
A1-Ch-05
A1-Ch-06
A1-Ch-07
A1-Ch-08
A1-Ch-09
A1-Ch-10
A1-Ch-11
A1-Ch-12
A1-Ch-13
A1 average
A1 SD
A2-Ch-01
A2-Ch-02
A2-Ch-03
A2-Ch-04
A2-Ch-05
A2-Ma-01
A2-Ma-02
A2 average
A2 SD
A3-Ch-01
A3-Ch-02
A3-Ch-03
A3-Ch-04
A3-Ch-05
A3-Ch-06
A3-Ch-07
A3-Ch-08
A3 average
A3 SD
A average
A SD
M1-Ch-01
M1-Ch-02
M1-Ch-03
M1-Ch-04
M1-Ch-05
M1-Ch-06
M1-Ch-07
M1-Ch-08
M1-Ch-09
M1-Ch-10
M1-Ch-11
M1-Ch-12
M1-Ch-13
M1-Ch-14
M1-Ma-01
M1 average
M1 SD
P21
P3
(10)8 cm3STP g)1 Ma)1)
1.89
2.05
2.05
2.02
1.99
1.87
1.87
0.37
0.53
0.53
0.48
0.46
0.30
0.30
2.03
2.10
2.02
1.97
2.06
2.04
1.91
1.92
0.53
0.58
0.49
0.44
0.55
0.53
0.40
0.42
1.93
1.77
1.97
1.82
1.98
1.67
1.95
1.95
1.94
1.87
1.84
1.98
1.94
1.96
1.79
0.42
0.30
0.51
0.33
0.52
0.21
0.50
0.49
0.49
0.38
0.35
0.51
0.48
0.50
0.18
T3
(Ma)
10.9 ±
4.7 ±
4.7 ±
2.88 ±
3.73 ±
3.5 ±
3.31 ±
3.03 ±
4.0 ±
3.2 ±
3.65 ±
3.05 ±
3.66 ±
3.4
0.4
3.54 ±
3.93 ±
3.93 ±
3.95 ±
3.90 ±
3.91 ±
3.91 ±
3.8
0.2
3.81 ±
4.02 ±
4.05 ±
3.95 ±
4.16 ±
3.78 ±
3.74 ±
4.00 ±
3.9
0.1
3.7
0.4
1.57 ±
0.44 ±
1.48 ±
1.31 ±
1.82 ±
0.36 ±
7.3 ±
1.49 ±
1.5 ±
1.25 ±
1.14 ±
1.63 ±
1.5 ±
1.23 ±
1.2
0.5
T21
0.4
0.3
0.2
0.08
0.08
0.1
0.09
0.04
0.1
0.1
0.05
0.07
0.05
0.05
0.03
0.03
0.01
0.01
0.02
0.02
0.05
0.05
0.03
0.03
0.02
0.01
0.02
0.01
0.03
0.01
0.03
0.06
0.07
0.02
0.1
0.02
0.1
0.02
0.01
0.02
0.1
0.05
9.0 ±
7.3 ±
7.7 ±
3.8 ±
5.9 ±
4.3 ±
5.3 ±
4.2 ±
5.4 ±
4.0 ±
5.2 ±
4.3 ±
5.3 ±
4.8
0.7
4.7 ±
4.23 ±
4.42 ±
4.63 ±
4.55 ±
4.43 ±
4.38 ±
4.5
0.2
5.2 ±
4.8 ±
5.38 ±
5.37 ±
5.15 ±
4.87 ±
4.88 ±
5.20 ±
5.1
0.2
4.8
0.5
2.2 ±
0.87 ±
1.64 ±
1.9 ±
2.1 ±
1.3 ±
9.8 ±
1.82 ±
2.0 ±
1.51 ±
1.60 ±
1.82 ±
3.4 ±
1.5 ±
1.48 ±
1.7
0.4
0.7
0.8
0.8
0.3
0.4
0.3
0.3
0.3
0.2
0.4
0.3
0.2
0.3
0.1
0.06
0.06
0.06
0.05
0.05
0.08
0.1
0.1
0.07
0.07
0.07
0.06
0.06
0.06
0.1
0.06
0.08
0.2
0.1
0.1
0.3
0.06
0.2
0.07
0.05
0.05
0.4
0.1
0.07
998
A. S. G. Roth et al.
Table 2. Continued. 3He and 21Ne production rates and exposure ages.
Sample
M2-Ch-01
M2-Ch-02
M2-Ch-03
M2-Ch-04
M2-Ch-05
M2-Ch-06
M2-Ch-07
M2-Ch-08
M2-Ch-09
M2-Ch-10
M2-Ch-11
M2-Ch-12
M2-Ch-13
M2-Ch-14
M2-Ch-15
M2-Ch-16
M2-Ch-17
M2-Ch-18
M2-Ch-19
M2-Ch-20
M2-Ch-21
M2-Ch-22
M2-Ch-23
M2-Ch-24
M2-Ma-01
M2 average
M2 SD
M average
M SD
P21
P3
(10)8 cm3STP g)1 Ma)1)
T3
(Ma)
1.96
1.94
1.98
1.85
1.90
1.91
1.99
1.92
1.95
1.89
1.92
1.94
1.85
1.90
1.94
1.86
1.91
1.90
1.96
1.92
1.70
1.91
1.97
1.92
1.79
1.15
1.13
1.37
1.42
1.27
1.23
1.43
1.60
1.46
2.50
1.77
8
4.4
1.31
1.26
1.40
0.91
22.1
1.42
1.32
21.9
1.26
10.9
6.33
0.43
0.46
0.52
0.38
0.40
0.44
0.54
0.40
0.43
0.38
0.44
0.43
0.37
0.38
0.41
0.45
0.43
0.44
0.43
0.44
0.24
0.39
0.51
0.44
0.18
1.4
0.2
1.3
0.3
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
T21
0.02
0.02
0.02
0.02
0.01
0.02
0.02
0.03
0.02
0.03
0.03
4
0.9
0.03
0.01
0.02
0.03
0.3
0.02
0.02
0.3
0.02
0.1
0.08
1.33
1.41
1.52
1.23
1.62
1.34
1.66
2.0
1.59
3.51
2.02
12
6
1.54
1.50
1.74
1.07
34.7
1.86
1.70
34.1
1.50
12.7
8.5
1.10
1.6
0.2
1.6
0.3
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
0.06
0.05
0.04
0.07
0.04
0.06
0.07
0.1
0.04
0.08
0.06
7
1
0.09
0.04
0.04
0.07
0.9
0.07
0.06
0.9
0.05
0.3
0.2
0.06
Notes: Production rates (P3, P21) and nominal exposure ages (T3, T21) are calculated as described in the text. Discarded samples are shown in
italics, preirradiated Murchison chondrules in bold face. Average values do not include matrix samples, and in the case of Murchison the
averages refer to the ‘‘normal’’ chondrules only (not in bold face).
samples from the same chip analyzed by IR-laser
extraction, the standard deviation of both 3He and
21
Ne ages is better than 5%. This spread is similar to
that reported for five Allende chondrules by Eugster
et al. (2007) but remarkably smaller than that
reported for seven Allende chondrules by Das and
Murty (2009).
2. The majority of the Murchison chondrules have also
similar concentrations of cosmogenic noble gases and
hence similar exposure ages, with an overall average
of approximately 1.5 Ma. We will call these the
‘‘normal chondrules’’ of Murchison. The standard
deviation of the exposure ages of these chondrules
from within the same chip is about 12–14% or about
0.20–0.25 Ma. As an aside, note that the two
Murchison chondrules M2-Ch-02 and M2-Ch-06
have strikingly lower 3He ages than all other
Murchison chondrules and also the 21Ne ages of these
two chondrules are on the low end of the observed
range. The low 3He ages are presumably due to loss of
cosmogenic 3He, perhaps because they originated
from close to fusion crust, although this is not
documented. Some loss of cosmogenic 21Ne could
also be the reason for their rather low 21Ne ages.
However, these two chondrules have remarkably low
Mg concentrations, by far the most important
element for cosmogenic Ne production (Table S1).
Uncertainties in the relative elemental production
rates might thus also explain their comparatively low
21
Ne ages. Note furthermore that the very similar 3He
exposure ages of the normal Murchison chondrules
are by themselves good evidence that all 3He in these
chondrules is indeed cosmogenic. Therefore, our
assumption of a negligible solar 3He contribution is
correct at least for the normal Murchison chondrules.
3. The most remarkable observation of this study is
that five of the 32 Murchison chondrules considered
herein show strikingly higher concentrations of both
cosmogenic 3He and 21Ne than the normal
chondrules. Two further Murchison chondrules
Cosmogenic He and Ne in chondrules
999
Fig. 2. Cosmogenic 3He and 21Ne concentrations (upper panels) and the corresponding nominal 3He (T3) and 21Ne (T21) exposure
ages (lower panels) of chondrules analyzed in this study. The data are grouped according to the different meteorite chips (Allende:
A1–A3; Murchison: M1 and M2). Nominal exposure ages are calculated as described in the text. The five Murchison chondrules
with unequivocal pre-exposure (bars extending above the axis break on the ordinate) and the two additional Murchison chondrules
showing a likely pre-exposure are highlighted by filled bars.
show smaller 21Necos excesses. We call these seven
chondrules in the following the ‘‘pre-exposed
chondrules’’ of Murchison. They are marked by
black symbols and black bars, respectively, in Figs.
1 (lower panel) and 2. The highest nominal exposure
ages of these chondrules are about 34 Ma, more
than an order of magnitude higher than those of the
‘‘normal’’ chondrules. Because all 3He ages of the
pre-exposed chondrules are similar to the respective
21
Ne ages, the assumption that all measured 3He is
cosmogenic is very likely to be also true for the preexposed chondrules.
4. In five of the seven pre-exposed chondrules the noble
gases acquired during the pre-exposure amount to
about 80–94% of the total cosmogenic gases. These
chondrules therefore allow an unambiguous
(21Ne ⁄ 22Ne)cos ratio of the Ne produced during the
pre-exposure to be determined (Fig. 2). These values
of about 0.90 are typical for GCR Ne. This is in
strong contrast to the 21Ne ⁄ 22Ne ratio as low as 0.38
inferred by Das and Murty (2009) for the excess
component of two chondrules from another
meteorite (Dhajala), which they suggested to be
indicative for a SCR production of the pre-exposed
1000
A. S. G. Roth et al.
fraction. However, their values have considerably
larger uncertainties than ours, as they were derived
from much smaller relative Ne excesses.
Further important points to be noted from Table 2
and Fig. 2 are:
5. For all Allende chondrules and all normal
Murchison chondrules from a given fragment, the
relative standard deviation of 21Ne exposure ages is
more than two times lower than the relative
standard deviation of the respective concentrations
of 21Necos (except for normal chondrules from
fragment M2, where the difference is only about
20%). This illustrates the reliability of the 21Ne
production rates based on the chemical composition
measured by EPMA. For 3He, the difference
between the relative scatter of gas concentrations
and respective exposure ages is lower than that for
21
Ne. This is to be expected since the 3He
production rates depend less on the chemical
composition of the target than 21Ne production
rates. Only for Allende chips A2 and A3 a
noticeable difference exists, ages scattering by a
factor of 1.5 less than gas concentrations.
6. Calculated 21Ne exposure ages of Allende
chondrules from chip A2 are on average some 10%
lower than values from Allende chip A3. This very
likely reflects a difference in shielding of the two
chips in the Allende meteoroid. The mean value of
the shielding-sensitive 22Ne ⁄ 21Necos ratio is indeed
slightly lower for A3 chondrules (1.10) than for A2
chondrules (1.13), suggesting an approximately 10–
15% higher 21Ne production rate in chip A3. This
can explain the slightly different nominal exposure
ages. We do not attempt to correct for this presumed
shielding difference, because the shielding dependence
of the 22Ne ⁄ 21Ne ratio in chondrules embedded in a
meteorite with carbonaceous chondrite bulk
composition is not very well constrained (cf. Leya and
Masarik 2009). Helium-3 ages of A3 chondrules are
on average only some 3% higher than the respective
A2 chondrule ages, since the 3He production rate
depends less on shielding than the 21Ne production
rate. The average ages of the normal chondrules from
Murchison chip M2 are some 10% higher than the
average of the normal chondrules from chip M1, but
this difference is within the standard deviation of the
individual ages of the normal chondrules from each
chip.
7. Ages based on 3He are systematically lower than
21
Ne ages by about 20% for the Allende and the
normal Murchison chondrules, and between about
15–35% for the pre-exposed Murchison chondrules.
Because also the pre-exposed Murchison chondrules
show lower 3He ages, it is highly unlikely that the
systematic difference is the result of a diffusive loss
of He during the meteoroid exposure (which,
moreover, would have to be of similar magnitude
for both meteoroids). Probable exceptions are the
two Murchison chondrules with very low 3He ages,
which we ascribed above to diffusive He loss. More
likely explanations for the systematic differences
between 3He and 21Ne exposure ages of chondrules
are a systematic overall uncertainty in the elemental
production rates given by Leya and Masarik (2009)
or incorrect assumptions of shielding depths.
Average 3He- and 21Ne-ages would differ from each
other by <15% if we were to assume our Allende
samples to be from close to the center of a
r = 65 cm carbonaceous chondrite meteoroid,
whereas the difference would reach some 35% on
average if we were to assume all samples to be from
a near-surface location. The adopted shielding of
20 cm is a compromise, because it is unlikely that
our samples are from the small part of the total
mass possibly stemming from near the center of the
Allende meteoroid. Similar considerations have led
to our choice of the shielding depths of the
Murchison chips. These adopted shielding depths
are irrelevant anyway for the pre-exposed
Murchison chondrules, which may explain the larger
discrepancy between 3He and 21Ne ages for these
chondrules. We note again that uncertainties in the
absolute values of the exposure ages are not critical
for our data interpretation, whether these
uncertainties be due to incorrect shielding
assumptions or systematic overall uncertainties of
the adopted elemental production rates.
8. Helium-3 and 21Ne exposure ages of the two Allende
matrix samples are almost identical to the mean
respective chondrule ages (Table 2). The 21Ne ages
of the two Murchsion matrix samples are somewhat
lower than most chondrule ages of this meteorite,
apart from M2-Ch-02, which possibly lost some
cosmogenic 21Ne (see above). No cosmogenic 3He
can be derived for Murchison matrix samples due to
the presence of solar He. The comparison between
chondrule and matrix ages is further discussed in the
next section.
DISCUSSION
The purpose of this article is to search for
cosmogenic noble gases produced during exposure
episodes of chondrules to energetic particles prior to the
final compaction of the host meteorite parent body or
the parent body regolith from which a studied meteorite
derives. As shown above, very clear-cut evidence for such
‘‘pre-exposures’’ is preserved in about 20% of the
Cosmogenic He and Ne in chondrules
Murchison chondrules considered herein. Before we
interpret these data, we first examine the Allende data
more closely.
Allende
The foremost observation on the Allende data set is
that all chondrules have very similar noble gas cosmicray exposure ages. When potential systematic
uncertainties are avoided, i.e., the comparison is
restricted to chondrules from within the same chip and
only to either 3He or 21Ne ages, and when the less precise
values from chip A1 are neglected, ages of all Allende
chondrules within one chip have a standard deviation of
about 200,000 yr or 4–5%. This scatter can easily be
explained by analytical uncertainties. For Allende, the
clear-cut conclusion of this work is thus that cosmic ray–
produced noble gases reveal no evidence of a differential
irradiation of chondrules by high-energy particles prior
to the compaction of the meteorite.
The exposure ages of Allende matrix samples are
also similar to the chondrule ages. The slight difference
between 21Ne ages of Allende chondrule and matrix
samples on the one hand and the slight difference
between 3He and 21Ne matrix ages on the other may well
be due to production rate uncertainties. We therefore
conclude that there is neither any evidence that all
chondrules might have experienced a precompaction
exposure of similar duration relative to the Allende
matrix. The overall data pattern therefore constrains a
putative precompaction exposure of Allende chondrules
to a few hundred thousand years of irradiation by GCR
at present-day intensity (or an equivalent SCR fluence),
well within uncertainty of the data.
Above we considered a loss of cosmogenic noble
gases from chondrules during the meteoroid exposure to
be unlikely. Similarly, it seems unlikely that Allende
chondrules would have largely or completely lost any
cosmogenic Ne acquired early, prior to parent body
metamorphism. Although Allende suffered mild thermal
metamorphism (McSween 1977), peak metamorphic
temperatures in the parent body may not have exceeded
230 C (Green et al. 1971), too low for minerals like
olivine to have lost cosmogenic He or Ne. We therefore
conclude that they very likely never were exposed to a
recognizable flux of energetic particles in the solar
nebula, at least not after they had cooled for the last
time below the closure temperature for cosmogenic Ne,
and that all cosmogenic 3He and 21Ne in Allende
chondrules and matrix was produced during some
4–5 Myr of exposure of the Allende meteoroid to GCR
on its recent journey from the asteroid belt to the Earth.
Based on smaller data sets, Eugster et al. (2007) and Das
and Murty (2009) arrived at the same conclusion.
1001
Another way to recognize an (early) precompaction
exposure of all chondrules to a common particle fluence
is to compare noble gas exposure ages with radionuclide
exposure ages, since the latter only record recent
exposures during the last few half-lives of the respective
radionuclide. A 53Mn exposure age of Allende of 5–6 Ma
can be deduced based on measured activities of various
samples and an assumed 53Mn saturation activity
(Nishiizumi 1978; Nishiizumi, personal communication).
The nominal 53Mn age is slightly higher than the noble
gas ages reported herein, contrary to what would be the
case if the noble gases had partly been produced in an
early exposure. The difference between 53Mn and noble
gas ages thus very likely reflects inaccurate shielding
corrections or inaccurate absolute production rates. This
is further supported by the fact that the average noble
gas exposure age of Allende chondrules reported by
Eugster et al. (2007) of about 5.2 Ma is slightly higher
than the value given herein and in agreement with the
53
Mn age. Radionuclides, therefore, provide no evidence
for a precompaction exposure of Allende samples either.
Murchison
Apart from the seven Murchison chondrules with
clear-cut evidence for a pre-exposure, which will be
discussed below, all others (the ‘‘normal’’ Murchison
chondrules) show a similar picture as the Allende
chondrules, i.e., the normal chondrules have very similar
cosmogenic noble gas concentrations, corresponding to a
mean exposure age of about 1.5 Ma. However, the
conclusion that therefore the normal Murchison
chondrules show no evidence for a precompaction
exposure requires a close inspection of the data. First, we
concluded above that the two exceptional chondrules
with very low 3He ages of only approximately 0.4 Ma
suffered a loss of cosmogenic noble gases. If one would
argue instead that these lowest ages define the meteoroid
exposure age, all other normal chondrules would have
suffered precompaction exposures on the order of 1 Myr.
However, a comparison with radionuclide exposure ages
makes this notion untenable. Because of its smaller preatmospheric size and lower exposure age, Murchison has
a better-constrained radionuclide exposure age than
Allende. Herzog et al. (1997) report a 26Al-10Be
exposure age of 1.6 ± 0.3 Ma and Nishiizumi (personal
communication) a 53Mn age of 2 Ma. These values are in
good agreement with the mean 3He and 21Ne ages of the
normal Murchison chondrules of approximately 1.4–
1.8 Ma, in particular given the inherent uncertainties of
such a comparison mentioned in the previous subsection.
One may further argue that also the two Murchison
matrix samples show slightly lower 21Ne ages than
almost all chondrules. However, these ages of
1002
A. S. G. Roth et al.
approximately 1.1–1.5 Ma (Table 2) again are lower
than what is expected from radionuclide data. They are
based on average chemical composition of Murchison
matrix taken from the literature, and we may also not
exclude that the fine-grained Murchison matrix samples
suffered some noble gas loss. In summary, there is no
evidence that the normal Murchison chondrules suffered
a precompaction irradiation. On the other hand, in view
of the unequivocal cosmogenic noble gas excesses
recorded by seven Murchison chondrules we cannot
definitely exclude that part of the age scatter of the
normal chondrules may also be due to minor
precompaction exposure, though unresolvable within
limits of uncertainty. This notwithstanding, we will only
consider the group of seven clear-cut cases as evidence
for an irradiation of chondrules prior the final
compaction of the material eventually becoming the
Murchison meteorite. This is our reasoning to call these
seven chondrules the ‘‘pre-exposed chondrules.’’
The excesses of cosmogenic 3He and 21Ne of the preexposed Murchison chondrules correspond to formal
pre-exposure durations of between about 1–2 and 20–
35 Ma in a 4p irradiation at current GCR fluxes (Fig. 2).
These ages would roughly double if the pre-exposure had
occurred in a 2p geometry, e.g., in a regolith of a larger
body.
Where Did the Pre-Exposure Happen?
The results of our study differ in important aspects
from previous reports on cosmogenic noble gases in
chondrules. Earlier work reported evidence for
precompaction exposures of chondrules corresponding to
between one and a few million years of irradiation to
GCR (Polnau et al. 1999, 2001; Eugster et al. 2007; Das
and Murty 2009). These results were mainly based on
rather modest relative differences between noble gas
concentrations in matrix and chondrules, while a
preliminary report (Das et al. 2010) of occasionally large
formal age differences between various fragments of the
same chondrule needs confirmation. Herein we present
unequivocal evidence for a substantial irradiation of a
sizeable fraction of the chondrules of one of the two
meteorites studied—Murchison—prior to its final
compaction. It is remarkable that the cosmogenic 21Ne
concentrations in the pre-exposed Murchison chondrules
are in the same range as the concentrations found by
Hohenberg et al. (1990) in individual track-rich mineral
grains extracted from Murchison and other CM
chondrites. On the other hand, based on a larger data set
we confirm the conclusion by Eugster et al. (2007) and
Das and Murty (2009) that in the other meteorite we
studied—Allende—none of the chondrules had suffered a
precompaction irradiation, at least not one exceeding the
limits of uncertainty equivalent to a few hundred
thousand years of GCR exposure at present-day
intensity.
The two main—interrelated—questions suggested by
these observations are:
1. Where, when, and by what type of irradiation did
the about 20% pre-exposed Murchison chondrules
acquire their excess cosmogenic noble gases?
2. Why do some chondrules in Murchison but none in
Allende show large precompaction exposure effects?
We compare first in more detail the results of this
study with the data reported by Hohenberg et al. (1990)
on individual olivine grains from Murchison. All grains
free of solar flare tracks showed similar cosmogenic Ne
concentrations as the normal chondrules measured here.
Our pre-exposed chondrules also have a similar
distribution of 21Necos concentrations as the few percent
of track- and Necos-rich olivine grains measured by
Hohenberg and coworkers. It seems thus very likely that
many or most olivines studied by Hohenberg et al.
(1990) were chondrule fragments broken apart either in
the regolith of the parent asteroid or during sample
preparation. The latter explanation is supported by the
fact that the single olivine grains contained little solar
Ne. This is to be expected for chondrules but not for
olivine grains that resided in the fine-grained matrix. We
therefore conclude that the noble gas data published by
Hohenberg and coworkers and those herein are in fact
largely equivalent to each other.
It seems clear that the large 3He and 21Ne excesses in
the pre-exposed Murchison chondrules cannot be the
result of an early irradiation to GCR while the
chondrules were freely floating. Even if such an
irradiation would have occurred at essentially zero
shielding by gas and dust in the solar nebula, required
free-floating times of up to several ten million years ago
are not consistent with chondrule and chondrite
formation scenarios. For this reason, lifetimes of
chondrules as small objects in the nebula cannot be
dated assuming a GCR origin for the observed excess
cosmogenic noble gases.
The pre-exposed Murchison chondrules thus had to
be exposed either to a high fluence of SCR or an
irradiation to GCR in a regolithic environment for up to
several ten million years ago (roughly twice the formal
pre-exposure ages shown in Fig. 2). Regolith evolution
and formation scenarios of CM meteorite parent bodies
are discussed by Price et al. (1975) and Goswami and Lal
(1979) in the context of their exposure to cosmic rays. A
solar irradiation might have occurred either while the
chondrules were freely floating in the nebula or also while
residing in a parent body regolith. However, with current
solar activity, noble gas production by SCR in a
vertically well-mixed regolith contributes only a very few
Cosmogenic He and Ne in chondrules
percents of that due to concurrent GCR production (cf.
Hohenberg et al. 1978). Also an exposure of chondrules
in the nebula to SCR at approximately present-day fluxes
and at distances from the Sun where chondrules would
not have been heated too much would have had to last on
the order of several million years at least to cause the
highest observed excesses, even if these chondrules had
been completely unshielded by other nebular matter.
Therefore, scenarios involving a solar irradiation of the
pre-exposed chondrules and olivine grains postulate a
much higher SCR flux than today, hence an early active
Sun. Hohenberg and coworkers (Hohenberg et al. 1990;
Woolum and Hohenberg 1993) preferred this possibility
for CM chondrites, because they considered very long
regolith exposures of up to a few hundred million years to
be unlikely. Caffee et al. (1987) and Rao et al. (1997)
arrived at similar conclusions from data of solar gas–rich
ordinary chondrites and achondrites. Wieler et al. (2000)
contested this view. They concluded that the data by
Caffee et al. (1987) and Rao et al. (1997) can
straightforwardly be explained by a regolith exposure and
therefore represent no convincing evidence for an early
active Sun. Wieler et al. (2000) also expressed
reservations with respect to a SCR origin of the noble gas
excesses in single olivines from CM chondrites, because
solar flare track densities in the gas–rich olivines
(Hohenberg et al. 1990) are much lower than those
expected for a concurrent solar production of the
cosmogenic 21Ne.
We propose that the data presented herein provide
additional support for the hypothesis that pre-exposure
effects in chondrules were acquired on a parent body
regolith—not necessarily early in solar system
history—rather than being the result of an early active
Sun. First, the observation that chondrules from the
meteorite containing implanted solar noble gases
(Murchison) do contain cosmogenic noble gases from a
preirradiation, whereas the chondrules from chips free of
implanted solar gases of the other studied meteorite
(Allende) do not show preirradiation signatures is in line
with earlier observations. The most clear-cut preirradiation signatures in individual olivine grains and
chondrules have predominantly been observed in
regolithic solar-gas–rich meteorites such as Kapoeta,
Weston, Fayetteville, and several CM chondrites (e.g.,
Pellas et al. 1969; Lal and Rajan 1969; Caffee et al. 1987;
Hohenberg et al. 1990; Eugster et al. 2007; Das and
Murty 2009). This is to be expected if the preirradiation
happened in a parent body regolith. No straightforward
scenarios have been proposed to explain why chondrules
or other matter once irradiated by the early Sun in the
nebula should end up preferentially in regolithic
meteorites. Some pieces of the Allende meteorite also
contain relatively minor amounts of implanted solar
1003
noble gases (Palma and Heymann 1988), whereas others
such as those studied herein do not. Therefore, although
Allende appears to be a regolith breccia, it is a much less
mature one than Murchison. It is thus well conceivable
that the Allende material studied herein never resided
high enough in its parent regolith to have acquired
sizeable amounts of GCR-induced noble gases before
being ejected as meteoroid. The simultaneous absence in
the analyzed Allende samples of preirradiated chondrules
and implanted solar noble gases and the simultaneous
presence of both these features in the analyzed
Murchison fragments can therefore easily be explained
by different residence histories of the sampled meteorites
in their respective parent body regolith.
A further strong argument for a GCR origin of the
excess noble gases in the Murchison chondrules studied
herein is provided by the 21Ne ⁄ 22Ne ratio of the preirradiation component, which is around 0.90. As shown
in the Results section, this ratio could unambiguously be
determined for those chondrules where the preirradiation component dominates the cosmogenic noble
gas inventory. Values of around 0.90 for 21Ne ⁄ 22Ne are
typical for GCR Ne and similar to the values found in
the Murchison chondrules without Ne excess. In
contrast, irradiation of chondritic matter by protons with
a (present-day) SCR energy spectrum would lead to
lower 21Ne ⁄ 22Ne ratios on the order of 0.6 only
(Garrison et al. 1995).
Observations by Metzler (2004) in our view also
provide support for the regolith-irradiation scenario. He
studied solar flare tracks in olivine grains from several
CM chondrites, including Murchison. Tracks were found
only in the clastic matrix of each meteorite but not in
millimeter- to centimeter-sized lithic clasts, termed
‘‘primary rocks’’ by Metzler. This strongly indicates that
also the grains rich in tracks and cosmogenic Ne studied
by Hohenberg et al. (1990) originated exclusively from
matrix. Since we concluded above that the track- and
gas–rich single olivines also stem from chondrules, we
infer that also all pre-exposed chondrules studied in this
work originally resided in the clastic matrix of
Murchison and not in lithic clasts. In fact, the matrix of
Murchison does contain intact chondrules (Metzler 2004;
Metzler, personal communication). A scenario where
clastic matrix material was irradiated in a mixed regolith
by solar and galactic energetic particles, both more or
less with current fluxes, while the lithic clasts were
shielded from galactic particles prior to meteoroid
ejection, and hence contain only normal (i.e., not
preirradiated) chondrules can explain the data of this
work, those by Hohenberg and coworkers as well as the
observations by Metzler (2004). In such a scenario, the
Murchison meteorite would thus be a mixture of finegrained regolith material from near the parent body
1004
A. S. G. Roth et al.
surface with coarser material from below a few meters,
the maximum penetration depth of GCR particles.
Conversely, if the pre-exposed chondrules had been
irradiated by a high fluence of solar particles in the
nebula prior to parent body accretion, such chondrules
would be expected to now be present in lithic clasts also
and not exclusively in matrix, contrary to the track
observations by Metzler (2004). The possibility that both
tracks and excess cosmogenic noble gases were the result
of an intense irradiation of chondrules by SCR in the
regolith is also very unlikely. In this case, only
chondrules at the very surface of the regolith would have
accumulated tracks and excess noble gas, but the track
densities observed in olivines would then be far lower
than those expected from the cosmogenic noble gas
excesses (cf. Wieler et al. 2000).
Actual pre-exposure times of gas–rich chondrules in
a regolith depend on the integrated shielding history of
each chondrule but would at least be about twice as high
as the nominal ages to be read from Fig. 2, which
assumes a GCR irradiation in a 4p geometry. Thus,
some chondrules measured herein resided in the
uncompacted regolith for at least some 70 Ma, and some
of the single olivine grains for even more than 100 Ma
(Hohenberg et al. 1990). Woolum and Hohenberg (1993)
doubted whether such long pre-exposure times were
compatible with compaction ages based on Pu ⁄ U-fissiontracks in U-poor grains determined by MacDougall and
Kothari (1976). However, Metzler (2004) proposed that
only the youngest of these compaction ages (which show
a wide spread) date the real compaction of the host
breccias. In our view, even the youngest fission-track
ages do not necessarily date the time since the entire
regolith portion from which a meteorite originates was
compacted.
CONCLUSIONS
This work is a continuation of efforts by several
groups to look for evidence of a high activity of the early
Sun in the form of possible traces left by a high fluence
of energetic solar particles in meteorites. The irradiation
environment in the early solar system is, among other
issues, important for an understanding of isotopic
anomalies in meteorites. We concentrated on cosmic-rayproduced noble gases in chondrules. Chondrules may
well be envisaged to once have been exposed as
individual objects to an intense radiation emitted by the
early Sun. We searched for differences in the nominal
3
He and 21Ne cosmic-ray exposure ages within large sets
of chondrules from the two carbonaceous chondrites
Allende and Murchison. The selected meteorites both
acquired only low concentrations of cosmogenic noble
gases on their recent journey from the asteroid belt to
Earth, facilitating the detection of noble gases from a
potential pre-exposure. This approach reduces
ambiguities that may result when comparing cosmogenic
nuclide concentrations in small sets of chondrules with
concentrations in matrix samples. Differences in the
production rates of cosmogenic noble gases due to
different chemical compositions of analyzed chondrules
were accounted for by major element analyses on splits
of the analyzed chondrules.
Unlike earlier reports of rather minor effects of
precompaction exposures on chondrules from a variety
of meteorites, we find large and unambiguous 3Hecos and
21
Necos excesses in about 20% of the chondrules studied
from Murchison (the ‘‘pre-exposed’’ chondrules). In
agreement with, and extending two recent studies
(Eugster et al. 2007; Das and Murty 2009), we also find
good evidence that none of the chondrules from Allende
has been exposed to a sizeable fluence of energetic
particles except during the journey of the Allende
meteoroid to Earth. On the other hand, the excesses of
cosmogenic 21Ne we find in the pre-exposed Murchison
chondrules are of the same size as excesses previously
observed by Hohenberg et al. (1990) in single olivine
grains extracted from Murchison and other CM
chondrites. These 21Ne-rich olivine grains also were rich
in tracks induced by solar energetic particles. Hohenberg
and coworkers therefore suggested that these grains are
evidence for an early active Sun. We suggest instead that
our pre-exposed Murchison chondrules acquired their
cosmogenic noble gas excesses in a parent body regolith
during several ten million years of exposure to GCR, a
scenario already proposed by Wieler et al. (2000) as an
alternative explanation for the 21Ne- and track-rich
individual olivine grains. Our argumentation relies on (1)
the isotopic composition of the cosmogenic Ne in the
pre-exposed chondrules, (2) the fact that mainly
meteoritic regolith breccias show unambiguous preexposure effects, and (3) observations by Metzler (2004)
about the distribution of solar flare tracks in matrix
samples and lithic clasts in CM chondrites. We presume
that future studies, carefully distinguishing chondrules
from matrix and lithic clasts will eventually allow a
definitive distinction between the early active Sun and
the regolith irradiation hypotheses. In any case, the data
also strongly suggest that cosmogenic noble gases are not
a viable tool to constrain lifetimes of chondrules as
individual objects in the solar nebula.
Acknowledgments––This work has been supported by
the Swiss National Science Foundation. Advice from
Carmen Sanchez on sample preparation is appreciated.
Reviews by Bernard Marty and an anonymous referee as
well as comments by Associate Editor G. Srinivasan are
acknowledged.
Cosmogenic He and Ne in chondrules
Editorial Handling––Dr. Gopalan Srinivasan
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Table S1. Major element concentrations (wt%) in
Allende and Murchison chondrules.
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