Meteoritics & Planetary Science 40, Nr 2, 315–327 (2005)
Abstract available online at http://meteoritics.org
Regolith history of lunar meteorites
S. LORENZETTI,† H. BUSEMANN,‡ and O. EUGSTER*
†Present
Physikalisches Institut, Universität Bern, 3012 Bern, Switzerland
address: Laboratorium für Biomechanik, Eidgenössische Technische Hochschule Zürich, 8093 Zürich, Switzerland
‡Present address: Carnegie Institution, Washington, D.C. 20015, USA
*Corresponding author. E-mail: [email protected]
(Received 1 December 2003; revision accepted 12 December 2004)
Abstract–The regolith evolution of the lunar meteorites Dhofar (Dho) 081, Northwest Africa
(NWA) 032, NWA 482, NWA 773, Sayh al Uhaymir (SaU) 169, and Yamato (Y-) 981031 was
investigated by measuring the light noble gases He, Ne, and Ar. The presence of trapped solar neon
in Dho 081, NWA 773, and Y-981031 indicates an exposure at the lunar surface. A neon three-isotope
diagram for lunar meteorites yields an average solar 20Ne/22Ne ratio of 12.48 ± 0.07 representing a
mixture of solar energetic particles neon at a ratio of 11.2 and solar wind neon at a ratio of 13.8. Based
on the production rate ratio of 21Ne and 38Ar, the shielding depth in the lunar regolith of NWA 032,
NWA 482, SaU 169, and Y-981031 was obtained. The shielding depth of these samples was between
10.5 g/cm2 and >500 g/cm2. Based on spallogenic Kr and Xe, the shielding depth of Dho 081 was
estimated to be most likely between 120 and 180 g/cm2. Assuming a mean density of the lunar
regolith of 1.8 g/cm3, 10.5 g/cm2 corresponds to a depth of 5.8 cm and 500 g/cm2 to 280 cm below the
lunar surface. The range of regolith residence time observed in this study is 100 Ma up to 2070 Ma.
INTRODUCTION
Lunar samples collected during the Apollo and Luna
missions represent just a small part of the lunar surface.
Numerous lunar meteorites have been recovered in cold or hot
desert areas in the last two decades. Lunar meteorites were
ejected by asteroidal and cometary impacts spread over the
entire lunar surface. Hence, the lunar meteorites can be a
source of extensive information about the nature of the Moon
and, particularly, the lunar crust. The average lunar regolith
layer thickness can be estimated with radar and optical data.
The thickness is 5 m for maria and 12 m for highlands
(Shkuratov and Bondarenko 2001).
The transfer time for lunar meteorites from the parent
body to Earth is rather short compared to other meteorite
classes. Only Calcalong Creek, Yamato (Y-) 82192/3/86032,
and Dhofar (Dho) 025 were in space for more than 1.5 Ma. As
summarized in this work (Table 1), the range for the MoonEarth transfer time of the lunar meteorites is between 0.01 and
10.8 Ma. The short transfer times imply that most of the
cosmogenic noble gases in lunar meteorites were produced
during the residence time in the regolith at the lunar surface.
The material from which lunar meteorites were formed
was exposed to cosmic rays in the lunar regolith. Solar gases
are trapped by the material in the top layer of the Moon. The
penetration depth of solar wind particles (SW) and solar
energetic particles (SEP) is only tens of nanometers and a
few micrometers, respectively (Rao et al. 1991). Galactic
cosmic ray (GCR) particles and their secondary particles
penetrate lunar material down to a depth on the order of a few
meters. Thus, lunar rocks may be irradiated by cosmic rays,
but do not trap any solar gases. During the regolith residence
time TR, the lunar material is exposed in a 2π-irradiation
geometry.
The ejection age from the Moon is Tej = Ttrans + Tterr,
where Ttrans is the Moon-Earth transfer time and Tterr the
terrestrial residence time since the meteorites fell on Earth.
Tej, Ttrans, and Tterr and are listed in Table 1. During the
transfer from the Moon to Earth, the meteorite is exposed in a
4π-irradiation geometry. To determine Ttrans, the cosmic-rayproduced radionuclides such as 10Be, 14C, and 36Cl were used
(cf., Nishiizumi et al. 2001a, b, 2004). For many lunar
meteorites, Ttrans is sufficiently short that the irradiation in the
lunar regolith is clearly visible in the radionuclides, and the
radionuclide concentration can in fact be used to determine
the ejection depth. An irradiation in the lunar regolith is not
visible in the radionuclides if the time since the irradiation is
much longer than the half-life of the respective radionuclide.
The stable cosmogenic noble gases 3He, 21Ne, and 38Ar in
lunar meteorites are, in general, predominantly produced
315
© The Meteoritical Society, 2005. Printed in USA.
316
S. Lorenzetti et al.
Fig. 1. Ca/Ti versus lunar ejection time for lunar meteorites.
during the first exposure in the lunar regolith and during the
second one in free space during the transfer to Earth.
Meteorites that were ejected by the same cratering event
are called “source crater paired.” These meteorites must have
the same Tej. On the other hand, it is possible that meteorites
with similar Tej were ejected separately at different ejection
events. Thus, the number of ejection events as obtained from
Tej is a lower limit. Of course, an impactor can hit a nonuniform region with different material. To count the ejection
events on the Moon, we considered both Tej and the ratio Ca/
Ti that characterizes the type of meteorite (Fig. 1). A ratio Ca/
Ti >60 represents the lunar highlands and Ca/Ti <15 lunar
mare material. We assume that it is unlikely that material from
both regions were ejected by the same event. Consequently,
highland breccias and mare basalts are not considered to be
crater paired. For the lunar meteorites collected until now, we
propose 3–5 ejection events in the highlands and 5–6 events
in the mare regions.
The aim of this study of lunar meteorites was to
determine the residence time of the meteoritic material and its
shielding depth in the regolith. Here, we present the noble gas
data of six recently recovered lunar meteorites and will
discuss their regolith exposure history.
Preliminary or incomplete data of the investigated
meteorites were published earlier: Dho 081 (Eugster et al.
2001), NWA 032 (Fagan et al. 2002); NWA 773 (Fagan et al.
2003); SaU 169 (Gnos et al. 2004); and Y-981031 in the
framework of a consortium study published in an NIPR
journal (Lorenzetti et al. 2002).
SAMPLES AND EXPERIMENTAL PROCEDURE
For each meteorite, we selected samples without fusion
crust. Dho 081 was found in 1999 as a single fragment in the
desert Dhofar in Oman. It is a shocked feldspatic fragmental
highland breccia dominated by anorthosite-rich lithic and
mineral clasts embedded into a fine-grained mostly shock
melted clastic matrix (Greshake et al. 2001). Cahill et al. (2002,
2004) conclude that Dho 081 may originate from a highland
terrain on the far side of the Moon. Dho 081 is similar in
composition and weathering to Dho 280 (Demidova et al.
2003). These two meteorites appear to be fall paired (Nazarov
et al. 2003). Dho 081 was bought from J. Nauber (Meteorite
Laboratory, Zürich). Two samples were analyzed: 5.48 mg (for
He, Ne, and Ar) and 4.22 mg (for Kr and Xe).
NWA 032 is the third unbrecciated meteorite from the
lunar maria (Fagan et al. 2000) and is geochemically and
petrographically distinct from the previously identified lunar
mare meteorites Y-793169 and Asuka-881757. The trace
element concentrations of NWA 032 were measured by
Korotev et al. (2001). The sample of NWA 032 was obtained
from T. Fagan (University of Hawai’i). The sample was
crushed to a grain size <750 µm to obtain average bulk
material. We measured two splits of 22.31 mg and 22.80 mg.
Daubar et al. (2002) concluded that NWA 482 is a
crystalline impact melt breccia from the highlands of the
Moon, and furthermore, that its short terrestrial age, combined
with the unique lithology, preclude any pairing with other
NWA lunar meteorites. The chemical composition was
measured by Warren and Kallemeyn (2001). Two samples of
NWA 482 (obtained from P. Warren, UCLA) weighing
4.98 mg and 4.87 mg were crushed to <750 µm and measured
to determine the He, Ne, and Ar isotope abundances.
NWA 773 consists of two lithologies: a green olivine-rich
cumulate and a black impact breccia (Fagan et al. 2001). A
part of our noble gas data was published in a work on lunar
origin, iron-enrichment trend, and brecciation of NWA 773
(Fagan et al. 2003). Nishiizumi et al. (2004b) determined the
radionuclides. The proposed models to explain the data will
be discussed later. The sample was separated by P. Warren
(UCLA) into a cataclastic breccia fraction (cbf, black
lithology) and into a peridotite fraction (p, green lithology).
For the breccia fraction, we prepared three grain size splits
(350–750 µm, 15.84 mg; 35–350 µm, 15.01 mg; <35 µm,
14.65 mg) to obtain a separation of the surface-correlated
trapped solar gases from the volume-correlated cosmic-rayproduced gases and radiogenic gases. For the peridotite
fraction, we analyzed a sample of 18.40 mg.
SaU 169 was found by a group of Swiss scientists in the
Omani desert. It consists of an extremely KREEP-rich
anorthosite-free impact-melt breccia and adherent KREEP-rich
regolith (Gnos et al. 2004). A slice of the impact-melt breccia
(imp) without fusion crust was crushed into pieces. Two
samples of 5.24 mg and 5.14 mg were used for the He, Ne, and
Ar analyses. From the lithologies IIE and IIF (Gnos et al. 2004),
both regolith breccia samples (reg), 5.31 mg and 10.20 mg,
respectively, were measured. Of the lithology IIH, part of the
KREEP clast (Gnos et al. 2004), 11.92 mg was analyzed.
Regolith history of lunar meteorites
317
Table 1. Ca/Ti ratios, lunar regolith residence times TR, Moon-Earth transfer times Ttrans, terrestrial ages Tterr, and lunar
ejection times Tej, for lunar meteorites. All ages in million years.
Meteorite
Mass (g)
Ca/Ti
TR
Ttrans
Highland breccias
ALH A81005
DaG 262
DaG 400
Dhofar 025
Dhofar 301
Dhofar 304
Dhofar 081
Dhofar 280
Dhofar 026
Dhofar 302
Dhofar 303
Dhofar 305
Dhofar 306
Dhofar 307
Dhofar 310
Dhofar 489
MET 01210
MAC 88104
MAC 88105
NWA 482
QUE 93069
QUE 94269
Yamato-791197
Yamato-82192
Yamato-82193
Yamato-86032
31.4
513
1425
751
9
10
174
251
148
3.8
4.2
7
2.6
50
–
34.4
22.8
61.2
662.5
1015
–
24.6
52.4
36.7
27.0
648.4
66.9a
90.5e
112.7h
–
–
52P
~90c
–
–
–
–
119P
123P
230P
–
–
–
–
84.4a
68.7F
–
94.7k
64.0a
55.8a
99.1a
97.9a
580 ± 180
100−1000f, K
<3i
–
–
–
680 ± 140D
–
–
–
–
–
–
–
–
~20W
–
–
630 ± 200b
2070 ± 420D
–
1000 ± 400k
910l
–
11j
–
<0.05r
0.15e
≤1i
>10U
–
–
≤0.01E
–
<0.01R
–
–
–
–
–
–
–
–
–
0.04−0.05r
0.9E
–
0.02−0.15k, n
<<0.1r
10.6 +0.6/−5.6j
10.6 +0.6/−5.6j
10.8 +0.7/−5.8j
19
23.4
8.7
186
290
22.3u
23.4y
24.3o
21.5J
31.5L
~600A
400 ± 60y
700 ± 200w
–
–
3 ± 1p
<0.1z
<0.01x
–
<0.02x
442
154
30.7
paired
53.0
–
252.5
244.1
59.0
156
~300
633
22.4
206.4
6.1
6.9o
–
15.4o
19.3f
–
–
–
–
–
4.2B
~14G
–
5.5N
8.2o
<1k
–
28.5f
–
–
–
–
–
–
207 ± 43D
100 ± 20H
–
<200D
50 ± 10k
0.9 ± 0.1p
<0.05p
–
–
<0.01s
0.01−0.08s, t
–
–
–
–
–
–
–
–
–
–
–
–
0.042 ± 0.005 <0.01C
<0.03d
–
–
–
≤0.34N
0.0097 ± 0.0013N
1.1 ± 0.2p
<0.05p
paired
paired
paired
paired
paired
paired
Highland/mare rocks
Calcalong Creek
QUE 94281
Yamato-793274
paired
Yamato-981031
Yamato-983885
Mare basalts
Asuka-881757
Dhofar 287
EET 87521
EET 96008
LAP 02205
LAP 02224
LAP 02226
LAP 02436
NWA 479
NWA 032
NWA 773
PCA 02007
SaU 169
Yamato-793169
Tterr
0.04−0.10r
0.05−0.06g
–
0.5−0.6E
–
–
0.04 ± 0.02E
–
–
–
–
–
–
–
–
–
–
–
0.21−0.35j, r
0.06−0.12E
–
0.005−0.01n
0.03−0.09r
–
0.072−0.083j
–
<0.03p
0.25 ± 0.10z
0.02−0.035x
–
0.045 ± 0.01x
Tej
0.040−0.15
0.05−0.075
≤1
>10U
–
–
0.04 ± 0.02
–
–
–
–
–
–
–
–
several kyW
–
–
0.25−0.40
0.28
–
0.025−0.16
0.03−0.19
–
8±3
–
3±1
0.35 ± 0.10
0.032–0.003x
–
0.045 ± 0.01x
0.9 ± 0.1
–
0.02–0.09
–
–
–
–
–
–
0.05 ± 0.01
0.017 ± 0.001d
–
≤0.35N
1.1 ± 0.2
References: a = Warren and Kallemeyn (1991); b = Eugster et al. (1991); c = Nishiizumi et al. (1989); d = Nishiizumi et al. (2004b); e = Bischoff et al. (1998);
f = Eugster et al. (2000); g = Nishiizumi et al. (1998); h = Zipfel et al. (1998); i = Scherer et al. (1998); j = Eugster et al. (1989); k = Thalmann et al. (1996);
l = Takaoka (1986); n = Nishiizumi et al. (1996b); o = Arai et al. (1996); p = Nishiizumi et al. (1992); r = Nishiizumi et al. (1991a); s = Nishiizumi et al. (1991b);
t = Vogt et al. (1993); u = Hill et al. (1991); w = Eugster et al. (1992); x = Nishiizumi et al. (2004a); y = Polnau et al. (1998); z = Nishiizumi et al. (1996a); A
= Swindle et al. (1995), adopting P38(2π) = 0.12 × 10−8 cm3 STP/g Ma; B = Fagan et al. (2002); C = Nishiizumi et al. (2001a); D = This work; E = Nishiizumi
et al. (2001b); F = Grossman et al. (2001); G = Mittlefehldt (2001), personal communication; H = This work, peridotite fraction; J = Kojima (2000); K =
Fernandes et al. (2000); L = Kaiden et al. (2002); N = Gnos et al. (2004); P = Demidova et al. (2003); R = Shukolyukov et al. (2001); U = Nishiizumi (2003);
W = Nishiizumi (2004).
318
S. Lorenzetti et al.
Fig. 2. Three-isotope plot for Ne containing all available data of lunar meteorites. SW and SEP data from Wieler (2002), noble gas data from
Schultz and Franke (2002) and from this work. The mixing line yields a solar ratio Ne/Ne = 12.48 ± 0.07.
Y-981031 is the seventh Antarctic lunar meteorite of mare
origin (Arai et al. 2002). These authors conclude that the overall
texture and the presence and composition of pyroclastic/impact
glasses suggest “source-crater pairing” of Y-981031 with Y793274 and QUE 94281, and “source-basalt pairing” with Y793169 and Asuka-881757 because of the similarity of the
pyroxene zoning trend, the presence of symplectic pyroxene,
and the mineral composition of the mesostases in the mare
basalt component. Y-981031 was allocated to us by the
Antarctic Meteorite Research Center of the National Institute of
Polar Research, Tokyo and the first data were published in an
NIPR journal (Lorenzetti et al. 2002). For the analyses, we used
a 5.05 mg and a 5.10 mg sample.
To desorb terrestrial atmospheric gases, we heated the
samples in the storage arm of the extraction system for about
a week at 90 °C. The measurements of He, Ne, and Ar were
performed using our mass spectrometer system B, described
in detail by Eugster et al. (1993). Typical blank corrections
are <1% for He and Ne and <2% for Ar. The Kr and Xe
measurements of Dhofar 081 were done according to the
procedures described by Busemann et al. (2002). Blank
corrections amount to 6 and 12% of the measured Kr and Xe
concentrations, respectively.
RESULTS AND DISCUSSION
The analytical results are given in Tables 2–4.
Experimental errors are 2σ mean for all analyses. The grainsize splits from NWA 773 show no differences neither in the
absolute amounts nor in the ratios. Therefore, we calculated
average values for these splits.
Neon is a mixture of trapped (tr) and in situ produced
cosmogenic (c) Ne. For argon, an additional component is
present: the radiogenic (r) component is a product of 40K decay
to 40Ar. For the calculation of the Ne and Ar components, we
adopted the following ratios: (20Ne/22Ne)c = 0.8; (21Ne/22Ne)tr
= 0.033; (36Ar/38Ar)c = 0.65; (36Ar/38Ar)tr = 5.32; and (20Ne/
22Ne) = 12.48 (as determined below), except for NWA 773,
tr
SaU 169, and Y-981031. Four fractions of NWA 773 were
measured. They contain solar gases and yield a solar ratio of
(20Ne/22Ne)tr = 13.09. SaU 169 and Y-981031 contain trapped
Ne with a solar ratio of (20Ne/22Ne)tr = 11.21 and 12.84,
respectively. The noble gas components are given in Table 5.
Isotopic Abundances of Trapped Ne
The (20Ne/22Ne)tr ratio in a meteorite can be derived from
a Ne three-isotope plot. If no fractionation during the trapping
process occurred and the noble gas isotopes were not
disturbed by a secondary process, such as differentiation or
mixing with another gas source, all extractions form a specific
sample plot on a mixing line between the trapped and the
cosmogenic components. Possible sources for the trapped
component are the SW, SEPs, or planetary sources.
Contamination with terrestrial atmospheric light noble gases,
as observed in the lunar meteorite Dho 026 (Shukolyukov
et al. 2001) and in chondrites (Scherer et al. 1994), was not
observed in our samples. Lunar meteorites contain much less
5.48
22.31
22.80
average
4.98
4.87
average
15.84
15.01
14.65
average
18.40
5.24
5.14
average
5.31
10.20
11.92
5.05
5.10
average
Sample
Dho 081
NWA 032
NWA 032
NWA 032
NWA 482
NWA 482
NWA 482
NWA 773 cbf 340–750 µm
NWA 773 cbf 35–340 µm
NWA 773 cbf <35 µm
NWA 773 cbf
NWA 773 p
SaU 169 imp
SaU 169 imp
SaU 169 imp
SaU 169 IIE
SaU 169 IIF
SaU 169 IIH
Y-981031.66
Y-981031.66
Y-981031.66
Weight
(mg)
4.22
Sample
Dho 081
407 ± 13
3676 ± 118
5168 ± 206
4422 ± 119
1148 ± 35
1161 ± 35
1154 ± 25
181071 ± 6922
191541 ± 16030
175248 ± 6972
182620 ± 6267
16516 ± 609
41266 ± 1415
42651 ± 1500
41939 ± 1500
2330 ± 70
5000 ± 155
2590 ± 79
100610 ± 4561
146206 ± 5695
123408 ± 3648
4He
1184 ± 178
(10−12 cm3 STP/g)
86Kr
Table 3. Results of Kr measurements.
Weight
(mg)
Table 2. Results of He, Ne, and Ar measurements.
cm3
STP/g)
20 ± 0.4
80Kr
7635 ± 167
2345 ± 54
2398 ± 57
2371 ± 39
1141 ± 29
973 ± 24
1057 ± 19
5469 ± 121
5962 ± 129
5048 ± 115
5493 ± 70
1537 ± 34
7303 ± 230
7595 ± 400
7449 ± 25
5806 ± 125
5124 ± 207
10875 ± 335
30526 ± 923
39207 ± 1363
34866 ± 823
40Ar
3.43 ± 0.06
78Kr
426 ± 13
17.61 ± 0.54
19.00 ± 0.62
18.31 ± 0.41
13.69 ± 0.56
13.72 ± 0.52
13.71 ± 0.38
20419 ± 614
22249 ± 670
16909 ± 508
19859 ± 347
8.75 ± 0.33
23.42 ± 0.92
23.36 ± 0.77
23.4 ± 0.9
97.1 ± 3.1
325 ± 10
14.4 ± 0.4
30945 ± 1049
41102 ± 2008
36023 ± 1133
(10−8
20Ne
86Kr
= 100
4.485 ± 0.033
0.681 ± 0.005
0.667 ± 0.005
0.674 ± 0.004
0.661 ± 0.006
0.673 ± 0.008
0.667 ± 0.005
5.27 ± 0.04
5.27 ± 0.04
5.28 ± 0.04
5.27 ± 0.02
1.85 ± 0.04
0.960 ± 0.020
1.001 ± 0.026
0.981 ± 0.026
2.24 ± 0.02
3.59 ± 0.04
1.46 ± 0.02
5.17 ± 0.05
5.22 ± 0.05
5.20 ± 0.04
36Ar/ 38Ar
78.1 ± 1.5
83Kr
3.76 ± 0.044
1.138 ± 0.012
1.154 ± 0.015
1.146 ± 0.010
1.230 ± 0.029
1.251 ± 0.030
1.240 ± 0.021
23.68 ± 0.34
23.41 ± 0.24
22.77 ± 0.24
23.29 ± 0.16
1.42 ± 0.23
1.197 ± 0.030
1.197 ± 0.013
1.197 ± 0.030
1.57 ± 0.03
2.29 ± 0.03
1.155 ± 0.016
20.47 ± 0.22
22.08 ± 0.22
21.28 ± 0.16
22Ne/ 21Ne
74.1 ± 1.5
82Kr
8.66 ± 0.09
0.832 ± 0.009
0.806 ± 0.008
0.819 ± 0.006
0.814 ± 0.024
0.800 ± 0.020
0.807 ± 0.016
12.93 ± 0.13
12.89 ± 0.13
12.94 ± 0.13
12.92 ± 0.08
3.53 ± 0.23
0.789 ± 0.022
0.815 ± 0.012
0.802 ± 0.020
3.87 ± 0.06
6.346 ± 0.070
0.825 ± 0.012
12.63 ± 0.15
12.63 ± 0.13
12.63 ± 0.10
20Ne/ 22Ne
0.0002 ± 0.01
81Kr
587 ± 72
177 ± 2
226 ± 2
201 ± 2
40.95 ± 0.71
41.33 ± 0.45
41.14 ± 0.42
2576 ± 66
2472 ± 44
2244 ± 51
2431 ± 31
3413 ± 266
723 ± 21
795.5 ± 8.1
744.5 ± 8.1
277 ± 35
253.3 ± 5.9
474.5 ± 8.4
1941 ± 65
2045 ± 21
1993 ± 34
4He/3He
334 ±7
84Kr
10.76 ± 0.09
176 ± 2.2
179.8 ± 1.4
177.7 ± 1.3
48.44 ± 1.00
48.27 ± 0.94
48.35 ± 0.69
2.21 ± 0.02
2.25 ± 0.02
2.30 ± 0.03
2.25 ± 0.01
1338 ± 48
411 ± 7
439 ± 10
425 ± 15
87.1 ± 1.1
24.3 ± 0.3
389 ± 4
2.36 ± 0.03
2.34 ± 0.05
2.35 ± 0.03
40Ar/ 36Ar
Regolith history of lunar meteorites
319
4.22
Sample
Dho 081
597 ± 90
(10−12 cm3 STP/g)
132Xe
5.48
average
average
average
18.40
average
5.31
10.20
11.92
average
Sample
Dho 081
NWA 032
NWA 482
NWA 773
NWA 773 p
SaU 169 imp
SaU 169 IIE
SaU 169 IIF
SaU 169 IIH
Y-981031.66
c
12.0 ± 0.5
20 ± 1
13.7 ± 0.6
17 ± 2
1.73 ± 0.31
24.4 ± 1.3
15.7 ± 0.6
21.6 ± 0.7
15.1 ± 0.5
44 ± 2
21Ne
SW (lunar soil 71501)
SEP (lunar soil 71501)
Modern SW
Aubrite parent body
Lunar meteorites
(10−8
13.8 ± 0.1
11.2 ± 0.2
13.8 ± 0.1
12.1 ± 0.2
12.48 ± 0.07
20Ne/22Ne
28.3 ± 1.6
19.7 ± 1.0
32.2 ± 1.2
–
0.46 ± 0.06
16.6 ± 1.0
19.6 ± 0.5
21.7 ± 0.8
15.8 ± 0.4
75 ± 8
c
0.95 ± 0.05
124Xe
38Ar
Table 6. Solar Ne in different solar system reservoirs.
Weight
(mg)
Table 5. Noble gas components of Ne and Ar.
Weight
(mg)
Table 4. Results of Xe measurements.
cm3
tr
tr
132Xe
100.4 ± 3.8
129Xe
691 ± 16
<0.5
<0.1
–
0.85 ± 0.05
6.73 ± 1.2
54.0 ± 1.7
197 ± 7
17.6 ± 0.7
14800 ± 2500
36Ar
9.57 ± 0.32
128Xe
13 ± 13
<0.7
<0.3
19850 ± 1985
7.21 ± 0.35
–
82.0 ± 3.2
306 ± 10
<0.4
36000 ± 6000
STP/g)
20Ne
1.46 ± 0.06
126Xe
–
–
–
13.08 ± 0.19
13.08 ± 0.19
–
11.21 ± 0.30
11.21 ± 0.30
11.21 ± 0.30
12.84 ± 0.60
33.0 ± 1.3
136Xe
0.058 ± 0.016
1.10 ± 0.06
2.05 ± 0.11
–
2.80 ± 0.61
2.27 ± 0.17
0.53 ± 0.10
0.87 ± 0.05
0.349 ± 0.021
0.57 ± 0.15
(3He/21Ne)c
38.7 ± 1.7
134Xe
(20Ne/22Ne)tr
86.0 ± 3.2
131Xe
Wieler (2002)
Wieler (2002)
Benkert et al. (1993)
Lorenzetti et al. (2003)
This work
Reference
1.33 ± 0.08
1.14 ± 0.01
1.25 ± 0.03
1.08 ± 0.02
1.08 ± 0.02
1.198 ± 0.060
1.207 ± 0.063
1.110 ± 0.055
1.152 ± 0.058
–
(22Ne/21Ne)c
16.5 ± 0.7
= 100
130Xe
320
S. Lorenzetti et al.
Regolith history of lunar meteorites
Fe than chondrites and are, therefore, less affected by
weathering. The cosmogenic component varies with the
chemical composition of the meteorite and its shielding depth.
Different meteorites lie on the mixing line if they have
cosmogenic and trapped components with the same
composition of Ne.
To compare the average trapped Ne in lunar meteorites
with the SW and the trapped Ne of other parent bodies, we
estimate the mean solar ratio of Ne for lunar meteorites by
plotting all available data from the compilation by Schultz
and Franke (2002) in a three-isotope plot (Fig. 2). Using the
Least-Squares Fit to calculate the mixing line, we obtain a
(20Ne/22Ne)tr ratio of 12.48 ± 0.07 for (21Ne/22Ne)tr = 0.03.
The given error is the standard deviation. Ninety percent of
the lunar meteorites lie within 10% of this mixing line.
In Table 6, the 20Ne/22Ne ratios in various solar system
reservoirs are summarized. The value of the lunar meteorites
lies between that for the SW and the SEPs.
Kr and Xe Components
We assume a priori that the composition of the trapped
Kr and Xe in Dho 081 is unknown and preliminarily
decompose the Xe data (Table 4) using both possible trapped
Xe components SW (Pepin et al. 1995) and air (see
Busemann et al. 2002). For the spallogenic (sp) Xe
composition, we used Xesp spectra (Hohenberg et al. 1981)
based on the Ba and REE concentrations in Dho 081 (Warren
et al. 2001). The pure trapped Xe isotope ratios for Dho 081,
corrected for the expected Xesp, are identical to those of air,
except for the ratios 128Xe/132Xe and 131Xe/132Xe. Solar Xe is
clearly absent, as is fissiogenic Xe. This is consistent with the
low concentrations of U and Th in Dho 081, compared to
other lunar meteorites (Warren et al. 2001). Thus, we
corrected for trapped Xe assuming that 136Xe is entirely of
trapped origin and Xetr = Xeair. The subtraction of air yields
(124Xe/126Xe)sp = 0.53 ± 0.06, (128Xe/126Xe)sp = 2.15 ± 0.40,
(130Xe/126Xe)sp = 1.2 ± 0.8, (131Xe/126Xe)sp = 6.1 ± 4.0,
126Xe = (6.7 ± 0.4) × 10−12 cm3 STP/g and 131Xe = (41 ±
sp
sp
27) × 10−12 cm3 STP/g. (128Xe/126Xe)sp and (131Xe/126Xe)sp
are high and indicate an origin of Dho 081 from a large depth,
in agreement with the absence of SW (this work) and the lack
of a “genuine regolith component” in this meteorite
(Greshake et al. 2001). Adopting (131Xe/126Xe)sp ratio = 6.1,
the Ba and La concentrations from Warren et al. (2001), and
the production rate systematics given in Hohenberg et al.
(1978), the shielding depth of Dho 081 would be ~80 g/cm2.
Taking in account the uncertainty of the (131Xe/126Xe)sp ratio,
shielding depths lie within a range of 0 to 175 g/cm2. The
transfer time from the Moon to Earth of <10 ka implies that
126Xe was essentially produced during the 2π irradiation of
sp
Dho 081 on the lunar surface. Thus, here we use the
production rates P126 = 0.012 × 10−12 cm3 STP/(g Ma) and
P131 = 0.075 × 10−12 cm3 STP/(g Ma), which are given for a
321
shielding depth of 80 g/cm2 (Hohenberg et al. 1978) and the
Ba and La concentrations in Dho 081 (Warren et al. 2001).
Assuming a simple exposure history for Dho 081, the Xe
isotope systematics alone would yield a residence time in the
lunar surface of ~550 Ma.
Kr in Dho 081 (Table 3) was partitioned assuming Krtr =
Krair and 86Kr to be entirely of trapped origin, because Xe in
Dho 081 did not indicate the presence of SW. The lack of
fissiogenic Xe in Dho 081 also excludes the presence of
fissiogenic 86Kr. 81Kr in Dho 081 was below detection limit,
precluding the determination of a 81Kr-83Kr cosmic ray
exposure age. A small excess of 80Kr due to neutron-induced
production on 79Br may be visible, whereas an excess on 82Kr
is within the uncertainty absent. The subtraction of trapped Kr
yields the Krsp composition of Dho 081 (78Kr: 80Kr: 82Kr: 83Kr:
84Kr: 86Kr = 0.118 ± 0.015, 0.579 ± 0.079, 0.653 ± 0.146, 1,
0.509 ± 0.548, 0). The low value for (78Kr/83Kr)sp of 0.118
implies, similar to a high (131Xe/126Xe)sp, a large shielding
depth of Dho 081 on the lunar surface. The depth can be
estimated with the production rates given by Regnier et al.
(1979) and the target element concentrations for Kr in Dho 081.
Only the concentration for Sr (240 ppm), the main target
element for Kr, is known (Warren et al. 2001), whereas the
concentrations of Rb (0.74 ± 0.15 ppm), Y (8.8 ± 0.7 ppm), and
Zr (34.5 ± 0.9 ppm) are adopted mean concentrations from
values given for MAC 88104/5 (Eugster et al. 1991; Jolliff et
al. 1991; Koeberl et al. 1991; Neal et al. 1991; Palme et al.
1991; Warren and Kallemeyn 1991). We have chosen this lunar
highland anorthosite because it might be crater paired with Dho
081 (Greshake et al. 2001). The choice of the lunar highland
anorthosite data set for Rb, Y, and Zr is not critical. The
shielding depth deduced for Dho 081 is ~180 g/cm2.
Considering the uncertainty of the (78Kr/83Kr)sp ratio, we
obtain a range from 120 to 260 g/cm2, which overlaps with the
depth range obtained for Xe. Both data sets imply a most
probable shielding depth for Dho 081 of 120 up to 180 g/cm2.
Nishiizumi et al. (2004b) calculated an ejection depth of 200–
230 g/cm2 based on the cosmogenic radionuclides. Adopting
the depth of 180 g/cm2 and assuming a simple irradiation in the
lunar surface for Dho 081, we obtain the production rates P78 =
0.0266 × 10−12 cm3 STP/(g Ma) and P83 = 0.2256 × 10−12 cm3
STP/(g Ma), respectively. These yield a residence time for
Dho 081 of ~640 Ma.
The Kr and Xe concentrations in Dho 081 are ~50%
higher than those given by Greshake et al. (2001).
Nevertheless, based on the Kr and Xe isotopic compositions,
we cannot confirm the presence of solar Kr and Xe in this
meteorite as found by Greshake et al. (2001).
Elemental Ratios of the Trapped Noble Gases
A (4He/20Ne)tr versus (20Ne/36Ar)tr is very illustrative
(Fig. 3) to compare the trapped solar noble gases in lunar
meteorites with those in other, so called “gas-rich”
322
S. Lorenzetti et al.
Fig. 3. Trapped noble gas ratios for all available data of lunar meteorites containing >2 × 10−5 cm3 STP/g 4He, of all available data of other
meteorites containing >100,000 × 10−8 cm3 STP/g 4He, and of the lunar soils from Apollo 11 and Apollo 12 (Eberhardt et al. 1972). The noble
gas data are from Schultz and Franke (2002) and from this work.
Table 7. Chemical composition in weight percent.
Sample
081a
Dho
NWA 032b
NWA 482c
NWA 773 cbf f
NWA 773 pf
SaU 169 impd
SaU 169 regd
SaU 169 KREEPd
Y-981031e
Ti
Cr
Mn
Fe
Ni
Mg
Al
Si
S
Ca
K
Na
0.060
1.85
0.078
0.5
0.24
1.32
1.5
0.88
0.43
0.052
0.27
0.048
0.29
0.62
0.10
0.13
0.08
0.18
0.039
0.26
–
0.21
0.15
0.11
0.11
0.09
0.16
2.49
17.9
2.72
15.4
15.9
8.29
8.62
6.84
9.66
0.009
0.005
–
–
–
0.02
0.008
0.006
–
1.448
5.10
2.17
8.54
15.3
6.69
4.79
4.17
5.68
16.9
4.63
15.5
4.32
2.9
8.41
9.28
8.65
9.76
21.0
20.9
20.9
21.4
19.6
21.1
21.9
23.4
21.0
–
–
–
0.06
–
0.33
–
–
–
11.4
7.79
12.5
6.37
3.6
7.26
8.38
7.58
9.18
0.050
0.091
0.025
0.091
0.17
0.45
0.38
0.73
0.033
0.52
0.28
0.30
0.16
0.11
0.73
0.58
0.88
0.25
a Greshake
et al. (2001).
et al. (2002).
c Warren and Kallemeyn (2001).
d Gnos et al. (2004).
e Kojima (2000).
fFagan et al. (2003).
b Fagan
meteorites (trapped 4He >100,000 × 10−8 cm3 STP/g). These
are supposed to originate from regoliths of parent bodies. To
calculate the trapped component of 4He, we assumed 4Her =
2 × 10−5 cm3 STP/g, which results in a correction of less than
5% in all cases. All lunar meteorites are in the range of (4He/
20Ne) = 1 to 11 and (20Ne/36Ar) = 1 to 9. The gas-rich nontr
tr
lunar meteorites clearly differ from the lunar meteorites.
They all have ratios (4He/20Ne)tr >50 and, in most cases,
(20Ne/36Ar)tr >9. We can deduce, therefore, that a meteorite
containing trapped noble gases, with elemental ratios in the
range defined above, most likely comes from the Moon.
Surprisingly, the lunar soils of Apollo 11 (A11) and
Apollo 12 (A12) yield about a factor of 10 higher (4He/20Ne)tr
ratios than the lunar meteorites. Compared to the lunar soils
A11 and A12, the 4Hetr in lunar meteorites seems to be
depleted. Perhaps this depletion is caused by the ejection
process from the Moon. Non-lunar meteorites do not show
this 4He depletion, as less energy is required to eject
meteoroids from asteroidal parent bodies than from the
Moon. Alternatively, 4He could have been lost during the
compaction process of the regolith into the solid rock material
of the meteorite.
Regolith history of lunar meteorites
323
Fig. 4. Production rate ratio P21/P38 versus shielding depth. The large open symbols represent the measured 21Ne/38Ar ratios.
Table 8. Production rates (10−8cm3STP/[g Ma]), cosmogenic ratios, shielding depths, and regolith residence times TR.a
Sample
4π irradiation
P21
P38
Measured in
the sample
(3He/21Ne)c
Produced on the
Moon
(21Ne/38Ar)c
(g/cm2)b
2π irradiation
P21
P38
TR (Ma)
Dho 081
NWA 032
NWA 482
NWA 773 cbf
NWA 773 p
SaU 169 imp
SaU 169 IIE
SaU 169 IIF
SaU 169 IIH
Y-981031
0.2011
0.1869
0.2041
0.2420
0.3354
0.2346
0.2116
0.2116
0.2051
0.2268
0.058
1.1
2.05
–
2.80
2.27
0.53
0.871
0.349
0.57
0.42 ± 0.03
0.99 ± 0.07
0.42 ± 0.02
–
3.8 ± 0.8
1.47 ± 0.12
0.80 ± 0.04
0.99 ± 0.05
0.95 ± 0.04
0.58 ± 0.07
180c
42
>500
–
>500
10.5
>500
64.1
129
>500
0.0894
0.0949
0.0150
–
0.0046
0.0776
0.0106
0.1032
0.0787
0.0115
275 ± 40
207 ± 43
2070 ± 420
–
100 ± 20
216 ± 45
1840 ± 370
210 ± 40
200 ± 40
4500 ± 1500
0.2115
0.1702
0.2301
0.1366
0.0895
0.1584
0.1778
0.1778
0.1691
0.1882
0.0514
0.0947
0.0068
–
0.0174
0.1120
0.0086
0.1033
0.0756
0.0093
a The
production rates for a 4π irradiation were calculated according to Eugster et al. (1995), those for a 2π irradiation according to Hohenberg et al. (1978).
depth.
c Based on Kr and Xe systematics.
b Shielding
Shielding Depth in the Lunar Regolith
To calculate the amount of the cosmogenic component
produced on the Moon, we subtracted the amount produced in
free space as calculated from Ttrans and the 4π production rate
according to Eugster et al. (1995). The chemical composition
of the investigated meteorites is summarized in Table 7.
The production rates in the lunar regolith vary with the
target material and the shielding depth. For stable isotopes
produced by cosmic rays, the ratio of the production rates is
equal to the ratio of the concentrations: 21Nec/38Arc = P21/P38.
Model calculations for the production rates of Ne and Ar,
based on nuclear systemactics, were performed by Hohenberg
et al. (1978) and Leya et al. (2001). Leya et al. (2001) give the
production rates for the Ar isotopes from Fe and Ni and
Hohenberg et al. (1978) from K, Ca, Ti, and Fe. Because Ca is
a significant target element in lunar meteorites for the
production of Ar isotopes, we used the production rates of
Hohenberg et al. (1978). For an Apollo drill core, Leya et al.
(2001) found an up to 60% higher production rate for 21Ne
compared to the model of Hohenberg et al. (1978). Neon
production rates, based on the chemistry in our samples, are
10–50% lower than those given by Leya et al. (2001). A direct
comparison for the ratio P21/P38 between the two different
data sets is only possible for Fe. Using the Leya et al. (2001)
data set, the ratio of the production rates is a factor of ~3 and
324
S. Lorenzetti et al.
a factor 1.1 higher at low shielding and at 500 g/cm2,
respectively. Obviously, this is not representative for bulk
composition of lunar meteorites. However, a part of the
systematic error disappears by using the ratio of P21 and P38
based on the same model. We assumed an error of the
production rates of 20%. The ratio of the production rates is
very sensitive to the elemental composition of the meteorite.
At a shielding depth of 65 g/cm2 the ratio P21/P38 varies from
0.65 in NWA 482 up to 3.83 in the peridotite fraction of
NWA 773. The variation of the ratio (21Ne/38Ar)c in duplex
samples is smaller than the experimental error of an single
measurement. Therefore, we conclude that our crushed
sample used for the noble gas analyses represents bulk
material.
In Fig. 4, the ratio of the production rates versus the
shielding depth is plotted. This ratio depends on the
chemical composition and the shielding depth of the
analyzed sample. To calculate the shielding depth from
(21Ne/38Ar)c, we fitted the values for the ratio of production
rates with a power function. The calculated shielding depths
are given in Table 8. This method is applicable if the
meteorite did not suffer any loss of 21Nec and 38Arc due to
diffusion, shock, or heat. For these processes, the loss of the
light noble gases can be assumed to be larger than the loss of
the heavy ones. As an indicator for such a diffusion process,
the ratio (3He/21Ne)c can be used. We assume that if (3He/
21Ne) is >1, the diffusion losses of 21Ne and 38Ar are
c
c
c
negligible. This is the case for NWA 032, NWA 482, and the
impact melt sample of SaU 169. For samples with diffusion
loss, the ratio of the production rates P21/P38 seems to be
low. Hence, the calculated shielding depth is too high and
the values for the shielding depth and the regolith residence
time are upper limits. The adopted error for the production
rate is 20%.
Dho 081 has suffered a loss of cosmogenic gases.
Therefore, the method described above is not suitable for this
meteorite. For the shielding depth, we assumed 180 g/cm2
based on the Kr measurements, which appears more precise
than the Xe measurement. The TR for the different isotopes
are: TR (21Ne) = 233 Ma, TR (38Ar) = 316 Ma, TR (83Kr) =
640 Ma, and TR (Xe) = 830 Ma (based on a shielding depth of
80 g/cm2, as determined with the Xe isotope systematics
alone, we would obtain ~550 Ma). The light noble gases 21Nec
and 38Arc yield lower ages than the heavy ones, indicating
diffusion loss of the light noble gases. The error-weighted
“best guess” residence time based on the heavy noble gases,
Kr and Xe, for Dho 081 is 680 ± 140 Ma.
NWA 032 has a shielding depth of 42 g/cm2 and a TR =
207 ± 43 Ma. Nishiizumi et al. (2001a) measured the
radionuclides and calculated exposure at depths of 660–
730 g/cm2 for 10Be, 520–630 g/cm2 for 26Al, and 520–540 g/
cm2 for 36Cl, and an ejection depth of 1100 g/cm2. The
authors exclude a production of these radionuclides in the
regolith. Therefore, we suggest that the exposure in the
regolith occurred 5–10 Ma ago. One or several impacts buried
NWA 032 deep enough, so that the radionuclides decayed to
the low levels presently observed. The shielding depth of
NWA 482 is >500 g/cm2 and the TR = 2070 ± 420 Ma,
representing the longest regolith residence time observed in
lunar meteorites.
The cataclastic breccia fraction of NWA 773 contains
solar gases and 17 ± 2 × 10−8 cm3 STP/g 21Nec. This fraction
was exposed in the lunar regolith, and most of the cosmogenic
Ne was produced at the lunar surface. To the contrary, the
peridotite fraction of NWA 773 contains 1.73 ± 0.31 × 10−8
cm3 STP/g 21Nec, and no solar wind noble gases. To produce
this amount of cosmogenic Ne, the sample must be irradiated
for 5.2 Ma in free space. This is a rather long transfer time for
lunar meteorites. Nishiizumi et al. (2004) proposed two
models to explain the low levels of 10Be and 41Ca in
NWA 773: 1) the meteorite was ejected from a depth of about
800 g/cm2 on the Moon and the transfer time was ~1 ka; and
2) the ejection depth was 1000 g/cm2 and the transfer time
was 30 ± 3 ka with and preatmospheric size ≥30 cm. Both
models include a short transfer time. Assuming a transfer time
of 30 ka, 99% of the cosmogenic noble gases in the peridotite
fraction was produced at the lunar surface. The measured
ratio (21Ne/38Ar)c can be explained adopting a shielding depth
>500 g/cm2 and a corresponding TR of 100 ± 20 Ma. The
cataclastic breccia fraction suffered an about 10 times longer
exposure to cosmic rays. We propose the following model.
The material of the cataclasic breccia fraction trapped solar
gases and was exposed to cosmic rays at the very top layer of
the Moon. During an impact event, the material was mixed
with the peridotite material and deposited at a depth of >500
g/cm2 for 100 Ma. After ejection, the exposure in space was
less than 30 ka.
The impact melt sample of SaU 169 has no gas loss. The
shielding depth of the impact melt sample in the lunar regolith
is 10.5 g/cm2 and TR is 216 ± 45 Ma. This sample is the oldest
material in SaU 169 with an U-Pb age of 4.24 ± 0.11 Ga
(Gnos et al. 2004). The regolith breccia fraction IIE of
SaU 169 has suffered more gas loss than the regolith breccia
fraction IIF. Therefore, the calculated values for the shielding
depth of the fraction IIE of >500 g/cm2 and TR of 1840 ±
370 Ma are too high. The fraction IIF yields a shielding depth
of 64.1 g/cm2 and a TR of 210 ± 43 Ma. In the KREEP clast,
we also found a low ratio (3He/21Ne)c. This sample has a
maximum shielding depth of 129 g/cm2 and an upper limit of
the TR of 200 ± 40 Ma.
We suggest that the impact melt sample of SaU 169
yields the most reliable value for TR of this meteorite. With
the assumption that the 38Arc concentration is reliable and the
residence time is 216 Ma, the shielding depth of the regolith
breccia is close to the surface. TR of the regolith fraction IIF
and the KREEPy sample IIH are within the error bars alike.
The ratio (3He/21Ne)c in Y-981031 indicates gas loss. The
shielding depth is below 500 g/cm2.
Regolith history of lunar meteorites
CONCLUSIONS
Dho 081, NWA 773, the regolith breccia fraction of SaU
169, and Y-981031 contain trapped solar gases. The other
lunar meteorites investigated here do not contain trapped
solar wind. Compared to the mean solar ratio (20Ne/22Ne)tr for
lunar meteorites of 12.48 ± 0.07, the trapped component in
NWA 773 and Y-891031 contains a larger proportion of SW.
On the other hand, SaU 169 contains more SEPs. The material
of NWA 032, NWA 482, and the peridotite fraction of
NWA 773 contains no solar gases and we conclude that this
material never resided in the very top layer (few millimeters)
of the lunar regolith.
Lunar meteorites without any gas loss have a regolith
residence time in the range of 207 ± 43 Ma for NWA 032 and
up to 2070 ± 420 Ma for NWA 482. The shortest TR of
100 ± 20 Ma was measured for the peridotite fraction of NWA
773. The most reliable residence time obtained for Dho 081
(680 Ma) is based on the heavy noble gases Kr and Xe.
NWA 032, NWA 773, and SaU 169 show a complex
regolith history. Material of NWA 032 suffered a multistage
exposure in different shielding depths. Materials of different
depths were also mixed together in NWA 773 and SaU 169. In
SaU 169, various impact events were dated (Gnos et al. 2004).
Assuming a lunar regolith mean density of 1.8 g/cm3
(Costes et al. 1969), the shielding depths observed for the
investigated meteorites correspond to a range of 5.8 cm to
>2.8 m. This is consistent with the observed lunar regolith
thickness of a few meters (Shkuratov and Bondarenko 2001).
Acknowledgments–We thank the National Institute of Polar
Research (Tokyo) for providing the Y-981031 sample, Tim
Fagan (University of Hawai’i) for the NWA 032 sample and
Paul Warren (University of California, Los Angeles) for the
NWA 482 and NWA 773 samples. We also thank our
colleagues Edwin Gnos, Beda Hofmann and Ali al Khatiri
(University of Bern) for collecting the SaU 169 meteorite in
Oman. We gratefully acknowledge the technical support by
Hans-Erich Jenni, Armin Schaller and Markus Zuber. Helpful
reviews by Kees Welten (University of California, Berkeley)
and an anonymous reviewer and the editorial handling by Marc
Caffee (Purdue University) are greatly appreciated. This work
was supported by the Swiss National Science Foundation.
Editorial Handling—Dr. Marc Caffee
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