41st Lunar and Planetary Science Conference (2009) - Page 1: (Click here for Official PDF Version)
EXPOSURE HISTORY OF LUNAR METEORITE NORTHWEST AFRICA 5000. K. Nishiizumi1, M. W.
Caffee2, N. Vogel3, R. Wieler3, M. D. Leclerc4, and A. J. T. Jull4, 1Space Sciences Laboratory, University of California,
Berkeley, CA 94720-7450, USA (email@example.com), 2Department of Physics, Purdue University, West
Lafayette, IN 47907-1396, USA (firstname.lastname@example.org), 3ETH-Zürich, CH-8092, Switzerland (email@example.com,
firstname.lastname@example.org), 4NSF Arizona AMS Facility, University of Arizona, Tucson, AZ 85721, USA (email@example.com, firstname.lastname@example.org).
Introduction: A lunar meteorite Northwest Africa
5000 (NWA 5000) was found in southern Morocco
in 2007. The recovered mass and size of the
meteorite, 11.528 kg and 27 x 24 x 20 cm, respectively,
make it the second largest known lunar meteorite
. The meteorite is a highland feldspathic leucogabbroic
breccia. It does not appear to be paired
with any other NWA lunar meteorites [1, 2].
Cosmogenic nuclide studies of lunar meteorites
contribute significantly to our understanding of these
objects. Using a combination of cosmogenic stableand
radionuclides it is possible to determine a number
of important properties of the meteorites. Most lunar
meteorites have complex cosmic ray exposure histories,
having been exposed both at some depth on the
lunar surface (2π irradiation) and after their ejection as
small bodies in space during transport from the Moon
to Earth (4π irradiation). Following these exposures is
a period of residence on Earth’s surface, a time commonly
referred to as the terrestrial age. The maximum
terrestrial age for hot desert meteorites was found to be
~0.6 Myr . Unraveling the complex history of these
objects requires the measurement of at least four cosmogenic
radionuclides. Noble gases in lunar meteorites
also provide useful information on their exposure
histories and conditions, in particular for samples having
resided for a very long time in the lunar regolith.
The specific goals of these measurements are to constrain
the following shielding or exposure parameters:
(1) the time a sample had spent near the lunar surface;
(2) the depth of the sample at the time of ejection from
the Moon; (3) the transit time from ejection off the
lunar surface until capture by Earth; and (4) the terrestrial
age. The sum of the transit time and terrestrial
age yields the ejection age, which is critical to recognize
launch pairing of lunar meteorites. The ejection
age, in conjunction with the sample depth on the
Moon, can then be used to model impact and ejection
mechanisms. In this study, we measured cosmogenic
radionuclides and noble gases in NWA 5000.
10Be, 26Al, 36Cl, and 41Ca measurements. We received
an exterior chip with white and brownish terrestrial
weathering products. To eliminate them, the sample
was leached twice with a 1.5 N HNO3 solution in
an ultrasonic bath for 15 and 10 minutes. The weight
loss was 23%, indicating severe terrestrial contamination.
Although this lunar meteorite is not heavily
weathered and it seems to be relatively fresh our fragment
had considerable terrestrial contamination and is
one of the most contaminated we have studied. 70.1
mg of ground sample was dissolved in an HF-HNO3
mixture along with Be and Cl carriers. Be, Al, Cl, and
Ca were separated for accelerator mass spectrometry
(AMS) measurements. The 10Be and 36Cl AMS measurements
were performed at PRIME Lab, Purdue University.
Measurements of 26Al and 41Ca are in progress.
14C measurement. Samples were pretreated with
100% H3PO4 to remove weathering products. The
residue was then washed and dried before melting in a
flow of oxygen to recover 14CO2 in presence of a carrier.
97.4 mg of NWA 5000 was used for the 14C
measurements. The AMS measurements were performed
at the University of Arizona NSF-AMS facility.
Noble gas measurements. A chip of 92 mg was
pre-heated in vacuum at ~120°C over night. Noble
gases were released in a single step at 1800°C, followed
by mass spectrometric analyses of He, Ne, and
Ar according to procedures outlined in ref. .
Results and Discussion: Preliminary results of
10Be, 14C, and 36Cl concentrations (±1σ) in NWA 5000
are shown in Table 1. Preliminary concentrations and
isotopic compositions of He, Ne, and Ar are shown in
Radionuclides. A maximum 14C terrestrial age of
10.4 ± 1.3 kyr was calculated assuming the 14C was
produced entirely by a 4π exposure during the Moonto-
Earth transit; a 14C saturation activity of 65 dpm/kg
was also assumed. This short terrestrial age produces
negligible decay corrections for 36Cl (< 2%) and 10Be
(< 1%). If all cosmogenic radionuclides were produced
by build-up during a 4π exposure, a minimum
14C exposure age would be 2.8 kyr. Likewise, the 36Cl
and 10Be ages would be 80 kyr and 310 kyr, respectively.
The large discrepancy between 4π exposure
ages implies that a large portion of the 36Cl and 10Be
was produced on the Moon during a 2π exposure before
the ejection. The end-member exposure model in
which all cosmogenic nuclides are produced in space
41st Lunar and Planetary Science Conference (2009) - Page 2: (Click here for Official PDF Version)
would not be correct. The alternative end-member
model is that most of the inventory of long-lived cosmogenic
nuclides was produced on the lunar surface.
Production rates as a function of depth on the Moon
are required to determine a 2π regolith exposure; these
are typically obtained from comparison of the measured
activities to those in the Apollo 15 drill core,
which extends to a depth of about 400 g/cm2. We
compared measured 10Be and 36Cl activities to those of
the Apollo 15 drill core depth profiles , normalizing
for differences in the target elemental composition.
The measured activities match the production at 325 ±
20 g/cm2 for 10Be and 350 ± 40 g/cm2 for 36Cl. The
measured 14C activities are also compared to the
Apollo 15 drill core . The observed 14C activity is
equivalent to a much shallower depth of 245 ± 15
g/cm2. The 14C activity can be reconciled with the
10Be- and 36Cl-derived ejection depth of 335 ± 20
g/cm2, and with the assumption that the balance of the
14C was produced during the post-ejection 4π exposure.
For this exposure scenario, the minimum 4π exposure
age is 1.3 kyr, assuming negligible terrestrial
Noble gases. NWA 5000 contains substantial
amounts of solar noble gases as demonstrated by the
isotopic ratios 3He/4He, 20Ne/22Ne and 36Ar/38Ar, which
are all typical for solar gas-rich lunar regolith samples
(Table 2). Elemental ratios (4He/20Ne = 14.3;
20Ne/36Ar = 3.70) are typical for plagioclase-rich samples,
which have poor retentivity for solar He and Ne
. Much of the measured 40Ar is likely radiogenic,
hence the 40Ar/36Ar ratio cannot be used to constrain
the time of solar-wind implantation ("antiquity") of the
sample. The regolithic origin of NWA 5000 is in
agreement with observations on other lunar meteorites
 and suggests by itself a shallow depth of ejection.
The solar He and Ar inhibit a determination of
cosmogenic 3He and 38Ar (although the rather high
measured 3He/4He ratio suggests that a sizeable fraction
of the 3He is cosmogenic). The concentration of
cosmogenic 21Ne is 1.57x10-7 cm3 STP/g (assuming
trapped Ne composition to fall in-between the pure and
fractionated SW end-members [e. g., 9]. This value is
typical for lunar meteorites . Obviously, most of the
cosmogenic 21Ne in NWA 5000 was produced on the
lunar surface. Taking the 21Ne production rate on the
lunar surface of 0.051x10-8 cm3 STP/(g·Myr) given in
 for lunar highland breccia Dhofar 081 (with 180
g/cm2 shielding depth) and correcting for the larger
shielding experienced by NWA 5000 results in a
P(21Ne) value of about 0.02x10-8 cm3 STP/(g·Myr).
This yields a residence time on the lunar surface of
NWA 5000 of about 600 Myr, if we assume that
throughout its entire regolith history the sample resided
at the same depth as during the past few Myr before
ejection. Such a long pre-exposure time on the lunar
surface is again typical for lunar meteorites.
Table 1: Cosmogenic radionuclide in NWA 5000.
||2.93 ± 0.03
||18.6 ± 0.6
||3.95 ± 0.13
Table 2: He, Ne, and Ar in NWA 5000.
Gas concentrations in 10-5 cm3 STP/g Uncertainties: 4% for concentrations, 3% for 3He/4He, 1% for other isotope ratios
Based on 10Be, 14C, 36Cl and cosmogenic 21Ne concentrations,
the most likely exposure history of NWA
5000 was as follows: After a roughly 600 Myr residence
in the lunar regolith, the meteorite was ejected
from a depth of 335 ± 20 g/cm2 on the Moon. The
minimum transition time from the Moon to Earth was
1.3 kyr with only a short terrestrial age < 1 kyr. The
short terrestrial age is consistent with the presence of
fresh, translucent fusion crust on part of the meteorite
. We cannot completely rule out a longer terrestrial
age up to ~10.4 kyr. 26Al and 41Ca measurements are
required to further constrain the history. Although the
petrologic observation and the trace element chemical
compositions [1, 2] do not support pairing, the exposure
history of NWA 5000 is similar to that of NWA
Acknowledgments: We thank A. C. Hupé and A.
J. Irving for providing samples. This work was supported
by NASA SRLIDAP and Cosmochemistry program
and the Swiss NF.
References:  Irving A. J. et al. (2008) LPS
XXXIX, Abstract #2168.  Korotev R. L. et al. (2008)
LPS XXXIX, Abstract #1209.  Nishiizumi K. et al.
(2002) LPS XXXIII, Abstract #1366.  Wieler R. et
al. (1989) GCA, 53, 1449-1459.  Nishiizumi K. et
al. (1984) EPSL, 70, 157-163.  Jull A. J. T. et al.
(1998) GCA, 62, 3025-3036.  Signer P. et al. (1977)
Proc. Lunar Sci. Conf., 8th, 3657-3683.  Lorenzetti
S. et al. (2005) Meteoritics & Planet. Sci., 40, 315-327.
 Grimberg A. et al. (2008) GCA, 72, 626-645. 
Nishiizumi K. and Caffee M. W. (2006) Meteoritics &
Planet. Sci., 41, A133.