Download Geochemical and isotopic systematics of the youngest dated lunar

Lunar and Planetary Science XXXVI (2005)
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GEOCHEMICAL AND ISOTOPIC SYSTEMATICS OF THE YOUNGEST DATED LUNAR IGNEOUS
ROCK, NORTHWEST AFRICA 773
L. E. Borg1, C. K. Shearer1, Y. Asmerom2, and J. J. Papike1, 1Institute of Meteoritics University of New Mexico, Albuquerque, NM 87131 USA; 2Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque NM 87131.
Sm-Nd Isochron Plot: The Sm-Nd isochron diagram
of mineral and whole rock fractions from NWA773
yields an age of 2.865 ± 0.031 Ga and an initial εNd of
-7.84 ± 0.22. (Fig. 1). This age is concordant with the
NWA 773 Gabbro-norite clast
(youngest dated lunar igneous rock)
0.5145
Ol (R)
0.5140
143Nd/144Nd
Introduction: Many lunar samples are strongly enriched in potassium (K), rare earth elements (REE),
and phosphorous (P). This KREEP geochemical signature is interpreted to indicate the involvement of
late-stage liquids derived from the crystallization of a
magma ocean in the petrogenesis of KREEP-rich samples. Most KREEP-rich samples, including Mg-suite,
alkali suite rocks, as well as KREEP-basalts, have ages
greater than 3.8 Ga. This has led to the hypothesis that
KREEP magmatism was confined to the earliest periods of lunar history. Below we present Rb-Sr and SmNd isotopic evidence that an olivine-rich clast in the
lunar meteorite Northwest Africa 773 (NWA773) is
not only the youngest lunar sample yet dated, but has
the largest KREEP geochemical signature.
Petrology and geochemistry of NWA 773: Northwest Africa 773 is a brecciated lunar meteorite containing a large igneous clast. This clast is interpreted
to be an olivine-gabbro cumulate and contains 48-66%
olivine, 19-27% pigeonite, 8-16% plagioclase, 5-11%
augite, ~2% orthopyroxene, trace – 2% K-feldspar,
and trace amounts of ilmenite, merrillite, troilite, Crspinel, Fe-Ni metal, and calcite [1-4]. The Mg#’s of
mafic phases in NWA773 are relatively high (68-77),
indicating that the parental magma has not undergone
large amounts of fractional crystallization [1]. Jolliff
et al. [1] have also demonstrated that the olivine-rich
clast, like KREEP, has an elevated LREE/HREE ratio.
Analytical Techniques: A 100 mg aliquot of the
olivine gabbro clast from NWA773 was obtained from
M. Grady at the Natural History Museum, London.
The sample was washed and sonicated in four-times
quartz distilled water, followed by 0.5M acetic acid.
The sample was crushed and sieved at 100-200 and
200-325 mesh. Mineral separations were begun using
heavy liquids on both size fractions. Plagioclase
floated in 2.85 g/cm3, whereas the mafic minerals
sank. Hand-picking was used to separate pyroxene
(brown) and olivine (green) mineral grains, as well as
to purify the plagioclase fraction. Individual mineral
fractions were then leached in 1N HCl for 10 minutes
in a sonicator prior to digestion. Chemical separations
and isotope ratio measurements were done at the Radiogenic Isotope Laboratory, University of New Mexico, following standard silicate dissolution procedures
and involved cation chromatography using a combination of HCl and methalactic acids. Isotopic ratios were
measured on a Micromass Sector 54 multi-collector
thermal ionization mass spectrometer.
0.5135
Ol + Px (R)
Age = 2.865 ± 0.031 Ga
εNd = -7.84 ± 0.22
Wr (R)
0.5130
0.4
0.5125
+31 Ma
0.2
Plag (R)
εNd 0.0
0.5120
-0.2
-31 Ma
-0.4
0.15
0.20
0.25
0.30
0.5115
0.15
0.19
0.23
0.27
0.31
147Sm/144Nd
Figure 1. Sm-Nd isochron plot of NWA773 whole rock and
mineral fractions. The age is calculated using the Isoplot
program of Ken Ludwig [5]. The MSWD = 0.92. Nd normalization is 146Nd/144Nd = 0.7219. R = residues.
Ar-Ar age determined by [6] indicating that NWA773
has not undergone significant thermal metamorphism.
NWA773 is the youngest lunar sample dated (Fig. 2).
The youngest crystallization age previously determined for a lunar sample is ~3.1 Ga [7], whereas the
youngest crystallization of a KREEP-rich sample
(15382) reported previously was 3.83 ± 0.02 Ga [8].
The 2.865 Ga crystallization age of NWA 773, therefore extends the range of dated lunar samples by 250
Ma, and the period of KREEP-magmatism by 1 Ga.
The Rb-Sr isochron is highly disturbed as a result of
terrestrial weathering. Nevertheless, an initial 87Sr/86Sr
ratio of 0.703568 ± 0.000032 can be estimated from
the plagioclase mineral fraction assuming a crystallization age of 2.865 Ga (Fig. 3). This initial Sr isotopic
composition indicates that NWA773 is derived from a
source region with an extremely high 87Rb/86Sr ratio of
0.07, assuming the source region formed at 4.56 Ga.
Figure 4 is an age versus initial Nd isotopic composition plot. It demonstrates that NWA773 is derived
from a source region with a 147Sm/144Nd ratio of ~0.16.
By comparison to other KREEP-rich samples plotted
on Figure 4 it is apparent that NWA773 is derived from the
Lunar and Planetary Science XXXVI (2005)
1026.pdf
Crystallization Ages of Lunar Rocks
T-I Diagram for Evolved Lunar Rocks
2
76535
Low-Ti-basalts
15445 ,17
78236
14321
CHUR (147Sm/144Nd = 0.1967)
0
Hi-Ti-basalts
15386
72275
15455 ,247
14304
-2
εNd
KREEP-basalts
NWA 773
Hi-Al basalts
147Sm/144Nd
= 0.181
-4
147Sm/144Nd
= 0.166
Alkali suite rock
+Granites/felsites
-6
147Sm/144Nd
= 0.158
Mg-suite
-8
FANs
2.5
3.0
3.5
NWA773
2.5
3
3.5
4.56
4
Age (Ga)
4.56
4.0
Age (Billion years)
Figure 4. Age - initial εNd plot of evolved lunar rocks.
Figure 2. Crystallization ages of lunar igneous rock.
0.712
Ol + Px (R)
0.710
T = 4.045 ± 58 Ga
87Rb/86Sr
87Sr/86Sr
0.711
Ol (R)
0.709
0.708
0.707
0.706
0.05
20
0.30
0.25
5
2
NWA773
0.20
0.15
Mixing before
KREEP
leaving source
g
avin )
n
er le
g aft similatio
in
ix
s
M
ce (a
r
u
o
s
40
1
Range of 87Rb/86Sr of KREEP
-rich rocks and their sources
0.10
0.05
Wr (R)
0.00
2865 Ma ref. line
ISr = 0.703568 ± 32
Plag (R)
0.07
0.35
mixture
NWA 773 Gabbro-norite clast
0.09
0.11
0.13
A15 Green glass
0.0
0.15
87Rb/86Sr
Figure 3. Rb-Sr diagram used to estimate the initial Sr isotopic composition of NWA773 to be 0.703568 ± 32.
most LREE-enriched source region of any lunar sample for which Sm-Nd isotopic analyses have been
completed. This observation is consistent with the
calculation of Jolliff et al. [1] in which they demonstrated that the parental magma of NWA773 has a
higher LREE/HREE ratio than estimates of KREEP.
An outstanding question regarding KREEP magmatism is when the KREEP-rich component was added to
the magma. On one hand it could be present in the
source region during melting, whereas on the other
hand, it could be assimilated by mafic melts derived
from the mantle as they rise to the surface. These possibilities are modeled (Fig. 5) with mixing relationships between (1) KREEP and green glass (lower
curve), and (2) KREEP and green glass/10 (upper
curve). The former model is representative of assimilation of KREEP by a primitive lunar melt, whereas
the latter model represents mixing between KREEP
and a primitive, green glass source, estimated to have
10 times lower incompatible element abundances than
20
40
60
80
100
Percent KREEP
Figure 5. KREEP- green glass mixing models.
the green glass. The 87Rb/86Sr ratios of KREEP-rich
sources and parental magmas derived from these
sources are calculated from the measured initial Sr
isotopic compositions and are represented by the
shaded area in Figure 5. These models demonstrate
that in order to produce the 87Rb/86Sr ratio of
NWA773, 22% KREEP must be assimilated by a
green glass parental magma, whereas only 2% KREEP
must be added to the green glass source region to produce the required 87Rb/86Sr ratio. The smaller proportion of KREEP required by the source mixing model is
more consistent with the relatively low Fe content and
high Mg#’s of the phases in NWA773. This is because
KREEP is expected to have a relatively high Fe content [9].
References: [1] Jolliff et al., (2003) GCA 67, 4857-4879. [2]
Fagan et al. (2003) MAPS 38, 529-554. [3] Grossman &
Zipfel (2001) MAPS 36, A300. [4] Bridges et al. (2002)
MAPS 37, A24. [5] Ludwig (2001) Berkeley Geochronology
Special Publication 1a. [6] Frenandez et al. (2003) MAPS 38,
555-564. [7] Nyquist et al. (1977) Proc. 8th LPSC 13831415. [8] Papanastassiou et al. (1976) Proc. 7th LPSC 20352054. [9] Warren (1988) Proc. 18th LPSC 233-241.