NASA Technical Reports Server (NTRS) 20070003760: Sm-Nd and Rb-Sr Ages for MIL 05035: Implications for Surface and Mantle Sources

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Sm-Nd AND Rb-Sr AGES FOR MIL 05035: IMPLICATIONS FOR SURFACE AND MANTLE SOURCES.
L. E. Nyquist1, C-Y. Shih2, and Y. D. Reese3, 1Mail Code KR, NASA Johnson Space Center, Houston, TX 77058,
[email protected], 2Mail Code JE-23, ESCG/Jacobs Sverdrup, P.O. Box 58477, Houston, TX 77058,
[email protected], 3Mail Code JE-23, ESCG/Muniz Engineering, Houston, TX 77058, [email protected].

Introduction: The Sm-Nd and Rb-Sr ages and Lunar Mare Basalt Meteorite MIL 05035
also the initial Nd and Sr isotopic compositions of MIL
Ilm2
05035 are the same as those of A-881757 [1]. Compar- 0.706 Px2(r)
T=3.90±0.04 Ga
ing the radiometric ages of these meteorites to lunar I(Sr)=0.699089±14
surface ages as modeled from crater size-frequency Sr 0.704
dinisittiraibl uStri-oinsos t[o2p,i3c] caosm wpeolsl iatiso nthse o Tf ioOth2 earb buansdaalntsc epsl aacneds 786Sr/ 0.702 (AWsuRk2a) 87Rb/86Sr
their likely place of origin as within the Australe or 8 Px1(r) 10.00 0.05 0.10
+35Ma
Humboldtianum basins. If so, a fundamental west-east 0.700 WR2 εSr0
lunar assymmetry in compositional and isotopic pa- Plag2 -35Ma
-1
0.698
rameters that likely is due to the PKT is implied.
0.00 0.05 0.10
Sm-Nd age: The Sm-Nd age (T ) = 3.80±0.05
Sm-Nd 87Rb/86Sr
Ga for MIL 05035 (Fig. 1) and agrees within mutual
Figure 2. Rb-Sr isochron for MIL 05035. A whole rock
error limits with T = 3.87±0.06 Ga for A-881757
Sm-Nd
analysis of A-881757 falls on the same isochron.
[1]. Initial ε = +7.2±0.4 for MIL 05035 compared to
Nd
793169 shows strong evidence for Ar degassing in a
+7.4±0.5 [1] for A-881757.
major impact ~430 Ma ago [4]. We use ~3.80 Ga as
Mare Basalt - MIL 05035 the age of MIL 05035 and the other “YAM” (Ya-
0.516 T=3.80±0.05 Ga Px1(r) mato\Asuka\Miller Range) basalts, but note that the
ε =+7.2±0.4 ~3.90 Ga Rb-Sr age, or the ~3.94 Pb-Pb age of A-
Nd
0.515 Px2(r) 881757 [1] may more accurately give the crystalliza-
d Ilm(r)
144Nd/N 0.514 WR(AsukWa)R WR(r) 0.2147Sm0/1.344Nd 0.4 tionO algde ((s~)3. . 7-3.9 Ga) mare basalts are found among
143 0.513 0.8 +51Ma the Apollo 11 and 17 Hi-Ti basalts, but the low TiO2
Plag(r) 0.4
ε 0.0 abundances of the YAMs make a Tranquilitatis or
0.512 Nd
WR(l) -0.4 Serenitatis origin unlikely. Crater size-frequency ages
0.511 Leach -0.8 -51Ma [2,3] make the maria Humorum, Humboldtianum, and
0.15 0.20 0.25 0.30 0.35 Australe their most probable places of origin among
147Sm/144Nd the nearside maria [4]. We prefer the pre-Nectarian
Australe basin, specifically units A1 or A2 (3.80-3.88
Figure 1. Sm-Nd isochron for MIL 05035, excluding the
Ga) of [2]. Our second preference is for units HU2 and
Plag(r) analysis. A whole rock analysis of A-881757 also lies
HU3 (~3.77 Ga [2]) in Mare Humboldtianum. An ori-
very close to the MIL 05035 isochron.
Rb-Sr age: The isochron values are T = gin within units H6 (3.46/3.75 Ga [2]) or H7
Rb-Sr
3.90±0.04 Ga and I (initial 87Sr/86Sr) = (3.45/3.94 Ga [2]) of Mare Humorum is permitted by
Sr
0.699089±0.000014 (Fig. 2). The Rb-Sr age reported
by [1] for A-881757 is 3.89±0.03 Ga when adjusted to Lunar Basalts
λ(87Rb) = 1.402 x 10-11 y-1 in excellent agreement with 4.4 Western Eastern
the MIL 05035 value. ISr = 0.69910±0.00002 for A- 4.0 A-14 KREEPA-1A1-17HtuiMamnaburoemld- AuMsatrreale
881757 [1] also agrees well with the MIL 05035
value. Ga) 3.6 LAP 0220A5-12 A-15 L-1L6-24 ?
Discussion: The Sm-Nd and Rb-Sr data as well as e ( ? MIL 05035
Sm-isotopic data not given here suggest that MIL Ag 3.2 A881757
05035 and A-881757 are isotopically identical. The Mare
2.8 Procellarum
internal Pb-Pb isochron age reported by [1] for A- PKT
881757 was 3.94±0.03 Ga, whereas the 39Ar-40Ar age 2.4
was 3.80±0.01 Ga. Recent 39Ar-40Ar age measure- -80 -60 -40 -20 0 20 40 60 80 100
West Longitude (°) East
ments [4] gave younger ages of 3.69±0.07 Ga for A-
Figure 3. Lunar basalt ages vs. lunar longitude of known
881757 and 3.71±0.11 Ga for Yamato-793169,
or estimated sampling sites.
thought to be launch-paired with A-881757. Y-
the age data, but seems less likely for reasons given mantle source of the YAM basalts to be very LREE-
below. depleted. Thus, the YAM source was deficient in
Lunar basalt ages [5] are plotted vs. the longitude
Lunar Basalts
of the known or estimated (YAM, LAP 02205 [6])
0.07 Western Eastern
sampling sites in Fig. 3. Comparing Fig. 3 to Fig. 12
of [2] summarizing mare basalt ages by the crater size- 0.06 LAP 02205 KREEP(=0.15)
A-14
frequency method shows both similarities and differ- urce 0.05 ?
etamonnamcdre iaasLr. i uaCF ncreaaoc true1rnre6 sds piiatzoanentd-idsf ir nLeagqun untdeoa n sc2Coy4mr iasesgi aueAmms1 pa,4 rl iebrn erlgsae pccsekciciitaneti sgvc, elfaloiys.ret. . ca, rgTytehhpsee-, 8786(Rb/Sr)So 000...000234 ProcMeallraerAum-12 A-15A-11A-L1-716L-2MA4I8L8 015?705375
0.01
sampled L-24 basalts are VLT basalts with TiO2 abun- PKT HuMmabroeld- Mare
dances about half the TiO abundances of the YAM 0.00 tianum Australe
2
basalts (Fig. 4.). TiO in Mare Crisium ranges ~1-8% -80 -60 -40 -20 0 20 40 60 80 100
2 West Longitude (°) East
[7]. Candidate surface units for the YAMs in Mare
Figure 5. Estimated source 87Rb/86Sr for lunar basalts vs.
Humorum [2] correspond to spectral units hDSP and
longitude of known or estimated sampling sites.
mISP of [8] with estimated TiO of ~3.5-5.0 and <~3
2
wt. %, resp. More recent estimates for the same areas LREE as well as K-correlated Rb, both characteristic
[7] are ~8-9 and ~5-8 wt. %, resp.; higher than TiO ~ of the urKREEP lunar differentiate. Also, the YAM
2
2 wt. % for the YAMs [9]. Also, the Humorum basin source is characterized by very low 238U/204Pb [1].
lies within the boundaries of the Procellarum KREEP Conclusions: The YAM basalts are the products of
Terrain (PKT) [10], and basalts from the PKT have early melting of sources composed mainly of olivine
relatively high I values in contrast to the YAM and and orthopyroxene [1], early cumulates in a magma
Sr
L-24 basalts. ocean model. The absence of urKREEP from their
sources suggests that melting was not due to radio-
Lunar Basalts
15 Western Eastern genic heating. The probable absence of urKREEP-
A-17
A-11 enriched reservoirs beneath the eastern maria suggests
an assymmetry in lunar mantle compositions related to
10 LAP 02205 the PKT.
%) A-15 MIL 05035 References: [1] Misawa K. et al. (1993) GCA, 57, 4687-4702.
wt ? A-12 L-16 A881757 [2] Hiesinger H. et al (2000) JGR, 105, 29,239-29,275. [3] Hi-
O (2 5 A-1K4REEP ? esinger H. et al. (2003) JGR, 108 (E7) 5065, 1-1 to 1-27. [4] Fernan-
Ti L-24 des V. A. et al. (2005) LPS XXXVI, Abstract #1002. [5] Nyquist L. E.
Mare
et al. (2001) The Century of Space Science, Kluwer, 1325-1376. [6]
0 Procellarum Mare Nyquist L. E. et al. (2005) LPS XXXVI, Abstract #1374. [7] Bussey
PKT Humbold- Mare
tianum Australe D. B. J. and Spudis P. D. (2000) JGR 105, 4235-4243. [8] Pieters C.
(1978) PLPSC9, 2825-2849. [9] Arai T. (1996) Meteoritics &
-80 -60 -40 -20 0 20 40 60 80 100 Planet. Sci., 31, 877-892. [10] Jolliff B. L. et al. (2000) JGR, 105,
West Longitude (°) East 4197-4216. [11] Papanastassiou D. A. and Wasserburg G. J. (1972)
Figure 4. TiO2 contents of lunar basalts vs. longitude EPSL, 13, 368-374. [12] Wasserburg G. J. et al. (1978) Mare
of known or estimated sampling sites. Crisium: The View from Luna 24, 675-678.

Fig. 5 summarizes information obtained by con- Lunar Basalts
Western Eastern
verting I values to source region 87Rb/86Sr ratios via
Sr Mare Mare
a 2-stage model. Low I for MIL 05035 and A-881757 0.30 Humbold- Australe
Sr A-11 tianum
shows derivation from a lunar mantle source with a urce A-12 A-15 A-17
o
low Rb/Sr ratio compared to the sources of basalts S ?
d) 0.25 LAP 02205
sampled during the Apollo missions. Similarly low N
source region Rb/Sr ratios were found only for basalts 144m/ ? A-14 L-24 MAI8L8 015705375
fprloemd btyh eth eea Lstuenrna 1m6a rainad F Leucunna d2i4ta mtisi sasniodn sC [r1is1iu, m12 ]s.a m- 147(S 0.20 Mare Chondrites
Procellarum
The YAM basalts differ from the L-24 basalts by PKT KREEP
having higher ε values. As for the I data, the ε 0.15
Nd Sr Nd -80 -60 -40 -20 0 20 40 60 80 100
values may be used to estimate 2-stage model source
West Longitude (°) East
region 147Sm/144Nd ratios (Fig. 6). Those data show the
Figure 6. Estimated source region 147Sm/144Nd for lunar
basalts vs. longitude of known or estimated sampling sites.