A synthesis of the early evolution of the moon and of the lunar highland crust is made on the basis of U-Th-Pb, Rb-Sr, K-Ar and Sm-Nd isotopic data from lunar highland breccias

Huneke, J. C.

McCulloch, M. T.

Oberli, F.

Papanastassiou, D. A.

Wasserburg, G. J.

Caltech Authors - Main

EARLY DIFFERENTIATION, LATE MAGMATISM, AND 
RECENT BOMBARDMENT ON THE SHERGOTTITE PARENT PLANET 
L.E. Nyquist and D.O. Bogard, NASA Johnson Space Center 
J . L. Wooden, H. Wiesmann, C.-Y. Shih, and B.M. Bansal, Lockheed 
Electronics Corp. Inc. 
G. McKay, Nationa!Research Council, Houston, TX 77058 
Rb-Sr ages (T) and initial 87Sr/86Sr (I) for the shergottites, Shergotty 
and Zagami, and for the related achondrite ALHA 77005 are : Shergotty: T::: 
165 ± 12 m.y. , I= 0.72261 ± 13 ; Zagami: T = 184 ± 8 m.y., I= 0.72139 ± 5; 
ALHA 77005: T = 183 ± 12 m.y. , I = 71039 ± 5. The equality of these ages 
and the occurrence of maskelynite in these meteorites, coupled with older 
39Ar-40Ar and Sm- d ages of Shergotty, imply that the Rb-Sr ages were reset 
by a single major impact on the shergottite parent body. 
A pyroxene-whole rock Sm-Nd age of 620 ± 170 m.y. and a maximum 
whole rock 39 Ar-40 Ar age of 626 ± 1 5 m.y. are probably good estimates of the 
crystallization age of Shergotty Sm- d model ages relative to an eucritic 
initial 143 d/ 144Nd of3.6, 3 .5, and 2.8 g.y. for Shergotty, Zagami, and ALHA 
77005, respectively , are upper age limits. Sm-Nd and Rb-Sr whole rock 
isochrons c~ 1.2 g.y. and~ 5 g.y ., respectively) are discordant and show that 
the meteorites cannot be comagmatic. An early differentiation of the 
Shergottite parent body is indicated by BABI Rb-Sr model ages of 4.8-5.1 
g.y. for the three meteorites. A two-stage model yields (Sm/Nd)r/ (Sm d)s > 
1.27 , 1.33, and 1.68 for Shergotty, Zagami, and ALHA 77005 , respectively 
for T x > 0 where r = rock, s = source, and x = crystallization. For 
(Sm/Nd)rf(Sm/ d)s ~ 2.2, the upper limit achievable via pyroxene accumula-
tion, the model yields T x ~ 2.8, 2.6, and 1.2 g.y .. respectively. Corresponding 
(Rb/Sr)rf(Rb /Sr)s are 0.80-0.96. Thus, the two-fold variation in Rb/Sr 
between ALHA 77005 and the shergottites is inherited from distinct sources 
which were established early in the parent b ody's history. Early differentia-
tion plus late magmatism imply a sizable parent planet. A portion of the 
planet's surface must be young. Mars fits these criteria. 
ISOTOPIC CONSTRAINTS FOR THE 
EARLY EVOLUTION OF THE MOON 
F. Oberli, J.C. Huneke, M.T. McCulloch, D.A. Papanastassiou, and G.J. 
Wasserburg, The Lunatic A sy lum, Division of Geological and Planetary 
Sciences, Cal tech, Pasadena, CA 91125 
A synthesis of the early evolution of the moon and of the lunar 
highland crust is made on the basis of U-Th-Pb, Rb-Sr, K-Ar and Sm-Nd 
isotopic data from lunar highland breccias. These data indicate that most of 
the lunar highland rocks were metamorphosed during a short period of giant 
502 
impacts at 3.8-4.0 AE (Terminal Lunar Cataclysm). The existence of an older 
major impact event at 4.17 AE is documented by precise , identical U-Pb and 
40Ar-39Ar ages obtained from breccia 78155. This older event may be 
connected with the formation of a basin on the eastern part of the 
earth-facing side of the moon or on the lunar far-side. 
Both 3.9 and 4.17 AE old breccias yield nearly identical primary U-Pb 
ages of 4.46 and 4.51 AE suggesting that the moon differentiated on a 
planetary scale at that time. Derivation of the highland breccias from 
differentiated sources is indicated by variations of initial Sr, Nd and Pb 
isotopic ratios at the times of impact metamo rphism. 
Rb-Sr model ages for KREEPUTh-rich breccias require Rb/Sr fractiona-
tion at times ,.;;; 4.2 AE. Most likely the fractionation occurred during the 
cataclysm at ~ 3.9 AE. A volatile/ refractory element fractionation mecha-
nism does not appear to be the principal process involved. It is most plausible 
that the KREEP component was generated by partial melting of a source with 
high Rb/Sr. 
CORRELATED ANOMALIES OF TELLURIUM 
AND XENON IN ALLENDE 
L.L. Oliver, R.V. Ballad, J.F. Richardson, and O.K. Manuel, Department of 
Chemistry, University of Missouri, R olla, MO 65401 
The relative abundances of 12<Te, 12~e, 12~e, 12"Te, 12&re, 12&re and 
13<Te in carbon- and spinel-rich fractions of Allende's acid etched residues 
have been redetermined by neutron activation and -y-ray spectrometry. 
Changes from the procedure described by Ballad eta/. (1979) are (i) 
irradiation to a higher neutron fluence , (ii) analysis of Te from a sample of 
natural sylvanite, Cripple Creek, CO, (iii) use of the ingrowth activity of 1311 
to monitor levels of 131Te in the irradiated samples and monitors, and (iv) use 
of a computerized peak-stripping program for data reduction. No differences 
were observed between the isotopic compositions of Te in the monitor of 
pu rified Te02 and the sample of sylvanite. 
The spectra of even mass Te isotopes in the carbon- and spinel-rich 
frac tions of Allende are shown in Figure l, normalized to the second lighest 
stable isotope, 12~e. 
(!) 
In addition to the six isotopes shown there , the activity produced by double 
neutron capture on 12~e indicates values of gm = 2.5 ± 0.8 and - 0.6 ± 0.8 
for the carbon and spinel fractions, respectively. 
To iJ!ustrate similarities in the anomaly patterns of Te and Xe in these 
residues, the Xe spectra reported by Ballad eta/. (1979) have been 
503 


Isotopic Constraints for the Early Evolution of the Moon

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Isotopic Constraints for the Early Evolution of the Moon

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