Other
Scientific paper
Jul 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59.2817n&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 59, Issue 13, pp.2817-2837
Other
45
Scientific paper
Now-extinct 146 Sm ( t 1/2 = 103 Ma ) was present in the early solar system, and lunar basalts might be expected to have inherited small anomalies in 142 Nd abundances from mantle sources with variable Sm/Nd ratios established during the early global differentiation of the Moon. 142 Nd/ 144 Nd ratios were measured for several lunar basalts, and values of Nd 142 , (deviations from terrestrial 142 Nd/ 144 Nd in parts in 10 4 ) calculated for them. The Nd 142 ENa values show very small variations attributable to 146 Sm decay. Neutron capture near the lunar surface also modified both the samarium and neodymium isotopic compositions of the basalt samples. The calculated neutron fluences range from ~0 to ~7.8 × 10 16 n/cm 2 . Both thermal and epithermal neutron fluences were calculated from the Sm isotopic compositions of the basalts and used to correct the Nd 142 values for neutron capture by neodymium. Well-resolved radiogenic enrichments Nd 142 = +0.28 ± 0.11 and +0.17 ± 0.08 (2 ), respectively, were measured for lunar meteorite Asuka 881757 and Apollo 17 basalt 74255, respectively, for which the neutron fluences were nil. Basalts 70135 and 75075, which were exposed to small, measureable neutron fluences of ~2.5 × 10 15 and ~7.7 × 10 15 n/cm 2 , respectively, have neutron-corrected Nd 142 values of +0.25± 0.15 and +0.29 ± 0.11, respectively. The average value of radiogenic Nd 142 for the three high-Ti basalts from Apollo 17 is +0.22 ± 0.06. Neutron capture effects were greater for the other basalts, but can be corrected using the measured neutron fluences derived from the samarium isotopic data. The neutron-corrected Nd 142 = +0.19 ± 0.20 for Apollo 12 ilmenite basalt 12056, consistent with its derivation from a highly depleted mantle source also. Three low-Ti basalts from Apollo 12 and Apollo 15 (12038, 15076, 15555), have neutron corrected Nd 142 which are not resolved from zero, and average +0.04 ± 0.06. A fourth low-Ti basalt, 12039, has neutron-corrected Nd 142 = +0.25 ± 0.12, but is subject to the largest neutron correction of ~0.32 -units. KREEP basalt 14078 was exposed to only a small neutron fluence of ~6.4 × 10 15 n/cm 2 , and has a small deficit of 142 Nd, corresponding to Nd 142 = -0.11 ± 0.08. The average neutron-corrected Nd 142 of three KREEP basalts is -0.05 ± 0.04. The Nd 142 values of the basalts correlate with 147 Sm/ 144 Nd values for their source regions as calculated from their ages and initial Nd,churi,i 142 values. 147 Sm- 143 Nd and 146 Sm- 142 Nd systematics were combined in a three-stage model yielding an isochron equation for the lunar mantle formation interval, which was calculated to be 238 -40 +56 Ma (2 ). Sm/Nd ratios for the source regions also are derived from the model and compare favorably with values from conventional geochemical models of mare basalt petrogenesis. If the Moon were formed by impact of a large, Mars-sized planetesimal with the Earth, the impact was early enough that the lunar mantle cooled to neodymium isotopic closure by ~4.32 Ga ago. The bulk lunar Moon 142 evaluated from the mantle isochron at ( 147 Sm/ 144 Nd) = 0.1967 is -0.01 ± 0.03 (2 ), consistent with derivation of lunar and terrestrial neodymium from a common reservoir.
Bansal Bhavtosh
Harper Charles L.
Keith James E.
Nyquist Larry E.
Shih Chia-You
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