146SM-142ND FORMATION INTERVAL FOR THE LUNAR MANTLE

Abstract

Now-extinct 146Sm (t1 = 103 Ma) was present in the early solar system, and lunar basalts might be expected to have inherited small anomalies in 142Nd abundances from mantle sources with variable Sm/Nd ratios established during the early global differentiation of the Moon. 142Nd/144Nd ratios were measured for several lunar basalts, and values of ε142Nd, (deviations from terrestrial 142Nd/144Nd in parts in 104) calculated for them. The ε142Nd, values show very small variations attributable to 146Sm 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 x 1016 n/cm2. Both thermal and epithermal neutron fluences were calculated from the Sm isotopic compositions of the basalts and used to correct the ε142Nd values for neutron capture by neodymium. Well-resolved radiogenic enrichments ε142Nd = +0.28 +/- 0.11 and +0.17 +/- 0.08 (2σ), 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, measurable neutron fluences of #2.5 x 1015 and #7.7 x 1015 n/cm2, respectively, have neutron-corrected ε142Nd values of +0.25 +/- 0.15 and +0.29 +/- 0.11, respectively. The average value of radiogenic ε142Nd 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 ε142Nd = +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 ε142Nd which are not resolved from zero, and average +0.04 +/- 0.06. A fourth low-Ti basalt, 12039, has neutron-corrected ε142Nd = +0.25 +/- 0.12, but is subject to the largest neutron correction of #0.32 ε-units. KREEP basalt 14078 was exposed to only a small neutron fluence of #6.4 x 1015 n/cm2, and has a small deficit of 142Nd, corresponding to ε142Nd = -0.11 +/- 0.08. The average neutron-corrected ε142Nd of three KREEP basalts is -0.05 +/- 0.04. The ε142Nd values of the basalts correlate with 147Sm/144Nd values for their source regions as calculated from their ages and initial ε143Nd,CHUR,I values. 147Sm- 143Nd and 146Sm- 142Nd systematics were combined in a three-stage model yielding an isochron equation for the lunar mantle formation interval, which was calculated to be 238+56-40 Ma (2σ). 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.