SM-146-ND-142 FORMATION INTERVAL FOR THE LUNAR MANTLE

Now-extinct Sm-146 (t(1/2) = 103 Ma) was present in the early solar sy stem, and lunar basalts might be expected to have inherited small anom alies in Nd-142 abundances from mantle sources with variable Sm/Nd rat ios established during the early global differentiation of the Moon. N d-142/Nd-144 ratios were measured for several lunar basalts, and value s of epsilon(Nd)(142), (deviations from terrestrial Nd-142/Nd-144 in p arts in 10(4)) calculated for them. The epsilon(Nd)(142) values show v ery small variations attributable to Sm-146 decay. Neutron capture nea r the lunar surface also modified both the samarium and neodymium isot opic compositions of the basalt samples. The calculated neutron fluenc es range from similar to 0 to similar to 7.8 X 10(16) n/cm(2). Both th ermal and epithermal neutron fluences were calculated from the Sm isot opic compositions of the basalts and used to correct the epsilon(Nd)(1 42) values for neutron capture by neodymium. Well-resolved radiogenic enrichments epsilon(Nd)(142) = + 0.28 +/- 0.11 and + 0.17 +/- 0.08 (2 sigma), respectively, were measured for lunar meteorite Asuka 881757 a nd Apollo 17 basalt 74255, respectively, for which the neutron fluence s were nil. Basalts 70135 and 75075, which were exposed to small, meas ureable neutron fluences of similar to 2.5 x 10(15) and similar to 7.7 x 10(15) n/Cm-2, respectively, have neutron-corrected epsilon(Nd)(142 ) values of + 0.25 +/- 0.15 and + 0.29 +/- 0.11, respectively. The ave rage value of radiogenic epsilon(Nd)(142) for the three high-Ti basalt s from Apollo 17 is + 0.22 +/- 0.06. Neutron capture effects were grea ter for the other basalts, but can be corrected using the measured neu tron fluences derived from the samarium isotopic data. The neutron-cor rected epsilon(Nd)(142) = + 0.19 +/- 0.20 for Apollo 12 ilmenite basal t 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 epsilon(Nd)(142) which are not resolved from zero, and average + 0.04 +/- 0.06. A fourth low-Ti basal t, 12039, has neutron-corrected epsilon(Nd)(142) = +/- 0.25 Nd +/- 0.1 2, but is subject to the largest neutron correction of similar to 0.32 epsilon-units. KREEP basalt 14078 was exposed to only a small neutron fluence of similar to 6.4 X 10(15) n/cm(2), and has a small deficit o f Nd-142, corresponding to epsilon(Nd)(142) = -0.11 +/- 0.08. The aver age neutron-corrected epsilon(Nd)(142) of three KREEP basalts is -0.05 +/- 0.04. The epsilon(Nd)(142) values of the basalts correlate with S m-147/Nd-144 values for their source regions as calculated from their ages and initial epsilon(NdCHUR,I)(143) values. Sm-147/Nd-143 and Sm-1 46-Nd-142 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 sigma). Sm/Nd ratios for the sour ce regions also are derived from the model and compare favorably with values from conventional geochemical models of mare basalt petrogenesi s. If the Moon were formed by impact of a large, Mars-sized planetesim al with the Earth, the impact was early enough that the lunar mantle c ooled to neodymium isotopic closure by similar to 4. 32 Ga ago. The bu lk lunar epsilon(Moon)(142) evaluated from the mantle isochron at (Sm- 147/Nd-144)(CHUR) = 0.1967 is -0.01 +/- 0.03 (2 sigma), consistent wit h derivation of lunar and terrestrial neodymium from a common reservoi r.