Sm--Nd systematics of extraterrestrial objects and the question of excess 142Nd

The Sm-Nd systematics of the eucritic basaltic achondrites Juvinas and Stannern were studied. Juvinas has a crystallization age of 4.56 +- 0.08 AE, in excellent agreement with reported 87Rb-87Sr and 206Pb-207Pb ages. The currently observed values of 147Sm/144Nd = 0.1936 and 143Nd/144Nd = 0.512636 for Juvinas are taken as the best estimates for ''chondritic'' ratios. The shocked meteorite Stannern did not yield a precise linear array of data points, but the best fit line corresponds to an age of 4.52 +- 0.08 AE. This is the oldest and most precise value obtained for this meteorite by any dating technique. Comparison of the Stannern data points with the Juvinas isochron as reference shows that Stannern, as a whole, apparently remained a closed system during the shock event. Pyroxene appears not to have been affected by this disturbance, while plagioclase and a rare earth element rich, HCl soluble phase were. Mass balance-type calculations indicate that a third disturbed component with high rare earth element content may also be present. Comparison of these samples is seen to imply that their respective complementary Sm/Nd parent reservoirs could have formed contemporaneously, and that the physical event associated with this could have been the differentiation of the outer region of the moon into crust and mantle