| Abstract: | Abstract— The Sm-Nd systematics of whole-rock and mineral separate samples from nakhlite Governador Valadares define a good 147Sm-143Nd mineral isochron age of 1.37 ± 0.02 Ga. This age is in excellent agreement with the 39Ar-40Ar and Rb-Sr ages obtained previously for this meteorite. However, the Rb-Sr isotopic data for our sample show that the isotopic system is disturbed. The lack of isotopic equilibrium is probably caused by the weathering of the sample as indicated by the presence of secondary alteration phases. The whole-rock and acid-washed mineral data yield a Rb-Sr age of 1.20 ± 0.05 Ga, which probably represents a lower limit to the crystallization age of the rock. The petrographic evidence indicates that this meteorite is a clinopyroxene cumulate that probably crystallized in a subsurface sill (McSween, 1994). Thus, the Sm-Nd isotopic age probably represents the age of such a magmatic event. The initial ε143Nd value determined for the rock at 1.37 Ga is +17 ± 1, indicating that the parent magma of the rock came from a light-rare-earth-element-depleted source of 147Sm/144Nd = ～0.237 based on a simple two-stage evolution model. Results of the same model calculation for the initial 87Sr/86Sr ratio of the rock suggest that its source material was depleted in 87Rb/86Sr by ～50% relative to the estimated martian value at 1.37 Ga. Both the high Sm/Nd and low Rb/Sr values support a clinopyroxene-rich cumulate source for the genesis of the nakhlite Governador Valadares. Furthermore, our Sm-Nd age and ε143Nd data and the previously published ε142Nd datum for the rock (Harper et al., 1995) are consistent with early differentiation of the parent planet, formation of cumulate sources ～4.56 Ga ago, and late melting of the sources and formation of the rock ～1.37 Ga ago. The good agreement of isotopic ages and petrographic features among Governador Valadares, Nakhla, and Lafayette strongly suggests that all three nakhlites have undergone similar evolutionary histories. The nakhlite age data suggest that isotopic heterogeneity in the martian mantle sources existed up to ～1.37 Ga ago and early mantle structures probably have not been disturbed for a significant portion of martian history. |
