Samarium-neodymium and rubidium-strontium systematics of nakhlite Governador Valadares

The Sm-Nd systematics of whole-rock and mineral separate samples from nakhl ite Governador Valadares define a good Sm-147-Nd-143 mineral isochron age o f 1.37 +/- 0.02 Ga. This age is in excellent agreement with the Ar-39-Ar-40 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. Th e lack of isotopic equilibrium is probably caused by the weathering of the sample as indicated by the presence of secondary alteration phases. The who le-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 clinopyr oxene cumulate that probably crystallized in a subsurface sill (McSween, 19 94). Thus, the Sm-Nd isotopic age probably represents the age of such a mag matic event. The initial epsilon(143)Nd 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 Sm-147/Nd-144 = similar to 0. 237 based on a simple two-stage evolution model. Results of the same model calculation for the initial Sr-87/Sr-86 ratio of the rock suggest that its source material was depleted in Rb-87/Sr-86 by similar to 50% relative to t he estimated martian value at 1.37 Ga. Both the high Sm/Nd and low Rb/Sr va lues support a clinopyroxene-rich cumulate source for the genesis of the na khlite Governador Valadares. Furthermore, our Sm-Nd age and epsilon(143)Nd data and the previously published epsilon(142)Nd datum for the rock (Harper et al., 1995) are consistent with early differentiation of the parent plan et, formation of cumulate sources similar to 4.56 Ga ago, and late melting of the sources and formation of the rock similar to 1.37 Ga ago. The good a greement of isotopic ages and petrographic features among Governador Valada res, Nakhla, and Lafayette strongly suggests that all three nakhlites have undergone similar evolutionary histories. The nakhlite age data suggest tha t isotopic heterogeneity in the martian mantle sources existed up to simila r to 1.37 Ga ago and early mantle structures probably have not been disturb ed for a significant portion of martian history.