The early differentiation history of Mars from W-182-Nd-142 isotope systematics in the SNC meteorites

摘要

We report here the results of an investigation of W and Nd isotopes in the SNC (Shergottite-Nakhlite-Chassignite (martian)) meteorites. We have determined that epsilon(182)W values in the nakhlites are uniform within analytical uncertainties and have an average value of similar to 3. Also, while epsilon(182)W values in the shergottites have a limited range (from 0.3-0.7), their epsilon(142)Nd values vary considerably (from -0.2-0.9). There appears to be no correlation between epsilon(182)W and epsilon(142)Nd in the nakhlites and shergottites. These results shed new light on early differentiation processes on Mars, particularly on the timing and nature of fractionation in silicate reservoirs. Assuming a two-stage model, the metallic core is estimated to have formed at similar to 12 Myr after the beginning of the solar system. Major silicate differentiation established the nakhlite source reservoir before similar to 4542 Ma and the shergottite source reservoirs at 4525 [(+19)(-21)] Ma. These ages imply that, within the uncertainties afforded by the Hf-182-W-182 and (SM)-S-146-Nd-142 chronometers, the silicate differentiation events that established the source reservoirs of the nakhlites and shergottites may have occurred contemporaneously, possibly during crystallization of a global magma ocean. The distinct W-182-Nd-142 isotope systematics in the nakhlites and the shergottites imply the presence of at least three isotopically distinct silicate reservoirs on Mars, two of which are depleted in incompatible lithophile elements relative to chondrites, and the third is enriched. The two depleted silicate reservoirs most likely reside in the Martian mantle, while the enriched reservoir could be either in the crust or the mantle. Therefore, the W-182-Nd-142 isotope systematics indicate that the nakhlites and the shergottites originated from distinct source reservoirs and cannot be petrogenetically related. A further implication is that the source reservoirs of the nakhlites and shergottites on Mars have been isolated since their establishment before similar to 4.5 Ga. Therefore, there has been no giant impact or efficient global mantle convection to thoroughly homogenize the Martian mantle following the establishment of the SNC Source reservoirs. Copyright (c) 2005 Elsevier Ltd.