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

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 182 W values in the nakhlites are uniform within analytical uncertainties and have an average value of ϳ3. Also, while 182 W values in the shergottites have a limited range (from 0.3-0.7), their 142 Nd values vary considerably (from Ϫ0.2-0.9). There appears to be no correlation between 182 W and 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 ϳ12 Myr after the beginning of the solar system. Major silicate differentiation established the nakhlite source reservoir before ϳ4542 Ma and the shergottite source reservoirs at 4525 ͫ ϩ19 Ϫ21 ͬ Ma. These ages imply that, within the uncertainties afforded by the 182 Hf-182 W and 146 Sm-142 Nd 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 182 W-142 Nd 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 182 W-142 Nd 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 ϳ4.5 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.