Hf-182-W-182 isotope systematics of chondrites, eucrites, and martian meteorites: Chronology of core formation and early mantle differentiation in Vesta and Mars

摘要

The timescale of accretion and differentiation of asteroids and the terrestrial planets can be constrained using the extinct Hf-182-W-182 isotope system. We present new Hf-W data for seven carbonaceous chondrites, five eucrites, and three shergottites. The W isotope data for the carbonaceous chondrites agree with the previously revised W-182/W-184 of chondrites, and the combined chondrite data yield an improved epsilon(W) value for chondrites of -1.9 +/- 0.1 relative to the terrestrial standard. New Hf-W data for the eucrites, in combination with published results, indicate that mantle differentiation in the eucrite parent body (Vesta) occurred at 4563.2 +/- 1.4 Ma and suggest that core formation took place 0.9 +/- 0.3 Myr before mantle differentiation. Core formation in asteroids within the first similar to5 Myr of the solar system is consistent with the timescales deduced from W isotope data of iron meteorites. New W isotope data for the three basaltic shergottites EETA 79001, DaG 476, and SAU 051, in combination with published W-182 and Nd-142 data for Martian meteorites reveal the preservation of three early formed mantle reservoirs in Mars. One reservoir (Shergottite group), represented by Zagami, ALH77005, Shergotty, EETA 79001, and possibly SAU 05 1, is characterized by chondritic Nd-142 abundances and elevated epsilon(W) values of similar to0.4. The W-182 excess of this mantle reservoir results from core formation. Another mantle reservoir (NC group) is sampled by Nakhla, Lafayette, and Chassigny and shows coupled Nd-142-W-182 excesses of 0.5-1 and 2-3 epsilon units, respectively. Formation of this mantle reservoir occurred 10-20 Myr after CAI condensation. Since the end of core formation is constrained to 7-15 Myr, a time difference between early silicate mantle differentiation and core formation is not resolyable for Mars. A third early formed mantle reservoir (DaG group) is represented by DaG 476 (and possibly SAU 051) and shows elevated Nd-142/Nd-144 ratios of 0.5-0.7 epsilon units and epsilon(W) values that are indistinguishable from the Shergottite group. The time of separation of this third reservoir can be constrained to 50-150 Myr after the start of the solar system. Preservation of these early formed mantle reservoirs indicates limited convective mixing in the Martian mantle as early as similar to 15 Myr after CAI condensation and suggests that since this time no giant impact occur-red on Mars that could have led to mantle homogenization. Given that core formation in planetesimals was completed within the first similar to5 Myr of the solar system, it is most likely that Mars and Earth accreted from pre-differentiated planetesimals. The metal cores of Mars and Earth, however, cannot have formed by simply combining cores from these pre-differentiated planetesimals. The W-182/W-184 ratios of the Martian and terrestrial mantles require late effective removal of radiogenic W-182, strongly suggesting the existence of magma oceans on both planets. Large impacts were probably the main heat source that generated magma oceans and led to the formation metallic cores in the terrestrial planets. In contrast, decay of short-lived Al-26 and Fe-60 were important heat sources for melting and core formation in asteroids. Copyright (C) 2004 Elsevier Ltd.