The state of Hf-W clock and the planetary formation timescale

Details The state of Hf-W clock and the planetary formation timescale The state of Hf-W clock and the planetary formation timescale

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja....13619y&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #13619

Astronomy and Astrophysics

Astronomy

Scientific paper

The rapid formation of planetary systems implied by the new Hf-W data (Yin et al., 2002) solves a number of problems with the old data. (1) The new data are consistent with astronomical observations of young stellar objects with planetary system formation within a few million years; (2) dynamic modeling of planetary formation; (3) 26Al-26Mg and 53Mn-53Cr age constraints; (4) rare gas (solar He and Ne) observation for the Earth's mantle, implying accretion of the Earth before complete dissipation of nebula gas. The new Hf-W data require that bulk of metal-silicate separation in the entire Solar System is completed by 30 Ma (no data plots with shallower slope than chondrite-Earth-Moon line, see Fig 1a of Yin et al., 2002). We show that the main growth stage (63%) for the Earth is largely completed in ˜10 Ma and the Moon-forming giant impact is dated at 29 Ma, with a fully-grown Earth. Interpreting the new Hf-W results as Earth formed twice as fast (30 Ma vs. 60 Ma) is incorrect. Old chondrite data being identical with the silicate Earth in W isotope composition implied that core-mantle segregation occurred after 182Hf was extinct. There was never a precise age constraint on the problem with the old data. The core formation could have happened anytime between 60Ma-4506Ma (today). There was great confusion with "at 60 Ma" vs. "after 60 Ma", even among the practitioners. The new data for the first time record clear evidence that the silicate Earth is radiogenic compared to the bulk solar system (be it represented by L, LL, H, E, or C chondrites), thus core-mantle segregation must have happened while 182Hf was live (9 Ma half-life). This is the first order observation. The exact timescale depends on what happens to the Hf-W system during the accretion processes. The N-body simulations of stage 3 in planetary accretion with no gas disk (e.g. Chambers 2001) all seem to be finding timescales in the 10 to 50 Ma ranges for forming terrestrial sized planets. In order to damp the high eccentricity of the planets orbits typically observed in the stage 3 models, different from the near circular orbits of Earth and Venus, a remnant of the gas nebula still present during the final stage (Agnor &Ward 2002; Kominami &Ida 2002) seem to be required. Having some gas around during stage 3 may further decrease the time it takes to get through stage 3. This is because the lower planetary eccentricities make gravitational focusing more effective, which acts to shorten collision times (Canup, personal communication). It is interesting to see that dynamics and geochemistry are both independently moving in the same direction!

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