Hf-W CHRONOMETRY AND THE TIMING OF THE GIANT MOON-FORMING IMPACT ON EARTH.

182 Hf- 182 W chronometry. The key to resolving this disagreement is to obtain experimental data on the scale of physical and chemical mixing and equilibra- tion of metal and silicate in the post-giant impact Earth. Because the extreme conditions that prevailed in the Earth during and shortly after the giant Moon-forming impact (5) are inaccessible for conventional tech- niques, we use the results of high power laser shock- induced melting of metal-silicate targets at high pres- sures (100's GPa) and temperatures (10 4 's K). Evaluation of the late Moon formation model: Since planetary accretion is a stochastic process, the last giant impact does not necessarily have to occur on an exponentially decreasing accretion rate curve; it could happen either before or after. It has recently been argued that the Moon formed late at about 70-110 Myr (6). Because the W isotopic compositions of the mod- ern Earth's mantle (W(CHUR) (tf) = 1.9) and the bulk impactor (W(CHUR) = 0 by definition) are well known, the recently suggested formation of the Moon by a late impact ~ 70-110 Myr after the Solar System formation places rather tight constraints on the W isotopic com- position and the accretion time of the silicate proto- Earth, if the mass ratio of the impactor to the total sys- tem is known. Simulations of the Moon-forming impact (5) require the mass fraction of the impactor to be ~0.13 of the final Earth-Moon system in order to match its astronomical characteristics. Neglecting the small mass of the Moon, the mass ratio of the pre-impact mantle to the current mantle is 0.87. For this ratio the isotopic composition of the pre-impact Earth's mantle of W(CHUR) (ti) = 11.6 was calculated from the equation 76 of (1) using the Hf-W fractionation factor f Hf/W =