The age and accretion of the earth

Culminating a long series of effort, the monumental work of Patterson [Geochim. Cosmochim. Acta 10 (1956) 230] showed that the age of the earth was close to that of most meteorites at 4.55 Ga. Later refinements have consistently arrived at a younger age for the earth, shedding light on the accretion history of the earth. A review of progresses after Pattersonʹs work is presented on ages for core formation, Xe closure, and formation of the earliest crust using U–Pb, Hf–W, I-Pu-U-Xe, Sm–Nd, and Nb–Zr systems; consistency among the systems is examined; and discrepancies are decoded. The combination of U–Pb and Hf–W systems can rule out some models of rapid earth accretion (at ∼4.55 Ga) followed by smooth and continuous core formation, but allow at least two different models. I-Pu-U-Xe systematics reveals a consistent and young age of 4.45±0.02 Ga for Xe closure. The systematics also allows an estimation of primordial 130Xe concentration in the bulk silicate earth to be 0.034×÷3 ppt, and I concentration to be 15.5×÷2.8 ppb. Earliest crustal formation age constrained by U–Pb ages of detrital zircon, coupled Sm–Nd system, and Nb–Zr system is about 4.45±0.05 Ga. The combination of all the isotopic constraints shows that they are consistent with either one of the following two scenarios for the accretion and differentiation of the earth: single age of 4.45±0.02 Ga for all events in the context of instantaneous differentiation, younger than Pattersonʹs 4.55 Ga by about 100 Myr. This age would most likely represent the time of the last giant impact by an impactor of the size of Mars or greater, from which the earth was rehomogenized and reborn. The age would probably also signify the time when the earth reached about 80–90% of its present mass. In this scenario, the history of the proto-earth before 4.45 Ga was obliterated by the giant impact at ∼4.45 Ga. ontinuous earth accretion and simultaneous core formation with a mean age of 4.53 Ga (mean accretion time of 30 Myr). The continuous accretion was infrequently disrupted by giant impacts that were not powerful enough to rehomogenize the whole earth. The last of such impacts (by a body the size of the moon or greater) occurred at about 4.45 Ga, which stripped the atmosphere from the earth and remelted the crust of the earth. In this scenario, some history of the proto-earth before 4.45 Ga is still preserved in the isotopic records. measurement precision of 182W/184W ratio can be improved by a factor of 10, or if earliest crust formation age can be further constrained, it will be possible to rule out one of the scenarios and further constrain the accretion history of the earth.