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The partitioning of rare earth elements (REE) between zircon, garnet and silicate melt was determined using synthetic compositions designed to represent partial melts formed in the lower crust during anatexis. The experiments, performed using internally heated gas pressure vessels at 7 kbar and 900-1000°C, represent equilibrium partitioning of the middle to heavy REE between zircon and garnet during high-grade metamorphism in the mid to lower crust. The D REE (zircon/garnet) values show a clear partitioning signature close to unity from Gd to Lu. Because the light REE have low concentrations in both minerals, values are calculated from strain modelling of the middle to heavy REE experimental data; these results show that zircon is favoured over garnet by up to two orders of magnitude. The resulting general concave-up shape to the partitioning pattern across the REE reflects the preferential incorporation of middle REE into garnet, with D Gd (zircon/garnet) ranging from 0.7 to 1.1, D Ho (zircon/garnet) from 0.4 to 0.7 and D Lu (zircon/garnet) from 0.6 to 1.3. There is no significant temperature dependence in the zircon-garnet REE partitioning at 7 kbar and 900-1000°C, suggesting that these values can be applied to the interpretation of zircon-garnet equilibrium and timing relationships in the ultrahigh-T metamorphism of low-Ca pelitic and aluminous granulites.

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Immediately after their formation, the terrestrial planets experienced intense impact bombardment by comets, left-over planetesimals from primary accretion, and asteroids. This temporal interval in solar system evolution, termed late accretion, thermally and chemically modified solid planetary surfaces and may have impeded life's emergence on the Hadean (pre-3.85 Ga) Earth. The sources and tempo of late accretion are, however, vague. Here, we present a timeline that relates variably retentive radiometric ages from asteroidal meteorites, to new dynamical models of late accretion that invokes giant planet migration. Reconciliation of the geochronological data with dynamical models shows that giant planet migration immediately leads to an intense ~30 Myr influx of comets to the entire solar system. The absence of whole-sale crustal reset ages after ~4450 Ma for the most resilient chronometers from Earth, Moon, Mars, Vesta and various meteorite parent bodies confines the onset of giant planet migration to no later than ca. 4480 Ma. Waning impacts from planetesimals, asteroids (and a minor cometary component) continue to strike the inner planets through a protracted monotonic decline in impactor flux; this is in agreement with predictions from crater chronology. Amended global 3-D thermal analytical bombardment models derived from our new impact mass-production functions show that persistent niches for prebiotic chemistry on the early Hadean Earth could endure late accretion for at least the last 4400 Myr. (222 words) Main text: 46564 characters with spaces/7549 words