THE USE OF EXTINCT RADIONUCLIDES TO DEFINE THE PRECISE CHRONOLOGY OF PLANET FORMATION

A number of radionuclides with half-lives from under 100 ka to about 100 Ma were present in the Solar System at the beginning of planet formation. Incorporation of these short-lived radionuclides into growing planetismals provided both an energy source to drive initial planetesimal differentiation and a chronometer to track the processes involved in planet growth and differentiation. The diverse geochemical behavior of the various radionuclides and their daughter isotopes allow their application to investigations of the primary mechanisms that control planetary composition – condensation, core formation, and silicate differentiation. Extinct radionuclides provide only relative ages that must be calibrated with an absolute chronometer, such as U-Pb, in order to transform the relative timescales into absolute timescales. Using U-Pb ages of 4567.5 Ma for refractory inclusions from primitive meteorites as the starting date for Solar System formation, Earth’s depletion in volatile elements is dated to within <2 Ma of this time. Core formation on planetesimals started within <100 ka of this date with crust-mantle differentiation on the parent bodies of the eucrite and angrite meteorites starting within the next 2 to 4 million years. Rapid formation and differentiation of planetesimals suggests that Earth grew primarily through accretion of already differentiated objects from which it likely inherited some of its compositional features, particularly volatile depletion. 146Sm-142Nd systematics place differentiation of the lunar mantle near 4.4 Ga, an age similar to that determined by this system for mafic amphibolites from the Nuvvuagittuq greenstone belt of northern Quebec. The Nuvvuagittuq application demonstrates another unique feature of the short-lived radiometric systems, their relative insensitivity to metamorphic resetting compared to traditional long-lived chronometers. Ages near 4.4 Ga also are found in the oldest lunar zircons and lunar crustal rocks, the U-Pb “age of the Earth”, the I-Xe age of terrestrial atmosphere formation, and the oldest ages for zircons from western Australia. The similarity of these ages is another line of evidence that supports Moon formation from material ejected from Earth during a giant impact that occurred relatively late (4.4 Ga) in Solar System history.