Boyet, M - Laboratoire Magmas et Volcans, UMR CNRS 6524, 5 rue Kessler, Clermont-Ferrand, 63038, France
Moynier, F - Department of Geology, University of California Davis, One Shields Avenue, Davis, CA 95616, United States
Yin, Q - Department of Geology, University of California Davis, One Shields Avenue, Davis, CA 95616, United States
Irisawa, K - Department of Geology, University of California Davis, One Shields Avenue, Davis, CA 95616, United States; Dep. Earth and Planet. Sciences, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo, Tokyo, 152-8551, Japan
Carlson, R W - Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd., N.W., Washington, DC 20015, United States
Evidence for rapid differentiation of the planet Earth comes from studies of short-lived chronometers, in particular 182Hf-182W (T1/2=9Ma) and 146Sm-142Nd (T1/2=103Ma) used to constrain the age of core formation and the silicate differentiation, respectively. Although the major fractionation between Hf and W occurred during the core formation, differentiation within the silicate earth is expected to create Hf/W fractionation, W being more incompatible than Hf. Boyet and Carlson (Science 309, 2005) showed that all terrestrial samples present 20 ppm excess of 142Nd relative to chondrites. Assuming that the Earth formed by accretion of material similar in refractory element concentrations to chondrites, they connect this signature to a global differentiation of the silicate earth during the first 30 Ma of the solar system history. Old mafic rocks from Isua, Greenland have a 142Nd excess 15 ppm higher than found in most other terrestrial rocks suggesting that the early-Earth's mantle was even more depleted than the post-3.8 Ga mantle. These anomalies provided the first indisputable evidence that differentiation of the Earth occurred while 146Sm was present and even when 182Hf was extant. W isotopic composition has been measured by MC-ICPMS at UC Davis in Isua samples for which excess in 142Nd were reported. No 182W deviation has been measured outside the error of 30 ppm (external precision). We will use these data to bring more constraints on the timing of the silicate Earth differentiation. Using a 3-stage model, if the core/mantle differentiation occurred before 25 Ma, the decoupling between 142Nd and 182W anomalies provides evidence of an early mantle differentiation (40 Ma) produced probably before the Moon's formation. Finally, we will discuss the importance of early events for the Earth's mantle evolution, trying to reconcile the modern view we have of the mantle with its long-term evolution.
