New Isotopic Constraints Support Early Plate Tectonics on Earth and Rapid Mantle Mixing

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The differentiation of silicate mantles in the terrestrial planets can be traced by examining the Sm-Nd (samarium-neodymium) isotopic system in available mantle-derived samples. For example, the decay of 146Sm yields 142Nd with a half-life of ~100 Ma, making the 146Sm-142Nd system ideal to trace differentiation events within the first 500 Ma of solar system formation. On Earth, such a differentiation event within the Hadean (4.6 – 4.0 Ga) has been determined based on the large variability of ancient rocks with 142Nd/144Nd values both greater and less than the present-day mantle. Because production of 142Nd stops after the rapid decay of 146Sm, subsequent mantle mixing works to homogenize any 142Nd variability, and precise measurements of 142Nd in modern samples can, therefore, constrain the rate of mantle mixing through time. A new study by Eugenia Hyung and Stein B. Jacobsen of the Department of Earth and Planetary Sciences at Harvard University used thermal ionization mass spectrometry to obtain ultrahigh-precision 142Nd/144Nd measurements on modern-day mid-ocean ridge basalts, ocean island basalts, and continental crustal rocks dating to 2 Ga. The authors find that the post-Archean (more recent than 2.5 Ga) mantle is homogeneous with respect to 142Nd/144Nd. In contrast to previous hypotheses of sluggish mantle convection, the authors determine that a more rapid mantle stirring time of 400 My since the early Hadean is needed in order to reproduce a homogeneous 142Nd/144Nd signature as early as 2.4 Ga. These results are consistent with plate tectonics beginning shortly after the end of the Hadean, adding new data to address the highly debated issue of when persistent plate tectonics began on Earth. READ MORE