资源环境科技发展态势分析平台

Early silicate differentiation events for the terrestrial planets can be traced with the short-lived Sm-146-Nd-142 system (similar to 100-My halflife). Resulting early Earth-produced Nd-142/Nd-144 variations are an excellent tracer of the rate of mantle mixing and thus a potential tracer of plate tectonics through time. Evidence for early silicate differentiation in the Hadean (4.6 to 4.0 Ga) has been provided by Nd-142/Nd-144 measurements of rocks that show both higher and lower (+/- 20 ppm) values than the present-day mantle, demonstrating major silicate Earth differentiation within the first 100 My of solar system formation. We have obtained an external 2s uncertainty at 1.7 ppm for Nd-142/Nd-144 measurements to constrain its homogeneity/heterogeneity in the mantle for the last 2 Ga. We report that most modern-day mid-ocean ridge basalt and ocean island basalt samples as well as continental crustal rocks going back to 2 Ga are within 1.7 ppm of the average Earth Nd-142/Nd-144 value. Considering mafic and ultramafic compositions, we use a mantle-mixing model to show that this trend is consistent with a mantle stirring time of about 400 My since the early Hadean. Such a fast mantle stirring rate supports the notion that Earth's thermal and chemical evolution is likely to have been largely regulated by plate tectonics for most of its history. Some young rocks have Nd-142/Nd-144 signatures marginally resolved (similar to 3 ppm), suggesting that the entire mantle is not equally well homogenized and that some silicate mantle signatures from an early differentiated mantle (>4.1 Ga ago) are preserved in the modern mantle.