A two-reservoir recycling model for mantle-crust evolution

The 142Nd/144Nd variations in mantle-derived rocks provide constraints on the stirring rate of the mantle from the Hadean to the present

Early silicate differentiation events for the terrestrial planets can be traced with the short-lived 146Sm-142Nd system (∼100-My half-life). Resulting early Earth-produced 142Nd/144Nd 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 142Nd/144Nd 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 2σ uncertainty at 1.7 ppm for 142Nd/144Nd 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 142Nd/144Nd 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 142Nd/144Nd signatures marginally resolved (∼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.

The evolution of mantle mixing

We present a geochemical overview of the canonic model that suggests a two-layer mantle for most of the Earth's history. A change in the Rayleigh number may have modified the convection and now allows the subduction of oceanic plates into the lower mantle, which was not the case in the past. The measurement of stirring time in the source of mid-ocean-ridge basalt, together with Xe- and Pb-isotopic ratios in the mid-ocean-ridge-basalt source, suggests that the upper mantle is separated into two domains, one above the 400 km discontinuity (asthenosphere) with rapid mixing and short residence time, and another between the 400 and 670 km discontinuities with sluggish mixing and a residence time of ca. 1.5 x 10(9) yr.