Mantle Evolution: Integrating Dynamics, Chemistry and Tectonics
Geoffrey F. Davies1 and Jinshui Huang2
1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui, 230026 China
The mantle may have undergone episodic layering punctuated by dramatic overturns that would have been major magmatic and tectonic events, according to new results of numerical modelling of evolving mantle dynamics incorporating a "basalt barrier" mechanism. The overturns died out after 1.5-1.8 Ga in the models because tectonic plates become thicker and heavier as the mantle cools. The overturn events may be recorded in the D" region of the bottom of the mantle by trace element and isotope signatures of the mantle melting that would have accompanied the overturns. It is possible that overturn events like these could account for the continental crust having been formed in bursts rather than continuously. During an overturn hotter and more fertile material from the lower mantle floods into the upper mantle and generates enough primary melt to resurface the Earth to a depth of 10 km in basaltic lava flows.
The new results were obtained in the continuation of a project using numerical modelling to understand the dynamics of the early mantle. The mantle controls surface tectonics and the present geochemistry of the mantle. Previously we have reported how models of this type, in both two and three dimensions, can account for geochemical observations of mantle heterogeneity, stratification and an apparent age of around 1.8 Ga. The new models extend previous models by computing the complete evolution of the models over the age of the Earth, with declining internal radioactive heating and by incorporating the basalt barrier. The barrier occurs because subducted oceanic crust is expected, on the basis of laboratory experiments, to be buoyant just below the mantle transition zone at 660 km, down to a depth of about 750 km.
The new results raise the prospect of a single mantle model being able to explain tectonic history and the present chemical and physical structure of the mantle, even including the noble gases, which have been quite enigmatic. Until now it has not been clear how all of the relevant observations might be reconciled.
Figure 1. The beginning of a mantle overturn, in which cold upper-mantle material
sinks into the lower mantle (right), while hotter material from the lower mantle rises into to upper mantle (left).
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Davies, G. F., Episodic layering of the early mantle by the 'basalt barrier' mechanism: implications for tectonic and geochemical evolution, submitted, Geochem. Geophys. Geosystems (2007).