Metasomatism and physical properties of the lithosphere-asthenosphere boundary: a combined petrological and numerical study

Abstract:

Lithosphere−asthenosphere boundary (LAB) marks a critical zone that decouples the rigid and conducting lithosphere from the weak and convective asthenosphere. A change in physical properties across the LAB is observed through geophysical methods. Various studies image a low seismic velocity zone (LVZ) and a high conductivity layer (HCL) at similar depths and suggest that these anomalies correspond to the top of the asthenosphere^1,2. Various explanations for the LVZ and HCL have been proposed with the most popular being the presence of melt pockets². However, the occurrence of melt is problematic given that the temperatures at these depths are inferred to be lower than the dry solidus of peridotite and questions remain as to how the melt migrates through the lithosphere. Similar anomalies are observed in the continental lithosphere, which is known as the mid-lithosphere discontinuity (MLD), and explained by the presence of metasomatic cumulates.

The aim of this project is to create a model of melt transport that accounts for the evolution of magma as it percolates through the mantle. The model includes thermal aspects, crystallization and melt−peridotite reaction in order to fit what is observed in mantle xenoliths and from mantle outcrops. First, the velocity of ascending magma must be constrained to determine the thermal path and the consequential effect on melt chemistry. The mode of the crystallizing phases, including amphibole and phlogopite, is also determined. For example, a body of magma with 5 m radius rising from 80 km to 50 km through lithosphere will cool by more than 200°C even with a high velocity of 10 km/yr. At a slower velocity (1 km/yr), magma will cool to 1150°C after rising 17 km and will be within the P-T stability range of amphibole. Combining this thermal model with experimental data for the differentiation of melt at high pressure, we created a thermo-chemical model. The next step will be to add the mechanics of magma transport to produce a fully integrated model and to evaluate the geophysical implications of this model. A crucial aspect will be to understand how low degree partial melts from the asthenosphere are able to rise within the ductile lithospheric mantle to form the alkaline magmas observed in intraplate volcanoes.

¹ Naif et al. (2013). Nature 495, 356-359

² Kawakatsu et al. (2009). Science 324, 499-502