Rates of Magma Ascent: Constraints from Mantle-Derived Xenoliths

The rates of ascent of magmas through the lithosphere can be estimated by a variety of methods using physical and compositional parameters of lithospheric mantle fragments brought to the surface in these magmas. The xenoliths are relatively dense, and the magma must be ascending more rapidly than the xenoliths can sink through it. These methods suggest an average ascent rate through the whole lithosphere (mantle and crust) in the range of 0.2 to 0.5 m s-1 (about 0.5 to 2 km/ hour). The ascent rates through the shallow crust may be much higher: 20 m s-1 and up to supersonic speeds ( 300 m s-1) in the uppermost crust. Residence times derived from microstructural observations in recrystallised minerals in the xenoliths and element diffusion profiles in xenolith minerals suggest ascent rates in the range of 0.2 to 0.4 m s-1. These methods all provide minimum velocities ranging from about 0.2 to 2 m s-1 for relatively low-volume melts such as alkali basalts, to 4 to 40 m s-1 (and up to supersonic) for volatilecharged ultramafic melts such as kimberlites. In summary, magmas carrying mantle xenoliths much reach the surface within a maximum of about 8 to 60 hours of picking up these dense fragments from depths of about 200 to 80 km depth.