Ascent rates of mantle xenoliths in low-viscosity magmas in eruptive phases: new insights from a quiescent volcano

Abstract

This study provides new insights into magma ascent based on mantle xenoliths erupted during the last explosive volcanic activity of Mt. Vulture volcano (southern Italy), combining H diffusion chronometry with fluid-mechanical models. Olivine crystals within lherzolite, wehrlite and dunite xenoliths are Mg-rich (Fo88-90), with NiO contents up to 0.30 wt%. They show hydrogen loss profiles between 0.5 and 2.5 ppm, and the resulting xenolith ascent rates are up to 5 m s−1. The fluid mechanical model also generates high magma ascent rates, with values up to three times faster (17 m s−1) than those computed with the H diffusion chronometry. Lag times between the ascent rates of the low-viscosity magma that transported the mantle xenoliths and the xenoliths themselves during the last eruption of Mt. Vulture (140 ka) are constrained to a few tens of minutes. To make the fluid mechanical and the H diffusion chronometry models compatible, a low density contrast between the ascending magma and the surrounding crust (Δϱ = 20–40 kg m−3) and a conduit diameter of 2–5 m are required. Our study highlights the importance of using independent models based on multiple parameters to constrain magma ascent rates. This could provide new means for reconstructing the volcanic activity of currently quiescent volcanoes, as carried out here for Mt. Vulture.