MAGMA ASCENT RATES OF TERRESTRIAL AND LUNAR MAGMAS

Magma ascent rate, linked to mass discharge rate, controls the mode of volcanic eruptions. Magma ascent rate is the ascending speed of silicate melts before disrupting into a mixture of gas and liquid droplets. Several methods have been developed to estimate terrestrial magma ascent rates of diverse compositions (kimberlitic, basaltic, to rhyolitic), including xenoliths-melt reaction, mass discharge rates, mineral-melt reactions, groundmass crystallization, and diffusive loss of volatiles. Magma ascent rates have been found to range from cm/s to 10s m/s. Because the presence of abundant dissolved volatiles (H₂O and CO₂) in terrestrial magmas, the interplay of the different solubility and diffusivity of these volatiles is advantageous for estimating the magma ascent rate.

Basaltic lava-fountain eruptions likely occurred on the Moon on the basis of volcanic glass beads in Apollo collections as well as dark-mantle deposits observed by remote sensing techniques on the surface of the Moon. Different models were proposed for the fire-fountain eruptions on the Moon including hydrostatic pressure driven (e.g., Wilson and Head, 1981), buoyancy control (Wieczorek et al., 2001), and gas driven (Sato, 1979; Rutherford and Papale, 2009). Evidence for gas-driven eruptions comes mainly from the volatile-rich rims of glass beads (see Heiken et al., 1974). The volatile phase caused the eruption is commonly attributed to CO-rich gas formed by oxidation of dissolved carbon in the melt (Sato, 1979). Using CO-rich gas as the driving force, magma rising rates were estimated to be <10 m/s through numerical modeling (e.g., Rutherford and Papale, 2009). However, the recent finding of significant H inside volcanic glass beads (<46.4 ppm H₂O; Saal et al., 2008) and magmatic apatite (1550 ± 110 ppm H₂O, Boyce et al., 2010; 220 ± 40 to 7000 ± 1000 ppm H₂O, McCubbin et al., 2010) implies that the effects of H₂O on lunar basaltic eruption need to be considered. Here we review how volatile contents can be used to estimate magma ascent rates in terrestrial and lunar volcanoes.