The change of eruption styles of Martian volcanoes and estimates of the water content of the Martian mantle

Estimated water contents in the Martian mantle range from 36 ppm to more th an 1%. These values are based on the chemical analyses such as hydrous mine rals in SNC meteorites and formation models of Mars. This study evaluates t he water content of the Martian mantle using the change with time of volcan ic eruption style on Mars as an observational constraint. Styles of volcani c activity depend on the volatile content of the magma and the atmospheric pressure. Because a low atmospheric pressure leads to a more explosive volc anic eruption, it has been believed that the volcanism on the current Marti an environment would be very explosive. Our calculations, however, show tha t, under the current Martian atmospheric conditions, erupted magma cannot e ntrain the ambient air effectively, so the decrease in temperature of the m agma during ascent is small. Consequently, the erupted magma may form a lav a-like deposit when it falls back on the ground. This effusive-like style o f eruption is a counterpart of clastogenic lava on Mars. On the other hand, numerical calculations under a thick CO2 atmosphere, which may correspond to an ancient Martian atmosphere, reveal a rather explosive eruption style. Geological features of earlier stages of Martian history in the Noachian a nd Hesperian eras suggest that the volcanic eruptions on Mars were explosiv e then. Effusive eruptions, however, became dominant in more recent times. It has been widely accepted that Mars experienced a major climate change. i n addition, the release factor of volatiles on Mars has been suggested to b e as small as 0.017-0.112. This may imply that the volatile content has bee n almost constant throughout Martian history. Consequently, we assume that this change in eruption style was caused by the change in atmospheric press ure. For a given water content of magma, a major climatic change may lead t o a transition in eruption style. If we know the atmospheric pressure at th e time of this transition, we can calculate the possible range of the volat ile content of the mantle using our numerical simulations. If the atmospher ic pressure on Mars around late Hesperian era is about 1 bar, the estimated values for a typical Martian magma are 0.05-0.25 wt.%, which is within the range of the water content of typical terrestrial basaltic magmas, (C) 200 0 Elsevier Science B,V. All rights reserved.