Mantle Plume Magmatism on Present-day Mars

Two independent types of evidence demonstrate the existence of very young volcanism on Mars. The shergottites are a type of igneous meteorite from Mars, many of which have radiometric ages of just 180 million years. High resolution images of some lava flows have such a paucity of small impact craters that the flows must be quite young, perhaps just 10-30 million years. The concentrated nature of young volcanic activity in just two provinces of Mars, Tharsis and Elysium, is best understood as a result of upwelling mantle plumes which originate deep in the martian mantle. Each plume feeds a single large volcano, such as Olympus Mons. Thus, Tharsis consists of several distinct plumes, set within a broader zone of internally heated upwelling. Numerical models of plume magmatism have been developed which use melting relationships appropriate for martian mantle compositions, as inferred from the shergottite meteorites. These models can explain the geologically inferred magma production rate and the geochemically inferred mean melt fraction, provided that the martian mantle has retained about half of its original content of radioactive elements, with the remainder of the heat production now in the crust. The recently recognized shergottite Yamato 980459 is more magnesian than previously known martian meteorites and has a significantly higher melting temperature. The required high temperature further enhances the requirement for hot mantle plumes on Mars. Previous models of martian plume volcanism assumed a depth-dependent rheology. In these models, the thickness of the upper, high viscosity layer was adjusted to produce a heat flux that is consistent with the elastic lithosphere thickness inferred from gravity modeling. New models are now in development using a more realistic, temperature-dependent olivine rheology. The improved rheology model may modify previous results in several ways. The low viscosity in the plume conduit will permit faster ascent of material through the mantle and may reduce the amount of cooling of the plume by the surrounding mantle. Also, the new models will permit local thinning of the lithosphere in the center of the plume. These effects may be crucial in explaining the high melting temperature of Yamato 980459.