Basaltic Pyroclastic Deposits on Earth and Mars: Constraints for Robotic Exploration of Martian Pyroclastic Deposits

Introduction: Basaltic pyroclastic rocks are uncommon on Earth because most mafic magmas are too volatile-poor to drive explosive eruptions [3-4]. However, basaltic pyroclastic deposits maybe much more common on Mars than on Earth [4-7] because Mars’ lower gravity and thinner atmosphere permit explosive eruptions from magmas with lower volatile contents. Though rare, basaltic pyroclastic rock on Earth provide a crucial way to help understand the formation of pyroclastic deposits on Mars, and to help identify them from orbital and lander data. We are investigating one such deposit, the Trailbridge basaltic ignimbrite near McKenzie Bridge, OR. Here, we give a preliminary description of an outcrop of the Trailbridge ignimbrite, and compare it to several possible basaltic pyroclastics on Mars [3-7]. Trailbridge Resevoir Ignimbrite: The Trailbridge ignimbrite, described only in preliminary reports [8-9], is best exposed in road cuts along OR highway 126 (Fig. 1). The ignimbrite unit is horizontally extensive, ~10 m thick, overlies vesicular basalt and laminated sediments, and is overlain by a conglomerate. The ignimbrite is dark gray and massive, and contains many angular, light-colored clasts and mineral fragments, mmto cm-sized. The rock basaltic or basaltic andesite, with SiO2 of 52-54%, like many other lavas of the High Cascades. The unit is interpreted as a welded ash flow (ignimbrite), because of its: compostion; lack of basalt flow structures; and abundance, preferred orientations, and zoned ditribution of clasts [9]. Representative samples were taken and analyzed in thin section by electron microprobe. Basal portion: The lower few meters of the unit are massive, gray, and rich in lithic clasts up to several cm long. The clasts are accompanied by euhedra and fragments of plagioclase with pyroxene. The matrix is microcrystalline, with small pockets of brown basaltic glass and rare pores. These textures suggest hot deposition and welding, but rapid cooling so that some glass was preserved. Middle portion: The middle meters of the unit are dark gray, massive, dense, fine grained and fracturetough. This rock consists of a crystalline matrix of plagioclase, pyroxene and small oxide grains. These minerals are similar to, but coarser-grained than the minerals in basal portion. No fresh basaltic glass remains in this sample. Within the matrix are rare mineral euhedra (olivine and plagioclase) up to several mm in length and very rare lithic clasts (Fig. 1). There is almost no porosity in this sample. The coarser, crystalline matrix of this portion suggests that it cooled relatively slowly and was likely the most intensively welded. Upper portion: The upper few meters of the ignimbrite are similar to the basal portion in containing abundant rounded and angular lithic clasts in a finegrained matrix with uncommon mineral fragments (mostly plagioclase with pyroxene and olivine). As in the basal unit, the matrix is microcrystalline (probably devitrified glass), basaltic glass, and significant porosity. These features suggest that the upper portion of the unit cooled relatively rapidly.