THE INFLUENCE OF TRACE AMOUNTS OF WATER ON THE VISCOSITY OF RHYOLITES

As a major volatile in volcanic systems, water has a significant influ ence on the rheological properties of silicic magmas. This is especial ly so at minor water contents relevant to the emplacement of silicic l avas. To investigate the influence of water on the viscosity of natura l rhyolitic obsidians, a novel strategy has been adopted employing par allel-plate and micropenetration techniques. Viscosities have been det ermined on three types of material: (a) raw water-bearing obsidians; ( b) remelted (1650 degrees C, 1 atm) degassed glasses of the obsidians; and (c) hydrothermally hydrated (1300 degrees C, 3 kbar) obsidians. T en natural rhyolitic obsidians (peraluminous, calc-alkaline and peralk aline) were employed: seven originated from lava flows and contained < 0.2 wt.% H2O, two samples were F-rich from pyroclastic successions, an d one was an obsidian cobble with 1.5 wt.% water also associated with pyroclastic units. Melt compositions and water contents were stable du ring viscometry. The measured decreases in activation energies of visc ous flow and viscosity with small amounts of water are much greater th an the Shaw calculation scheme predicts. In addition, a marked nonline ar decrease in eta exists with increasing water content. In contrast t o the case for peralkaline rhyolites, 0.1-0.2 wt.% water decreases act ivation energies significantly (up to 30%) for calc-alkaline compositi ons. These results have important implications for the ease of near-su rface degassing of silicic magmas during emplacement and permit the te sting of calculational models for viscosity, largely based on syntheti c systems.