GRADIENTS IN PHYSICAL PARAMETERS IN ZONED FELSIC MAGMA BODIES - IMPLICATIONS FOR EVOLUTION AND ERUPTIVE WITHDRAWAL

Five diverse, well documented, chemically zoned magmas have been chosen from the literature to demonstrate the extent and patterns of density and viscosity gradients in zoned magma chambers. The patterns are used to assess implications for development of zonation, and withdrawal dynamics and preservation of systematic chemical variations in the final pyroclastic deposit. These examples are: Bishop Tuff, California (high-silica rhyolite); Los Humeros, Mexico (calc-alkaline rhyolite to andesite); Fogo A, Azores (trachyte); Laacher See, Eifel (phonolite) and Tenerife, Canary Islands (phonolite). It was necessary to make several simplifying assumptions in order to calculate viscosity and density profiles through each system; results are particularly sensitive to magmatic water and crystal contents. Nevertheless, the following conclusions can be drawn: 1. (1) Small, strongly zoned, alkaline magma systems which evolved through fractional crystallisation of a basaltic parent (Fogo A, Laacher See) have suffered a partial time-integrated volatile depletion prior to eruption. The most likely mechanism of volatile loss is degassing of the uppermost, highly differentiated, "cupola" magma layer. 2. (2) Eruption withdrawal dynamics are critically dependent on density gradients (and therefore on volatile content and phenocryst abundance), while viscosity variations play a subordinate rôle in the chosen examples. 3. (3) Formation of a chemically zoned tephra sequence by eruption of chemically zoned felsic magma requires a pre-eruptive volatile gradient in the magma chamber. 4. (4) Withdrawal-layer thicknesses during eruptions from naturally zoned magma chambers are of the order of 100 m. 5. (5) The quantitative treatment of gravitational liquid segregation processes by Nilson et al. (1985) successfully predicts times required for zonation of magma bodies: typically 103-104 years for small alkaline systems, and > 105 years for large silicic systems. © 1990.