HR: 09:30h
AN: V11F-07 [Abstracts]
TI: A Model of Emitted Gas Ratios From Persistently-Degassing Volcanoes and Implications for Equilibrium Closed-System Degassing
AU: * Witham, F
EM: fred.witham@bristol.ac.uk
AF: Dept. Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol,
Avo BS8 1RJ, United Kingdom
AU: Phillips, J C
EM: j.c.phillips@bristol.ac.uk
AF: Dept. Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol,
Avo BS8 1RJ, United Kingdom
AU: Blundy, J D
EM: jon.blundy@bristol.ac.uk
AF: Dept. Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol,
Avo BS8 1RJ, United Kingdom
AB:
Many passively degassing volcanoes exhibit remarkably constant ratios of different emitted gas species.
Examples, with measured sulphur : chlorine ratios (S/Clemitted) include Stromboli, Italy (0.8 -– 0.9);
Masaya, Nicaragua (1.4 –- 1.5) and Nyiragongo, Dem. Rep. Congo (0.034 –- 0.035). These ratios
persist over periods of years despite variations in absolute gas fluxes of over 200%. We present a simple
model for the evolution of gas ratios during volcanic degassing which shows that a constant gas ratio is exactly
what one would expect if degassing processes are dominated by equilibrium, closed-system degassing. The
model assumes a simplified configuration consisting of a single magma reservoir connected to the surface by a
conduit in which magma convection occurs, with degassing taking place rapidly as the magma reaches the
surface, followed by return of degassed magma to the deep reservoir. Using measured `primitive' and
`degassed' volatile concentrations from these volcanoes, we find excellent quantitative agreement between the
simple model and observed values at Stromboli and Masaya.
We show that the emitted gas ratios are controlled by the initial (S0, Cl0) and degassed (Sd, Cld)
volatile concentrations of the source magmas and the degassed magmas, according to the expression
S/Clemitted = (S0 - Sd) / (Cl0 - Cld). The ratios are not affected by the extent of degassing, or by an
influx of fresh, volatile-rich magma of the same initial volatile composition. We infer that volcanoes that exhibit
strongly variable gas ratios (e.g. Etna, Italy, S/Clemitted ~ 0 –- 7) must either be controlled by open-
system degassing, or tap several magmas with distinct initial volatile compositions.
DE: 8419 Volcano monitoring (7280)
DE: 8425 Effusive volcanism
DE: 8430 Volcanic gases
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting