RESIDENCE TIME ANALYSIS OF GEOCHEMICAL FLUCTUATIONS IN VOLCANIC SERIES

A model to estimate magma residence times in volcanic reservoirs from time series of elemental and isotopic data is developed. Isotopic or incompatible element ratios are preferred to concentration data as they are much less sensitive to the phenocryst abundance in the lavas. A magma reservoir is considered as a well-stirred reactor fed by magmas from distinct sources which mix and fractionate crystals. By making a few assumptions, a first-order estimate of residence times can be retrieved from the rate of change of ratios. Strontium isotope ratios in the lavas erupted by Vesuvius from 1752 to 1882 suggest a magma residence time shorter than 1000 years. An alternative method involves a Fourier analysis of the geochemical signal damped and delayed by the magma reservoir. The damping of the Ce/Yb fluctuations in the Piton de la Fournaise lavas (Reunion Island) over the 1931-1986 period brackets the magma residence time between 10 and 30 years. From Sr isotope data on Mount Etna (Sicily), the residence time of magmas in the plumbing system of this volcano can be bracketed between 14 and 80 years. These figures combined with effusion rates suggest that the maximum size of actually tapped magma chambers under these volcanoes may hardly exceed one cubic kilometer. The magma chambers in which the processes responsible for the geochemical variability of volcanic products take place are likely to be distinct from the geophysically identified magma bodies.