Magma transport and storage at Piton de La Fournaise (La Reunion) between 1972 and 2007: A review of geophysical and geochemical data

摘要

Since the middle of the 20th century, improvement in volcano monitoring techniques has provided large data sets that can be used to evaluate the evolution of magma plumbing systems. At Piton de La Fournaise volcano, abundant eruptive activity and a dense monitoring network are especially conducive to such research. Analysis of the extensive set of geophysical and geochemical data associated with the last 35 years of activity provides new insights into magma transport and storage at Piton de La Fournaise. Two periods of frequent eruptions separated by 6 years of inactivity can be distinguished: 1972-1992 and 1998-2007. Considering these two periods, we show evidence of major changes in the shallow plumbing system in 2000. During 1972-1992 and 1998-2000, lava compositions and weak long term eruptive precursors (lasting few days to three weeks), characterized by no significant summit inflation and about ten volcano-tectonic earthquakes per day, suggest that eruptions were fed by the progressive drainage of an occasionally recharged shallow magma reservoir. Geophysical evidence of shallow magma reservoir recharge was only recorded in 1986 and 1998. In contrast, from 2000 to 2007, geophysical data highlight the appearance of long-term precursors (1 to 5 months of summit inflation and strong seismicity, up to one hundred events per day) preceding cycles of successive eruptions. Each cycle of eruptions was characterized by a sequence (3 to 10 months in duration) of summit and near-summit, proximal, eruptions, and ended with a distal, low-altitude oceanite (olivine abundances >20 vol.%) eruption on the eastern flank of the volcano. Within each of these eruptive cycles, lavas became progressively more magnesian over time. The preferential motion of the eastern flank caused by continuous recharging of the shallow reservoir since 2000 would favour the occurrence of distal eruptions towards this flank at the end of a cycle.