Conditions for the growth of a long-lived shallow crustal magma chamber below Mount Pelee volcano (Martinique, Lesser Antilles Arc)

The compositional homogeneity of silicic andesites emitted during the 13,500-year-long last eruptive cycle of Mount Pelee suggests that the physical state of the magma chamber was largely unmodified during this period. Experimental phase equilibria on Mount Pelee recent products indicate that pre-eruptive magma temperatures and pressures were in the range of 875-900 degrees C and 2 +/- 0.5 kbar, respectively. The estimated average eruption rate was 7.5 x 10(-4) km(3)/a with an average volume of about 0.3 km(3) per eruption. An analytical model for a spherical magma chamber indicates that a magma flux of 4-5 x 10(-4) km(3)/a can maintain a magma chamber of 0.3 km(3) above 875 degrees C below Mount Pelee. However, observations of plutons suggest that magma chambers may grow by addition of sheet-like intrusions. With numerical simulation we show that a minimum sheet accretion rate of a few centimeters per year is required to grow a persistently active magma chamber independently of the intruded volumetric flux. This minimum injection rate is higher if the heat transfer is enhanced by convection processes. The limited ability of an arc crust to extend and accommodate dikes suggests that magma injections are sills or, if they are dikes, that most of the volume injected in the magma chamber is eventually erupted. In a conductively cooling igneous body formed by sills injected at a rate of a few centimeters per year, most of the injected magma completely solidifies and only a small part of the body (about 10-20%) is above 875 degrees C and able to feed eruptions.