Activity at Vesuvius stopped abruptly in 1944 after about 300 years of semi-persistent activity. The 1637-1944 period of activity brings the testimony of primitive and water-rich tephritic magmas (e.g., the 1822 eruption), trapped as melt inclusions in spinel-bearing high pressure olivines. Through time, the extents of melt differentiation increase as well as the relative proportion between carbon dioxide and water dissolved in magmas, as recorded as olivine melt inclusions. Lowering the H2O/CO2 ratio in magmas, particularly in the 1944 samples, would enhance crystallization of K-tephritic magma, possibly modifying their rheology and ascent rates. It could be one of the processes leading to the transition to the present resting state. A decrease in the feeding rate of magmas from the mantle source is another possible cause, but there is no evidence of change in magma chemistry and thus of mantle source process, as discussed above. A further possibility is that Vesuvius since 1944, according to its past working mode, has moved to a pre-plinian stage in which a shallow magma chamber of medium-high volume is developing.
As a matter of fact, the melt inclusions data for Vesuvius point to the existence of a volatile-rich magma storage zones, located in the upper crust, where primary magmas crystallize and differentiate, modifying their volatile content. This zone is arranged as a mush column located at a depth of >8 km that appears to be the source zone for volatile-rich magma batches. The studied eruptions (VEI 2–3) have a distinct behavior with respect to the mainly effusive or weakly explosive events occurred during the open conduit activity at Vesuvius in the last 300 years.
In these eruptions and in particular in deposits related to the more intense phases of lava fountains, abundant olivine containing volatile-rich melt inclusions is distinctive, clearly testifying to the involvement of deep-seated volatile-rich magmas. We propose that the uprising of deep CO2-H2O-rich magma blobs and the sudden release of their volatiles triggers the high VEI eruptions and controls the dynamics of these eruptions with the transition between the lava effusions of degassed magmas, residing in open conduit shallow reservoirs, and intense lava fountains and sustained columns.
1. Dipartimento di Scienze della Terra, Università di Pisa, Italy
2. Laboratoire Pierre Süe, CNRS-CEA, Saclay, France
Geophysical Research Letters 2005, 32, L02306, doi:10.1029/2004GL021667