226 Ra-excess during the 1631-1944 activity period of Mt. Vesuvius (Italy): a model of alpha recoil enrichment in a metasomatized mantle and implications on the current state of the magmatic system

Abstract--The origin of the 226Ra-excess during the last cycle of Vesuvius activity was investigated by high-resolution -spectrometry, TIMS and EDXRF. Lavas display high initial 226Ra-excess (500 -1000%),similar (230Th/232Th) activity ratios(0.87- 0.91) and most samples show significant 238U-excess. During the period 1631-1944 the initial absolute 226Ra-excess reached the highest values (19 - 44 dpm g-1) recorded for earth volcanoes. Crystal fractionation and particularly leucite floating did not cause the 226Ra-excess in spite of the high226Ra activity (21- 85 dpm g-1) in leucite. The presence of phlogopite in the mantle source, documented by field and petrological evidences on local mantle-derived xenoliths, rules out that equilibrium partial melting can be responsible for the 226Ra-excess. This primary feature may be explained by a multistage process involving metasomatic mantle fluids (MMFs) flowing through a mantle wedge where U is concentrated as U-accessory minerals deposited along microfractures. Fluids, passing through the mantle wedge, are supplied of226Ra,230Th and 234U by -decay recoil of parent nuclides from U-enriched microfractures. This model calculates that the ascent time of fluids through the mantle wedge was 12 ka. Successively MMFs mixed with mantle-derived melts, giving rise to2 26Ra-enriched magmas, which entered the Vesuvius plumbing system less than 7 ka. Crystal fractionation did not affect extensively the initial 226Ra/Ba ratio,which varied in the 1631-1944 period according to a pattern reflecting periodic inputs of 226Ra-enriched magma, variable reservoir volumes and residence times in magmatic chamber(s). The temporal trend of the reservoir volumes, extrapolated to the present time, indicates a volume of magma of 0.021 km3, stored most probably in a shallow chamber.