The origin of the Ra-226-excess during the last cycle of Vesuvius activity was investigated by high-resolution gamma-spectrometry, TIMS and EDXRF. Lavas display high initial Ra-226-excess (500-1000%), similar (Th-230/Th-232) activity ratios (0.87-0.91) and most samples show significant U-238-excess. During the period 1631-1944 the initial absolute Ra-226-excess reached the highest values (19-44 dpm g(-1)) recorded for earth volcanoes. Crystal fractionation and particularly leucite floating did not cause the Ra-226-excess in spite of the high Ra-226 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 Ra-226-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 of Ra-226, Th-230 and U-234 by alpha-decay recoil of parent nuclides from U-enriched microfractures. This model calculates that the ascent time of fluids through the mantle wedge was less than or equal to 12 ka. Successively MMFs mixed with mantle-derived melts, giving rise to Ra-226-enriched magmas, which entered the Vesuvius plumbing system less than 7 ka. Crystal fractionation did not affect extensively the initial Ra-226/Ba ratio, which varied in the 1631-1944 period according to a pattern reflecting periodic inputs of Ra-226-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 similar to0.021 km(3), stored most probably in a shallow chamber. Copyright (C) 2004 Elsevier Ltd.
Ra-excess during the 1631-1944 activity period of Vesuvius (Italy): A model of alpha-recoil enrichment in a metasomatized mantle and implications on the current state of the magmatic system
Tedesco, D
;
2004
Abstract
The origin of the Ra-226-excess during the last cycle of Vesuvius activity was investigated by high-resolution gamma-spectrometry, TIMS and EDXRF. Lavas display high initial Ra-226-excess (500-1000%), similar (Th-230/Th-232) activity ratios (0.87-0.91) and most samples show significant U-238-excess. During the period 1631-1944 the initial absolute Ra-226-excess reached the highest values (19-44 dpm g(-1)) recorded for earth volcanoes. Crystal fractionation and particularly leucite floating did not cause the Ra-226-excess in spite of the high Ra-226 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 Ra-226-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 of Ra-226, Th-230 and U-234 by alpha-decay recoil of parent nuclides from U-enriched microfractures. This model calculates that the ascent time of fluids through the mantle wedge was less than or equal to 12 ka. Successively MMFs mixed with mantle-derived melts, giving rise to Ra-226-enriched magmas, which entered the Vesuvius plumbing system less than 7 ka. Crystal fractionation did not affect extensively the initial Ra-226/Ba ratio, which varied in the 1631-1944 period according to a pattern reflecting periodic inputs of Ra-226-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 similar to0.021 km(3), stored most probably in a shallow chamber. Copyright (C) 2004 Elsevier Ltd.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
