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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 |
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| Authors: | Mario Voltaggio Paola Tuccimei |
| Institution: | 1 Istituto di Geologia Ambientale e Geoingegneria, CNR, Rome, Italy 2 Dipartimento di Scienze Ambientali, Università Napoli 2, Caserta, Italy 3 Dipartimento di Scienze Geologiche, Università Roma Tre, Rome, Italy 4 Dipartimento di Scienze della Terra, Università “La Sapienza,” Rome, Italy |
| Abstract: | The origin of the 226Ra-excess during the last cycle of Vesuvius activity was investigated by high-resolution γ-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 high 226Ra 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 of 226Ra, 230Th and 234U by α-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 to 226Ra-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. |
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