Mario Voltaggio

Leaching procedure used in dating young potassic volcanic rocks by the 226Ra/230Th method

A method for determining the ages of shoshonitic volcanics erupted within the last 5000 years is presented and assessed. This method is based on 226Ra/230Th radioactive disequilibrium and on a leaching procedure and γ-counting of volcanics containing Th-enriched, Rainitial-depleted mineral phases detected by microprobe analyses. K is used as a Ra-stable chemical analogue. The data are plotted on (226Ra/228Ra)− K(228Ra) Osmond-type diagrams in order to evaluate the 226Raradiogenic230Th activity ratio of the Th-enriched, Rainitial-depleted phases. Four samples of volcanics from Vulcano Island (southern Italy) gave ages consistent with historical reports andK/Ar chronology.

Estimation of diffusion constants by observations of isokinetic effects: a test for radiogenic argon and strontium

Abstract A method of estimating unknown diffusion constants, by testing isokinetic effects, is briefly outlined. The calculated Ar rad and Sr rad diffusion constants agree well with experimental data. The employment of the method is suggested for the setting up of diffusion experiments.

226Ra-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

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.

Dating of newly formed minerals in geothermal fields through 232Th series short lived isotopes: Check on mineral of known age and implications to fluid-rock interactions

The 228Th/228Ra activity ratio can provide a tool for age measurements of newly formed minerals, down to about 15 yr. A general discussion of the method is followed by an experimental check on an aphthitalite brine, crystallized from a geothermal well at Cesano (Rome) in July, 1978 (the real age is 85 ± 5 months). It yielded an age of 75 + 8/-5 months through alpha-spectrometry activity measurements, and 85 ± 3 months through gamma-spectrometry activity measurements. A residence time of the fluid in the rock ranging from 3 to 10 centuries has been estimated by comparing the Ra isotopic composition of the fluid with the U/Th ratio of the wall rocks, according to a model proposed by Cherdyntsev (1971).

U-series disequilibrium radionuclides in sulphur incrustations from the fumarolic field of Vulcano Island

Abstract U-series disequilibrium radionuclides ( 238 U, 234 U, 210 Pb, 210 Po) in sulphur incrustations deposited at La Fossa cone (Vulcano Island, Italy) were measured over the period 1990–1995 to collect information about changes in the feeding system of the fumarolic field. The 210 Po/ 210 Pb and 210 Pb /Pb ratios of the sulphur incrustations allowed the identification of a fumarolic vent which is free from magmatic contribution and remobilized sublimate contamination. In spite of the low temperature of the selected fumarole (280–350°C), kinetic data show that 210 Po is almost fully volatilized due to the relatively high velocity of the gas; as a result, the 210 Po/ 222 Rn activity ratio of the gas can be used to estimate fluid rising times (about 100 days). 234 U/ 238 U activity ratios of sulphur incrustations deposited in 1991–1993 showed a shift toward high values typical of nonmagmatic fluids. Both short-lived and long-lived U-series disequilibrium radionuclides contribute to volcanic forecasting and give information on fluid sources and rising times as well as the residence times of the aquifers involved in the degassing process of the fumarolic field.

GROTTA ROMANELLI (SOUTHERN ITALY, APULIA): LEGACIES AND ISSUES IN EXCAVATING A KEY SITE FOR THE PLEISTOCENE OF THE MEDITERRANEAN

Grotta Romanelli, located on the Adriatic coast of southern Apulia (Italy), is considered a key site for the Mediterranean Pleistocene for its archaeological and palaeontological contents. The site, discovered in 1874, was re-evaluated only in 1900, when P. E. Stasi realised that it contained the first evidence of the Palaeolithic in Italy. Starting in 1914, G.A. Blanc led a pioneering excavation campaign, for the first-time using scientific methods applied to systematic paleontological and stratigraphical studies. Blanc proposed a stratigraphic framework for the cave. Different dating methods (C 14 and U/Th) were used to temporally constrain the deposits. The extensive studies of the cave and its contents were mostly published in journals with limited distribution and access, until the end of the 1970s, when the site became forgotten. In 2015, with the permission of the authorities, a new excavation campaign began, led by a team from Sapienza University of Rome in collaboration with IGAG CNR and other research institutions. The research team had to deal with the consequences of more than 40 years of inactivity in the field and the combined effect of erosion and legal, as well as illegal, excavations. In this paper, we provide a database of all the information published during the first 70 years of excavations and highlight the outstanding problems and contradictions between the chronological and geomorphological evidence, the features of the faunal assemblages and the limestone artefacts.