M.F. Le Cloarec

Volcanic emission of radionuclides and magma dynamics

210Pb,210Bi and210Po, the last decay products of the238U series, are highly enriched in volcanic plumes, relative to the magma composition. Moreover this enrichment varies over time and from volcano to volcano. A model is proposed to describe 8 years of measurements of Mt. Etna gaseous emissions. The lead and bismuth coefficients of partition between gaseous and condensed phases in the magma are determined by comparing their concentrations in lava flows and condensated volatiles. In the case of volatile radionuclides, an escaping time is calculated which appears to be related to the volcanic activity. Finally, it is shown that that magma which is degassing can already be partly degassed; it should be considered as a mixture of a few to 50% of deep non-degassed magma with a well degassed superficial magma cell.

Fractionation of metals in volcanic emissions

Abstract The enrichment of some metals (Al, Mg, Na, K, Cu, Zn, Cd) in volcanic gases is measured by an emanation coefficient ex relating the amount of the element x in lavas and in aerosols.ex is determined in the volcanic emissions of Mount Etna (Sicily), with regard to that of210Pb. The accuracy of the results is limited by the geochemical behavior of common lead compared to that of210Pb. Concerning the most volatile species, it appears that practically all the volcanic aerosols are produced by evaporation followed by gas-to-particle conversion and the spattered fraction appears to be negligible.

Fractionation of families of major, minor, and trace metals across the melt-vapor interface in volcanic exhalations☆

Chemical families of metals fractionate systematically as they pass from a silicate melt across the interface with the vapor phase and on into a cooled volcanic plume. We measured three groups of metals in a small suite of samples collected on filters from the plumes of Kilauea (Hawaii, USA), Etna (Sicily), and Merapi (Java) volcanoes. These were the major, minor, and trace metals of the alkali and alkaline earth families (K, Rb, Cs, Ca, Sr, Ba), a group of ordinarily rare metals (Cd, Cu, In, Pb, Tl) that are related by their chalcophile affinities, and the radon daughter nuclides 210Po, 210Bi, and 210Pb. The measurements show the range and some details of systematic melt-vapor fractionation within and between these groups of metals. In the plumes of all three volcanoes, the alkali metals are much more abundant than the alkaline earth metals. In the Kilauea plume, the alkali metals are at least six times more abundant than the alkaline earth metals, relative to abundances in the melt; at Etna, the factor is at least 300. Fractionations within each family are, commonly, also distinctive; in the Kilauea plume, in addition to the whole alkaline earth family being depleted, the heaviest metals of the family (Sr, Ba) are progressively more depleted than the light metal Ca. In plumes of fumaroles at Merapi, K/Cs ratios were approximately three orders of magnitude smaller than found in other earth materials. This may represent the largest observed enrichment of the “light ion lithophile” (LIL) metals. Changes in metal ratios were seen through the time of eruption in the plumes of Kilauea and Etna. This may reflect degree of degassing of volatiles, with which metals complex, from the magma bodies. At Kilauea, the changes in fractionation were seen over about three years; fractionation within the alkaline earth family increased, and that between the two families decreased, over that time. All of the ordinarily rare chalcophile metals measured are extremely abundant in volcanic plumes, and Cd and Tl are enriched relative to the others. Indium is much more abundant in the plume of the hotspot volcano Kilauea than in the Etna plume (probably non-hotspot in character). It may be a useful indicator of the tapping of deep mantle zones, or could aid in the interpretation of reports of Pt group metals in exhalations from hot spot volcanoes. Indium in old glacial ice strata could help assess magnitude and variability of exhalations from hotspot volcanoes in past time. Strong melt-vapor fractionation of the alkali and alkaline earth metals may only be observed in plumes during quiescent degassing of volcanoes; when large amounts of ash or spatter (undifferentiated lava) enter the plume, its alkali and alkaline earth metal composition may approach that of the melt. Ratios among the chalcophile metals may not be much changed by addition of ash, because their concentrations in melt are so small, and masses of them in any plume may remain dominated by transfer across the melt-vapor interface. Radon daughter nuclides give information about state of volcanic activity at time of sampling. The precisely known origins, ultratrace detectability, decay systematics, and wide variations in volatility of these species provide information about residence times, degassing and travel histories, and identities of melt bodies in volcanic systems.

Methane, carbon monoxide and light non-methane hydrocarbon emissions from African savanna burnings during the FOS/DECAFE experiment

Atmospheric samples from savanna burnings were collected in the Ivory Coast during two campaigns in January 1989 and January 1991. About 30 nonmethane hydrocarbons from C2 to C6, carbon monoxide, carbon dioxide and methane were measured from the background and also at various distances from the burning. Concentrations in the fire plume reached ppmv levels for C2-C4 hydrocarbons, and 5300, 500 and 93 ppmv for CO2, CO and CH4 respectively. The excess in the mixing ratios of these gases above their background level is used to derive emission factors relative to CO and CO2. For the samples collected immediately in the fire plume, a differentiation between high and low combustion efficiency conditions is made by considering the CO/CO2 ratio. Ethene (C2H4), acetylene (C2H2), ethane (C2H6) and propene (C3H6) are the major NMHC produced in the flaming stage, whereas a different pattern with an increasing contribution of alkanes is observed in samples typical of post flaming processes. A strong correlation between methane and carbon monoxide suggests that these compounds are produced during the same stage of the combustion. In samples collected at a distance from the fire and integrated over a period of 30 minutes, the composition is very similar to that of flaming. ΔNMHC/ΔCO2 is of the order of 0.7%, ΔCH4/ΔCO2 of the order of 0.4% and ΔCO/ΔCO2 of the order of 6.3%. From this study, a global production by African savanna fires is derived: 65 Tg of CO-C, 4.2 Tg of CH4-C and 6.7 Tg of NMHC-C. Whereas acetylene can be used as a conservative tracer of the fire plumes, only ethene, propene and butenes can be considered in terms of their direct photochemical impact.

Radioactive isotopes and trace elements in gaseous emissions from White Island, New Zealand

Abstract Measurements of the radioactivity of gases released by White Island volcano show variations in 210 Po-concentrations with temperature in fumarolic gases, and this is interpreted to be due to dilution of magmatic gases with vapours derived from an envelope of acid brines surrounding the vent system. The degassing model derived for Mount Etna emissions is tested on White Island magmatic gases. The results are a short degassing time (2 days), small degassing magma volume ( 5 × 10 6 m 3 ), and the following estimations of emanation coefficients: Pb= 1% , Bi= 2.7% , Cd= 0.05% and Cu= 0.07/%. The initial content of S in the magma is estimated to be within the range 300–600 mg/kg. Fluxes of trace metals (kg/day) emitted by the volcano are estimated to be Pb= 125 , Bi= 6 , Cu= 300 and Cd= 9 .

Origins of 210Po in the atmosphere at lamto, ivory coast: Biomass burning and saharan dusts

Abstract Measurements of 210 Po and 210 Pb activities in surface air at Lamto (Ivory Coast) during 18 months are presented and discussed in relation to the regional sources of 210 Po. High seasonal variation is observed with maximum in winter, reaching an activity of 210 Po as high as I Bq per 10 3 m 3 . Our estimation shows that the high 210 Po activity in winter is not closely related to the local biomass burning emissions but rather due to the particular atmospheric circulation patterns and geographical location of this region: in winter, surface winds having continental origin bring soil dusts, enriched in 210 Po and 210 Pb, from Sahara desert. However, biomass burning and the growth from 222Rn within the atmosphere appear to be the major sources of unsupported 210 Po in the other seasons.

210Po in savanna burning plumes

During the FOS-DECAFE experiment at Lamto (Ivory Coast) in January 1991 aerosols samples were collected at ground level above fires in order to investigate the possibility of using210Po as a tracer of biomass burning. The concentration of this radionuclide in plants is studied as a function of its content in soils and in the atmospheric background. It is shown that it depends strongly on the atmospheric content in210Po, due to dry deposition of the aerosols. The mean concentration of plants at Lamto is found to be about 4.4 pCi of210Po/gC during the fire season and falls down to less than 1pCi/gC outside this period. The budget of210Po is evaluated taking into account its complete volatilization during the flaming phase, the (210Po)ash/(210Po)plants ratio, which is measured to be about 14% and the percentage of submicron particles in the plume, about 91%. The inferred flux of210Po is 3850 Ci/yr for the African savanna, and 5800 Ci/yr for the global savanna. From this flux, fluxes of Ct and Cs are estimated to be 8.4 and 1.1 Tg of C/yr for the worldwide savanna.

Origin of fumarolic fluids emitted from a nonerupting volcano: Radionuclide constraints at Vulcano (Aeolian Islands, Italy)

The behaviour of 210Po and 222Rn in hot fumaroles on the crater rim at Vulcano Island was studied over 12 years (1980–1992) in order to infer the origin of the fluids. The decreasing activity of 222Rn observed since 1985 results from the mixing of the magmatic component with shallow water. A part of 222Rn in the gases is emitted by the surrounding rocks and carried along by the shallow water. 210Po is completely volatilized at T > 450°C. Above this temperature its activity remains unchanged with increasing temperature. 210Po has two different origins: one is magmatic, correlated with 222Rn; the other one is produced by the sublimates deposited in the volcanic edifice. From the magmatic 210Po component, the volume of degassing magma is estimated to be about either 170,000 m3 or 90,000 m3/day, according to the model utilized. In the first case, the associated degassing time is about twelve days.

Long-lived radon decay products in Mount St. Helens emissions: An estimation of the magma reservoir volume

Abstract The size of the Mount St. Helens magma reservoir is estimated from long-lived radon decay products in fumarole emissions. Samples were collected in September 1981 on filters and by a new sampling technique developed for fumarole condensate. The Po-210/Pb-210 activity ratios were significantly higher in the condensate than on the filters. The difference is attributed to Po-210 losses from the filters by a variety of processes and to trapping of Po-210 in sulfur precipitate that prevents detection of alpha particles. Model calculations based on Po-210, Pb-210, and Bi-210 activities give volume estimates for the degassing magma of about either 1 × 10 8 m 3 or 4 × 10 5 m 3 /d according to the model utilized.

Radionuclide behavior in high-temperature gases from Satsuma Iwojima volcano, Japan

Radionuclide and sulfur measurements were performed on samples from plume and hot fumarolic gas discharged from Satsuma Iwojima volcano, Japan. Such measurements in volcanic plumes contribute to a better understanding of degassing mechanisms (emanation coefficient of metals, degassing magma volume, residence time of the magma). At Satsuma Iwojima, inferred emanation coefficients of 210Pb and 210Po were estimated to be 0.07 and 2.5%, respectively, the lowest values obtained to date in volcanic gases. These values may be “apparent” emanation coefficients, due to the high viscosity of the degassing magma, which prevents efficient degassing of such low concentration components. The volume of the degassing reservoir (rhyolite layer) is estimated to be 0.24 km3, assuming no radionuclide recharge from the underlying basaltic reservoir to the degassing rhyolite.