Lead isotopes behavior in the fumarolic environment of the Piton de la Fournaise volcano (Réunion Island)

Abstract

The recent activity of the Piton de la Fournaise volcano offers a rare opportunity to address the issue of Pb isotope behavior in volcanic fumaroles, as the composition of the degassing source is accurately and precisely known. Gas sublimates formed between 2007 and 2011 at temperature ranging from 400 to ca. 100°C include Na-K sulfate (aphthitalite), Ca-Cu sulfate (e.g., gypsum), Na sulfate (thenardite), Ca-Mg-Al-Fe fluoride (e.g., ralstonite) and native sulfur. The high-temperature deposits show trace element patterns typical of volcanic gas (with Pb concentration up to 836 ppm) while the lowtemperature deposits are depleted in most volatile elements (Pb <1 ppm) with the exception of Pd and Tl (in fluorides) and Se (in native sulfur).Only for low-temperature fluoride samples do Pb isotope compositions plot significantly outside the field of lavas. The isotopic shift is ascribed to leaching ubiquitous unradiogenic phases (e.g., sulfides) by acidic gas condensates. The similarity in Pb isotope signature between lavas and sublimate samples more representative of the gas phase (sulfates) indicates that the net fractionation of Pb isotopes resulting from volatilization and condensation processes is smaller than the precision of Pb isotope measurements (better than 60 ppm/a.m.u.). The absence of net fractionation could result from negligible isotope fractionation during Pb volatilization followed by extensive condensation of gaseous Pb, with possibly significant isotopic fractionation at this stage. Although this scenario has to be refined by more direct measurement of the gas phase, and its general applicability tested, it suggests that a small fraction (<10%) of initially volatilized Pb ultimately escapes to the atmosphere, while the remaining dominant fraction is trapped in sublimates. As sublimates are rapidly dissolved and entrained by runoff, the fumarolic environment appears as a factory efficiently transferring isotopically unfractionated Pb from magmas towards the hydrological system and seawater.Resolving very small isotopic differences between magmas and their gaseous products remains an analytical challenge. High-precision Pb isotope measurements rest not only on instrumental performance but also on high-yield chemistry, as Pb isotopes drastically fractionate (800 ppm/a.m.u.) upon elution on anionic resin. For 50% Pb recovery, the estimated isotopic bias is plus or minus 60-80 ppm/a.m.u., depending on which of the early (isotopically light) or late (isotopically heavy) Pb fraction is lost.

Rapid, simultaneous separation of Sr, Pb, and Nd by extraction chromatography prior to isotope ratios determination by TIMS and MC-ICP-MS

“…Finally, it is emphasized that the nearly quantitative recovery makes the potential issue of isotopic fractionation during column separation irrelevant, as already pointed out by Gale 26 and Smet et al 30 This is a distinct advantage over conventional separation methods based on anion-exchange chromatography which have been reported to induce large isotopic fractionation of lead when relatively poor (as low as 50%) chemical yields are achieved. 34 3.1.3. Neodymium.…”

Across‐arc versus along‐arc Sr‐Nd‐Pb isotope variations in the Ecuadorian volcanic arc

“…Indeed, this is necessary to convert all the lead present in bromide form, suitable for our chemical separation scheme. Samples were centrifugated and loaded on 100 µL teflon columns filled with an anionic resin (Bio‐Rad AG1‐X8 resin 100–200 mesh) for lead purification [ Vlastelic et al ., ]. The separation process was carried out twice consecutively.…”

Origin and fate of sulfide liquids in hotspot volcanism (La Réunion): Pb isotope constraints from residual Fe–Cu oxides

“…The separation method used solutions with varying HBr concentration (0.2-0.03M) in 0.5M HNO3 media to control Pb retention on anionic resin (AG1-X8; Vlastélic et al, 2013). This method has been improved in two ways.…”

“…Cerium is also amongst the less volatile elements at Piton de la Fournaise (Vlastélic et al, 2013). Elements commonly enriched in leachates include Pb (0.93<EFPb<5.7), Li (0.97<EFLi<14.1), As (0.97<EFAs<14.7), Cu…”

“…Fumarolic incrustations (sulfates and fluorides) that formed between one and three years after the eruption at temperatures between 400 and 100°C display unradiogenic Pb and Os isotope signatures that indicate a sulfide contribution of 0.004-0.06% to magmatic fluids (Vlastélic et al, 2013;Gannoun et al, 2015). Thus, sulfide breakdown might also occur during the progressive oxidation of lava flow interiors, and contribute to late SO2 degassing.…”