首页|Isotopic constraints on selenium degassing from basaltic magma and near-surface capture by fumarolic deposits: Implications for Se redistribution onto the Earth's surface
Isotopic constraints on selenium degassing from basaltic magma and near-surface capture by fumarolic deposits: Implications for Se redistribution onto the Earth's surface
扫码查看
点击上方二维码区域，可以放大扫码查看
原文链接
NSTL
Elsevier
Volcanic emanations from cooling basaltic lava represent a diffuse and relatively poorly constrained source of metallic and non-metallic compounds to the Earth's surface. These compounds become incorporated in fumarolic minerals and redeposit at the surface of lava flows before entering the environmental cycle. The semi-volatile and chalcophile element selenium (Se) can be either vital or toxic to animals and humans. Thus, understanding the pathways of Se capture and relative concentrations in fumaroles is imperative for estimating their contributions to soils and aquifers in volcanically active regions, with implications for animal and human health. In this study, we report Se concentrations and Se stable isotope composition in a sample suite comprising degassed and undegassed basaltic rocks and various fumarolic deposits (thenardite, Na-K sulfate, gypsum, fluoride, and native sulfur) fed from degassing lava flows at Piton de la Fournaise volcano, Reunion Island. Erupted basaltic lavas (136-58 ng.g(-1) Se) lost up to more than half of their pre-eruptive Se due to subaerial degassing and retained a heavier isotope composition (delta Se-82/76 = 0.11 +/- 0.17 parts per thousand, 2 s.d.) compared to less undegassed volcanic glass (138 ng.g(-1) Se and delta Se-82/76 = -0.19 +/- 0.04 parts per thousand, 2 s.e.). Fumarolic deposits that formed over a temperature range of similar to 800-100 degrees C and captured Se from the degassing lava show higher to very high Se concentrations ranging from 0.54 mu g.g(-1) to 1578 mu g.g(-1) and significant Se isotope fractionation (delta(82/76) Se = +0.6 to -2.08 parts per thousand). We propose two separate models that can explain the relative concentration and Se isotope composition of the deposits: 1) A compound oxidation state-dependent Se incorporation into the various fumarolic minerals, or 2) Temperature-dependent Rayleigh condensation from a cooling gas triggered by compound saturation. The Rayleigh condensation model can entirely explain the Se concentration and isotope composition of the fumaroles and predicts that up to 80% of the Se released from the lava is likely to be captured by precipitation to form solid phases within the lava pile, most dramatically in the coldest deposits below the sublimation temperature of SeO2 (< 315 degrees C). In contrast, mineral-dependent isotope pathways cannot fully explain our data, including the lighter Se isotope compositions in the more oxidized compounds compared to more reduced ones. Such a mineralogical effect cannot be excluded but further investigations and experimental studies are required in order to scrutinize and invoke its role. Finally, the environmental impact of these degassing-induced secondary products will be dependent on the relative compound water solubilities resulting in either Se mobilization towards aquifers or accumulation onto developing soils and plants. Such studies could prove useful for developing risk assessments in volcanically active regions on our planet, and for reaching a better understanding of the global continent-ocean Se isotope budget and signature.
Low-temperature volcanic emissionsFumarolesSeleniumSe isotopesPiton de la FournaiseReunion islandLA-FOURNAISE VOLCANOMASS-DEPENDENT FRACTIONATIONATMOSPHERIC SELENIUMSULFIDE SATURATIONTRACE-ELEMENTSINDIAN-OCEANPITONREUNIONADSORPTIONSELENATE
Rosca, Carolina、Vlastelic, Ivan、Varas-Reus, Maria Isabel、Koenig, Stephan
NSTL
Elsevier
