Elemental assay of the fumarolic microecology of the Los Azufres (México) volcanic complex using nuclear analytical techniques

Abstract

The elemental composition of the mantle is a key factor for the understanding of the science of the earth’s interior. However, the direct sampling of the mantle is a difficult and involved task. Fumarolic vapours carry a certain imprint of the mantle composition but are also relatively difficult to sample directly given their high temperature and acidity. Many terrestrial fu- marolic vents are colonized by extremophilic mosses that absorb the vapours and become repositories of the mantle elements brought up by the vapours. The elemental composition of the niche ecology of the fumarole therefore may serve as a near “proxy” for the deep-earth elemental profile of the region. Our study concerns the elemental composition of fumarolic plants, their rhizospheric soil, hydrothermal sediment and volcanic substrate collected from three geothermal microecological sites of the Los Azufres volcanic complex of Mexico. This work presents the results for the heavy elements (HE) Cd, Hg, Pb, Th and U that have been determined using the nuclear analytical techniques of Polarized Energy Dispersive Xray Fluorescence (PEDXRF) spectrometry, Instrumental Neutron Activation Analysis (INAA) as well as Inductively Coupled Plasma Mass spectrometry (ICPMS). The distributions of these HE in the aforesaid biogeochemical matrices are examined and rationalized with respect to possible mechanisms of their deposition. Comparisons with literature reports of the HE composition of fumarolic sulphur deposits and fumarolic vapours of the volcanoes of the Kuril Islands and geothermal aquifers of Iceland, are made. The HE composition is found to vary considerably by the geographic location of the fumarole. However, the trend of the concentrations of these 5 HE is found to be approximately the same for the fumarolic plants of Los Azufres as with the fumarolic vapours of the Kuril volcanoes the displacement being only in the magnitudes. Consequently, the concentrations of these 5 HE in the fumarolic plants are direct qualitative analogues of the vapours of those particular fumaroles of Los Azufres and thereby of the deep-earth geochemistry of that particular geothermal zone.