Overview of hazardous elements present in incrustations resulting from the 2014-15 eruption of Fogo volcano, Cape Verde


Silva T Pa, De Oliveira D P Sb, Veiga J Pb, Ávila Pc, Candeias Cd, Salas-Colera Ee, Caldeira Rf

aMineral Resources and Geophysics Research Unit, National Laboratory for Energy and Geology (LNEG), Estrada da Portela, Apartado 7586, 2610-999 Amadora, Portugal

 bCENIMAT/I3N, Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal

 cMineral Science and Technology Unit, National Laboratory for Energy and Geology (LNEG), S. Mamede de Infesta, Portugal

 dEpiUnit - Epidemiology Research Unit, Environmental Health Department, Institute of Public Health of the University of Porto, Porto, Portugal / GeoBioTec - Geobiosciences, Geotechnologies e Geoengineering Research Center, Geosciences Department, University of Aveiro, Aveiro, Portugal

eSpLine, Spanish CRG Beamline, European Synchrotron Radiation Facility (ESRF), Grenoble, France / Instituto de Ciencia de Materiales de Madrid-CSIC, Madrid, Spain

 fGeology, Hydrogeology and Coastal Geology Unit, National Laboratory for Energy and Geology (LNEG), Amadora, Portugal

The last eruption of the Fogo volcano (Cape Verde) occurred between November 23, 2014 and February 7, 2015, producing extensive lava flow fields (a’a and pahoehoe), that destroyed houses and agriculture (mainly vine and fruit plantations). Furthermore, the hot volcanic gases (pure or resulting from their interaction with surroundings) emitted through vents and fissures for a long time, transport volatile species that are released to the atmosphere or deposited by cooling and condensation or sublimation - fumarole incrustations. Minor and trace elements carried by minerals on these materials, can be hazardous to health, depending mainly on its nature, concentration and speciation. Therefore, the chemical characterization of incrustations and altered rocks were undertaken to identify possible harmful elements.

Two techniques were used: X-ray fluorescence spectrometry with wavelength dispersive system (XRF-WDS), performed at the laboratory to obtain a semi-quantitative analysis, and energy dispersive X-ray fluorescence (EDXRF) used at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The high brilliance of synchrotron X-rays, allows for remarkably low limits of detection for most chemical elements, thus enabling the analysis of trace and sub-trace species hosted by a mineral.

A large span of minor and trace elements, some of them potentially toxic, were identified - Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Fe, Ni, Cu, Zn, Ga, As, Se, Br, Rb, Sr, Y, Zr, Nb, Mo, Ba, Ce, Pt, Hg, Tl, Pb, Bi, U. The crystal structure of the carrier mineral phase strongly constrains the range and content of elements that may be hosted in the various structural sites available, however the presence of some heavy metals, e.g. Hg, Tl, Bi, U, plays a major environmental concern as local populations use sulphur and white materials as treatment for some diseases. The crystal structure of common sulphur is built up by the packing of S8 molecules restraining the diadochic replacements to Se and As in solid solution. Results to be presented will show the identified phases and detected concentrations of hazardous heavy elements. The highest content obtained for Se by XRF-WDS, was 1000 ppm. The white materials are mainly sulphates (anhydrite, bassanite, gypsum, thenardite) incorporating As, Ba, Pt, Hg, Pb, Bi and U, while the toxic metal Tl (highest content, 2000 ppm) is carried by ralstonite, an hydrated aluminum fluoride. The presence of these geohazardous elements was also noticed following the 1995 eruption of the same volcano endangering the local population.

Abstract presented in the Conference 27th Colloquium of African Geology, 21-28 July 2018, Aveiro, Portugal