Geochemical evolution of soils developed from pyroclastic rocks of Trindade Island, South Atlantic

Ana Carolina Campos Mateus,Sabine Petit,Angélica Fortes Drummond Chicarino Varajão,Fábio Soares De Oliveira,Carlos Ernesto Gonçalves Reynaud Schaefer

doi:10.1590/2317-4889202120200073

Abstract

The geochemical behavior of the major, minor, trace and rare earths elements (REEs) in soil profiles from ultramafic volcanoclastic rocks of the Vulcão do Paredão and Morro Vermelho Formation from Trindade Island (TI) was analyzed in this study. Losses and gains of chemical elements were calculated through the mass balance for two profiles along the slope: one located at higher altitude (460 m) and the other at lower altitude (258 m). In all profiles, Al, Fe and Ti accumulate due to their low mobility, whereas Ca, Na, K and Mg are the most intensely leached. Soils located at lower altitude show higher K and Mg values in the surface due to the contribution of saline sprays. Leaching of the REEs from higher to the lower slope led to the enrichment of these elements, especially the light REEs, in the soil at the lowest altitude (258 m). The high altitude profile showed Ce positive anomaly due to longer exposure to weathering. The geochemical balance shows a relative enrichment of Ti, Mn, Fe, Co, Cr, Ni, V, Zr, S related to the loss of mobile elements during the soil formation process, despite the youthful nature of these volcanic rocks.

Highlights

The distribution and migration of the chemical elements, and the chemical reactions during weathering and soil formation processes are essential in pedogeochemical studies (Carvalho 1995)
The chemical reactions occurring during the soil formation determine the mobilization and redistribution of elements within the soil profile, and a significant correlation can be recorded between the total concentration of elements in soils and parent material (Roca et al 2008)
The present study aims to bring more information about the geochemical evolution from the pyroclasts to the soil and to evaluate the distribution and quantification of losses and gains of major, minor, trace and rare earth elements (REEs)

Summary

Introduction

The distribution and migration of the chemical elements, and the chemical reactions during weathering and soil formation processes are essential in pedogeochemical studies (Carvalho 1995). The meteoric water reacts with most rock minerals, and new stable mineral phases are formed under new physicochemical conditions (Carvalho 1995, Putnis 2009) In this process, some chemical elements are leached out, while others are concentrated. There are the studies of Oliveira et al (2014) that showed the soil phosphatization process of Rata Island (Fernando de Noronha Archipelago) due to the interaction of bird excrement with olivine nephelinites (ankaratritic lavas), and of Oliveira et al (2009) that report high contents of Cu, Pb, Zn, As, U, and Sr in besides soils of this island due to the interaction of guano with mafic rocks, and anomalous concentrations of Ba, Nb, Ta, Cr, Hf, V and Zr due to the geochemical inheritance of local basalts. In the Fernando de Noronha Island, Oliveira et al (2011b) showed that soils developed from mafic rocks (nepheline basalts) presented higher contents of Fe, Co, Ni and Cr, and Oliveira et al (2011a) showed that soils from these rocks concentrate rare earth elements (REEs), preferably the heavy REEs (HREEs)

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Conclusion