Abstract

ABSTRACT: Regolith characterization in its entirety is particularly difficult in gneissic regoliths due to the heterogeneity of their rock structure. Rare earth elements (REE) are a useful tool in helping understand the evolution of regoliths. This study relates the mineralogy and weathering indexes of three gneisses regoliths (P1-leucocratic, P2-mesocratic and P3-melanocratic gneisses) to the distribution of REE at depth. In soil, clay activity, iron and manganese oxides, CaO, SiO2, P2O5, TiO2, Fe2O3, and MgO showed high positive correlation with REE. The absolute content of REE was enriched in mafic minerals. At the interface between the soil and saprolite, the sum of absolute REE content was greater in soil than in saprolite in P1, while the opposite pattern was found in P2 and P3. The sums of absolute REE in the whole profiles did not overlap between P1, P2 and P3, and the absolute concentration of Gadolinium (Gd) differentiated the three gneisses in all and every horizon/layer of their regoliths without overlapping values. Normalized REE content was greater in the subsurface of P1 due to Eu content in plagioclase, and fractionation had less variation when estimated by Light REE / Heavy REE (LREE/HREE) than by La/Yb, since the variation in REE is great in gneisses (due to the segregation of minerals into bands), and had low levels of association with the Chemical Index of Alteration (CIA) and the Weathering Index of Parker (WIP).

Highlights

  • Rare earth elements (REE) are the set of 14 chemical elements with atomic numbers between 57 and 71 and are frequently grouped into light REE (LREE: La, Ce, Pr, Nd, Sm, Eu; Lanthanium, Cerium, Praseodymium, Neodymium, Samarium, Europium) and heavy REE (HREE: Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; Gadolinium, Terbium, Dysprosium, Holmium, Erbium, Thulium, Ytterbium, Lutetium)

  • At the interface between the soil and saprolite, the sum of absolute REE content was greater in soil than in saprolite in P1, while the opposite pattern was found in P2 and P3

  • Normalized REE content was greater in the subsurface of P1 due to Eu content in plagioclase, and fractionation had less variation when estimated by Light REE / Heavy REE (LREE/HREE) than by La/Yb, since the variation in REE is great in gneisses, and had low levels of association with the Chemical Index of Alteration (CIA) and the Weathering Index of Parker (WIP)

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Summary

Introduction

Rare earth elements (REE) are the set of 14 chemical elements with atomic numbers between 57 and 71 and are frequently grouped into light REE (LREE: La, Ce, Pr, Nd, Sm, Eu; Lanthanium, Cerium, Praseodymium, Neodymium, Samarium, Europium) and heavy REE (HREE: Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; Gadolinium, Terbium, Dysprosium, Holmium, Erbium, Thulium, Ytterbium, Lutetium). Since REE have similar atomic properties, their distribution along the regolith can help to understand the weathering and pedogenic processes, their stages of development and even the contribution of an external source of materials (alloctonism) (Hu et al, 2006; Jin et al, 2017; Laveuf and Cornu, 2009; Yusoff et al, 2013). The morphological distinction between soil and saprolite is not always clear, in profiles developed from metamorphic, banded rocks (Santos et al, 2019b). Santos et al (2019a) compared the soil-saprolite boundary assigned in the field by pedologists with the boundary assigned by the highest contrast in the chemical and physical properties in 25 regolith profiles. The greatest discrepancy was found in profiles derived from metamorphic, banded rocks

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