Trace metals distribution from a serpentinite weathering at the scales of the weathering profile and its related weathering microsystems and clay minerals.
Résumé
The distribution of four trace metals - nickel (Ni), chromium (Cr), manganese (Mn) and cobalt (Co) - and some major elements is studied in a soil profile derived from a natural trace metals-bearing rock (serpentinite), at the scales of (1) the weathering profile (C and B horizons) and (2) its related weathering microsystems and clay minerals. All trace elements are relatively enriched in weathered horizons compared to the rock, in the same way as iron and aluminium whereas magnesium is largely leaching. An evolutive sequence of trace metals host phases is formed along the weathering profile from magnesian at the bottom to iron rich clay minerals, and then aluminous clay minerals and oxy-hydroxides at the top. Different chemical forms of trace metals in weathering profile have been quantified by chemical extractions. Ni and Cr remain mainly in the silicates. In contrast, Mn and Co are mainly distributed into amorphous oxy-hydroxides. In addition, trace metals contents are measured (using electron microprobe) in individual rock-forming minerals and in their specific weathering microsystem each with its specific clay mineral. Each rock-forming mineral serves as the host phase for a specific trace metal. Silicates (serpentine and clinochlore) contain Cr, Ni and Mn whereas oxides concentrate Co and Cr. In the first weathering stages, Ni and Cr are concentrated in secondary clays, especially in Fe-montmorillonite (derived from mesh serpentine minerals) and in trioctahedral vermiculite (derived from chlorite) respectively, whereas Mn and Co are mainly used to constitute secondary oxides. At the top of the profile, Mn, Co and Ni host secondary oxy-hydroxides have developed due to iron segregation.