Geochemical profile of soil formation from serpentinite in Sekinomiya massif, southwest Japan

Weathering geochemical profile of serpentinite of the Sekinomiya massif of the Maizuru Terrane, southwest Japan was examined to describe element mobility of bed rock to weathered products (soil). The geochemical compositions of the vertical profile (about 3 m length) from bed rock serpentinite to soil via relatively weathered matter showed changes of element concentrations mostly related to the decrease in Mg contents during the weathering processes. The profile can be divided into three units; C, B and A in ascending order. The unit C (over 80 cm) is serpentinite and can be subdivided into two subunits (C1 and C2) according to the development of the fractures. The unit B (90 cm) is characterized by refining of the rocks with appearance of matrix clays. This unit is characterized by loss of the original structure and by color change to dark appearance. The unit A (1.2 m) is dark color soil, of which upper part is characterized by enrichment of organic matter. Weathering trends of serpentinite to soils were examined A-CN-K and A-C-M diagrams. The CIA values change from 72.57 to 84.70 with some variation. The trend of samples on the A-CN-K diagram apparently indicates systematic change in CaO, Na_2O and K_2O, although these elements show increasing in vertical profile. The linear array in A-C-M diagram also shows change in M component (Fe_2O_3 and MgO) during weathering process. Contents of TiO_2, Al_2O_3, P_2O_5 gradually increased upward from the upper part sample of subunit C2. Significant increase of Fe_2O_3 from the unit C (9.58 wt%) to unit B (30 wt%) was apparent. Decrease of MgO contents from the unit C (almost 45 wt% in this unit) to the unit B is great, and much decrease in the unit A (7.5 wt% at the top of this unit）is characteristic of the serpentinite weathering. In accordance to this decrease in MgO, Zr, Y, Sc, Th and As contents show increasing upward. Abrupt increase in concentrations of Cr and V in the unit B is characteristic feature in this profile. This may be due to concentrations of chromian spinel during weathering process.