Geochemical study of Al–Fe–Ti enrichment in rock weathering: Implications for the recognizing of igneous protolith and the enrichment of REE in soil profile

Abstract

Chemical weathering of rocks has been investigated since 1960, and lithogeochemistry has commonly been used since 1980 to determine the protoliths of heavily weathered igneous rocks. However, it is still quite difficult to uniquely identify the laterite originating from a potassic basaltic rock, which formed in the anorogenic/within-plate environment (e.g., low silica trachytic rock, phonolite, and tephri-phonolite). These difficulties occur because the within-plate basaltic rocks contain a relatively high Zr concentration; therefore, laterites with high Zr/Ti ratios are often identified as dacite or rhyolite. Besides that, using Zr/Ti is also difficult to distinguish between ultramafic and basaltic rocks. This study has developed the “overlaid ternary DOS (Degree of Saprolitization)" to solve those problems by overlaying the existing ternary DOS to recognize the igneous rock lithology on lateritic profiles released from ultramafic, mafic/basaltic, intermediate, and felsic rocks.The study of drill core laterites from Western Sulawesi/WS (Mamuju district) in hole-A and hole-B characterized by high Zr (1170–2553 ppm), relatively high TiO2 (1.68–2.99%), and high Zr/Ti ratio values (Zr/Ti > 0.0833); and they are recognized as weathering of basaltic rock. While laterite from West Java (Dago Atas Bandung/DAB) released from weathering of andesitic rock (Zr: 148 ppm; TiO2: 1.49%; Zr/Ti: 0.0167). The enrichment of REE (rare earth element) in the hole-B laterite profile occurred on the surface of topsoil (1st meters) to a depth of the 3rd meters (TRE2O3(avg): 2359 ppm; TREY: 2005 ppm), and a slight enrichment in the 5th to 7th meters (TRE2O3: 1688–1933 ppm; TREY: 1437–1647 ppm). This study shows that the enrichment of REE in the soil horizon does not directly correspond to the Ce-anomaly.