Rare earth element behavior in soils and salt pan sediments of a semi-arid granitic terrain in the Western Cape, South Africa

The rare earth element (REE) contents of soils and pan sediments of the Darling Hills granitic terrain of southwestern South Africa were determined to study the behavior of REE during weathering in a semi-arid, Mediterranean climate. The P-rich, S-type bedrock granite has a middle REE (MREE) enrichment relative to post-Archean Australian sedimentary rocks (PAAS) that primarily reflects the REE content of feldspar and accessory apatite. Initial chemical weathering of the granite results in the loss of REE, particularly Ce, Eu and the heavy REE (HREE). The enrichment of REE in soils relative to bedrock granite corresponds to clay mineral and amorphous Fe oxide content in the case of the light REE (LREE) and precipitation of pedogenic calcite in the case of the HREE. Quartz sand dilutes the REE content, and concentration of K-feldspar produces positive Eu anomalies. Cerium is rapidly precipitated during weathering and retained in clayey, upper soil horizons as insoluble CeO2. Most pan sediments have a bowl-shaped REE pattern that is derived from the accumulation of eroded catchment soils modified by precipitation of evaporative carbonates. REE enrichment, particularly LREE, and the large negative Ce anomaly of oxidized basal pan sediment relative to the bedrock granite suggest that REE entering the pan by groundwater flow are sorbed onto amorphous Fe oxides. In contrast, goethite-rich nodules from the pan have a low REE content and show HREE enrichment relative to the granite. Precipitation of pedogenic calcite and evaporative pan carbonates greatly increases the retention of the HREE in these semi-arid soils. Diagenetic pan dolomite has a REE pattern similar to pedogenic and evaporative calcite, but with a significantly lower REE content. The REE patterns indicate that, although most abundant in the bedrock granite, the MREE are least retained by soils during weathering. The LREE are generally retained by clayey and amorphous Fe-oxide-rich soils and sediments, whereas HREE are retained by precipitation of soil and pan carbonates.