Mineralogical control of rare earth elements in acid sulfate soils

Major, trace and rare earth element concentrations were measured in porewater, surface water and sediments at an acid sulfate soil site. The concentrations of La and Ce in porewater are up to 1-3 ppm. There is a strong correlation between REE concentration and acidity, except that the maximum concentrations were consistently found below the horizon of maximum acidity, associated with an increase in pH (to ca. 4) and change in mineralogy from jarosite-dominated to goethite-dominated mottles. Jarosite replacement by goethite is as expected with the rise in pH, which in turn is due to the occurrence of a fossil shell bed just below. The rare earth element patterns in the porewaters are enriched in the MREE with respect to Post-Archaean Australian Shale (PAAS). Measurements and calculations show that this is in accord with experiments on low-degree partial dissolution of jarosite, even when the jarosite itself is highly enriched in LREE. There is a clear fractionation in the patterns between the clay-rich soil matrix, which is slightly depleted in the LREE when normalized to PAAS (La/Yb_PAAS ∼0.5), and the secondary mineral phase jarosite, which is enriched in the LREE (La/Yb_PAAS = 15-50). The REE pattern in the porewater changes with the transition from jarosite- to goethite-rich mottles, becoming relatively more enriched in the LREE compared to the HREE, which is consistent with the incongruent dissolution of jarosite to form goethite and the release of greater amounts of jarosite REE to solution, including proportionately more of the jarosite-compatible LREE. Maximum surface water REE concentrations in acidic water were 100-200 ppb La and Ce. REE patterns in surface water were very similar to the porewater transition zone, enriched in the MREE, but asymmetric, relatively enriched in the LREE compared to the HREE.