Rare earth elements in stream waters from the Rokko granite area, Japan: Effect of weathering degree of watershed rocks.

Abstract

The concentrations of major elements and rare earth elements (REE) of the stream waters from the Rokko granite area, Japan, were analyzed to examine the relationship between the degree of weathering and chemistry of the waters. The clay minerals in the related soils from the watershed areas were also examined. It was found that the relative proportion of kaolinite to total clay minerals is a good measure for the degree of weathering of the area. The Ca concentrations of the stream waters are rather high in comparison with other cations such as Na, K and Mg, and are well correlated with HCO 3 -. These facts indicate that the Ca concentrations in the stream waters are largely controlled by dissolution of calcite that occurs as disseminated crystals in granites. The Ca/Na molar ratio in the waters decreases with increasing kaolinite/ clay ratio in the related soils. The high Ca waters tend to occur in the fresh or weakly weathered areas, which is attributed to abundant calcite in these granites. By contrast, in the strongly weathered areas, where calcite had already been consumed via previous weathering processes, low Ca waters occur. The REE abundances, especially light REE, of the stream waters tend to be high in the strongly weathered watershed areas, and low in the fresh or weakly weathered watershed areas. This is attributed to either enhanced REE leaching from rocks in strongly weathered watershed areas due to larger solid surface areas or increased weathering of accessory minerals such as apatite. Interestingly, the magnitude of negative Ce anomalies appears to be inversely related to the degree of weathering. We suggest that the magnitude of Ce anomalies in those waters is largely controlled by mechanical filtration of CeO 2 during infiltration in micro-cracks or pores rather than by Eh-pH conditions.

Rare earth element abundances in some seawaters and related river waters from the Osaka Bay area, Japan: Significance of anthropogenic Gd

“…4) show light REE depleted patterns similar to those of the seawater samples, but their Ce anomalies are very small. The smallness of these Ce anomalies is conspicuous when they are compared with those of the stream waters in the Rokko Mountain area (cf., Nakajima and Terakado, 2003). The Yodo river samples are also characterized by obvious Gd anomalies.…”

Reply to the comment by T. P. Hutchinson on “Rare earth elements in stream waters from the Rokko granite area, Japan: Effect of weathering degree of watershed rocks”

“…However, the geographical positions of these samples are cut by valleys and ridges. Specially, #5′ is an intermediate sample between #3 and #8 in the mixing model, but their sample positions do not support such mixing (see figure 1 of Nakajima and Terakado, 2003).…”

“…addressed significance of mixing of two components together with subsequent concentration or dilution for the interpretation of the REE data of Nakajima and Terakado (2003). It is true that mixing of two or more components might more or less occur during water flows in the earth's surface or in the underground aquifer, but emphasis of such mixing processes should miss the important scientific points arisen in the nature.…”

Comment on “Rare earth elements in stream waters from the Rokko granite area, Japan: Effect of weathering degree of watershed rocks” by Nakajima and Terakado, Geochemical Journal, 37, 181–198, 2003

“…Is it practicable to adapt the method for use when there are many sets of concentrations of REEs in the dataset? Using the eleven samples of Nakajima and Terakado (2003), I will show that it is.…”

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Nakajima and Terakado (2003) measured rare earth elements in stream waters from eleven locations in the Rokko granite area. The present paper examines their data in the light of a model in which each sample arises from mixing two sources, followed by concentration or dilution.

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