EXPERIMENTAL STUDIES ON THE MOBILITY OF LANTHANIDES ACCOMPANYING ALTERATION OF ANDESITE BY ACIDIC HOT SPRING WATER

Abstract

The mobility of lanthanides (Lns) accompanying interactions of powdered andesitic rocks with acidic hot spring water and acidic hot spring water vapor/steam was studied in the experiments set up in the field. At the early stage of rock alteration, light Lns were slightly easier to be leached out by acidic hot spring waters from andesitic rocks than heavy Lns. This result qualitatively agreed with the result obtained in the previous experiments carried out in the laboratory. It was also found that the Ln abundance patterns (Ln patterns) obtained by normalization with Ln contents of the original rock showed slightly positive Eu anomalies at the early stage of rock alteration and turned to show negative Eu anomalies and the extent of negative anomaly monotonously increased as rock alteration proceeded. This trend in the direction of Eu anomaly was correlated to the degrees of decompositions of major rock-forming minerals and interstitial materials in the rock. At the earliest stage of rock alteration in acidic environments, the interstitial materials with relatively low Eu contents were decomposed, which resulted in the positive Eu anomaly. In the following stage, feldspar that had a relatively high Eu content was decomposed, which led to the negative Eu anomaly.