Europium anomaly variation under low-temperature water-rock interaction: A new thermometer

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The positive europium (Eu) anomaly, enrichment of Eu abundance relative to the neighboring elements, is often observed for water interacted with the rocks. Not only high temperature (~400°C) water-rock interaction such as seafloor hydrothermal fluids, but also relatively lower temperature interaction, less than 100°C, cause positive Eu anomaly. However, relationship between the degree of Eu anomaly and interaction temperature has not been investigated. Water-rock interaction experiments at three different reaction temperatures were performed in this study to reveal the cause of positive Eu anomaly. Comparison of the results under different solution chemistry and temperature conditions showed that the basalt containing plagioclase released larger abundances of REEs than the basaltic glass. The degree of Eu anomaly assessed by Eu/Eu* value was smaller when 0.7 M NaCl solution was used for liquid phase for both solid phases. On the other hand, the Eu/Eu* became larger with increasing reaction temperature for basalts interacted with ultra-pure water. Therefore, it is suggested that the Eu anomaly is potentially used as a fluid-rock interaction thermometer under low salinity condition.