Eu isotope fractionation and hydrothermal alteration

Europium (Eu) has two naturally stable isotopes (¹⁵¹Eu and ¹⁵³Eu). The Eu anomalies produced by feldspar crystallization during magma differentiation have provided valuable geochemical information for understanding the evolutionary history of magma in the crust-mantle system. Recently, the correlation between the degree of Eu isotope fractionation (δ¹⁵³Eu) and the magnitude of the Eu anomaly produced by magma differentiation has received much attention as a new research field for understanding the evolution of the Earth system^1-4. Here, we report large variation of Eu isotope ratio for geochemically mafic volcanic rocks from the Korean Peninsula and Ocean Drilling Program (ODP) site 801 C, western Pacific Ocean. The Eu isotope ratio in this work was determined precisely via multicollector inductively coupled plasma mass spectrometry using a Sm internal standard^5-7. We find that the δ¹⁵³Eu values of hydrothermally altered alkaline mafic volcanic rocks vary from - 0.65‰ to -0.1‰, in contrast to the narrow range that characterizes the primary fresh subalkaline mafic volcanic rocks (from - 0.1‰ to 0.1‰). Our findings indicate that Eu isotope fractionation can occur through not only magmatic differentiation but also hydrothermal reactions in the Earth system.