<p>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^{<CitationRef AdditionalCitationIDS="CR2 CR3" CitationID="CR1">1</CitationRef>–<CitationRef CitationID="CR4">4</CitationRef>}. 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&#xa0;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^{<CitationRef AdditionalCitationIDS="CR6" CitationID="CR5">5</CitationRef>–<CitationRef CitationID="CR7">7</CitationRef>}. 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.</p>