Seung-Gu Lee, Tsuyoshi Tanaka, Mijung Lee

Rare earth element geochemistry can provide critical information on the evolution of the crust-mantle system. Europium (Eu) exists in divalent and trivalent states, and Eu2+ can substitute for Ca2+ during plagioclase feldspar crystallization in reducing magmas. This leads to positive Eu anomaly in Ca-plagioclase-rich anorthosite derived from the mantle and negative Eu anomalies in fractionated silica-rich crustal rocks. While many studies have addressed Eu anomalies in REE data, especially in igneous rocs, almost none have evaluated ratios of Eu’s two stable isotopes (151Eu and 153Eu) alongside Eu anomalies. Here we report systematic variation of the Eu isotopic ratio (δ153Eu) from igneous rocks including anorthosite. This study detected a linear relationship between Eu anomalies and Eu isotopic ratios. Rhyolites and highly fractionated granites exhibited large negative Eu anomalies and negative δ153Eu values while anorthosites exhibited large positive Eu anomalies and positive δ153Eu values. In the case of the highly fractionated igneous rocks with negative Eu anomaly, the Eu isotope fractionation slope varied according to the degree of magmatic differentiation for both extrusive and intrusive rocks. Our finding reveals that Eu isotope fractionation in igneous rocks can provide new information related to magmatic differentiation and plagioclase feldspar fractional crystallization including anorthosite formation.