Geochemical implication of Eu isotopic ratio in anorthosite: new evidence of Eu isotope fractionation during feldspar crystallization

Abstract

Rare earth element geochemistry (REE) can provide critical information on the evolution of the crust-mantle system. Europium (Eu) exists in divalent and trivalent states, and Eu-2+ can substitute for Ca-2+ 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 rocks, almost none have evaluated ratios of Eu's two stable isotopes (Eu-151 and Eu-153) alongside Eu anomalies. Here we report systematic variation of the Eu isotopic ratio (delta Eu-153) from igneous rocks including anorthosite. This study detected a correlation between Eu anomalies and Eu isotopic ratios. Rhyolites and highly fractionated granites exhibited large negative Eu anomalies and negative delta Eu-153 values while anorthosites exhibited large positive Eu anomalies and positive delta Eu-153 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 volcanic (extrusive) and plutonic (intrusive) rocks. Our finding reveals that Eu isotope fractionation in igneous rocks can provide new information related to magmatic differentiation and plagioclase feldspar crystallization including anorthosite formation.