Europium isotope fractionation in highly fractionated igneous rocks with large Eu negative anomaly

Abstract

highly fractionated granite is geologically related to postorogenic events and also to mineralization of rare-metals such as W, Sn, Sb, REEs etc. Another one of the geochemical characteristics of such highly fractionated granite is chondrite-normalized REE pattern with large Eu negative anomaly. In addition, the rhyolite is the most evolved volcanic rock. Since the chemical composition of major elements and chondrite-normalized REE patterns in rhyolite are similar to those of highly fractionated granites, rhyolite was considered highly fractionated igneous rock for comparison of Eu isotope ratio in highly fractionated granite. Eu is a very interesting element of REEs because it has two oxidation states (Eu2+ and Eu3+) whereas most of the rare earth elements exist as the trivalent state (3+) in the nature. The divalent oxidation state of Eu becomes a cause of elemental fractionation during magma evolution in the Earth and planetary system and can be used as an indicator of oxygen fugacity of magma system (Burnham et al., 2015). Eu has only two isotopes, 151Eu and 153Eu. Though Belli et al. (2007) reported that 151Eu decayed to 147Pm by α decay with the half-life T1/2 = 5 × 10 18 yr, apparently, 151Eu can be considered as a stable isotope in earth and solar system. In addition, Eu isotope ratio in geological rocks may vary due to redox condition in Earth system (Dauphas et al., 2014). Europium isotope fractionation in highly fractionated igneous rocks with large Eu negative anomaly