Factors controlling the chromium isotope compositions in podiform chromitites

Abstract

The application of Cr isotope compositions to the investigation of magmatic and post-magmatic effects on chromitites is unexplored. This study presents and compiles the first Cr stable isotope data (δ53Cr values) with major and trace element, contents from the Balkan Peninsula, aiming to provide an overview of the compositional variations of δ53Cr values in ophiolite-hosted chromitites and to delineate geochemical constraints controlling the composition of chromitites. The studied chromitites exhibit δ53Cr values ranging from −0.184‰ to +0.159‰, falling in the range of so-called “igneous Earth” or “Earth’s mantle inventory” with values [−0.12 ± 0.11‰ to 0.079 ± 0.129‰ (2sd). A characteristic feature is the slightly positively fractionated δ53Cr values of all chromitite samples from Othrys (+0.043 ± 0.03‰), and the occurrence of a wide range of δ53Cr values spanning from positively, slightly negatively to the most negatively fractionated signatures (Pindos, δ53Cr = −0.147 to +0.009‰; Skyros, δ53Cr = −0.078 to +0.159‰). The observed negative trend between δ53Cr values and Cr/(Cr + Al) ratios may reflect a decrease in the δ53Cr values of chromitites with increasing partial melting degree. Alternatively, it may point to processes related to magmatic differentiation, as can be seen in our data from Mikrokleisoura (Vourinos).