High-temperature chromium isotope fractionation and its implications: Constraints from the Kızıldağ ophiolite, SE Turkey

Abstract

Chromium isotope data were obtained from olivine, orthopyroxene and chromite separates of the Kızıldağ ophiolite, SE Turkey, to investigate the effects of high temperature magma processes on Cr isotope fractionation. Harzburgite in the Kızıldağ ophiolite has the δ53Cr values of −0.14 to −0.12‰ in chromite, and −0.08 to −0.01‰ in bulk rocks. These Cr isotope fractionations could be driven by partial melting and metasomatism. The dunite and chromitite samples from the mantle-crust transition zone of the Kızıldağ ophiolite are cumulates, and their chromite and olivine have δ53Cr values of −0.29 to −0.06‰ and −0.11 to 0.41‰, respectively. The δ53Cr values of chromite are negatively correlated with the chemical indices of fractional crystallization (e.g., Fe# of chromite), suggesting that Cr isotopes were fractionated during fractional crystallization. In the chromitite, the degree of Cr isotope fractionation increases with fractional crystallization, with 0.23‰ Cr isotope fractionation occurring at mineral scale. During fractional crystallization, solid phases preferentially incorporate Cr, particularly heavy Cr isotopes, driving the depletion of Cr and enrichment of light Cr isotopes in the evolved melts. The δ53Cr values of olivine are higher than the coexisting chromite, which could be explained by the fact that the olivine grains were probably formed earlier than the chromite. The Cr isotopic features of chromite in the podiform chromitite (−0.22 to −0.04‰) from the mantle sequence are consistent with the modelled fractional crystallization trend, confirming the magmatic origin of the podiform chromitite. Therefore, a significant Cr isotope fractionation at chromite from the Kızıldağ ophiolite could be induced by the high-temperature fractional crystallization.