Chromium isotope fractionation during magmatic processes of island arc basalts

Abstract

We present the first stable chromium isotope dataset for a suite of island arc basalts (IABs) from Kamchatka to investigate chromium isotope fractionation during magmatic processes in a subduction setting. The chromium isotopic data (δ53Cr) for Kamchatka arc magmas range from –0.09 ± 0.02 ‰ to –0.16 ± 0.02 ‰ (2σ, n = 22). These magmatic rocks exhibit a positive correlation between MgO content and Ni, Cr contents, as well as CaO/Al2O3 ratios, which suggests that they undergo olivine and pyroxene fractional crystallization. However, δ53Cr shows no correlation with Cr, Ni, or MgO contents, nor with CaO/Al2O3 ratios. Ionic modeling and Rayleigh fractionation modeling reveals minimal effects of fractional crystallization on the δ53Cr factors in these arc magmas (Δ53Crcrystal-melt: +0.0001 ‰ to + 0.009 ‰), and probably IABs in general. Furthermore, the lack of correlation between δ53Cr and the melting fraction suggests that partial melting also has a limited influence on Cr isotopic compositions. The δ53Cr values also show no correlation with 87Sr/86Sr ratios, chromium contents, or fluid-mobile elements (e.g., Ba/Th, B/Nb, As*/Ce), indicating that slab-derived fluids have a minimal effect on the chromium isotope composition of the mantle source of arc magmas. Notably, however, the majority of these samples show δ53Cr similar to Bulk Silicate Earth, and are statistically higher compared to those observed in mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs). Melting and Rayleigh fractionation models suggest that the high δ53Cr values in these arc magmas may be attributed to partial melting and fractional crystallization under higher magmatic oxygen fugacity conditions compared to those in MORBs and OIBs. This is consistent with their higher Fe3+/Σ Fe ratios and Δ log fO2 (QFM) values, emphasizing the role of varying oxygen fugacity on chromium isotope fractionation in terrestrial basalts. This study highlights the potential of chromium isotopes as a powerful tool for tracing planetary oxygen fugacity environments.