Chromium isotope fractionation during magmatic processes of island arc basalts

Abstract

<p>We present the first stable <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/chromium-isotope" title="Learn more about chromium isotope from ScienceDirect's AI-generated Topic Pages">chromium isotope</a> dataset for a suite of <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/island-arcs" title="Learn more about island arc from ScienceDirect's AI-generated Topic Pages">island arc</a> <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/basalt" title="Learn more about basalts from ScienceDirect's AI-generated Topic Pages">basalts</a> (IABs) from Kamchatka to investigate chromium isotope fractionation during magmatic processes in a subduction setting. The chromium isotopic data (δ⁵³Cr) for Kamchatka arc <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/magma" title="Learn more about magmas from ScienceDirect's AI-generated Topic Pages">magmas</a> 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 <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/pyroxene" title="Learn more about pyroxene from ScienceDirect's AI-generated Topic Pages">pyroxene</a> <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/fractional-crystallization" title="Learn more about fractional crystallization from ScienceDirect's AI-generated Topic Pages">fractional crystallization</a>. 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 (Δ⁵³Cr_crystal-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 <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/isotopic-composition" title="Learn more about isotopic compositions from ScienceDirect's AI-generated Topic Pages">isotopic compositions</a>. 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 <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/mantle-source" title="Learn more about mantle source from ScienceDirect's AI-generated Topic Pages">mantle source</a> 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 <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/ocean-island-basalt" title="Learn more about ocean island basalts from ScienceDirect's AI-generated Topic Pages">ocean island basalts</a> (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.<br></p>