Arc magma evolution: Insights from magnesium and iron isotope in mafic–ultramafic rocks, Eastern Kunlun, NW China

The origin of magnesium and iron isotope variations in mafic–ultramafic rocks has long been debated. In particular, a substantial lack of understanding exists regarding how the variation in the Mg and Fe isotopic compositions of arc magmas relates to fractional crystallization. Here, we report new Mg and Fe isotopic analyses of mafic–ultramafic rocks formed by co-genetic magmatic evolution in the Eastern Kunlun orogenic belt. The ultramafic and gabbro samples present mantle-like Mg isotopic compositions, with δ²⁶Mg values ranging from -0.279‰ to -0.266‰ for wehrlite, -0.266‰ to -0.243‰ for clinopyroxenite, and -0.284‰ to -0.253‰ for gabbro. In contrast, the δ²⁶Mg values of the hornblende gabbro samples are significantly higher (-0.195‰ to -0.176‰). These rocks have similar heavy Fe isotopic compositions, with δ⁵⁶Fe values ranging from 0.020‰ to 0.157‰. The high δ²⁶Mg values observed in the hornblende gabbro are interpreted as indicating a maximum of 6% olivine fractionation, whereas the slightly high δ²⁶Mg values in the clinopyroxenite are ascribed to the accumulation of titanomagnetite. The Fe isotopic signatures of these rocks are inherited from magmatic sources. A comparison of the studied rocks with other mafic-ultramafic rocks implies that partial melting of mantle wedges induced by metasomatism of marine sediment-derived melts could generate “dry” arc magmatism, which would necessitate a more comprehensive and detailed analysis in further investigations.