Variation of oxygen fugacity during magmatic evolution: in-situ geochemistry of mafic dikes from Jiaodong, southeastern North China Craton

Mafic lava is generally used to study the oxygen fugacity of lithospheric mantle. However, some studies confirmed that the high oxygen fugacity of mafic lava from lithospheric mantle may be result of auto-oxidation during the late magma evolution. Whether mafic lava reflects the oxygen fugacity of source area remains controversial. The major and trace element compositions of biotite, amphibole, and plagioclase as well as the Sr isotopes of plagioclase were in-situ examined aim to evaluate the variation in oxygen fugacity of mafic magma during transportation from depth to current location. Mineralogical characteristics show that the dikes originate from an enriched lithospheric mantle source and experienced very little or no crustal contamination during the magma transfer process. Clinopyroxene, plagioclase, Al-rich amphibole, biotite, and Mg-rich amphibole crystallized successively as the depth of magma reduced from 75.9 to 0.8 km. Fayalite-magnetite-quartz (△FMQ) and Ni-NiO (△NNO) are used as reference oxygen fugacity buffers in this study. The average oxygen fugacity of mafic magma gradually increases from △NNO-1.50 (lithospheric mantle source) to △NNO + 2.74 (near the surface) during magma evolution. Calculation results from clinopyroxene, amphibole and biotite show that the oxygen fugacity of magma increases during fractionation of clinopyroxene, plagioclase, amphibole, and biotite. However, the above fractionation cannot generate such a wide variation in oxygen fugacity. Moreover, it is not caused by degassing in terms of the low carbonate mineral content and increasing S6+/Stot ratios during fractionation. Previous studies have shown a rapid cooling and a high H2O content of the Jiaodong mafic magma. Combine with the garnet-spinel transition zone of mantle lithosphere and magnetite observed in petrography, we tend to propose that the variation in oxygen fugacity of mafic magma were caused by magnetite-spinel fractional crystallization at depth. Low-pressure metamorphic amphibole and the country rocks of Late Mesozoic monzodiorite, which has a much higher oxygen fugacity than the stable range of magnetite-spinel, indicated that the secondary alteration in last stage of magma evolution may cause additional variation in oxygen fugacity. In that case, mafic lava cannot be used to determine the oxygen fugacity of source rock.