Iron Isotope Compositions of the Late Mesozoic Bimodal Volcanic Rocks from NE China

Abstract:

Although it is well known that δ⁵⁶Fe of igneous rocks dramatically increase when the SiO₂ content is above 71wt. %, there are still debates on the reason for Fe isotope fractionation at high temperature conditions. Previous studies proposed that crystal fractionation, partial melting, volatile exsolution, and thermal diffusion could result in the enrichment of heavy Fe isotopes in high-SiO₂magma. In order to better understand the mechanism for Fe isotope fractionation in magmatism, here we present a systematic Fe isotope study for a suit of well-characterized bimodal volcanic rocks. The samples are from the late Mesozoic Hailar and Songliao basin, NE China, including basaltic-trachyandesites, I-type Trachytes and rhyodacites, and A-type rhyolites. Based on major and trace elements, radiogenic isotopes and geochronological data, it was suggested that the I-type rocks were produced by patial melting of the basaltic-trachyandesites, while the A-type rhyolites resulted from melting of the I-type rocks.

Fe isotope measurements were conducted using a Neptune Plus MC- ICP- MS in the USTC. The long-term external precision of δ⁵⁶Fe is ±0.05‰ (2SD).The δ⁵⁶Fe values for all the three types of rocks are positively correlated with SiO₂ contents and inversely correlated with total Fe contents, consistent with previous studies. This suggests that partial melting can produce isotopically heavier partial melt than their source rocks. Notably, rhyolite samples have the highest δ⁵⁶Fe (up to 0.63± 0.05 ‰) among the previously reported data for igneous rocks at a given silica content. Wet chemical analysis reveals high Fe³⁺/Fe_T in all samples, likely suggesting that partial melting occurred at an oxidizing condition. This can explain the high δ⁵⁶Fe in rhyolite rocks. In summary, our results indicate that partitioning of ferric iron between mineral and melt during partial melting is critical for enrichment of heavy Fe isotopes in high silica igneous rocks.