IRON ISOTOPIC COMPOSITIONS OF ADAKITES: CONSTRAINTS ON FRACTIONATION DURING ADAKITIC MAGMATISM AND RECYCLING OF CRUSTAL MATERIALS

It is debated whether the bulk silicate earth (BSE) has a chondritic iron isotope composition or not. δ⁵⁶Fe variations observed in peridotite xenoliths and basalts indicate that the mantle may be heterogeneous due to melt extraction and recycling of crustal materials. An accurate estimate of BSE δ⁵⁶Fe thus calls for knowledge about crustal materials recycled to the mantle by subduction, delamination, and intra-crustal differentiation processes. Here we present iron isotope compositions of 43 early Cretaceous Dabie granites from east-central China, which represent partial melts of thickened and foundering crust during the mountain root removal process. Eleven modern adakites from Panama, which are true slab melts that suffered melt-mantle interaction, were also measured. High-Mg adakites from the Dabie orogen and Panama have mean δ⁵⁶Fe as 0.098±0.038‰ (N = 11) and 0.085±0.045‰ (N = 11) respectively, indistinguishable from MORBs. Iron isotope compositions of high-Mg adakites may record equilibrium with the upper mantle. No chromatography effect of oxidized melts (Fe³⁺/∑Fe ~35-79%) penetrating the upper mantle has been observed. Normal Dabie granites with no adakitic features have increasing δ⁵⁶Fe with decreasing FeOt, yielding a trend of δ⁵⁶Fe = 0.189‰ – 0.024×FeOt. This trend can be explained by either fractional crystallization of biotite and amphibole and/or changing Fe isotope fractionation factors between mineral and melt. Low-Mg adakites (δ⁵⁶Fe from 0.092 to 0.253‰), which formed by partial melting of thickened crust with an eclogitic residue, have a mean composition-corrected δ⁵⁶Fe*(FeOt=3.5) as 0.128±0.069‰ (2SD, N = 17), comparable to normal granites (0.108±0.053‰, 2SD, N = 15 of 16). No correlation between δ⁵⁶Fe*(FeOt=3.5) and Sr/Y, La/Yb, Dy/Yb is identified. These observations suggest that iron isotope fractionation is same during partial melting of lower continental crust with a garnet-rich versus garnet-poor residue. Combined with estimated degrees of partial melting, mass balance calculations indicate that the mean recycled lower continental crust may have δ⁵⁶Fe as low as 0.009‰. Therefore, iron isotope compositions may evolve during intra-crustal differentiation and delamination and thus can provide new constraints on crustal growth history.