2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 189-7
Presentation Time: 10:10 AM

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


HE, Yongsheng, School of Earth Sciences and Resources, China University of Geosciences, Beijing, 29 Xueyuan Road, Haidian District, China University of Geosciences, Beijing, 100083, China and WÖRNER, Gerhard, Department of Geosciences, University of Göttingen, 1 Goldschmidtstr.1, Göttingen, 37077, Germany

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.