Iron isotopic compositions of adakitic and non-adakitic granitic magmas: Magma compositional control and subtle residual garnet effect

摘要

Here we present iron (Fe) isotopic compositions of 51 well-characterized adakitic and non-adakitic igneous rocks from the Dabie orogen, Central China and Panama/Costa Rica, Central America. Twelve I-type non-adakitic granitoid samples from the Dabie orogen yield delta(56) Fe ranging from -0.015% to 0.184%. The good correlations between delta(56) Fe and indices of magma differentiation (e. g., SiO2, FeOt, Mg#, and Fe3+/Sigma Fe) suggest Fe2+- rich silicate and oxide minerals dominated fractional crystallization with Delta Fe-56(melt-crystal) similar to 0.06% may account for the delta Fe-56 variation in these samples. One A-type granite sample from the Dabie orogen has delta Fe-56 as high as 0.447%, likely indicating less magnetite crystallization and an increase in 10(3) ln beta(melt) with magma (Na + K)/(Ca + Mg). Combined with the literature data, most high silica (SiO2 >= 71 wt.%) granitic rocks define a good positive linear correlation between delta Fe-56 and (Na + K)/(Ca + Mg): delta Fe-56 = 0.0062% x (Na + K)/(Ca + Mg) + 0.130% (R-2 = 0.66). Given that fractional crystallization also tends to increase delta Fe-56 with (Na + K)/(Ca + Mg), this correlation can serve as the maximum estimate of the magma compositional control on Fe isotope fractionation. Low-Mg adakitic samples (LMA) have delta Fe-56 ranging from 0.114% to 0.253%. The melt compositional control on LMA delta Fe-56 could be insignificant due to their limited (Na + K)/(Ca + Mg) variation. Except for one sample that may be affected by late differentiation, 14 out of 15 LMA have delta Fe-56 increasing with (Dy/Yb) N, reflecting a subtle but significant effect of residual garnet proportion. This serves as evidence for that source mineralogy may play an important role in fyractionating Fe isotopes during partial melting. Dabie and Central America high-Mg adakitic samples have homogeneous Fe isotopic compositions with mean delta Fe-56 of 0.098 +/- 0.038% (2SD, N = 11) and 0.085 +/- 0.045% (2SD, N = 11), respectively. These samples have undergone melt-mantle interaction, which may neutralize the possible delta Fe-56 variation of their pristine magmas derived by partial melting of eclogitic rocks. (C) 2017 Elsevier Ltd. All rights reserved.