In-situ Fe isotope ratio determination in Fe–Ti oxides and sulfides from drilled gabbros and basalt from the IODP Hole 1256D in the eastern equatorial Pacific

Abstract

In-situ Fe isotope measurements have been carried out to estimate the impact of the hydrothermal metamorphic overprint on the Fe isotopic composition of Fe–Ti-oxides and Fe-sulfides of the different lithologies of the drilled rocks from IODP Hole 1256D (eastern equatorial Pacific; 15Ma crust formed at the East Pacific Rise). Most igneous rocks normally have a very restricted range in their 56Fe/54Fe ratio. In contrast, Fe isotope compositions of hot fluids (>300°C) from mid-ocean-ridge spreading centers define a narrow range that is shifted to lower δ56Fe values by 0.2‰–0.5‰ as compared to igneous rocks. Therefore, it is expected that mineral phases that contain large amounts of Fe are especially affected by the interaction with a fluid that fractionates Fe isotopes during exsolution/precipitation of those minerals.We have used a femtosecond UV-Laser ablation system to determine mineral 56Fe/54Fe ratios of selected samples with a precision of <0.1‰ (2σ level) at micrometer-scale. We have found significant variations of the δ56FeIRMM-014 values in the minerals between different samples as well as within samples and mineral grains. The overall observed scale of δ56Femagnetite in 1256D rocks ranges from −0.12 to +0.64‰, and of δ56Feilmenite from −0.77 to +0.01‰. Pyrite in the lowermost sheeted dike section is clearly distinguishable from the other investigated lithological units, having positive δ56Fe values between +0.29 and +0.56‰, whereas pyrite in the other samples has generally negative δ56Fe values from −1.10 to −0.59‰.One key observation is that the temperature dependent inter-mineral fractionations of Fe isotopes between magnetite and ilmenite are systematically shifted towards higher values when compared to theoretically expected values, while synthesized, well equilibrated magnetite–ilmenite pairs are compatible with the theoretical predictions. Theoretical considerations including β-factors of different aqueous Fe-chlorides and Rayleigh-type fractionations in the presence of a hydrous, chlorine-bearing fluid can explain this observation. The disagreement between observed and theoretical equilibrium fractionation, the fact that magnetite, in contrast to ilmenite shows a slight downhole trend in the δ56Fe values, and the observation of small scale heterogeneities within single mineral grains imply that a general re-equilibration of the magnetite–ilmenite pairs is overprinted by kinetic fractionation effects, caused by the interaction of magnetite/ilmenite with hydrothermal fluids penetrating the upper oceanic crust during cooling, or incomplete re-equilibration at low temperatures.Furthermore, the observation of significant small-scale variations in the 56Fe/54Fe ratios of single minerals in this study highlights the importance of high spatial-resolution-analyses of stable isotope ratios for further investigations.