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

• Verfasst von: Dziony, Wanja [VerfasserIn]
• Verfasst von: Horn, Ingo [VerfasserIn]
• Verfasst von: Lattard, Dominique [VerfasserIn]
• Verfasst von: Koepke, Jürgen [VerfasserIn]
• Verfasst von: Steinhoefel, Grit [VerfasserIn]
• Verfasst von: Schuessler, Jan A. [VerfasserIn]
• Verfasst von: Holtz, François [VerfasserIn]
• Titel: 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
• Verf.angabe: Wanja Dziony, Ingo Horn, Dominique Lattard, Jürgen Koepke, Grit Steinhoefel, Jan A. Schuessler, François Holtz
• Jahr: 2014
• Jahr des Originals: 2013
• Umfang: 13 S.
• Fussnoten: Available online 7 November 2013 ; Gesehen am 29.10.2020
• Titel Quelle: Enthalten in: Chemical geology
• Ort Quelle: New York, NY [u.a.] : Elsevier, 1966
• Jahr Quelle: 2014
• Band/Heft Quelle: 363(2014), Seite 101-113
• ISSN Quelle: 1872-6836
• DOI: doi:10.1016/j.chemgeo.2013.10.035
• Datenträger: Online-Ressource
• Sprache: eng
• Sach-SW: Fe-isotopes
• Sach-SW: Fe-Ti-oxides
• Sach-SW: Hydrothermal alteration
• Sach-SW: IODP
• Sach-SW: Laser ablation
• Sach-SW: Mid-ocean ridge
• K10plus-PPN: 1737344866

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.