Fe-oxide fracture fillings as a palaeo-redox indicator: Structure, crystal form and Fe isotope composition

Penetration of oxygenated waters to the deeper sub-surface may occur in association with deglaciation events and significantly change the geochemical processes acting at depth. Such a scenario may compromise long-term storage of radioactive waste in underground repositories where copper canisters would corrode in the presence of oxygen.

In this study, Fe-oxides from fractures in granite drill-cores and from drilling debris were investigated and a method developed to trace the low-temperature, oxidising conditions which may have been caused by prior deglaciations. X-ray diffraction and Mössbauer spectroscopy showed that all the examined fracture fillings contained Fe-oxides.

Based on their structure and form, three genetic types of Fe-oxides were identified: (I) coarse-grained (∼ 100 nm) hydrothermal hematite; (II) very fine-grained (∼ 10 nm) amorphous Fe-oxides that precipitated during drilling; (III) Intermediate grain-size crystalline Fe-oxides, that are interpreted to have formed naturally at low-temperatures (∼ 10 °C).Fe isotope composition of the Fe-oxides was determined by multiple-collector inductively coupled plasma mass spectrometry using a 58Fe–54Fe double-spike. δ56Fe of the Fe-oxides ranges from −0.8 to +0.8‰ (relative to the IRMM-14 standard).

Hydrothermal samples have intermediate δ56Fe (−0.3 to 0.0‰), whereas natural low-temperature samples may be isotopically lighter (−0.8 to 0.0‰), and samples precipitated from drilling activity are isotopically heavier (−0.2 to 0.8‰). Within the three genetic suites, δ56Fe correlates with the relative proportion of Fe(III).

For the hydrothermal samples, Fe isotope composition likely reflects input of partially dissolved chlorite. The Fe isotope composition and Fe redox state of the low-temperature and drill-induced samples is consistent with a conceptual model where Fe isotope fractionation occurs during dissolution and oxidation.

In our study, the deepest sample showing evidence of natural low-temperature oxidation, putatively as a result of penetration of oxidising surface waters during deglaciation, is from a depth of ∼ 100 m.