TY - JOUR
T1 - Fe-oxide fracture fillings as a palæo-redox indicator
T2 - Structure, crystal form and Fe isotope composition
AU - Dideriksen, K.
AU - Christiansen, B.C.
AU - Baker, J.A.
AU - Frandsen, C.
AU - Balic-Zunic, T.
AU - Tullborg, E.
AU - Mørup, S.
AU - Stipp, S.L.S.
N1 - Funding Information:
We thank Helge Rasmussen and Daniel E. Madsen for performing Mössbauer spectroscopy on some of the samples, Helene Almind for help with X-ray diffraction, Birgit Damgaard for analyses by atomic absorption spectroscopy, and Bjarne Bisballe for access to scanning electron microscopy facilities. Martin Bizzarro and David Ulfbeck contributed to valuable discussions about the Fe isotope results, and Helen Williams kindly provided the ETH Zürich in-house Fe standard. We are grateful for the valuable input and discussions on Fe-oxide genesis offered by John Smellie and Ignasi Puigdomenech, as well as Peter Wikberg who initiated this project and comments offered by Donald Canfield, Mark Rehkamper, Clark Johnson and Derek Vance. Funding was provided by SKB (Svensk Kärnbränslehantering AB). The NanoGeoScience laboratory was established by a grant from the Danish Natural Sciences Research Council.
PY - 2007/9/30
Y1 - 2007/9/30
N2 - 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.
AB - 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.
KW - Fe isotope
KW - Fe-oxide
KW - Fractures
KW - Granite
KW - Oxidation
KW - Radioactive waste repository
UR - http://www.scopus.com/inward/record.url?scp=34548463795&partnerID=8YFLogxK
U2 - 10.1016/j.chemgeo.2007.06.027
DO - 10.1016/j.chemgeo.2007.06.027
M3 - Article
AN - SCOPUS:34548463795
SN - 0009-2541
VL - 244
SP - 330
EP - 343
JO - Chemical Geology
JF - Chemical Geology
IS - 1-2
ER -