Comparison of iron isotope variations in modern and Ordovician siliceous Fe oxyhydroxide deposits

Möller, Kirsten; Schoenberg, Ronny; Grenne, Tor; Thorseth, Ingunn H.; Drost, Kerstin; Pedersen, Rolf B. (2014). Comparison of iron isotope variations in modern and Ordovician siliceous Fe oxyhydroxide deposits. Geochimica et cosmochimica acta, 126, pp. 422-440. Elsevier Science 10.1016/j.gca.2013.11.018

[img] Text
moeller_geochim.pdf - Published Version
Restricted to registered users only
Available under License Publisher holds Copyright.

Download (3MB) | Request a copy

Formation pathways of ancient siliceous iron formations and related Fe isotopic fractionation are still not completely understood. Investigating these processes, however, is difficult as good modern analogues to ancient iron formations are scarce. Modern siliceous Fe oxyhydroxide deposits are found at marine hydrothermal vent sites, where they precipitate from diffuse, low temperature fluids along faults and fissures on the seafloor. These deposits exhibit textural and chemical features that are similar to some Phanerozoic iron formations, raising the question as to whether the latter could have precipitated from diffuse hydrothermal fluids rather than from hydrothermal plumes.
In this study, we present the first data on modern Fe oxyhydroxide deposits from the Jan Mayen hydrothermal vent fields, Norwegian-Greenland Sea. The samples we investigated exhibited very low δ56Fe values between -2.09‰ and -0.66‰. Due to various degrees of partial oxidation, the Fe oxyhydroxides are with one exception either indistinguishable from low-temperature hydrothermal fluids from which they precipitated (-1.84‰ and -1.53‰ in δ56Fe) or are enriched in the heavy Fe isotopes. In addition, we investigated Fe isotope variations in Ordovician jasper beds from the Løkken ophiolite complex, Norway, which have been interpreted to represent diagenetic products of siliceous ferrihydrite precursors that precipitated in a hydrothermal plume, in order to compare different formation pathways of Fe oxyhydroxide deposits. Iron isotopes in the jasper samples have higher δ56Fe values (-0.38‰ to +0.89‰) relative to modern, high-temperature hydrothermal vent fluids (ca. -0.40‰ on average), supporting the fallout model. However, formation of the Ordovician jaspers by diffuse venting cannot be excluded, due to lithological differences of the subsurface of the two investigated vent systems.
Our study shows that reliable interpretation of Fe isotope variations in modern and ancient marine Fe oxyhydroxide deposits depends on comprehensive knowledge of the geological context. Furthermore, we demonstrate that very negative δ56Fe values in such samples might not be the result of microbial dissimilatory iron reduction, but could be caused instead by inorganic reactions.

Item Type:

Journal Article (Original Article)

Division/Institute:

08 Faculty of Science > Institute of Geological Sciences
08 Faculty of Science > Institute of Geological Sciences > Isotope Geology

UniBE Contributor:

Möller, Kirsten

Subjects:

500 Science > 550 Earth sciences & geology

ISSN:

0016-7037

Publisher:

Elsevier Science

Language:

English

Submitter:

Kirsten Möller

Date Deposited:

26 Mar 2014 13:38

Last Modified:

14 Jan 2015 22:51

Publisher DOI:

10.1016/j.gca.2013.11.018

BORIS DOI:

10.7892/boris.45296

URI:

https://boris.unibe.ch/id/eprint/45296

Actions (login required)

Edit item Edit item
Provide Feedback