Coupled Fe and S isotope variations in pyrite nodules from Archean shale
|Author(s)||Marin-Carbonne Johanna1, 2, Rollion-Bard Claire3, Bekker Andrey4, Rouxel Olivier5, Agangi Andrea6, Cavalazzi Barbara6, 7, Wohlgemuth-Ueberwasser Cora C.6, 8, Hofmann Axel6, McKeegan Kevin D.1|
|Affiliation(s)||1 : Univ Calif Los Angeles, Dept Earth Planetary & Space Sci, Los Angeles, CA 90095 USA.
2 : Univ Paris 07, CNRS, Inst Phys Globe, Sorbonne Paris Cite, F-75238 Paris, France.
3 : Univ Lorraine, CNRS, CRPG, F-54501 Vandoeuvre Les Nancy, France.
4 : Univ Calif Riverside, Dept Earth Sci, Riverside, CA 92521 USA.
5 : IFREMER, Ctr Brest, Dept Marine Geosci, F-29280 Plouzane, France.
6 : Univ Johannesburg, Dept Geol, Johannesburg, South Africa.
7 : Univ Bologna, Dipartimento Sci Biol Geol & Ambientali, I-40127 Bologna, Italy.
8 : Stockholm Univ, PetroTecton Ctr, Dept Geol Sci, S-1069 Stockholm, Sweden.
|Source||Earth And Planetary Science Letters (0012-821X) (Elsevier Science Bv), 2014-04 , Vol. 392 , P. 67-79|
|WOS© Times Cited||58|
|Keyword(s)||pyrite nodules, Fe and S isotopes, Archean, SIMS|
|Abstract||Iron and sulfur isotope compositions recorded in ancient rocks and minerals such as pyrite (FeS2) have been widely used as a proxy for early microbial metabolisms and redox evolution of the oceans. However, most previous studies focused on only one of these isotopic systems. Herein, we illustrate the importance of in-situ and coupled study of Fe and S isotopes on two pyrite nodules in a c. 2.7 Ga shale from the Bubi Greenstone Belt (Zimbabwe). Fe and S isotope compositions were measured both by bulk-sample mass spectrometry techniques and by ion microprobe in-situ methods (Secondary Ion Mass Spectrometry, SIMS). Spatially-resolved analysis across the nodules shows a large range of variations at micrometer-scale for both Fe and S isotope compositions, with δ56Feδ56Fe and δ34Sδ34S values from −2.1 to +0.7‰+0.7‰ and from −0.5 to +8.2‰+8.2‰, respectively, and Δ33SΔ33S values from −1.6 to +2.9‰+2.9‰. The Fe and S isotope variations in these nodules cannot be explained by tandem operation of Dissimilatory Iron Reduction (DIR) and Bacterial Sulfate Reduction (BSR) as was previously proposed, but rather they reflect the contributions of different Fe and S sources during a complex diagenetic history. Pyrite formed from two different mineral precursors: (1) mackinawite precipitated in the water column, and (2) greigite formed in the sediment during early diagenesis. The in-situ analytical approach reveals a complex history of the pyrite nodule growth and allows us to better constrain environmental conditions during the Archean.|