THE IRON ISOTOPE VARIATIONS IN THE ~ 2.5 GA MT. MCRAE SHALE

We have analyzed δ⁵⁶Fe at high stratigraphic resolution in the ~ 2.5 Ga McRae Shale, sampling ~ 100 m of continuous drill core recovered from the Hamersley Basin, Western Australia by the NASA Astrobiology Institute in 2004. The core contains two intervals of carbonaceous, pyritic shale, termed S1 and S2. δ⁵⁶Fe (relative to IRMM-14) of total Fe in 36 bulk samples spans a range of ~ 2.5 ‰. Most of this variation occurs in the upper shale unit (S1). δ⁵⁶Fe is lowest at ~ 140 m depth, in the middle of S1. In contrast, δ⁵⁶Fe values from the lower pyritic shale unit (S2) are less variable and less negative, clustering near ~ -0.5 ‰. In addition to these measurements of total Fe, we measured pyrite-associated Fe in 18 samples from S1 and S2, using selective chemical extraction. Pyrite Fe isotopes roughly mirror total Fe isotopes in S1, while in S2 pyrite Fe is persistently lighter than total Fe by ~ 1 ‰.

Previous research into Fe isotopes in sedimentary pyrites from Archean and Proterozoic anoxic basins demonstrated an overall ~ 4.5 ‰ variation in δ⁵⁶Fe values (-3.5 ‰ ~ 1 ‰) linked to changes in the ocean Fe budget and the rise of atmospheric oxygen (1). Thus, the variation of pyrite δ⁵⁶Fe values in S1 could reflect variations in environmental redox. Intriguingly, the depleted δ⁵⁶Fe at ~ 140 m coincides with previously observed authigenic Mo and Re enrichments in S1 that were interpreted as indicating a period of mild oxygenation in the late Archean, prior to the Great Oxygenation Event (2, 3). Consistent with this interpretation, we hypothesize that the depleted δ⁵⁶Fe reflects increased precipitation of isotopically heavy Fe oxides under oxidizing conditions around 2.5 Ga.

In S2, the decoupling of pyrite and total δ⁵⁶Fe may indicate that S2 pyrites were imprinted by fractionation between reactive and pyrite Fe. Such decoupling might have been common in anoxic, Fe-rich, S-poor Archean oceans, in which the extent of Fe conversion to pyrite was limited by S availability. Sedimentary Fe speciation data are consistent with this hypothesis. Therefore, δ⁵⁶Fe in Archean pyrites do not necessarily record δ⁵⁶Fe of contemporaneous seawater.