HR: 09:15h
AN: PP41D-06    [Abstracts]
TI: Fe Isotope Composition of Neoproterozoic Post-Glacial "Cap Dolostones"
AU: * Halverson, G P
EM: galen.halverson@lmtg.obs-mip.fr
AF: LMTG UniversitAc Paul Sabatier - IRD, 14 avenue Edouard Belin, Toulouse, 31400 France
AB: The largest variations in the Fe isotope composition in the geological record are found in sedimentary rocks, presumably as the result of redox transformations of iron during mineral precipitation, microbial processing, and diagenesis (Johnson et al., Cont. Min. Petrol., 2003). Systematic trends in the variability of the Fe isotope composition of sulfide minerals formed in ancient marine black shales broadly mirror patterns in sulfur isotope data (Δ³³S, Δ³⁴S), which are consistent with geological and other geochemical evidence for the progressive oxidation of the earth's surface during the Precambrian (Rouxel et al., Science, 2005). Therefore, the record of the Fe isotope composition of minerals formed in the marine environment appears to be a promising proxy for the redox evolution of the ocean. We have developed a method to extract the marine Fe isotope composition from carbonates in an attempt to establish higher resolution records of changes in marine redox changes than permitted by black shale geochemistry. We have applied this method to the study of ca. 635 Ma iron-rich dolostones, which are found in Neoproterozoic successions worldwide and immediately post-date a purported snowball (Marinoan) glaciation during which time the deep ocean is thought to have become anoxic (Hoffman et al., Science, 1998), allowing its Fe isotopic composition to evolve towards the composition of relatively light (δ⁵⁷Fe vs. IRMM-14 ~ -0.6‰) hydrothermal iron (Beard et al., Geology, 2003). Fe isotope compositions were measured relative to IRMM-14 in medium-resolution mode on a Neptune MC-ICP-MS with a long-term external (2σ) reproducibility of < 0.04‰/amu. Preliminary data on dolomite samples from Svalbard, northern Namibia and northwest Canada show a range in δ⁵⁷Fe values from -0.65 to 0.04‰, similar to the range found in siderite and Fe-rich dolomite in ancient BIFs (Johsnon et al., 2003) and to values for the Namibian cap dolostone reported by Leighton et al. (Goldschmidt abstract, 2005), but distinctly lower than altered dolostones (δ⁵⁷Fe = 0.10 - 1.02‰) in a Jurassic, organic-rich mudstone (Matthews et al., GCA, 2004). It is difficult to conclude at this time whether or not the relatively low δ⁵⁷Fe composition of the cap dolostones is consistent with the Beard et al. (Geology, 2003) hypothesis due to large uncertainties in the solution-mineral fractionation factors for carbonates, the potential effects of diagenesis and biological influences on dolomite precipitation, and the possibility that the Marinoan deep ocean was euxinic.
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 1051 Sedimentary geochemistry
DE: 3344 Paleoclimatology (0473, 4900)
DE: 4802 Anoxic environments (0404, 1803, 4834, 4902)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005