Publication Type:
Journal ArticleSource:
Chemical GeologyChemical Geology, Elsevier, Amsterdam, Netherlands, Volume 249, Number 1-2, p.113-128 (2008)ISBN:
0009-2541Keywords:
alkaline earth metals, Archean, biogenic structures, carbonate rocks, carbonatization, Cenozoic, chemical composition, Coral Sea, diagenesis, dolomitization, Fe-54, Fe-56, geochemistry, Great Barrier Reef, Holocene, ICP mass spectra, isotope ratios, Isotopes, limestone, magnesium, mass spectra, metals, microbialite, mobilization, Neoarchean, Pacific Ocean, patterns, Precambrian, Quaternary, rare earths, sea water, Sedimentary rocks, sedimentary structures, South Pacific, Southwest Pacific, spectra, Stable isotopes, stromatolites, strontium, Trace elements, West PacificAbstract:
In a comparative study of fossil and recent stromatolites and microbialites we have investigated whether reefal stromatolitic limestones are able to preserve Fe isotope compositions that potentially serve as proxies of seawater chemistry at the time of their formation. It was found that delta (super 56) Fe values of Archean stromatolites vary between -2.1 ppm and -0.5 ppm, Devonian and Carboniferous microbialites vary between 0 ppm and -1 ppm, and modern microbialites from the Great Barrier Reef show a range from -0.12 ppm to +0.15 ppm. Five lines of evidence support the possibility that these compositions are potentially pristine. 1) Fe concentrations and Fe isotope ratios are not correlated. 2) The concentrations of other elements that are potentially mobile during carbonate diagenesis (Mg, Sr) do not correlate with delta (super 56) Fe. 3) Mn concentrations are inversely correlated with delta (super 56) Fe, which might hint at meteoric alteration. However, if Mn concentrations and Fe isotope signatures were pristine features, they might serve as a record of the reduction of these two metals in seawater. 4) Dolomitisation of two Devonian limestones does not shift their Fe isotope compositions. 5) Rare earth element and yttrium (REY) patterns in all limestones are similar to seawater REY sources. This suggests that the bulk of the Fe in the samples occurs in hydrogenous carbonates, not in primary or secondary Fe and Mn hydroxides, because the latter have different REY patterns. The modern microbialites are unfractionated in Fe relative to crustal rocks. Thus, these hydrogenous precipitates either incorporated unfractionated Fe of the ambient seawater composition, or they modified the Fe composition upon incorporation from a fractionated ocean Fe reservoir. In either case, provided diagenetic alteration has not modified the Fe isotope composition in a major way, a potential open ocean Fe isotope proxy may exist that may allow reconstruction of secular variations in seawater Fe isotope compositions. Our preliminary results suggest that Neoarchean microbial carbonate Fe may have been lighter than today's microbialite Fe, and that the carbonates' Fe isotopes potentially record the efficiency of transition metal reduction in ocean basins through geologic time. Abstract Copyright (2008) Elsevier, B.V.
Notes:
GeoRef, Copyright 2018, American Geological Institute.<br/>2008-115711
