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44141-AC2
The Development of the Sulfur Isotopic Composition of Carbonate Associated Sulfate as an Indicator of Diagenetic Environment in the Formation of Dolomite
Progress on the Completion of the Grant.
The proposed work involved the assessment of the use of Non-Conventional Geochemical Indicators (NCGIs) to aid with the interpretation of diagenesis in carbonate systems, in particular concentrating on the processes of dolomitization. In addition it was proposed to use stable sulfur isotopes (34S and 32S) in carbonate associated sulfate (CAS). Carbonate associated sulfate, is sulfate which is trapped within the crystal structure of the mineral as opposed to sulfate which adheres to the outside if the carbonate minerals. In order to assess the use of NCGIs and the δ34S of CAS as diagenetic indicators a series of four criteria were established
1- Is it possible to remove external contamination from materials which are known to have formed in one environment, but are now situated in a second fluid with distinctly different geochemistry as regards the NGCI elements?
2- Does the geochemical signature of the NCGI elements in carbonates with well constrained diagenetic and paragenetic histories reflect the composition of those fluids?
3- What are the distribution coefficients for the NCGI elements in aragonite, low-Mg calcite, and dolomite?
4- Does the δ34S of the CAS reflect the δ34S of primary environments.
After establishing these criteria, the plan of action was to apply the lessons learned to two sets of dolomites, one in which the diagenetic history was well constrained and one in which the diagenetic history was still enigmatic. The samples chosen for the second part of the study were dolomites of Plio-Miocene age from the subsurface of Great Bahama Bank and dolomites from the lower-Carboniferous Madison formation in Wyoming.
Removal of Contamination
In order to assess methods for the removal of contamination from the carbonate material, modern carbonate organisms growing in seawater were subjected to a series of leaching experiments using distilled water and more aggressive cleaning solutions, including sodium hypochlorite, and various organic solvents. Samples were ground into size fractions of less than 63 um and repeatedly rinsed in distilled water (treatment 1), bleached and repeatedly rinsed (treatment 2), bleached, rinsed, treated with organic solvent, and repeatedly rinsed (treatment 3). After each treatment the solid material and the rinsed solution was analyzed for its concentration of sulfur, potassium, and sodium. The experiments were repeated on bulk marine and freshwater sediments, as well as sediments which were original formed in the marine environment, but later were deposited in hypersaline situations. These experiments showed that through progressively harsher treatments it was possible to reduce the concentration of NCGI elements to progressively lower values. For example, the concentration of sodium within coral skeletons has been quoted as being approximately 4000 ppm. After treatment 3, the concentration of sodium falls below 2000 ppm. As it is assumed that lower concentrations more accurately represent the ‘correct' value, work is continuing to ascertain whether it is possible to reduce the concentration further. Analysis of the sediments deposited in marine environments and then subjected to different environment with significantly different geochemical characteristics is continuing. These environments include sediments from Florida Bay and from Leg 182 of the Ocean Drilling Program. We are also exploring methods of analyzing the concentration of chloride using ion chromatography.
Analysis of the Sulfur Isotopic Composition (δ34S)
The sulfur isotopic composition (δ34S) will be carried out using a modified automated carbon/nitrogen analyzer (ANCA) interfaced to a Europa 20-20. Although we have one system which is dedicated to N and C, we had to acquire a second, as it is not proactical to continually convert the system from C & N to S and then back again. Once the second ANCA system was obtained, the combustion system was modified to be specific to oxidize sulfur to sulfur dioxide, remove excess water (using a magnesium perchlorate and nafion drier), and removed any influence of 18O and 17O by passing the stream of gas over SiO2 at 800oC. This system, which is now operational is termed the ASA (Automated Sulfur Analyzer). Both ANCA and the ASA are interfaced to the same Europa 20-20. Sulfur reference gas is supplied to a dedicated reference gas injector. Samples of CAS are converted to barium sulfate and these are processed using the ASA.
Initial work on the cleaning of the samples and preliminary analyses of the concentration of the non-conventional geochemical indices in samples from the Bahamas was undertaken by a graduate student early in the history of the grant. Unfortunately the work performed by this student proved to be unreliable and it has determined that this needs to be carried out anew. She is no longer involved in the project. A postdoctoral research associate supported by the grant was hired in June 2007 and she is progressing with the work outlined in this report. It is anticipated that a no-cost extension will be requested so that the work can be completed.
