GEOCHEMISTRY OF PRECAMBRIAN CARBONATES .4. EARLY PALEOPROTEROZOIC (2.25 +/- 0.25 GA) SEAWATER

In an effort to narrow down the first- and second-order variations in isotopic composition of the Early Paleoproterozoic (2.25 +/- 0.25 Ga) seawater, we report a comprehensive study of mineralogy, chemistry, and isotopic composition of the Malmani Dolomite (Transvaal Supergroup, South Africa), Duck Creek Dolomite (Wyloo Group, Australia), and Bruce "Limestone" Member of the Espanola Formation (Huronian Supergroup, Canada). Based on trace element data, their diagenetic rank increases in the order Duck Creek < Bruce < Malmani. The interpolation of alteration trends to "best" values yields an estimate of 0.70550 for Sr-87/Sr-86, comparable to our previously published value of 0.70496. For delta-C-13, the measured range of 0 +/- 1.5 parts per thousand PDB is similar to that observed for Phanerozoic marine carbonates. In contrast, the "best" delta-O-18 value for dolostones is -5 parts per thousand PDB, depleted in O-18 relative to Phanerozoic counterparts, but comparable to the estimates obtained by similar approach for Archean facies. The isotope geochemistry and mineralogy of Bruce "Limestone" Member is consistent with the proposition that the sequence may have been deposited in a lacustrine environment, possibly related to the recession of the Bruce glaciers. If such interpretation proves unacceptable from a sedimentological standpoint, the origin of the unusual isotopic signatures must be sought, despite the above trace element interpretation, in as yet unspecified post-depositional alteration phenomena. In contrast to Phanerozoic carbonates, the Early Paleoproterozoic-as well as the previously described Archean-counterparts are enriched in Mn and Fe.