GEOCHEMISTRY OF PRECAMBRIAN CARBONATES .5. LATE PALEOPROTEROZOIC SEAWATER

In an effort to obtain better constrained data for the first- and second-order variations in the isotopic composition of late Paleoproterozoic (1.8 +/- 0.2 Ga) seawater, we report a study of mineralogy, chemistry, and isotopic composition of the Coronation Supergroup (approximately 1.9 Ga, NWT), Canada, and the McArthur Group (approximately 1.65 Ga, NT), Australia. Petrologically, both carbonate sequences are mostly dolostones. The McArthur population, however, contains more abundant textural features that attest to the former presence of sulfates and halite, and the facies investigated herein probably represent ancient equivalents of modern evaporitic sabkhas and lacustrine playa lakes. This is reflected in O-18 enrichment (up to -2.9 parts per thousand PDB) and high Mn, Fe, and Sr concentrations. Nonetheless, the available delta-O-18 data indicate that the "best estimate" for the Archean to late Paleoproterozoic limestones is about -7 parts per thousand PDB, with dolostones some 3 +/- 25 parts per thousand heavier. The observed DELTA-O-18(dolomite-calcite) is thus within the range of the proposed fractionation factor for a cogenetic mineral pair. This, together with the consistency of isotopic patterns for sedimentological facies over almost 2 billion years, from the late Paleoproterozoic to the present, suggest that dolomitization was an early diagenetic event and that the O-18 depletion of the Archean to late Paleoproterozoic carbonates is not an artefact of post-depositional alteration. The delta-C-13, although within the range of other Precambrian and Phanerozoic carbonates, indicates a possible secular evolution from 0 +/- 2 parts per thousand PDB at 1.9 Ga ago to -1 +/- 1.5 parts per thousand at 1.65 Ga ago. The secular evolution for Sr-87/Sr-86 reflects the Sr-87 increase from 0.7031 +/- 0.0008 at 3.5 +/- 0.1 Ga ago, through 0.7025 +/- 0.0015 at 2.8 +/- 0.2 Ga, 0.7033 at 2.5 Ga, 0.7050 at 2.25 +/- 0.25 Ga, 0.7047 at 1.91, to less-than-or-equal-to 0.706 at 1.65 Ga ago.