EarthRef.org Reference Database (ERR) -- Mirota & Veizer 1994

The approximately 1.85 + 0.15 Ga old Albanel Formations of northern Quebec, Canada, an example of Paleoproterozoic carbonate buildups, are lithologically composed of cherty dolostones. Subordinate limestones are encountered only near the base of the sequence, coincident with maximal transgression. The chemistry of the bulk rocks is almost entirely controlled by noncarbonate phases, such as chert, quartz, plagioclase, illite (+/-K-spar), pyrite, and Fe-Mn oxides, all abundant particularly in the basal part of the section. The chemistry of the carbonate portion of limestones is strongly reset by post-depositional phenomena, but the relict high Sr content suggest that aragonite may have been an original precursor phase. Geochemically, the carbonate portion of dolostones can be subdivided into two populations, reflecting the stratigraphic bottom and top of the sequence. Trace element and oxygen isotope criteria suggest that the entire sequence was subjected to meteoric and subsequent burial diagenesis, with the less deeply buried top portion of the sequence having recrystallized and stabilized at lower temperatures, but in a more open system. The latter reflects the greater lithological variability and permeability of the younger strata. The delta C-13 varies up-section from about +3 to + parts per thousands perhaps a reflection of progressively more open marine circulation. The associated four generations of post-depositional pyrites have overlapping delta S-34, from -7.1 to + 54 parts per thousand; with a mean of + 24 parts per thousand; a reflection of dissimilatory bacterial sulfate reduction in a relatively closed diagenetic system with perhaps a limited sulfate availability and/or replenishment. The best measured Sr-87/Sr-86 of 0.70600, although possibly not a direct reflection of the coeval seawater, may still be a near-original value, as indicated by its coherency with the roughly co-stratigraphic Paleoproterozoic sequences. Evaluation of the existing data indicates that strontium isotopic composition of the Precambrian seawater was characterized by oscillations similar to those of the Phanerozoic, the interval of approximately 0.8-1.5 Ga ago being dominated by relatively nonradiogenic values, followed by a radiogenic period at approximately 1.6-2.2 Ga ago, near-mantle values during the late Archean, and possibly another radiogenic peak at approximately 3.5 Ga ago.