Mathematics – Logic
Scientific paper
Apr 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.1735m&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 7, pp.1735-1745
Mathematics
Logic
11
Scientific paper
The ~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 contents suggest that aragonite may have been an original precursor phase. Geochemically, the carbonate portion of dolostones can be subdivided into two populations, reflecting the stratigraphie 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 13 C varies up-section from about +3 to + 1 , perhaps a reflection of progressively more open marine circulation. The associated four generations of post-depositional pyrites have overlapping 34 S, from -7.1 to +54 , with a mean of +24 ; 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 87 Sr / 86 Sr 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 Paleo-proterozoic 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 ~0.8-1.5 Ga ago being dominated by relatively nonradiogenic values, followed by a radiogenic period at ~ 1.6-2.2 Ga ago, near-mantle values during the late Archean, and possibly another radiogenic peak at ~3.5 Ga ago.
Mirota Michael D.
Veizer Ján
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