SYSTEMATIC DECREASE OF HIGH DELTA-C-13 VALUES WITH BURIAL IN LATE ARCHEAN (2.8-GA) DIAGENETIC DOLOMITE - EVIDENCE FOR METHANOGENESIS FROM THE CRIXAS GREENSTONE-BELT, BRAZIL

Remnants of Fe-poor (2700-4400 ppm) dolomite crystals, crystal aggrega tes, and microconcretions are cryptically distributed within stratifor m bodies (tens of metres across) of metamorphic Fe-rich (3-6 wt% Fe) d olomite that occur in graphitic schists of the 2.8 Ga Ribeirao das Ant as Formation in the Crixas Greenstone Belt, Brazil. Fe-poor dolomite i s a diagenetic carbonate formed during burial of organic-bearing, mudd y sediments. Using cathodoluminescence microscopy, a consistent sequen ce of dolomite growth stages is well defined within individual crystal s, crystal aggregates, and microconcretions. From comparison with simi lar textures in Phanerozoic carbonates, we suggest that the early dolo mite formed during progressive burial, aggregates having developed fro m mergent growth of individual crystals during mechanical and then che mical compaction. The delta(13)C(dolomite) values of crystals and conc retion cores are very high, 12-14 parts per thousand (PDB), and a nega tive shift of about 7 parts per thousand occurs over millimetres in su bsequent dolomite generations. Early diagenetic Fe-poor dolomite and l ate-stage Fe-rich dolomite show overlapping ranges of initial Sr-87/Sr -86 ratios, between 0.7080 and 0.7164, and delta(18)O values, -14 to - 16 parts per thousand (PDB). These similarities may indicate isotopic exchange between the early dolomite and subsequent metamorphic fluids, yet the well preserved growth zonation of the Fe-poor dolomite sugges ts that such exchange did not follow a process of recrystallization. T he decrease in the delta(13)C of dolomite is correlated with crystal a nd concretion growth during burial. This behaviour is interpreted to i ndicate a change in the delta(13)C of pore-water bicarbonate resulting from bacterially mediated organic diagenesis within the lower part of the zone of methanogenesis. The Crixas dolomite represents direct evi dence that methanogenic bacteria were important contributors in shallo w subsurface geochemical systems in the early Precambrian. The pattern of delta(13)C values found for the Crixas dolomite is also documented in other early to middle Precambrian dolomites and limestones, Previo us interpretations have considered positive delta(13)C values to be th e result of primary (oceanographic) controls, whereas variability in d elta(13)C is commonly linked to metamorphic alteration. We suggest tha t the role played by organic diagenesis in controlling the delta(13)C values of diagenetic carbonates has been underestimated.