Systematic decrease of high δ13C values with burial in late Archaean (2.8 Ga) diagenetic dolomite: evidence for methanogenesis from the Crixás Greenstone Belt, Brazil

Remnants of Fe-poor (2700–4400 ppm) dolomite crystals, crystal aggregates, and microconcretions are cryptically distributed within stratiform bodies (tens of metres across) of metamorphic Fe-rich (3–6 wt% Fe) dolomite that occur in graphitic schists of the 2.8 Ga Ribeira˜o das Antas Formation in the Crixás Greenstone Belt, Brazil. Fe-poor dolomite is a diagenetic carbonate formed during burial of organic-bearing, muddy sediments. Using cathodoluminescence microscopy, a consistent sequence of dolomite growth stages is well defined within individual crystals, crystal aggregates, and microconcretions. From comparison with similar textures in Phanerozoic carbonates, we suggest that the early dolomite formed during progressive burial, aggregates having developed from mergent growth of individual crystals during mechanical and then chemical compaction. The δ13Cdolomite values of crystals and concretion cores are very high, 12–14‰ (PDB), and a negative shift of about 7‰ occurs over millimetres in subsequent dolomite generations. Early diagenetic Fe-poor dolomite and late-stage Fe-rich dolomite show overlapping ranges of initial 87Sr86Sr ratios, between 0.7080 and 0.7164, and δ18O values, −14 to −16‰ (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 suggests that such exchange did not follow a process of recrystallization. The decrease in the δ13C of dolomite is correlated with crystal and concretion growth during burial. This behaviour is interpreted to indicate a change in the δ13C of pore-water bicarbonate resulting from bacterially mediated organic diagenesis within the lower part of the zone of methanogenesis. The Crixás dolomite represents direct evidence that methanogenic bacteria were important contributors in shallow subsurface geochemical systems in the early Precambrian.The pattern of δ13C values found for the Crixás dolomite is also documented in other early to middle Precambrian dolomites and limestones. Previous interpretations have considered positive δ13C values to be the result of primary (oceanographic) controls, whereas variability in δ13C is commonly linked to metamorphic alteration. We suggest that the role played by organic diagenesis in controlling the δ13C values of diagenetic carbonates has been underestimated.