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Mg Isotope Fractionation in Microbial Carbonates From the 2.5 Ga Gamohaan Formation, South Africa
Jacobsen, B - Department of Geology University of California, Davis, One Shields Avenue, Davis, CA 95616 United States
Chavdarian, G V - Department of Geology University of California, Davis, One Shields Avenue, Davis, CA 95616 United States
Yin, Q - Department of Geology University of California, Davis, One Shields Avenue, Davis, CA 95616 United States
Sumner, D Y - Department of Geology University of California, Davis, One Shields Avenue, Davis, CA 95616 United States
Mg isotopic fractionation in calcite and dolomite from microbialites in the 2.5 Ga Gamohaan Formation, South Africa, were explored to study the possible use of Mg isotopes as biomarkers. We measured Mg isotopes in microdrilled fibrous marine calcite and dolomite from four different samples. Between 10-200 μg of powdered mineral were dissolved in ~6N HCl. Mg was subsequently purified on a cation-exchange column. Mg isotopes were measured at UC Davis using the Nu Plasma HR multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) in medium resolution mode. Samples are depleted in 26Mg and 25Mg relative to DSM 3, the standard reference material, by -1.5 to -3.7‰ and -0.8 to -2‰ respectively. Dolomite is systematically heavier in δ26Mg and δ25Mg by 0.5-2‰ and 0.3-1‰ respectively, than the calcite. Fibrous marine calcite values from nearly contemporaneous samples vary by ~1.8‰, suggesting that Mg isotopic values were significantly influenced by local processes. The abundance of evidence for microbial influences on carbonate precipitation in these rocks suggests a microbial influence on the preserved Mg isotopes, consistent with a recent study that showed that microbial activity can cause significant
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