BACTERIAL AND SEDIMENT GRAIN-SIZE CONTROL OF METAMORPHIC MINERAL ASSEMBLAGES

Mineral assemblages in greenschist- to amphibolite-faces metamorphic schists depend on bulk chemistry, e.g., LREE-content affects monazite abundances, whereas high Mn stabilizes garnet-chorite assemblages relative to biotite-staurolite. Petrologists may often assume that rocks passively inherit their bulk chemistry from source materials, e.g., LREE-rich source rocks => LREE-rich sediments => monazite-rich schist. Yet, in fact the chemistry of marine sediments is strongly controlled by trace metal adsorption and bacterially-mediated redox reactions involving Fe, Mn, and S, rather than source material alone. Two examples from high-grade rocks of the Appalachians support a dominant control of grain size and bacterial redox reactions in influencing original sediment compositions and hence metamorphic mineral assemblages.

1) Grain size and REE: Amphibolite-facies schists from Great Smoky Mountains, North Carolina show poor correlations of REE, Th, V, and U with either P or Zr, suggesting that original sediment sources exert little control over trace metals, yet strong positive correlations with Al. Assuming Al proxies for clay content, these data implicate grain size and adsorptive surface abundance as principal controls on REE, Th, and U concentrations. Elevated LREE/HREE ratios reflect stronger adsorption of LREE vs. HREE and are consistent with this hypothesis. Redox reactions alone are not responsible because they would yield positive correlations between V and U (as observed), but negative correlations with REE and Th (not observed).

2) Bacteria and Mn – Fe/Mg systematics: Amphibolite-facies schists from west-central New Hampshire show a consistent correlation among Fe/(Fe+Mg), Mn, and mineral assemblage; graphitic rocks with simple Grt+Bt+Chl assemblages typically have the highest Mn and lowest Fe/(Fe+Mg) compared to graphite-free rocks. Bacterially-mediated redox reactions strongly control Fe and Mn contents of sediments, e.g., increasing Mn and decreasing Fe from suboxic to anoxic conditions. The correspondence between graphite- and Mn-enrichment suggests bacteria principally define original sediment Fe-Mn-Mg ratios and, ultimately, amphibolite-facies mineral assemblages.