Diagenetic calcite from the Chazyan group (Vermont): an example of aragonite alteration in a greenhouse ocean

Marine diagenetic calcite with both a calcitic (low-to-intermediate Mg) and aragonitic origin was examined from the middle Ordovician buildups of the Chazyan Group in Vermont. All marine phases have elevated Sr (up to 1800 pp m) compared with that observed from marine precipitates in other middle Ord ovician units. Stromatoporoids (Iabechiids), which were originally aragonit ic, have higher Sr values than phases with an original calcite mineralogy ( trilobites, marine cement). Additional evidence supporting precursor minera logy interpretations includes elevated Mg values (up to 3.6 mole% MgCO3) an d the presence of microdolomite in interpreted calcitic phases. Originally aragonitic precipitates have lower Mg Values and most significantly lack mi crodolomite. This study demonstrates the presence of elevated Sr values in marine precipitates that formed during a period when calcite, not aragonite , was the dominant physiochemically precipitated calcium carbonate mineralo gy that formed from sea water. Elevated Sr is attributable to at least a pa rtially open system diagenetic stabilization of biogenic aragonite. (C) 199 8 Elsevier Science B.V. All rights reserved.