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Gonzalez, Luis A.1, Vionette De Choudens1, Jennifer R. Rogers1
(1) University of Kansas, Lawrence, KS

ABSTRACT: Controlled Instantaneous Supersaturation: Experimental Carbonate Precipitation Under Geologically Plausible Scenarios

Over half-a-century of experimental precipitation has failed to provide clear answers to fundamental questions in carbonate geochemistry such as: What controls the precipitation of calcite vs. aragonite? What controls the incorporation of Mg into calcite? Is the Mg partition coefficient for calcite rate dependent? How do temperature, ionic strength, and changes PCO2 affect precipitation rates and cation incorporation into calcite? Can dolomite be precipitated inorganically at low-temperature? The answers to these and many other fundamental questions of carbonate geochemistry are necessary for the proper interpretation of the environmental information, original or diagenetic, encoded in the mineralogy, morphology, and elemental composition of carbonate minerals.
Precipitation of carbonate minerals in natural systems is most often the result of the sudden loss/removal of CO2 from the fluids, resulting in instantaneous supersaturation with respect to carbonate minerals. We have devised an instantaneous supersaturation setup designed for systematic replication of carbonate mineral precipitation in natural systems. Initial experiments have addressed the role of supersaturation on the precipitation of high magnesium calcite (HMC) and aragonite from solutions with Mg/Ca ratios of ~ 1. Experiments have resulted in co-precipitation of aragonite and HMC with ~ 8 mol % MgCO3 , a composition never before attained from fluids with Mg/Ca of less than 5. Importantly, HMC appears to be dominant at higher supersaturations with no crystal elongation detectable.
Ongoing experimentation is investigating precipitation from solutions of Mg/Ca > 1, and extreme supersaturation. Future experimentation will introduce microbes to document difference in precipitation styles resulting from microbial mediation.

 

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.