Type I Water-Serpentinized Harzburgite Interactions: Implications on Geologic Carbon Cycle

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Presenter

Dr. Pouyan Asem
Researcher
The University of Texas Permian Basin

Description

The interaction of Type I, Mg²⁺-HCO₃^– waters with the fractured, partially serpentinized mantle peridotites is responsible for the evolution of their bulk mineralogy, chemical composition, and microstructural features. Type I waters are enriched in Mg and HCO₃^–, and therefore it is of broad interest to understand which secondary minerals – phyllosilicates or carbonates – dominate the alteration of the intact matrix of a partially serpentinized mantle peridotite during the present-day processes within the shallow aquifers where Type I waters are present. To investigate the fundamental processes within the CaO-FeO-MgO-SiO₂-CO₂-H₂O system, an intact serpentinized harzburgite with 38.5 wt% lizardite from the Semail ophiolite, Oman was reacted with synthetic Type I water at 20 °C and 2 MPa under closed conditions. Over a 2-year period, the specimen volumetric deformation was measured and water samples were periodically obtained. The analysis of the water composition showed (i) slight and persistent supersaturation with respect to carbonate minerals (e.g. calcite, dolomite); (ii) no major changes in the carbonate alkalinity (HCO₃^–, CO₃^2–); (iii) buffering of the rock-water system by the stability of phyllosilicate minerals (e.g. chrysotile, lizardite); (iv) a net Mg consumption; and (v) influx of Si by dissolution was faster than its withdrawal by precipitation. The analysis of volumetric strain indicated a net volume increase for the specimen. The analysis of the reaction products indicated: (i) the percentage of phyllosilicate minerals (lizardite, chrysotile) increased; (ii) quartz polymorphs precipitated; and (iii) the minute amount of carbonate minerals (pyroaurite, hydrotalcite) dissolved with no net carbonate mineralization. The evolution of the bulk mineralogy was consistent with (i) the thermodynamics of solid solutions and their predictions, and (ii) the observed volume increase associated with serpentinization reactions and microcrack filling mechanisms by formation of silica polymorphs and serpentine minerals. The formation of carbonate minerals did not dominate the alteration of the intact specimen, which is consistent with the fact that they usually form in conductive fractures potentially by mixing with hyperalkaline, Type II waters and not in intact matrix during natural processes.