Dolomite microstructures and reaction mechanisms of dolomitization on the triassic Latemar buildup, Dolomites, northern Italy

Abstract

The wavelengths of nanometer-scale structural modulations in calcium-rich dolomite on the Latemar buildup, Dolomites, northern Italy vary as a function of composition, which is controlled by the temperature of dolomitizing fluids, and contact time with these fluids. Dolomites formed at the highest temperatures are the most magnesian and those formed at the lowest temperatures are the most calcian. The wavelengths of structural modulations, which are visible in brightfield and high-resolution transmission electron microscope images, increase with increasing temperature of formation. Long wavelength modulations are associated with more magnesium rich dolomite that formed at high temperatures. Three generations of dolomite, formed under subaerial sedimentary to subsurface hydrothermal conditions, partially replace Triassic Latemar limestones. Early exposure cap dolomite (52 to 57 mole % CaCO3) formed at earth surface temperatures and is host to mottles and diffuse modulations with wavelengths of 2 to 7.5 nm. Subsurface hydrothermal dolomite, formed by platform-scale circulation of hot Triassic seawater, comprises a kilometer-scale mushroom-shaped body of massive replacement dolomite and saddle dolomite cement that crosscuts platform sediments (Wilson et al. 1990; Hardie et al. 1991). Massive replacement dolomite from the stem consists of dolomite breccia and saddle dolomite cement. Saddle dolomite cements (49 to 55 mole % CaCO3) formed at high temperatures are host to sharp modulations with wavelengths of 7.5 to 15 nm and ribbon microstructures (1 to 2.5 nm across). Dolomite breccias (52 to 56 mole % CaCO3) are modulated; modulations wavelengths are 5 to 20 nm. Dolomites from the cap of the dolomite body, formed at lower temperatures than those in the stem, are more calcium rich, (51 to 58 mole % CaCO3), and are host to modulations with wavelengths of 0.5 to 12.5 nm. Incommensurate superstructure reflections have been recognized in SAED patterns of calcium rich dolomite from the Latemar buildup. Incommensurate c-reflections have been observed approximately halfway between the principal reflections in the [110]* and [014]* directions. Superstructure reflections overlap the a- and b-reflections of the host dolomite and extend asymmetrically toward the center of the diffraction pattern. The c-reflections are elongated perpendicular to the structural modulations and are produced by quasiperiodic domains (less than approximately 2.5 nm wide) with wavelengths of 5 to 20 nm. The superstructure phase associated with these extra reflections has smaller reciprocal lattice dimensions than the sublattice, hence larger unit cell dimensions. The superstructure phase is metrically monoclinic and is interpreted to incorporate more calcium than the host dolomite, as suggested by previous workers (cf. Reeder and Wenk 1979; Gunderson and Wenk 1981; Reeder 1981, 1983; Wenk and Zhang 1985; Van Tendeloo et al. 1985; Reeder and Prosky 1986; Reeder et al. 1990; Reksten 1990a; Wenk et al. 1991; Reeder 1992; Ward and Reeder 1992).