Morphologic and Mineralogic Transitions From Opal-A to Opal-CT in Low-Temperature Siliceous Sinter Diagenesis, Taupo Volcanic Zone, New Zealand

摘要

The opal-A to opal-CT silica phase transformation in New Zealand's siliceous hot-spring deposits (sinter) is accompanied by recurring textural changes at the micron scale. Recognition of these changes is essential to extract paleohydrological, paleoenvironmental, and paleobiological signatures from ancient hydrothermal systems, and to recognize diagenetic overprints upon primary depositional signals. We examined 39 samples of siliceous sinter from the low-temperature (< 35° C) palisade microfacies at Orakei Korako and Te Kopia geothermal fields, Taupo Volcanic Zone, New Zealand, in three different postdepositional environments (least-disturbed, heat-affected, weathered). Friable to indurated samples were collected from modern, presently silicifying microbial mats in the outflow of nearly neutral alkali chloride springs, to sinter 3,500 years old. The progressive mineralogical change from noncrystalline opal-A to paracrystalline opal-CT was traced by X-ray powder diffraction and scanning electron microscopy. One diagenetic sequence with two morphological pathways, spherical and smooth (polymeric and monomeric deposition, respectively), was identified at the micron scale. Both pathways ultimately lead to formation of typical, opal-CT bladed lepispheres. Along the spherical pathway, initial morphological restructuring includes formation of circular holes (< 0.1 to 1.0 µm in diameter) in opal-A spheres (< 3.0 µm in diameter). These changes correlate with a shift in the maximum intensity of the opal-A X-ray scattering broadband, from 4.0 Å to 4.09 Å, the characteristic position for opal-CT. An early transitional diagenetic stage is indicated in both sphere and smooth silica pathways by development of hexagonal platelets and a composite opal-A/-CT scattering broadband that is sharp-peaked in comparison, and centered at 4.09 Å. A still sharper-peaked, composite opal-CT/-A X-ray diffraction band follows, which has developed an incipient tridymite shoulder; it is found in samples that display both hexagonal platelets and incipient "fuzzy" opal-CT lepispheres. At this late transitional stage, patchy replacement textures are visible at the macroscale. Only with the appearance of numerous, well-developed, opal-CT bladed lepispheres is a typical, sharp-peaked (4.09 Å), opal-CT X-ray trace evident, with a well-defined tridymite shoulder. This diagenetic process can produce either porous or vitreous fabrics, depending on the degree to which infill of silica has reduced porosity and increased density of the deposit. This study provides a textural-mineralogical context for understanding silicification of microbial communities in geothermal settings and their subsequent diagenetic modifications. These modifications continue long after geothermal activity has ceased, with little or no burial, and occur in horizons or localized patches of a given deposit. Moreover, postdepositional conditions, such as heat overprinting or weathering, influence the diagenesis of siliceous sinter by increasing its maturation rate.