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

ABSTRACT 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 A to 4.09 A, 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 A. 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 A), 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.

[1]  D. Lowe,et al.  Microstructure of high-temperature (>73 °C) siliceous sinter deposited around hot springs and geysers, Yellowstone National Park: the role of biological and abiological processes in sedimentation , 2003 .

[2]  N. Trewin,et al.  Subaqueous silicification of the contents of small ponds in an Early Devonian hot-spring complex, Rhynie, Scotland , 2003 .

[3]  L. Benning,et al.  Experimental studies on New Zealand hot spring sinters: rates of growth and textural development , 2003 .

[4]  K. Campbell,et al.  Diagenetic transformations (opal-A to quartz) of low- and mid-temperature microbial textures in siliceous hot-spring deposits, Taupo Volcanic Zone, New Zealand , 2003 .

[5]  H. Chafetz,et al.  Depositional Facies and Diagenetic Alteration in a Relict Siliceous Hot-Spring Accumulation: Examples from Yellowstone National Park, U.S.A. , 2003 .

[6]  K. Konhauser,et al.  The effect of cyanobacteria on silica precipitation at neutral pH: implications for bacterial silicification in geothermal hot springs , 2003 .

[7]  K. A. Rodgers,et al.  Opal-A and associated microbes from Wairakei, New Zealand: the first 300 days , 2003, Mineralogical Magazine.

[8]  H. Chafetz,et al.  Anatomy of siliceous hot springs: examples from Yellowstone National Park, Wyoming, USA , 2003 .

[9]  H. Chafetz,et al.  Factors governing subaqueous siliceous sinter precipitation in hot springs: examples from Yellowstone National Park, USA , 2002 .

[10]  K. Campbell,et al.  An unusual modern silica–carbonate sinter from Pavlova spring, Ngatamariki, New Zealand , 2002 .

[11]  K. Campbell,et al.  The mineralogy, texture and significance of silica derived from alteration by steam condensate in three New Zealand geothermal fields , 2002, Clay Minerals.

[12]  Richard A. Kerr.  Reversals Reveal Pitfalls in Spotting Ancient and E.T. Life , 2002, Science.

[13]  K. Campbell,et al.  Sedimentary Facies and Mineralogy of the Late Pleistocene Umukuri Silica Sinter, Taupo Volcanic Zone, New Zealand , 2001 .

[14]  D. G. Adams,et al.  Microbial–silica interactions in Icelandic hot spring sinter: possible analogues for some Precambrian siliceous stromatolites , 2001 .

[15]  K. A. Rodgers,et al.  The age and significance of in-situ sinter at the Te Kopia thermal area, Taupo Volcanic Zone, New Zealand , 2000 .

[16]  K. A. Rodgers,et al.  Routine instrumental procedures to characterise the mineralogy of modern and ancient silica sinters , 2000 .

[17]  K. Campbell,et al.  Mineralogical and textural changes accompanying ageing of silica sinter , 2000 .

[18]  G. Bock,et al.  Evolution of hydrothermal ecosystems on Earth (and Mars , 1998 .

[19]  Deane K. Smith,et al.  Opal, cristobalite, and tridymite: Noncrystallinity versus crystallinity, nomenclature of the silica minerals and bibliography , 1998, Powder Diffraction.

[20]  Deane K. Smith.  Evaluation of the detectability and quantification of respirable crystalline silica by X-ray powder diffraction methods , 1997, Powder Diffraction.

[21]  B. Jones,et al.  Vertical zonation of biota in microstromatolites associated with hot springs, North Island, New Zealand , 1997 .

[22]  J. Bartley.  Actualistic taphonomy of cyanobacteria; implications for the Precambrian fossil record , 1996 .

[23]  N. Hinman,et al.  Seasonal changes in silica deposition in hot spring systems , 1996 .

[24]  B. Jones,et al.  Primary silica oncoids from Orakeikorako Hot Springs, North Island, New Zealand , 1996 .

[25]  K. Konhauser,et al.  DIVERSITY OF IRON AND SILICA PRECIPITATION BY MICROBIAL MATS IN HYDROTHERMAL WATERS, ICELAND : IMPLICATIONS FOR PRECAMBRIAN IRON FORMATIONS , 1996 .

[26]  K. Konhauser,et al.  In situ silicification of an Icelandic hot spring microbial mat: implications for microfossil formation , 1995 .

[27]  H. Graetsch.  Structural characteristics of opaline and microcrystalline silica minerals , 1994 .

[28]  G. Bignall,et al.  Surface hydrothermal alteration and evolution of the Te Kopia Thermal Area, New Zealand , 1994 .

[29]  George A. Parks,et al.  Silica Diagenesis, I. Solubility Controls , 1985 .

[30]  B. Houghton,et al.  Caldera Volcanoes of the Taupo Volcanic Zone, New Zealand , 1984 .

[31]  D. Cole,et al.  Geothermal mineralization. I. The mechanism of formation of the Beowawe, Nevada, Siliceous sinter deposit , 1983 .

[32]  K. Kano.  Ordering of Opal-CT in diagenesis , 1983 .

[33]  C. L. Stein.  Silica Recrystallization in Petrified Wood , 1982 .

[34]  J. Hein,et al.  Diagenesis of late Cenozoic diatomaceous deposits and formation of the bottom simulating reflector in the southern Bering Sea , 1978 .

[35]  J. Oehler.  Origin and distribution of silica lepispheres in porcelanite from the Monterey Formation of California , 1975 .

[36]  J. K. Nakata,et al.  Cristobalitic Stage in the Diagenesis of Diatomaceous Shale , 1974, Science.

[37]  J. B. Jones,et al.  The nature of opal I. nomenclature and constituent phases , 1971 .

[38]  R. Fournier,et al.  Estimation of underground temperatures from the silica content of water from hot springs and wet-steam wells , 1966 .

[39]  D. White,et al.  Silica in hot-spring waters , 1956 .

[40]  W. H. Weed.  On the formation of siliceous sinter by the vegetation of thermal springs , 1889, American Journal of Science.

[41]  Kai S. Anderson,et al.  The Zonation and Structuring of Siliceous Sinter around Hot Springs, Yellowstone National Park, and the Role of Thermophilic Bacteria in Its Deposition , 2001 .

[42]  A. Knoll,et al.  Stromatolites in Precambrian carbonates: evolutionary mileposts or environmental dipsticks? , 1999, Annual review of earth and planetary sciences.

[43]  J. Farmer,et al.  Fossilization processes in siliceous thermal springs: trends in preservation along thermal gradients. , 1996, Ciba Foundation symposium.

[44]  J. Farmer,et al.  Lithofacies and biofacies of mid-Paleozoic thermal spring deposits in the Drummond Basin, Queensland, Australia. , 1996, Palaios.

[45]  Jack D. Farmer,et al.  Biological versus inorganic processes in stromatolite morphogenesis: Observations from mineralizing sedimentary systems , 1994 .

[46]  N. Trewin.  Depositional environment and preservation of biota in the Lower Devonian hot-springs of Rhynie, Aberdeenshire, Scotland , 1993, Earth and Environmental Science Transactions of the Royal Society of Edinburgh.

[47]  J. Apps,et al.  Prediction of Chemical Problems in the Reinjection of Geothermal Brines , 1982 .

[48]  C. Isaacs.  Porosity Reduction During Diagenesis of the Monterey Formation, Santa Barbara Coastal Area, California , 1981 .

[49]  Lesile Glasser.  The chemistry of silica: By Ralph K. Iller. Pp. vii+ 866. Wiley, Chichester. 1979, £39.50 , 1980 .

[50]  Thomas D. Brock,et al.  Chapter 6.2 Microbiology and Morphogenesis of Columnar Stromatolites (Conophyton, Vacerrilla) from Hot Springs in Yellowstone National Park , 1976 .

[51]  James C. Taylor.  Geologic appraisal of the petroleum potential of offshore southern California; the borderland compared to onshore coastal basins , 1976 .

[52]  R. R. Larson,et al.  Diagenesis of Miocene siliceous shales, Temblor Range, California , 1975 .

[53]  M. Ichikuni.  Incorporation of aluminum and iron into siliceous sinters , 1970 .

[54]  C. H. Sandberg,et al.  Rocks, structure, and geologic history of Steamboat Springs thermal area, Washoe County, Nevada , 1964 .