combining in situ isotopic trace element and textural analyses of quartz from four magmatic hydrothermal ore deposits this study couples in situ 16o 17o and 18o isotope and in situ trace element analyses to investigate and characterize the geochemical and textural complexity of magmatic hydrothermal quartz crystals euhedral quartz crystals contemporaneous with mineralization were obtained from four magmatic hydrothermal ore deposits el indio au ag cu deposit summitville au ag cu deposit north parkes cu au deposit and kingsgate quartz mo bi w deposit the internal features of the crystals were imaged using cathodoluminescence and qualitative electron microprobe maps quantitative isotopic data were collected in situ using 157 nm laser ablation inductively coupled plasma mass spectrometry for 40 trace elements in quartz and sensitive high resolution ion microprobe for 3 isotopes in quartz imaging revealed fine oscillatory zoning sector zoning complex macromosaic textures and hidden xenocrystic cores in situ oxygen isotope analyses revealed a 18o range of up to 12 4 0 3 in a single crystal the largest isotopic range ever ascribed to oscillatory zonation in quartz some of these crystals contain a heavier 18o signature than expected by existing models while sector zoned crystals exhibited strong trace element variations between faces no evidence for anisotropic isotope fractionation was found we found 1 isotopic heterogeneity in hydrothermal quartz crystals is common and precludes provenance analysis e g d 18o using bulk analytical techniques 2 the trace element signature of quartz is not an effective pathfinder toward noble metal mineralization and 3 in three of the four samples both textural and isotopic data indicate non equilibrium deposition of quartz

baking black opal in the desert sun the importance of silica in desert varnish desert varnish a widespread black manganese rich rock coating contains labile organic compounds but a mechanism for its formation and for their preservation remains unproven using raman spectroscopy x ray diffraction and scanning transmission electron microscopy we analyzed varnish and found amorphous hydrated silica opal and the silica mineral moganite similar to findings we have reported from siliceous hot spring deposits we suggest that the slow dissolution of silica from anhydrous and hydrous minerals and its subsequent gelling condensation and hardening provides a simple explanation of a formation mechanism for desert varnish and silica glazes and the incorporation of organic material from local environments these chemical signatures sequestered in silica provide valuable information about terrestrial and extraterrestrial paleoenvironments

physico chemical and environmental controls on siliceous sinter formation at the high altitude el tatio geothermal field chile abstract el tatio geothermal field is located 4270 m above sea level in the altiplano northern chile siliceous sinter deposits from el tatio were studied to understand the influence of water chemistry and the extreme climatic conditions on their textures and mineralogy the results of this study show that the mineralogy of el tatio sinters include of opal a and accessory minerals such as halite gypsum and cahnite ca 4 b 2 as 2 o 12 4h 2 o which precipitate by full evaporation of high arsenic boron and calcium thermal waters el tatio sinters show a high degree of structural disorder probably linked to cation incorporation into the silica structure and or the occurrence of micro to nano scale accessory minerals the high content of cations in the thermal waters is strongly tied to relatively high silica precipitation rates considering silica concentration in water 147 285 mg l sio 2 precipitation rate reach 2 5 kg m 2 per year based on in situ precipitation experiments the particular environmental conditions of this high altitude geothermal area that produce high water cooling rate and high evaporation rate may also be responsible for the fast silica precipitation low environmental temperatures create freezing related sinter textures i e silica platelets and micro columns ridges silicified microbial filaments are also characteristic of el tatio sinters and they are indicative of water temperature and hydrodynamic conditions at the moment of sinter formation however sinter textural interpretation in a high altitude andean context must be done carefully as specific relationships between microbial and hydrodynamic textures are found

factors governing subaqueous siliceous sinter precipitation in hot springs examples from yellowstone national park usa siliceous sinter precipitation within hot spring systems has been attributed to a variety of mechanisms evaporative concentration cooling changes in ph and cation effects repetitive in situ t ph alkalinity etc and laboratory major minor and trace elemental stable isotopic analyses of the waters plus observations of silica precipitation on natural e g twigs pine cones as well as artificial substrates glass slides and copper plates in the waters substantiate that subaqueous precipitation is occurring throughout the vent to distal end of flow in both cistern spring norris geyser basin and deerbone spring lower geyser basin yellowstone national park wyoming usa quartz and sodium potassium geothermometers indicate that cistern spring is fed by a subsurface reservoir that is between 232 and 272 c calculated reservoir temperatures are significantly lower at deerbone spring 182 197 c based on a suite of measured and theoretical saturation indices downflow changes in the system resulting from evaporative concentration e g cl increases 10 changes in ph e g 5 6 7 1 and cation effects al and fe are of negligible importance in the subaqueous precipitation of hot spring opal a similarly at the macroenvironmental scale potential biotic effects on opal a precipitation appear to be minimal modelling of the two active siliceous sinter precipitating systems indicates that cooling e g 80 17 c is the predominant process governing subaqueous mineral precipitation

late pleistocene and holocene large magnitude earthquakes along himalayan frontal thrust in the central seismic gap in nw himalaya kala amb india abstract the himalayan frontal thrust hft forms the southernmost active tectonic mountain front of the himalaya to understand the ongoing tectonics further paleoseismological study has been carried out in the vicinity of the hft system along the himalayan front near kala amb india the trench excavation survey conducted across an explicit surface exposure of the hft exhibits two distinct faults considered to be associated with the reactivation of the hft where the middle siwalik rocks late miocene have repeatedly thrust over the late pleistocene and holocene sediments presence of large sized coseismically induced sand injection feature and its disposition recognized in the trench also suggest occurrence of large magnitude earthquakes in this region an uplifted and upwarped strath terrace 3 to 5 m thick alluvium resting over the 15 m high middle siwaliks abruptly truncated by the hft indicates its latest activity optically stimulated luminescence dating techniques were employed to constrain the chronology of events the long term slip rate of the abandoned terraces due to the activity of the hft is estimated to be 3 4 mm yr or greater since late holocene the paleoseismological investigations have provided unambiguous evidences of at least two large magnitude earthquakes occurred in this region where an earthquake with 12 m or larger surface displacement and magnitude 7 5 or greater hit this region in the period between 29 3 ka and 17 ka in the late pleistocene and another great earthquake recurred with 20 22 m or more surface displacement and magnitude of 7 7 or greater between 5 8 ka and 2 ka in the holocene the present study is the first time report of multiple large magnitude paleoearthquakes in the northwestern part of the frontal himalaya during late pleistocene and holocene the repeated reactivation of hft substantiates high seismic potential of the frontal himalaya and calls for more extensive study of paleoearthquakes of this vastly populous mountainous region

evolution of a dynamic paleo hydrothermal system at mangatete taupo volcanic zone new zealand abstract recent quarrying and active faulting at mangatete taupo volcanic zone tvz new zealand illuminate a rare spatial and temporal window on a dynamic late quaternary geothermal system detailed geological mapping stratigraphic logging ams 14 c dating and textural and mineralogical analyses were used to construct a complex history of hydrothermal volcanological and tectonic activity from 36 to 2 ka extinct surface hydrothermal manifestations occur over a 2 km 2 area and include in situ siliceous sinters distributed on normal fault terraces an inferred hydrothermal eruption breccia heb containing acid etched sinter blocks another probable heb that was bathed in silicifying thermal fluids and sinter clasts that were entrained in a debris flow associated with a volcanic ash event preserved sinter textures typical of near neutral ph alkali chloride spring discharge channels aprons terraces and affiliated marshes comprise plant rich palisade tufted bubble mat and domal stromatolitic fabrics in addition a packed fragmental sinter facies is shown herein to constitute silicified microbial mats that were broken transported and deposited as point bar deposits in thermal spring fed streams moreover four unusual siliceous sinter fabrics vuggy globular spongy scalloped and arcuate wavy layered are interpreted to have formed from local acid sulfate chloride thermal springs possibly associated with paleo fumaroles the reconstructed history of paleo hydrothermal activity indicates that the oldest sinters 36 ka at mangatete developed in alkali chloride hot springs but then underwent post depositional alterion overprinting by acid sulfate steam condensate and were dismembered possibly by a hydrothermal eruption low ph hot spring discharges forming the unusual inferred acid sinter fabrics were localized in the same area a shift in paleo hydrology is evidenced by unaltered alkali chloride sinters dated between 22 and 3 ka a cluster of sinter dates between 22 7 and 20 7 ka denote thermal fluid emissions around the time of the okareka eruption 22 1 ka an 11 kyr gap in sinter ages followed which overlaps with a time of relative quiescence in local fault movement in between the rotorua and rotoma eruptions as determined from previous paleo seismicity studies on the whirinaki fault in the mangatete area at 9 4 ka hot spring activity in thermally fed streams slightly post dated the rotoma eruption 9 5 ka the locations and ages of the youngest post 9 4 ka alkali chloride sinters and silicified paleosols in relation to whirinaki fault movements indicate an overall westward migration of siliceous spring discharge dated charcoal 1 8 kyr in ash at the base of a locally extensive volcaniclastic debris flow containing sinter blocks of a range of ages and mineralogical maturity suggests its affiliation with the taupo eruption mangatete sinters occur along sw ne striking faults parallel to the regional trend of the tvz which dissect the topography into stepped terraces inferred basement cross faults in the study area i e structural lineaments orthogonal to this trend may have served as fluid pathways at depth initiating and maintaining hydrothermal activity at mangatete for more than 30 kyr within the larger ngakuru graben 150 km 2 the ages and distributions of sinters and hydrothermal eruption breccias including those at mangatete show that significant hydrothermal activity occurred for more than 60 kyr in an area lacking such activity today thus mangatete retains a rare stratigraphic record within the tvz sinter archive of changing geothermal fluids resulting from shifts in paleo phreatic water levels moreover although limited the presented age data imply a relationship among seismic activity volcanic ash fall events and the occurrence of permeable pathways for upward migration of thermal fluids finally whereas currently active tvz geothermal fields appear to be stable over tens of thousands of years or more the fossil hydrothermal systems in the wider mangatete area demonstrate the possibility of much greater temporal variability in the lateral expression of local surface manifestations of thermal fluid discharge through time

impact of three common post depositional environmental settings on siliceous sinter diagenesis an eight year experiment abstract samples of newly formed opal a filamentous siliceous sinter were placed inside three contrasting environments to assess their effect on sinter diagenesis 1 an actively discharging fumarole 2 buried in steaming vegetation 3 buried in soil in a non thermal area each setting was chosen to represent common sinter post depositional environments the sinter remained in these settings for eight years during which time samples were taken from the sinter and examined using x ray powder diffraction xrpd and scanning electron microscopy sem these techniques enabled changes in the silica phase mineralogy and morphology to be tracked over time while all three sinters remained as opal a during the eight years minor but significant silica phase changes did take place and were reflected by the increase in maximum intensity value and shift in apex position on the xrpd trace silica phase transitions were greatest in the sinter buried in the steaming vegetation followed by the sinter in the fumarole the sinter buried in the non thermal garden revealed almost no change throughout the duration of the experiment in all three settings the filamentous nature of the sinter was preserved botryoidal clusters and new generations of opal a spheres were observed in samples from all three sites at various time periods in all environments silicified filaments formed smooth non porous ridges separated by a more open arrangement of silicified filaments major minor and no dissolution textures were observed in the sinter buried in the steaming vegetation non thermal soil and the fumarole settings respectively rows on aligned opal a spheres were only present in the fumarole sinter at 33 weeks tracking sinter diagenesis in three commonly found post sinter formation environments contributes to our understanding of the preservation potential of siliceous sinters and the influence steam meteoric water and organics have on sinter diagenesis

tracing the continental diagenetic loop of the opal a to opal ct transformation with x ray diffraction abstract opaline silica forms in a variety of continental deposits on the earth s surface the maturation from opaline silica to quartz is traditionally traced with x ray diffraction that suggests a gradual phase transformation and structural ordering however the identification and categorization of the gradual opal a to opal ct transformation remains challenging due to potential mineral impurities and the transient structure of opaline silica here we use x ray powder diffraction to identify peak parameters that characterize the maturation from opal a to opal ct in continental settings we analyzed inclusion free opals from non sinter continental deposits and the products of opal mixing and heating experiments twenty five samples were analyzed from australia ethiopia slovakia usa mexico and madagascar and represent opal from a range of host rocks and formation conditions xrd analysis reveals that opal peak parameters constitute a diagenetic loop and that the structural order is decoupled from the low tridymite cristobalite ratio at large d values systematic changes of the width and position of a subsidiary peak at ca 2 5 a accurately reflect the low tridymite cristobalite ratio and degree of ordering our results suggest that the identification of individual stages in the opal a to opal ct transformation sequence requires the application of multiple peak parameters that complement or replace existing maturation indicators this information is essential for a precise silica identification and genetic reconstruction especially of open continental systems with internal re distribution and external input of aqueous silica

silica recrystallization in petrified wood abstract the mineralogy of thirty three samples of silicified wood was examined by x ray diffraction and each sample was found to consist of one of the following opal a opal ct or quartz the correlation between ordering of the silica phases and sample age suggests that the opal a opal ct quartz conversion sequence occurs in much the same manner and at approximately the same rates as in biogenic siliceous marine sediments in addition a poorly crystalline clay phase illite was identified in most of these samples groundwater leaching of volcanogenic sediments commonly thought to be the silica source may also provide aluminum and other cations that are deposited simultaneously in wood tissue consequently silica recrystallization may be accompanied by authigenic clay formation

acceleration of sinter diagenesis in an active fumarole taupo volcanic zone new zealand siliceous sinters form where nearly neutral ph alkali chloride waters discharge at the surface 100 c they may preserve biogenic and abiogenic material and therefore archive paleoenvironmental settings freshly precipitated sinters undergo diagenesis through a five step series of silica mineral phase changes from opal a to opal a ct to opal ct to opal c to quartz transformation rates vary among sinters because postde positional conditions can accelerate or retard diagenesis meanwhile preserving or destroy ing biosignals we monitored alteration and diagenesis of newly precipitated filamentous microbe rich sinter during a two year field experiment where sinter was suspended inside a fumarole at orakei korako taupo volcanic zone new zealand patchy and complex diagenesis resulted from changes in environmental conditions including variations in tem perature ph and the intermittent deposition of sulfur throughout the experiment opal a was dissolved by acidic steam condensate and reprecipitated locally quartz crystals grew on the sinter surface within 21 weeks previous reports of transformation rates from opal to quartz are on the order of thousands of years in duration thus our results show that fumarolic overprinting accelerates diagenesis microbial preservation was not favor able because primary filamentous fabrics were obscured by deposition of opal a micro spheres smooth silica infill and sulfur if ancient hydrothermal systems were among the likely places where early life flourished it is necessary to distinguish between depositional features and those inherited during diagenesis this near real time experiment enabled observations on environmental controls of diagenetic change in silica minerals and illus trated the variability of conditions that can occur in nature during this complex process

silica diagenesis i solubility controls abstract the diagenetic sequence of silica is examined in terms of several thermodynamic and kinetic concepts the relationship between solubility and surface area or particle size is sufficient to explain simple opal a opal ct quartz transformations suggesting that the role of temperature and other physical parameters may not be as critical as previously thought complexation and adsorption in an impure system when added to the model allow for possibilities of opal a transformation directly to quartz of clay and zeolite formation from an opaline silica source and of variations in silica transformation rates due to presence of impurities in the system

thermophiles early biosphere evolution and the origin of life on earth implications for the exobiological exploration of mars mars holds great interest for exobiology and presently stands center stage in our plans to explore the inner solar system for signs of past or present life with the spectacular success of the pathfinder mission golombek 1997 and upcoming mapping efforts by the mars global surveyor orbiter albee et al 1998 it is an especially appropriate time to revisit some important issues in mars exobiology the papers making up this journal of geophysical research planets special issue on mars were the outgrowth of an american geophysical union special session entitled exploring for a martian biosphere which was presented at the fall meeting in san francisco in 1995 farmer 1995a together these papers provide a context for evaluating some fundamental issues in the exobiological exploration of mars topics covered fall into the following broad categories 1 environments and conditions required for the origin of life hydrothermal environments as sites for prebiotic chemical synthesis shock and schulte this issue 2 early martian environments as a context for the evolution and diversification of life on mars early impact hazards sleep and zahnle this issue early volatile and climate history of mars haberle this issue and ice covered lakes as habitats for early life doran et al this issue 3 exploring for extant martian life results of the viking missions klein this issue environmental extremes for martian life clark this issue and planetary protection and mars sample return devincenzi et al this issue and 4 exploration strategies phosphate minerals as targets for chemofossils mojzsis and arrhenius this issue and the role of robotic surface missions stoker this issue

exploring for a record of ancient martian life the immediate task facing exopaleontology is to define a strategy to explore mars for a fossil record during the decade long exploration program that lies ahead consideration of the quality of paleontological information preserved under different geological conditions is important if we are to develop a strategy with broad applicability the preservation of microbial fossils is strongly influenced by the physical chemical and biological factors of the environment which acting together determine the types of information that will be captured and retained in the rock record in detrital sedimentary systems preservation is favored by rapid burial in fine grained clay rich sediments in chemical sedimentary systems preservation is enhanced by rapid entombment in fine grained chemical precipitates for long term preservation host rocks must be composed of stable minerals that are resistant to chemical weathering and that form an impermeable matrix and closed chemical system to protect biosignatures from alteration during subsequent diagenesis or metamorphism in this context host rocks composed of highly ordered chemically stable mineral phases like silica e g cherts or phosphate e g phosphorites are especially favored such lithologies tend to have very long crustal residence times and along with carbonates and shales are the most common host rocks for the precambrian microfossil record on earth although we make the defensible assumption that mars was more like the earth early in its history clearly the geological and historical differences between the two planets are many such differences must be carefully considered when adapting an earth based strategy to mars

silica hydrate preserved with δ18o rich quartz in high temperature hydrothermal quartz in the high sulfidation copper gold deposit at el indio chile quartz microcrystals from the el indio au ag cu deposit chile preserve a rare glimpse into the high temperature evolution of silica here we show for the first time that aggregates of euhedral quartz microcrystals preserve cryptocrystalline cores that contain silica hydrates opal and moganite we propose that these phases are metastable remnants of progressive dehydration from a precursor silica hydrate phase evidence for sequential dehydration to from silica hydrate to quartz silica hydrate opal moganite quartz is provided by shrimp 18o microanalytical data that show oscillatory isotopic zoning from 3 6 to 16 2 18o 0 5 coupled with k and al variations we estimate that the precursor silica hydrate deposited between 480 680 c and contained 32 63 wt h2o silica hydrate is metastable with respect to quartz and forms during rapid deposition of silica at high silica supersaturation a consequence of rapid expansion of magmatic fluid into the fracture array that hosts the el indio copper gold deposit modern understanding of ore forming fluids in hydrothermal ore deposits is largely underpinned by the assumption that quartz and its included fluids faithfully record depositional conditions the discovery of silica hydrate affects the paragenetic and geochemical interpretation of quartz and included fluids quartz matured from silica hydrate would record pseudo primary fluid inclusions such that homogenization temperatures record retrograde rather than depositional conditions while 18o data may bias fluid provenance interpretation within sub volcanic systems

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 m in diameter in opal a spheres 3 0 m 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

effect of diagenetic phase transformation on the silicon isotope composition of opaline sinter deposits of geysir iceland detectible 30si variations in present day chemical silica deposits have stimulated the application of silicon isotopes to infer environmental conditions from ancient equivalents interpretations of 30si signatures remain problematic in view of potential post depositional changes of which magnitudes and underlying mechanisms are largely unknown a critical issue in the interpretation of isotope data from cherts concerns the extent to which early diagenetic processes modify original 30si signatures here we report 30si variations in opal a opal a ct and opal ct from fossil sinter deposits in an active discharge apron in the geysir geothermal field iceland opal a samples show an average 30si of 0 7 0 2 while opal ct samples are isotopically lighter with an average 30si of 2 0 0 4 implying a sizable shift of approximately 1 3 between the different phases this shift can be explained by repetitive dissolution re precipitation processes diffusive transport or temperature differences during phase transitions on average the fossil opal a tends to be less negative in 30si than amorphous silica that recently precipitated from the hydrothermal water the difference can be attributed to primary variability in isotopic fractionation that accompanies precipitation out of spring water at the surface or to a post depositional release of surface 28si at the onset of diagenetic formation our results corroborate the perception that original silicon isotope signatures of silica acquired during chemical precipitation from a saturated solution may not be preserved in the geological record and that post depositional changes must be taken into account when interpreting data from ancient chert deposits

secondary minerals associated with lassen fumaroles and hot springs implications for martian hydrothermal deposits the active hot springs fumaroles and mud pots of the southwestern lassen hydrothermal system include various alteration environments which produce a range of hydrothermal mineral assemblages analysis of water mineral precipitates altered sediment and rock samples collected at and near these features at sulphur works bumpass hell little hot springs valley and growler and morgan hot springs reveals conditions ranging from 100 c acid sulfate fumaroles e g sulphur works and bumpass hell to near neutral hot springs e g growler and morgan and includes both oxidizing and reducing conditions resulting hydrothermal minerals include a wide variety of sulfates dominated by al sulfates but also including fe 2 fe 3 ca mg and mixed cation sulfates sulfides pyrite and marcasite elemental sulfur and smectite and kaolinite clays most altered samples contain at least one silica phase most commonly quartz but also including cristobalite tridymite and or amorphous silica quartz and other silica phases are not as abundant in the less altered rock samples thus their abundance in some hydrothermally altered sediment samples suggests a detrital origin or formation by hydrothermal alteration either modern or pleistocene this requires a high degree of diagenetic or epigenetic maturation these results support a previously identified model that the lassen hydrothermal system involves the de coupling of a vapor phase which becomes acidic as it oxidizes near the surface producing acid sulfate fumaroles at higher elevations at sulphur works and bumpass hell from the residual near neutral thermal waters that emerge as hot springs at lower elevations growler and morgan because both acid sulfate fumarole and near neutral sinter producing hot springs have been invoked to explain the silica rich deposits observed by the mars exploration rover spirit near home plate in the columbia hills on mars lassen can serve as a useful terrestrial analog for comparison

upper jurassic travertine at el macanudo argentine patagonia a fossil geothermal field modified by hydrothermal silicification and acid overprinting the deseado massif hosts numerous late jurassic 150 ma fossil geothermal systems related to an extensive volcanic event developed in a diffuse extensional back arc setting detailed mapping petrography and mineralogical observations of el macanudo outcrops verify that it represents a hot spring related travertine partially replaced by silica and delineated by six sedimentary facies these are large concentric cones f1 laminated vertical columnar structures f2 porous layers f3 shrubby and irregular lamination f4 low amplitude wavy bedding f5 and mounds and breccias f6 the macanudo norte outcrop rocks constitute a silica replaced travertine sequence with development of large conical stromatolites in a deep pool or geothermally influenced shallow lacustrine environment surrounded by a subaerial travertine apron terrace whereas the macanudo sur outcrop is a subaerial travertine mound sequence structurally controlled vent areas occur in both northern f1 and southern f6 outcrops mainly located along regional nne and ene trending faults the other sedimentary units display a concentric distribution of travertine facies with respect to the interpreted vent areas the el macanudo palaeo hot spring deposit is situated in an eroded jurassic volcanic centre and records a complex evolutionary fluid history the sediments archived three different jurassic events when large and long lasting hydrothermal systems were active across the region this relative temporal sequence was formed by 1 travertine precipitation 2 development of a silica cap where early silicification was responsible for exceptional preservation of some stromatolitic fabrics and 3 acid alteration recorded by dissolution textures and clay formation and caused by a palaeo phreatic water level drop

sedimentary facies and mineralogy of the late pleistocene umukuri silica sinter taupo volcanic zone new zealand abstract the umukuri silica sinter is a large late pleistocene hot spring deposit exposed along the eastern upthrown block of the umukuri fault one kilometer southwest of the active orakei korako geothermal area taupo volcanic zone new zealand uplift along the fault has frozen sinter maturation to produce a characteristic silica phase stratigraphy revealed by x ray diffraction paracrystalline opal ct dominates upper layers opal c occurs throughout middle and lower horizons and recrystallized fabrics of microcrystalline quartz constitute lowermost exposed layers original and secondary matrix fabrics in the sinter include fine grained porous friable dense vitreous and massive mottled diffusely layered original fabrics combine with silicified plant matter peloids pisoids sinter fragments detrital grains diatoms ostracodes and filamentous to tufted microbial remains to form nine depositional microfacies thinly laminated plant rich and palisade types dominate sinter breccia and wavy laminated varieties also are common whereas curved laminae with lenticular voids bubble mats clotted peloidal and pisolitic microfacies are minor umukuri microfacies represent silicification in mid to low temperature waters on sinter apron terrace and slope areas and in distal marshy settings closely spaced lateral and vertical intercalation of various microfacies in outcrop implies changing local flow and temperature conditions no facies typical of high temperature proximal vent areas have been identified comparison with modern thermal spring analogs suggests that original umukuri sinter fabrics recorded varying degrees of polymerization vs monomerization of juvenile opal a mineralogical and textural modification of matrix fabrics reflects a microscale incremental continuum following granular or fibrous habits along solution precipitation pathways late stage quartz rims and infills pores throughout the sinter

quantifying seismic vulnerability dynamical shear strain and liquefaction of the quaternary deposits in the doon valley near the main boundary thrust in the northwest himalaya india abstract erosion and deposition are two conjugate surface dynamical processes that affect differently to the soft soil site characteristics of quaternary deposits in the doon valley and at its northern topographic transition zone near the main boundary thrust mbt during an impending large magnitude earthquake what would be the seismic vulnerability and liquefaction susceptibility of the soft soil at both the regions we quantified seismic vulnerability index effective dynamical shear strain and the expected ground deformation in the valley soft soil through h v site characteristics using seismic ambient noise data at the in accessible topographic transition zone near the mbt site effects were studied using moderate and light magnitude earthquakes recorded by three broad band seismographs bbs results show unique dual peak mean resonant frequencies 1 11 and 1 38 hz and amplification factors 1 69 and 1 51 at the topographic transition zone which highlights a bimodal distribution of site effects where the top soil formation is influenced by erosion and deposition due to topographic slope instability apart from fluvial re working here relatively compact oldest doon gravel is present in the foothill terraces with the re worked fluvial deposition of the youngest doon gravel however in the valley and its terraces the soft soil formed under fluvial environ is relatively thick 10 to 2 and mean resonant frequency of 1 1 hz the spot resonant frequencies increases from 1 hz to more than 6 hz from the hft region to the mbt while the amplification factor decreases from 14 to 1 in general the valley soft soil is susceptible for liquefaction because the estimated dynamical shear strain is 15 25 millistrains under medium 2 2 m s 2 225 years and long 4 6 m s 2 2500 years term strong motion conditions which is much higher than the soil plasticity limit of ten millistrains that lead to liquefaction dynamical shear strain distribution with depth at eight boreholes in the valley indicate 20 m is the liquefaction potential depth where the pore pressure induced peak shear strain exceeds the threshold shear strain of 0 01 during the passage of seismic waves isolated locations in the valley like that at dharmawala h1 where all site parameters are very high and would be hazardously vulnerable to liquefaction however near the mbt the liquefaction possibility of soft soil is less but would experience ground deformation of more than 50 cm

time temperature burial significance of devonian anthracite implies former great 6 5 km depth of burial of catskill mountains new york specimens of coalified plant debris in tully correlative strata of the gilboa formation uppermost middle devonian within the eastern catskill mountains of new york state have been converted to anthracite having a vitrinite reflectance of 2 5 this implies a level of organic metamorphism lom of 16 the specimens are about 350 m y old if 200 m y is taken as the duration of the time of exposure to the maximum geothermal temperature then the lom of 16 and other thermal indicators imply a maximum temperature of 190 sup 0 c using a geothermal gradient of 26 sup 0 c km sup 1 17 sup 0 f 1 000 ft sup 1 a former depth of burial of 6 5 km is implied such former deep burial is not usually inferred for the catskills but it is consistent with the idea that the thick about 6 4 km or 21 000 ft carboniferous strata of northeastern pennsylvania formerly extended northeast far enough to bury the catskills the lack of metamorphism of the paleozoic strata lying about 4 5 km beneath the tully correlative rocks and exposed in the adjacent hudson valley places low limits on the former geothermal gradient this supports the concept of great depth of former burial of the catskills for example 6 5 km of former burialmore and a geothermal gradient of 26 sup 0 c km sup 1 imply a temperature of 307 sup 0 c for the base of the paleozoic by contrast only 1 km of former burial requires a geothermal gradient of 170 sup 0 c km sup 1 which would have subjected the base of the paleozoic to a temperature of 955 sup 0 gamma which is far higher than the 600 to 650 sup 0 c recently inferred for the acadian age metamorphism of the taconic allochthon in southwestern massachusetts and adjoining areas less

source and fate of inorganic solutes in the gibbon river yellowstone national park wyoming usa i low flow discharge and major solute chemistry abstract the gibbon river in yellowstone national park ynp is an important natural resource and habitat for fisheries and wildlife however the gibbon river differs from most other mountain rivers because its chemistry is affected by several geothermal sources including norris geyser basin chocolate pots gibbon geyser basin beryl spring and terrace spring norris geyser basin is one of the most dynamic geothermal areas in ynp and the water discharging from norris is much more acidic ph 3 than other geothermal basins in the upper madison drainage gibbon and firehole rivers water samples and discharge data were obtained from the gibbon river and its major tributaries near norris geyser basin under the low flow conditions of september 2006 surface inflows from norris geyser basin were sampled to identify point sources and to quantify solute loading to the gibbon river the source and fate of the major solutes ca mg na k sio 2 cl f hco 3 so 4 no 3 and nh 4 in the gibbon river were determined in this study and these results may provide an important link in understanding the health of the ecosystem and the behavior of many trace solutes norris geyser basin is the primary source of na k cl so 4 and n loads 35 58 in the gibbon river the largest source of hco 3 and f is in the lower gibbon river reach most of the ca and mg originate in the gibbon river upstream from norris geyser basin all the major solutes behave conservatively except for nh 4 which decreased substantially downstream from gibbon geyser basin and sio 2 small amounts of which precipitated on mixing of thermal drainage with the river as much as 9 14 of the river discharge at the gage is from thermal flows during this period

geochemistry of metals and metalloids in siliceous sinter deposits implications for elemental partitioning into silica phases abstract sinter deposits are formed by precipitation of silica from hydrothermal fluids that have reached the surface environment they are commonly found around hot springs and represent surface expressions of underlying geothermal systems and or low sulfidation epithermal gold silver hydrothermal deposits several studies have reported ppm to weight percent concentrations of metals e g au ag cu and metalloids e g as sb b in sinters capping geothermal systems and epithermal gold silver deposits however the relation between the maturity of the siliceous sinter and its metal enrichment remains unknown here we use geochemical and mineralogical data that link the silica crystallinity degree with trace metal and metalloid contents in sinter in this paper we provide in situ trace element data in metal rich silica sinter samples from the puchuldiza geothermal field in the altiplano of northern chile that record the complete diagenetic sequence from non crystalline opal a to microcrystalline quartz combined sem xrd and la icp ms data show that the concentration of metals and metalloids in sinters from puchuldiza display a strong correspondence with silica crystallinity while arsenic and boron are predominantly enriched in the more amorphous silica phases opal a ct gold and silver show higher concentrations in the more crystalline phases opal c quartz silica structural morphological and geochemical transformations from its initial precipitation to its final maturation after diagenesis are responsible for this differential enrichment during the initial stages gold and silver are incorporated into silica microspheres as cationic species and or metal nanoparticles or colloids while arsenic and boron incorporation is controlled by as bearing accessory minerals and fe oxyhydroxides as diagenesis progresses and the crystallinity of silica increases diffusion driven processes such as ostwald ripening may progressively enrich gold and silver in the sinter while metalloids are depleted owing to the low retention of arsenic by silica these findings indicate that the diagenetic transitions of silica defined by significant structural changes that involve generation of surface defects and the creation of reactive sites may play an important role in elemental uptake by silica in near surface environments

dating silica sinter geyserite a cautionary tale abstract we describe a new effort to date hydrothermal silica sinter deposits geyserite from the upper geyser basin of yellowstone national park using 14c of co deposited organic matter u series and cosmogenic 10be methods a majority of the samples were collected from stratigraphic sections mainly at riverside giant and castle geysers ages obtained from 41 14c analyses range from modern to 12 1 cal ka bp nearly all the 14c ages show inconsistencies with their stratigraphic positions and several replicate 14c analyses from the same sample result in significantly different ages the 13c values of the organic material in the sinter range from 26 6 to 12 7 the more enriched values are attributed to microbial fixation of dissolved inorganic carbon dic which has heavier 13c values and is 14c depleted relative to atmospheric co2 leading to an apparent older age u series analyses on 4 samples yielded ages between 2 2 and 7 4 ka large 230th u age uncertainties in the sinter due to low uranium concentrations along with elevated 232th and associated initial 230th make these ages imprecise for use on holocene deposits a single cosmogenic 10be exposure age of 596 18 ka is considerably older than the age of underlying rhyolite and is thus unreliable this apparent old age results from contamination by meteoric 10be trapped in the opal that overprints the very small amount of cosmogenic 10be by presenting the problems we encountered and discussing their probable cause this paper highlights the difficulty in obtaining reliable high precision geochronological data necessary to use sinter deposits as paleoenvironmental and paleo hydrothermal archives

fluid mineral equilibria in a hydrothermal system roosevelt hot springs utah the availability of fluids and drill cuttings from the active hydrothermal system at roosevelt hot springs allows a quantitative comparison between the observed and predicted alteration mineralogy calculated from fluid mineral equilibria relationships comparison of all wells and springs in the thermal area indicates a common reservoir source and geothermometer calculations predict its temperature to be higher 288 c 10 than the maximum measured temperature of 268 c the composition of the deep reservoir fluid was estimated from surface well samples allowing for steam loss gas release mineral precipitation and ground water mixing in the well bore this deep fluid is sodium chloride in character with approximately 9700 ppm dissolved solids a ph of 6 0 and gas partial pressures of o2 ranging from 10 32 to 10 35 atm co2 of 11 atm h2s of 0 020 atm and ch4 of 0 001 atm comparison of the alteration mineralogy from producing and nonproducing wells allowed delineation of an alteration pattern characteristic of the reservoir rock theoretical alteration mineral assemblages in equilibrium with the deep reservoir fluid between 150 and 300 c in the system na2o k2o cao mgo feo fe2o3 al2o3 h4sio4 h2o h2s co2 hcl were calculated minerals theoretically in equilibrium with the calculated reservoir fluid at 240 c include sericite k feldspar quartz chalcedony hematite magnetite and pyrite this assemblage corresponds with observed higher temperature 210 c alteration assemblage in the deeper parts of the producing wells the presence of montmorillonite and mixed layer clays with the above assemblage observed at temperatures 210 c corresponds with minerals predicted to be in equilibrium with the fluid below 240 c alteration minerals present in the reservoir rock that do not exhibit equilibrium with respect to the reservoir fluid include epidote anhydrite calcite and chlorite these may be products of an earlier hydrothermal event or processes such as boiling and mixing or a result of errors in the equilibrium calculations as a result of inadequate thermochemical data

trace element distribution in an active hydrothermal system roosevelt hot springs thermal area utah abstract chemical interaction of thermal fluids with reservoir rock in the roosevelt hot springs thermal area utah has resulted in the development of characteristic trace element dispersion patterns multielement analyses of surface rock samples soil samples and drill cuttings from deep exploration wells provide a three dimensional perspective of chemical redistribution within this structurally controlled hot water geothermal system five distinctive elemental suites of chemical enrichment are recognized each characteristic of a particular combination of physical and chemical conditions within the geothermal system these are 1 concentrations of as sb be and hg associated with siliceous material at locations of liquid discharge fluid mixing or boiling 2 concentrations of mn ba w be cu co as sb and hg in manganese and iron oxide deposits 3 high concentrations of hg in argillized rock near fumaroles and lower concentrations in a broad diffuse halo surrounding the thermal center 4 concentrations of as in sulfides and li in silicate alteration minerals immediately surrounding high temperature fluid flow controlling fractures 5 deposits of caco 3 at depth where flashing of brine to steam has occurred due to pressure release the geochemical enrichments are not in general widespread pervasively developed zones of regular form and dimension as are typical in many ore forming hydrothermal systems as the geothermal system develops changes and eventually declines through time the chemical deposits are developed remobilized or superimposed upon each other thus preserving within the rocks a record of the history of the geothermal system recognition of trace element distribution patterns during the exploration of a geothermal system may aid definition of the present geometry and interpretation of the history of the system

subaqueous silicification of the contents of small ponds in an early devonian hot spring complex rhynie scotland the early devonian rhynie and windyfield cherts of northeast scotland originated as siliceous sinters deposited by hot springs silicification affected both subaerial and subaqueous environments preserving a diverse terrestrial and freshwater biota cherts originally deposited in small shallow pools can be recognised on the basis of both texture and fossil content textures comprise clotted and microcoprolitic textures bacterial coatings on plant axes that can be stromatolitic and a variety of bacterial and fungal meshworks the crustacean lepidocaris the euthycarcinoid heterocrania the charophyte alga palaeonitella and chytrid fungi are typical elements of the aquatic biota observations of modern hot springs in yellowstone national park wyoming u s a demonstrate that shallow ponds and streams on low angle outwash areas and dormant vent orifices provide suitable environmental analogues textures comparable to those described from rhynie are recorded from yellowstone sinters but examples of the

a devonian auriferous hot spring system rhynie scotland the early devonian rhynie hot spring system is the oldest known and is of the low sulphidation type it extends for at least 1 5 km along a major fault zone defining the western margin of an outlier of fluvial and lacustrine sediments plant bearing sinters and andesitic lavas the age of sedimentation and hydrothermal activity has been determined by palynological pragian and radio metric 396 12 ma techniques the outlier is a half graben with a complex stepped western margin the devonian rocks show intense hydrothermal alteration along the fault zone the main alteration minerals are quartz k feldspar calcite hematite and illitic and chloritic clays multiple chert veining and brecciation are widely developed and geyserite and vent material are also present pyrite occurs in veins and all alteration facies sinters and altered rocks contain high concentrations of au as sb hg w and mo gold occurs in arsenian pyrite and as sub micron particles in oxidized rocks the fluid s responsible for most hydrothermal alteration were near neutral with low sulphur and oxygen activities and dominated by meteoric water however incursions of high temperature 300 440 c magmatic fluids occurred with d 65 and 18 o around 8 5 34 s pyrite and initial 87 sr 86 sr ratios vein calcite lie mainly within the ranges 3 4 to 8 5 and 0 71138 to 0 71402 respectively these data indicate that late proterozoic dalradian metasediments are a likely source for s and sr but other sources are possible 13 c values for caliche and vein calcite imply derivation of carbon from non organic sources the rhynie cherts were deposited from a low salinity fluid of probable meteoric origin 18 o chert 13 1 to 16 5 which had interacted with the basement rocks and sediments high xe ar br cl and i cl ratios plant bearing chert yielded an 40 ar 39 ar ratio 292 1 0 6 significantly less than that of modem air and may be the first valid determination of a sample of ancient atmosphere

cryptoachneliths hidden glassy ash in composite spheroidal lapilli abstract cryptoachneliths perceptible by means of electron microscopy but unresolved under the optical microscope occur unnoticed inside spheroidal lapilli of ultrabasic composition of the cabezo segura volcano calatrava volcanic province spain the cryptoachneliths are glassy spherical particles that have compositions of al rich silicate with minor amounts of fe ca and other elements the smallest cryptoachneliths of 1 m microachneliths and homogeneous less regular masses of similar composition nano and microachneliths welded each other or to other types of volcanic particles crystals crystal fragments spinning droplets cognate lithic clasts etc to form spheroidal lapilli and even bomb size clasts within proximal fall deposits of the cabezo segura volcano the welding processes took place inside the eruptive column previous to the fall of the spheroidal lapilli on top of the volcanic cone the presence of the cryptoachneliths implies that lapilli and even bomb size tephra within deposits formed during explosive eruptions of low viscosity basic to ultrabasic magmas should be carefully examined in order to establish key parameters of eruption dynamics like size amount and distribution of juvenile fine particles

diagenetic transformations opal a to quartz of low and mid temperature microbial textures in siliceous hot spring deposits taupo volcanic zone new zealand silica sinter is a subaerial hot spring deposit formed upon cooling 100 c of discharging alkali chloride waters silica deposition traps and fossilizes living microbes in low temperature 35 c to mid temperature 35 59 c apron terrace outflow channels and pools which record distinctive macrotextures and microtextures along a thermal gradient sinters from four geothermal fields orakei korako northern waiotapu te kopia and umukuri within the taupo volcanic zone new zealand were sampled from two common microbe rich microfacies low temperature palisade mid temperature bubble mat through a range of ages modern to 40 000 years bp we observed morphologic changes in microbial silicification and stepwise transitions in silica phase mineralogy throughout diagenesis opal a to quartz x ray powder diffractometry analysis of taupo volcanic zone sinter samples revealed that mode of microbial fossilization is controlled by silica phase mineralogy which also determines the preservation potent

geothermal mineralization i the mechanism of formation of the beowawe nevada siliceous sinter deposit the siliceous sinter deposit at beowawe nev has a volume of 7 71 x 10 sup 7 m sup 3 and contains 1 28 x 10 sup 11 kg silica this silica precipitated from geothermal solutions that welled up along the malpais fault a graben bounding fault sinter formation involved three steps 1 geothermal solutions saturated with quartz in the reservoir moved to the surface where they cooled and became supersaturated with amorphous silica 2 amorphous silica particles nucleated heterogeneously to produce a colloidal suspension 3 the amorphous silica particles agglomerated and were cemented together by amorphous silica precipitating in the embayments between the particles at least 5 58 x 10 sup 14 kg of geothermal solution must have been produced over time to account for the mass of silica in the beowawe deposit this fluid would have carried at least 4 95 x 10 sup 17 kj of heat to the surface 21 references 6 figures 1 table

rock solute reaction mass balance of water flowing within an aquifer system with geochemical stratification abstract aquifers possess distinct mineralogy diagenetic history and geochemical constraints which control the pore water s hydrochemistry whenever aquifers exchange groundwater the incoming groundwater establishes a new chemical driving force imposing new reactions related to rock water interactions this study developed a simple approach to assess the mass balance of the rock water interaction of the groundwater exchanged between interconnected aquifers to illustrate the suitability of this approach it was applied to the bauru aquifer system located in sao paulo state in southeastern brazil the hydrochemical characterization identified three major stratified hydrochemical facies with typical chemical compositions as well as the authigenic mineralogy assemblages the first and shallowest facies was composed of a variable groundwater composition along with highly diluted water from a recent recharge the second facies was composed of unconfined bicarbonate calcium rich water with a mineralogy mainly composed of k feldspar mg montmorillonite and calcite the third facies which was confined and the deepest was composed of bicarbonate sodium rich water and mineralogically dominated by na mg and ca montmorillonite calcite and analcime although the replacement of na by ca suggests an extensive cation exchange in the deep confined portion sodium rich silicate dissolution and calcium rich clay precipitation were identified as the operating mechanisms promoting na enrichment and ca consumption in the water of the closed system this finding is supported by the authigenic mineral characterization 13c of the dissolved inorganic compounds and numerical analysis the approach developed for this study which used numerical simulations to calculate the mass balance was capable of consistently reproducing the water composition and mineralogy changes

tracking crystallinity in siliceous hot spring deposits siliceous hot spring deposits sinters entrap paleoenvironmentally significant components and are used as extreme environment analogs in the search for early earth and extraterrestrial life however sinters undergo a series of textural and mineralogical changes during diagenesis that can modify and overprint original environmental signals for ancient hydrothermal settings including those close to the dawn of life these transformations have long since occurred so that study of diagenetic processes and effects is best undertaken in much younger deposits still undergoing change three young sinters preserve the entire diagenetic sequence of silica phases from opal a to quartz the 6000 to 11 500 years bp 70 years sinter at steamboat springs nevada the 1600 1900 160 years bp opal mound sinter at roosevelt hot springs utah and the 456 35 years bp deposit at sinter island taupo volcanic zone new zealand provide an opportunity to track crystallographic mineralogic and morphologic transitions of sinter diagenesis using standard and new analytical approaches worldwide sinter forms from cooling alkali chloride waters as noncrystalline opal a transforming first into noncrystalline opal a ct then paracrystalline opal ct moganite paracrystalline opal c and eventually to microcrystalline quartz in this study these changes were identified by the novel and combined application of electron backscatter diffraction x ray powder diffraction and scanning electron and optical microscopy techniques we show that mineralogical changes precede morphological and accompanied crystallographic transformations during this modification silica particles grow and shrink several times from the micron to nano meter scales via dissolution reprecipitation and recrystallization and diagenesis follows the ostwald step rule all deposits followed nearly identical diagenetic pathways with time as the only variable in the march toward physicochemically stable quartz crystals diagenesis alters original environmental signatures trapped within sinters after five silica phase changes filamentous microfossils are modified but still remain recognizable within sinter from the opal mound and steamboat springs deposits and during the opal a to opal ct silica phase transformations at sinter island therefore delineating diagenetic components and how they affect sinters is necessary to accurately identify biosignals from ancient hot spring deposits

stromatolites in precambrian carbonates evolutionary mileposts or environmental dipsticks stromatolites are attached lithified sedimentary growth structures accretionary away from a point or limited surface of initiation though the accretion process is commonly regarded to result from the sediment trapping or precipitation inducing activities of microbial mats little evidence of this process is preserved in most precambrian stromatolites the successful study and interpretation of stromatolites requires a process based approach oriented toward deconvolving the replacement textures of ancient stromatolites the effects of diagenetic recrystallization first must be accounted for followed by analysis of lamination textures and deduction of possible accretion mechanisms accretion hypotheses can be tested using numerical simulations based on modern stromatolite growth processes application of this approach has shown that stromatolites were originally formed largely through in situ precipitation of laminae during archean and older proterozoic times but that younger proterozoic stromatolites grew largely through the accretion of carbonate sediments most likely through the physical process of microbial trapping and binding this trend most likely reflects long term evolution of the earth s environment rather than microbial communities

preservation of biogenic opal a in earliest cretaceous radiolarian claystone from the western pacific abstract tuffaceous claystone with anomalously well preserved radiolarian tests of berriasian age was collected from the oceanward slope of the mariana trench using the japanese submersible shinkai 6500 the rock is porous and light slightly calcareous and has experienced neither significant compaction nor deformation younger cretaceous cherts are composed of recrystallized opal ct and quartz silica phases in which tests were destroyed dissolved and deformed however the oldest claystone still contains radiolarian tests of biogenic opal a and dehydrated opal a opal a showing slight dissolution this anomalous preservation of reactive biosilica required isolation from ordinary chemical and physical diagenetic conditions since deposition the unusually weak diagenesis might occur from either avoidance of deep burial or unique chemical and physical microenvironments controlled by the clay rich composition the latter would include the effect of high pore fluid pressure due to low permeability which may favor the anomalous preservation found in this rock

occurrence and distribution of moganite in agate chalcedony a combined micro raman rietveld and cathodoluminescence study agate chalcedony samples of different origin were investigated by performing raman x ray diffraction using rietveld refinement and cathodoluminescence measurements these analyses were performed to measure the content and spatial distribution of the silica polymorph moganite which is considered to represent periodic brazil law twinning of quartz at the unit cell scale in agate chalcedonies homogeneous standard samples including the nearly quartz free moganite type material from gran canaria were analysed in order to compare results of the x ray diffractometry and raman spectroscopy techniques and to provide a calibration curve for the raman results however due to the different length scales analysed by the two techniques the moganite content in microcrystalline sio2 samples measured by raman spectroscopy short range order was found to be considerably higher than the moganite content measured by x ray diffractometry long range order the difference is explained by the presence of moganite nanocrystals nano range moganite lamellae and single brazil law twin planes that are detected by vibrational spectroscopy but that are not large enough in the sense of coherently scattering lattice domains to be detected by x ray diffractometry high resolution raman analysis provides a measure of the moganite content and its spatial variation in microcrystalline silica samples with a lateral resolution in the m range variations in the moganite to quartz ratio are revealed by varying intensity ratios of the main symmetric stretching bending vibrations a1 modes of quartz 465 cm 1 and moganite 502 cm 1 respectively traces of raman microprobe analyses perpendicular to the rhythmic zoning of agates revealed that the moganite to quartz ratio is often not uniform but shows a cyclic pattern that correlates with the observed cathodoluminescence pattern colour and intensity data obtained from an agate sample from a fluorite deposit near okorusu namibia and from a volcanic agate from los indios cuba were selected for detailed presentation variations of cathodoluminescence and raman data between single bands in agates suggest alternating formation of fine grained highly defective chalcedony intergrown with moganite and coarse grained low defect quartz multiple zones indicate dynamic internal growth during a self organizational crystallization process from silica rich fluids

influence of rock composition on kinetics of silica phase changes in the monterey formation santa barbara area california the diatomaceous deposits of the monterey formation along the santa barbara coast were subjected to temperatures and burial depths that increased westward after deposition as a result the silica phase in originally similar laterally equivalent strata generally changed westward from biogenous opal a to diagenetic opal ct to diagenetic quartz in individual beds at the same stratigraphic level however silica phases changed at various temperatures producing a transformation or kinetic sequence related to variations in the ratio of silica to detrital minerals compared to silica in rocks with minor detrital minerals in rocks with progressively more abundant detrital minerals 1 opal ct formed progressively later 2 the initial d spacing of opal ct was progressively smaller and 3 quartz formed progressively earlier comparison of associated calcareous dolomitic and carbonate free rocks with equal silica detrital ratios shows that carbonates did not significantly affect rates of silica diagenesis in the vast majority of rocks in carbonate bearing rocks with silica detrital ratios 8 however some 2 to 10 quartz formed early in diagenesis probably prior to opal ct formation in addition atypical quartz cherts formed locally in carbonate bearing strata by pore filling and replacement early in diagenesis possibly before widespread opal ct formation

dissolution precipitation processes governing the carbonation and silicification of the serpentinite sole of the new caledonia ophiolite the weathering of mantle peridotite tectoni cally exposed to the atmosphere leads commonly to natural carbonation processes extensive cryptocrystalline mag nesite veins and stock work are widespread in the ser pentinite sole of the new caledonia ophiolite silica is systematically associated with magnesite it is commonly admitted that mg and si are released during the laterization of overlying peridotites thus the occurrence of these veins is generally attributed to a per descensum mechanism that involves the infiltration of meteoric waters enriched in dissolved atmospheric co 2 in this study we investigate serpentinite carbonation processes and related silicifica tion based on a detailed petrographic and crystal chemical study of serpentinites the relationships between serpen tine and alteration products are described using an original method for the analysis of micro x ray fluorescence images performed at the centimeter scale our investigations highlight a carbonation mechanism together with precipitation of amorphous silica and sepiolite based on a dis solution precipitation process in contrast with the per descensum mg si enrichment model that is mainly concentrated in rock fractures dissolution precipitation process is much more pervasive thus although the texture of rocks remains relatively preserved this process extends more widely into the rock and may represent a major part of total carbonation of the ophiolite

microbial silica interactions in icelandic hot spring sinter possible analogues for some precambrian siliceous stromatolites silicified deposits such as sinters occur in several modern geothermal environments but the mechanisms of silicification and crucially the role of microorganisms in their construction are still largely unresolved detailed examination of siliceous sinter in particular sections of microstromatolites growing at the krisuvik hot spring iceland reveals that biomineralization contributes a major component to the overall structure with approximately half the sinter thickness attributed to silicified microorganisms almost all microorganisms observed under the scanning electron microscope sem are mineralized with epicellular silica ranging in thickness from 5 lm coatings on individual cells to regions where entire colonies are cemented together in an amorphous silica matrix tens of micrometres thick within the overall profile there appears to be two very distinct types of laminae that alternate repeatedly throughout the microstromatolite microbial layers are predominantly consisting of filamentous intact vertically aligned biomineralized cyanobacteria identified as calothrix and fischerella sp and weakly laminated silica layers which appear to be devoid of any microbial component the microbial layers commonly have a sharply defined base overlying the weakly laminated silica and a gradational upper surface merging into the weakly laminated silica these cyclic laminations are probably explained by variations in microbial activity active growth during spring summer allows the microorganisms to keep pace with silicification with the cell surfaces facilitating silicification while during their natural slow growth phase in the dark autumn winter months silicification exceeds the bacteria s ability to compensate i e grow upwards at this stage the microbial colony is probably not essential to microstromatolite formation with silicification presumably occurring abiogenically when conditions once again become favourable for growth recolonization of the solid silica surface by free living bacteria occurs cell motility is not responsible for the laminations we have also observed that microbial populations within the microstromatolite some several mm in depth appear viable i e they still have their pigmentation the trichomes are not collapsed cell walls are unbroken cytoplasm is still present and they proved culturable this suggests that the bulk of silicification occurred rapidly probably while the cells were still alive surprisingly however measurements of light transmittance through sections of the microstromatolite revealed that photosynthetically active light pal only transmitted through the uppermost 2 mm therefore the deeper microbial

depositional facies and diagenetic alteration in a relict siliceous hot spring accumulation examples from yellowstone national park u s a abstract siliceous precipitates from portions of an 11 m long core drilled through a relict yellowstone hot spring deposit provide valuable insights into the diagenetic facies in this hydrothermal environment most of the core consists of relatively low temperature siliceous sinter containing abundant plant diatom and microbial fossilized remains derived from the distal debris apron and discharge channel flowpath of the hot spring a few intervals contain stromatolitic horizons analogous to modern siliceous pool margin and discharge channel flowpath facies despite the abundance of silicified microbial remains e g silicified stromatolitic horizons as well as individual filamentous microbes diatoms distinctive organic biomarkers are not preserved in the core the apparent lack of distinctive organic biomarkers is attributable to pervasive diagenetic alteration in the presence of high temperature chemically reactive fluids numerous diagenetic features have been superimposed on the siliceous sinter especially considering the relatively shallow depths of burial the uppermost interval 0 to 1 2 m consists of highly porous siliceous sinter opal a and interbedded travertine amorphous manganese oxides and sparry calcite cements are present and are irregularly distributed throughout this interval below is a diagenetic facies 4 8 to 5 3 m that consists of opal ct and minor chalcedony the most pervasive diagenetic alteration in the core is the 6 8 to 11 2 m depth interval where abundant zeolites mordenite heulandite bladed cristobalite lepispheres chalcedony cements and calcite veins are present judging by the assemblage of plant and diatom remains most of this pervasive alteration is superimposed on relatively low temperature siliceous sinters and is likely the result of migration of hot waters through this interval the paragenesis of all these intervals is very complex involving numerous episodes of mineral precipitation and dissolution this diagenetic alteration is facilitated by the porous nature of the siliceous sinter thermal convection of superheated fluids or steam strongly oxidizing spring waters and fluctuating hydrochemical regimes rapid dissolution and or replacement of organic matter indicates that standard morphological means e g petrographic identification of body fossils may be the best method by which to identify that organisms were previously present in ancient hot spring accumulations

himalayan seismic hazard the devastating earthquake in bhuj on 26 january 2001 highlighted the high earthquake risk in the indian region of gujarat in their perspective bilham et al warn that major earthquakes may be expected in another part of india the himalayan arc an analysis of historic earthquakes suggests that several major earthquakes are overdue in this region but that recent low activity has focused attention away from the himalayan arc projections from earlier earthquakes suggest that just one of the possibly overdue himalayan earthquakes may lead to 200 000 predictable fatalities

seasonal changes in silica deposition in hot spring systems abstract monthly monitoring of water chemistry at octopus spring yellowstone national park reveals no changes in the chemical composition over this eight month study differences in silica deposition are attributed to flow dynamics and seasonal air temperatures colder air temperatures in winter promote visible accumulation of siliceous sinter probably as a result of decreased solubility periodic drying promotes accumulation and subsequent dehydration of the silica this combination of intermittent supply and periodic dehydration may account for the laminated siliceous deposits observed in the geological record the processes described herein are important in the siliceous preservation of microbial fossils but the following observations underscore the difficulty in preserving microbial fabrics in the geological record pools populated by cyanobacteria are bounded by margins built of silicified microbial mats silicification of the pool margins occurs through an evaporative process called wicking silica rich water is transported upward through the microbial mats by capillary action depositing silica at the surface of the mat as water evaporates the upper surface of such wicking deposits is extremely resistant to brecciation but the underlying support is highly friable and would be difficult to preserve intact in winter months temporary isolation of some pools from active flow causes fragile silica deposition around biological and other debris film coated gas bubbles on the surfaces of microbial mats are observed during all seasons and probably are filled with oxygen released during photosynthesis although this has not been confirmed they become coated with silica during winter months presumably because of increased cooling possibly forming thin discontinuous silica sheets as observed in the geological record