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

electromagnetic induction in an inhomogeneous conductive thin sheet distinguishing between the electromagnetic em response of a subsurface conductor and the em response of an overburden whose conductivity and or thickness varies laterally requires a capability to calculate the em response of both types of conductor while methods for calculating the response of some simple subsurface conductors such as dipping rectangular sheets are already available methods for computing the response of an irregular overburden are not common using price s analysis we have formulated two numerical techniques for calculating the response of a laterally varying overburden which is thin and flat and which lies on a perfectly resistive subspace the first technique is a frequency domain method in which a large matrix equation is solved to find the horizontal wavenumber components of the secondary vertical magnetic field the method is best suited to calculating the response of the overburden when the em source and receiver are located above the sheet such as in airborne em systems helicopter em profiles calculated using this technique have been checked against a simple scale model the second method calculates the time domain step response of the overburden by time stepping the vertical component of the magnetic field the method is suitable for calculating the response of the overburden when the em source is a large transmitter loop close to the overburden using the time domain method to investigate the response of simple conductance structures illustrates that the zero crossing of the vertical magnetic field moves more slowly across conductive regions than across resistive regions this is because the rate of decay of the vertical field in a region varies in proportion to the resistance of the region a response profile from a utem survey shows a response that could be interpreted as due to a dipping subsurface conductor this response has been modeled using the time domain method and a geologically acceptable pattern of lateral variations in the overburden conductance yields a response close to the measured em response thus a subsurface conductor need not lie below the profile line to explain the response

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

breaks in lithology interpretation problems when handling 2d structures with a 1d approximation most airborne electromagnetic aem data are processed using successive 1d approximations to produce stitched conductivity depthsections becausethecurrentinducedinthenearsurface by an aem system preferentially circulates at some radial distancefromahorizontallooptransmittersometimescalledthe footprint the section plotted directly below a concentric transmitter receiver system actually arises from currents induced in the vicinity rather than directly underneath detection of paleochannels as conduits for groundwater flow is a common geo

the inversion of time domain airborne electromagnetic data using the plate model airborne electromagnetic em methods were developed in the early 1950s mostly by canadian mining exploration companies as a means of economically carrying out prospecting for sulfide deposits associated with volcanics in resistive shield areas present interpretation techniques are based on the use of nomograms but the approach is easily amenable to digital processing for highly accurate interpretation however it is necessary to develop quantitative interpretation techniques that can make full use of all the data available inverse theory has been used for interpretation with great success in most geophysical disciplines however in airborne em surveying inversion has only been used for the interpretation of airborne em data using half space and one layer models by introducing some approximations to the rectangular thin plate model it is now possible to apply inverse theory to the interpretation of time domain em data this approach provides estimates of the parameter errors the correlation matr

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

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

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

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

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

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

probabilistic inversion of airborne electromagnetic data for basement conductors abstractprobabilistic inversion of airborne electromagnetic data is often approximated by a layered earth using a computationally efficient 1d kernel if the underlying framework accounts for prior beliefs on spatial correlation the inversion will be able to recover spatially coherent interfaces and associated uncertainties greenfield exploration using airborne electromagnetic data however often seeks to identify discrete economical targets in mature exploration provinces such bodies are frequently obscured by thick conductive regolith and the response of such economic basement conductors presents a challenge to any layered earth inversion a well known computationally efficient way to approximate the response of a basement conductor is to use a thin plate here we have extended a bayesian parametric bootstrap approach so that the basement of a spatially varying layered earth can contain a thin plate the resulting bayesian framework allowed for the inversion of basement conductors and associated

airborne em footprints as frequency domain airborne electromagnetic aem studies move towards more detailed assessments of the near surface the behaviour of system footprints and hence the spatial averages involved in the measurement becomes important published estimates suffer from two main limitations first they are based on perfectly conducting thin sheet models and secondly they are system specific the present study is a revision of footprint estimates based on i a finitely conducting half space and ii an at surface scale estimate that uses the spatial equivalent of the conventional electromagnetic skin depth in order to remove the system dependence a transmitter footprint is defined in terms of electromagnetic skin distance only the limiting cases of vertical and horizontal magnetic dipole sources then require analysis electromagnetic skin distances two for each of the coil orientations are defined the revised definition makes it possible to investigate the footprint behaviour of both towed bird and fixed wing aem systems over an altitude range from 20 to 100 m the footprint altitude ratio has a primary dependence on altitude and a secondary dependence on both resistivity and frequency the analysis covers a frequency range from 1 to 100 khz and results are presented for two specific resistivity values that represent conductive 10 m and resistive 1000 m environments the revised footprint parameters display a quasi linear behaviour with altitude particularly for mid range frequencies this behaviour enables the coefficients of linear least squares relationships to be obtained thus assisting with the prediction of footprint estimates for survey planning and interpretation a comparison of the new estimates with published values suggests that existing footprint values for a vertical magnetic dipole should be revised downward

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

3d parametric hybrid inversion of time domain airborne electromagnetic data we have developed a method to invert time domain airborne electromagnetic aem data using a parametric level set approach combined with a conventional voxel based technique to form a parametric hybrid inversion the approach was designed for situations in which a voxel based inversion alone may struggle such an example is where a distinct anomaly is present with sharp boundaries and there is a large contrast between a low resistivity target and a high resistivity background the first step of the proposed hybrid method used our novel parametric inversion to recover a best fitting skewed gaussian ellipsoid that represented the target of interest subsequently the parametric result was set as an initial and reference model for the second stage where smooth features with smaller resistivity contrasts were introduced into the model through a conventional voxel based approach the approach was tested with synthetic and field data in the synthetic case we recovered the size and dip of a conductive thin dipping plate with better accuracy compared with a voxel based inversion in the field example we inverted aem data over the caber volcanogenic massive sulfide deposit based on information from past drilling our results improve upon previous parametric plate inversions of the deposit itself while additionally imaging the conductive cover over the deposit these findings showcased how our parametric hybrid method can improve the accuracy of time domain aem inversions for thin dipping targets with large resistivity contrasts compared with the background

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

quasi 3d modeling of airborne tem data by spatially constrained inversion we present a new methodology spatially constrained inversion sci that produces quasi 3d conductivity modeling of electromagnetic em data using a 1d forward solution spatial constraints are set between the model parameters of nearest neighboring soundings data sets models and spatial constraints are inverted as one system the constraints are built using delaunay triangulation which ensures automatic adaptation to data density variations model parameter information migrates horizontally through spatial constraints increasing the resolution of layers that would be poorly resolved locally sci produces laterally smooth results with sharp layer boundaries that respect the 3d geological variations of sedimentary settings sci also suppresses the elongated artifacts commonly seen in interpretation results of profile oriented data sets in this study sci is applied to airborne time domain em data but it can also be implemented with other ground based or airborne data types

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

tropical intraseasonal oscillation super cloud clusters and cumulus convection schemes abstract a new framework for interpreting the origin of the tropical intraseasonal oscillation tiso which avoids the speed and scale selection problems in the previous theories is proposed in this study in this interpretation tiso is viewed as an oscillation driven by an eastward moving convective region this convective region consists of one or more super cloud clusters originating in the indian ocean and terminating in mid pacific and is then followed by another convective region arising in the indian ocean in a period of 40 50 days additionally a formal analogy is pointed out between super cloud clusters and the middle latitude baroclinic wave packets this study includes a simulation of tiso in a 2d model to support our interpretation experiments were conducted with four different convection schemes the authors advocate that the successful simulation of tiso depends on the successful simulation of super cloud clusters which in turn depends on the successful simulation of the life cycle of

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

3d inversion of airborne electromagnetic data abstracttime domain airborne surveys gather hundreds of thousands of multichannel multicomponent samples the volume of data and other complications have made 1d inversions and transforms the only viable method to interpret these data in spite of their limitations we have developed a practical methodology to perform full 3d inversions of entire time or frequency domain airborne electromagnetic aem surveys our methodology is based on the concept of a moving footprint that reduces the computation requirements by several orders of magnitude the 3d aem responses and sensitivities are computed using a frequency domain total field integral equation technique for time domain aem responses and sensitivities the frequency domain responses and sensitivities are transformed to the time domain via a cosine transform and convolution with the system waveform we demonstrate the efficiency of our methodology with a model study relevant to the abitibi greenstone belt and a case study from the reid mahaffy test

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

the harmonic and transient electromagnetic response of a thin dipping dike an exact solution is given for the electromagnetic induction in a dipping dike of finite conductivity represented as a thin half sheet in a nonconducting surrounding the problem is formulated for arbitrary dipole or circular loop tx rx configurations the formal solution obtained by the wiener hopf technique is cast into a rapidly convergent triple integral suitable for an effective numerical treatment a good agreement is found between numerical results and analog measurements available for harmonic excitation the transient response is obtained as a superposition of the half sheet free decay modes and is illustrated by some numerical examples for coincident loops including a diagram for the approximate determination of conductance and depth of a vertical dike

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

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