the role of porosity in the development of authigenic dolomite and palygorskite an example in siliceous sandstones of continental origin in western spain during the tertiary the hercynian basement of the iberian peninsula was uneven due to alpine tectonism which led to the development of a set of horsts and grabens forming its main geological structure one of these grabens is the ciudad rodrigo basin located close to the portuguese border the south western margin of this graben is filled with arkosic sandstones of continental origin that once deposited underwent important low grade diagenetic processes related to the surface processes the main diagenetic processes were hydrolysis replacement by carbonate and argillization to palygorskite these three processes led to the differentiation of three main levels within the outcrops studied 1 the lower level where the original clayey sandstone outcrops displaying fairly hydromorphic traits 2 the middle level where part of the clayey matrix has been replaced by dolomitic carbonate and 3 the upper level where palygorskite is the new neoformed clay the change in porosity seems to have been the controlling factor since it governs the dynamics of the solutions inside the rocks these conditions changed from being diffusion driven at the start of the diagenesis to flow driven as porosity increased

oxygen isotope evidence for hydrothermal alteration within a quaternary stratovolcano lassen volcanic national park california brokeoff volcano a quaternary stratocone located in the lassen volcanic center in northern california has been deeply eroded exposing a 10 km2 meteoric hydrothermal alteration zone at the core of the volcano portions of the former volcanic edifice are sufficiently well preserved that an unusual opportunity exists wherein the alteration pattern can be correlated with the position of the volcanic cone the 18o analyses of more than 100 whole rock samples consisting primarily of andesitic lavas vary from 9 8 to 0 6 per mil the highest 18o values occur in bleached solfatarically altered rocks that have interacted with low ph fumarolic hot springs associated with the present day hydrothermal system low 18o values are found in propylitically altered rocks that underwent isotopic exchange with meteoric hydrothermal fluids at elevated temperatures mostly during the stratovolcanic stage 650 400ka of the hydrothermal system but probably continuing today at depth electron microprobe analyses of secondary layer silicate minerals in strongly propylitized samples 18o 5 0 revealed the presence of discrete chlorite suggesting that temperatures up to 200 to 250 c were attained in the shallow levels of the system two zones of pervasive meteoric hydrothermal alteration defined by concentric 18o contours that are probably interconnected at depth are located within the original topographic edifice of the volcano the most intensely altered rocks within these equant zones of alteration define nnw trends that coincide with stream valleys and with regional structural patterns a comparison of the characteristics of the 18o depleted zone at brokeoff with those of more deeply eroded volcanic centers such as the comstock lode mining district criss and champion 1991 permits the construction of composite 18o cross sections through a hypothetical intact stratovolcano at both brokeoff and comstock hydrothermal fluids were strongly focused into plumelike zones of intense 18o depletion at comstock these low 18o plumes are associated with faults although major fault displacements are not observed at brokeoff the topographic and alteration patterns are consistent with the presence of a linear array of faults that acted as conduits for fluid flow up into the shallow levels of the volcano

thermal structure beneath kuju volcano central kyushu japan abstract thermal conditions beneath kuju volcano central kyushu japan are simulated in two manners simulation of the regional high heat flow around kuju volcano shows that the underlying magma reservoir is not molten but maintains high temperatures of 400 700 c simulation of the thermal processes beneath the solfatara field in the central part of kuju volcano suggests that the geothermal reservoir is a two phase reservoir heated by magmatic steam

pedogenesis along a thermal gradient in a geothermal region of the southern cascades california abstract hydrothermal alteration is an important soil forming factor in the vicinity of active volcanic centers yet we are aware of no studies that have addressed the role of active fumaroles on soil development this paper examines a soil developmental sequence of five pedons established across a thermal gradient 100 m induced by an active fumarole in lassen volcanic national park in the southern cascades of california the soil temperature gradient at the time of sampling ranged from 100 to 15 c with a corresponding change in vegetation from barren areas near the fumarole to pygmy conifer conifer and meadow grasses and herbs with increasing distance from the fumarole the primary objectives of this study were to characterize morphological chemical and mineralogical soil properties across the thermal gradient and document pedogenic processes within this exotic soil forming environment field morphological properties standard soil characterization soil solution chemistry and detailed mineralogical investigations were conducted the hydrothermally altered parent material from which the soils formed was relatively clay rich 18 48 clay with a mineralogical composition dominated by mica smectite chlorite and montmorillonite soils near the fumarole were strongly acidified ph 6 high soil temperatures near the fumarole resulted in iron hydr oxides being primarily crystalline while non crystalline iron hydr oxides were generally dominant in the upper horizons of the non thermal soils steam heated alteration in the vicinity of the fumarole resulted in the transformation of the mica smectite and chlorite to smectite kaolinite and alunite kal so 4 2 a similar but less intense alteration appeared in the conifer pedons resulting in both smectite and kaolinite as weathering products the meadow pedon displayed the least alteration with appreciable mica smectite and or chlorite smectite still present there was no evidence of poorly crystalline aluminosilicates materials e g allophane imogolite in any of the soils the lack of vegetation establishment on soils near the fumarole appears to result primarily from the high soil temperatures 59 c at 50 cm with strong soil acidity ph 3 generally 10 cmol c kg 1 low base saturation and low nutrients further impairing soil fertility

melanges magmatiques a la montagne pelee martinique origine des eruptions de type saint vincent au cours des 40 000 dernieres annees l activite de la montagne pelee a consiste en l emission de coulees de ponces nuees ardentes peleennes et de type merapi mettant en place des produits de composition andesitique a dacitique ainsi que de quelques nuees ardentes de type saint vincent responsables de l emission de produits plus basiques andesites basaltiques basaltes accompagnes par des phenomenes de melanges magmatiques deux eruptions de ce type sv1 et sv2 datees a 25700 1200 et 22300 1200 ans b p ont ete etudiees au point de vue du deroulement de l eruption de la composition chimique des produits et de la mineralogie elles suivent toutes les deux un meme scenario eruptif evoluant depuis un pole andesitique vers un pole plus basique avec un stade intermediaire de melange magmatique les volumes emis sont tres importants de l ordre de 1 km3 ou plus les differences chimiques peu marquees 3 a 6 sio2 les variations moderees et progressives de la mineralogie ainsi que des conditions physiques de cristallisation t 920 930 a 950 970 c indiquent le cogenetisme des differents magmas

the use of mixing models and chemical geothermometers for estimating underground temperatures in geothermal systems abstract application of various chemical geothermometers and mixing models indicate underground temperatures of 260 c 280 c and 265 c in the geysir hveravellir and landmannalaugar geothermal fields in iceland respectively mixing of the hot water with cold water occurs in the upflow zones of all these geothermal systems linear relations between chloride boron and 18o constitute the main evidence for mixing which is further substantiated by chloride silica and sulphate relations in the geysir and hveravellir fields a new carbonate silica mixing model is proposed which is useful in distinguishing boiled and non boiled geothermal waters this model can also be used to estimate underground temperatures using data from warm springs this model as well as the chloride enthalpy model and the na li and co2 gas geothermometers invariably yield similar results as the quartz geothermometer sometimes also does by contrast the na k and the na k ca geothermometers yield low values in the case of boiling hot springs largely due to loss of potassium from solution in the upflow the results of these geothermometers are unreliable for mixed waters due to leaching subsequent to mixing

formation and preservation of pedogenic carbonates in south india links with paleo monsoon and pedological conditions clues from sr isotopes u th series and rees the influence of the pedogenic and climatic contexts on the formation and preservation of pedogenic carbonates in a climosequence in the western ghats karnataka plateau south west india has been studied along the climosequence the current mean annual rainfall mar varies within a 80 km transect from 6000 mm at the edge of the plateau to 500 mm inland pedogenic carbonates occur in the mar range of 500 1200 mm in the semi arid zone mar 500 900 mm carbonates occur i as rhick hardpan calcretes on pediment slopes and ii as nodular horizons in polygenic black soils i e vertisols in the sub humid zone mar 900 1500 mm pedogenic carbonates are disseminated in the black soil matrices either as loose irregular and friable nodules of millimetric size or as indurated botryoidal nodules of centimetric to pluricentimetric size they also occur at the top layers of the saprolite either as disseminated pluricentimetric indurated nodules or carbonate cemented lumps of centimetric to decimetric size chemical and isotopic sr 87 sr 86 compositions of the carbonate fraction were determined after leaching with 0 25 n hcl the corresponding residual fractions containing both primary minerals and authigenic clays were digested separately and analyzed the trend defined by the sr 87 sr 86 signatures of both labile carbonate fractions and corresponding residual fractions indicates that a part of the labile carbonate fraction is genetically linked to the local soil composition considering the residual fraction of each sample as the most likely lithogenic source of ca in carbonates it is estimated that from 24 to 82 55 on average of ca is derived from local bedrock weathering leading to a consumption of an equivalent proportion of atmospheric co2 these values indicate that climatic conditions were humid enough to allow silicate weathering mar at the time of carbonate formation likely ranged from 400 to 700 mm which is 2 to 3 fold less than the current mar at these locations the sr u and mg contents and the u 234 u 238 activity ratio in the labile carbonate fraction help to understand the conditions of carbonate formation the relatively high concentrations of sr u and mg in black soil carbonates may indicate fast growth and accumulation compared to carbonates in saprolite possibly due to a better confinement of the pore waters which is supported by their high u 234 u 238 signatures and or to higher content of dissolved carbonates in the pore waters the occurrence of ce mn and fe oxides in the cracks of carbonate reflects the existence of relatively humid periods after carbonate formation the carbonate ages determined by the u th method range from 1 33 0 84 kyr to 7 5 2 7 kyr and to a cluster of five ages around 20 kyr i e the last glacial maximum period the young occurrences are only located in the black soils which therefore constitute sensitive environments for trapping and retaining atmospheric co2 even on short time scales the maximum age of carbonates depends on their location in the climatic gradient from about 20 kyr for centimetric nodules at mule hole mar 1100 mm yr to 200 kyr for the calcrete at gundlupet mar 700 mm yr durand et al 2007 the intensity of rainfall during wet periods would indeed control the lifetime of pedogenic carbonates and thus the duration of inorganic carbon storage in soils c 2010 elsevier ltd all rights reserved

opening of the grenada back arc basin and evolution of the caribbean plate during the mesozoic and early paleogene abstract geological and geophysical data indicate that the grenada basin was presumably created during the paleocene by sundering of a proto eastern caribbean arc into a remnant arc to the west aves swell and an active arc to the east lesser antilles ridge grenada basin spreading is thought to have been penecontemporaneous with the creation of the yucatan basin located at the opposite side of the caribbean sea i suggest that a continuous mesozoic caribbean arc m c a including the greater antilles the aves lesser antilles system and the aruba blanquilla chain netherland venezuelan antilles was initiated in the pacific probably about 130 120 ma ago its arrival in front of and its subsequent motion inside the central atlantic tethyan seaway caused the opening of both yucatan and grenada basins which occurred at the two initial points of contact with the north and south american cratons in contrast to the style of many other island arcs this back arc spreading event occurred only once in the long history of the m c a the lesser antilles appear to be the oldest currently active intra oceanic island arc

magmatology of mt pelee martinique f w i iii fractional crystallization versus magma mixing the mineralogical and chemical compositions of the magmas emplaced during the three successive stages of mt pelee s activity ca 0 4 0 2 ma ca 0 1 ma 19 500 y b p 13 500 y b p remained relatively constant all magmas are cogenetic in a broad sense and result from the combination of two major petrogenetic processes fractional crystallization witnessed by gabbroic and dioritic cumulates fichaut et al 1989 this issue and magma mixing evidenced by textural mineralogical and chemical heterogeneities gourgaud et al 1989 this issue the study of heterogeneous eruptions e g the ash and scoria flows second stage and the 1902 1929 block and ash flows demonstrates that the basic and acid magmas involved have small but significant geochemical differences the distribution of compatible mg sc co and hygromagmaphile th la ta hf elements indicates that fractional crystallization played a dominant role in the genesis of the second stage products whereas magma mixing was dominant in the 1902 1929 lavas geological mineralogical and geochemical constraints are used to propose models of magma chambers the second stage deposits are considered to be the result of the discharge of the magmas stored in a chemically stratified ca 2 3 km3 magma chamber tentatively described as a cylinder or a parallelepiped of large vertical extent tens of km but small horizontal section some hundreds of meters by contrast the young 13 500 y b p deposits of mt pelee are thought to originate from a small 1 km3 and shallow ca 10 km in depth chamber filled with andesitic to dacitic magmas the periodic intrusion of basic magmas presumably from a deeper chamber causes magma mixing which triggers the eruption of heterogeneous and hybrid products

oxygen isotope aeromagnetic and gravity anomalies associated with hydrothermally altered zones in the yankee fork mining district custer county idaho epithermal ag au vein and disseminated deposits in the yankee fork district are hosted in altered volcanic rocks having low delta 18 o values low magnetic susceptibilities low remanent magnetizations and relatively high densities these isotopic and physical quantities provide sensitive indices of rock alteration that can be contoured over areas that are much larger than zones of strong visible alteration such contour maps reveal a marked spatial association between the ore deposits the steepest delta 18 o gradients and the perimeter of zones where the rocks have extremely low total magnetizations the oxygen isotope results indicate that the ore zones coincide with steep gradients in temperature and water rock ratio in two fossil hydrothermal convective systems that involved fluids derived from tertiary meteoric waters our magnetic measurements suggest that these same high gradient zones also coincide with oxidation reduction boundaries low aeromagnetic intensities and positive bouguer anomalies are also associated with the altered zones in the yankee fork district the intensities of these geophysical features are consistent with the size of the altered zones and with the contrast in physical properties between altered and unaltered rocks contour maps of the magnetic and isotopic properties of host rocks should find application in exploration for a variety of types of hydrothermal ore deposits as well as in the study of ore controls and conditions of mineralization

carbonate accumulation and weathering of the granitic substratum of tanecherfi basin northeastern morocco abstract in the tanecherfi basin a granitic body is partly overlain by a calcrete which only seems superimposed morphologically the profiles consist of three horizons the lowest one corresponds to a filling of carbonates into the weathered granitic rocks microscopically the carbonates cut the primary minerals along open cracks and voids in the second horizon the increased carbonation results in calcite septa that gradually isolate rock cells inclosed in a micritic matrix forming a cortex gradually the rock residues vanish and glaebules appear the upper horizon is characterized by a micritic matrix associated with the disappearance of the nodules these morphologic relations are confirmed by chemical data the primary minerals are weathered silica is locally released and precipitates partly around the glaebules magnesium combines into the carbonate structure but locally combines with the silica of the cortex of the glaebules aluminium seems to be totally removed the granite calcrete relationships indicate a weathering process rather to a simple superposition

case study of a volcanic geothermal system mount pelee martinique abstract mount pelee is one of the active volcanoes of the lesser antilles arc with more than 20 eruptions during the last 5000 years despite this abundant potential heat flow there are few surface manifestations indicative of the existence of a large high temperature hydrothermal system a few na hco 3 so 4 type hot springs with emergence temperatures not exceeding 65 c occur on the south and west flanks of the volcano two small hydrothermal eruptions occurred in 1792 and 1851 on the southwestern flank on the site of now extinct sulfur springs the heat source provided by the mount pelee magma chamber should occur according to petrologic data at shallow depth 5 8 km beneath the summit region we assume abundant groundwater recharge to take place within the central calderas where faults and magmatic conduits provide vertical channelways for deep infiltration of cold meteoric water this would be expected to depress a presumed nacl hydrothermal system located deep in the core of the volcano upwelling of high temperature fluids and gases is thought to be found at the periphery of the central calderas along the arcuate caldera related faults and or along regional faults and any permeable stratigraphic levels connected to the central caldera structure steam heating of shallow meteoric waters by a degassing nacl system could produce a na hco 3 so 4 type system which would develop preferentially on the periphery of the central caldera structure or even outside it in an adjacent zone of fracture permeability caldera or regional faults where cold meteoric flows are less abundant this na hco 3 so 4 system is assumed to be located beneath the south flank of the volcano and the major upflow zone seems to be the southern limit of the intermediate caldera as evidenced by the location of hot springs and hydrothermal eruptions the scarcity of thermal manifestations on mount pelee is not surprising in view of its groundwater hydrogeology the na hco 3 so 4 fluids which ascend will be masked wherever a highly permeable subhorizontal aquifer occurs near the surface and they commonly mix with cold waters before discharging

first results of the moses experiment sea sediment conductivity and thickness determination bute inlet british columbia by magnetometric offshore electrical sounding a static electrical method has been developed to determine the electrical resistivity of crustal rock beneath the sea the transmitter is a vertical long wire bipole extending from the sea surface to the sea floor a commutated current generated on the ship is fed to two large electrodes one near the sea surface the other at the end of a long insulated wire the return path for the current is through the sea and the subjacent crust the receiver is a self contained remote microprocessor controlled magnetometer which is deployed from the ship to the sea floor and subsequently recovered the data are measurements of the azimuthal component of the magnetic field as a function of transmitter receiver horizontal separation the acronym moses has been coined for the method the choice of the name moses is appropriate because the system geometry is carefully arranged to remove many of the adverse effects of the relatively conductive sea water in particular accurate estimates of sea floor resistivity ar

hydrothermal circulation beneath mount pelee inferred by self potential surveying structural and tectonic implications abstract self potential sp surveys were made on mount pelee volcano martinique island french west indies in 1991 and 1992 in order to recognize its hydrothermal system the associated groundwater channeling and the main superficial structures of the massif almost 70 km of profiles were carried out with an average sample spacing of 50 m measurements essentially reveal negative sp anomalies down to 1700 mv with high gradients 1 83 mv m due to the infiltration of meteoric water into the massif rims of summit calderas morne macouba and etang sec present sharp negative sp anomalies on the western northern and eastern flanks negative sp anomalies indicate no upward water flow beneath mount pelee summit on the southwestern volcano flank a 3 5 6 km horseshoe shaped structure corresponding to a southwest flank collapse event older than 25 000 years bp is clearly identified by the sp mapping high gradients border the inner southern rim from morne calebasse to st pierre town and the caribbean sea along the northern rim of the horseshoe shaped structure the negative sp anomalies give place to a positive sp anomaly up to 200 mv of sw ne trend this zone covers the area of two active hot springs sources chaudes and puits chaud 40 65 c marine magnetic surveys and bathymetry show that the horseshoe shaped structure spreads into the caribbean sea up to about 10 km from the coast buried structural discontinuities are evidenced inside the flank collapse structure the upper one deviates the groundwater flow coming from the summit toward the south flank where the flow finds an indentation to expand again downwards this discontinuity is either an old hypothetical caldera rim partly destroyed by the collapse of the south southwestern flank and covered by recent pyroclastic deposits or more probably the trace of a bulge landslide a circulation model of the hydrothermal waters is proposed rainfall 5 6 m year is partly drained inside the summital calderas and the flank collapse zone through pyroclastic flows down to an impermeable basement there the groundwater constitutes perched aquifers at the contact of the bulge landslide or of the hypothetical old caldera rim along the inner northern border of the flank collapse structure the phreatic water is reheated warm groundwater flows along the northern avalanche structure rim and discharges near the coast in ground and marine outcrops of medium temperature finally the main part of the meteoric water is channeled along the old caldera rim or along the bulge landslide towards the south flank of mount pelee where some gaps in the rim exist there the groundwater finds again a subhorizontal gravitational circulation along mount pelee slopes into the caribbean sea

strontium and carbon isotope tracers and the origins of soil carbonate in south australia and victoria abstract calcium carbonate occurs in soils in association with a variety of rock types in coastal and inland south australia and victoria the 87 sr 86 sr ratios of the coastal soil carbonates fall between 0 7094 and 0 7098 showing that the ocean is the principal source of calcium to nearly all soils near the coast and that contributions from bedrock are generally small an exception is the soil formed on the young volcano at mt gambier in which the average ratio for carbonate is about 0 7070 compared to 0 7041 for the volcanic ash the basaltic ash of the volcano is readily weathered and rich in calcium roughly 40 of strontium and 10 to 20 of the calcium in the soil carbonate derives from the ash and the balance from marine sources the 87 sr 86 sr ratios from soil carbonate gradually increase inland reaching values of 0 7152 these higher ratios reflect at least in part the increasing 87 sr 86 sr ratio of inland dust in contrast the ocean does not contribute significantly to the carbon pool of the soils we studied the 13 c pdb values of soil carbonates on kangaroo island and the higher rainfall areas of eastern victoria fall between 11 0 and 8 7 indicating that the carbonate formed in equilibrium with the c3 dominated vegetation typical of the region in the warmer inland areas to the north such as the eyre peninsula the 13 c value of soil carbonate averages 5 3 reflecting the higher proportion of c4 plants in the local biomass mixing with marine sources of carbonate cannot account for the observed carbon isotopic patterns the flux of carbon into soils from the ocean therefore must be small compared to that derived from plant respiration and decay

a model for simulating rock water interactions in a weathering profile subjected to frequent alternations of wetting and drying abstract physically based equations for unsaturated groundwater flow and solute transport have been coupled with kinetic rate laws for mineral dissolution precipitation and mass balance mass action equations for aqueous species in a numerical model that is capable of simulating rock water interactions in a weathering profile subjected to fluctuating boundary conditions a numerical experiment was conducted to demonstrate how incipient soil development may proceed in a warm subhumid environment the simulation involved a hypothetical coarse textured parent material that was subjected to frequent wetting and drying during an annual water cycle the hypothetical weathering profile evolved rapidly dissolution of primary minerals enstatite forsterite and diopside and precipitation of secondary clay minerals kaolinite and ca montmorillonite occurred monotonically despite the abrupt fluctuations in soil moisture content in contrast the activities of aqueous species and dissolution precipitation rates of calcite were very sensitive to the changing moisture conditions in the upper part of the profile although the simulation involved numerous simplifying assumptions reasonable results were achieved and the calculated from the model rate of chemical denudation fell within the range of contemporary denudation rates determined from the dissolved loads of rivers

the chemical and hydrologic structure of poa s volcano costa rica comparison of the chemical characteristics of spring and river water draining the flanks of poa s volcano costa rica indicates that acid chloride sulfate springs of the northwestern flank of the volcano are derived by leakage and mixing of acid brines formed in the summit hydrothermal system with dilute flank groundwater acid chloride sulfate waters of the rio agrio drainage basin on the northwestern flank are the only waters on poa s that are affected by leakage of acid brines from the summit hydrothermal system acid sulfate waters found on the northwestern flank are produced by the interaction of surface and shallow groundwater with dry and wet acid deposition of so2 and h2so4 aerosols respectively the acid deposition is caused by a plume of acid gases that is released by a shallow magma body located beneath the active crater of poa s no evidence for a deep reservoir of neutral ph sodium chloride brine is found at poa s the lack of discharge of sodium chloride waters at poa s is attributed to two factors 1 the presence of a relatively volatile rich magma body degassing at shallow depths 1 km into a high level summit groundwater system and 2 the hydrologic structure of the volcano in which high rates of recharge combine with rapid lateral flow of shallow groundwater to prevent deep seated sodium chloride fluids from ascending to the surface the shallow depth of the volatile rich magma results in the degassing of large quantities of so2 and hcl these gases are readily hydrolyzed and quickly mix with meteoric water to form a reservoir of acid chloride sulfate brine in the summit hydrothermal system high recharge rates and steep hydraulic gradients associated with elevated topographic features of the summit region promote lateral flow of acid brines generated in the summit hydrothermal system however the same high recharge rates and steep hydraulic gradients prevent lateral flow of deep seated fluids thereby masking the presence of any sodium chloride brines that may exist in deeper parts of the volcanic edifice structural stratigraphic and topographic features of poa s volcano are critical in restricting flow of acid brines to the northwestern flank of the volcano a permeable lava lahar sequence that outcrops in the rio agrio drainage basin forms a hydraulic conduit between the crater lake and acid chloride sulfate springs spring water residence times are estimated from tritium data and indicate that flow of acid brines from the active crater to the rio agrio source springs is relatively rapid 3 to 17 years hydraulic conductivity values of the lava lahar sequence calculated from residence time estimates range from 10 5 to 10 7 m s these values are consistent with hydraulic conductivity values determined by aquifer tests of fractured and porous lava pyroclastic sequences at the base of the northwestern flank of the volcano fluxes of dissolved rock forming elements in rio agrio indicate that approximately 4300 and 1650 m3 of rock are removed annually from the northwest flank aquifer and the active crater hydrothermal system respectively over the lifetime of the hydrothermal system 100 s to 1000 s of years significant increases in aquifer porosity and permeability should occur in marked contrast to the reduction in permeability that often accompanies hydrothermal alteration in less acidic systems average fluxes of fluoride chloride and sulfur calculated from discharge and compositional data collected in the rio agrio drainage basin over the period 1988 1990 are approximately 2 38 and 30 metric tons day these fluxes should be representative of minimum volatile release rates at poa s in the last 10 to 20 years

diversity of eruptive styles inferred from the microlites of mt pelee andesite martinique lesser antilles abstract we have studied the compositional and textural characteristics of plagioclase microlites from several dome forming eruptions dome block and ash flows and surges and plinian fallouts of the recent period of activity of mt pelee in order to infer magma ascent dynamics dome forming and plinian samples display a wide range of plagioclase microlite compositions from an 35 to an 90 and an 55 to an 90 respectively microlites with compositions an 50 60 crystallized from a mafic melt that intruded the andesitic reservoir in proportion of 10 vol during magma ascent in the conduit the fraction number density and morphology of the decompression induced microlites compositions 50 60 suggest that the plinian magmas experienced a nucleation dominated crystallization regime with high undercooling in contrast magmas from domes and block and ash flows were dominated by a growth regime with low undercooling and the surges were influenced by both regimes according to these crystallization regimes and new experimental data on h 2 o content of the matrix glasses we propose magma relative ascent rates and fragmentation pressures for the different eruptive styles observed at mt pelee that may explain why surges are explosive events and why they may or may not be followed by a plinian event

phyllosilicates in hydrothermally altered basalts from dsdp hole 504b leg 83 a tem and aem study phyllosilicates occurring as replacements of olivine clinopyroxene and interstitial materials and as veins or fracture fillings in hydrothermally altered basalts from dsdp hole 504b leg 83 have been studied using transmission and analytical electron microscopy the parageneses of phyllosilicates generally change systematically with depth and with the degree of alteration which in turn is related to permeability of basalts saponite and some mixed layer chlorite smectite are the dominant phyllosilicates at the top of the transition zone chlorite corrensite and mixed layer chlorite corrensite occur mainly in the lower transition zone and upper levels of the sheeted dike zone chlorite talc and mixed layer talc chlorite are the major phyllosilicates in the sheeted dike zone although replacement of talc or ohvine by saponite is observed the phyllosilicates consist of parallel or subparallel discrete packets of coherent layers with packet thicknesses generally ranging from 100 a to a few hundred a the packets of saponite layers are much smaller or less well defined than those of chlorite corrensite and talc indicating poorer crystal linity of saponite by contrast chlorite and talc from the lower transition zone and the sheeted dike zone occur in packets up to thousands of a thick the si si al ratio of these trioctahedral phyllosilicates increases and fe fe mg decreases in the order chlorite corrensite saponite and talc these relations reflect optimal solid solution consistent with minimum misfit of articulated octahedral and tetrahedral sheets variations in composition of hydrothermal fluids and precursor minerals especially in si si al and fe fe mg ratios are thus important factors in controlling the parageneses of phyllosilicates the phyllosilicates are generally well crystallized discrete phases rather than mixed layered phases where they have been affected by relatively high fluid rock ratios as in high permeability basalts in veins or areas adjacent to veins intense alteration in basalts with high permeability indicating high fluid rock ratios is characterized by pervasive albitization and zeolitization minimal alteration in the basalts without significant albitization and zeolitization is characterized by the occurrence of saponite mixed layer chlorite smectite in the low temperature alteration zone and mixed layer chlorite corrensite or mixed layer talc chlorite in the high temperature alteration zone textural non equilibrium for phyllosilicates is represented by mixed layering and poorly defined packets of partially incoherent layers the approach to textural equilibrium was controlled largely by the availability of fluid or permeability

on measuring the electrical conductivity of the oceanic crust by a modified magnetometric resistivity method magnetotelluric sounding reveals a conductive zone beneath the pacific ocean at depths in excess of 60 km but does not resolve the resistivity of the lithosphere above this zone further resolution can be obtained by controlled source electrical methods the simplest of these are the galvanic techniques dipole dipole resistivity sounding is not suitable because dipole separations of thousands of kilometers would be required to obtain values of the resistivity a viable alternative is to measure on the ocean floor the magnetic field of a vertical bipolar source extending from the sea surface to the seafloor magnetometer transmitter separations of only a few kilometers are sufficient to determine the resistivity of a half space beneath the ocean sounding curves similar to those of the resistivity method may be constructed to resolve the resistivity of a layered lithosphere the curves constructed are valid at alternating frequencies which are small compared with a skin frequency not in the ocean but in the lithosphere the depth of penetration is of the order of half the transmitter receiver separation magnetic field amplitudes are in the range of picotesla for reasonable lithospheric resistivities and separations up to 10 times the length of the bipole modern instrumentation modified for the ocean floor can detect such signals at a range of 20 km at a frequency of about 0 02 hz averaged over several hours

properties of the lateral electromagnetic field of a vertical dipole and their applications the formullas for the electromagnetic field in region 1 z 0 k1 due to a vertical electric dipole also in region 1 near its plane boundary with region 2 z 0 k2 k2 2 k1 2 are reviewed the corresponding formulas for the field in region 2 are then determined use is made of this field to determine the locus of the poynting vector and the maximum depth of penetration into region 2 of the lateral wave part of the field application is made to the use of the vertical antenna in the sea for the measurement of the conductivity of the oceanic crust

implications of replacement for reaction transport modeling abstract mineral replacement in rocks consists of growth of guest mineral and dissolution of host the two reactions are coupled by the grain grain stress generated by growth of the guest grain in a rigid or very viscous rock in which it has initially no available room this stress self adjusts to make the volumetric rates of guest growth and host dissolution equal to each other which accounts for the volume conservation typical of replacement volume preservation during replacement is required by conservation of mass and momentum as well the two reactions are also simultaneous and proceed by a sequence of many tiny alternating increments where these increments are small enough internal textural details of the host grain or grain aggregate are morphologically preserved as ghost textures by the replacement the strong variation of mineral reaction rates with stress required by replacement is evidenced also experimentally reported widespread replacement in many rocks laterites diagenesis of siliciclastics and carbonates metasomatic ores metamorphic rocks and hydrothermal alteration warrants creating water rock reactive transport models that predict when and where in a system replacement should take place current geochemical models implicitly view replacement as sequential uncoupled dissolution of host and growth of guest this sequence of reactions may also occur in rocks but crucially differs from replacement implications of replacement for water rock reaction modeling include 1 balancing of replacement reactions on volume 2 use of helmholtz rather than gibbs free energies of minerals involved in replacement 3 taking account of the strong stress dependence of mineral growth and dissolution rates in rocks and 4 taking account of possible accommodation of growth by local deformation as well as pressure solution of the host rock and of the effect of strain rate growth rate on local rock viscosity

oxygen isotopic composition of carbonate in the nakhla meteorite implications for the hydrosphere and atmosphere of mars abstract we have located small areas of siderite within the mesostasis of the nakhla meteorite high concentrations of mn up to 50 rhodochrosite and elevated d h ratios indicate that the siderite is not a terrestrial alteration product the isotopic composition of oxygen in the siderite has been determined with the manchester isolab 54 ion microprobe the siderite has 18 o 34 1 which is higher than any other martian carbonate yet reported if the 18 o value is the result of equilibration with water at 2 h 2 o produced during the degassing of mars and not modified subsequently isotopically formation from a water rich fluid at 60 c requires that the fluid was heavy isotope enriched relative to fluids produced during planetary degassing an enrichment of 8 15 is consistent with theoretical outgassing models that are able to account for enhancements of 17 o in martian alteration products estimated deposition temperatures would be raised to 80 170 c the effect of a global scale fixation of martian co 2 as carbonate operates in the opposite direction and could lead to a reduction in 18 o of the martian hydrosphere of a few permil

diagenetic replacement controlled by force of crystallization many diagenetic replacements in sedimentary rocks occur along thin solution films between the authigenic and host phases rather than conventionally invoking bulk pore water undersaturation for host phase dissolution we propose that many diagenetic replacements occur by a force of crystallization controlled replacement mechanism whereby nonhydrostatic stresses resulting from authigenic crystal growth are principally responsible for host phase dissolution this model explains diagenetic features such as the restriction of host phase dissolution to authigenic host crystal contacts and euhedral authigenic crystal faces in planar contact with unreplaced host phases as well as the relative replacement tendencies of some common authigenic minerals

three dimensional oxygen isotope imaging of convective fluid flow around the big bonanza comstock lode mining district nevada oxygen isotope analyses of propylitized andesites from the con virginia and california mines allow construction of a detailed three dimensional image of the isotopic surfaces produced by the convective fluid flows that deposited the famous big bonanza orebody on a set of intersecting maps and sections the 18o isopleths clearly show the intricate and conformable relationship of the orebody to a deep 500 m gyre of meteoric hydrothermal fluid that circulated along and above the comstock fault near the contact of the davidson granodiorite the core of this gyre 18o 0 to 3 8 encompasses the bonanza and is almost totally surrounded by rocks having much lower 18o values 1 0 to 4 4 this deep gyre may represent a convective longitudinal roll superimposed on a large unicellular meteoric hydrothermal system producing a complex flow field with both radial and longitudinal components that is consistent with experimentally observed patterns of fluid convection in permeable media

offshore electrical exploration of sedimentary basins the effects of anisotropy in horizontally isotropic layered media our electrical method for exploring beneath the sea consists of a long vertical bipolar ac source extending downward from the sea surface to the bottom of the sea and a remote encapsulated microprocessor controlled magnetometer located on the sea floor the amplitude and phase of the magnetic field are measured over a range of suitable frequencies and transmitter magnetometer separations at the low frequency static limit apparent resistivity curves similar to standard schlumberger resistivity sounding curves are constructed as an aid in the direct interpretation of isotropic crustal resistivity an intermediate relatively resistive or relatively conductive zone is detectable when the transmitter receiver separation exceeds the order of twice the depth to the zone the physical property resolved by the method in an anisotropic crust which has different horizontal and vertical resistivities is the geometric mean of the two independent resistivities the thickness of a layer is indeterminate a la

shrinkage and sediment supply control on multiple calcrete profile development a case study from the maastrichtian of central india multiple calcrete profiles of the mottled nodular beds form part of an extensive dinosaur bearing infratrappean regolith in central india the profiles have clear variations in the distribution of carbonate morphologies powdery nodular and sheet platy calcretes sheet calcrete is however distinctive and resembles to some extent a k horizon these calcretes are interpreted as having formed in the soil vadose zone profile development was governed mainly by the pattern and depth of shrinkage it is possible that the mature stage 3 to incipient stage 4 calcrete sheets of these shrinkage related profiles formed quite rapidly and certainly in no more than about 400 000 years prominent shrinkage cracking appears to have favoured quite rapid calcrete formation and associated rhizocretion formation was also likely to have been rapid a combination of climatic conditions carbonate availability and volcanism induced sediment starved conditions resulted in the development of a stack of fourteen calcrete profiles sediment starvation allowed most of the stratigraphic interval to be affected to some degree by calcareous pedogenesis this contrasts with other phanerozoic calcrete stacks where nodular carbonate typically occurs in discrete horizons caused by relatively higher sediment supply

mafic dike swarms in the south shetland islands volcanic arc unravelling multiepisodic magmatism related to subduction and continental rifting eight groups of mafic dikes and related high level stocks cut triassic accretionary complex and mesozoic magmatic arc formations on livingston island some are affected by silicic sericitic alteration related to cretaceous hydrothermal activity and propylitic epidosite alteration analogous to that in ocean floor sheeted dikes alteration was accompanied by major and trace element metasomatism ar ar analysis of the freshest rocks indicates five intrusive events some of which are unexpectedly young groups 1 3 were intruded in the mid to late cretaceous 108 74 ma and were coeval with the calc alkaline arc between 70 and 50 ma relatively rapid and oblique plate convergence led to strike slip tectonism and a pause in magmatism at 52 ma orthogonal slow convergence resulted in extensional faulting and emplacement of calc alkaline group 2 and primitive tholeiitic dikes groups 4 6 between 51 and 45 ma extension of antarctic peninsula southern south american crust culminated in emplacement of mafic to intermediate medium grained plutons and group c porphyries between 44 and 36 ma localized hydrothermal flow along fault zones resulted in partial to complete argon loss from nearby cretaceous lavas and ar ar reset ages of 40 ma in mid cretaceous hydrothermal k feldspar primitive olivine basalts group d and epithermal carbonate veins 31 29 ma were emplaced during along arc extension accompanying the opening of drake passage and powell basin excess argon occurs in two forms strongly held in melt inclusions in the primitive tholeiites and weakly held in some secondary alteration there is no radiometric evidence in the area studied for magmatism related to late cenozoic subduction nor to the pleistocene recent opening of the back arc bransfield rift

the past 5 000 years of volcanic activity at mt pelee martinique f w i implications for assessment of volcanic hazards the history of mt pelee martinique was subdivided into three stages based on field geology and 14c data the two first stages constructed an ancient mt pelee and an intermediate cone between 0 4 m y and 19 500 y b p the third or present stage started 13 500 years ago after a repose of 6 000 years this paper focuses on the activity of mt pelee during the past 5 000 years as a means to assess and zone volcanic hazards of the 23 magmatic eruptions during the past 5 000 years the ages of 21 eruptions of this period are based on 75 new 14c dates the types of phenomena and distribution of pyroclasts relate to four main types of activity the first type consists of pumice and ash flows that are not preceded by a plinian fall two eruptions named p6 and p4 illustrate this type for which the mixture of gas ash and pumice simply overflow the vent and flood several valleys the second type differs from the first by the occurrence of a preliminary moderate plinian fall stage four eruptions p5 p31 p2 and p1 illustrate this type two eruptions p32 and p33 experienced cataclysmic plinian explosions and pumiceous surges the third type is related to dome growth with the rise of viscous spines and the production of related block and ash flows five eruptions 1929 sept 1902 1904 npm nab2 and nmp illustrate this type the fourth type is characterized by violent ejection of more or less heterogeneous nuees ardentes the direction of the blast dictated by the morphology of the crater has been towards the south several times at mt pelee four eruptions may 1902 nab1 nrp2 and nrp3 belong to this type future magmatic eruptions at mt pelee will very likely belong to one of these four types assessment of hazards at mt pelee is based upon the behavior of the volcano during the past 5 000 years because 1 recognition of past magmatic eruptions is quite complete and well dated and 2 no structural change has occurred in the volcano a probabilistic and statistical approach has been tentatively followed stochastic models of wickman indicate a the chance that contemporary abnormal seismic and or phreatic activity at the volcano would signal an impending magmatic eruption is 20 percent b mt pelee exhibits a complex loading time behavior that may be related to the buffering effect of a magma chamber time is needed following a single or several closely spaced eruptions to establish conditions to initiate a new eruption and c if the 1929 event can be regarded as the last episode of the major 1902 eruption the volcano could remain in a dormant stage with respect to magmatic events for another century

interactions between volcanism and geodynamics in the southern termination of the ecuadorian arc abstract this study focuses on the construction and evolution through time of volcanic edifices located in the southern part of the ecuadorian arc with the objective to contribute to a better understanding of the interactions between magmatism slab geometry and the activity of tectonic faults our new groundmass k ar ages obtained for a dozen volcanoes from the southern quaternary arc are rather young without ages older than 800 ka and highlight an increasing volcanic activity between 300 and 100 ka these new temporal constraints suggest that a southward migration of the ecuadorian arc occurred during the last 600 ka we interpret this evolution as the result of major and recent geodynamic reorganization in ecuador related to the activation of lithospheric faults and to the flexure of the slab at depth following the inland prolongation of the grijalva fracture zone both phenomena could have been induced by the oblique subduction of the nazca plate towards the convex margin of the south american continent and the coupling of these two plates along the wadati benioff zone progressively intensified by the carnegie ridge subduction southward migration of the slab flexure at depth could have changed pressure and temperature conditions favoring the partial melting of the mantle wedge and magma genesis thereby inducing a southward migration of the ecuadorian arc volcanoes these results contribute therefore to a better knowledge of the current arc dynamics in the northern andes and provide insights into the use of geochronological data applied to volcanic rocks for studies of past tectonic activity in ecuador

development of magnesian carbonates in quaternary soils on the island of hawaii abstract pedogenic carbonate minerals have progressively developed on basalt in a quaternary weathering chronosequence 5 350 ka on the island of hawaii the sites were isolated from direct contact with seawater or the water table and were minimally influenced by eolian input throughout their history the carbonates consist chiefly of high magnesium calcite hmc and well ordered stoichiometric dolomite pedogenic development of such minerals is rare and the conditions of their formation bear on interpretations of paleosols and the development of sedimentary magnesian carbonates 87sr 86sr ratios of chronosequence carbonates range from 0 7038 to 0 7048 the data indicate that 75 of the strontium was derived from the weathering of volcanic parent material even in the oldest soils where few primary minerals are available to provide ca and mg this is in contrast to continental calcretes which are commonly dominated by atmospheric inputs and are generally composed of low mg calcite lmc stable isotope data are consistent with carbonate precipitation from soil waters in an arid environment the magnesium content of the carbonate increases with substrate age and textural and mineralogical changes suggest direct precipitation of dolomite as well as dolomitization of early formed lmc and hmc opal was detected in young 5 10 ka samples and gibbsite is present in older samples indicating a shift from silica dominated to aluminum dominated soil solution high concentrations of dissolved silica and aluminum during early weathering likely precluded formation of mg rich secondary clay minerals such as smectite or palygorskite weathering of basalt with a molar mg ca ratio of 0 8 1 5 and the absence of secondary magnesian silicates can account for elevated soil water mg ca ratios 0 7 1 0 in soils 70 ka that led to magnesian carbonate formation over time the formation of a clay rich soil profile prevented rapid evaporation of soil water possibly facilitating crystallization of stoichiometric dolomite

controlled electromagnetic sources for measuring electrical conductivity beneath the oceans 1 forward problem and model study exact closed form expressions for the electromagnetic induction fields produced by vertical and horizontal current sources in the conducting ocean overlying a one dimensional earth are derived from the maxwell equations numerical methods for the evaluation of the solutions are given including correction for the finite size of real sources simple models of the electrical conductivity structure of the ocean crust and lithosphere are deduced from geologic petrologic and laboratory data and their electromagnetic response is modeled horizontal electric dipole sources produce much larger field amplitudes than their vertical counterparts for a given frequency and range and the horizontal electric field offers superior received signal performance reflections of electromagnetic waves from the sea surface and thermocline must be considered for low enough frequencies or long ranges estimates of the ambient noise level from natural electromagnetic sources in the frequency range 0 01 10 hz are presented the ability of controlled sources to determine features of the conductivity of the ocean crust and upper mantle especially low conductivity zones is demonstrated if the mantle conductivity is low enough horizontal ranges of 50 km and conductivity estimates to over 20 km depth can be achieved

magmatology of mt pelee martinique f w i ii petrology of gabbroic and dioritic cumulates abstract the study of 24 plutonic xenoliths collected from mt pelee pyroclastic deposits shows that they can be subdivided into anorthite pargasite salite bearing gabbroic adcumulates type i olivine bearing type ii magnetite bearing and bytownite hornblende hypersthene augite bearing dioritic orthocumulates additionally olivine diopside pargasite and anorthite from a basaltic megacryst rich ash layer have been considered mineralogical data show continuous and progressive variations in the compositions of the solid phases with increase of fe fe mg ratios during the petrographic evolution of the investigated cumulate xenoliths thermobarometric data on minerals or mineral liquid pairs indicate a decrease of crystallization temperatures from about 1090 c in gabbroic assemblages to about 850 c in dioritic assemblages the gabbroic assemblage is believed to form in a water bearing environment from a derivative calc alkaline liquid at sufficient depth to stabilize al rich pargasite i e 8 9 kbar we suggest that the materials investigated may represent respectively the bottom megacrysts the walls type i and ii gabbroic adcumulates and the roof dioritic orthocumulates of stratified magma chamber s underlying mt pelee volcano

distribution of calcium carbonate in desert soils a model a model that describes the distribution of calcium carbonate in desert soils as a function of dust flux time climate and other soil forming factors shows which factors most strongly influence the accumulation of carbonate and can be used to evaluate carbonate based soil age estimates or paleoclimatic reconstructions models for late holocene soils have produced carbonate distributions that are very similar to those of well dated soils in new mexico and southern california these results suggest that 1 present climate is a fair representation of late holocene climate 2 carbonate dust flux can be approximated by its holocene rate and 3 changes in climate and or dust flux at the end of the pleistocene effected profound and complex changes in soil carbonate distributions both higher carbonate dust flux and greater effective precipitation are required during the latest pleistocene early holocene to explain carbonate distributions in latest pleistocene soils 21 refs 4 figs 1 tab

volcanic landforms and ore deposits metals and lithophile element deposits related to four types of high level magmatic systems each characterized at the surface by a distinct volcanic landform stratovolcano flow dome complex caldera and maar porphyry copper bearing stocks and base metal rich precious metal veins are known from the deeper central zones of andesitic stratovolcanoes precious and base metal veins and breccias as well as cu pb zn ag replacement orebodies and lithophile element mineralization were emplaced in the shallow parts of flow dome complexes tin beryllium uranium and fluorine mineralization in rhyolite flow domes may be a manifestation of subjacent porphyry molybdenum systems precious metal vein deposits are associated with ring fracture domes around resurgent calderas and moat sediments are known to carry dispersed silver or mercury uranium mineralization many large calderas appear to be relatively deficient in temporally related ore deposits possibly as a result of volatile dissipation during pyroclastic eruption modified journal abstract

the character and genesis of calcrete in late quaternary alluvial deposits gujarat western india and its bearing on the interpretation of ancient climates late quaternary deposits in mainland gujarat contain sediments deposited in subhumid and semi arid climates the 30 35 m succession shows the presence of vertisols at the base and a red bed horizon ferric calcisol when pedogenic in origin that roughly bisects the succession a widespread development of calcretes is observed throughout the succession the various varieties of calcrete include pedogenic calcrete groundwater calcrete calcrete conglomerate transported calcrete and rhizogenic calcrete pedogenic calcrete nodules associated with vertisols and the red soil show marked differences in morphology dimensions and the distribution of microscopic features these differences arise due to contrasting climate controlled physicochemical environs under which they formed pedogenic calcrete nodules associated with vertisols acquire large 5 10 cm dimensions and are characterised by either showing the presence of a nucleus of soil matter or showing a dense micritic groundmass cut by thick sparitic veins in contrast calcrete nodules that formed in the red soil are 3 cm in size and do not exhibit dense networks of sparitic veins calcretes associated with vertisols and the ferric calcisol also exhibit differences in the morphology of rhizoliths these differences also show up in the distribution of microfabrics of calcretes grain coats are present in rhizogenic and pedogenic calcretes but absent in groundwater calcretes and less profuse in hydromorphic soil calcretes clotted micrite is present in all types except groundwater calcretes sparitic veins however are observed in each type but are relatively less developed in groundwater calcretes a similar distribution of displacive and replacive textures is also seen although some grains in groundwater calcretes showed signs of corrosion the vertisol associated calcretes represent a subhumid 500 700 mm climate whereas the red soil calcretes suggest a semi arid 100 500 mm climate calcretes from the vertisol association show a range in 13c composition constrained between 9 and 5 whereas the red soil calcretes exhibit the whole spectrum of values from 9 to 1 based on mineralogical associations calcretes usually taken to reflect semi arid episodes in the earth s history may be classified further calcretes when associated with sepiolite palygorskite suggest an arid climate mean annual rainfall 50 100 mm when associated with smectite haematite and the absence of hydromorphism a semi arid climate mean annual rainfall 100 500 mm and when found in smectitic vertisols subhumid climates mean annual rainfall 500 700 mm

calcrete and palustrine assemblages on a distal alluvial floodplain a response to local subsidence miocene of the duero basin spain abstract in the upper miocene of the eastern duero basin two carbonate units cu 1 and cu 2 display a lateral transition from distal alluvial floodplain red mudstones to calcrete and an upwards transition from calcrete to palustrine facies the carbonate content in the calcrete increases upwards as shown in sequences from red or reddish brown mudstones with diffuse calcification to nodular calcrete facies to massive calcretes the palustrine facies display a broad range of textures related to the degree of exposure peloidal intraclast textures formed during the most prolonged phases of exposure and are those most commonly seen in the palustrine facies the transition from calcrete to palustrine facies occurs in two ways 1 as an indurated limestone level with mixed features of calcrete and palustrine facies and 2 a well delimited boundary between the two facies which show very similar characteristics in the vicinity of this boundary the 13cpdb and 18opdb values of the calcretes are similar to those of the palustrine facies this is because their genesis was related to groundwaters which inherited the isotopic signature of meteoric waters good covariance between 13c and 18o palustrine values r 0 78 is typical of lakes in semi arid regions with long water residence times in closed basins the calcrete palustrine association records a progressive decrease in the terrigenous supply and a continuous elevation of the groundwater table until it intersected with the surface to create a pond the rise in the groundwater table resulted from local subsidence that was not compensated by terrigenous supply leading to a lowering of the floodplain changes in the thickness of palustrine facies from central to marginal pond areas with ubiquitous exposure features suggest that the pond was progressively filled up by bio chemical carbonate precipitation balancing the available space created by subsidence this maintained the shallow character of the pond deposits

calc silicate mineralization in active geothermal systems detailed study of the calc silicate mineralogy in the cerro prieto geothermal system was used to demonstrate examples for thermodynamic evaluation of phase relations among minerals of variable composition and to calculate the chemical characteristics of hydrothermal solutions compatible with the observed calc silicate assemblages in general there is a close correlation between calculated and observed fluid compositions calculated fugacities of o 2 at about 320 degrees c in the cerro prieto geothermal system are about five orders of magnitude less than those at the nearby salton sea geothermal system this observation is consistent with the occurrence of fe super 3 rich epidotes and andradite garnets in the latter system and the presence of prehnite at cerro prieto modified journal abstract

stratigraphic lithologic and major element geochemical constraints on magmatic evolution at lassen volcanic center california the lassen volcanic center is the most recent of several long lived volcanic centers in the southernmost cascade range these centers have erupted products ranging from basaltic andesite to rhyolite and are superimposed on a background of regional basaltic to andesitic volcanism the evolution of the lassen volcanic center is described in three stages stages i and ii comprise the brokeoff volcano and 80 km3 andesitic stratocone active from 600 to 400 ka brokeoff volcano is compositionally equivalent to the regional basaltic andesite to andesite volcanism in the lassen region and is the result of structurally controlled focusing of the diffuse regional mafic magmatism stage iii comprises a silicic dome field and adjacent area of hybrid andesites and has a total volume of about 100 km3 volcanism during stage iii was episodic and is subdivided into four sequences of lithologically and temporally distinct lavas stage iii began at 400 ka with a rhyolitic caldera forming pyroclastic eruption and chemically related lavas additional sequences of dacite erupted between 250 200 ka and 100 0 ka hybrid andesites erupted adjacent to the silicic dome field between 300 and 0 ka porphyritic andesite and dacite with high al2o3 low tio2 medium k2o and feo mgo ratios of 1 5 2 0 are the most abundant rock types in the lassen volcanic center however the single most voluminous unit is sparsely phyric rhyolite pumice in general the lavas of lassen volcanic center form a single coherent trend on major element variation diagrams and in pseudo quaternary phase space consistant with an origin either by fractional crystallization or magma mixing in detail however the lack of systematic temporal change in silica and subtly crossing trends indicate a complex origin a variety of statistically successful fractional crystallization models can be constructed that derive brokeoff andesites from regional magmas an important conclusion of the modeling is that if fractional crystallization is the process responsible for generation of brokeoff andesite then the parent magma must be low to medium k in geochemical affinity in order to explain the variation in k2o however although major element variation can be modeled by fractional crystallization petrographic and stratigraphic evidence indicates that magma mixing is an important but subtle process in brokeoff lavas and suggests that lavas evolved in small independent batches lavas erupted during stage iii while predominantly silicic range from 53 to 75 sio2 disequilibrium mineral assemblages in the stage iii lavas indicate that they are not directly derived from brokeoff andesite by fractional crystallization mixing of silicic magma with regional mafic magma and disaggregation of andesitic quenched magmatic inclusions play dominant roles in the compositional diversity of stage iii lavas

dolomitized calcrete in the middle devonian winnipegosis carbonate mounds subsurface of south central saskatchewan canada dolomitized calcrete is a widespread diagenetic feature in the uppermost portions of the middle devonian winnipegosis mud mounds in the subsurface of south central saskatchewan individual calcrete profiles are composed of 2 to 5 horizons including in descending order laminar crust massive pisolitic breccia chalky and transitional horizons the calcrete profiles are interpreted to be pedogenic in origin formed by diagenetic alteration of the host carbonate deposits during subaerial exposure of the mud mounds this interpretation is based on the occurrence of an orderly set of well differentiated horizons disconformities vadose pisoids micritic stringers circumgranular cracks linkage coatings and gradational contacts between the calcrete and underlying host carbonate rocks lack of root related structures such as rhizoconcretion and alveolar septal fabric in the winnipegosis calcrete suggests a limited influence of macrophytes on the formation of calcrete the occurrence of up to three discrete pedogenic calcrete profiles in a single winnipegosis calcrete succession indicates the mounds underwent three periods of subaerial exposure that resulted from drops of water level in the elk point basin of saskatchewan during middle devonian time d 2004 elsevier b v all rights reserved

late cenozoic volcanism subduction and extension in the lassen region of california southern cascade range hundreds of short lived small to moderate volume mostly mafic volcanoes occur throughout the lassen region of ne california and surround five longer lived large volume intermediate to silicic volcanic centers younger than 3 ma volcanic rocks older than 7 ma are scarce in the lassen region we identify 537 volcanic vents younger than 7 ma and we classify these into five age intervals and five compositional categories based on sio2 content maps of vents by age and composition illustrate regionally representative volcanic trends by 2 ma the eastern limit of voicanism had contracted westward toward the late quaternary arc late quaternary volcanism is concentrated around and north of the silicic lassen volcanic center the belt of most recent volcanism 25 0 ka has been active since at least 2 ma most mafic volcanism is cakalkaline basalt and basaltic andesite however lesser volume of low potassium olivine tholeiite lkot a geochemically distinctive basalt type found in the northern basin and range province also has erupted throughout the lassen segment of the cascade arc since the pliocene thus models of the mantle source and tectonic control of lkot magmatism should be applicable both within and behind the subduction related arc normal faults and linear groups of vents are evidence of widespread crustal extension throughout most of the lassen region nnw alignments of these features indicate nnw orientation of maximum horizontal stress ene extension which is similar to the stress regime in the adjacent northwestern basin and range and northern sierra nevada provinces the large long lived volcanic centers developed just west of a zone of closely spaced nnw trending normal faults within that zone of faulting pervasive ene extension has precluded growth of large long lived crustal magma systems we interpret the western limit of the zone of nnw trending normal faults as the western boundary of the basin and range province where it overlaps the lassen segment of the cascade arc in our view the lassen volcanic region occurs above the subducting gorda north plate but also lies within a broad zone of distributed extension that occurs in the north american lithosphere east and southeast of the present cascadia subduction zone an episode of ene extension that began in the late miocene in the northwestern basin and range province appears to have triggered widespread late miocene to quaternary mafic volcanism in the lassen region the scarcity of volcanic rocks older than 7 ma suggests that a more compressive lithospheric stress regime prior to the late miocene extensional episode may have suppressed volcanism even though subduction probably was occurring beneath the lassen region

isotope geochemistry of minerals and fluids from newberry volcano oregon isotopic compositions were determined for hydrothermal quartz calcite and siderite from core samples of the newberry 2 drill hole oregon the 15o values for these minerals decrease with increasing temperatures the values indicate that these hydrothermal minerals precipitated in isotopic equilibrium with water currently present in the reservoirs the 18o values of quartz and calcite from the andesite and basalt flows 700 932 m have isotopic values which require that the equilibrated water 18o values increase slightly 11 3 to 9 2 with increasing measured temperatures 150 265 c the lithic tuffs and brecciated lava flows 300 700 m contain widespread siderite calculated oxygen isotopic compositions of waters in equilibrium with siderite generally increase with increasing temperatures 76 100 c the 18o values of siderite probably result from precipitation in water produced by mixing various amounts of the deep hydrothermal water 10 5 with meteoric water 15 5 recharged within the caldera the 13c values of calcite and siderite decrease with increasing temperatures and show that these minerals precipitated in isotopic equilibrium with co2 of about 8 the 18o values of weakly altered 5 alteration of plagioclase whole rock samples decrease with increasing temperatures above 100 c indicating that exchange between water and rock is kinetically controlled the water rock mass ratios decrease with decreasing temperatures the 18o values of rocks from the bottom of newberry 2 show about 40 isotopic exchange with the reservoir water the calculated 18o and d values of bottom hole water determined from the fluid produced during the 20 hour flow test are 10 2 and 109 respectively the d value of the hydrothermal water indicates recharge from outside the caldera

the evolution of the waiotapu geothermal system new zealand based on the chemical and isotopic composition of its fluids minerals and rocks abstract the waiotapu geothermal system is hosted by silicic rocks of the taupo volcanic zone new zealand exploration drilling in the late 1950s down to 1100 m provided physical information on the system measured temperatures show a boiling profile to 295 c with shallow inversions particularly in the north total discharge fluid samples were collected the geothermometry and measured temperatures show that fluids derive mainly from a shallow 400 m deep reservoir at about 225 c petrologic study of drillcore samples recovered from seven wells reveals an alteration assemblage of quartz and albite adularia with a variable distribution of chlorite pyrite calcite zeolites epidote pyrrhotite sphene leucoxene apatite and minor base metal sulfides white mica is a late overprint particularly well developed at shallow depths surficial alteration of kaolin cristobalite alunite and smectite clays reflect alteration by acid sulfate steam heated waters the activities of components in minerals determined from microprobe analyses and composition activity relations and fluids speciated to reservoir conditions indicate equilibrium now exists between the fluids and white mica the na k ratio of the fluid is being controlled by dissolution of albite and adularia while its h 2 h 2 s ratio is buffered by pyrite replacing pyrrhotite the fluids are now slightly undersaturated with respect to calcite the present deep fluids boil adiabatically from at least 300 c to 230 c at depths of 500 m this ascending chloride fluid is variably diluted by a steam heated water of zero chloride that lies over and occurs on the margin of the system like a discontinuous umbrella the steam heated water is relatively co2 rich 0 1 m the cooling at shallow levels by this mixing has shifted the alteration from albite adularia stability to white mica stability this shift is enhanced by the co2 rich nature of the diluent dilution of ascending chloride fluids by shallow waters is also supported by the oxygen and hydrogen isotopic composition of the mixture with the deep fluid being enriched in 18o and d from local meteoric by 7 and 10 respectively the patterns in whole rock 18o indicate that they were largely shifted in isotopic composition prior to incursion of steam heated waters possibly induced by a series of hydrothermal eruptions 900 years ago in contrast the 18o composition of late vug calcite indicates its formation is related to the initial incursion of steam heated groundwater and subsequent cooling this is supported by fluid inclusion evidence the d shift from local groundwater composition and the 13c composition of co2 determined from calcite 4 to 6 may be evidence for a magmatic input to the meteoric convection cell the shallow portion of the waiotapu geothermal system has recently evolved both chemically and physically by incursion of fluids from a steam heated carapace continued refluxing of these relatively cool hybrid fluids progressively deeper with their recycled co2 content will hasten hydrolytic leaching in contrast to a single pass of adiabatically cooling deep fluids this action accompanied by argillic alteration may eventually seal the deeper portions of the system hastening its demise there is evidence for similar events occurring in the fossil environment at epithermal depths

chlorine isotopes of thermal springs in arc volcanoes for tracing shallow magmatic activity the evaluation of the status of shallow magma body i e from the final intrusion stage to quiescence and back to activity one of the key parameters that trigger and sustain volcanic eruptions has been challenging in modern volcanology among volatile tracers chlorine cl uniquely exsolves at shallow depths and is highly hydrophilic consequently cl enrichment in volcanic gases and thermal springs has been proposed as a sign for shallow magmatic activities however such enrichment could also result from numerous other processes e g water evaporation dissolution of old chloride mineral deposits seawater contamination that are unrelated to magmatic activity here based on stable isotope compositions of chloride and dissolved inorganic carbon as well as previous published 3he 4he data obtained in thermal springs from two recently erupted volcanoes la soufriere in guadeloupe and montagne pelee in martinique in the lesser antilles arc we show that the magmatic cl efficiently trapped in thermal springs displays negative 37cl values 0 65 0 65 consistent with a slab derived origin but distinct from the isotope compositions of chloride in surface reservoirs e g seawater local meteoric waters rivers and cold springs displaying common 37cl values of around 0 using this 37cl difference as an index of magmatic cl we further examined thermal spring samples including a 30 year archive from two thermal springs in guadeloupe covering samples from its last eruption in 1976 1977 to 2008 and an island wide sampling event in martinique in 2008 to trace the evolution of magmatic cl in the volcanic hydrothermal systems over time the results show that magmatic cl can be rapidly flushed out of the hydrothermal systems within 30 to 80 years after the eruption much quicker than other volatile tracers such as co2 and noble gases which can exsolve at greater depths and constantly migrate to the surface because arc volcanoes often have well developed hydrothermal systems where magmatic cl is easily transferred to the surface following its exsolution from shallow magma body we suggest that 37cl has great potential to be a unique proxy to monitor the cessation and revival of infrequent arc volcanoes particularly at centennial time scales

regional 18o depletion of neoproterozoic igneous rocks from avalonia cape breton island and southern new brunswick canada neoproterozoic igneous rocks of the avalonian mira terrane cape breton island and caledonia terrane southern new brunswick have experienced regional 18 o depletion the majority of these rocks have 18 o wr values between 1 and 6 markedly different from igneous rocks of the inboard ganderian terranes which have normal high 18 o wr values of 7 to 12 the 18 o depletion of these avalonian terranes resulted from pervasive hydrothermal alteration all of the igneous rocks contain classic propylitic alteration assemblages of turbid feldspar chlorite epidote sericite and calcite and host multiple generations of veins containing the same phases the 18 o values of the rock forming minerals show strong disequilibrium with qtz fs ranging from 2 5 to 11 0 and kfs plg from 0 5 to 1 0 such values are indicative of post magmatic hydrothermal alteration the hydrothermal alteration occurred at ca 560 550 ma during initial transcurrent rifting of avalonia at the gondwanan margin the development of rift wrench basins with associated bimodal volcanism allowed large volumes of hydrothermal fluids to penetrate and circulate within mira and caledonia terrane crust before its submergence in the early cambrian ca 540 530 ma the hydrothermal fluids were predominantly meteoric although the presence of a significant seawater component cannot be ruled out the almost ubiquitous 18 o depletion exhibited by the neoproterozoic rocks in these avalonian terranes is absent in the associated ganderian terranes suggesting that avalonia remained separate from ganderia until at least the cambrian ordovician this 18 o depletion provides an additional geochemical tool for identification of avalonian crust in other areas of the appalachians and europe

the smectite chlorite transition in drillhole nj 15 nesjavellir geothermal field iceland xrd bse and electron microprobe investigations abstract in well nj 15 of the nesjavellir geothermal field iceland the transition of discrete smectite into discrete chlorite has been studied from drill cuttings recovered at depths of less than 1714 m and over a continuous range of temperatures between 60 and 300 c at temperatures below 180 c the clay fractions contain mixtures of di and trioctahedral smectites whose layer charge increases with depth between 200 and 240 c discrete smectites have transformed into smectite rich randomly interstratified chlorite and smecite r0 c s because the abundance of chlorite interlayers in this c s is generally 20 its presence can be detected only by electron microprobe techniques and not by x ray diffraction between 245 and 265 c both regularly r1 and randomly interstratified c s are the predominant layer silicates discrete chlorite first appears at approximately 270 c and coexists with minor amounts of r0 c s at higher temperatures r0 and r1 c s form a nearly complete compositional series between trioctahedral saponite and discrete chlorite end members the interlayer cation and si content of smectites and c s decrease with increasing temperature the mg mg fe content of smectite c s and chlorite is unrelated to temperature the percentage of chlorite in c s as determined by electron microprobe analyses increases continuously with increasing temperature except for occurrences of smectite rich c s in fresh basaltic dykes which have not thermally equilibrated with the higher grade country rocks

systematics of u th disequilibrium in calcrete profiles lessons from southwest india pedogenic carbonates calcretes have often been proposed as possible markers of the pedogenetic processes in soils under semi arid climate however precise chronological constraints on their formation are required in order to investigate the climatic and paleo environmental conditions that prevailed during and after their formation and to improve our understanding of the physical and chemical conditions that promoted their development and preservation moreover these authigenic calcium carbonate precipitates provide us with an interesting test of the u th radioactive disequilibrium dating method the reliability of which has been demonstrated extensively in aragonitic corals and calcitic continental speleothems but which remains much more questionable in highly porous chemically and mineralogically complex media such as soil profiles we report here 238u 234u 230th radioactive disequilibrium analyses measured by thermo ionization mass spectrometry in calcretes from two sites in southwest india since calcretes are impure carbonates mixed with various amounts of parent rock and weathered minerals all ages must be computed using the isochron technique involving the extraction of several coeval subsamples from the same soil horizon and u th analyses after total dissolution tsd method in order to correct for the detrital component 232th is usually used as index of the detrital contamination and the isochron age is derived from the isotopic composition 230th 238u 234u 238u of the pure authigenic carbonate end member of the mixing line our results show that each set of samples taken from decimetric blocks is characterized by a well defined isochron line in a 3 d osmond diagram 232th 238u 230th 238u 234u 238u the regression lines obtained for most of the samples show probabilities of fit satisfying the mathematical assumptions of the isochron model in most cases 0 05 and mean square weighted deviations mswd values around 1 the compositions of the end members are highly variable between the different isochrons both for the carbonate phase indicating various ages of cristallisation and for the detrital phase indicating various origins and compositions notably our data demonstrate that the detrital component is in all cases out of secular equilibrium by contrast with the standard paradigm of dirty carbonate dating which is based on the assumption of average continental crust th u elemental ratio and secular equilibrium values of the 230th 238u and 234u 238u ratios at the first site coimbatore we obtain u th ages between 304 38 ka and 44 2 9 ka 2 from the base to the top of the profile these suggest discontinuous precipitation of the different facies in good agreement with micromorphological observations at the second study site gundlupet the carbonate nodules sampled from two different profiles yield consistent ages of 200 ka which suggests that precipitation of nodular carbonate occurred across the entire pediment toposequence during a single event surprisingly in mere contradiction with the standard model of impure carbonate dating two puzzling samples display vertical isochrons i e variable 232th 238u without any variation of the 230th 238u and 234u 238u ratios such patterns require extremely high 232th 238u ratios in an as yet unidentified detrital phase taken at face value these results suggest that no clear correlation can be inferred between the precipitation of these pedogenic carbonates and major orbitally driven climate variations recorded in other archives during the last glacial interglacial cycles

pedogenic replacement of aluminosilicate grains by caco3 in ustollic haplargids south central montana u s a abstract a chronosequence of calcic soils formed on granitic glaciofluvial terrace deposits of rock creek and the clarks fork in south central montana shows progressive replacement of aluminosilicate parent material grains by calcium magnesium carbonate the terraces range from late pliocene to holocene in age as dated by tephrochronology correlation and stream incision rates replacement is first seen in soils that are as old as 120 000 yr the amount and degree of replacement increase in soils older than 120 000 yr along with the development of calcic horizons under the petrographic microscope carbonate replacement of quartz feldspars and the groundmass of andesite grains in rock creek soils is shown by embayed grains networks of carbonate along cracks and between parts of polycrystalline grains and optically aligned grain fragments within carbonate masses microprobe data suggest that silica is released by replacement because it is absent from carbonate filled spaces and is depleted in corrosion pits little microscopic evidence exists to support displacement of framework grains by carbonate because fragments of a single grain are rarely rotated out of optical alignment in the calcic soils of rock creek k fabric grains floating in a carbonate matrix may form by both replacement and displacement

calcrete as a source of heterogeneity in triassic fluvial sandstone aquifers otter sandstone formation sw england carbonate is not generally considered as a potential source of large scale heterogeneity in permo triassic sandstone aquifers this study shows that carbonate in the form of early diagenetic calcrete forms an abundant component of the triassic otter sandstone formation in south devon three types of calcrete are described from the outcrop of this fluvial sandstone dominated aquifer rhizocretions calcrete sheets and calcrete conglomerates data obtained from core plugs show that calcrete reduces sandstone permeability to less than 10 15 m2 and porosity to less than 12 calcrete can be sufficiently abundant to produce a significant up to 30 reduction in the total effective porosity of the aquifer moreover calcrete conglomerates can form laterally extensive up to 1 km low permeability sheets that will represent major baffles to vertical flow

sources of strontium and calcium in desert soil and calcrete the carbon cycle significance of soil carbonate fluxes is subject to large uncertainties because it is not clear precisely how much calcium is derived from atmospheric sources compared with that from the chemical weathering of silicate minerals in the petrocalcic horizon calcrete of a pleistocene soil from the usda scs desert project area near las cruces nm approximately 1 5 g ca cm 3 has been added with an associated expansion of the profile of200 strontium isotope values for the labile cations and carbonate from the a b and k soil horizons have 87 sr 86 sr values that range from 0 7087 to 0 7093 similar to the values for easily soluble local dust and rain the parent material non calcareous camp rice alluvial sediment has a 87 sr 86 sr ratio of0 7165 mixing calculations indicate a minimum atmospheric contribution to soil carbonate calcium of 94 the more likely scenarios indicate at least 98 of the ca originated from atmospheric input the variations in 87 sr 86 sr ratios of soil silicate 0 7131 to 0 7173 are consistent with weathering of volcanogenic sediments and neoformation of clay minerals in the petrocalcic horizon moreover the sr isotope data suggest that 50 70 of silicate in the uppermost 25 cm of the profile could be atmosphere derived the isotopic composition of labile strontium in the a horizon and the mass distribution of silicon and calcium indicate that the uppermost portion of the profile is the present zone for the release of cations due to silicate weathering steady state models of the whole profile yield a sr weathering flux ranging from200 to 400 m gc m 2 ma 1 the results indicate that both the present day and long term contribution of calcium from silicate weathering is less than 2 of that supplied from the atmosphere and confirm that desert soil formation is not a significant sink for atmospheric carbon 1999 elsevier science b v all rights reserved

petrology of philippine geothermal systems and the application of alteration mineralogy to their assessment abstract philippine geothermal systems occur in the vicinity of large holocene calc alkaline volcanic complexes wells drilled in these areas encountered multiple intrusions the latest dikes are the subsurface manifestations of the youngest heat source commonly at least two hydrothermal regimes are juxtaposed in a single area with the latest being in equilibrium with the present temperature and chemical regime alteration by neutral ph water is pervasive and abundant a contact metamorphic aureole also occurs near intrusives alteration due to acid sulfate fluids is generally confined to permeable structures neutral ph alteration is divided into four zones on the basis of key clay minerals and two subzones are defined by calc silicates these are the smectite ambient to 180 c transition 180 230 c illite 230 320 c and biotite 270 340 c zones subzones are defined by epidote 250 340 c and amphibole 280 340 c the four main zones of acid alteration are kaolinite ambient to 120 c dickite kaolinite 120 200 c dickite pyrophyllite 200 250 c and pyrophyllite illite 230 320 c where relict high temperature alteration reaches the surface the area being drilled is usually the outflow zone of the present system these hydrothermal mineral assemblages are used 1 as geothermometers 2 to assist in determining the depth at which the production casing will be set during drilling 3 to estimate fluid ph and other chemical parameters 4 to predict possible corrosion and scaling tendencies of the fluids 5 as a measure of permeability and possible cold water influx into wells 6 as a guide to field hydrology and 7 to estimate roughly the thickness of the eroded overburden

overview of the structure and geothermal potential of newberry volcano oregon evidence from a broad range of geological and geophysical investigations of newberry volcano in centeral oregon suggests the presence of a small magma body beneath the summit caldera drilling has encountered temperatures indicative of a hydrothermal system above the hypothesized magma body on the basis of hydrothermal mineralogy the temperature of the hydrothermal system is at or near its maximum value indicating that the hydrothermal system is young basaltic underplating is postulated to replenish the magma body the results of interdisciplinary studies at newberry volcano document that young silicic volcanism in the cascade range can have geothermal potential these results should be applied with caution to the rest of the cascade range because of the strong influence of basin and range extensional tectonics on newberry volcano s style of volcanism

conditions for the growth of a long lived shallow crustal magma chamber below mount pelee volcano martinique lesser antilles arc 1 the compositional homogeneity of silicic andesites emitted during the 13 500 year long last eruptive cycle of mount pelee suggests that the physical state of the magma chamber was largely unmodified during this period experimental phase equilibria on mount pelee recent products indicate that pre eruptive magma temperatures and pressures were in the range of 875 900 c and 2 0 5 kbar respectively the estimated average eruption rate was 7 5 10 4 km3 a with an average volume of about 0 3 km3 per eruption an analytical model for a spherical magma chamber indicates that a magma flux of 4 5 10 4 km3 a can maintain a magma chamber of 0 3 km3 above 875 c below mount pelee however observations of plutons suggest that magma chambers may grow by addition of sheet like intrusions with numerical simulation we show that a minimum sheet accretion rate of a few centimeters per year is required to grow a persistently active magma chamber independently of the intruded volumetric flux this minimum injection rate is higher if the heat transfer is enhanced by convection processes the limited ability of an arc crust to extend and accommodate dikes suggests that magma injections are sills or if they are dikes that most of the volume injected in the magma chamber is eventually erupted in a conductively cooling igneous body formed by sills injected at a rate of a few centimeters per year most of the injected magma completely solidifies and only a small part of the body about 10 20 is above 875 c and able to feed eruptions

dynamic model of the genesis of calcretes replacing silicate rocks in semi arid regions abstract in both pedogenic and groundwater calcretes calcium carbonate precipitates in voids or displacing other grains or replacing underlying parent silicates replacement textures are widespread in pedogenic calcrete many calcretes also contain magnesium layer silicates and minor chert we present a reaction transport model that accounts for the genesis of replacement in calcretes and for their mineralogy replacement is difficult to account for geochemically because it requires simultaneous removal of large amounts of silicates and import of also large amounts of caco 3 in the model the genesis of replacement is directly related to seasonally alternating dry wet climates and to appropriate groundwater or circulating soil water compositions in a dry season water evaporation causes caco 3 and sepiolite or attapulgite to precipitate if groundwater contains enough mg 2 sepiolite precipitation by the chemical divide mechanism depletes sio 2 aq resulting in the dissolution of parent silicates in the following wet season sepiolite dissolves fast and silica and cations are flushed away by rainwater making room for caco 3 precipitation in the next dry season as climate cycles repeat caco 3 is accumulated and silicates are removed the sepiolite or attapulgite or mg smectite serves as a temporary storage of silica between seasons if the groundwater contains too little aqueous mg then the model predicts growth of calcium carbonate without removing silicates thus producing void filling and or displacive textures instead of replacement the model consists of a set of nonlinear partial differential equations taking account of mass conservation dispersion advection rainwater infiltration evaporation and the kinetics of mineral reactions the hydrodynamics of unsaturated media is applied in determining water flow in calcrete profiles wet dry seasonal changes are incorporated by alternating the upper boundary conditions the model successfully produces the mineral and textural zonation observed in many calcretes namely at the bottom of the profile the parent rock is first replaced by sepiolite only part of which is in turn replaced by calcite whereas at the top of the calcrete the sepiolite is itself completely replaced by the calcite which appears to directly replace the parent rock the model can produce calcrete near the earth s surface well above the water table low p co 2 intensive evaporation and long dry seasons all are predicted to produce thicker calcretes calcretes constitute an effective geochemical tool that via replacement removes parent silicate rocks and shapes the landscape of semi arid countries the model herein provides a mechanism that accounts for the efficiency of replacement in removing silicates

effects of normal faulting on fluid flow in an ore producing hydrothermal system comstock lode nevada abstract normal faults in the comstock lode mining district nv usa are delineated by low 18o belts and strong isotopic gradients perpendicular to strike these faults perturb and also offset the regional pattern of 18o contours produced by pervasive hydrothermal alteration of the alta formation and have preserved the down dropped hanging wall rocks while exposing the footwall rocks to greater erosion fluid rock interaction was greatly enhanced within 200 m wide zones centered on the normal faults where permeability was increased and fluids were focused parallel to the fault plane the effects of displacement and permeability enhancement around the faults are investigated using oxygen isotope profiles through the comstock and occidental faults and across steeper secondary normal faults such as the coryell fault that may have formed after the principal faults were rotated to more shallow dips oxygen isotope profiles record both post hydrothermal displacement and syn deformational enhancement of fluid flow around the faults fluid focusing along normal faults perturbs the 18o values by increasing the water rock ratio this isotopic effect can be mathematically described for any distance x from the fault using the formula f a e bx 2 which is analogous to the fundamental solution for diffusion away from a plane source the maximum magnitude of 18o depletion a depends on the effects of permeability temperature mineralogy fluid isotopic composition and integrated fluid flux the constant b is related to the scale of exchange enhancement around the fault the characteristic width w 1 b of the fluid focusing effects is remarkably similar at 100 m for all three faults examined coupling this isotope exchange effect with a step function which accounts for fault displacement provides a good fit to the observed 18o fault profiles the 18o distribution around faults can be used to infer relationships and timing among the processes of structural deformation hydrothermal convection and ore deposition in the comstock district

oxygen isotope geochemistry of the lassen volcanic center california resolving crustal and mantle contributions to continental arc magmatism this study reports oxygen isotope ratios determined by laser fluorination of mineral separates mainly plagioclase from basaltic andesitic to rhyolitic composition volcanic rocks erupted from the lassen volcanic center lvc northern california plagioclase separates from nearly all rocks have o values 6 1 8 4o higher than expected for production of the magmas by partial melting of little evolved basaltic lavas erupted in the arc front and back arc regions of the southernmost cascades during the late cenozoic most lvc magmas must therefore contain high o crustal material in this regard the o values of the volcanic rocks show strong spatial patterns particularly for young rhyodacitic rocks that best represent unmodified partial melts of the continental crust rhyodacitic magmas erupted from vents located within 3 5 km of the inferred center of the lvc have consistently lower o values average 6 3o 0 1o at given sio2 contents relative to rocks erupted from distal vents 47 0 km average 7 1o 0 1o further magmas erupted from vents situated at transitional distances have intermediate values and span a larger range average 6 8o 0 2o basaltic andesitic to andesitic composition rocks show similar spatial variations although as a group the o values of these rocks are more variable and extend to higher values than the rhyodacitic rocks these features are interpreted to reflect assimilation of heterogeneous lower continental crust by mafic magmas followed by mixing or mingling with silicic magmas formed by partial melting of initially high o continental crust 9 0o increasingly hybridized by lower o 6 0o mantle derived basaltic magmas toward the center of the system mixing calculations using estimated endmember source o values imply that lvc magmas contain on a molar oxygen basis approximately 42 to 4 isotopically heavy continental crust with proportions declining in a broadly regular fashion toward the center of the lvc conversely the o values of the rhyodacitic rocks suggest that the continental crust in the melt generation zones beneath the lvc has been substantially modified by intrusion of mantlederived basaltic magmas with the degree of hybridization ranging on a molar oxygen basis from approximately 60 at distances up to 12 km from the center of the system to 97 directly beneath the focus region these results demonstrate on a relatively small scale the strong influence that intrusion of mantle derived mafic magmas can have on modifying the composition of pre existing continental crust in regions of melt production given this result similar but larger scale regional trends in magma compositions may reflect an analogous but more extensive process wherein the continental crust becomes progressively hybridized beneath frontal arc localities as a result of protracted intrusion of subduction related basaltic magmas

does burial diagenesis reset pristine isotopic compositions in paleosol carbonates abstract sedimentological study of early oligocene continental carbonates from the fluvial dagshai formation of the himalayan foreland basin india resulted in the recognition of four different types namely soil palustrine pedogenically modified palustrine and groundwater carbonates stable oxygen and carbon isotopic 18 o and 13 c analyses of fabric selective carbonate microsamples show that although the pristine isotopic compositions are largely altered during deep burial diagenesis complete isotopic homogenization does not occur 18 o and 13 c analyses of 200 calcrete and palustrine carbonates from different stratigraphic horizons and comparison with 18 o of more robust bioapatite fossil vertebrate tooth phase show that dense micrites 70 carbonate invariably preserve the pristine 18 o value mean of 9 8 while altered carbonates show much lower 18 o value 13 8 such inhomogeneity causes large intra sample and intra soil profile variability as high as 5 suggesting that soils behave like a closed system where diagenetic overprinting occurs in local domains a simple fluid rock interaction model suggests active participation of clay minerals to enhance the effect of fluid rock ratio in local domains during diagenesis this places an upper limit of 70 micrite concentration above which the effect of diagenetic alteration is minimal careful sampling of dense micritic part of the soil carbonate nodules therefore does provide pristine isotopic composition and it is inappropriate as proposed recently to reject the paleoclimatic potential of all paleosol carbonates affected by burial diagenesis based on pristine 13 c value of 8 8 0 2 in soil carbonates an atmospheric co 2 concentration between 764 and 306 ppmv is estimated for the early oligocene 31 ma dagshai time these data show excellent agreement between two independent proxy records viz soil carbonate and marine alkenone and support early oligocene survival of the antarctic ice sheet

atmospheric input vs in situ weathering in the genesis of calcretes an sr isotope study at galvez central spain strontium isotopes have been analyzed in a typical calcrete profile developed on granite in the toledo mountains central spain the pedogenic carbonates show clear petrographic evidence of pseudomorphic replacement of the weathered parent granite calcretes display 87sr 86sr ratios between 0 70961 and 0 71059 in sharp contrast to the granite whole rock 0 72856 and minerals 0 71359 to 0 91351 this difference shows that the contribution of sr from the granite to the calcretes is at most 33 and may be as low as 3 direct measurements in rains and aerosols show that the allochtonous source of ca and sr is clearly related to the atmospheric input mainly as dry deposit a slight decrease of sr concentration is observed from the upper horizon composed of continuous calcrete to the deeper calcrete veins in the saprolite this may be due to a kinetic control of the sr ca fractionation and different crystallization rates of the carbonates in the different units of the profile finally local groundwaters have sr isotopic compositions similar to the calcretes and the atmospheric input very different from waters running on the granite

why allanite may swindle about its true age allanite from the tertiary rieserferner pluton austrian alps is texturally and chemically heterogeneous continuous covariation trends reflect coupled substitution of ca al vs fe ree th in allanite whereas systematic variations in la nd demonstrate the increasingly stronger depletion of lree in the melt during its crystallization allanite samples corrected for 206pbexcess from two rocks scatter in the concordia diagram and define discordias from 31 8 0 4 ma and 32 2 0 4 ma to ca 540 ma the apparent inheritance does not originate from the inclusion of older allanite or a high phase such as monazite xenotime or zircon but from the incorporation of radiogenic pb originating from a precursor since allanite requires a high enrichment of th u and lree it may form at the expense of a th lree rich precursor in metamorphic rocks or where such a phase had dissolved in melts likely precursors acquire with time radiogenic pb isotopic compositions this pb if incorporated in the product mineral may give the illusion of inheritance the allanite samples from the rieserferner pluton show a tendency from high th ucalc 30 50 in samples with an alpine age to low th ucalc 4 12 in samples with distinct inheritance this relation between extent of inheritance and apparent th ucalc indicates a lower th ucalc for a possible precursor falling into the th u range commonly encountered for monazite precursor monazite would have originated from assimilated palaeozoic rocks and give rise to localized enrichments of th and lree in the melt thus eventually enabling the growth of allanite

an oxygen isotope study of hydrothermal alteration in the lake city caldera san juan mountains colorado abstract a 23 m y old fossil meteoric hydrothermal system in the lake city caldera 11 14 km has been mapped out by measuring 18o values of 300 rock and mineral samples 18o varies systematically throughout the caldera reaching values as low as 2 great topographic relief regional tilting and variable degrees of erosion within the caldera all combine to give us a very complete section through the hydrothermal system from the surface down to a depth of more than 2000 m the initial 18o value of the caldera fill sunshine peak tuff was very uniform 7 2 0 1 making it easy to determine the exact amount of 18o depletion experienced by each sample during hydrothermal alteration also we have excellent stratigraphic control on depths beneath the mid tertiary surface quantitative information on mineralogical alteration products and accurate data on the shape of the central resurgent intrusion which was the principal heat engine that drove the hydrothermal circulation major conclusions are 1 although pristine mid tertiary meteoric waters in this area had 18 o 14 these fluids were 18o shifted upward to about 18o 8 to 5 prior to entering the shallow convective system associated with the resurgent intrusive rocks although there was undoubtedly radial inflow toward the caldera from all directions the highly fractured eureka graben southwest of the caldera was probably the principal source of recharge groundwater for the lake city system 2 fluid flow within the caldera was dominated by three major categories of permeable zones the porous megabreccia units which dip outward from the resurgent dome vertical fractures and faults related to resurgence and the caldera ring fault itself all of these zones exhibit marked 18o depletions and they are also typically intensely mineralogically altered 3 the resurgent intrusive stock and its contact metamorphic aureole of hornfels both experienced water rock ratios lower than the permeable zones however they have similarly low 18o values because they were altered at higher temperatures 4 throughout the caldera the 18o of sunshine peak tuff decreases with increasing depth about 6 per mil km indicative of a shallow thermal gradient typical of a convective hydrothermal system the near surface portion of this gradient was controlled by the temperature drop associated with boiling in the uprising fluid 5 deeply circulating meteoric water rose along permeable ring fractures 3 to 5 km beneath the mid tertiary surface these fluids were drawn into the shallow convective system through the lower porous megabreccia units near the resurgent intrusions fluid flow was again directed upward where resurgence related near vertical fractures intersect the megabreccia units

pathways of calcrete development on weathered silicate rocks in tamil nadu india mineralogy chemistry and paleoenvironmental implications abstract poorly documented yet spectacularly thick and extensive outcrops of calcrete hardpan occur on gneiss in the semiarid region of coimbatore south india the hardpan caps a series of residual plateaux forming the present day continental divide and grades into large expanses of vertisols characteristic calcrete and vertisol profiles were logged along toposequences and sampled for macro and micromorphological study and for chemical and mineralogical composition strontium isotopic analyses revealed that the calcrete is derived from in situ weathering of ca bearing primary minerals of the saprolite which is rich in ankerite ca amphiboles and ca plagioclase the macroscale analysis revealed a range of facies developed within the gneiss saprolite but in terms of relative chronology the nodular hardpan has the longest history two evolutionary pathways leading to nodular hardpan formation have been established the first occurs entirely within a vadose environment whereas the second begins within a phreatic environment before continuing to develop in vadose conditions the ability to identify and map these generic categories of calcrete constitutes a potential tool for reconstructing paleotopography and paleogroundwater levels the bedrock weathering derived nodular hardpan is blanketed by a laminar facies that correlates with an eolian event with marine sr signatures this suggests influx of ca dust from the arabian sea continental shelf during a pleistocene sea level low stand it defines an important benchmark in the chronology of the area and highlights the potential antiquity of the thick calcrete profiles

mechanism of palygorskite formation in the red clay formation on the chinese loess plateau northwest china abstract the morphological characteristics and microstructures of the authigenic palygorskite occurring with smectite and carbonate minerals in the late miocene red clay formation on the chinese loess plateau have been investigated with the powder x ray diffraction xrd scanning electron microscopy sem and high resolution transmission electron microscopy tem the xrd results indicate that the red clay sediments contain quartz feldspar calcite dolomite illite kaolinite chlorite palygorskite and smectite and sem analyses show that palygorskite is a common component delicate palygorskite fibers radiate from the margins of platy smectite suggesting a pedogenic origin during the weathering of pre existing smectite a process that involved intense dissolution of the smectite and an adjustment in its structure moreover palygorskite occurs as coatings pore fillings and pore bridging cements among dolomite rhombs suggesting that the fibrous palygorskite was formed by a direct dolomite precipitation from pore fluids mostly after dolomite crystallization there is evidence to show that these two mechanisms took place synchronously in the red clay formation and that the two processes are related under alkaline conditions with high ph 8 0 fluids rich in magnesium magnesium ions occupy the interlayer positions in the smectite and this results in a misfit between the mg octahedral sheet and the smectite layer thereby allowing interaction between the magnesium ions and smectite layers and a reorganization of the structure to form palygorskite then with this kind of palygorskite as a crystal nucleus more palygorskite grows through a direct chemical deposition process according to previous studies the red clay sediments experienced a steady dry warm climate with periods of high rainfall and evaporation the warm climate would have increased the rate of weathering of minerals and subsequently enhanced the ions content and ph in the soil solutions during dry periods this soil environment is favorable for smectite transformation and palygorskite deposition this study has improved our understanding of the environmental and paleoclimatic changes during the time that palygorskite formed in the arid soils of the chinese loess plateau

the association of gold with calcrete abstract the discovery that au accumulates in calcrete pedogenic carbonate or caliche was made in 1987 by csiro calcrete is a general term describing accumulation of alkaline earth metals in soils of arid and semi arid terrains around the world the principal constituent of calcrete is calcite while au is a noble metal calcrete has been a significant tool in a number of au deposit discoveries so understanding the mechanisms by which these diametrically different components come together is valuable for enhancing future discovery numerous laboratory experiments case histories and exploration models have been published most from australia yet we do not fully understand the mechanisms involved it is timely therefore twenty five years on since the first publication of this phenomenon to review this highly unusual but economically important association critical to any review on au in calcrete is to first consider calcretes themselves the nature of a particular calcrete where it has formed and mode of formation is relevant to how where and why au accumulates within it this review commences with a background nomenclature history classification and some examples of calcrete types found near au deposits how calcretes form their origins and the role of biota is considered their locations in the regolith and landscape as well as exploration models for au in calcrete are discussed a section on the chemistry of au in calcretes details what we know about possible mechanisms of formation and considers what laboratory experiments on microorganisms and abiotic experiments tell us following on is a summary of practical aspects of identifying collecting and analysing samples for exploration purposes selected mineral exploration case histories are described and how they fit into models of exploration and different regolith settings concluding sections include a summary and implications of this accumulated knowledge to discovering au deposits

direct evidence for the origin of low 18o silicic magmas quenched samples of a magma chamber s partially fused granitoid walls crater lake oregon abstract partially fused granitoid blocks were ejected in the climactic eruption of mount mazama which was accompanied by collapse of crater lake caldera quartz plagioclase and glass in the granitoids have much lower 18 o values 3 4 to 4 9 than any fresh lavas of mount mazama and the surrounding region 5 8 to 7 0 oxygen isotope fractionation between phases in granitoids is consistent with equilibrium at t 900 c following subsolidus exchange with hydrothermal fluids of meteoric origin assimilation of 10 20 of material similar to these granitoids can account for the o and sr isotopic compositions of lavas and juvenile pyroclasts derived from the climactic magma chamber many of which have 18 o values 0 5 or more lower than comparable lavas of mount mazama the o isotope data provide the only clear evidence for such assimilation because the mineralogy and chemical and radiogenic isotopic compositions of the granitoids dominantly granodiorite are similar to those of erupted juvenile magmas the granitoid blocks from crater lake serve as direct evidence for the origin of 18 o depletion in large shallow silicic magma bodies

magma mixing in a main stage of formation of montagne pelee the saint vincent type scoria flow sequence martinique f w i contrasting with the first stage of activity at montagne pelee which built an andesitic volcano chiefly formed of brecciated lava flows and pyroclastic breccias the second main stage 40 000 to 20 000 years b p is characterized by the prevalence of scoria flows of more mafic composition many cross sections of the deposits of stage 2 show an upward magmatic evolution from early acid andesites plinian air fall and pumice flow deposits to late basaltic andesites scoria flow deposits heterogeneous banded rocks complex mineralogical associations and mineralogical disequilibrium features in the whole series are explained by mechanical mixing of contrasting magmas reversed zoning characterized plagioclase phenocrysts of both the acid and basic terms and also pyroxene phenocrysts of the mafic terms grossly uniform composition of residual glass in most macroscopically homogeneous rocks implies that chemical exchanges between liquids and local hybridization occurred plutonic rocks with cumulate textures are found in upper units of the sequence their phenocrysts anorthite salitic clinopyroxene pargasitic hornblende and ti magnetite are in equilibrium with a subalkaline basaltic interstitial liquid the same phases are commonly present as xenocrysts in the basaltic andesites it is inferred that basaltic andesites of stage 2 were firstly derived from a subalkaline basalt by crystal fractionation mixing of the basaltic andesites 52 sio2 with acid andesites 62 sio2 occurred in late stage of evolution of the series producing heterogeneous rocks of intermediate composition successive eruption of acid up to mafic magmas dominant amount of mafic rocks in the sequence and short duration of the magma mixing event shown by narrow rims of the reversely zoned phenocrysts suggest that mixing was produced by intrusion of mafic magma into a differentiated magma body we propose that hydrostatic overpressure induced by a high rate of intrusion triggered the eruption thus magma mixing would be due to a large volume of mafic magma passing through the resident magma rapid admixture of the two batches increased gas exsolution resulting in a highly explosive eruption magma mixing can thus explain the explosive dynamics of the basaltic andesites eruptions which surprisingly contrast with the relatively effusive style of the preceding andesitic stage

evolution of calcrete in palaeodrainages of the lake napperby area central australia abstract carbonate lithofacies developed along the trunk valley of palaeodrainage channels as well as those adjacent to gypsite facies in the lake napperby drainage basin in central australia show wide ranges of sedimentary characteristics reflecting significant variation in depositional environments sedimentological and geochemical characterization of the calcrete lithofacies in this drainage area indicate that whereas variations in bedrock morphology are important in overall distribution of the phreatic and vadose calcrete varieties faulting and basin adjustments since the early quaternary may also have played a significant role in redistribution of the hydrologic zones and hence the evolution of these calcretes accordingly phreatic calcretes which initially precipitated from laterally moving groundwaters along the palaeodrainage channels undergo systematic mineralogical changes due to direct evaporative precipitation and diagenetic displacement replacement processes mainly dolomitization and silicification under present hydro chemical conditions within the zone of grounwater fluctuations however lateral and downward shifts in groundwater flow due to excessive volumetric expansion and reduction in effective permeability of calcrete may ultimately result in isolation of the calcrete like those observed from the eastern margins of the lake the evolution of the vadose calcretes is thereafter topographically controlled and relates to mobilization and redistribution of earlier deposited calcretes under present meteorological conditions

geophysical evidence for the locations shapes and sizes and internal structures of magma chambers beneath regions of quaternary volcanism this paper is a review of seismic gravity magnetic and electromagnetic techniques to detect and delineate magma chambers of a few cubic kilometres to several thousand cubic kilometres volume a dramatic decrease in density and seismic velocity and an increase in seismic attenuation and electrical conductivity occurs at the onset of partial melting in rocks the geophysical techniques are based on detecting these differences in physical properties between solid and partially molten rock although seismic refraction techniques with sophisticated instrumentation and analytical procedures are routinely used for detailed studies of crustal structure in volcanic regions their application for magma detection has been quite limited in one study in yellowstone national park u s a fan shooting and time term techniques have been used to detect an upper crustal magma chamber attenuation and velocity changes in seismic waves from explosions and earthquakes diffracted around magma chambers are observed near some volcanoes in kamchatka strong attenuation of shear waves from regional earthquakes interpreted as a diffraction effect has been used to model magma chambers in alaska kamchatka iceland and new zealand one of the most powerful techniques in modern seismology the seismic reflection technique with vibrators was used to confirm the existence of a strong reflector in the crust near socorro new mexico in the rio grande rift this reflector discovered earlier from data from local earthquakes is interpreted as a sill like magma body in the kilauea volcano hawaii mapping seismicity patterns in the upper crust has enabled the modelling of the complex magma conduits in the crust and upper mantle on the other hand in the usu volcano japan the magma conduits are delineated by zones of seismic quiescence three dimensional modelling of laterally varying structures using teleseismic residuals is proving to be a very promising technique for detecting and delineating magma chambers with minimum horizontal and vertical dimensions of about 6 km this technique has been used successfully to detect low velocity anomalies interpreted as magma bodies in the volume range 10 3 10 6 km 3 in several volcanic centres in the u s a and in mt etna sicily velocity models developed using teleseismic residuals of the cascades volcanoes of oregon and california and kilauea volcano hawaii do not show appreciable storage of magma in the crust however regional models imply that large volumes of parental magma may be present in the upper mantle of these regions in some volcanic centres teleseismic delays are accompanied by p wave attenuation and linear inversion of spectral data have enabled computation of three dimensional q models for these areas the use of gravity data for magma chamber studies is illustrated by a study in the geysers clear lake volcanic field in california where a strong gravity low has been modelled as a low density body in the upper crust this body is approximately in the same location as the low velocity body delineated with teleseismic delays and is interpreted as a magma body in yellowstone national park magnetic field data have been used to map the depth to the curie isotherm and the results show that high temperatures may be present at shallow depths beneath the yellowstone caldera the main application of electrical techniques in magma related studies has been to understand the deep structure of continental rifts electromagnetic studies in several rift zones of the world provide constraints on the thermal structure and magma storage beneath these regions