a brine interface in the salton sea geothermal system california fluid geochemical and isotopic characteristics data from 71 geothermal production intervals in 48 wells from the salton sea geothermal system ssgs indicate that fluids in that system cluster into two distinct populations in terms of their salinity and their stable isotopic compositions the distinctive hot hypersaline brine typically 20 wt total dissolved solids for which the ssgs is known is overlain by a cooler 260 c fluid with distinctly lower salinity typically 10 wt total dissolved solids hypersaline brines have high and rather consistent 18o shifts produced by water rock interaction and have a very narrow range in d values low tds fluids on the other hand show a wide range in both d and 18o production of both types of fluid from closely spaced geothermal wells in many regions of the ssgs indicates that a relatively sharp salinity interface exists over much of the field the fluid interface typically cross cuts sedimentary bedding but is consistently found where reservoir temperatures are approximately 260 c at these temperatures hypersaline brines have densities of approximately 1 0 gm cm3 while the low tds fluids have densities as low as 0 85 gm cm3 this stable density stratified interface acts as a barrier to convective heat and mass transfer in the ssgs isolating the hypersaline reservoir from overlying dilute fluids a lithologic cap implied by previous ssgs models is unnecessary in such a stratified system since heat and mass transfer across the interface must occur by slow conductive diffusional and interface mixing processes regardless of local permeability

a view of the lower crustal component of hydrothermal systems at the mid atlantic ridge an extensive suite of hydrothermally altered gabbros was recovered by alvin and dredging from the mid atlantic ridge in the vicinity of the eastern ridge transform intersection rti of the kane fracture zone mark where tectonic extension has provided a window into the lower crustal component of hydrothermal cells four alteration types are distinguished on the basis of metamorphic assemblage mineral composition and deformation textures a conceptual model is presented that places temperature fluid rock ratio relationships in conjunction with styles of deformation into a spatial and temporal framework exsolution of late stage magmatic fluids in the vicinity of the magma hydrothermal interface at temperatures 700 c marks the onset of fluid rock interaction in the mark gabbros kelley et al this issue with subsequent alteration involving seawater derived fluids being strongly influenced by deformation mechanisms in zones of ductile shear hydration was initiated at temperatures between 550 and 700 c and moderate fluid rock ratios elsewhere in the lower crust the onset of seawater penetration occurred at temperatures of 400 550 c and was facilitated by brittle fracturing early vein networks indicate very low initial fluid rock ratios and are interpreted as being related to the solidification of plutons later fractures indicate higher water rock ratios and may be related to the downward propagation of shallow fault systems throughout the crust fluid rock interaction ceased at temperatures 180 300 c subsequent cataclastic deformation associated with the unroofing and emplacement of crustal blocks in the rti massif produced moderate fluid rock ratios within localized zones hydrothermal alteration in the crustal section exposed at mark was initiated at lower temperatures and water rock interaction proceeded to lower temperatures than many other plutonic suites these differences must reflect the temporal and spatial evolution of magmatic and tectonic extension that is particular to each section of sampled crust

water rock interaction in the magmatic hydrothermal system of nisyros island greece abstract in this work we investigated the water rock interaction processes taking place in the hydrothermal reservoir of nisyros through both 1 a review of the hydrothermal mineralogy encountered in the deep geothermal borehole nisyros 2 and 2 a comparison of the analytically derived redox potentials and acidities of fumarolic related liquids with those controlled by redox buffers and ph buffers involving hydrothermal mineral phases the propylitic zone met in the deep geothermal borehole nisyros 2 from 950 to 1547 m total depth is characterised by abundant well crystallised epidote adularia albite quartz pyrite chlorite and sericite muscovite accompanied by less abundant anhydrite stilpnomelane wairakite garnet tremolite and pyroxene these hydrothermal minerals were produced in a comparatively wide temperature range from 230 to 300 c approximately hydrothermal assemblages are well developed from 950 to 1360 m whereas they are less developed below this depth probably due to low permeability based on the r h values calculated for fumarolic gases and for the deep geothermal fluids of nisyros 1 and nisyros 2 wells redox equilibrium with the feo feo 1 5 rock buffer appears to be closely attained throughout the hydrothermal reservoir of nisyros this conclusion may be easily reconciled with the nearly ubiquitous occurrence of anhydrite and pyrite since r h values controlled by coexistence of anhydrite and pyrite can be achieved by gas separation the ph of the liquids feeding the fumarolic vents of stephanos and polybote micros craters was computed by means of the eq3 code based on the cl d relationship which is constrained by the seawater magmatic water mixing occurring at depth in the hydrothermal magmatic system of nisyros the temperature dependence of analytically derived ph values for the reservoir liquids feeding the fumarolic vents of stephanos and polybote micros craters suggests that some unspecified ph buffer fixes the acidity of these reservoir liquids at values of 4 72 4 85 and 4 88 5 23 respectively many of these ph values are lower than those expected for the full equilibrium condition although they are close to those of the reservoir liquids of nisyros 1 5 16 and nisyros 2 4 87 it is likely that this excess of acidity producing species chiefly co 2 promotes release of fe ii and fe iii to the reservoir liquids through rock dissolution permitting the attainment of redox equilibrium with the feo feo 1 5 rock buffer as already suggested by the late werner giggenbach

geothermal type alteration in a burial metamorphosed volcanic pile central chile the upper cretaceous and tertiary volcanic sequences in the chilean andes are affected by burial metamorphism for example in central chile between 32 30 and 35 s the abanico formation a folded upper cretaceous to palaeogene unit composed of basic lavas tuffs and ash flows of intermediate composition and volcaniclastic sandstones is characterized by heulandite to laumontite bearing zeolite facies assemblages in its upper part passing with depth to prehnite pumpellyite facies assemblages however at c 33 30 s in the abanico hill area located just east of a graben at santiago the longitudinal central valley the alteration pattern is unrelated to stratigraphic depth it is characterized by a lateral increase in grade defined by assemblages with yugawaralite reported for the first time in the andes laumontite then wairakite epidote and finally with abundant epidote successively closer to the graben boundary this pattern was formed in a palaeogeothermal system with a high t and a very low p gradient geothermal alteration has also been inferred from the border zone of a neogene caldera in the abanico hill area and from the western fault boundary of the graben these expressions of geothermal alteration together with the occurrence of caldera structures the huge volume of ignimbrites and numerous epithermal precious metal deposits of late cretaceous and tertiary age in central and northern chile suggest that fossil geothermal systems of this age are probably common features in the chilean andes

comparison of rio tinto spain and guaymas basin gulf of california an explanation of a supergiant massive sulfide deposit in an ancient sill sediment complex the volcano sedimentary unit that hosts the stratiform massive sulfide mineralization at rio tinto is reinterpreted as a sill sediment complex that in many respects is analogous to the example that is currently forming in the guaymas basin gulf of california however rio tinto is distinctive because 1 it is a product of bimodal magmatism 2 it has a much higher sill sediment ratio 4 1 lessening the extent of ore forming metal depletion during interaction between hydrothermal fluid and wet sediment and 3 most of the sills were emplaced penecontemporaneously as shown by the ubiquitous development of peperitic sill margins despite the narrowness of the sedimentary screens these data together with evidence for local extrusive behavior imply rapid formation of the complex at exceptionally high levels of emplacement it follows that in the complex variations in thickness of 400 m were directly expressed as basin floor deformation and that troughs in the resultant topography were able to trap exhalative hydrothermal plumes thus preserving the sulfides from dispersion also crucial to the mineralization process was the sheetlike form of the sill complex which by acting as a cap prevented regular convective cell patterns from forming and focused considerable discharge at areas of weakness the intense magmatic pulse the creation of pronounced topography and the capping of the hydrothermal system combined to make a 500 mt supergiant deposit

mineralogical and isotopic characteristics of sodic calcic alteration in the highland valley copper district british columbia canada implications for fluid sources in porphyry cu systems

ngatamariki geothermal system magmatic to epithermal transition in the taupo volcanic zone new zealand the ngatamariki geothermal system is one of more than 20 high enthalpy 225 c geothermal systems in the taupo volcanic zone north island new zealand at shallow levels 2 km they are analogous to low intermediate sulfidation state epithermal ore forming environments unique to ngatamariki is the occurrence of an intrusive complex with an associated magmatic hydrothermal alteration halo that was intersected by deep geothermal drilling 3 km depth and that resembles hydrothermal alteration associated with high intermediate sulfidation state epithermal mineralization this paper describes the results of a study involving alteration petrography x ray diffraction shortwave infrared swir reflectance spectroscopy backscattered electron scanning electron microscopy energy dispersive spectroscopy bse sem eds plus whole rock and trace element geochemistry to document and characterize distinct hydrothermal alteration found nowhere else in the taupo volcanic zone two separate phases of hydrothermal activity are distinguished old and modern as defined by a paleosurface that is dated at 0 68 ma which occurs 500 m below sea level a composite plutonic body comprising intrusions of diorite to tonalite was encountered in three adjacent drill holes nm4 nm8 and nm9 between 2 000 and 3 200 m below sea level 2 300 3 500 m depth below the surface in the northern part of the ngatamariki system the associated hydrothermal alteration is zoned and made up of potassic advanced argillic phyllic and propylitic mineral assemblages that occur between 500 and 2 500 m below sea level subtle potassic alteration consisting of biotite magnetite k feldspar mantles the intrusive complex it is crosscut by a hypogene advanced argillic alteration containing pyrophyllite minor andalusite topaz anhydrite rare aluminophosphates ap and fluorine bearing minerals but lacks alunite the deep formed advanced argillic alteration appears in some samples to be overprinted by phyllic alteration made up of quartz muscovite pyrite between 500 and 1 000 m below sea level the intense phyllic alteration is less pervasive and the hydrothermal alteration is dominated by kaolinite rare dickite and localized occurrences of highly silicified rocks that resemble vuggy quartz which is bounded at the top by the paleosurface defined as the unconformity at the base of the overlying whakamaru group ignimbrite in the central and southern part of the system below the paleosurface propylitic hydrothermal alteration consisting of chlorite calcite epidote wairakite actinolite along with albite and illite is widespread and could equally have formed during old hydrothermal activity associated with emplacement of the intrusive complex or in the deep hot parts of the modern hydrothermal system the mineralogy and geochemistry of advanced argillic altered rock indicates that acidic magmatic hydrothermal fluids leached base cations resulting in the loss of elements typically considered immobile al ti y zr nb as well as rare earth elements to form cation depleted minerals such as pyrophyllite andalusite and topaz the strongest enrichments in au 0 6 g t ag 4 6 g t te as sb and bi coincide with intense acid alteration at 250 m depth beneath the paleosurface the results of this study reveal a complex history of intrusion and hydrothermal activity that provides a modern example of successive development 600 000 years apart of acid and neutral ph hydrothermal alteration assemblages which are associated with the two end member types of epithermal mineralization

metamorphosed plio pleistocene evaporites and the origins of hypersaline brines in the salton sea geothermal system california fluid inclusion evidence abstract the salton sea geothermal system ssgs occurs in plio pleistocene deltaic lacustrine evaporite sediments deposited in the salton trough an active continental rift zone temperatures up to 365 c and hypersaline brines with up to 26 wt tds are encountered at 1 3 km depth in the sediments which are undergoing active greenschist facies hydrothermal metamorphism previous models for the origins of the na ca k cl brines have assumed that the high salinities were derived mainly from the downward percolation of cold dense brines formed by low temperature dissolution of shallow non marine evaporites new drillcores from the central part of the geothermal field contain metamorphosed bedded evaporites at 1 km depth consisting largely of hornfelsic anhydrite interbedded with anhydrite cemented solution collapse shale breccias fluid inclusions trapped within the bedded and breccia cementing anhydrite homogenize at 300 c identical to the measured downhole temperature and contain saline na ca k cl brines some of the inclusions contain up to 50 vol halite sylvite and carbonate crystals at room temperature and some halite crystals persist to above 300 c upon laboratory heating the data are consistent with the trapping of halite saturated na ca k cl fluids during hydrothermal metamorphism of the evaporites and accompanying solution collapse of interbedded shales we conclude that many of the salt crystals in inclusions are the residuum of bedded evaporitic salt that was dissolved during metamorphism by heated connate fluids therefore the high salinities of the salton sea geothermal brines are derived in part from the in situ hydrothermal metamorphism and dissolution of halides and caso 4 from relatively deeply buried lacustrine evaporites this fact places important constraints on modeling fluid flow in the ssgs as brines need not have migrated over great distances the brines have been further modified to their present complex na ca k fe mn cl compositions by on going sediment metamorphism and water rock interaction

actinolite forming reaction at low pressure and the role of fe 2 mg substitution the 6km thick karmutsen metabasites exposed over much of vancouver island were thermally metamorphosed by intrusions of jurassic granodiorite and granite observation of about 800 thin sections shows that the metabasites provide a complete succession of mineral assemblages ranging from the zeolite to pyroxene hornfels facies around the intrusion the reaction leading to the appearance of actinolite which is the facies boundary between prehnite pumpellyite and prehnite actinolite facies was examined using calcite free karmutsen metabasites collected from the route along the elk river in the prehnite pumpellyite facies xfe3 fe3 fe3 al in prehnite pumpellyite and epidote buffered by the four phase assemblage prehnite pumpellyite epidote chlorite systematically decreases with increasing metamorphic grade such a trend is the reverse of that proposed by cho et al 1986 this may be related to the higher f o 2 in the mt menzies area the actinolite forming reaction depends on the value of xfe3 in pumpellyite if using a low value of fe3 3 89 pr 0 06 0 48 ep 0 26 0 60 chl h2o 2 10 pm 0 08 0 17 act 0 88 qz is delineated the number in parentheses stands for the xfe3 value in ca al silicates on the other hand replacing the xfe3 of 0 08 in pumpellyite with a higher xfe3 value 0 24 changes the reaction to 0 41 pm 0 02 chl 0 42 qz 0 11 pr 0 62 ep 0 10 act h2o the first hydration reaction forms pumpellyite and actinolite on the high temperature side whereas the second dehydration reaction consumes pumpellyite to form prehnite epidote and actinolite the former reaction seems to explain the textural relationship of ca al silicates in the study area however actinolite forming reaction changes to a different reaction depending on the compositions of the participating minerals although in the other area even physical conditions may be similar to those in the study area chemographic analysis of phase relations in the pra facies indicates that the appearance of prehnite depends strongly on the bulk feo mgo ratio this may explain the rarity of prehnite in common metabasites in spite of the expected dominant occurrence in the conventional pseudo quaternary ca al fe3 fm system an increasing feo mgo ratio stabilizes the pr act assemblage and reduces the stability of the pm act one therefore the definition of pumpellyite actinolite facies should include not only pm act but also the absence of pr act assemblages in addition to the possible role of high x co 2 cho and liou 1987 and or high f o 2 to mask the appearance of prehnite the effect of the feo mgo ratio is emphasized

a field chemical and stable isotope study of subseafloor metamorphism of the josephine ophiolite california oregon the josephine ophiolite is a large complete ophiolite generated in a late jurassic back arc basin along western north america the ophiolite underwent subseafloor metamorphism under a steep thermal gradient as indicated by downward changes in mineralogy and 18o values characteristic mineral zonation includes mica orthoclase and hematite in the lower extrusives greenschist facies assemblages in the upper sheeted dikes and amphibolite facies assemblages in the lower sheeted dikes and high level gabbro cumulate gabbros are only incipiently altered but are depleted in 18o indicating high temperature alteration with fluids that evolved at low water w rock r ratios a second off axis circulation system is recorded by metalliferous sediments that occur 8 21 m above the ophiolite alteration in the upper sheeted dike complex and especially the extrusive sequence is heterogeneous at outcrop scale as indicated by large variations in mineralogy chemistry and 18o values within individual pillows and dikes the heterogenity is due to 1 variations in w r ratio and starting material during alteration by downwelling seawater e g pillow cores versus glassy rims and 2 localized discharge mineralization mostly epidosites superimposed on the background recharge alteration epidosites occur largely as dike parallel stringers irregular replacement of pillow and massive lava and replacement of interpillow and pillow breccia matrices the chemistry of most extrusives and sheeted dikes is characterized by loss of ca and gain in na and mg consistent with alteration by downwelling fluids in addition the sheeted dikes are depleted in k2o and the lower sheeted dikes and high level gabbros are depleted in zn and cu the 18o and d values for most samples from the extrusive sequence indicate alteration by seawater at 50 rotation of the entire crustal sequence at the spreading axis suggests that the ophiolite formed at a slow spreading center where magma chambers were episodic

water rock interaction processes in the aluto langano geothermal field ethiopia aluto langano is a water dominated geothermal field in the main ethiopian rift system the hydrothermal mineral assemblage is characterized by the presence of illite montmorillonite t 150 c and illite chlorite 150 c t 310 c in the hottest part of the wells up to 335 c the mineral assemblage is characterized by the presence of ca al silicates epidote garnet actinolite however these phases also occur as relics in wells where lower temperatures are measured calcite quartz and chlorite are widespread minerals in all the wells both at low and high temperatures fluid inclusion data from quartz and calcite of two wells la 3 and la 6 reveal fluids with an apparent salinity of 1 5 2 0 for la 3 and 0 85 1 36 wt nacl eq for la 6 homogenization temperatures tb range from 240 c to 350 c the calculated co2 concentration in the liquid phase ranges from 0 07 to 0 66 mol thermodynamic properties and phase relations of calc silicates show that the presence or absence of these minerals is mainly controlled by the pco2 in the system even where temperature has played an important role fluid inclusion data from two deep wells la 3 and la 6 indicate an increase in temperature and co2 content of the geothermal fluid after the trapping of the inclusions the wells produce an alkali chloride bicarbonate water enthalpy chloride and na 1000 k 100 mg diagrams indicate that none of the discharge waters can be taken as representative of the deep fluid in full equilibrium with the reservoir rock and that important mixing processes characterize even the hottest zone of the system these mixing processes could be responsible for the continuous dynamic evolution of the hydrothermal system as evidenced by the authigenic mineralogy

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

patterns and processes of alteration in the lavas and dykes of the troodos ophiolite cyprus alteration patterns in the lavas and dykes of the troodos ophiolite cyprus record a complex history of axial hydrothermal alteration crustal aging and subsequent uplift and emplacement of the ophiolite field mapping shows that distribution of five alteration zones each with distinct mineralogical geochemical and hydrologie characteristics is influenced by igneous stratigraphy structure and the nature and thickness of the overlying sediments paragenetic sequences of secondary minerals indicate that alteration conditions changed progressively as the crust cooled and moved off axis along spreading axes low temperatures 50 c were maintained by the rapid flow of seawater in and out of the lavas and only minimal alteration took place in contrast lower water rock ratios and higher temperatures 200 c in the dykes promoted extensive seawater rock interaction although the sharp rise in temperature between the two regimes generally coincides with the lava dyke transition late stage intrusions or hydrothermal upwelling zones locally cause high temperature alteration to extend upward into the lavas as a segment of crust moved off axis temperatures remained low in the lavas and progressively decreased from 250 to 80 c in the dykes high permeability in the uppermost lavas led to the downward migration of an oxidative alteration front whose thickness and spatial distribution was dependent upon the rate and nature of sedimentation and thus the original seafloor morphology although field relations show that alteration has a consistent vertical pattern in troodos the alteration zones are not laterally continuous and the stratigraphie depth of their boundaries varies considerably

an association between organic matter and localised prehnite pumpellyite alteration at builth wells wales u k metcalfe r banks d and bottrell s h 1992 an association between organic matter and localised prehnite pumpellyite alteration at builth wells wales u k chem geol 102 1 21 the builth volcanic group of the builth wells ordovician inlier wales is a sequence of basic intermediate lavas and volcaniclastics which shows a unique association between pyrobitumen and localised domainal calc silicate alteration the pyrobitumens occur in calcite veins and disseminated throughout alteration domains which form haloes to these veins the pyrobitumens were emplaced by ordovician hydrothermal fluids which were also responsible for the domainal alteration combined petrographic gas chromatography and stable isotopic studies show that light organic gases c2 c5 were liberated into the hydrothermal fluids by devolatilisation of liquid petroleum to form the pyrobitumens these light organic gases mixed with methane characterized by 13c 13 o which was already present in the fluids this methane was most likely mantle derived or produced from inorganic synthesis the organic content of the fluids exerted an important control on the inorganic secondary assemblages a similar control of inorganic assemblages by organic matter might possibly occur deep within modern petroleum bearing hydrothermal systems such as the guaymas basin and escanaba trough hydrothermal systems

magnetization of the la palma seamount series implications for seamount paleopoles paleopoles determined from seamount magnetic anomalies constitute the major data source for the pacific apparent polar wander path but relatively little is known about the processes of remanence acquisition in seamounts since magnetic anomalies reflect both natural remanence nrm and the induced field it is important first to assess whether the nrm is likely to represent an original field direction and second to constrain the magnitude of the induced component to this end we present paleomagnetic data from an uplifted subaerially exposed section through a seamount on la palma canary islands the pliocene seamount series of la palma comprises a 6 km sequence of alkalic extrusives and intrusives which includes all lithologies likely to be volumetrically important in seamounts the structural tilt of the seamount series allows separation of early thermal or chemical remanence from magnetization components acquired after tilting e g viscous remanence the nrm provides a poor indication of the original magnetization direction although the characteristic magnetization of many la palma samples is compatible with the original pretilt direction hydrothermal alteration has resulted in the production of ti poor magnetite and an increasing contribution of hematite with increasing degree of alteration more importantly well defined magnetization directions which deviate from any reasonable geomagnetic direction at la palma can be attributed to hydrothermal alteration in a different polarity than prevalent during the original magnetization based on a comparison of the magnitude of low stability components of magnetization and laboratory acquisition of viscous remanence and previous estimates of the induced magnetization we conclude that viscous and induced magnetization probably account for 15 25 of the total magnetization of seamounts the resulting paleopole bias is a function of the polarity and paleolatitude of the seamount and ranges from 4o to 16 o for cretaceous seamounts in the pacific

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

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

role of upslope soil pore pressure on lateral subsurface storm flow dynamics 1 the role of upslope soil pore water pressure on lateral subsurface storm flow dynamics is poorly understood further development of hillslope hydrologic models requires new understanding from field understanding in particular we need new quantifiable measures that link upslope soil pore pressure and water table dynamics to the timing and volume of subsurface storm flow here we examine the relationship between hillslope scale pore pressure and lateral outflow from slope base using the fine temporal resolution hydrometric data 10 min interval from two steep unchanneled concave hillslopes one hillslope fudoji covered by relatively high hydraulic conductivity sandy soil and the other toinotani covered by relatively low hydraulic conductivity clay soil in both hillslopes pore pressures in the area close to the slope base were only weakly related to subsurface storm flow dynamics during periods of storm flow production hillslope discharge was strongly related to the cross sectional area of the upslope saturated layer during slope seepage periods between events hillslope discharge from the highly permeable hillslope was still related to the upslope cross sectional subsurface saturated area however during this same period at the low permeability site hillslope discharge was not related to the upslope subsurface saturated area through intersite comparison we show that the soil matrix permeability has a large impact on the hydrological extension of preferential flow and hence the linkage between upslope pore pressure and subsurface storm flow dynamics

pre middle jurassic accretionary metamorphism in the southern klamath mountains of northern california usa high p t metamorphic parageneses are preserved within two late palaeozoic to early mesozoic assemblages of the southern klamath mountains that show contrasting structural styles and mineral parageneses reflecting formation in different parts of a subduction zone regime blueschist facies tectonites of the stuart fork terrane represent a coherent subduction complex formed at relatively deep crustal levels whereas the chaotic metasedimentary melange of the eastern hayfork terrane contains a diverse range of metamorphic parageneses reflecting complex structural mixing of metamorphic components at shallower levels the convergent margin type accretionary metamorphism evident in both terranes pre dates middle jurassic low p t metamorphism resulting from regional tectonic contraction and magmatism the epidote to lawsonite zone stuart fork blueschists and eclogites locally formed at pressures of about 6 11 kbar and temperatures of 250 400 c deformed matrix material of the eastern hayfork melange formed at similar temperatures but lower pressures on the order of 3 6 kbar the melange contains a diverse assemblage of tectonic blocks that formed under a range of p t conditions including those of the blueschist pumpellyite actinolite greenschist and upper greenschist to amphibolite facies the succession of mineral parageneses and inferred p t conditions of the eastern hayfork blocks reflect those of igneous protolith formation structural mixing subduction zone metamorphism olistolith transport and tectonic and erosional denudation although temporal relations are not well constrained the evolution of these terranes is consistent with formation within a single convergent margin system

compositional order disorder and stable isotope characteristics of al fe epidote state 2 14 drill hole salton sea geothermal system epidote ca2fe3si3o12 oh ca2al3si3o12 oh is a common hydrothermal mineral in metasediments and veins at depths 900 m in the state 2 14 drill hole of the salton sea geothermal system the mole fraction of ca2fe3si3o12 oh in epidotes xps from this drill hole ranges from 0 11 to 0 42 and complex compositional zoning of octahedral fe3 and al is typical within single grains with increasing depth there is an overall but irregular decrease in the fe3 content of epidotes in metasandstones in most samples vein epidotes are more fe3 rich and exhibit a wider compositional range than metasandstone epidotes from the same depth octahedral fe3 in the m 1 sites in four epidotes evaluated by 57fe mossbauer spectroscopy ranges from 7 5 1 7 to 11 4 1 5 of the total iron the most ordered epidote formed in veins at 2618 m in the biotite zone this epidote has an ordering parameter 1 2xfe3 m 1 of 0 85 0 03 which corresponds to a calculated state of equilibrium order disorder at 390 60 c in close agreement with the probable downhole temperature of 340 c two epidote samples from the chlorite calcite zone 1420 m 265 c 1867 m 300 c are more disordered corresponding to calculated states of equilibrium order disorder of 450 c these data suggest that the analyzed epidotes from the chlorite calcite zone are in a metastable state of substitutional order disorder the measured value of depidote from 1420 m is 96 which is 5 6 lighter than epidotes from 1867 m depidote 90 2227 m depidote 91 and 2618 m depidote 90 when compared to the reservoir fluid from the december 1985 flow test the derived fractionation of hydrogen isotopes between epidote and the geothermal brine is 19 at 1867 m and 340 c this value is in accord with published experimental hydrogen isotope fractionations between iron rich epidote and aqueous electrolyte solutions therefore considering the complexity of the salton sea brine the isotopic temperature is similar to the inferred downhole temperature

hydrothermal alteration in a modern suprasubduction zone the tonga forearc crust an extensive suite of hydrothermally altered basalts gabbros and plagiogranites was recovered from the trench facing slope of the tonga forearc the tectonic setting lithology and geochemistry of these samples make them a unique collection for comparison with suprasubduction zone ssz ophiolites petrography mineral chemistry and geothermometry are used to constrain the metamorphic evolution of ocean crust formed in a modern ssz setting seawater derived hydrothermal fluids first penetrated the lower crust along grain boundaries and microscopic fracture networks at temperatures 800 c as the plutonic sequence cooled amphibole progressively replaced the mafic phases followed by chlorite and epidote below 550 c basalts record peak alteration temperatures up to 773 c however most were altered at lower temperatures typical of mid ocean ridge mor volcanic sequences kelman 1998 epidosites formed by pervasive alteration of basalt and plagiogranite at greenschist facies conditions and at high water rock ratios pervasively altered gabbros and basalts display evidence of late cataclastic deformation and or contain veins that is likely related to a later tectonic event such as trench rollback the range of alteration temperatures and mineral assemblages in basalts and gabbros are similar to those described from both ssz ophiolites and mors however the degree of alteration in basalts and the presence of epidosites in the tonga collection are most similar to alteration characteristics in ssz ophiolites the initiation of high temperature brittle deformation in the absence of ductile deformation suggests that the tonga forearc crust was constructed in a magma rich environment similar to fast spreading mors