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

chloritisation along the thanewasna shear zone western bastar craton central india its genetic linkage to cu au mineralisation abstract the cu au mineralisation of thanewasna bastar craton central india is hosted in quartz chlorite veins traversing granitoid rocks mineralisation and associated alteration are controlled by a nw se trending brittle ductile shear zone large scale alkali magmatism in the form of k rich granite emplacement is found in the vicinity of the shear zone the emplacement of this granite 1587 14 ma along the northern shoulder of the godavari rift produced a high geothermal gradient resulting in intense chloritisation and mineralisation field and petrographic studies show that the chlorite formed in two possible modes viz a replacement of ferromagnesian minerals of the host rock and or b precipitated directly from hydrothermal solutions mineral chemistry epma coupled with sem and xrd studies reveals that chamosite with minor clinochlore varieties of chlorite is commonly associated with cu and au mineralisation chlorite shows variation in fe fe mg ratios from 0 54 to 0 60 0 55 to 0 64 and 0 49 to 0 55 and al iv from 1 88 to 2 40 1 91 to 2 50 and 1 80 to 1 94 apfu respectively at the surface and in the sub surface mineralisation and host rock empirical and thermodynamic geothermometers have been used to determine the temperature of chlorite formation based on chemical composition six empirical methods based on al iv al vi and fe fe mg ratio showed a temperature range of 187 c to 381 c temperatures of chlorite formation have also been determined by the thermodynamic solid solution method the thermodynamic equations of three intra crystalline equilibria at 2 kbar pressure show that temperature ranges from 215 to 454 c for low si contents 2 8 apfu the thermodynamic method of vidal et al 2001 and 2005 was not so compatible for an estimation of temperature of hydrothermal fe rich chlorites associated with cu au mineralisation at the thanewasna copper deposit tcd the temperature of chlorite formation increases from the host rock mean 201 c to proximal mineralisation zone mean 234 c to distal k na alteration zone mean 251 c integrating field and laboratory results suggested that tcd chlorite is part of a structurally controlled high sulphidation epithermal system this characterization would help in the regional exploration for base and precious metals and the results are applicable for the entire western bastar craton wbc of central india and similar deposit elsewhere

alteration of basalt hyaloclastite at the off axis sea cliff hydrothermal field gorda ridge abstract the sea cliff hydrothermal field on the northern segment of the gorda ridge is situated along a rift bounding normal fault about 2 6 km east of the neovolcanic zone and approximately 300 m above the spreading axis the structural setting of this hydrothermal field differs from that of most other active seafloor hydrothermal sites investigated to date which are typically situated in the neovolcanic zone mineralization occurs in basaltic talus covering a large normal fault scarp hydrothermal crusts cover much of the seafloor in the area of the active hydrothermal field these crusts form by extensive alteration of basaltic hyaloclastite in a zone of mixing between ascending hydrothermal fluid and entrained seawater the initial stage of alteration is magnesian metasomatism of both crystalline basalt and basaltic glass converting the rock to mg rich smectite and smectite chlorite further alteration removes nearly all cations and ultimately leads to silicification preservation of basaltic texture in the silicified rocks provides evidence that even such sparingly soluble elements as a1 and ti have been removed oxygen isotopic ratios of the altered rocks constrain initial alteration to temperatures near 220 c close to the maximum measured vent temperatures of 247 c silicification proceeded to much lower temperatures and most amorphous silica deposition occurred at temperatures below 100 c sulfur strontium and lead isotopic data all indicate a predominantly basaltic source with important contributions from seawater but no significant contribution from sedimentary sources comparison with ophiolite hosted massive sulfide deposits shows that the structural setting alteration sequence and depositional environment are all similar and suggests that mineralization and replacement of basaltic breccia in the sea cliff hydrothermal field likely occur in the subsurface beneath a capping layer of silicified hyaloclastite

low grade metamorphism of mafic rocks through most of this past century metamorphic petrologists in the united states have paid their greatest attention to high grade rocks especially those which constitute the core zones of exhumed mountain belts the pioneering studies of the 50 s through the 80 s those which applied the principles of thermodynamics to metamorphic rocks focused almost exclusively on high temperature systems for which equilibrium processes could be demonstrated by the 1980 s metamorphic petrologists had developed the methodologies for deciphering the thermal and baric histories of mountain belts through the study of high grade rocks of course low grade metamorphic rocks here defined as those which form at pressures and temperatures up to and including the greenschist facies had been well known and described as well initially through the efforts of alpine and circum pacific geologists who recognized that they constituted an integral and contiguous portion of mountain belts and that they underlay large portions of accreted terranes many of oceanic origins but until the mid 80 s much of the effort in studying low grade rocks for a comprehensive review of the literature to that point see frey 1987 had been concentrated on mudstones volcanoclastic rocks and associated lithologies common to continental mountain belts and arcs in the mid 80 s results of the deep sea drilling project dsdp rather dramatically mitigated a shift in the study of low grade metamorphic rocks

the rootzone of an ancient hydrothermal system exposed in the troodos ophiolite cyprus abstract a well exposed oceanic hydrothermal rootzone in the troodos ophiolite cyprus is composed of the basal sheeted dikes the upper 50 100 m of the gabbroic sequence and a contact aureole interpreted as a fossil conductive boundary layer the basal sheeted dikes and upper gabbros were pervasively altered at high temperatures 400 775 c and cut by a series of broadly distributed mineralized faults that are oriented parallel to the strike of the sheeted dike complex fluid inclusion and mineralogical data for fault breccias show that circulating fluids were 350 400 c fairly oxidizing and relatively h2s rich with a range in salinity 1 5 7 8 wt nacl equivalent field and petrological evidence suggest that the rootzone migrated from the basal sheeted dikes into the upper gabbros and back again perhaps for several cycles these migrations and p t oscillations in the conductive boundary layer were likely caused by fluctuations in thermal conditions imposed by the magmatic system at a spreadin

controls on hydrothermal alteration in a section of fast spreading oceanic crust abstract young oceanic crustal sections that formed at the fast spreading east pacific rise epr were mapped by submersible along steep fault scarps that bound the hess deep rift valley excellent exposures allowed the documentation of the spatial distribution of alteration within the lava sequence sheeted dike complex and uppermost plutonic sequence alteration assemblages record the initiation and subsequent cooling of an axial hydrothermal system s that was periodically perturbed by the injection of sheeted dikes mineralogical and geochemical data indicate that temperatures increased across the lava dike transition 50 180 c and that the grade of metamorphism was heterogeneous within the sheeted dike complex in that temperatures varied laterally from 100 300 c to 600 800 c within the upper dikes and were 400 450 c within the lower sheeted dikes during vigorous hydrothermal circulation at the epr the base of the hydrothermal system was located within or at the top of a sequence of gabbros that is interpreted to be the root zone of the sheeted dike complex the base of the hydrothermal cell did not penetrate into the subjacent plutonic sequence until the section of crust had migrated off axis and the magma lens had solidified the vigor of the hydrothermal system would have waned by this time and hydrothermal fluids migrated into the lower crust along broadly distributed microfracture networks at temperatures between 450 and 650 c although locally temperatures may have reached 800 c the highest temperature alteration assemblages within the sheeted dikes formed in response to the injection of a dike or swarm of dikes that initiated a short lived hydrothermal system the sheeted dikes and plutonic rocks experienced very little retrograde alteration and there is no evidence for a discrete off axis hydrothermal event s incipient alteration conditions in crustal sections that formed at mid ocean ridges with slower rates of spreading than the epr are similar to hess deep although there are differences in the distribution and mineralogical expression of incipient alteration in plutonic sequences the evolution of alteration as a section of crust cools and moves off axis reflects its particular magmatic tectonic history and thus may vary significantly in ridges with all spreading rates

fluid flow triggered migration of events in the 1989 dobi earthquake sequence of central afar during the august 1989 sequence of earthquakes beneath the dobi graben in central afar principal events including 10 shocks with ml 5 5 propagated 50 km north westwards in 50 h the sequence is analyzed in terms of triggering by propagation of a critical pore pressure wave in a compressible fluid through a fractured medium the macroscopic fluid diffusion tensor d in the crust beneath the graben is estimated its principal eigenvector strikes n115 e nearly parallel to the graben axis with a value it is about 10 times larger than the other two consistent with fissure anisotropy in the graben the order of magnitude of the permeability 10 8 m is compatible with characteristic fissure widths ranging from 1 mm to 3 cm estimating such parameters in other extensional regions might help to understand better the temporal evolution of seismic swarms

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

reaction pathways and reaction progress for the smectite to chlorite transformation evidence from hydrothermally altered metabasites the transformation from smectite to chlorite has been interpreted as involving either a disequilibrium chlorite smectite mixed layering sequence or an equilibrated discontinuous sequence involving smectite corrensite chlorite here analysis of the smectite to chlorite transition in different geothermal systems leads us to propose that the transformation proceeds via three contrasting reaction pathways involving i a continuous mixed layer chlorite smectite series ii a discontinuous smectite corrensite chlorite series and iii a direct smectite to chlorite transition such contrasting pathways are not in accord with an equilibrium mineral reaction series suggesting that these pathways record kinetically controlled reaction progress in the geothermal systems reviewed the style of reaction pathway and degree of reaction progress is closely correlated with intensity of recrystallization and not to differences in thermal gradients or clay grain size this suggests a kinetic effect linked to variation in fluid rock ratios and or a contrast between advective or diffusive fluid transport the mode of fluid transport provides a means by which the rates of dissolution nucleation growth can control the reaction style and the reaction progress of the smectite to chlorite transition slow rates of growth are linked to the first reaction pathway involving mixed layering while increasing rates of growth relative to nucleation promote the generation of more ordered structures and ultimately lead to the direct smectite to chlorite transition representative of the third pathway

cap rock efficiency of geothermal systems in fold and thrust belts evidence from paleo thermal and structural analyses in rosario de la frontera geothermal area nw argentina abstract cap rock characterization of geothermal systems is often neglected despite fracturing may reduce its efficiency and favours fluid migration we investigated the siliciclastic cap rock of rosario de la frontera geothermal system nw argentina in order to assess its quality as a function of fracture patterns and related thermal alteration paleothermal investigations xrd on fine grained fraction of sediments organic matter optical analysis and fluid inclusions on veins and 1d thermal modelling allowed us to distinguish the thermal fingerprint associated to sedimentary burial from that related to fluid migration the geothermal system is hosted in a neogene n s anticline dissected by high angle nnw and ene striking faults its cap rock can be grouped into two quality categories rocks acting as good insulators deformed by nnw sse and e w shear fractures nne ssw gypsum and n s striking calcite filled veins that developed during the initial stage of anticline growth maximum paleo temperatures rocks acting as bad insulators deformed by nnw sse fault planes and nnw and wnw striking sets of fractures associated to late transpressive kinematics maximum paleo temperatures higher than about 115 c are linked to fluid migration from the reservoir to surface with a reservoir top at maximum depths of 2 5 km along fault damage zones this multi method approach turned out to be particularly useful to trace the main pathways of hot fluids and can be applied in blind geothermal systems where either subsurface data are scarce or surface thermal anomalies are lacking

multi stage zeolite facies mineralization in the hvalfjordur area iceland the hvalfjordur area 30 km north of iceland s capital reykjavik belongs to the sequence of late tertiary to early quaternary flood basalts with minor intercalations of hyaloclastites and rhyolites the basalts are affected by progressive low temperature metamorphism caused by the burial of the lava succession and higher heat flow from nearby central volcanoes low grade zeolite facies metamorphism of basaltic lavas in the hvalfjordur area results in two distinct mineral parageneses that can be correlated to events in the burial and hydrothermal history of the lava pile stage ia represents syn eruptive near surface alteration in which celadonite and silica were precipitated along primary pores during regional burial metamorphism stage ib hydrolysis of olivine and glass led to the formation of mixed layer chlorite smectite clays the chlorite content of stage ib phyllosilicate vesicle rims increases with increasing burial depth and temperature stage ii occurred after the burial and is marked by zeolite mineralization caused by higher heat flow from the laxarvogur and hvalfjordur central volcanoes altogether 11 different zeolites were found in the hvalfjordur area analcime chabazite epistilbite heulandite laumontite levyne mesolite stilbite stellerite thomsonite and yugawaralite in total three separate depth and temperature controlled zeolite zones occur in the hvalfjordur area

petrology of the dykes from the waziristan ophiolite nw pakistan abstract the waziristan ophiolite is located in the suture zone between the indian plate to the east and afghan block to the west it is highly dismembered and divisible into three main sheets or nappes which from east to west are the vezhda sar nappe entirely comprised of pillow basalts the boya nappe made up of ophiolitic melange with an intact section in its basal part and the datta khel nappe consisting mainly of sheeted dykes with smaller proportions of other components faunal evidence suggests that the ophiolite is of tithonian valanginian age it was thrust over the mesozoic shelf slope sediments of the indian plate to the east during the paleocene and is unconformably overlain by sedimentary rocks of early to middle eocene age to the west beside the sheeted dykes best exposed in the hanging wall of the datta khel thrust ene of datta khel the ophiolite also contains isolated dykes these are doleritic and basaltic in composition the dykes contain high na 2 o contents and high feo t mgo and lile hfse ratios and low tio 2 2 o contents non depletion of nb and high lile hfse ratio negate respectively an island arc or mid ocean ridge setting for these dykes enrichment in the lile suggests the involvement of a crustal component driven by fluids along the subduction zone several geochemical parameters suggest that the dykes of waziristan ophiolite have transitional characteristics between mid ocean ridge basalt and island arc tholeiite it is therefore proposed that these dykes may have originated in a back arc basin tectonic setting

chemical transport in geothermal systems in iceland evidence from hydrothermal alteration abstract this study focuses on the chemical changes in basaltic rocks in fossil low and high temperature hydrothermal systems in iceland the method used takes into account the amount of dilution caused by vesicle and vein fillings in the rocks the amount of dilution allows a calculation of the primary concentration of the immobile element zr and by multiplying the composition of the altered rock by the ratio of zr protolith zr altered rock one can compute the mass addition caused by the dilution of the void fillings and also make a direct comparison with the likely protoliths from the same areas the samples were divided into three groups two from tertiary fossil high temperature systems hafnarfjall geitafell and the third group from a low temperature zeolite altered plateau basalt succession the results show that hydrothermally altered rocks are enriched in si al fe mg and mn and that na k and ca are mobile but show either depletion or enrichment the elements that are immobile include zr y nb and probably ti the two high temperature systems show quite similar chemical alteration trends an observation which may apply to icelandic fresh water high temperature systems in general the geochemical data show that the major changes in the altered rocks from icelandic geothermal systems may be attributed to addition of elements during deposition of pore filling alteration minerals a comparison with seawater dominated basalt hosted hydrothermal systems shows much greater mass flux within the seawater systems even though both systems have similar alteration assemblages the secondary mineral assemblages seem to be controlled predominantly by the thermal stability of the alteration phases and secondarily by the composition of the hydrothermal fluids

fluid flow patterns in fast spreading east pacific rise crust exposed at hess deep tectonic exposures of a volcanic sequence and sheeted dike complex over a 4 km wide region at hess deep equatorial pacific reveal significant spatial heterogeneity 10 103 m in the extent and nature of hydrothermal alteration in young fast spreading east pacific rise crust the volcanic sequence is fairly uniformly altered with only minor oxidation and alteration to clay minerals sheeted dikes in the eastern part of the field area are highly fractured with narrow intervals of intact dikes that dip up to 60 their alteration characteristics show a simple depth trend such that with increasing depth the dominant secondary mafic mineral changes from chlorite to amphibole clinopyroxene replacement increases 40 whole rock 18o values decreases 4 4 5 5 to 3 5 4 5 and calculated peak metamorphic temperatures increase 250 c to 450 700 c within the deepest dikes localized zones up to 400 m wide are chlorite rich and have low 18o 2 9 4 1 and low peak metamorphic temperatures 345 c these alteration patterns likely formed within broad recharge zones whereby the low 18o zones developed in the regions with the highest fluid flux in the west massive slightly rotated sheeted dikes near the volcanic sheeted dike transition are 18o and cu depleted and display higher peak temperatures 345 c than elsewhere in the shallow dikes these characteristics are consistent with formation within a high temperature hydrothermal discharge zone we propose that the spreading history of a fast spreading ridge segment can create significant spatial heterogeneity in fluid flow and alteration patterns within sheeted dike complexes similar to those preserved in many ophiolites

integrated tem xrd and electron microprobe investigation of mixed layer chlorite smectite from the point sal ophiolite california five basalt samples from the point sal ophiolite california were examined using hrtem and aem in order to compare observations with interpretations of xrd patterns and microprobe analyses xrd data from ethylene glycol saturated samples indicate the following percentages of chlorite in mixed layer chlorite smectite identified for each specimen i l2036 almost equal to 50 ii l2035 almost equal to 70 and 20 iii 1a 13 almost equal to 70 iv 1b 42 almost equal to 70 and v 1b 55 100 detailed electron microprobe analyses show that chlorite analyses with high si k na and ca contents are the result of interlayering with smectite like layers the fe fe mg ratios of mixed layer phyllosilicates from point sal samples are influenced by the bulk rock composition not by the percentage of chlorite nor the structure of the phyllosilicate measurements of lattice fringe images indicate that both smectite and chlorite layers are present in the point sal samples in abundances similar to those predicted with xrd techniques and that regular alternation of chlorite and smectite occurs at the unit cell scale both 10 and 14 angstrom layers were recorded with hrtem and interpreted to be smectite and chlorite respectively regular alternation of chlorite and smectite 24 angstrom periodicity occurs in upper lava samples l2036 and 1a 13 and lower lava sample 1b 42 for as many as seven alternations per crystallite with local layer mistakes sample l2035 shows disordered alternation of chlorite and smectite with juxtaposition of smectite like layers suggesting that randomly interlayered chlorite 0 5 smectite exists images of lower lava sample 1b 55 show predominantly 14 angstrom layers units of 24 angstrom tend to cluster in what may otherwise appear to be disordered mixtures suggesting the existence of a corrensite end member having thermodynamic significance

hydrothermal alteration of the ediacaran doushantuo formation in the yangtze gorges area south china abstract the geochemical and fossil record preserved in the ediacaran age 635 551 ma doushantuo formation of south china has been extensively examined to explore the impact of changing climate and the oxidation state of the oceans on the development and distribution of early multicellular life in the yangtze gorges area this formation shows many of the geochemical trends and features thought to typify global ocean chemistry in the ediacaran period but there are indications that post sedimentary processes modified these signals this study of clay minerals and organic matter builds a more detailed picture of the type and degree of post sedimentary alteration at different stratigraphic levels of the formation and focuses on how this alteration influenced stable carbon and oxygen isotope records in the cratonward jiulongwan and huajipo sections of the doushantuo formation its lower part members 1 and 2 consists largely of dolomitic shale rich in authigenic saponite that crystallized in an alkaline sedimentary basin saponite has been altered to chlorite via corrensite across tens of meters of strata in lower member 2 with increased alteration downward toward the cap dolostone the greater chloritization is accompanied by lower 18 o and higher d values of trioctahedral clays this pattern of alteration of trioctahedral clays is likely due to hydrothermal fluid activity in the underlying relatively permeable nantuo formation and cap dolostone a concomitant increase of solid bitumen reflectance toward the base of the formation supports this idea in the uppermost part of the formation in the yangtze gorges area member 4 a typical open water marine dolomitic shale rich in illite and organic matter increases in the methylphenanthrenes ratio index and solid bitumen reflectance correlate with decrease of the bulk rock k al ratio upward providing evidence for hot fluid migration above the nearly impermeable shale clay from the upper part of the formation is enriched in 18 o but not in d relative to clay from the lower parts indicating progressive 18 o enrichment of hydrothermal fluids that percolated upward and laterally through permeable 18 o rich carbonates a maximum hydrothermal alteration temperature of 200 c is estimated from a calibration curve for illitization during burial diagenesis but given that the hydrothermal activity probably occurred in short pulses the temperature could have been much higher k ar ages are consistent across different size fractions of fine illite from member 4 shale 430 ma and from a k bentonite bed near the base of member 2 in the jiuqunao section 325 ma 25 km from jiulongwan and huajipo these age values show that the diagenetic illite of the doushantuo formation is a product of either deep burial diagenesis overprinted by spatially limited hydrothermal activity or of two localized hydrothermal events patterns of carbonate 13 c and 18 o depletion in the basal doushantuo formation are similar to chloritization trends and 18 o variation in diagenetic clay minerals given independent evidence for 13 c depletion of hydrothermal fluids these trends indicate carbonate fluid isotope exchange commensurate with the degree of post sedimentary alteration supporting a model of lithologically controlled differential diagenesis induced by hydrothermal fluids as the main control on c and o isotope variability in this stratigraphic interval this model could potentially explain other notable 13 c excursions higher up in member 3

reaction path modelling of in situ mineralisation of co2 at the carbfix site at hellisheidi sw iceland abstract results from injection of 175 tonnes of co2 into the basaltic subsurface rocks at the carbfix site in sw iceland in 2012 show almost complete mineralisation of the injected carbon in less than two years matter et al 2016 snaebjornsdottir et al 2017 reaction path modelling was performed to illuminate the rate and extent of co2 water rock reactions during and after the injection the modelling calculations were constrained by the compositions of fluids sampled prior to during and after the injection as reported by alfredsson et al 2013 and snaebjornsdottir et al 2017 the ph of the injected fluid prior to co2 dissolution was 9 5 whereas the ph of the background waters in the first monitoring well prior to the injections was 9 4 the ph of the sampled fluids used in the modelling ranged from 3 7 at the injection well to as high as 8 2 in the first monitoring well modelling results suggest that co2 rich water basalt interaction is dominated by crystalline basalt dissolution along a faster high permeability flow path but by basaltic glass dissolution along a slower pervasive flow path through which the bulk of the injected fluid flows dissolution of pre existing calcite at the onset of the injection does not have a net effect on the carbonation but does contribute to a rapid early ph rise during the injection and influences which carbonate minerals precipitate at low ph mg and fe are preferentially released from crystalline basalts due to the higher dissolution rates of olivine and to lesser extent pyroxene compared to plagioclase and glass gudbrandsson et al 2011 this favours the formation of siderite and fe mg carbonates over calcite during early mineralisation the model suggests the formation of the following carbonate mineral sequences siderite at ph 5 and calcite at higher ph other minerals forming with the carbonates are al and fe hydroxides and chalcedony and zeolites and smectites at elevated ph the most efficient carbonate formation is when the ph is high enough for formation of carbonates but not so high that zeolites and smectites start to form which compete with carbonates over both cations and pore space the results of reaction path modelling at the carbfix site in sw iceland indicate that this sweet spot for mineralisation of co2 is at ph from 5 2 to 6 5 in basalts at low temperature 20 50 c

evolution of clay mineral assemblages in the tinguiririca geothermal field andean cordillera of central chile an xrd and hrtem aem study abstract hrtem textural evidence shows that clay minerals in the tinguiririca geothermal field andean cordillera central chile are the result of direct alteration of former volcanic glass and minerals by hydrothermal fluids at similar temperatures to the present day they show the classical pattern of diagenetic transformation from smectite at the top to illite at the bottom with the progressive formation of corrensite and chlorite the high fluid rock ratio disposability of necessary cations and absence of previous detrital phyllosilicates allow the consideration of this area as a natural laboratory to establish the extreme ideal conditions for very low t reactions transformations from smectite to r1 illite smectite i s and from these to r3 mixed layers occur respectively at 80 120 c and 125 180 c in spite of ideal genetic conditions the new formed minerals show all the defective character and lack of textural and chemical equilibrium previously described in the literature for diagenetic and hydrothermal low temperature assemblages chemistry of smectite illite phases evolves basically through a diminution of the pyrophyllitic component toward a theoretical muscovite si 4 al 3 k however a second chemical vector si 4 mg 2 al 3 al 3 that is decreasing of the tschermack component also contributes to the evolution toward the less si more al rich muscovite in relation to the original smectite residual mg and fe from the latter reaction is consumed in the genesis of chloritic phases nevertheless as a consequence of the lack of chemical equilibrium probably because of the short time scale of the geothermal alteration processes the composition of clay minerals is highly heterogeneous at the level of a single sample consequently the respective fields of smectite r1 i s and r3 i s overlap each other making the distinction among these three phases impossible based exclusively on chemical data

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

atacamite inclusions in rock forming feldspars and copper bearing smectites from the radomiro tomic mine chile copper insoluble mineral occurrences recovery of copper from the radomiro tomic deposit in chile employs innovative large scale hydrometallurgical extraction methods with sulfuric acid used for heap leaching here we report on improvements in understanding mineral components of copper oxide ores which are insoluble in acid leach solutions in order to advance leaching technology using optical electron microprobe x ray diffraction energy dispersive spectroscopy and least squares inversion methods two different occluded mineral occurrences were discovered in oxide ores which present two distinct metallurgical challenges for heap leaching but offer important new opportunities in process mineralogy first in a high chloride to sulfate ratio feldspar stable portion of the orebody at radomiro tomic disseminated atacamite cu4cl2 oh 6 inclusions 1 to 8 m in diameter occur in microfractured feldspars and biotites these account for the acid insoluble fraction often as high as 30 percent of the total contained copper about 70 percent of the total copper occurs generally as atacamite in cracks which upon crushing is exposed along surfaces of the rock fragments and is hence soluble in contrast in more intensely hydrothermally altered feldspar destructive weak argillic alteration zones insoluble copper exists more probably within the crystal structure of well crystallized saponitic smectite clays in nonexchangeable octahedral crystallographic sites the two different forms of applied acid insoluble components are products of contrasting microchemical environments one in a reactive potassic alteration gangue and the second in a nonreactive gangue in both cases however the nature of hydrothermal alteration though pervasive was relatively weak and left intact rock mineral buffers capable of neutralizing acidic and reducing fluids hence the insoluble components with respect to the applied sulfuric acid reflect strong wall rock mineral assemblage control on the behavior of copper on all scales the macrofield zonal scale the microscale of individual crystals in cracks and the atomic scale of octahedral sites in the clay structure the metallurgical implications of these atacamite microinclusions in rock forming minerals and structurally bound copper in smectites are different in the former case exposing ore inclusions on fragment surfaces would require extremely costly grinding to a grain size well below 400 mesh secondly chemical extraction of microinclusion copper from feldspar host phases would entail high acid consumption as hydrolysis of feldspars by chemical reaction with applied acid would occur causing neutralization in the case of cu smectites since the copper occurs in structurally bound nonexchangeable sites processes to remove the copper via ion exchange or acidic leach cannot work alternatives to ion exchange could involve destroying the smectite host mineral perhaps by its conversion to kaolinite that may only incorporate copper or other divalent cations in negligible amounts

tuffisitic kimberlites from the wesselton mine south africa mineralogical characteristics relevant to their formation abstract tuffisitic kimberlites from the wesselton mine consist in order of formation of the following primary components chloritized olivine macrocrysts and phenocrysts magmaclasts cryptocrystalline diopside phlogopite rich mantles and a smectite chlorite interclast matrix magmaclasts consist of one to several crystals of chloritized olivine set in a microcrystalline groundmass of diopside apatite perovskite spinel and chloritized and fresh phlogopite the latter commonly rimming chloritized olivines magmaclasts have some similarities to holocrystalline hypabyssal kimberlite but lack monticellite carbonate carbonate serpentine segregations and atoll spinels spinels in the magmaclasts show only a limited compositional evolution relative to spinels in spatially associated hypabyssal kimberlite pre existing solids including discrete olivine grains magmaclasts and most xenoliths are mantled by acicular diopside and phlogopite the interclast matrix is now represented by mixed layer phyllosilicates chlorite smectites that are poorer in alumina and iron than chlorite pseudomorphing olivine and microlitic phlogopite and diopside the interclast chlorite smectite is considered to represent former phlogopite which has undergone late stage deuteric hydrothermal like modification the interclast matrix crystallized from the volatile rich remnants of the magma none of the constituents of tuffisitic kimberlites including the chlorite or chlorite smectites represent material formed from externally derived fluids these primary textures are unique to kimberlites and only form in certain circumstances tuffisitic kimberlites formed by progressive crystallization and volatile exsolution induced segregation and or disruption within a kimberlite magma magmaclasts mantles interclast matrix during a continuum of rapidly changing conditions in a subsurface subvolcanic magmatic system this continuum represents a transition from a degassing magmatic system to a magmatic derived hydrothermal like fluid rapid drops in temperature with extensive exsolution of co 2 and incorporation of large amounts of locally derived country rock xenoliths resulted in explosive diatreme formation and within diatreme modification of the crystallization and deuteric replacement sequence of typical kimberlite magma resulting in a distinctive mineralogy and texture

miocene fossil hydrothermal system associated with a volcanic complex in the andes of central chile abstract cenozoic deposits in the andes of central chile have been affected by very low grade burial metamorphism at about 33 s in the cuesta de chacabuco area approximately 53 km north of santiago two oligocene and miocene volcanic units form a ca 1300 m thick rock pile the miocene unit corresponds to a volcanic complex composed of two eroded stratovolcanoes secondary mineral assemblages in both units were studied petrographically and using x ray diffraction and electron microprobe analyses most of the igneous minerals are wholly or partially preserved and the ubiquitous secondary minerals are zeolites and mafic phyllosilicates the alteration pattern observed is characterized by a lateral zonation in secondary mineralogy related to a lateral increase in temperature but not to stratigraphic depth the following three zones were established mainly based on the distribution of zeolites zone i comprises heulandite thomsonite mesolite stilbite and tri smectite zone ii contains laumontite yugawaralite prehnite epidote and chlorite and zone iii comprises wairakite epidote chlorite diopside biotite and titanite for each zone the following temperature ranges were estimated zone i 100 180 c zone ii 180 270 c and zone iii 245 310 c the alteration episode was characterized by a high p fluid p total ratio ca 1 0 although slightly variable a high geothermal gradient of ca 160 c km 1 and fluid pressures below 500 bars although temperature was the main control on the mineral zonation several interrelated parameters mainly fluid composition porosity and permeability were also important hot near neutral to slightly alkaline ph alkali chloride hydrothermal fluids with very low dissolved co 2 contents deposited the secondary minerals the alteration pattern is the result of depositing fluids in outflow regions from a hydrothermal system developed inside a volcanic complex during the miocene the hydrothermal system has been eroded to a maximum depth of 1 7 km

volcaniclastic rocks of the reydarfjordur drill hole eastern iceland 2 alteration volcaniclastic rocks of basaltic to rhyolitic composition interlayered with basaltic to icelanditic lava flows in a 1918 m drill hole in eastern iceland are altered to zeolite and pumpellyite prehnite facies assemblages major alteration minerals are chlorites illite chlorite mixed minerals illites k feldspar laumontite quartz and sphene others include julgoldite and secondary allanite smectites as well as mordenite and heulandite are restricted to the upper 900 m epidote and minor pumpellyite prehnite wairakite and anhydrite are restricted to the core below 900 m the boundary between zeolite and prehnite pumpellyite facies is marked by the appearance of chlorite epidote quartz assemblages at around 900 m the typical paragenesis prehnite pumpellyite epidote chlorite occurs near the base of the drill hole at 1855 m textures indicate generally prograde metamorphic conditions the broad compositional spectrum ranging from basalt to rhyolite abundance of vitric fragments and high initial porosity and permeability of the volcaniclastic rocks were major factors governing alteration the site of growth crystal shard vesicle pore and vein of a secondary phase strongly influences composition habit amount paragenesis and time of crystallization for example sphene and to a lesser degree epidote are less rich in fe and better crystallized in plagioclase than in the adjacent matrix the different mineral assemblages developed within plagioclase crystals through the section most clearly reflect the metamorphic changes the abundance of vitric rhyolitic clasts shards and pumice favored k al si enriched fluids from which phases like illite and adularia crystallized these phases were not found in interlayered basalt flows mehegan et al this issue low ca activity is also indicated by the occurrence of wairakite and pumpellyite rather than prehnite which is common in the basalts secular changes during metamorphism in pore fluid composition are shown by zonations in mineral compositions for example the depletion of epidote and juldoldite pumpellyite in fe with time appears to reflect decreasing o2 bulk rock chemical changes are more intense in the volcaniclastic rocks compared to the basalt lavas h2o fe3 fe2 and in part co2 values are higher in the upper part of the hole reflecting the higher porosity and glass content of the clastic rocks they decrease markedly downhole approaching values of the lava flows due to more pervasive metamorphic alteration and thus similar mineral assemblages and physical properties

the chemistry of trace elements in surface geothermal waters and steam iceland abstract the geochemistry of trace elements in surface geothermal fluids in iceland was studied the sampled fluids included hot springs mud pots steam vents and soil solutions with temperatures ranging from 4 to 100 c ph between 2 01 and 9 10 and total dissolved solids between 86 and 4375 ppm the surface geothermal waters may be categorized into three groups based on their chemical composition namely nacl waters steam heated acid sulfate waters and mixed waters nacl waters with ph 8 are considered to represent aquifer geothermal fluids that have undergone boiling in the upflow they contained only low concentrations of most metals

alteration and mineralization of an oceanic forearc and the ophiolite ocean crust analogy mineralogical chemical and isotopic o c s and sr analyses were performed on minerals and bulk rocks from a forearc basement section to understand alteration processes and compare with mid ocean ridges mor and ophiolites ocean drilling program hole 786b in the izu bonin forearc penetrates 103 m of sediment and 725 m into volcanic flows breccias and basal dikes the rocks comprise boninites and andesites to rhyolites most of the section was affected by low temperature 100 c seawater alteration with temperatures increasing downward the rocks are partly 5 25 altered to smectite fe oxyhydroxide calcite and phillipsite and exhibit gains of k rb and p loss of ca variable changes in si na mg fe sr and y and elevated 18o and 87sr 86sr higher temperatures 150 c in the basal dikes below 750 m led to more intense alteration and formation of chlorite smectite corrensite albite k feldspar and quartz chlorite a 5 m thick hydrothermally altered and pyritized zone at 815 m in the basal dikes reacted with mixtures of seawater and hydrothermal fluids to mg chlorite albite and pyrite and gained mg and s and lost si and ca focused flow of hydrothermal fluids produced serialization halos na k sericite quartz pyrophyllite k feldspar and pyrite along quartz veins at temperatures of 200 250 c high 87sr 86sr ratios of chloritized 0 7055 and sericitized 0 7065 rocks indicate involvement of seawater via mixing with hydrothermal fluids low 34s of sulfide 2 to 5 5 and sulfate 12 5 are consistent with input of magmatic so2 into hydrothermal fluids and disproportionation to sulfide and sulfate alteration processes were generally similar to those at mors but the arc section is more intensively altered in part because of the presence of abundant glassy rocks and mafic phases the increase in alteration grade below 750 m and the mineralization in the basal dikes are analogous to changes that occur near the base of the volcanic section in mor and the troodos ophiolite

nature of mixed layer clays and mechanisms of their formation and alteration abstract mixed layer clay minerals are intermediate products of reactions involving pure end member clays they come from natural environments ranging from surface to low grade metamorphic and hydrothermal conditions most often mixed layering is essentially two component but more complicated interstratifications have also been documented variable tendency to form regular 1 1 interstratifications has been observed and explanations of this phenomenon have been proposed mixed layer clays are either di or trioctahedral di trioctahedral interstratifications are rare most mixed layer clays contain smectite or vermiculte as a swelling component exceptions are all trioctahedral serpentine chlorite in low temperature environments and mica chlorite and talc chlorite at high temperatures solid state transformation and dissolution crystallization are the two mechanisms responsible for the formation of different mixed layer clays in general the weathering reactions that produce mixed layering are revers

melanges in southern mexico geochemistry and metamorphism of las ollas complex guerrero terrane las ollas complex loc is a subduction complex spatially associated with the early cretaceous zihuatanejo huetamo subterrane guerrero terrane in southern mexico loc tectonic melanges compose of a stack of east dipping west vergent tectonic sheets containing blocks of metabasalt metadolerite metagabbro ultramafics volcaniclastics quartz rich sandstone and chert enveloped in a highly sheared clastic or serpentinitic matrix most ig neous and igneous derived metamorphic blocks show geochemical and isotopic features typical of island arc tholeiitic suites i l ow tio 2 0 13 to 0 91 and zr 5 to 57 ppm contents ii high lfse hfse n ratios low lan ybn 0 5 to 4 values and high end t 7 9 to 8 0 ratios petrographical and mineral chemistry evidence indicates that blocks underwent early recrystallization under high pressure and low temperature hp lt blueschist facies conditions during subduction typical assemblages include blue sodic through calco sodic to na rich calcic amphibole lawsonite tremolite mg chlorite white mica albite quartz phase relations and chlorite thermometry suggest temperatures of about 200 330 o c and pressures of 5 7 kbar it is proposed that sedimentary blocks were generated by in situ remobilization and mixing whereas igneous blocks most probably derived from the chemically and isotopically identi cal zihuatanejo island arc suite our data suggest that loc represents part of a subduction complex formed by east ward directed subduction related with the evolution of the early cretaceous zihuatanejo island arc resume le complexe de las ollas loc est un complexe de subduction associe dans l espace au sous terrane zihuatanejo huetamo datant du cretace et localise dans le sud du mexique les melanges tectoniques du loc com prennent un empilement de feuillets tectoniques a vergence ouest et a pendage est qui contiennent des blocs de meta basalte metadolerite metagabbro roches ultramafiques roches volcanoclastiques gres riches en quartz et du chert enveloppes dans une matrice clastique ou serpentineuse hautement cisaillee la plupart des blocs ignes et metamorphi ques provenant de roches ignees presentent des caracteristiques geochimiques et isotopiques typiques de suites tholeiiti ques d arc insulaire i faibles concentrations de tio 2 0 13 a 0 91 et de zr 5 a 57 ppm ii rapports eleves

the pantelleria caldera geothermal system data from the hydrothermal minerals abstract this paper proposes on the basis of petrographic and mineralogic data on cutting and cores from two deep wells pantelleria 1 and pantelleria 2 the first model of the active hydrothermal system of the island of pantelleria phyllosilicates were studied in detail because they are considered key minerals in the identification of hydrothermal processes the results of these studies emphasize differences between the intracaldera and pericaldera areas of the island within the 45 ka caldera there is a high temperature 240 260 c at 600 800 m depth active hydrothermal system with five zones of characteristic alteration minerals with increasing depth rocks are unaltered to a depth of 200 m contain smectite and mixed layer chlorite smectite c s between 200 and 380 m chlorite illite chalcedony and quartz from 380 to 500 m albite adularia and saponite from 500 to 680 m mixed layer biotite vermiculite from 680 m to the depth drilled 1100 outside the caldera but near the rim a low temperature and low permeability

occurrence of fe mg rich smectites and corrensite in the morron de mateo bentonite deposit cabo de gata region spain a natural analogue of the bentonite barrier in a radwaste repository the morron de mateo bentonite deposit is being studied as a natural analogue of the thermal and geochemical effects on a bentonite barrier in a deep geological repository of high level radioactive wastes this bentonite deposit and its host rocks were intruded by a rhyodacitic volcanic dome that induced a hydrothermal metasomatic process affecting the biocalcarenite beds close to the dome in this work the mineralogical and chemical features of the clay minerals of the hydrothermally altered pyroclastic white tuffs and epiclastic rocks mass flow located in the ne sector of the morron de mateo deposit are described white tuffs have a high content of phyllosilicates mainly composed of dioctahedral smectites while mass flow have a higher proportion of inherited minerals the neoformed phyllosilicates are dioctahedral smectites and an interlayer chlorite smectite mineral of corrensite type the chemical composition of smectites reflects the different nature of the parent rocks in such a way that smectites from white tuffs have a quite homogeneous chemical composition and their structural formulae correspond to montmorillonite type while smectites from mass flow show more chemical variability higher fe and mg contents and a mean structural formulae corresponding to fe mg rich beidellite and or to an intermediate smectite member between beidellite and saponite in addition chemical composition and textural features of corrensite like clay minerals in relation to fe mg rich smectites in the samples have also been studied suggesting that the former seems to be formed from fe mg rich smectites the presence of corrensite in the epiclastic rocks suggests that in the morron de mateo area a hydrothermal alteration process occurred after bentonite formation which transformed fe mg rich smectites into corrensite this transformation was probably favoured by the intrusion of the morron de mateo volcanic dome which produced a temperature increase in the geological media and a supply of fe mg rich solutions these physicochemical conditions were also responsible for the metasomatic transformations observed in the biocalcarenite beds located on the top of the bentonite deposit all these data suggest that the morron de mateo natural system could be a good natural analogue of both thermal and chemical effects on a bentonite barrier related to the radioactive decay of fission products and the interaction between the corrosion products of steel over packs and the bentonite these circumstances would favour the transformation of the candidate al rich smectites into fe mg rich smectites and corrensite as steps prior to formation of chlorite in this case all the physicochemical and mechanical properties of al rich smectites would disappear and the clayey barrier would fail

the iceland deep drilling project at reykjanes drilling into the root zone of a black smoker analog the aim of the iceland deep drilling project is to drill into supercritical geothermal systems and examine their economic potential the exploratory well iddp 2 was drilled in the reykjanes geothermal field in sw iceland on the landward extension of the mid atlantic ridge the reykjanes geothermal field produces from a 300 c reservoir at 1 to 2 5 km depth and is unusual because it is recharged by seawater the well was cased to 3000 m depth and then angled towards the main up flow zone of the system to a total slant depth of 4659 m 4500 m vertical depth based on alteration mineral assemblages joint inversion of wireline logging and rate of heating measurements the bottom hole temperature is estimated to be about 535 c the major problem encountered during drilling was the total loss of circulation below 3 km depth and continuing to the final depth drilling continued without recovering drill cuttings consequently spot coring provided the only deep rock samples from the well these cores are characteristic of a basaltic sheeted dike complex with hydrothermal alteration mineral assemblages that range from greenschist to amphibolite facies hornblende hornfels and pyroxene hornfels allowing the opportunity to investigate water rock interaction in the active roots of an analog of a submarine hydrothermal system as they have not yet been sampled the composition of the deep fluids at reykjanes is unknown at present cold water is currently being injected with the aim of enhancing permeability at depth before allowing the well to heat up prior to flow tests planned for early 2019 the well has at least two fluid feed zones a dominant one at 3 4 km depth and a second smaller one at 4 5 km extensive geophysical surveys of the reykjanes peninsula completed recently allow correlation of geophysical signals with rocks properties and in situ conditions in the subsurface earthquake activity monitored with a local seismic network during drilling the iddp 2 drilling detected abundant small earthquakes ml 2 within the depth range of 3 5 km a zone at 3 5 km depth below the producing geothermal field that was generally aseismic prior to drilling but became seismically active during the drilling the drilling of the iddp 2 has achieved number of scientific and engineering firsts it is the deepest and hottest drill hole so far sited on an active mid ocean spreading center it penetrated an active supercritical hydrothermal environment at depths analogous to those postulated as the high temperature reaction zones feeding black smoker systems

the structure of the temsamane fold and thrust stack eastern rif morocco evolution of a transpressional orogenic wedge abstract the structure of the temsamane fold and thrust stack corresponds to four units limited by anastomosing ductile shear zones cutting a trend of south verging recumbent folds this ductile stack was formed in an inclined left handed transpressional zone at the north african paleomargin during chattian to langhian times producing two main deformational events the first event d p produced a s p l p planar linear fabric generated in a non coaxial deformation with a top to the wsw sense of movement and was associated to metamorphic p t conditions varying from late diagenesis in the southernmost temsamane outcrops to epizone in the north according to the 40 ar 39 ar ages this deformation occurred at chattian aquitanian times the second deformational event d c event generated ene wsw trending folds with sse vergence and a set of anastomosing shear zones with s m l m planar linear fabric the latter units were generated at around 15 ma langhian and indicate a strong localization of the simple shear component of the transpression moreover this orientation is compatible with the kinematics of the temsamane detachment which can explain most of the uplift of the temsamane rocks from the middle to the uppermost crust the described evolution indicates that collision between the western mediterranean terranes and the north african paleomargin and the formation of the rifean orogenic wedge occurred at chattian to langhian times

metabasalts from the mid atlantic ridge new insights into hydrothermal systems in slow spreading crust an extensive suite of hydrothermally altered rocks were recovered byalvin and dredging along the mark mid atlantic ridge south of the kane fracture zone 23 24 n where detachment faulting has provided a window into the crustal component of hydrothermal systems rocks of basaltic composition are altered to two assemblages with these characteristics i type i albitic plagioclase an02 10 mixed layer smectite chlorite or chlorite actinolite quartz sphene 20 of the clinopyroxene is altered and cu and zn are leached the geochemical signature of these alteration types reflects the relative proportion and composition of secondary minerals and the degree of alteration of primary phases and does not show simple predictive relationships element mobilities indicate that both alteration types formed at low water rock ratios the mark assemblages are typical of the greenschist and transition to the amphibolite facies and represent two distinct albeit overlapping temperature regimes type i 180 to 300 c and type ii 250 to 450 c by analogy with dsdp odp hole 504b and many ophiolites the mark metabasalts were altered within the downwelling limb of a hydrothermal cell and type i and ii samples formed in the upper and lower portions of the sheeted like complex respectively episodic magmatic and hydrothermal events at slow spreading ridges suggest that these observed mineral assemblages represent the cumulative effects of more than one hydrothermal event groundmass and vein assemblages in the mark metabasalts indicate either that alteration conditions did not change during successive hydrothermal events or that these assemblages record only the highest temperature event lack of retrograde reactions or overprinting of lower temperature assemblages e g zeolites suggests that there is a continuum in alteration conditions while crustal segments remain in the ridge axis environment the type ii samples may be representative of thereaction zone where compositions of hydrothermal fluids actively venting at the seafloor today become fixed this prediction necessitates interaction between hydrothermal fluids and intersertal glass and or mafic phases in addition to plagioclase in order to produce the observed range in vented fluid ph

hydrothermal frictional strengths of rock and mineral samples relevant to the creeping section of the san andreas fault abstract we compare frictional strengths in the temperature range 25 250 c of fault gouge from safod cdz and sdz with quartzofeldspathic wall rocks typical of the central creeping section of the san andreas fault great valley sequence and franciscan complex the great valley and franciscan samples have coefficients of friction 0 35 at all experimental conditions strength is unchanged between 25 and 150 c but increases at higher temperatures exceeding 0 50 at 250 c both samples are velocity strengthening at room temperature but show velocity weakening behavior beginning at 150 c and stick slip motion at 250 c these rocks therefore have the potential for unstable seismic slip at depth the cdz gouge with a high saponite content is weak 0 09 0 17 and velocity strengthening in all experiments and decreases at temperatures above 150 c behavior of the sdz is intermediate between the cdz and wall rocks

tectonometamorphic evolution of the acatlan complex eclogites southern mexico the acatlan complex of southern mexico is linked to the evolution of the appalachian caledonian chains and records events related to the taconian acadian and alleghanian orogenies of northeastern north america mafic eclogites and garnet amphibolites from two selected localities are used to partially reconstruct the tectonometamorphic evolution of this complex eclogites contain garnet almandine ca na pyroxene phengitic mica zoisite clinozoisite quartz ca na amphibole barroisite katophorite albitic plagioclase rutile phase and textural relationships thermobarometric determinations and available radiometric ages indicate that eclogite facies metamorphism took place during the ordovician at temperatures around 560 60 c and pressures between 11 and 15 kbar 1 kbar 100 mpa eclogites underwent widespread retrogression to epidote amphibolite then greenschist facies during exhumation most probably during devonian times epidote amphibolite facies include the critical assemblage cal

hydrothermal alteration of a 1 km section through the upper oceanic crust deep sea drilling project hole 504b mineralogy chemistry and evolution of seawater basalt interactions deep sea drilling project hole 504b penetrates 1076 m into oceanic layer 2 and is the first hole to pass through the transition from pillow basalts altered at low temperatures into hydrothermally metamorphosed sheeted dikes alteration of the crust at site 504 occurred in four stages related to the movement of the crust away from the spreading axis 1 dikes reacted with seawater 200 300 c in the upwelling zone of an axial convection cell at the spreading axis resulting in the formation of greenschist facies parageneses in veins and host rocks mixing of the upwelling hydrothermal fluids with seawater circulating in the overlying more permeable pillow section occurred in the upper part of the lithologic transition zone causing a steep temperature gradient at the base of the pillow section secondary minerals formed in the lower pillow section from the resultant reducing mixed fluids at temperatures of around 100 c at the same time the initial effects of seafloor weathering began in the upper 320 m of the pillow section at low temperatures 50 c and under conditions of open circulation of oxidizing seawater 2 following refracturing of the dikes a second stage of axial upwelling occurred hydrothermal fluids 200 380 c were probably similar to those presently sampled from spreading ridges on the seafloor mixing of these fluids with seawater in the lithologic transition zone caused deposition of quartz epidote and sulfides in veins and formation of a sulfide rich stockworklike zone within this transition zone alteration of the pillow section proceeded under the prior conditions with the effects of seafloor weathering extending progressively downward into the crust 3 seawater recharge penetrated to depths of at least 1075 5 m subbasement and deposited anhydrite locally in veins 4 off axis alteration of the dikes was characterized by formation of zeolites calcite and prehnite in veins and host rocks from more highly evolved and lower temperature 100 250 c fluids alteration in the upper pillow section evolved from seafloor weathering conditions to more reducing and rock dominated as cracks in the basalt were sealed with secondary minerals and the basement was covered with a layer of sediment zeolites and calcite were the last phases to form throughout the pillow section sealing of the crust to convective cooling also allowed conductive reheating of the crust from below calculations of seawater crustal chemical fluxes from whole rock data are complicated by the low recovery and heterogeneity of alteration effects but indicate that basalt seawater interactions are a sink for seawater mg and k and a source for si and ca

compositions and parageneses of secondary minerals in the onikobe geothermal system japan abstract parageneses and chemical compositions of secondary minerals in many core samples of the onikobe geothermal system japan were investigated in detail in order to evaluate the controls of geothermal waters on alterations of andesitic dacitic rocks depth zonations of secondary minerals were delineated for cazeolites as mordenite laumontite yugawaralite wairakite and clay minerals as smectite alkaline smectite chlorite smectite chlorite compositional variations with depth hence temperature are apparent for smectite chlorite and illite chlorite interstratified clay minerals but not for wairakite both prehnite and epidote at depth have fe rich compositions and become more aluminous with increasing depth the ubiquitous occurrence of pyrite and rare magnetite and the absence of hematite suggest that the onikobe system had fs2 fo2 conditions within the pyrite stability field the occurrence of abundant calcite and ca zeolites indicates that thermal waters at depth are neutral to slightly alkaline and xco2 is very low to stabilize ca al silicates this conclusion is consistent with the production of a remarkable amount of alkaline high temperature steam and thermal waters in a newly drilled production well

a chlorite solid solution geothermometer the los azufres mexico geothermal system chlorite constitutes a major hydrothermal alteration product of metamorphism of andesites in the active geothermal system of los azufres mexico electron microprobe analyses performed on a set of crystals from each sample show wide variations in composition correlation coefficients among chemical constituents were calculated it is shown that the tetrahedral charge is positively correlated with the octahedral vacancy and negatively with the iron content and there is almost no correlation with the octahedral aluminium and magnesium content a procedure is proposed to select end members and substitution vectors and to give a general formula for these chlorites

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

corrensite and mixed layer chlorite corrensite in metabasalt from northern taiwan tem aem empa xrd and optical studies many chloritic minerals in low grade metamorphic or hydrothermally altered mafic rocks exhibit abnormal optical properties expand slightly upon glycolation expandable chlorite and or have excess alvi relative to aliv as well as significant ca k and na contents chloritic minerals with these properties fill vesicles and interstitial void space in low grade metabasalt from northern taiwan and have been studied with a combination of tem aem empa xrd and optical microscopy the chloritic minerals include corrensite which is an ordered 1 1 mixed layer chlorite smectite and expandable chlorite which is shown to be a mixed layer chlorite corrensite corrensite and some mixed layer chlorite corrensite occur as rims of vesicles and other cavities while later formed mixed layer chlorite corrensite occupies the vesicle cores the tem observations show that the mixed layer chlorite corrensite has ca 20 and the corrensite has ca 50 expandable smectite like layers consistent with xrd observations and with their abnormal optical properties the aem analyses show that high si and ca contents high alvi aliv and low fevi fe mg vi ratios of chlorites are correlated with interstratification of corrensite or smectite like layers in chlorite the aem analyses obtained from 200 500 a thick packets of nearly pure corrensite or chlorite layers always show that corrensite has low aliv siiv and low fevi fe mg vi while chlorite has high aliv siiv and high fevi fe mg vi this implies that the trioctahedral smectite like component of corrensite has significantly lower aliv siiv and fevi fe mg vi the ratios of fevi fe mg vi and aliv siiv thus decrease in the order chlorite corrensite smectite the proportions of corrensite or smectite like layers relative to chlorite layers in low grade rocks are inferred to be controlled principally by fe mg ratio in the fluid or the bulk rock and by temperature compositional variations of chlorites in low grade rocks which appear to correlate with temperature or metamorphic grade more likely reflect variable proportions of mixed layered components the assemblages of trioctahedral phyllosilicates tend to occur as intergrown discrete phases such as chlorite corrensite corrensite smectite or chlorite corrensite smectite a model for the corrensite crystal structure suggests that corrensite should be treated as a unique phase rather than as a 1 1 ordered mixed layer chlorite smectite

hydrothermal alteration of plagioclase and growth of secondary feldspar in the hengill volcanic centre sw iceland dissolution of igneous feldspar and the formation and occurrence of secondary feldspar in tholeiitic basalts from the hengill volcanic centre in sw iceland was studied by microprobe analysis of cuttings from two ca 2000 m deep geothermal wells well ng 7 in nesjavellir represents a geothermal system in a rift zone where the intensity of young insignificantly altered intrusions increases with depth well khg 1 in kolvioarholl represents the margin of a rift zone where the intensity of intrusives is lower and the intensity of alteration higher this marginal well represents altered basaltic crust in an early retrograde state the secondary plagioclase in both wells is mainly oligoclase occurring in association with k feldspar and chlorite actinolite the texture of this assemblage depends on the lithology and intensity of alteration in nesjavellir ng 7 the composition of secondary albite oligoclase is correlated with the host rock composition this connection is not apparent in more intensely altered samples from kolvioarholl khg 1 the influence of temperature on composition of secondary na feldspar is unclear in both wells although ca is expected to increase with temperature any temperature dependence may be suppressed by the influence of rock composition in nesjavellir and by retrograde conditions at kolvioarhall the absence of clear compositional gradients between igneous plagioclase and secondary feldspar and between na feldspar and k feldspar suggests that secondary feldspars formed by dissolution precipitation reactions c 2002 elsevier science b v all rights reserved less

contribution of long term hydrothermal experiments for understanding the smectite to chlorite conversion in geological environments the smectite to chlorite conversion is investigated through long duration experiments up to 9 years conducted at 300 c the starting products were the wyoming bentonite mx80 79 smectite metallic iron and magnetite in contact with a na ca chloride solution the predominant minerals in the run products were an iron rich chlorite chamosite like and interstratified clays interpreted to be chlorite smectite and or corrensite smectite accompanied by euhedral crystals of quartz albite and zeolite the formation of pure corrensite was not observed in the long duration experiments the conversion of smectite into chlorite over time appears to take place in several steps and through several successive mechanisms a solid state transformation significant dissolution of the smectite and direct precipitation from the solution which is over saturated with respect to chlorite allowing the formation of a chamosite like mineral the reaction mechanisms are confirmed by x ray patterns and data obtained on the experimental solutions ph contents of si mg na and ca because of the availability of some nutrients in the solution total dissolution of the starting smectite does not lead to 100 crystallization of chlorite but to a mixture of two dominant clays chamosite and interstratified chlorite smectite and or corrensite smectite poor in smectite the role of fe fe mg in the experimental medium is highlighted by chemical data obtained on newly formed clay particles alongside previously published data the newly formed iron rich chlorite has the same composition as that predicted by the geothermometer for diagenetic to low grade metamorphic conditions and the quartz fe chlorite albite experimental assemblage in the 9 year experiment is close to that fixed by water rock equilibrium

correlation of very low grade data to calibrate a thermal maturity model in a nappe tectonic setting a case study from the alps abstract vitrinite and bituminite reflectance data xrd studies on clay minerals radiometric data and geochemical data have been used to investigate metamorphism of very low and low grade sediments from the lower austroalpine and south penninic nappes in the oberhalbstein valley central alps numerical maturity modelling methods are calibrated with petrological structural and new geochronological data to reconstruct temperature histories even in nappe tectonic settings the combination of low grade data and maturity modelling shows that the duration of peak temperature metamorphism plays an important role in the alteration of organic matter vitrinite reflectance vr often does not correlate with other low grade parameters but in the oberhalbstein equilibrium between the different parameters is evident because heating near the peak conditions for more than 20 ma took place it is proposed to use the comparison to evaluate equilibrium conditions for the temperature parameters in general in addition from petrological work and vr modelling the pressure conditions are estimated the results yield an eroded overburden of 6 1 km and a temperature gradient of 30 2 c km 1 at the base of the nappe stack pressure conditions of 4 1 kbar are determined and therefore no evidences for a high pressure event the main heating is dated between 90 and 70 5 ma and is attributed to nappe stacking in the upper crust

water rock interaction and hydrothermal mineral equilibria in the tendaho geothermal system abstract the tendaho geothermal system occurs within a nw se trending rift basin filled with quaternary volcanics mainly basalts and fluvio lacustrine sediments three deep td 1 td 2 and td 3 and one shallow td 4 geothermal wells have been drilled the waters of productive wells td 1 td 2 and td 4 are typical na cl geothermal waters with reservoir temperature ranging between 220 and 270 c chemical analysis of core samples altered basalts shows increased ca fe mg al content owing to the dissolution of plagioclase and femics and the precipitation of wairakite laumontite epidote garnet calcite and clay minerals and decreased na k si and ti owing mostly to the dissolution of glass matrix petrographic study of cuttings and core samples shows 1 evidence of an early stage of calcite zeolite wairakite or laumontite and quartz crystallization while calcite underwent different stages of dissolution precipitation possibly due to abrupt changes in ph and co 2 partial pressure 2 that epidote garnet prehnite pyroxene and amphibole crystallization occurred after wairakite or laumontite chlorite is the main layered silicate in the basaltic rocks in wells td 1 and td 2 temperatures beyond the stability of smectite and chlorite smectite c s interlayers must have been reached the smectites and c s interlayers in association with chlorite in well td 3 indicate that this well has had a more complex thermal history with variable temperatures the smectites occur at temperatures above 120 c which is considered to be the threshold for the transformation of smectite to illite no evidence of disequilibrium conditions of smectites has been found at reservoir temperatures currently present at tendaho fluid inclusion data indicate heating in the well td 1 and thermal conditions similar to the ones measured in the deepest part of well td 2 while the uppermost part of this latter has undergone cooling intense cooling has affected well td 3 drilled far from the upflow zone of the field probably in an area characterized by self sealing cooling and very low permeability

implications for early hydrothermal environments on mars through the spectral evidence for carbonation and chloritization reactions in the nili fossae region 1 previous identification of serpentine and magnesium carbonate in the eastern nili fossae region of mars indicates hydrothermal alteration of an olivine rich protolith here we characterize fe mg phyllosilicates associated with these units and present spectral evidence for the presence of a talc component distinguishable from saponite locations with magnesium carbonate are exclusively associated with talc related phyllosilicates in the westernmost portions of the nili fossae region where a mafic protolith dominates fe mg phyllosilicates display spectral evidence for a wide degree of chloritization we propose that noachian fe mg smectites were uniformly buried by hesperian lava flows that initiated hydrothermal alteration in the eastern nili fossae region the chloritization of smectites may have produced silica rich fluids necessary for the serpentinization of olivine temperature and depth constraints indicated by their distribution also suggest a hydrothermal system was present the subsequent carbonation of serpentine and or olivine in eastern nili fossae while requiring an additional co2 source provides an explanation for the limited occurrence of serpentine and the colocation of carbonate and talc bearing material throughout this area the consequence of the hypothesized carbonation reaction and the presence of serpentine provides geochemical constraints for the proportion of co2 present in the fluids that interacted with the protolith if this carbonation reaction was a widespread phenomenon it may have been an important process in the ancient martian carbon cycle and could have provided a sink for co2 in the past

mineralogical geochemical and isotopic characteristics of hydrothermal alteration processes in the active submarine felsic hosted pacmanus field manus basin papua new guinea during odp leg 193 4 sites were drilled in the active pacmanus hydrothermal field on the crest of the felsic pual ridge to examine the vertical and lateral variations in mineralization and alteration patterns we present new data on clay mineral assemblages clay and whole rock chemistry and clay mineral strontium and oxygen isotopic compositions of altered rocks from a site of diffuse low temperature venting snowcap site 1188 and a site of high temperature venting roman ruins site 1189 in order to investigate the water rock reactions and associated elemental exchanges the volcanic succession at snowcap has been hydrothermally altered producing five alteration zones 1 chlorite illite cristobalite plagioclase alteration apparently overprinted locally by pyrophyllite bleaching at temperatures of 260 310degreesc 2 chlorite mixed layer clay alteration at temperatures of 230degreesc 3 chlorite and illite alteration 4 illite and chlorite illite mixed layer alteration at temperatures of 250 260degreesc and 5 illite chlorite alteration at 290 300degreesc felsic rocks recovered from two holes 1189a and 1189b at roman ruins although very close together show differing alteration features hole 1189a is characterized by a uniform chlorite illite alteration formed at similar to250degreesc overprinted by quartz veining at 350degreesc in contrast four alteration zones occur in hole 1189b 1 illite chlorite alteration formed at similar to300degreesc 2 chlorite illite alteration at 235degreesc 3 chlorite illite and mixed layer clay alteration and 4 chlorite illite alteration at 220degreesc mass balance calculations indicate that the chloritization illitization and bleaching silica pyrophyllite assemblages alteration stages are accompanied by different chemical changes relative to a calculated pristine precursor lava the element cr appears to have a general enrichment in the altered samples from pacmanus the clay concentrate data show that cr and cu are predominantly present in the pyrophyllites illite shows a significant enrichment for cs and cu relative to the bulk altered samples considerations of mineral stability allow us to place some constraints on fluid chemistry hydrothermal fluid ph for the chloritization and illitization was neutral to slightly acidic and relatively acidic for the pyrophyllite alteration in general the fluids especially from roman ruins and at intermediate depths below snowcap show only a small proportion of seawater mixing 10 fluids in shallow and deep parts of the snowcap holes in contrast show stronger seawater influence copyright c 2004 elsevier ltd

distribution and mobilization of sn in silicate minerals from the mesozoic shizhuyuan w dominated polymetallic deposit south china abstract although mobilization and concentration of elements in super large deposits have become hot topics in recent years studies of the mobilization of metals between minerals using in situ analyses are rare the shizhuyuan w dominated polymetallic deposit in south china is one of the largest non ferrous metal deposits in the world and contains large metal reserves of sn w bi and rare earth elements using laser ablation inductively couple plasma mass spectrometry la icp ms we analyzed trace elements in major silicate minerals and magnetite from granites and skarns of the shizhuyuan deposit tin concentrations of muscovitized biotite range from 196 to 558 ppm muscovite from 57 7 to 332 ppm chloritized biotite from 1 62 to 527 ppm k feldspar from 4 16 to 6 97 ppm kaolinized k feldspar from 1 15 to 9 38 ppm plagioclase from 1 87 to 5 09 ppm sericitic plagioclase from 2 53 to 28 7 ppm kaolinized plagioclase from 0 99 to 17 7 ppm epidote from 1199 to 3382 ppm garnet from 797 to 2860 ppm chloritized garnet from 72 5 to 2548 ppm chlorite from 19 6 to 159 ppm and magnetite from 23 7 to 872 ppm thus sn is mainly hosted in biotite muscovitized biotite garnet and epidote sn occurs as an isomorphous substitution and is predominantly associated with fe3 for fe3 poor minerals sn can isomorphously substitute al in the form of simple cation due to similar radius at the octahedral site such as in k feldspar and kaolinized k feldspar however sn cannot substitute al in the si o tetrahedrons therefore sn and al correlate different in different silicate minerals alteration of biotite to muscovite and chlorite can release a large amount of sn to the magmatic hydrothermal fluids which may be already enriched in sn during the formation of skarn deposits sn f complexes can be converted to cassiterite sno2 to free up f by hydrolysis a significant amount of sn enters the lattices of garnet and epidote therefore release of sn during alteration of garnet to chlorite in a retrograde stage of alteration may have contributed to the mineralization

the smectite to chlorite transition in the chipilapa geothermal system el salvador clay mineralogical x ray diffraction and electron microprobe studies have been carried out on separated 2 m fractions from cutting and core material from three wells in the chipilapa geothermal system in el salvador the data indicate that the smectite to chlorite transition is prevalent but a secondary smectite to illite transition is also present at depths approximately 750 m smectite with very minor chlorite mixed layers approximately 15 is dominant and has a composition midway between a di and tri smectite at 750 m there is a very clear distinction and sharp transition into discrete chlorite with very minor smectite mixed layers approximately 10 corrensite is recorded only as a rare and minor phase smectite occurs in abundance at temperatures up to 200 c and the transition from a smectitedominant to chlorite dominant assemblage takes place over a narrow temperature range 150 to 200 c the stability range of smectite is very similar to that recorded in other geothermal systems whereas the smectite to chlorite transition differs greatly from that recorded in other systems the transition does not involve continuous chlorite smectite mixed layering but a marked step it is the sharpest and most discontinuous stepped sequence of this mineralogical transition recorded

comparison of diagenetic and low grade metamorphic evolution of chlorite in associated metapelites and metabasites an integrated tem and xrd study chlorite is a common sheet silicate that occurs in various lithologies over a wide grade range involving diagenesis and low grade metamorphism thus the reaction progress of chlorite overs a unique oppor tunity for direct correlation of zonal classification of metasedimentary rocks based on illite crystallinity with metabasite mineral facies to provide such correlation chlorite crystallinity indices apparent mean crystallite sizes and lattice strains crystallite size distributions and compositions of chlorite from coexisting metapelites and metabasites were determined by x ray divraction xrd scanning electron microscopy sem transmission electron microscopy tem analytical electron microscopy aem and electron microprobe emp methods samples were from palaeozoic and mesozoic formations of the bukkium innermost western carpathians hungary that underwent alpine cretaceous orogenic metamorphism metapelites range in grade from late diagenesis to epizone whereas metabasites vary from prehnite pumpellyite through pumpellyite actinolite to greenschist facies despite significant diverences in composition mineral assemblages and textures reaction progress as measured in part by chlorite crystallinity in metapelites paralleled that in metabasites chlorite crystal linity and mean crystallite size increase and the proportion of mixed layers in chlorite decreases whereas the calculated lattice strain does not change significantly with increasing metamorphic grade similar trends but especially at higher grades significant diverences were found in mean crystallite size values using various methods for xrd line profile analyses the increase in crystallite size with increasing grade was demonstrated also by direct tem measurements on ion milled whole rock samples but with a larger scatter of data at higher grades in spite of the diverent kinds of mixed layering in chlorite mg rich smectitic mostly random local corrensite like units in metabasites and fe rich berthierine and diocta hedral smectite in metapelites xrd calculated and tem measured parameters were found to be reliable tools for measuring reaction progress and metamorphic grade of the same degree in both lithotypes

the smectite to chlorite transition in a fossil seamount hydrothermal system the basement complex of la palma canary islands the hydrothermal metamorphism of a sequence of pliocene aged seamount extrusive and volcanoclastic rocks on la palma includes a relatively complete low p t facies series encompassing the zeolite prehnite pumpellyite and greenschist facies the observed mineral zonations imply metamorphic gradients of 200 300 c km 1 the transition from smectite to chlorite in the la palma seamount series is characterized by discontinuous steps between discrete smectite corrensite and chlorite which occur ubiquitously as vesicles and to a much lesser extent vein in fillings trioctahedral smectites mg fe mg 0 4 0 75 occur with palagonite and na ca zeolites such as analcime and a thompsonite natrolite solid solution corrensite mg fe mg 0 5 0 65 first appears at stratigraphic depths closely corresponding to the disappearance of analcime and first appearance of pumpellyite discrete chlorite mg fe mg 0 4 0 6 becomes the dominant layer silicate mineral coincident with the appearance of epidote and andraditic garnet within the stratigraphic section there is some overlap in the distribution of the three discrete layer silicate phases although random interstratifications of these phases have not been observed although smectite occurs as both low and high charge forms the la palma corrensite is a compositionally restricted 1 1 mixture of low charge trioctahedral smectite and chlorite electron microprobe analyses of coarse grained corrensite yield structural formulae close to ideal values based on 50 negative charge recalculations calcium average 0 20 cations formula unit is the dominant interlayer cation with lesser mg k and na the absence of randomly interlayered chlorite smectite in the la palma seamount series may reflect high time integrated fluid fluxes through the seamount sequence this is consistent with the ubiquity of high variance metamorphic mineral assemblages and the general absence of relict igneous minerals in these samples

tem and aem constraints on the origin and significance of chlorite mica stacks in slates an example from central wales u k chlorite mica stacks grains of intergrown chlorite and white mica in a matrix of fine grained white mica and chlorite have been studied using xrd sem empa tem and aem methods the stacks occur in a weakly cleaved llandoverian mudstone central wales which has a white mica illite crystallinity index of 0 35 a20 corresponding to the lower anchizone white mica occurs as packets 100 to 8 m thick interleaved with dominant chlorite packets in stacks with both apparent coherent interfaces or cross cutting small angle boundaries with chlorite layers it is well crystallized 2m polytype with phengitic composition and low paragonite component na na k 0 0 07 chlorite in stacks is fe rich and relatively homogeneous tio2 crystals surround stacks and occur within chlorite and white mica is ti rich compared to matrix white mica fine grained white mica and chlorite in the matrix have two different orientations one parallel to bedding and one parallel to cleavage which is approximately 30 50 to bedding matrix white mica is predominantly a 2m1 polytype but some cleavage parallel white mica is 3 t and some bedding parallel white mica is 1md it is na rich na na k 0 14 0 42 and relatively heterogeneous some discrete paragonite and phengitic muscovite are observed to coexist in the cleavage orientation matrix white mica and chlorite each contain less fe than corresponding white mica and chlorite in stacks both matrix and stack chlorite are one layer polytypes the data imply that chlorite in the stacks is largely derived from the replacement of volcanogenic biotite and other ferromagnesian minerals probably via intermediate expandable trioctahedral phyllosilicates most intergrowths of chlorite and mica in stacks formed by mica replacement of chlorite and altered biotite along cleavage fissures subsequent deformation caused further modification of pre existing chlorite mica stacks whereas partial dissolution of stacks and bedding parallel matrix phyllosilicates resulted in the formation of cleavage parallel phyllosilicates

textural and mineralogical changes associated with the incipient hydrothermal alteration of glassy dacite at the submarine pacmanus hydrothermal system eastern manus basin variably altered dacite from the pacmanus vent field in the eastern manus back arc basin papua new guinea was studied to determine the textural and mineralogical characteristics of hydrothermal alteration taking place in the immediate subsurface of this modern seafloor hydrothermal system detailed textural investigations show that fluid flow through the glassy dacite has been strongly controlled by the primary volcanic textures quench fractures and networks of interconnected perlitic cracks linking vesicles provided pathways for hydrothermal fluids flowing up to the seafloor hydrothermal alteration along these pathways resulted in the formation of pseudoclastic textures textural evidence suggests that alteration of the glassy dacite has not been sustained the samples have been affected by incipient hydrothermal alteration that is typically not preserved in ancient volcanic rock hosted massive sulfide deposits interaction of the glassy dacite with hydrothermal fluids primarily resulted in the conversion of volcanic glass to dioctahedral smectite only minor amounts of trioctahedral smectite were formed destruction of the volcanic glass and the formation of smectite caused pronounced changes in the chemistry of the dacite samples in particular a decrease in the sio2 whole rock content and the na2o k2o ratio the two alkali elements behaved differently during hydrothermal alteration due to preferential incorporation of k into the interlayer position of the newly formed dioctahedral smectite smectite formation occurred under rock dominated conditions although the addition of mg was required to form trioctahedral smectite from the silicic volcanic glass primary plagioclase was resistant to hydrothermal alteration highlighting the fact that the destruction of volcanic glass and feldspar are not necessarily contemporaneous in massive sulfide forming hydrothermal systems incipient alteration of the glassy dacite close to the seafloor occurred at temperatures below 150 c in an environment that allowed the development of steep temperature gradients comparison of the new data to the findings of deep drilling during odp leg 193 suggests that the smectite rich alteration in the immediate subsurface of the pacmanus hydrothermal vent field represents the low temperature equivalent of illite and chlorite rich alteration associations forming in the upflow zones of the hydrothermal fluids in the deeper portion of the volcanic sequence

incipient hydrothermal alteration of basalts and the origin of martian soil the martian soil is a fine grained regolith that is chemically basaltic in character with evidence for both gains and losses of volatile and mobile elements compared to martian basalt compositions these chemical fractionations provide clues to geochemical processes on the surface of mars geochemical processes contributing to the soil proposed in the past include the chemical and mechanical breakdown of rocks under surface conditions the addition of volcanic aerosols containing s and cl compounds and the alteration of basaltic glass to palagonite our studies of terrestrial analogs suggest that hydrothermal alteration processes involving impact craters and volcanism could also contribute to the major element trends observed in martian soil data from viking pathfinder and the current mer missions consistently show that relative to basaltic martian meteorite compositions the major element compositions of the soils are 1 depleted in the fluid mobile element calcium 2 generally similar or somewhat enriched in iron oxide and magnesium but mgo depleted compared to gusev rocks 3 locally variable in potassium 4 possibly poorer in aluminum and 6 very enriched in chlorine and sulfur the major element trends aside from the cl and s enrichment could be explained by the formation or addition of palagonite according to mcsween and keil 2000 but the missing cao remains a problem the chlorine and sulfur are probably derived from other processes such as volcanic aerosols and hydrothermal fluids mcsween and keil 2000 also argued that hydrothermal alteration of basalts produce alteration trends that are inconsistent with the mars soil but this study concludes otherwise we have used quantitative mass balance mixing models to investigate possible models involving mixtures of basaltic compositions with different types of alteration materials including palagonite we show that the mars soil composition can be matched with a combination of unweathered basaltic martian meteorites with basaltic feo rich cao poor alteration products palagonite is a possible but not a necessary component of successful model mixtures the hydrothermal alteration materials that form successful model mixtures are formed in low temperature low water rock ratio environments and they can reproduce the required geochemical trends because they are poorer in cao but not in feo compared to their respective protoliths these results argue that material altered by hydrothermal processes could be a plausible component of the soil and that removal of cao from the soil into some undiscovered reservoir after its formation is not required the current soil on mars therefore did not have to undergo an episode of in situ aqueous alteration but could represent a sink for materials that experienced aqueous processes in a different setting before erosion to form the soil the soil can also represent a sink for mobile elements e g s cl and br derived from other sources such as volcanic aerosols and hydrothermal fluids

low temperature fluid rock interaction an isotopic and mineralogical perspective of upper crustal evolution eastern flank of the juan de fuca ridge jdfr odp leg 168 abstract during odp leg 168 10 sites were drilled across the eastern flank of the juan de fuca ridge jdfr to examine the conditions of fluid rock interaction in three distinct hydrothermal regimes referred to as the hydrothermal transition ht buried basement bb and rough basement rb transects extending over a 120 km linear transect perpendicular to the spreading ridge this was carried out in an attempt to constrain the conditions and processes that control the location style and magnitude of low temperature 87 sr 86 sr and 18 o analyses of basalts from across the eastern flank of the jdfr reinforce petrological observations with 87 sr 86 sr and 18 o values slightly elevated above accepted pristine morb values for this region these results are consistent with an increase in the amount of fluid rock interaction with time across the flank enrichment in the 87 sr 86 sr and 18 o relative to morb is influenced by a number of factors including local and regional variations in the crustal lithology and structure the age of the crust the extent of bulk rock alteration and theoretically the relative abundance of different isotopically enriched secondary mineral phases in the crust

mineral carbonation reactions under water saturated hydrothermal like conditions and numerical simulations of co2 sequestration in tholeiitic basalt of the eastern deccan volcanic province india abstract previous water co 2 interaction studies have studied few basalt types rich in ca mg and fe cations to determine pyroxene olivine and plagioclase reaction rates however limited research has examined the deccan basalt therefore in this study basalt co 2 water saturated interaction experiments and numerical simulations were performed under hydrothermal like conditions x ray diffraction xrd data revealed the appearance of calcite aragonite ankerite huntite and siderite additionally smectite chlorite smectite chlorite mixed layers and chabazite were also formed sem images showed that tiny calcite crystals developed over larger calcite crystals incipient disordered calcite formed with imperfections on its crystal faces and cubic chabazite crystals were bounded by smectite grains furthermore these observations were confirmed by eds analyses and mineral formulae calculations major shifts in the carbonate peaks observed at 155 and 282 178 and 849 cm 1 on raman spectra confirmed the formation of calcite dolomite and aragonite respectively mixing trends between basalt and ca mg fe carbonates and chlorite smectite were observed in the case of phyllosilicates however ankerite calcite and siderite recorded the enrichment of i ca ii fe mg and iii fe high degrees of carbonation and progressive mineral growth were observed with the increasing ph of the solution the formation of secondary carbonates predominated over that of silicates in short term experiments however with increasing reaction time the carbonates no longer persisted in the system as they were dissolved and replaced by silicates however the degree of carbonation increased with the increasing pco 2 and ph of the solution transition state theory tst based numerical simulation models do not agree well with the results of experiments as carbonate growth was greater in the former case these models work well to predict the dissolution rates of most minerals but overpredict the growth of non hydrous mg carbonates at low temperatures

low grade regional metamorphism in the mesozoic cenozoic volcanic sequences of the central andes all the mesozoic and cenozoic volcanic rocks of the central andes from southern ecuador to central chile except recent ones have been affected by episodes of regional metamorphism without change in texture and structure the metamorphism which ranges from low zeolite to greenschist facies can be classified as burial metamorphism because there is an overall increase in metamorphic grade with stratigraphic depth in the individual volcanic sequences separated by regional unconformities some sequences display metamorphic patterns transitional to ocean floor and to geothermal field types reflecting variations along and across the andes in tectonic setting and thermal gradients volcanism was closely followed by metamorphism during each cycle characterizing the geological history of the central andes the episodic nature of the metamorphism has led to breaks in metamorphic grade at regional unconformities and repetition of facies series where strata of higher grade may even overlie those of lower grade the existence of permeability controlled distribution patterns of secondary minerals within individual flows shows that gradients of chemical activity rate of reaction and pfluid were acting in addition to temperature and p tot overall gradients during the regional metamorphism the alteration is accompanied by chemical changes and disturbances of the k ar and rb sr isotope systems similarities between mesozoic facies series in the western and eastern flanks of the andes are consistent with a mechanism of ensialic spreading subsidence

subaqueous basaltic lava dome and carapace breccia on king george island south shetland islands antarctica on king george island during latest oligocene earliest miocene time submarine eruptions resulted in the emplacement of a small ca 500 m estimated original diameter basalt lava dome at low head the dome contains a central mass of columnar rock enveloped by fractured basalt and basalt breccia the breccia is crystalline and is a joint block deposit lithic orthobreccia interpreted as an unusually thick dome carapace breccia cogenetic with the columnar rock it was formed in situ by a combination of intense dilation fracturing and shattering caused by natural hydrofracturing during initial dome effusion and subsequent endogenous emplacement of further basalt melt now preserved as the columnar rock muddy matrix with dispersed hyaloclastite and microfossils fills fractures and diffuse patches in part of the fractured basalt and breccia lithofacies the sparse glass rich clasts formed by cooling contraction granulation during interaction between chilled basalt crust and surrounding water together with muddy sediment they were injected into the dome by hydrofracturing local steam fluidisation and likely explosive bulk interaction the basalt lava was highly crystallised and degassed prior to extrusion together with a low effusion temperature and rapid convective heat loss in a submarine setting these properties significantly affected the magma rheology increased the viscosity and shear strength and influenced the final dome like form of the extrusion conversely high heat retention was favoured by the degassed state of the magma minimal undercooling a thick breccia carapace and viscous shear heating which helped to sustain magmatic eruption temperatures and enhanced the mobility of the flow

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