isotope and trace element evidence for late stage intra crustal melting in the high andes abstract purico chascon is an acid igneous complex less than 1 5 ma old rising to 5800 m in the north chilean andes and consisting of andesite dacite cones and dacite domes on an ignimbrite shield the rocks are subdivided into two groups those from chascon appear to exhibit evidence for magma mixing with more basic material now preserved as xenoliths whereas among those at purico no xenoliths have been found 87sr 86sr 0 7095 0 7081 at purico 0 7079 0 7069 at chascon and 0 7061 0 7057 in the xenoliths from the chascon lavas 143nd 144nd 0 51222 0 51236 overall the purico lavas are characterised by higher sio2 rb sr 87sr 86sr and ree abundances and lower sr nd sr ba and143nd 144nd than most andean igneous suites there is no indication ofselective crustal contamination of sr or any systematic change in isotope ratios during differentiation nonetheless the trend of for example high sr nd and sr contents in rocks with low87sr 86sr 0 704 ecuador to low sr nd and sr and high sio2 in rocks with87sr 86sr 0 7081 0 7095 at purico is interpreted as a shift from subduction zone related magmatism to one with greater crustal affinity the formation of the least evolved purico lavas 60 sio2 is discussed in terms of bulk assimilation of crustal material mixing between crustal and mantle derived magmas and partial melting of pre existing crust although such models are still extremely primitive the simplest explanation of the observed chemical variations is that the purico rocks evolved from parental magmas derived by crustal anatexies thermal considerations suggest that such late stage crustal anatexis is a predictable response to crustal thickening which in the andes is thought to have taken place during the cenozoic

calcic amphibole equilibria and a new amphibole plagioclase geothermometer there is currently a dearth of reliable thermobarometers for many hornblende and plagioclase bearing rocks such as granitoids and amphibolites a semi empirical thermodynamic evaluation of the available experimental data on amphibole plagioclase assemblages leads to a new thermometer based on the aliv content of amphibole coexisting with plagioclase in silica saturated rocks the principal exchange vector in amphiboles as a function of temperature in both the natural and experimental studies is left na square 1 right a left alsi 1 right t1 we have analysed the data using 3 different amphibole activity models to calibrate the thermometer reactions 1 edenite 4 quartz tremolite albite 1 2 pargasite 4 quartz hornblende albite 2 the equilibrium relation for both 1 and 2 leads to the proposed new thermometer t frac 0 677p 48 98 y 0 0429 0 008314 ln k and k left frac si 4 8 si right x ab plag where si is the number of atoms per formula unit in amphiboles with p in kbar and t in k the term y represents plagioclase non ideality rtln ab from darken s quadratic formalism dqf with y 0 for xab 0 5 and y 8 06 25 5 1 xab 2 for xab 0 5 the best fits to the data were obtained by assuming complete coupling between al on the t1 site and na in the a site of amphibole and the standard deviation of residuals in the fit is 38 c the thermometer is robust to ferric iron recalculation procedures from electron probe data and should yield temperatures of equilibration for hornblende plagioclase assemblages with uncertainties of around 75 c for rocks equilibrated at temperatures in the range 500 1100 c the thermometer should only be used in this temperature range and for assemblages with plagioclase less calcic than an92 and with amphiboles containing less than 7 8 si atoms pfu good results have been attained on natural examples from greenschist to granulite facies metamorphic rocks as well as from a variety of mafic to acid intrusive and extrusive igneous rocks our analysis shows that the pressure dependence is poorly constrained and the equilibria are not suitable for barometry

boron lithium relationships in rhyolites and associated thermal waters of young silicic calderas with comments on incompatible element behaviour abstract this study had three goals 1 1 to study b distribution in a rhyolitic volcanic sequence already extensively investigated for other elements 2 2 to interpret the joint behaviour of b and li during the interaction of such rocks with subsurface waters 3 3 to assess the manner in which water affects the behaviour of incompatible elements such as b and gd new b gd and sm analyses have been made on a suite of yellowstone rhyolites including fresh and partially devitrified glassy obsidian from surface exposures of several flows drill core of increasing degrees of alteration in the biscuit basin flow and two drill cores from other flows within the biscuit basin flow the sm and gd concentrations remain rather constant 9 12 and 8 10 ppm respectively and behave conservatively independent of alteration boron decreases from about 10 to 3 ppm with progressive alteration and li increases from about 40 ppm by a factor of 2 3 in the most altered rocks thermal waters reflect leaching from the fresh rocks but contain much less li than b b li 3 compared with 0 8 in fresh obsidian retention of li was favoured by abundant illitic alteration obsidians from the valles and long valley calderas show greater li loss during alteration all the rhyolitic rocks lose b during aqueous alteration the waters acquire both b and li but proportionately much more b natural waters of all kinds including those from the three calderas show six orders of magnitude range in aqueous b and li with a high degree of linear correlation and an average ratio b li essentially constant at 4 0 the linearity mainly expresses processes of dilution and concentration reactions specific to b or li engender waters with deviating b li the patterns of sm gd b li variation in rocks exemplify two kinds of behaviour in petrological processes with no hydrous phase lunar rocks mantle fresh seafloor basalt the lower continental crust these elements are incompatible and behave similarly when an aqueous phase is present b and li behave independently of sm and gd

time stratigraphic framework for the eocene oligocene mogollon datil volcanic field southwest new mexico a time stratigraphic framework for discontinuously exposed regional ignimbrites in the eocene oligocene mogollon datil volcanic field has been established using correlations aided by 40ar 39ar age determinations and paleomagnetic analyses 40ar 39ar age spectra from sanidine separates 25 regional ignimbrites 85 samples 97 spectra yield well defined plateau ages that are precise within sample and within unit 1 0 5 and agree closely with independently established stratigraphic order paleomagnetic remanence directions 404 sites from individual ignimbrite outflow sheets are generally vertically and horizontally uniform throughout facies ranging from thick 100 500 m densely welded proximal ignimbrites to thin 1 5 30 m unwelded distal fringes between unit differences in paleomagnetic directions provide useful correlation criteria particularly for units having ages too close to be resolved using 40ar 39ar dating the mogollon datil time stratigraphic framework clarifies ignimbrite history and provides improved age control for intercalated lavas and sedimentary rocks ignimbrite activity was strongly episodic outflow sheets were primarily erupted in four discrete pulses representing synchronized activity of two separate cauldron complexes activity in the southern complex began at 36 2 ma near las cruces new mexico and subsequently migrated 220 km northwest culminating in the 28 0 ma bursum cauldron activity in the northern complex located west of socorro new mexico underwent a less defined and more modest 40 km westward migration over its 32 0 to 24 3 ma life span the four pulses of ignimbrite activity were 1 36 2 24 3 ma 12 major units 1 500 km3 total volume 2 32 0 31 4 ma three major units 1 500 km3 volume 3 29 1 27 4 ma nine major units 6 000 km3 and 4 24 3 ma one major unit the third and largest ignimbrite pulse was accompanied by extensive rhyolitic dome and flow eruptions in the area between the two main cauldron complexes

k feldspar megacrysts in granites phenocrysts not porphyroblasts abstract k feldspar megacrysts in granitoid plutons have been interpreted as either phenocrysts or porphyroblasts most of the microstructural mineralogical and chemical evidence e g shape alignment concentration ba content zoning inclusions and twinning favours a phenocryst origin the main features that have been used to support a porphyroblast origin are occurrence of megacrysts 1 across aplite vein boundaries 2 in country rocks and 3 in or across boundaries of microgranitoid enclaves mafic inclusions however these features can be explained by the phenocryst hypothesis in particular megacrysts in microgranitoid enclaves can be explained by growth or mixing in magma before a globule of that magma or a fragment of the resulting igneous rock was incorporated as an enclave all available evidence favours or is consistent with a phenocryst origin for k feldspar megacrysts in granitoid rocks and their enclaves the large size of the megacrysts is evidently due to nucleation difficulties for k feldspar in granitic melts though k feldspar is commonly the last mineral to begin crystallizing in granitic magmas abundant melt is still present at that stage allowing sufficient space for the megacrysts to grow the reason for the common lack of megacrysts in volcanic rocks may be that the phenocrysts do not grow large enough to be called megacrysts until the magma contains such a high proportion of crystals that it cannot erupt

hydrochemical and isotopic tracing of mixing dynamics and water quality evolution under pumping conditions in the mine shaft of the abandoned frances colliery scotland abstract since 1995 when pumps were withdrawn from deep mines in east fife scotland mine waters have been rebounding throughout the coalfield recently it has become necessary to pump and treat these waters to prevent their uncontrolled emergence at the surface however even relatively shallow pumping to surface treatment lagoons of the initially chemically stratified mine water from a shaft in the coastal frances colliery during two dynamic step drawdown tests to establish the hydraulic characteristics of the system resulted in rapid breakdown of the stratification within 24 h and a poor pumped water quality with high dissolved fe loading further data are presented here of hydrochemical and isotopic sampling of the extended pump testing lasting up to several weeks the use in particular of the environmental isotopes 18 o 2 h 34 s 3 h 13 c and 14 c alongside hydrochemical and hydraulic pump test data allowed characterisation of the frances system dynamics mixing patterns and water quality sources feeding into this mineshaft under continuously pumped conditions the pumped water quality reflects three significant components of mixing shallow freshwater seawater and leakage from the surface treatment lagoons in spite of the early impact of recirculating lagoon waters on the hydrochemistries the highest fe loadings in the longer term pumped waters are identified with a mixed freshwater seawater component affected by pyrite oxidation melanterite dissolution in the subsurface system

environmental problems at the nevsehir kozakli geothermal field central turkey the kozakli nevsehir geothermal field extends a long a nw se direction at se of the centrum of kozakli the area is not rugged and average elevation is 1 000 m the kozanozu creek flows towards north of the area in the kozakli thermal spa area thermal waters are manifested along a valley with a length of 1 5 km and 200 m width in this resort some hot waters are discharged with no use the thermal water used in the area comes from wells drilled by mta in addition these waters from wells are also utilized by hotels baths and motels belonging to city private management municipality and private sector the measured temperature of kozakli waters ranges from 43 51 c in springs and 80 96 c in wells waters are issued in a wide swampy area as a small group of springs through buried faults electrical conductivity values of thermal spring and well waters are 1 650 3 595 s cm and ph values are 6 72 7 36 kozakli cold water has an electrical conductivity value of 450 s cm and ph of 7 56 all thermal waters are dominated by na and cl so4 while cold waters are dominated by ca 2 and hco3 the aim of this study was to investigate the environmental problems around the kozakli geothermal field and explain the mechanisms of karstic depression which was formed by uncontrolled use of thermal waters in this area and bring up its possible environmental threats at the kozakli geothermal field a sinkhole with 30 m diameter and 15 m depth occurred in january 17th 2007 at the recreation area located 20 m west of the geothermal well which belongs to the government of nevsehir province the management of the geothermal wells should be controlled by a single official institution in order to avoid the creation of such karstic structures affecting the environment at the source area

integrated hydraulic hydrogeochemical assessment of flooded deep mine voids by test pumping at the deerplay lancashire and frances fife collieries

saline brines and metallogenesis in a modern sediment filled rift the salton sea geothermal system california u s a the salton sea geothermal system ssgs is the site of active hydrothermal metamorphism and metallogenesis in the delta of the colorado river which partially fills the salton trough rift zone at the head of the gulf of california growth of the delta across the rift has isolated the northern part of the salton trough since the pleistocene forming the evaporative salton sea basin whose sediments host the ssgs more than 70 commercial geothermal wells including a 3 2 km deep research borehole drilled as part of the salton sea scientific drilling project sssdp are yielding a wealth of new data from this system within the ssgs active greenschist facies metamorphism is occuring at temperatures 365 c at only 2 3 km depth by reaction of nacakfemncl brines with the deltaic and lacustrine pilo pleistocene sediments two kinds of base metal ore mineralization occur at depth stratabound diagenetic fe sulfides and epigenetic vertical veins containing fezncupb sulfides and fex oxides the vein mineralization occurs as two types an older reduced carbonate sulfide assemblage and a modern oxidized silicate hematite sulfide assemblage the earlier assemblage formed at temperatures up to 100 c higher than the ambient temperatures measured in the wells today implying that cooler oxidized fluids have displaced hot reduced fluids a sharp interface between shallow fluids containing 12 wt tds and deep hypersaline brines containing 15 27 wt tds exists in the ssgs the deeper hypersaline brines are rich in base metals fe 1500 ppm mn 1000 ppm zn 500 ppm whereas the overlying lower salinity fluids contain less than 100 ppm each of fe mn and zn the modern silicate hematite sulfide vein assemblage is precipitating where the two fluids appear to be mixing the two fluid types also have distincty different 18o and d relations the shallow lower salinity fluids have only partially exchanged oxygen with deltaic and lacustrine host rocls the deep hypersaline brines have exchanged oxygen extensively at 250 c with the deltaic sediments d values of the hypersaline brines are typically lighter than either the lower salinity fluids or the modern evaporating groundwaters occupying the salton trough implying a different source for the original fluids the high salinities of the geothermal brines are derived from a combination of evaporation of fossil lakewaters groundwater dissolution of shallow lacustrine evaporites and subsurface hydrothermal metamorphism of buried lacustrine evaporites episodic filling and desiccation of the closed salton sea basin has allowed cold saline brines to form and percolate down into the sedimentary section in some wells anyhydrite meta evaporites and interbedded solution collapse breccias occur at 1 km depth the anyhydrite contains fluid inclusions that are saturated in halite at their homogenization temperatures of 300 c recording the hydrothermal dissolution of bedded salt based on sr and pb isotopic data and whole rock chemical data the source of the metals in the hypersaline brines is from leaching of the host sediments however the origin of reduced s for ore mineralization remains an enigma 34s values for vein sulfides cluster uniformly around zero per mil implying that an isotopically homogeneous source of reduced sulfide exists in the brines it is possible that the vein sulfides receive a constribution from a magmatic s source movement and mixing of brines of different chemistry and oxidation states play a major role in ore genesis our model envisages an early stage in which a deep brine pool accumulated at depth in the sedimentary section by partial section by partial evaporation of basin and dissolution of bedded salts intrusion of rift related basaltic magma into the base of the sedimentary caused heatingand fracturing of the sediments resulting in precipitation of the carbonate sulfide veins during pore fluid expulsion heating also caused a diapir of the hypersaline brine to rise and displace colder less saline shallower pore fluids this brine intrusion was accompanied by pervasive and extensive mineralization as this diapir cooled it began to move downward drawing in shallow more oxidized fluids and causing the formation of the modern silicate hematite sulfide vein ore zone

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

petrology of calc alkaline lavas at volc n ollague and the origin of compositional diversity at central andean stratovolcanoes

chloride depletions and enrichments in seafloor hydrothermal fluids constraints from experimental basalt alteration studies na k ca cl fluid was reacted with diabase at 400 and 425 c 400 bars and fluidrock mass ratio of 0 5 to assess the relative mobility of dissolved cl fluids from the present experiments reveal relatively large time and temperature dependent decreases in cl temperature increase enhances cl removal whereas reaction progress has the opposite effect these changes are not due to boiling and or phase separation at 400 and 425 c 400 bars cl removal from solution is accompanied by ca and na fixation respectively both experiments produced mixed layer chlorite smectite clinozoisite and albite olivine was not detected by x ray diffraction or petrographic analysis of run products from either experiment however we propose that the time dependent changes in dissolved cl result from olivine replacement by an fe hydroxy chloride phase followed by hydration effects quenched and washed alteration products however did not reveal anomalous cl which suggests that the cl bearing phase is characterized by retrograde solubility this is consistent with the relative magnitude of cl fixation observed for experiments at both temperatures and from results of an isobaric cooling experiment the experiments provide evidence for the non conservative behavior of cl during hydrothermal alteration of basalt the data are particularly important in light of the cl depleted nature of some ridge crest hot spring fluids and suggest temperatures of formation for these fluids of approximately 410 c assuming sub seafloor pressure of 400 bars in addition the retrograde solubility of the cl bearing phase responsible for cl fixation during high temperature basalt alteration may help to explain cl enrichment in hot spring fluids which have conductively cooled below 350 c

the boron isotope systematics of icelandic geothermal waters 1 meteoric water charged systems we have measured the boron isotope composition and boron and chloride concentrations of 27 icelandic geothermal fluids from both high and low temperature systems the 11b values range from 6 7 in the krafla system to 25 0 in a warm spring from the southern lowlands in addition we have also determined the 11b values of basaltic glass from nesjavellir 5 3 1 4 and travertine from snaefellsnes 22 0 5 the b isotope and cl b systematics of the high temperature systems are dominated by the composition of the local basalts the lower temperature systems show evidence for mixing with b and cl of a marine origin together with some uptake of b into secondary mineral phases the data from the snaefellsnes geothermal system indicate that the fluids have undergone interaction with basalts that have undergone significant low temperature alteration by seawater

magmatic gases in well fluids aid the mapping of the flow pattern in a geothermal system abstract gas composition and silica concentrations of well fluids are used in conjunction with pressure temperature and enthalpy data to obtain a model of the drilled part of the krafla geothermal field northeast iceland a magma chamber is located at 3 8 km depth under the field magmatic gases emanate from the chamber and travel via a channel reaching the surface at the hveragil eruptive fissure the composition of the gases is apparently modified on the way in that sulphur which presumably is in the form of sulphur dioxide to begin with is removed on the way and what remains is in the form of hydrogen sulphide at the end it is suggested that the major removal mechanism is the deposition of pyrite and pyrrhotite during the passage of the gases through the hydrothermal system the hydrothermal system is divided into a lower part whose temperature exceeds 300 c and whose fluids are to a varying extent affected by the magmatic gases and an upper part whose temperature is ca 200 c this upper part is a run off from the lower part but the magmatic effects have mostly disappeared there

catastrophic caldera forming eruptions thermomechanics and implications for eruption triggering and maximum caldera dimensions on earth abstract approximately every 100 000 years the earth experiences catastrophic caldera forming supereruptions that are considered to be one of the most hazardous natural events on earth utilizing new temperature dependent viscoelastic numerical models that incorporate a mohr coulomb failure criterion we find that eruptive failure of the largest magma chambers is a function of the geometry of the overlying roof and the location of the brittle ductile transition in particular the ductile halo created around the hot magma chamber buffers increasing overpressures and prevents pressure relief via magmatic injection from the magma chamber the numerical results indicate that as chamber volume increases the higher temperatures in the host rock and the decrease in the roof aspect ratio cause a shift from reservoir triggered eruption to an external roof triggered mechanism specifically as overpressure increases within the largest magma chambers extensive uplift in the overlying roof promotes the development of through going faults that may trigger eruption and caldera collapse from above we find that for magma chamber volumes 103 km3 and roof aspect ratios depth width

primary and secondary alkali and halogen element distribution in iceland research drilling project basalts from eastern iceland the degree of oxidation and hydration and the variation of fe mg li na k rb f cl and i are reported for hydrothermally altered basaltic lavas and dikes intersected in the iceland research drilling project irdp borehole sequence from eastern iceland the concentrations of water soluble na cl and i have also been determined and represent approximately 1 40 and 53 respectively of the total content of each element hydration and oxidation in both lavas and dikes generally increase downward in the total lava pile to maxima toward the base of the drilled section three lava groups and two dike groups are recognized at different depths on the basis of the relative distribution of zr and y each group exhibits a trend toward more primitive compositions upward in the pile within group and vertical chemical variations reflect both 1 primary chemical features such as parental magma composition and degree of crystal fractionation and 2 secondary mobilization during hydrothermal alteration the latter caused variable alkali and halogen element dispersion but initial differences between the chemical groups appear still to be discernible mobility sequences were rb k li na and cl f i although all the analyzed elements were mobilized at the relatively low levels of hydration few systematic trends with progressive hydration are observed e g k and rb were leached and li was sometimes enriched the main effect of alteration was a localized redistribution of mobile elements into stable secondary phases apparent depletions of mobile elements at the base of the sequence reflect primary variations rather than extensive leaching

magmatic differentiation at la poruna scoria cone central andes northern chile evidence for assimilation during turbulent ascent processes and genetic links with mafic eruptions at adjacent san pedro volcano abstract la poruna 21 53 s 68 30 w is a 140 m high scoria cone composed of pyroclastic material and an extensive basaltic andesite to andesite lava flow that is up to 8 km in length automated mineralogical analysis describes a suite of porphyritic mafic samples comprising olivine and pyroxene bearing rocks well defined major element compositional trends as well as trace and rare earth element characteristics e g sr y 96 ka from san pedro volcano exhibit similar isotopic characteristics therefore evidence of similar magmatic processes this new dataset clearly defines magma compositional changes during the la poruna eruption ca 100 ka revealing an increase in crustal contamination at shallow crustal levels for the younger san pedro lavas

boron isotope geochemistry of the oceanic crust from dsdp odp hole 504b boron contents and boron isotopic compositions were determined for the uppermost 1 3 km section of typical 6 2 ma oceanic crust from dsdp odp hole 504b costa rica rift galapagos spreading center both the boron content and the 11b value in the oceanic crust are controlled by two types of alteration 1 1 low temperature alteration 0 to 60 c zones i and ii and 2 2 high temperature hydrothermal alteration 200 to 400 c zones iii and iv basalts subjected to the low temperature alteration are characterized by their relatively high boron contents 0 69 to 19 3 ppm and high 11b values 2 2 to 10 6 indicating uptake of boron into secondary phases in equilibrium with seawater or evolved seawater hydrothermally altered basalts contain less abundant boron 0 17 to 0 52 ppm and relatively constant 11b values 0 1 to 1 0 although basalts from the upper part of these hydrothermal zones 1300 mbsf original boron content and 11b value of the hole 504b morb were 0 35 ppm and 0 2 respectively the present data provide fundamental information in understanding of the distribution of boron and boron isotopes in the oceanic crust

hydrogeochemical and isotopic study of thermal and mineralized waters from the nevsehir kozakli area central turkey abstract in the kozakli geothermal province thermal waters are manifested along a valley 1 5 km long and 200 m in width thermal waters utilised by the resort and some other hotels are mostly discharged from bore wells the issuing temperatures of the thermal waters vary from 40 50 c in thermal springs and 45 96 c in bores and open wells the geochemical and isotopic signatures of the thermal water suggest mixing of thermal waters with formation waters and cold near surface groundwaters before emerging to the surface and hence geochemical indicators fail to indicate the near true reservoir temperatures however the oxygen and hydrogen isotopic signatures strongly suggest a high temperature reservoir 220 c in the crystalline basement rocks long circulation of meteoric waters within the basement rocks is indicated by low tritium values in the thermal waters major involvement of miocene marls in modifying the chemical signatures of the thermal waters is inferred from the trace element concentrations

the upper crustal magma plumbing system of the pleistocene apacheta aguilucho volcanic complex area altiplano puna northern chile as inferred from the erupted lavas and their enclaves abstract the evolution of the magma plumbing system of the pleistocene apacheta aguilucho volcanic complex area altiplano puna volcanic complex northern chile was investigated through petrographic geochemical and isotopic studies of representative lavas and related enclaves updated available dates of these products both from the apacheta and aguilucho stratovolcanoes and nearby domes allow us to define the activity during the last 1 ma this investigation shows that the andesitic magmas were affected by processes of assimilation plus fractional crystallization afc with a significant role played by amphibole fractionation during their ascent through the upper crust presumably by the interaction with the altiplano puna magma body 15 20 km these andesitic magmas were erupted with no or minor additional contamination at shallower levels or experienced plagioclase dominated fractional crystallization fc to dacite within shallower crustal magma chambers 4 8 km depth the constructional phase of the apacheta and aguilucho stratovolcanoes 1 to ca 0 6 ma reflect a transition from high flux i e flare ups to steady state magmatism as also documented in other pleistocene volcanic complexes of the altiplano puna volcanic complex during this stage the mafic magma recharge was high enough to permit a large spectrum of hybridization of the resident magmas in the upper crust to form the abundant andesites and dacites lavas in contrast at 150 100 ka the magmatism turned to a new stage of recharge waning stage and the episodic intrusion of small volumes of andesitic magmas permitted the remobilization of the crystal rich dacites triggering the extrusions of the chanka chac inca and cerro pabellon domes the andesitic enclaves in the domes studied here represent a snapshot of the magmatic processes of interaction that occurred in the shallower reservoir at the interface of the resident dacite with the ascending andesitic magma nevertheless as there are no dated volcanic products from the area between 0 6 and 0 1 ma the youngest dacitic domes could also be interpreted as the beginning of a new magmatic pulse of the altiplano puna volcanic complex independently from the significance of the mafic recharge at 150 100 ka waning stage vs new pulse the youngest investigated domes share similar geochemical features and crustal depth constraints 4 8 km with the ignimbrites of the altiplano puna volcanic complex therefore suggesting that the remobilized magmas erupted as the domes are possibly remains of older plumbing systems left over from the last magmatic flare up of the altiplano puna volcanic complex

the ion microprobe as a tool for obtaining strontium isotopes in magmatic plagioclase a case study at okataina volcanic centre new zealand abstract we investigated the potential of multi collector secondary ion mass spectrometry mc sims as a tool for obtaining sr isotopic compositions in plagioclase a ubiquitous mineral in igneous rocks that serves as a recorder of crystallization history mc sims allows for high spatial resolution analysis 12 m in this study of isotopes and therefore improves the temporal scale at which fluctuations in crystallization conditions can be recognized ultimately improving our understanding of rates of magmatic processes plagioclase crystals from two young rhyolitic deposits from two major eruptive complexes tarawera and haroharo of the okataina volcanic centre in new zealand were analysed results were corrected for matrix effects using linear modelling of mc sims data versus an contents as well as 87sr 86sr ratios acquired via laser ablation inductively coupled plasma mass spectrometry la mc icp ms corrected mc sims sr isotopic ratios had an average 2 uncertainty of 0 0008 per spot and were homogeneous in okataina plagioclase at high spatial resolutions average la mc icp ms 87sr 86sr ratios of plagioclase from both intra caldera volcanic complexes tarawera 87sr 86sr 0 7056 and haroharo 87sr 86sr 0 7054 suggest similar magma sources and similar assimilation and fractional crystallization processes for the two complexes overall homogeneity of plagioclase excluding relict cores indicates no significant changes in contributions i e crustal assimilation mafic influx to the system during the majority of plagioclase crystal growth furthermore lack of 87sr 86sr ratio fluctuations in plagioclase rims suggest interaction between the resident silicic magma and the intruding mafic magma that triggered the eruptions was largely limited to volatile and heat transfer using appropriate standards and analysis this mc sims method can be used to detect short lived open system events in magma reservoirs where differences in 87sr 86sr isotopic ratios are significant

magma evolution in the purico ignimbrite complex northern chile evidence for zoning of a dacitic magma by injection of rhyolitic melts following mafic recharge the 1 3 ma purico complex is part of an extensive neogene pleistocene ignimbrite province in the central andes like most other silicic complexes in the province purico is dominated by monotonous intermediate ash flow sheets and has volumetrically minor lava domes the purico ignimbrites total volume 80 100 km3 are divided into a lower purico ignimbrite lpi with two extensive flow units lpi i and lpi ii and a smaller upper purico ignimbrite upi unit crystal rich dacite is the dominant lithology in all the purico ignimbrites and in the lava domes it is essentially the only lithology present in the first lpi flow unit lpi i and in the upper purico ignimbrite but the lpi ii flow unit is unusual for its compositional diversity it constitutes a stratigraphic sequence with a basal fall out deposit containing rhyolitic pumice 68 74 wt sio2 overlain by ignimbrite with dominant crystal rich dacitic pumice 64 66 wt sio2 rare andesitic and banded pumice 60 61 wt sio2 are also present in the uppermost part of the flow unit the different compositional groups of pumice in lpi ii flow unit rhyolite andesite dacite have initial nd and sr isotopic compositions that are indistinguishable from each other and from the dominant dacitic pumice end 6 7 to 7 2 and 87sr 86sr 0 7085 0 7090 however two lines of evidence show that the andesite dacite and rhyolite pumices do not represent a simple fractionation series first melt inclusions trapped in sequential growth zones of zoned plagioclase grains in the rhyolite record fractionation trends in the melt that diverge from those shown by dacite samples second mineral equilibrium geothermometry reveals that dacites from all ignimbrite flow units and from the domes had relatively uniform and moderate pre eruptive temperatures 780 800 c whereas the rhyolites and andesites yield consistently higher temperatures 850 950 c hornblende geobarometry and pressure constraints from h2o and co2 contents in melt inclusions indicate upper crustal 4 8 km magma storage conditions the petrologic evidence from the lpi ii system thus indicates an anomalously zoned magma chamber with a rhyolitic cap that was hotter than and chemically unrelated to the underlying dacite we suggest that the hotter rhyolite and andesite magmas are both related to an episode of replenishment in the dacitic purico magma chamber rapid and effective crystal fractionation of the fresh andesite produced a hot rhyolitic melt whose low density and viscosity permitted ascent through the chamber without significant thermal and chemical equilibration with the resident dacite isotopic and compositional variations in the purico system are typical of those seen throughout the neogene ignimbrite complexes of the central andes these characteristics were generated at moderate crustal depths 30 km by crustal melting mixing and homogenization involving mantle derived basalts for the purico system assimilation of at least 30 mantle derived material is required

small scale disequilibrium in a magmatic inclusion and its more silicic host basaltic andesite inclusions and their host dacite from the purico chascon complex in northern chile are isotopically distinct textural characteristics of the inclusions are typical of those resulting from magma mingling serial sectioning across the interface of an inclusion and its host dacite complemented by microdrill sampling and detailed microprobe work has enabled us to examine the scale of mixing and chemical isotopic and trace element disequilibrium the results of this work show that 1 the composition of the inclusion is relatively homogeneous 2 the dacite host is generally higher in 87sr 86sr and lower in 143nd 144nd than the enclave but it is heterogeneous on a small scale and probably a hybrid 3 the isotopic composition in the marginal zone apparently on both host and inclusion sides of the weakly chilled interface actually shows the highest 87sr 86sr and lowest 143nd 144nd 4 large plagioclase crystals in the inclusions and host are xenocrystic with higher 87sr 86sr than any of the other samples these observations are reconciled with a model of magma evolution in a crustal magma chamber in such a scenario the mafic magma is overlain by a cap of rhyolite a partial melt of the dacitic ignimbrites which now underlie the purico chascon complex overturn of such a magma system gives rise to a hybrid dacite containing discrete mafic inclusions

timing of tectonic events in the menderes massif western turkey implications for tectonic evolution and evidence for pan african basement in turkey the menderes massif forms the western of the two large metamorphic culminations within the turkish alpide orogen it has three major lithologic units with a gneissic core at the base and a schist and a marble envelope overlying it successively although relations between them have been largely obscured by the polymetamorphic and structurally complex history of the massif we present a review of the available stratigraphic evidence and combine it with new isotopic ages from the central and southern parts of the massif to constrain the timing of major tectonic events that shaped the massif since the late proterozoic pt3 in the southern part of the massif sensu stricto three episodes of deformation and metamorphism are distinguished whereas in its northern part there are four the first episode occurred at 500 10 ma with intense deformation and metamorphism at high grade amphibolite facies with local anatexis in the central part of the massif the 470 9 ma old now highly deformed tonalitic and granitic intrusions mark the end of this episode on a cambrian reconstruction of continents around the eastern mediterranean the menderes massif forms the southern end of the pan african orogenic collage of northeastern africa and arabia the area of earliest palaeozoic deformation in the menderes may connect with the northwest african orogenic collage along the strike via the bozburun and saricicek diabases arkoses and schists in the center of the karacahisar dome interpreted herein as fragments of a pan african suture the earliest palaeozoic deformations to affect the rocks were probably related to the last pan african collisions and associated postcollisional convergence the southern part of the massif was undeformed from early ordovician to the early later eocene whereas the northern part was deformed metamorphosed and intruded possibly during the late triassic related to the closure of the karakaya marginal basin of palaeo tethys the next major event affecting the whole of the massif was the intense deformation and widespread metamorphism that reached high amphibolite grade in the structurally lower parts and only greenschist grade in the outermost envelope this metamorphism here called the main menderes metamorphism mmm is biostratigraphically constrained between early eocene and early oligocene time rb sr isotopic data show a spread of ages between 60 ma and 25 ma with the greatest number of determinations around 35 5 ma this number is in excellent agreement with the stratigraphic evidence and shows that mmm took place during latest eocene time or at the transition from eocene to oligocene time along the northern border of the massif deformation and metamorphism had already taken place during the late cretaceous in a high pressure low temperature hp lt metamorphic belt northernmost part of menderes massif sensu lato which was then covered by palaeocene molasse the evolution of the hp lt belt was probably related to the obduction of the bozkir ophiolites from the izmir ankara branch of the neo tethys ocean and preceded terminal collision in the evolution of the massif the mmm was a product of the latest palaeocene collision across neo tethys and the consequent internal imbrication of the menderes taurus block that resulted in the burial of the menderes massif area beneath the lycian nappe complex a low grade metamorphic event dated at 10 to 5 ma is interpreted to be related to the extensional deformation in the aegean and western turkey which has been dated stratigraphically to have begun during the tortonian 11 5 ma

the role of hydrogeology in solution subsidence development and its environmental impacts a case study for sazlica nigde turkey solution subsidence and other types of collapse failures developed in the vicinity of sazlica have been a major source of environmental problems such as groundwater pollution soil erosion and foundation failures extensive pumping of water from the lake dipsiz during irrigation seasons creates landslide developments at the shores of the lake five stages of landslide development were observed in the unconsolidated soils around lake dipsiz bowl shaped solution subsidence features with depths from 1 to 3 m and diameters between 1 and 60 m were formed due to excess pumping and recharge and discharge features of unconfined aquifer the pumping activities must be controlled to prevent further environmental problems lake dipsiz and other subsidence developments are the groundwater exposure areas highly vulnerable to contamination the proposed remedies for overcoming the environmental problems are the establishment of conservation areas around such natural features ceasing the removal of top soil controlling the urbanization and keeping the livestock farming away from the areas susceptible to pollution

volcanic rocks from the bolivian altiplano insights into crustal structure contamination and magma genesis in the central andes lavas from quaternary minor volcanic centers behind the central andean volcanic arc in bolivia form a compositional continuum with rocks from the arc front extending to more primitive compositions the lavas show petrographic and geochemical evidence for deep crustal contamination during differentiation sr nd and pb isotopic data from centers within 30 km of each other encompass the entire range found in the central volcanic zone of the andes the most mafic rocks are largely typical of arc magmas elsewhere and may represent parental magmas to the porphyritic andesites and dacites erupted from stratovolcanoes along the arc to the west andean arc magmas therefore undergo considerable contamination during ascent through the thick crust the only basalts lacking petrographic evidence for contamination are picrobasalts from chiar kkollu that form the most primitive end member of the compositional spectrum these rocks resemble ocean island basalts in some respects indicating that variations in crustal contamination across the strike of the arc may be superimposed on variations in the mantle source composition

generation of a crust mantle magma mixture magma sources and contamination at cerro panizos central andes cerro panizos a large caldera in the central andes mountains produced two large dacitic ignimbrites at 7 9 ma and 6 7 ma and many andesitic and dacitic lava flows and domes the older rhyodacitic cienago ignimbrite represents the most silicic magma erupted by the system the younger much larger volume dacitic cerro panizos ignimbrite is very crystal rich containing up to 50 biotite plagioclase and quartz crystals in the pumice it is weakly zoned with most of the zoning apparent between two main cooling units major and most trace elements show little variation through the cerro panizos ignimbrite but the small range of composition is consistent with typical fractionation trends sr nd and pb isotopic ratios are very crustal with initial 87sr 86sr values of 0 711 to 0 715 nd values of 7 5 to 10 2 and nearly invariant pb isotopic ratios 206pb 204pb 18 85 207pb 204pb 15 67 and 208pb 204pb 38 80 the limited zonation observed in the cerro panizos ignimbrite is explained by impeded crystal settling due to high crystal content the magma body was a crystal liquid mush before ascent to the pre eruption crustal levels crystals formed but did not separate easily from the magma limited fractionation of plagioclase and biotite may have occurred but the composition was largely controlled by lower crustal mash processes afc modeling shows that the cerro panizos magmas resulted from a mixture of roughly equal proportions of late miocene mantle derived basalts and melts from 1 0 ga grenville age lower crust this occurred in a mash zone in the lower crust and set the crustal isotopic ratios observed in the cerro panizos magmas the great thickening of the crust beneath the central andes mountains sent upper and middle crustal rock types to lower crustal and deeper depths and this explains the upper crustal isotopic signatures of the cerro panizos rocks minor upper crustal assimilation of early miocene volcanic or subvolcanic rocks produced much of the isotopic variation seen in the system the nearly invariant high pb isotopic values and high pb concentrations indicate that pb came almost entirely from the crustal source and was little altered by any subsequent upper crustal assimilation this pb signature is isotopically similar to that of the southern bolivian tin belt suggesting a widely distributed pb source the great difference between compositions of miocene and quaternary central andean volcanic rocks is explained by crustal thickening in early miocene time leading to abundant lower crustal water and associated fluxed melting during the time of the earlier eruptions the lower crust dried out considerably by quaternary time so less crustal component is present

clay minerals of four soils formed in eolian and tephra materials in iceland abstract clay minerals in a vitricryand and three haplocryands derived from eolian and tephra materials in different parts of iceland and in glacial till samples representative of underlying strata were studied by a combination of methods the soils are primarily used as rangelands and are highly susceptible to wind erosion allophanes imogolite and poorly crystalline ferrihydrite are abundant 700 g kg of the clay throughout the soils including a horizons this clay mineralogy would give rise to stable low density aggregates that influence the susceptibility of the soils to wind erosion it also affects chemical properties such as high phosphate retention of the soils even of the cryands with relatively low clay contents 160 275 g kg imogolite was found in some but not all horizons of all soils no particular relationship of imogolite to measured si al molar ratios of the oxalate oxalic acid soluble fractions of the clays 0 51 to 1 13 was evident layer silicate minerals and laminar opaline silicas are either not present or occur in small quantities plagioclase augite and or their weathering residues were found in the clay and silt separates dark colored glass is predominant in the fine sand throughout the soils and the tills one till sample had a clay mineral composition similar to the eolian andic soils while the other contained smectite and possibly a spherical halloysite smectite intermediate but not allophane and imogolite as major weathering products the clay mineral composition of these soils and tills was compared with those derived from basic tephras in other regions

basalt triggering of the c ad 1305 kaharoa rhyolite eruption tarawera volcanic complex new zealand abstract the ad 1305 kaharoa eruption episode of tarawera volcanic complex was the largest 4 km3 magma eruption in new zealand in the last 1000 years high silica 76 sio2 rhyolite was erupted from seven vents along an 8 km linear zone to form pumiceous pyroclastic deposits and lava domes initial vent clearing explosions were followed by a series of plinian pumice eruptions that spread tephra downwind and pyroclastic density current deposits over the volcano slopes the early pyroclastic eruptions were followed by extrusion of a dome in the summit vent and the migration of further plinian activity to adjacent vents the episode finished with the extrusion of three summit lava domes accompanied by block and ash flows two main types of rhyolite were erupted 1 a low zr low sr high rb type comprising the early plinian pyroclastics and 2 a high zr high sr low rb type comprising the late eruptives including the summit domes basaltic material basalt and basaltic andesite is commonly present as free clasts within the kaharoa pyroclastic deposits along with rare clasts of granodiorite diorite gabbro and olivine clinopyroxenite the basaltic clasts are often veneered by kaharoa rhyolite basaltic material is also common as inclusions within rhyolite pumices along with olivine and augite xenocrysts derived from the basalt the basaltic clasts and inclusions contain complexly zoned plagioclase and corroded olivine commonly with a reaction rim consistent with disequilibrium caused by magma mixing some basaltic clasts and inclusions are hornblende free others contain groundmass hornblende plumes of brown glass often extend from hornblende bearing basaltic inclusions into the surrounding rhyolite these plumes demonstrate magma mixing as does the presence of rhyolitic glass and quartz xenocrysts in some basaltic inclusions most basaltic inclusions have crenulate boundaries suggesting they were fluidal when coming into contact with the rhyolite magma diorite gabbro and olivine clinopyroxenite clasts have mineralogy similar to the basalts commonly contain patches of fine grained basaltic material and are considered co magmatic with the basalts intrusion of basalt magma into the base of the rhyolite magma chamber appears to have occurred for some time before the kaharoa eruptions began allowing considerable mixing with the rhyolite magma transfer of water to some basalt inclusions and formation of hornblende in their groundmass conversely the hornblende free basalt inclusions did not have time to react with the rhyolite magma and must have been intruded shortly before the start of the eruption intrusion of this hornblende free basalt appears to have triggered the kaharoa eruption after the magma chamber had been primed by the earlier intrusions

recharge in volcanic systems evidence from isotope profiles of phenocrysts strontium isotope ratios measured from core to rim across plagioclase feldspar crystals can be used to monitor changes in the isotope composition of the magma from which they grew in samples from three magma systems from convergent margin volcanoes sudden changes in major element composition petrographic features and strontium isotope composition were found to correspond to discrete magmatic events most likely repeated recharge of more mafic magma with lower ratios of strontium 87 to strontium 86 into a crustally contaminated magma

element redistribution during hydrothermal alteration of rhyolite in an active geothermal system yellowstone drill cores y 7 and y 8 abstract hydrothermal alteration of the 0 54 ma biscuit basin rhyolite flow in yellowstone drill cores y 7 and y 8 has involved significant changes in ti normalized whole rock concentrations of li mg ca na k mn rb sr sb cs ba and u whereas such changes have generally not occurred for al fe sc co y zr ree hf ta and th the relatively mobile elements have been redistributed over distances from 2 5 cm to more than 10 100 m and are associated mainly with zeolites and clays in thoroughly altered samples average whole rock changes in ti normalized mass range from about 0 5 17 g kg 9 49 for major elements and 0 1 280 mg kg 19 4260 for minor and trace elements compositional changes correlate with hydrothermal mineralogy the relatively immobile elements have been redistributed over a distance of at least 10 100 m but less than 2 5 cm and are associated mainly with clays in thoroughly altered samples addition of sio2 ranges from 110 890 g kg in samples where porosity has been thoroughly sealed by silica deposition thermal water is not in oxygen isotopic equilibrium with any primary or secondary phases analyzed except calcite modest depletions of 18o 1 2 due to exchange with thermal water apparently have occurred in y 8 plagioclase and obsidian oxygen isotope ratios in obsidian replacing smectite and in veinlet celadonite and cristobalite suggest that these phases precipitated from water locally enriched in 18o up to 8 heavier than present thermal water the minimum integrated water rock mass ratio in y 7 and y 8 is 103 104 assuming advective transport

recording the transition from flare up to steady state arc magmatism at the purico chascon volcanic complex northern chile abstract the long term evolution of continental magmatic arcs is episodic where a few transient events of high magmatic flux or flare ups punctuate the low flux magmatism or steady state that makes up most of the arc history how this duality manifests in terms of differences in crustal architecture magma dynamics and chemistry and the time scale over which transitions occur is poorly known herein we use multiscale geochemical and isotopic characteristics coupled with geothermobarometry at the purico chascon volcanic complex pcvc in the central andes to identify a transition from flare up to steady state arc magmatism over 800 kyr during which significant changes in upper crustal magmatic dynamics are recorded the pcvc is one of the youngest volcanic centers related to a 10 1 ma ignimbrite flare up in the altiplano puna volcanic complex of the central andes activity at the pcvc initiated 0 98 0 03 ma with the eruption of a large 80 100 km3 crystal rich dacite ignimbrite high restricted 87sr 86sr isotope ratios between 0 7085 and 0 7090 in the bulk rock and plagioclase crystals from the purico ignimbrite combined with mineral chemistry and phase relationships indicate the dacite magma accumulated and evolved at relatively low temperatures around 800 850 c in the upper crust at 4 8 km depth minor andesite pumice erupted late in the ignimbrite sequence records a second higher temperature 965 c higher pressure environment 17 20 km but with similar restricted radiogenic bulk rock 87sr 86sr 0 7089 0 7091 to the dacites the compositional and isotopic characteristics of the purico ignimbrite implicate an extensive zone of upper crustal mixing assimilation storage and homogenization mash between 30 and 4 km beneath the pcvc 1 ma the final eruptions at the pcvc 0 18 0 02 ma suggest a change in the magmatic architecture beneath the pcvc these eruptions produced three small the textural and compositional characteristics of the pcvc attest to two distinct stages in its history at 1 ma the system was broadly homogeneous and dominantly dacitic recording extensive upper crustal magmatism by 0 2 ma the pcvc had transitioned to a more compositionally heterogeneous smaller volume mixed dacite to basaltic andesite system coinciding with the appearance of less enriched baseline compositions the evolution of pcvc is a microcosm of the central andean arc in this region where from 10 to 1 ma upper crustal mash processes resulted in the production and eruption of large volumes of homogeneous crystal rich dacite during a regional ignimbrite flare up since 1 ma decreasing explosivity smaller eruptive volumes increasing heterogeneity and the emergence of less isotopically enriched basaltic andesite to dacite composite volcanoes signal a return to steady state arc volcanism we posit that the transition from flare up to steady state captured at the pcvc tracks the waning of the arc scale thermal engine high magmatic fluxes during the flare up would lead to elevated geothermal gradients and efficient crustal processing leading to a dominantly crustal magmatism feeding the large volume purico ignimbrite this upper crustal mash zone would act as an efficient filter to any parental compositions precluding them from the eruption record as magmatic flux and thermal energy wanes crustal isotherms would relax leading to greater thermal contrast between parental magmas upper crust and remnant felsic magmas stored in the upper crust these changes are manifested in the preservation of textural and compositional heterogeneity and the survival of less isotopically enriched magmas in the upper crust the chemical imprint of these arc scale changes in magma dynamics is recorded at all scales from bulk rock to intra crystalline the distinct magma dynamics and chemical signatures of the two modes of arc magmatism detailed here should provide a model for investigations of mature continental arc evolution through time and space

description and interpretation of the composition of fluid and alteration mineralogy in the geothermal system at svartsengi iceland abstract the bulk composition and mineralogy of hydrothermally altered tholeiite along with the composition and speciation of fluid have been determined for a well defined alteration zone at 240 c and 110 bars at svartsengi iceland mass balances between the geothermal fluid and altered tholeiite relative to a seawater fresh water mixture and unaltered tholeiite indicate the overall reaction per 1000 cm 3 is 1325 gm plagioclase 1228 gm pyroxene 215 gm oxide minerals break down to form 685 gm chlorite 636 gm albite 441 gm quartz 249 gm epidote 266 gm calcite 201 gm oxide minerals 15 gm pyrite requiring an influx of 123 gm co 2 10 gm h 2 s and 4 gm na 2 o and a release of 57 gm sio 2 35 gm feo 21 gm cao 8 gm mgo and 4 gm k 2 o principal reactions deduced from textural evidence include na ca exchange in plagioclase precipitation of quartz calcite and anhydrite and formation of chlorite and epidote by reactions between groundmass minerals and fluid thermodynamic analyses of authigenic minerals and downhole fluid indicate that the fluid maintains a state close to equilibrium with the secondary mineral phases chlorite epidote albite quartz calcite prehnite anhydrite pyrite and magnetite whereas remnant primary labradorite and augite are out of equilibrium with the fluid water rock ratios for the system are determined under a variety of assumptions however the open nature of the system makes comparisons with experimental and theoretical closed system studies ambiguous

tectonic and magmatic evolution of the andes of northern argentina and chile abstract two orogenic cycles both with different evolution are developed in the western margin of the south american continent in northern argentina and chile the paleozoic hercynic cycle and the meso cenozoic andean cycle the hercynic cycle a wide marine basin extending westward of the cordillera oriental which developed in cambrian ordovician times marks the beginning of this cycle in contrast to peru and bolivia where this basin developed between two precambrian blocks the western margin of this basin in northern argentina and chile is still unknown the ordovician sedimentation and accompanied volcanism ends with the ocloyic deformation phase and its synkinematic granitic plutonism two basins were developed in the silurian devonian separated by the puna arc uplifted during this ocloyic phase the shallow water marine terrigeneous sediments which were deposited in them were deformed by a new tectono magmatic associated phase chanic phase carboniferous to lower permian marine carbonates were deposited west of the puna arc and red beds east of it later on during the permian to triassic a magmatic belt was developed along the cordillera occidental the rhyolites ignimbrites and the granitic to granodioritic related intrusives are well represented in chile although the overall geologic history of this period is known many problems concerning its origin and relations to plate tectonics are still unsolved the andean cycle during this cycle a series of magmatic arc systems related to the subduction of the pacific crust was built up along the western margin of south america huge volumes of calc alkaline lavas and related plutons were emplaced since the jurassic along belts parallel to the present coastline showing a general eastward migration trend up to the lower cretaceous an ensialic back arc basin was formed east of this magmatic arc thousands of meters of marine and continental sediments were deposited in it this basin disappeared during the middle cretaceous probably as a result of the final opening of the atlantic and the active westward movement of the south american plate since middle cretaceous times the magmatic arc has been the fundamental paleogeographic element the arc progressively migrated stepwise eastward each step marked by a tectonic phase subduction related crustal erosion could explain the lack of fore arc petrotectonic assemblages the eastward magmatic polarity which characterizes this section of the andes could also be explained by such a mechanism

meteoric water basalt interactions ii a field study in n e iceland the compositions of rain snow melt spring and geothermal waters from the rift zone of n e iceland can be explained by seaspray addition chemical fractionation at the seawater air interface burning of fossil fuel farming activities purification by partial melting of snow and ice dissolution of basalts and buffering by alteration minerals the dissolution of the rocks appears to be incongruent during solute acquisition spring compositions move through the stability fields of kaolinite and smectite to the laumontite and illite fields all but four of the springs are undersaturated with respect to calcite silica concentrations are compatible with the solubility of basaltic glass the reactions reflected in the spring waters appear to have taken place sealed off from atmospheric co2 after initial saturation the geothermal waters which are recharged by the spring waters are depleted in mg and ca but enriched in carbon and sulfur with respect to dissolution of primary rocks expressions are derived relating dissolution rates of rocks age of groundwaters physical properties of groundwaters and mass transfer the characteristic rock particle radii in the cold water aquifers range from 0 2 to 2 cm and the characteristic crack openings are of the order 0 04 to 0 4 cm using laboratory studies on the icelandic lavas as a guide the residence times of the cold waters in the aquifers can be estimated at 60 days to 4 years the average active surface area of the aquifers enclosing 1000 g of spring water is of the order of 0 6 to 6 m2 and these 1000 g of water have reacted with 0 1 to 1 g of basaltic rocks the same mass of thermal water has interacted with 100 to 300 g of unaltered rocks

the sedimentary cycle of the boron isotopes abstract marine sediments contain two isotopically distinct components a non desorbable fraction with 11b between 4 3 and 2 8 per mil n 10 and a desorbable component with 11b between 13 9 and 15 8 per mil n 6 the adsorption coefficient k for the uptake of b from seawater by fluvial suspended material mississippi has an experimentally determined value of 1 54 0 05 the associated isotope fractionation factor is 0 974 0 003 empirical values for k determined from the analysis of marine sediments n 6 were between 3 and 4 the amount of b adsorbed onto fluvial suspended material during estuarine mixing is about 9 109 moles yr based on pore water data adsorption experiments and comparative size fraction analyses suspended sediments from the mississippi river vs bottom deposits from the delta there is no evidence for the incorporation of b into detrital sediments at low temperature however the high b content of bauer deep metalliferous sediments must be due to incorporation of seawater b during formation of authigenic silicates the 11b of these minerals is 2 3 per mil hydrothermally altered sediments from dsdp hole 477 guaymas basin gulf of california have variable b contents from 33 ppm similar to the unaltered detritus to 1 2 ppm in the recrystallized material the latter has 11b as low as 9 0 per mil substantially lower than the unaltered material hydrothermal solutions collected from vents adjacent to the hole have boron contents elevated by about a factor of 4 relative to seawater and 11b between 16 5 and 23 2 per mil the b mobilized from the sediments is isotopically fractionated with the fluids being preferentially enriched in the heavy isotope analysis of an oxisol profile developed over granite in the guyana shield showed that boron is partially mobilized during weathering with an isotopic offset of 2 5 per mil between bedrock and soil the observed enrichment of b in shales relative to igneous rocks does not occur during weathering or during exposure to seawater at low temperature incorporation occurs only during burial diagenesis at temperatures greater than about 60 c however the enrichments cannot be attained during a single cycle of primary weathering and burial but must be cumulative over many cycles

on the longevity of large upper crustal silicic magma reservoirs understanding the processes involved in the formation and maturation of upper crustal magma reservoirs ultimately sourcing the largest volcanic eruptions on earth is one of the most fundamental aspects of volcanology while such reservoirs are known to assemble incrementally over extended periods of time debate persists regarding the time scales of melt preservation in the cold upper crust if rapid cooling individually freezes incoming replenishing intrusions accumulations of eruptible magma are impossible precluding the construction of voluminous volcanic reservoirs for all but the highest magma emplacement rates recent numerical thermal models have been used to assess the viability of upper crustal silicic magma survival and have suggested that supervolcanic reservoirs must form on geologically short time scales with anomalously high injection rates and subsist only ephemerally making their long term evolution less predictable motivated by geological observations suggesting the contrary we have improved upon these models by incorporating two fundamental features of natural systems not previously considered 1 a nonlinear crystallization temperature relationship adapted for upper crustal silicic magmas and 2 a temperature dependent thermal conductivity we demonstrate that the incorporation of both of these properties can allow an upper crustal reservoir to remain above its solidus for hundreds of thousands of years when fed by magma fluxes typical of large magmatic provinces while the crystallization temperature path plays the most significant role in maintaining a large pool of eruptible magma the incorporation of temperature dependent thermal properties significantly extends the lifetime of such reservoirs furthermore while deeper emplacement levels e g 10 km depth can both extend magma survival and increase melt availability we show that a 5 km depth can also provide an adequate magma storage environment these results provide strong support for long lived upper crustal mushes as a staging ground for accumulation of highly eruptible crystal poor silicic magmas and further assert the evolutionary link between volcanic and plutonic systems

a geochemical traverse across the north chilean andes evidence for crust generation from the mantle wedge major and trace element and sr and nd isotope analyses are presented on 186 0 ma magmatic rocks along an east west traverse across north chile at 22 s esr ranges from 25 to 100 and end from 6 to 9 but the low end and high esr values are in rocks generated in the last 15 ma it is argued that previous discussions of petrogenesis in north chile have been hampered because the changes in magma chemistry in this area of unusually thick crust reflect not one but two processes one results in a progressive shift of esr from 25 to 20 and end from 6 to 6 in jurassic to recent rocks which is accompanied by increasing ta sm and sr and decreasing th ta the second is largely confined to the younger rocks and it is characterised by esr increasing up to 100 with increasing sio2 and decreasing sr and it results in relatively shallow trends on an end esr diagram the preferred interpretation is that trend 1 is due to the mobilisation of old late proterozoic mantle lithosphere as magmatism migrated eastwards and that trend 2 is due to crustal melting and contamination with differentiation in this area of thickened continental crust it follows that the mantle wedge is the principal site of crust generation and it is argued that 20 of the sr in the recent northern chile rocks is derived from the subducted ocean crust

fluid inclusion research proceedings of cofel volume 14 1981 edited by edwin roedder and andrej kozlowski 1985 the university of michigan press ann arbor mi 15 00

the central andean altiplano puna magma body receiver function analysis of 14 teleseismic events recorded by 6 temporary passcal broadband stations within the altiplano puna volcanic complex apvc shows a consistent 2 s negative polarity p to s conversion for all stations for all available azimuths forward modeling of the largest amplitudes suggests that this conversion is produced by the top of a very low velocity zone at a depth of 19 km with a vs 0 5 km s and a thickness of 750 810 m we interpret the characteristics of the low velocity zone low vs areal extent and flatness to be consistent with a sill like magma body on the basis of additional data from the german ancorp experiment the altiplano puna magma body appears to underlie much of the apvc and it may therefore be the largest known active continental crustal magma body

alteration of the oceanic crust implications for geochemical cycles of lithium and boron abstract fresh tholeiitic basalt glass has been reacted with seawater at 150 c water rock mass ratio of 10 and fresh diabase has been reacted with a na k ca cl fluid at 375 c water rock mass ratios of 1 2 and 5 to understand better the role of temperature basalt composition and water rock mass ratio on the direction and magnitude of b and li exchange during basalt alteration at 150 c slight but nevertheless significant amounts of b and li were removed from seawater and incorporated into a dominantly smectite alteration phase at 375 c however b and li were leached from basalt b behaved as a soluble element and attained concentrations in solution limited only by the b concentration in basalt and the water rock mass ratio li however was less mobile for example at water rock mass ratios of 1 2 and 5 the percent of li leached from basalt was 58 70 and 92 respectively this suggests some mineralogic control on li mobility during hydrothermal alteration of basalt especially at low water rock mass ratios in general these results as well as those for b are consistent with the temperature dependent chemistry of altered seafloor basalt and the chemistry of ridge crest hydrothermal fluids based on the distribution and chemistry of products of seafloor weathering low 150 c and high temperature hydrothermal alteration of basalt and the chemistry of ridge crest hydrothermal fluids it was estimated that alteration of the oceanic crust is a li source for seawater this is not true for b however since the hot spring flux estimated for b is balanced by low temperature basalt alteration these data coupled with b and li flux estimates for other processes e g continental weathering clay mineral adsorption authigenic silicate formation and formation of siliceous skeletal material yield new insight into the b and li geochemical cycles calculations performed here indicate relatively good agreement between the magnitude of b and li sources and sinks the geochemical cycle of b however may be affected by serpentinization of mantle derived peridotite in oceanic fracture zones serpentinites are conspicuously enriched in b and if the b source for these rocks is seawater then an additional b sink exists which must be integrated into the b geochemical cycle however until more data are available in terms of areal extent of serpentinization serpentite chemistry and isotopic composition the importance of b in these rocks with respect to the b geochemical cycle remains speculative at best

effusive eruption of viscous silicic magma triggered and driven by recharge a case study of the cerro chascon runtu jarita dome complex in southwest bolivia the cerro chascon runtu jarita complex is a group of ten late pleistocene 85 ka lava domes located in the andean central volcanic zone of bolivia these domes display considerable macroscopic and microscopic evidence of magma mixing two groups of domes are defined chemically and geographically a northern group the chascon consists of four lava bodies of dominantly rhyodacite composition these bodies contain 43 48 phenocrysts of plagioclase quartz sanidine biotite and amphibole in a microlite poor rhyolitic glass rare mafic enclaves and selvages are present mineral equilibria yield temperatures from 640 to 750 c and log o2 of 16 geochemical data indicate that the pre eruption magma chamber was zoned from a dominant volume of 68 to minor amounts of 76 sio2 this zonation is best explained by fractional crystallization and some mixing between rhyodacite and more evolved compositions the mafic enclaves represent magma that intruded but did not chemically interact much with the evolved magmas a southern group the runtu jarita is a linear chain of six small domes 1 km3 total volume that probably is the surface expression of a dike the five most northerly domes are composites of dacitic and rhyolitic compositions the southernmost dome is dominantly rhyolite with rare mafic enclaves the composite domes have lower flanks of porphyritic dacite with 35 vol phenocrysts of plagioclase orthopyroxene and hornblende in a microlite rich rhyodacitic glass sieve textured plagioclase mixed populations of disequilibrium plagioclase compositions xenocrystic quartz and sanidine with ternary composition reaction rims indicate that the dacite is a hybrid the central cores of the composite domes are rhyolitic and contain up to 48 vol phenocrysts of plagioclase quartz sanidine biotite and amphibole this is separated from the dacitic flanks by a banded zone of mingled lava macroscopic microscopic and petrologic evidence suggest scavenging of phenocrysts from the silicic lava mineral equilibria yield temperatures of 625 727 c and log o2 of 16 for the rhyolite and 926 1000 c and log o2 of 9 5 for the dacite the rhyolite is zoned from 73 to 76 sio2 and fractionation within the rhyolite composition produced this variation most of the 63 73 sio2 compositional range of the lava in this group is the result of mixing between the hybrid dacite and the rhyolite eruption of both groups of lavas apparently was triggered by mafic recharge a paucity of explosive activity suggests that volatile and thermal exchanges between reservoir and recharge magmas were less important than volume increase and the lubricating effects of recharge by mafic magmas for the runtu jarita group the eruption is best explained by intrusion of a dike of dacite into a chamber of crystal rich rhyolite close to its solidus the rhyolite was encapsulated and transported to the surface by the less viscous dacite magma which also acted as a lubricant simultaneous effusion of the lavas produced the composite domes and their zonation reflects the subsurface zonation the role of recharge by hotter more fluid mafic magma appears to be critical to the eruption of some highly viscous silicic magmas

the chemistry of geothermal waters in iceland i calculation of aqueous speciation from 0 to 370 c abstract a computer programme has been developed to calculate the composition and aqueous speciation of geothermal reservoir waters including ph redox potential and gas partial pressures the programme is specifically suited to handle geochemical data from wet steam wells hot water wells and boiling hot springs but it may also be used for non thermal waters solubility data for selected geothermal minerals are incorporated to facilitate the study of solution mineral equilibria the programme may also be used to study chemical changes in water chemistry accompanying boiling variable degassing and cooling and how these changes disturb solution mineral equilibria

magmatic interactions as recorded in plagioclase phenocrysts of chaos crags lassen volcanic center california the silicic lava domes of chaos crags in lassen volcanic national and mingling of this material into the silicic host these processes park contain a suite of variably quenched hybrid basaltic andesite are commonplace in some orogenic magma systems and may be magmatic inclusions the inclusions represent thorough mixing elucidated by isotopic microsampling and analysis of the plagioclases between rhyodacite and basalt recharge liquids accompanied by some crystallizing from them mechanical disaggregation of the inclusions resulting in crystals mixing into the rhyodacite host preserved by quenching on dome emplacement sr sr ratios 0 7037 0 7038 of the inclusions are distinctly lower than those of the host rhyodacite

processes controlling the distribution of boron and chlorine in natural waters in iceland abstract the concentrations of b and cl in natural waters in iceland lie in the range 0 001 10 and 1 20 000 ppm respectively lowest concentrations occur in surface and groundwaters in the central highlands highest cl concentrations are found in high temperature waters on the reykjanes peninsula and highest b concentrations in well waters from high temperature geothermal systems in the axial zones of the active volcanic belts the b and cl contents of icelandic basalts are 0 1 6 6 and 75 750 ppm respectively these large variations are considered to result from variable degassing during consolidation as well as concentration variations in the initial magma tholeftes are lowest in both elements av 1 2 ppm b and 170 ppm ci transitional basalts intermediate av 2 1 ppm b and 240 ppm ci and alkali basalts highest av 3 8 ppm b and 340 ppm cl cl b ratios in tholeiites are most often within 25 50 but somewhat lower in alkali basalts most geothermal waters contain b and cl within 0 05 1 and 10 100 ppm respectively these rather low values are attributed to the low content of b and cl in the basaltic rock the b and cl distribution in the icelandic natural waters indicate that these elements act as essentially incompatible their sources of supply include 1 the atmosphere seawater spray and aerosols 2 the rock with which the water interacts 3 seawater groundwater and 4 magma intrusions in surface water and nonthermal groundwater the dominant source of supply of cl is seawater spray and aerosols but the soil and rock contribute often significantly to the b in these waters their cl b ratio is similar to that of seawater 1330 or somewhat lower boron and cl concentrations generally increase with water temperature warm waters cl b ratios intermediate between those of seawater and basalt with increasing temperature this ratio decreases and gradually approaches that of the rock some high temperature geothermal waters have as low cl b ratios as 1 and b concentrations as high as 10 ppm the cause is either b degassing of the magmatic intrusive heat source or phase separation in the producing aquifer some low temperature waters

monitoring of fumarole discharge during the 1975 1982 rifting in krafla volcanic center north iceland abstract fumarole discharge chemistry in the krafla geothermal field changed regionally during the 1975 1982 rifting activity the discharge chemistry previously encountered in the krafla fumarole grounds was masked by a carbon dioxide rich gas during the first weeks of rifting the new discharge composition remained unchanged until 1983 when the first signs of recovery of the previous equilibrium composition appeared at the margins of the area the outgassing carbon dioxide is released from the deep aquifers beneath the area by the interaction of magmatic gas with the hydrothermal system in addition to juvenile magmatic carbon the outgassing contains carbon released from the hydrothermal systed upon reaction with acid magmatic gases increased boiling of the hydrothermal fumaroles was induced by the lowering of the partial pressure of steam due to increased gas content in the fumarole conduits at the center of rifting activity above the magma chamber the fumarole discharge was temporarily mixed with magmatic gases during local effusive activity hydrogen was the dominating magmatic gas in that discharge due to the preferred degassing of hydrogen from magma at shallow levels these hydrogen pulses increased in magnitude and duration towards the end of rifting in 1982 the discharge chemistry correlates with the expansion of the magma reservoir of the volcano regional change of long duration and local volcanism short lived change hydrogen pulses the chemical monitoring of fumaroles in krafla shows that the chemical surveillance of volcanos needs rapid methods for sampling and complete chemical analysis which can be interpreted in terms of reactions and magmatic processes

subsurface thermal conductivity assessment in calabria southern italy a regional case study determining the suitability of a local area at a regional or local scale for the geothermal application of low enthalpy systems requires the knowledge of rock thermal conductivity values to evaluate the possibility of low enthalpy heat exchange a digital cartographic tool is also needed to synthesize the thermal properties of the underground this tool should be easily accessible and upgradeable and thus suitable for territorial planning and environmental control in order to address these key issues a methodological approach was developed within the framework of the national vigor project dedicated to evaluating the geothermal potential in southern italy in this paper the region of calabria was selected as a case study around 70 samples that were representative of the main geological formations were collected from all over the area thermal property tests were carried out both in dry and wet conditions using a thermal device in accordance with the modified transient plane source method the thermal conductivity values were then compared with data from the international literature in order to consider the influence of the entire stratigraphic sequence on the thermal conductivity parameters a geostatistical analysis of the available lithostratigraphic data was performed using the matlab toolbox modalstrata specially developed for this purpose a comprehensive geothermal subsurface characterization of calabria was thus obtained

revised age of aleutian island arc formation implies high rate of magma production radioisotopic dating of subaerial and submarine volcanic and plutonic rocks from the aleutian island arc provides insight into the timing of arc formation in the middle eocene twenty eight 40 ar 39 ar ages constrain the duration of arc magmatism to the last 46 m y basaltic lavas from the finger bay volcanics the oldest exposed rocks in the arc gave an isochron age of 37 4 0 6 ma which is 12 17 m y younger than a widely cited age of 55 50 ma three main pulses of arc wide magmatism occurred at 38 29 16 11 and 6 0 ma which coincide with periods of intense magmatism in other western pacific island arcs using the geochronology and volumetric estimates of crust generated and eroded over the last 46 m y we calculate a time averaged magma production rate for the entire arc that exceeds previous estimates by almost an order of magnitude

the thermal springs of jordan the thermal springs alongside wadi araba dead sea and jordan valley jordan were investigated and sampled in a comprehensive field study of 2 weeks conventional physicochemical properties were measured in situ dissolved solids and isotopic composition were analyzed two main spring locations on the east side of the dead sea were mapped it became evident that the hottest springs are among the closest to prominent faults some springs are controlled by gas lift jordan s hot springs are described by means of a broad hydrochemical and physicochemical data set and several figures are presented based on their hydrochemistry the thermal springs are classified into four thermal provinces water genesis is discussed several geothermometers are applied to estimate reservoir temperatures observed discharge rate water temperature and isotopic composition are compared with data from the literature although discharge and water temperature are reported to be constant over the last decades groundwater overexploitation led to a shift of the isotopic composition what is documented for the first time in jordan thus the effect of groundwater mining on jordan s hot springs can be stated as a fact