a new estimate for present day cocos caribbean plate motion implications for slip along the central american volcanic arc velocities from 153 continuously operatinggps sites on the caribbean north american and pacic plates arecombinedwith61newlyestimatedpacic cocosseafloor spreading rates and additional marine geophysical data to deriveanewestimate of present daycocos caribbean plate motion acomparisonofthepredictedcocos caribbean di rectiontoslipdirectionsofnumerousshallow thrustsubduc tion earthquakes from the middle america trench between costa rica and guatemala shows the slip directions to be deflected 10 clockwise from the plate convergence direc tion supporting thehypothesisthatfrequentdextralstrike slip earthquakes along the central american volcanic arc result from partitioning of oblique cocos caribbean plate convergence linear velocity analysis for forearc locations in nicaragua and guatemala predicts 14 2m m yr 1 of northwestward trench parallel slip of the forearc relative to thecaribbean plate possibly decreasing inmagnitude inel salvador and guatemala where extension east of the vol canic arc complicates the tectonic setting

hydrothermal hydrocarbon gases in the sediments of the king george basin bransfield strait antarctica thermogenic hydrocarbons formed by the thermal alteration of organic matter are encountered in several piston core stations in the king george basin anatarctica these hemipelagic sediments are being deposited in an area of active hydrothermalism associated with the back arc spreading in the bransfield strait the lateral extent of sediments infiltrated by the hydrothermally influenced interstitial fluids is characterized by basalt diapirix intrusions and is delineated by an acoustically turbid zone in the sediments of the eastern part of the basin iron sulphide bearing veins and fractures cut across the sediment in several cores they appear to be conduits for flow of hydrothermally altered fluids these zones have the highest c2 and ethene contents the thermogenic hydrocarbons have molecular c1 c2 c3 ratios typically 50 and 13ch4 values between 38 and 48 indicating an organic source which has undergone strong thermal stress several sediment cores also have mixed gas signatures which indicate the presence of substantial amounts of bacterial gas predominantly methane hydrocarbon generation in the king george basin is thought to be a local phenomenon resulting from submarine volcanism with temperatures in the range 70 150 c there are no apparent seepages of hydrocarbons into the water column and it is not believed that significant accumulation of thermogenic hydrocarbons reside in the basin

monitoring of active but quiescent volcanoes using light hydrocarbon distribution in volcanic gases the results of 4 years of discontinuous monitoring in the campi flegrei italy abstract during the period from january 1991 to january 1994 the 10 yr geochemical monitoring in the campi flegrei southern italy usually performed using the inorganic gas fraction was implemented by discontinuous monitoring of the hydrocarbon gas fraction the measured compositions have revealed the presence of significant amounts of at least 11 hydrocarbon species related to alkane alkene and aromatic classes thermodynamic calculations showed the possible existence of partially independent equilibria within the c1 c2 c3 and c1 c2 c3 c4 alkane systems as well as among alkene alkane pairs at redox conditions controlled by the geothermal feo fe1 5o redox buffer system at temperatures between 200 and 400 c and ph2o controlled by the presence of a saline brine unlike the constancy of the measured and calculated inorganic temperature equilibrium temperature based on organic species has displayed fluctuations and a significant increase starting from the second half of 1993 according to their apparent correlation with other independent observations such as the measured increases of vapor fraction h2s co2 and h2 n2 ratios on the monitored fumarole in addition to a general increase of dissolved co2 in the campi flegrei groundwaters the observed variations of the organic parameters were interpreted as the result of a general increase in acid bearing gas flux throughout the re equilibration zone

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

alpe arami a peridotite massif from depths of more than 300 kilometers the abundance of fetio 3 and chromite precipitates in olivine of the alpe arami peridotite massif switzerland requires a much higher solubility for highly charged cations than is found in mantle xenoliths from depths of 250 kilometers three previously unknown crystal structures of fetio 3 were identified that indicate that the originally exsolved phase was the high pressure perovskite polymorph of ilmenite implying a minimum depth of origin of 300 kilometers these observations coupled with the unique lattice preferred orientation of olivine further suggest that the original composition of olivine may reflect that of a precursor phase perhaps wadsleyite that is stable only at depths greater than 400 kilometers

geochemistry of some gases in hydrothermal fluids from the southern juan de fuca ridge five samples of hydrothermal fluids from two vent areas on the southern juan de fuca ridge were analyzed for dissolved gases concentrations in the end member hydrothermal fluid of h2 270 527 mol kg ch4 82 118 mol kg and co2 3920 4460 mol kg are well above values in ambient seawater and are similar to concentrations reported for other ridge crest hydrothermal systems the carbon isotopic ratios of the ch4 13c 17 8 to 20 8 and co2 13c 3 6 to 4 7 suggest that at least some of the ch4 and co2 in the fluids is basalt derived the range of 13c values for the basalt derived co2 is 6 8 to 9 7 calculated by assuming conservation of recharge co2 during hydrothermal circulation apparent temperatures of equilibration between the ch4 and the basalt derived co2 range from 640 c to 750 c small amounts of ethane c2h6 ch4 0 9 10 3 2 2 10 3 propane and butane detected in the samples may also have formed in the basalt one sample of almost pure 95 5 hydrothermal fluid contained a significant fraction up to 63 and 74 respectively of the recharge ar and n2 this suggests that the fluid has not undergone extensive vapor liquid phase separation

the hydrothermal volcanic system of rincon de la vieja volcano costa rica a combined inorganic and organic geochemical approach to understanding the origin of the fluid discharges and its possible application to volcanic surveillance abstract in the period 1998 2002 thermal spring discharges of rincon de la vieja volcano nw costa rica have been sampled and analyzed for major trace and isotopic 18 o 16 o and d h in waters and 3 he 4 he and 13 c 12 c in co 2 in gases composition the boiling pools hosted inside the summit crater active crater are characterized by high contents of magmatic related compounds so 2 hcl and hf that strongly affect the chemistry of the crater lake these chemical features are not shown by the thermal discharges seeping out in the surrounding area of the volcano here the shallow aquifer apparently masks any possible clues related to the magmatic system this suggests that the fluid vents located inside the active crater are likely to represent the most appropriate sampling sites for geochemical surveillance purposes although the high gas discharge rate from the lake may occasionally prevent any fluid sampling alternatively as already suggested by recent studies on the behavior of light hydrocarbons in different volcanic environments the compositional features of the organic gas fraction in the more accessible outer flank thermal discharges could usefully be utilized at least to assess the thermodynamic conditions of the volcanic hydrothermal system since these compounds are affected by secondary processes only at limited extent

evolution of fluid geochemistry at the turrialba volcano costa rica from 1998 to 2008 turrialba 10 02 n 83 45 w is a 3 349 m high stratovolcano belonging to the holocene cordillera central volcanic belt of costa rica the summit consists of three ew oriented craters east central and west since its last eruptive phase 1864 1866 the central and west craters have displayed modest fumarolic activity with outlet temperatures clustering around 90 c in 2001 seismic swarms ground deformation and increasing fumarolic activity occurred from 2005 to 2008 new fumarolic vents opened between and within the central and west craters and along the western and southwestern outer flanks of the volcanic edifice these physical changes were accompanied by a drastic modification in the gas chemistry that can be divided in three stages 1 hydrothermal from 1998 to autumn 2001 characterized by the presence of h2o co2 h2s and to a very minor extent hcl and hf 2 hydrothermal magmatic autumn 2001 2007 with the appearance of so2 and a significant increase of hcl and hf and 3 magmatic dominated 2007 2008 characterized by increased so2 content so2 h2s 100 and temperatures up to 282 c accordingly gas equilibrium in the co2 ch4 h2 system suggests a progressive evolution of the deep fluid reservoir toward higher temperatures and more oxidizing conditions the chemical physical modifications of turrialba in the last decade can be interpreted as part of a cyclic mechanism controlling the balance between the hydrothermal and the magmatic systems nevertheless the risk of rejuvenation of the volcanic activity cannot be excluded and an appropriate seismic ground deformation and geochemical monitoring program is highly recommended turrialba lies at a distance of 35 and 15 km from san jose and cartago respectively the two largest cities in costa rica

joining up land and marine seismic data case study from procida and ventotene islands tyrrhenian sea italy abstractthe seismic refraction survey routinely used to obtain information on subsurface geology has a limited use in the foreshore zone we have evaluated a nonstandard approach to acquire seismic data in the backshore foreshore area joining up land and marine seismic data the innovative part was the acquisition of a continuous seismic profile that allowed the recording of high resolution data in a transition zone tz to establish an integrated approach for improving the land marine seismic model in areas characterized by a complex near surface geology to achieve this objective two seismic tests were carried out along the beaches of the tyrrhenian islands of procida and ventotene we defined the p wave vp and s wave velocities vs with depth by integrating the tomography of the refracted first arrivals vp velocities versus depth with the inversion of the phase velocities dispersion curves extracted from the surface waves 1d vs velocities models working with both seismic methods allowed us to o

ultrahigh pressure minerals and metamorphic terranes the view from china abstract ultrahigh pressure uhp metamorphism refers to mineralogical modifications of continental and oceanic crustal protoliths associated mafic ultramafic rocks initially formed or emplaced in shallow levels of the lithosphere but which subsequently have experienced p t conditions within or above the coesite stability field 2 7 gpa 700 c typical products include eclogite garnet peridotite and uhp varieties of metapelite quartzite marble paragneiss and orthogneiss uhp metamorphic assemblages require relatively cold lithospheric subduction to mantle depths some recrystallization even occurs under forbidden p t conditions characterized by a geotherm of the current explosion of research on continental uhp terranes reflects their significance for mantle dynamics and the tectonics of continental subduction collision exhumation mantle slab interactions and geochemical recycling a further characterization of uhp phases and positive identification of uhp minerals requires new experimental studies coupled with state of the art analyses for example the very rare occurrence of microdiamond inclusions in zircons from dabie sulu uhp rocks may reflect high f o 2 attending recrystallization inasmuch as epidote is rather common rutile needles within garnets from sulu uhp eclogitic rocks may not be the result of exsolution so in such cases the apparent uhp pressure may have been over estimated hadean igneous microdiamond inclusions in jack hills detrital zircons could have originated from mantle xenoliths whereas abundant detrital phanerozoic diamonds containing inclusions of coesite and other eclogitic minerals from new south wales might have been derived from unexposed uhp metamorphic terranes micro mineral intergrowth and nano size minerals may hold important key to deciphering the actual p t paths of subduction and mantle return flow although most exhumed terranes have returned surfaceward relatively rapidly after short time of uhp condition the long duration of storage at great depth and slow exhumation for the largest uhp terranes remain as major problems

diamond from the dabie shan metamorphic rocks and its implication for tectonic setting diamond occurs in ultrahigh pressure metamorphic rocks from dabie shan anhui province eastern china diamond bearing rocks include eclogite gamet pyroxenite and jadeitite diamond occurs in a mineral assemblage with coesite and jadeite the diamonds and diamondiferous rocks of dabie shan are interpreted to be the products of ultrahigh pressure metamorphism in the underthrust basement of the yangtze continental plate during the early mesozoic at greater than 4 0 gigapascals and 900 c this interpretation is based on the distribution of rock units the stability field of diamond and isotopic data indicating a crustal origin for the rocks most diamonds occur as euhedral inclusions in garnets and are 10 to 60 micrometers across although some are up to 700 micrometers across

catastrophic collapse at stratovolcanoes induced by gradual volcano spreading unlike ordinary mountains which are formed by slow uplift and erosion volcanoes are constructed rapidly as a consequence many are liable to massive flank failures leading to debris avalanches for example at mount st helens in 1980 such failures occur worldwide about once every 25 years ref 1 and even small ones can present a major hazard in particular if far reaching tsunamis are generated as at mayu yama in 1792 ref 2 previous work has tended to emphasize differences in eruption style associated with flank failure2 but here we focus on the fundamental structural causes of failure most volcanic failures are generated by magmatic intrusion and flank spreading3 we present evidence however that mombacho volcano in nicaragua experienced a previously unrecognized type of failure triggered by sub volcanic basement spreading notably collapses related to basement spreading do not require that the volcano be magma tically active and thus flank failure may pose a significant risk even at inactive volcanoes which are rarely monitored

radiocarbon ages for the timing of debris avalanches at mombacho volcano nicaragua radiocarbon dated lake sediments provide minimum limiting ages for two major debris avalanches originating from mombacho volcano in nicaragua a basal age from lake el gancho indicates that the northeast debris avalanche las isletas occurred sometime before 140 to 345 a d basal ages from lakes blanca and verde indicate that the southern el crater debris avalanche occurred sometime before 270 to 650 a d both events therefore occurred in the space of a few centuries yet there is strong evidence that the mechanisms varied for destabilization of each flank possibly the influence of a developing hydrothermal system lead first to deeper structural failure in the substrata to produce the las isletas sector collapse progressing to higher level destabilization within the edifice and the el crater collapse

difference in n2 ar ratio of magmatic gases from northeast and southwest japan new evidence for different states of plate subduction two different plates are subductiog beneath the japanese island arcs the pacific plate in the north and the philippine sea plate in the south geophysical evidence indicates that the philippine sea plate subducts without extending beneath many of the volcanoes in the southwest japan arc the n 2 ar ratio 424 of a common magmatic gas from geothermal and volcanic areas in southwestern japan is an order of magnitude lower than the value 4250 in northeastern japan because the high n 2 ar values reflect input from subducted sediments the large difference in the n 2 ar ratios of magmatic gases indicates that the contribution of subducted sediment to the source of magma in southwestern japan is much lower than that in northeastern japan the n 2 ar ratios off magmatic gases from island arcs elsewhere may also yield information regarding the degree of contribution of subducted sediments to arc magma sources and differences in the states of plate subduction

clinoenstatite exsolution in diopsidic augite of dabieshan garnet peridotite from depth of 300 km clinoenstatite exsolution lamellae have been discovered in clinopyroxene grains of garnet peridotite from the bixiling massif of the ultrahigh pressure metamorphic uhpm terrane in the dabie mountains china using transmission electron microscopy we show here that although the lamellae are low clinoenstatite lclen with p 2 1 c space group symmetry they contain nanometer scale antiphase domains indicating that the originally precipitating phase had c 2 c space group symmetry the lamellae are oriented parallel to 010 and 18 to 001 of the host approximately parallel to 401 both the very cold exhumation path of the dabie uhpm terrane and the orientation of the lamellae preclude the possibility that the originally precipitating phase could have been high temperature clinoenstatite htclen we conclude that the lamellae precipitated as high pressure clinoenstatite hpclen and subsequently inverted to lclen analysis of the incomplete high pressure crystallographic data available for compositions approximating those of the lamellae and host suggests a minimum pressure of precipitation of 9 gpa for dabie and about 12 gpa for similar exsolution previously observed in the alpe arami peridotite switzerland the bixiling complex is a crustal cumulate hence this result extends the minimum depth of subduction of continental rocks in the dabie sulu orogen to 300 km p 9 gpa

delineation of potential groundwater zones using shear wave velocity in eastern deccan volcanic province india a comprehensive study based on multichannel analysis of surface waves masw a near surface geophysical technique was carried out for the first time in the mandla lobe of the eastern deccan volcanic province the study was conducted to demarcate fractures which are essential for finding potential groundwater zones in the hard rock region and for the delineation of shallow subsurface geological features in terms of shear wave velocity the masw surveys were carried out at different selected locations in the study area to delineate fractures in the basaltic terrain covered with thin weathering and alluvium cover these basalts are mainly comprised of massive vesicular and amygdaloidal varieties and interpreted as different basaltic flows of mandla dhuma and pipardahi formations the shear wave velocity sections divulge the layered nature of the basalt sequences in some of the massive flows there are low velocity vertical zones that can be deduced to be either vertical columnar joints or fracture zones and the low velocity weathered flows can be the good groundwater reservoirs the shear wave velocities vs obtained are in the range from 300 to 2500 m s at different locations and the thicknesses of flows are varied in the masw sections the obtained shear wave data are compared with the borehole data and the results are well correlated the study clearly differentiates the variations within basaltic formations which are interpreted as potential groundwater zones

very high pressure orogenic garnet peridotites mantle derived garnet peridotites are a minor component in many very high pressure metamorphic terranes that formed during continental subduction and collision some of these mantle rocks contain trace amounts of zircon and micrometer sized inclusions the constituent minerals exhibit pre and postsubduction microstructures including polymorphic transformation and mineral exsolution experimental mineralogical petrochemical and geochronological characterizations using novel techniques with high spatial temporal and energy resolutions are resulting in unexpected discoveries of new phases providing better constraints on deep mantle processes

extension and rotation of crustal blocks in northern central america and effect on the volcanic arc sinistral displacements across the north american caribbean plate boundary in northern central america are distributed among major arcuate faults that have been active in neogene time south of the jocotan and motagua faults in guatemala extensional tectonics has accompanied rotation of the trailing edge of the caribbean plate around these faults segmentation of the volcanic arc in northern central america hitherto attributed to transverse breaks in the subducting cocos plate may instead be a result of this block rotation accompanying changes along the arc are observed in seismicity gravity anomaly patterns volume and composition of volcanic products and topography therefore the complex volcano tectonic geology south of the main boundary faults may be explained by interaction and rotation of crustal blocks in the overriding caribbean plate above the magma production zone along the downgoing cocos slab

a palaeomagnetic study and age determinations of tertiary rocks in nicaragua central america summary palaeomagnetic k ar and ar40 ar39 measurements have been made to elucidate nicaragua s plate tectonic history these show that the nicaraguan highland rotated 90 counterclockwise between 30 and 18 myr ago but the pacific coastal plain has not rotated a lack of data has prevented a direct comparison with the tectonic history of adjacent areas in particular the chortis block which is the part of the caribbean plate that is geologically most closely related to the nicaraguan highland however our data suggest that the caribbean plate in this part of central america was not consolidated until c 15 ma

new evidence for coesite in eclogite and gneisses defining an ultrahigh pressure province in the western gneiss region of norway widespread evidence of ultrahigh pressure metamorphism is reported from the nordfjord and stadtlandet area of the western gneiss region in norway outlining a distinct ultrahigh pressure province coesite has been found as inclusions in garnet omphacite or kyanite in eclogite from 5 new localities and quartz pseudomorphs after coesite occur in eclogite minerals at an additional 12 localities quartz pseudomorphs after coesite within garnet and clinozoisite are reported for the first time from associated gneisses which is evidence for in situ ultrahigh pressure metamorphism at least at a local scale the boundary to an ultrahigh pressure province is defined by a metamorphic break from lower pressure 2 8 gpa within a 10 km wide mixed zone in the nordfjord area this zone may mark the disrupted tectonic boundary to a small ultrahigh pressure unit and casts doubt on the assumption that the western gneiss region behaved as a coherent terrane during burial and exhumation

oriented inclusions of pyroxene amphibole and rutile in garnet from the luliangshan garnet peridotite massif north qaidam uhpm belt nw china an electron backscatter diffraction study

field investigation of love waves in near surface seismology we examine subsurface conditions and survey parameters suitable for successful exploitation of love waves in nearsurface investigations love wave generation requires the existence of a low shear velocity surface layer we examined the minimum thickness of the near surface layer necessary to generate and record usable love wave data sets in the frequency range of 5 50 hz we acquired field data on a hillside with flat lying limestone and shale layers that allowed for the direct testing of varying overburden thicknesses as well as varying acquisition geometry the resulting seismic records and dispersion images were analyzed and the love wave dispersion relation for two layers was examined analytically we concluded through theoretical and field data analysis that a minimum thickness of 1 m of low velocity material is needed to record usable data in the frequency range of interest in near surface love wave surveys the results of this study indicate that existing guidelines for rayleigh wave data acquisition such as receiver interval and line length are also applicable to love wave data acquisition

coesite and clinopyroxene exsolution lamellae in chromites in situ ultrahigh pressure evidence from podiform chromitites in the luobusa ophiolite southern tibet abstract we report in situ petrological evidence of deep mantle origin from podiform chromitites in the luobusa ophiolite southern tibet analytical transmission electron microscopy measurements reveal that chromites in podiform chromitites from the luobusa ophiolite have numerous exsolution lamellae of diopsidic clinopyroxene and coesite which indicate an ultrahigh pressure origin of over 3 gpa 100 km deep the presence of these lamellae coupled with abundant micro inclusions of clinopyroxene requires high solubility of sio2 and cao in the host chromite and suggests a precursor of chromite a cafe2o4 structured high pressure polymorph stable at pressures over 12 5 gpa 380 km deep these nano scale observations and geological occurrence indicate that the mantle peridotite under the tibetan mid ocean ridge was transported from the deep mantle at least 100 km probably more than 380 km deep by the mantle convection it implies that the root of mantle upwelling has a much deeper origin than previously believed

precipitation of pyroxenes and mg2sio4 from majoritic garnet simulation of peridotite exhumation from great depth our experimental simulations of the exhumation path of mantle peridotites show that high temperature 1400 c decompression of lherzolite from 14 to 13 and 12 gpa results in exsolution of interstitial blebs of diopside and mg2sio4 wadsleyite lamellae from majoritic garnet at lower pressures from 8 to 5 gpa at t 1400 c only enstatite exsolves as blebs at garnet boundaries continuous high temperature decompression from 14 to 7 gpa produces zoned majoritic garnet containing blebs of exsolved pyroxenes inside garnet rims no intracrystalline precipitation of pyroxene was observed in garnet although such lamellae are found in some natural garnet peridotites the explanation appears to be the three orders of magnitude difference in grain size between experimental and natural specimens our data suggest that mg2sio4 and diopside exsolutions reflect the deepest point of the exhumation path of garnet peridotites whereas enstatite precipitation may be restricted to garnets with less majoritic component at shallower depths

hydrothermal processes governing the geochemistry of the crater fumaroles at mount etna volcano italy article i nfo we investigated the geochemistry of the fumaroles at the summit area of mt etna including sulfur speciation and the content of acidic gases the carbon isotope composition of the etnean plume was also measured in order to compare it to that of fumaroles two types of fumaroles were identified i low temperature fumaroles which are dominated by co2 with minor amounts of so2 and h2s and negligible chlorine contents and ii high temperature fumaroles which are strongly air contaminated and characterized by appreciable amounts of volcanogenic carbon sulfur and chlorine as recognized by martelli et al 2008 both groups of fumaroles are fed by the degassing of an underlying magma nevertheless compositional data clearly show that secondary processes affect the composition of the fluids once they leave the magma body here a model of cooling and condensation of fluids is proposed to explore such postmagmatic processes the model which uses etnean plume geochemistry as starting composition of fluids exsolved from magma shows that so2 and h2s control the redox conditions of the gas mixture during the cooling until the reactions involving co co2 and h2 h2o ratios are fully quenched at temperatures around 350 450 c the dissolution of gases in water subsequent to condensation must occur at thermobaric conditions over 50 bar and 260 c which allows a total removal of hcl b partial removal of sulfur species while preserving the so2 h2s ratio and c the c s ratio to increase by almost 10 fold relative to that in the plume the observed ch4 co2 ratios are higher than those calculated for the etnean magmatic gas and hence they provide evidence of modest contributions from peripheral hydrothermal fluids during the migration of magmatic gases toward the surface in both low and high temperature fumaroles due to the peculiar thermodynamic conditions the model predicts that carbon isotopes do not experience any postmagmatic fractionation and hence the isotopic composition of the fumaroles is representative of magmatic carbon measurements of the carbon isotope composition of the plume corroborate these findings

coesite in clinopyroxene in the caledonides and its implications for geodynamics coesite is reported for the first time from the caledonide orogen it occurs as inclusions in clinopyroxene in the dolomite eclogite at grytting norway and provides valuable new evidence to support the hypothesis of extremely high pressure 30 kbar in certain norwegian eclogites this finding has implications for the geodynamics and kinetics of the concepts of microscale intracrystalline tectonic overpressure and of macroscale local or regional tectonic overpressure and for the hypotheses of subduction and obduction involving a more than 100km vertical component of transport

discovery of metamorphic diamonds in central china an indication of a 4000 km long zone of deep subduction resulting from multiple continental collisions the central orogenic belt cob of china is a major continental collision zone that contains extensive outcrops of deeply subducted and exhumed rocks at both the eastern and the western end of the belt here we report discovery of microdiamonds from both eclogites and felsic gneisses in the north qinling zone in the central portion of the cob this discovery demonstrates that the country rocks of continental affinity shared in the ultra high pressure metamorphic uhpm event and provides a bridge connecting the two previously recognized uhpm terranes thereby establishing the existence of a uhpm belt extending more than 4000 km geochronological dating yields early palaeozoic ages in the west and early mesozoic ages in the east recording two separate continental collisions overprinted within the cob occurrence of uhp metamorphism during recurrent continental collision here and in the alps suggests that deep subduction of continental material during such collisions is probably common rather than exceptional with significant implications for processes of plate tectonic reorganization and mantle mixing over time

pyroxene exsolution topotaxy in majoritic garnet from 250 to 300 km depth we report here the detailed microstructure and chemistry of pyroxene exsolution from a polycrystalline garnet porphyroblast of the western gneiss region wgr garnet peridotite otroy norway for both clinopyroxene cpx and orthopyroxene opx the same basic crystallographic relationship is found with the host garnet 100 py 112 grt 010 px 110 grt and 001 px 111 grt for the majority 90 of its intracrystalline pyroxene rods in addition this pattern is also exhibited by some interstitial opx and a subpopulation of both pyroxenes shows a different pattern or no discernible pattern the results provide quantitative microstructural evidence demonstrating an exsolution precipitation origin of both the intracrystalline cpx and opx and the small interstitial opx crystals the reconstructed precursor majoritic garnet taking into account both the intracrystalline pyroxenes and interstitial opx was characterized by si 3 07 cation per formula unit that corresponds to a minimum pressure of 7 7 gpa 250 km depth we also deduce from the observation of opx being the majority of intracrystalline precipitates and 100 of the interstitial ones that the precursor majoritic garnet probably originated from a pressure less than 10 gpa 300 km depth a multistage decomposition hypothesis is proposed for this wgr majoritic garnet during exhumation of the peridotite from 250 to 300 km depth to explain the topotaxy and chemistry of the exsolved pyroxenes

microdiamond in high grade metamorphic rocks of the western gneiss region norway three grains of microdiamond were recovered from high grade gneiss exposed in the western gneiss region norway identification and characterization of the diamond grains by raman and infrared spectroscopy indicate the presence of substitutional impurities of h and n primary fluid inclusions in garnet and quartz in the diamond bearing rock demonstrate the evolution of metamorphic volatile fluids from reduced n 2 co 2 compositions during the peak phase of metamorphism to n 2 ch 4 h 2 o bearing compositions during retrograde metamorphism compatible geologic petrologic and fluid composition data imply a metamorphic origin for the microdiamonds if so the metamorphic and fluid conditions recorded by the microdiamonds and gneissic host may be applicable to microdiamond investigations in other high pressure regionally metamorphosed orogens

neotectonic development of western nicaragua this study presents a reconstruction of the neotectonic evolution of western nicaragua by means of structural analysis of outcrop scale joints faults and bedding planes integrated with macroscale structures the results indicate three clearly separable deformation phases which are closely related to specific volcanic events and account for the major tectonic features of nicaragua the first deformation phase was a late miocene to early pliocene shortening that produced the pacific plain anticlines and controlled the contemporaneous extrusion of the el coyol group volcanic rocks the second phase active between the pliocene and the pleistocene resulted in extensional structures such as the 500 km long and 50 km wide nw se trending nicaragua depression this phase was accompanied by a gradual trenchward migration of the volcanic activity responsible for the formation of the las sierras group the third deformation phase is still active and is responsible for the present shallow focus seismic activity associated with n s trending volcanic vents and the n s managua pull apart graben the neotectonic development of western nicaragua as interpreted here is closely related to the contemporaneous evolution of the neighboring areas of guatemala south of the jocotan fault honduras el salvador and costa rica north of the valle central the valle central of costa rica represents the southern boundary of this structurally homogeneous domain and may indicate the southern boundary of the neogene chortis block changes in deformation style and the trenchward migration of volcanism which occurred during the pliocene may indicate an increase in the subduction angle of the cocos plate and may also be correlated to pliocene shortening of jamaica and hispaniola

erratum to the solubility of tio2 in olivine implications for the mantle wedge environment chemical geology 160 1999 357 370

cation deficient phlogopitic mica exsolution in diopside from garnet peridotite in sulu china article i nfo deep subduction and return to the surface of sediments and other continental rocks is now well known forming terranes of ultrahigh pressure metamorphism uhpm a common constituent of these terranes is garnet peridotite mantle rock that has been brought to the surface along with the returning subducted material microstructures of these peridotites especially precipitates within their constituent minerals offer promise of providing information not only about their depth of origin but also about the subduction environment here we report exsolution lamellae of cation deficient phlogopitic mica within diopside of a drill core from the uhpm terrane of sulu china detailed analysis by transmission electron microscopy tem shows these precipitates to be phlogopitic mica that is highly complex both structurally and compositionally with a significant component of the 10 a phase a talc like phase stable only at quite high pressures several mica polytypes are present the simplest one 1m being the most cation deficient the presence of a significant 30 10 a phase component in the mica suggests a means of expanding the 10 a phase bridge for subduction of h2o to deep mantle depths published by elsevier b v

experimental studies of mineralogical assemblages of metasedimentary rocks at earth s mantle transition zone conditions metasedimentary rocks a major component of the continental crust are abundant within ultra high pressure uhp metamorphic terranes related to continental collisions the presence of diamond coesite and relics of decompressed minerals in these rocks suggests that they were subducted to a depth of more than 150 250 km reconnaissance experiments at 9 12 gpa and 1000 1300 c on composi tions corresponding to felsic rocks from diamond bearing uhp terranes of germany and kazakhstan show that at higher pressures they consist of majoritic garnet al na rich clinopyroxene stishovite solid solution of kalsi3o8 naalsi3o8 hollandite topaz oh and tio2 with a pbo2 structure comparison of our data with experiments conducted by others at similar p t conditions shows differences which are due to variations in bulk chemistry and the type of starting material gel oxides minerals these differences may affect correct establishment of the point of no return of subducted continental lithologies this paper discusses the implication of the experimental data with regard to naturally existing uhp metamorphic rocks and their significance for our understanding of the deep subduction of continental material

focused ion beam technique and transmission electron microscope studies of microdiamonds from the saxonian erzgebirge germany a focused ion beam of ga ions is a relatively new technique that has been developed for microelectronic industries now researchers of the earth sciences find it to be a promising tool for studying various geological materials using the fib technique and an fei strata db 235 dual beam system we have successfully prepared several electrontransparent foils which crossed wm sized diamonds included in host minerals such as zircon and garnet from quartzofeldspathic rocks of the saxonian erzgebirge germany scanning and transmission electron microscopy applied to these foils revealed that the diamonds contain crystalline nanometric inclusions these inclusions consist of minerals of known stoichiometries such as sio2 and al2sio5 whereas others are characterized by different combinations of si k p ti and fe in the presence of oxygen stoichiometries are not clear at this stage of research one suite of inclusions is assumed to be represented by archerite kh2po4 which is known to be stable at pressures of 4 22 gpa and one nanocrystal containing pb oxygen and carbon is interpreted to be pbxoy or pbco3 along with solid crystalline inclusions the diamonds contain cavities filled by liquid gas that escaped during sample preparation these are associated with dislocations of diamond growth our data are consistent with the concept of diamond crystallization from a coh rich multicomponent supercritical fluid and suggest that the composition of such a fluid is more consistent with a local crustal source rather than that of a mantle origin 2003 elsevier science b v all rights reserved

phase relations on the diopside jadeite hedenbergite join up to 24 gpa and stability of na bearing majoritic garnet abstract phase relations on the diopside di hedenbergite hd jadeite jd system modeling mineral associations of natural eclogites were studied for the compositions mol di70jd30 di50jd50 di30jd70 di20hd80 and di40hd10jd50 using a toroidal anvil with hole 7 gpa and a kawai type 6 8 multianvil apparatus 12 24 gpa we established that di hd and jd form complete series of solid solutions at 7 gpa and melting temperatures of pure di 1980 c and jd 1870 c for that pressure were estimated experimentally the melting temperature for the di50jd50 composition at 15 5 gpa is 2270 c the appearance of garnet is clearly dependent on initial clinopyroxene composition at 1600 c the first garnet crystals are observed at 13 5 gpa in the jadeite rich part of the system di30jd70 whereas diopside rich starting material di70jd30 produces garnet only above 17 gpa the proportion of garnet increases rapidly above 18 gpa as pyroxene dissolves in the garnet structure and pyroxene free garnetites are produced from diopside rich starting materials in all experiments garnet coexists with stishovite st at a pressure above 18 gpa pyroxene is completely replaced by an assemblage of majorite maj st casio3 perovskite ca pv in ca rich systems whereas maj is associated with almost pure jd up to a pressure of 21 5 gpa above 22 gpa maj and st are associated with naalsio4 with calcium ferrite structure cf we established that an hd component also spreads the range of pyroxene stability up to 20 gpa in the di70jd30 system at 24 gpa an assemblage of maj ca pv mgsio3 with ilmenite structure mg il was obtained the experimentally established correlation between na si and al contents in maj and pressure in grt maj pyroxene assemblages may be the basis for a majorite geobarometer the results of our experiments are applicable to the upper mantle and the transition zone of the earth 400 670 km and demonstrate a wide range of transformations from eclogite to perovskite bearing garnetite in addition the mineral associations obtained from the experiments allowed us to simulate parageneses of inclusions in diamonds formed under the conditions of the transition zone and the lower mantle

ultradeep 300 kilometers ultramafic xenoliths petrological evidence from the transition zone the seismologically delineated transition zone at depths between 400 and 670 kilometers is a fundamental discontinuity in the earth that separates the upper mantle from the lower mantle xenoliths from within or close to the transition zone are dominated by pyropic garnet and associated pyroxene or mineralogically heterogeneous garnet lherzolite these xenoliths show evidence for the high pressure 90 to 120 kilobars transformation of pyroxene to a solid solution of pyroxene in garnet majorite and silicon in octahedral coordination low pressure less than 80 kilobars exsolution of clinopyroxene or orthopyroxene from the original majorite is preserved although mineral modes and rock proportions below the transition zone and the relative amount of eclogite present cannot be accurately assessed from the xenoliths it is likely that both majorite and beta spinel help produce the observed seismic gradient of the transition zone

tectonic interpretation of the morphology of honduras the morphotectonics of honduras have been studied using existing geologic and topographic maps and imaging done from satellites and the following interpretations have been made during the late oligocene to early miocene a wide strike slip boundary existed in western honduras and eastern guatemala and produced east west trending basins containing the coarse clastic rocks of the subinal formation marine geophysical studies of the continental shelf of honduras show that during the rest of the miocene and the early pliocene the boundary was inactive but motion resumed in the late pliocene to pleistocene and produced horsts on the north coast of honduras in northwestern honduras the east northeast trending alignment of the jocotan copan and charnelecon river valleys may result from crustal flexuring rather than from faulting as previously proposed because the postulated fault the jocotan chamelecon fault cannot be reconciled with available geologic maps the northwest and northeast flowing rivers of central and eastern honduras define domains of strike slip faulting that have probably rotated 30 from an original set of conjugate fractures widespread deposition of conglomerates suggests that strike slip movements began in the mid cretaceous and reoccurred at intervals culminating in the development of pull apart basins in western honduras during the late pliocene at the present time no further strike slip movement is possible as the faults defining each domain lie parallel and perpendicular to the middle america trench new conjugate fractures however cut honduras along north south and east northeast trends and have also separated into domains in the domain of north south fractures easterly directed ductile extension of the lithosphere beneath western honduras and guatemala is opening up rifts but the domain of east northeast fractures is characterized by strike slip motion that has produced left lateral pull aparts in central honduras early workers identified no rift valley extending from the gulf of fonseca to the caribbean and there is no fresh evidence upon which to base the existence of one today the notion of the honduras depression which has persisted over the last half century stems from misinterpretation of the maps and writings of karl sapper

the geometrical evolution of normal fault systems abstract the purpose of this paper is to examine the kinematic behaviour of normal fault systems and see what general conditions govern their geometrical evolution we pay particular attention to seismological and surface data from regions of present day active normal faulting as the instantaneous three dimensional geometry at the time of fault movement is better known in active regions than in areas where the faults are now static most normal faults are concave upward or listric this shape can be produced by geometric constraints either because the faults reactivate curved thrusts or because they must be curved to accommodate rotations another effect which will produce curved faults is the variation of rheology with depth brittle failure at shallow depths produces less fault rotation than does distributed creep in the lower part of the crust an important geometric feature of normal faulting is the uplift of the footwall the amount of such uplift is related not only to the elastic properties of the lithosphere but also to the throw and dip of the fault a striking feature of active normal faults is that they occur in groups in which all the faults dip in the same direction this behaviour arises because the faults cannot intersect if they do one must cease to be active the rotation which such fault systems produce reduces the dip of the faults until a new steeply dipping fault is formed once a new fault cuts pre existing faults the earlier faults become locked and a new set of faults must propagate rapidly across the whole region involved many of these geometric constraints also apply to thrust faulting

diffuse co2 efflux from iwojima volcano izu ogasawara arc japan abstract iwojima volcano located on the southernmost part of the izu ogasawara arc is characterized by the extrusion of trachyte or trachy andesite lavas and pyroclastic rocks of holocene and surface thermal manifestations small phreatic explosions have been recorded frequently during the last 100 years with the most recent in 1999 and 2001 in order to elucidate the behavior of volcanic volatiles and to assess the potential activity of this volcano diffuse co 2 efflux co 2 content and 13 c co 2 in soil gas and soil temperature at 30 cm depth were measured at 272 sites in march 2000 112 sites in december 2000 and 40 sites in december 2001 we found that high co 2 efflux values of more than 100 g m 2 day 1 occurred at several locations on motoyama volcano corresponding with high soil temperatures more than 60 c at 30 cm depth region and with areas where co 2 with magmatic 13 c was observed here the magmatic 13 c determined for fumarolic co 2 data ranged from 2 to 3 which is clearly higher than magmatic gas values 8 to 2 typically found in island arc settings around the world however this can be explained in terms of carbon isotope fractionation between calcite and co 2 under subsurface temperature and pressure conditions at iwojima a total efflux of co 2 for iwojima volcano is estimated to be 760 t day 1 with a magmatic contribution of about 450 t day 1 this value is rather high compared with other volcanoes in island arc settings since iwojima has no visible plume almost all volcanic co 2 is released as diffuse efflux through the volcanic edifice

an experimental investigation of the pyroxene garnet transformation in a pyrolite composition and its bearing on the constitution of the mantle abstract the pyroxene garnet transformation in a model pyrolite composition has been investigated over the pressure interval 4 0 to 17 5 gpa at temperatures of 1200 1400 c the transformation is largely completed in the pressure interval 14 16 gpa and a single phase aluminum deficient garnet majorite becomes stable above 16 gpa the mineral assemblages and the density and seismic velocity distributions for pyrolite are calculated as a function of depth on the basis of this and other recent experimental data density and seismic velocity profiles calculated for pyrolite are consistent with models based on seismic observations to a depth of 550 km if the existence of a high density velocity gradient between 550 km and 650 km is confirmed the presence of further phase transformations in majorite and or some degree of chemical heterogeneity in this depth interval would be indicated

the solubility of tio2 in olivine implications for the mantle wedge environment abstract one characteristic of many subduction zone garnet peridotites is that they contain titanium bearing phases not otherwise found in mantle rocks in particular titanoclinohumite tich and or its breakdown assemblage consisting of symplectic intergrowths of olivine and ilmenite is common in many of these bodies the alpe arami garnet lherzolite of the lepontine alps while lacking tich displays instead large numbers of fetio 3 rod shaped precipitates in the oldest generation of olivine amounting to approximately 1 by volume indicating that at some time in its past the peridotite experienced conditions under which the solubility of tio 2 in olivine was greater than 0 6 wt in order to test the hypothesis that the environment of very high solubility of tio 2 in olivine is to be found at very high pressures we have conducted experiments on lherzolite compositions with added ilmenite at pressures between 5 and 12 gpa and temperatures of 1350 1700 k our results on anhydrous compositions show that whereas solubility of tio 2 was not detected in olivine at 5 gpa 1400 k where it coexists with rutile when rutile disappeared from the paragenesis the solubility climbed to 0 5 wt at 8 and 10 gpa and to 1 wt at 12 gpa all at 1600 k these results support our previous interpretations from titanate morphology and abundance that the alpe arami massif has surfaced from depths 300 km corresponding to pressures 10 gpa but remove the need to suggest a deeper origin and possible precursor phase such as wadsleyite they also support the hypothesis that garnet peridotites with unusual ti bearing phases reflect a unique mantle environment occurring in the mantle wedge overlying subduction zones 2000 elsevier science b v all rights reserved

horizontal resolution of multichannel analysis of surface waves abstractthe multichannel analysis of surface wave masw method has been effectively and widely used to determine near surface shear wave velocity horizontal resolution of the masw method represents the minimum horizontal length of recognizable geologic anomalous bodies on a pseudo 2d s wave velocity vs section accurately assessing the achievable lateral resolution is one of the main issues in lateral variation reconstruction using the masw method it is difficult to quantitatively estimate the horizontal resolution of the masw method because of the many influencing factors such as parameters of the observation system the depth of an anomalous body and the velocity contrast between the anomalous body and the surrounding rocks we first analyzed the horizontal resolution of the masw method based on numerical simulation experiments according to different influencing factors of the horizontal resolution we established different laterally heterogeneous models and observation systems and then simulated

multichannel analysis of surface waves to map bedrock in many geologic settings topographic variations and discontinuities in the surface of bedrock can influence the transport and eventual fate of contaminants introduced at or near the ground surface determining the nature and location of anomalous bedrock can be an essential component of hydrologic characterization preliminary analysis of the hydrologic characteristics of a site in olathe kansas u s based primarily on borehole data alone suggested that a cluster of fractures and or an unmapped buried stream channel may influence fluid movement along the drill defined bedrock surface

geological petrochemical and metallogenic characteristics of the costa rican gold belt contribution to new explorations the costa rican gold belt is located in the northwestern part of costa rica the geology of this region is characterized by tectonically juxtaposed volcanic sedimentary and igneous rock units of tertiary age

sources and flux of natural gases from mono lake california abstract the ability to identify a formation mechanism for natural gas in a particular environment requires consideration of several geochemical factors when there are multiple sources present four primary sources of methane have been identified in mono lake two of these sources were associated with numerous natural gas seeps which occur at various locations in the lake and extend beyond its present boundary the two other gas sources result from current microbiological processes in the natural gas seeps we observed flow rates as high as 160 moles ch4 day 1 and estimate total lakewide annual seep flux to be 2 1 106 moles ch4 geochemical parameters 13ch4 dch4 ch4 c2h6 c3h8 and 14ch4measurements revealed that most of the seeps originate from a paleo biogenic 13ch4 about 70 natural gas deposit of pleistocene age which underlies the current and former lakebed gas seeps in the vicinity of hot springs had in combination with the biogenic gas a prominent thermogenic gas component resulting from hydrothermal alteration of buried organic matter current microbiological processes responsible for sources of natural gas in the lake included pelagic meth anogenesis and decomposition of terrestrial grasses in the littoral zone methanogenesis in the pelagic sediments resulted in methane saturation 2 3 mm at 50 cm 13ch4 about 85 interstitial sulfate decreased from 133 mm at the surface to 35 mm by 110 cm depth indicating that sulfate reduction and methanogenesis operated concurrently methane diffused out of the sediments resulting in concentrations of about 50 m in the anoxic bottom waters methane oxidation in the oxic anoxic boundry lowered the concentration by 98 but values in surface waters 0 1 1 3 m were supersaturated with respect to the atmosphere the 13ch4 range 21 8 to 71 8 of this unoxidized residual methane was enriched in 13c relative to methane in the bottom water and sediments average outward flux of this methane was 2 77 107 moles yr 1 a fourth but minor source of methane 13ch4 55 2 was associated with the decomposition of terrestrial grasses taking place in the lake s recently expanded littoral zone

dynamic site characterization and correlation of shear wave velocity with standard penetration test n values for the city of agartala tripura state india seismic site characterization is the basic requirement for seismic microzonation and site response studies of an area site characterization helps to gauge the average dynamic properties of soil deposits and thus helps to evaluate the surface level response this paper presents a seismic site characterization of agartala city the capital of tripura state in the northeast of india seismically agartala city is situated in the bengal basin zone which is classified as a highly active seismic zone assigned by indian seismic code bis 1893 indian standard criteria for earthquake resistant design of structures part 1 general provisions and buildings according to the bureau of indian standards new delhi 2002 it is the highest seismic level zone v in the country the city is very close to the sylhet fault bangladesh where two major earthquakes m w 7 have occurred in the past and affected severely this city and the whole of northeast india in order to perform site response evaluation a series of geophysical tests at 27 locations were conducted using the multichannel analysis of surface waves masw technique which is an advanced method for obtaining shear wave velocity v s profiles from in situ measurements similarly standard penetration test spt n bore log data sets have been obtained from the urban development department govt of tripura in the collected data sets out of 50 bore logs 27 were selected which are close to the masw test locations and used for further study both the data sets v s profiles with depth and spt n bore log profiles have been used to calculate the average shear wave velocity v s30 and average spt n values for the upper 30 m depth of the subsurface soil profiles these were used for site classification of the study area recommended by the national earthquake hazard reduction program nehrp manual the average v s30 and spt n classified the study area as seismic site class d and e categories indicating that the city is susceptible to site effects and liquefaction further the different data set combinations between v s and spt n corrected and uncorrected values have been used to develop site specific correlation equations by statistical regression as v s is a function of spt n value corrected and uncorrected considered with or without depth however after considering the data set pairs a probabilistic approach has also been presented to develop a correlation using a quantile quantile q q plot a comparison has also been made with the well known published correlations for all soils available in the literature the present correlations closely agree with the other equations but comparatively the correlation of shear wave velocity with the variation of depth and uncorrected spt n values provides a more suitable predicting model also the q q plot agrees with all the other equations in the absence of in situ measurements the present correlations could be used to measure v s profiles of the study area for site response studies

the degassing character of a young volcanic center cerro negro nicaragua cerro negro volcano is a young basaltic cinder cone which is part of the nicaraguan volcanic arc eruptive activity at cerro negro is characterized by explosive strombolian to subplinian eruptions driven by volatile rich basaltic magma ascending rapidly from various crustal depths 15 to 6 km resulting in the onset of precursory activity only 30 min before an eruption in this paper we present a comprehensive degassing characterization of the volcano over a 4 year period aimed at improving our understanding of the magmatic plumbing network and its relationship with regional tectonics a total of 124 individual soil gas samples were collected between 2010 and 2013 and analyzed for stable carbon isotopes 13c from co2 high temperature fumaroles were sampled for 18o d and 3he 4he isotope analysis and major degassing zones were mapped using soil co2 flux measurements gases at cerro negro are characterized by a strong 3he 4he mantle signature 6 3 to 7 3 ra magmatic 13c ratios 2 3 to 3 0 meteoric 18o and d ratios and stable co2 fluxes 31 t d 1 the lack of 13c fractionation and an increase in the relative mantle component from 2002 to 2012 suggest that the volatile flux at cerro negro originates from the mantle and ascends to the surface via a series of crustal fractures that act as permeable conduits despite the lack of new eruptions the hydrothermal system of cerro negro continues to evolve due to seasonal inputs of meteoric water slope failures that expose and bury sites of active degassing and bursts of regional seismicity that have the potential to open up new conduits for gas release as well as magma continuing geophysical and geochemical monitoring of the main edifice and the recently formed south zone is essential as the volcano remains overdue to erupt

light hydrocarbons in hydrothermal and magmatic fumaroles hints of catalytic and thermal reactions volcanic gaseous mixtures emitted from active volcanoes frequently show variable amounts of saturated alkanes unsaturated alkenes and aromatic volatile hydrocarbons three major patterns of distributions can be recognized apparently related to the chemical physical environment of formation of the gas exhalations alkane rich low temperature gas emissions from recently active volcanic areas aromatic rich hydrothermal manifestations and alkene rich magmatic fumaroles on active volcanoes thermodynamic data together with theoretical and practical findings from the petroleum industry point to two main types of reactions occurring in these volcanic environments cracking and reforming cracking processes mainly caused by thermal effects occur when hydrocarbon bearing hydrothermal fluids enter and mix with a hot and dry rapidly rising magmatic gas phase the most probable products are light alkenes with carbon numbers decreasing with increasing reaction temperatures the presence of aromatic species in hydrothermal fluids can be linked to reforming processes catalysed by several possible agents such as smectites and zeolites generally present in the hydrothermally altered volcanic terranes and facilitated by long residence times in a hydrothermal envelope

ultradeep greater than 300 kilometers ultramafic upper mantle xenoliths geophysical discontinuities in earth s upper mantle and experimental data predict the structural transformation of pyroxene to garnet and the solid state dissolution of pyroxene into garnet with increasing depth these predictions are indirectly verified by omphacitic pyroxene exsolution in pyropic garnet bearing xenoliths from a diamondiferous kimberlite conditions for silicon in octahedral sites in the original garnets are met at pressures greater than 130 kilobars placing the origin of these xenoliths at depths of 300 to 400 kilometers these ultradeep xenoliths support the theory that the 400 km seismic discontinuity is marked by a transition from peridotite to eclogite

a new stratigraphy for the tertiary volcanic rocks of the nicaraguan highland three major volcanic events have occurred in nicaragua since middle tertiary time first the oligocene was dominated by rhyolite shield volcanism producing the highland ignimbrite second creation of northeast and later northwest striking dilatational fracture zones allowed the extrusion of basaltic to andesitic magmas along the pacific coast constructing the northwest oriented coyol volcanic arc this miocene arc can be followed all along the pacific coast of the chortis block third a pliocene pleistocene southwestward shift of arc volcanism created the modern volcanic arc this volcanic history is supported by a new stratigraphy based on volcanic centers and related facies the old cyclic layer cake stratigraphy of the coyol group has been abandoned

light hydrocarbons as redox and temperature indicators in the geothermal field of el tatio northern chile abstract el tatio northern chile one of the largest geothermal fields of south america is presently undergoing a new program of geothermal exploration after the failure of the first exploration phase in the early 1970s the geochemical features of the fluid discharges characterizing this system mainly consist of boiling pools and fumaroles and represent the result of a complex mixing process involving 3 main components i hydrothermal ii atmospheric iii magmatic chemical reactions involving light hydrocarbons equilibrate at higher temperature than those directly measured in the geothermal wells and calculated on the basis of the composition of the inorganic gas species this suggests that in the deeper parts of the hydrothermal system temperatures higher than 300 c may be achieved such results can have a strong impact for the evaluation of the potential resources of this geothermal system moreover the chemical characteristics of the organic gas fraction allow the assessment of the chemical physical conditions governing the geochemical processes acting on geothermal fluids at depth

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

the selection of field acquisition parameters for dispersion images from multichannel surface wave data the accuracy and resolution of surface wave dispersion results depend on the parameters used for acquiring data in the field the optimized field parameters for acquiring multichannel analysis of surface wave masw dispersion images can be determined if preliminary information on the phase velocity range and interface depth is available in a case study on a fill slope in hong kong the optimal acquisition parameters were first determined from a preliminary seismic survey prior to a masw survey field tests using different sets of receiver distances and array lengths showed that the most consistent and useful dispersion images were obtained from the optimal acquisition parameters predicted the inverted s wave velocities from the dispersion curve obtained at the optimal offset distance range also agreed with those obtained by using direct refraction survey

caribbean tectonics and relative plate motions during the last century three different ways of interpreting the tectonic evolution of the gulf of mexico and the caribbean have been proposed taking into account the bailey willis school of a permanent pre jurassic deep sea basin the edward suess school of a subsided continental terrain and the alfred wegener school of continental separation the present investigation is concerned with an outline of an interpretation which follows that of pindell and dewey 1982 an attempt is made to point out ways in which the advanced hypotheses can be tested the fit of africa north america and south america is considered along with aspects of relative motion between north and south america since the early jurasic attention is given to a framework for reconstructing caribbean plate evolution the evolution of the caribbean the plate boundary zones of the northern and southern caribbean and the active deformation of the caribbean plate

fumarolic gases at mombacho volcano nicaragua presence of magmatic gas species and implications for volcanic surveillance mombacho is a deeply dissected volcano belonging to the quaternary volcanic chain of nicaragua the southern historic collapse crater el crater currently hosts a fumarolic field with a maximum temperature of 121 c chemical and isotopic data from five gas sampling field campaigns carried out in 2002 2003 and 2005 highlight the presence of high temperature gas components e g so2 hcl and hf which indicate a significant contribution of juvenile magmatic fluids to the hydrothermal system feeding the gas discharges this is strongly supported by the mantle derived helium and carbon isotopic signatures although the latter is partly masked by either a sedimentary subduction related or a shallow carbonate component the observed chemical and isotopic composition of the mombacho fluids seems to indicate that this volcanic system although it has not experienced eruptive events during the last centuries can be considered active and possibly dangerous in agreement with the geophysical data recorded in the region systematic geochemical monitoring of the fumarolic gas discharges coupled with a seismic and ground deformation network is highly recommended in order to monitor a possible new eruptive phase

bookshelf faulting in nicaragua oblique subduction at a high rate of convergence along much of the middle america trench results in northwest directed trench parallel block motion accommodation of this motion along northwest striking dextral strike slip faults has been postulated however in nicaragua such faults are not well developed we suggest instead that this motion is accommodated by bookshelf faulting that includes northeast striking left lateral faults we present earthquake epicenter and focal mechanism data and mapped fracture and fault data consistent with this model trenchward migration of the volcanic arc since the miocene and reactivation of northeast striking miocene structures may have led to the development of this arc and trench normal fault system

spatial variability of the depth of weathered and engineering bedrock using multichannel analysis of surface wave method in this paper an attempt has been made to evaluate the spatial variability of the depth of weathered and engineering bedrock in bangalore south india using multichannel analysis of surface wave masw survey one dimensional masw survey has been carried out at 58 locations and shear wave velocities are measured using velocity profiles the depth of weathered rock and engineering rock surface levels has been determined based on the literature shear wave velocity of 330 30 m s for weathered rock or soft rock and 760 60 m s for engineering rock or hard rock has been considered depths corresponding to these velocity ranges are evaluated with respect to ground contour levels and top surface levels have been mapped with an interpolation technique using natural neighborhood the depth of weathered rock varies from 1 m to about 21 m in 58 testing locations only 42 locations reached the depths which have a shear wave velocity of more than 760 60 m s the depth of engineering rock is evaluated from these data and it varies from 1 m to about 50 m further these rock depths have been compared with a subsurface profile obtained from a two dimensional 2 d masw survey at 20 locations and a few selected available bore logs from the deep geotechnical boreholes

chemical composition of fumarolic gases and spring discharges from el chichon volcano mexico causes and implications of the changes detected over the period 1998 2000 abstract since the march april 1982 eruption of el chichon volcano intense hydrothermal activity has characterised the 1 km wide summit crater this mainly consists of mud and boiling pools fumaroles which are mainly located in the northwestern bank of the crater lake during the period 1998 2000 hot springs and fumaroles discharging inside the crater and from the southeastern outer flank agua caliente were collected for chemical analyses the observed chemical fluctuations suggest that the physico chemical boundary conditions regulating the thermodynamic equilibria of the deep rock fluid interactions have changed with time the chemical composition of the lake water characterised in the period 1983 1997 by high na cl ca 2 and so 4 2 contents experienced a dramatic change in 1998 1999 turning from a na cl to a ca 2 so 4 2 rich composition in june 2000 a relatively sharp increase in na and cl contents was observed at the same time so 2 h 2 s ratios and h 2 and co contents in most gas discharges increased with respect to the previous two years of observations suggesting either a new input of deep seated fluids or local variations of the more surficial hydrothermal system migration of gas manifestations enhanced number of emission spots and variations in both gas discharge flux and outlet temperatures of the main fluid manifestations were also recorded the magmatic hydrothermal system of el chichon is probably related to interaction processes between a deep magmatic source and a surficial cold aquifer an important role may also be played by the interaction of the deep fluids with the volcanic rocks and the sedimentary limestone and evaporites basement the chemical and physical changes recorded in 1998 2000 were possibly due to variations in the permeability of the conduit system feeding the fluid discharges at surface as testified by the migration of gas and water emanations two different scenarios can be put forward for the volcanic evolution of el chichon 1 build up of an infra crater dome that may imply a future eruption in terms of tens to hundreds of years 2 minor phreatic phreatomagmatic events whose prediction and timing is more difficult to constrain this suggests that unlike the diminished volcanic activity at el chichon after the 1982 paroxistic event the volcano hydrothermal fluid discharges need to be more constantly monitored with regular and more frequent geochemical sampling and at the same time a permanent network of seismic stations should be installed