contrasting compositions of mariana trough fallout tephra and mariana island arc volcanics a fractional crystallization link discrete quaternary 400 ka tephra fallout layers mostly 1 cm thick within the siliceous oozes of the central mariana trough at 18 n are characterized by medium k to high k subalkalic volcanic glasses k2o 0 8 3 2 wt with high large ion lithophile elements lile high field strength elements hfse ratios and nb depletion ba la 35 ba zr 3 5 la nb 4 typical for convergent margin volcanic rocks compositional zoning within layers ranges from basaltic to dacitic sio2 48 71 wt mgo 0 7 6 5 wt all layers contain basaltic andesites the tephra layers are interpreted as single explosive eruptive events tapping chemically zoned reservoirs the sources being the mariana arc volcanoes mav due to their proximity 100 400 km and similar element ratios mav ba la 36 7 ba zr 3 5 0 9 the glasses investigated however contrast with the contemporaneous basaltic to dacitic lavas of the mav by being more enriched in tio2 1 2 wt mav 0 8 wt feo 10 wt mav 8 9 wt k2o 1 1 wt mav 0 8 wt and p2o5 0 4 wt mav 0 2 wt semi incompatible trace elements including rare earth elements ree of the basaltic andesitic and dacitic glasses match those of the dacitic mav lavas which became enriched by fractional crystallization moreover the glasses follow a tholeiitic trend of fractionation in contrast to mav transitional trends and have a characteristic p2o5 trend that reaches a maximum of 0 6 wt p2o5 at 57 wt sio2 whereas mav lavas increase linearly in p2o5 from 0 1 to 0 3 wt with increasing silica both explosive and effusive series are interpreted to have evolved in common magma reservoirs by convective fractionation similar parental magmas are suggested to have separated into coexisting si andesitic to dacitic and basaltic melts by in situ crystallization the differentiated melt is interstitial in an apatite saturated crystalline mush of plag px ox ol at the cooler chamber margins in contrast to the less differentiated basaltic to basaltic andesitic magmas which are not yet saturated in apatite and occupy the chamber interior reinjection of interstitial melt into the chamber interior and mixing with larger melt fractions of the interior liquid mixing ratios about 1 8 9 can explain the paradoxical behavior of apatite controlled p and mree variation in the basaltic andesite glasses and their mav dacite like fractionation patterns the process may also account for the exclusively tholeiitic trend of fractionation of the glass shard series but in situ crystallization alone cannot cause their absolute enrichment in semi incompatible elements the newly mixed melt is suggested to form the basaltic end member of the glass shard series however it must have become physically separated from the main mav magma body possibly by density driven convective fractionation in order to allow for further evolution of the contrasting geochemical paths as well as differentiation

trace element transport rates in subduction zones evidence from th sr and pb isotope data for tonga kermadec arc lavas abstract trace element and th sr and pb isotope data for young lavas from the tonga kermadec arc in the southwest pacific suggest that geochemical variations in the lavas along the arc are linked to differences in the material being subducted beneath the arc lavas from the southern kermadec segment of the arc have relatively radiogenic pb isotope compositions which reflects a contribution from subducted sediment in contrast much of the pb in tonga lavas is derived from the altered oceanic crust in the subducting pacific plate and lavas from the northernmost tonga islands of tafahi and niuatoputapu contain pb and sr derived from the subducted part of the louisville seamount chain the origin of the pb in the lavas from these two islands can thus be traced to a point on the subducting slab and this observation is used to estimate the rate at which trace elements are transported beneath the arc our calculations suggest that fluid soluble elements such as u sr and pb are transported from the subducted slab across the mantle wedge and back to the surface in lavas over a period of approximately 2 3 ma and that magmas are erupted at the surface less than 350 ka after the melts are generated in the mantle wedge

changing techniques and paradigms for the evaluation of magmatic processes the study of magmatic processes is at a juncture new continuum models for the complex physics of crystallization and melting with convection and crystal melt segregation are now available and the models are amenable to exploration by numerical methods these models which were largely developed in the fields of material science and mechanical engineering will allow petrologists to consider a wide range of transport phenomena crystal settling with reaction as fully multiphase flow combined thermal and compositional buoyancy with realistic material property variations open system and eruptive behavior and the appropriate system geometry and boundary conditions the traditional notions of magma chambers as large vats of near liquidus material is being replaced by models where a substantial portion of the chamber may consist of a crystal melt mush subject to reintrusion or mobilization percolative flow and reaction in this mush including contaminants may be as important as near liquidus processes in driving petrological diversity thus simple dimensionless numbers are of little utility in describing the vigor or style of magmatic processes melting of the crust following intrusion by basalt can be considered in light of two end members the large sill or tabular mafic magma body and intrusion of basalt as sequences of spatially and temporally over lapping dikes although more commonly cartooned the large sill type may not be nearly as efficient as repeated diking in driving crustal melting magma mixing and mingling the greatest challenge for those studying magma dynamics will be to key the models to well constrained geological examples and to raise the sophistication of the content of these models in light of well defined tests that have geological significance

evolution of geothermal systems around the red sea the current thermal regime around the red sea was initiated by a mantle plume beneath ethiopia eretria yemen and sw of saudi arabia at about 31 ma before the present the evolution of the geothermal systems around the red sea is coeval to the initial onset of volcanism over the separated land masses eritrea ethiopia djibouti yemen saudi arabia and egypt at 14 ma the entire tectono magmatic activities around the red sea gave rise to several geothermal provinces over the continents surrounding the red sea represented by thermal springs and fumaroles at several locations in eretria djibouti ethiopia yemen and saudi arabia these countries have the potential to develop these resources like ethiopia to mitigate co2 emissions in countries like saudi arabia and to enhance the gdp gross domestic product of the economically backward countries the geothermal resources have the potential to make these countries energy independent in future

mariana trough basalts mtb trace element and srnd isotopic evidence for mixing between morb like and arc like melts abstract mariana trough basalt mtb glasses from zones of of active seafloor volcanism have incompatible trace element compositions which are intermediate between normal morb and basaltic rocks from the active northern mariana island arc miab the chemical variation is observed in trace elemental abundances and ratios such as lil lil and lil hfs mtb glasses with high lil hfs and ba sm ratios and low k rb k ba and sm nd ratios have more enriched sr and nd isotopic compositions comparison of the sr nd isotopic compositions of mtb and miab suggests that the source region within the mantle wedge is heterogeneous the diverse trace element and isotopic compositions of mtb glasses both within and between dredge sites near 18 n imply small scale source heterogeneity correlation between sm nd and143nd 144nd of the mtb glasses is interpreted as due to recent binary mixing rather than closed system evolution of a common homogeneous source mixing of melts at or near the source region between a mantle component with long term lree and lil element depletion morb like and a relatively enriched component with lower integrated143nd 144nd arc like is suggested by trends of the mtb data on ratio ratio ratio element and element element plots

geothermal potential of egypt abstract one hundred and sixty samples of groundwater from nearly all parts of egypt have been collected and chemically analyzed in order to assess the country s geothermal potential the samples considered to be thermal include 20 wells t 35 c 4 springs t 30 c and 1 spring not included in the present inventory the remaining samples together with data from the literature establish background chemistry the hottest springs are located along the east shore of the gulf of suez uyun musa 48 c and ain hammam faraoun 70 c additional warm springs are located along both shores of the gulf of suez and this region is the most promising for geothermal development the eastern desert of egypt particularly the coastal area adjacent to the red sea has above normal heat flow 72 0 mw m 2 and therefore some geothermal potential although only one thermal well umm kharga 35 8 c could be located in the major oases of the western desert kharga dakhla farafra and bahariya the regional temperature gradient is low c km but many of the wells tap deep artesian aquifers and produce large volumes of water in the 35 43 c range such wells constitute a low temperature geothermal resource none of our samples in northern egypt can be considered thermal including several reported hot springs application of the silica nakca and nakcamg geothermometers does not indicate the presence of a high temperature geothermal resource at any area we visited

exploration and assessment of the geothermal resources in the hammam faraun hot spring sinai peninsula egypt abstract the tectonic position of egypt in the northeastern corner of the african continent suggests that it may possess significant geothermal resources especially along its eastern margin the most promising areas for geothermal development in the northwest red sea gulf of suez rift system are located where the eastern shore of the gulf of suez is characterized by superficial thermal manifestations including a cluster of hot springs with varied temperatures magnetotelluric and gravity reconnaissance surveys were carried out over the geothermal region of hammam faraun to determine the subsurface electric resistivity and the densities that are related to rock units these surveys were conducted along profiles one dimensional 1d and two dimensional 2d inversion model techniques were applied on the mt data integrating the 2d inversion of gravity data the objectives of these surveys were to determine and parameterize the subsurface source of the hammam faraun hot spring and to determine the origin of this spring based on this data a conceptual model and numerical simulation were made of the geothermal area of hammam faraun the numerical simulation succeeded in determining the characteristics of the heat sources beneath the hammam faraun hot spring and showed that the hot spring originates from a high heat flow and deep ground water circulation in the subsurface reservoir that are controlled by faults these studies were followed by an assessment of the geothermal potential for electric generation from the hammam faraun hot spring the value of the estimated potential is 28 34 mw as the reservoir is assumed to be only 500 m thick this value would be enough for the desalination of water for both human and agricultural consumption

on the statistics of the phase of microwave backscatter from the ocean surface in this paper we describe what we believe is the first study of the distribution of the phase of microwave fields backscattered from the ocean surface scatterometry data from the synthetic aperture radar and x band ocean nonlinearities experiment conducted on the german north sea research platform forschungsplatform nordsee in november 1990 have been analyzed to reveal the distribution of phase differences as a function of time lag a theoretical model for these statistics is presented based on modulated gaussian fields this theoretical model is shown to be in good agreement with the measured statistics from this agreement we conclude that the backscattered fields have a gaussian distribution on short time scales but are modulated in amplitude and frequency by the long surface waves these results are of more than purely academic interest with direct applications to the design and analysis of interferometric synthetic aperture radars a relatively new class of instruments that may be capable of providing high resolution maps of ocean surface currents from aircraft or satellites

230 th 238 u disequilibrium in island arcs evidence from the aleutians and the marianas complex models for magma generation in island arcs have recently been proposed which require varying contributions from three possible sources 1 the mantle 2 subducted altered oceanic crust and 3 sediments see for example refs 1 6 these models involve partial melting of metasomatized mantle the metasomatizing liquids derive from the subducted slab and are large ion lithophile lil element bearing fluids or are themselves partial melts further modification of the chemical composition of the lavas may occur by processes such as shallow level fractional crystallization because the th u ratio is quite different for each of the three possible magma sources mentioned above and as th and u may behave differently during fractionation processes particularly if volatile transport is involved studies of 230 th 238 u disequilibrium have great potential in investigations of arc lavas our data given here imply that the th u ratio in the sources of mariana and aleutian island arc lavas is similar to ocean ridge basalt sources although there is a hint of a somewhat lower th u component in the aleutian source the data also confirm and extend the observations by gill and allegre and condomines that oceanic arc lavas are distinguished from magmas erupted in virtually all other volcanic environments by activity ratios 230 th 238 u 1 implying relative enrichment of uranium

episodic development of a convergent margin inferred from structures and processes along the southern chile margin seismic reflection data acquired in the vicinity of isla mocha across the southern coast of chile image structures formed along the continental margin and reveal an episodic history of accretion nonaccretion and possibly erosion structures formed at the toe of the continental slope suggest frontal accretion of to 1 km of trench fill seismic images also reveal that a small accretionary wedge 20 30 km wide abuts the truncated continental metamorphic basement that extends seaward from beneath the shelf the small size of the accretionary wedge on three profiles examined here is not consistent with a long history of accretion with the current deformational style as current rates of frontal accretion could have accumulated all of the existing accretionary wedge in less than 1 2 m y this is a small fraction of convergence history along this margin and the current accretionary mode has not been consistently maintained in the past the isla mocha region is located between the temperate climate of central chile and the glacial climate of southern chile and climatic conditions in this region have likely fluctuated sufficiently to cause significant variation in trench sediment supply accretionary and nonaccretionary or erosional episodes are probably linked to temporal variations in trench sediment thickness as suggested by observations along the chile margin currently thick trench sediment correlates with accretion along the southern chile margin and thin trench sediment correlates with nonaccretion tectonic erosion as near the chile ridge and from the juan fernandez ridge to northern chile the isla mocha region also lies 900 1000 km north of the chile triple junction and the chile ridge lies approximately 2000 km to the west and has not yet collided and affected the margin near isla mocha this part of the precollision zone provides an excellent reference to examine the effects of chile ridge collision in the development of the chile margin the most apparent effect of subduction of the buoyant young crust of the chile ridge is a shallow trench that is nearly devoid of sediment consequently the triple junction is undergoing nonaccretion or erosion and the accretionary complex near the triple junction remains smaller than to the north or south because the current phase of rapid accretion elsewhere in the trench has bypassed the triple junction region the interplay of subduction zone processes such as trench sedimentation and ridge collision has resulted in an episodic development of the margin and produced a discontinuous record of convergence history within the accretionary wedge

geology and geochemistry of gunung subang gold deposit tanggeung cianjur west java indonesia abstract the gunung subang gold deposit is located in the sunda arc volcanic belt in west java indonesia the ore bodies are associated with volcanic rocks from the miocene koleberes and bentang formation to constrain the significance of the ore genesis and mineralization this study was carried out with the aim of examining the deposit using methods such as detailed field observation petrography petrochemistry zircon u pb dating hf isotope fluid inclusion analysis and sulfur and in situ pb isotope analyses the results suggest the following 1 the volcanic rocks in gunung subang belong to the basalt andesite group with 48 66 79 17 average 61 04 sio2 the trace element patterns of the volcanic rocks exhibit relatively low hree content flat hree patterns and weak negative or positive eu anomalies which are similar to those of typical arc magmatic rocks 2 most zircons selected from the volcanic rocks are magmatic and only a few are inherited the magmatic zircons have a weighted mean u pb age of 17 0 0 4 ma and the inherited zircons are from the precambrian dated at 887 2379 ma the magmatic zircons have ehf t values ranging from 1 24 to 13 59 indicating that the magmatism may be contaminated predominantly with juvenile crust but also with a small percentage of ancient crust 3 the sulfur isotopic compositions are fairly uniform and the 34s values of pyrite galena and sphalerite range from 0 53 to 2 97 suggesting a magmatic origin 4 the pb isotopic compositions of pyrite and galena are clustered and some include relatively high amounts of radiogenic lead the mean ratios of 206pb 204pb 207pb 204pb and 208pb 204pb are 18 59 0 007 15 65 0 007 and 38 89 0 017 respectively indicating that the ore forming material is highly related to miocene to pliocene volcanic rocks 5 fluid inclusions in the quartz from the ores are liquid and vapor liquid phases with components of the nacl h2o system with homogenization temperatures of 240 320 c and salinities of 0 2 6 2 wt nacl the coexistence of vapor and liquid rich fluid inclusions suggests that boiling occurred throughout the deposition of the ore 6 the results of electron probe microanalysis epma indicate that pyrite is the major au bearing mineral combined with the observation of scanning electron microscope sem gold is primarily present as au ag telluride minerals and there may also be native gold au0 finally we consider the gunung subang is an epithermal gold deposit related to the mid miocene magmatism

regional o sr and pb isotope relationships in late cenozoic calc alkaline lavas of the andean cordillera quaternary recent volcanism in the andes has occurred in three regions 45 33 s 28 16 s and 2 s 5 n each of which has a distinct plate tectonic setting and contains volcanic suites with different chemical and isotopic characteristics isotope ratios of o and sr are lowest and those of pb least variable in the southern volcanic zone svz where medium k lavas have isotopic characteristics equivalent to volcanics from intraoceanic arcs where continental crust is absent the svz lavas were probably derived from an asthenospheric mantle source above a shallow benioff zone parental basaltic magmas rose largely unmodified through thin continental crust where differentiation occurred by low pressure crystal fractionation without concurrent modification of isotopic composition the slight enrichment of 206 pb in the northern volcanic zone nvz where the crust is thin and the benioff zone deep suggests a greater subduction zone component for parental medium k magmas the slight 18 o enrichment suggests a small amount of lower crustal interaction isotope ratios of o and sr are highest and those of pb most variable in the central volcanic zone cvz parental magmas in the cvz were probably generated within the same mantle source region as those in the svz and nvz subsequently during transit through the exceptionally thick continental crust of the central andes high k magmas were produced by a combination of bulk contamination in the lower crust and later low pressure fractional crystallization assimilation processes in the upper crust which altered both chemical and isotopic compositions

erosion of coastal drainages in the mendocino triple junction region mtj northern california abstract the mendocino triple junction mtj region in northern california is an archetype for studying landscape response to varying rock uplift rates as they increase toward the triple junction underpinning these studies lies the assumption that this landscape has reached a dynamic equilibrium in which rock uplift and erosion rates are equal however no study has shown that such an equilibrium actually exists around the mtj region here we report 10be and 26al derived erosion rates calculated from isotope concentrations in detrital fluvial sediment from coastal drainage basins we then compare these rates with uplift rates inferred from marine terraces that were formed and preserved by uplift during the last 305 ka erosion rates in the more slowly uplifting southern part of the region range from 0 21 0 32 mm yr and are consistent with rock uplift rates since 305 ka however in the northern transition zone where uplift rates apparently started to increase about 100 ka due to northward migration of the mtj erosion rates are higher 0 43 to 0 69 mm yr these rates are similar to uplift rates from 96 305 ka but substantially less than recent uplift rates of 3 5 4 mm yr inferred for the past 72 ka in the central part of the king range finite erosion rates cannot be determined due to i the presence of excess 10be that does not appear to have originated from in situ cosmic ray production during erosion and transport of the sediment and ii 26al concentrations below detection limits the difference between catchment wide erosion and recent uplift rates in coastal catchments may be related to a lag in response of hillslopes and tributaries to changes in rates of tectonically driven relative base level fall or the uncertainties in rock uplift rates inferred from marine terraces this study suggests that measurements of both erosion and uplift rates are crucial in assessing the equilibrium state of landscapes and in understanding the topographic features made by surface and tectonic processes

geothermal measurements in the northern red sea implications for lithospheric thermal structure and mode of extension during continental rifting the northern red sea is a continental rift in the process of transition from continental to oceanic rifting we present 191 new heat flow measurements from the northern red sea forming three traverses across the water covered portion of the rift the heat flow across the rift systematically increases from values of about 125 mw m2 seaward of the coasts to average values greater than 250 mw m2 in the axial depression the heat flow measurements are evaluated for environmental disturbances these are found to be generally small the largest estimated disturbance results from the relief of the seafloor and of the top of a subbottom evaporite layer the relief on these surfaces can account for the 20 point to point scatter typically observed in the heat flow measurements limits are placed on systematic disturbances to the heat flow pattern across the rift the estimated largest systematic disturbance results from sediment blanketing which may cause a reduction in the heat flow on the order of 10 the heat flow variation across the rift is then utilized to examine the lithospheric thermal structure and the geometry and mechanism of extension we employ a two dimensional time dependent numerical technique to follow the advection and diffusion of heat for a simple shear model and various pure shear models of lithospheric extension the model of simple shear lithospheric extension along a planar shallow dip detachment produces significantly lower than observed heat flow results in relatively small amounts of thermal thinning of the lithosphere compared to pure shear extension and does not result in conditions likely to generate melt the pure shear models with widening or constant width zones of extension also do not match the observed heat flow nor generate partial melt the modeling study indicates that the actively extending region must become narrower through time in order to match the heat flow high in the center of the rift generate some degree of partial melt and produce crustal subsidence within the observed width of the rift a narrowing of the zone of active extension is also consistent with the pattern of tectonic activity inferred from sedimentary structures observed on seismic reflection profiles which indicate an abatement of activity in the marginal areas and intense present day activity in the axial depression in all of the models an additional component of lithospheric heating other than that produced by extension appears necessary to match heat flow values in the marginal and near coastal areas as well as explain the observed magnitude of rift flank uplift several scenarios in which the zone of extension eventually becomes narrow are consistent with the heat flow data alone including those resulting in seafloor spreading for the past several million years however additional information provided by gravity magnetic and seismic reflection and refraction data appear to be most consistent with a history in which a period of relatively uniform extension across the rift is followed by a concentration of extension toward the center of the rift such a model retains a small thickness of continental crust in the center of the rift and does not necessitate active seafloor spreading in the northern red sea the three heat flow traverses indicate that maximum lithospheric thinning is occurring within the axial depression in the northern red sea which is very close to initiating seafloor spreading there is evidence that upper crustal extension is locally concentrated in specific areas along the strike of the axial depression forming deeps associated with large recent intrusions these deeps may constitute a precursory stage to seafloor spreading and appear to be nucleating at discrete and regularly spaced sites along the axis of the rift the deeps are located midway between regularly spaced cross trending fault zones or accommodation zones which in the marginal areas separate sets of fault blocks along strike crustal extension appears to be taken up more diffusely in the accommodation zones than in the intervening fault block domains which are characterized by fewer larger faults as a result initial crustal rupturing and emplacement of large intrusive bodies occurs halfway between the accommodation zones where more focused crustal extension forms the deeps comparison with other rifts suggests that the pattern of fault block domains separated by cross trending fault zones is a common feature that it originates early in the development of a rift and that it may continue to influence the development of the rift up to the establishment of a continuous seafloor spreading axis

u series disequilibrium in arc magmas induced by water magma interaction abstract volcanic rocks from subduction zones are widely believed to originate by partial melting of mantle lherzolite modified by the addition of a fluid or melt extracted from the down going slab u series disequilibrium in such magmas is commonly attributed to this particular melting process a detailed study of u series isotopes in the 650 y b p eruptive sequence of mt pelee martinique shows that plinian products are in radioactive equilibrium whereas dome forming products of the same eruption are characterized by 238u 230th disequilibrium the same features apply to other plinian and dome forming products of this volcano and systematically correspond to different eruptive styles we attribute these characteristics to variable superficial interaction of magmas with the hydrothermal system during the final stages of eruption rather than to deep magma genesis processes this conclusion might be generally applicable to arc magmas

230th 238u radioactive disequilibria in tholeiites from the famous zone mid atlantic ridge 36 50 n th and sr isotopic geochemistry we analyzed u th and230th 232th activity ratios for a few tholeiites from the mid atlantic ridge famous zone at 36 50 n the results show a fairly wide scatter for both th u and 230th 232th ratios seawater contamination appears to be responsible for this scatter and for the uranium produces an increase in content yielding a 234u 238u ratio greater than 1 and for the th an increase of the 230th 232th ratio which is a very sensitive indicator for contamination also the latter often is selective u th and sr are not affected in the same manner when discarding all data for contaminated samples the famous zone appears to be very homogeneous with ath u ratio value of 3 05 and a 230th 232th ratio value of 1 24 comparison with other active volcanic areas reveals a negative correlation between 230th 232th and87sr 86sr ratios for present lavas which is indicative of a consistency in th u and rb sr fractionation in the source regions of these magmas the th isotopic geochemistry can thus provide useful information for the study of present volcanism information as valuable as that from sr pb or nd isotopes

fluid melt rock interaction in mafic eclogites and coesite bearing metasediments constraints on volatile recycling during subduction the question as to whether the metasomatizing slab agent needed to form island arc magmas is an aqueous fluid or a hydrous melt requires a knowledge of fluid melt rock interaction during high pressure low temperature metamorphism in the western alps typical anhydrous mafic eclogites monviso massif and coesite bearing metasediments dora maira massif have experienced a prograde p t path characteristic of a mature cold subduction zone microstructural petrologic geochemical and fluid inclusion studies show that fluid flow was limited during eclogite and high grade blueschist facies metamorphism most of the fluids were driven off the rocks at pressures lower than 1 and 1 6 gpa respectively with increasing pressure and temperature the remaining fluid phase was released during crystal plastic flow processes or devolatilisation reactions but retain the host rocks to form veins partial melts or dense hydrous silicates tectonic erosion at modern convergent margins delivers large amounts of terrigenous sediments kmash system in subduction zones in contrast to the basalt and peridotite systems the kmash system is characterized by a wide range of pressure sensitive reactions which allow the reconstruction of dehydration hydration depths in the subducted slab independently of the thermal structure of subduction zones terrigenous sediments will begin to melt at pressures 2 gpa the melt phase can absorb tremendeous quantities of water and represent major sinks for devolatilisation fluids dense hydrous silicates will form in any subducted sediments at all depths these silicates can store significant amount of water in their structure in the subducted oceanic crust and upper mantle heterogeneous distribution of fluids inherited from hydrothermal alteration at mid ocean ridges will cause contemporaneous dehydration and hydration reactions on a local scale rather than large scale fluid infiltration in the overlying mantle wedge the situation where a cold competent and fluid rich subducted slab is bounded above by a hot relatively weak and anhydrous material is likely to act against buoyant rise of devolatilisation fluids by forcing the fluids to flow downwards at sub arc depths 50 10 to 80150 km and for any geothermal gradient continuous internal buffering of volatile activities will retain the fluid phase in the slab this is in contrast with shallow sub forearc depths 20 to 50 10 km where tectonic vein arrays localized thrust faults serpentine diapirs mud volcanoes and boninite magmas attest to the release of large volume of fluids into the overriding mantle wedge

multiple subduction components in the mantle wedge evidence from eruptive centers in the central southern volcanic zone chile in the andean central southern volcanic zone basalts from small eruptive centers near the large composite center volcan villarrica are poor in fluid mobile elements such as b cs rb k pb ba and u compared with concurrently erupted villarrica basalts new 1 0 be and u series isotopic data for these centers show that fluid mobile element poor small eruptive center basalts have small 1 0 be 9 be ratios 1 6 1 9 10 1 1 and 2 3 8 u 2 3 0 th activity ratios near 1 0 whereas basalts from villarrica show 2 3 8 u enrichment and larger 1 0 be 9 be ratios 4 0 6 4 10 1 1 these results suggest that small eruptive center basalts include materials derived from the subducted lithosphere that were stored in the mantle wedge for 350 k y to 3 m y that these materials are poor in fluid mobile elements may reflect fluid expulsion during solidification or their formation in an initially hotter subduction setting in contrast the composite center basalts sample materials rich in fluid mobile elements that were recently transferred into the mantle wedge from the subducted lithosphere the results confirm that mantle wedges in subduction zones include subducted materials added to the wedge over both long and short time scales

hot stuff under southern chile isotopes of the uranium decay series can be used to explore geological processes that have taken place on and in the earth such an analysis has now been applied to find out more about a slab of oceanic plate being subducted under southwestern chile in effect this work provides an image of the plate at depth and helps to relate plate melting to volcanic activity at the surface

correlation of aerogravity and bht data to develop a geothermal gradient map of the northern western desert of egypt using an artificial neural network the northern part of the western desert of egypt represents the second most promising area of hydrocarbon potential after the gulf of suez province an artificial neural network ann approach was used to develop a new predictive model for calculation of the geothermal gradients in this region based on gravity and corrected bottom hole temperature bht data the best training data set was obtained with an ann architecture composed of seven neurons in the hidden layer which made it possible to predict the geothermal gradient with satisfactory efficiency the bht records of 116 deep oil wells 2 000 4 500 m were used to evaluate the geothermal resources in the northern western desert corrections were applied to the bht data to obtain the true formation equilibrium temperatures which can provide useful constraints on the subsurface thermal regime on the basis of these corrected data the thermal gradient was computed for the linear sections of the temperature versus depth data at each well the calculated geothermal gradient using temperature log data was generally 30 c km with a few local high geothermal gradients in the northwestern parts of the study area explained by potential local geothermal fields the bouguer gravity values from the study area ranged from 60 mgal in the southern parts to 120 mgal in the northern areas and exhibited ne sw and e w trends associated with geological structures although the northern western desert of egypt has low regional temperature gradients 30 c km several potential local geothermal fields were found 40 c km the heat flow at each well was also computed by combining sets of temperature gradients and thermal conductivity data aerogravity data were used to delineate the subsurface structures and tectonic framework of the region the result of this study is a new geothermal gradient map of the northern western desert developed from gravity and bht log data

rapid magma ascent and generation of 230 th excesses in the lower crust at puyehue cordon caulle southern volcanic zone chile basaltic to rhyolitic lavas and tephras erupted over the last 70 kyr at the puyehue cordon caulle volcanic complex in the andean southern volcanic zone svz were analyzed for major and trace element sr isotope and u th isotope compositions to constrain the timescales of magmatic processes and identify the subducted and crustal components involved in magma genesis internal u th mineral isochrons from five lavas and three tephra fall deposits are indistinguishable from their eruption ages indicating a short period b1000 yr of crystal residence in the magma prior to eruption the 230 th 232 th ratios define a narrow range 0 80 0 83 compared to that of all svz lavas 0 72 0 97 suggesting that puyehue basalt was derived from a relatively uniform mantle source dacites and rhyolites have the largest u excesses and likely evolved via fractional crystallization of a plagioclase dominated mineral assemblage in contrast basalts have 1 to 6 230 th excesses a characteristic not previously observed in frontal arc stratovolcanoes of the andean svz the 230 th excesses are interpreted to reflect a relatively small degree of fluid flux melting coupled with assimilation and melting of the lower crust lower crustal processes therefore have dampened the 238 u excesses that were generated during fluid addition to the mantle wedge although prior 238 u 230 th 226 ra studies of lavas from other southern svz stratovolcanoes 36 to 41 s have inferred that slab additions and the extent of mantle melting were nearly constant along strike of the arc our results suggest that mash processes envisioned by hildreth and moorbath w hildreth s moorbath crustal contributions to arc magmatism in the andes of central chile contrib mineral petrol 98 1988 455 489 in the northern svz also occur in the southern svz where the crust is relatively thin

the origin of10be in island arc volcanic rocks the presence of cosmogenic10be t1 2 1 5 106 y in island arc volcanic rocks has been interpreted as indicating that sediments are subducted to the depths of island arc magma genesis 1 2 we have measured10be and9be the stable common isotope in phenocryst and groundmass fractions separated from four aleutian volcanic rocks 10be and9be covary linearly in three of the rocks proving that the10be was incorporated prior to the formation of phenocrysts in the other rock10be and9be do not covary the lack of covariation indicates either that the rock incorporated10be during weathering or that9be rich xenocrysts were incorporated by mechanical mixing our results strongly support the hypothesis of brown et al 1 and tera et al 2 that sediments containing10be are an integral part of island arc magmatic systems as we cannot identify the depth at which the10be was incorporated we still cannot completely rule out the possibility of assimilation of sediments or altered crust near the earth s surface as a source of10be

the multistage tectonic evolution of the gulf of suez and northern red sea continental rift from field observations field surveys in the gulf of suez northern red sea area revealed the structural pattern and evolution of this tertiary continental rift and provided constraints on the continental rifting process the structure has been controlled at every scale by four major trends of faults n0 20 e n100 120 e n140 e 160 e and n40 60 e observations of sealed faults and folds and superposed structures and the detailed analysis of the fault surfaces are coherent and indicate that as least two different stages of deformation characterize the evolution of the rift the initiation of the rift late oligocene aquitanian corresponds to the creation of large rhomboidal basins weakly subsident and partly bounded by oblique strike slip faults this framework which resulted from the reactivation of inherited weakness zone in the basement controlled the subsequent creation of large tilted blocks upper aquitanian lower burdigalian these structures were trangressed by the upper burdigalian deposits then the blocks were sliced by a secondary horst and graben pattern which was progressively sealed by middle miocene units since the upper miocene the deformation has been focused along some major faults located in the central part of the rift the determination of paleostresses from measurements of striae confirms that the initiation of the rift was controlled by a tectonic regime characterized by an horizontal compressive stress parallel to the rift and its subsequent evolution was controlled by triaxial and radial extensional regimes with horizontal minimum stress 3 predominantly trending n70 80 e the rifting process was not induced by regional doming but by a new distribution of intraplate stresses at approximately oligocene miocene time passive rift uplift of the rift shoulders began in the upper burdigalian cross sections of the rift emphasize its asymmetry large basement blocks are tilted toward the edge of the rift where a major recently active fault and a distinct morphological unit which we call a banquette are located several homogeneous tilt direction provinces can be defined they are separated by transverse hinge zones that correspond to inherited basement trends rifting deformation is expressed in the basement by steep faults arranged in a zig zag pattern and in neogene quaternary sediments by steep faults and listric faults associated with decollements therefore the style of shallow deformation cannot be extrapolated to deeper levels structures formed during the beginning of the rift show different charateristics according to their orientation but their initial shape was generally complicated by later deformation

a test of a quartz eclogite source for parental aleutian magmas a mass balance approach the observation that magmatic conduits evolve with time has provided a new framework within which to interpret aleutian compositional and isotopic data accordingly a two stage physical model of aleutian magma genesis has been developed and used to model quantitatively geochemical and isotopic mass balance relations parental magmas approximated by high alumina basalts are presumed to be generated by partial melting of eclogite and subducted sediment the incorporation of sediment produces elevated parental magma sr and pb isotopic ratios but lowered epsilon nd relative to morb if these magmas ascend along well established conduits their isotopic characteristics are little changed during ascent in immature conduits interaction with oceanic lithosphere produces significant isotopic variability according to this model the isotopic and geochemical characteristics of parental magmas are fixed by mixing eclogite and sediment melts as a test we have quantitatively examined the mass balance rel

deep mantle serpentinization in subduction zones insight from in situ b isotopes in slab and mantle wedge serpentinites abstract serpentinization occurs both within the seafloor when seawater percolates through fractures to alter underlying peridotite and in subduction zones when aqueous fluids from the dehydration of the slab are released within the subduction channel the main difference between these two environments is therefore the source of serpentinizing fluid however distinguishing between the various types of serpentinites encountered in suture zones is difficult essentially because of the lack of significant differences in their mineralogy and major and trace element chemistry eight samples from corsica and the western alps representing slab serpentinites i e serpentinites embedded within the subducting slab which are fragments of former abyssal serpentinites and seven samples from guatemala and nicaragua representing mantle wedge serpentinites i e serpentinites that form in situ above the subducting slab by hydration of the overlying mantle wedge have been selected to study their fluid mobile elements ree concentrations and boron b isotopic signatures the trace element results do not allow a conclusion about the source of fluid s but the b isotopic signatures do permit identification of the different fluid sources responsible for the serpentinization the 11b values of slab serpentinites are significantly positive ranging from 12 to 34 in the same range as present day abyssal serpentinites 5 5 to 40 5 which are altered by seawater therefore these positive signatures indicate serpentinization by seawater derived fluid the b isotopic signature acquired during oceanic residency is therefore preserved even after subduction processes subjected the serpentinites to eclogite facies conditions by contrast 11b in samples from the mantle wedge range from 14 to 10 supporting the interpretation of interactions with fluids derived from the progressive dehydration of the slab which releases fluids with slightly positive and negative 11b it indicates that mantle wedge serpentinite isotopic signature is acquired via the infiltration of subducted crust derived metamorphic fluids during the subduction processes

lead lead age of komatiitic lavas and limitations on the structure and evolution of the precambrian mantle abstract using lead isotopes we have studied komatiites and associated sulfides from three complexes at barberton the lead lead isochron gives an age of 3 46 0 07gy in good agreement with the sm nd age at munro township three distinct komatiitic flows yield significantly different ages pyke hill is 2 72 0 02gy theo s flow is 2 47 0 13gy and fred s flow is 2 58 0 02gy old 2 72 gy is the emplacement age while the alignment obtained on theo s flow and fred s flow clearly points out the effect of a metamorphism posterior to 2 7 gy at cape smith the age obtained by the lead lead method is 1 6 0 13gy this age is younger than that obtained by other methods in contrast the isochron obtained on sulfide gives an age of 1 88 0 17gy in agreement with sm nd methods these results together with others from the literature are used to describe the evolution of pb in the mantle the important result inferred is that the mantle 2 70 gy ago was very different from the primitive mantle according to both th u and u pb values indeed the latter are lower than the planetary values this evolution is the result of the formation of the continental crust

statistical analysis and intercomparison of wam model data with global ers 1 sar wave mode spectral retrievals over 3 years ocean wave spectra were retrieved from a set of ers i synthetic aperture radar sar wave mode swm spectra between january 1993 and december 1995 an assessment is given of the swm data quality and the retrieval performance as well as the operational feasibility of the retrieval algorithm sensitivity studies are performed to demonstrate the weak residual dependence of the retrieval on the first guess input spectrum the mean spectral parameters of the swm retrievals are compared with spectral parameters from collocated wave model wam spectra the time series of swm retrieved and wam derived monthly mean significant wave heights hs in various ocean basins show good overall agreement but with a small systematic underestimation of hs by the wam a decomposition of the wave spectra into wind sea and swell reveals an average 10 overprediction of the wind sea by the wam while swell is underpredicted by 20 30 the positive wind sea bias exhibits no clear wave height dependence while the negative swell bias decreases with swell wave height this could be due to a too strong damping in the wam at low frequencies detailed regional investigations point to the existence of smaller scale phenomena which may not be adequately reproduced by the wam at the present resolution of the wind forcing finally an intercomparison is made of the observed and modeled azimuthal cutoff length scales and global distributions are investigated ratios of the observed azimuthal cutoff wavenumber to the mean azimuthal wavenumber component indicate that about 75 of the swell can be directly resolved by the sar while about 70 of the wind sea lies at least partially beyond the cutoff

230th 238u disequilibrium systematics in oceanic tholeiites from 21 n on the east pacific rise significant disequilibrium occurs between 230 th and its parent 238 u in a suite of fresh basalt glasses from the rise project study area at 21 n on the east pacific rise the 230 th 232 th activity ratios observed for eight of nine samples from the crest of the axis at this site are constant within analytical uncertainty with a value of 1 22 this observed homogeneity of 230 th 232 th has two possible interpretations first the measured 230 th 232 th can be considered to indicate a mantle source for the rise basalts with th u of 2 5 this interpretation however conflicts with the proposed correlation between 230 th 232 th and 87 sr 86 sr 1 which predicts that 230 th 232 th should equal 1 33 at the rise site a second possible interpretation is that radioactive decay of 230 th in the basalts themselves or in a magma chamber has decreased 230 th 232 th from 1 33 to the observed values the required time span is 11 000 to 100 000 years however petrologic arguments rule against long residence time in a magma chamber and the spreading rate of this section of the east pacific rise 6 cm yr predicts that the maximum age for axis basalts is 27 000 years both interpretations of the measured 230 th 232 th imply a low th u ratio for the rise basalt source and suggest that the morb source at this location is depleted in th with respect to u relative to primitive mantle or bulk earth in spite of their constant 230 th 232 th the basalts from 21 n have wide ranges of measured th u and thorium and uranium concentrations in apparent conflict with the common assumption that these two elements have very small similar crystal liquid partition coefficients participation of an accessory phase with high th and u concentrations during partial melting or fractional crystallization appears to be required to explain this anomaly major and other trace element compositions provide more information about petrogenetic processes although fractional crystallization can explain the variations in major element composition a more complex process such as continuous melting is required to account for the observed trends in trace element compositions in addition to its use as a tracer the 230 th 238 u disequilibrium technique is capable of putting some constraints on the timing of fractionation events and it may eventually lead to a method for dating very young mid ocean ridge basalts

curie point depths of the island of kyushu and surrounding areas japan as part of a comprehensive nationwide evaluation of geothermal resources for japan the first of the curie point depth maps covering the island of kyushu has been prepared the map was created by inverting gridded regional aeromagnetic data two satisfactory algorithms were developed to invert the gridded data based upon a distribution of point dipoles the first algorithm estimates x0 y0 and z0 the coordinates of the centroid of the distribution by computing a least squares fit to the radial frequency of the fourier transform the second algorithm estimates centroid depth only by computing a least squares fit to the squared amplitude of the frequency estimates the average depth to the top zt of the collection of point dipoles was estimated by a variation of the second algorithm the depth to the bottom of the dipoles inferred curie point depth is zb 2z0 zt the depth estimates are hand contoured to produce the final map the curie point depth map is then compared to regional geology and heat

foreland basin magmatism in the western moroccan meseta and geodynamic inferences 1 basaltic magmatism occurs in the hercynian foreland basin of the western moroccan meseta as pillow lavas flows interbedded with flysch deposits and sills of dolerite and gabbro a sedimentological and tectonic study shows that the deposition of the flysch and thus the intrusion of the lava flows and sills was controlled by the northwestward propagation of thrust related folds in the wedge top depozone of a syncontractional foreland basin system the pillow lavas appear as either massive sheets of stacked lava flows or thin lava flows interlayered with the syntectonic turbiditic deposits the sills are composed of several mafic units up to six in the most evolved marziqallal sill cut through by granophyric veins which can be interpreted as a result of in situ crystallization the pillows lava basalts and the dolerites gabbros are calc alkaline and cogenetic a discussion of the possible mechanisms of magma production in subduction related orogens leads us to conclude that this foreland basin magmatism was generated in a retrolithospheric setting a comparison with the other early carboniferous volcanic and plutonic rocks of the western and eastern mesetas shows that calc alkaline foreland basin magmatism was widespread in the entire mesetan domain and was probably a result of wet melting of the metasomatised mantle lithosphere of the overriding plate because the moroccan hercynides are likely to have been a result of continental subduction and the magmatism is observed at a distance of more than 500 km from the suture it is suggested that the retroforeland basin magmatism was a result of either dehydration of a shallow dipping 15 foregoing oceanic slab or underplating of the continental lithosphere ending with slab breakoff beneath the foreland basin

melting of a subducting oceanic crust from u th disequilibria in austral andean lavas understanding crustal genesis at convergent plate boundaries is important for determining mass transfer between different geochemical reservoirs in the earth s mantle and for deciphering the long term growth of the continental crust most arc magmas are thought to be generated from fluid induced melting of the mantle wedge above slabs of subducting oceanic crust1 such magmas frequently display 238u enrichments or radioactive equilibrium2 3 between 238u and its radiogenic product 230th but where a young and hot oceanic crust is being subducted it may itself partially melt and produce calc alkaline andesites and dacites termed adakites4 here we report a uniform excess of 230th over 238u but variable th isotope ratios in young adakites from the andean austral volcanic zone south of the triple junction where the chile ridge subducts beneath south america we show that these results are compatible with the adakites having been formed by approximately 20 equilibrium melting due to amphibole decomposition in a heterogeneous5 oceanic crust moreover both the degree of melting of the oceanic crust and its thermal structure appear to be uniform under most of the andean austral volcanic zone such partial melting of subducted oceanic slabs may have occurred throughout the earth s history where young oceanic plates were subducted

u series disequilibria in morb from the garrett transform and implications for mantle melting abstract here we report 238u 230th 231pa 226ra disequilibria and 87sr 86sr measurements in 11 mid ocean ridge basalt morb glasses from the garrett transform 13 30 s latitude on the east pacific rise epr whose compositions range from primitive depleted high mgo basalts to evolved basalts u and th concentrations of samples range between 3 and 75 ppb and between 6 and 220 ppb respectively with a corresponding large variation in th u 1 5 2 9 230th 232th varies from 1 2 to 1 6 such that 230th 238u range from 15 excess 230th in a high th u evolved sample to 25 excess 238u in a high mgo sample with low th u out of 11 samples 7 have 238u excess an unusual feature for morb all samples have 226ra excesses with 226ra 230th varying between 1 3 and 3 8 constraining ages since eruption to comparing the garrett transform to the siqueiros transform shows a remarkable correspondence between sample setting composition and disequilibria systematics both settings produce linear trends of 230th 238u as a function of th u consistent with mixing between two melts derived from different depths in the melting column the mixing relationships are identical in both locations the most incompatible rich samples with the highest th u and 230th excess come from the ridge transform intersection rti whereas the most incompatible element poor basalts with the lowest 230th 238u and th u are erupted along leaky transform faults samples with intermediate th u and 230th 238u all come from within intra transform spreading centers consistent with the spreading centers acting to homogenize these diverse magmas the cause of variation in th u could reflect either melting processes or different long lived sources no clear indication exists within these data

geothermal resources in egypt integrated with gis based analysis abstract the energy of egypt is generally derived from petroleum products however advancements in the development of the generation capacity using sustainable and renewable resources for example solar wind and geothermal resources and investment in nuclear power plants are planned the geology and structure setting of egypt affirms that reasonable geothermal resources are available particularly along the gulf of suez the red sea the western desert and parts of the nile valley the main objective of this research is to develop a geothermal favourability map of egypt which may be considered as a screening tool for the assessment of optimal areas for geothermal development digital data layers and maps were employed in a gis model to select the most promising areas for geothermal potentiality the arcgis model includes six thematic layers distance to faults bouguer anomaly distance to seismic activity curie depth heat flow and temperatures at different depths the validation of the generated geothermal favourability map was performed by a comparison with thermal water wells and hot springs generally the most encouraging zones for geothermal probability in egypt are located near the shorelines of the gulf of suez and the red sea

recycling of crystal mush derived melts and short magma residence times revealed by u series disequilibria at stromboli volcano abstract the presence of crystal mushes in the feeding system of active volcanoes is generally revealed by antecrysts representing the crystalline portion of old magmas recycled in the juvenile material but very little is known about the fate of interstitial liquid hosted within the crystal rich mush i e antemelt u series disequilibria measured in magmas erupted in the past 18 years at stromboli volcano provide the first geochemical evidence of the involvement of antemelt and help constraining the timescales of the processes occurring in the plumbing system of the volcano despite almost constant major and trace element composition significant variations in isotope ratios are observed 230 th 232 th decreases with time whilst 238 u 232 th 226 ra 230 th and 87 sr 86 sr are different in the two types of magma erupted magma with low phenocryst content lp is erupted as pumices during paroxysm and is thought to belong to a deep reservoir highly porphyritic magma hp is erupted during the normal strombolian activity as scoria and during the effusive events as lavas and it is considered to derive from the former one within a shallow reservoir through degassing driven crystallisation mixing and incorporation of antecrysts the distinct 238 u 232 th of lp and hp magma requires the involvement of a component with high 87 sr 86 sr and 238 u 232 th deriving from older magmas erupted earlier in the volcano history up to 2 5 ka the incompatibility of u and th in major mineral phases limits the possible effect of antecrysts hence requiring the involvement of a u and th rich antemelt the decrease of 226 ra excess from lp to hp magmas provides further and independent evidence for the involvement of a few thousands years old antemelt the variation with time of 230 th 232 th within lp and hp magmas is exploited to constrain the residence time of magmas in the deep and shallow reservoir of the volcano to 55 yrs inferred reservoir volume 0 5 km 3 and 2 10 yrs inferred reservoir volume 0 02 0 09 km 3 respectively our results show the occurrence of magmatic processes operating at different timescale within the feeding system of a so called steady state volcano such as stromboli we show that while most of the magma is erupted within few years a portion of it made of both crystals antecrysts and residual liquid antemelt can be stored in the plumbing system for thousands of years to be eventually rejuvenated and mixed back into the shallow reservoir the presence of antemelts may also affect the eruptive mechanism by promoting heat transfer in locked crystal mushes hence favouring their remobilisation

origin of 226ra 230th disequilibria in arc lavas from southern chile and implications for magma transfer time abstract improved understanding of mantle melting processes and melt transport requires knowledge of how fast magma is generated and transferred from source region to surface the rate of magma transfer can in favorable cases be estimated from radioactive disequilibria between nuclides of the 238u series young lavas from southern chile in which 238u 230th disequilibria have been measured sigmarsson et al nature 346 1990 163 165 sigmarsson et al nature 394 1998 566 569 were analyzed for 226ra abundances the disequilibrium between 226ra and 230th in these lavas is found to correlate with 238u 230th disequilibria and 10be be morris et al nature 344 1990 31 36 these correlations strongly suggest that the excess of 226ra over 230th is due to the addition of a slab derived fluid to the magma source since ra and u are fluid mobile elements and the cosmogenic 10be is most likely derived from the subducting nazca plate beneath the andes the largest slab signature is observed in the lavas of villarrica volcano which is the most active volcano in south america a model for subduction fluxing is discussed in which the u series disequilibria in arc lavas will reflect the integrated dehydration process during metamorphism of the subducting plate and the metasomatized mantle but be principally controlled by the latest hydrous mineral breakdown in the mantle wedge repeated precipitation and dehydration mineral reactions of the hydrated mantle could be the homogenization process of the slab input needed to explain the 10be be b be correlation for different arcs morris et al nature 344 1990 31 36 the fact that excesses of 226ra and 238u over 230th are correlated indicates that linear arrays on the 230th 232th 238u 232th diagram are not isochrons reflecting time elapsed since a fluid addition but rather mixing lines between a fluid phase and melts the 226ra 230th disequilibrium in arc lavas suggests significantly shorter timescales for magma transfer or less than 8000 years this disequilibrium is consistent with minimum magma transfer rate through the mantle wedge on the order of 10 m year finally the correlations of 226ra 230th with 238u 232th and 10be be in andean magmas imply that magma chamber residence time is of the same order of magnitude beneath the stratovolcanoes studied

uranium and 10 be enrichments by fluids in andean arc magmas volcanic rocks from subduction settings frequently contain 10be enrichments that have been attributed to sediment subduction1 6 and are often characterized by 238u 230th ratios greater than unity in contrast to mid ocean ridge and ocean island basalts7 8 here we report such an excess of 238u over 230th in historic lavas from the southern volcanic zone of chile 33 42 s the greatest uranium excess is found south of 39 s where 10be 9 be ratios are highest and the trench is filled with sediment these features suggest that both u and 10be enrichments result from preferential partitioning of u and be into a fluid phase during dehydration of the recently subducted sediments the presence of strong 238u 230th disequilibria in basaltic magmas of the southern volcanic zone suggests that the timescale for dehydration melting and eruption in this arc is probably less than 20 000 yr

recent stress field of the sinai subplate region abstract quaternary and recent tectonic stress data for the sinai subplate region have been compiled including focal mechanism solutions in situ stress measurements fault slip data and alignments of young volcanic feeders the average direction of maximum horizontal stress sh has been determined by means of a linear interpolation method the results indicate that the direction of the average maximum horizontal stress is closely parallel to the direction of the absolute motion of africa analysis of the orientation of principal stress directions suggests that a large part of the sinai subplate region is subjected to a strike slip regime characterized by an average maximum horizontal stress trend of nw 54 w for example out of 50 earthquake focal mechanism solutions 52 are pure strike slip and 10 are either normal or thrust fault with a remarkable strike slip component the remaining are pure normal fault 18 thrust fault 12 or undetermined 8 three different tectonic subprovinces can be delineated northwestern egypt the eastern mediterranean israel lebanon and northern syria are characterized by strike slip often with a remarkable thrust faulting component stress regime the northern red sea the sinai peninsula and a significant part of the arabian plate are characterized by strike slip often with a remarkable normal faulting component stress regime this is also the case for southwestern turkey

u series isotope data on lau basin glasses the role of subduction related fluids during melt generation in back arc basins new thermal ionization mass spectrometry u series disequilibrium data are presented for 24 basaltic to dacitic glasses from active spreading centres in the back arc lau basin sw pacific together with additional inductively coupled plasma mass spectrometry trace element analyses and sr nd pb isotope data valu fa ridge samples adjacent to the arc front have high u th and 230th 238u 1 implying a recent 350 ka addition of a u rich slab derived fluid the valu fa data can be combined with existing 230th 238u data for the central tonga arc to infer a fluid addition event at 50 ka the similar sources and time scales for fluid transfer beneath the valu fa ridge and beneath the arc itself suggest that the valu fa ridge is propagating into the arc front region central lau basin samples further behind the arc have lower u th and 230th 238u 1 similar to typical mid ocean ridge basalts morb within the central lau basin a water rich subduction component is seen only in samples closest to the arc and this fluid does not have the high u th composition of the fluid at valu fa melt generation in the central lau basin appears to be dominated by normal ridge type processes but the relatively low 230th 238u for these shallow ridges compared with global morb could be a consequence of increased melt productivity as a result of the elevated water contents the transition from 238u to 230th excesses within the back arc basin is not a smooth function of decreasing addition of a u rich fluid moving away from the arc front but also reflects the complex dynamics between two major mantle domains within the mantle wedge pacific beneath valu fa ridge indian beneath the central lau basin

mobilization of beryllium in the sedimentary column at convergent margins studies of be distributions in subduction zone sediments will help to understand questions regarding the enrichments of cosmogenic be 10 in arc volcanic rocks analyses of be 10 and be 9 in sediments of ocean drilling program site 808 nankai trough and be 9 in porewaters of site 808 and sites 671 and 672 barbados ridge complex show significant decreases in solid phase be 10 and large increases of porewater be 9 at the location of the decollement zone and below or at potential flow conduits these data imply the potential mobilization of be during pore fluid expulsion upon sediment burial experiments involving reaction between a decollement sediment and a synthetic nacl cacl2 solution at elevated pressure and temperatures were conducted in an attempt to mimic early subduction zone processes the results demonstrate that be is mobilized under elevated pressure and temperature with a strong ph dependence the be mobilization provides an explanation of be 10 enrichment in arc volcanic rocks and supports the argument of the importance of the fluid processes in subduction zones at convergent margins

time scales of magma formation ascent and storage beneath subduction zone volcanoes there is now sucient information to attempt an integrated model for melt gener ation transfer and storage beneath subduction zone volcanoes fluid release from the subducting oceanic crust into the mantle wedge may occur over a period ranging from a few hundred kyr to as little as less than 1 kyr before eruption this supports models in which uid addition is closely linked to partial melting though there may also be evidence for a component of decompression melting the timing of the onset of uid addition may be linked to the rate of subduction i e water supply rate and the angle of subduction and consequently the thermal structure of the mantle wedge in contrast contributions from subducted sediments to subduction zone lava sources appear to occur some 350 kyr 4 myr before eruption evidence for partial melting of the sediment component combined with the short uid transfer times phenocryst equilibration temperatures and other observations all point to quite high mantle wedge temperatures close to the interface with the subducting plate new 226 ra data permit only a short period of time between uid addition and eruption this requires rapid melt segregation magma ascent by channelled ow and minimal residence time within the lithosphere typically the evolution from basalt to andesite occurs rapidly during ascent or in magma reservoirs inferred from some geophysical data to lie within the lithospheric mantle mineral isochron data suggest that some andesitic magmas subsequently stall in more shallow crustal level magma chambers where they can evolve to dacitic compositions via fractionation typically combined with assimilation on time scales of a few thousand years or less

238u decay series systematics of young lavas from batur volcano sunda arc abstract activities of 238u decay series radioisotopes have been determined for both postcaldera basalts erupted between 1849 and 1974 and genetically related young precaldera dacites from batur volcano bali sunda arc all rocks possess 230th 238u 1 within 2 sigma error indicating that little if any fractionation between th and u occurred during their genesis or in their source regions within approximately the last 350 ka both the basaltic and the dacitic rocks possess 230th 232u 0 71 consistent with both having a common mantle source with an unusually high th u weight ratio of 4 4 and with the dacitic magma having formed by closed system convective fractionation of the same basaltic magma from which the postcaldera lavas were derived the historical basalts possess 226 ra 230 th 1 indicating a significant ra enrichment relative to th and u within approximately the last 8 ka the highest degree of 226ra enrichment occurs in the most mafic basaltic lavas which form a cycle of mafic volcanism between 1888 and 1926 if ra enrichments were produced during mantle melting as a result of its greater incompatibility relative to th then variations in 236ra 230th may reflect the presence at batur of a periodically replenished but poorly mixed open system magma chamber in addition this result implies that the primitive batur basaltic magmas take considerably less than about 8 ka to travel from their mantle source through the batur magma chamber to the surface whole rock 210 pb 226 ra 1 in the youngest historical basalts indicates that pb has been enriched relative to ra within about 100 years of their eruption because pb is expected to be less incompatible then ra during mantle melting the enrichments are probably due to late stage processes such as accumulation of 210pb rich plagioclase phenocrysts or post eruptive condensation of volatile pb salts in vesicles

aleutian lead isotopic data additional evidence for the evolution of lithospheric plumbing systems abstract lead isotopic ratios and concentrations have been measured in lavas from the aleutian volcanic centers of adak 12 and atka 12 whereas lavas from atka have very small pb isotopic ranges 206 pb 204 pb 18 78 18 86 207 pb 204 pb 15 55 15 62 208 pb 204 pb 38 31 38 52 those from adak have ranges nearly twice as large 206 pb 204 pb 18 34 18 86 207 pb 204 pb 15 47 15 63 208 pb 204 pb 37 81 38 56 on lead lead isotopic diagrams atka data form tight clusters whereas those from adaak define a well developed linear trend extending from the atka field toward less radiogenic values at a given 206 pb 204 pb the lavas from both islands plot in the morb trend for 208 pb 204 pb but atka lavas plot above the morb 207 pb 204 pb 206pb 204pb trend and those from adak fall both within and above the trend lead contents in lavas from atka increase four fold 5 21 ppm over the compositional range of the volcanic suite 49 68 sio2 in contrast actak lavas 49 65 have concentration levels of 0 6 13 ppm and display no simple correlation with sio2 the lead isotopic data alone can be explained by three different processes model 1 assigns lead isotopic differences to original magma source heterogeneity according to model 2 the isotopic ratios of a primary non radiogenic component from the mantle are elevated by the addition of an isotopically enriched slab derived component in contrast model 3 assumes a primary radiogenic magma produced by melting of the slab is contaminated by a non radiogenic lithospheric component during conduit formation because these models all adequately explain the lead isotopic data supplementary geologic petrographic geochemical and isotopic data must be used to select the most likely model careful consideration of the evidence suggests model 3 best explains our extensive lead isotopic data as well as other characteristics of aleutian lavas our study suggests detailed isotopic studies of individual volcanic centers arc or otherwise can be extremely useful in understanding the complex processes of magma generation extraction ascent and evolution

contrasting timescales of crystallization and magma storage beneath the aleutian island arc abstract geologic chronologic and u th isotope data from pleistocene recent basaltic to rhyolitic lavas and their phenocrysts constrain the long term evolution of magmatic processes at seguam island aleutian island arc and suggest a model in which erupted magmas were derived from a single deep seated reservoir for 130 kyrs of its eruptive history the monotonic evolution in 230 th 232 th 0 ratios is consistent with radiogenic ingrowth of 230 th in a long lived magma reservoir between 142 and 9 ka internal u th mineral isochrons from seven lavas and one ignimbrite are indistinguishable from their eruption ages as constrained by 40 ar 39 ar dating which implies a short period i e 10 3 yrs or less of crystal residence in the magma prior to eruption these results can be reconciled if small batches of magma are repeatedly extracted from a deep thermally buffered basaltic reservoir followed by decompression driven crystallization and differentiation in small chambers or conduits in the upper crust immediately prior to eruption stratovolcano collapse at 9 ka produced a 4 km diameter caldera and a 70 m thick dacitic ignimbrite covering 12 km 2 the ignimbrite as well as subsequent rhyolitic and basaltic lava flows signal a decrease in 230 th 232 th 0 and 87 sr 86 sr ratios the abrupt change in magma composition at 9 ka most likely reflects disruption of the long lived reservoir by rapid ascent of basaltic magma with low 230 th 232 th and 87 sr 86 sr ratios and excess 226 ra while the physiochemical connection between cone collapse and rise of new magma into the shallow crust remains to be explored the shift in isotopic composition and evidence for a protracted 100 kyr period of magma storage would have been missed had the focus been on only the most recent historical eruptions our results suggest that not all arc basalt produced in the mantle wedge separates and ascends rapidly magma ascent rates and storage times may vary significantly over periods of 10 4 to 10 5 yrs at a single arc volcano and u th isotopes offer a means of better understanding relationships between explosive eruptions and changes in magma reservoir dynamics in island arcs provided a sufficiently long period of volcanic activity is examined

the sm nd age of kambalda volcanics is 500 ma too old although sm nd isotopic analyses of volcanics from kambalda western australia form a 3 2 ga linear array pb pb analyses of the same suite give an isochron age of 2 73 0 03 ga and a 2 7 ga model age for the associated sulphides we suggest that the 3 2 ga age is incorrect and that the sm nd array results from mixing between depleted mantle and either older continental crust or enriched mantle

an analytical model for the description of the full polarimetric sea surface doppler signature this paper describes an analytical model of the full polarimetric sea surface scattering and doppler signature the model combines the small slope approximation theory at the second order with a weak nonlinear sea surface representation such a model is used to examine the variation of the doppler central frequency bandwidth and of the normalized radar cross section as function of wind speed and direction the results suggest that the model can be a valuable tool for the accurate observation of sea surface currents

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

basaltic rocks from the andean southern volcanic zone insights from the comparison of along strike and small scale geochemical variations and their sources abstract the origin of spatial variations in the geochemical characteristics of volcanic rocks erupted in the andean southern volcanic zone svz has been studied by numerous researchers over the past 40 years diverse interpretations for along strike across strike and small scale variations have been proposed in this paper we review geochemical data showing along strike geochemical variations and address the processes causing such trends we compare large and small scale changes of the same geochemical parameters in basaltic rocks in order to use spatial scale as a tool for isolating processes that may have the same result along strike geochemical variations in the svz are expected due to 1 greater thickness or age of the sub arc continental crust and mantle lithosphere in the northern svz nsvz 33 s 34 30 s and transitional svz tsvz 34 30 s 37 s compared with the central svz csvz 37 s 41 5 s and southern svz ssvz 41 5 s 46 s and 2 along strike changes of the subducting nazca plate and overlying asthenosphere basalts and basaltic andesites erupted at volcanic front stratovolcanoes define several along strike geochemical trends 1 higher 87sr 86sr and lower 143nd 144nd at volcanoes in the nsvz compared with the tsvz csvz and ssvz 2 higher and more variable la yb at volcanoes in the nsvz and tsvz compared with the csvz and ssvz 3 lower 87sr 86sr for a given 143nd 144nd at volcanoes in the tsvz compared with the csvz and ssvz and 4 large values for time sensitive subduction tracers such as 10be 9be and 238u 230th at some volcanoes in the csvz but not in the nsvz and tsvz geochemical parameters that distinguish the tsvz from the csvz and ssvz are also found within the csvz at small basaltic eruptive centers sec aligned with the liquine ofqui fault system lofs which extends from 38 s to the southernmost svz our interpretation is that csvz magmas with strong time sensitive subduction tracers represent the ambient subduction modified asthenosphere beneath the svz and that tsvz magmas and some sec in the csvz are modified by interaction with pyroxenite during ascent through the mantle lithosphere in the csvz and probably the ssvz enhanced crustal permeability created by the lofs allows the eruption of geochemically diverse magmas on a small scale in the tsvz and the nsvz the composition of magma from the subduction modified asthenosphere is overprinted by interaction with the mantle lithosphere

an introduction to saxon fpn the synthetic aperture radar and x band ocean nonlinear it ies saxon forschungsplatform nordsee fpn program was a 3 year effort to investigate radar backscatter from the ocean and synthetic aperture radar sar imagery of the ocean a secondary objective of the program was to explore the relationship between acoustic and microwave scattering from the ocean surface the program was a joint effort between the united states and the federal republic of germany it consisted primarily of a major field experiment phase i in the north sea on and around the german forschungs platform nordsee fpn during november 1990 a second smaller field experiment phase ii on the same platform in november 1991 and a series of four data analysis workshops

smnd geochronology of greenstone belts in the yilgarn block western australia abstract sm nd geochronology has been used to date 3 widely spaced metavolcanic greenstone belts in the yilgarn block the measured isochron ages are 2 78 0 03 ga initial nd 2 5 0 3 for greenstones at kanowna in the eastern goldfields province 3 05 0 10 ga initial nd 0 9 0 7 for diemals marda in the southern cross province and 2 98 0 12 ga initial nd 0 7 1 2 for the warriedar fold belt in the murchison province these data show that substantial crustal components of age 3 0 ga exist in the southern cross and murchison provinces and support published evidence that the eastern goldfields province contains no crustal material 2 8 ga these ages are all significantly younger than ages commonly observed in the western gneiss terrain and so support the suggestion of a progressive crustal age trend across the yilgarn block the measured age values depend strongly on the inclusion of data for felsic components of the greenstone belts data for mafic and ultramafic units allow that eruption in the 3 localities could have been contemporaneous at 2 8 ga the age range of metavolcanic rocks within each province is possibly comparable to the maximum possible age difference between study areas 0 2 ga so the documented lithological variations between the greenstone successions of the 3 provinces cannot be attributed to depositional age differences leaching experiments on altered warriedar samples indicate that both alteration and isotopic homogenisation occurred early in the evolution of the system probably prior to the 2 6 ga regional metamorphic event the initial nd values do not conform to previously suggested mantle depletion curves when considered with published data for greenstone belts they suggest complex mantle heterogeneity with end diverging over time to both positive and negative values

dopscat a mission concept for simultaneous measurements of marine winds and surface currents a radar scatterometer operates by transmitting a pulse of microwave energy toward the ocean s surface and measuring the normalized per unit surface radar backscatter coef cient r8 the primary application of scatterometry is the measurement of near surface ocean winds by combining r 8 measurements from different azimuth angles the 10 m vector wind can be determined through a geophys ical model function gmf which relates wind and backscatter this paper proposes a mission concept for the measurement of both oceanic winds and surface currents which makes full use of earlier c band radar remote sensing experience for the determination of ocean currents in particular the novel idea of using two chirps of opposite slope is introduced the fundamental processing steps required to retrieve surface currents are given together with their associated accuracies a detailed description of the mission proposal and comparisons between real and retrieved surface currents are presented the proposed ocean doppler scatterometer can be used to generate global surface ocean current maps with accuracies better than 0 2 m s at a spatial resolution better than 25 km i e 12 5 km spatial sampling on a daily basis these maps will allow gaining some insights on the upper ocean mesoscale dynamics the work lies at a frontier given that the present inability to measure ocean currents from space in a consistent and synoptic manner repre sents one of the greatest weaknesses in ocean remote sensing

structural controls of volcanism in transversal chains resheared faults and neotectonics in the cordon caulle puyehue area 40 5 s southern andes abstract the nature of the relationship between tectonics and volcanism in a specific area is the result of the interaction between the crustal structures of the basement and ongoing regional stress field in the southern andes different kinematic environments represented by two nearly orthogonal active fissure systems show apparently incompatible arrays of volcanic chains moreover a local quaternary compressive regime is inferred from basement faults however these mutually orthogonal chains which are also oblique to the volcanic front have contrasting magma evolution and eruptive styles that can be related with their tectonic setting whereas northeast trending monogenetic cones carran los venados can be interpreted as resulting from magma upwelling along regional tension cracks in a quaternary dextral transpressive compressive domain northwest striking fissure system cordon caulle volcanic complex can be the result of extensional fault reactivation and long magma residence in the upper crust different kinematic environments cause contrasting magma evolution and eruptive styles a two way coupling mechanism is suggested for to explain local depart of the regional stress field geometric array of extensional features and final magmatic evolution in these volcanic domains of the southern andes

helium strontium isotope constraints on mantle evolution beneath the roman comagmatic province italy european social fund scottish universities carnegie trust for the universities of scotland

regional sediment sources versus the indus river system the plio pleistocene of the peshawar basin nw pakistan abstract sedimentary archives within the peshawar basin in northwest pakistan preserve a record of siliciclastic supply patterns during the plio pleistocene epoch the basin fill consists of fluvial floodplain clays overlain by thick lacustrine glaciofluvial rhythmites with deposits of multiple alluvial fans in the southern parts of the basin and topped by loess deposits in ideal vertical successions framework mineralogical composition and bulk sediment geochemical proxies of these sediments indicate detritus derivation from dominantly acidic igneous sources with contribution from medium grade metamorphic rocks there appears to be no significant contribution from ultramafic mafic or very high grade metamorphic sources in addition sediments from the southern part of the basin support supply from low grade metamorphic rocks and carbonate sediments overall sedimentation occurred in an active continental margin setting under conditions of insignificant chemical weathering the uplift of the attock cherat range during the late pliocene 2 8 ma restricted the indus river from shedding its detritus into the basin therefore the sediments of the peshawar basin do not reflect signatures of the indus river siliciclastics and thus lack the basic igneous geochemical signal which is otherwise typical for the himalayas hinterland on the contrary the geochemical signatures of these sediments favour predominant detritus flux from the acidic rocks of the peshawar plain alkaline igneous complex ppaic and associated metasediments therefore the kabul swat river and other local tributaries of this smaller river system that originated within the ppaic and associated mountains are considered the major transporting agents which contributed siliciclastics into the peshawar basin during the plio pleistocene additionally in the southern part of the basin the uplifting attock cherat range appeared to have contributed detritus in the form of alluvial fluvial fan deposits

seismic evidence for water deep in earth s upper mantle water in the deep upper mantle can influence the properties of seismic discontinuities in the mantle transition zone observations of converted seismic waves provide evidence of a 20 to 35 kilometer thick discontinuity near a depth of 410 kilometers most likely explained by as much as 700 parts per million of water by weight

wind wave induced velocity in ati sar ocean surface currents first experimental evidence from an airborne campaign conventional and along track interferometric ati synthetic aperture radar sar sense the motion of the ocean surface by measuring the doppler shift of reflected signals measurements are affected by a wind wave induced artefact surface velocity wasv which was modelled theoretically in past studies and has been estimated empirically only once before with envisat asar by mouche et al 2012 an airborne campaign in the tidally dominated irish sea served to evaluate this effect and the current retrieval capabilities of a dual beam sar interferometer known as wavemill a comprehensive collection of wavemill airborne data acquired in a star pattern over a well instrumented validation site made it possible for the first time to estimate the magnitude of the wasv and its dependence on azimuth and incidence angle from data alone in light wind 5 5 m s and moderate current 0 7 m s conditions the wind wave induced contribution to the measured ocean surface motion reaches up to 1 6 m s upwind with a well defined 2nd order harmonic dependence on direction to the wind the magnitude of the wasv is found to be larger at lower incidence angles the airborne wasv results show excellent consistency with the empirical wasv estimated from envisat asar these results confirm that sar and ati surface velocity estimates are strongly affected by wasv and that the wasv can be well characterized with knowledge of the wind knowledge and of the geometry these airborne results provide the first independent validation of mouche et al 2012 and confirm that the empirical model they propose provides the means to correct airborne and spaceborne sar and ati sar data for wasv to obtain accurate ocean surface current measurements after removing the wasv the airborne wavemill retrieved currents show very good agreement against adcp measurements with a root mean square error rmse typically around 0 1 m s in velocity and 10 in direction this article is protected by copyright all rights reserved

thorium uranium ratio of the earth through the study of komatiites and ophiolites remnants of the old mantle we review the evolution of the lead system in the depleted mantle through time using the 230th232th vs 208pb 206pb diagram 208pb 206pb radiogenic lead ratio we constrain the th u ratio of the bulk silicate earth to 4 2 this value is also in full agreement with ratios inferred from the study of the various types of meteorites using this value in the 208pb 206pb 143nd144nd diagram bulk earth plots on the oceanic basalt correlation line as previously guessed a comparison with sr and nd data shows that the th u fractionation correlates with that of rb sr and nd sm whereas the u pb fractionation is independent of other ratios

curie point depth from spectral analysis of magnetic data in taiwan abstract this study constructs an integrated magnetic anomaly map of taiwan and its vicinity that combines land magnetic data and marine magnetic data the reduction to pole rtp correction method was applied to the integrated magnetic anomaly data the corrected magnetic anomaly results for a region roughly 880 km in width and 660 km in length fit well with geological tectonic signatures and are used to estimate the curie point isotherm the adopted curie point depth method provides a relationship between the 2 d fft power spectrum of the magnetic anomalies and the depth of magnetic sources by transforming the spatial data into the frequency domain the basal depth of the magnetic sources is calculated from the magnetic data finally the curie point depth of taiwan is obtained the highest value of 17 km is located in northern taiwan and the lowest value of 6 km is located at southern taiwan and also has the highest thermal gradient of 88 c km the curie point depth is consistent with heat flow measurements with correlation coefficient 0 62

neogene sedimentary and mass wasting processes on the continental margin off south central chile inferred from dredge samples the active continental margin off south central chile 36 to 40 s is transitional between the tectonically erosive empty trench margin north of juan fernandez ridge and the accretionary trench filled margin south of the chile triple junction the small width of the presently active accretionary wedge maximum width of 25 to 50 km argues for past phases of tectonic erosion at present this sector shows indications of contemporaneous accretion subduction and underplating of sediment as well as readjustment of the slope by various mass wasting processes in this context this study aims to examine the neogene sedimentary processes on the continental margin from dredge samples recovered during r v sonne cruise so161 5 within this transitional domain using lithology sandstone petrology shale mineralogy and analysis of sedimentary structures our results yield that the principal transport of material occurs in high energy turbidity currents and debris flows via submarine canyons deeply cutting the continental slope whereas sediment on the shelf is transported by strong coast parallel bottom currents and trapped by submarine canyons cutting into the shelf a wide range of mass wasting processes including slumping debris flows evolving to low density turbidity currents and mud flows rework the slope sediments in contrast thick undisturbed sequences of mostly hemipelagic sediments accumulate in active slope basins which are largely protected from mass movements xrd analyses revealed early diagenetic lithification and overall burial depths of up to 230 mbsf suggesting a shallow subsurface cycle of sedimentation subsidence diagenesis uplift erosion and resedimentation the composition of sandstones is dominated by volcanic rock fragments of andean provenance along strike modal changes reflect a southward increase in glacially denudation and rainfall the combination of which caused more intense erosion of volcanic rocks and exposure weathering and as a result increased fluvial transport of metamorphic and plutonic rocks to the sea

usercalc a web based uranium series calculator for magma migration problems 1 measured departures from secular equilibrium in the uranium series decay chains provide important constraints on the rates of recent mantle and crustal processes nevertheless the inferences drawn from these observations depend upon the models used to interpret them while several useful models for u series exist they are not all equally easy to use and it is often the ease of use rather than the utility of the model that determines which models are applied the purpose of this paper is to level the playing field by making some of the more general u series transport models accessible to a wider community through the usercalc web site available from lamont doherty earth observatory at www ldeo columbia edu mspieg usercalc these models calculate the effects of both melting and melt transport on two u series chains 238u 230th 226ra and 235u 231pa and allow the user to input their own petrological insight and data to explore the behavior of this model the purpose of the site is to provide a tool that can spur additional collaborations between geophysicists and geochemists as well as drive more rapid exploration evolution of u series models

regional geothermal exploration in egypt although egypt is not characterized by abundant cenozoic igneous activity its location in the northeastern corner of the african plate suggests that it may possess geothermal resources especially along its eastern margin regional geothermal exploration has been carried out in egypt using the thermal gradient heat flow technique and groundwater temperature chemistry technique in the thermal gradient heat flow study existing oil well bottom hole temperature data as well as subsurface temperature measurements in existing boreholes were utilized before special thermal gradient holes were drilled groundwater temperature and chemistry data were used to extend the geographic range of the direct subsurface thermal measurements on a very modest budget a regional thermal high has been discovered along the eastern margin of egypt and a local thermal anomaly has been discovered in this zone published geological information suggests that the sandstones of the nubian formation may be a suitable reservoir for geothermal fluids the new data indicate that temperatures of 150 sup 0 c or higher may be found in this reservoir in the gulf of suez and red sea coastal zone where it lies at a depth of 4 km and deeper

magmatic evolution of the puyehue cordon caulle volcanic complex 40 s southern andean volcanic zone from shield to unusual rhyolitic fissure volcanism abstract magmas erupted from quaternary volcanoes of southern andes between 37 and 46 s latitude are mainly basaltic to andesitic however pccvc 40 s shows a singular magmatic evolution due to the abnormal evacuation of rhyolites especially in the last 100 ka in addition pccvc is the result of juxtaposing products from the nw trending alignment of cordillera nevada caldera cordon caulle fissure volcano and the puyehue stratocone using 40 ar 39 ar and 14 c geochronology it can be established that they evolved since ca 500 ka as coeval but separated vents with a first stage of shield volcanism followed by repeated collapses that formed an internal nw elongated graben from ca 100 ka volcanic activity occurred in both a fissure system cordon caulle and a central volcano puyehue holocene explosive eruptions mainly in the puyehue crater accompanied the dome growing along a nw trending fissure system last historical eruptions were in 1921 and 1960 when nw fissures of cordon caulle fed rhyodacitic lava flows in 1960 the fissure eruption was triggered by a remote mw 9 5 thrust earthquake cordillera nevada caldera presents a reduced compositional range 52 63 sio 2 and geochemical features of low pressure magma mixing and assimilation instead cordon caulle and puyehue volcanoes have a wide silica range 48 71 sio 2 and an outstanding affinity which can be modelled with initial high pressure fractional crystallization moderate magma mixing and subsequent low pressure fractional crystallization from a common parental source the exceptional magmatic evolution and eruptive style of pccvc in southern andes could be related with the physics of the plumbing system which in turn can be controlled by external factors as the structure of the continental crust and the ongoing stress regime

magma cumulate mixing identified by u th disequilibrium dating magma mixing is believed to be important in magmagenesis1 particularly in volcanic arcs2 where its influence is revealed by heterogeneous phenocryst populations3 and quenched inclusions4 nevertheless isotope studies particularly of uranium series dis equilibria have concentrated on the analysis of bulk rock samples5 9 comprehensive u th mineral separate data from arc volcanics are scarce10 14 here we present the first u th isotopic evidence for mixing between a crystal mush and a magma in an andesite from the island of santorini in the aegean arc the lava contains crystal populations from two sources of distinct thorium isotope composition one from a basic cumulate the other phenocrysts from a dacite magma the crystal populations have statistically indistinguishable crystallization ages of 79 14 12kyr and 93 29 22kyr and are thought to have physically intermingled just before eruption the crystals did not re equilibrate and hence retain the isotopic compositions and ages of the parental materials

experimental constraints on recycling of potassium from subducted oceanic crust petrological experiments on oceanic crust samples characterize the recycling of potassium from mid ocean ridge basalts and sediments metasomatism could develop directly and continuously from subducted potassium bearing crust from shallow levels to a maximum depth of 300 kilometers phengite a potassium rich mica is the principal potassium host at subsolidus conditions it transports potassium and water to depths of up to 300 kilometers and could yield over the entire depth range potassium rich fluids or melts depending on the specific geotherm which are likely to constitute one of the primary metasomatic agents for generation of calc alkaline magmas

isotopic and incompatible element constraints on the genesis of island arc volcanics from cold bay and amak island aleutians and implications for mantle structure reply to a critical comment by m r perfit and r w kay

seismicity and active tectonics of the egyptian red sea margin and the northern red sea abstract an active median spreading center has been identified in the red sea south of 22 n but from published reports the northern red sea appears to be essentially aseismic we have used microearthquake monitoring techniques along the egyptian red sea margin to investigate the active tectonics in the region and have found that the northern red sea is not aseismic events recorded with epicenters in the red sea define an active zone extending south southeast from the gulf of suez into the axial region of the red sea down to 25 75 n with additional microseismicity between 24 and 25 n suggesting active median spreading in the northern red sea two areas of intense microearthquake activity have also been identified the first at the southern end of the gulf of suez and the second in the egyptian red sea hills clustered at approximately 25 28 n 34 52 e the gulf of suez seismicity results from the adjustments in motion at the triple junction between the african and arabian plates and the sinai subplate the source of the seismicity in the egyptian red sea hills is unknown but a uniform fault plane mechanism is not indicated by first motion studies or the spatial distribution of the hypocenters

annual validation of significant wave heights of ers 1 synthetic aperture radar wave mode spectra using topex poseidon and ers 1 altimeter data significant wave heights retrieved globally during 1994 from low bit rate imagette spectra of the synthetic aperture radar sar operating in the intermittent wave mode swm onboard the first european remote sensing ers 1 satellite hsswm are validated using independent buoy validated satellite altimeter data from topex poseidon hstop and ers 1 hsers hsswm is retrieved using the extended inversion algorithm of the fully nonlinear wave to sar spectral integral transform hasselmann et al 1996 the statistical comparison shows that globally retrieved hsswm agree remarkably well with collocated data of both altimeters histograms of global hsswm are well approximated by the universal lognormal distribution function a small but systematic underestimation of hsswm with respect to both altimeters by 0 1 m is noticed this bias is small compared to the uncorrelated root mean square rms deviation of 0 5 m the underestimation originates from underestimations of the high sea states dominated by wind sea the entire range of swell waves of hsswm however corresponds very closely to hstop thus the high accuracy of swm retrieved swell wave heights substantiates their suitability for model validation and for operational wave data assimilation to achieve also reliable estimates of the high sea states the combined analysis of wave and wind data from both modeling and observation is the most promising approach