magma extraction from the mantle wedge at convergent margins through dikes a parametric sensitivity analysis 1 we address under what conditions a magma generated by partial melting at 100 km depth in the mantle wedge above a subduction zone can reach the crust in dikes before stalling we also address under what conditions primitive basaltic magma mg 60 can be delivered from this depth to the crust we employ linear elastic fracture mechanics with magma solidification theory and perform a parametric sensitivity analysis all dikes are initiated at a depth of 100 km in the thermal core of the wedge and the moho is fixed at 35 km depth we consider a range of melt solidus temperatures 800 1100c viscosities 10 100 pa s and densities 2400 2700 kg m 3 we also consider a range of host rock fracture toughness values 50 300 mpa m 1 2 and dike lengths 2 5 km and two thermal structures for the mantle wedge 1260 and 1400c at 100 km depth and 760 and 900c at 35 km depth for the given parameter space many dikes can reach the moho in less than a few hundred hours well within the time constraints provided by u series isotope disequilibria studies increasing the temperature in the mantle wedge or increasing the dike length allows additional dikes to propagate to the moho we conclude that some dikes with vertical lengths near their critical lengths and relatively high solidus temperatures will stall in the mantle before reaching the moho and these may be returned by corner flow to depths where they can melt under hydrous conditions thus a chemical signature in arc lavas suggesting partial melting of slab basalts may be partly influenced by these recycled dikes alternatively dikes with lengths well above their critical lengths can easily deliver primitive magmas to the crust particularly if the mantle wedge is relatively hot dike transport remains a viable primary mechanism of magma ascent in convergent tectonic settings but the potential for less rapid mechanisms making an important contribution increases as the mantle temperature at the moho approaches the solidus temperature of the magma

evidence for thin oceanic crust on the extinct aegir ridge norwegian basin ne atlantic derived from satellite gravity inversion 1 satellite gravity inversion incorporating a lithosphere thermal gravity correction has been used to map crustal thickness and lithosphere thinning factor for the ne atlantic predicted oceanic crustal thicknesses in the norwegian basin are between 4 and 7 km on the extinct aegir ridge increasing to 9 14 km at the margins consistent with volcanic margin continental breakup at the end of the paleocene the observation from gravity inversion and seismic refraction studies of thin oceanic crust produced by the aegir ridge in the oligocene may have implications for the temporal evolution of asthenosphere temperature under the ne atlantic during the tertiary thin oligocene oceanic crust may imply cool normal asthenosphere temperatures during the oligocene in contrast to elevated asthenosphere temperatures in the paleocene and miocene recent as indicated by the formation of volcanic margins and iceland respectively

boninitic and tholeiitic dykes in the lewis hills mantle section of the bay of islands ophiolite implications for magmatism adjacent to a fracture zone in a back arc spreading environment a detailed integrated field petrographic and geochemical study of the springers hill area of the bay of islands ophiolite exposed in the lewis hills was undertaken to explain the anomalously high abundance of veins and dykes of chromitite orthopyroxenite and clinopyroxenite and their associated dunites hosted by a refractory harzburgite dunite mixture a geodynamic situation is presented which is constrained by previous studies requiring formation of the springers hill mantle section at a ridge fracture zone intersection and the whole of the bay of islands ophiolite within a back arc spreading environment the veins and dykes formed during magmatism at the ridge fracture zone intersection and along the fracture zone as progressively hotter more fertile richer in clinopyroxene asthenosphere ascended and was channelled up and along the fracture zone wall shallow melting of refractory harzburgite in the presence of subduction derived hydrous fluids produced light rare earth element lree enric

dehydration melting of amphibolite at 10 kbar the effects of temperature and time we have simulated the dehydration melting of a natural low k calcic amphibolite 67 4 hornblende 32 5 anorthite in piston cylinder experiments at 10 kbar and 750 1000 c for 1 9 days the solidus temperature is lower than 750 c garnet appears at 850 c the overall reaction is hb p l cpx al hb ca hb ga opx three stages of reaction are 1 melting dominated by the growth of clinopyroxene and garnet with little change in composition of liquid or garnet 2 a reversal of this reaction between 875 c and 900 c with decreases in the amounts of liquid and garnet and 3 a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase garnet is enriched in pyrope and zoned from fe cores to mg edges range 3 mol pyrope liquid composition is enriched first in an to 950 c and then in ab the liquids are more calcic and aluminous than natural tonalites which is attributed to the plagioclase composition an90 the formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite not h2o saturated conditions the results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet absent field 900 1000 c but with a solidus backbend at 7 9 kbar coincident with the garnet in boundary hornblende breakdown due to garnet formation in a closed system must make h2o available for h2o undersaturated melting right down to the h2o saturated solidus below 700 c which defines a large low temperature pt area where hydrous granitoid melts can be generated with residual garnet and hornblende

k ar analyses of the post caldera lavas of bratan volcano in bali island indonesia ar isotope mass fractionation to light isotope enrichment abstract the post caldera lavas of bratan volcano in bali island indonesia were collected for whole rock chemical analyses and k ar analyses major and trace element chemistry shows that the lavas are basalts to andesites and typical of subduction related tectonic setting the 38 ar 36 ar ratios are 0 1851 3 0 1875 2 and the 40 ar 36 ar 294 3 0 3 301 6 0 1 which strongly suggest that the mass fractionation to light isotope enrichment took place the effect of the groundwater on magma is common on the basis of systematic mass fractionation of the atmospheric ar enriched in lighter isotopes this case was under the mass fractionation law analyzed numerically giving the mass fractionation correction ages 14 15 31 6 55 22 66 23 94 32 and 125 51 ka consistent with the volcano stratigraphy though the magma composition that changed frequently in time

global marine gravity from retracked geosat and ers 1 altimetry ridge segmentation versus spreading rate 1 three approaches are used to reduce the error in the satellite derived marine gravity anomalies first we have retracked the raw waveforms from the ers 1 and geosat gm missions resulting in improvements in range precision of 40 and 27 respectively second we have used the recently published egm2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean third we have used a biharmonic spline interpolation method to construct residual vertical deflection grids comparisons between shipboard gravity and the global gravity grid show errors ranging from 2 0 mgal in the gulf of mexico to 4 0 mgal in areas with rugged seafloor topography the largest errors of up to 20 mgal occur on the crests of narrow large seamounts the global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate for rates less than about 60 mm a the typical ridge segment is 50 80 km long while it increases dramatically at higher rates 100 1000 km this transition spreading rate of 60 mm a also marks the transition from axial valley to axial high we speculate that a single mechanism controls both transitions candidates include both lithospheric and asthenospheric processes

alpine and pacific styles of phanerozoic mountain building subduction zone petrogenesis of continental crust a broad continuum exists between two distinct end member types of mountain building alpine type orogenic belts develop during subduction of an ocean basin between two continental blocks resulting in collision they are characterized by an imbricate sequence of oceanward verging nappes some alpine belts exhibit superimposed late stage backthrusting sediments are chiefly platform carbonates and siliciclastics in some cases associated with minor amounts of bimodal volcanics pre existing granitic gneisses and related continental rocks constitute an autochthonous parautochthonous basement metamorphism of deeply subducted portions of the orogen ranges from relatively high pressure hp to ultrahigh pressure uhp calcalkaline volcanic plutonic rocks are rare and have peraluminous s type bulk compositions in contrast pacific type orogens develop within and landward from long sustained oceanic subduction zones they consist of an outboard oceanic trench accretionary prism and an inboard continental margin island arc the oceanic assemblage consists of first cycle in part melanged volcaniclastics and minor but widespread cherts deep water carbonates intimately mixed with disaggregated ophiolites the section recrystallized under hp conditions recumbent fold vergence is oceanward a massive slightly older to coeval calcalkaline arc is sited landward from the trench complex on the stable non subducted plate it consists of abundant dominantly intermediate metaluminous i type volcanics resting on old crust both assemblages are thrown into open folds intruded by comagmatic i type granitoids and metamorphosed locally to regionally under high t low p conditions in the subduction channel of collisional and outboard circumpacific terranes combined extension above and subduction below allows buoyancy driven ascent of ductile thin aspect ratio slices of hp uhp complexes to midcrustal levels where most closely approached neutral buoyancy exposure of rising sheets caused by erosion and gravitational collapse results in moderate amounts of sedimentary debris because exhumed sialic slivers are of modest volume at massive sialic buildups associated with convergent plate cusps syntaxes tectonic aneurysms may help transport hp uhp complexes from mid to upper crustal levels the closure of relatively small ocean basins that typify many intracratonic suture zones provides only limited production of intermediate and silicic melts so volcanic plutonic belts are poorly developed in alpine orogens compared with circumpacific convergent plate junctions generation of a calcalkaline arc mainly depends on volatile evolution at the depth of magma generation phase equilibrium studies show that under typical subduction zone p t trajectories clinoamphibole ca al hydrous silicates constitute the major hydroxyl bearing phases in deep seated metamorphic rocks of morb composition other hydrous minerals are of minor abundance ca and na clinoamphiboles dehydrate at pressures of above approximately 2 gpa but low temperature devolatilization may be delayed by pressure overstepping thus metabasaltic blueschists and amphibolites expel h2o at melt generation depths and commonly achieve stable eclogitic assemblages partly serpentinized mantle beneath the oceanic crust dehydrates at roughly comparable conditions for reasonable subduction zone geothermal gradients however white micas biotites remain stable to pressures 3 gpa accordingly attending descent to depths of 100 km mica rich quartzofeldspathic lithologies that constitute much of the continental crust fail to evolve substantial amounts of h2o and transform incompletely to stable eclogite facies assemblages underflow of amphibolitized oceanic lithosphere thus generates most of the deep seated volatile flux and the consequent partial melting to produce the calcalkaline suite along and above a subduction zone where large volumes of micaceous intermediate and felsic crustal materials are carried down to great depths volatile flux severely diminishes thus continental collision in general does not produce a volcanic plutonic arc whereas in contrast the long continued contemporaneous underflow of oceanic lithosphere does

water in h2o saturated magma fluid systems solubility behavior in k2o al2o3 sio2 h2o to 2 0 gpa and 1300 c abstract the distribution of h 2 o alkali silicate and alkali aluminosilicate components between coexisting h 2 o saturated melt and silicate and aluminosilicate saturated aqueous fluids has been determined in the pressure and temperature range 0 8 2 0 gpa and 700 1300 c respectively the solubility of silicate and aluminosilicate materials in aqueous fluid is between 1 and 12 mol this solubility is a positive function of both pressure and temperature the solubility increases with increasing alkali content and diminishes as the system becomes more aluminous the h 2 o solubility in coexisting h 2 o saturated peralkaline silicate and aluminosilicate melt ranges between 10 and 50 mol the h 2 o solubility is a positive and near linear function increasing pressure and of decreasing temperature the h 2 o saturation values are marginally sensitive to aluminum content the si k and al k ratios in the aqueous fluid are less than those of the coexisting aluminosilicate melt the partial molar volume of h 2 o in the melts v h 2 o melt is 14 cm 3 mol and decreases slightly with increasing temperature at the same temperature v h 2 o melt decreases as the melts become more aluminous the partial molar volume of h 2 o in the silicate and aluminosilicate saturated aqueous fluids v h 2 o fluid 16 7 22 cm 3 mol generally is less than that of pure h 2 o at the same pressure and temperature the v h 2 o fluid is insensitive to pressure in the 0 8 to 2 0 pressure range the v h 2 o fluid p t is much smaller than that of pure h 2 o in the 0 8 2 0 gpa pressure range the partial molar volumes of h 2 o in silicate melts and silicate saturated aqueous solutions were combined with published data to estimate the energy release during exsolution of h 2 o from h 2 o saturated magma in shallow magma chambers such as those feeding explosive dacitic eruptions 0 2 gpa and 800 1000 c about 1 9 10 9 ergs g h 2 o were obtained at 800 c and 0 2 gpa this value decreases by about 10 between 0 2 gpa and ambient pressure thus the exact pressure during such eruptions is not needed to evaluate the effect of exsolved h 2 o on the energy budget the energy contribution from exsolved h 2 o appears to be 2 to 4 of the total energy budget of such volcanic eruptions

lower crustal intrusion on the north atlantic continental margin near the faroe islands in the north atlantic the majority of basalt is intruded into the continent ocean transition rather than extruded onto the surface this melt is intruded into the lower crust as sills which cross cut the continental fabric rather than as an underplate of 100 melt as has often been assumed when continents break apart the rifting is sometimes accompanied by the production of large volumes of molten rock1 2 3 the total melt volume however is uncertain because only part of it has erupted at the surface furthermore the cause of the magmatism is still disputed specifically whether or not it is due to increased mantle temperatures we recorded deep penetration normal incidence and wide angle seismic profiles across the faroe and hatton bank volcanic margins in the northeast atlantic here we show that near the faroe islands for every 1 km along strike 360 400 km3 of basalt is extruded while 540 600 km3 is intruded into the continent ocean transition we find that lower crustal intrusions are focused mainly into a narrow zone 50 km wide on the transition although extruded basalts flow more than 100 km from the rift seismic profiles show that the melt is intruded into the lower crust as sills which cross cut the continental fabric rather than as an underplate of 100 per cent melt as has often been assumed evidence from the measured seismic velocities and from igneous thicknesses are consistent with the dominant control on melt production being increased mantle temperatures with no requirement for either significant active small scale mantle convection under the rift or the presence of fertile mantle at the time of continental break up as has previously been suggested for the north atlantic ocean4 5 6

aleutian terranes from nd isotopes nd isotope ratios substantiate the identification of oceanic crustal terranes within the continental crustal basement of the aleutian island arc the oceanic terranes are exposed in the westernmost aleutians but to the east they are completely buried by isotopically distinct arc volcanic rocks analogous oceanic terranes may be important components of the terrane collages that comprise the continents

the arc lavas of the shirahama group japan sr and nd isotopic data indicate mantle derived bimodal magmatism new sr and nd isotopic data are presented and integrated with previous data for the shirahama group mio pliocene mediumk volcanic arc suite of south central honshu japan main results are 1 the shirahama lavas range in sr sr from 0 70315 to 0 70337 and in mnd 144nd from 0 51298 to 0 51306 the sr and nd isotopic data cluster tightly within the mantle array and all lie within an overlapping field of midocean ridge basalt and ocean island basalt 2 small differences exist among the shirahama tholeiitic series calc alkaline series and mixed lavas the present isotopic data are consistent with a previously published model which proposes that chemical variations in magmas of coexisting tholeiitic and calc alkaline series are produced through crystal fractionation from mantlederived magmas of basalt and magnesian andesite respectively moreover the tholeiitic series and the calc alkaline series are isotopically identical thus both magma series can be derived from a source mantle with the same isotopic composition supporting the hypothesis of simultaneous generation of basalt and magnesian andesite magmas from a single diapir rising through the mantle wedge above the subduction zone the differences of water content and temperature within the diapir are again thought to have been produced through dehydration and heating of an isotopically homogeneous hydrous diapir the isotopic data show that the high sio lavas have the same isotopic compositions as more mafic lavas these data and liquid lines of descent of the shirahama magmas suggest that even rhyolites can be produced by differentiation from mantle derived magmas without crustal contamination analyses from 38 other arc volcanoes have been compiled to investigate the intravolcano variability of sr sr twelve of these display no intravolcano strontium isotopic variability as is the case with the shirahama group but others show greater variation of sr 86sr from individual volcanic centers presumably reflecting crustal contamination most of the latter volcanoes are underlain by thick continental crust it is noteworthy however that the greater variations of sr sr correlate with sio2 content andesites or dacites not basalts from the same volcano have the lowest sr sr and these rocks are calc alkaline in terms of feo mgo and sio theoretically assimilation of continental crust by the isotopically uniform shirahama magmas could produce these relationships given that mantle derived basalt and magnesian andesite both encounter continental crust on their ascent to the surface the hotter basalt magma would assimilate more crustal wallrocks than the cooler andesite resulting in the basalt being more radiogenic fractional crystallization magma mixing and or assimilation fractional crystallization of these magmas in crustal magma chambers could produce large compositional variations but the derivatives of the hotter basaltic magmas tholeiitic series in the broad sense would display greater contamination than those derived from the cooler andesitic magmas calc alkaline series

asthenospheric injection and back arc opening isotopic evidence from northeast japan nd and sr isotopic compositions were determined for the tertiary volcanics from the back arc side of the ne japan arc sr isotopes show a linear trend through time from an enriched signature 87sr86sr 0 705437 to a depleted signature 87sr86sr 0 70270 in a complementary fashion nd isotopes start at low value nd 0 80 and show a gradual increase nd 8 3 with decreasing age isotopic change of nd and sr from the enriched signature to the depleted one is synchronous with the opening of the japan sea at 15 ma this synchronism indicates that the opening of the japan sea was initiated by the injection of the asthenosphere during the pre opening stage the mantle wedge was composed of a two layered structure the sub continental lithosphere and the underlying asthenosphere the volcanics of this stage characterized by an enriched isotopic signature were derived from a source with a higher proportion of sub continental lithosphere the sub continental lithosphere of the back arc side was thinned by the injection of the depleted asthenosphere which accelerated the growth of the morb source within the mantle wedge of the back arc side and resulted in magma generation with the depleted isotopic signature of nd and sr at the post opening stage

southeast baffin volcanic margin and the north american greenland plate separation plate breakup over plumes is characterized by the development of margins showing extensive magma production both underplated at moho level and extruded as thick piles of seaward dipping lava formations the disko svartenhuk area in west greenland is one of the few places in the world with exposed seaward dipping basalts forming a prism whose thickness increases seaward we present a quantitative tectonic study of this margin which we tentatively restored in its geodynamic position during the different stages of plate separation between greenland and north america our structural data are constrained with recently published 39ar 40ar and new and coherent 40k 40ar geochronology in dikes of different orientations the first order structure is that of a tectonically driven seaward crustal flexure linked to definitive plate breakup between greenland and baffin island during the eocene coeval with the formation of the upper part of the exposed seaward dipping volcanic prism this flexing is associated with a significant crustal stretching associated with arrays of continentward dipping normal faults this across strike structure is correlated to a fundamental along strike segmentation with the three segments adopting a zigzag strike there is a clear increase of extensional strain at the extremities of the segments eocene extension trended n060 on average in the northern and southern segments but was nw trending in the central ne trending nugssuaq segment we discuss the interpretation of such an extension perpendicular to the different margin segments from a regional point of view the n060 extension is distinct in orientation from the approximately n s trend of plate separation between north america and greenland in the baffin bay during the eocene however the extension recorded in the margin is nearly perpendicular to the obliquely spreading eocene accretion axis in the baffin bay this latter point suggests a mantle or ridge control of the development of the margin over regional far field lithospheric stress models

comparing conservative and nonconservative tracers in karst and using them to estimate flow path geometry summary a controlled recharge event was conducted with multiple tracers in a karst aquifer in southeastern minnesota a pool adjacent to a sinkhole was filled with approximately 13 000 l of water after tracers were added and thoroughly mixed the pool was emptied into the sinkhole data were collected at freiheit spring approximately 95 m north of the sinkhole to monitor spring responses flow peaked first at the spring and suspended sediment peaked next then nearly identical uranine chloride and d peaks occurred temperature was the last of the tracers to peak the initial increase in flow at the spring recorded the time at which the water reached a submerged conduit sending a pressure pulse to the spring at approximately the speed of sound in open water the initial increase in uranine chloride and d at the spring recorded the arrival of the recharge water the initial change in temperature and its peak occurred later than the same features in the uranine chloride and d breakthrough curves as water flowed along this flow path water exchanged heat with the aquifer producing a lagged damped thermal peak at the spring the combination of hydraulic response and conservative and nonconservative tracers illustrates unique pressure advective and nonconservative processes geometrical properties of the flow system may be estimated using these tracers by summing discharge between the time of the initial increase in stage produced by a pressure pulse in a fully phreatic flow path and the time of the chloride peak the conduit volume is estimated as 47 10 m3 heat transport simulations were used to reproduce the modified thermal signal and simulations with planar flow paths and hydraulic diameters of 7 and 8 cm produced the best fits to the observed temperature breakthrough curve these volume and hydraulic diameter estimates together predict a bedding plane flow path that is 3 5 cm high by 9 m wide or 4 cm high by 8 m wide the different tracers provide complementary information and the combination of parameters provides useful constraints on flow path geometry

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

wet and dry basalt magma evolution at torishima volcano izu bonin arc japan the possible role of phengite in the downgoing slab the arc front volcanoes of sumisu 31 58n 1408e andtorishima 30 58n 140 38e in the central izu bonin arc are similar in size and rise as relatively isolated edifices from the seafloor together they provide valuable along arc information about magma generation processes the volcanoes have erupted low k basalts originating from both wet and dry parental basaltic magmas low zr basalts and high zr basalts respectively based on models involving fluid immobile incompatible element ratios la sm the parental basalts appear to result from different degrees of partial melting of the same source mantle 20 and 10 for wet and dry basalt magmas respectively assuming that the wet basalts contain greater abundances of slab derived components than their dry counterparts geochemical comparison of these two basalt types permits the identification of the specific elements involved in fluid transport from the subducting slab using an extensive set of new geochemical data fromtorishima where the top of the downgoing slab is about 100 km deep we find that cs pb and sr are variably enriched in the low zr basalts which cannot be accounted for by fractional crystallization or by differences in the degree of mantle melting these elements are interpreted to be selectively concentrated in slab derived metasomatic fluids variations in k high field strength element and rare earth element concentrations are readily explained by variations in the degree of melting between the lowand high zr basalts these elements are not contained in the slab derived fluids rb and ba exhibit variable behaviour in the low zr basalts ranging from immobile similar to k to mildly enriched in some low zr basalts we suggest that the k rich mica phengite plays an important role in determining the composition of fluids released from the downgoing slab in arcfront settings where slab depth is 100 km phengite is stable and the fluids released from the slab contain little k in back arc settings however where the slab is at 100 140 km depth phengite is unstable and k rich fluids are released we conclude that cross arc variations in the k content of arc basalts are probably related to differing compositions of released fluids or melts rather than the widely held view that such variations are controlled by the degree of partial melting

formation of tonalite from basaltic magma at the komahashi daini seamount northern kyushu palau ridge in the philippine sea and growth of izu ogasawara bonin mariana arc crust tonalitic rocks dredged from the komahashi daini seamount northern kyushu palau ridge are classified as biotite hornblende tonalites and hornblende tonalites these rocks have radiometric ages of 37 38 ma indicating that felsic plutonic activity occurred during the early stages of izu ogasawara bonin mariana ibm arc volcanism therefore this tonalite complex has great importance for understanding the initial processes of island arc and continental crust formation these tonalitic rocks exhibit the following petrological and geochemical characteristics 1 common lamellar twins and oscillatory zoning patterns in plagioclase phenocrysts throughout the compositional range 2 hornblende tonalite shows parallel ree patterns and increasing total ree content with increasing sio2 except for an increasingly strong negative eu anomaly at higher sio2 levels and 3 isotopic composition remains constant over a wide silica variation we compare this tonalite with younger tonalities of the same arc from the tanzawa complex 10 5 ma central japan considered to represent the lower middle crust of the ibm arc and find the following differences 1 cumulate textures found in tanzawa tonalites are not observed in samples from the komahashi daini seamount and 2 komahashi daini seamount tonalites unlike those from tanzawa exhibit linear variations of zr and rees vs sio2 plots these data and other observations support the interpretation that tonalite in the komahashi daini seamount was produced by crystal fractionation from basaltic magma we suggest that fractional crystallization operated during the early stage of oceanic island arc formation to produce tonalite whereas tonalities in later stages formed largely by partial melting of basaltic lower crust as represented by the tonalites in the tanzawa complex

chemical stratigraphy of the parana lavas south america classification of magma types and their spatial distribution a new classification scheme has been developed to assign the lava flows of the parana continental flood basalt province south america into geochemically distinct magma types with six basaltic major and trace element abundances and or ratios by mapping out the spatial distribution of these magma types within the lava sequences it has been possible to determine the internal stratigraphy of the lava pile on a regional scale previous studies on road profiles traversing the well exposed coastal serra geral escarpment of southern brazil are summarised together with results from some new sampled sections more widespread stratigraphical investigations of the parana lavas have been hampered by the lack of sufficient topographic relief and the cover of sedimentary rocks however access to drill core chippings from nine boreholes in the central parana region has provided a unique opportunity to investigate the stratigraphy of the otherwise inaccessible deeper levels of the lava pile and to map out stratigraphic variations in three dimensions the borehole samples have indicated cated a stacking of units of different magma types all overlapping towards the north which suggests that the main locus of magmatism moved northwards with time within the parana basin this migration could be related to the northward propagation of rifting during the initiation of the south atlantic ocean maps of the surface distribution of samples of each magma type show a pattern consistent with the stratigraphy inferred from the boreholes although suggesting that the shift in magmatism may have been towards the northwest on the basis of geochemical similarities between magma types and their inferred stratigraphical relationships it is proposed that the parana can be divided into two principal magmatic centres 1 an older one in the south comprising the gramado esmeralda and urubici magma types and 2 a younger one developed about 750 km to the north formed by the pitanga paranapanema and ribeira magma types

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

partial melting of mantle wedge and evolution of island arc crust evolution of the crust in the northeast honshu arc is discussed on the basis of the melting experiments of a peridotite under hydrous conditions and the crustal compositions estimated from deep crustal xenoliths and seismic velocity data the chemical compositions of melts formed by partial melting of a hydrous peridotite within the pressure temperature conditions for shallow parts of the mantle wedge 1 2 2 0 gpa have been experimentally determined with the sandwich experiment technique the melts range from olivine tholeiitic to magnesian andesitic compositions sio2 48 56 wt mgo 9 14 wt beneath the volcanic front in the northeast honshu arc and are slightly less silicic beneath the coast of the sea of japan the crustal compositions of the volcanic zone in the northeast honshu arc estimated from deep seated crustal inclusions in volcanic rocks and the velocity structures of the crust are andesitic in the central zone and high alumina basaltic near the coast of the sea of japan the compositions of possible primary melts formed in the mantle wedge are significantly more mafic than these crustal compositions consequently if the major part of the crustal materials in the volcanic zone was formed by magmas generated in the mantle wedge thick up to 30 km ultramafic cumulates including olivine pyroxenite must have been formed beneath moho during fractional crystallization and would constitute the upper part of the mantle wedge the volume of the mantle wedge consisting entirely of even fertile peridotite was not sufficient to produce crustal materials and cumulates in the volcanic zone by such processes in the northeast honshu arc a static mantle wedge model is therefore not satisfactory for the growth of the crust in northeast honshu arc a dynamic model is suggested in which the mantle materials have been supplied into the mantle wedge from the back arc side by a convective flow

seismic reflections from the moho the effect of isostasy summary deep seismic reflection profiling has become an established technique for studying the continental lithosphere the base of the crust can be identified on many deep profiles and it correlates closely with the moho determined by refraction studies on many reflection profiles the moho appears at a rather constant two way travel time despite the highly variable structure of the crust above it is shown that the apparent flatness of the moho on reflection time sections is an artefact produced when the crust is in local isostatic equilibrium the true geometry of the moho can only be determined if accurate and spatially well resolved seismic velocities are obtained from the lower crust the degree of isostatic compensation can be assessed from deep reflection data and an equivalent elastic thickness calculated the elastic thickness around the uk is very small only 2 3 km it is suggested that the many major fault zones visible on the seismic data weaken the lithosphere to produce these low values

volcanism on the rockall continental margin continental break up in the northern north atlantic was accompanied by massive volcanism this produced sequences of basalt flows erupted close to sea level which are imaged as seaward dipping reflectors on seismic reflection profiles they comprise the upper 4 5 km of transitional crust in both the edoras bank and hatton bank areas of the rockall continental margin detailed surveys show that the reflectors are convex upward on dip lines but are sub horizontal on strike lines they are probably produced by aerially extensive basalt flows extruded from linear fissure vents aligned parallel to the rift axis along the rockall margin two sequences of seaward dipping reflectors occur separated by an 9outer high9 near the foot of the continental margin the landward set of basalt flows are emplaced above stretched and intruded continental crust while the seaward set span the region from highly distended continental crust to fully oceanic crust the outer high is capped by basalt flows but exhibits little coherent internal reflectivity we discuss possible origins of the outer high as a residual block of thinned and intruded continental crust or as a central volcano cut by fissure vents similar to those found in the neovolcanic zones of iceland

shifts in the source of the parana basalts through time magmatism occurred intermittently in the parana region of south america between 240 50 ma with the peak of magmatic activity between 138 126 ma comprising the parana continental flood basalts cfb detailed chemical subdivision of the parana basalts into six units allowed stratigraphic correlations to be made implying a northward migration of magmatism following the rifting of the south atlantic however 40ar 39ar ages do not uphold the chronostratigraphic nature of such magmatic units instead they indicate a southeastwards migration major elements extrapolated to 8 wt mgo are used to investigate changes in magma type with time and are interpreted in terms of parameters such as pressure depth of melting and the nature of the source region fertile refractory and water content the oldest parana unit originated by melting at the greatest depths 90 120 km shifting to shallow levels 30 60 km for the younger units accompanying this decrease in pressure the source material undergoing partial melting became progressively more refractory in major elements and also more hydrous this hydrous metasomatism occurred after major element depletion of the lithosphere at shallow levels the data is consistent with conductive heating of a stratified source region inferred to lie within the lithospheric mantle which is relatively fertile but anhydrous at depth and also appears to be younger in age than the relatively major element refractory hydrous lithosphere at shallower depths thus the interaction of asthenosphere and lithosphere can be determined within an integrated age tectonic and thermal framework

generation and polybaric differentiation of east greenland early tertiary flood basalts major element trace element isotope and petrographic studies of magma diverentiation mantle melting the basal volcanics in the kangerdlugssuaq fjord region of the east greenland margin provide constraints on the generation and subsequent diverentiation and contamination histories of magmas during the early stage of continental rifting creating the north atlantic ocean basin most primitive lavas of the succession are introduction shown to have accumulated olivine corrections for crystal ac the distinctive geochemical features of continental flood cumulation yield estimates for erupted primitive liquids with mgo basalts cfbs are strongly controlled by the composition contents between 9 and 13 wt the occurrence of suspended and structure of the continental lithosphere and underfo88 olivine phenocrysts further requires the existence of parental lying mantle significant controversy remains concerning magmas with mgo contents up to 17 wt and feo contents up the balance of contributions from the lithosphere and to 14 wt quantitative modeling of fractionation indicates the mantle and the mechanisms by which the various that kangerdlugssuaq magmas primarily diverentiated at moderate source components contribute to basalt petrogenesis pressures 8 kbar to form dunitic and wehrlitic cumulates at how are basalt compositions influenced by generation depths of 25 km evolved east greenland flood basalts from the in anomalous regions in the deep mantle plumes by scoresby sund region record a more complex fractionation history interaction between rising magmas and the continental involving olivine clinopyroxene fractionation in the lower crust lithospheric mantle and by contamination with conas found for the lower basalts followed by olivine plagioclase tinental crust how are the diverentiation con clinopyroxene fractionation at shallow crustal levels cor tamination magma mixing and storage histories of cfbs relations between sio2 incompatible elements and sr nd isotopic controlled by the structure of the continental lithosphere ratios for the lower basalts are consistent with bulk assimilation and how do the thermal and chemical features of the of up to 15 20 archean leucogneiss with unradiogenic sr and mantle and the extensional history of the continental nd isotopic ratios characteristic of lewisian type lower crust lithosphere avect mantle melting and compositions of comparisons between inferred magma types for lower basalts and primary liquids estimated primary liquids from elsewhere in the north atlantic these questions are best addressed in the context of show complex relationships among feo mgo tio2 contents a geologically and geochemically well constrained cfb and cao al2o3 ratios that relate to regional and temporal changes province such as the north atlantic tertiary province in mantle temperatures lithospheric thickness and involvement of separation of east greenland from northern europe in at least two end member mantle sources we postulate that the the early tertiary at 60 myr ago was accompanied by lower basalts of east greenland were derived from iron poor voluminous volcanic activity the blosseville coast of mantle depleted by prior melt extraction and distinct from morb east greenland extending from kangerdlugssuaq fjord source mantle that was enriched in tio2 and lrees by a to scoresby sund fig 1 contains spectacularly exposed metasomatic event shortly before early tertiary magmatism we sections of lavas these sections begin at the basal contact associate this depleted mantle source with the ancestral iceland of the lavas with underlying cretaceous tertiary sediments and archean basement and continue to the plume anomaly

constraining south atlantic growth with seafloor spreading data recent models of south atlantic opening history focus on early plate divergence by incorporating intracontinental deformation which is poorly constrained aiming to avoid the uncertainties in this approach we model the entire divergence history with a joint inversion for seafloor spreading data for this history the pre campanian motion parameters are the first to feature formal uncertainty estimates we date the onset of spreading at 138 ma with movement along intracontinental accommodation zones leading to the assembly of south america by 123 ma and africa by 106 ma part of the ridge in the agulhas basin jumped westward soon afterward toward the bouvet plume initiating the motion of a short lived malvinas plate the ne georgia and maud rises and agulhas plateau formed as a large igneous province over the plume farther north part of the ridge jumped eastward toward the tristan plume around 94 93 ma but seems not to have resulted in independent plate motion our results show that the south atlantic grew by diachronous breakup of continents on just two plates cretaceous intracontinental deformation in south america and africa can be interpreted in terms of the accommodation of stress associated with northward propagation of this process the pattern of accommodation is usually envisaged as focusing all of the strain in narrow belts with our rotations a commonly used set of such belts accounts instead for just 42 67 of the implied total strain we suggest that the remainder was accommodated at all scales within the continental interiors and the extended continental margins

petrology of arc volcanic rocks and their origin by mantle diapirs abstract calc alkalic chemical trends characteristic of arc volcanic rocks mainly result from three mechanisms which act additively 1 fractional crystallization involving separation of titanomagnetite 2 selective concentration of plagioclase phenocrysts and selective depletion of titanomagnetite phenocryst compared with the actually fractionated proportion and 3 mixing of magmas on continuous fractionation trends the association of calc alkalic and tholeiitic trends in a single composite volcano may not represent different fractional crystallization processes or different chemistries of primary magmas but the calc alkalic chemical trend can be considered as a mixing trend resulting from mixing of various magmas on associated tholeiitic chemical trends chemical variations of most arc volcanic rocks including calc alkalic ones can accordingly be essentially accounted for by the low pressure fractional crystallization of phenocrystic phases from primary basaltic magmas crystallization sequences of arc magmas which are strongly dependent on water content in magmas are deduced from the phenocryst assemblages the crystallization sequence changes laterally across arc suggesting increasing water contents in magmas toward the back arc side as is also seen for other incompatible elements such as k and rb systematic differences in the characteristic crystallization sequence are also observed among arcs roughly correlating with the crustal thickness water content in magma like other incompatible elements tends to increase with increasing crustal thickness the variation of incompatible elements including water roughly represents that of the degree of partial melting of the upper mantle which is broadly controlled by the crustal thickness the variation of water content indicates that arc magmas are not saturated with water during differentiation to late differentiates such as dacite or rhyolite this strongly constrains the maximum water contents in primary basaltic magma at most 2 5 wt this value suggests that magma generation beneath arcs is dependent on dry solidus of peridotite diapiric uprise of the hot deeper mantle and associated adiabatic decompression would be necessary for mantle peridotite to attain the temperature as high as dry solidus diapirs that begin to rise from the subduction zone may stop at or near the crust mantle boundary because of the surrounding density change and their degree of partial melting is roughly controlled by their stopped depth assuming their similar temperature across arc variation is also explained by the stopped depth of diapirs but is not controlled by crustal thickness

crustal contamination of late neogene basalts in the dien bien phu basin nw vietnam some insights from petrological and geochronological studies early pliocene zanclean basalts in the dien bien phu pull apart basin in nw vietnam associated with the presently sinistral dien bien phu fault zone have been dated by the k ar method at 4 4 4 9 and 5 4 5 2 ma rapid migration of basaltic magma to the surface in the dien bien phu fault zone may be due to pliocene transtension of the crust in this region resulting from asthenospheric upwelling induced by lateral displacement of the mantle the basalts are moderately phyric 10 and consist of olivine hyalosiderite plagioclase bytownite labradorite and orthopyroxene bytownite labradorite phenocrysts and a fine grained crystalline matrix olivine hortonolite plagioclase labradorite clinopyroxene pigeonite and augite k feldspar the presence of fe rich olivine and orthopyroxene phenocrysts indicates that the basalts are sio2 saturated oversaturated olivine tholeiites which formed under water undersaturated conditions the dien bien phu basalts contain both mantle derived pyroxenites dunites gabbros and crustal sillimanite mullite mg fe spinel wallrock xenoliths indicative of crustal contamination during the ascent of the basaltic magma the basalts show selective enrichment in some mobile elements k rb sr and th a feature considered to be a result of metasomatism these rocks classified on the basis of their normative composition as quartz tholeiites could represent primary olivine tholeiites basalts in which the geochemical signatures were modified by the processes of contamination 2006 elsevier ltd all rights reserved

petrogenic implications of ree and ba data on mafic and ultramafic inclusions from itinome gata japan ultramafic and mafic inclusions from itinome gata in japan were studied systematically from a viewpoint of earth element ree and barium abundances mineral separates from lherzolites and their host basalt as well as andesite from an adjacent volcano were also analyzed for ree and ba garnet lherzolites which are believed to have formed at a depth of 50 to 70 km have a convex upwards chondrite normalized ree patterns similar to those of alpine type lherzolites spinel lherzolites which are believed to be derived from shallower depths than garnet lherzolites have various ree patterns and negative eu anomalies spinel lherzolites that are high in basaltic components have convex upwards ree patterns as do garnet lherzolites whereas mg rich strongly basalt depleted spinel lherzolites have v shaped ree patterns the ree variation among spinel lherzolites reflects the differing degrees of liquid phase extraction of basaltic components in times past the ree pattern of the liquid phase extracted from t

diffusion reaction in a thermal gradient implications for the genesis of anorthitic plagioclase high alumina basalt and igneous mineral layering abstract piston cylinder experiments investigating the interaction between basaltic andesite melt and partially molten gabbro in a thermal gradient provide insight into melt rock reaction processes occurring during magma differentiation in the crust in two experiments juxtaposing basaltic andesite and gabbro at 0 5 gpa pressure for durations of either 13 or 26 days diffusive chemical exchange between the two materials results in mineral layering and notable mineral compositions such as anorthitic plagioclase specifically the basaltic andesite gains al2o3 mgo and cao from the gabbro and loses na2o k2o sio2 and feo to it with a plagioclase rich layer developing at the interface between the two materials in a process termed diffusion reaction the percent crystallinity of the basaltic andesite increases during the process and the plagioclase crystals within the interface region develop anorthitic cores up to an90 that abruptly shift in composition to thin rims that are in na ca exchange equilibrium with the co existing melt both the mineralogical layering and bulk compositional change occurring at the interface are reproduced in model simulations of diffusion reaction isotopic tracers 45ca 6li 84sr and 136ba initially deposited at the basaltic andesite gabbro interface in the 13 day experiment were detected in the cores of the anorthitic plagioclase after the experiment demonstrating that the melt chemically communicates with the plagioclase cores over the duration of the diffusion reaction experiment the formation of anorthitic plagioclase during diffusion reaction may explain its widespread occurrence in terrestrial volcanic rocks without requiring the presence of ultra calcic melts textures and mineralogical changes in the gabbro indicate that chemical transport occurs throughout the experiments despite temperatures at the cold end of the experimental capsule approaching 500 c for instance apatite fenis olivine and almost pure albite occur at distinct specific horizons in the gabbro within the 26 day experiment because the bulk element profiles indicating chemical transport reflect analyses of almost completely solid gabbro equilibration between minerals and fluids melts must be rapid the overall effect of the diffusion reaction process is to make an ascending magma more primitive in composition and in this case produce anorthitic plagioclase while making surrounding crustal wall rocks more evolved several observations within igneous rocks support the occurrence of this process suggesting that the genesis of porphyritic high alumina basalt ubiquitously observed at convergent margins could reflect a diffusion reaction process in the crust

the genesis of silicic arc magmas in shallow crustal cold zones abstract a number of currently popular models for the genesis of evolved arc magmas from basaltic andesite to dacite invoke repeated intrusion partial melting and differentiation at the base of the crust however several observations suggest that this may be the exception rather than the norm 1 geobarometry often indicates shallow pressure 0 1 0 3 gpa evolution 2 incongruent melting of amphibolite at elevated pressures should yield magmas in equilibrium with high pressure phases like garnet but rare earth element patterns almost ubiquiously preclude this 3 compositionally zoned caldera forming eruptions suggest differentiation at near surface depths 4 u series data most commonly indicate differentiation over millennia time scales this requires rapid cooling that in turn is most easily explained by relatively small magma volumes undergoing crystal fractionation within the shallow i e cool crust to further test these ideas we combined published experimental data for liquidus equilibria with appropriate silicic arc magma compositions on projections of the ternary liquidus system nepheline silica olivine recent data for tongan silicic lavas plot either on or close to low pressure 1 atm cotectics for the rocks phenocryst phases suggesting low pressure differentiation using our own and published data from arc volcanoes around the world we find that the majority are consistent with differentiation at shallow depths regardless of total crustal thickness combined with the typical timescales of differentiation we estimate that the volumes of magma stored during differentiation in shallow crustal zones are usually on the order of only a few km 3 there is also a clear role for mixing and recharge that involves magmas that are more deeply sourced and primitive in character typically evolved basalts and basaltic andesites whether the latter differentiated in the lower crust or at the crust mantle boundary has important implications for the constitution and average composition of arc crust at present we can only conclude that evidence for more silicic arc magma generation at these depths is generally lacking

combined tracer tests in the karst aquifer of the artesian mineral springs of stuttgart germany chlorinated solvents have been detected at low concentrations in some of the mineral and medicinal springs spas of stuttgart since 1984 these springs discharge from a confined karst aquifer in order to investigate both the properties of the aquifer and the mechanisms of contaminant transport two multi tracer tests were carried out in 1998 and 1999 both fluorescent tracers naphthionate eosin pyranine and particle tracers clubmoss spores microspheres were used all available wells and springs were sampled for at least 12 months in these experiments naphthionate produced the best results maximum flow velocities were established to be within the range of 53 and 230 m day the breakthrough curves demonstrated a heterogeneous aquifer the results identified flow to the springs from the west and south west it was possible to prove an assumed boundary between the northern zone of low mineralised water and the southern zone of highly mineralised water

densities and porosities in the oceanic crust and their variations with depth and age we have used drilling and downhole logging results from deep sea drilling project holes 395a and 418a measured properties of core samples from the seafloor and from ophiolites and seismic velocity structure models of the oceanic crust to constrain empirical statistical relationships between in situ densities porosities and seismic velocities and to estimate densities and porosities in the oceanic crust with the following principal results our review of drilling results and studies of ophiolites suggests that the velocity structure of the oceanic crust is more closely related to changes in porosity and alteration or metamorphic grade than to its igneous structure the transition from layer 2a 3 7 0 4 km s 1 to 2b 5 2 0 4 km s 1 occurs within the extrusive pile the top of layer 2c 6 2 0 2 km s 1 is associated with the top of the sheeted dike complex and with the first appearance of greenschist facies alteration products ophiolite studies have shown that the top of layer 3 occurs within the sheeted dikes where the metamorphic grade of alteration products increases from greenschist to amphibolite facies a comparison of reprocessed and corrected downhole logs from holes 395a and 418a indicates that in situ densities and fractional porosities differ by 0 2 mg m 3 and 0 07 respectively these differences are consistent with prevailing views of the progressive alteration of the upper oceanic crust statistical relationships between in situ density porosity and slowness are highly linear and indicate that the density slowness relation estimated from laboratory samples pb 3 81 0 02 6 0 0 1 s applies to in situ properties as well we obtained a similar empirical relationship for porosities from the hole 418a log data f 0 35 0 03 2 37 0 15 s r2 0 84 s e 0 015 applying these empirical models to seismic velocity structure models yields the following crustal densities and porosities layer 2 2 62 2 69 mg m 3 0 10 0 12 and layer 3 2 92 2 97 mg m 3 the estimated average density of the oceanic crust is 2 86 0 03 mg m 3 densities and porosities estimated from a seismic crustal structure model for old atlantic crust agree very well with the logging data from hole 418a suggesting that in situ densities and porosities can be estimated from seismic velocities high density and porosity gradients are characteristic of the upper oceanic crust increasing densities reflect both a progressive decrease of porosity and increasing grain density with depth based on the seismic properties of layer 2a we estimate that the layer thins at an approximate rate 20 to 45 m m y 1 while the velocity increases by 0 040 km s 1 m y 1 the corresponding rates of change of density and porosity are 0 016 0 009 mg m 3 m y 1 and 0 005 0 003 m y 1 respectively

garnet growth during amphibolite anatexis implications of a garnetiferous restite formation of a garnet clinopyroxenite restite may have important physicochemical consequences for deep crustal and mantle processes we have experimentally simulated the dehydration melting of amphibolite and report the modal and compositional evoltuion of the mafic granulitic mineral assemblage the restite with an emphasis on garnet a natural low k calcic amphibolite was powdered and run in sealed gold capsules in a piston cylinder apparatus at 10 kbar 750 1000 c 1 to 9 day runs and fo 2 ni nio garnet first appears in 8 day runs at 825 25 c above 850 c clinopyroxene and poikilitic garnet comprise most of the restite garnet peaks at 950 c at 32 vol of the material within the capsule or 41 vol of the restite assemblage and disappears between 975 1000 c there is no cross over of fe mg partitioning between coexisting liquids and garnets liquids remain mg poorer than garnets throughout the melting interval but do increase in mg with increasing temperature thus the hotter liquids may be less likely to react with mantle wedge peridotite the modes are consistent with ree patterns of archean tonalitic trondhjemitic rocks generated by the partial melting of garnet bearing mafic crust a garnet clinopyroxenite with 41 vol garnet has a density of 3 5g cm 3 which may induce lower crustal delamination in hot nonthickened amphibolitic crust this crust is seismically indistinguishable from lithosphere or mantle because garnet growth traps water bearing hornblende inclusions the restite may contain up to 0 3 wt water thus delivery of poikilitic garnet to the mantle possibly by delamination or by subduction may be an important mechanism for adding water to the mantle

origin composition and relative timing of seaward dipping reflectors on the pelotas rifted margin abstract the mechanism by which seaward dipping reflectors sdrs are formed is a topic of debate two contrasting models exist for their formation the volcanic faulting model and the volcanic loading model each of these models has important implications for the processes which control the structure and formation of magma rich rifted continental margins we have examined high quality deep seismic reflection data across the pelotas basin offshore brazil these data reveal a remarkable set of sdrs for which we have investigated the likely nature of their formation the total package of sdrs has an across strike width of 200 km and a variable vertical thickness of 10 17 km previously interpreted as volcanic flows detailed observations however show changes in seismic character and geometry within the sdr package which suggest a complex and varied evolution we have used gravity anomaly inversion and seismic observations together to investigate the likely composition of the sdrs by determining the proportion of basaltic material to sedimentary volcaniclastic material basalt fraction within the sdrs this has been achieved by minimising the difference between the depth of the gravity moho and seismic moho in order to quantify the lateral variation in basalt fraction taken to be proportional to the bulk density of the package the density of the sdr package together with seismic interpretation is then used to infer the composition depositional environment source and time of formation relative to breakup our analysis suggests that the overall sdr basalt fraction and bulk density decrease oceanwards possibly due a change in the type of volcanic deposits from predominantly subaerial to volcaniclastics possibly deposited subaqueously the sdrs can be split into three sub packages the two inner sdr packages are interpreted to consist of lava flows sourced from syn tectonic subaerial eruptions associated with the onshore parana large igneous province flowing eastwards into an extensional basin the outer sdr package shows reflector geometries that progressively offlap oceanwards interpreted as extrusives sourced from an eastwards migrating newly formed ocean ridge our analysis suggests that both the volcanic faulting and volcanic loading models for sdr formation are applicable to the pelotas rifted margin recording distinct syn rift and syn breakup magmatic events we show that both sdr formation models can be recognised in a naturally occurring example and can coexist on the same margin

seismic response and construction of seaward dipping wedges of flood basalts voring volcanic margin geological and geophysical data from the 900 m of volcanic basement drilled at ocean drilling program site 642 provide the framework for studying seismic properties of huge extrusive constructions on volcanic margins the main part of the drilled section corresponding to a prominent seaward dipping reflector sequence consists of subaerially emplaced tholeiitic basalt flows and thin interbedded sediments the basalts exhibit a characteristic velocity and density lava flow distribution reflecting changing porosity pore aspect ratio distribution and alteration stacks of laterally continuous basalt flows appear to have thin layer transverse isotropic properties for typical wavelengths in multichannel seismic data vertical seismic profiling and average sonic log velocities are similar 3 77 km s and 3 88 km s respectively while comparable refraction velocities are 10 20 higher synthetic seismogram modeling based on downhole logs shows that basement reflectors originate from interference and tuning effects of numerous basalt flow and interbedded sediment interfaces though the most continuous reflectors are related to thick flows seismic models based on the characteristic velocity and density basalt flow distributions and site 642 stratigraphy show that reflector truncation and onlap may be caused by seismic interference phenomena in a sequence of landward thinning flows the base of the dipping reflector sequence reflector k correlates with flows in the lower part of upper series basalts while the transition from basaltic to underlying dacitic andesitic lavas correlates with a locally defined reflector a model for the breakup related volcanism includes 1 prebreakup dacitic andesitic volcanism 2 early breakup basaltic volcanism infilling the prebreakup relief 3 main breakup stage with intense focused volcanism large subsidence and lava pounding and 4 late breakup volcanism during a period of decreased subsidence and local off axis activity

residence time and crystallization history of nickeliferous olivine phenocrysts from the northern yatsugatake volcanoes central japan application of a growth and diffusion model in the system mg fe ni abstract the chemical zoning of nickeliferous olivine phenocrysts in calc alkaline andesites from the northern yatsugatake volcanoes central japan is examined by a growth and diffusion model in the system mg fe ni nickeliferous olivine phenocrysts are included in the high alumina basaltic end member magma which forms the mixed calc alkaline andesites in the northern yatsugatake volcanoes the olivine phenocrysts can be divided into three types on the basis of chemical composition zoning profile and occurrence type 1 olivine is rich in ca and mg and the zoning profile of type 1 forms a slightly more fe rich trend than the maximum fractionation trend in the nio versus forsterite content fo diagram the most mg and ni rich and mn poor core of type 1 fo 91 5 mol nio 0 40 wt mno 0 13 wt can be in equilibrium with peridotite in the upper mantle by contrast type 2 olivine is ca poor and rich in fe and ni compared to type 1 the zoning patterns of type 2 are more variable the nio content in fe rich type 2 phenocrysts decreases rapidly from core to rim within a narrow range of fo the maximum nio content of type 2 is 0 53 wt the transitional type olivine has an intermediate composition between type 1 and type 2 and a similar zoning profile as that of type 1 type 1 olivine mainly occurs as isolated phenocrysts while fe rich type 2 occurs as crystal clots the growth and diffusion calculation in the system mg fe ni revealed that the composition of normally zoned olivine is enriched in fe by diffusion without a marked depression in the ni content due to a larger fe mg interdiffusion coefficient than the ni tracer diffusion coefficient in olivine the zoning patterns of both type 1 and type 2 can be reproduced by calculation the residence time of type 1 olivine in the magma is estimated to be less than 10 0 5 1 y at 1150 1050 c whereas the residence time of fe enriched type 2 olivine is from ten to one hundred times longer than that of type 1 the long residence time and occurrence as crystal clots suggest that the fe enriched type 2 olivine phenocrysts were captured and crystallized in the crystal mush at the roof and bottom of a magma chamber

an algorithm to calculate the gravity anomaly of sedimentary basins with exponential density depth relationships we derive wavenumber domain expressions to calculate the gravity anomaly of a body with irregular bounding surfaces and an exponential density depth relationship we apply the method to sedimentary basins which commonly have this type of geometry and density distribution the mathematical formulation also allows the exponential density depth relationship to be measured from an arbitrary irregular surface rather than the top surface using this arrangement the gravity anomaly of exhumed sedimentary basins can be predicted if the amount of eroded section can be estimated the corresponding inverse algorithms are also derived examples of the use of the forward algorithms from the galicia interior basin and the central irish sea basin are used to illustrate these methods

geophysical and petrological modelling of the structure and composition of the crust and upper mantle in complex geodynamic settings the tyrrhenian sea and surroundings abstract information on the physical and chemical properties of the lithosphere asthenosphere system las can be obtained by geophysical investigation and by studies of petrology geochemistry of magmatic rocks and entrained xenoliths integration of petrological and geophysical studies is particularly useful in geodynamically complex areas characterised by abundant and compositionally variable young magmatism such as in the tyrrhenian sea and surroundings a thin crust less than 10 km overlying a soft mantle where partial melting can reach about 10 is observed for magnaghi vavilov and marsili which belong to the central tyrrhenian sea backarc volcanism where subalkaline rocks dominate similar characteristics are seen for the uppermost crust of ischia a crust about 20 km thick is observed for the majority of the continental volcanoes including amiata vulsini roccamonfina phlegraean fields vesuvius vulture stromboli vulcano lipari etna and ustica a thicker crust is present at albani about 25 km and at cimino vico sabatini about 30 km the structure of the upper mantle in contrast shows striking differences among various volcanic provinces volcanoes of the roman region vulsini sabatini alban hills sit over an upper mantle characterised by vs mostly ranging from about 4 2 to 4 4 km s at the alban hills however slightly lower vs values of about 4 1 km s are detected between 60 and 120 km of depth this parallels the similar and rather homogeneous compositional features of the roman volcanoes whereas the lower vs values detected at the alban hills may reflect the occurrence of small amounts of melts within the mantle in agreement with the younger age of this volcano the axial zone of the apennines where ultrapotassic kamafugitic volcanoes are present has a mantle structure with high velocity lid vs 4 5 km s occurring at the base of a 40 km thick crust beneath the campanian volcanoes of vesuvius and phlegraean fields the mantle structure shows a rigid body dipping westward a feature that continues southward up to the eastern aeolian arc in contrast at ischia the upper mantle contains a shallow low velocity layer vs 3 5 4 0 km s just beneath a thin but complex crust the western aeolian arc and ustica sit over an upper mantle with vs 4 2 4 4 km s although a rigid layer vs 4 55 km s from about 80 to 150 km occurs beneath the western aeolian arc in sardinia no significant differences in the las structure are detected from north to south the petrological geochemical signatures of italian volcanoes show strong variations that allow us to distinguish several magmatic provinces these often coincide with mantle sectors identified by vs tomography for instance the roman volcanoes show remarkable similar petrological and geochemical characteristics mirroring similar structure of the las the structure and geochemical isotopic composition of the upper mantle change significantly when we move to the stromboli campanian volcanoes the geochemical signatures of ischia and procida volcanoes are similar to other campanian centres but sr pb isotopic ratios are lower marking a transition to the backarc mantle of the central tyrrhenian sea the structural variations from stromboli to the central vulcano and lipari and western aeolian arc are accompanied by strong variations of geochemical signatures such as a decrease of sr isotope ratios and an increase of nd pb isotope and lile hfse ratios the dominance of mafic subalkaline magmatism in the tyrrhenian sea basin denotes large degrees of partial melting well in agreement with the soft characteristics of the uppermost mantle in this area in contrast striking isotopic differences of plio quaternary volcanic rocks from southern to northern sardinia does not find a match in the las geophysical characteristics the combination of petrological and geophysical constraints allows us to propose a 3d schematic geodynamic model of the tyrrhenian basin and bordering volcanic areas including the subduction of the ionian adria lithosphere in the southern tyrrhenian sea and to place constraints on the geodynamic evolution of the whole region

tectonic controls on magmatism associated with continental break up an example from the parana etendeka province the high and low ti basalts of the parana etendeka province were primarily derived from old trace element enriched source regions in the lithospheric mantle and they are associated with dyke swarms of different orientations these swarms appear to reflect different amounts of extension and it is inferred that the high and low ti magma types were characterised by different melt generation rates of 0 15 km3 yr 1 and 0 4 km3 yr 1 respectively stewart et al earth planet sci lett 143 1996 95 109 there is probably a gap of 2 myr between the end of the main phase of cfb magmatism and the oldest rocks on the adjacent ocean floor a simple numerical model has been used to constrain the amounts and rates of melt generated from the continental lithosphere and asthenosphere under finite duration extension melting in the mantle is assumed to be controlled by the dry peridotite solidus in the asthenosphere and the hydrous 0 2 h2o peridotite solidus in the lithosphere for a maximum of 4 and a duration of extension of 10 myr the derivation of melt from the asthenosphere by dry peridotite melting depends primarily on potential temperature tp and is relatively insensitive to the thickness of the mbl while the converse is the case for melt derived from the lithosphere by hydrous peridotite melting for a tp of 1450 50 c inferred from the crustal thickness estimates along the rio grande rise and walvis ridge the model successfully generates 2 4 km of lithosphere derived melt before producing significant volumes of asthenosphere derived melt it is concluded that increases of melt volume with time can be generated by decompression melting of the mantle lithosphere critically in areas of significant melt generation within the mantle lithosphere during extension and break up there is likely to be a gap in the volcanic record between the end of melt generation in the lithosphere and the onset of melting in the underlying asthenosphere no such gap is present if all melts are generated within the mantle plume and thus these models may in principle be tested in the geologic record

island arc magmatism in relation to the evolution of the crust and mantle abstract one aspect of the evolution of the earth s mantle is the development through geologic time of the continental crust a popular hypothesis is that the earth s crust has developed through the magmatic events associated with plate movements a two stage melting process involving generation of ocean ridge basalts followed by subduction and partial remelting of the oceanic lithosphere to form andesitic suites has been invoked as the major continental crust forming magmatic cycle however there is chemical and isotopic evidence for the composition of continental crust and arcs that is in conflict with the broad consequences predicted by this hypothesis the average composition of the upper continental crust has been fairly readily determined as granodioritic but the composition of the lower crust is less certain seismological and petrological evidence suggest that a substantial portion of the lower crust is basaltic in character therefore it cannot be concluded that the bulk average crust corresponds to andesite again the rock type most frequently collected in many arcs is andesite but the conclusion that the whole arc pile is of this composition is illogical and not supported by much field and petrologic evidence the peripheral accretion of arcs to continents is well documented for post palaeozoic epochs so that the nature of arc magma sources is critically relevant to the processes of continental crustal growth a large body of data on arc magmas can be explained by the hypothesis that the superjacent upper mantle wedge rather than the subducted lithosphere itself is the major source of arc erupted melts for many oceanic arcs isotopic evidence 87 sr 86 sr 143 nd 144 nd rules out the possibility that they constitute appropriate bulk crustal additions for the development of continents continental crustal growth may therefore either have changed in essential chemical isotopic character since the cambrian or arc magmas derived from chondritic type or lil enriched sources may be the only volumetrically significant additions to continental crust

basalt magma genesis and fractionation in collision and extension related provinces a comparison between eastern central and western anatolia experimental studies of synthetic and natural basalt systems suggest that conditions of magma genesis and fractionation depend fundamentally on mantle temperatures and lithospheric stress fields in general compressional settings are more conducive to polybaric fractionation than extensional settings and in this regard the anatolian magmatic province offers a natural laboratory for comparing near coeval basalt eruptions as a function of regional tectonics compressional collision related regimes dominating in eastern anatolia and extensional tectonics characterizing a western province related to aegean sea opening projection of plio quaternary basalt normative compositions from the western anatolia extensional province waep the central anatolian ova province caop and eastern anatolia compressional province eacp are projected onto ol ne cpx and pl cpx ol planes in the simplified basalt system ne cpx ol qz each showing distinctive liquid lines of descent waep basalts are mostly constrained by low pressure 0 5 gpa cotectics while caop and eacp compositions conform to moderate and or high pressure 0 8 3 0 gpa cotectics overall a quasi linear shift from moderate and or high pressure to low pressure equilibria matches the westward transgression from compressional east anatolia to the extensional west anatolian aegean region comparison of their respective primary mantle equilibrated magmas simulated by normalizing their compositions to mgo 15 wt mg 15 with parameterized anhydrous and h2o undersaturated experimental melts suggests they segregated from spinel to garnet lherzolite mantle facies at pressures between c 2 and 3 gpa c 70 100 km depth under h2o undersaturated conditions interpolated potential temperatures tp and lithospheric stretching factors range as follows 1 eastern anatolian basalts associated with the arabian foreland show tp varying between 1250 and 1400 c except for the karacalidag alkali basalts south of the bitlis zagros fracture zone for which tp ranges up to 1450 c for values of 1 2 1 8 tp values for central anatolia e g sivas range between 1300 and 1375 c except for karapinar egrikuyu and hasandag which show 1150 c and values of 1 3 1 4 for western anatolian basalts tp range mostly between 1250 and 1330 c except for a single value for canakkale of 1400 c and kula sample showing tp 1200 c and values of 1 3 2 0 variation of these conditions is as great or greater than that between provinces although there are clearly significant constraints on the inferred polybaric to low pressure isobaric fractionation regimes covariation of total feo tio2 la yb ce sm zr y and zr nb reflects small but significant differences in bulk composition and ambient melt fraction while the covariance of ce sm and sm yb is consistent with the segregation of primitive melts at the spinel to garnet lherzolite transition

intracrustal controls on the coexistence of tholeiitic and calc alkaline magma series at aso volcano sw japan aso caldera sw japan was formed incrementally over a period of 300 kyr by the eruption of four basaltic to dacitic pyroclastic flow and fall deposits aso 1 4 these units are collectively referred to as the aso pyroclastic flow deposit apfd in which tholeiitic and calc alkaline magmas have been erupted in close temporal and spatial proximity detailed petrological and geochemical studies indicate that a major shift in magmatic composition occurred between aso 1 and 4 as the caldera forming stage evolved changing from a predominantly tholeiitic to a calc alkaline signature the mineralogical and geochemical characteristics of aso 1 4 suggest that magmatic evolution within each unit is controlled by fractional crystallization associated with magma mixing crustal assimilation by altering the fractional crystallization mineral assemblage as well as the relative amounts of fractional crystallization magma mixing and crustal assimilation the systematic shifts in differentiation trends and magmatic series observed between the four eruptive units can be reproduced this suggests that the coexistence of tholeiitic and calc alkaline magma series can be attributed to shallow level intracrustal processes

on the lateral variations in chemical composition and volume of quaternary volcanic rocks across japanese arcs the fe mg fe ratios xfe of the quaternary basalts sio2 53 wt in the japanese arcs were examined the xxfe of relatively magnesian basalts decreases from the volcanic front toward the japan sea across the arcs based on the partition coefficient of mg fe2 between olivine and liquid it is suggested that all the basalts near the volcanic front which are mostly tholeiitic basalts are significantly fractionated whereas many basalts near the japan sea which are mostly alkali basalts are little fractionated the k2 o content in the primary basalt magmas increases toward the japan sea combining the xfe and k2 o data it is suggested that relatively large amounts of tholeiitic magmas are produced near the volcanic front but they fractionate during their ascent whereas smaller amounts of alkali basalt magmas are formed near the japan sea but they can ascend with less fractionation the density of primary tholeiite magma is significantly larger than that of primary alkali basalt magmas it is most likely that primary tholeiite magmas cannot ascend beyond the upper crust and would fractionate to produce less dense tholeiitic magmas near the volcanic front whereas primary alkali basalt magmas can ascend through the upper crust without fractionation as far as buoyancy is the principal ascending force in the japanese arcs the stress field may be less compressional near the japan sea than near the volcanic front so that magmas can ascend more rapidly in the latter region than in the former these two factors may be responsible for the above mentioned chemical variations of basalt magmas across the arcs the variation in volume of the quaternary volcanic rocks across the arcs can be explained by the presence of a melt rich zone above but nearly parallel to the subducted slab

anomalous intra plate high mg andesites in the choshi area chiba central japan produced during early stages of japan sea opening abstract geochemical and isotopic data are reported for 20 ma high magnesian andesite hma lavas erupted on the forearc side of central japan in the choshi coastal area chiba rarely olivine phyric 345 ppm high sr 520 ppm high sr y ca 30 and high la yb n 8 conforming to the definition of low silica adakites transitional in character to hma however despite slight negative nb and ta troughs in a few samples their incompatible element distribution patterns appear closer to those of intra plate basalts apparently confirming previous suggestions that terms such as hma and adakite cover a diverse range of compositions enriched in radiogenic sr and nd ca 0 7040 to 0 7043 and 0 5128 to 0 51275 respectively the choshi hma shows relatively low 206pb 204pb 18 25 to 18 35 and 208pb 204pb 38 37 to 38 48 ratios resembling those of late miocene intra plate basalts in the japan sea and sw japan in this regard the choshi hma differs from those of the classic middle miocene 14 to 12 ma hma described from the setouchi volcanic belt e g tatsumi 1981 a short distance to the west which also shows relative high k contents although hmas have been generally attributed to subduction related processes the lack of high field strength element hfse depletions cf large ionic lithophile elements lile in most choshi hma suggests an absence of so called subduction signatures accordingly we suggest that the choshi hma probably tapped a fertile asthenospheric region of the convecting mantle wedge primitive melt products having acquired hma like character as a result of shallow level modification by interaction with lithospheric components aided by thermal input resulting from mantle extrusion during the early stages of japan sea opening 23 15 ma

metagabbros of the gangdese arc root south tibet implications for the growth of continental crust abstract the gangdese batholith of the southern tibet is regarded as a principal component of the pre himalayan collision andean type continental margin however the gangdese arc root remains poorly understood because most of previous studies concentrated on granitoids from the middle and upper crust of the arc in this paper we report new petrochemical and geochronological characteristics of metagabbros occurred as a part of the late cretaceous lilong batholith in the eastern gangdese arc the gabbros with protolith ages of 82 95 ma underwent slightly late 68 77 ma granulite facies metamorphism and nearly coeval anatexis typical of lower arc crusts the metagabbros and diorites of the lilong batholith show geochemical features of juvenile crust in continental magmatic arcs and an original magmatic differentiation trend with gabbros formed as the cumulates whereas diorites as the crystalline products of evolved magmas the lilong batholith forms an arc crustal section with a possible thickness of 20 30 km and may have an overall andesitic composition providing a support for the andesite model of continental crust growth

origin of seaward dipping reflectors in oceanic crust off the norwegian margin by subaerial sea floor spreading a remarkable layered acoustic stratification is observed in the upper oceanic crust adjacent to the norwegian continental margin it comprises a seaward dipping complex of reflectors in the form of a wedge we suggest that it is a layered igneous sequence that results when crustal accretion occurs at a subaerial spreading axis and that this phenomenon may commonly occur during the earliest phase of ocean basin genesis

age of the ponta grossa dike swarm brazil and implications to parana flood volcanism the ponta grossa dike swarm pgds occurs in a nw trending 200 km wide zone exposed just east of the parana basin in southeastern brazil the predominantly basaltic dikes intrude crystalline basement paleozoic mesozoic sediments and rarely flows of the parana etendeka flood volcanic province pep the pgds resembles the failed arm of a rift rift rift triple junction related to the separation of south america and africa detailed geochemical studies of the dikes including major minor trace element and sr nd isotopic analyses indicate that they probably represent feeders for the voluminous phase of flood volcanism represented by relatively uncontaminated predominantly high tio2 lavas of the northern pep where lava accumulations reach 1700 m thick 40ar 39ar stepwise degassing data using both laser and radiofrequency induction furnace on plagioclase separates from eighteen dikes and one sill yield seventeen plateau ages three are between 120 7 1 3 ma and 125 8 0 6 ma and fourteen are clustered between 129 2 0 4 ma and 131 4 0 5 ma isochron ages are not significantly different from the plateau ages and plateau ages are adopted in all but two cases as being the most accurate age estimates the age probability distribution for the dominant pulse 131 4 0 4 to 129 2 0 4 ma shows a pronounced peak at 130 5 ma this distribution probably reflects the magma production history in the region the new geochronologic data are consistent with conclusions based on paleomagnetic and chemical stratigraphic data that the pgds is younger than the volumetrically dominant majority of volcanism in the southern pep which occurred at 133 132 ma the younger commonly ne trending dikes may reflect the initiation of full drift which was coincident with major basin development in the adjacent continental borderland at 125 120 ma the pgds may indeed represent the failed arm of a rift rift rift triple junction but the triple junction did not coincide exactly in time or space with the site that would be inferred for plume impact

karst geomorphology of carbonatic conglomerates in the folded molasse zone of the northern alps austria germany abstract the folded molasse zone of the northern alps consists of clastic sedimentary rocks that are usually not considered to be karstifiable however large areas within this zone are composed of carbonatic conglomerates numerous karst landforms have recently been discovered but are not recorded on official maps and in the literature therefore a research programme was initiated at the hochgrat site austria germany that included geomorphological mapping and characterisation of the karst phenomena both fracture controlled and hydrodynamically controlled karren were observed on conglomerate outcrops the predominant karst landforms dolines are typically circular funnel shaped most often 2 to 9 m in diameter 1 to 6 m deep and frequently act as swallow holes poljes that are atypically small 1 ha occur in either glacial cirques or syncline depressions are flat floored and lined with sediment and soil and drain underground via swallow holes short caves springs with marked discharge variations and estavelles are further evidence for karst development karstic landforms are widespread in carbonatic conglomerate terrains but their dimensions are smaller than in typical limestone karst the practical implications of these findings are also briefly mentioned in this paper

magmatism and continental break up in the south atlantic high precision40ar 39ar geochronology abstract a detailed ar ar study of the parana etendeka continental flood basalts cfb has been undertaken using laser spot analysis data provide information not only on the age of the samples but also on the variability of the non radiogenic ar component and state of alteration the results indicate the parana etendeka cfb were erupted over 10 million years between 137 and 127 ma providing a minimum overall eruption rate of 0 1 km3 yr 1 this is an order of magnitude less than that previously proposed for this the deccan and siberian cfb provinces but it is similar to estimates for hawaii iceland and the columbia river cfb in detail the new ar ar analyses indicate that magmatism within the parana etendeka province migrated from nw to se prior to and during the opening of the south atlantic providing an explanation for the asymmetry of the cfb lavas about the south atlantic moreover chemically defined magma types were erupted at different times in different places and so within the paranathey may not be used as reliable chronostratigraphic units rather such magmatic units may reflect the extent of compositionally distinct source regions in the uppermost mantle and indicate that partial melting took place over a wide area under the paranabasin the nw se migration of the onset of magmatism might be interpreted as a plume trace but the inferred rate of movement is 3 times faster than that inferred from subsequent magmatism on the rio grande rise it is argued that the onset of magmatism reflected extensional tectonics normal to the ponta grossa dyke swarm given that magmatism preceded the main phase of rifting and that extension across the ponta grossa was moderate current plume models have difficulty in predicting sufficient melting within the asthenosphere

palaeogene continental to oceanic magmatism on the se greenland continental margin at 63 n a review of the results of ocean drilling program legs 152 and 163 mantle melting zone accompanied by an increase in the average drilling along a 63 n transect off se greenland during ocean degree of melting with time from 4 to 12 these modest drilling program odp legs 152 and 163 recovered a succession degrees of melting imply mantle temperatures only 100 c hotter of volcanic rocks representing all stages in the break up of the than normal upper mantle upwelling mantle must therefore have volcanic rifted margin the rocks range from pre break up continental been fed dynamically to the melt zone to generate the igneous crust tholeiitic flood basalt through syn break up picrite to truly oceanic of 18 km thickness deduced from seismic and gravity studies nbasalt forming the main part of the seaward dipping reflector morb like magmas dominated the earliest part of the succession sequence sdrs all the lava flows recovered from the transect although a few flows of icelandic basalt were erupted in the prewere erupted in a subaerial environment ar ar dating shows break up phase in contrast the post break up magmas had an that the earliest magmas were erupted at 61 ma and has icelandic mantle source this suggests that the developing head of confirmed that the main part of the sdrs was erupted during c24r the ancestral iceland plume was compositionally zoned with a core 56 53 ma following continental break up magma represented by of icelandic mantle surrounded by a thick outer zone of hot depleted the pre break up lava flows was stored in crustal reservoirs where upper mantle it evolved by fractional crystallization and assimilation of continental crust trace element and radiogenic isotope data show that the contaminant changed through time from lower crustal granulite to a mixture of granulite and amphibolite suggesting storage of magma at progressively shallower levels in the crust the degree of contamination declined rapidly as break up proceeded and the youngest rocks sampled in the transect are uncontaminated by continental basement variation of for example sc zr and sm

pathogen and chemical transport in the karst limestone of the biscayne aquifer 3 use of microspheres to estimate the transport potential of cryptosporidium parvum oocysts 1 the vulnerability of a municipal well in the northwest well field in southeastern florida to potential contamination by cryptosporidium parvum oocysts was assessed in a large scale forced gradient convergent injection and recovery test the field study involved a simultaneous pulse introduction of a nonreactive tracer sf6 an inert gas and oocyst sized 1 6 2 9 and 4 9 m diameter carboxylated polystyrene microspheres into karst limestone of the biscayne aquifer characterized by a complex triple matrix touching vug and conduit porosity fractional recoveries 97 m down gradient were inversely related to diameter and ranged from 2 9 for the 4 9 m microspheres to 5 8 for 1 6 m microspheres their centers of mass arrived at the pumping well approximately threefold earlier than that of the nonreactive tracer sf6 gas underscoring the need for use of colloid tracers and field scale tracer tests for these kinds of evaluations in a modified triaxial cell using near in situ chemical conditions 2 9 and 4 9 m microspheres underestimated by fourfold to sixfold the attachment potential of the less electronegative 2 9 4 1 m oocysts in the matrix porosity of limestone core samples the field and laboratory results collectively suggested that it may take 200 300 m of transport to ensure even a 1 log unit removal of oocysts even though the limestone surfaces exhibited a substantive capability for their sorptive removal the study further demonstrated the utility of microspheres as oocyst surrogates in field scale assessments of well vulnerability in limestone provided that differences in attachment behaviors between oocysts and microspheres are taken into account

melting experiments on a high magnesian andesite abstract melting experiments were conducted on a high magnesian bronzite olivine andesite teraga ike andesite which is considered to be a primary andesite the high magnesian andesite magma is in equilibrium with both olivine and orthopyroxene at about 15 5 kbar and 1080 c under h2o saturated conditions and at lower pressure and higher temperature under h2o undersaturated conditions this suggests that high magnesian andesites could be generated by the partial melting of upper mantle peridotite containing a small amount of h2o

boron partitioning in the upper mantle an experimental and ion probe study piston cylinder experiments were performed on two natural spinel lherzolites from lherz french pyrenees and kilbourne hole usa in order to determine the distribution coefficients of boron between minerals and melt for upper mantle conditions additionally a morb glass and its phenocrysts were taken as a low pressure analogue boron concentration measurements were made by ion probe the only technique suitable for in situ 10 m boron concentration measurements in the range 0 1 1 ppm the two starting peridotites samples have low bulk boron concentrations of 0 96 and 0 81 ppm boron distribution coefficients were determined from 1 direct measurement of melt and crystals in contact and 2 calculations from profiles of several tens of measurements made using 10 m diameter spots containing variable proportions of crystals and melt distribution coefficients between minerals and melt decrease in the following order dcpx melt 0 117 dsp melt 0 08 dol melt 0 034 dopx melt 0 027 the limited ranges of composition pressure and temperature investigated mimic natural conditions for basalt genesis but sharply limit the present data for determining variations of partition coefficients with p t and x however boron behavior seems to follow roughly that of aluminum since the al richest minerals are the ones with the highest boron distribution coefficients the dol melt values increase with pressure which is similar to what has been observed for the distribution coefficient of aluminum between olivine and melt boron is therefore strongly incompatible during partial melting in the mantle using these distribution coefficients and assuming that morb have been produced by 5 20 partial melting the depleted mantle source of morb is estimated to contain 0 05 0 3 ppm b these values correspond to the lower limit for the mean boron content of chondrites 0 55 0 3 ppm but are in accordance with a simple budget of boron between mantle crust and seawater in fact a simple mass balance calculation where all the boron hosted in the crust and in seawater is presumed to have been extracted from an upper mantle of chondritic original boron content yields a boron content for the depleted upper mantle of 0 3 ppm at present because in situ boron concentration measurements under the ppm level at the m scale are now possible with the ion probe and because of its strong partitioning in melts and possible partitioning in fluids boron is a potentially very powerful geochemical tracer for the study of mantle processes such as basalt genesis or metasomatism

the argentine continental margin north of 48 s sedimentary successions volcanic activity during breakup abstract new seismic data and geophysical results from the outer argentine continental margin and from seismic flow line transects across the oceanic argentine brazil and cape angola basins are presented the results document that the early cretaceous south atlantic continental breakup and initial sea floor spreading were accompanied by large scale transient volcanism emplacing voluminous extrusive constructions on the conjugate outer continental margins of the south atlantic on the argentine margin we interpret three major tectono volcanic crustal units beneath a thick and tectonically undisturbed sedimentary succession of cretaceous and tertiary age 1 syn rift basins and even pre rift features are present on the outer shelf 2 a deeply buried 60 120 km wide and several thousand metres thick volcanic wedge occupies most of the slope the wedge is characterized by seaward dipping reflectors and is associated with the distinct magnetic anomaly g 3 the seaward adjacent oceanic crust of cretaceous age shows isochronous changes of the seismic images along our flow line transects suggesting some form of episodicity of rich magma and poor to moderate magma supply at the pre existing mid ocean ridge of the south atlantic during cretaceous time

south atlantic volcanic margins the early cretaceous south atlantic continental break up and initial sea floor spreading were accompanied by large scale transient volcanism emplacing the parana etendeka continental flood basalts and voluminous extrusive constructions on the conjugate margins south of the torres arch abutment plateau on the north namibia margin we interpret four main tectono magmatic crustal units 1 oceanic crust 2 thickened oceanic crust covered by huge seaward dipping wedges 3 a c 150 km wide break up related rift zone partly covered by the dipping wedges and 4 thicker continental crust partly deformed by palaeozoic extension east of the early cretaceous rift similar settings also characterize other south atlantic margin segments we infer an up to 300 km wide and 2400 km long rift zone representing lithospheric extension leading to breakup and formation of the south atlantic volcanic margins comparison with other volcanic margins demonstrates in spite of local and regional differences gross similarities in tectono magmatic style crustal units and dimensions

three dimensional gravity inversion for moho depth at rifted continental margins incorporating a lithosphere thermal gravity anomaly correction summary this paper describes a method for determining moho depth lithosphere thinning factor 1 1 and the location of the ocean continent transition at rifted continental margins using 3 d gravity inversion which includes a correction for the large negative lithosphere thermal gravity anomaly within continental margin lithosphere the lateral density changes caused by the elevated geotherm in thinned continental margin and adjacent ocean basin lithosphere produce a significant lithosphere thermal gravity anomaly which may be in excess of 100 mgal and for which a correction must be made in order to determine moho depth accurately from gravity inversion we describe a method of iteratively calculating the lithosphere thermal gravity anomaly using a lithosphere thermal model to give the present day temperature field from which we calculate the lithosphere thermal density and gravity anomalies for continental margin lithosphere the lithosphere thermal perturbation is calculated from the lithosphere thinning factor 1 1 obtained from crustal thinning determined by gravity inversion and breakup age for thermal re equilibration time for oceanic lithosphere the lithosphere thermal model used to predict the lithosphere thermal gravity anomaly may be conditioned using ocean isochrons from plate reconstruction models to provide the age and location of oceanic lithosphere a correction is made for crustal melt addition due to decompression melting during continental breakup and seafloor spreading we investigate the sensitivity of the lithosphere thermal gravity anomaly and the predicted moho depth from gravity inversion at continental rifted margins to the methods used to calculate and condition the lithosphere thermal model using both synthetic models and examples from the north atlantic

isotopic and hydrochemical data as indicators of recharge areas flow paths and water rock interaction in the caldas da rainha quinta das janelas thermomineral carbonate rock aquifer central portugal summary an updated conceptual circulation model for the caldas da rainha and quinta das janelas thermomineral waters was developed these thermomineral waters t 33 c are related to a huge syncline ascribed to the regional flow paths two diapiric structures were responsible for the uplift and subsequent folding of regional jurassic carbonate rocks environmental isotopic 2h and 18o data indicates that the main recharge area of the thermomineral system is linked to the jurassic limestones candeeiros mountains e border of the syncline the thermomineral waters belong to the cl na sulphurous type with a total mineralization of about 3000 mg l the thermomineral aquifer system seems to be isolated from anthropogenic contamination which is typical for the local shallow groundwater systems due to the existence of impermeable layers composed of a series of loamy and detritic rocks of the upper jurassic the presence of 3h in some thermomineral borehole waters not accompanied by an increase in so 4 2 and no 3 could be ascribed to different underground flow paths and different mean residence time the 34s so4 and 18o so4 values of dissolved sulphate of groundwaters of the caldas da rainha spas indicate that the sulphate is the result of water rock interaction with evaporitic rocks e g gypsum and anhydrite ascribed to the regional synclinal structure

island arc processes relevant to crustal and mantle evolution abstract recycling of continental crust occurs at convergent plate margins recycling must be investigated at several structural levels and time scales shallow intermediate and deep surface recycling sedimentary igneous and structural redistribution of crustal mass is not included in this hierarchy although it controls the composition of the recycled crust crustal growth structural and geochemical evolution is directly tied to the larger less complex mantle crustal recycling at the deepest structural level must be understood better in order to choose between alternative interpretations of the isotopic evolution of the mantle crustal recycling at intermediate structural levels by subduction of crust and its subsequent reappearance as a component of volcanic arc magmas has important implications for the question of long term crustal growth crustal recycling at shallow structural levels is important in understanding the development of forearc regions the mass flux at present day convergent plate margins has been diagramed in order to systematize future investigations and to clarify the distinction between the recycling at different levels and the observable quantities in the present day earth that are relevant to recycling disputes