stability of chlorite in the upper mantle assrracr stability of chlorite in the presence of orthopyroxene under upper mantle conditions has been thermodynamically examined in the model system mgo alror sio2 hro mas all minerals were assumed to be pure in the mash system calculation of the reaction curve took into account heat capacity c thermal expansion ai and isothermal compressibility n if i is assumed ihat lrrv api 0 the stability of chlorite in the upper mantle is slightly pressure dependent the upper stability curve for chlorite is located at 800 c 650 c and 500 c at 2 9 gpa 4 2 gpa and 5 0 gpa respectively on the other hand the assumption that aav 0 and apv 0 yields a more temperature dependent curve for chlorite stability 800 c at 3 9 gpa and 700 c at 5 5 gpa the present calculation indicates that chlorite in down dragged hydrous peridotite at the base of a mantle wedge decomposes at pressures of 3 0 3 5 gpa therefore hro released from chlorite may play a partial role in producing magmas beneath a volcanic front that lies about i l0 km above a subducted slab inrnooucrron chlorite must crystallize during hydration processes in an ultramafic system and may be one of the main hydrous phases in both the lower crust and the upper mantle phase relations ofchlorite have been examined in natural metamorphosed ultramafic rocks formed at low pressures or under lower crustal conditions e g springer 1974 however very little is known about the stability of chlorite in the upper mantle probably because chlorite is relatively uncommon in high pressure hydrous ultramafic rocks that commonly contain amphibole and or phlogopite the stability of chlorite which carries a large amount of water ca l3 wto o should yield important information pertinent to the understanding of the distribution and circulation of water in the upper mantle in this paper we examine the stability limit of chlorite in the upper mantle in terms of thermodynamic calculations and discuss the role of chlorite in magma genesis in subduction zones mineral abbreviations in the text and figures are after kretz 1983

effects of h2o on elastic wave velocities in ultrabasic rocks at 900 c under 1 gpa simultaneous measurements of compressional and shear wave velocities vp and vs in five ultrabasic rocks with hydrous minerals were carried out from room temperature to 900 c under 1gpa in a closed system both vp and vs in all five rocks decreased markedly near 600 c and or 800 c following the dehydration of hydrous minerals in the rocks considering velocities only in the anhydrous matrix which were estimated from the volumetric ratio of forsterite and enstatite in each sample the effects of water produced by the dehydration reactions were estimated the ratio of v at 900 c under 1gpa is represented by the following equation v 1 0 0027 w to both p and s wave where v is the velocity in the rock including the extricated water w kg m 3 and is the velocity in the sample excluding the extricated water the low velocity layer lies between the descending lithosphere and the overlying asthenosphere beneath the japanese island arcs and the ratio of vp drop for the low velocity layer to that of the underlying descending lithosphere is approximately 5 if the observed velocity drop is wholly attributed to the free water produced by dehydration of serpentine minerals in the descending slab the required volumetric ratio of serpentine minerals in the descending slab would be 5 10

back arc spreading trench migration continental pull or induced convection abstract mechanisms of the opening of back arc systems are analyzed limited focal mechanisms of intraplate earthquakes are used to determine the stress regime of an overriding plate preliminary analyses show that compressive deviatoric stresses exist in the plate except near the spreading center based on this observation trench suction does not appear to be the primary force that drives back arc spreading since it will result in tensional deviatoric stresses within the overriding plate even though continental pull is able to satisfy the stress requirements it does not appear to be a likely mechanism either because of the initiation and subsequent symmetric spreading difficulty associated with such a mechanism the mechanism we favor is the one that involves the induced convective current in the mantle wedge immediately above the slab calculations show that the induced flow is able to generate sufficient stress to break up the overriding lithosphere if the tectonic stresses of the region are favorable both trench suction and continental pull may help to provide such a favorable tectonic stress regime

geology of the don manuel igneous complex central chile implications for igneous processes in porphyry copper systems the don manuel igneous complex and associated porphyry copper mineralization in the andean cordillera of central chile demonstrate similarities between intrusive complexes associated with porphyry copper deposits and arc volcanoes that generate porphyritic volcanics the don manuel igneous complex intrusions progressed from quartz monzonite through rhyolite and biotite tonalite to intermediate porphyritic and basaltic andesite dikes which intrude the older units mineralization is associated with the biotite tonalite and intermediate porphyries which also contain the greatest abundance of mafic enclaves zoning patterns within plagioclase phenocrysts suggest that the later intermediate porphyries comprise a hybridized suite formed by magma mixing new zircon u pb ages and whole rock ar ar ages indicate that the don manuel igneous complex was emplaced between ca 4 and 3 6 ma the time scale for the episodic intrusion of the don manuel igneous complex units is similar to observed episodicity of eruption and degassing events in active arc volcanoes observations from the don manuel igneous complex are consistent with the close spatial and temporal association of mineralization with episodic intrusion and interaction between silicic and mafic magmas during emplacement the observations are also consistent with the hypothesis that mafic magma provides a source of sulfur for porphyry copper deposit formation

constraints on the timing and duration of orogenic events by combined lu hf and sm nd geochronology an example from the grenville orogeny abstract grenville age basement rocks of the virginia blue ridge province preserve a record of over 150 million years of tectonism and crustal reworking associated with the construction of the supercontinent rodinia although the igneous origins of these rocks have been the subject of much previous work few studies have examined the complex metamorphic history of these rocks during the grenville orogeny to better constrain the timing and duration of metamorphism we determined lu hf and sm nd garnet and u pb zircon rim dates of garnet bearing rocks exposed in the virginia blue ridge multiple periods of mesoproterozoic metamorphism and deformation have been previously recognized with a major deformational event occurring before 1065 ma and subsequent local episodes occurring at least until 1000 ma zircon overgrowths and monazite dates suggest metamorphic events between 1050 and 960 ma our new lu hf garnet dates range from 1032 3 to 1019 3 ma new sm nd dates determined on the same garnet dissolutions define a younger range from 965 6 to 949 6 ma our zircon dates are consistent with previously measured ages 1090 940 ma and often overlap with the lu hf garnet dates from the same samples there is a large and systematic difference 60 80 m y between the garnet dates from the two isotopic systems although sm nd dates slightly younger than those recorded by lu hf are not uncommon the large discrepancies between the two systems that we see here are unusual the dated garnets are unzoned and yield lu hf dates that coincide with the zircon overgrowth dates sm nd ages overlap with the younger monazite dates we interpret the discrepancy in garnet dates as reflecting the difference in closure temperature between the two isotopic systems further these data require slow cooling following large scale crustal thickening after the assembly of rodinia during grenville orogenesis these data illustrate that the use of the coupled lu hf and sm nd chronometers in garnet can provide important constraints on the conditions of metamorphism during past orogenic events

non stoichiometry of interlayer cations in micas from low to middle grade metamorphic rocks in the ryoke and the sanbagawa belts japan micas in 17 pelitic k feldspar free and 8 psammitic k feldspar bearing rocks from the wazuka and the asemi gawa areas in the ryoke and the sanbagawa metamorphic terrains respectively were analyzed on an electron probe microanalyzer the deficiency of alkali cations in the low to middle grade metamorphic micas is ascribed to the illite substitution kxii aliv xii vacancy siiv

attenuation anisotropy beneath the subduction zones in japan we analyze high quality digital waveform data at stations in japan from the poseidon and iris broad band seismograph networks the analysis of multiple scs waves of the deep event beneath sakhalin on 12 may 1990 shows that the orientation of the principal axis of horizontal scsn particle motion is rotated clockwise from nwn ses to n s as the number of reflections increases this suggests that the attenuation of the e w component is higher than that of the n s component of scs waves the spectral ratio scs2 scs1 differs between the two horizontal components we estimate the quality factor qscs of the whole mantle for each horizontal component using the decay rate of spectral ratio of band passed scs1 and scs2 phases in the frequency range from 0 01 to 0 03 hz we obtain a higher average qscs for the n s component 141 8 than that for the e w component 91 5 that is the e w component attenuates more than the n s component for upcoming scs waves the n s direction is nearly parallel to the strike of the subducting slab beneath the mantle wedge in the japanese islands and the e w is perpendicular to the strike the up welling and downgoing flow in the mantle wedge due to drag caused by the subduction is likely to cause the difference in attenuation between the two directions in other words azimuthal attenuation anisotropy

timescales of collisional metamorphism from sm nd lu hf and u pb thermochronology a case from the proterozoic putumayo orogen of amazonia abstract the construction of mountain belts by continent continent collision is a fundamental phase of the supercontinent cycle and an inevitable consequence of subduction driven plate tectonics on earth yet quantitative estimation of the rate and duration of prograde orogenic metamorphism in high grade collisional belts has proven elusive to constrain differences in the chemical partitioning and diffusive behavior of elements associated with the 147sm 143nd and 176lu 176hf isotopic systems in metamorphic garnet can provide valuable insights on the time scales over which this mineral grows and cools in regionally metamorphosed terranes here we present new sm nd and lu hf isotopic results from multiple garnet and whole rock aliquots from a granulite grade metasedimentary rock from the proterozoic putumayo orogen of south america using a combination of internal isochron dating by these two decay systems combined with apatite u pb dating and pressure temperature time p t t constraints from mineral equilibria and chemical diffusion geospeedometry we provide first order insights into the time scales of a proterozoic continent continent collisional metamorphic event in amazonia associated with its incorporation to the supercontinent rodinia our results indicate that these metasediments attained peak granulite grade metamorphic conditions of nearly 0 62 gpa and 680 c at approximately 1035 ma and started to cool slowly presumably during exhumation at a rate of 5 k myr simple diffusion scaling relationships and numerical modeling of sm nd and lu hf in garnet indicate that while sm and nd were both fully re equilibrated at peak temperatures and therefore date cooling lu and hf underwent diffusive decoupling due to incomplete hf re equilibration by modeling the impact of this differential re equilibration in the development of the sm nd and lu hf isochrons we constraint the duration of the prograde metamorphic path and argue that garnet growth in this sample took place over millions of years approximately 25 myr according to our preferred interpretation the determined apatite u pb date vs grain size relationship cannot be reconciled with the thermal history described above when modeled using the existing experimental diffusion data and instead suggest that much slower pb diffusivities in this mineral compared to those predicted by the existing experimental data may have inhibited its resetting we note that the inferred temperature time history for the metamorphic loop of this segment of the putumayo basement is comparable to that proposed for the metamorphism and exhumation of metasediments in the india asia collision zone suggesting that similar tectonic processes and rates likely operated during the mesoproterozoic putumayo orogeny

submarine hydrothermal activity along the mid kermadec arc new zealand large scale effects on venting 1 the 2 500 km kermadec tonga arc is the longest submarine arc on the planet here we report on the second of a series of cruises designed to investigate large scale controls on active hydrothermal venting on this arc the 2002 nzaplume ii cruise surveyed 12 submarine volcanic centers along 580 km of the middle kermadec arc mka extending a 1999 cruise that surveyed 260 km of the southern kermadec arc ska average spacing between volcanic centers increases northward from 30 km on backarc crust along the ska to 45 km on backarc crust along the southern mka to 58 km where the mka joins the kermadec ridge volcanic cones dominate in the backarc and calderas dominate the kermadec ridge the incidence of venting is higher along the mka 83 10 of 12 volcanic centers than the ska 67 8 of 12 but the relative intensity of venting as given by plume thickness areal extent and concentration of dissolved gases and ionic species is generally weaker in the mka this pattern may reflect subduction of the 17 km thick oceanic hikurangi plateau beneath the ska subduction of this basaltic mass should greatly increase fluid loss from the downgoing slab initiating extensive melting in the upper mantle wedge and invigorating the hydrothermal systems of the ska conversely volcanic centers in the southern mka are starved of magma replenishment and so their hydrothermal systems are waning farther north where the mka centers merge with the kermadec ridge fewer but larger magma bodies accumulate in the thicker older crust ensuring more widely separated caldera dominated volcanic centers

seismic vp and vp vs structure of the geothermal area around tarutung north sumatra indonesia derived from local earthquake tomography abstract a passive seismic experiment with 42 short period stations was setup for 10 months from may 2011 within a 20 30 km radius covering the tarutung basin and the northern part of the sarulla graben along the seismically active sumatran fault north sumatra indonesia the objective of the project is to investigate the geothermal setting based on the vp and vp vs structures and the distribution of seismicity we recorded 2856 local earthquakes by at least 8 stations which were reduced to 809 events having gap angle less than 180 the initial earthquake locations were determined using simultaneous inversion for hypocenters and 1d velocity structure followed by 3d tomographic inversion the resulting seismicity distribution vp and vp vs structure reveal systematic differences between the tarutung basin and the sarulla graben the tarutung pull apart basin is characterized by a complex structure reflected by a broader distribution of seismicity compared with the very narrow rather vertical hypocenter distribution in the sarulla graben which was formed by fault perpendicular extension and not by pull apart tectonics high vp vs values near to the surface are imaged within the sarulla graben and northeast of the tarutung basin these features are interpreted as fluid bearing sediments with fracturing indicating potentially favorable conditions for geothermal exploitation the localized fluid flow northeast of tarutung could be explained by the releasing step over structure of the pull apart basin and resulting dilatational stress regime in this part of the basin heat sources are assumed to be located at greater depth in association with magmatic activities accompanying the subduction process and the heat is transported along the sumatran fault to shallow depth

migration of fluid phases and genesis of basalt magmas in subduction zones dehydration and hydration reactions in both the downgoing lithosphere and the overlying mantle wedge have been examined in order to understand the role of h2o in the production of magmas at convergent plate boundaries the subduction of oceanic lithosphere occurring with increasing pressures and rising temperatures causes liberation of h2o from the slab amphibole which can be stable to the highest pt conditions among hydrous phases in the slab decomposes at around 90 km depth it follows that the subducted lithosphere is essentially anhydrous beneath volcanic arcs lying more than 110 km above the slab and that the supply of slab derived h2o is not a direct trigger for the production of arc magmas instead the h2o released from downgoing lithosphere reacts with the forearc mantle wedge to crystallize hydrous minerals serpentine talc amphibole chlorite and phlogopite this hydrated peridotite is dragged downward on the slab toward higher pt regions and releases h2o to shallower potential magma source regions in the mantle wedge combining experimental data on the stability of serpentine and talc with the thermal structure in the mantle wedge it is concluded that those minerals decompose beneath the forearc region on the other hand high pt experimental and thermodynamic data suggest that dehydration of amphibole and chlorite in the downdragged hydrated peridotite can take place just beneath a volcanic front phlogopite in the peridotite decomposes to release h2o at a deeper level about 200 km h2o liberated from the hydrated peridotite causes partial melting of overlying mantle wedge peridotites along with the migration of h2o through the above processes subduction components especially large ion lithophile elements can be overprinted on the magma source region which governs the geochemical characteristics of arc magmas

contrasting seismic reflectivity of the lower crust and uppermost mantle between ne japan and sw japan as illustrated by petrophysical analyses of mafic and ultramafic xenoliths 1 petrophysical analyses of mafic and ultramafic xenoliths reveal large contrasts in acoustic impedance between the lower crustal rocks and uppermost mantle rocks both beneath ichinomegata crater in ne japan and beneath oki dogo island in sw japan in contrast to minor contrasts in acoustic impedance both among the lower crustal rocks and among the uppermost mantle rocks beneath ichinomegata crater relatively large contrasts in acoustic impedance are present beneath oki dogo island between granulite and olivine gabbro in the lower crustal rocks as well as between plagioclase bearing wehrlite and other cumulus peridotites in the uppermost mantle rocks based on the existing petrologic models of the lower crust and uppermost mantle beneath ichinomegata crater and oki dogo island we then constructed model structures of the lower crust and uppermost mantle beneath these two areas composed of units with randomly arranged 100 m thick foliation parallel horizontal layers of xenolith samples and used these model structures to examine the seismic reflectivity beneath these two areas the results suggest the presence of strong seismic reflectors between the lower crust and the uppermost mantle in both areas although no other recognizable seismic reflector is expected beneath ichinomegata crater many seismic reflectors due to the above contrasts in acoustic impedance are expected both in the lower crust and in the uppermost mantle beneath oki dogo island thus a remarkable contrast in seismic reflectivity in the lower crust and uppermost mantle is likely present between these two areas as well as between ne japan and sw japan

hydrothermal alteration of hemi pelagic sediments experimental evaluation of geochemical processes in shallow subduction zones abstract laboratory hydrothermal experiments provide unique information regarding the fate of volatile and or incompatibles e g b li and as during oceanic crust subduction examination of chemical redistribution between the subducted slab mantle wedge arc volcanics and overlying ocean water during subduction is critical to gain further insight into arc volcanism and chemical oceanic budgets for instance efficient mobilization of b at shallow depths may be a key aspect of its oceanic budget and can help to explain the systematics of b 11b and b 10be in arc lavas fluid rock interactions at elevated temperatures and pressures in accretionary prisms were studied using a rocking autoclave hydrothermal apparatus to monitor sediment porewater interaction over the range of 25 to 350 c at 800 bars clay rich hemi pelagic sediments from the decollement zone of ocean drilling program site 808 nankai trough were reacted with nacl cacl2 solutions at water rock ratios of 3 5 to 1 5 w w to mimic alteration processes in the shallow subduction zone fluids were extracted at 25 50 c intervals and were analyzed for major and trace chemical constituents the fluid chemistry changed significantly during the course of these experiments but there was generally only minor modification of the solid phase only b li as br and pb are significantly depleted during the heating cycle dissolved na mg and so4 decreased sharply and nh4 sio2 k b and li increased at t 300 c calcium drops gradually at low temperatures but concentrations rise sharply at t 300 c decomposition of organic matter so42 depletion and mg fixation dominate at low temperatures however albitization of calcic plagioclase leads to marked na depletions and ca enrichments at t 300 c dissolved sio2 remained below saturation with respect to quartz and amorphous silica throughout the entire experiment b and li mobilization with large isotopic fractionations occurred at low temperature exchangeable b 11b 15 is completely leached before reaching 150 c substantial o2 exchange between fluids and the solid phase occurred at t 200 c in the spiked experiment where 18o varies more than 100 in the fluids during retrograde cooling dissolved mg so4 ca si k and sr are released as a result of carbonate or anhydrite dissolution and marked b re adsorption occurred at temperatures below 60 c

isotopic evolution of the tonga arc during lau basin rifting evidence from the volcaniclastic record new pb sr and nd isotope data from volcaniclastic sediments recovered from the tonga forearc and lau basin permit the isotopic evolution of a section of this arc system next to the modern island of ata to be traced through the backarc basin rifting process from 7 0 ma to the present the new data suggest that the isotopic character of the mantle wedge remains constant and of pacific mid ocean ridge basalt morb character during the early rifting phase the isotopic evidence supports trace element data in showing an increase in the sediment contribution to arc petrogenesis about 2 3 my after the start of lau basin rifting since 0 45 ma the sediment contribution decreased to pre rift values with the initiation of spreading in the adjacent backarc basin where the high sediment influence is not seen in the isotopes the isotopes show a relative increase in the volcaniclastic compared with pelagic sediment involvement during rifting the inferred peak in sediment subduction is probably the result of a decoupling of the two plates owing to roll back of the pacific lithosphere at the time of arc rifting

regional variations of amplitudes of scsp phases observed in the japanese islands summary scsp phases are examined with special reference to their amplitude variations over seismograph networks in the japanese islands scsp of rela tively large amplitude are observed along the pacific side of the japanese islands amplitude ratios of the scsp with respect to scs are explained in terms of scs to p conversion at the upper boundary of a descending pacific plate which dips at about 30 and whose seismic wave velocity is between 5 and 10 per cent higher than that of the surrounding mantle large amplitude scsp are observed at stations nearest the kurile or japan trench the ampli tude ratios of scsp scs are found to decrease rapidly with distance inland from the trench an estimate is made of the effects of sharpness of the con version interface the incident angle of scs upon it and lateral variations of anelasticity within the asthenosphere overlying the descending pacific plate upon the scsp scs ratios we show that the sharpness of the conversion inter face is the dominant factor this result implies that the nature of the scs to p conversion interface varies with distance from the trench or equivalently with depth the presence of a low velocity zone in the vicinity of the conversion interface is inferred from the relative polarity between scsp and scs observed at the stations on the outermost side of north eastern japan scsp arrivals from shallower conversions at depths less than about 80 km observed in central and south western japan are also examined to elucidate the structure of the philippine sea plate descending beneath this region a discussion is made on the correlations between dipping subcrustal seismicity and the scs to p conversion interfaces except for the chugoku region the scs to p conversion interfaces correlate well with the dipping subcrustal seismicity our scsp observations indicate the presence of a dipping dis continuity in the uppermost mantle beneath the chugoku region where no subcrustal earthquakes have been detected

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

pressure temperature trajectory of the sanbagawa metamorphism deduced from garnet zoning abstract most garnets in a small volume of the pelitic sanbagawa schists nucleated simultaneously grew and then ceased with their growth maintaining near equilibrium with coexisting minerals such as chlorite and biotite during the prograde metamorphism chemical trends of zoned garnets systematically shift from the pyrope poor to pyrope rich portions in a ternary mn fe mg system in order of ascending metamorphic grade a ternary mn fe mg model system suggests that the higher grade garnets started to grow at lower pressures and higher temperatures than the lower grade ones during prograde metamorphism it follows that high grade metamorphism took place at lower pressures in the garnet to oligoclase biotite zones of the sanbagawa metamorphic terrain in central shikoku

wind and tidal influence on water circulation in a galician ria nw spain abstract the wind effect on water circulation was described by means of hydrodynamical and thermohaline measurements carried out in the ria of pontevedra nw spain from february to july 1998 the samples were measured both at an anchored station situated in the inner part of the ria and at other stations located in the main channel in this way temporal and spatial events were related thermohaline variables salinity and temperature were used to clarify the current patterns measured at the anchored station it has been observed that there is a clear predominance of wind in the main estuary axis direction the wind direction is strongly dependent on topography wind speeds higher than 4 ms 1 are able to dominate the current at surface layers even against tidal effect easterly winds force water to leave the estuary at surface layers while westerly winds force water to enter it diurnal breezes associated with westerly winds were rarely recorded bottom layers were controlled by tide except during an upwelling event

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

opening of the sea of japan back arc basin by asthenospheric injection abstract the position of a volcanic front on an arc is governed by dehydration reactions of amphibole chlorite in down dragged hydrated peridotite at the base of the mantle wedge as these reactions are essentially pressure dependent ca 3 5 gpa the paleo position of a volcanic front offers an estimate for the depth of the subducted oceanic lithosphere beneath it this paper evaluates volcanism in the northeast japan arc since the oligocene and discusses the mass transportation in the mantle wedge during the episode of back arc extension in the sea of japan with reference to the migration of the volcanic front the volcanic front during the oligocene ca 30 ma was located along the western coast of the present northeast japan are and migrated about 200 km trenchwards at 23 ma before the opening of the sea of japan back arc basin it follows that the angle of subduction became steeper during the period of 30 23 ma triggered by the injection of the asthenosphere into the mantle wedge the back arc extension of the sea of japan between 21 and 14 ma is a manifestation of the dynamic process of the lithosphere due to the asthenospheric injection the temperature of the injecting asthenospheric mantle was higher than that of the surrounding upper mantle or lithosphere this caused the characteristic volcanism at 23 22 ma 1 high magnesian andesite volcanism took place in the near trench region 2 across arc compositional variation was not observed 3 the volume of volcanic materials increases towards the back arc side of volcanic arc the volcanic front has migrated over a short distance inwards less than 100 km since 15 ma possible as a result of cooling in the mantle wedge because both amphibole and chlorite break down at slightly higher pressures at lower temperatures the upper mantle materials which injected into the mantle wedge and caused back arc spreading are identical to those of the mid oceanic ridge basalt source in sr nd isotopic compositions because volcanic rocks in the back arc region of the northeast japan arc show secular variation in isotopic compositions from enriched to depleted isotopic characteristics during the last 30 ma

lithospheric contributions to arc magmatism isotope variations along strike in volcanoes of honshu japan major chemical exchange between the crust and mantle occurs in subduction zone environments profoundly affecting the chemical evolution of earth the relative contributions of the subducting slab mantle wedge and arc lithosphere to the generation of island arc magmas and ultimately new continental crust are controversial isotopic data for lavas from a transect of volcanoes in a single arc segment of northern honshu japan have distinct variations coincident with changes in crustal lithology these data imply that the relatively thin crustal lithosphere is an active geochemical filter for all traversing magmas and is responsible for significant modification of primary mantle melts

the southern tyrrhenian subduction zone deep geometry magmatism and plio pleistocene evolution abstract we report on a high resolution vp vp vs and qp model of the southern tyrrhenian subduction zone obtained by the inversion of p and s wave arrival times and t values from intraslab seismicity the arcuate shape of the southern apennines calabrian arc sicilian maghrebides is perfectly mirrored by two rather continuous low and high vp bands lying beneath the belt system at ca 25 and 100 km respectively between 100 and 300 km two independent high vp slabs lie beneath the neapolitan region and the southern tyrrhenian sea separated by unperturbed mantle we suggest that the ca 150 km wide slab window beneath the southern apennines opened after a tear occurring within a composite subduction system formed by the apulian continental lithosphere and the ionian oceanic slab the abrupt slab rupture induced ultrafast southeastward retreat of the ionian slab and the 19 cm yr spreading of the back arc oceanic marsili basin between ca 2 1 and 1 6 ma ago the 25 km low vp zone beneath the arc denotes continental upper crustal rocks below the chain its striking continuity requires a unique orogenic wedge at 25 km depth below the southern apennines the calabrian arc and the sicilian maghrebides the alternative explanation would imply the ubiquitous occurrence of autochthonous lower plate rocks at 25 km depth i e a puzzling autochthonous continental calabria the ionian slab beneath calabria shows high vp high qp and low vp vs anomalies typical of old oceanic lithosphere intermediate depth seismicity is concentrated within its thin oceanic crust suggesting the occurrence of vigorous metamorphism the slab dehydration promotes the melting of the overlying mantle as testified by high vp vs and low qp anomalies between the slab and the aeolian magmatic arc

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

upper mantle velocity structure estimated from ps converted wave beneath the north eastern japan arc summary the upper boundary of the descending oceanic plate is located by using ps waves converted from p to s at the boundary in the tohoku district the north eastern part of honshu japan the observed ps p time data are well explained by a two layered oceanic plate model composed of a thin low velocity upper layer whose thickness is less than 10 km and a thick high velocity lower layer the upper and lower layers respectively have 6 per cent lower and 6 per cent higher velocity than the overriding mantle the estimated location of the upper boundary is just above the upper seismic plane of the double planed deep seismic zone this result indicates that events in the upper seismic plane at least in the depth range from 60 to 150 km occur within the thin low velocity layer on the surface of the oceanic plate

elastic wave velocities in isochemical granulite and amphibolite origin of a low velocity layer at the slab mantle wedge interface simultaneous measurements of compressional and shear wave velocities vp and vs in granulite and amphibolite with identical chemistry were carried out from room temperature to 850 c at 1 gpa both vp and vs in granulite and amphibolite decreased linearly with increasing temperature the values of vp and vs at 700 c are 7 75 km s and 4 37 km s for granulite and 7 40 km s and 4 21 km s for amphibolite respectively 3 4 lowering of elastic wave velocities in hydrous amphibolite compared with anhydrous granulites may be consistent with seismological observations indicating the presence of a low velocity layer at the slab mantle wedge interface in subduction zones

seismological constraints on the mechanism of deep earthquakes temperature dependence of deep earthquake source properties abstract seismological observations are of vital importance for understanding the mechanism of deep earthquakes most of the seismological observables for earthquakes deeper than 300 km are similar to shallow earthquakes deep earthquakes clearly represent shear failure on a planar surface as shown by their double couple mechanisms deep earthquake aftershock sequences show temporal decay rates and magnitude frequency relations b values that are similar to shallow earthquakes with the aftershocks occurring preferentially along the mainshock fault planes in addition deep earthquake rupture velocities are similar to those of shallow earthquakes however there are also some observations that are clearly distinctive relative to shallow earthquakes stress drops of deep earthquakes show a large variation but are larger on average than shallow earthquakes different deep seismic zones show very different b values in contrast to shallow earthquakes which show similar b values worldwide in addition deep earthquakes show fewer aftershocks than shallow earthquakes a variety of observations suggest that deep earthquakes are highly sensitive to the temperature of the slab both deep earthquake b values and the rate of deep earthquake aftershock occurrence are inversely correlated with the temperature of the deep slabs suggesting that these factors are temperature controlled to an extent much greater than with shallow earthquakes large deep earthquakes in warm slabs show slower rupture velocities larger stress drops and lower seismic efficiencies than similar earthquakes in cold slabs the width of deep seismic zones is also probably temperature controlled but deep earthquake rupture can propagate outside the normal limits of benioff zone seismicity simple thermal models for the tonga slab near the 1994 deep earthquake suggest that the temperature at the rupture termination point was at least 200 c warmer than the temperature that limits smaller earthquakes in the slab these observations can be used to evaluate physical models for deep earthquakes including brittle slip along fluid weakened faults transformational faulting and thermal and perhaps melt lubricated shear instabilities it is difficult to explain the large fault widths of some deep earthquakes using a fluid weakening model since hydrated materials are expected in only a narrow depth zone at the top of the slab and the fault planes do not have the expected orientations for reactivated faults the lateral extent of the largest deep earthquakes cast doubt on the transformational faulting model in which events should be confined within a narrow metastable wedge seismological studies have also failed to find evidence for the existence of metastable olivine in slabs the temperature dependence of deep earthquakes argues in favor of a temperature activated phenomenon such as thermal shear instabilities and perhaps fault zone melting

hydrochemical variability associated with rainfall regime a case study in the coastal wetland of the outer rio de la plata estuary argentina in coastal wetlands the interaction between groundwater surface water and marine or estuarine water determines the water chemistry variations in the water balance may be of influence as well the objective of this work was to assess how the rainfall regime affects the hydrological and geochemical processes that regulate the quality of surface and groundwater in the samborombon bay coastal wetland we analyzed the water balances and chemical data of surface and groundwater during a dry and a wet period results show that the rainfall regime regulates water quality in the three hydrological units defined for the region i e groundwater in the coastal plain groundwater in the shell ridges and surface water in the coastal plain in the water balance corresponding to a dry period real evapotranspiration is similar to precipitation in such conditions in the coastal plain the halite and gypsum salts precipitate subsequently the rain or the tidal flow dissolves them and they enter the water table by infiltration the lowering of the water table favors the oxidation of sedimentary pyrite and the contribution of sulfates the oversaturation of calcium carbonates in groundwater causes their precipitation both in the coastal plain and in the shell ridges the chemistry of surface water depends on the influence generated by the tidal inflow and groundwater discharge during the wet periods precipitation is higher than real evapotranspiration leading to the infiltration of water excess toward the water table and the increase in surface runoff this causes a decrease in the concentration of ions dissolved by dilution this rainfall regime hinders the formation of evaporite salts gypsum and halite and leads to the undersaturation of groundwater with respect to calcite allowing the dissolution of carbonates as regards the chemical composition of the surface water in the coastal plain the surface runoff in the drainage basin is more significant than the groundwater discharge and the tidal inflow it can be concluded that the alternation of dry and wet periods in the samborombon bay wetland causes the rainfall regime to be a determining factor in the different hydrological and geochemical processes affecting the quality of surface and groundwater

mantle layering from scs reverberations 1 waveform inversion of zeroth order reverberations this is the first paper in a four part sequence that examines the nature of mantle layering required by the multiple scs phases and internal reflections observed within the reverberative interval of sh polarized seismograms here we present path averaged estimates of the two way travel times through the crust mantle scs and crust alone m the whole mantle quality factor qscs and the reflection coefficient at the m discontinuity r0 zm obtained by iterative waveform inversion of scs reverberations we show that qscs estimates derived from direct waveform inversion can be biased significantly if reverberations originating within the crust are ignored especially in continental regions where the crust is thick applying our technique which explicitly models crustal reverberations we substantially expand the geographical coverage of scs and qscs measurements available from previous studies the scs data correlate well with whole mantle travel times predicted by the harvard tomographic models both the scs data and the qscs data support the hypothesis that variations in velocity and attenuation structure between old continents and oceans persist to depths of several hundred kilometers in particular the value of qscs derived for paths crossing stable continent is very high 280 30 at 30 mhz compared with that for mature ocean basin 189 16 consistent with the thick plate model of cratons requiring that differences in thermal structure extend to several hundred kilometers the crustal thicknesses implied by the observations of path averaged m are consistent with those inferred from short range refraction data gravity measurements and surface topography the path averaged reflectivity of the m discontinuity is also tectonically correlated becoming brighter as the percentage of continental crust encountered along the path increases this result implies that on average the impedance at the base of the continental crust is significantly lower than at the base of the oceanic crust

origin of high magnesian andesites in the setouchi volcanic belt southwest japan i petrographical and chemical characteristics high magnesian andesites of middle miocene age occur in southwest japan forming an obvious volcanic belt these andesites have low feo mgo ratios 0 546 0 931 and are rich in ni 101 312 ppm co 30 0 45 1 ppm and cr 208 756 ppm they are relatively aphyric phenocrysts 10 vol and the phenocrysts of magnesian olivine fo88 are in equilibrium with the host high magnesian andesite magmas on the basis of the fe mg exchange partitioning these features suggest that the high magnesian andesites are not differentiated or accumulative they appear to represent primary andesites generated in the upper mantle these southwest japanese high magnesian andesites are rich in incompatible elements and show light rare earth enrichment relative to boninites suggesting that the former is derived from a less depleted mantle source than the latter

evidence for forearc seafloor spreading from the betts cove ophiolite newfoundland oceanic crust of boninitic affinity abstract the ordovician betts cove ophiolite of newfoundland has a well developed cumulate sequence in which is rooted a sheeted dyke complex that grades up into pillow lavas dominant chromite olivine orthopyroxene cumulate peridotites and orthopyroxenites have phase assemblages and mineral chemistries consistent with crystallization from boninitic magmas dykes and lavas have phenocrysts of olivine high cr al chromite orthopyroxene low tio2 clinopyroxene they have high sio2 and mgo contents and depleted u shaped trace element profiles indistinguishable from those of bonin islands boninites field data imply that cumulates dykes and lavas all are comagmatic while geochemical and mineralogical data indicate that all are of boninitic affinity since boninites are only found in forearcs this implies that the betts cove ophiolitic crust formed in a forearc since the entire oceanic crustal section at betts cove is of boninitic affinity then this implies that true seafloor spreading can initiate in forearc

the boron isotopic composition of altered oceanic crust samples of drilled oceanic crust from dsdp holes 417a 417d and 418a and odp hole 735b and oceanic crust from the oman and cyprus ophiolites were analyzed for b contents and 11b composite samples from dsdp holes 417a 417d and 418a were used to represent the upper 550 m of altered oceanic crustal layer 2a whole rock samples from the troodos ophiolite cyprus and the oman ophiolite wer selected to represent crustal layer 2b dikes composite samples from odp hole 735b were used to represent crustal layer 3 the b content of the dsdp composites ranges from 7 2 ppm to 104 ppm and correlates with both 18o and k showing that it is a good indicator of the extent of low temperature alteration the 11b of the dsdp composites varies between 2 5 and 5 4 the b content of the samples from the troodos ophiolite ranges from 2 4 ppm to 8 1 ppm 11b varies from 0 9 to 7 8 the b content of the oman ophiolite samples ranges from 5 0 ppm to 11 1 ppm 11b varies from 1 6 to 16 9 the b content of the samples from odp hole 735b ranges from 1 1 ppm to 7 1 ppm 11b varies from 4 3 to 24 9 the general pattern displayed by these samples is one of greatest and most variable b enrichment at the top of the crust and least enrichment at the bottom of the section all of these samples are enriched compared to unaltered morb which is believed to have a b content of approximately 0 5 ppm the 11b values of deeper samples from layers 2b and 3 are more variable and generally higher than those from layer 2a boron contents and 11b are not correlated the data from the dsdp site 417418 composites indicate that the 11b of fluid circulating in the upper crust changes only slightly during alteration increasing by an average of 5 1 with an accompanying decrease in b concentration of 7 low temperature alteration appears to be a water dominated process resulting in minor modification of circulating seawater a minimum water rock ratio of 400 is calculated for these samples implying a minimum low temperature seawater flux through the upper oceanic crust of 3 4 1014 l y the average b content of altered oceanic crust as represented by these samples is 5 2 1 7 ppm and the average 11b is 3 4 1 1 this average isotopic composition is measurably different from the apparent average of oceanic sediments supporting the idea that 11b could be useful for identifying the source s of b in island arcs

geochemistry of the quaternary volcanic rocks of the northeast japan arc major and trace element data are presented for a series of lavas from 17 volcanic centres in the ne japan arc these represent a transect of the quaternary arc from its volcanic front type a volcanoes in the east across its central zone type b to its western margin type c rocks range from basalt to dacite in composition and the variation is attributed to fractionation of plagioclase mafic silicates and titanomagnetite for comparative purposes trace element data for each volcano are presented on a 55 wt sio2 normalised basis the volcanoes display the characteristic features of arc volcanism viz enrichment in large ion lithophile elements rb sr ba k and depletion in high field strength elements zr nb hf ta using these data and derived primary magma compositions tatsumi et al 1983 mixing calculations give the percentages of fractionating mineral phases necessary to derive the 55 wt sio2 liquids these data together with published distribution coefficients allow the calculation of trace element abundances in the primary magmas reciprocal trace element plots indicate that irrespective of position with respect to the volcanic front the elements la ba k sr nb y and zr of the primary magmas all lie on a common line strongly suggesting that the melts are the products of varying degrees of melting of a common homogeneous source however those melts close to the volcanic front type a show anomalous rb th and pb values suggesting that their mantle source was enriched in these elements further calculations suggest that the hfs depletion is not due to retention by residual mineral phases but is a feature of the source thus the data suggest a that the source of the arc volcanism was homogeneous and b that the characteristic chemical features were an inherent feature of the source since zr nb ratios in the source melts are close to those found in n type morb it is suggested that a major component of arc volcanic source rocks was a depleted mantle of the type which has given melts of n type morb composition lil elements added to such a depleted source are derived by the loss of silica rich aqueous fluids from the descending subducting slab such losses occur within the first 100 km and are fixed in the overlying mantle wedge this contaminated material is transported down by drag induced convection producing a homogeneous but metasomatised mantle within the melting zone diapiric uprise produced by small scale melting produces further melting with the major control of melt chemistry being the degree of partial melting superimposed on this chemistry is a localised addition of pb rb and th in the source of those volcanoes nearest the volcanic front

trace elements in anorthite megacrysts from the kurile island arc a window to across arc geochemical variations in magma compositions abstract anorthite megacrysts from twelve volcanic centers of the kurile island arc were analyzed by the ion microprobe for concentrations of li be b f mg p cl k ti fe co rb sr y zr nb cs ba la ce pr nd sm eu and pb these 1 2 cm megacrysts are mainly found as early phenocrysts in basalts but are also present in andesites rhyolites and hybrid rocks from almost every studied volcanic center both subaerial and submarine electron and ion microprobe studies show that the cores of these megacrysts are homogeneous and unzoned whereas rims often result from overgrowth of sodium richer plagioclase in equilibrium with its host silica richer melt the cores of these megacrysts can be used to see back through fractional crystallization and magma mixing and serve as a window into an earlier basaltic stage of magmatic evolution petrological observations and numerical crystallization modeling indicate that highly anorthitic plagioclase is the first or second after olivine liquidus phase in high al basalts regardless of varying k2o contents across the arc given the narrow compositional range of anorthite megacrysts the across arc chemical variations are well displayed by direct comparison of trace element concentrations in the megacrysts across the arc the concentration of k in megacrysts increases linearly by a factor of 11 ba 13 sr eu 4 be ti p 3 li 2 ree 5 2 whereas concentrations of pb b and halogens and their ratios to hfse either remain constant or decrease with deepening of the benioff zone from 110 to 220 km we used ion microprobe derived plagioclase basalt partition coefficients to convert trace element concentrations in megacrysts to parental magmatic values the reconstructed concentrations of several key trace elements and their correlation with benioff zone depth are consistent with whole rock basalt data but all give systematically more primitive compositions therefore anorthite megacrysts are helpful for retrieving the chemistry of parental basalts which have subsequently suffered variable differentiation and mixing they can be used to assess delicate variations in primary basaltic magma chemistry as a function of benioff zone depth we also report new results for sr nd and pb isotopes for anorthite bearing rocks and other rocks from the same volcanoes across arc zoning with respect to trace element concentrations ratios and isotopic values is largely linear and requires superposition of several petrogenetic processes trace element transfer by fluids variation in degree of partial melting and progressive change of the melting source

subduction products according to experimental prediction knowledge of the thermal structure and processes in subduction zones is so uncertain that our more extensive knowledge of source materials and phase relationships remains insufficient for prediction of magmatic products the phase relationships do provide important constraints for testing geophysical and petrogenetic models especially when considered along with geochemical constraints physical questions such as the behavior of partially molten rocks are intimately related to the compositions of magmas from source to eruption site tentative predictions and conclusions include 1 andesite is not a primary magma from oceanic crust 2 it is unlikely that liquids from subducted oceanic crust yield andesites by fractionation except by melting of amphibolite in an especially warm subducted crust 3 it is more likely that hydrous siliceous magmas from the crust leak into the overlying mantle 4 andesite magmas can be generated in peridotite only under exceptional circumstances 5 h2o undersaturated basic magmas from peridotite modified by aqueous fluids or hydrous magma may yield andesite by fractionation 6 phlogopite peridotite could escape melting and yield alkalic magmas at deeper levels 7 andesite magmas could be produced in continental crust only by extreme heating through underplating of basalts or by mixing of basalt and rhyolite derived from crust most of these conclusions need revision if convection in the asthenosphere wedge raises the temperature of the subducted slab to 1250 c at 100 km as proposed by marsh 1979a this demonstrates the dependence of experimental predictions on geophysical models

sediment recycling and crustal growth in the central asian orogenic belt evidence from sr nd hf isotopes and trace elements in granitoids of the chinese altay felsic igneous rocks are common constituents of volcanic arcs and contain valuable information about subduction related magmatism in this study we investigate nine granitoids with s type volcanic arc affinity from the chinese altay emplaced from 507 to 391 ma in an active subduction zone during the early middle paleozoic these granitoids are characterized by moderate to high sio2 contents 61 01 75 30 wt moderate total alkalis na2o k2o 3 43 7 64 wt and high al2o3 contents 13 29 17 18 wt negative epsilon nd t values 6 1 to 1 0 the wide range of epsilon hf t values 7 0 to 9 0 and enrichment of liles such as pb th and u all suggest that the granitoids were probably derived from the partial melting of subducting oceanic sediments and the associated mantle wedge this inference is further supported by the nd isotope data the high initial sr 87 sr 86 ratios 0 703963 0 719428 low ba th ratios 7 00 118 93 and uniformly negative end t values 6 1 to 1 0 indicate that slab derived aqueous fluids were vital in generating the initial magma of these granitoids and assimilation played only a minor role our data demonstrate that residual zircon retains a substantial amount of hf during the partial melting of oceanic sediments therefore hf may not be an effective tracer for the input of recycled sediments we conclude that sediment recycling played an important role in the generation of arc magmatism and the growth of the central asian orogenic belt caob c 2016 international association for gondwana research published by elsevier b v all rights reserved

the mafic layered complex of the kabye massif north togo and north benin evidence of a pan african granulitic continental arc root abstract between the predominantly neoarchean paleoproterozoic west african craton wac and the saharian metacraton smc the dahomeyides suture zone represents a valuable witness of continental amalgamation during pan african times in the kabye massif northern togo and northern benin mainly granulitic metagabbros associated with al rich kyanite and garnet bearing felsic dykes are exposed as tectonic lenses within the pre pan african gneisses of the smc new geochemical data suggest that the high grade rocks granulites in the kabye massif originated in a mature continental arc setting afc calculations constrain the amount of contamination of mantle wedge derived magmas by crustal metasediments to about 10 reconstruction of initial magmatic stratigraphy has been carried out using cipw norms for cumulitic sequences within the massif indicating a normal igneous polarity from west to east published geochemical signatures along the pan african suture zone of the dahomeyides in ghana togo and benin constrain the origin of mafic granulitic and eclogitic complexes a distinction is made between bodies with mainly morb signatures originated from the subducted wac plate passive margin and those with magmatic arc signatures originated from the over riding plate mantle wedge this suggests that the closure of the oceanic domain between the wac and the smc from 640 to 610 ma was mainly accommodated by oceanic subduction beneath the active continental margin before pan african collision

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

transportation of beryllium with h2o at high pressures implication for magma genesis in subduction zones the degree of transportation of beryllium with h2o is measured under the upper mantle p t conditions in order to examine the incorporation of 10be in subduction zone magmas 5 4 0 3 of original beryllium in the starting serpentinite is extracted with h2o from the system through the dehydration reaction of serpentine at 850 c and 1 5 gpa it follows that the partial melting of oceanic lithosphere including 10be rich sediment layer is not necessarily required for inclusion of the isotope in island arc lavas we suggest that the dehydration processes in the subducted slab and mantle wedge materials dragged downward by the slab subduction govern the concentration of 10be in arc lavas

alteration of oceanic crust and geologic cycling of chlorine and water we report new estimates of transport rates for h2o and cl between the mantle and surface reservoirs our estimates take into consideration alteration of oceanic crust especially that of plutonic rocks and possible subduction of sediments the effect of hydrothermal alteration on the cl budget seems to be negligible but the effect on the h2o budget is significant altered oceanic crust excluding sediments contains about 10 times as much h2o as the unaltered crust and its subduction may result in a net transport of h2o to the upper mantle in subduction zones however the rate of expulsion of h2o from the mantle by subduction zone magmatism is comparable to the amount released by ridge magmatism and is only about 10 of the amount subducted therefore about 90 of the subducted h2o must be returned to the mantle or returned to the crust by other processes in addition subduction of oceanic sediments to mantle depths will result in 1 1 a further increase in the return rate of h2o to the mantle reservoir 2 2 possible net transfer of cl to the mantle depending on the rate of pore water expulsion

a seismological constraint on the depth of basalt eclogite transition in a subducting oceanic crust the oceanic crust primarily has a basaltic composition1 2 which must transform into eclogite on subduction of the oceanic crust beneath continents3 4 there has been a considerable debate about the role of the basalt eclogite transformation in driving lithospheric plate motion5 and in the thermal regieme of subducting plates6 how important this transformation is in tectonics critically depends on the depth of transformation which is affected by factors such as temperature7 8 bulk rock chemical composition2 water content9 and perhaps stress environment becuase it would be practically impossible to evaluate all these factors correctly we attempt here to constrain the transformation depth by a more direct seismological method

corrigendum to lu hf and sm nd garnet geochronology chronometric closure and implications for dating petrological processes earth planet sci lett 381 2013 222 233

volatility of se and te during subduction related distillation and the geochemistry of epithermal ores of the western united states subduction partial melting of mantle and magma generation and ultimately magma degassing can lead to the preferential mobilization of volatile elements and compounds research indicates that devolatilization reactions in a subducted slab can cause the overlying mantle to be enriched in volatile ore forming constituents water co2 sulfur metal loid s this process of mantle enrichment plays an important role in subsequent magma generation and initiation of ore genesis furthermore the relative volatility of metal loid s appears to play a role in the ultimate geochemistry of shallow epithermal epizonal hydrothermal ores in the western united states for example there is a general west to east geographic zonation of tertiary ores from se rich to te rich it is proposed that this geographic geochemical zonation is the direct result of the higher geochemical volatility of se as compared to te previously au ag te se hg as sb and tl have been referred to as the epithermal suite of elements it is suggested here that their geochemical cycles are linked by their similar volatile behavior through subduction partial melting in the mantle and subsequent magma degassing processes essentially specific tectonomagmatic events that affected the western united states e g oligocene eocene magmatism in the great basin and southern rocky mountains mid miocene magmatism associated with the inception of the yellowstone hotspot in the pacific northwest were distillation events that drove devolatilization of fertile mantle after the laramide shallow slab event furthermore the metal loid volatility behavior likely occurs in other regions affected by subduction and may play a fundamental role in the metal endowment of these regions

kinematics and correlation of the surface wind field in the south atlantic bight surface winds observed at charleston south carolina and at an offshore buoy located some 300 km to the east of charleston are analyzed to determine operational transformations for estimating the offshore winds from the coastal winds both the statistics describing the wind field at each station along with those describing the correlation between stations are found to be seasonally modulated ensemble averaged by season transfer functions computed by using the first 3 years of available data are applied to a fourth independent data year for assessing their predictive utility the transfer functions and predictability are both seasonally modulated and the transformation matrix is nondiagonal each offshore wind velocity component depends upon both coastal components

compositions of aqueous fluids in equilibrium with pyroxenes and olivines at mantle pressures and temperatures solubility experiments were performed at 30 kbars in the system mg2sio4 sio2 h2o and at 20 and 30 kbars on omphacitic pyroxene water mixtures they confirm that the solubility of the forsterite component in aqueous fluids remains rather low up to 5 wt whereas the solubility of the sio2 component from solids of appropriate sio2 rich compositions in the system mg2sio4 sio2 h2o increases with temperature up to some 75 at 1 100 c at this temperature a simplified harzburgite consisting of forsterite and enstatite coexists with a fluid containing about 35 mgo sio2 hydrous fluids coexisting with omphacitic clinopyroxenes leach sodium silicate component from the solid leaving less jadeitic pyroxenes behind most interestingly the amount of sodium leached at constant temperature increases with decreasing pressure comparison of the results with previous solubility studies in the system k2o mgo al2o3 sio2 h2o indicates that hydrous fluids in the mantle must be alkaline rather than silicanormative alkali metasomatism caused by such fluids would lead to potassium enrichment in deeper portions of the upper mantle and to sodium enrichment at shallower levels where amphiboles become stable this k na fractionation in the upper mantle may explain the generation of k rich or of na rich magmas through partial melting at different depths

the subduction of young lithosphere studies using a variety of techniques of the subduction beneath western south america and the southwest honshu kyushu region of japan indicate that volcano free segments occur where the subducted and continental lithospheres remain in contact without intervening asthenosphere the subduction is initiated at normal dip angles but the plate deforms at some depth 100 km under central peru to travel horizontally immediately beneath continental lithosphere the most plausible reason for this geometry is that the subducted plate is buoyant a model is developed constrained by age of the plate bathymetry and heat flow estimates of the density of oceanic plates as a function of age show that younger ocean floor may be less dense than the asthenosphere into which it subducts if the high density tranformation of crustal basalt to eclogite is retarded by low temperatures the plate can remain buoyant to considerable depth for long periods of time heat flow data from western south america are consistent with this model

variation in noble gas isotopic composition of gas samples from the aegean arc greece abstract in contrast to most other arcs with oceanic plate subduction the aegean arc is characterized by continent continent subduction noble gas abundances and isotopic compositions of 45 gas samples have been determined from 6 volcanoes along the arc 2 islands in the back arc region and 7 sites in the surrounding areas the 3he 4he ratios of the samples ranged from 0 027ra to 6 2ra ra denotes the atmospheric 3he 4he ratio of 1 4 10 6 demonstrating that even the maximum 3he 4he ratio in the region is significantly lower than the maximum ratios of most oceanic subduction systems which are equal to the morb value of 8 1 ra regional variations in the 3he 4he ratio were observed both along and across the arc the maximum 3he 4he ratio was obtained from nisyros volcano located in the eastern end of the arc and the ratio decreased westward possibly reflecting the difference in potential degree of crustal assimilation or the present magmatic activity in each volcano across the volcanic arc the 3he 4he ratio decreased with an increasing distance from the arc front reaching a low ratio of 0 063ra in macedonia which suggested a major contribution of radiogenic helium derived from the continental crust at nisyros a temporal increase in 3he 4he ratio due to ascending subsurface magma was observed after the seismic crisis of 1995 1998 and mantle neon was possibly detected the maximum 3he 4he ratio 6 2ra in the aegean region which is significantly lower than the morb value is not probably due to crustal assimilation at shallow depth or addition of slab derived helium to morb like mantle wedge but inherent characteristics of the subcontinental lithospheric mantle sclm beneath the aegean arc

magma in forearcs implication for ophiolite generation abstract forearc areas non volcanic arcs of contemporary island arcs at convergent plate boundaries contain magmatic rocks geological evidence seismic profiles heat flow data density considerations and petrological and geochemical arguments suggest that a forearc tholeiitic association fat containing high mg calc alkaline andesites is present in non volcanic arcs at some stage of island arc development the fractionated as well as primitive magma is unable to penetrate low density sediments and underplates thick piles of unconsolidated accreting rocks the underplating causes upwelling the occurrence of magma in forearcs provides an alternative interpretation for the tectonic setting of some ophiolitic masses rather than ocean ridge formation and later obduction it offers an autochthonous island arc bound and geologically substantiated interpretation for the ophiolite suite

influence of artificial channels on the source and extent of saline water intrusion in the wind tide dominated wetlands of the southern albemarle estuarine system usa saline water intrusion is degrading water quality in the channelized coastal wetlands of the southern albemarle estuarine system aes the source transport and fate of the saline water in the southern aes was determined by monitoring specific conductivity and water levels in small artificial channels the alligator river the alligator pungo canal and the groundwater system for 12 months results indicate that water levels are affected by wind tides which trigger the movement of saline water into the interior of the wetlands via the small canals the wind tides are mostly driven by episodic southerly winds pushing saline water into the canal network and the groundwater regime proximal to the alligator river the saline waters persist in the canals as long as the wind tide events last specific conductivities from canals and groundwater are shown to be unexpectedly higher closer to the source of the alligator river than toward the albemarle sound suggesting that the large alligator pungo canal facilitates the northward migration of saline water from the pamlico sound to the alligator river overwash and reversals in the flow directions between groundwater and surface water bodies suggest that saline water that is present during wind tide events may migrate into the groundwater system from surface water bodies the results of this study reveal that whereas the large alligator pungo canal channels saline water to the aes small artificial channels may also play significant roles in degrading water quality in the interior of channelized coastal wetlands

velocity structure of the pamir hindu kush region possible evidence of subducted crust the arrival times of compressional p and shear s waves from approximately 580 microearthquakes recorded by a temporary array in the pamir hindu kush region in central asia are used to deduce one and three dimensional velocity structures of this region the results for one dimensional structures imply that the moho is at 70 5 depth also there is a velocity reversal near 160 km depth which is inferred to be the beginning of the low velocity zone this reversal continues to depths of approximately 230 km below 230 km velocities are somewhat higher than those of normal mantle at similar depths 9 3 km s versus 8 4 km s for p waves the outstanding feature of the results for three dimensional velocity structures is a broad 40 km centrally located region with 8 to 10 lower velocities than those in the surrounding regions this low velocity region envelopes the seismic zone at depths between 70 and 150 km the region may actually extend beyond these depths but the results for shallower and deeper structure lack sufficient resolution to decide several tests using both hypothetical and real data were performed to estimate the reliability of the three dimensional solutions the results of these tests suggest that the inferred velocities are reasonably accurate representations of the average velocities in the blocks although one must be cautious of the effects of averaging in interpreting the solution the low velocity region is inferred to be a manifestation of substantial quantities of subducted continental crust therefore while subduction has occurred in the pamir hindu kush the results of the three dimensional inversions suggest that continental rather than only oceanic lithosphere has been subducted to depths of at least 150 km appendices are available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j81 013 1 00 payment must accompany order

structure of the benioff zone beneath the shumagin islands alaska relocation of local earthquakes using three dimensional ray tracing seismic rays are traced through a prescribed three dimensional inhomogeneity that simulates the subducted slab below the shumagin islands region to calculate local station delays for a given hypocenter and a slab model hypocenters determined by the shumagin seismic network are then relocated using the station delays a flat layered velocity structure and a standard earthquake location computer program station delays are calculated for 12 hypocenters with respect to six different slab models to identify the slab model that is most consistent with the available arrival time and waveform data a set of path corrections that is calculated for each grid point station pair on a preliminary grid of 36 points in the depth range from 60 to 300 km is used to recalculate the hypocenters for all of the 1982 earthquakes with depths greater than 50 km application of this method to data from 1982 for the digitally recording shumagin seismic network shows the following results 1 a previously observed apparent increase in dip of the subducted slab at depths of 100 km disappears 2 the subducted slab can be modeled as a dipping structure that dips at a constant angle of 45 toward north northwest at depths between 80 and 250 300 km and has a 7 higher velocity than the surrounding mantle 3 hypocenters determined from shumagin network data are located only 10 20 km south of high quality hypocenters determined from teleseismic data alone 4 qualitative comparison of digitally recorded seismograms with calculated ray paths shows enrichment of high frequency coda possible converted phases and low amplitudes of first p arrivals for rays that travel mostly along the slab conversely for rays that travel almost vertically through the upper plate the seismograms show a high amplitude of first p arrivals that are followed by an insignificant coda and low amplitude s waves

origin of the granodiorite in the forearc region of southwest japan melting of the shimanto accretionary prism abstract in the forearc region of southwest japan felsic to intermediate volcano plutonic complexes of middle miocene age intruded into the shimanto accretionary complex possibly in association with megatectonic events such as the opening of the japan sea and resultant underthrusting of the newly borne shikoku basin philippine sea plate beneath the southwest japan arc activities of mantle derived magmas are also distinct in the forearc of middle miocene southwest japan tholeftic basalts are closely related to igneous activity of the shikoku basin and high mg andesites probably formed under anomalously high temperature conditions caused by the opening of japan sea two plutons uwajima and miuchi were chosen for understanding the origin of felsic magmas in the forearc region of southwest japan all granitic rocks of the plutons show enriched isotopic characteristics with higher 87 sr 86 sr and lower 143 nd 144 nd tha bulk earth which suggests that the contribution of crustal materials has a magma source however they have lower 87 sr 86 sr and higher 143 nd 144 nd than shimanto sediments and hence an additional component with lower 87 sr 86 sr and higher 143 nd 144 nd than granitic rocks must be required possible candidates for such components are coeval mantle derived magmas mass balance modelling between mantle derived magmas and shimanto sediments on the basis of their sr nd isotopic compositions suggested that mixing of anatectic melt of the shimanto sediments and porphyritic andesite magma evolved from high mg andesite as a possible mechanism of the origin of the granodiorites of the uwajima pluton injection of high mg andesite into the shimanto accretionary prism may triggered the melting of sediments calculated composition of anatectic melt of the shimanto sediment and p t condition of the gneissose enclave in the granodiorite suggest that the melting of sediments may have occurred at higher temperature conditions than those of the melting reactions involving biotite breakdown

on the origin of high alumina arc basalt and the mechanics of melt extraction most models of high alumina arc basalt petrogenesis rely heavily on the supposition that the abundances of certain trace elements in particular the relatively unfractionated rare earth element ree patterns and the unusually high concentrations of k rb sr and ba are incompatible with a garnet bearing subducted oceanic crustal quartz eclogite source rock we have carefully examined this apparently unequivocal evidence in light of recent progress on the physics of melt extraction and the heat transfer and mechanics of magma ascent the weakest element of all trace element models involving a quartz eclogite source is the assumption that the element concentrations are fixed at the source and only later modified in the near surface environment we expand on such models by monitoring the concentrations of r ee and major and trace elements during initial melting ascent and extraction of magma this is done by combining calculated cooling curves for ascending magmatic bodies with high pressure phase equilibria the amount that each phase contributes to the melt is monitored along with the composition of the melt and residual solids with quartz eclogite initial melting initiates gravitational instability of the entire source material melt plus solids before melt extraction can occur during ascent of this mush melting increases until the solids can be repacked to free the melt this extraction takes place some 15 20 km above the slab after about 50 per cent melting at which point the melt has a pattern of ree and other trace element concentrations almost identical to those observed in high alumina arc basalts assuming an initial composition equivalent to altered oceanic crust plus 5 per cent pelagic sediment sr abundances are the only ones which are not well matched by this process the major element concentrations of the extracted melt also closely match those of high alumina arc basalt a similar but less detailed evaluation of both fertile and depleted peridotite source rocks yields good agreement for the ree and other trace element concentrations assuming a lree enriched source rock strongly enriched in k rb sr and ba ni cr and co abundances are satisfied only through substantial low pressure fractionation of mafic phases in particular olivine though not rigorously tested such a process may be compatible with the observed major element concentrations of high alumina basalt however the experimentally verified fact that high alumina basalts could never have been in equilibrium with either an olivine beari ng magma or source rock eliminates this possibility altogether thus the simultaneous consideration of the mechanics of ascent and melt extraction along with phase equilibria clearly shows that partial melting of quartz eclogite best satisfies the chemical constraints of major trace and ree characteristics of high alumina arc basalts